COAL ZIMB[1]

www.elsevier.com/locate/ijcoalgeo

International Journal of Coal

The Lower Karoo coal (k2–3) of the Mid-Zambezi basin,

Zimbabwe: depositional analysis, coal genesis and

palaeogeographic implications

P.M. Oesterlena,*, J. Lepperb

aGirlitzpark 49, 30627 Hannover, GermanybNieders7chsisches Landesamt fur Bodenforschung, Stilleweg 2, 30655 Hannover, Germany

Received 17 March 2004; accepted 19 July 2004

Available online 13 October 2004

Abstract

Intensive lithological study and correlation of borehole records from the k2–3 coalfields or coal occurrences in the Mid-

Zambezi basin led to the identification of two sedimentological types of coal: the Alluvial plain coal and the freshwater-lake

shoreline coal. The Alluvial plain coal was found only at Gokwe and in the Nyamandlovu area. Its depth of more than 200 m

below surface, the thinness and discontinuous nature of the seams, and the high ash content of the coal make the economic

significance extremely small. In clear contrast, lithologically and economically, stand the lake shoreline coal fields at Wankie,

Lubimbi, Lusulu, Lubu, Busi, and Sengwa. The pay-zone is the basal Main Seam, up to 17-m thick. The shoreline coal is either

more or less massive (Wankie, Lusulu-Lubu) or is thin coal bands alternating with carbonaceous mudstone (Lubimbi, Sengwa).

The clearest evidence for a lake shoreline environment comes from the lateral lithofacies change of the coal, e.g., at Wankie

where it turns down-dip into sapropelic mudstone of the lake, and up-dip into terrestrial sediments of the coastal plain. The lake

shoreline interpretation results finally in the delineation of a 20- to 40-km-wide coal-belt stretching from Wankie in the W to

Sengwa in the E. The new model also opens up new perspectives for more coal within and between the coalfields.

The study of quality and petrography of the shoreline coal supports the above depositional environment and reveals a

standard maceral profile characterized by a basal vitrinite-rich coal passing upwards into inertinite-rich coal forming the

major upper part of the sequence (typical Gondwana coal). The profile reflects an initial swamp phase generating a wet-

forest swamp with Glossopteris trees, but this turned soon to a dry-forest swamp, with oxidation and decomposition of the

vegetation, before it was finally overlain by fluviodeltaic sandstones of k4. The paludification is referred to an eustatic rise

of the water-table caused by post-ice-age meltwater, but soon the water level dropped, due to the warmer climate. The

local and regional controls of the peatswamp formation were considered, as well as the autochthonous and diachronous

nature of the coal.

The two coal types led to a new palaeogeographic setting for the Mid-Zambezi basin which is in agreement with the new rift

concept. It was more of a trough having a SW–NE trend axis which was in the centre filled by a shallow freshwater lake. The

0166-5162/$ - s

doi:10.1016/j.co

* Correspon

E-mail addr

Geology 61 (2005) 97–118

ee front matter D 2004 Elsevier B.V. All rights reserved.

al.2004.07.002

ding author.

esses: [email protected] (P.M. Oesterlen)8 [email protected] (J. Lepper).

P.M. Oesterlen, J. Lepper / International Journal of Coal Geology 61 (2005) 97–11898

above coal-belt was formed out of a peatswamp zone along its palaeo-shoreline. South of this stretched a ca. 100-km-wide

shallow alluvial plain drained towards the NW by some meandering rivers, with adjacent flood plains temporarily occupied by

local swamps. The alluvial plain was bounded on the SE by crystalline highlands representing the source of clastic sediments

for the basin.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Lower Karoo coal; Mid-Zambezi basin; Depositional environment; Coalfields; Coal petrography; Palaeogeography

1. Introduction

Coals are delicate indicators for a certain environ-

ment in sedimentation and regional tectonics: the

water-table must remain at or near the ground

surface of the swamp, the area must subside to keep

pace with the vertical growth of the swamp mat, and

the site has to be protected from detrital input. These

special conditions were fulfilled during the Lower

Karoo in the Mid-Zambezi basin, as manifested by a

number of coalfields and occurrences in the Mid-

Zambezi basin (Fig. 1). The main fields, such as the

active coal mine of Wankie and the coalfields

Sengwa-South, Lubimbi, etc., occur in the Black

Shale and Coal Member of the Lower Permian

Wankie Formation (k2–3, see Table 1). Some

occurrences, however, such as Marowa and Nebiri,

are now known to belong to the Upper Permian

Madumabisa Formation (k5).

This paper focuses on the k2–3 coal sequence, as

this coal represents the prime energy resource for

Zimbabwe and, secondly, its main conditions of

formation, the depositional environment and palae-

ogeographic setting, are, to date, poorly understood.

The knowledge on the Mid-Zambezi k2 coal is rather

limited, the most recent publications being from

Duguid (1986). The general Lower Karoo lithostra-

tigraphy of the Mid-Zambezi was covered by Lepper

(1992). But since then, new ideas have been put

forward on the formation and evolution of the

Zambezi basin and rift in general, as well as on

the Mid-Zambezi basin which shed a new light on

the Lower Karoo coal of Zimbabwe (e.g., Oesterlen

and Blenkinsop, 1994; Oesterlen, 1998, 1999).

The previous models of k2 coal genesis and

palaeogeographic setting are summarized below and

are discussed in the light of the new ideas on the

Mid-Zambezi basin formation and evolution. Then,

the lithology of the main k2 coalfields and

occurrences is reviewed applying sedimentological

criteria, and depositional environments and trends

are established. Subsequently, these results are

viewed in the light of coal quality and maceral

data from the individual coalfields so far published.

Finally, a new model for the coal formation and the

palaeogeographic setting for the k2–3 sequence of

the Wankie Formation is proposed.

For the stratigraphic subdivision of the Mid-

Zambezi basin, the new classification of Oesterlen

(1999) is used (Table 1).

2. Previous models of coal origin and their

palaeogeographic settings—a summary

Lightfoot, mapping the Wankie coalfield in 1912

and again in 1923, was the first to suggest a detrital

origin for the Wankie Main Seam (Lightfoot, 1914,

1929). He used as main arguments the lack of

underclay for the Main Seam, its common composi-

tional alternation of bright and dull bands, the high

ash content of the coal (compared to the coals of

the UK), the exclusive clastic nature of organic

remains in the coal and also borehole results

(Watson, 1960, p. 15 ff.). Watson (1960), following

a mapping survey of the Wankie coalfield between

1950 and 1956, opposed all these arguments in

detail and instead came to the conclusion that the

coal was formed din situT, dat the shoreline of a

lakeT occupying much the same area as the Middle

Zambezi Valley of today (Fig. 2). Bond (1967)

accepted, in general, the arguments of Watson

(1960) for the in situ origin of the coal, and

pointed out the dfundamental differences between

Wankie coal and Northern Hemisphere (Europe–

USA) Upper Carboniferous coalsT (p. 185).

Fig. 1. Location of coalfields and coal-occurrences in the Mid-Zambezi basin, Zimbabwe (I=Insuza, S=Sawmills, T=Tjolotjo).

P.M. Oesterlen, J. Lepper / International Journal of Coal Geology 61 (2005) 97–118 99

Duguid (1977, 1986) changed the generally

accepted palaeogeographic picture of the basin by

introducing the dKamativi–Sijarira inlierT as a

dpalaeo-islandT during Lower Karoo time. Thus, he

subdivided the Lower Karoo basin into a northern or

dWankie intrabasinT and a southern or dLusuluintrabasinT (Fig. 3), allowing him to apply the dk2shoreline modelT not only for Wankie, Sebungwe and

Sengwa-North but also for Lubimbi, Lusulu,

Sengwa-South, and even for the k2 coal intersected

in the boreholes at Gokwe and Sawmills, Tjolotjo,

Insuza (Fig. 3). Hosking (1981), in studies of the

stratigraphy and sedimentation of the Karoo Super-

group in the Mid-Zambezi Valley, agreed with the

palaeogeographic setting of Duguid (1977), mainly

on grounds of palaeocurrent data. However, he

Table 1

Stratigraphic correlation of the Karoo System for the Mid-Zambezi basin (MZB, from Oesterlen, 1999)

Period Group Gair (1959) Bond (1967) Sutton (1979) Hosking (1981) Oesterlen (1999)

Cretaceous Late Post Gokwe White sandstone M. Gokwe Gokwe

Middle Karoo Gokwe

Early Calcareous M.

