FIELD EXCURSION GUIDE KACHCHH 2021 Dinosaur graveyard discovered in 1999 from Kuar Bet APG Vadodara Regional Chapter Western Onshore Basin, ONGC Ltd. Vadodara, Gujarat: 390009 Pic Credits: Dr. Alok Dave Professor Emeritus, Indian Institute of Science Education and Research, Pune [Executive Director (retd.), ONGC, Mumbai]
FOREWORD Dear APG Member, Association of Petroleum Geologists is a confluence of professionals of the E&P industry and academia, striving together to exchange knowledge and ideas pertaining to petroleum geology. Kutch Basin falls under Category-II Sedimentary Basins of India. Hence, it urges for rigorous efforts in terms of hydrocarbon exploration and exploitation. Western Onshore (WON) Basin, as of now, is synonymous with Cambay Basin. It is worth mentioning that WON Basin has already ventured into Kutch Onshore Basin, where it has been allotted OALP acreage and PEL obtained from the State authorities. In Kutch offshore discoveries from Tertiary and Mesozoic sequences has already been established. Monetisation of these discoveries are underway through DST & FDP processes. In the onshore part H/C indications have been observed from the Mesozoics. Therefore, upgradation of Kutch & Saurashtra Basin from Category – II Basin to Category – I Basin would remain our main focus for the future exploration strategy. Kutch Basin is famous for its fossils of Mesozoic and it is Type sections with various structural elements like Kutch Mainland Fault, Type sections of Harudi Fm.,Naredi Fm , the Inliers of Jumara Dome, Kaladungar Fm etc are worth visiting and a must see features for G & G scientists. This geological field workshop Integrating outcrops to subsurface rocks of Cenozoic & Mesozoic – A perspective on Kachchh Basin will encompass study of Cenozoic & Mesozoic clastic and other outcrops exhibiting various types of Geological features to understand the processes 1
of the sedimentation, basin evolution and its hydrocarbon significance through outcrop based studies. It is my privilege that APG Vadodara Regional Chapter has taken the initiative to organize this workshop. I am hopeful that the young geo-scientists participating in this field workshop will benefit from this Field Excursion Guide and the guidance of the accompanying experts. I wish you an enlightening, successful and safe trip. Kush Rajkeshari Singh Vice-President APG India 2
ACKNOWLEDGEMENTS A workshop of this magnitude is possible only due to invaluable support of many people to the organising team. The help rendered by all must be acknowledged without fail. The team is grateful to Patron, APG, Shri R K Srivastava, Director (E) for being the guiding light throughout the workshop. We are also indebted to the President, APG, Shri SN Chitnis, ED, HOI, KDMIPE for his constant support and encouragement. With his support and guidance in meticulous planning of workshop, Vice President, APG, Shri K R K Singh, ED, Basin Manager, WON Basin is acknowledged gratefully. This entire field trip would not have been possible without the guidance of Dr. Alok Dave, ED (Retd.), ONGC Ltd. and the team is grateful to him. We are also thankful to all the HOIs, Asset Managers for their support by sending nominations enthusiastically. The assistance rendered by Sh. Jayanta Sarkar, Secretary, APG and Sh. Shadab Raza, Treasurer, APG in organizing this field workshop was invaluable and is gratefully appreciated. Support of our internal faculty Sh. J P Tiwari, GM (Geology) is gratefully acknowledged. Last but not the least, the enthusiastic participation of all the G & G executives made the trip memorable. APG, VRC Executive Body 3
PROGRAMME 24th December, 2021: Arrival at Bhuj Introduction to the field trip by Dr. Alok Dave 25th December, 2021: Field work: Early –Middle Eocene type sections of Naredi, Harudi; Fulra Limestone (Rakdi River section); Eocene / Oligocene unconformity at Kharay; Early Miocene carbonate litho- biofacies at Rampar, depositional setup (lagoon, open shelf facies), sequences, sequence stratigraphic surfaces, Systems Tracts, Sedimentogical scale cycles, cyclothems 26th December, 2021: Field work : Breakup unconformity (Rift to Passive margin) and fluvial Early Cretaceous Bhuj sandstone at Airport road section; Cenozoic euxinic paleo lagoon condensed section at Matanomadh; Paleocene sequence at Matanomadh, fluvial processes and products, concepts of upstream sequence stratigraphy beyond the controls of shoreline shifts ; Middle Eocene at Lakhpat; Recent tidal flats at Narayan Sarovar; 27th December, 2021: Field work: Intrusives (Reunion Hot spot?) in the Early Cretaceous Bhuj Sandstone; Early-Middle Miocene at Kankawati ; foreshore processes and products at Mandvi, sub-environments of shallow shoreface and foreshore; Early Cretaceous Highstand delta succession at Mandvi- Bhuj road 4
28th December, 2021: Field work : Middle Jurassic (Half Dome) (Jumara) sequence near Bhuj, Katrol fault; Late Jurassic(Jhuran) sequence at Rudramata, progradations and sedimentary features; Middle Jurassic Rift fill sequence at Babia Hill,Pachcham Island across Banni Rann; Kutch Mainland fault; Rann of Kachchh 29th December, 2021: Departure from Bhuj Note: daily departures for field at sharp 7.30 AM from Hotel. Participants to assemble in Hotel lobby at 7.15 AM for briefings 5
Mandatory requirements for the field trip On account of Kachchh district being a sensitive border district and some of the outcrops are logistically challenging, safety and security of participants are non- negotiable. Therefore, following mandatory requirements are suggested for compliance. ▪ All participants must carry valid identity cards in field ▪ Proper field shoes to negotiate tough terrains and steep climbs ▪ Full sleeve shirts and jeans are strongly recommended to avoid thorny bushes ▪ Field caps ▪ As we expect chilly weather in morning and evening, proper woollens are recommended ▪ The team should carry first aid medical kit ▪ Participants must carry geological hammers, sample bags etc. if they wish to collect samples Wish you all a safe and successful field trip! 6
