Chapter 1

Introduction and Geology

1 Introduction & Geology

1.1 The Himalaya

The “Himalaya” (“abode of snow” in Sanskrit) is one of the most prominent, youngest (c. 30 Ma

old) and highest mountain ranges on the earth with an average elevation of 6100 m in the Higher

(Great) Himalayan belt, and comprises 9 of the 14 tallest peaks of the world (e.g., Everest 8,848m,

Kanchenjunga 8,598m, etc.). It is one of the most active intracontinental mountain range in the

world (Lave and Avouac, 2000). The “Himalayan Terrain” also called the ‘Asian upland’ is spread

over an area of 3.4 million km2 (between latitudes 20°-38°N, and longitudes 63°-104°E) covering

Afghanistan, Pakistan, India, China, Nepal, Bhutan, Bangladesh and Myanmar (Fig. 1.1). It forms a

massive and conspicuous 2400 km long and 250-300 km wide arcuate mountain belt (framing the

Indian subcontinent) with peculiar northwest (Nanga Parbat) and northeast (Namche Barwa)

‘syntaxial bends’ (Wadia, 1931; Valdiya, 1980 & Sorkhabi, 1999).

The Himalaya has aroused the scientific curiosity and inquiry of geologists for centuries, world-

wide, amply demonstrated by the pioneering studies undertaken by Strachey (1851), Medlicott

(1864), Oldham (1883) and Middlemiss (1885, 1887 & 1890) amongst others. In later period

critically acclaimed scientific work by Pilgrim and West (1928), Auden (1934), Heim and Gansser

(1939), Carey (1955), Gansser (1964 & 1966) and Valdiya (1964) heralded the future of Himalayan

studies.

The advent of Plate Tectonics catalysed the Himalayan research, sparking a renewed interest in the

understanding of the Himalayan geology with prominent scientific contributions during the last few

decades by Saklani (1970 & 1971); Dewey and Burke (1973); Crawford (1974); Pande (1974);

Valdiya (1975, 1978 & 1980); Le Fort (1975); Powell and Conaghan (1973 & 1975); Fuchs (1975

& 1981); Molnar and Tapponier (1975); Stocklin (1977); Raina (1978); Klootwijk (1980) &

Klootwijk et al. (1986); Seeber (1980); Andrews-speed and Brookfield (1981); Jain (1987); Ranga

Rao et al. (1988) and more recently by Thakur (1992); Powers et al. (1998); Myrow et al. (2003);

DiPietro (2004); Célérier et al. (part 1 & 2, 2009); Brookfield et al. (2010); Meigs et al. (2010); and

Shellnutt et al. (2011, 2012, 2013) which have yielded valuable scientific information and are of

immense importance in the Himalayan studies.

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Figure 1.1: Tectonic map of Himalaya (modified after DiPietro, 2004)

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The Himalayan range is divisible into distinct litho-tectonic (tectono-stratigraphic/ tectono-

geomorphic) zones descernible from the southernmost extremity in contact with Indo-Gangetic

alluvial plain to the northern contact delimited by the Indus Tsangpo Suture Zone (ITSZ) with the

Tibetan Block towards the north. From south to north the designated zones are; 1) Sub Himalayan

Zone (SHZ), which overrides the Indo-Gangetic alluvial plain along the Main Frontal Thrust (MFT)

= Himalayan Frontal Fault (HFF) or Himalayan Frontal Thrust (HFT) and Salt Range Thrust (SRT)

in Pakistan, 2) Lesser Himalayan Zone (LHZ) delimited from the SHZ by Main Boundary Thrust

(MBT), 3) Central Crystalline Zone (CCZ) or Higher Himalayan Crystalline Zone (HHCZ)

separated from the LHZ by the Main Central Thrust (MCT) and 4) Tethyan Sedimentary Zone

(TSZ) overlying the CCZ towards north, the contact between the two marked by a low angle normal

fault, i.e., South Tibetan Detachment System (STDS) (Burchfiel et al., 1984) (Figs 1.1 & 1.2). The

MBT, MCT and MFT are considered to be splays from the STDS (Hirn et al., 1984; Ni &

Barazangi 1984; Seeber et al., 1981). Valdiya (1980) considered the MCT and MBT to be two

longitudinally continuous intra-continental boundary thrusts in the Himalaya. In the frontal part of

the Himalayan mountain belt, the SHZ and LHZ together constitute a fold-thrust belt (FTB). The

geology of this area has been known for a long time (Medlicott 1864; Pilgrim and

Figure 1.2: Tectonostratigraphic/ tectonogeomorphic zones of the Himalaya. “Ages in brackets depicting the peak activity time along the thrusts”

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West 1928 and others), but the structural evolution and stratigraphy has not been well understood.

