Sakai(1983, 1985) have divided a west central lesser Himalayan zone into following two main geological and tectonic units, which are outlined as follows:
- Tansen Group
- Kali Gandaki Supergroup
The Tansen Group consists of Post Gondawanas and Gondwanas rock and has following formations namely
- Sisne Formation
- Taltung Formation
- Amile
- Bhainskati Formation
- Dumri Formation
The Kali GandakiSupergroup is further divided in to upper, middle and lower Group and consist of following Formation namely
- Andhi Formation
- Naudanda Quartzite
- Heklang Formation
- Virkot Formation
- Chappani Formation
- Khoraidi Formation
- SaidiKhola Formation
- Ramdighat Formation
- Kerabari Formation
The table of sub-division of above Groups is given in Table 2 according to Sakai (1983, 1985).
Stratigraphy and Lithology:
The stratigraphy of study area can be discussed under two major heads, the Tansen Group and Kali GandakiSupergroup which is further divided into five and nine formation respectively.
Tansen Group
Geological map of the study area is shown in fig.6, and lithostratigraphy in Table.2.
The geology of the Tansen Group is important from the view point that at a number of places fossiliferous sequence ranging age from Permo- Carboniferous to Lower Miocene(Gondwana and Post Gondwana) rocks have been mapped.
The Gondwanas
The Gondawanas sequence is sequence, unconformably overlying the older metasedimentary formation is largely a nonmarine sequence though some formation of marine origin have been identified.Gondwana rock in western Nepal may be divided into lower and Upper Gondwanas depending upen their fossil assemblages and other characters.
Lower Gondwanas
The Lower Gondwanas are well developed in western Nepal in Tansen, Phalabang and Sallyan, areas. In the Tansen area, this formation has been described as Sisne Formation by (Sakai 1983,85). The Sisne Formation (1983) represtents the oldest rocks of Tansen Group and well exposed in TansenSynclinorium. This formation rests disconformably on the Kerabari Formation of the Kali Gandaki Super Group (Sakai 1985) with a basal conglomerate derived from the underlying Kerabari Formation.
Sisne Formation
It is the lowest unit of the Tansen Group and belongs to Gondawanas class. The name 'Sisne' was derived from the Sisnegaun(Sakai 1983), is distributed from the Sisnegaun in the west to Charchere village to the east, more or less along the Aulis Khola.
This formation is disconformably overlain by the Taltung formation and mainly consists of diamictite and bioturbated mudstone or mudstone converted into slates with some beds of sandstones and conglomerates. Diamictite and black shale influence and is correlated with the glaciofluvial lower gondwana. Diamictite is unstratified scattered clasts of various shapes and sizes. Conglomerate is the diamictite whose proportion of the clasts is large. The clasts are made up of gnesis, limestones, granites, dolomites, sandstones and less of slate and phyllites. The clasts vary in size from pebble to cobble to sand size but sometimes exceeds. Dark grey to black Claystone forms a major constituent of Sisne Formation.
The thickness of this formation is about 1000m and attitude of bedding plane is N85E/75SW
Taltung Formation
The name 'Taltung' was derived from the Taltung village (Sakai 1983). The Taltung Formation is extended from north of Sisnegaun in west to Gautul in the east.
This formation consist of olive green sandstone,siltstone and conglomerate with volcanic detritus.In the basal part basaltic lava are intercalated and have supplied enormous amount of pebbles to the charchare Conglomerate. The conglomerate consists of densly packed well sorted pebbles and cobble. Compositionally, the conglomerates are mainly made up of volcanic rocks and quartzites. Basic volcanic rocks (Aulis volcans) make a distinct member of Taltung Formation. Wood fragments are common in the sandstones.
Taltung represents an upward fining fluvial facied characterized by cyclic sedimentation in lowerpart and rhythemic one in the upper part and yields plant fossil of upper gondwana indicating the Jurassic to early cretaseous age.
The thickness of this formation is about 300m and attitude of the bedding plane is N65W/59SE.
Amile Formation
The name of this Formation was derived from the Amilekhola running west near Taltung (Sakai, 1983). This formation is extended in the both limbs of the TansenSynclinorium.
This formation lies disconformably on the Taltung, is composed of quartose sandstone and lesser amount of shale accompanying bioturbuted siltstone and lime stone which yields poorly preserved corals and molluscs.Also, carbonaceous shales containing coalified wood fragments is found.
Its total thickness of this formation is about 280m and is interpreted as the coastal plain to intertidal sediments of Paleocene to late Cretaceous age.
The attitude of the foliation plane is N88E/80SE.
Bhainskati Formation
The name of this formation was derived from the Bhainskati Khola (Sakai, 1983).This formation is well exposed along the Bhainskati Khola.
This formation consist largely of black shale with many thin calcareous and fissilliferousinter beds in the middle part and caricolored mainly green and red purple shale and red iron oolitic hematite beds in upper part. It consist of Black shale and is accompanied in the lower part with many thin calcareous interbeds yielding foraminifers of Eocene age along with abundant marine molluscs and some vertebrate. Hematite beds occur at several horizons in the upper part and extensively traceable, although the thickness of the beds and the iron content are locally varied.
