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CHAPTER II:

GEOLOGICAL SETTING

2.1 REGIONAL HIMALAYAN SETTING

The mighty Himalaya is a part of the Alpine-Himalayan chain of mountains

stretching from Spain to Indonesia. As introduced earlier it is the resultant of

plates collision (Indian and Eurasian plates) during the Cenozoic era which

resulted in crustal shortening, largely accommodated by folding and thrusting and

is still active orogenic belt. Palaeomagnetic, magnetic anomaly and volumetric

balancing studies revealed about 1800-2100 km convergence between India and

Asia in the western sector, 2475 km in the central and 2750-2800 km in the

eastern sector (Johnson, 2002).

South to southwest directed thrusting is manifested by progressively

developed thrust planes MCT, MBT and HFT (Fig. 1.1) (Gansser, 1964; Molnar

and Tapponeir, 1975; Valdiya, 1980a, 1980b; Johnson, 2002).These thrust zones

are very significant for studying the high strain sheared zone in the frontal part of

the Indian plate. The Himalaya may be divided geologically into six zones, (Heim

and Gansser, 1939; Gansser, 1964; Molnar and Tapponeir, 1975; Valdiya 1980b,

2010; Johnson, 2002) which are described in detail from south to north as follows

(Fig. 1.1).

Sub-Himalaya consisting of mollasse sediments (18 to 1 Ma) exposed in

the Siwalik Hills together with their intermontane valleys called the duns

(e.g., Dehra Dun). The Sub-Himalaya is separated by the HFT from the

vast Holocene Indo-Gangetic Alluvial plane.

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The Lesser Himalaya contains Palaeoproterozoic (~1800 Ma) to lower

Palaeozoic (~500 Ma) sedimentaries, some of which are mildly

metamorphosed. It is bounded to the south by the MBT, along which the

Lesser Himalaya thrusts over the Sub-Himalaya.

Higher Himalaya with its rugged scarps consists of Central Crystallines,

representing the metamorphic core affected by intense ductile deformation

and extensive migmatisation. The Central Crystallines (2200-1800Ma)

consist of a 10-15 km thick assemblage of mica schists, quartzites, calc-

silicate rocks, orthogneisses, paragneisses, migmatites and Miocene

luecogranites (~20Ma). The Higher Himalaya is separated by MCT from

the Lesser Himalayan sequences in south.

The Tethys or Trans Himalaya is separated from the Higher Himalaya by

northerly directed low angle normal fault called the Trans Himadri Fault

(THF) or South Tibetan Detachment (STD) system. The Tethyan Himalaya

has 10-17km thick succession of mostly unmetamorphosed, highly

fossiliferous marine sediments of Neoproterozoic (~600Ma) to Eocene

(~65Ma) ages.

The Indus-Tsangpo Suture Zone (ITSZ) limits the Tethys Himalaya in its

south. The suture zone consists of deep-sea flysch sediments, blue

schists and ophiolite mélange (Frank et al., 1977; Jan and Symour, 1977;

Virdi et al., 1977).

The Trans-Himalaya batholiths (Karakoram, Ladakh and Kailas-

Mansrovar), forming a large plutonic complex of I-type plutons of age 110-

40 Ma, occurs to the north of the Indus-Tsangpo suture zone.

Along the Himalayan strike the change in stratigraphic juxtaposition and

metamorphic grade across the MCT indicates a westward decrease in its slip

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magnitude, possibly a result of a westward decrease in total crustal shortening

along the Himalayan orogeny (Yin, 2006).

2.2 LESSER HIMALAYA

2.2.1 TECTONIC SETTING

The Lesser Himalaya is characterized by number of thrust-fold belts and a very

complex geological and tectonic setting. Attempting to unravel the complex

features of the Lesser Himalayan region, various views have been put forth on its

geological setting (e.g., Middlemiss, 1885; Auden, 1935; Heim and Gansser,

1939; Misra and Sharma, 1967; Jain, 1971; Rupke, 1974; Valdiya, 1980;

Srivastava and Mitra, 1994; Valdiya, 1995; Richards et al., 2005; Yin, 2006;

Celerier et al., 2009; Joshi and Tiwari, 2009). The sheets of highly

metamorphosed crystalline rocks lying over the Lesser Himalaya supposed to be

transported thrust sheets of Higher Himalayan Crystallines (Heim and Gansser,

1939; Valdiya, 1980b).