The region of Kachchh in Western India is conspicuously featured by uplands and surrounded by lowlands. The uplands are rugged hilly terrains with sparse vegetation, exposing the Mesozoic rocks (Middle Jurassic to Early Cretaceous) bordered by thin strips of gently dipping Cenozoic rocks (Paleocene to Pleistocene/Recent), which form the coastal plains. The lowlands are extensive plains, alluvial or mud and salt flats (Rann) and grassy flats (Banni).The highlands are the areas of uplift and the plains are intervening basins. The marginal hill ranges of the highlands with escarpments facing the vast flatlands are sharp marginal flexures of Mesozoic rocks that are well exposed along the lengths of the highlands. No outcrop is seen within the featureless plains and consequently subsurface information is lacking for these areas excepting for some geophysical and well data. The sedimentary basin of Kachchh occupies the entire district of Kachchh in the state of Gujarat close to India’s western border with Pakistan, between Lat. 220 30’ and 240 30’N ; Long. 680 and 720 E (Figs.1-2). It is an east-west oriented pericratonic rift basin at the western most periphery of Indian craton. The fossil rift of Thar and Southern Indus basin of Pakistan borders the basin to the north. To the south occurs another parallel rift basin, the western offshore extension of Narmada rift, with Saurashtra horst between them. The north – south trending Cambay rift basin, crosses the two parallel rifts. Together, the three rifts form an intersecting rift system around the foundered cratonic block of Saurashtra at the trailing edge of Indian continental plate. Mumbai offshore shelf extends south of the Narmada rift along the west 7
coast fault. This shelf is also rifted and featured by north-south trending grabens/half grabens and horsts. Mesozoic, Cenozoic and Quarternary sediments fill up the Kachchh basin. The sediment fill thickens from 500m in north to over 4000m in south and from 200m in east to over 2500m in west indicating paleoslope of the basin to the southwest. To the north, Pre Cambrian basement rocks are exposed in Meruda Takkar and Nagar Parkar hill in Pakistan. To the east, the Kachchh rift basin ends up against a basement arch – Radhanpur-Barmer arch, which is the western rift shoulder of the N-S oriented Cambay rift. Further east Pre - Cambrian rocks under cover of alluviam extend over the North Gujarat plains. To the west the basin extends across the continental shelf while to the south the uplifted Saurashtra platform covered by Late Cretaceous sediments and Deccan lava delimits the basin. KACHCHH - SAURASHTRA BASINS The Kachchh–Saurashtra basins are composite contiguous basins extending from land to offshore. The Kachchh basin exposes classic Jurassic and Cretaceous sections amidst the vast plains of alluvium covered Rann (Fig.3). In the offshore, the basin extends into offshore with a wide shelf platform. In Saurashtra, the outcropping area is covered by Deccan Trap with few Mesozic exposures, coastal Cenozoic and Quaternary sediments. The northern limit of Kachchh-Saurashtra basins continues to the north beyond the Indo-Pakistan border. The rock formations are 8
continuous to the north of this boundary. Further south, the basin is contiguous with the Mumbai offshore basin and some of the formations are seen to continue uninterrupted towards south. The surface outcrop mapping and stratigraphy have both been worked in detail in the Mesozoic-Cenozoic succession of Kachchh. In the offshore of Kachchh, it is possible to extend these units through excellent biostratigraphic correlation especially in the Cenozoic. The extensive development of the Deccan Trap covers the highest Mesozoic sequence of Bhuj Formation in the outcrops and continues in the subsurface of the western shelf adjacent to Kachchh as also in the Gulf of Kachchh. To the north, it is not seen in the \"island belt\" of Patcham,Khadir, Bela, Chorar. The infra-trappean sequence is essentially clastic ranging in age from Middle Jurassic to Late Cretaceous in the Gulf of Kachchh and mainly a carbonate facies of Late Cretaceous till the depths penetrated, i.e. around four kilometers. In the Western Saurashtra and adjacent coastal region, the Deccan Trap continues almost up to the shore line in most parts. In the near coastal belt alone some sedimentaries develop during the Neogene. They range in age from Early to Middle Miocene, followed, thereafter, by a Plio-Pleistocene sequence generally referred as 'Dwarka Beds' and 'Miliolites'. Exploration in Kachchh commenced with the geological mapping in early sixties, the first exploratory well was drilled in Banni area in 1972. A small discovery was made subsequently in the offshore in 1984. As on date, more than 60 exploratory wells have been drilled 9
in the basin (including 5 onland by ONGC and a few by Focus Energy with success in the Mesozoic) and commercial gas is also known from the Cenozoic and Mesozoic sequence in offshore shelf area especially the GK-28-GK42 areas. Wells have also been drilled on Saurashtra shelf adding to a wealth of information on the Cenozoic and Mesozoic succession. Biswas (1977, 1982) and Pandey and Dave (1993) have summed up the contributions to the stratigraphy and geology of Kachchh and Saurashtra. Large volumes of geoscientific data on various aspects of geology have got accumulated by now which have improved our perceptions and knowledge regarding tectonic and sedimentation history. The updation of the lithostratigraphic model was subsequently carried out by Zutshi et al (1993), which is presently in vogue in ONGC. Both these basins have a composite geological history, broadly differentiated into Mesozoic and Cenozoic. The extensive Deccan Trap sequence actually constitutes a tectono-stratigraphic unit which differentiates the older and younger sedimentation episodes. Within the Cenozoic, Paleogene and Neogene depositional histories also stand apart. Based on the well-known structural and tectonic features, these basins have been further divided into blocks for the purpose of better appreciation of stratigraphy (Zutshi et al., 1993). These are :Banni Graben; Kachchh Mainland; Kachchh shelf; Saurashtra Plateau and Saurashtra shelf. 10
Banni Graben The Alluvium and the Rann covered area in the northern part of Kachchh, extends from the international border to the mainland fault in the south. Westward it opens over the Indus Shelf, into the Arabian Sea. Towards east, it merges into Radhanpur trend of Cambay basin. Several inliers of Mesozoic sequence appear in the form of 'Islands' in North; Patcham, Bela, Kadir in the North and Wagad in the east. A major east west fault in the north and the mainland fault, shape it into a graben deepening due west. Mesozoic sequence constituting the coarse clastics, limestone, marlstone and shales, rest on the pre- Cambrian basement followed by Cenozoic sequence. The absence of Deccan Trap in the subsurface or in exposure is a conspicuous feature of this block. The oldest known Mesozoic is exposed in this tectonic block. Kachchh Mainland Physiographically a rugged terrain, it extends from the mainland fault in the north to the Gulf of Kachchh and Arabian sea in the south and west respectively. East of it, is the little Rann, an extention of Kachchh Gulf. Apparently Kachchh Mainland also appears to look like a large island, surrounded by the Rann and the sea. Rock exposures of Mesozoic (Jurassic and Cretaceous) having a southerly plunge, have been well studied and documented with an established Rock Stratigraphy (Biswas, 1977). Deccan Trap and rocks strata of Cenozoic section, which are overlying the Mesozoic section build an overlapping prism and extended into the offshore. The east-west 11
