REPUBLIC OF KENYA

...

MINISTRY OF NATURAL RESOURCES

Geological Survey of Kenya

Report No. 84

GEOLOGY OF THE LAISAMIS~ AREA

DEGREE SHEET 28, NE. QUARTER

(with coloured geologicalmap)

by

R. P. Randel, B.A.Geologist

Price: She 13/50 - 1970

PRINTED BY TIlE GOVERNMENT PRINTER, NAIROBI

r ~--

."

= =~::::.~

REPUBLIC OF KENYA

...

MINISTRY OF NATURAL RESOURCES

Geological Survey of Kenya

Report No. 84

GEOLOGY OF THE LAISAMIS~ AREA

DEGREE SHEET 28, NE. QUARTER

(with coloured geologicalmap)

by

R. P. Randel, B.A.Geologist

Price: She 13/50 - 1970

PRINTED BY TIlE GOVERNMENT PRINTER, NAIROBI

r ~--

."

= =~::::.~

Report No. 84

REPUBLIC OF KENYA

MINISTRY OF NATURAL RESOURCESGeological Survey of Kenya

GEOLOGY OF THE LAISAMISAREA

DEGREE SHEET 28, NE. QUARTER(with coloured geological map)

by

R. P. Randel, B.A.Geologist

Price: Sh. 13/50 — 1970

inififiB—a—Y—THE GOVERNMENT yawn-m, NAIROBI

"\

CONTENTSABSTRACT

I INTRODUCTION

PAGE

II PREVIOUS GEOLOGICAL WORK

III PHYSIOGRAPHY

2

3

IV SUMMARY OF GEOLOGY

V DETAILS OF GEOLOGY..

6

7

I. Basement System. .1. METAMORPHOSED CALCAREOUS SEDIMENTS. .

2. METAMORPHOSED SEMI-PELITIC SEDIMENTS. .

3. METAMORPHOSED PSAMMITIC SEDIMENTS..

4. METAMORPHOSED CARBONACEOUS SEDIMENTS

7889

10

10

13

13

19

2. Intrusives into the Basement System

3. Tertiary Sediments . . . .

4. Pleistocene and Recent Volcanic Rocks. .

5. Superficial Deposits

VI METAMORPHISM..

VII STRUCTURE

20

20

VIII ECONOMIC GEOLOGY..

IX REFERENCES

21

23

ILLUSTRATIONS

Fig. I-Physiographic map of the Laisamis area

Fig. 2-Sketch-map of the volcanic history of the Laisamis area

4

15

MAP

Geological map of the Laisamis area (degree sheet 28, N.E. quarter) scale 1:125,000 . . at end.

I

"\

CONTENTSABSTRACT

I INTRODUCTION

PAGE

II PREVIOUS GEOLOGICAL WORK

III PHYSIOGRAPHY

2

3

IV SUMMARY OF GEOLOGY

V DETAILS OF GEOLOGY..

6

7

I. Basement System. .1. METAMORPHOSED CALCAREOUS SEDIMENTS. .

2. METAMORPHOSED SEMI-PELITIC SEDIMENTS. .

3. METAMORPHOSED PSAMMITIC SEDIMENTS..

4. METAMORPHOSED CARBONACEOUS SEDIMENTS

7889

10

10

13

13

19

2. Intrusives into the Basement System

3. Tertiary Sediments . . . .

4. Pleistocene and Recent Volcanic Rocks. .

5. Superficial Deposits

VI METAMORPHISM..

VII STRUCTURE

20

20

VIII ECONOMIC GEOLOGY..

IX REFERENCES

21

23

ILLUSTRATIONS

Fig. I-Physiographic map of the Laisamis area

Fig. 2-Sketch-map of the volcanic history of the Laisamis area

4

15

MAP

Geological map of the Laisamis area (degree sheet 28, N.E. quarter) scale 1:125,000 . . at end.

I

CONTENTSABSTRACT

I INTRODUCTION

II PREVIOUS GEOLOGICAL “'ORK

III PHYSIOGRAPHY

l\ SL'NIMARY OF GEOLOGY

\' DETAILS OF GEOLOGY ..

I, Basement System1‘ .\-'It-:I_AMURPH051[) CALCARLEJL'S SLDIMLMS . .I \‘ILIAMURPHUSLD SLVII—PLLIIIC SLDI\1L\ES3. IN-ILIAMURPHUSLD PSAMMIIJL' Smmrws4 ML'MMURPHUHLD C;\l{li(,JNA(I-{)LES SE Dim-MS

Eu lntrusives into the Basement 53stem

DJ Tertiary Sediments

Pleistocene and Recent Volcanic Rocks . ,

111.4-

Superficial Deposits

VI METANIORPHISNI ..

\'Il STRLJC’I'L'RE

VIII [C(‘UNUNIH' GEULUG‘I

[X REFERENCES

ILLI'SFRAHUNS

Hg. 1,, Phyx’iographie map of the Lainamis area

Fig. 2 Sketch-map of the volcanic history of the Laisamis area

M A P

(Ecological map of the Lajsamis area {degree ShC‘t 18‘ NF. quarter 1 scale 1 115.000 . .

PAGE

atencl.

ABSTRACf

The report describes an area in northern Kenya, approximately 1,200 square milesin extent, bounded by the meridians 37°30'E and 38°00'E and by latitudes 1°30'Nand 2°00'N. Physiographica1ly the area is divided into three units, the lava-cappedplateaux:; the dissected plain surface; the residual hill masses some of which have levelsapproximating to the sub-Miocene erosion bevel.

The rocks of the area include Precambrian gneisses and granulites; premetamorphicbasic, ultra basic and acid intrusions; Tertiary sediments; Pleistocene volcanic rocks;Recent volcanic rocks; superficial deposits of Pleistocene and Recent age. The petro-graphy of the various rock types is described and the structure and metamorphismof the Basement System rocks is discussed. A chapter is devoted to the economicminerals and water supplies of the area.

[

ABSTRACf

The report describes an area in northern Kenya, approximately 1,200 square milesin extent, bounded by the meridians 37°30'E and 38°00'E and by latitudes 1°30'Nand 2°00'N. Physiographica1ly the area is divided into three units, the lava-cappedplateaux:; the dissected plain surface; the residual hill masses some of which have levelsapproximating to the sub-Miocene erosion bevel.

The rocks of the area include Precambrian gneisses and granulites; premetamorphicbasic, ultra basic and acid intrusions; Tertiary sediments; Pleistocene volcanic rocks;Recent volcanic rocks; superficial deposits of Pleistocene and Recent age. The petro-graphy of the various rock types is described and the structure and metamorphismof the Basement System rocks is discussed. A chapter is devoted to the economicminerals and water supplies of the area.

[

A BSTRA CI‘

The report describes an area in northern Kenya‘ approximately 1.200 square milesin extent. bounded by the meridians 37 ME and 38:00’E and by latitudes 1"30’Nand E'OO‘N. Physiographieally the area is divided into three units. the lava—cappedplateaux: the dissected plain surface: the residual hill masses some of which have levelsapproximating to the sub—Miocene erosion bevel.

The rocks of the area include Precambrian gneisses and granulites; premetamorphichasic. ultrabasic and acid intrusions; Tertiary sediments: Pleistocene volcanic rocks:Recent volcanic rocks: superficial deposits of Pleistocene and Recent age. The petro-graphy of the various rock types is described and the structure and metamorphismof the Basement System roclu is discussed. A chapter is devoted to the economicminerals and water supplies of the area.

1-INTRODUCflONGeneral

The area described in this repart is appraximately 1,200 square miles in extent andis baunded by latitudes. 1°30'N and rOO'N, and by meridians 37°30'E and 38°00'E(Directarate af Overseas Surveys sheet NO'. 67). The Laisamis area is part af theEastern Pravince af Kenya, with administrative headquarters at Embu, being directlycantrolled by the District Cammissianer, Marsabit.

The gealagical survey was carried aut between Navember 1962 and February 1963as part af the reconnaissance mapping pragramme af Kenya carried aut by the Minesand Gealagical Department.

Communications

The main Isiala-Marsabit raad which runs thraugh the area is kept in a gaad stateaf repair. It is, hawever, a dry-weather raad anly and mavement af vehicles is natpermitted during the rainy peri ads. The ather tracks in the area are nat maintainedbut it is passible to' matar alang them in faur-wheel-drive vehicles withaut any greatdifficulty. Far thase portians af the area withaut tracks the relatively flat nature afthe graund and the thinness af the bush caver make mavement camparatively easy.

Rainfall and Vegetation

NO' rainfall recards exist far this area, but it is abviaus from the barren nature afthe terrain that the amaunt af rain in any ane year is very small and is probably inthe regian af 10 inches ar less per year, precipitatian accurring in twO' shart periadsin Navember and April.

As a result af the paucity af rain the vegetatian is sparse and is campased mainlyaf stunted tharn trees. Alang the banks af the larger sand rivers there is a mareprolific growth af acacia and daum palms.

Map.I'

The anly map available at the time af the survey was the 1: 250,000 sheet MaralalN.E., E.AR. NO'. 1169, campiled by the E,A. Survey Graup in 1941 and revised inOctaber 1942, which proved inadequate far the gea1agical survey. The tapography afthe printed map accampanying this report is based an air phatagraphs at a scale afapproximately 1: 40,000 taken in 1956 by Hunting Aerosurveys Ltd. A base map wascampiled by the authar at a scale af 1: 83,333 using these phatagraphs cantrolled byexisting triganametrical statians and secandarypoints surveyed by plane table. Farmlines an the map are based an spot heights taken with an aneraid barometer, thereadings being based an the heights af the triganametrical statians and suitably carrectedfar diurnal variatians, and must be regarded as anly appraximate. Geolagical infar-matian was platted in the field directly an to' air phatagraphs and trartsferred to' thebase map, the final draft af which was reduced mechanically to' the printed scale af1: 125,000.

Population

The majarity of the papulatian belang to' the RendiIle tribe whO' are a namadicpea pie and whase wanderings are controlled by the locatian af water far their stack.In the western partian af the area there are a few members af the Samburu tribe.

1-INTRODUCflONGeneral

The area described in this repart is appraximately 1,200 square miles in extent andis baunded by latitudes. 1°30'N and rOO'N, and by meridians 37°30'E and 38°00'E(Directarate af Overseas Surveys sheet NO'. 67). The Laisamis area is part af theEastern Pravince af Kenya, with administrative headquarters at Embu, being directlycantrolled by the District Cammissianer, Marsabit.

The gealagical survey was carried aut between Navember 1962 and February 1963as part af the reconnaissance mapping pragramme af Kenya carried aut by the Minesand Gealagical Department.

Communications

The main Isiala-Marsabit raad which runs thraugh the area is kept in a gaad stateaf repair. It is, hawever, a dry-weather raad anly and mavement af vehicles is natpermitted during the rainy peri ads. The ather tracks in the area are nat maintainedbut it is passible to' matar alang them in faur-wheel-drive vehicles withaut any greatdifficulty. Far thase portians af the area withaut tracks the relatively flat nature afthe graund and the thinness af the bush caver make mavement camparatively easy.

Rainfall and Vegetation

NO' rainfall recards exist far this area, but it is abviaus from the barren nature afthe terrain that the amaunt af rain in any ane year is very small and is probably inthe regian af 10 inches ar less per year, precipitatian accurring in twO' shart periadsin Navember and April.

As a result af the paucity af rain the vegetatian is sparse and is campased mainlyaf stunted tharn trees. Alang the banks af the larger sand rivers there is a mareprolific growth af acacia and daum palms.

Map.I'

The anly map available at the time af the survey was the 1: 250,000 sheet MaralalN.E., E.AR. NO'. 1169, campiled by the E,A. Survey Graup in 1941 and revised inOctaber 1942, which proved inadequate far the gea1agical survey. The tapography afthe printed map accampanying this report is based an air phatagraphs at a scale afapproximately 1: 40,000 taken in 1956 by Hunting Aerosurveys Ltd. A base map wascampiled by the authar at a scale af 1: 83,333 using these phatagraphs cantrolled byexisting triganametrical statians and secandarypoints surveyed by plane table. Farmlines an the map are based an spot heights taken with an aneraid barometer, thereadings being based an the heights af the triganametrical statians and suitably carrectedfar diurnal variatians, and must be regarded as anly appraximate. Geolagical infar-matian was platted in the field directly an to' air phatagraphs and trartsferred to' thebase map, the final draft af which was reduced mechanically to' the printed scale af1: 125,000.

Population

The majarity of the papulatian belang to' the RendiIle tribe whO' are a namadicpea pie and whase wanderings are controlled by the locatian af water far their stack.In the western partian af the area there are a few members af the Samburu tribe.

I—INTRODUCTIONGeneral

The area described in this report is approximately 1.200 square miles in extent andis bounded by latitudes. 1‘30’N and Z'OU’N. and by meridians 37‘30’11'. and 38"(_l()’l:‘(Directorate of Overseas Surveys sheet .\'o. 67). The Laisamis area is part of theEastern Province of Kenya. with administrative headquarters at 'i-lmbu, being directlycontrolled by the District Commissioner. Marsabit.

The geological survey was carried out between November 1962 and February 1963as part of the reconnaissance mapping programme of Kenya carried out by the Minesand Geological Department.

C.'tmummit'urimrt'

The main lsiolo-Marsabit road which runs through the area is kept in a good stateof repair. It is. however. a dry-weather road only and movement of vehicles is notpermitted during the rainy periods. The other tracks in the area are not maintainedbut it is possible to motor along them in four-wheel-drive vehicles without any greatdiliiculty. For those portions of. the area without. tracks the relative]y flat nature of.the ground and the thinness of the bush cover make movement comparatively easy.

Rainfall and l“"egcmiioriN'o rainfall records exist for this area. but it. is obvious from the barren nature of

the terrain that the amount of rain in any one year is very small and is probably inthe region of 11) inches or less per year. precipitation occurring in two short periodsin November and April.

As a result of the paucity of rain the vegetation is sparse and is composed mainlyof stunted thorn trees. Along the banks of the larger sand rivers there is a moreprolific growth of acacia and doum palms.

.MrtptThe only map available at the time of the surVey was the 1150.000 sheet Maralal

NE" E.A.R. No. 1169. compiled by the EA. Survey Group in 1941 and revised inOctober 1942. which proved inadequate for the geological survey: The topography ofthe printed map accompanying this report is based on air photographs at a scale ofapproximately 1:4(l,()(l() taken in 1956 by Hunting Aerosurveys Ltd. A base map wascompiled by the author at a scale of 1:83.333 using these photographs controlled byexisting trigonometrical stations and secondary points surveyed by plane table. Formlines on the map are based on spot heights taken with an aneroid barometer. thereadings being based on the heights of the trigonometrical stations and suitably correctedfor diurnal variations. and must be regarded as only approximate. Geological infor—mation was plotted in the field directly on to air photographs and transferred to thebase map, the final draft of which was reduced mechanically to the printed scale ofI : 125,0“0.

Pnprrt'urionThe majority of the population belong to the Rendille tribe who are a nomadic

people and whose wanderings are controlled by the location of water for their stock.In the western portion of the area there are a few members of the Samburu tribe.

Both the Rendille and the Samburu have large herds of cattle and goats with a fewsheep, and as a result of these herds the available pasture is greatly over grazed.Camels and donkeys are used as beasts of burden but due to the aridity of the landthe camel is the more popular.

There are two small trading centres in the area, at Laisamis and at Lokuloko, theshops in both cases being owned by Somalis. At Laisamis there is also a Tribal PolicePost.

The Marsabit Game Reserve

That portion of the area to the west of the Isiolo- Marsabit road is part of theMarsabit Game Reserve which eX'tends northwards to Marsabit and southwards toArcher's Post. Grevy's zebra, Grant's gazelle and giraffe are common and several rhinowere seen during the course of the survey.

II-PREVIOUS GEOLOGICAL WORK

Prior to this survey practically no geological work had been done on the area andthe first accounts were those of the early explorers who passed through the region.On his expedition of 1894-95, A. Donaldson-Smith (1900)* went through the area onhis way south and marked Laisamis on his map.

In 1901 Count Wickenberg (1903) on his way to Lamu noticed the lava plateauxbetween Laisamis and Marsabit.

G.F. Archer (1913) travelled north from the Uaso Nyiro River in 1909 to Marsabitin order to establish a Government Post there. The account of his journey is the mostdetailed of those of the early travellers. He noted that "Laisamis presents a lonely andarid scene. . ." (p. 423) and that at Laisamis the line of water holes that he had beenusing on his journey northwards ended. To the east of his route he recorded" . . . asuccession of low plateaux or tablelands which are bounded by abrupt lava scarps.For the most part, the higher ground, some 200 or 300 feet above the surroundinglevel is very broken and is composed of black cotton soil freely strewn with lava. . . ".Of the lower ground Archer said" . .. the surface is of hard sand, covered with openthorn bush and freely intersected by dry channels, the courses of which are indicatedby the lines of flat topped acacia trees. .." Beyond Laisamis, to the west of the track,he noticed Lodermurt (Halilugum Nder) , which he said "stands. .. towering above theplain like an immense obelisk, 2,000 feet high, and forming an unmistakable landmark, while to the south stands the solid granite mass of Moille, surmounted by asharp and ragged peak".

J. Parkinson (1920) in a report on the geology and geography of the northern partof the East African Protectorate noted that around Laisamis the greatly eroded surfaceof the Basement System rocks was overlain by lava, and that examples of puys wereconspiouous to the north of Laisamis, and he ooncluded that the final phase of vulcani-city was of the explosive type. Discussing the drainage in the Kauro-Merille area tothe south Parkinson went on to say that "at Laisamis a second large tributary fromthe west trends eastwards on ridges of felspathic gneiss and presumably goes on tojoin the Kauro-Merille system". Parkinson noticed that the dominant rock type alongthe caravan route from Laisamis southwards to Kinya was a compact felspathicbiotite gneiss.

In a further paper (1924) Parkinson suggested that the lavas capping the old rockssouth of Marsabit were not greatly different in age from those of the Dida Galgallaflows which occur to the north of Marsabit and that these were clearly older than

*References are quoted on p. 23 and 24.

2

i""

Both the Rendille and the Samburu have large herds of cattle and goats with a fewsheep, and as a result of these herds the available pasture is greatly over grazed.Camels and donkeys are used as beasts of burden but due to the aridity of the landthe camel is the more popular.

There are two small trading centres in the area, at Laisamis and at Lokuloko, theshops in both cases being owned by Somalis. At Laisamis there is also a Tribal PolicePost.

The Marsabit Game Reserve

That portion of the area to the west of the Isiolo- Marsabit road is part of theMarsabit Game Reserve which eX'tends northwards to Marsabit and southwards toArcher's Post. Grevy's zebra, Grant's gazelle and giraffe are common and several rhinowere seen during the course of the survey.

II-PREVIOUS GEOLOGICAL WORK

Prior to this survey practically no geological work had been done on the area andthe first accounts were those of the early explorers who passed through the region.On his expedition of 1894-95, A. Donaldson-Smith (1900)* went through the area onhis way south and marked Laisamis on his map.

In 1901 Count Wickenberg (1903) on his way to Lamu noticed the lava plateauxbetween Laisamis and Marsabit.

G.F. Archer (1913) travelled north from the Uaso Nyiro River in 1909 to Marsabitin order to establish a Government Post there. The account of his journey is the mostdetailed of those of the early travellers. He noted that "Laisamis presents a lonely andarid scene. . ." (p. 423) and that at Laisamis the line of water holes that he had beenusing on his journey northwards ended. To the east of his route he recorded" . . . asuccession of low plateaux or tablelands which are bounded by abrupt lava scarps.For the most part, the higher ground, some 200 or 300 feet above the surroundinglevel is very broken and is composed of black cotton soil freely strewn with lava. . . ".Of the lower ground Archer said" . .. the surface is of hard sand, covered with openthorn bush and freely intersected by dry channels, the courses of which are indicatedby the lines of flat topped acacia trees. .." Beyond Laisamis, to the west of the track,he noticed Lodermurt (Halilugum Nder) , which he said "stands. .. towering above theplain like an immense obelisk, 2,000 feet high, and forming an unmistakable landmark, while to the south stands the solid granite mass of Moille, surmounted by asharp and ragged peak".

J. Parkinson (1920) in a report on the geology and geography of the northern partof the East African Protectorate noted that around Laisamis the greatly eroded surfaceof the Basement System rocks was overlain by lava, and that examples of puys wereconspiouous to the north of Laisamis, and he ooncluded that the final phase of vulcani-city was of the explosive type. Discussing the drainage in the Kauro-Merille area tothe south Parkinson went on to say that "at Laisamis a second large tributary fromthe west trends eastwards on ridges of felspathic gneiss and presumably goes on tojoin the Kauro-Merille system". Parkinson noticed that the dominant rock type alongthe caravan route from Laisamis southwards to Kinya was a compact felspathicbiotite gneiss.

In a further paper (1924) Parkinson suggested that the lavas capping the old rockssouth of Marsabit were not greatly different in age from those of the Dida Galgallaflows which occur to the north of Marsabit and that these were clearly older than

*References are quoted on p. 23 and 24.

2

i""

Both the Rendille and the Samburu have large herds of cattle and goats with a fewsheep. and as a result. of these herds the available pasture is greatly over grazed.("amels and donkeys are used as beasts of burden but due to the aridity of the landthe camel is the more popular.

There are two small trading centres in the area. at Laisamis and at. Lokuloko. theshops in both cases being owned by Somalis, At Laisamis there is also a 'l‘ribal PolicePost.

The Marsahi! Game ReserveThat portion of the area to the west of the lsiolo-Marsabit road is part of the

Marsabit Game Reserve which extends northwards to Marsabit and southwards toArcher‘s Post. Grevy‘s zebra. Grant‘s gazelle and girall‘e are common and several rhinowere seen during the course of the survey.

II—PREV’IOUS GEOLOGICAL WORK

Prior to this survey practically no geological work had been done on the area andlhe first accounts were those of the early explorers who passed through the region.On his expedition of 1894—95. A. Donaldson-Smith (1900)* went through the area onhis way south and marked Laisamis on his map.

in I901 Count Wickenberg I19(_l3) on his way to Lamu noticed the lava plateausbetween Laisamis and Marsabit.

CLF. Archer (1913) travelled north from the Llaso Nyiro River in 1909 to Marsabitin order to establish a Government Post there. The account of his journey is the mostdetailed ol‘ those of the early travellers. He noted that “liaisamis presents a lonely andarid scene . . f” (p. 423) and that at Laisamis the line of water holes that he had beenusing on his journey northwards ended. To the east. of his route he recorded . .. asuccession of low plateaux or tablelands which are bounded by abrupt. lava scarps.For the most part. the higher ground. some 200 or 300 feet above the surroundinglevel is very broken and is composed of black cotton soil freely strewn with lava . i . ”.Of the lower ground Archer said “ . .. the surface is of hard sand, covered with openthorn bush and freely intersected by dry channels. the courses of which are indicatedby the lines of llat topped acacia trees Beyond Laisamis. to the west of the track.he noticed Lodermurt (Halilugum Xder). which he said "stands. .. towering above theplain like an immense obelisk. 2.000 feet high. and forming an unmistakable landmark, while to the south stands the solid granite mass of Moille. surmounted by asharp and ragged peak".

J. Parkinson (1920) in a report on the geology and geography of the northern partof the East African Protectorate noted that around Laisamis the greatly eroded surfaceof the Basement System rocks was overlain by lava. and that examples of puys wereconspicuous to the north of I.aisamis. and he concluded that the final phase of vulcani—city was of the explosive type. Discussing the drainage in the Kauro-Merille area tothe south Parkinson went on to say that "at l.aisamis a second large tributary fromthe west trends eastwards on ridges of felspathic gneiss and presumably goes on tojoin the 'Kauro-Merille system“. Parkinson noticed that the dominant rock type alongthe caravan route from Laisamis southwards to Kinya was a compact felspathicbiotite gneiss.

In a further paper (1924) Parkinson suggested that the lavas capping the old rockssouth of Marsabit were not greatly difierent in age from those of the Dida Galgallaflows which occur to the north of Marsabit and that these were clearly older than

*Refcrences are quoted on p. 23 and 24.

Ix)

the Marsabit volcano. He regarded the volcano as being "a final effort of the upwellingmagma, a piled up series of flows from a single vent, the activity ending with theformation of a noticeable number of parasitic puys; a feebly developed explosivephase".

In a third paper Parkinson (1939) again emphasised the two stages of volcanicactivity, first the plateau lavas and second the formation of a Hawaiian type phasewhich produced Marsabit volcano. He also postulated the northwards continuationof the Merti Plateau to include those plateaux lying to the east of the old routefrom Laisamis to Langaia. He considered that those puys to the west of Laisamis aredistinctly younger than the plateau lavas as they are only very slightly eroded.

H. D. Roberts, in a report of a geological reconnaissance survey of the MertiPlateau (1941), also said that the plateaux to the north of the Merti area have a similarstructure to that of the Merti Plateau, i.e. gneiss, overlain by arkosic sediments andcapped by lavas, and he thought that these plateaux were at one time probably allcontinuous.

F. Dixey made a hydrogeological survey of the Northern Province in 1943 (Dixey,1944), and in 1948 wrote a general report on the geology of this part of Kenya. Hecommented on the presence of basalt flows capping isolated hills of gneiss, or ofsediments resting on gneiss and themselves being capped by basalt and thus beingpreserved.