The central part of the Lesser Himalaya is characterized by the largest

crystalline thrust sheet i.e. Almora crystallines and further east through the

Dandeldhura region of western Nepal (Figs. 1.1 and 1.2) (Valdiya, 1980b;

Decelles et al., 2001). Heim and Gansser (1939) were the first to put forward the

concept of nappe hypothesis for this thrust sheet in the Lesser Himalaya.

Valdiya (1980b) suggested that the Almora Nappe is an integrated termed

of imbricated thrust sheets i.e. Ramgarh thrust sheet and overlying Almora thrust

sheet. These bodies mainly occupy the core of a major synformal part of the

Lesser Himalaya and implying that the antiformal part has been eroded off, now

due to continuous compression the low angle thrust fault of the nappe has been

converted into high angle fault (Fig. 2.1). Valdiya (1980b) explained that the

Almora Nappe is the southeastern extension of the Jutogh Nappe of Himachal

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Pradesh and the Ramgarh Nappe has been correlated with the Chail Nappe of

Pilgrim and West (1928) in Himachal Pradesh. Jain (1971), Mehdi et al. (1972),

Agarwal and Kumar (1973), Kumar et al. (1974) and Kumar and Agarwal (1975)

have ruled out the possibility of a nexus between NAT and SAT and believe it to

be continuing westwards upto Jammu valley, with various local names such as

Naulpani Fault (Dhoundial and Ali, 1967), Dharkot dislocation (Saklani and

Pande, 1970), Dharasu Thrust (Jain, 1971), Srinagar Thrust (Mehta, 1971) and

Srinagar Shear (Bhargava, 1972).

Figure 2.1: Reactivation of the Almora Thrusts and faults is attributed to the compression experienced by the Almora Nappe due to the under thrusting of the Indian plate beneath the Himalaya (after Valdiya, 2001).

According to Saklani (1970, 1971 and 1984) the NAT is a high angle

reverse fault and the zone is characterized by mylonitised rocks and many

mesoscopic structures and petrofabric elements supporting the thrust.

Molnar et al. (1977) described in detail the structure and tectonics of the

Himalaya based on relevant geophysical observations. Misra and Sharma

(1966a, 1966b, 1967, 1972) have established the presence of three phases of

deformation and also elucidate by the petro chemistry of the Almora crystallines.

Whereas according to the Joshi and Tiwari (2009) the rocks of the Almora Nappe

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have been subjected to two metamorphic events, viz. the regional metamorphism

(M1) and dynamic metamorphism (M2) where regional metamorphism (M1) relates

to a pre-Himalayan event while the dynamic metamorphism (M2) relates to the

Himalayan orogeny.

Powar et al. (1969) established a relationship between plutonism,

volcanism, regional metamorphism and tectonism of the Himalaya orogeny as

recorded by the Almora crystallines. According to Bhattacharya (2000) the flow

characteristics and the deformation pattern of the rocks of a ductile shear zone

depend to a large extent of the mineralogical constitution of the rocks. The large

scale shear zone of NAT is characterized by the presence of thick mylonite

sequence (Joshi, 1999; Joshi and Tiwari, 2004).

Valdiya (1962a, 1962b, 1980a, 1980b, 1981, 1987, 1988a, 2001, 2003,

2010) has contributed a lot to the Himalayan geology on various aspects of

origin, stratigraphy and tectonic sequence (Fig.2.3) of the Kumaun Lesser

Himalaya. Tectonic evolution of the Kumaun Lesser Himalaya is also outlined by

Mehdi et al. (1972), Agarwal and Bhattacharya (1987), Agarwal (1994), Agarwal

and Bali (2008).

