Geology & Structure

Tectonic Features

Tectonic Evolution

 

The Kumaon Himalayas

The Kumaon Himalayas, lying between the Kali River in the east and Sutlej in the west, include a 320 km stretch of mountainous terrain.  Much pioneer work was done at the turn of the century followed by more modern investigations between 1930 and 1940.

The almost complete absence of fossils in the Lesser Himalaya leaves many structural and stratigraphic problems unsolved, since the correlation is based on stratigraphy alone.  This casts some doubts on certain stratigraphic interpretations.  The Lesser Kumaon Himalaya include a thrust-bound sector delineated by two tectonic planes - the Main Boundary Fault to the south and the Main Central Thrust to the north.  Many workers have postulated existence of the regional inversion of sedimentary pile.  There are two elongate tectonic belts of sedimentary/metasedimentary rocks separated by an ESE-WNW trending Almora-Dudhatoli Crystalline zone.  T he Outer Sedimentary Belt to the south of the crystalline mass is the Krol Belt while the Inner Sedimentary Belt to the north constitutes the Deoban-Tejam zone (Gansser, 1964), or the Jaunsar-Berinag nappe (Valdiya, 1978).  A generalised tectonic sequence for the Lesser Kumaon Himalaya is tabulated below:

vaikrita Group
Vaikrita Thrust
Munsiari Formation
Main Central Thrust
Almora Dudhatoli Nappe
(and Askote, Baijnath, Chiplakot & Satpuli Klippen)

----------- Almora Thrust ---------

 Outer Sedimentary Belt
Inner Sedimentary Belt
Main Boundary Fault
Siwalik Group

 

Inner Sedimentary Belt

This extends from the Garhwal region in the west to beyond the Kali River marking the eastern boundary of the Kumaon Himalaya.  The oldest rock unit of this belt is termed the Hatsila Formation by Misra and Bhattacharya (1972, 1973).  This formation is correlated with the Rautgara Quartzites (Valdiya, 1964) and Rameshwar Formation (Ahmad, 1975).  The Hatsila Formation consists of slate, siltstone, greywacke, protoquartzite, phyllite, and limestone.  The arenaceous beds sometimes show sedimentary structures.

The rock unit overlying the Hatsila Formation is termed the Kapkot Formation.  Apparently it is the same as the main calcareous unit of the Calc Zone of Pithoragarh (Valdiya, 1962; Misra and Banerjee, 1968; Misra and Valdiya, 1969), the Calc Zone of Tejam (Heim and Gansser, 1939; Gansser, 1964), and the Kotaga Banali of Saklani (1971, 1978), as well as the Doya Dolomite (Misra and Bhattacharya, 1972) of Pugar valley and the Jhatkwali Formation (Mehdi et al., 1972).  The Formation includes stromatolite-bearing dolomitic limestone with magnesite, talc, chert, pebble beds and some slate/calcareous slate.  Dolomitization and recrystallization are common features of the limestone.  On the basis of the discovery of stromatolites (Valdiya, 1969; Kumar and Tiwari, 1977, 1978; Kumar, 1978; Bhattacharya, 1976a), a Middle Riphean age has been assigned to the rocks of this Formation.

Overlying the Kapkot Formation, the Saling Formation (Bhattacharya, 1980) is the same as, or directly correlateable with the Kanalichina Formation (Mehdi et al., 1972) of the Pithoragarh district and the Betalghat Formation (Raina and Dungrakoti, 1975) of the Bhimtal-Bhowali area in Nainital district.  The rock types include gray, green, and black slate and phyllite with subordinate calcareous interbeds.

The topmost horizon of the Inner Sedimentary Belt is the Berinag Formation which is seen surrounding the crystalline masses of the Almora, Askote, and Baijnath units.  The lithology consists of fine to coarse grained massive quartzite, often sericitic and schistose with pebble beds, chlorite beds, and interbedded metabasites.  Kumar (1978) has correlated the Berinag Formation with the Kaimur Formation of Upper Vindhyans.  Jain (1971) classified this unit as the Garhwal Group.  Based on field evidences, some workers (Valdiya, 1962, 1964, 1969, 1973; Misra and Kumar, 1968; Misra and Banerjee, 1968; Misra and Bhattacharya, 1972; Saklani, 1971, 1972; Jain, 1971; Pachauri, 1972; and Bhattacharya, 1976b) are of the opinion that a thrust plane separates the Berinag Formation from the underlying units, and that this sedimentary sequence is inverted.  Other workers (Heim and Gansser, 1939; Gansser, 1964; Mehdi et al., 1972; Banerjee and Bisaria, 1975; Ramji, 1976; Kumar and Tiwari, 1977; Kumar, 1978; and Bhattacharya, 1980) maintain that the entire sedimentary pile is in a normal position except for locally inverted sequences.

The Inner Sedimentary Belt has been folded into a few E-W to ESE-WNW trending folds.  At places there is strong evidence of thrusting and dislocation by a number of faults.

Outer Sedimentary Belt

Delineated in the south by the MBF (= Krol Thrust) and in the north by the Ramgarh Thrust, the Outer Sedimentary Belt includes a relatively less thick pile of sediments referable to the Krol Belt of the Garhwal-Simla Himalaya.  It occurs as a detached outlier, which widens westwards and extends upto Himachal Pradesh through Garhwal.  Bhattacharya (1980) considers the Outer Sedimentary Belt to rest conformably over the Inner Sedimentary Belt.  Hence the Outer Sedimentary Belt is considered the younger of the two.