trending Katrol fault, constitutes a major structural element in the block, along which some translatory motion is also anticipated. The Mainland fault to the North is basement controlled with increasing displacement from east to west. Kachchh Shelf The Paleogene sedimentary prism happens to thicken gradually from the Kachchh coast to the outer and deeper shelf. Underlying this mega sequence are the upper Mesozoic sediments which have been partially penetrated. The geological details of the shelf platform are, therefore, available from the drilled well data only. The most conspicuous observation is that of the continuation of Deccan Trap sequence, with varying thickness and age into the offshore. The deeper shelf is differentiated by a major fault and tectonic subsidence during late Cenozoic. Thick Miocene sediments (clastic and carbonate facies), extend atleast up to the 200 m isobath line. Saurashtra Plateau This block is marked by Gulf of Kachchh and little Rann in the north, Cambay rift in the east, Surat depression due south and the western shelf extension of it into the Arabian sea. As already mentioned, it is covered in major part by Deccan Trap and outcrops of Mesozoic in northeast. 12
Saurashtra Shelf Adjoining the plateau is the narrow shelf which carries a thick prism of Tertiary-Quaternary sediments both in clastic and calcareous facies, similar to Shelf Platform of Kachchh offshore. The prominent structural element distinguished is the east-west trending, Saurashtra Arch, which also marks the offshore 'boundary' of the Kachchh with Saurashtra. In the deep shelf part of Saurashtra a considerable tectonic subsidence and structural complexity of a higher order than Kachchh is noted. Well OS-II-AI drilled continued to remain in the Middle Miocene even upto 4406 m depth. BASIN ARCHITECTURE The Kachchh basin has the typical geometry of an evolved rift basin. The rift is oriented E-W. The Nagar Parkar Pre Cambrian block and the Saurashtra block are the northern and southern rift shoulders, respectively. The Radhanpur arch is the hinterland high, which limits the rift extension to the east. To the west, the rift is open merging with the continental shelf. The rift is asymmetrical with maximum tilt towards southwest. The most striking feature of the basin is the occurrence of a meridonial high, referred as Median High, across the middle of the basin. It passes transversely across both positive and negative structural elements of the basin so that the uplifts plunge bilaterally and the intervening basins have a central high and shallower region. The Median High (MH) trends NNE-SSW. To the west of the high, the basin 13
is deeper with thicker accumulation of sediments showing change of facies from shallow to deeper marine. To the east of the high, the basement is shallower with less thickness of sediments and the facies varies from shallow marine to littoral and fluvial. This part of the basin is more tectonized with most of the uplifts occurring in this eastern part. This Median High, which controlled the facies and thickness of sediments is, thus, a tectonized zone along the hinge line of the basin and is featured by a network of low amplitude faults (mostly externsional bending fractures in crestal region) and intrusions. The shoulder highs and the hinterlands were the source areas for sediment supply. Structurally, the basin is styled by footwall uplifts and half grabens along longitudinal master faults.The uplifts are the outcropping areas and the grabens/ half grabens form extensive plains covered by Quarternary sediments. The plains (structural lows) around the uplifts (structural highs) form residual depressions. The grabens/ half grabens are different depositional domains in a rifted setup. Rift –fill sediments show variations of lithofacies in these sub-basins. Seismic and well data indicate that the uplifts are unexposed tilted basement blocks bounded by planer faults and draped by thin (1500-2000m) sediment cover. GEOLOGY AND STRATIGRAPHY OF KACHCHH BASIN The Kachchh Basin preserves almost a complete record of sedimentary succession from Mesozoic to Recent punctuated by several stratigraphic breaks (Figs.7-13). Stratigraphically, the northern limit of Kachchh outcrops continue north beyond Indo-Pakistan border. In the offshore of Kachchh and further south, the basin is contiguous 14
with Mumbai Offshore basin. The extensive development of Deccan Traps covers the highest Mesozoic clastics sequence (Bhuj Formation) in the outcrops and continues in the subsurface of the Western shelf adjacent to Kachchh. The infra-trappean sequence is essentially clastic ranging in age from Middle Jurassic to earliest Late Cretaceous and mainly a carbonate facies of Late Cretaceous in offshore till the depth penetrated by wells. Rifting and consequent subsidence of the block between the Nagar Parkar hills and the Saurashtra Peninsula in the Late Triassic, as evidenced by continental Rhaetic sediments in wells Banni-2 and Nirona-1 (Koshal,1975), initiated development of the Kachchh basin. The first occurrence of marine sediments of Bathonian age indicates that the basin became a fully marine during the Middle Jurassic. The basin formed the site for westerly deepening epi- continental sea, probably an extension of the Tethys, in which thick pile of sediments, ranging in age from Middle Jurassic to Early Cretaceous, were deposited in shallow marine to deltaic environments. The sediments were deposited in two major cycles - a Middle Jurassic transgressive cycle and a Late Jurassic-Early Cretaceous regressive cycle (Biswas, 1983). During the transgressive cycle mainly carbonates and shales were deposited, whereas deltaic clastics constitute the regressive deposits. In Early Cretaceous time, the sea began to recede in response to thermo tectonic uparching of the western continental margin and deposited thick pile of fluvial clastics (Bhuj Formation). The oldest recorded Mesozoic sediments in offshore are from well GK-29- A-1, where Late Jurassic (Oxfordian-Kimmeridgian) sediments are homotaxial with the outcropping sediments in Jumara Dome. The Mesozoic sediments were intruded and covered by Deccan Trap 15