2.2.2 LITHOLOGICAL SETTING

The geology of the Kumaun and Garhwal Himalaya has been studied for over a

century (e.g., Middlemiss, 1885; Auden, 1935; Heim and Gansser, 1939; Misra

and Sharma, 1967; Jain, 1971; Rupke, 1974; Valdiya, 1980a; Valdiya, 1995;

Srivastava and Mitra, 1994; Richards et al., 2005; Celerier et al., 2009). A large

part of the Lesser Himalayan province occupying a position between MCT and

MBT in North and South respectively is made up of meta-sedimentary rock

formations and contains crystalline lithounits of tectonic thrust sheets. Broadly,

the area can be divided into two major units:

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i. The sedimentary successions comprising calcareous, arenaceous and

argillaceous rocks with rare fossils and thus not ordinarily amenable to

palaeontological dating.

ii. The overlying crystalline nappe of Almora- Dudatoli and associated klippen

viz., Baijnath, Askot etc.

Figure 2.2: Litho-stratigraphic column of (a) Rautgara Formation and (b) Almora Group (after

Valdiya, 1980b).

Sedimentary sequence of the Lesser Himalaya comprises five litho-

stratigraphic groups i.e. conformable and relatively older Damtha and Tejam

groups stretching from East to West in the Inner Lesser Himalaya and the rest

three relatively younger viz., the Jaunsar, Mussoorie and Sirmur groups make the

Krol nappe of the outer Lesser Himalaya. The Damtha and Tejam groups, made

up of respectively of Chakrata-Rautgara and Deoban-Mandali formations,

constitute the autochthonous zone in the inner Lesser Himalaya (Fig. 2.2). The

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Jaunsar comprises Chandpur and Nagthat formations, and the Mussoorie group

consists of Blaini, Krol and Tal formations. The Jaunsar and Mussoorie groups

with crumpled capping of the Bansi and Subathu Formation of Sirmur Group,

make the Krol Nappe of the outer Lesser Himalaya in the northern limit of

Siwaliks (Heim and Gansser, 1939; Valdiya, 1980b).

Allochthonous unit (Nappe) in the Kumaun Lesser Himalaya mainly

comprises three formations, viz. Saryu Formation, Champawat Formation and

Gumalikhet Formation belong to the Almora Group (Heim and Gansser, 1939;

Valdiya, 1980b) (Fig.2.2). Joshi and Tiwari (2009) suggests the roof of the basal

shear zone of the Almora thrust sheet show dynamic metamorphism reaching

upto greenschist facies (450°C/ 4 kbar) and in the central part of nappe the

unmylonitized schists and gneisses are affected by regional metamorphism (M1)

reaching upper amphibolite facies (600-709°C and 4.0-7.0 kbar) of four zones

from Chlorite-biotite to sillimanite-K-feldspar zone.

Present investigation is carried out in and around the northern flank of the

Almora Nappe i.e., the NAT, which lies in the central Kumaun Lesser Himalaya

(Fig.1.2 a). The NASZ comprises sheared quartzite and slates of Rautgara

formation of Damtha Group in the footwall and mylonitised and sheared basal

lithounits (Saryu Formation) of the Almora Group in the hanging wall (Heim and

Gansser, 1939; Gansser, 1964; Prakash, et al., 1978; Valdiya, 1980b) (Fig. 2.2).

2.2.2.1 RAUTGARA FORMATION (DAMTHA GROUP)

In the investigated area, the meta-sedimentary sequence exposed in footwall of

the NAT is described as Rautgara Formation of the Damtha Group (Valdiya,

1980b). It was explained as Saryu valley quartzites by Valdiya (1962b). Rautgara

Formation consists of fine to medium-grained muddy quartzarenite (sub-

greywacke to sublitharenite) of creamy white, pink, purple, grey and brown, and

purple slates (often superficially oxidized to deep red soils), some time it is

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characterized by presence of primary structures such as cross bedding (Fig. 2.3

a and b).

The upper line of demarcation of this unit is defined by almost abrupt

change of slates into limestones and dolomites of Deoban Formation. The lower

limit is marked by the presence of a lenticular conglomeratic horizon, can be

correlated with the Muth-quartzites of Devonian age (Valdiya, 1980b). The

Rautgara and Chakrata formations, being at the lower litho-stratigraphical level,

definitely belong to the precambrians of the Lesser Himalaya. The lowermost

Chakrata Formation is not exposed in the southeastern Kumaun Himalaya. The

Rautgara Formation represents the lower most lithounit of the eastern Kumaun

Lesser Himalaya (Valdiya, 1962b, 1980b) and well exposed through narrow linear

belt, immediately north of the NAT and defined as the footwall unit of the NAT.