The stratigraphic unit forming the base of the Outer Sedimentary Belt is the Nagthat Formation, which appears to correlate well with the Berinag Formation (Valdiya, 1978).  It consists of conglomeratic and pebbly protoquartzites, shales, slates, and basic rocks.

Overlying the Nagthat is the Blaini Formation which consists of two typical rock types; one is the boulder beds, the other the limestones, overlying the former.  The rock association is not constant and often the boulder beds or the limestones can occur alone, or several horizons of boulder beds are found together with the limestones.  These complications may be due to tectonic imbrication (Gansser, 1964).

The Infra-Krol Formation conformably overlying the Blainis consists of gray to black slates and shales with siltstone and quartzite bands.  Rarely blue limestone beds are seen.  The Krol Formation, which has been divided into three sub units viz., the Lower, Middle, and Upper Krols overlie the Infra-Krols.  The Lower Krols consist of shales and slates with dolomitic intercalations, gray shales with intercalations of limestone, dolomites, and minor red shale.  The Middle Krols have green to red, medium to fine grained sandstone with shaly and silty intercalations.  The Upper Krols include massive and thickly bedded dolomites, cherty and oolitic dolomite, and subordinate shale and quartzite interbeds.

The strata of the Outer Sedimentary Belt have been folded into a syncline on an E-W axis, the northern limb of which is open as compared with the southern limb.  The northern limb exposes all the rock units of the Outer Sedimentary Belt and is best exposed near Nainital (Bhattacharya, 1980).  The Outer Sedimentary Belt has been affected by a number of tear and normal faults of varying dimensions.

The Crystalline Units

The occurrence of scattered outcrops of older crystalline rocks is a typical feature of the Lesser Himalaya of Kumaon.  The crystallines consist of Almora, Baijnath, Askote, Chiplakot, and Satpuli units (Figure 2) and are characterised by a set of general features

  • Of the various younger sedimentary units, it is usually the Berinag Formation that underlies the crystallines.

  • Each of these units is dispersed in a synform, though severed by later folds, faults, and thrusts.

  • Each unit consists of meso- to kata-grade of metamorphic rocks together with granites and migmatites.

  • The rocks are metamorphosed equivalents of argillaceous sediments and display the phenomenon of inverse metamorphism in which rocks with low metamorphic grades occur at the base, while higher grade rocks lie higher up the sequence.

  • Metavolcanic rocks are generally seen at the contact of the crystallines with the sedimentaries.

  • Granitic gneisses and porphyries occur in the core of all these synclinal crystalline units.

  • A zone of chlorite schists is quite common along the base of all these crystalline units indicating retrogression (to epi-grade).

 The Almora Crystalline unit is delineated along the two flanks by what are known as the South Almora Thrust and the North Almora Thrust.  The rocks of the Almora Crystallines and other lithologically correlatable units of the Baijnath, Askote, Chiplakot and Satpuli units, have been considered by most workers, including some pioneers in Himalayan geology, to constitute thrust sheets or nappes pushed over the younger rocks from the north with their roots in the Central Crystallines of the Higher Himalaya (Heim and Gansser, 1939; Gansser, 1964; Ghose et al., 1974; Misra and Bhattacharya, 1976; Bhanot et al., 1977).  However, Saxena (1974), Saxena and Rao (1975), Misra and Sharma (1972), Misra et al., (1973), and Bhattacharya (1980) suggest that the crystallines do not occur as nappes with their roots in the Central Crystallines: instead they are autochthonous in nature with their root zone along the North Almora Thrust, and their present disposition is due to vertical uplift.  The various structural and tectonic considerations, supported by the correlation of radiometric age determinations of the granitic gneisses of the Central Crystalline Zone with those of the Crystalline sheets of the Lesser Himalaya (Bhanot et al., 1977), however, indicate that a great nappe of crystalline rocks was pushed southwards over the sedimentary rocks of the Kumaon Himalaya.  This crystalline sheet possibly extended as far as the Main Boundary Fault because the Satpuli Klippe is exposed very close to it (Figure 2).  Saxena (1977) considers a Permo-carboniferous age for this thrusting.  Raina et al., (1980) favour a Middle Miocene age.

The South Almora Thrust is a very low dipping feature, and convergence of overlying and underlying formations of almost similar lithological units has made its recognition and delineation quite difficult.  Some workers (Kumar et al., 1974; Saxena and Rao, 1975) deny its very existence.  Detailed structural studies by Merh and Vashi (1965, 1976), Vashi and Merh (1974), Ghose (1973), and other workers have conclusively established the existence of the South Almora Thrust which joins up with the North Almora Thrust at the northwestern closure of the vast plunging syncline in the western Nayar valley, west of Dudhatoli massif.  Valdiya (1978) and Bhattacharya (1980) are of the opinion that the metasedimentary sequence of the southern limb of the Almora synform is constituted of two thrust sheets, the upper one is called the Almora Nappe and the lower the Ramgarh Nappe (Pande, 1950) overlying the Outer Sedimentary Belt.  The Ramgarh Nappe is composed of mildly metamorphosed flyschoid rocks.  The delimiting Ramgarh Thrust is not discernible in the northern flank, being overlapped by the rocks of the Almora Nappe with which the Ramgarh Nappe forms an imbricating pair.  However, Raina and Dungrakoti (1975) deny the existence of the Raingarh Thrust.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


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