basaltic flows in Late Cretaceous-Early-Paleocene time. The onland part of the basin experienced a period of non-deposition during Late Cretaceous time. Occurrence of over 2,000 m thick limestone unit of Late Cretaceous age in the offshore wells KD-1 and KI-1A indicates that shallow marine conditions continued to prevail in the offshore part of the basin during this time, and withdrawal of the sea from the offshore part of the basin was for a very short duration represented probably by the period of Deccan Trap activity and the development of the Early Paleocene continental clastics. The continental Early-Middle Cretaceous clastics, below the datable Coniacian-Turonian carbonates are recorded in offshore wells. These unfossiliferous fluvial clastics may represent extension of outcropping Bhuj Formation in the subsurface. Coniacian to Maastrichtian carbonates, representing Late Cretaceous marine transgression are recorded in several offshore wells below the Deccan Trap. The Cenozoic sedimentation began in Kachchh in a stable shelf environment. Thus, the strata have remarkable lateral continuity from onland to offshore. These sediments have been dated based on their faunal assemblages and several unconformities have been recorded which continue to the offshore area over the continental shelf. The Early Paleocene sediments (variegated lithologies of Matanomadh Formation) exposed in the onland are not encountered in the subsurface of Kachchh Offshore. The Late Paleocene represents the onset of first marine transgression of the Cenozoic. Early Eocene transgression extended over the entire western margin of India, marking the beginning of post-rift history of development of the western 16
continental margin of India. Sedimentary sequences of the subsequent time represent deposition contemporaneous with passive subsidence of the continental margin during drift of the Indian plate away from the spreading centre. The environmental framework of the Kachchh basin and Saurashtra basin and those of the other southern basins got differentiated into shelf, slope and basin floor. The shelfal horst-graben complex, including the lowlands of the onland part of the Kachchh basin, formed the site for development of shelf depositional systems, whereas the Kori-Comorin depression, the Kori-Comorin ridge, the Laxmi-Laccadive depression and the Laxmi-Laccadive ridge lay in slope and basinal region. The Mesozoic highlands, in the onland part of the Kachchh basin, behaved as stable horst blocks throughout the Tertiary time restricting Tertiary deposition in peripheral lows bordering them. The shelf depositional systems comprised deltas, clastic and carbonate tidal flats, strand plains and extensive carbonate ramps. The slope-basin regions, viz., the Kori-Comorin depression, the Kori- Comorin ridge and the area west of it, formed the site for development of submarine fans and hemipelagic and pelagic deposits. Regions of favourable paleobathymetry over the Kori-Comorin ridge formed the sites for development of carbonate buildups (pinnacle reefs), as suggested by seismic data and observed in the well GKH-1 drilled in the northwestern part of the basin. Several breaks in sedimentation are recorded in the Cenozoic sedimentary history of the basin. The significant ones are Early/Middle Eocene unconformity, Eocene/Oligocene unconformity, Early/Late Oligocene unconformity, and minor hiatus at Oligo-Miocene and Early/Middle Miocene boundaries. The clastic influx, drowning of carbonate platform and 17
deepening of the basin characterizes Post-Middle Miocene history of the basin, when the western margin as a whole experienced heavy influx of clastics manifested in basin-ward rapid shift of the shelf edge over considerably long distance to its present position. In the region of the Kachchh basin, the structural differentiation between the shelfal horst-graben complex, the Kori-Comorin depression and the Kori- Comorin ridge got lost as the Kori-Comorin depression was covered up with rapidly prograding slope facies of the Neogene time. Kachchh offshore basin covers an area of about 28,000sq.km up to 200 m isobaths. This category-II basin has been classified as \"Medium-High Risk and Low-Medium Reward basin, under the risk- reward categorization. While presence of Oil in KD and Gas in GK-29A structures prove the hydrocarbon generation and entrapment potential of the Paleogene sequence, the gas find in GK-22C-1 has established the occurrence of hydrocarbon in Cretaceous sequence and gave fillip to Mesozoic exploration. The basin is strategically straddled between the prolific petroliferous Mumbai offshore basin in the south and southeast and the significant petroliferous Sind-lndus geological province in the north. Exploratory efforts in the basin were initiated with 2D seismic and gravity/ magnetic surveys in early seventies. Since then 2D seismic data were acquired over most of the area. 53 structural and 13 stratigraphic prospects have so far been identified based on interpretation of seismic data over the shelf part of the basin (up to 200 m isobaths). A total of about 50 prospects have been confirmed so far upto 200 m isobaths. Exploratory drilling and leads obtained from geological/ geophysical data, have established several discoveries in 18
the basin. Middle to Late Eocene, Early Eocene and Early Cretaceous sequences contain main pay zones in the basin. Kachchh Basin is characterized by structurally controlled dispersed hydrocarbons. Mesozoic Stratigraphy The Mesozoic sedimentary succession in Kachchh comprises of Late Triassic? (Rhaetic) continental, Middle to Late Jurassic marine and Early Cretaceous fluvio-deltaic sediments. Mesozoic sediments are the rift fill sediments and constitute major part of the basin fill. Syn-rift sediments were accommodated in the half grabens by block tilting, sea level changes and regional subsidence, two major sedimentation domains are Banni half-graben and Gulf of Kachchh half graben. The lithostratigraphic classification of Biswas (1977) is presented below. Kaladongar Formation (Aalenian to Bathonian, Middle Jurassic) A thick succession of conglomerate, sandstone and shale fully developed in the Kaladongar (Black hills) Range of Pachham Island constitutes this oldest stratigraphic unit of Kachchh. The section exposed in the lofty scarp facing the Rann, below the highest Babia peak is designated as the type section. It outcrops extensively in the north face of Kaladongar escarpment along the Rann from Kuran village. Within the hills, these rocks are also exposed in deep incised valleys. The oldest beds are exposed in Dingi hill and Kuar Bet Island (the last outcrop in Indian Territory). The formation is divided into three informal members. The lowest Dingi Hill Member consists of green and red siltstone, brown and grey, hard calcareous sandstone overlain by 19