Figure 2.3: Field photographs showing lithounits of Rautgara Formation (a) thickly bedded quartzarenite with cross bedding near Kaphligair and (b) purple slates near Seraghat.

2.2.2.2 ALMORA GROUP

The “Crystalline zone of Almora” is explained under the Almora Group originally

referred by Heim and Gansser (1939) and known as “Almora Crystallines”. This

entire thrusted package with a variety of schists, micaceous quartzites and

gneisses has reached upto the lower to upper amphibolites facies of regional

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metamorphism (~4.0 to 7.kbar and ~500 to 709°C) (Joshi and Tiwari, 2009) and

with emplaced plutonic bodies of granodiorite and granite.

The root zone of the Almora Nappe is believed to be Munsiari Fm. of the

Great Himalaya (Valdiya 1973a). Between the root zone and overthrusted Almora

sheet, lies the chain of klippen (detached pieces of presumably once continuous

sheet) i.e. Nandprayag, Baijnath, Dharamghar, Askot and Chiplakot (Heim and

Gansser, 1939; Valdiya, 1962b; Valdiya and Gupta, 1972). The lower limit

demarcated by the basal thrust i.e. Almora Thrust separates the autochthonous

metasedimentary below from the crystallines above (Gansser, 1964; Merh, 1968;

Gairola and Joshi, 1980; Valdiya, 1980; Joshi, 1999; Joshi and Tiwari, 2009).

Almora Group builds the upper part of the Dudatoli-Ranikhet-Almora-Champawat

range embraces three lithologic units:

i The basal Saryu Formation

ii Champawat Granodiorite

iii The upper Gumalikhet Formation

i. The Saryu Formation

This lithological unit is named after the Saryu River along which it is well

exposed. It is the lowermost part comprising chlorite-sericite schist, followed by at

higher level garnetiferous-mica-schists alternating with micaceous quartzites and

pophyritic granitic gneiss (Fig. 2.4 a, b, c).

At the basal part of Almora nappe, all along the NAT, rocks of Saryu Fm.

are intensely sheared and got mylonitised which is clearly reflected by the

presence of shear sense indicators and understood by looking the gradual

reduction in grain size towards the NAT contact. At the NAT contact the strongly

mylonitised granitic gneiss and ultramylonites are seen persistently (Fig. 2.4 d).

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The degree of mylonitization gradually decreases towards the central part of the

Almora Nappe till the signatures of shearing fade away and the mylonites grade

into the unmylonitized schists and gneisses (Joshi and Tiwari, 2009).

Figure 2.4: Field photographs of the lithounits of Almora Group (a) garnet mica schist near Someshwar, (b) schist with alternating micaceous quartzite near Kaphligair, (c) porphyritic granitic gneiss away from the NAT near Panduakhal and (d) mylonitic bands in ultra mylonitic granitic gneiss, near Chaukhutiya.

Towards the upper end of the Saryu Formation there are lenticular bodies of

pophyritic granite converting into augen gneiss at its upper margin.

ii. The Champawat Granodiorite

Saryu Formation is followed by the batholithic and sill-like bodies of syn-kinematic

granodiorite suite intruded by later leucocratic granites the so called Champawat

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granodiorite (Valdiya, 1980b). It is first studied in the Khetikhan-Khark-

Champawat area by Kharkwal (1951) and named after the township of

Champawat (29°21′′:80°71′′) by Valdiya 1962a, who regards it a composite body

constituted predominantly of granodiorite that grades on the one hand into

tonalite (quartz-diorite) and on the other into adamellite (quartz-monzonite).

The main body is massive non-foliated, coarse-grained equigranular to

locally porphyritic in the central part and becoming progressively foliated or

gneissose towards the margins (Valdiya, 1962a).The northwestern extension of

the Champawat Granodiorite in the Devidhura-Mornaula belt has been studied by

Misra and Sharma (1967). It is characterized in the NE by granitic body, which is

marginally gneissose and described as Almora Granite by Heim and Gansser

(1939) and Power (1970).

iii. The Gumalikhet Formation

The uppermost unit of the Almora Group termed as Gumalikhet Formation

consisting of schistose and carbonaceous phyllite alternating with black, fine

grained biotite-rich greywacke and carbonaceous or graphitic schists alternating

rhythmically with fine-grained micaceous often garnetiferous meta-greywacke

(Valdiya, 1962a and 1980b). Gumalikhet Formation has been named after the

village Gumalikhet (29°24'':80°13'') in the Kali valley, South of Pancheshwar.