The geological investigations in the Himalayan FTB have gained immense scientific significance

especially for the purpose of conventional and unconventional hydrocarbon exploration (Bhat et al.,

2012).

1.1.1 The Sirban Limestone Formation

In the Sub-Himalayan foothill fold-thrust belt of the Jammu region (Jammu and Kashmir State),

Proterozoic Sirban Limestone Formation (Fm) [also known as Great Limestone (Medlicott, 1864 &

1876); Vaishno Devi Limestone (Rao et al., 1968); Shali Dolomite (Fuchs, 1975), Trikuta

Limestone (Chadha, 1979 & 1992) and Jammu Limestone (Raha, 1972 &1980; Raha and Sastri,

1973)] is an allochthonous unit cropping out in detached inliers (viz. (i) Dandili-Devigarh, (ii)

Kalakot-Mahogala, (iii) Riasi and (iv) Dhansal-Sawalkot (Lopri) inliers, (Fig. 1.3) and forms

prominent high mountain ranges amongst the low-lying Cenozoic hills. The Sirban Limestone Fm

has its coeval equivalents laterally disposed in the west in Salt Range, in the northwest in

Muzzafrabad-Punch area of Pakistan and in the northeast in Dharamshala and Himachal Pradesh,

India (Bhat et al., 2009; Dey, 2011; Hakhoo et al., 2011 & Craig et al., 2013) (Fig. 1.3).

Figure 1.3: Regional geological map of the foothills of the NW Himalaya of India and adjacent areas of

Pakistan showing the distribution of inliers of the Sirban Limestone Formation (Bhat et al., 2009 and

Hakhoo et al., 2011).

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In the NW Himalaya, the Sirban Limestone Fm crops out (south of the MBT) over a length of about

80 km and a width of about 8-20 km, and forms the ‘basement’ to the Sub-Himalayan Cenozoic

sedimentary sequence (Murree and Siwalik strata). These Cenozoic strata have been folded and

faulted during Middle Miocene and Late Pleistocene orogenic movements (Raha, 1976). On the

basis of lithological similarity and their unfossiliferous nature, the Sirban Limestone Fm was

correlated with the limestones of Mt. Sirban in Abbottabad (Hazara, Pakistan) and named as

“Sirban Limestone” (Wright, 1906 and Middlemiss, 1929) -- the term deeply engraved in literature

and in vogue at present.

In Jammu, the best sections of the Sirban Limestone Fm occur in the vicinity of Anji, Bakkal,

Bhawan, Bidda, Darabi, Dhyangarh, Jyotipuram, Katra, Muttal, Ransuh, Riasi, Sangar, Salal and

Talwara localities in the Riasi Inlier, c. 70 km north of Jammu city (Fig. 1.4). The Sirban Limestone

succession consists of thickly bedded, highly jointed, hard and dark to light grey and pinkish-blue

(silicified) dolostone, limestones characterised by microbial mats and stromatolites (quite resistant

to erosion and physical weathering) and are interbedded with thin chert and shale beds and

occasional 15-20 cm thick oolitic limestone and tempestite (storm deposit) beds along with fine

laminations and lenticular arenaceous layers. There are also quartz-arenitic and quartzitic beds at

some stratigraphic levels. Some siliceous chert bands, nodules and lenses also occur in this thick

Figure 1.4: Local geology of the Riasi area and the key outcrop localities of the Sirban Limestone Fm.

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dolostone sequence. Slightly metamorphosed carbonaceous shale beds (c. 10m thick) (reported

previously as intercalated dark marls, shales or slates, e.g., Fuchs, 1975) are also present in some of

the sections (e.g., Anji, Bidda-Talwara and Panthal) and siliceous ‘Red-bed’ and mineralization

zones (lenses) occur in others (e.g., ‘Red bed’ in Salal-Kanthan, Anji-Garadhar and Batalgala-

Bakkal areas/ sections; Magnesite in Panthal area).