Jurassic Late

Middle Upper

Early Karoo Batoka basalt Batoka basalt Batoka basalt Batoka basalt Batoka basalt

Triassic Late Red sandstone Forest sandstone Forest sandstone Forest sandstone Forest sandstone

Sandstone and Pebbly arkose Pebbly arkose Pebbly arkose

Interbedded

mudstone

Fine red marly

sandstone

Fine red sandstone Tashinga

Ripple-marked

flagstone

Escarpment grit Escarpment grit Escarpment grit Escarpment Escarpment

Middle

Early

Permian Late Lower

Karoo

Madumabisa

mudstone

Upper

Madumabisa

mudstone

Madumabisa Madumabisa (k 5)

Middle

Madumabisa

mudstone

Early Lower

Madumabisa

mudstone

Lower

Madumabisa

mudstone

Upper Wankie

sandstone

Upper Wankie

sandstone

Wankie Upper Wankie

sandstone (k 4)

Gwembe coal Black shale

and coal

Black shale

and coal

Wankie Black shale

and coal (k 2–3)

Red mudstone

and Basal sandstone

Lower Wankie

sandstone

Lower Wankie

sandstone

Lower Wankie

sandstone (k 1)

Carbonifer. Late Basal beds Tillites and

varved shales

Tillites and

varved shales

Dwyka Dwyka (k 0)

Region Gwembe area/

Zambia

MZB, Zimbawe Gokwe area,

Zimbabwe

MZB,

Zimbabwe

MZB, Zimbabwe


changed the existing stratigraphy of Bond (1967) for

the Lower Karoo of the Mid-Zambezi Valley by

defining the Wankie Formation as constituted by the

three members Lower Wankie Sandstone, Black

Shale and Coal, and Upper Wankie Sandstone (Table

2). His argument was that these three units replace

each other laterally. The introduction of the Wankie

Formation was accepted by Oesterlen (1999) and is

used in this paper.

From the isopach pattern of the various units and

subunits of the Lower Karoo Group, the lack of any

coarse clastic marginal facies adjacent to the inlier,

the palaeocurrent directions, and the pebble size

distribution pattern Lepper (1992) concluded a

postsedimentary (post-Lower Karoo) uplift of the

central Kamativi–Sijarira horst, but did not review

the palaeogeographic setting for the k2–3 member in

detail.

3. The new rift concept of the Mid-Zambezi basin

The kinematic–thermal models of McKenzie

(1978) and others for rift basins were applied to

the intracratonic Lower Zambezi Basin (Orpen et

al., 1989; Oesterlen and Blenkinsop, 1994) and to

the Mid-Zambezi basin (Oesterlen, 1999): the non-

deformational phase of a sag basin originated by

Fig. 2. Apparent shape of the Early Karoo Mid-Zambezi basin (from Watson, 1960).

Fig. 3. The Lower Karoo Mid-Zambezi basin (from Duguid, 1993).


Table 2

Subseam development of borehole No. 1067, Wankie Concession

(from Palloks, 1987)

Depth (m) Ash (%) Volatile

matter (%)

Subseam Definition

of Subseam

From To

3.96 4.19 58.2 13.4 Roof

4.19 4.42 35.5 16.9 3 Cumulative

AshN25%4.42 5.03 30.9 16.9

5.03 5.64 22.8 18.3

5.64 6.25 25.1 18.7

6.25 6.86 23.3 18.4

6.86 7.47 16.9 20.3 2 Cumulative

Ashb20%7.47 8.08 12.9 20.9

8.08 8.69 13.9 21.7

8.69 9.30 13.3 22.0

9.30 9.91 13.6 21.4

9.91 10.52 13.3 21.1

10.52 11.13 11.7 21.0

11.13 11.74 12.9 22.1

11.74 12.32 10.0 24.3

12.32 12.93 7.4 26.5 1 Cumulative

Ash up to 15%12.93 13.24 8.6 26.3

13.24 13.85 6.2 30.3

13.85 14.46 7.0 29.2

14.46 15.07 8.5 28.4

15.07 15.68 5.4 32.3

15.58 15.98 8.9 32.5

Floor


kinematic stretching of crust and lithosphere during

the Lower Karoo (Permian; pre-rift phase), the

phase of formation of a subsiding central rift zone

along boundary faults, due to crustal breakdown,

accompanied by rising rift shoulders during the

Upper Karoo (syn-rift phase during Triassic time),

and the phase of widening of the Mid-Zambezi

basin in Jurassic time, due to thermal subsidence on

the margins of the basin (post-rift phase). Evidence

for all these processes, which resulted in the so-

called dTexas longhorn cross-sectionT of a typical

rift basin, was recognized in the Mid-Zambezi basin.

For further details on the evolution of the Mid-

Zambezi basin, see Oesterlen (1999).

For the Lower Karoo time, the new model contra-

dicts substantially the palaeogeographic settings

described by Duguid (1977, 1986) and Hosking

(1981) mentioned above, as it describes a large

shallow sag basin which was not subdivided by an

intrabasinal high. The Kamativi–Sijarira inliers were

uplifted in conjunction with the rifting and subsidence

of the graben which happened only in Upper Karoo

time, from Lower Triassic onwards. This model is

supported by a number of arguments:

(1) The inliers consisting of Precambrian rocks of

various ages still carry a number of Lower Karoo

erosional remnants, being witnesses of the

original Lower Karoo roof sediments overlying

the Precambrian base (Chappell, 1969; Hum-

phreys, 1969).

(2) Isopach trends for the k2 Main Seam from

Wankie to Sengwa coalfields and also the ash

content isolines, both calculated by Palloks

(1984), do not reflect the boundary faults of

the inliers.

(3) The coal seams at Sengwa, Lusulu, and Wankie

Concession show a distinct increase in thickness

towards the uplifted blocks (Lepper, 1992, p.

23–24).

The new model of a large shallow Lower Karoo

basin calls urgently for a new palaeogeographic and

depositional picture, in particular, for the k2 coal–

mudstone member of the Wankie Formation, as the

palaeo-shoreline model of Duguid (1986) is no longer

suitable. This new picture has also to consider the k2

coal findings in the boreholes of Tjolotjo, Sawmills,

and Insuza (Thompson, 1977; Harrison, 1978). The

answer to all these questions comes only from the k2–

3 coal–mudstone sequence and from the sediments

below and above the Wankie Formation.

4. Lithology and coal quality related to the

environment of deposition of the Black Shale and

Coal member (k2–3), Wankie formation

All the available literature and selected borehole

records of the k2–3 coalfields from the Mid-Zambezi

basin were studied concerning lithology, lithofacies,

thickness, and coal quality and were tentatively

interpreted with regard to the environment of depo-

sition. The most substantial document for this paper

was the borehole correlation of the main coalfields

and coal occurrences (Wankie Concession with

Entuba and Western Areas, Lubimbi, Lusulu, Lubu,

Busi, Sengwa-South and -North, and Gokwe) done by

Lepper (1985) who had selected and reinterpreted 480


key boreholes out of approximately 5000 drilling

records.

The coalfields of Lusulu and Wankie Concession

contain medium volatile bituminous coal, whereas the

rank of all the other fields ranges from subbituminous

to high-volatile bituminous coal (Lepper, 1992).

4.1. Wankie coalfields (Wankie Concession, Western

Areas, and Entuba)

More than 3000 boreholes have been drilled

altogether in the fields since the beginning of mining

activities in 1903; thus, the database for sedimento-

logical interpretation is excellent. The coalfields

consist of the Wankie Concession in the centre,

Entuba to the east, and the Western Areas to the

west, all located south of Hwange town and stretching

over 40 km in WSW–ENE direction (Fig. 4). The

boreholes of the coalfields revealed striking litholog-

ical differences, as Duguid has already pointed out

Fig. 4. The coal localities of Wankie

(Duguid, 1986, 1993, and Fig. 5). The Wankie

Concession k2–3 sequence typically consists of the

Main Seam at the base, up to 14-m thick, which is

overlain by a ca. 20-m carbonaceous mudstone

succession, in places intersected in the upper part by

a thin coal seam, Seam No. 1, and a 6-m-thick fireclay

horizon (see hole W 1539, Fig. 5). This pelite–coal

lithology changes in the Western Areas gradationally

replacing the coal by clastic intercalations in the Main

Seam and in the hanging mudstones and fireclay, until

finally the Main Seam tapers out, having only

siltstone and fireclay resting on the k1 sandstone

(borehole M 92, Fig. 5). Towards the east, the dblackshale and coalT lithology is replaced increasingly by

carbonaceous mudstone (borehole SE 33, Fig. 5). This

general trend from sand–silt facies in the far WSW-

part via a peatswamp–mud facies in the centre

towards a pure mud facies in the far ENE was

recognized by Duguid (1986) and interpreted as a lake

shoreline peatswamp environment, with its up-dip

coalfields (from Palloks, 1987).

