GEOLOGICAL SETTING C H Ap Ter II
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Chapter EH GEOLOGICAL SETTING C h ap ter II GEOLOGICAL SETTING Introduction : The Dras-Kargil area, which has been studied during the course of the present investigation, constitutes a part of the Ladakh Himalaya. It falls within the Indus-Tsangpo Suture Zone which for brevity is hereafter referred to as the Indus Suture Zone. In this chapter the geology of Ladakh is briefly reviewed and the field characteristics of the varied lithological units occuring in the Dras- Kargil area are detailed. Tectonic Framework of Ladakh: In the Kashmir region of northwestern Himalaya the NE-SW trending Indus Suture Zone lies between the Ladakh Range and the Zanskar Mountains. The tectonic framework and stratigraphy of this region has been described amongst others, by Thakur (1981), Honegger et al. (1982), Searle (1983). Three lithotectonic units, namely, the Zanskar Zone, the Indus Suture Zone and the Ladakh Plutonic Complex are recognised from the southwest to northeast in the Ladakh region. To the northeast of the Ladakh Plutonic Complex lies the Shyok Suture Zone and the Karakorum Batholith. To the southwest of the Zanskar Zone is the Central Crystalline Zone of the Higher Himalaya. The spatial 7 distribution of these tectonic zones is shown in the regional geological map (Fig.2.1) which has been modified after Thakur (1981). The tectono-stratigraphy of the Ladakh Himalaya, as reconstructed by Thakur (1981, op. cit.), is given in Table 2.1. It is to be noted that in this table Thakur has shown the Ladakh Plutonic Complex to be a part of the Indus Suture Zone; a position that is not generally accepted. In the Dras-Kargil area lithounits of the Zanskar Zone, the Indus Suture Zone and the Ladakh Plutonic Complex are exposed. Hence a brief review of these three tectonic units is given below. The Zanskar Zone : The Zanskar Zone, which corresponds to the Higher Himalaya of Kashmir comprises of the Zanskar Crystalline Complex, the Zanskar Supergroup and the Tso Morari Crystalline Complex. The Zanskar Crystalline Complex consists of metasedimentary rocks, gneisses and migmatites, which show greenschist - granulite facies metamorphism and granites. The Rb-Sr ages of upto 1830 Ma suggest that they represent a reactivated crystalline basement (Thakur, 1981). The Zanskar Supergroup (Nanda and Singh, 1977; Srikantia et al., 1978), having a thickness of over 9000m, consist of a fossiliferous sequence of Upper Proterozoic and Paalaeozoic sediments with Permian volcanics and Mesozoic Eocene carbonates. The Tso Morari Crystalline Complex includes metamorphosed Palaeozoic sediments of the Zanskar Supergroup. 8 Fig. 2.1 MAP SHOWING REGIONAL GEOLOGY OF LADAKH HIMALAYA. (modified after Thakur. 1981 ) The Indus Suture Zone : This is a narrow belt of rocks averaging 20 kms in width and extending over a distance of about 400 kms in Ladakh, that is quite distinct from the crystalline units on either side. It comprises of flysch and molasse sediments, plutonic-volcanic rock associations that are characteristic of a magmatic arc, and ophiolite and ophiolitic melanges. The rocks of this belt were first studied by Stoliczka (1866) and Lydekker (1883). Subsequently their main characteristics were described by Dainelli (1934), Auden (1935), Wadia (1937) and Berthelsen (1953). More recently a wealth of information has been made available through the publications of Mamgain and Itao (1965), Pande et al., (1969), Raiverman and Mishra (1974), Gansser (1974, 1977, 1979), Shah et al., (1976), Varadarajan et al., (1980), Thakur (1981), Honegger et al., (1982), Rai and Pande (1982, 1983), Searle (1983), Radhakrishna et al., (1984, 1987), Varadrajan and Vashist (1986), Dessai et al., (1986), Anand (1989), Karmalkar et al., (1989), and Varadarajan and Powar (1989). The litho-stratigraphy of this belt has been recently described by Thakur (1981) and Searle (1983). Their classifications are given in Table 2.2. The stratigraphy proposed by Thakur (1981) is followed here. Lamayuru Formation : It comprises of shales, siltstones and graded sediments with flute casts which sometimes have intercalations of red radiolarian cherts. It has a thickness of about 1000 - 1500 m. Large blocks and lenticular bodies of limestones of Permian age, (Tewari and Pande, 1970), occur as exotic blocks 9 Tabic 2.1 : Stratigraphy of the Indus Suture Zone, Ladakh Himalaya. Thakur(1981) Searle (1983) Age Kargil Formation Indus Group Conglomerates and Kargil Formation Neogene sandstones with occasional Fine grained sandstones, shales siltstones and shales --------------- T h ru st-------------- Bazgo Formation : Red Cretaceous to Early sandstones and shales Eocene Indus Formation : Clastic Hemis Formation : Well sediments, conglomerates, bedded conglomerate