In the order of succession, the Sirban Limestone Fm represents the oldest sequence in the region,

but its age cannot be assigned on account of two factors; the absence of any age diagnostic fossil

(as of today) and the absence of exposure of the base of the sequence, creating problems in placing

this succession in the proper stratigraphic order. Formerly, in Abbottabad (Hazara, Pakistan) the

Sirban Limestone Fm was known as belonging to “Infra-Trias series” (Middlemiss, 1896).

However, Wadia (1937) assigned a Permo-Carboniferous age to these limestones on the basis of

some volcanic rocks he found in the basal part of the Devigarh Inlier (Jammu), which he correlated

with the 'Agglomeratic Slate' of Kashmir associated with fossiliferous Permo-Carboniferous

horizons. Additionally, the upper part (cherty limestones) of the Sirban Limestone Fm in

Abbottabad was assigned to “Infra-Trias” Group (Marks & Ali, 1961). Mostler (in Fuchs &

Mostler, 1972) suggested a Cambrian age of the fossils found in the Hazira Formation, implying a

Precambrian (or Early Cambrian?) age for the underlying Sirban Dolomite. In the Riasi Inlier, Raha

(1980a) suggested the age of this succession to range from Palaeoproterozoic to Neoproterozoic on

the basis of stromatolite biostratigraphy and an imprecise date of c. 967 Ma (Early Tonian) for a

galena sample from intercalated quartz arenites has also been proposed (Raha et al., 1978).

However, recently, Venkatachala and Kumar (1996) and Koul (2006) reported a Late

Neoproterozoic microflora assemblage from these sediments, which support the age assigned by

Raha et al. (1978) and Raha (1980b). The recovery of Melanocyrillium sp. and other palynomorphs

from different sections of the Sirban Limestone Fm in the Riasi Inlier suggest that the Sirban

Limestone Fm is Neoproterozoic (c. 850 Ma) (Bhat et al., 2009) in age. To place the palynomorph

bearing intervals into stratigraphic order is of paramount importance for correlation of these

sections. However, the structural complexity and the monotonous nature of the lithology

(limestones and dolostones interbedded with occasional black shale) limit the scope for correlation.

These carbonates have a stratigraphic significance both from the abruptness of their relief and form.

The structural style and tectonic evolution of the Riasi Inlier has posed a riddle for the last 130

years and has been analysed by many scientists, notable among others are Medlicott (1876),

Lydekker (1876), Simpson (1904), Middlemiss (1928), Wadia (1937), Rao et al. (1968),

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Chadha (1979, 1992). The direction and amount of dip varies within the inlier. Raha (1976)

reported various structural features including simple folds, complex curvatures of anticlines and

synclines in the Sirban Limestone Fm and near Katra he reported tightly compressed limbs of

anticlines and synclines associated with reclined folds. Wadia (1928 & 1937) recognised the major

thrust in the region separating the Eocene Subathu Group (Gp), the Mio-Pliocene Murree Group

(Gp) and the Sirban Limestone Fm inliers from the Plio-Pliestocene Siwalik Gp as the Main

Boundary Fault (MBF), consisting of an en echelon array of more or less parallel faults within the

Tertiary Zone of the Outer Himalaya. Wadia (1928 & 1931) also placed Boundary Fault (BF)

between the Murree Gp and the Siwalik Gp in Jammu–Punch region. Between Muzzafarabad and

Nahan (Himachal Pradesh, India), later workers have designated the MBF as the Jammu Fault

(Gansser, 1964), Vaishno Devi Thrust (Rao et al., 1968), Riasi Thrust (Karunakaran et al., 1979,

Raiverman et al., 1994 & Hakhoo et al., 2011) in Jammu, Palampur Thrust (Karunakaran et al.,

1979 & Raiverman et al., 1994) in Kangra, Bilaspur Thrust (Raiverman, 2002) in the Simla Hills

and Nahan Thrust in Sirmur. However, Thakur et al. (2010) advocated to collectively designate the

MBF and related faults as the Medlicott–Wadia Thrust (MWT) extending from east of the Hazara–