According to the Valdiya (1962a) the border gneiss of the Champawat Formation,

is a product of granitization while the main body is undoubtedly magmatic.

As our study is confined in the adjoining area of the NAT so study is

carried out in the rock units present in the area, which belong to the Saryu

Formation of Almora Group and Rautgara Formation of Damtha Group.

Best descriptive geological map and cross sections in the area, to explain

the lithological and structural variation across the NAT, are prepared after the

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detailed lithological and structural mapping in four sectors, along which oriented

sample were collected for lab study.

2.3 PANCHESWAR–SERI SECTOR

Pancheswar- Seri sector falls in the eastern part of the study area in the

Pithoragarh district lying between 29o23′-29o35′N Latitude and 80o18′-80°5′E

Longitude (Fig. 2.5a). Detail mapping is completed by studying number of

traverses across the NAT in the sector.

Near the Rautgara-Laddang region transect was taken from NE-SW and

then continued towards SE direction along the western bank of the Kali River.

The NAT contact is characterized by the sharp contact between mylonitized

quartz-porphyry or granitic gneiss of the Saryu Fm. and quartzarenite of the

Rautgara Fm. Lithounits are near vertical to subvertical with steeply inclined

folded thrust plane (NAT), towards NE, (Fig. 2.6 a). Mylonitized gneiss is followed

by garnet mica schist and granite-granodiorite as go away from the NAT in the

area.

From Melti though Simli to Netra villages a NE-SW cross traverse through

the NAT was taken. The lithounits exposed at the NAT contact are similar to the

Rautgara- Laddang section with vertical to subvertical attitude. Quartzarenite with

intercalated grey to black colored slates at the vicinity of the NAT are highly

folded and sheared (Fig. 2.6 b). Similarly the lithounits encountered in the area

from Ghat to Chyurani and Rameshwar to Batuli, are steeply inclined with steep

thrust contact. Ductile to brittle deformation can be seen in intensely sheared

units (Fig. 2.6 c).

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Fig

ure

2.5

: G

eo

log

ica

l m

ap

s o

f all f

ou

r secto

rs a

lon

g t

he N

AS

Z (

a)

Pan

ch

esh

war-

Se

ri s

ecto

r, (

b)

Seri

-Sera

gh

at

secto

r, (

c)

Sera

gh

at-

Dw

ara

hat

secto

r an

d (

d)

Dw

ara

hat-

Gair

sen

secto

r.

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Figure 2.6: Lithological cross sections (a) Rautgara-Laddang area and (b) Melti to Netra area, across

the NAT in the Pancheshwar-Seri sector.

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General lithological description: Varying types of lithounits encountered in the

Pancheshwar- Seri are following:

_____________________________Top _______________________________

Saryu Fm. Highly sheared mylonitised quartzporphyry (granitic

gneiss) (locally ultramylonitised) alternated with chlorite biotite schist.

Granite gneiss changing into porphyroblastic granitic gneiss away

from the NAT contact.

Garnetiferous mica schist interbedded with flaggy grey-white

sericite-quartzite.

Porphyritic granite-granodiorite of the Champawat Formation

exposed away from the NAT (in Pancheshwar)

______________________________NAT_______________________________

Rautgara Fm. Fine grained light to dark brown quartzarenite with

subordinate intercalated purple and predominant olivegreen slates

weathered to deep red colour (near Rautgara village).