As a result of Dixey's hydrogeological report a scheme was instituted in 1950 forthe purpose of implementing his recommendations. The results of this scheme areembodied in a report by the consulting engineers to the Kenya Government (Humphries,1958) and this and the Dixey report will be discussed more fully in the water sectionof this publication.

III-PHYSIOGRAPHY

The area can be divided into three physiographic units (fig. 1):-

(a) The lava-capped plateaux which rise from 50 to 200 feet above the plain.

(b) The dissected plain surface, which slopes downwards in a north-easterly direction.

(c) The residual hills standing well above the level of the plain, with summit heightsranging from 2,645 feet to 5,746 feet.

3

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II

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I

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the Marsabit volcano. He regarded the volcano as being "a final effort of the upwellingmagma, a piled up series of flows from a single vent, the activity ending with theformation of a noticeable number of parasitic puys; a feebly developed explosivephase".

In a third paper Parkinson (1939) again emphasised the two stages of volcanicactivity, first the plateau lavas and second the formation of a Hawaiian type phasewhich produced Marsabit volcano. He also postulated the northwards continuationof the Merti Plateau to include those plateaux lying to the east of the old routefrom Laisamis to Langaia. He considered that those puys to the west of Laisamis aredistinctly younger than the plateau lavas as they are only very slightly eroded.

H. D. Roberts, in a report of a geological reconnaissance survey of the MertiPlateau (1941), also said that the plateaux to the north of the Merti area have a similarstructure to that of the Merti Plateau, i.e. gneiss, overlain by arkosic sediments andcapped by lavas, and he thought that these plateaux were at one time probably allcontinuous.

F. Dixey made a hydrogeological survey of the Northern Province in 1943 (Dixey,1944), and in 1948 wrote a general report on the geology of this part of Kenya. Hecommented on the presence of basalt flows capping isolated hills of gneiss, or ofsediments resting on gneiss and themselves being capped by basalt and thus beingpreserved.

As a result of Dixey's hydrogeological report a scheme was instituted in 1950 forthe purpose of implementing his recommendations. The results of this scheme areembodied in a report by the consulting engineers to the Kenya Government (Humphries,1958) and this and the Dixey report will be discussed more fully in the water sectionof this publication.

III-PHYSIOGRAPHY

The area can be divided into three physiographic units (fig. 1):-

(a) The lava-capped plateaux which rise from 50 to 200 feet above the plain.

(b) The dissected plain surface, which slopes downwards in a north-easterly direction.

(c) The residual hills standing well above the level of the plain, with summit heightsranging from 2,645 feet to 5,746 feet.

3

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II

I

I

- --

the Marsabit volcano. llc regarded the volcano as being “a final effort of the upwellingmagma. a piled up series of flows from a single vent. the activity ending with theformation of a noticeable number of parasitic puys; a fecbly developed explosivephase".

In a third paper Parkinson (1939) again emphasised the two stages of volcanicactivity, lirst the plateau lavas and second the formation of a 'H'axxaiian type phasewhich produced Vlarsabit volcano. He also postulated the northwards continttationof the Merti Plateau to include those plateaux lying to the east of the old routefrom laisamis to Langaia. He considered that those puvs to the nest of Laisamis aredistinctly younger than the plateau lznas as they at: only \ery slightly” eroded.

II. D. Roberts. in a report of a geological reconnaissance survey of the MertiPlateau [1941). also said that the plateaus to the north of the Mctti area have a similarstructure to that of the N-‘lerti Plateau. i.e. gnciss. overlain by arkosic sediments andcapped by lavas, and he thought that. these plateaux were at one time probably allcontinuous.

F. Dixey made a hydrogeological survey of the \‘orthern Province in 1943 ('Dixey.19-1-1), and in 1948 wrote a general report on the geology Of this part of Kenya. Hecommented on the presence of basalt. tlows capping. isolated hills of gneiss. or of.sediments resting on gneiss and themselves being capped by basalt and thus beingpreserved.

As a result of Dixey's hydrogeological report a scheme was instituted in 1950 forthe purpose of implementing his recommendations. The results of this scheme areembodied in a report by the consulting engineers to the Kenya Government (Humphries,WES) and this and the Dixey report will be discussed more fully in the water sectionof this publication.

III—PHYSIOGRAPHY

The area can be divided into three physiographic units dig. l):—-ta)'[‘hc lava-capped plateaux which rise from F“ to 2H0 feet above the plain.tbl'l'hc dissected plain surface. \shich slopes dovtnvxzirds in a nm‘th—eas‘terly direction.(e) The residual hills standing w ell above the level of. the plain. with summit heights

ranging. from 2.645 feet to 5.746 feet.

loaO'N.0;

'0"'

0::; KAISUT-PLATEAU

~ llhm

KA ISUT

DESERT

-AMBARAPLATEAU-

( ----

\ (

tarti~r\

8J(\

'\\(

(

----;f

~~~/ )

" --e--!1 Ii0~ ---" ,1'7./ /1-

~ r' )

;/ /NOON YO \MARA /,

PLATEAU /1(\

i) ( I

( ~ ( \( Kot". \ It r7 J.,

( a (( I) /

( (r ,,-.J

SAGERERUAPLATEAU

~

'"0,0':0

1°3rYf["'

0L

51

SCALE 101

15 MILESI

1====1 Lavo plateaux capping

t:::::=j end -Tertiary erosion bevel

~ Possible remnants of the~ sub-Miocene ero"on bevel

c=J Post Miocene dissection

Fig. I-Physiographic map of the Laisamis Area,

4

r---

loaO'N.0;

'0"'

0::; KAISUT-PLATEAU

~ llhm

KA ISUT

DESERT

-AMBARAPLATEAU-

( ----

\ (

tarti~r\

8J(\

'\\(

(

----;f

~~~/ )

" --e--!1 Ii0~ ---" ,1'7./ /1-

~ r' )

;/ /NOON YO \MARA /,

PLATEAU /1(\

i) ( I

( ~ ( \( Kot". \ It r7 J.,

( a (( I) /

( (r ,,-.J

SAGERERUAPLATEAU

~

'"0,0':0

1°3rYf["'

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51

SCALE 101

15 MILESI

1====1 Lavo plateaux capping

t:::::=j end -Tertiary erosion bevel

~ Possible remnants of the~ sub-Miocene ero"on bevel

c=J Post Miocene dissection

Fig. I-Physiographic map of the Laisamis Area,

4

r---

u. x.-E 5 L ‘hm g

v — __hl‘ \17'

.‘ aw .—- - “a."\a -\d_..

KflmSL‘TJ'FSFRT

PG hinm

?—A-‘sa.u—-—"'\ ‘ ‘ ‘—m.47.FAu “7-3) 1“ k

I _ i n ‘. Ff“ ‘L —L/\ "71 ‘1‘

I S is“?, f n.;— _®— 5\

=‘-- we 15 mi:-

q :7" LT Fowl-.19. T'"."V'.:'gl"‘t5 0‘ :re.n beve‘ M :q ”Momma e'c'xu' Dma

Pr.-1 P“ c.:u'_e ' c due-chemi;

Fig l l’h‘vsiograpl'fic map of {he l..aés:1mis Area.

(a) The lava-capped plateaux

These plateaux in general consist of a thin sheet of lava overlying metamorphic rocks.Exceptions to this occur in the south-eastern portion of the area where a sedimentarysequence underlies the lava capping, and in the Kaisut Plateau where there are noindications of either sediments or metamorphic rocks and the lava rests directly onthe plain. From ground level these plateaux give the impression of maintaining afairly constant height, but when viewed from some point above their surface, suchas Limirina, it can be seen that the effects of drainage have caused local undulations.

The surface underlying the lava, which has been preserved by the tough, homogeneousnature of the capping basalt, is considered to be remnants of the last erosion bevelin this part of Kenya. The cycle of erosion which led to the formation of this bevel,the end-Tertiary cycle, probably commenced in late Miocene or earliest Pliocene timesand had ended by the Middle Pliocene. By placing the end of this erosion cycle inthe Middle Pliocene the sediments which occur in the south-eastern portion of thearea can be dated as Upper Pliocene and the overlying lava Lower Pleistocene. Theseplateau lavas are obviously older than the small local flows which in places overliethem and they are also older than the craters and vents of the area, which are wellpreserved and show little sign of erosion.

In the south-eastern portion of the area the lava caps a sedimentary series on partof the northern and southern margins of the Rusarus Plateau, but along the westernportion of this plateau there is no indication of any sediments, the rocks underlyingthe lava being metamorphic. The sediments also underlie the lava capping of theTirkamo Plateau, the line joining the sedimentary outcrops at the southern marginof the Rusarus Plateau and those of the Tirkamo Plateau being aligned in a north-north-westerly direction. These sediments have a very local distribution, as Rix (ReportNo. 92) has noted that in the Kauro-Merille area to the south no sediments are foundat Kurato Hill, which lies only two. miles south of the Rusarus Plateau, and that therethe lava rests on a surface of gneiss.

(b) The dissected plain surface

This surface has resulted from dissection of the end-Tertiary bevel which is nowonly represented by those remnants which have a lava capping. The lack of protectionby a tough homogeneous basaltic lava has led to the erosion of the heterogeneousand often well foliated metamorphic rocks.

The main river in the area is the Milgis which drains eastwards from the NdotoMountains. This river is seasonal and contains water only in rainy periods. Over allits length it has a flat profile and nowhere are any solid rocks exposed. Except in thesouth-eastern and northern portions of the area the drainage all flows northwards tojoin the Milgis. To a very large e)l!tent this northerly direction of the drainage is con-trolled by the structure of the Basement System rocks, which have a dominantly north-south strike. Eastwards from the northern part of the Ambara Plateau there is noexposure of rock with the exceptions of the northern parts of the lava plateaux ofNdonyo Mara and Sagererua, and the tributary streams flow in a north-easted~direction until they join the Milgis.

In the south-eastern portion of the area the drainage is eastwards. It is clear that atone time the lavas of the Rusarus and Sagererua plateaux formed a continuous sheet,but incision by the Seralatubi River has now separated the two.

It is at Laisamis that the only flowing water in the area occurs, but this extendsfor a distance of merely 150 yards before disappearing again beneath the sand. Thepresence of water here is due to the fact that outcrops of Basement System rocks formsub-surface dams across the river. At this point the sand cover above the metamorphic

5

I-

(a) The lava-capped plateaux

These plateaux in general consist of a thin sheet of lava overlying metamorphic rocks.Exceptions to this occur in the south-eastern portion of the area where a sedimentarysequence underlies the lava capping, and in the Kaisut Plateau where there are noindications of either sediments or metamorphic rocks and the lava rests directly onthe plain. From ground level these plateaux give the impression of maintaining afairly constant height, but when viewed from some point above their surface, suchas Limirina, it can be seen that the effects of drainage have caused local undulations.

The surface underlying the lava, which has been preserved by the tough, homogeneousnature of the capping basalt, is considered to be remnants of the last erosion bevelin this part of Kenya. The cycle of erosion which led to the formation of this bevel,the end-Tertiary cycle, probably commenced in late Miocene or earliest Pliocene timesand had ended by the Middle Pliocene. By placing the end of this erosion cycle inthe Middle Pliocene the sediments which occur in the south-eastern portion of thearea can be dated as Upper Pliocene and the overlying lava Lower Pleistocene. Theseplateau lavas are obviously older than the small local flows which in places overliethem and they are also older than the craters and vents of the area, which are wellpreserved and show little sign of erosion.

In the south-eastern portion of the area the lava caps a sedimentary series on partof the northern and southern margins of the Rusarus Plateau, but along the westernportion of this plateau there is no indication of any sediments, the rocks underlyingthe lava being metamorphic. The sediments also underlie the lava capping of theTirkamo Plateau, the line joining the sedimentary outcrops at the southern marginof the Rusarus Plateau and those of the Tirkamo Plateau being aligned in a north-north-westerly direction. These sediments have a very local distribution, as Rix (ReportNo. 92) has noted that in the Kauro-Merille area to the south no sediments are foundat Kurato Hill, which lies only two. miles south of the Rusarus Plateau, and that therethe lava rests on a surface of gneiss.

(b) The dissected plain surface

This surface has resulted from dissection of the end-Tertiary bevel which is nowonly represented by those remnants which have a lava capping. The lack of protectionby a tough homogeneous basaltic lava has led to the erosion of the heterogeneousand often well foliated metamorphic rocks.

The main river in the area is the Milgis which drains eastwards from the NdotoMountains. This river is seasonal and contains water only in rainy periods. Over allits length it has a flat profile and nowhere are any solid rocks exposed. Except in thesouth-eastern and northern portions of the area the drainage all flows northwards tojoin the Milgis. To a very large e)l!tent this northerly direction of the drainage is con-trolled by the structure of the Basement System rocks, which have a dominantly north-south strike. Eastwards from the northern part of the Ambara Plateau there is noexposure of rock with the exceptions of the northern parts of the lava plateaux ofNdonyo Mara and Sagererua, and the tributary streams flow in a north-easted~direction until they join the Milgis.

In the south-eastern portion of the area the drainage is eastwards. It is clear that atone time the lavas of the Rusarus and Sagererua plateaux formed a continuous sheet,but incision by the Seralatubi River has now separated the two.

It is at Laisamis that the only flowing water in the area occurs, but this extendsfor a distance of merely 150 yards before disappearing again beneath the sand. Thepresence of water here is due to the fact that outcrops of Basement System rocks formsub-surface dams across the river. At this point the sand cover above the metamorphic

5

I-

in} the trivia-topped plan-tarry'l'hese plateaus irt general consist of a thin sheet of lava overlying metamorphic rocks.

liseeptions to this occur in the south-eastern portion of the area where a sedimentarysequence underlies the lava capping. and in the Kaisttt Plateau where there are noindications of either sediments or metamorphic rocks and the lava rests directly onthe plain. l‘ron't ground level these plateaus give the impression of maintaining afairly constant height. but when viewed from some point above their surface. suchas Limirina. it can be seen that tlte etl'eets of drainage have caused local undulations.

'l he surface unt'lerlying the lava. which has been preserved by tlte tough. homogeneousnature of the capping basalt. is considered to he remnants ot‘ the last erosion bevelin this part of Kenya. 'lhe cycle of erosion which led to the formt‘ttion of this bevel.the end-Tertiary eyele. probably commenced in late N‘tioeene or earliest Pliocene timesand had ended by the Middle Pliocene. By placing the end of this erosion cycle inthe Middle Pliocene the sediments which occur in the south-eastern portion of thearea can be dated as Upper Pliocene and the overlying laya Loo'er Pleistocene. Theseplateau lavas are obviously older than the small local flows which in places overliethem and they are also older than the craters and vents of the area. which are wellpreserved and shoyv little sign of erosion.

In the south-eastern portion of the area the lava caps a sedimentary series on partot‘ the northern and southern margins of the Rusartts Plateau. but along the westernportion of this plateau there is to indication of any sediments. the rocks underlyingthe lava being metamorphic. The sediments also underlie the lava capping of the'l'irltamo Plateau. the line joining the sedimentary outcrops at the southern marginof the Rttsartts Plateau and those of the 'l'irltamo Plateau being aligned in a north—north—westeriy direction. These sediments have a Very local distribution. as Ris [Report\o. 92) has noted that in the Kauro-l’vtcrille area to the south no sediments are foundat Kurato Hill. which lies only twomiles south of the Rttsarus Plateau. and that therethe lava rests on a surface of gneiss.

In} The tt’is's't't'tt-‘(i plain surfaceThis surface has resulted front dissection of the end-Tertiary bevel which is now

t.-nly represented by those remnants uhieh have a lava. capping. The laelt of protectionby a tough homogeneous basaltic lava has led to the erosion olf the heterogeneousand often well foliated metamorphic roelss.

The main river in tlte area is the \lilgis \\hieh drains eastWt'trds from the Ndotoyiount'ains. this river is seasonal and contains water only in rainy periods. Over allits length it has a hat profile and nowhere are any solid rocks exposed. Except in thesottth eastern and northern portions of the area the drainage all tlow's northwards tojoin the Milgis. To a very large extent this northerly direction of the. drainage is con-trolled by the structure of the Basement System I'oclss. \\hich have a dominantly north-south strike. Fastwards from the northern part. of the Ambara Plateau there is noesposttre or rock with the exceptions of the northern parts of the lava plateaus of\tlonyo Mara and Sagererua. and the tributary streams flow in a north-easterlydirection until they join the Milgis.

lo the south-eastern portion of the area the drainage is eastwards. It is. clear that, at:one time the lavas of the Rusarus and Sager-erua plateaux formed a continuous sheet,but incision by the Seralatttbi River has no“ separated the two.

It. is at Laisamis that the only flowing water in the area occurs. but this extendsfor a distance of merely 15h yards before disappearing again beneath the sand. The

once of water here is due to the fact that outcrops of Basement System rocks formsub surface dams across the river. .-\t this point the sand coy er above the metamorphic

racks is thin and the water table sufficiently high that the water rises and flaws an thesurface until the sand caver is sufficiently thick far it to become sub-surface wateragain.

In the narthern part of the Kaisut Desert streams draining aff the lava af the KaisutPlateau soon disappear beneath the sands. The drainage is more permanent in thenarth-east where streams caming off the lava run eastwards.

TO' the sauth-east af Halisiruwa, alang the margin af the lava plateau near the bare-hale, there is an expanse of pale brown alluvium, the remains of an aId lake bedfarmed by the lava damming the original drainage. At the present time this area is stillliable to' flaad and will retain water for same time after the rainy seasan.

The watershed between the Milgis and MeriJIe drainage systems fallaws fairly closdythe sauthern baundary af the present area.

(c) The residuals above the plain

All the majar hill masses in the area are composed af Basement System racks, withsummit heights ranging from 5,746 feet at Baia dawn to' 2,645 feet at Nalawuru. Thehills rise abruptly fram the plain and farm impressive steep-sided features. The heightaf the sub-Miacene erosian bevel in the area has been pastulated by Pulfrey (1960)at approximately 2,500 feet in the centre af the area, rising to' 3,000 feet in thesauth-west carner. Several features in the area appear to' represent remnants af thissurface: the ridge immediately narth and sauth af the trigonometrical point afKatira, at a height of 2,800 feet; a bevel at 2,600 feet an Martiagar (which has asummit height af 2,781 feet); twO' law hills an the Arnbara Plateau whose summitsare just below 3,000 feet; the ridge narth af the peak af Illim. The remainder af theresidual hill masses such as Baia, Illim, Halilugum Nder and Maile perhaps representeraded remnants af alder erasian surfaces.

IV-SUMMARY OF GEOLOGY

The racks which occur in this area can be classified under five main headings:-

1. Basement System metamarphic rocks af Precambrian age.

2. Basic, uItrabasic and acid igneous rocks intrusive intO' the Basement System.3. Tertiary sediments.4. Valcanic rocks ranging in age fram Lawer Pleistocene to' Recent.

5. Superficial depasits.

The metamorphic rocks af the Basement System can be sub-divided as fallaws:-

Semi-{ A finely banded successian af biatite and harnblende gneisses with

pelitic calc-silicate lenses.

~l

Semi-peli ticwithpsammiticharizans

Well foliated biatite gneisses and biatite-muscovite gneisses, withbands af harnblende gneiss, crystalline limestane and quartza-felspathic granulites.

The rocks intrusive into the Basement System can be classed under five headings:AmphibalitesPyraxenitesSerpentinitesGranitesQuartz veins

6

racks is thin and the water table sufficiently high that the water rises and flaws an thesurface until the sand caver is sufficiently thick far it to become sub-surface wateragain.

In the narthern part of the Kaisut Desert streams draining aff the lava af the KaisutPlateau soon disappear beneath the sands. The drainage is more permanent in thenarth-east where streams caming off the lava run eastwards.

TO' the sauth-east af Halisiruwa, alang the margin af the lava plateau near the bare-hale, there is an expanse of pale brown alluvium, the remains of an aId lake bedfarmed by the lava damming the original drainage. At the present time this area is stillliable to' flaad and will retain water for same time after the rainy seasan.

The watershed between the Milgis and MeriJIe drainage systems fallaws fairly closdythe sauthern baundary af the present area.

(c) The residuals above the plain

All the majar hill masses in the area are composed af Basement System racks, withsummit heights ranging from 5,746 feet at Baia dawn to' 2,645 feet at Nalawuru. Thehills rise abruptly fram the plain and farm impressive steep-sided features. The heightaf the sub-Miacene erosian bevel in the area has been pastulated by Pulfrey (1960)at approximately 2,500 feet in the centre af the area, rising to' 3,000 feet in thesauth-west carner. Several features in the area appear to' represent remnants af thissurface: the ridge immediately narth and sauth af the trigonometrical point afKatira, at a height of 2,800 feet; a bevel at 2,600 feet an Martiagar (which has asummit height af 2,781 feet); twO' law hills an the Arnbara Plateau whose summitsare just below 3,000 feet; the ridge narth af the peak af Illim. The remainder af theresidual hill masses such as Baia, Illim, Halilugum Nder and Maile perhaps representeraded remnants af alder erasian surfaces.

IV-SUMMARY OF GEOLOGY

The racks which occur in this area can be classified under five main headings:-

1. Basement System metamarphic rocks af Precambrian age.

2. Basic, uItrabasic and acid igneous rocks intrusive intO' the Basement System.3. Tertiary sediments.4. Valcanic rocks ranging in age fram Lawer Pleistocene to' Recent.

5. Superficial depasits.

The metamorphic rocks af the Basement System can be sub-divided as fallaws:-

Semi-{ A finely banded successian af biatite and harnblende gneisses with

pelitic calc-silicate lenses.

~l

Semi-peli ticwithpsammiticharizans

Well foliated biatite gneisses and biatite-muscovite gneisses, withbands af harnblende gneiss, crystalline limestane and quartza-felspathic granulites.

The rocks intrusive into the Basement System can be classed under five headings:AmphibalitesPyraxenitesSerpentinitesGranitesQuartz veins

6

rocks is thin and the water table sufi'leiently high that the water rises and flows on thesurface until the sand cover is suifieiently thick. tor it to become sub-surface wateragain.

In the northern part of the Kaisut Desert streams draining ofi“ the lava of the Kaisut.Plat‘au soon disappear beneath the sands. lhe drainage is more permanent in thenorth—east \there streams coming ml the lava run eastwards.

lo the south—east of Halisirtova. along the margin ot‘ the lava plateau near the bore—hole, there is an expanse ot‘ pale brown alluvium. the remains of an old lake bedformed by the lava darnming the original drainage. At the present. time this area is stillliable to flood and will retain water for some time alter the rainy season.

The Watershed between the Milgls and Merille drainage systems rollowx fairly closelythe southern boundary ol‘ the present area.

{c} The residuals uhm'e the plainAll the major hill masses in the area are compOsed or Basement System rocks, with

summit heights ranging from 5.746 feet at Baio down to 2.645 feet at. Nalawuru. Thehills rise abruptly from the plain and form impressive steep-sided features The heightof the sub-Miocene erosion bevel in the area has been postulated by Pulfrey (1960)at approximately 3.5t feet in the centre of the area‘ rising to 3.000 feet in thesouth—west corner. Several features in the area appear to represent remnants of thissurface: the ridge immediately north and south of the trigonometrieal point ofKotira. at a height. of 2.800 feet; a bevel at 2.60%) feet on N‘lartiagar (which has asummit height of 2,781 feet): two low hills on the Ambara Plateau whose summitsare just below 10t feet; the ridge north otf the peak of Illim. The remainder of theresidual hill masses such as Baio. Illim. .l-lalilugurn l\'der and Moile perhaps representeroded remnants of older erosion surfaces.

lV—SUMMARY OF GEOLOGY

The rocks which occur in this area can be elassilied under five main headings:—I. Basement. System metamorphic rocks of Precambrian age.

Ix) Basie. ultrabasic and aeid igneous rocks intrusive into the Basement System.

'l'ertiary sediments.

44.3

..-

Volcanic roeks ranging in age from lower Pleistoeene to Recent.

'JI . Superficial deposits.

The metamorphic rocks of the Basement System can be sub-divided as follows:

Semi- f A finely banded succession of biotite and hornblende gneisses withpelitie l (sale-silicate lenses.

Semi- ' Well foliated biotite gneisses and biotite-museovite gneisses. withpelitie bands of hornblende gneiss. crystalline limestone and quartzo-with S felspathie granulites.psammitic .horirons ‘

The rocks intrusive into the Basement System can be classed under the headings:AmphibolitesPyroxenitesSerpentinitesGranitesQuartz veins

Three intrusions of basic and ultra basic igneous rocks occur in the area. At Halisiruwain the north-west of the area the dominant rock type is a garnetiferous pyroxenitebut other types are also represented. At Chaparkrom in the south-east the intrusivebody is an amphibolite which locally contains small amounts of garnet and epidote.The serpentinite at Chaparkrom is younger than the amphibolite intrusion as it hasbeen emplaced in it. On aerial photographs it shows up as a light-coloured patchtowards the centre of the amphibolite. Associated with it are veins of magnesite.

Outcrops of granite occur on the eastern side of Baio, but are of limited extent, andthere is also a small outcrop to the west of Halisiruwa in the north-west of the area.

Numerous quartz veins of small extent occur in the older rocks of the area.

Tertiary sediments occur in the south-east of the area and extend in a north-westerlydirection. It is considered that they are an infilling of a warp in the end-Tertiaryerosion bevel and have been preserved by a capping of olivine basalt. Due to theirunconsolidated and unsorted state it is thought that they are the products of a rapidmode of deposition.