pink to brown, massive current bedded sandstone, with interbeds of shale, siltstone and thin fossiliferous calcareous sandstone. The overlying Kaladongar Sandstone Member comprises grey to brown, massive, medium to coarse grained, quartz arenite (calcareous towards top), with thin bands of fossiliferous, calcareous sandstone, and occasional cobble-conglomerate. The upper most Babia Cliff Sandstone Member with similar sandstone is separated from the former by an olive green fossiliferous siltstone bed. The lower part of this member is brown to pink sandstone with thin bands of silty shale overlain by yellow, hard calcareous and fine grained sandstone. The formation is poorly fossiliferous with pelecypods and gastropods – Carbula, Astarte, Turritella, Natica etc. Rolled pebbles of corals are common in the upper beds. Petrified tree trunks with Rhynchonella have been recovered from conglomerates of Dingi Hill. It’s lithologic, structural and fossil characteristics indicate a littoral environment of slowly transgressing sea over a granitic terrain. Based on the occurrence of Corbula and other fossils, the formation has been dated as Aalenian to Bathonian (Middle Jurassic) in age. The formation is correlatable with the overlying Goradongar Formation, with a change of facies from sandstone to shale and limestone. Goradongar Formation (Bathonian to Callovian, Middle Jurassic) A succession of limestone shale and sandstone above the Kaladongar Formation is grouped under this formation, which is named after its type locality Goradongar (white hills) ranges of Pachham Island. The formation has been divided into four informal members. These are 20
(in ascending order): Goradongar Flagstone Member consisting of grey and yellow fossiliferous limestone; Gadaputa Sandstone Member pale brown to pink, massive, current bedded, medium to coarse grained quartz arenite ; Raimalro Limestone forming scarp tops and grey to yellow, thickly bedded limestone and Modar Hill Member comprising basal olive grey fossiliferous gypseous shale (Khavda Shale) with fossiliferous limestone bands overlain by grey, coarse grained, massive, current bedded sandstones. The fossiliferous bands of Goradongar Flagstone Member and Khavda Shale are full of pelecypods, mostly Corbula, Astarte, Trigonia, Nucula besides annelid tubes. Rhynchonella, among brachiopods are seen in flagstone and Raimalro Limestone. Benthic foraminifera have been recorded from the Khavda shale and Raimalro Limestone (Pandey and Dave, 1993) indicating a Bathonian age. The Khavda shale also contains ammonites including Kamptokephalites and Indocephalites. As stated above, the lower boundary of the formation is conformable, while the upper part is not exposed. it is unconformable overlain by Neogene strata. The lithologic association, biota including ammonites and benthic foraminifera, indicate an infra-littoral environment. The ammonite assemblage in the Khavda Shale shows that this horizon corresponds to the middle macrocephalus zone of Callovian age. The foraminiferal assemblage comprising Epistomina and other benthics also suggests a Bathonian–Callovian age to this formation. 21
Jhurio Formation (Bathonian to Callovian, Middle Jurassic) A thick succession of limestone and shale with bands of “Golden oolites”, in the lower part of the Mainland stratigraphy, has been named as the Jhurio Formation after the type section of Jhurio (Jhura) Hill in north central mainland The formation has been subdivided into three units – lower, middle and upper. It comprises of bedded white limestone in the upper part. Oolitic bands with shale and limestone constitute the middle and lower sections. The lower boundary is not exposed in the type section. The upper contact is conformable with the overlying Jumara Formation. The formation is rich in fossils including brachiopods, pelycepods and ammonites including Rhynconella, Terebratula, Ostrea, Astarte, Trigonia, Belemnites, and ammonites. The presence of benthic foraminifera referable to Epistomina regularis – E. ghoshi Assemblage zone, Lenticulina dilectaformis Partial-Range zone, Tewaria kutchensis Partial-Range zone in the stratotype indicates Bathonian–Callovian age (Pandey and Dave, 1993).The physical and biological aspects indicate a neritic – infra littoral depositional environment. Jumara Formation (Callovian to Oxfordian, Middle Jurassic) A thick argillaceous formation overlying the Jhurio Formation has been named after its type section in Jumara hill, near the Rann, north of Jumara village. The formation is exposed as inliers at the centre of the domal and anticlinal hill along the northern edge of the Mainland. 22
The formation is characterised by monotonous olive grey, gypseous, laminated shale with thin, red ferruginous bands, alternating beds of limestone and occasional sandstone inter beds. In Jumara type section, the formation has been subdivided into four informal members I to IV from bottom on the basis of limestone and sandstone interbeds dividing the continuous shale succession. Thin, fossiliferous oolitic limestone bands occur in the shale near the top of member IV, named as “Dhosa Oolite”. These are very characteristic beds and used as the main key- beds in the Mainland stratigraphy. The lower boundary of the formation is defined by conformable limestone shale contact and the upper boundary is marked by Dhosa Oolite member which is followed by Jhuran Formation. The boundary is generally conformable excepting local disconformity observed at a few places where the Jhuran shales are seen resting over the eroded Dhosa Oolite Member.Besides, megafossils of ammonites, brachiopods, lamellibranchs, gastropods and corals, the formation is rich in foraminifera. The benthic foraminifera recorded from the type section are referred to Tewaris kutchensis Partial-Range zone, Proteonina difflugiformis - Astacolus anceps Assemblage zone and Epistomina majungaensis Range–zone (Pandey and Dave, 1993). The fossil assemblage gives an age of Callovian – Oxfordian to the formation.The litologic and faunal aspects suggest the deposition below wave base in shallow marine set up. Jhuran Formation (Kimmeridgian to Neocomian, Late Jurassic to Early Cretaceous) The formation comprises a thick succession of alternating beds of sandstone and shale, it is defined by the Dhosa Oolite Member below 23