_____________________________bottom______________________________

2.3 SERI–SERAGHAT SECTOR

The section is lying between Seri to Seraghat (29o 35′-29o45′N Latitude and 80o5′-

79o55′E Longitude) taken along the Saryu River in the central Kumaun Lesser

Himalaya (Fig. 2.5 b). Mapping is done by taking the three significant transects

across the Saryu River, which are explained below:

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The Sartola-Khanyari area near Seraghat region the rock units are gently

dipping (30º-40o) towards SW with gentle thrust plane. Mylonitised

quartzporphyry (granitic gneiss) of Saryu Fm. and mylonitised calcareous slate

and quartzarenite of the Rautgara Fm. represent the contact plane of the NAT

(Fig. 2.7 a). Grain size reduction is visible in the mylonitized granitic gneiss as

moves towards the NAT contact.

SE of the Seraghat area, in the Naichun-Harara transect rock units show

vertical to subvertical attitute with steeply inclined NAT plane (Fig. 2.7 b). The

rock units exposed near the NAT contact are same as in the Sartola-Khanyari

area. The rock units are highly sheared, folded and faulted.

General lithological description: Varying types of lithounits encountered in the

Seri- Seraghat sector which are following:

________________________________Top_____________________________

Saryu Fm.: Ultramylonitised and mylonitised quartzporphyry

(granitic gneiss) with alternate bands of chlorite-phyllonite, chlorite-

sericite schist.

Increasing in grain size from medium to very coarse porphyritic grains of feldspar and quartz in granitic gneiss can be seen away from the NAT contact.

Porphyritic granitic gneiss at the margin of the NASZ is visible.

________________________________NAT_____________________________

Rautgara Fm. Fine grained white and reddish quartzarenite with

intercalated calcareous slates. The upper limit of the Rautgara Fm. is

bounded by the Deoban lime stone in the NE part of transects.

________________________________bottom___________________________

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Figure 2.7: Lithological cross sections (a) Sartola-Khanyari area and (b) Naichun-Harara area, across the NAT in Seri-Seraghat sector.

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Figure 2.8: Lithological cross sections (a) Kanarichhina-Dhaulchhina, (b) Kaphligair-Bansauli, (c) Supakot-Manan and (d) Binta-Bagwalipokhar regions, across the NAT in Seraghat-Dwarahat sector.

2.5 SERAGHAT–DWARAHAT SECTOR

The sector is bounded between 29o38′-29o45′N Latitude and 79º45'-79º25'E

Longitude in the central Kumaun Lesser Himalaya all along the Kosi River and

Jagan Gad. Detail mapping was done by taking SW-NE transects across the Kosi

River and Jagan Gad (Figs. 2.5. c and 2.8 a, b, c, d, e).

In the Kanarichhina-Dhaulchhina area, transect was taken (NE-SW)

across the Jagan gad. Quartzarenite with interbedded slates are juxtaposed to

mylonitized gneiss and schist (Fig. 2.8 a). At the vicinity of the NAT ultramylonites

are exposed. Rock units are steeply inclined with steep NAT plane. Similarly

Kaphligair-Bansauli region the NAT contact is characterized by mylonitized

gneiss, schist and quartzarenite with interbedded slates and having gentle dip

(Fig. 2.8b).

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Rocks of Saryu Formation and metasedimentaries of Rautgara Formation

are folded, faulted and highly deformed. Superimposed folding was observed in

mica schist the Saryu Formation near the Kaphligair. Third transect is taken from

Someshwar to Manan across the NAT (Fig. 2.8 c). The lithounits of Saryu and

Rautgara formations are highly sheared at the NAT contact. Lithounits are gently

dipping with gently dipping NAT plane. Near Supakot and Dhumangaon (near

Someshwar area) the lithounits of the Rautgara Formation are characterized

superimposed folding similar to the Saryu Formation.

Near the Lodh area, the NAT contact lies between ultra-mylonitised granitic

gneiss and quartzarenite of phyllitic nature with interbedded greenish grey slates.

Here quartzarenite is showing reddish colour representing oxidizing condition.

Garnetiferous mica schist and chlorite schist are exposed in the area. Mylonitized

granitic gneiss exhibits grain size reduction as moves towards NAT contact in the

area. Folding and faulting are observed in the rock units of the area.