The volcanic rocks range from Lower Pleistocene to Recent in age. The volcanichistory of the area can be divided into three stages:-

(i) Olivine basalt flows in the Lower Pleistocene forming cappings to the end-Tertiary erosion bevel.

(ii) A broad flow of olivine-augite basalt in the western part of the Kaisut Plateau,and small flows of analcime basalt on all the olivine basalt plateaux of thearea. It is difficult to differentiate between the ages of these two sets of activitybut the age range is probably Middle to Upper Pleistocene.

(iii) The formation of small vents and lava flows of limited extent and the productionof explosion craters with widespread emission of ash. From the well-preservedcharacters of these features it is considered that they are Recent in age.

Large tracts of the area are covered by reddish-brown sandy soils derived from thedisintegration of the metamorphic rocks. On close examination fragments of quartz,felspar and ferromagnesian minerals can be seen. Small patches of secondary (kunkar)limestone occur and ferruginous secondary deposits are present locally, but generallythe latter are of too limited extent to be shown on the map. The olivine basaltplateau lavas have a dusty, reddish brown soil cover through which blocks of the lavaprotrude.

A pale brown alluvium occurs on the south-western edge of the Kaisut Plateau. Thisis thought to be due to the damming of the drainage by the lava with the productionof a lake after periods of heavy rain.

V-DETAILS OF GEOLOGY

1. Basement System

The Basement System rocks of the Laisamis area are considered to be a metasedi-mentary series and are classified under the following headings: -

(1) Metamorphosed calcareous sediments

(a) Crystalline limestones.

(b) Calc-silicate granulites.

(2) Metamorphosed semi-pelitic sediments

(a) Foliated biotite gneisses, biotite-muscovite gneisses and hornblende gneisses.

(b) Banded succession of biotite and hornblende gneisses.

7

I:

1-

I

,.

~.

II

I.I

II

I;

I;

J!

I

I

I

~

Three intrusions of basic and ultra basic igneous rocks occur in the area. At Halisiruwain the north-west of the area the dominant rock type is a garnetiferous pyroxenitebut other types are also represented. At Chaparkrom in the south-east the intrusivebody is an amphibolite which locally contains small amounts of garnet and epidote.The serpentinite at Chaparkrom is younger than the amphibolite intrusion as it hasbeen emplaced in it. On aerial photographs it shows up as a light-coloured patchtowards the centre of the amphibolite. Associated with it are veins of magnesite.

Outcrops of granite occur on the eastern side of Baio, but are of limited extent, andthere is also a small outcrop to the west of Halisiruwa in the north-west of the area.

Numerous quartz veins of small extent occur in the older rocks of the area.

Tertiary sediments occur in the south-east of the area and extend in a north-westerlydirection. It is considered that they are an infilling of a warp in the end-Tertiaryerosion bevel and have been preserved by a capping of olivine basalt. Due to theirunconsolidated and unsorted state it is thought that they are the products of a rapidmode of deposition.

The volcanic rocks range from Lower Pleistocene to Recent in age. The volcanichistory of the area can be divided into three stages:-

(i) Olivine basalt flows in the Lower Pleistocene forming cappings to the end-Tertiary erosion bevel.

(ii) A broad flow of olivine-augite basalt in the western part of the Kaisut Plateau,and small flows of analcime basalt on all the olivine basalt plateaux of thearea. It is difficult to differentiate between the ages of these two sets of activitybut the age range is probably Middle to Upper Pleistocene.

(iii) The formation of small vents and lava flows of limited extent and the productionof explosion craters with widespread emission of ash. From the well-preservedcharacters of these features it is considered that they are Recent in age.

Large tracts of the area are covered by reddish-brown sandy soils derived from thedisintegration of the metamorphic rocks. On close examination fragments of quartz,felspar and ferromagnesian minerals can be seen. Small patches of secondary (kunkar)limestone occur and ferruginous secondary deposits are present locally, but generallythe latter are of too limited extent to be shown on the map. The olivine basaltplateau lavas have a dusty, reddish brown soil cover through which blocks of the lavaprotrude.

A pale brown alluvium occurs on the south-western edge of the Kaisut Plateau. Thisis thought to be due to the damming of the drainage by the lava with the productionof a lake after periods of heavy rain.

V-DETAILS OF GEOLOGY

1. Basement System

The Basement System rocks of the Laisamis area are considered to be a metasedi-mentary series and are classified under the following headings: -

(1) Metamorphosed calcareous sediments

(a) Crystalline limestones.

(b) Calc-silicate granulites.

(2) Metamorphosed semi-pelitic sediments

(a) Foliated biotite gneisses, biotite-muscovite gneisses and hornblende gneisses.

(b) Banded succession of biotite and hornblende gneisses.

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Three intrusions of basic and ultrabasic igneous rocks occur in the area. At llalisiruwain the north—west of the area the dominant rock type is a garnetiferous pyroxcnitebut other types are also represented. At (‘haparkrom in the south east the intrusivebody is an amphibolite which locally contains small amounts of garnet and epidote,'l'hc serpentinite at Chaparkrorn is younger than the amphiholite intrusion as it hasbeen crnplaccd in it. On aerial photographs it shows up as a light—coloured patchtowards the centre of the amphibolite, Associated with it are veins of magnesite.

Outerops of granite occur on the eastern side oi Baio. but are of limited extent. andthere is also a small outcrop to the west of Halisiruwa in the north-west of the area.

\iurnert'ius quartz veins of small extent occtir in the older rocks of the area.

'l'erliary sediments occur in the southeast of the area and extend in a nortliiwesterlydirection. It is considered that they are an inlilling of a warp in the end-Tertiaryerosion bevel and have been preserved by a capping of olivine basalt. Due to theirunct'insolidatcd and unsorted state it is thought that they are the products of a rapidmode of deposition.

The volcanic rocks range from Lower Pleistocene to Recent in age. The volcanichistory of the area can be divided into three stages:7

tiJOlivinc basalt lloxss in the lower Pleistocene forming cappings to the end-'i‘ertiary erosion bevel.

iii) A broad tloo ot olivine augite basalt in the western part of the Kaisut Plateau.and small flows of analcimc basalt on all the olivine basalt plateaus of thearea. It is ditlicult to diliercntiate between the ages of these two sets of activitybut the age range is probably Middle to Upper Pleistocene.

I'iii) lhe formation of small vents and lava firms of limited cstent and the productionof explosion craters \sith Widespread emission of ash. l-rom the \vcll~prcscrvcdcharacters of these features it is considered that they are Recent in age.

large tracts of the area are covered by reddishebrown sandy soils derived from thedisintegration of the metamorphic rocks, On close examination fragments of quartz.fclspar and ferromagncsian minerals can be seen. Small patches of secondary tkunkar]limestone occur and t‘crruginous secondary deposits are present locally. but generallythe latter are of too limited extent to be shown on the map. The olivine basaltplateau lavas have a dusty. reddish brown soil cover through which blocks of the lavaprotrude.

A pale brown alluvium occurs on the southwestern edge of the Kaisut Plateau. Thisis thought to be due to the damming of the drainage by the lava with the productionot a lake after periods of heavy rain.

V—IHC'I‘AILS ()F GEOLOGY1. Basement System

The Basement System rocks of the Laisamis area are considered to be a metasedi—mentary series and are classified under the following headings:

llJ Metamorphosed calcareous sedimentsif!) ("rystalline limestones.{it} Cale-silicate granulites.

I22} Metamorphosed scmi-pelitic sedimentsta) Foliated biotite gneisses. biotite-muscovite gneisses and hornblende gneisses.to) Banded succession of biotite and hornblende gneisses.

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(3) Metamorphosed psammitic sediments.

(a) Quartzo-felspathic granulites.

(4) Metamorphosed carbonaceous sediments.

(a) Graphite schists.

(1) METAMORPHOSED CALCAREOUSSEDIMENTS

(a) Crystalline Limestones

These form impersistent bands, varying in colour and thickness, in the biotite gneissand biotite-muscovite gneiss succession. In hand specimen the limestones are typicallywhite or light grey in colour, though one outcrop of pinkish crystalline limestone wasalso mapped.

Specimen 28/295* from Illim is typical of these crystalline limestones, and in thinsection shows a mosaic of calcite crystals with tiny flakes of graphite scattered throughthe slide. Specimen 28/375 from an outcrop approximately three miles south-westof Kotira is an example of the pink-coloured limestone and in thin section shows calcitewith typical rhombohedral cleavage. Also present in the thin section are small flakesof muscovite, small anhedral crystals of quartz, and a crystal of andesine.

(b) Calc-silicate Granulites

The term granulite is used in this report in the textural sense only, and has noconnexion with the granulitic facies of metamorphism. The calc-silicate gneisses andgranulites 'Occur sporadically throughout the area but attain no !great size. They arethe result of the metamorphism of lime-rich patches of sediments.

In hand specimen the colour of these rocks varies from light green to black, depend-ing on the proportions of the various minerals present. Specimen 28/361 from theslopes of the more northerly of the two hills in the south of the Ambara Plateau isa medium-grained melanocratic rock with patches of light green epidote. Tiny crystalsof garnet are also scattered through the rock. In thin section it is seen to containdiopside pleochroic from light green to dark green, prisms of epidote and plagioclasefelspar in the form of oligoclase. Sphene occurs as an accessory mineral in crystals ofvarying size.

Specimen 28/364 from five and a half miles south of Martiagar is a melanocraticrock with a finely banded appearance. Thin section examination reveals that thedominant mineral is again diopside, with epidote also present. Andesine is the plagio-clase felspar and sphene occurs as an accessory mineral.

Specimen 28/371, from five miles west of Moile, is dark green in colour and granu-litic in texture. In thin section it is seen to contain diopside, epidote and andesine withaccessory sphene.

(2) METAMORPHOSED SEMI-PELITIC SEDIMENTS

(a) Foliated Biotite Gneisses, Biotite-muscovite Gneisses and Hornblende Gneisses

This group of metasediments occur in the eastern and central portion of the areaand have a large width of outcrop. Commonly these gneisses form low ground butwithin the area of their outcrop there are several hills which stand well above the

* Numbers prefixed by 28/ refer to specimens in the regicmal collection of the Mines ac:d GeologicalDepartment, Nairobi.

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(3) Metamorphosed psammitic sediments.

(a) Quartzo-felspathic granulites.

(4) Metamorphosed carbonaceous sediments.

(a) Graphite schists.

(1) METAMORPHOSED CALCAREOUSSEDIMENTS

(a) Crystalline Limestones

These form impersistent bands, varying in colour and thickness, in the biotite gneissand biotite-muscovite gneiss succession. In hand specimen the limestones are typicallywhite or light grey in colour, though one outcrop of pinkish crystalline limestone wasalso mapped.

Specimen 28/295* from Illim is typical of these crystalline limestones, and in thinsection shows a mosaic of calcite crystals with tiny flakes of graphite scattered throughthe slide. Specimen 28/375 from an outcrop approximately three miles south-westof Kotira is an example of the pink-coloured limestone and in thin section shows calcitewith typical rhombohedral cleavage. Also present in the thin section are small flakesof muscovite, small anhedral crystals of quartz, and a crystal of andesine.

(b) Calc-silicate Granulites

The term granulite is used in this report in the textural sense only, and has noconnexion with the granulitic facies of metamorphism. The calc-silicate gneisses andgranulites 'Occur sporadically throughout the area but attain no !great size. They arethe result of the metamorphism of lime-rich patches of sediments.

In hand specimen the colour of these rocks varies from light green to black, depend-ing on the proportions of the various minerals present. Specimen 28/361 from theslopes of the more northerly of the two hills in the south of the Ambara Plateau isa medium-grained melanocratic rock with patches of light green epidote. Tiny crystalsof garnet are also scattered through the rock. In thin section it is seen to containdiopside pleochroic from light green to dark green, prisms of epidote and plagioclasefelspar in the form of oligoclase. Sphene occurs as an accessory mineral in crystals ofvarying size.

Specimen 28/364 from five and a half miles south of Martiagar is a melanocraticrock with a finely banded appearance. Thin section examination reveals that thedominant mineral is again diopside, with epidote also present. Andesine is the plagio-clase felspar and sphene occurs as an accessory mineral.

Specimen 28/371, from five miles west of Moile, is dark green in colour and granu-litic in texture. In thin section it is seen to contain diopside, epidote and andesine withaccessory sphene.

(2) METAMORPHOSED SEMI-PELITIC SEDIMENTS

(a) Foliated Biotite Gneisses, Biotite-muscovite Gneisses and Hornblende Gneisses

This group of metasediments occur in the eastern and central portion of the areaand have a large width of outcrop. Commonly these gneisses form low ground butwithin the area of their outcrop there are several hills which stand well above the

* Numbers prefixed by 28/ refer to specimens in the regicmal collection of the Mines ac:d GeologicalDepartment, Nairobi.

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t3) A-{etanmrphosed psammitic sediments.in) Quartzo-felspathic eranulites.

l4) Metamorpltmed cai'honacemts sediments.m) Graphite sehists.

tl) Me I \\lt)l{l’llt_).‘.-$l;l) C\l__C\I{F.OL‘3 SEDIVFNTS

(a) C rytmliitie Limt'xruum‘These form impersistcnt bands. varying in colour and thicknesg in the biotite gneiss

and hiutite-mt.t~'eo\'ite gneiss \UL‘L“C\\lUTL ln hand specimen the lime-stones are typicallywhite or light grey in colour. though one outcrop of pinkish crystalline limestone “asalso mapped.

Specimen 283395”: from lllim is typical olf these crystalline limestone» and in thinsection shows a mosaic ol‘ Calcite crystals \titlt tiny illtl-tes of graphite. scattered throughthe slide. Specimen IN 375 from an outcrop approximately three miles south-nestol' Kotiz'a is ttn example of the pink—coloured limestone and in thin section shows calcitewith typical rhtnnhohedral cleavage. Also present in the thin section are small flakesof n'uts'cmlte. .s'mull unhedrul crystttis of quartz. and 1t Crystal of ttndosinc,

lit) (.‘u!t‘-<i.lit‘ttlt' ('It‘mzulz'nwThe term granulite is used in this report in the tex‘tural sense only. and has no

connexion \xith the granulitic l‘acies or metamorphism. The calc-silicate LTTWiSF-Bfi andgrantllites occur .soorculfcztll} throughout the area hut t'ttiairi {to great si/c, They arethe result or the metamorphism oil lime-rich patches of sediments.

In hand specimen the Colour ol‘ these rocks \uz‘ies from light green to hinck. depend—ingr on the proportions of the various minerals present, Specimen Zfi‘firil from theslopes of the more northerly ol‘ the mo hills in the south or the Amhara I’iateau isa metlEton-grained melanocratlc rock with patches or” light gr:;:n epitlots. Tiny cl‘telilhof garnet are also scattered through the rock. ln thin section it is .s'een to containdiopsitle pleoehroie from light green to dark "reen. prisms ol' epidole and plagioclttseicls‘par in the term of oligoclase. Sphene occurs as an accessory mineral in crystals ofvarying sire.

Specimen 2V 3.64 t'rom tive and a half miles south or Martiagar is a melttuoeraticrock. with a linely banded appearance. Thin section emonination reveals that thedominant mineral is again tlitipsitlc. with cpidote also present. Andesine is the plagio-clas'e telspar and sphene occurs as an acccssory mineral.

Sipeeimen IS 371. from five miles nest ot‘ Mode is? dark green in colour and granu-llllC in texture. ln thin section it is seen to contain diopside. epidote and Ijl‘ILlCSlnC withaccessory spliene.

l2) Mt-i.\.\tt.t‘itt:t.~;tao SFVl-l’i-‘I'lTlC‘ SF.|'}I\ll-\T~;

it!) frills-tried [i'irn'itt' Crier-wet. Him‘irt-lllt1r(wt‘[tt_‘ iwyc (“hf Ilnr):r‘“IL-ndy (JiJh‘t'lt'H’X'

This group of metasedintcnts occur in the eastern and central portion. of the areaand lime a large width of outcrop. (jtmnnonly these gneisses form low ground butWithin the area of their outcrop there are se\er;tl hills- tthich stand uell above the

'* .\-lllI‘.l1“"~¢ prelixed h): ."rl lctcr ta \llc'c‘ll'lft‘l'ln it tut :‘cgfnnal c.\Il.‘.I:o-i of1l‘c \‘l.l'lt‘-s a '.-:l (igoluj: ':-.IDellnttncrtt. _\Ll1['ul1j_

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level of the plain. These rocks are the metamorphic equivalents of an original sedi-mentary sequence. In hand specimen the biotite gneisses are typically leucocratic, semi-pelitic rocks composed dominantly of quartz and felspar, but flakes of biotite scatteredthrough the rock impart a well-defined foliation to it. The degree of foliation dependson the amount of biotite present and with higher proportions of this mineral the foliationbecomes more marked and the rock has less resistance to erosion and weathering.

In thin section the biotite gneisses are seen to contain biotite which is pleochroicfrom yellowish brown to dark brown, quartz, micro cline and andesine felspar. Someof the specimens contain small quantities of magnetite. Typical sections showing theseassemblages are 28/348 from the eastern slopes of Baio and 28/360 from near theeastern margin of the Ambara Plateau.

Included in the pelitic metasediments are the biotite-muscovite gneisses which formbands in the biotite gneiss succession but are not of sufficient size to be delineatedseparately on the map. In both hand specimen and thin section (e.g. specimen 28/365from south of Martiagar) they are very similar to the biotite gneisses in that they arecomposed of quartz, felspar and biotite, but in addition flakes of muscovite occur.

Hornblende gneisses occur as discrete bands in the biotite gneiss and biotite-muscovitegneiss succession. In hand specimen they are generally leucocratic rocks with alternat-ing bands of hornblende and quartz and felspar. The colour of the hornblende variesfrom black to dark green and occasionally this mineral occurs in clots. A good exampleof this is specimen 28/337 from a point about two and a half miles north-east ofNalawuru. In thin section the minerals present are seen to be hornblende, pleochroicfrom light green to dark green, quartz, micro cline, andesine and magnetite.

(b) Banded Succession of Biotite and Hornblende Gneisses

In many cases these gneisses are similar to the foliated biotite gneisses which havebeen described in the previous section. They occur in the west central portion of thearea and have been mapped as an alternating series of biotite and hornblende gneisses.Except where it has been necessary to show the structure these rocks have not beendifferentiated as the change from one rock type to another is very rapid. The hornblendegneisses in this succession are similar petrologically to those already described as occur-ring as discrete bands in the biotite gneiss and biotite-muscovite gneiss succession.

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(3) METAMORPHOSED PSAMMITIC SEDIMENTS

(a) Quartzo-felspathic Granulites

The quartzo-felspathic granulites are thought to be the end product of the metamor-phism of arkosic horizons in the original sedimentary series. In the field they formfeatures which are resistant to weathering due to the fact that they are largely composedof quartz and felspar and have only minor amounts of mica. Outcrops are massive,occasionally with jointing at right angles to their strike, and are relatively short andnarrow, suggesting that they were derived from impersistent bands of arkosic sedi-ments.

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In hand specimen the granulites are unfoliated, leucocratic, holocrystalline rocksand vary in colour from white to buff. The dominant minerals are quartz and felspar,but small flakes of biotite and crystals of magnetite can be seen to be scattered throughthe rock in random orientation. In thin section there is a general uniformity of mineralcomposition and very little variation from one specimen to another. A typical exampleof these granulites is 28/312, from the Isiolo- Marsabit road five miles north of Laisamis,which shows a mosaic of quartz, micro cline and andesine crystals with biotite flakesin random orientation. Magnetite crystals are also present in small amounts scatteredthrough the slide. Sphene is present as an accessory mineral in some of the specimens,

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level of the plain. These rocks are the metamorphic equivalents of an original sedi-mentary sequence. In hand specimen the biotite gneisses are typically leucocratic, semi-pelitic rocks composed dominantly of quartz and felspar, but flakes of biotite scatteredthrough the rock impart a well-defined foliation to it. The degree of foliation dependson the amount of biotite present and with higher proportions of this mineral the foliationbecomes more marked and the rock has less resistance to erosion and weathering.

In thin section the biotite gneisses are seen to contain biotite which is pleochroicfrom yellowish brown to dark brown, quartz, micro cline and andesine felspar. Someof the specimens contain small quantities of magnetite. Typical sections showing theseassemblages are 28/348 from the eastern slopes of Baio and 28/360 from near theeastern margin of the Ambara Plateau.

Included in the pelitic metasediments are the biotite-muscovite gneisses which formbands in the biotite gneiss succession but are not of sufficient size to be delineatedseparately on the map. In both hand specimen and thin section (e.g. specimen 28/365from south of Martiagar) they are very similar to the biotite gneisses in that they arecomposed of quartz, felspar and biotite, but in addition flakes of muscovite occur.

Hornblende gneisses occur as discrete bands in the biotite gneiss and biotite-muscovitegneiss succession. In hand specimen they are generally leucocratic rocks with alternat-ing bands of hornblende and quartz and felspar. The colour of the hornblende variesfrom black to dark green and occasionally this mineral occurs in clots. A good exampleof this is specimen 28/337 from a point about two and a half miles north-east ofNalawuru. In thin section the minerals present are seen to be hornblende, pleochroicfrom light green to dark green, quartz, micro cline, andesine and magnetite.

(b) Banded Succession of Biotite and Hornblende Gneisses

In many cases these gneisses are similar to the foliated biotite gneisses which havebeen described in the previous section. They occur in the west central portion of thearea and have been mapped as an alternating series of biotite and hornblende gneisses.Except where it has been necessary to show the structure these rocks have not beendifferentiated as the change from one rock type to another is very rapid. The hornblendegneisses in this succession are similar petrologically to those already described as occur-ring as discrete bands in the biotite gneiss and biotite-muscovite gneiss succession.

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(3) METAMORPHOSED PSAMMITIC SEDIMENTS

(a) Quartzo-felspathic Granulites

The quartzo-felspathic granulites are thought to be the end product of the metamor-phism of arkosic horizons in the original sedimentary series. In the field they formfeatures which are resistant to weathering due to the fact that they are largely composedof quartz and felspar and have only minor amounts of mica. Outcrops are massive,occasionally with jointing at right angles to their strike, and are relatively short andnarrow, suggesting that they were derived from impersistent bands of arkosic sedi-ments.

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In hand specimen the granulites are unfoliated, leucocratic, holocrystalline rocksand vary in colour from white to buff. The dominant minerals are quartz and felspar,but small flakes of biotite and crystals of magnetite can be seen to be scattered throughthe rock in random orientation. In thin section there is a general uniformity of mineralcomposition and very little variation from one specimen to another. A typical exampleof these granulites is 28/312, from the Isiolo- Marsabit road five miles north of Laisamis,which shows a mosaic of quartz, micro cline and andesine crystals with biotite flakesin random orientation. Magnetite crystals are also present in small amounts scatteredthrough the slide. Sphene is present as an accessory mineral in some of the specimens,

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level of the plain. These rocks- atre the metunmt'phie equivalent: at an original sedi-nzentart “L'L|L[t‘|']t.‘t'. In ltLiI‘lLI speeinten the biotite gneixses LU'L' typically mime-crude. r«2mi—pelitte rock; eomposed dominantly oi LlLiLtrlf. Lind fc‘IiPLil'. but ilitkes 0f biotit'e scatteredthrough the rock impart £1 \\'ell»deiined I‘olitttion to it. Hit: degree UI foliattfon dependson the ttnttitlttt 0t biotite prefient and with higher prupot‘EionRoE‘ [lilRt‘t‘til‘lL‘FLtI the IDIELLIIOIII‘JL‘L‘UIHL’H more lttt'd LLIMI the rock has leie reaixtanet’: to erosion and \x'eatltering.

In thin Heetion the bioiite gneix‘xek 'dt'c seen to enntttin hintite which is plenehruie{rum yellmt'ik'h bronn to dark brown. quartz. mieroeline Land LLFItIL'KIIlE: It'lHPiti'. Someoi" the Apeeimena eontuin xntull quantities or magnetite. '|'_\'pie-.1l section; mowing theseLiH\L'nti‘I£l};L'H LIFE JR 345 from the ettxtern SIOPCS of Rain and 2.54 Phil from near theeastern margin of the Arnhttrtt I’I;tte:ttt.

Inelttded in the pelitie metakediments are the IWIUIIIL‘elTILIHL‘UVIlt‘ gneissex which formtnds' in the hiotite gneiw‘ \‘tleeewinn httt are not till \ttllieien! xi/e to be delinented\epttrntelt' sin the map. In both ilitTILI upeeiinen and thin i'eetion tug. speeinien 3b 365from (ttLlLIt or \‘Iurtingur; thet LtI'L’ very \‘itniILLr to the biotite gneiwes in thttt they areeotnpoxed o!" ttttttrtL t'elxpur ILHLI hiotite. b:tt in :idditzon IILtkeK or museovite oCL'ur.