Kashmir syntaxis to Yamuna River, covering a distance of c. 700 km. But, for simplicity and to

avoid confusion we have retained the term Riasi Thrust, which has significant structural bearing on

the Riasi Inlier, where the Riasi thrust has brought the southern flank of the Sirban Limestone Fm in

direct contact with younger Siwalik Gp, resulting in stratigraphic inversion. The pattern of joints in

the limestone is parallel, at right angles and inclined to the bedding. Middlemiss (1891 & 1928),

Wadia (1937) and Srivastava (1956) primarily studied and observed several unconformities (now

recognised as tectonic contacts, Hakhoo et al., 2011, Bhat et al., 2012), both of local and regional

importance, in these limestones. Numerous views have been expressed by early workers regarding

the structural form of the Riasi Inlier; Middlemiss (1929) considered it as an elongated anticlinal

dome, Wadia (1937) considered it an anticline with a system of folding with broad and open types

of anticlines and synclines without any considerable compression, Auden (1944) regarded this

structure as an asymmetrical dome, Banerjee (1959) discarded the domal character of the inlier,

Mehta and Srivastava (1959) mapped the area and strongly supported its non-domal structure,

Gupta and Kapoor (1962) considered it a WNW plunging anticline, the southern limb of which is

traversed by a thrust, which was supported by Gupta and Dhall (1962) and then Dhall and

Srivastava (1964a and 1964b). Until now, the field observations have suggested that it is not a

symmetrical dome but an elongated (doubly-plunging) anticlinal dome (Chadha, 1992).

Investigations regarding the emplacement of the inlier have been carried out

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by many workers. Raina (1964) considered the Riasi Inlier to represent a klippe which has been

formed either due to erosion or gliding or detachment of the mass from the original nappe. Valdiya

(1964), in Chadha, 1992, considered it to be a “Tectonic Window”; Rao et al. (1968) thought it to

be a Parautochthon (named as Vaishnodevi Parautochthon). The inlier might have occupied its

present position through sliding over the folded Neogene and hence obliterating and masking the

Neogene structure, as advocated by Dasarathi (1968). However, analysis and synthesis of structural

data and deformation structures observed in this study are indicative/ suggestive of compressional

(thrust) tectonics and antiformal stacking that has generated the present configuration of the inlier.

As envisaged previously, the Sirban

Limestone Fm (capped by Orthoquartzite

unit) is thought to be unconformably

overlain by the Eocene Subathu Gp (coal

seams, carbonaceous shales, nummulitic

limestone) forming the autochthonous

basement of the Cenozoic sediments.

This suggests a period of non-deposition

and erosion spanning over c. 542 Ma

(marked by the presence of breccias,

iron-stone shale and bauxite, with the top

1-2m being pisolitic) between the two

(Table 1.1). The origin of breccias and bauxite is not well understood, albeit some views have been

put forth in this context. Wadia (1928) presented his views on the origin and occurrence of bauxite

and chert breccia between the Nummulitic Limestone (Eocene) and the Sirban Limestone Fm in

Jammu region. He recognized two facies within the Eocene strata, i.e., i) Hazara facies and ii)

Subathu facies. Little attempt has so far been made to sub-divide the Sirban Limestone Fm into

distinct units. All that has been done to date is the recognition of some lithofacies based on

petrological observations. A few of these studies are worth mentioning e.g., Gundu Rao (1973),

who classified the Sirban Limestone Fm into seven units. Raha (1974, 1984) identified three

stromatolitic bands within the Sirban Limestone Fm and termed them as Biostromes- BS- I, BS- II

and BS- III. Thappa et al. (1993) have placed Sirban Limestone under the Sirban Group comprising

of two distinct formations, i.e., Lower Trikuta Formation (calcareous sequence) and Upper

Khairikot (areno-argillaceous sequence) Formation (records, Vol. 127).

Table 1.1: Generalised stratigraphy of the Riasi area

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According to Thappa et al. (1993) the Sirban Group is unconformably (marked by 4-5 m thick

coarse chert breccia) overlain by the Jangalgali Formation (Cretaceo-Eocene?) and the latter is

unconformably (marked by bauxite with top 1-1.5m bearing pisolites) overlain by the Subathu

Group (Eocene age). Different views have been put forth to explain the origin and disposition of the

breccia and the bauxite units in the area (e.g., Siddaiah, 2011; Singh, 2012 amongst others). The

present stratigraphic disposition of the Sirban Limestone Fm is tenuous and its tectono-stratigraphic

relationship with younger successions in the area is equivocal and has been re-assessed in the light

of the present structural, stratigraphic, geochronological and sedimentological studies (this work).