and non-marine sandstones of Bhuj Formation above. The formation is divided into four informal members – Lower, Middle (Rudramata shale), Upper and Katesar members. The Lower Member consists of alternating yellow, red sandstone and shale bands in almost equal proportions with thin bands of hard, yellow, fossiliferous, pebbly, calcareous sandstone with large Bellemnites at places. The Middle Member is predominantly shaly comprising thick monotonous succession of dark grey to black well laminated gypseous shale weathering into olive – grey colour. Thin, red bands of ferruginous sandstone, laminated, micaceous siltstone and yellow ochreous mudstone are common in shale. The ferruginous bands are concretionary encasing ammonites at many places. The shales are highly fossiliferous in western part and less so in the eastern part. The unfossiliferous shales contain macerated plant fragments. The Upper Member is predominantly arenaceous and composed of red and yellow, massive current bedded sandstone with intercalations and alternations of shale, siltstone and calcareous sandstone in the middle. In its type section, three ridge forming, conglomeritic calcareous sandstone beds, 2-3 ft thick contain large shells of Trigonia (Trigonia Ridge Sandstone). The Katesar Member consists of greenish grey to yellow, massive, current bedded, sandstone with minor intercalations of shales. The formation is richly fossiliferous in the western Mainland and becomes less and less fossiliferous towards east. Common fossils include ammonites, Belemnites, pelecypods, gastropods and locally corals and echinoids. The formation contains rich ammonite fauna in Katrol area. A rich suite of benthic foraminifera referable to Lenticulina bulla Partial- Range zone, Dorothia kummi – Haplophragmoides pacilis Range-zone 24
of Kimmeridgian – Neocomian age is recorded from this formation (Pandey and Dave, 1993).The paralic facies, physical and faunal characteristics of the sediments of different members indicate that the environment shifted from sub-littoral to supra-littoral and finally to continental deposition of the overlying Bhuj Formation. Bhuj Formation (Neocomian to Santonian, Early-Late Cretaceous) A hugh thickness of non-marine sandstone of uniform character, best exposed around Bhuj town, and constituting youngest formation of Mesozoic stratigraphy of Kachchh, was named as Bhuj Formation. The lower contact with Jhuran Formation is gradational and defined by the boundary between marine and non-marine rocks. The first appearance of iron stone or the topmost calcareous sandstone of Jhuran Formation is the criterion to distinguish the two formations. The upper boundary with the overlying Deccan Trap is disconformable. The formation has been divided into three informal members – Ghuneri, Ukra and Upper members in east and central Mainland. The lower part is characterised by cyclic repetition of ferruginous or lateritic band, shales and sandstone. The Ghuneri Member consists of thick beds of red and yellow sandstone alternated by “laminate zone” comprising ferruginous bands, fissile or laminated sandstone and shale. The Ukra Member contains olive green glauconitic sandstone “green sands” and green and grey shales with thin fossiliferous bands of purple ironstone, ferruginous mudstone and grey limestone. These bands pinch out laterally into the sandstones of Ghuneri and Upper members. The Upper Member consists of whitish to pale brown, massive, current 25
bedded coarse-grained, well sorted sandstone with kaolinitic shale and ferruginous band alternations at thick intervals. The sandstones of all members are pale brown to buff, soft, friable, usually current bedded (large scale tubular), fine to coarse grained, well sorted and loosely cemented quartz arenite which are usually micaceous, ferruginous and/or calcareous. Some coarse grained varieties are feldspathic arenite. Some sandstones in Ghuneri Member are glauconitic and locally contain ferruginous oolite bands. Ferruginous bands in these sandstones are very characteristic, usually concretionary, hematitic or lateritic with spongy or nodular weathering and locally ripple marked. Shales are grey, silty, laminated with limonitic partings and locally carbonaceous in the Lower and Ghuneri members with occasional thin coal bands as seen near Ghuneri. The formation is devoid of any fossil fauna except in the Ukra Member in which the fossiliferous bands are full of ammonites and pelecypods. It is rich in fossil flora, containing several plant beds in the shales of Ghuneri and Upper members. Important plant beds are seen near Lakhapar, Dharesi, Jakh, Kurbi, Nangar and Manjal. The flora is typically Ptilophyllum flora of Upper Gondwana. Beside leaf impressions, large chunk of fossil wood are also seen. Based on the plant fossil and marine Ukra bed, the age of the formation is given as Neocomian to Santonian (Early to Late Cretaceous). The lithology, absence of fauna, rich flora, sedimentary structures, patterns of current bedding and marine tongues in down basin direction, suggest that the Bhuj sediments represent deltaic deposits with distal part towards west. 26
Cenozoic Stratigraphy The Cenozoic succession in Kachchh onland, overlying the Deccan Trap basalt, consists dominantly of limestone, shale and sandstone and can be grouped into discrete mappable formations based on vertical variations of gross lithologies and unconformities (Fig.4). Due to stable shelf environment, the formations have remarkable lateral continuity and could be mapped throughout the extent of the Cenozoic outcrops. The unconformities, mostly disconformities are generally marked by laterised undulating surfaces or by bioturbated cut and fill structures and regional overlaps. These unconformities are also recognised by distinct faunal changes across the formation boundaries. The lithostratigraphic framework of the Cenozoic was earlier proposed by Biswas and Raju (1971, 1973) and later formalised by Biswas (1992). Pandey and Dave (1998) described the formations in great details which is summarised below. Recently, Octavian and Dave (2017), have published a detailed account of Cenozoic sequence stratigraphy of Kachchh onland basin. Matanomadh Formation (Paleocene) A variety of rock types consisting of trap derived clastic sediments in various degree of admixture constitute a distinctive type of brightly coloured arenaceous litho unit named as Matanomadh Formation by Biswas and Raju (1971).The stratotype is named after Matanomadh village where it is exposed in road cuttings and in the badlands around the village. The formation has a non – conformable contact with the Deccan Traps and the upper contact is marked by a lignite band in the type locality. Elsewhere the contact is 27