Hornhlende gttcisHSH oeettt' :i\ dixet'ete bands in the bitiiite gneih zind hiL’diIC—t‘lttlxqtRi1:gnei'xs Ktteeession. In hand specimen they :1 re generally lettener'zttie melts with alternztt»Eng hands oJ‘ hornblende and Littr—“t/ and I'elxpur. 'lhe eolottr vi the hwnbiend: \:irie\from biztek to dark green (ind oee:i<it.in'.':lI} thii mineral oeettri. in Chile. A gnod eutnnileot‘ this ix \peeimen 25' 3}? from It point about [no and :t helf' miie~ [1t‘2Ili»(‘Ii\‘. ol'\:ti:i\\tirtt. In thin keetion the mineral: prefient are Keen to he hornblende. pleoehroieI'rom light green to dark green. ttuitrtV. miemeiine. (indesine and magnetite.

will] I'J'rt'tltllt'u'l .K‘JJfi‘r‘t'i.‘..tr 'H t-'-'- Ir}."‘-‘-'.-'I‘."r.' (HF-1.I Hr! PJ'IIII'IFL'It‘t'ft' (I‘MKI‘KM '\

In mutt} eases these ;[]CI\\:'\ Lire siniildr to tlte radiated biotite gneiwek \xhieh havebeen dexerihed in the pt‘c‘VittLh \eetion. they oeettr in the new L‘L‘UII‘LtI portion of theurea .ind have been mapped an an alternating \eries OI hiotite (ind hornblende gneisxes.Iixeept \\ here it has been neees's'ury to show the structure these roekq have not beenditi'erentiitted [N the Clutfigt’ from 0 ie melt type to [mother is very rapid. The hornblende{.{tte‘ixfie‘fi in this sneeeision .tre \intilar petrologieally to those already described (is tJL‘C‘t:'ng tix diwt'ete b.1nds in the biotite gnetxx and hintiteenttsxemit: gnu“ ‘LtL'e'C'~\IUII.

t1.) \-‘IFI‘\\IH|{I’I[£J.\'J t: P\\\f\tlt](' 5E.|!I'\II\IS

{sit Quart?“ .t't'iltputltfr Uriirmilitt'sIhe quart/.0 I‘eIdthie :2]‘LtIttlIiIt.‘\ Ltl'L‘ Ilit1!.l”Iti to he the end prodtiet of the metal-non

pltism of LH‘IxUSIL' hori/onx in the originril ‘iL‘LIiiTiL’ITI.tI'} Rer‘iex. In the tield the} t'ormIL".tlllt‘L'\' \thteh a re rexi<:;tnt to \xentherittg dtte to the titet that the_\ are largely eompmedot‘ titairt/ {and t'elspar and hme only minor untotzntx or mien. Otttempn Lite mufiive.oeemionully uith jointing at right angles to their strike and :tre relatively short andnttt'rotx. xttggexting thttt they were derived from impersistent bundx ut’ Ltrlx'oxie \edietTIL‘TtIS.

In hand aneeimen the grunulitex t'e .tnrolidted. IeLteoerutie. holoertxtullinc t'oel-csLIIILI \ur} in eolottr from white to hull. The dominant minerals LtI‘L‘ ttttttrtz LtJILI felspitr.but Rthilll liztkes or biotite and L'I'}'>tl.'dIh oi mgnetite L‘lLT] be seen to be settttered throughthe rnek in random orientation. In thin .xeetit‘n there is‘ :1 general ttnillormit ' ol‘ tTIITIL‘t’LLIeonipmition .md \ery little \-EU‘I;lEi0t‘t t'rom one apeeimen to another. A typieul examplent theae grttntilites ie ES 312. from the laiolorMursstbit road the milex north or Luisztntis.uhieh \IIi'I\\H at niosuie or tttiztt't/. ntieroeline and undextne ertwtaix \\ith hiotite tithesin rttndom orienttttion. Magnetite er} \tdlx are Ltlsu prew'ent in .‘JT‘ILLII LLHIL3LJEIIN scatteredthrough the slide. Sphene is present Ll'i an tteeemury mineral in Home or the speeimens,

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natably 28/336 fram twO' miles narth-west af Chaparkrom. This specimen daes natcantain biatite in the thin sectian, but atherwise the minerals present are the same asin 28/312. Epidate is present as a band appraximately ane millimetre thick in 28/338fram the western flank af Lasai. AlsO' present are quartz, micracline and andesinewith a few flakes af biatite.

(4) METAMORPHOSED CARBONACEOUSSEDIMENTS

(a) Graphite SchistsOne band af graphite schist was mapped an the eastern slapes af Baia. Specimen

28/347 is an ex:ample af this rack which in hand specimen is seen to' cantain flakesaf biatite and graphite. In thin sectian a certain amaunt af parallel orientatian af thegraphite and biatite flakes can be seen, with the spaces between these twO' mineralsfiHed by quartz. The graphite amounts to' abaut ane per cent by valume of the rack.

2. Intrusives into the Basement System

(1) Pre-metamorphic IntrusivesFive majar intrusians were faund in the area, a pyroxenite bady at Halisiruwa in

the narth-west, the Chaparkram-Saita Narok amphibalite in the sauth-east, a serpen-tinite lens intrusive intO' the Chaparkram bady, and twO' granitic intrusians, ane in thenarth-west and the secand to' the east af Baia.

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(a) The Halisiruwa PyroxeniteSeveral rack types make up this bady which cansists af Halisiruwa Hill, the twO'

hills immediately to' the sauth and the law ridge to' the west. The daminant rack isa garnetiferous pyroxenite but the narth-west face af the mast sautherly hill fromHalisiruwa is campased of garnetiferaus diorite. Diffuse lenses of quartz and quartz-felspar pegmatite occur but are rare. At the top of Halisiruwa Hill there is an actinoliteschist and a quartz-epidote rock with crystals of epidote up to' three centimetres inlength. On the ridge to' the west af Halisiruwa laminated garnetiferous harnblendegneisses are present in the lower part and pass upwards intO' a garnetiferaus pyraxenitewith epidiarite patches and lenses. On the eastern side af the ridge there is muchquartz flaat from quartz veins and small expasures af finely laminated harnblendegneiss with an equigranular texture. The laminatian appears to' be due to' ariginalcampasitianal variatians. There are alsO' same expasures af caarse gneissase harnblendegranite traversed by plagiaclase amphibalite sills.

(i) Garnetiferous. Pyroxenites

These farm the daminant rack type af the intrusian. In hand specimen they aremelanacratic halacrystalline racks with abundant garnets scattered thraugh them. Insame specimens the garnets make up as much as 30 per cent af the rack by valume.Specimen 28/289, a typical example af these pyraxenites, shaws in thin sectian amasaic af gamet, diapside and harnblende with very small quantities af plagiaclasefe1spar (labradorite). The harnblende was probably£mmed by the breaking dawn afdiapside. Specimen 28/307 is very similar to' 28/289 and carnes from a three-fa at lensin the garnetiferous diarite af the mast sautherly af the three hills. The main differencein thin sectian is that this specimen cantains nO' felspar and that hypersthene is alsO'present in large crystals.

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(ii) Garnetiferous DioritesThe whale af the narth-west face af the mast sautherly af the three hills is cam-

pasedaf a garnetiferous diarite, with diffuse bands varying from granitic to' gabbraicin campasitian. Quartz lenses and quartz-felspar pegmatites accur but are rare. Thisgarnetiferaus diarite has nat been differentiated an the map as it is nat passible to'shaw it an the scale used. A typical specimen, 28/306, is a leucacratic halacrystallinerack with quartz, andesine felspar, garnet and hornblende.

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natably 28/336 fram twO' miles narth-west af Chaparkrom. This specimen daes natcantain biatite in the thin sectian, but atherwise the minerals present are the same asin 28/312. Epidate is present as a band appraximately ane millimetre thick in 28/338fram the western flank af Lasai. AlsO' present are quartz, micracline and andesinewith a few flakes af biatite.

(4) METAMORPHOSED CARBONACEOUSSEDIMENTS

(a) Graphite SchistsOne band af graphite schist was mapped an the eastern slapes af Baia. Specimen

28/347 is an ex:ample af this rack which in hand specimen is seen to' cantain flakesaf biatite and graphite. In thin sectian a certain amaunt af parallel orientatian af thegraphite and biatite flakes can be seen, with the spaces between these twO' mineralsfiHed by quartz. The graphite amounts to' abaut ane per cent by valume of the rack.

2. Intrusives into the Basement System

(1) Pre-metamorphic IntrusivesFive majar intrusians were faund in the area, a pyroxenite bady at Halisiruwa in

the narth-west, the Chaparkram-Saita Narok amphibalite in the sauth-east, a serpen-tinite lens intrusive intO' the Chaparkram bady, and twO' granitic intrusians, ane in thenarth-west and the secand to' the east af Baia.

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(a) The Halisiruwa PyroxeniteSeveral rack types make up this bady which cansists af Halisiruwa Hill, the twO'

hills immediately to' the sauth and the law ridge to' the west. The daminant rack isa garnetiferous pyroxenite but the narth-west face af the mast sautherly hill fromHalisiruwa is campased of garnetiferaus diorite. Diffuse lenses of quartz and quartz-felspar pegmatite occur but are rare. At the top of Halisiruwa Hill there is an actinoliteschist and a quartz-epidote rock with crystals of epidote up to' three centimetres inlength. On the ridge to' the west af Halisiruwa laminated garnetiferous harnblendegneisses are present in the lower part and pass upwards intO' a garnetiferaus pyraxenitewith epidiarite patches and lenses. On the eastern side af the ridge there is muchquartz flaat from quartz veins and small expasures af finely laminated harnblendegneiss with an equigranular texture. The laminatian appears to' be due to' ariginalcampasitianal variatians. There are alsO' same expasures af caarse gneissase harnblendegranite traversed by plagiaclase amphibalite sills.

(i) Garnetiferous. Pyroxenites

These farm the daminant rack type af the intrusian. In hand specimen they aremelanacratic halacrystalline racks with abundant garnets scattered thraugh them. Insame specimens the garnets make up as much as 30 per cent af the rack by valume.Specimen 28/289, a typical example af these pyraxenites, shaws in thin sectian amasaic af gamet, diapside and harnblende with very small quantities af plagiaclasefe1spar (labradorite). The harnblende was probably£mmed by the breaking dawn afdiapside. Specimen 28/307 is very similar to' 28/289 and carnes from a three-fa at lensin the garnetiferous diarite af the mast sautherly af the three hills. The main differencein thin sectian is that this specimen cantains nO' felspar and that hypersthene is alsO'present in large crystals.

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(ii) Garnetiferous DioritesThe whale af the narth-west face af the mast sautherly af the three hills is cam-

pasedaf a garnetiferous diarite, with diffuse bands varying from granitic to' gabbraicin campasitian. Quartz lenses and quartz-felspar pegmatites accur but are rare. Thisgarnetiferaus diarite has nat been differentiated an the map as it is nat passible to'shaw it an the scale used. A typical specimen, 28/306, is a leucacratic halacrystallinerack with quartz, andesine felspar, garnet and hornblende.

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notably 28(336 from two miles north-west of Cliaparkrom. This specimen does notcontain biotite in the thin section. but otherwise the minerals present are the same asin 28 312. Epidote is present as a band approximately one millimetre thick in 28,338from the western flank oi." Losai. Also present are quartz. microeline and andesinewith a few llakes of biotite.

(4) Misrxioapuoseo CKRBL).\ACF(_)L'S SFDIMENTS

m) (Imp/tile SchismOne hand of graphite schist was mapped on the eastern slopes of Baio. Specimen

28 347 is an example of this rock which in hand specimen is seen to contain flakesot biotite and graphite. in thin section a certain amount oi.‘ parallel orientation of thegraphite and biotite flakes can be seen. with the spaces between. these two mineralsfilled by quartz. The graphite amounts to about one per cent by Volume of the rock.

2. Intrusives into the Basement System( l) PH" meltmwrphic IntrusiVes

Five major intrusions were found in the area. a pyroxenite body at. Halisiruwa inthe north—west. the Chaparkrom—Soito Narok amphibolite in the south—east. a serpen—tinite lens intrusive into the (.“haparkrom body. and two granitic intrusions, one in thenorth-west and the second to the east of Baio.

(at) The Huli.i'irim‘ct I’yrus‘erziieSeveral rock types make up this body which consists of Halisiruwa Hill. the two

hills immediately to the south and the low ridge to the west. The dominant rock isa garnetiferous pyroxenite but the north-west face of the most southerly hill fromHalisiruwa is composed of garnetiferous diorite, Dittuse lenses of quartz and quartz.-l‘elspar pegmatite occur but are rare. At the top of Halisiruwa Hill there is an actinoliteschist and a ouartx-epidote rock with crystals of epidote up to three centimetres inlength. On the ridge to the west of Halisiruwa laminated garnetitcrous hornblendegneisses are present. in the lower part and pass upwards into a garnetitet‘ous p_\_-’roxenitewith epitiiorite patches and lenses. On the eastern side of the ridge there is muchquart; float from quartz veins and small exposures ot‘ finely laminated hornblendegneiss with an equigranular texture. The lamination appears to be due to originalcompositional variations. There are also sortie exposures of coarse gneissose hornblendegranite traversed by plagioelase amphibt‘)lite sills.

ti) (im-Iteifft’r'ntn' P_Vrn.\'t’nift’.i'These form the dominant rock type of the intrusion. In hand specimen they are

melanocratic holoerystalline rocks with abundant garnets scattered through them. Insome specimens the garnets make up as much as 30 per Cent of the rock by volume.Specimen 283239. a typical example of these pyroxenites. shOWs in thin section amosaic of garnet. diopside and hornblende with very small quantities of plagioclaset'elspar [labradoritet The hornblende was probably formed by the breaking down ofdiopside. Specimen 28,307 is very similar to 281‘s”) and comes from a three—foot lensin the garnetiferous diorite of the most southerly of the three hills. The main differencein thin section is that this specimen contains no t'elspar and that hypersthene is alsopresent in large crystals.

iii) Gurneir'ft’rous [JinrirerThe whole of the north-west face of the most southerly of the three hills is com-

posed of a garnetiferous diorite. with dittuse bands varying from granitic to gabbroiein composition. Quartz lenses and quartz-felspar pegmatites occur but are rare. Thisgarnetiferous diorite has not been differentiated on the map as it is not possible toshow it on the scale used. A typical specimen. 28306. is a leucocratic holocrystallinerock with quartz. andesine felspar. garnet and hornblende.

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(iii) Actinolite Schists and Quartz-epidote Rocks

Both these rocks occur at the top of Halisiruwa Hill but do not form outcrops ofany appreciable size, covering an area of only approximately 50 sq. feet. The actinoliteschist, 28/287, is green in colour and shows a network of fibrous crystals of actinolite.Thin sections show fibrous actinolite and large actinolite crystals with a sub-parallelalignment. No other minerals are present. It is likely that the actinolite has beenderived from the metamorphism of original pyroxenes. The quartz-epidote rock, 28/288,shows crystals of green epidote up to three centimetres in length together with smallquantities of crystalline quartz.

(iv) The Origin of the Halisiruwa Intrusion

There are two possible hypotheses as to the origin of this pyroxenite. The first isthat the body was emplaced as an eclogite and suffered retrogressive metamorphism toform a pyroxenite. Eclogites are rocks of high specific gravity composed of omphaciteand garnet. Omphacite differs from the pyroxenes of normal igneous rocks in that ithas a high content of Na20 and Al203 and negligible amounts of Ti02. The garnetscharacteristic of this rock are lime-bearing almandine-pyropes. In chemical compositioneclogite is equivalent to an olivine gabbro. The origin of eclogites is still a matter ofdispute, but it is generally considered that they are products of high-grade regionalmetamorphism and have been formed at depth at high temperatures and pressures.

If then the Halisiruwa pyroxenite body were derived from an eclogite, it wouldseem probable that it was emplaced prior to the deposition of the sediments that nowmake up the Basement System rocks of the area, and is the product of a much earliermetamorphism which was of the eclogite facies and of which now the only remnantsare at Halisiruwa. With the onset of the metamorphism of lower grade that succeededthe deposition of the sediments which now form the gneisses and granulites of thearea the eclogite body suffered retrogressive metamorphism. Eclogites are verysusceptible to diaphthoresis and the omphacite crystals break down in the initial stagesto diopside and plagioclase felspar, and finally go to hornblende. The garnets changeto hornblende and epidote. In the thin sections of the rocks collected from the Hali-siruwa pyroX'enite it is seen that both diopside and garnet are present but felspar ispresent only in very small quantities in one specimen. Epidote occurs as large crystalson the top of Halisiruwa Hill, as does an actinolite schist.

The second hypothesis is that the body was emplaced as a pyroxenite prior to orduring the first stages of the metamorphism of the sediments and that this metamor-phism caused the breakdown of the pyroxenes to hornblende, the garnets being formedat the same time. The presence of the granitizing fluids associated with the metamor-phism led to the formation of the quartz and pegmatite veins and to the productionof the garnetiferous diorite.

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(b) The Chaparkrom-Soito Narok AmphiboliteThis intrusion has an arcuate form and extends into the area to the south. It is

concordant with the Basement System rocks of the south-eastern part of the area andhas been folded with them. The intrusion varies in width from about 1,200 feet inthe south to about 6,500 feet at Chaparkrom. At Soito Narok the low hill formingthe outcrop is practically bare rock and the amphibolite appears as a black rock withglistening prisms of amphiboles. In places this intrusion bears a resemblance to theHalisiruwa body in that garnet and epidote are present locally, but the former neverreaches the proportions in which it is present at Halisiruwa.

The thin section of one of the specimens collected from Chaparkrom, 28/325, showshornblende, pleochroic from bright green to dark green, with quartz and epidote.Felspar is rather rare but is present in this section as labradorite, and pyroxene relicsare also present. This section shows a roughly developed alignment of the hornblende

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(iii) Actinolite Schists and Quartz-epidote Rocks

Both these rocks occur at the top of Halisiruwa Hill but do not form outcrops ofany appreciable size, covering an area of only approximately 50 sq. feet. The actinoliteschist, 28/287, is green in colour and shows a network of fibrous crystals of actinolite.Thin sections show fibrous actinolite and large actinolite crystals with a sub-parallelalignment. No other minerals are present. It is likely that the actinolite has beenderived from the metamorphism of original pyroxenes. The quartz-epidote rock, 28/288,shows crystals of green epidote up to three centimetres in length together with smallquantities of crystalline quartz.

(iv) The Origin of the Halisiruwa Intrusion

There are two possible hypotheses as to the origin of this pyroxenite. The first isthat the body was emplaced as an eclogite and suffered retrogressive metamorphism toform a pyroxenite. Eclogites are rocks of high specific gravity composed of omphaciteand garnet. Omphacite differs from the pyroxenes of normal igneous rocks in that ithas a high content of Na20 and Al203 and negligible amounts of Ti02. The garnetscharacteristic of this rock are lime-bearing almandine-pyropes. In chemical compositioneclogite is equivalent to an olivine gabbro. The origin of eclogites is still a matter ofdispute, but it is generally considered that they are products of high-grade regionalmetamorphism and have been formed at depth at high temperatures and pressures.

If then the Halisiruwa pyroxenite body were derived from an eclogite, it wouldseem probable that it was emplaced prior to the deposition of the sediments that nowmake up the Basement System rocks of the area, and is the product of a much earliermetamorphism which was of the eclogite facies and of which now the only remnantsare at Halisiruwa. With the onset of the metamorphism of lower grade that succeededthe deposition of the sediments which now form the gneisses and granulites of thearea the eclogite body suffered retrogressive metamorphism. Eclogites are verysusceptible to diaphthoresis and the omphacite crystals break down in the initial stagesto diopside and plagioclase felspar, and finally go to hornblende. The garnets changeto hornblende and epidote. In the thin sections of the rocks collected from the Hali-siruwa pyroX'enite it is seen that both diopside and garnet are present but felspar ispresent only in very small quantities in one specimen. Epidote occurs as large crystalson the top of Halisiruwa Hill, as does an actinolite schist.

The second hypothesis is that the body was emplaced as a pyroxenite prior to orduring the first stages of the metamorphism of the sediments and that this metamor-phism caused the breakdown of the pyroxenes to hornblende, the garnets being formedat the same time. The presence of the granitizing fluids associated with the metamor-phism led to the formation of the quartz and pegmatite veins and to the productionof the garnetiferous diorite.

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(b) The Chaparkrom-Soito Narok AmphiboliteThis intrusion has an arcuate form and extends into the area to the south. It is

concordant with the Basement System rocks of the south-eastern part of the area andhas been folded with them. The intrusion varies in width from about 1,200 feet inthe south to about 6,500 feet at Chaparkrom. At Soito Narok the low hill formingthe outcrop is practically bare rock and the amphibolite appears as a black rock withglistening prisms of amphiboles. In places this intrusion bears a resemblance to theHalisiruwa body in that garnet and epidote are present locally, but the former neverreaches the proportions in which it is present at Halisiruwa.

The thin section of one of the specimens collected from Chaparkrom, 28/325, showshornblende, pleochroic from bright green to dark green, with quartz and epidote.Felspar is rather rare but is present in this section as labradorite, and pyroxene relicsare also present. This section shows a roughly developed alignment of the hornblende

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{iii} At‘rirmlite Sc/‘tis‘ts' and Quart: epidote RocksBoth these rocks occur :it the top oi Halisit‘ttwa Hill but do not form outcrops of

any appreciable size, covering an area oi only approsimately it) sq. feet. The tlctinoiiteschist, 28 28'}. is green in colour and shows a network of fibrous crystals of actinolite.Thin sections show fibrous actinolite and large actinolite crystals with a sub—parallelalignment. .\'o other minerals are present. it is likely that the actinolite has beenderived from the metamorphism of original pyroxenes. The quartz-epidote rock. 28,5288.shows crystals of green epidote up to three centimetres in length together uith smallquantities of crystalline quartz.

tiv) The Origin of the Iluiisirmx'u IntrusionThere are two possible hypotheses as to the origin of this pyroxenite. The first is

that the body was emplaced as an eclogite and sutTered retrogressive metamorphism toform a pyroxenite. .Eclogites are rocks of high specific gravity composed of omphaciteand garnet. Omphacite difi‘ers from the pyroxenes of normal igneous rocks in that ithas a high content of Nat) and Argo: and negligible amounts ot‘ 'l‘it)._., 'I‘he garnetscharacteristic of this rock are lime-hearing almandine-pyropes. In chemical compositioncclogite is equivalent to an olivine gabbro. The origin or eclogites is still a matter ofdispute. but it is generally considered that they are products of high—grade regionalmetamorphism anti have been formed at depth at high temperatures and pressures.

if then the l-[alisiruwa pyroxenite body xtere derived from an eclogite. it wouldseem probable that it was emplaced prior to the deposition of the sediments that nowmake up the Basement. System rocks of the area. and is the product of a much earliermetamorphism which was or the eclogite facies and of which now the only remnantsare at Halisiruwa. W'ith the onset of the n'tetamorphism of lots or grade that succeededthe deposition of the sediments which now form the gneisses and granulites of thearea the eclogite body sutl‘ered retrogressive metamorphism. Eclogites are verysusceptible to diapl'ithoresis anti the omphacite crystals break down in the initial stagesto diOpside and plagioclase lelspar. and tinally go to hornblende. The garnets changeto hornblende and epidote. In the thin sections of the rocks collected from the Hali-siruwa pyroxenite it is seen that both diopside and garnet are present but felspar ispresent. only in very small quantities in one specimen. Epidotc occurs as large crystalson the top of Halisiruua Hill. as does an actinolite schist.

The second hypothesis is that the body was emplaced as a pyroxenite prior to orduring the first Stages of the metamorpl'tism of the sediments and that this metamor—phism caused the breakdown of the pyroxenes to hornblende. the garnets being formedat the same time. The presence of the graniti/ing tluids associated with the metamor—phism led to the formation of the qttartl. and pegmatite veins and to the productionof the garnetiferous diorite.

lit) The C‘lmparkrrmtagain: Nurok Amp/tihulitc'l'his intrusion has an arctlate form and extends into the area to the south. It is

concordant with the Basement System rocks of the south-eastern part. of the area andhas been folded with them. The intrusion varies in width from about 1.200 feet inthe south to about (3,3500 feet at. Chaparkrom. At Soito Narok the low hill formingthe outcrop is practically bare rock and the amphibolite appears as a black rock withglistening prisms ot‘ amphiboles. In places this intrusion bears a resemblance to theHalisiruwa body in that garnet and epidote are present locally. but the former neverreaches the proportions in which it is present at 'Halisiruwa.

The thin section of one of the specimens collected from Chaparkrom. 28(325. showshornblende, pleoehroic from bright green to dark green. with quartz and epidote.Felspar is rather rare but is present in this section as labradorite. and pyroxene relicsare also present. This section shows a roughly developed alignment of the hornblende

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prisms. From the outcrop at Soito Narok, specimen 28/327 shows felspar present ingreater proportions, and in this specimen is andesine. Hornblende, epidote and quartzare again present as are relics of pyroxenes. Specimen 28/332 is a green holocrystallinerock composed of hornblende with needles of epidote.

From the evidence of relics of pyroxenes which are present in some slides it is likelythat this intrusion was emplaced as a gabbro. During the metamorphism and foldingof the Basement System the pyroxenes were changed to amphiboles and a crudefoliation was impressed on the rock by the parallel or sub-parallel alignment of theamphibole prisms.

(c) Serpentinite

This forms a lens-like body within the amphibolites of Chaparkrom aligned parallelto the length of the intrusion and hence parallel to the regional strike, which at thispoint is east-west. In length the lens reaches nearly three-quarters of a mile, with anoutcrop width of about 200 yards. Along the contact between the serpentinite and theamphibolite there is no indication of any metamorphic effects and it is assumed thatthe serpentinite was intruded at a temperature which was low enough not to produceany contact phenomena. In hand specimen (28/333) it is dark green in colour, homo-geneous and unfoliated, and thin section examination shows olivine crystals surroundedby fibrous antigorite and accessory magnetite. This body is cut by veins of magnesiteseldom more than three or four inches thick, which have a random orientation.