The thickness is about 160m thick and is supposed to be of Eocene age. Because of its unique litho facies consisting largely of black shale containing abundant marine fossils, this formation is one of the most distinctive beds among the geologic sequence in the lesser Himalayas.
Attitude of the bedding plane is N88E/74SE.
Dumri Formation
The name Dumri was derived from the Dumre village (Sakai, 1983). This Formation represents the youngest unit, not only the Tansen Group but also in whole Lesser Himalayan rock sequences. This formation is well exposed along the Bhainskati Khola, Tinau Khola around Dumre, Masyam, Jorpipal, Olesaetc.Sakai (1983).
This formation occupies the top of the Tansen Group, overlies disconformily over the eroded surface of BhainskatiFormation.It is a thick series of fluvial deposits consisting inter bed of medium-grained , bluish grey to greenish grey sandstone and lesser amount of purple and green shale. Sandstone becomes much predominant and massive upwards. Strong bioturbation is common in shale and can be easily recognized by their mottled structures.The sandstone are quartzose wacke in composition. Several intraformtional shale-pebble conglomerate is also noted.Tree trunk is also found in is formation. (plate 2).
The thickness of this formation is about 725 m and the age assigned to these rocks is Oligocene to early Miocene.
Attitude of the bedding plane is N60W/77NE.
Kali Gandaki Supergroup
Geological map of the study area is shown in fig.6, and lithostratigraphy in Table
The Kali gandakiSupergroup consists of pre- Gondwana rocks, is divided into three groups; the Lower, Middle and Upper Groups(table.2) and in to nine Formations.The Lower Group is monotonous , thick sequence of predominantly argillite and argillaceous gritstone with lesser amount of quartzite and meta-basic rocks. The Middle is extremely varied sequence composed of quartzite, dolomite, limestone and phyllitic slate. The Upper is made up of thick dolomite , limestone and varicolourted calcareous slate.
The stratigraphic correlation of western Nepal Lesser Himalaya (Sakai, 1983) with that of central Nepal (Stocklin and Bhattarai, 1978) is shown in Table.
1.The Lower Kali Gandaki Group
This formation consists of monotonous, thick succession of alteration of phyllite, phylliticgritstoneand as lesser amount of quartzite. In Kali Gandaki area, they are more than 2 km thick and are composed of slate and phyllite. The rock are regarded as the lowest sequence of the Lesser Himalayan rocks in the lithological similarities to the Kuncha Formation, its great thickness incomparable to other formation and is having carbonate rocks which have enough thickness to be compared with the carbonate formation in the overlying two groups.
2.The Middle Kali Gandaki Group
The middle kali Gandaki Group is distributed in both the outer and inner belts. In the outer belt, this group crops out in three areas. The First is in the area between the Bari Gad Fault and the NBF of the TansenSynclonorium. The second is in a narrow belt between the MBT and the SBF, where the group forms a homoclinical structure of the southern limb of the TansenSynclonorium. The third is the palpaKlippe that tectonically overlies the Tansen Group in the central part of the Tansensynclonorium. The Middle Group is subdivided into five formation as compared with the monotonous sequence of the upper and Lower Grpup, and is characterized by arenaceous and calcareous rocks that are considered to have been deposited under arid shoreline environments.
3.The Upper Kali Gandaki Group
The Upper kali Gandaki Group is distributed around the Tansensynclonorium in the outer belt. It is normally distributed in the lower valley of the AndhiKhola in the inner belt.
This group consists of two formations; the Ramdhighat and the Kerabari in ascending order is a monotonous beds of dolomite attaining 2 km in thickness. The basal black limestone of the KerabariForaion is separated from the main part as the Ri-Rilimestine Member. The upper group ,as a whole characterized by thick carbonate rocks and its total thickness attains 2900m.
Andhi Formation
The name of this Formation is derived from the AndhiKhola (Sakai, 1985).
It is the oldest lithounit of Kali Gandaki Supergroup.This formation consist of thick monotonous sequence of greenis- grey to brownish-grey phyllitic slate with silver luster and with occasional interbeds of thin, calcareous siltstone up to 10 cm thick. The phyllite are thinly foliated. The attitude of the beds of phyllite and quartzite were more or less similar.
The thickness of this formation is about 2000 m belongs to Late Precambrian age is correlated with Kunchha Formation of Central Nepal (Stocklin and Bhattarai, 1978).
Attitude of the foliation plane is N40W/67NE.
Naudanda Quartzite
The name of this Formation is derived from the Naudada cliffs of Sangja district (Sakai, 1985).
This formation comprises a fine- to coarse-grained , white quartzite arenite with several interbeds of phyllite, conglomerate and metabasite. The quartzite are strongly rippled and cross-bedded. Oscillation type ripple mark suggest shallow marine environment. This formation is not visible due to our limited time.
The total thickness of this formation is about 400 m belongs to Late Precambrian age and is correlated with Fagfog quartzite of Central Nepal ( Stocklin and Bhattarai, 1977 and 19 ).
Attitude of the foliation plane is N85W/68NE.