Fig. 5. The k2-3 lithofacies changes at Wankie coalfields including lithological explanation (borehole data from Lepper, 1985).


margin on one side representing the shore of the Mid-

Zambezi lake, and its down-dip lacustrine facies in the

other direction.

Watson (1960) described for the Main Seam of

Wankie Concession the increase of ash contents

upwards, from 5–7% at the base to 30% at the top

of the Main Seam. The same picture was given by

Thompson et al. (1982b) for the Entuba Coalfield and

Palloks (1984) for the Western Areas. However,

Palloks (1987) found out, by vertical quality varia-

tions in ash content and volatile matter, that appa-

rently the Main Seam of Wankie Concession consists

of three subseams: subseam No. 1 revealing a

cumulative ash yield of maximum 15%, subseam

No. 2b20%, and subseam No. 3N25% (see Table 2).

The roof of subseam No. 3 is again marked by a

sudden increase in ash content (N30%). Palloks (1987,

p. 10) explained these djumpsT by either a sudden

subsidence of the floor or rise of the water level.

Another characteristic found often, and not only at

Wankie coalfields, is the striking high ash content of

about 20% in the first 50 cm at the footwall contact of

the Main Seam, before it drops abruptly to b10%

(Palloks, 1987, Table 5). This is considered as

evidence of the previous fluviodeltaic clastic deposi-

tional environment of k1, before this was replaced by

a shoreline peatswamp–mud environment of k2–3.

Duguid (1993) pointed out that the Main Seam in

the Wankie district becomes progressively higher in

ash and lower in basal coking coal (Duguid, 1993,

Table 1) towards the edges of distribution, the up-dip

margin at the Western Areas, and the down-dip edge

at Entuba. This is also in full agreement with the

general lithofacies trend of the k2–3 sequence


described above, where the coal was replaced by

clastic or pelitic sediments. These environments

brought along a large amount of inorganic material

deposited in the peatswamp, forming mudstone

partings and resulting in higher ash contents of the

raw coal.

4.2. Lubimbi coalfields

The Lubimbi coalfields are located about 100 km

east of Hwange town and consist of the four

individual fields Dahlia, Hankano, Lubimbi, and

Lubimbi-East (Fig. 6), arranged from SW to NE.

They altogether cover an area at least 40-km long and

5-km wide at Dahlia and 25-km wide at Lubimbi-

East. More than 300 boreholes were drilled up to

1975, which is the database available for the authors.

The main publications are from Taupitz (1976) and

Thompson (1981).

The lithology of the k2–3 succession at Lubimbi

coalfields is rather different from that of Wankie

coalfields. The 40- to 50-m-thick succession consist-

ing, in general, of bright and dull coal, carbonaceous

mudstone, mudstone, and the dGrey Shale MarkerT is

Fig. 6. The coal localities o

traditionally subdivided into the six coal-bearing

horizons B, C, D, E, F, and G, and the sequence

was called dBira Coal MeasuresT (Thompson, 1981).

The A-coal-horizon occurs interbedded in the k1

sequence. The Grey Shale Marker, on average is 4-m

thick and is petrographically the same as the Fireclay

of Wankie, thus representing its eastern extension,

ubiquitously occurs between the E- and F-horizon.

Comparing the Lubimbi lithology with Wankie the

Main Seam of Wankie can be correlated with the B-

and C-coal horizons from Lubimbi, and the A- or No.

1 Seam with the E–F-horizons.

Fig. 7 displays the lithology, coal horizons and

thickness of one typical borehole from each of the

four coalfields arranged from SW to NE. The basal B-

horizon consists of dull coal with a higher ash content,

rarely bearing bright coal band. The overlying C-

horizon, the main dpay-zoneT, is predominantly a low-

ash bright-banded coal alternating with dull coal and

carbonaceous mudstone layers. The D-horizon is

composed not of coal but of stratified carbonaceous

mudstone with bright coal bands, and an ash content

of N50%. The E-horizon is an alternation of bright

coal with bituminous mudstone. Above the Grey

f Lubimbi coalfields.

Fig. 7. The k2-3 lithofacies changes at Lubimbi coalfields (data from Lepper, 1985, modified). For explanation, see Fig. 5.


Shale marker occur the F- and G-horizons; the lower,

a bright coal with mudstone partings and the upper,

again a carbonaceous mudstone.

The typical features of the Lubimbi coal, in

contrast to the Wankie coal, are the alternation of

bright coal sequences with dull coal horizons, the

alternation of coal with bituminous mudstone layers,

the common stratification of the coal (dbanded coalT)with bands displaying a lenticular shape on a larger

scale (Thompson, 1981), the relatively high amount of

silty mineral matter of the coal, and the considerable

increase in thickness of the k2–3 member, from 32 m

at Dahlia to 53.5 m at Lubimbi-East (Fig. 7). The

higher thickness accounts for Lubimbi and Lubimbi-

East, but not for the Dahlia sequence, which compares

better with the Wankie Concession lithology and

thickness. The Hankano lithology, however, appears

to be a transition between the Dahlia type in the SW

and the Lubimbi types in the NE.

All the lithological criteria cited above do not point

to a deltaic environment, as suggested by Thompson

(1981), but to a lacustrine shoreline–swamp environ-

ment of deposition. The thick alternation of coal with

mudstones (D- to G-horizons) is best explained by a

long period of water-level fluctuation, leading to a

temporary drowning of the swamp and deposition of

sapropelic mud. The greater thickness of this sequence

towards the northeast might indicate the growing

influence of a delta system nearby delivering higher

mineral input into the adjacent swamp in times of

flooding.

However, the thickness decrease of the coal layers

of horizons C and D to the north, where they are

replaced by carbonaceous mudstone, looks more like

the change from shoreline swamp into sapropelic

lacustrine sediment. This depositional environment is

also found in the northern part of Dahlia coalfield

which is composed completely of carbonaceous

mudstone (boreholes 504 and 509, Lepper, 1985).

Another interesting hint to the palaeogeographic

setting is the fact that the Lubimbi coalfields did not

give any evidence for shore-face sediments in the

southern part (Dahlia-South), such as in the Wankie

coalfields. Instead, carbonaceous mudstone was found

which could have come from a sapropelic lake

environment or an alluvial plain environment. For

palaeogeographic reasons, the latter suggestion

appears more probable.

The coal quality data for Lubimbi are scarce, but

Thompson (1981, p. 60) compiled some figures for all

horizons of each coalfield (Table 3). In general, it can

be stated (1) that the ash content increases upwards

Table 3

Averages of raw coal ash-content for the k2–3 coal horizons,

Lubimbi coalfields (from Thompson, 1981)

Horizon of

coalfield

Bright banded

coal fraction

(Ash %)

Dull coal/

carbonaceous

mudstone

fraction (Ash %)

Lubimbi

F or F/G 37.9 62.9

E or E/F 30.7 65.1

D or D/E 32.0 61.2

C or C/D 20.4 48.4

B or B/D 23.6 36.4

Lubimbi East

F or F/G 36.6 69.5

E or E/F 34.0 65.9

D or D/E 29.0 61.5

C or C/D 19.0 40.8

B or B/C 15.0 24.9

Hankano

F or F/G 24.9 73.2

E or E/F 30.9 66.3

D or D/E 28.9 61.4

C or C/D 27.3 55.7

B or B/C (one borehole) 26.5 56.2

Dahlia

F or F/G (two boreholes) 37.8 71.4

E or E/F 33.2 63.0

D or D/E 33.3 60.3

C or C/D 28.9 55.3

B or B/D 26.9 43.3


throughout the horizons of k2–3, as in Wankie

coalfields and (2) that the amount of mineral matter

or ash content for the Lubimbi coal is higher than for

Wankie—which again is in line with the hypothesis of

a delta system not far to the northeast of the Lubimbi

coalfields.