(d) Granites.

Two occurrences of granite were mapped in the area. The first is in the north-westof the area, to the west of Halisiruwa at the southern end of a short river exposure.It is a faintly gneissose porphyritic granite with offshoots which penetrate the surround-ing hornblendic rocks. In hand specimen (28/309) it is a mesocratic holocrystallinerock, the thin section showing quartz, andesine felspar, biotite and small quantities ofmuscovite.

The second exposure of granite is on the plain below the eastern slopes of Baio.Several small outcrops are exposed there and form impressive features which riseabruptly from the plain and present a bare rock surface with little or no vegetation.They bave the form of exfoliation domes, being elliptical or round in plan and posses-sing a crude concentric foliation, probably due to temperature variations.

In hand specimen this rock is leucocratic and holocrystalline with visible quartz,felspar and abundant biotite. Specimen 28/352, a typical example, in thin section isseen to be composed of quartz, albite felspar, biotite and small quantities of hornblende.There is no indication of any contact metamorphic phenomena between the graniteand the country rock, but it has been noted that at Rosguill, Co. Donegal, Ireland(Knill, 1961), that the emplacement of a granodiorite into regionally metamorphosedsediments has only produced porphyroblastic biotite and small quantities of cordieriteand muscovite in the contact aureole.

(e) Pegmatites

Pegmatites are common in the Basement System rocks of the area, and vary in sizefrom a few inches up to siX' feet in width. The majority of the larger pegmatites areconcordant with the strike of the Basement System rocks, but the smaller ones tend tocross-cut. All are leucocratic, coarse-grained rocks consisting dominantly of quartzand microcline felspar. Small amounts of other minerals are present in some, notablymuscovite and magnetite, but neither occurs in economic concentrations.

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prisms. From the outcrop at Soito Narok, specimen 28/327 shows felspar present ingreater proportions, and in this specimen is andesine. Hornblende, epidote and quartzare again present as are relics of pyroxenes. Specimen 28/332 is a green holocrystallinerock composed of hornblende with needles of epidote.

From the evidence of relics of pyroxenes which are present in some slides it is likelythat this intrusion was emplaced as a gabbro. During the metamorphism and foldingof the Basement System the pyroxenes were changed to amphiboles and a crudefoliation was impressed on the rock by the parallel or sub-parallel alignment of theamphibole prisms.

(c) Serpentinite

This forms a lens-like body within the amphibolites of Chaparkrom aligned parallelto the length of the intrusion and hence parallel to the regional strike, which at thispoint is east-west. In length the lens reaches nearly three-quarters of a mile, with anoutcrop width of about 200 yards. Along the contact between the serpentinite and theamphibolite there is no indication of any metamorphic effects and it is assumed thatthe serpentinite was intruded at a temperature which was low enough not to produceany contact phenomena. In hand specimen (28/333) it is dark green in colour, homo-geneous and unfoliated, and thin section examination shows olivine crystals surroundedby fibrous antigorite and accessory magnetite. This body is cut by veins of magnesiteseldom more than three or four inches thick, which have a random orientation.

(d) Granites.

Two occurrences of granite were mapped in the area. The first is in the north-westof the area, to the west of Halisiruwa at the southern end of a short river exposure.It is a faintly gneissose porphyritic granite with offshoots which penetrate the surround-ing hornblendic rocks. In hand specimen (28/309) it is a mesocratic holocrystallinerock, the thin section showing quartz, andesine felspar, biotite and small quantities ofmuscovite.

The second exposure of granite is on the plain below the eastern slopes of Baio.Several small outcrops are exposed there and form impressive features which riseabruptly from the plain and present a bare rock surface with little or no vegetation.They bave the form of exfoliation domes, being elliptical or round in plan and posses-sing a crude concentric foliation, probably due to temperature variations.

In hand specimen this rock is leucocratic and holocrystalline with visible quartz,felspar and abundant biotite. Specimen 28/352, a typical example, in thin section isseen to be composed of quartz, albite felspar, biotite and small quantities of hornblende.There is no indication of any contact metamorphic phenomena between the graniteand the country rock, but it has been noted that at Rosguill, Co. Donegal, Ireland(Knill, 1961), that the emplacement of a granodiorite into regionally metamorphosedsediments has only produced porphyroblastic biotite and small quantities of cordieriteand muscovite in the contact aureole.

(e) Pegmatites

Pegmatites are common in the Basement System rocks of the area, and vary in sizefrom a few inches up to siX' feet in width. The majority of the larger pegmatites areconcordant with the strike of the Basement System rocks, but the smaller ones tend tocross-cut. All are leucocratic, coarse-grained rocks consisting dominantly of quartzand microcline felspar. Small amounts of other minerals are present in some, notablymuscovite and magnetite, but neither occurs in economic concentrations.

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prism» l-‘rom the outcrop at Sotto Varok. specimen 28 *2? shows t'elspttr present ingreater t‘ut'oportiom. and in this specimen is Lill‘clL—Nll'tc. llornblende. epitiotc Zi‘flLl quartz

3are again present are are relicx ot' pyroxene» Specimen 3V 3 2 is a green holocrystallinerock cornpmcti ot' hornblende with neetilex‘ of cpitlotc.

From the evidence ol' relics of pyroxenek which are present in some SllLlCS it is likelythat this intrusion was ernplaccti a: a gttbhro. During. the n‘tetamorphimt and foldingot' the Basement System the pyroxenes were changed to amphiboles and a crudefoliation was imprexsed on the l'tVL'lx hy the parallel or sub-parallel alignment. ol‘ theamphibole prifirns.

I. ("l .Vt'rpt rtlittitt'

This t‘m‘nta a lens-like body within the antphibolitci 0t Chapt'trkrom aligned parallelto the length otw the intrusion and hence parallel to the regional strike. which at thispoint is east went. ln length the lens l'L‘LtL‘l'tL‘s‘ nearly three tguat'tcra oti {1 mile. with anoutcrop witlth ot' about Zttlt _\'Jt"kl'\ Along ll'lt.‘ contact between the \crpentinite am! theamphibolite there is no indication of any metamorphic cll'ccte and it i< assumed thatthe serpentinite \yttR intt'utieti at a temperature which VH1." low enough not to produceany contact phenomena. In hantl specimen 423‘ I???) it is dark green in colour. homo-genentm and unioliated. and thin section examination shows olivine crystals surroundedby tibs'otts antigorite and ac " gory magnetite, This bocly is cut by veins of magnesiteseldom more than three or four inches thick. which h_'-tye ..t random orientation.

it!) ('tmiter.

'l‘wo occurrences of granite were mapped in the area. the tirst is in the mirth-westoi the area. to the west of Httlisiruwa at the southern end in :1 <hot't river exposure.it i: a faintly gneismsc porphyritic granite with otls‘ltooti which penetrate the Stll't'tttll'lfil-int; liornblentlic rocks. In hantl specimen tZN MU) it is a mesoertttic l'ttllt.)Cl'}'\'l'illiil'lCrock. the thin section showing quartz. tintiesinc lielspttt'. biotite and small uuantitiefi ofrl'lllRC'JVlt'c‘.

”the RL‘LIOIltl L-‘tpoture of granite is on the plain below the eastern Slopes ol‘ tio.Seyernl antull UtllL‘t't‘pR are expoxecl there .tnti form impreviyc l‘eatu‘ex' which riseabruptly from the plain and prexcnt a bare rock xurte r: with little o; no \e_t{e:;.‘.ion.'i'hcy‘ have the form ot' exfoliation domes. being elliptiLctl or round in plan and pox-sex-sing. a crude concentric toliation. probably LlLlC to temperature yariatitms.

In ltElTlL'l flpecintcn this rock is lcucocrwtic anti hulocryx‘tiflline with Visible quartz.i'cl'ipar and abundant biotite. Specimen IN “‘52. a typical example. in thin SCCEiOI] isseen to he composed of quartz. albite l'clspa r. biotite and small quantities oi hornblende.There i; no indication of any contact metamorphic phenomena between the graniteand the country rock. but it ha: been noted that 111 Roigtull. (To. [)onegttl. Irelantillx'nill. 19M). that the ei'nplaccmcnt of a fll'itl‘ltllllt'tl'llc into regionally n‘tetamorphusetiseetlimett'ts haq only prottucctl [tut'taltyt'tthlax'tic hiotitc and small quantities ot‘ cortiicritcatntl rnuscoyite in the contac aureolc.

tt'] [’r'gititttitr’t‘

I’egt‘ntttites are common in the Btucmcnt Syfitcnt rocks of the area. and vary in silt:item a few inches up to \i\ feet in wi hit. The majority oi the larger pegmettites arecrmcortlztnt with the strike ot‘ the Baaement S}Ntct't't Ttl‘c‘lx‘. but the «matter onee tend tocrow-cut. All are lcucoct'utic. coat‘Re-graineti l'\\L'l\\ coneis‘ting. dominantly of quart?and micrttcline felnpar'. Swell entountx of other ntinct'alx' arc. prevent in some. notablyrrtuscmite anti magnetite. but neither occurs in econot‘nic concentrations.

I 1

- --1

(2) QUARTZ VEINS

These are numerous in the metamorphic rocks of the area but never attain a largesize and none have well-formed crystals of quartz. In only one case is there any mine-ralisation associated with them, in the O'Iltcrop to the east of Halisiruwa Hill, wherethere are needles of tourmaline.

3. Tertiary Sediments

These occur in the south-eastern portion of the area and extend in a north-westerlydirection. They underlie the Tirkamo Plateau and the eas-tem side of the RusarusPlateau. A vertical section at the northern end of the Rusarus Plateau shows thefollowing succession:-

9. Olivine basalt ...

8. Brown sand with quartz grains

7. Red and green sand with quartz pebbles...

6. White sand with quartz pebbles ..' ...

5. Local wedges of quartz conglomerate and sand

4. Red and green sand with quartz and felspar

3. Brown sand with quartz and quartzo-felspathic pebbles

2. Reddish sand and quartz grains . .. .. . . ..

1. Red and green sand with quartz, felspar and asbestos fibres

approx. thickness (feet)

(25)20

12

6

1.5

8

12

9

9

II

III

77.5I,

--

The mineral assemblages and fragments of rocks present indicate derivation fromBasement System rocks. There is no sorting in the individual units of the sedimentsand the pebbles and sand grains are intimately mixed. The sand is poorly washed andunconsolidated, and in attitude the sediments are horizontal to sub-horizontal.

The succession of the sediments is much thicker at the southern end of the RusarusPlateau than at the northern end, and is also thicker under the olivine basalt of the

Tirkamo Plateau than at the latter locality. These sediments are probably equivalentto the Merti Beds of the Chanler's Falls area (Williams, 1966, p. 36), and the sedimentsunderlying the Barchuma Plateau of the Barchuma-Kom area (Dodson, Report No. 93,in preparation). The sediments of both these areas are similar lithologically to thesediments of the present area. The sediments were deposited in a trough in the end-Tertiary erosion surface and it is likely that this trough was produced by local down-warping of the surface, so that the sediments are probably late Tertiary in age. Sincethe sediments are ungraded and unsorted a rapid mode of deposition is indicated. Itis probable that the material of the sediments was transported by water, possiblyduring periods of heavy rain, and swept into the trough from the surrounding higherground. The angularity of the fragments precludes their having been transported overgreat distances. No traces of organic remains were found in the sediments.

I.

II

II

II

'I

III

,I

4. Pleistocene and Recent Volcanic Rocks

The lack of fossils in the sediments underlying the Rusarus and Tirkamo plateauxmakes it difficult to date the overlying volcanics with any degree of certainty. The agerange is probably from Pleistocene to Recent, although the lower age limit of theolivine basalt plateau lavas could possibly be put in the late Tertiary since they preservethe sediments which accumulated during the end-Tertiary cycle of peneplanation.

II

\1

13 I'

~,

I-- - - - - - - - -

i

II~J

- --1

(2) QUARTZ VEINS

These are numerous in the metamorphic rocks of the area but never attain a largesize and none have well-formed crystals of quartz. In only one case is there any mine-ralisation associated with them, in the O'Iltcrop to the east of Halisiruwa Hill, wherethere are needles of tourmaline.

3. Tertiary Sediments

These occur in the south-eastern portion of the area and extend in a north-westerlydirection. They underlie the Tirkamo Plateau and the eas-tem side of the RusarusPlateau. A vertical section at the northern end of the Rusarus Plateau shows thefollowing succession:-

9. Olivine basalt ...

8. Brown sand with quartz grains

7. Red and green sand with quartz pebbles...

6. White sand with quartz pebbles ..' ...

5. Local wedges of quartz conglomerate and sand

4. Red and green sand with quartz and felspar

3. Brown sand with quartz and quartzo-felspathic pebbles

2. Reddish sand and quartz grains . .. .. . . ..

1. Red and green sand with quartz, felspar and asbestos fibres

approx. thickness (feet)

(25)20

12

6

1.5

8

12

9

9

II

III

77.5I,

--

The mineral assemblages and fragments of rocks present indicate derivation fromBasement System rocks. There is no sorting in the individual units of the sedimentsand the pebbles and sand grains are intimately mixed. The sand is poorly washed andunconsolidated, and in attitude the sediments are horizontal to sub-horizontal.

The succession of the sediments is much thicker at the southern end of the RusarusPlateau than at the northern end, and is also thicker under the olivine basalt of the

Tirkamo Plateau than at the latter locality. These sediments are probably equivalentto the Merti Beds of the Chanler's Falls area (Williams, 1966, p. 36), and the sedimentsunderlying the Barchuma Plateau of the Barchuma-Kom area (Dodson, Report No. 93,in preparation). The sediments of both these areas are similar lithologically to thesediments of the present area. The sediments were deposited in a trough in the end-Tertiary erosion surface and it is likely that this trough was produced by local down-warping of the surface, so that the sediments are probably late Tertiary in age. Sincethe sediments are ungraded and unsorted a rapid mode of deposition is indicated. Itis probable that the material of the sediments was transported by water, possiblyduring periods of heavy rain, and swept into the trough from the surrounding higherground. The angularity of the fragments precludes their having been transported overgreat distances. No traces of organic remains were found in the sediments.

I.

II

II

II

'I

III

,I

4. Pleistocene and Recent Volcanic Rocks

The lack of fossils in the sediments underlying the Rusarus and Tirkamo plateauxmakes it difficult to date the overlying volcanics with any degree of certainty. The agerange is probably from Pleistocene to Recent, although the lower age limit of theolivine basalt plateau lavas could possibly be put in the late Tertiary since they preservethe sediments which accumulated during the end-Tertiary cycle of peneplanation.

II

\1

13 I'

~,

I-- - - - - - - - -

i

II~J

.; 2} Qt '.\t<']';/. Viass

'I'hese are numerous in the n'tetamor; tit; rocks ol‘ the area but never attain a largesire and none have well-I‘ormed crystals of quartz. In only one ease is there any mineraiisation associated with them. in the outcrop to the east ol' Haiisfran a Hill. wherethere are needles of tourmaline.

3. Tertiary Sediments

'l hese occur in the south-eastern portion oi the area and extend in a north-westerlydirection. 'lhcy underlie ilte 'Iir's'atno Plateau and 1h: eastc'n site ol' the RttsarusPlateau. A vertical section at. the northern end of the Rusarus l’lateat; shows thefollowing succession:

uppity-g. .thif‘ilt'ur'ss five!)

L).('Jliy'inel_\..'ts.1|: .. .. . IIF'iS. Brown sand w'ith quart? grains . . El}7". Red and green sand saith uuartx pebbles . . . ll(.1. White sand with quartz. pebbles . . 65. Local wedges or quartz conglom ’rate and sand . 1.5-t. Red and green sand with quartz. and tel-spar . . . H3. Bronn sand \tith quartx and cluart/u-t'clspathic pebbles I:2. Rcdtlish sand and quartz grains .. . 91. Red and green sand with quartz. i'elspar and asbestos tihres 9

77.5

The mineral assemblages and fragments ol‘ rocks present indicate derivation fromBasement System rocks. 'lhere is no sorting in the Endiyidual units ol‘ the sedimentsand the pebbles and sand grains are intimately mixed. the sand is poorly washed andunconsolidated. and in attitude the sediments are horiyontal to sub-liorimnliil.

The succession of the sediments is much thicker at the southern end of the RusarusPlateau than at the northern end. and is also thicker under the olivine basalt of the'l'irkamo Plateau than at the latter locaiity. 'I'hese sediments are probably equivalent.to the \ilerti Beds ol‘ the C'hanler's Falls area {\‘t-"illiants. [066. p. 36}. and the sedimentsunderlying the Barchuma Plateau of the Barchurna-lx‘oiti area tDndson. Report No. 93.in l‘t‘i’ptkrationt. The sedin'tents of both these areas are similar lithologically to thescdin'tents oi the present area. 'Ihe sediments were deposited in a trough in the end-l'ertiary? erosion surface and it is likely that this trough was produced by local down-“ilt't‘tiiné Of the surface. so that the sediments are probably late 'l‘ertiary in age. Sincethe sediments are ungraded and unsorted a rapid mode 01' deposition is indicated. iti“ Probable that the n'iaterial of the sediments nets transported by water. possiblyduring periods of heavy rain. and swept. into the trough from the surrounding higherETULH‘IU. 'l'he angularity of the fragments precludes their haying been transported oyergreat distances. \o traces ol‘ organic remain-s were lound in the sediments.

4. Pleistocene and Recent Volcanic Rocks

'llie lack of fossils in the sediments underlying the Rusat'us and 'I'irkanto piaicauxmalt-es it ditlicait to date the overlying volcanies nitlt any degree at certainty. The agerange is probably from Pleistocene to Recent, although the loner age limit of theolivine basalt plateau lavas could possibly be put in the late 'ifiertiary since they preservethe sediments which accumulated during the end l'ertiary cycle of penetflanation.

~ -- --

For the purpose of petrographic description the volcanic rocks of the area aredescribed as follows:-

(1) Olivine basalt plateruu lavas.

(2) Olivine-augite basalts of the Kaisut Plateau.

(3) Analcime basalts.

(4) Vents and associated olivine basalt lavas.

(4) Tuffs and agglomerates.

(6) Explosion craters.

Figure 2 shows the distribution of the volcanic rocks in the area and it can be seenthat the last three groups are concentrated in the eastern part of the area. The explo-sion craters are particularly well preserved, as are the volcanic vents north-west ofLaisamis.

14

~ -- --

For the purpose of petrographic description the volcanic rocks of the area aredescribed as follows:-

(1) Olivine basalt plateruu lavas.

(2) Olivine-augite basalts of the Kaisut Plateau.

(3) Analcime basalts.

(4) Vents and associated olivine basalt lavas.

(4) Tuffs and agglomerates.

(6) Explosion craters.

Figure 2 shows the distribution of the volcanic rocks in the area and it can be seenthat the last three groups are concentrated in the eastern part of the area. The explo-sion craters are particularly well preserved, as are the volcanic vents north-west ofLaisamis.

14

For the purpose of petrographic description the volcanic rocks of. the area aredescribed as follows:—

(1] Olivine basalt plateau lavas.

(2) Olivine-augite hasalts of the Kaistit Plateau,

H) Analcime basalts.(4; Vents and associated olivine basalt lavas.

(4‘) Tutfs and agglomerates.i6) Explosion craters,

Figure 7 shows the distribution of the volcanic rocks in the area and it can be seen,4.

that the last three groups are concentrated in the eastern part of the area. The explo-sion craters are particularly well preserved, as are the voleanie vents north-west oflaisamis.

14

l' OO'N.

.....;<:.0'"......, '--

~~ ~~

~

KAISUT

DESERTIii:""

\ r--

0

~--r" /----I"'''~/'Y-j ,.-'

/'-' r""~--"'-"'-X' /

-:;? ~~ I-~, r- NDONYO

. Q ( MAU

\ r .PLATEAU

)Ma'ti.s.,

(ijr-!

\~)

(

....<>

~'"

IIIIIII

~I

'I

I

SCALE,030'N

0 5L

10 15 MILES

F==1 LowerPleistoc,

ene

t::=:::J olivine basalts -Recent lovas

V///1 M,ddle Upper Pleistocene~ oJivine augite basalts r':-:-:'>I Recent ashes and ash: ::: :>::: and agglomerate cones

II Upper Pleistoceneanalc,me, basalts ~ Explosion craters"

Fig. 2-Sketch-map of the volcanic history of the Laisamis area.

15

I

l' OO'N.

.....;<:.0'"......, '--

~~ ~~

~

KAISUT

DESERTIii:""

\ r--

0

~--r" /----I"'''~/'Y-j ,.-'

/'-' r""~--"'-"'-X' /

-:;? ~~ I-~, r- NDONYO

. Q ( MAU

\ r .PLATEAU

)Ma'ti.s.,

(ijr-!

\~)

(

....<>

~'"

IIIIIII

~I

'I

I

SCALE,030'N

0 5L

10 15 MILES

F==1 LowerPleistoc,

ene

t::=:::J olivine basalts -Recent lovas

V///1 M,ddle Upper Pleistocene~ oJivine augite basalts r':-:-:'>I Recent ashes and ash: ::: :>::: and agglomerate cones

II Upper Pleistoceneanalc,me, basalts ~ Explosion craters"

Fig. 2-Sketch-map of the volcanic history of the Laisamis area.

15

I

57°5

0'E

2° DO’N,

_ \ *fafifl

__ J__ *— »._. H Chat,“1 —-</‘“ "-1fi-j‘hx— \_,_‘

KA‘SL‘TZJESEW ’;:____ \ (J

1m" __J_J \' \ ___.—.._,— 2'_‘.- \ ~—L___ __H,‘ . ‘

g Wfi—x‘

— '* \._,-—-'\‘___ —\d-- —+ 1_..o§

_ _ EAGERFRUA ___ .PLATFAUH — I

__'b_'

Rus/aeusFLATEAU 1

STOU

T

ISMLES

L Lower Plexstccene___ ' ohvine basalts

:- . A ../ , . Mddle Upper ”‘erstocene’ A‘JoJivma nuglte basalts

Upper Ple‘stoceneanalcwme basalts

Fig. 2—Sketch-map of the volcanic history

Recent 'e as

-. .J aem ashes and ashand aggwrr‘erarc cones

w Explcsxan crater»

of the Laisamis area.

(1) OLIVINE BASALT PLATEAU LAVAS

These lavas are widespread in the area and form cappings, varying in thicknessfrom 10 to 20 feet, on remnants of the end-Tertiary erosion bevel. In hand specimenthey are melanocratic, aphanitic rocks with phenocrysts of olivine present in some andsparse microphenocrysts of felspar in one specimen. In most cases the lavas are massive,but some specimens are scoriaceous while others show a tendency to vesicularity.

In thin section these lavas are seen to have a uniform mineral composition. Speci-men 28/300 from the Sagererua Plateau is a typical example and shows olivinephenocrysts marginally altered to iddingsite. The groundmass consists of augite, lathsof labradorite felspar and magnetite. Calcite is present as an infiIling in the vesicles.Similar specimens are 28/310 from the Kaisut Plateau, 28/340 from the small outlierby the Isiolo-Marsabit road three miles south-west of Laisamis and 28/368 from theAmbara Plateau. Slight variations in these specimens consist of differences in theiddingsite alteration of the olivine phenocrysts; in some thin sections crystals of olivinehave undergone complete alteration while in others there is very little change. Thegroundmass also shows textural variations, that of 28/368 being very much coarserthan any of the others.

The olivine basalt lavas of these plateaux originated from fissures, though nonewere seen, and are probably penecontemporaneous with the earliest Marsabit vulcani-city (Parkinson 1924, p. 102). In a later paper Parkinson (1939, p. 165) postulated theoriginal northward continuation of the Merti Plateau to include those plateaux whichlie to the east of the old road from Laisamis to Langaia. The lava-capped Merti Plateaulies nearly 40 miles south-east of the nearest plateau of the present group, a distancewhich would appear to be too great for there to be any direct connexion between thetwo, and it seems that these lava plateaux had their origins in a series of separatefissure eruptions. It is more than likely however, that the Rusarus and Sagereruaplateaux were at one. time a single plateau and that subsequent dissection by theSeralatubi River led to their present separation. None of the remaining plateaux inthe area have any apparent connexion with each other.

(2) OLIVINE-AUGITE BASALTS OF THE KAISUT PLATEAU

The olivine-augite basalts show up very distinctly on aerial photographs, and aredearly separate from the olivine basalts of the Kaisut Plateau which they overlie inthe western part. The lava is devoid of vegetation of any kind but has a thin coveringof soiL Since this lava is younger than the olivine basalt plateau lava its age is probablyMiddle to Upper Pleistocene.

III

IiI

The plateau formed by the lava rises 10 to 15 feet above the surrounding country.The flow shows a crudely developed form of columnar jointing, being composed oflarge polygonal blocks with vertical long axes and generally flat tops. The columnsdip inwards at the sides of the plateau and the flattish upper surfaces define themarginal top of the flow which slopes down at the plateau margins at angles of 15 to20 degrees, but locally reaches 45 degrees. Well-developed radial and marginal fissuresup to 10 feet deep occur at the plateau margins. The fissures from which this lava waserupted are not seen.

In hand specimen the lava is a melanocratic holocrystaIline basalt with olivine micro-phenocrysts, and is highly vesicular. A secondary calcite infilIing of the vesicles isoccasionally present. In thin section the three specimens collected, 28/303, 28/304and 28/305, show a uniform mineral composition with olivine and augite being themajor components. Some of the olivine crystals have alteration rims of iddingsite.The plagioclase felspar is labradorite and small quantities of magnetite are alsopresent.

