SEDIMENTOLOGICAL STUDIES OF THE INDUS SUTURE ZONE ROCKS, LADAKH, NW-HIMALAYA, INDIA, WITH SPECIAL REFERENCE TO THEIR SOURCE ROCK EVALUATION FOR PETROLEUM

Abstract

The Indus Suture Zone is a major crustal lineament between Himalayan range and Tibet. It runs for about 2500 km from Hindukush Mountain of Afghanistan to the Mishmi Hills of Assam in India. This important tectonic belt has been interpreted as a collisiion boundary between the Indian plate in the south and the Eurasian plate in the north and is considered to be the site of Cretaceous subduction zone along which a large portion of pre-Tertiary Tethyan ocean crust was consumed (Gansser, 1977; Bally et al., 1980; Shackleton, 1981). Although it has attracted many earth scientists in the recent years, only very limited sedimentological studies have been done so far. This is mainly due to very poor and difficult accessibility together with very inclement cold and arid climatic conditions of this difficult terrain which is covered for most part of the year by snow. On the basis of these limited sedimentological studies, various workers have assigned to these sediments different depositional environments such as shelf and littoral (Pal at al.,1975), shallow marine and gentle slope (Srivastava et al., 1978) and the upper part of submarine fan and distal turbidites (Brookfield and Andrew-speed, 1984). There exists, therefore, controversy among the geoscientists regarding the depositional environments of these sediments. Pre-Teriary Tethys ocean is believed to have an important influence on the accumulation and distribution of pertroleum in the world (Bois et al.,1982). It appears that the major oil fields of the world are associated with the subduction zones. This subduction zone, associated with the Indus Suture, therefore, becomes attractive from this angle also. The present studies were, therefore, taken up to bridge the gap in our knowledge about the sedimentational environments with a view to evaluate source rock potential for petroleum in the Ladakh area of the Indus Suture Zone in the NW Himalaya, Kashmir, India. These investigations are based on detailed field and laboratory work, which include outcrop mapping, vertical and lateral variation in lithology, sedimentary structures, paleocurrent, provenance, petrographic studies of sandstone and clay mineralogical studies of shale using X-ray technique, and petroleum source rock evaluation using various geochemical and geoptical techniques. Based on lithological character, vertical and lateral continuity and lithological associations, various lithostratigraphic units namely, the Lamayuru Formation, Nindum Formation, Indus Formation, Kargil Formation, Ladakh plutonic complex and Khardung Formation, have been identified. The present study is confined to the sediments of the Lamayuru, Nindum and Indus formations exposed in this area of investigation. The Lamayuru Formation (Triassic - Jurassic) consists mainly of 1000 - 1500 m thick calcareous and non^-calcareous greenish, greenish grey coloured shale, interlayered with thinly bedded, fine grained, grey coloured sandstone.

The Nindum Formation (Cretaceous) is stratigraphically 3000 m thick. It overlies the Lamayuru Formation with a thrust contact. It consists of volcanic and associated sedimentary sequence of medium to fine grained sandstone alternating with grey coloured shale. The proportion of sedimentary rocks in the Nindum Formation increases eastward. The Indus Formation (Early Cretaceous to Early Eocene), with stratigraphic thickness of about 4000 m consists mainly of grey coloured, medium grained sandstone, grey coloured siltstone, dark grey coloured shale and conglomerate. Sandstone and shale form the bulk of sedimentary rocks in the area. Sandstones found in various formations have similar characters. Petrographic studies of these sandstones indicate that they consist of quartz, feldspar and rock fragments, with chloritic and quartz - rich matrix material. Quartz occurs both as monocrystalline and polycrystalline grains. Secondary overgrowth is observed in some grains. Quartz is most abundant in the sandstones of the oldest Lamayuru Formation, followed by Nindum Formation and the youngest Indus Formation. Feldspar is the second most abundant detrital constitutent of the sandstones of various formations and is mainly potassic. The plagioclase feldspars are more sodic (Ab80 - Ab90). The feldspars are altered, in general to sericite. The grains show Vll varying degrees of alteration, majority of these are sericitized. Rock fragments form the third major constituents of these formations, specially the Nindum and Indus. Rock fragments found in these formations are volcanic, metavolcanic, sedimentary, metasedimentary and plutonic igneous rocks. The volcanic and metavolcanic rock fragments are felsic, microlitic, lathwork, vitric and glass shards of zeolite and feldspar occuring in microcrystalline aggregates. Sedimentary and metasedimentary rock fragments are mainly shale, carbonates, siltstone, chert along with slate, phyllite, quartz-mica tectonite and the Plutonic igneous rock fragments are of mainly granite. The sandstones of the Lamayuru Formation are virtually devoid of rock fragments. When present, they occur in traces as sedimentary rock fragments. Sandstones of the Nindum Formation are richer in sedimentary and metasedimentary rock fragments with little volcanic rock constituents. The Indus Formation has a host of rock fragments of various types - volcanic, sedimentary, metasedimentary and granitic. Howver the proportion of volcanic to sedimentary rock fragments increases from eastern to western sections of the Indus Formation. The sandstones from the Lamayuru Formation are mainly quartzose, subarkose and arkosic wacke. Sandstones of the Nindum Formation are classified as feldspathic (arkosic) wacke, lithic wacke and lithic arenite and the sandstones of the Indus Formation are classified as feldspathic (arkosic) wacke viii feldspathic arenite, lithic wacke and lithic arenite in various sections. Shales of various formations have been studied for their clay minerals. Based on X - ray studies, it has been observed that kaolinite, illite and chlorite are the main clay minerals of various formations. Kaolinite is the chief mineral in all formations. it increases from older Lamayuru Formation through the Nindum, to the youngest Indus Formation. These sediments exhibit four catagories of sedimentary structures namely erosional, depositional, depositional - defomrational and deformational. Erositional structures consists of different types of sole marks such as flute cast, groov cast and longitudinal furrow and ridges. Depositional structures consists mainly of graded bedding, parallel and cross laminations and cross bedding. Deformational structures include, load casts, ball and pillow and slump structures. Convolute laminations are the main depositional - deformational structures. In general, the paleocurrent direction is found mainly to be of unimodal azimuthal pattern, indicating unidirectional paleoflow, mainly towards NNW and NNE with low variability. Following the approaches of Blatt and Christie (1963), Young (1976) and Dickenson (1979) based on petrographic constituents of sandstones, together with the paleocurrent directions, it is inferred that the Lamayuru Formation received sediments mainly from southerly - located argillites, arenite and carbonate rocks