4.3. Lusulu coalfield

The Lusulu coalfield is situated ca. 90 km north-

east of Lubimbi and ca. 50 km southwest of Sengwa-

South. It stretches 45 km in a NE–SW direction and

has an average width of 5 km (Fig. 1). The main

source of data is Palloks (1984) who interpreted 185

boreholes and other results from exploration by Shell

Developments Zimbabwe and others during 1975–

1976 and 1980–1982 (Shell Developments Zimbabwe

(Pvt.) Ltd., 1983). Further information was obtained

from the research borehole MT 1, which is 368-m

deep and located about 5 km south of the southwest-

ern corner of Lusulu coalfield (Falcon, 1973).

Taupitz (1976) recognized the lithological similarity

of the so-called Lusulu dLower coal shaleT to the

Lubimbi k2–3 lithology. Fig. 8 displays four borehole

records, where the two logs of boreholes 201 and 250

represent the typical lithology of central Lusulu. The

sequence starts with the two main coal horizons, the

Main Seam, up to 10-m thick, and the A-Seam, 2.5- to

4.5-m thick and up to 3.5 m above the Main Seam. The

younger coal seams B, C, D, and E, of lesser quality,

occur higher up in the succession, interbedded with

carbonaceous mudstone. The Grey Shale Marker

horizon separates the D- and E-Seams (Fig. 8). In

borehole MT 1, the coal seams are widely replaced by

carbonaceous mudstone, and in borehole 256, the total

thickness is greater due to a thick pile of carbonaceous

mudstone overlying the E-Seam. The k2–3 sequence at

Lusulu is 40- to 60-m thick.

Palloks (1984) demonstrated that the thickness of

theMain Seam decreased from 10.0 m in the N to 4.0 m

in the SE and SW, the coal being replaced by

carbonaceous mudstone, as in borehole MT 1 (Fig.

8). The same trend was also found for the A-Seam. The

typical lithology of the dLower coal shaleT, i.e., thealternation of lenticular coal seams with carbonaceous

mudstone indicates, as with the Lubimbi coalfields, a

shoreline swamp environment which, towards the

south, grades rapidly into the carbonaceous floodplain

sediments of the adjacent alluvial plain (borehole MT

1). Although lithologically the same, the carbonaceous

mudstone of the uppermost sequence towards the NE

(C-section from Lepper, 1985, and borehole 256, Fig.

8) has probably an origin from a nearby delta system.

The northwestern continuation of the Lusulu coalfield

might be seen in the Lubu coalfield (see below).

The best quality coal (ash content b20%) occurs in

the lower portion of theMain Seam, towards the top the

ash content increases (Palloks, 1984). All the coal

seams above the Main Seam are of inferior quality.

Laterally, the quality of theMain Seam becomes poorer

towards the south, in agreement with the above

explanation that the shoreline sediments grade into

sediments of an alluvial plain. The Lusulu coal is

classified as a high-ash coal on average (Palloks, 1984,

p. 26).

Fig. 8. The k2-3 lithofacies changes at Lusulu coalfield (data from Lepper, 1985). For explanation, see Fig. 5.


4.4. Lubu coalfield

Lubu coalfield is located about 35 km in the

northwestern continuation of Lusulu (Fig. 1). It has an

extent of only 15 km in a SW–NE direction and a

maximum 8 km width, as it occurs on a down-faulted

block within the Sijarira Inlier. Palloks (1984)

compiled the results of the coal exploration done by

Messina–Transval Development (MTD) in 1982,

which included the drilling of 13 boreholes.

The k2–3 lithology is similar to that of central

Lusulu, a basal Main Seam averaging 12.5 m thick-

ness (maximum of 18 m), overlain by mudstone and

carbonaceous mudstone with two smaller intercalated

coal seams (A-seam with 2.5 m, B-seam with 2.0 m

thickness average, see Fig. 9). There is no fireclay or

Grey Shale marker horizon intersected in the

sequence.

The Lubu k2–3 sequence represents the north-

western down-dip extension of the Lusulu shoreline

sediments. The lower portion, including the Main

Seam, A-, and B-Seam, is the expression of a

persistent shoreline swamp environment which was

replaced twice by thick nonorganic mud intervals,

representing the lateral input of a nearby delta

system (Fig. 9). This explanation is supported by

the high ash content of the Main Seam (see below)

and the considerable thickening of the Main Seam

towards the SE (Palloks, 1984: from 6 m in the

NW to 18 m in the SE), and the unusual thickness

of the total succession (Fig. 9). However, the very

thick sapropelic mudstones, with few sandstone

tongues, of the upper portion, which was found

also in most of the other boreholes, most probably

are of lacustrine origin resulting from a trans-

gression of the lake, like at Wankie Concession.

Borehole 13, ca. 10 km to the W of borehole 6, has

much more mudstone in the succession than the

other boreholes, at the expense of coal, probably

indicating the lateral influence of another fluvial

delta towards the west.

The best quality coal is not found at the base of the

Main Seam as typical, but near its centre; the bottom

and upper portions being of inferior quality, as well as

the A- and B-coal seams (Palloks, 1984, p. 19). On

average, the Main Seam is high in ash content (27.3%

for raw coal).

Thompson (1980) described a 1.2-m thick coal

seam at Sebungu, located about 25 km west of Lubu

(see Fig. 1), but no sampling or drilling results were

reported. The lack of information does not allow a

sedimentological classification of the coal, but a

sapropelic origin is suggested.

4.5. Busi coalfield

Busi is situated between the Lusulu, about 40 km

towards the SW, and the Sengwa coalfields, ca. 20 km

Fig. 9. Some k2-3 lithofacies records of Lubu and Busi coalfields (from Lepper, 1985). For explanation, see Fig. 5.


towards the NE (Fig. 1). The data is very limited

(Lepper, 1985) and consists only of six borehole-logs

stretching over 18 km in a SW–NE direction, only

four of them having intersected the k2–3 sequence of

the Wankie Formation.

Nevertheless, the four litho-logs display two

distinct different sedimentological environments.

The two boreholes TB 1 and TB 3, with a pure

sand-facies, sometimes with pebble bands and in

places with a few coal bands in the k2–3 sequence

(see borehole TB 1, Fig. 9) represent a delta

environment of a river system, here at least 40-m

thick. The delta obviously follows the SE–NW

depositional trend, as expected, but is not wide

enough to affect the Lusulu coalfield. The other two

boreholes (TB 4 and TB 5), which consist of an

alternation of carbonaceous mudstone with coal

bands, in places with a Main Seam at the base (see

borehole TB 5, Fig. 9), represent a shoreline

environment affected by frequent flooding of the

adjacent delta system, leading to temporary swamps

before they were terminated by suspended sediment

load. This facies resembles the one found in the

Lubimbi coalfields.

Unfortunately, no coal quality data are available.

4.6. Sengwa coalfields

The Sengwa coalfields consist of Sengwa-South

and Sengwa-North, both situated along the northeast-

ern end of the Sijarira Inlier, the former on the

southeastern side, the latter on the northern side and

15 km NNW of Sengwa-South (Fig. 1). The main

source of information is again Palloks (1984), Lepper

(1985), and Oesterlen (1999). From 1973 to 1980, Rio

Tinto Rhodesia carried out coal exploration in this

region, of which 13 boreholes of each area were

recorded by Palloks (1984) and Lepper (1985). In

1994, Rio Tinto Zimbabwe drilled another 178

boreholes in Sengwa-South, but only very limited

data were available for the authors (Falcon Research

Laboratory (Pty.) Ltd., 1995). The area drilled at

Sengwa-North stretches ca. 15 km in the SW–NE

direction and 4 km in the perpendicular direction.

Sengwa-South area extends 7 km in the N–S direction

and 2 km in the E–W direction (Fig. 1).

The lithologies of both coalfields are nearly

identical. The k2–3 sequence starts with the Main

Coal Seam (MCL), the only economic seam of the

area, at the base, overlain by the Lower Carbonaceous

Shale (LCS), the Upper Coal Seam (UCL), and then


the Upper Carbonaceous Shale (UCS). Both the LCS

and UCS are in reality carbonaceous mudstones

intercalated with thin coal bands. The fireclay is

developed in Sengwa-North, but not in Sengwa-

South; the UCL is often missing in Sengwa-North

(see Fig. 10).

The Main Coal Seam of Sengwa-North averages

14.1 m thickness (17 m maximum), and 12.5 m

thickness (maximum 15 m) in Sengwa-South. As in

the other coalfields, the lithology of the entire

sequence and the high thickness of the Main Seam

suggest a shoreline swamp environment. Subse-

quently, the swamp vegetation was periodically

terminated by the deposition of suspension load.