16

r- --

(1) OLIVINE BASALT PLATEAU LAVAS

These lavas are widespread in the area and form cappings, varying in thicknessfrom 10 to 20 feet, on remnants of the end-Tertiary erosion bevel. In hand specimenthey are melanocratic, aphanitic rocks with phenocrysts of olivine present in some andsparse microphenocrysts of felspar in one specimen. In most cases the lavas are massive,but some specimens are scoriaceous while others show a tendency to vesicularity.

In thin section these lavas are seen to have a uniform mineral composition. Speci-men 28/300 from the Sagererua Plateau is a typical example and shows olivinephenocrysts marginally altered to iddingsite. The groundmass consists of augite, lathsof labradorite felspar and magnetite. Calcite is present as an infiIling in the vesicles.Similar specimens are 28/310 from the Kaisut Plateau, 28/340 from the small outlierby the Isiolo-Marsabit road three miles south-west of Laisamis and 28/368 from theAmbara Plateau. Slight variations in these specimens consist of differences in theiddingsite alteration of the olivine phenocrysts; in some thin sections crystals of olivinehave undergone complete alteration while in others there is very little change. Thegroundmass also shows textural variations, that of 28/368 being very much coarserthan any of the others.

The olivine basalt lavas of these plateaux originated from fissures, though nonewere seen, and are probably penecontemporaneous with the earliest Marsabit vulcani-city (Parkinson 1924, p. 102). In a later paper Parkinson (1939, p. 165) postulated theoriginal northward continuation of the Merti Plateau to include those plateaux whichlie to the east of the old road from Laisamis to Langaia. The lava-capped Merti Plateaulies nearly 40 miles south-east of the nearest plateau of the present group, a distancewhich would appear to be too great for there to be any direct connexion between thetwo, and it seems that these lava plateaux had their origins in a series of separatefissure eruptions. It is more than likely however, that the Rusarus and Sagereruaplateaux were at one. time a single plateau and that subsequent dissection by theSeralatubi River led to their present separation. None of the remaining plateaux inthe area have any apparent connexion with each other.

(2) OLIVINE-AUGITE BASALTS OF THE KAISUT PLATEAU

The olivine-augite basalts show up very distinctly on aerial photographs, and aredearly separate from the olivine basalts of the Kaisut Plateau which they overlie inthe western part. The lava is devoid of vegetation of any kind but has a thin coveringof soiL Since this lava is younger than the olivine basalt plateau lava its age is probablyMiddle to Upper Pleistocene.

III

IiI

The plateau formed by the lava rises 10 to 15 feet above the surrounding country.The flow shows a crudely developed form of columnar jointing, being composed oflarge polygonal blocks with vertical long axes and generally flat tops. The columnsdip inwards at the sides of the plateau and the flattish upper surfaces define themarginal top of the flow which slopes down at the plateau margins at angles of 15 to20 degrees, but locally reaches 45 degrees. Well-developed radial and marginal fissuresup to 10 feet deep occur at the plateau margins. The fissures from which this lava waserupted are not seen.

In hand specimen the lava is a melanocratic holocrystaIline basalt with olivine micro-phenocrysts, and is highly vesicular. A secondary calcite infilIing of the vesicles isoccasionally present. In thin section the three specimens collected, 28/303, 28/304and 28/305, show a uniform mineral composition with olivine and augite being themajor components. Some of the olivine crystals have alteration rims of iddingsite.The plagioclase felspar is labradorite and small quantities of magnetite are alsopresent.

16

r- --

tlt 0t t\'I\t Bxstt Pt \‘l'titt t_\\'\s

'Ihsse laws are lt‘riLlL‘\I‘l-Ull\i in th: ttt'et; and hunt Cttt‘t'ttttgs_ \.tt'_\'!‘lg :n {hi-.knessfrom I“ to I“ feeti on remnants of the end tertiary erosion hevel. ln hand specimenthe) rite nielanoei'atte. aphanitie rocks “ill‘. phenoertsls oi olivine present in swine andsparse mterophenoer}sts oi t’elspitr in one sgieeinten. in most eases the lLl\LtS are n‘ussiw.l‘tlt some specimens are scot'iaeeous \thile others short a tendency to \‘esiettlaritv,

In thin seetion these l;t\a\ are seen to lt‘aVL‘ a uniform mineral composition. Speei~men 25 NIH from the Sageret‘tta Plateau is a typical e‘x‘atttple and shows Olivinepin-neettsts marginally altered to itltlingsite. ‘lhe gt'uttndniztss consists of Ltttgite. lathsoi lal‘ratlortte t‘elspar anti magnetite (faleite is present as an intiltin; in the \esieles.Similar specimens are 25 ll” trom the K;tisttt Plateau. 25 NH from the small outlierh} the Isit‘tloJVlttt‘sahit roatl three miles south \test of luisttmis‘ and IN 365‘ from the'\tttl‘.tt'Lt l’latettti, Slight Variations in these speeiniens consist of tiitl'erenees in theititlingsite alteration of the olivine phenoet‘tsts: in some thin sections crystals of olivinehaw: undergone complete alteration \xhile in others there is \et'}; little ehange. Thegroantimitss also shtms tC‘xltlt’al \ariations. that ot 15‘ 36> heirs; \er} mueh eoarserthan any of the others.

the olivine hamlt laws of these plateatn originated from tisstu‘es. though none“ere seem and are prohahly peneeontetnporaneotts \tith the earliest \Iatstshit \‘Ltleunrcity (Parkinson 192-3. p. lttlt In a later paper Parkinson HUN). p, tftit postulated theoriginal nortimard continuation oi the Merti Plateau to inelude those plateatn uhiehlie to the east of the oltl road from laisaniis to lttngaia 'l he lava capped Merit Plateaulies nearly 4“ miles southeast of the nearest plateau of the present group it distance\thieh \multl appear to he too great for there to he any direct eonnexion hemeen theitm. and it seems that these lint plateatn hati their origins in it series. of separatetissttre eruptions. It is more than likely houewr. that the Rusat‘tts anti SugerertiaPluICLtLE\ “ere at one time a single plateau and that subsequent dissection hy theSet‘ulatuhi Ri\er leti to their present separation. None of the remaining plate-mu inthe area have any apparent eonnexion \\ith each other:

|1l(:)tl\l\l,\ttvlil Btstt is «it till. l\,\l\t'1’ Pt Hut

The t,tli\incilittgitc basalts show up \L‘fl tlistinetlv on aerial photographs. and areelearh separate from the olivine hasalts or the Kitisut Plateau \xhieh they overlie inthe \testern part. The lava is devoid of vegetation of any kind but has a thin enteringo1 soil, Sinee this lLt\Lt is younger than the oli\ine basalt plateau ltna its age is prohabh\Iitltile to l, ppet‘ Pleistoeenet

the plateau formed hy the lava rises ltt to 15 feet (move the surrounding eottntrvlhe :lott shtms a muddy timeloped tortn tit columnar iotnttng heing composed oftarge poltgt‘nai blocks with \ei‘tie.tl long mes anti genez'allt tlat tops. The enltttnnsLlip innards at the sides of the plateau and tlte tlattish ttppet sttrt‘aees define thenLtreiixtl top of the tiotx \xhieh slopes down at the plateau margins at angles nt‘ 15 toit? degrees. but [oettllt reaches 4,: degrees, \\‘(lirtlL‘\ClUpL\l radial and marginal fissuresrm to ill l'eet tieep oeeur :it the plateau margins The tissures from \\hlL‘lt this lava waset‘ttt‘tetl are not seen

In hanti speeimen the lava is :t melanoeratie holoert‘stitlline hasalt \\ith olivine microphent‘ertsts. and is highl} \esieulati ~\ seeont’ltr}; caleite fniilling of the \esicles isoeettsionttll) present. In thin seetion the three speeinzens eoileetett. Its 3M. 18 ,‘s!‘—tand 2N 3H5. .shotv Lt ttnit’orni mineral composition with oli\ine anti attgite heing themajor components. Sum: of the oli\ine er}stals haw alteration rims ol‘ ititiingsite.Alhe plagioelase i‘elspaz' is lahratfoi‘ite and small quantities of magnetite are alsopresent.

(3) ANALCIME BASALTS

On all the olivine basalt plateaux a series of younger fissure flows are present.Vegetation, apart from an occasional tree growing from soil trapped in crevices, isabsent as there is no surface soil cover present as there is on the olivine-augite basaltsof the Kaisut Plateau. These lavas rise up to 50 feet above the level of the olivinebasalt plateau and vary greatly in extent. Some of the flows consist of merely smallisolated ridges a quarter of a mile long while others cover quite appreciable areas.These flows are probably Middle to Upper Pleistocene in age.

On the Ambara Plateau in the west of the area these lavas are comparatively rare,but there is a gradual increase in the frequency of their occurrence towards the eastand it is in that part of the area, in the southern portions of the Rusarus and Sagereruaplateaux, that they have their maximum development. In outcrop these lavas, particularlythe smaller flows, consist of piles of angular blocks and it is only towards the centreof the !larger flows that there is any indication of columnar jointing, but this is onlyvery crudely developed.

In hand specimen these rocks are melanocratic holocrystalline basalts with olivinemicro phenocrysts and all are slightly vesicular, with the vesicles of some specimenslined with secondary calcite. Specimen 28/297 from the northern part of the SagereruaPlateau is a typical example, and shows olivine phenocrysts, many of them withiddingsite alterations, with augite, labradorite, magnetite and analcime in the ground-mass. Not all the specimens collected contain analcime but from their similarity bothin hand specimen and in outcrop all have been classified under the one heading.

(4) VENTS AND ASSOCIATED OLIVINE BASALT LAVAS

Three well-preserved vents lie to the north-west of Laisamis and are considered tobe of Recent age. The tuffs and agglomerates and explosion craters which will bedescribed in the two subsequent sections are also thought to be Recent. The twonorthern vents have not produced large quantities of lava, the flows being sufficientonly to build up the cones, but the southern one has produced flows from two vents,of which now only one remains, on the south-eastern side of the cone. The summitis composed of ash and ,agglomerate and the vent which produced the flows aroundthe northern and eastern sides of this volcano was presumably sited on the easternflank. The vents are comparatively shallow features and access to them is easy.

Other lavas of Recent age are present around the feet of cones, such as Sengerarua,on the eastern side of the explosion crater to the south of Sengerarua, and around thefoot of the cone on the south-eastern margin of the Sagererua Plateau. The trigono-metrical point of Limirina on the Rusarus Plateau is entirely composed of Recentlava.

All the Recent olivine basalt lavas show a marked similarity in hand specimen andin thin section. All are melanocratic, holocrystalline, fine grained rocks with olivinemicrophenocrysts. Specimen 28/314, from the middle vent of the group to the north-west of Laisamis, also has phenocrysts of felspar, but in thin section this rock is seento be a contaminated lava. It contains olivine and andesine felspar, and quartz andmicrocline. The two latter have been derived from fragments of Basement Systemgneisses caught up and partially assimilated by the lava. The groundmass, like thatof all these lavas, is fine-grained and heavily impregnated with magnetite.

Specimen 28/302 from the lava on the eastern side of the craters south of Sengeraruacontains olivine phenocrysts, some of which have iddingsite alterations. The fine-grained groundmass consists of augite, andesine felspar and magnetite, with secondarycalcite.

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(3) ANALCIME BASALTS

On all the olivine basalt plateaux a series of younger fissure flows are present.Vegetation, apart from an occasional tree growing from soil trapped in crevices, isabsent as there is no surface soil cover present as there is on the olivine-augite basaltsof the Kaisut Plateau. These lavas rise up to 50 feet above the level of the olivinebasalt plateau and vary greatly in extent. Some of the flows consist of merely smallisolated ridges a quarter of a mile long while others cover quite appreciable areas.These flows are probably Middle to Upper Pleistocene in age.

On the Ambara Plateau in the west of the area these lavas are comparatively rare,but there is a gradual increase in the frequency of their occurrence towards the eastand it is in that part of the area, in the southern portions of the Rusarus and Sagereruaplateaux, that they have their maximum development. In outcrop these lavas, particularlythe smaller flows, consist of piles of angular blocks and it is only towards the centreof the !larger flows that there is any indication of columnar jointing, but this is onlyvery crudely developed.

In hand specimen these rocks are melanocratic holocrystalline basalts with olivinemicro phenocrysts and all are slightly vesicular, with the vesicles of some specimenslined with secondary calcite. Specimen 28/297 from the northern part of the SagereruaPlateau is a typical example, and shows olivine phenocrysts, many of them withiddingsite alterations, with augite, labradorite, magnetite and analcime in the ground-mass. Not all the specimens collected contain analcime but from their similarity bothin hand specimen and in outcrop all have been classified under the one heading.

(4) VENTS AND ASSOCIATED OLIVINE BASALT LAVAS

Three well-preserved vents lie to the north-west of Laisamis and are considered tobe of Recent age. The tuffs and agglomerates and explosion craters which will bedescribed in the two subsequent sections are also thought to be Recent. The twonorthern vents have not produced large quantities of lava, the flows being sufficientonly to build up the cones, but the southern one has produced flows from two vents,of which now only one remains, on the south-eastern side of the cone. The summitis composed of ash and ,agglomerate and the vent which produced the flows aroundthe northern and eastern sides of this volcano was presumably sited on the easternflank. The vents are comparatively shallow features and access to them is easy.

Other lavas of Recent age are present around the feet of cones, such as Sengerarua,on the eastern side of the explosion crater to the south of Sengerarua, and around thefoot of the cone on the south-eastern margin of the Sagererua Plateau. The trigono-metrical point of Limirina on the Rusarus Plateau is entirely composed of Recentlava.

All the Recent olivine basalt lavas show a marked similarity in hand specimen andin thin section. All are melanocratic, holocrystalline, fine grained rocks with olivinemicrophenocrysts. Specimen 28/314, from the middle vent of the group to the north-west of Laisamis, also has phenocrysts of felspar, but in thin section this rock is seento be a contaminated lava. It contains olivine and andesine felspar, and quartz andmicrocline. The two latter have been derived from fragments of Basement Systemgneisses caught up and partially assimilated by the lava. The groundmass, like thatof all these lavas, is fine-grained and heavily impregnated with magnetite.

Specimen 28/302 from the lava on the eastern side of the craters south of Sengeraruacontains olivine phenocrysts, some of which have iddingsite alterations. The fine-grained groundmass consists of augite, andesine felspar and magnetite, with secondarycalcite.

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til 3t\'\t ("Hit-Z Btstt Ts

nresent.‘s-“egetatton, apart t‘rom an occasional tree growing irom soil trapped in crevices. isabsent as there. is no sttri‘aee soil eover present as there is on the olivine-augite basaltsof the lx't'tisut Plateau. These lavas rise up to ill t'eet :ihoVe the level ot‘ the olivinebasalt plateau and \ar}. greatly in e\tent. Some of the firms consist ot' merely smallisolated ridges a quarter of a mile long while others cover quite appreciable areas.'lhese lions are probably \‘liddle to [Inner Pleistocene in age.

()n all the olixine basalt plateatn a series of younger tissurc Eons are

On the Ambara Plateau in the west or the area these laws are comparatively rare.but there is a gradual increase in the frequency of their occurrence towards the eastand it is in that part oi the area. in the soathern portions of the Rusartts and Sagereruaplateaus. that they have their ma.\imum develotunent. In outcrop these lavas. partiettlarlythe smaller llovvs'. consist oi piles oi angular blocks and it is only totvtrds the centreoi the larger lions that there is any indication of coltttitnar jointing. “ut this is onlyyer} crudely dey eloned.

In hand specimen these roelss are melanoerattc holocrystalline basalts \sith olivinemieronhenoe' 'sts and all are slightly vesicular. with the vesicles of some specimenslined with secondary calcite. Specimen ZR 2‘}? from the northern part of the SageteruaPlateau is a tynieal example. and shows Oli\ine nhenocrysts. many of them \tithiddingsite alterations_ utth attgite. lahradorite. magnetite and t'tnalcime in the ground-mass. Not all the specimens eolleeted contain analcime but irom their similarity bothin hand specimen and in outcrop all have heen classilied under the one heading.

t—lt Vrsts .-\‘\tt .-\ssoc*tt'it.o Oi t\'I.\l. Bxsu r Ltyts

Three nell-nreserved vents lie to the north-nest ot' laisamis and are considered tohe ot‘ Recent age. The tufi's and agglomerates and esplosinn craters which will hedcseribed in the two subsequent sections are also thought to be Recent. The twonorthern Vents have not produced large quantities of lava. the flows being sLttiicientonly to build up the cones. but the southern one has produced flows from tho \ents.oi “him now only one remains. on the south eastern side of the cone. the summitis‘ Composed oi tlsh and agglomerttie and the tent uhich produced the tlons (trot.ndthe northern and eastern sides oi this \oicano “as hreseniahly sited on the easternIlanlx. 'l'he \‘ents are comparatiyely shallots ieatures and access to them is easy.

Other lavas oi Recent age are present around the feet of cones. such as Seneerai'ua.on the eastern side oi the e\nlosion crater to the south of Sertgerarua. and around theioot oi the cone on the south-eastern margin of the Sagererua l’lateau. 'l'he trigonometrical point of Limirina on the Ritsarits Plateau is entirely comnoseti oi Recentl;t\a.

All the Recent olivine basalt lLl‘.Lts shots a marked similarity in hand specimen andin thin section. All are melanocratic. holocrystailine. tine grained {'tie‘h‘s \sith olivinemicrophenocry'sts. Specimen 3* Fl—l. irom the middle \ent oi the group to the north-west oi laisamis. also has nhenoerysts ol ielspar. but in thin section this roels is seento he a contaminated lata. It contains oli\.ine anti andesine iclspat'. and quartz andmiert'aeline. The two latter have been derived l‘rom iragments or Basement Systemgneisscs eaugh: up and partially assimilated hy the lava. The groundntass. like that.n' all these laws. is tine-grained and lietoily impregnated \tith magnetite.

Sneeimen 25 Ni: from the l:i\a on the eastern side oi the craters south oi Senget'ttrutteontains olivine phenocrysts. some oi uliich hioe iddingsitc alterations. The tine-

illUL'l grotmdy‘nhss consists of u'_l‘:‘lle. Lil'ltlcslfle" l‘t‘lsz‘lltf Ltfiti magnetite. \\ll.'l1 secondary

calcite.

l‘.‘

The lava flow at the foot of Sengerarua from which specimen 28/301 was takenshows phenocrysts of olivine and augite. The ground mass is extremely fine-grained,almost glassy.

Specimen 28/319 from the western foot of Limirina has phenocrysts of olivineextensively altered to iddingsite, augite, and leucoxene probably derived from titani-ferous magnetite. Only magnetite and small crystals of augite are recognizable in thevery fine-grained groundmass.

(5) TUFFS AND AGGLOMERATES

These occur mainly on the eastern side of the area, particularly on the Sagereruaand Rusarus plateaux, but there is also a small hill of tuff in the north-central portionof the Kaisut Plateau. The highest hill in the east, Sengerarua which is 2,360 feet high,is conical with steep sides composed of ash and agglomerate which make climbingdifficult. The tuffs are generally weathered and unconsolidated. In the case of Sengeraruaand the hill three and a half miles to the south-south-west, the emission of the tuffsand agglomerates was preceded by lava flows which covered only small areas aroundthe feet of the cones, and which have been described in the preceding section.

On the Rusarus Plateau there are several small hills composed of tuffs and basalticagglomerates which have not been preceded by lava flows.

(6) EXPLOSION CRATERS

These are spectacular phenomena and occur only on the eastern plateaux. The maincentre of their occurrence is on the Rusarus Plateau but two small craters are alsopresent on the Sagererua Plateau. These two craters, 'One mile south-south-west 'OfSengerarua, are interconnected.

A total of seven craters are present on the Rusarus Plateau but from none of thesewas there emission of lava prior to the formation of the craters. Of the seven craterspresent on this plateau, four are interconnected to form one large one which is ellipticalin shape. The remaining three craters, the last two of which are also interconnected,lie to the north of this along a line trending north-north-east. The significance of thisline will be discussed later.

All the craters show a close similarity in form, the rims being composed of beddedtuffs which dip outwards from the craters at angles varying from 18° to 26°, and allhave steep inner walls, making a descent to the bottom virtually impossible. The highestpoints of all the craters are on the western sides, and tails of tuff extend in a north-westerly direction, indicating that the prevailing wind at that time was from the south-east.

Contained in the tuffs which fringe the craters are blocks of basalt and crystals ofolivine. Specimen 28/321 from the western rim of the largest crater immediately toto the north of Limirina is a cluster of pale green olivine crystals with small magnetitecrystals. No traces of metamorphic rocks were found around the rims of the cratersbut the thin section of a secondarily compacted tuff, 28/324 from a stream bed to thewest of Rusarus Plateau, shows quartz and micro cline accompanying augite, olivineand labradorite, indicating that Basement System rocks were involved in the explosions.

With regard to the origin of these craters there are two possible explanations. Thefirst, which applies particularly to those craters on the Sagererua Plateau, surmisesthat after the emission of the lava which flowed northwards the vent became blockedby the cooling lava, leading to a building-up of pressure in the magma chamber. Thispressure when sufficiently high overcame the blockage and removed the top 'Of thevolcano in the ensuing explosion, scattering showers of ash and basaltic fragmentsover the adjacent area. The second theory is that in the initial stages the magma wasemitted not as lava but as tuff, and a cone was built up. The discharge of the magmacreated a void in the magma chamber into which the cone collapsed.

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The lava flow at the foot of Sengerarua from which specimen 28/301 was takenshows phenocrysts of olivine and augite. The ground mass is extremely fine-grained,almost glassy.

Specimen 28/319 from the western foot of Limirina has phenocrysts of olivineextensively altered to iddingsite, augite, and leucoxene probably derived from titani-ferous magnetite. Only magnetite and small crystals of augite are recognizable in thevery fine-grained groundmass.

(5) TUFFS AND AGGLOMERATES

These occur mainly on the eastern side of the area, particularly on the Sagereruaand Rusarus plateaux, but there is also a small hill of tuff in the north-central portionof the Kaisut Plateau. The highest hill in the east, Sengerarua which is 2,360 feet high,is conical with steep sides composed of ash and agglomerate which make climbingdifficult. The tuffs are generally weathered and unconsolidated. In the case of Sengeraruaand the hill three and a half miles to the south-south-west, the emission of the tuffsand agglomerates was preceded by lava flows which covered only small areas aroundthe feet of the cones, and which have been described in the preceding section.

On the Rusarus Plateau there are several small hills composed of tuffs and basalticagglomerates which have not been preceded by lava flows.

(6) EXPLOSION CRATERS

These are spectacular phenomena and occur only on the eastern plateaux. The maincentre of their occurrence is on the Rusarus Plateau but two small craters are alsopresent on the Sagererua Plateau. These two craters, 'One mile south-south-west 'OfSengerarua, are interconnected.

A total of seven craters are present on the Rusarus Plateau but from none of thesewas there emission of lava prior to the formation of the craters. Of the seven craterspresent on this plateau, four are interconnected to form one large one which is ellipticalin shape. The remaining three craters, the last two of which are also interconnected,lie to the north of this along a line trending north-north-east. The significance of thisline will be discussed later.

All the craters show a close similarity in form, the rims being composed of beddedtuffs which dip outwards from the craters at angles varying from 18° to 26°, and allhave steep inner walls, making a descent to the bottom virtually impossible. The highestpoints of all the craters are on the western sides, and tails of tuff extend in a north-westerly direction, indicating that the prevailing wind at that time was from the south-east.

Contained in the tuffs which fringe the craters are blocks of basalt and crystals ofolivine. Specimen 28/321 from the western rim of the largest crater immediately toto the north of Limirina is a cluster of pale green olivine crystals with small magnetitecrystals. No traces of metamorphic rocks were found around the rims of the cratersbut the thin section of a secondarily compacted tuff, 28/324 from a stream bed to thewest of Rusarus Plateau, shows quartz and micro cline accompanying augite, olivineand labradorite, indicating that Basement System rocks were involved in the explosions.

With regard to the origin of these craters there are two possible explanations. Thefirst, which applies particularly to those craters on the Sagererua Plateau, surmisesthat after the emission of the lava which flowed northwards the vent became blockedby the cooling lava, leading to a building-up of pressure in the magma chamber. Thispressure when sufficiently high overcame the blockage and removed the top 'Of thevolcano in the ensuing explosion, scattering showers of ash and basaltic fragmentsover the adjacent area. The second theory is that in the initial stages the magma wasemitted not as lava but as tuff, and a cone was built up. The discharge of the magmacreated a void in the magma chamber into which the cone collapsed.

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'l‘he lava flow at the foot of Sengerarua from which specimen 28‘301 was takenshows phenocrysts of olivine and augite. The groundmass is extremely fine—grained."lmost “la v(A _. H‘- __.