disconformable. The lithologic succession is extremely variable and consists of a variety of bright colour rock types with admixture of clastics and volcanics, laterite and clay in the lower part, red laterite, bauxite, lateritic trap-pebble-conglomerate, trapwash, wacke, bentonitic clays, red and yellow ferruginous clays, grey and white tuffaceous shales and red, current bedded tuffaceous sandstone and occasional layers of lignite. The fluvial floodplain dominated succession comprising of variegated clays, in the road cuttings at Matanomadh locality have plant fossils, with isolated channel fills and crevasse splays. The overlying current bedded sandstone are medium to coarse grained, moderately sorted, representing upper delta plain set up. The planer beds in the lower part indicate relatively lower energy from the overlying higher energy cross bedded strata. These sands indicate a unidirectional flow regime. The Late Paleocene is represented by a thick lignite facies, although some authors have dated them to be of Early Eocene age. The formation is exposed extensively in the mainland of Kachchh bordering consistently the outcrops of Tertiary rocks. It also occurs as outliers in the Trap or Jurassic country occupying the topographic depressions. It covers a widespread laterised post-Trappean peneplaned surface and forms the floor of the Tertiary deposits. The spheroidal weathering of traps and laterisation is best observed in the Matanomadh locality. The thickness of the succession is extremely variable with a maximum of 50m.The formation is generally devoid of fossils but locally rich microflora of dicot leaf impressions, occasionally with fossil fruits and woods are seen. The floral assemblage suggests a Paleocene age. Abundance of fossil flora also indicates a warm and arid climate which is responsible for the bright colour. 28
Naredi Formation (Early Eocene) The brightly coloured lateritic argillaceous formation was named as Naredi Formation by Biswas and Raju (1971) after its stratotype in the cliff of Kakdi river near the village of Naredi. The type section is exposed in the cliff along the Kakdi river south of Naredi village and partly (upper part only) along the Guvar river NNW of Naredi. In the type section, the formation directly overlies the Deccan Trap with a non-conformity. In other places a well-marked disconformity separates this formation from the underlying Matanomadh Formation of Paleocene age. Its upper contact with the Harudi Formation (Middle Eocene) is also marked by a disconformity. This plane of disconformity is characterised by red ferricreted erosional surface exposed on the road side between Naredi and Baranda villages. In the type section at Naredi, the formation is dominantly argillaceous in the lower part, carbonate in the middle and clastic in the upper part. The lower part comprises gypseous shale consisting of grey, red and olive green glauconitic and splintery shale with thin layers of gypsum, limonite and bands of siderite concretions, which occasionally contain fossils. While the glauconitic green shale are rich in foraminifera including small size planktics including Guembelitria, Chiloguembelia and globigerines; the red shales are unfossiliferous. This is followed by thin bedded, dirty white limestone and yellowish grey marlite with abundant Assilina. The limestone occurs at the top of calcareous mud facies with a sharp contact. In upper part, the formation comprises of ferruginous claystone capped by hard limestone and ripple laminated sandstone. A red laterite bed caps the sequence and marks the unconformity surface. The lowermost part of the formation locally develops black shale facies 29
comprising black pyritic shale and lignite bands in the areas around Panadra in the north western mainland, where thick beds of lignite occur over the white, well sorted friable sandstone. The formation occurs in a narrow sinuous belt of outcrops in the mainland Kachchh. A huge thickness of the lignite is developed in the Babia syncline between Umarsar and Panadra.The formation is rich in microfauna including Assilina spinosa, Lockhartia, Nummulites burdigalensis and rare Guembelitria, Chiloguembelia, Globigerina and Globorotalia. Apart from foraminifera varieties of bivalves, gastropods and corals are seen. The Naredi section is dated as Early Eocene. The depositional environment ranges from lagoonal, restricted, swamp for the lignite beds to shallow inner shelf for the overlying succession. The Middle Eocene in the area is represented by two conformable formations viz., the lower Harudi and the upper Fulra Limestone. Harudi Formation (Middle Eocene) A unit of grey, calcareous highly fossiliferous shales overlying the Naredi Formation and underlying the scarp forming the Nummulitic limestone, was named as Harudi Formation after its type section in an escarpment to the west of village Harudi. In this section the formation is well exposed for a short distance of 300m.The lower contact is disconformable at the top of laterite bed exposed at the top of Naredi Formation in Kakdi River. The upper contact is conformable and placed at the base of lowest massive foraminiferal limestone bed containing characteristic larger foraminifera Nummulites and Discocyclina). Here, the formation consists of green to greenish grey, splintery shales with yellow limonitic partings in the lower part and 30
calcareous claystone and siltstone with occasional layers of gypsum and carbonaceous shale in the upper part. A thin coquina bed is occurs near the base. Occasionally concretionary, oolitic and fossiliferous limestone bands are seen in the lower part. A two feet thick ferrugineous, gypseous clayey marlite band with large Nummulites obtusus is a characteristic marker bed within the formation. This marker is popularly referred as N. Obtusus band. The Nummulites are easily recognised by their doubly convex tablet shape and ornamented surface with curved sutures resembling that of a thumb impression. This bed occurs about 4.5m below the top of the formation. It is extensive and can be traced from Lakhpat in west to Wagapathar in south. However, its absence in Babia Hill area is conspicuous. The formation outcrops throughout the Tertiary belt of western Kachchh mainland bordering the extensive outcrops of the overlying Nummulitic limestone.The characteristic foraminifera of the formation include Nummulites obtusus, N.acutus, Discocyclina and Truncorotaloides topilensis. The coquina bed near the base (transgressive lag) contains variety of pelecypods, gastropods and corals. The foraminiferal assemblage indicates a definite Middle Eocene age. The environment of deposition is littoral in the lower part to shallow inner shelf in the upper part. Fulra Limestone (Middle Eocene) A thick succession of bedded white, foraminiferal limestone conformably overlying the argillaceous Harudi Formation, forms a distinct litho-unit named as Fulra Limestone after the village Fulra in the western Kachchh. The type section is best 31