Heklang Formation
The name of this Formation is derived from the Heklang village (Sakai, 1985).
This formation is composed mainly of dark-green phyllitic slate or phyllite with brown and grey tints caused due to weathering intercaliated with fine marl beds. Thickness of individual beds of phyllite ranges from 1 cm to 4 cm.Thephyllite is intercalated at several horizons with up to 10 meters thick layer that frequently contain fine calcareous, laminae and bands.This layer are associated with calcareous sanstone, marl and dolomite. Foliation plane are developed in the phyllite. The carbonate rocks often weather a coffee brown color. This formation can be exposed in Madanpokhari and Telgha of Tansen area.
The total thickness of this formation is about 800 m belongs to Late Precambrian age and is correlated with DandagaunPhyllite of Central Nepal ( Stocklin and Bhattarai, 1977 ).
At Bhalebas, attitude of the foliation plane is N60W/77NE.
Virkot Formation
The name of this Formation is derived from the Virkot village (Sakai, 1983, 1984, 1985).This Formation is repeatedly distributed by folding in the are between the TansenSynclinorium and the Badi Gad Fault.
This formation consists of white to pink quartzite and reddish-purple phyllitic slate interlayering with each other giving characteristiccolourbandingThe lower part of the formation is predominantly reddish-purple phyllitic slate, and the upper part comprises predominantly quartzose sandstone..
The total thickness of this formation is about 510 m belongs to Late Precambrian age and is correlated withNourpulFormation. This formation exhibits an upward-coarsening and thickening sequence as a whole.
At the southern limb of the synclinorium, attitude of the foliation plane is N35E/15NE.
Chappani Formation
The name of this Formation is derived from the Chappani village (Sakai, 1883, 1984, 1985).This Formation is distributed in to two to four belts, having been repeated by folding , in the area between Badi Gad Fault and the TansenSynclinorium.
At this location red purple colored shale are found which contains mud cracks and ripple marks. In the beds of slate domed shaped stromatolites are present which indicates the beds are overturned..The lower and middle part of the Chappani Formation comprise of black, light green and pink, calcareous laminated slate with many thin inter beds of stromatolites. The upper part contains quartzite and reddish purple and light green shale with same bed of shale and pebble conglomerate.
The total thickness of this formation is about 400 m belongs to Late Precambrian age and is correlated with Nourpul formation .
Attitude of the foliation plane is N75E/86NE.
Khoraidi Formation
The name of this Formation is derived from the Khoraidi village (Sakai, 1883, 1984, 1985).
This formation is unique in having a large number of stromatolites. The stromatolites are dome shaped, sub-spherical and dolomitic. The dolomite is grey white to ash color. The stromatolites in Khoraidi Formation are concave upward sequence but in normal sequence stromatolites are concave downward which indicates beds are overturned. (plate 3)
The total thickness of this formation is about 350 m is correlated with Dhading Dolomite of Central Nepal ( Stocklin and Bhattarai, ).
Attitude of the bedding Plane is N15W/36NE.
SaidiKhola Formation
The name of this Formation is derived from the SaidiKhola (Sakai, 1883, 1984, 1985). The Saidi Khola Formation repeatedly crops out by folding in the area between the Bari Gad Fault and NBF of the Tansen synclinonium .
The bed rocks contain chiefly of coarsely inter layered purple sandstone and grey shale. The shale are bioturbeted, which means they were acted by micro-organisms during their formation. This alternating thin beds of shale and sandstone is called rythmite. This rythmitic sequence of shale and sandstone indicates tidal deposits. They were deposited during high tide and low tide. The boulders present there are of dolomites which contains stromatolites with different structures of faulting and folding. Some of they even have mud-cracks and ripple marks also.
The total thickness of this formation is about 400 m.
Attitude of the bedding plane is N60E/49SE
Ramdighat Formation
The Ramdighat Formation is named from a village Ramdighat on the southern bank of the Kaligandaki (Sakai, 1983, 1984, 1985).This formation is located in the area between the Bari Gad fault and NBF of the TansenSynclonorium.
This formation consists of calcareous argillite and minor amount of thin limestone and coarse clastic rocks are extremely rare.
The total thickness of this formation is about 750 m is correlated with Benighat Slate of Central Nepal (Stocklin and Bhattarai). The Ramdhighat slate has possibly been deposited in a lagoon or large bay that was fringed with an intertidal flat with an algal marsh zone. The lithofacieschanges from the Saidikhola Formation to RamdhighatFormatiob indicates a transgression or sbbsidence of the depositional basin.
Attitude of the foliation plane is N5E/40SE
Kerabari Formation
The name of this Formation is derived from the Kerabari village (Sakai, 1883, 1984, 1985).The Kerabari Formation, the uppermost unit of the Kali Gadali Super group, is most extremely distributed in Tansen area.
The kerabari Formation conformably rest on Ramdighat formation. The basal black limestone is designated as Riri Member.
The total thickness of this formation is about 2100 m is correlated with Malekhu limestone of Central Nepal (Stocklin and Bhattarai).
At Kerabari, the attitude of the bedding plane is N65W/51NE
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