The Sirban Limestone Fm is an extension of the oil & gas producing Potwar Basin in northeast

Pakistan (Hakhoo et al., 2011) where a complete stratigraphic succession from Proterozoic to

Eocene (Patala Shales = Subathu Group) has been documented (Wandrey et al., 2004). The present

investigation of the structural and tectono-stratigraphic elements of the Sirban Limestone Fm and

the Subathu Gp has revealed the existence of a tectonic (back-thrusted) contact between the two

around the Riasi Inlier (Hakhoo et al., 2011 & Bhat et al., 2012). However, this needs to be

supported by further investigations in the adjoining areas.

1.2 Rationale of the present study

The research undertaken for this thesis focuses primarily on the structural evaluation and

stratigraphy of the Sirban Limestone Fm in the Riasi Inlier with special emphasis on ascertaining

the existence of the petroleum system elements in it. Re-Os geochronological analysis of the

embedded shale unit was also undertaken to determine its age and so to correlate the Sirban

Limestone Fm with the coeval Neoproterozoic to Early Cambrian petroleum bearing formations in

India and other parts of the world [e.g., proven plays in Mali, Mauritania & Algeria (Taoudenni

Basin); Morocco & Algeria (Tindouf Basin); Libya (Cyrenaica-sirte rift basin margin); Pakistan

(Potwar Basin); India & Pakistan (Punjab Platform); Australia (Officer and Amadeus basins) and

potential plays in Libya (Kufra Basin); Pakistan (Miajalar Basin); Neoproterozoic to Early

Cambrian of North Africa and Neoproterozoic of Arabia & India (Fig. 1.5)]. This study is

important in view of the recognition of the Neoproterozoic successions as potentially large

petroleum systems with a few proven plays and a largely untapped resource. These Precambrian

and Neoproterozoic-Early Cambrian petroleum systems are relatively abundant and widespread.

The oldest live oil recovered to date is sourced from Mesoproterozoic rocks within the Velkerri

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Formation (Roper Group) of the McArthur Basin of northern Australia, dated at 1361 ± 21 Ma and

1417 ± 29 Ma (Re-Os dates) with atleast the initial phase of oil generation and migration having

taken place before 1280 Ma (Kendall et al., 2009). However, the geologically oldest commercial

production is currently from the somewhat younger mid to Late Neoproterozoic (Cryogenian-

Ediacaran) petroleum systems of the Lena-Tunguska province in East Siberia, southern China and

from the latest Neoproterozoic to Early Cambrian Huqf Supergroup in Oman (Craig et al., 2013)

where the discovery and exploitation of giant oil and gas fields has demonstrated the vast potential

of these systems. In the Indian Sub-continent, this potential has been proven with the discovery of

heavy oil in the Baghewala-1 well (Bikaner-Nagaur Basin, western India) (Bhat et al., 2012). The

Neoproterozoic-Cambrian rift basins in western India, Pakistan, southern Oman and South China

were in close proximity in the central portion of the Pannotia Supercontinent during the Late

Precambrian (Bhat et al., 2012).

1.2.1 Precambrian (Proterozoic) stratigraphy and hydrocarbons

‘Precambrian’ is an informal stratigraphic term used to define the geological time from the

establishment of the earth to the commencement of the Cambrian Period. It is preceded by the

informal time unit of the ‘Hadean’ and is subdivided into Archaean (4000-2500 Ma) and

Proterozoic (2500-542 Ma) Eons (Craig et al., 2013). The Archaean Eon is further sub-divided into

Figure 1.5: Proven/producing and ‘potential’ Precambrian petroleum systems of the world.