The interpretation of this portion could consider, as

described already for Lubimbi and Busi coalfields,

either an episodic flooding of the lake or a rhythmic

influx of suspension material from an adjacent delta

system. The overall configuration suggests the two

coalfields were originally part of one ca. 25-km wide

SW–NE-trending shoreline peatswamp, with its

down-dip side passing into the lake to the NW

(Sengwa-North) and the up-dip side into the alluvial

floodplain towards the SE (Sengwa-South, see also

borehole 4, Fig. 10). The central part was eroded by

the uplift of the Sijarira Inlier. This concept is

supported by the shaling-out of the Main Seam in

Fig. 10. The k2-3 lithofacies changes at Sengwa-North and Sengwa-So

Sengwa-North towards the east, and in Sengwa-South,

towards the south (Palloks, 1984, p. 35). Another

argument is the synchronous Bari coal occurrence, ca.

30 km to the east (see Fig. 1), which displayed only

four thin coal bands within a 50-m-thick carbonaceous

mudstone succession of k2–3, typical for an alluvial

plain environment. The unusually great thickness of

the k2–3 sequence at Sengwa, i.e. 65–80 m at

Sengwa-South and 40–100 m at Sengwa-North, is

probably explained by the particular palaeogeographic

position of Sengwa in a large bay of the eastern

margin of the lake, where the hinterland provided

large amounts of detrital load (see Fig. 12).

This concept is also supported by the high ash

contents of coals in both coalfields, about 22.5% on

average for both. The best coal quality is found in the

lower portion of the Main Seam (b10% ash), but from

2 m above the footwall, it deteriorates towards the top

of the seam (N20% ash, Palloks, 1984, p. 34 ff.)—

results which were already reported from the other

coalfields.

4.7. The Gokwe coal occurrences

Sessami and Kaongo are the two subsurface coal

occurrences of the Gokwe area, 14 km apart in a

northeasterly direction, and both located ca. 60 km

uth coalfields (data from Lepper, 1985). Explanation see Fig. 5.


southeast of Sengwa-South and 30 km north of

Gokwe town (Fig. 1). They were investigated by

reconnaissance drilling first in 1951–1952 and later in

1971–1974 by Rio Tinto Rhodesia (Boehmke and

Duncan, 1974). The results were compiled by Sutton

(1979), and 13 borehole logs, covering an area of 70

km in an W–E direction and 30 km in a N–S direction,

were correlated by Lepper (1985).

The k2–3 sequence intersected in depths of 200–

300 m below surface looks very different from the

ones described above. It is composed of various

lithologies changing rapidly in a lateral direction (see

Fig. 11). Sometimes the Main Seam occurs at the

base, with a maximum of 9 m thickness, overlain by

siltstone (e.g. borehole G 7 and G 10, Fig. 11);

sometimes the sequence is represented only by

carbonaceous mudstone (borehole SY 1) or siltstone

or sandstone (boreholes G 12, G 4), or is completely

missing (boreholes G 11; see Sutton, 1979). Its total

thickness is reduced to 15 m average (maximum 40

m). The coal quality is not good, with about 20% ash

content at the base of the Main Seam and N30% in the

upper portion (Sutton, 1979).

The rapid lateral change of lithology, the higher

proportion of silt- and sandstone in the sequence and

the higher ash content in the coal all indicate an

alluvial plain environment for the coal, with the entire

sequence composed of river systems and adjacent

flood plains. The thickness increase of the succession

Fig. 11. The k2-3 lithofacies changes at Gokwe coal occurre

towards the NW (Lepper, 1985, Section B) and a

stable thickness of ca. 15 m in E–W direction (see Fig.

11) point to drainage from a source area in the SE

towards the lake in the NW (Fig. 12).

4.8. Boreholes of Tjolotjo, Sawmills, and Insuza

Further information on the coal-bearing k2–3

succession comes from the three deep research bore-

holes at Tjolotjo, Sawmills, and Insuza; all located in

the Nyamandlovu district and arranged on a 50-km-

long NE–SW-trending line in an area situated ca. 200

km southeast of Wankie (Fig. 1). The boreholes were

correlated and interpreted by Harrison (1978).

The litho-logs of the k2–3 sequence intersected at

about 300 m depth in all three holes are similar in all

boreholes: one or several thin coal seams are

interbedded in an alternation of carbonaceous mud-

stone, the dominant lithology, with siltstone or, rarely,

sandstone. A general coarsening-up of grain size is

recognizable, as well as a progressive decrease of

organic material in the mudstone. The lithofacies and

the small thickness of 26–30 m resemble much the

Gokwe logs; thus, the sediments are also considered

typical sediments of a flood plain on an alluvial plain.

This environmental classification is supported by

the very high ash content of N30% for the coal seam at

the base (Thompson, 1977) and the increasing thick-

ness for the Lower Karoo succession towards the NW.

nces (data from Lepper, 1985). Explanation see Fig. 5.

Fig. 12. The k2-3 coal-belt and adjacent sedimentological boundaries in the Mid-Zambezi Lower Karoo basin, on the geological background (I=

Insuza, S=Sawmills, T=Tjolotjo).


The same principal lithostratigraphic subdivision of

the Wankie Formation also provides evidence, that the

area of the boreholes is still a part of the same Lower

Karoo basin of the Mid-Zambezi terrain.

5. Information from maceral analysis of the coal

The petrography of the k2–3 coal is not well-

studied yet; the data is scarce and in parts incomplete,

with no information at all from Busi and Lubu

coalfields, Gokwe occurrence, and the deep boreholes

of Nyamandlovu area. However, it appears that there

exists one dominant maceral model for the coalfields

of Wankie, Lusulu, and Sengwa, while Lubimbi

coalfields present a different one.

Watson (1960) reported on two maceral analyses

from the Main Seam coal of Wankie Concession No. 2

colliery, without giving any details of sampling. The

one sample is a bi-maceral coal (54.3% vitrinite (V),

44.3% inertinite (I), 0.2% liptinite (L) and 1.2% other

components), presumably sampled from the lower

portion of the Main Seam; the other a typical

Gondwana coal (61.7% I, 34.6% V, 0.5% L and

3.2% other components) which comes certainly from

the upper Main Seam. Thompson et al. (1982b)

Table 4

Maceral analyses from boreholes No. 101 and 107, Lubimbi

coalfields (from Thompson, 1981)

Bright banded

coal (vol.%)

Dull coal and carbonaceous

mudstone (vol.%)

Vitrinite 45–59 13–23

Liptinite 8–11 5–7

Inertinite 23–31 41–61

Minerals 10–13 19–31

All data refer to a float at S.G. of 1.9


produced some data from Entuba coalfield, from

washed coal with specific gravity of 1.4: the Main

Seam lowermost 1.3 m coal, classified as coking coal,

is composed of 47.3% V, 27.0% I and 2.3% L (and

23.4% minerals, as suggested by the authors), whereas

the overlying 1.4 m of coal classified as blend coking

coal revealed only 29.7% V, 18.0% I and 1.3% L (the

remaining 51.0% not reported). Similar maceral

values were given for the Main Seam 2.3 m bottom

coal (coking coal) of the Western Area coalfield

(40.6% V, 20.2% I, 2.7% L (the remaining 36.5% not

reported), Thompson et al., 1982a). No maceral

analyses were given for the main upper portion of

the Main Seam, classified as steam coal, but it is noted

that the bright coal changes to a dull coal accom-

panied by an abrupt drop of the vitrinite content to 0%

(Thompson et al., 1982a, Fig. 2). One result is known

for the No. 1 Seam above the Main Seam, which again

is a vitrinite-rich coal (71.1% V, 7.2% I, and 4.4% L

(the remaining 17.3% not reported)).

Similar results are available from Lusulu coalfield

(Shell Developments Zimbabwe (Pvt.) Ltd., 1983).

The base of the Main Seam is a vitrinite-rich coal

(64% V, 28% I, 6% L, and 2% minerals), but the most

common coal type is a Gondwana-type inertinite-rich

coal (ca. 50% I, 40% V, and 10% L) grading into the

less-common, strongly inertinitic coal (73% I, 12% V,

10% L, and 5% minerals). Falcon Research Labo-

ratory (Pty.) Ltd. (1995) reported the same results

from Sengwa-South coalfield based on the analysis of

three boreholes. The Main Seam coal was determined

to be dominantly an inertinite-rich coal (51–96% I,

1.5–35% V, and 1.3–13% L), a typical Gondwana

coal. The vitrinite-rich coal (50–60% V, 30–45% I,

and 0.5–12% L) was subordinate occurring only at the

base of the Main Seam and in the Upper Seam.