Specimen 28 3|?) from the western foot of limirina has phenoct'ysts of olivineextensively altered to iddingsite. augite. and leucoxenc probably derived from titani—lerous magnetite. Only magnetite and small crystals of augite are recognizable in thevery tinc grained grottndmass.

t5) Tt lit-‘5; awn At;<'.t.o\n;R.-\'1Ls'l'ltese occur mainly on the eastern side of the area. particularly on the Sagercrua

and Rusarus plateatlx. but there is also a small hill of ttilt in the north central portionof the Kaisut Plateau. The highest hill in the east. Serigerarua which is 2.360 feet high.is conical with steep sides composed of ash and agglomerate which make climbingditlicult. The tutl's are generally weathered and unconsolidated. In the case of Sengeraruaand the hill three and a half miles to the south-south west. the emission ot‘ the tutl‘santi agglomerates was preceded by lava flows which covered only small areas aroundthe feet of the cones. and which have been described in the preceding section.

On the Rusarus Plateau there are several small hills composed ot" tutl‘s and basalticagglomerates which have not been preceded by lava flows.

(6) Exptosiots CR ATERS

These are spectacular phenomena and occur only on the eastern plateaus. The maincentre of their occurrence is on the Rusarus Plateau but two small. craters are alsopresent on the Kagererua Plateau. These two craters. one mile south-south-west ofSengerarua. are interconnected.

A total of seven craters are present on the Rusarus Plateau but from none of thesewas there emission of lava prior to the formation of the craters. Of. the seven craterspresent on this plateau. four are interconnected to form one large one which is ellipticalin shape. The remaining three craters. the last two of which are also interconnected.lie to the north of this along a line trending north-north east. The significance of. thisline will be discussed later.

All the craters show a close similarity in form. the rims being composed of beddedlutl's which dip outwards from the craters at angles varying from 18 to 26'. and allhave steep inner walls. making a descent to the bottom virtually impossible. The highestpoints of all the craters are on the western sides, and tails of tuff extend in a north-westerly direction, indicating that the prevailing wind at that time was from the south-L’Eifil.

Contained in the tutfs which fringe the craters are blocks of basalt and crystals ot'olivine. Specimen 289321 from the western rim of. the largest crater immediately toto the north of .l.imirina is a cluster of pale green olivine crystals with small magnetitecrystals. No traces of metamorphic rocks were found around the rims of the cratersbut the thin section of a secondarily compacted tuft. 38 324 from a stream bed to thewest of Rusarus Plateau. shows quartz and microcline accompanying augite. olivineanti labradorite. indicating that Basement System rocks were involved in the explosions.

With regard to the origin of these craters there are two possible explanations. Thefirst. which applies particularly to those craters on the Sagererua Plateau. surmisesthat after the emission of the lava which tiowcd northwards the vent became blockedby the cooling lava. leading to a building-up of pressure in the magma chamber. Thispressure when sufficiently high overcame the blockage and removed the t0p of thevolcano in the ensuing explosion. scattering showers of ash and basaltic fragmentsover the adjacent area. The second theory is that in the initial stages the magma wasemitted not as lava but as tull, and a cone was built up. The discharge of the magmacreated a void in the magma chamber into which the cone collapsed.

l8

The alignment in a north-north-easterly direction of the craters on the RusarusPlateau has already been noted. This line can be extended southwards to include thecone of Limirina. The same alignment is seen in the craters on the Sagererua Plateau.Extension of this line to the north brings in the cone of Sengerarua, and an extensionin a southerly direction shows that the volcanic vents lying to the north-west of Laisamisalso lie along this line. It is obvious that these two north-north-easterly trending lineshave a structural significance and have to a great extent determined the situation of thecraters and volcanic vents. It is also significant that the large crater on the R'llsarusPlateau is sited on the intersection of one of these north-north-easterly trending linesand the continuation of a fault which runs in a south-easterly direction and whichdown throws to the south-west.. The intersection of this fault with the line joiningSengerarua and the volcanic vents to the north-west of Laisamis does not produce anyphenomenon at that point.

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5. Superficial Deposits(1) CALCAREOUSCRUSTALDEPOSITS

Small patches of secondary limestone (kunkar) occur in various parts of the area.The largest outcrop is on the eastern side of the Manderay River, to the south-eastof the Ndonyo Mara Plateau, which apparently overlies crystalline limestone. Inhand specimen 28/359 this buff-coloured limestone shows quartz inclusions, and thinsection examination reveals the presence of quartz and amphiboles set in a fine-grainedcalcareous matrix with patches of crystalline calcite. Another occurrence of secondarylimestone is present to the north of Laisamis, on the eastern side of the MarsabitRoad, where the limestone is present only as small fragments scattered about thesurface. This deposit, like other small deposits throughout the area, is too small to beshown on the map.

Kunkar limestone is characteristic of areas with limited rainfall and hence is usefulas a climatic indicator. It owes .its origin to capillary action which draws lime-richwater to the surface, where by evaporation a calcareous deposit is formed.

(2) FERRUGINOUSCRUSTALDEPOSITS

These occur to a limited extent in the area mapped, but none attain a large sizeand most are too small to be shown on the map. One outcrop exists on the southernside of the IlIaut-Laisamis track east of Losai. In outcrop the deposit is unconsolidatedand brown in colour and contains fragments of metamorphic rocks. A similar butmuch smaller deposit exists about a mile west of Moile.

The origin of these ferruginous deposits is similar to that of the calcareous deposits,i.e. capillary action which drew iron-rich water to the surface where it evaporated.

(3) PEBBLESHEETS

These were encountered only in one locality, close to the amphiboliticof Chaparkrom. The pebbles are of angular quartz, derived from quartzthe hill east of Chaparkrom.

intrusionveins on

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(4) RECENT SANDS AND ALLUVIUM

Reddish brown sands are extremely common and form a blanket over much ofthe region to the south of the Kaisut Plateau, a cover broken only by inselbergs suchas IlIim and Baio. Examination of these sands shows that they contain fragments ofquartz, felspar, hornblende and biotite and hence are derived entirely from metamorphicrocks.

Pale brown alluvium is present along the south-western portion of the Kaisut Plateau,marking the site of a small lake. Alluvium is also present along stretches of the MilgisRiver.

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The alignment in a north-north-easterly direction of the craters on the RusarusPlateau has already been noted. This line can be extended southwards to include thecone of Limirina. The same alignment is seen in the craters on the Sagererua Plateau.Extension of this line to the north brings in the cone of Sengerarua, and an extensionin a southerly direction shows that the volcanic vents lying to the north-west of Laisamisalso lie along this line. It is obvious that these two north-north-easterly trending lineshave a structural significance and have to a great extent determined the situation of thecraters and volcanic vents. It is also significant that the large crater on the R'llsarusPlateau is sited on the intersection of one of these north-north-easterly trending linesand the continuation of a fault which runs in a south-easterly direction and whichdown throws to the south-west.. The intersection of this fault with the line joiningSengerarua and the volcanic vents to the north-west of Laisamis does not produce anyphenomenon at that point.

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5. Superficial Deposits(1) CALCAREOUSCRUSTALDEPOSITS

Small patches of secondary limestone (kunkar) occur in various parts of the area.The largest outcrop is on the eastern side of the Manderay River, to the south-eastof the Ndonyo Mara Plateau, which apparently overlies crystalline limestone. Inhand specimen 28/359 this buff-coloured limestone shows quartz inclusions, and thinsection examination reveals the presence of quartz and amphiboles set in a fine-grainedcalcareous matrix with patches of crystalline calcite. Another occurrence of secondarylimestone is present to the north of Laisamis, on the eastern side of the MarsabitRoad, where the limestone is present only as small fragments scattered about thesurface. This deposit, like other small deposits throughout the area, is too small to beshown on the map.

Kunkar limestone is characteristic of areas with limited rainfall and hence is usefulas a climatic indicator. It owes .its origin to capillary action which draws lime-richwater to the surface, where by evaporation a calcareous deposit is formed.

(2) FERRUGINOUSCRUSTALDEPOSITS

These occur to a limited extent in the area mapped, but none attain a large sizeand most are too small to be shown on the map. One outcrop exists on the southernside of the IlIaut-Laisamis track east of Losai. In outcrop the deposit is unconsolidatedand brown in colour and contains fragments of metamorphic rocks. A similar butmuch smaller deposit exists about a mile west of Moile.

The origin of these ferruginous deposits is similar to that of the calcareous deposits,i.e. capillary action which drew iron-rich water to the surface where it evaporated.

(3) PEBBLESHEETS

These were encountered only in one locality, close to the amphiboliticof Chaparkrom. The pebbles are of angular quartz, derived from quartzthe hill east of Chaparkrom.

intrusionveins on

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(4) RECENT SANDS AND ALLUVIUM

Reddish brown sands are extremely common and form a blanket over much ofthe region to the south of the Kaisut Plateau, a cover broken only by inselbergs suchas IlIim and Baio. Examination of these sands shows that they contain fragments ofquartz, felspar, hornblende and biotite and hence are derived entirely from metamorphicrocks.

Pale brown alluvium is present along the south-western portion of the Kaisut Plateau,marking the site of a small lake. Alluvium is also present along stretches of the MilgisRiver.

I

11

I

1

19

I

The alignment in a north-north—easterly direction of the craters on the RusarusPlateau has already been noted. This line can be extended southwards to include thecone of Limirina. The same alignment is seen in the craters on the Sagererua Plateau.Extension of this line to the north brings in the cone of Sengerarua. and an extensionin a southerly direction shows that the volcanic vents lying to the north-west of Laisamisalso lie along this line, It is obvious that these two north-north-eastcrly trending lineshave a structural significance and have to a great extent determined the situation of thecraters and volcanic vents. It is also significant that the large crater on the RusarusPlateau is sited on the intersection of one of these northnorth-easterly trending linesanti the continuation of a fault which runs in a south-easterly direction and whichdownthrows to the south-west. The intersection of this fault with the line joiningSengerarua and the volcanic vents to the north-west of Laisamis does not produce anyphenomenon at that point.

5. Superficial Depositst1) CALCAkEOL'S C‘RUSTAI. DEPOSITS

Small patches of secondary limestone (kunkar) occur in various parts of the area.The largest outcrop is on the eastern side of the Manderay River. to the south—eastof the Ndonyo Mara Plateau. which apparently overlies crystalline limestone. Inhand specimen 289359 this bull-coloured limestone shows quartz inclusions. and thinsection examination reveals the presence of quartz and amphiboles set in a fine—grainedcalcareous matrix with patches of crystalline calcite. Another occurrence of secondarylimestone is present to the north of Laisamis. on the eastern side of the MarsabitRoad. where the limestone is present only as small fragments scattered about thesurface This deposit. like other small deposits throughout the area. is too small to beshown on the map.

Kunkar limestone is characteristic of areas with limited rainfall and hence is usefulas a climatic indicator. It owesits origin to capillary action which draws lime-richwater to the surface. where by evaporation a calcareous deposit is formed.

(2) FERRUUINOI'S CRuSTAL DEPOSITSThese oecur to a limited extent in the area mapped. but none attain a large size

and most are too small to be shown on the map. One outcrop exists on the southernside of the lllaut-Laisamis track east of Losai. ln outcrop the deposit is unconsolidatedand brown in colour and contains fragments of metamorphic rocks. A similar butmuch smaller deposit exists about a mile west of Moile,

The origin of these ferruginous deposits is similar to that of the calcareous deposits.i.e. capillary action which drew iron—rich water to the surface where it evaporated.

(3) PEBBLE SheersThese were encountered only in one locality. close to the amphibolitie intrusion

of ('Thaparkrom. The pebbles are of angular quartz. derived from quartz veins onthe hill east of Chaparkrom.

t4) RECENI‘ SANDS AND ALLIJVIUMReddish brown sands are extremely common and form a blanket over much of

the region to the south of the Kaisut Plateau. a cover broken only by inselbergs suchas [Him and Baio. Examination of these sands shows that they contain fragments ofquartz. felspar. hornblende and biotite and hence are derived entirely from metamorphicrocks.

Pale brown alluvium is present along the south-western portion of the Kaisut Plateau.marking the site of a small lake. Alluvium is also present along stretches of the MilgisRiver.

19

P'

VI-METAMORPHISM

The metamorphic rocks of the area have been derived from sediments which arethought to have been deposited in the Mozambique geosyncline (Holmes, 1951). Theperiod of sedimentation was followed by down-warping of the crust accompanied byfolding and regional metamorphism whereby the original arenaceous, argillaceous andcalcareous sediments were converted into psammitic, semi-pelitic and calcareous meta-morphicrocks,

It is probable that in the late stages of orogenesis the metamorphic rocks were sub-jected to metasomatism, a process which in this case involved the introduction ofalkalis. Metasomatism has been defined by Goldschmidt (1922, pp. 105-123) as "a processof alteration which involves enrichment of the rock by new substances brought infrom outside" and this enrichment occurs "by definite chemical reactions between theoriginal minerals and the enriching substance". The degree of metasomatism thereforeis largely controlled by the original composition of the host rock and by textural andstructural details, as joint planes for example greatly facilitate the movement of themetasomatic agents. Rocks of calcareous and quartzitic composition are little susceptibleto metasomatic processes and often provide barriers to the passage of the agents ofmetasomatism.

In the Laisamis area the ancient rocks have been subjected to potash metasomatismleading to the formation of the potash felspar microcline, which is greatly in evidencein the gneisses. The degree of metamorphism attained by the Basement System rocksis indicated by a comparison of the mineral assemblages present with the idealisedassemblages present in regionally metamorphosed rock as described by Turner (Fyfe,Turner and Verhoogen, 1958, pp. 215-239). In this area typical mineral assemblagesare:-

Semi-pelitic and quartzo-fejspathic assemblages:-

(a) Quartz-microcline-plagioclase-biotite.

(b) Quartz-microcline-plagioclase-muscovite.

Calc-silicate assemblages:-

(a) Garnet-epidote-plagioclase-diopside.

Basic assemblages:-

(a) Hornblende-plagioclase.

(b) Garnet-diopside-hornblende.

These assemblages show that the metamorphic grade of the rocks corresponds toa sub-facies of the almandine-amphibolite facies of metamorphism, either the kyanite-almandine or the sillimanite-almandine sub-facies, although neither kyanite nor sillima-nite were seen during the survey. The non-development of these minerals is probablydue to an insufficient quantity of alumina in the original sediments. A single occurrenceof sillimanite was found in the Kauro-Merille area (Rix, Report No. 92) some eightmilel\ south of Soito Narok.

VII-STRUcrURE

Over the major part of the area the structure is straightforward and shows a seriesof folds with dominantly north-south axes. Several of these folds are overturned tothe west. Foliation in the area is best developed in the biotite gneiss succession, andto a lesser degree the hornblende prisms of the hornblende gneisses cause a crudefoliation to be developed. The intrusion at Chaparkrom, being dominantly composedof hornblende, has a weakly developed foliation.

20

t.:,;

P'

VI-METAMORPHISM

The metamorphic rocks of the area have been derived from sediments which arethought to have been deposited in the Mozambique geosyncline (Holmes, 1951). Theperiod of sedimentation was followed by down-warping of the crust accompanied byfolding and regional metamorphism whereby the original arenaceous, argillaceous andcalcareous sediments were converted into psammitic, semi-pelitic and calcareous meta-morphicrocks,

It is probable that in the late stages of orogenesis the metamorphic rocks were sub-jected to metasomatism, a process which in this case involved the introduction ofalkalis. Metasomatism has been defined by Goldschmidt (1922, pp. 105-123) as "a processof alteration which involves enrichment of the rock by new substances brought infrom outside" and this enrichment occurs "by definite chemical reactions between theoriginal minerals and the enriching substance". The degree of metasomatism thereforeis largely controlled by the original composition of the host rock and by textural andstructural details, as joint planes for example greatly facilitate the movement of themetasomatic agents. Rocks of calcareous and quartzitic composition are little susceptibleto metasomatic processes and often provide barriers to the passage of the agents ofmetasomatism.

In the Laisamis area the ancient rocks have been subjected to potash metasomatismleading to the formation of the potash felspar microcline, which is greatly in evidencein the gneisses. The degree of metamorphism attained by the Basement System rocksis indicated by a comparison of the mineral assemblages present with the idealisedassemblages present in regionally metamorphosed rock as described by Turner (Fyfe,Turner and Verhoogen, 1958, pp. 215-239). In this area typical mineral assemblagesare:-

Semi-pelitic and quartzo-fejspathic assemblages:-

(a) Quartz-microcline-plagioclase-biotite.

(b) Quartz-microcline-plagioclase-muscovite.

Calc-silicate assemblages:-

(a) Garnet-epidote-plagioclase-diopside.

Basic assemblages:-

(a) Hornblende-plagioclase.

(b) Garnet-diopside-hornblende.

These assemblages show that the metamorphic grade of the rocks corresponds toa sub-facies of the almandine-amphibolite facies of metamorphism, either the kyanite-almandine or the sillimanite-almandine sub-facies, although neither kyanite nor sillima-nite were seen during the survey. The non-development of these minerals is probablydue to an insufficient quantity of alumina in the original sediments. A single occurrenceof sillimanite was found in the Kauro-Merille area (Rix, Report No. 92) some eightmilel\ south of Soito Narok.

VII-STRUcrURE

Over the major part of the area the structure is straightforward and shows a seriesof folds with dominantly north-south axes. Several of these folds are overturned tothe west. Foliation in the area is best developed in the biotite gneiss succession, andto a lesser degree the hornblende prisms of the hornblende gneisses cause a crudefoliation to be developed. The intrusion at Chaparkrom, being dominantly composedof hornblende, has a weakly developed foliation.

20

t.:,;

VI-—.\IETA MORPHISM

The metamorphic rocks of the area have been derived from sediments which arethought to have been deposited in the Mozambique geosyncline (Holmes. 1951). Theperiod of sedimentation was followed by downvwarping of the crust accompanied byfolding and regional metamorphism whereby the original arenaceous. argillaeeous andcalcareous sediments were converted into psammitic. semi—pelitic and calcareous meta—morphic rocks.

it is probable that in the late stages of orogenesis the metamorphic rocks were sub—jected to metasomatism. a process which in this case involved the introduction. ofalkalis. Metasomatism has been defined by (joldschmidt (1922. pp. 105-123) as “a processof alteration which involves enrichment of the rock by new substances brought. infrom outside" and this enrichment oecurs “by definite chemical reactions between theoriginal minerals and the enriching substance". The degree of metasomatism thereforeis largely controlled by the original composition of the host rock and by textural andstructural details. as joint planes for example greatly facilitate the movement of themetasomatic agents. Rocks of. calcareous and quartzitic composition are little susceptibleto metasomatic processes and often provide barriers to the passage of the agents ofmetasomatism.

In the l.aisamis area the ancient reeks have been subjected to potash metasomatismleading to the formation of the potash felspar microcline. which is greatly in evidencein the gneisses. The degree of metamorphism attained by the Basement System rocksis indicated by a comparison of the mineral assemblages present with the idealisedassemblages present in regionally metamorphosed rock as described by Turner (Fyfe.Turner and Verhoogen. 1958. pp. 215-239). In this area typical mineral assemblagesare:—

Semi-politic and quartzo-felspathic assemblages:ta) Quartz-microcline—plagioclase-biotite.(l7) Quartz-microcline-plagioclase-muscovite.

Cale-silicate assemblages: —{(7) Garnet-epidote-plagioclase-diopstde.

Basie assemblages:-(a) Hornblende—plagioelase.lb) Garnet-diopside-hornblende.

'l'hese assemblages show that the metamorphic grade of the rocks corresponds toa sub-facies of the almandine-ampbibolite facies of metamorphism. either the kyanite-almandinc or the sillimanite-almandine sub-facies. although neither kyanite nor sillima-nite were seen during the survey. The non-development of these minerals is probablydue to an insullicient quantity of alumina in the original sediments. A single occurrenceof sillimanite was found in the Kauro-Merille area (Rix. Report No, 92) some eightmile." south of Soito Narok.

VII—STRUCTURE

OVer the major part of the area the structure is straightforward and shows a seriesof folds with dominantly northAsouth axes. Several of these folds are overturned tothe west. Foliation in the area is best developed in the biotite gneiss succession. andto a lesser degree the hornblende prisms of the hornblende gneisses Gtuse a crudefoliation to be developed. The intrusion at Chaparkrom. being dominantly composedof hornblende has a weakly developed i'oliation.

20

r

Lineations are useful in the elucidation of the structure of an area because theyreflect the direction and magnitude of the plunge of a fold axis. They occur in theLaisamis area as alignments of biotite flakes, or more often as alignments of quartzcrystals on the foliation planes of the rocks of the Basement System. Measured lineationsplunge gently to the north over most of the area, but in the south-eastern portionthere is a steepening of the angle of plunge.

The Basement System rocks in the south-eastern portion of the area show a divergencefrom the north-south trend of the rest of the area, there the alignment being in anorth-westerly direction. As Laisamis is approached the rocks revert to the normalnorth-south trend. At first sight it would seem that cross-folding had taken place asat the extreme limit the trend is east-west. The lineation plunges are steeper here thanelsewhere in the area. This area of disturbance can be traced into the Kauro-Merillearea to the south, and to interpret the structure it is best to consider the north-easternpart of the Kauro-Merille area and the south-eastern part of the Laisamis area asone unit. In that part of the Kauro-Merille area (Rix, Report No. 92) the foliationsindicate that some closures which are difficult to interpret are in fact overturnedfolds. The lineations indicate that no second folding is involved and that the axes havea persistent north-south trend. A stereogram of foliations and lineations plotted byRix shows the lineations plunging both north and south. In the Laisamis area theoverturning js not so acute as in the Kauro-Merille area and the dip of foliationsvaries between east-south-east and north. The lineations also plunge to the north, atsteep angles in the south-east but becoming less steep closer to Laisamis. It thereforeappears that the local overturning of folds was a direct result of the main episode offolding.

Only one visible fault was mapped during the survey although it is possible thatother faults exist beneath the widespread cover of superficial deposits. The faultmapped is a post~Pleistocene feature as it cuts the olivine basalt lava of the SagereruaPlateau. It trends in a south-south-easterly direction, down throwing to the south-westwith a maximum throw of 50 feet. It continues across the south-west corner of theTirkamo Plateau, but there is no indication of it on the Rusarus Plateau.

Mention has already been made of the two north-north-easterly trending lines linkingthe explosion craters on the Rusarus Plateau and the craters and cones on the SagereruaPlateau and the vents to the north-west of Laisamis. These lines appear to be due tostructural weakness in the underlying Basement System rocks as the lines of featureson the Rusarus Plateau can be traced into the area to the south. It has also beennoted that the craters on both the Sagererua and Rusarus plateaux are situatedon or close to the intersection of these lines with the south-easterly trending fault line,and probably owe their positions to this cause.

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11

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VIII-ECONOMIC GEOLOGY

Local concentrations of economic minerals were found during the survey of thearea, but in none of the occurrences were the minerals present in sufficient quantitiesto warrant exploitation. The nearest railhead is Nanyuki, some 160 miles to the south,and unless a low-price mineral is present in sufficiently large amounts it will proveuneconomic to exploit the deposit due to the high costs of transportation.

Mica.-Mica was found associated with pegmatites in the area, but none of thebooks were bigger than two inches long and one inch thick.

Magnetite.-This was found in small quantities on the western slopes of HalilugumNder as float from a pegmatitic vein.

Magnesite.-Associated with the serpentinite intrusion on Chaparkrom are randomlyoriented veins of magnesite generally not more than a few inches in width. The totaltonnage available is too small to warrant exploitation.

il

I'III

)1

21

I,1

I

-- -

r

Lineations are useful in the elucidation of the structure of an area because theyreflect the direction and magnitude of the plunge of a fold axis. They occur in theLaisamis area as alignments of biotite flakes, or more often as alignments of quartzcrystals on the foliation planes of the rocks of the Basement System. Measured lineationsplunge gently to the north over most of the area, but in the south-eastern portionthere is a steepening of the angle of plunge.

The Basement System rocks in the south-eastern portion of the area show a divergencefrom the north-south trend of the rest of the area, there the alignment being in anorth-westerly direction. As Laisamis is approached the rocks revert to the normalnorth-south trend. At first sight it would seem that cross-folding had taken place asat the extreme limit the trend is east-west. The lineation plunges are steeper here thanelsewhere in the area. This area of disturbance can be traced into the Kauro-Merillearea to the south, and to interpret the structure it is best to consider the north-easternpart of the Kauro-Merille area and the south-eastern part of the Laisamis area asone unit. In that part of the Kauro-Merille area (Rix, Report No. 92) the foliationsindicate that some closures which are difficult to interpret are in fact overturnedfolds. The lineations indicate that no second folding is involved and that the axes havea persistent north-south trend. A stereogram of foliations and lineations plotted byRix shows the lineations plunging both north and south. In the Laisamis area theoverturning js not so acute as in the Kauro-Merille area and the dip of foliationsvaries between east-south-east and north. The lineations also plunge to the north, atsteep angles in the south-east but becoming less steep closer to Laisamis. It thereforeappears that the local overturning of folds was a direct result of the main episode offolding.

Only one visible fault was mapped during the survey although it is possible thatother faults exist beneath the widespread cover of superficial deposits. The faultmapped is a post~Pleistocene feature as it cuts the olivine basalt lava of the SagereruaPlateau. It trends in a south-south-easterly direction, down throwing to the south-westwith a maximum throw of 50 feet. It continues across the south-west corner of theTirkamo Plateau, but there is no indication of it on the Rusarus Plateau.

Mention has already been made of the two north-north-easterly trending lines linkingthe explosion craters on the Rusarus Plateau and the craters and cones on the SagereruaPlateau and the vents to the north-west of Laisamis. These lines appear to be due tostructural weakness in the underlying Basement System rocks as the lines of featureson the Rusarus Plateau can be traced into the area to the south. It has also beennoted that the craters on both the Sagererua and Rusarus plateaux are situatedon or close to the intersection of these lines with the south-easterly trending fault line,and probably owe their positions to this cause.