of Zanskar Supergroup. The Nindum Formation derived its sediments mainly from the southerly - situated older sediments of Zanskar Supergroup and the Lamayuru Formation. The Indus Formation has two provenances. Whereas the southerly located Dras volcanic is the main source of sediments of the Indus Formation, the northerly situated Ladakh Granites also contributed to it. The plate tectonic settings of the depositional areas have been attempted using petrographic approaches of Dickenson and Suczek (1979) and Ingersoll and Suczek (1979). Petrographic constituents in terms of framework mode (QFL) and partial mode (Qm, F, Lt) when analysed using above methodology indicate that the Lamayuru Formation had a continental block provenance, the Nindum Formation had sediments deposited as the accretionary prism with collision orogen provenance and the Indus Formation had back- arc and accretionary prism setting with magmatic arc provenance. The sediments of the Lamayuru Formation (Triarsic - Jurassic) were deposited in the outer slope of trench formed due to convergence of Indian plate towards the Eurassian plate. With the subduction and formation of Dras volcanic arc, the Tethys basin was subdivided into southern eugeosynclinal basin in which the sediments of the Nindum Formation (Cretaceous) were deposited and the northern miogeosynclinal basin in which Indus Formation (cretaceous - Eocene) was formed. Thick pile of sediments exhibiting alternation of sandstone with shale, their large areal extent, frequent occurrence of graded bedding, sole marks like flute cast and load cast and tool makrs like groove casts are common features of all the formations. On the basis of these characters together with the occurrence terrestrial organic sediments in the marine inorganic sediments and the unimodal nature and pattern of paleoccurrents, these sediments have been identified and recognised as submarine fan turbidites. In view of the association of major oil/gas fields with the subduction zones, namely the Zagros subduction zone of Middle East, the Indo-Burma subduction zone of Burma and the Indonesian subduction zone, together with the occurrence of petroleum in turbidite sediments in many parts of the world, an attempt has been made to evaluate the prospects of petroleum generation from the argillaceous sediments of turbidites found in this suture zone as well. Various geochemical and geooptical techniques, were used for characterization and evaluation of source rocks for petroleum by determining the amount of organic matter, the type and the nature of organic matter and thermal maturation of organic matter, found in the argillaceous sediments of various formations of Indus Suture Zone.

The mean total organic carbon (TOC) contents in the Lamayuru, Nindum and Indus formations are 1.24%, 0.94% and 1.91% by weight respectively. All these values are more than the minimum critical limit of 0.5% TOC, indicating thereby that these sediments have sufficient organic matter to be of importance in term of source rock potential for petroleum. The nature or the type of organic matter has been determined by analysis of extractable organic matter, gas chromatographic analysis of saturate (C 15+), vitrinite reflectance, optical examination of organic matter and stable carbon isotopic analysis. Higher percentage of aromatic as compared to saturated hydrobones, predomance of odd normal - alkane over even ones, (C.P.I more than 1) and ratio of pristance to paytane (mainly more than 1) indicate land derived type III with minor amount of type II kerogen for organic matter of these sediments. This is further suported by presence vitrinite, fusinite, cuticte and woody organic matter in shales of various formations. Maturation of organic matter found in these sediments was determined on the basis of vitrinite reflectance, thermal alteration index and gas chromatographic analysis of C 15+. The mean vitrinite reflectance values vary from 0.6% to 1.7% in the Indus Formation, from 1.44% to 1.51% in the Nindum Formation and from 1.34% to 1.44% in the Lamayuru Formation. These values indicate that organic matters in these formations have undergone sufficient thermal maturation to generate petroleum hydrocarbons, mainly the gas. Thermal alteration index varies from 3.0 to 3.5 in the Lamayuru, from 3.0 to 3.75 in the Nindum and from 2.25 to 3.5 in the Indus formations. These values also indicate that the organic matter of these sediments are thermally matured. Low (< 50 mg/g) extractable hydrocarbon to organic carbon ratio, predominence of resolved peaks of saturates over humps and isoprenoid peaks and dominance of prestane over phytane further corroborate the deductions that these sediments have undergone sufficient maturity to produce mainly hydrocarbon gas with little oil. A perusal of these source rock parameters indicate that the sediments of Indus Formation are more prospective than these of the Nindum and Lamayuru formations. This study, in short, indicates that the sediments of Indus Suture Zone are mainly submarine fan turbidites deposited in the outer slope of trench, eugeosynclinal and miogeosynclinal basins which were formed due to the convergence of Indian plate towards Eurasian plate. The sediments have generated petroleum hydrocarbons, specially natural gases and need further exploration of traps.