The maceral analyses of the Lubimbi coalfields

were carried out on seven boreholes (Thompson,

1981). The main two coal lithologies within the k2–3

sequence, i.e. the bright banded coal and the dull coal

differ in the maceral composition and the ash or

mineral content (see Tables 3 and 4). The bright-

banded coal is mainly a vitrinite-rich coal, while the

dull coal and carbonaceous mudstone coal are

dominantly inertinite-rich, with a higher mineral

content than the vitrinite-rich coal (Table 4).

The maceral profile of the entire k2–3 succession

at Lubimbi does not fit to the above standard maceral

model. The dull coals of the B-horizon are inertinite-

rich coals (N70% I) which are overlain by a vitrinite-

rich coal of the C-horizon in the south (51% V, 33% I,

11% L, and 5% minerals, see Table 5). The banded

coal of the D-horizon revealed similar results, while

its carbonaceous mudstone coals are typical inertinite-

rich coals (Gondwana coal). The banded coals of the

E/F-horizons, equivalent to the No. 1 Seam of

Wankie, are vitrinite-rich (64% V, 16% I, 7% L, and

13% minerals), with mudstone intercalations and

minor inertinite-rich coal bands.

The maceral data of the individual coalfields

described above are the result of depositional pro-

cesses. The swamp formation started as a paludifica-

tion process; that is, the fluviodeltaic sandstones of

the k1 sequence were replaced by a swamp, due to a

rising groundwater-table. The above-described stand-

ard maceral model for the Wankie, Lusulu, and

Sengwa coal was, for the lower ca. 2 m of the Main

Seam, characterized by a wet forest swamp with a

Glossopteris–Gangamopteris flora, having a high

water-table (vitrinite-dominated coal). The first stage

of coal formation changed gradationally by the fall of

the water level, leading to oxidation and decomposi-

tion of the plants in the swamp (dry forest swamp).

This second stage was more stable than the first and

produced the inertinite-dominated coal of the major

part of the Main Seam. The second stage was

followed at Wankie by the deposition of carbonaceous

mudstone, when the shoreline swamp was drowned,

either by higher subsidence of the ground or a rise of

the lake water level (transgression of the shoreline

towards the south). However, the vitrinite-rich coal of

the No. 1 Seam at Wankie and equivalent coal

horizons in Lusulu and Sengwa demonstrate that the

first stage of a dwet forest swampT could be

reestablished by a transient regression of the lake

Table 5

Maceral analyses of the coal horizons from Lubimbi coalfields

(from Thompson, 1981)

Banded coals Dull

coals

Carbonaceous

mudstones

Horizon E/F D C B E/F D

South North

Vitrinite

(vol.%)

64 51 51 37 6 35 14

Liptinite

(vol.%)

7 10 11 9 10 6 7

Inertinite

(vol.%)

16 27 33 44 71(?) 33(?) 53(?)

Minerals

(vol.%)

13 12 5 10 13 26 26

Ash

(wt.%)

23.7 25.8 14.9 22.4 28.7 42.6 41.5

All data refer to a float at S.G. of 1.9(?) Data suggested by the authors.


shoreline. This facies regression is also indicated by

the alternation of mudstone with coal bands, typical

for the upper portion of k2–3 at Sengwa, or further

coal seams in the upper sequence at Lusulu. Finally,

the deposition of organic and/or suspension material

was abruptly replaced by the fluviodeltaic environ-

ment of the k4 sandstones (regression again).

The maceral profile of the Lubimbi coal (see above)

differs from the standard model mainly in the bottom

part of the k2–3 sequence, which is an inertinite-rich

coal. The latter grades into vitrinite-coal of the C-

horizon, both horizons together being the equivalent of

the Main Seam of Wankie. The maceral composition

of the overlying carbonaceous mudstones and mud-

stone–coal band alternation of the horizons D to G is

similar to the maceral development of the standard

model including the vitrinite-rich coal seams in the

upper k2–3 sequence. Most probably, the dull coal at

the base is due to a local, temporary topographic high,

where the swamp started to develop as a dry forest

swamp. Irregularities of this type were already

reported by Palloks (1987) from the Wankie Con-

cession, where the lack of bright coal at the base of the

Main Seam, i.e., the subseam No. 1, were explained by

palaeotopographic conditions (Palloks, 1987).

With regard to the vitrinertite, the most dominant

coal microlithotype of k2–3 in this report, and a

characteristic microlithotype in Gondwana coals as

well, Styan and Bustin (1983) came to the conclusion

that it has formed from herbaceous peats after

flooding with oxygenated water and subsequent

desiccation. The flooding was deduced from the

higher liptinite content of the inertinite-rich coal.

Falcon Research Laboratories (Pty.) Ltd. (1995)

stated, in its report on the Sengwa-South coal, that

the liptinite macerals were dtypically concentrated in

layers which are sometimes associated with high

mineral contents, sometimes with detrital inertiniteT.This explanation corresponds well with the conclu-

sions drawn by the authors for the sedimentological

environments of the individual coalfields, and the

increase in mineral content of the inertinite-rich coals.

6. Discussion

6.1. Peatswamp formation and controlling

parameters

It is well known that swamps form only under

certain special conditions, i.e., a subsiding floor, a

stable water level, and a lack of clastic contamination.

During the time span of the upper k2–3 sequence, the

main coalfields presented a number of different

lithologies, more or less synchronously: sapropelic

lacustrine mudstone was deposited in Wankie Con-

cession, while in Entuba, the vitrinite coal of the No. 1

Seam formed. In Lubimbi, Lusulu and Sengwa-South

as well, the dull coal of mainly inertinite macerals was

dominant. It means that at least three different

depositional sub-environments were present along

the shoreline of the k2–3 Mid-Zambezi lake, at about

the same time, certainly due to local controls on

deposition. In Entuba, the favourable conditions for a

water-covered shoreline–swamp were established

again, whereas in Wankie Concession, the swamp

was drowned by a high water level or by suspension

load from a sedimentary source nearby; but for

Lubimbi, Lusulu, and Sengwa-South, the oxygen-rich

dry forest–swamp was still active.

Local controls were also the reason for the

unusual maceral development at Lubimbi and

Wankie Concession, where inertinite coal was found

either all over or in parts at the base of the Main

Seam or equivalent. These irregularities were most

probably caused by a temporary low water level for

these areas—which was already suggested by Palloks


(1987). On the other side, the general trend of the

maceral development for all the coalfields, i.e., the

vertical change from a low-ash vitrinite coal at the

base of the sequence to the high-ash inertinite coal

(Gondwana coal) in the upper portion is referred to a

regional control. A substantial controlling factor was

slow post-ice-age climatic warming, with meltwater

causing an eustatic rise of the water level from k 1

onwards, a process which slowly came to a standstill

at the end of k2–3. Another regional factor was the

annual rhythm of precipitation, characterized by high

precipitation in the rainy season followed by a cold

dry season, which led to periodic or episodic

flooding and then drying up of lake waters. Crustal

rebound following melting of ice in the highlands to

the southeast, which, from k1 onwards, became the

sedimentary source area for the Lower Karoo basin

of the Mid-Zambezi, may have been another

parameter. This factor could have played a role in

the onlap of the k4 fluviodeltaic clastic sediments

over the k2–3 shoreline coals and mudstones. With

the beginning of k5, the isostatic control became the

dominant factor—the crust of the basin began to

subside and formed a sag-basin, until it broke in the

central zone and generated the Zambezi-graben,

approximately at the Permian/Triassic boundary.

6.2. Diachronous coal

Duguid (1986) was the first to consider the

diachronous nature of the k2–3 coal. The lithological

facies changes in vertical and lateral direction are the

result of changes in the sedimentary environment, as

described above, and led of course to a lateral

migration of the shoreline peatswamp. The dwetforestT zone with its vitrain, clarain, and duroclarain

lithotypes moved up-dip, when the water level rose,

and down-dip into the lake, when the water level

dropped. The migration affected also the maceral

types of the peat, the vitrain-dominated peat replaced

by the durain-dominated peat, and vice versa.

Evidently, the general topographic relief of the area

was very low; this migration of the peatswamp on the

shoreline of the lake led eventually to an unusually

broad zone for a coal-belt. Based on the drilling data

available, the Wankie coal-belt attains a width of at

least 30 km, Lubimbi belt at least 20 km, Lusulu and

Lubu coalfields about 40 km, and Sengwa (North–

South) about 25 km. In the lateral direction, the coal-

belt having a thickness of maximum 18 m at Lubu,

stretches from Wankie coalfields in the west in a lobe

over more than 300 km towards Sengwa coalfields in

the northeast, reflecting the palaeo-shoreline of the

k2–3 lake (Fig. 12). However, it is interbedded with

several wide clastic zones of delta systems, as seen in

the Busi coalfield, for example.