I,

t:

11

.1

VIII-ECONOMIC GEOLOGY

Local concentrations of economic minerals were found during the survey of thearea, but in none of the occurrences were the minerals present in sufficient quantitiesto warrant exploitation. The nearest railhead is Nanyuki, some 160 miles to the south,and unless a low-price mineral is present in sufficiently large amounts it will proveuneconomic to exploit the deposit due to the high costs of transportation.

Mica.-Mica was found associated with pegmatites in the area, but none of thebooks were bigger than two inches long and one inch thick.

Magnetite.-This was found in small quantities on the western slopes of HalilugumNder as float from a pegmatitic vein.

Magnesite.-Associated with the serpentinite intrusion on Chaparkrom are randomlyoriented veins of magnesite generally not more than a few inches in width. The totaltonnage available is too small to warrant exploitation.

il

I'III

)1

21

I,1

I

-- -

Lineations are useful in the elucidation of the structure of an area because the).rerlect the direction and magnitude of the plunge of a fold axis. They occur in the[.aisamis area as alignments of biotite flakes. or more often as alignments of quartzcrystals on the foliation planes of the rocks of the Basement System. Measured lineationsplunge gently to the north over most of the area. but in the south-eastern portionthere is a steepening of the angle of plunge.

The Basement System rocks in the south-eastern portion of the area show a divergencefrom the north-south trend of the rest of the area. there the alignment being in anorth—westerly direction. As Laisamis is approached the rocks revert to the normalnorth-south trend. At first sight it would seem that cross-folding had taken place asat the extreme limit the trend is east-west. The lineation plunges are steeper here thanelsewhere in the area. This area of disturbance can be traced into the Kauro-Merillearea to the south. and to interpret the structure it is best to consider the northeasternpart of the Kauro-Merille area and the south-eastern part of the Laisamis area asone unit. In that part of the Kauro-Merille area lRix. Report No. 92) the foliationsindicate that some closures which are ditlicult to interpret are in fact overturnedfolds. 'I'he lineations indicate that no second folding is involved and that the axes havea persistent north—south trend. A stereogram of t‘oliations and lineations plotted b5Rix shows the lineations plunging both north and south. In the Laisamis area theoVerturning is not so acute as in the Kauro—N‘lerille area and the dip of foliationsvaries between east-southeast and north. The lineations also plunge to the north. atsteep angles in the south—east but becoming less steep closer to Laisamis. It thereforeappears that the local overturning of folds was a direct result of the main episode ol‘folding.

Only one visible fault was mapped during the survey although it is possible thatother faults exist beneath the widespread CoVet’ of superficial deposits. The faultmapped is a post-Pleistocene feature as it cuts the olivine basalt lava of the SagereruaPlateau. lt trends in a south-south-easterly direction. downthrowing to the south—westwith a maximum throw of St) feet. It continues across the south-west corner of the'I'irkamo Plateau. but there is no indication of it on the Rusarus Plateau.

Mention has already been made of the two north-north-easterly trending lines linkingthe explosion craters on the Rusarus Plateau and the craters and cones on the SagereruaPlateau and the tents to the north-west of Laisamis. These lines appear to be due tostructural weakness in the underlying Basement System rocks as the lines of featureson the Rusarus Plateau can be traced into the area to the south, It has also beennoted that the craters on both the Sagererua and Rusarus plateaus are situatedon or Close to the intersection of these lines with the south-easterly trending fault lint).and probably owe their positions to this cause.

VIII—FCONON‘IIC GEOLOGY

Local concentrations of economic minerals were found during the survey of thearea. but in none of the occurrences were the minerals present in sufficient quantitiesto warrant exploitation. The nearest railhead is Nanyuki. some 16‘) miles to the south.and unless a low-price mineral is present in sufficiently large amounts it will prove.uneconomic to exploit the deposit due to the high costs of transportation.

Mica—Mica was found associated with pegmatites in the area. but none of thebooks were bigger than two inches long and one inch thick.

Mammoth—this was found in small quantities on the western slopes of HalilugumVder as float from a pegmatitic vein.

Magnet-ire.—Assoeiated with the serpentinite intrusion on Chaparkrotn are randomlyoriented veins of magnesite generally not there than a few inches in width. The totaltonnage available is too small to warrant exploitation.

“i

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Garnet .-Quantities af almandine garnet are present in the intrusive bady at Hali-siruwa. Soils araund the base af the intrusian cantain eraded garnets, but these areaf limited extent and the garnets themselves are flawed.

Tourmaline'1.-Prisms O'f black taurmaline are develaped in quartz to' the east afHalisiruwa.

Graphite.-A graphite schist was mapped an the eastern slapes af Baia. The autcrapis anly O'f limited eJctent and the graphite cantent daes nO't exceed ane per cent.

River Sands.-Ample supplies af clean quartz sand are available from the sandrivers far use as building material. There is hawever nO' fareseeable demand in thearea for this material.

Road Stone.-Shauld the need arise far material far raads in the area there aresupplies af volcanic rack which cauld easily be braken up and used far this purpose.The present Isiala-Marsabit road can be used anly in dry weather but it is unlikelythat a tarmac road will be canstructed fO'rmany years.

Radioactive Minerals.-Radiametric manitaring was carried aut in canjunctian withthe daily wark af the survey, but nO' anO'malies were faund.

Water.-By far the mast preciaus cammadity in the area is water, and its presencear absence is O'ne af the contralling factars in the mavements af the peaple whO' livethere. Dr. F. Dixey (1944) made a hydrographical survey af the Narthern Province in1943 and nated that at that time there was nO' permanent water in the Kaisut regianas the barehale at Lakuloka which had been drilled by army engineers in the earlypart af the war had ceased to' aperate and had been filled with stanes. He recammendedthat this barehO'le shO'uld be reapened ar a new ane put dawn clase by, and suggestedthat drilling in the stream caurses to' the sauth af Lakulaka wauld affer a gaad chanceaf success. As a result af this survey a firm af cansulting engineers, Haward Humphriesand Sans, was appainted in 1950 to' put intO' effect Dr. Dixey's recammendatians inthe NO'rthern Province and Samburu Regian.

In the present area the anly flawing water accurs at Laisamis. This is due to' thepresence af Basement System racks which farm a natural sub-surface dam acrass theriver. Quantities af water can be held by the sand af river beds thraughaut the year,and where the sand caver abave the metamarphic rocks is thin the water table mayrise and water flaw an the surface until the sand caver is sufficiently thick far it to'became sub-surface water again. At Laisamis the water flaws an the surface far abaut150 yards and the am aunt is sufficient to' serve the needs af the peaple and their animals.Far this purpose a can crete well and drinking trO'ugh have been installed.

Elsewhere in the area water can be abtained by digging in same af the sand rivers.

Barehales have been drilled at twO' places in the narth af the area, at the tradingcentre O'f Lokulako and to' the sauth-east of Halisiruwa. At the latter locality a largeshallaw lake farms after rains and water is available there far shart periads. Thebarehale, drilled at the edge af the lava flaw, was campleted by the Ministry af Worksin August 1960.

The lag af this barehale is as fallaws:-

Halisiruwa-C. 3056.Feet0-10

10-.3232-7070- 130

Ferruginaus sand.Caarse sand af lava.Dark brawn lava.Brown sandy clay.

130-150 Quartz and felspar sand.150-170 Sandy depasit.170-252 Sandy clay.252-330 Basement System racks.

22

}-L --~--

....--

Garnet .-Quantities af almandine garnet are present in the intrusive bady at Hali-siruwa. Soils araund the base af the intrusian cantain eraded garnets, but these areaf limited extent and the garnets themselves are flawed.

Tourmaline'1.-Prisms O'f black taurmaline are develaped in quartz to' the east afHalisiruwa.

Graphite.-A graphite schist was mapped an the eastern slapes af Baia. The autcrapis anly O'f limited eJctent and the graphite cantent daes nO't exceed ane per cent.

River Sands.-Ample supplies af clean quartz sand are available from the sandrivers far use as building material. There is hawever nO' fareseeable demand in thearea for this material.

Road Stone.-Shauld the need arise far material far raads in the area there aresupplies af volcanic rack which cauld easily be braken up and used far this purpose.The present Isiala-Marsabit road can be used anly in dry weather but it is unlikelythat a tarmac road will be canstructed fO'rmany years.

Radioactive Minerals.-Radiametric manitaring was carried aut in canjunctian withthe daily wark af the survey, but nO' anO'malies were faund.

Water.-By far the mast preciaus cammadity in the area is water, and its presencear absence is O'ne af the contralling factars in the mavements af the peaple whO' livethere. Dr. F. Dixey (1944) made a hydrographical survey af the Narthern Province in1943 and nated that at that time there was nO' permanent water in the Kaisut regianas the barehale at Lakuloka which had been drilled by army engineers in the earlypart af the war had ceased to' aperate and had been filled with stanes. He recammendedthat this barehO'le shO'uld be reapened ar a new ane put dawn clase by, and suggestedthat drilling in the stream caurses to' the sauth af Lakulaka wauld affer a gaad chanceaf success. As a result af this survey a firm af cansulting engineers, Haward Humphriesand Sans, was appainted in 1950 to' put intO' effect Dr. Dixey's recammendatians inthe NO'rthern Province and Samburu Regian.

In the present area the anly flawing water accurs at Laisamis. This is due to' thepresence af Basement System racks which farm a natural sub-surface dam acrass theriver. Quantities af water can be held by the sand af river beds thraughaut the year,and where the sand caver abave the metamarphic rocks is thin the water table mayrise and water flaw an the surface until the sand caver is sufficiently thick far it to'became sub-surface water again. At Laisamis the water flaws an the surface far abaut150 yards and the am aunt is sufficient to' serve the needs af the peaple and their animals.Far this purpose a can crete well and drinking trO'ugh have been installed.

Elsewhere in the area water can be abtained by digging in same af the sand rivers.

Barehales have been drilled at twO' places in the narth af the area, at the tradingcentre O'f Lokulako and to' the sauth-east of Halisiruwa. At the latter locality a largeshallaw lake farms after rains and water is available there far shart periads. Thebarehale, drilled at the edge af the lava flaw, was campleted by the Ministry af Worksin August 1960.

The lag af this barehale is as fallaws:-

Halisiruwa-C. 3056.Feet0-10

10-.3232-7070- 130

Ferruginaus sand.Caarse sand af lava.Dark brawn lava.Brown sandy clay.

130-150 Quartz and felspar sand.150-170 Sandy depasit.170-252 Sandy clay.252-330 Basement System racks.

22

}-L --~--

Gurnwt.—Quantities of almandine garnet are present in the intrusive body at Hali-siruwa. Soils around the base of the intrusion contain eroded garnets. but these areof limited extent and the garnets themselves are flaw ed.

Tourmaline.Halisiruna.

Prisms of black tourmaline are developed in quartz to the east of

Graphite—A graphite schist was mapped on the eastern slopes of Baio. The outcropis only of limited extent and the graphite content does not exceed one per cent.

Rit'z'r Xmidt. iAmple supplies of clean quartz sand are available from the sandrivers for use as building material. There is however no foreseeable demand in thearea for this material.

Road Stony. Should the need arise for material for roads in the area there aresupplies of volcanic rock which could easily be broken up and used for this purpose.’I he present lsiolo-Marsabit road can be used only in dry weather but it is unlikelythat a tarmac road will be constructed for many years.

Radioactive .Mincru/LirRadiometric monitoring was carried out in conjunction withthe daily work of the survey. but no anomalies were found.

WatchiBy far the most precious commodity in the area is water. and its presenceor absence is one of the controlling factors in the movements of the people who livethere. Dr. F. [)ixey (1944) made a hydrographical survey of the Northern Province in19-13 and noted that at that time there was no permanent water in the Kaisut regionas the borehole at Lokuloko which had been drilled by army engineers in the earlypart of the war had ceased to operate and had been tilled with stones. He recommendedthat this borehole should be reopened or a new one put down close by, and suggestedthat drilling in the stream courses to the south of Lokuloko would otter a good chanceof success. As a result 01’ this survey a firm ot consulting engineers. Howard Humphriesand Sons. was appointed in 195” to put into eti‘ect Dr. Dixey‘s recommendations inthe Northern Province and Samburu Region.

lo the present area the only flowing water occurs at Laisamis. This is due to thepresence of Basement System rocks which form a natural sub-surface dam across theriver. Quantities of water can he held by the sand or" river beds throughout the year.and where the sand cover above the metamorphic rocks is thin the water table mayrise and water Flow on the surface until the sand cover is sufficiently thick for it tobecome sub-surface water again. At Laisamis the water flows on the surface for about[50 yards and the amount is sutficient to serve the needs of the people and their animals.For this purpose a concrete well and drinking trough have been installed.

Elsewhere in the area water can be obtained by digging in some of the sand rivers.

Boreholcs have been drilled at two places in the north of the area. at the tradingcentre of Lokuloko and to the south-east ot llalisiruwa. At the latter locality a largeshallow lake forms after rains and water is available there for short periods, Theborehole. drilled at the edge of the lava flow. was completed by the Ministry of VVorl-tsin August N60,

The log of this borehole is as follows:

Halisiruwa C. 3056.FEB!

0-H) Ferruginous sand. 130—150 Quartz and felspar sand.10-32 Coarse sand ol‘ lava, 150—170 Sandy deposit.32—""O Dark brown lava. 170 75‘ Sandy clay,7t} 30 Brown sandy clay. 252—330 Basement System rocks.

Total depth of borehole-330 feet.Water struck at 252 to 297 feet and at 320 feet.Water rest level-218 feet.Estimated yield,---400 to 800 gallons per hour.

The borehole at Lokuloko, as already mentioned, was drilled to a depth of 244 feet,with the water rest level at 209 feet, but had been filled in with stones prior to 1943.It was cleaned out and extended to a depth of 280 feet by Howard Humphries andSons and equipped with a mechanical pump.

The log of the borehole is as follows:-Lokuloko--c. 1758.

Feet0-33-38

38-4040---6868-9595-183

Red soil.Boulders.Clay.Lava.Clay and sand.Lava.

Feet

183-187187-216216-236236-245245-268268-280

Decomposed lava.Old land surface.Decomposed lava.Lava.Old land surface.Solid rock.

IX-REFERENCES

I

~II

~

j

Total depth of borehole-280 feet.Water struck at 229 feet.Water rest level-215 feet.Estimated yield-1,750 gallons per hour.

In their recommendations for further supplies of water the consulting engineerssuggest (Humphries, 1958) that boreholes should be drilled between Baio and HalilugumNder, and in the vicinity of the old airstrip to the west of the Isiolo-Marsabit roadand just to the north of the Sagererua Plateau, where they say that water is fairly sureto be struck at a depth of about 300 feet.

ARCHER, G. F. (1913) "Recent exploration and survey in the north of British EastAfrica" Geogrl. J. 42, 421-430.

DIXEY, F. (1944) "Hydrographical survey of the Northern Frontier District, Kenya"Public Works Dept., Nairobi (Unpublished).

(1948) "Geology of Northern Kenya" Rep. geol. Surv. Kenya 15.

DODSON,R. G. "Geology of the Barchuma-Kom area" Rep. geol. Surv. Kenya 93 (Inpreparation) .

DONALDSON-SMITH,A. (1900) "An expedition between Lake Rudolf and the Nile"Geogrl. J. 16, 600-625.

FYFE, W. S., F. 1. TURNER and J. VERHOOGEN(1958) "Metamorphic reactions andmetamorphic facies" Mem. geol. Soc. Am. 73.

GOLDSCHMIDT,V. M. (1922) "On the metasomatic processes in silicate rocks" Econ.Geol. 17, 105-123.

I.

}i

, II

I:

il

II

I,

I1I

HOLMES,A. (1951) "The sequence of Precambrian orogenic belts in South and CentralAfrica" xviii Int. geol. Congr. 14,254-269.

HUMPHRIES,HOWARDand SONS(1958) "The Northern Frontier Province and SamburuDistrict water development scheme, 1950-1958" Report to Governmentby the Consulting Engineers.

23

I--

Total depth of borehole-330 feet.Water struck at 252 to 297 feet and at 320 feet.Water rest level-218 feet.Estimated yield,---400 to 800 gallons per hour.

The borehole at Lokuloko, as already mentioned, was drilled to a depth of 244 feet,with the water rest level at 209 feet, but had been filled in with stones prior to 1943.It was cleaned out and extended to a depth of 280 feet by Howard Humphries andSons and equipped with a mechanical pump.

The log of the borehole is as follows:-Lokuloko--c. 1758.

Feet0-33-38

38-4040---6868-9595-183

Red soil.Boulders.Clay.Lava.Clay and sand.Lava.

Feet

183-187187-216216-236236-245245-268268-280

Decomposed lava.Old land surface.Decomposed lava.Lava.Old land surface.Solid rock.

IX-REFERENCES

I

~II

~

j

Total depth of borehole-280 feet.Water struck at 229 feet.Water rest level-215 feet.Estimated yield-1,750 gallons per hour.

In their recommendations for further supplies of water the consulting engineerssuggest (Humphries, 1958) that boreholes should be drilled between Baio and HalilugumNder, and in the vicinity of the old airstrip to the west of the Isiolo-Marsabit roadand just to the north of the Sagererua Plateau, where they say that water is fairly sureto be struck at a depth of about 300 feet.

ARCHER, G. F. (1913) "Recent exploration and survey in the north of British EastAfrica" Geogrl. J. 42, 421-430.

DIXEY, F. (1944) "Hydrographical survey of the Northern Frontier District, Kenya"Public Works Dept., Nairobi (Unpublished).

(1948) "Geology of Northern Kenya" Rep. geol. Surv. Kenya 15.

DODSON,R. G. "Geology of the Barchuma-Kom area" Rep. geol. Surv. Kenya 93 (Inpreparation) .

DONALDSON-SMITH,A. (1900) "An expedition between Lake Rudolf and the Nile"Geogrl. J. 16, 600-625.

FYFE, W. S., F. 1. TURNER and J. VERHOOGEN(1958) "Metamorphic reactions andmetamorphic facies" Mem. geol. Soc. Am. 73.

GOLDSCHMIDT,V. M. (1922) "On the metasomatic processes in silicate rocks" Econ.Geol. 17, 105-123.

I.

}i

, II

I:

il

II

I,

I1I

HOLMES,A. (1951) "The sequence of Precambrian orogenic belts in South and CentralAfrica" xviii Int. geol. Congr. 14,254-269.

HUMPHRIES,HOWARDand SONS(1958) "The Northern Frontier Province and SamburuDistrict water development scheme, 1950-1958" Report to Governmentby the Consulting Engineers.

23

I--

Total depth of. borehole 330 feet.Water struck at 252 to 297 feet and at. 320 feet.Water rest level 218 feet.Estimated yield 400 to 800 gallons per hour.

The borehole at Lokuloko. as already mentioned. was drilled to a depth of 244 feet.with the water rest level at 209 feet. but had been filled in with stones prior to 1943.,It was cleaned out and extended to a depth of 280 feet by Howard Humphries andSons and equipped with a mechanical pump.

The log or" the borehole is as follows:——Lokuloko—C 1758.

Fee! Ft’t’f

0—3 Red soil. 183—187 Decomposed lava.3—38 Boulders. 187—216 Old land surface.

38—40 Clay. 216—236 Decomposed lava.40—68 Lava. 236—245 Lava.68—95 Clay and sand. 245—268 Old land surface.95—183 Lava. 268—280 Solid rock.

Total depth of borehole - 280 feet.Water struck at 229 feet.Water rest level—215 feet.Estimated yield --l.750 gallons per hour.

In their recommendations fer further supplies of water the consulting engineersSuggest (llumphries, 1958) that boreholes should be drilled between Buio and HulilugumNder, and in the vicinity of the old airstrip to the west of the lsiolo-Marsabit roadand just to the north of the Sagererua Plateau, where they say that water is fairly sureto be struck at a depth of about 300 feet.

IX—REFERENCES

ARCHER. C}. F. (1913) "Recent exploration and survey in the north of British EastAfrica" Gengrl’. J. 42. 421.430.

DIXEY. 1-'. {1944) “Hydrographical survey of the Northern Frontier District, Kenya"Public Works Dept. Nairobi [Unpublished].

{1948) “Geology of Northern Kenya" Rep. ecu]. .S‘urv. Kenya 15.

DODSON, R. G. “Geology of the Barchuma-Kom area” Rep. gun]. .S'urv. Kenya 93 (Inpreparation).

DONALDSON-SMITH, A. (1900) “An expedition between Lake Rudolf and the Nile"Gmgrl. J. 16, 600-625.

l-‘vt-L', W. 8.. F. J. 'l't'RNuR and J, VIERHoooL-‘N (1958) "N‘letamorphic reactions andmetamorphic fades" il/Icm. grail. Sue. Am. 73.

GoLDscirnr. V. M. (1922) “On the metasomatic processes in silicate rocks" Eam.Gaul. 17, 103-123.

HOLMES, A, {1951) “l“he sequence of Precambrian orogenic belts in South and CentralAfrica" .rviii 1m. goo]. Cungr. 14. 254-269.

l-lt_'.\-1r’lu<n=.s. Howard) and Soxs‘ {1958) “The Northern Frontier Province and SamburuDistrict water development scheme, 1950-195S” Report to Governmentby the Consulting Engineers.

I "r~- -.-

KNILL, D. C. and J. L. (1961) "Time relations between folding, metamorphism and theemplacement of granite in Rosguill, County Donegal" Q. Jl. geol Soc.Land. 117, 273-306.

PARKINSON,J. (1920) "Report of the geology and geography of the northern part ofthe East African Protectorate" Colon. Rep. misc. Ser. 91.

(1924) "A note on the volcanic history of South and East Africa" Jl. R.Afr. Soc. 23, 96-105.

(1939) "Notes on the Northern Frontier Province, Kenya" Geogrl. J.94, 162-166.

PULFREY,W. (1960) "Shape of the sub-Miocene erosion bevel in Kenya" Bull. geol.Surv. Kenya 3.

RIX, P. "Geology of the Kauro-Merille area" Rep. geol. Surv. Kenya 92 (In prepa-ration).

ROBERTS,H. D. (1941) "Final report on a geological reconnaissance of Merti Plateauand roads north and south of the Uaso Nyiro River" Unpublished MilitaryReport.

WICKENBERG,COUNTE. (1903) "Von Dschibuti bis Lamu" Petermanns Mitt. 49, 193-199.

'WILLIAMS,L. A. J. (1966) "Geology of the Chanler's Falls area" Rep. geol. Surv.Kenya 75.

24

GBK 482-1m-2!70

r-

I "r~- -.-

KNILL, D. C. and J. L. (1961) "Time relations between folding, metamorphism and theemplacement of granite in Rosguill, County Donegal" Q. Jl. geol Soc.Land. 117, 273-306.

PARKINSON,J. (1920) "Report of the geology and geography of the northern part ofthe East African Protectorate" Colon. Rep. misc. Ser. 91.

(1924) "A note on the volcanic history of South and East Africa" Jl. R.Afr. Soc. 23, 96-105.

(1939) "Notes on the Northern Frontier Province, Kenya" Geogrl. J.94, 162-166.

PULFREY,W. (1960) "Shape of the sub-Miocene erosion bevel in Kenya" Bull. geol.Surv. Kenya 3.

RIX, P. "Geology of the Kauro-Merille area" Rep. geol. Surv. Kenya 92 (In prepa-ration).

ROBERTS,H. D. (1941) "Final report on a geological reconnaissance of Merti Plateauand roads north and south of the Uaso Nyiro River" Unpublished MilitaryReport.

WICKENBERG,COUNTE. (1903) "Von Dschibuti bis Lamu" Petermanns Mitt. 49, 193-199.

'WILLIAMS,L. A. J. (1966) "Geology of the Chanler's Falls area" Rep. geol. Surv.Kenya 75.

24

GBK 482-1m-2!70

r-

ed. 117, 273 306.

Kwnt. I). C. and J. l.. (1961) “Time relations between folding. metamorphism and theemplacement of granite in Rosgtiill. County Doncgal" Q. Jr‘. 5490! 5m;

PARKINSON. .l. (1920) “Report of the geology and geography of the northern part ofthe East African Protectorate" Colon. Rap. mixc'. Sc’r‘. 9|.

— — (192—1) ”A note on the volcanic history of South and East Africa" 1!. R.Afr. SUCK 23.. 96-105.

——— (1939) "Notes on the Northern Frontier Province. Kenya" Geogrl. J.94. 162-166.

l’l'H'RL‘i', \V. (1960‘) "Shape of this sub-Miocene erosion bevel in Kenya' Brill. gen].511W. Kenya 3_

Rix. P “Geology of the .lx'auro-Mcrille area” Rt'p. will. .S'm-t‘, [x't-nyu 92 (in prepa-ration).

ROBERTS. 'H. D. (1941) "Final rcport on a geological reconnaissance of Merti Plateau

Report.and roadq north and south of the Dam Nyiro Rixcr" Unpublished Military

Kwu‘u 75.

\VICKtERU. COL\T F.. (1903) “Von Dscl‘iibuti his lama" [’L’féfl'i‘lltllll'fl' Min, 49. l93-199.

'\-’V"IIIIAMF<. 1.. A. J. (1966) ”Geology of [ha C'hanlcr's i‘LlllS area" Rep. 9601. Srrrt'.

24

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