sandstones, siltstones and shales with bands of Upshi Formation : limestone and dolerite sills Conglomerate -------------- Thrust-------------- -------------- Thrust--------------- Shergol Melange: Shergol Ophiolitic Melange : Middle to Late Serpentinites, dunites, Serpentinised harzburgites Cretaceous peridotites within jasperoid and dunites, gabbros and shales, cherts, pillow lavas sheared volcanic rocks with and basic rocks deep sea sediments. _________ T h ru st__________ -------------- T h ru st-------------- Dras Formation : Andesitic Dras Volcanic Group Middle to Late and basaltic lavas with Tholeiitic to andesitic Cretaceous pillow structure, radiolarian volcanic and volcano-clastic chert, jasper and limestone sediments with limestone blocks -------------- T h ru st--------------- -------------- Thrust-------------- Lamayuru Division : Lamayuru Complex : Shales, Triassic to Cretaceous Shales, siltstones and sandstones, turbidites with graded sandstones, blocks intercalations of radiolarian of limestone of Permian cherts. Exotic limestone age. occurs within the complex Note: The Nidar Complex is the equivalent of Shergol Melange and not shown as a separate unit. 10 within the Lamayuru Formation and these have been designated as Mulbek 'exotic' limestones by Searle (1983). On the basis of reported Daonella sp., bivalves, crinoids and foraminifera recorded by various workers (Frank et al., 1977; Shah and Sharma, 1978; Fuchs, 1977), the Lamayuru Formation is assigned a Triassic to Cretaceous age. Dras Formation : This consists of a thick ( over 3000 m) sequence of volcanic rocks with associated volcano-clastic sediments. The volcanic rocks are mainly andesitic and basaltic flows that show local association of pillow lavas and also rare rhyolites. The volcano-clastic association includes agglomerates, radiolarian cherts, jaspers and limestones. The proportion of sedimentary rocks progressively increases from west to east, and in the eastern part shales and sandstones also occur. The limestones have yielded Orbitolina, Ilippurites and bryozoa which indicate a Middle to Late Cretaceous age. Shergol Melange : This unit consists of bodies of serpentinite, peridotite, dunites and gabbros that occur in association with shales, cherts, pillow lavas and basic rocks. Microfauna of Middle to Late Cretaceous age has been reported from the sedimentary horizons by Shah and Sharma (1 977). Equivalent horizons in the eastern part have been designated as the Nidar Ophiolite and Zildat Ophiolite Melange (Thakur and Virdi, 1979). Varadarajan and Vashist (1986) point out that the ultramafic-mafic rocks that are the most important constituent of the Shergol Melange, often occur as intrusive bodies within the Dras Volcanics, and probably represent the last phase in the evolution of the Indus Suture Zone. 11 Indus Formation : It consists of clastic sediments (conglomerate, sandstone, siltstone and shale) having a stratigraphic thicknes of about 4000 m. The sediments are intruded by sills of dolerite. This formation has been interpreted by Pal et al. (1978) as being a flysch sequence of Cretaceous to Early Eocene age. Kargil Formation : This formation which rests directly and transgressively on the Ladakh Plutonic Complex, consists of conglomerates and sandstones with occasional shale bands. It is a molasse sequence which has yielded fresh water molluscs and Plant remains (Sahani and Bhatnagar, 1958), and also vertebrate fossils (Dixit et al., 1971) indicative of Neogene age. The Ladakh Plutonic Complex : The Ladakh Plutonic Complex constitutes the 30 - 50 km wide Ladakh Range to the north of the Indus Suture Zone. It largely consists of granites, granodiorites, with which are associated pyroxenites, hornblendites, malic rich gabbros, norites and anorthosites. According to Honegger et al., (1982) the complex consists of 50% of rocks of granodioritic composition, 20 - 30% of intermediate rocks and 10 - 20% of mafic intrusives. The granodiorites are also found to intrude the Dras Volcanics of the Indus Suture Zone. Pyroxene bearing pegmatites and hornblende bearing micro-dioritic sills intrude the Ladakh Plutonic Complex. Radiometric dating of the Ladakh Plutonic Complex suggests a Late Cretaceous to Eocene age (Searle, 1983). 12 Geology of the Dras-Kargil Area: Geological mapping carried out by the author shows the presence of a variety of sedimentary and igneous rocks in the Dras-Kargil area. These include limestones of the Zanskar Zone, litho-units of the Indus Suture Zone and igneous rocks of the Ladakh Plutonic Complex. The spatial distribution of the important litho-units is shown in the geological map (Fig. 2.2). The field characteristics of the various rock types are described below. Zanskar Zone : In the area mapped during the course of the present investigations