The width of the coal-belt is, in principle, defined

by the coal seams which grade laterally into a

sapropelic mudstone of lacustrine origin on the

down-dip side, and bituminous mudstone intercalated

with clastic sediments of an alluvial floodplain on the

up-dip side. In the light of these criteria, it can be seen

that in particular the Lubimbi coalfields still have an

interesting potential for more coal towards the up-dip

side in the southeast.

6.3. Autochthonous or allochthonous origin of the

coal

This discussion was opened, after Lightfoot (1914,

1929) had decided for the drift coal theory, whereas

Watson (1960) disagreed and opted clearly for the in-

situ origin of the coal (see Section 2). Since then, coal

petrography in general has developed considerably,

especially with respect to the individual maceral types

of the coal and their origin. The main arguments of

Lightfoot (1929) for a detrital coal (composition, high

ash content and fragmentary nature of the coal) lost

their value, as coal petrography has investigated and

understood the substantial differences between the

Carboniferous coals in Europe and North America and

the Permian coals of Gondwana. The most common

coal type of the k2–3 sequence, the inertinite coal

consists mainly of decomposed and detrital macerals,

e.g., inertodetrinite, which, however, were generated

by aerobic or subaerobic decay during the process of

oxidation of the swamp vegetation in situ and were

not necessarily of detrital origin.

Of course, small reworking of plant remains or of

peat took place repeatedly within the peatswamps of

k2–3 during times of flooding, and led to the term of

dhypautochthonous coalT. On the other hand, allochth-

onous coals are in general much richer in mineral

content. Therefore, the authors consider the shoreline

coal of Wankie and the other coalfields to be a

hypautochthonous coal.


7. Conclusions—the new concept of k2–3 coal

genesis and related palaeogeographic setting

Intensive lithology and lithofacies interpretation of

the coal-bearing k2–3 sequences from the individual

coalfields and occurrences of the Mid-Zambezi Karoo

basin and of their coal quality and coal petrology has

led to the identification of two different sedimento-

logical types of coal and also a new palaeogeographic

setting for the coals. These new results are in full

agreement with the new rift concept for the Mid-

Zambezi basin of Oesterlen (1999).

The Alluvial plain coal was found only at Gokwe

and in the Nyamandlovu area (see Sections 4.7 and

4.8) and occurs embedded in fine-grained flood plain

sediments deposited along meandering river channels.

The drainage system of the gentle alluvial plain had its

catchment area in the southeastern highlands and was

running towards the NW where the channels emptied

into the Mid-Zambezi lake with delta systems (Fig.

12). The respective coal seams are in general thin and

discontinuous; the coal contains high ash contents and

is encountered only at depths of more than 200 m

below the surface. In other words, the economic

significance of this type of coal in the Mid-Zambezi

basin is extremely low.

This is in contrast to the other main type of coal,

the lake shoreline coal, of all the coalfields in the NW,

from Wankie over Lubimbi, Lusulu and Lubu, Busi to

Sengwa (see Section 4). The coal-bearing sequence

starts almost in all fields with the Main Seam at the

base, up to 17-m thick, which represents the main

dpay-zoneT, and continues upwards with up to five

thinner coal seams intercalated in carbonaceous mud-

stone. In some cases, the coal is massive and is not

split-up by mudstone partings, like the coals of

Wankie or Lusulu–Lubu coalfields, suggesting con-

tinuous peat formation without episodes of clastic

influx. In other places, the coal consists of thick

alternations of carbonaceous mudstone with thin coal

bands, manifested at Lubimbi and Sengwa coalfields,

where the peatland was contaminated by the deposi-

tion of suspension load, most probably coming from

nearby delta systems.

The thickness of the Main Seam is one indicator of

a shoreline environment of the coal. Other ones are

the lack of coarser-grained clastic contamination, and

the presence of suspension load sediments as the main

country rock. However, the strongest argument

derives from the lateral lithofacies and environment

change, e.g., at Wankie coalfields, where drilling has

proven the shoreline coal changes down-dip into

sapropelic mudstone of lacustrine origin and up-dip

into terrestrial sediments of the coastal plain of the

lake. A similar, but less clear dfacies cross-sectionT ofthe shoreline coal has been found at Lusulu, with

Lubu as the down-dip continuation, and at Sengwa,

where Sengwa-North represents the down-dip exten-

sion of the Sengwa-South shoreline coal (Fig. 12).

Even the Lubimbi coalfields show good indications of

the same facies change. This interpretation was

supported by the finding in the Busi coalfield of delta

sandstones laterally interfingering with the shoreline

coal. The interpretation results in the delineation of a

20- to 40-km-wide coal-belt along the palaeo-shore-

line of the lake which stretches in Zimbabwe from the

Wankie coalfields in the west to the Sengwa coalfields

in the east. The coal-belt is, over its course,

interrupted by some delta systems related to river

channels draining from the SE, e.g. at Busi (Fig. 12).

The belt’s width points to a synsedimentary migration

of the shoreline peatswamps, due to a change of the

water level. Finally, the new sedimentological model

for the coalfields opens new potential for more coal

within and between the coalfields explored so far.

The maceral profile of the shoreline coal which is,

in its main trend, similar in all coalfields, reveals some

more information on the coal genesis. It suggests an

initial paludification (term from Diessel, 1992) of the

shoreline of the newly formed lake—due to a trans-

gression on the fluviodeltaic k1 sediments caused by

post-ice-age meltwater. Wet forest swamps of mainly

Glossopteris trees established themselves along the

shoreline which had a high water-table (vitrinite-rich

coal). Subsequently, the water-table began to drop, the

wet forests changed slowly to dry forests where the

swamp vegetation was prone to oxidation and

decomposition, due to a regression of the lake (typical

Gondwana inertinite-rich coal, with higher ash-con-

tent). This type of coal was formed for most of the

time span of the k2–3 sequence, the wet forest swamp

returning only at the end temporarily in some coal-

fields, before the final regression of the lake took

place and fluviodeltaic sandstones of k4 (Upper

Wankie Sandstone, Wankie Formation) were depos-

ited over the shoreline peatswamps. Only one


exception to this standard maceral profile was found

at Lubimbi coalfields where the paludification phase

did not start with a wet, but a dry forest–swamp

(inertinite-rich coal). This may have been the con-

sequence of palaeo-topography, such as a local

shallow floor of the lake shoreline.

This leads to the question of the controls for the

lake–shoreline peatswamp formation. An essential

precondition for the regional paludification phase

was, firstly, the eustatic rise of the groundwater-table,

by meltwater production following the Dwyka ice age

(Latest Carboniferous–Earliest Permian). The grada-

tional change from the wet forest to the longer-lasting

dry forest–swamp and the final cessation of peat

formation with the deposition of the k4–fluviodeltaic

sandstones is explained mainly by the slow fall of the

water level, when all the ice was melted and the

warming climate resulted in higher evaporation. Only

from k5 onwards did isostatic controls take over; that

is, the crust subsided by extension to form a sag-

basin—again setting the depositional environment for

another paludification phase which produced the coal

of the lower k5 Madumabisa Formation (Table 1).

The depositional classification of the k2–3 Mid-

Zambezi coal results in a new palaeogeographic

model for the Karro basin—a model which is in

contrast to the hypothesis of Duguid (1986), but

conforms to the rift concept of Oesterlen (1999).

Accordingly, the basin was rather a trough elongated

along a SW–NE axis, having one central depositional

centre which was filled with a shallow freshwater

lake—located more or less coincident with the

modern Lake Kariba (Fig. 12). The lithofacies of the

various coalfields indicates that its palaeo-shoreline

coincided with the southern boundary of the coal-belt

(Fig. 12). The shoreline follows a SW–NE trend

parallel to the basin axis, but turns at Wankie towards

the NW, and at Sengwa towards the N. Southwards

stretched an approximately 100-km-wide shallow

alluvial plain comprised of meandering rivers with

accompanying flood plains. The alluvial plain was

drained towards the lake in the NW. The interfluvial

flood plains were in some places temporarily occupied

by swamps, but often, mainly in the rainy seasons,

flooding episodes brought a clastic splay covering the

peatground. The origin of the alluvial plain so far is

unknown, but it was generated most probably by

erosion of glaciers flowing downwards from the

southeastern highlands during the Dwyka ice age

(Bond, 1970).