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four eras and the transition to the Proterozoic is considered to be diachronous in all the cratons and

the formalisation of a ‘Transition Eon’ (Eoproterozoic) is under discussion by the Precambrian sub-

commission of IUGS (Craig et al., 2013). The Proterozoic Eon is divided into three Eras, i.e.,

Palaeoproterozoic, Mesoproterozoic and Neoproterozoic which are further sub-divided into ten

Periods (Table 1.2). The Neoproterozoic Era stretches from 1000 Ma to the base of Cambrian at 542

Ma. The Neoproterozoic Era was a period of extreme atmospheric, climatic and tectonic changes,

whose evidence has been recorded on the Indian Plate, especially between 750 and 550 Ma (Bhat et

al., 2012). During the Neoproterozoic, diversification of eukaryotic life, which includes

multicellular algae and primitive phytoplanktons (acritarchs), led to the deposition of sediments

containing rich algal organic matter known to generate hydrocarbons. Towards the end of

Proterozoic (Terminal Proterozoic-Ediacaran, c. 630-542Ma), soft bodied animals and other

invertebrate groups came into existence along with other biotic communities (Knoll, 2000), such as

sponges (spicules), trilobites, brachiopods and small shelly microfossils which are also significant

contributors to organic source material. This biotic evolution occurred alongside extreme events,

viz. the dismemberment and amalgamation of landmasses, glaciations, deposition of glacial

diamictites, cap carbonates and extreme tectonics. In north and NW Himalaya, the Neoproterozoic

and younger sedimentary successions have been subjected to multiple deformation episodes before,

during and after the Himalayan Orogeny, resulting in great structural complexity. These complex

fold and- thrust belts are emerging as challenging, frontier hydrocarbon targets similar to the

producing hydrocarbon province of the Zagros Fold Belt in Iran and Iraq, which accounts for 49%

of all reserves in fold and thrust belts (Cooper, 2007 & Bhat et al., 2012). These are commonly

perceived as ‘difficult’ places to explore and therefore are often avoided by the oil and gas

companies. However, fold and –thrust belts host large oil and gas fields and barriers to effective

exploration mean that substantial prospects still remain to be explored (Goffey et al., 2010).

Initially the Proterozoic basins were not considered as having hydrocarbon potential by “oil

explorationists” and the research for hydrocarbons was restricted to Phanerozoic basins. Now, well-

understood biological evolution of life, a precursor for the generation of source rocks has been

established in the Upper Neoproterozoic-Terminal Proterozoic and Lower Palaeozoic. Moreover,

living with the fact that most of the easy exploration and corresponding oil and gas deposits have

been already found, the focus is shifting towards the frontier areas perceived as difficult targets to

explore and exploit, but the success achieved with the discovery of oil and gas fields (as mentioned

before) has opened a new era of “Old Oil” exploration in the Indian Sub-continent with the

Proterozoic basins as primary targets. The occurrence of oil and gas has been reported from the

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Neoproterozoic carbonates in many areas globally and the hydrocarbon potential of the coeval

Sirban Limestone Fm therefore, cannot be ruled out. There is already a widespread and growing

perception that the Neoproterozoic succession will continue to form an important target for future

exploration all along the Peri-Gondwana Passive Margin, of which the Sirban Limestone Fm is a

part. This research has embarked on unveiling the structural make-up and stratigraphic disposition,

vis-a-vis, hydrocarbon potential of the Sirban Limestone Fm, which has been overwhelmed by the

myriad of uncertainties, thus obscuring the parameters which define a petroleum system. The

detailed work on these carbonates (in the direction of defining the parameters of a petroleum

system) is likely to help in exploring the potential within the Neoproterozoic-Cambrian basins

representing the Frontier areas of research and also make possible the availability of “Rupee Oil”

(Talukdar, 2001). In view of the very complex structural geometries (imbricate fans,

duplex/breached duplexes, stacked thrust sheets, out-of-sequence thrusting, reactivated ramps and

Table 1.2: A) The current International Stratigraphic Chart for the Precambrian and B) possible

changes to the Precambrian time scale under consideration (Craig et al., 2013).

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so on) and the presence of triangle zones, duplex structures and associated back thrusts in the Sirban

Limestone Fm (Riasi Inlier) forming potential hydrocarbon traps (Hakhoo et al., 2011), rigorous

structural modelling, combined with robust geochemical and geochronological data, are required to

calibrate petroleum system models and reduce exploration risk in the Himalayan frontal region

(Bhat et al., 2012). This type of study is of paramount importance for understanding Neoproterozoic

hydrocarbon system in the Sirban Limestone Fm particularly in the light of recent discoveries of oil

and gas reserves from the coeval Proterozoic carbonates in different parts of the world.

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