The area between Binta to Bagwalipokhar is characterized by a straight

wide Gagas valley. Rock units at the NAT contact are gentle and highly sheared

(Fig. 2.8 d). Mylonitised granitic gneiss is characterized by grain size reduction

(feldspar and quartz grains) as moving towards NAT contact and showing shear

bands and folding. Sericite schist and quartzite interbedded with chlorite schist of

the Saryu Formation are well exposed in the area. Similar to previous area

reddish and brown thickly bedded quartzarenites of the Rautgara Formation is

exposed all along the foot wall of the NAT in the area.

General lithological description: Varying types of lithounits encountered in

the Seraghat- Dwarahat sector which are following:

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______________________________Top______________________________

Saryu Fm.: Ultramylonitised and mylonitised black quartzporphyry

(granitic gneiss) with alternate bands of chlorite-biotite and chlorite-

sericite schist.

Garnetiferous mica schist interbedded with flaggy grey-white

sericite-quartzite and granitic gneiss.

Granitic gneiss becoming porphyroblastic with grains of feldspar

and quartz in granitic gneiss away from the NASZ.

_______________________________NAT______________________________

Rautgara Fm. Massive thickly bedded to phyllonitic quartzarenite,

white brown and reddish in color, weathered to deep red colour with

intercalated dark grey, purple and greenish grey slates and siliceous

shale.

_______________________________bottom____________________________

2.6 DWARAHAT– GAIRSEN SECTOR

NW terminal part of the study area is described though Dwarahat- Gairsen sector

enclosed between 29o45′- 30o10′N Latitude and 79º 25' -79º 15' Longitude.

Numbers of transects were taken for detail study in the sector (Figs. 2.5 d and 2.9

a, b, c, d).

In the SE terminal part of the sector along the Dwarahat– Agar transect,

the NATZ in Dwarahat Domain is marked by a zone of ultramylonite to

mylonitised quartzporphyry (granitic gneiss), mica schist, porphyritic granitic

gneiss (the Saryu Formation) and quartzarenites (the Rautgara Formation).

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Figure 2.9: Lithological cross sections (a) Chaukhutiya area, (b) Mahalchauri-Panduakhal area, (c) Gairsen area and (d) Dewalikhal area, across the NAT in Dwarahat-Gairsen sector.

A NE-SW transect was taken in the central part of the sector along the

Ramganga River. The NAT represent the ultramylonite to mylonite porphyry of

Saryu Formation with the contact of quartzarenite of Rautgara Formation.

Micaceous quartzite interbedded with garnet mica schist is well exposed in the

area. Rock units of both side of the NAT plane, are folded and faulted with

steeply dipping (>65º) foliation and bedding planes (Fig.2.9 a).

Transect was taken from Mahalchauri to Dharapani all along the road

represents the ultra-mylonites juxtaposed to the thickly bedded quartzarenites at

the NAT. Lithounits are steeply inclined and highly sheared and ultramylonitised

at the NAT contact (Fig. 2.9 b). Grain size reduction cab be easily observed

across the NAT on moving towards it.

In the NW terminal of the sector, the NATZ in the Gairsen region is marked

by a zone of gentle to steeply dipping ultra- mylonite, mylonites and

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quartzarenites similar to the other sections of the sector (Fig. 2.9 c). Mylonites

exhibit shearing and characterized by S-C structures in the area. Near Gairsen in

the Dewalikhal area, which is the NNW terminal part of the study area, a small

transect was taken (Fig. 2.9 d). In the area the NAT contact is in between

micaceous quartzite of the Saryu Formation and quartzarenites of the Rautgara

Formation.

General lithological description: Varying types of lithounits encountered in

the Dwarahat- Gairsen sector which are following:

______________________________Top_______________________________

Saryu Fm.: Ultramylonitised and mylonitised black quartz porphyry

(granitic gneiss) with alternate bands of chlorite-biotite phyllonite.

Thickly bedded metaquartzite/ micaceous quartzite with intercalated

sericite schist.

Chlorite, biotite and Garnet mica schist interbedded with flaggy

grey-white sericite-quartzite and granitic gneiss.

Granitic gneiss becoming porphyroblastic with grains of feldspar

and quartz in granitic gneiss away from the NAT.

_____________________________NAT________________________________

Rautgara Fm. Massive thickly bedded reddish, white, grey and

purple colored quartzarenite with intercalated dark grey, purple and olive-

green slates and greywacke.

____________________________bottom_______________________________

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