The alluvial plain of the Mid-Zambezi Karoo basin

was bounded in the south by the crystalline highlands,

representing the southern source area for the Karoo

basin (Fig. 12). The southern boundary runs in general

parallel to the lake shoreline, but in the NE it turns

sharply towards the NW and meets the lakeshore.

Probably, it reflects the northeastern end of the Mid-

Zambezi basin, an idea previously suggested by

Oesterlen (2001) in his report on the Mana Pools

basin. The southwestern end of the basin at Wankie is

less intensely studied, but some drill records of the

Western Areas of Wankie coalfields revealed terres-

trial sediments replacing the coal–mudstone sequence.

Lepper (1992) suggested that the k2–3 succession

tapered out towards the west, where the k5 mudstones

overlap on the k1 sequence. Consequently, there was

apparently no direct connection to the k2–3 deposits

of Botswana.

Acknowledgements

The authors thank Drs. W. Hiltmann and Th.

Thielemann, Federal Institute for Geosciences and

Natural Resources (BGR), Hannover/Germany, for

discussions and advice, also for reviewing the manu-

script. BGR supported also the publication substan-

tially, by bringing the original figures into a digitized

format. The manuscript has been greatly improved by

the comments from the editor and two reviewers of

IJCG, and the final reading by Dr. D. Bartholomew,

New Galloway/Scotland. The senior author is grateful

to Rio Tinto of Zimbabwe, Harare, which offered him

accommodation at Sengwa Mine during June–July

1996.

References

Boehmke, F.C., Duncan, R.G., 1974. Final Report on Exploration

for Coal and Natural Gas, E.P.O. 378 (Gokwe). Unpublished

report Rio Tinto (Rhodesia) Ltd., 18 pp.

Bond, G., 1967. A review of Karoo sedimentation and lithology

in southern Rhodesia. Review First Symposium Gondwana

Stratigraphy, pp. 173–195.

Bond, G., 1970. The Dwyka series in Rhodesia. Proceedings-

Geological Association 81 (3), 463–472.


Chappell, J., 1969. The Geology of the Eastern Portion of

the Chizarira Game Reserve and Adjacent Country,

Rhodesia. Unpublished report, PhD-thesis Univ. Zimbabwe,

216 pp.

Diessel, C.F.K., 1992. Coal-bearing Depositional Systems. Springer

Verlag. 721 pp.

Duguid, K.B., 1977. A cross-section through the Lusulu

coalfield. In: Thompson, A.C. (Ed.), The Lusulu (and Sengwe)

Coalfields. Zimbabwe Geological Survey, Records of Rhodesian

Coalfields.

Duguid, K.B., 1986. Wankie district coal measures. In: Anhaeusser,

C.R., Maske, S. (Eds.), Mineral Deposits of Southern Africa.

pp. 2099–2104.

Duguid, K.B., 1993. The potential for coalbed methane gasfields in

Zimbabwe. Proceed. 1993 Internat. Coalbed Methane Sympo-

sium, pp. 37–55.

Falcon, R., 1973. Palynology of the Lower Karoo succession in the

middle Zambezi basin. In: Bond, G. (Ed.), The Palaeontology of

Rhodesia, Bulletin-Rhodesia Geological Survey, vol. 70.

Falcon Research Laboratory (Pty.) Ltd., 1995. Petrographic analyses

and report on three borehole cores M 94 G, M 129 G, and M

178, Sengwa Coalfield Exploration Project, for Rio Tinto

Zimbabwe. Unpublished report.

Gair, H.S., 1959. The Karoo system and coal resources of the

Gwembe District, north-east section. Bulletin-Geological Sur-

vey Northern Rhodesia 1. 88 pp.

Harrison, N.M., 1978. The Karoo succession at Tjolotjo, Nya-

mandhlovu District. Annals of the Rhodesia Geological Survey

III, 41–50.

Hosking, B.C., 1981. The stratigraphy and sedimentation of the

Karoo Supergroup in the Mid-Zambezi Valley, Zimbabwe.

Unpublished M.Sc thesis Univ. Zimbabwe, 194 pp.

Humphreys, M., 1969. The geology of the western portion of the

Chizarira Game Reserve and adjacent country, Rhodesia.

Unpublished report PhD thesis Univ. Rhodesia, 297 pp.

Lepper, J., 1985. Study on the Lower Karoo in the Mid-Zambezi

Basin. Unpublished open file, archive Bundesanst Geowiss.

Rohstoffe, Hannover.

Lepper, J., 1992. The Lower Karoo in the Mid-Zambezi basin

(Zimbabwe). Geologisches Jahrbuch, Reihe B, Regionale Geo-

logie Ausland 82. 38 pp.

Lightfoot, B., 1914. The geology of the north-western part of

the Wankie coalfield. Bulletin-Rhodesia Geological Survey 4.

49 pp.

Lightfoot, B., 1929. The geology of the central part of the

wankie coalfield. Bulletin-Rhodesia Geological Survey 15.

61 pp.

McKenzie, D.P., 1978. Some remarks on the development of

sedimentary basins. Earth and Planetary Science Letters 40,

25–32.

Oesterlen, P.M., 1998. The geology of the Dande-West area,

Lower-Zambezi valley. Bulletin-Zimbabwe Geological Survey

98. 85 pp.

Oesterlen, P.M., 1999. Some new results from the Mid-Zambezi

basin. Annals of the Zimbabwe Geological Survey XIX, 16–20.

Oesterlen, P.M., 2001. New geological results from the Mana Pools

basin of Zimbabwe, Lower-Zambezi Graben. Annals of the

Zimbabwe Geological Survey XX, 1–9.

Oesterlen, P.M., Blenkinsop, T.G., 1994. Extension direction and

strain near the failed triple junction of the Zambezi and

Luangwa rift zones, Zimbabwe. Journal of African Earth

Sciences 18, 175–180.

Orpen, J.L., Swain, C.J., Nugent, C., Zhou, P.P., 1989. Wrench fault

and halfgraben tectonics in the development of the Palaeozoic

Zambezi Karoo basins in Zimbabwe: the dLower-ZambeziT anddMid-ZambeziT basins respectively and regional implications.

Journal of African Earth Sciences 8 (2/3/4), 215–229.

Palloks, H.-H., 1984. An assessment of some of the coal deposits in

north-west Zimbabwe. Zimbabwe Geological Survey, Mineral

Resources Series 19. 39 pp.

Palloks, H.-H., 1987. The Wankie Concession area. Zimbabwe

Geol. Survey, Records of Zimbabwe Coalfields V. 14 pp.

Shell Developments Zimbabwe (Pvt.) Ltd., 1983. Summary Geo-

logical Report Lusulu Project, E.P.Os No. 573, 606 and 607.

Unpublished report, 69 pp.

Styan, W.B., Bustin, R.M., 1983. Sedimentology of Frazer river

delta peat: a modern analogue for some ancient deltaic coals.

International Journal of Coal Geology 3, 101–143.

Sutton, E.R., 1979. The geology of the Mafungabusi area. Bulletin-

Rhodesia Geological Survey 81. 318 pp.

Taupitz, K.C., 1976. Lubimbi Coal Project. Unpublished report.

Industrial Development Corp Rhodesia Ltd., 22 pp.

Thompson, A.O., 1977. The Insuza borehole, Special Grant 555,

Bembezi forest reserve, Nyamandhlovu District. Annals of the

Rhodesia Geological Survey 2, 22–39.

Thompson, A.O., 1980. The Lubu and Sebungu coalfields.

Zimbabwe Geological Survey, Records of Zimbabwe Coalfields

X. 6 pp.

Thompson, A.O., 1981. The geology of the Lubimbi, Dahlia and

Hankano coalfields, Wankie and Lupane districts. Bulletin-

Zimbabwe Geological Survey 88. 88 pp.

Thompson, A.O., Broderick, T.J., Palloks, H.-H., 1982. The

Western Areas coalfield. Zimbabwe Geological Survey, Records

of Zimbabwe Coalfields IX. 9 pp.

Thompson, A.O., Broderick, T.J., Palloks, H.-H., 1982. The Entuba

coalfield. Zimbabwe Geological Survey, Records of Zimbabwe

Coalfields VIII. 12 pp.

Watson, R.L.A., 1960. The geology and coal resources of the

country around Wankie, Southern Rhodesia. Bulletin-South

Rhodesia Geological Survey 48. 52 pp.

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