exposed near the village Fulra on the southern flank of Babia Hill. The entire formation is made up of massive to thickly bedded, white and buff coloured foraminiferal limestone. The limestone are foraminiferal packstone and grainstone locally silty. Large saddle to undulated Discocyclina, ellipsoidal Fasciolites are abundant, giving a characteristic, distinct appearance to this formation. The upper part contains a number of echinoides. In the present study the formation has been studied from escarpment west of Harudi village, Rakdi river on the Naliya-Narayan Sarovar road, Wagapathar Dam section and type section of Babia Hill. The lower contact is conformable and is fixed at the base of escarpment at near Harudi village, the upper contact is a wave ravinement surface/reworked subaerial unconformity between Middle Eocene and Early Oligocene with biostratigraphic hiatus of c.6 my. In the type section at Babia cliff, a thin lateritic band marks the Middle Eocene /Early Oligocene unconformity. Maximum thickness of the formation in Berwali stream is about 60m. The formation is made up of foraminiferal limestone. Its planktic foraminifera belong to two zones: the lower Orbulinoides beckmanni Zone and the upper Truncorotaloides rohri Zone. The larger foraminifera belong to Discocyclina sowerbyi Zone. Besides foraminifera, oysters, turritellids, Pecten, echinoides, corals and crabs are common. Vertebrate fossils like whales, sea cow, and fish have also been reported. The fauna suggest a Middle Eocene age to this formation. The faunal assemblage and lithology suggest low energy, shallow inner shelf environment of deposition. 32
Maniyara Fort Formation (Oligocene) A group of bedded, yellow to ochre coloured foraminiferal limestone with a basal greyish green glauconitic siltstone overlying the Middle Eocene Fulra Limestone has been named as Maniyara Fort Formation after its designated type section. The type section is continuously exposed along the Bermoti river flowing between Maniyara Fort and Bermoti village. The lower contact is marked by ochre colour and sudden appearance of glauconite pellets, making it easy to recognise the boundary in field. Cut and fill structures,packed with glauconite and Cidaris spines, mark the lower boundary very sharp at places. The upper boundary is a small magnitude erosional unconformity with the overlying Early Miocene Khari Nadi Formation. It is discernible by a regional overlap and occurrence of inliers of upper part of the succession (Bermoti Member) at many places. An erosional diastem between the Coral Limestone member and Bermoti member (Early/Late Oligocene unconformity) is clearly seen in Waior river. The Late Oligocene glauconitic sandstone of Bermoti Member is characterised by fossil bones and other reworked fossils. This formation is divided into four members. The Basal member consists of alternating beds of foraminiferal, glauconitic, brownish to yellowish siltstone and calcareous, gypseous claystone. The Lumpy clay member consists of cement coloured to brownish calcareous, lumpy claystone, occasionally containing thin limestone and marlite bed. The Coral Limestone member consists of dirty white nodular limestone alternating with calcareous claystone in the lower part. The upper part is grey to dirty white massive limestone with abundant corals, frequently forming small bioherms. The limestone are glauconitic 33
biomicrites and biosparites. All the above three members are well exposed in the Wagapathar Dam and river near Waior village. The upper Bermoti member is best developed in the Bermoti stream and in the cliff near the village of Waior. The lower part is best exposed in Waior, consisting of rusty brown, friable glauconitic argillaceous sandstone overlying the limestone of Coral limestone member. The upper part is composed of thinly bedded, very hard, grey to yellowish foraminiferal limestone with interbeds of silty marlite full of Spiroclypeus. The formation is richly fossiliferous with variety of echinoides, pelecypods, gastropods, corals, foraminifera and crabs. Nummulites fichteli and Miogypsinoides are the characteristic foraminifera present. The lower three members are dated as Early Oligocene in age while the upper member is of Late Oligocene age. Marginal marine to shallow inner shelf environment is inferred for this unit. Marine transgressive environment shifted from lagoonal to high energy open shelf, when coral bioherms formed. Khari Nadi Formation (Early Miocene, Aquitanian) A distinctive sequence of fine grained to silty, verigated sandstone overlying the white foraminiferal limestone of Middle Eocene Fulra and Oligocene Maniyara Fort formations is defined as Khari Nadi Formation after the river Khari Nadi.The type section is exposed along the cliffs and banks of Khari Nadi. In the present work the formation has been studied from Kankawati river. The lower contact is unconformable over the Late Oligocene Spiroclypeus limestone bed and at the base of bluish grey claystone bed. However, 34
a feeble erosional unconformity is indicated by a transgressive overlap. The upper contact is conformable and gradational. It is fixed at the base of marlite bed full of Turritella, Lepidocyclina and echinoides. In the Kankawati river a 5m red zone with gypseous and ferruginous band suggests a break in sedimentation at the top of the formation. The lithology consists predominantly of laminated to very thin bedded red and yellowish mottled to variegated siltstone and fine grained sandstone with occasional grey and brown gypseous claystone. A bluish grey claystone bed occurs consistently near the base in every section. Cross bedded, fine grained micaceous sandstone is present in the middle part, while a few thin fossiliferous marl and limestone beds are seen in the middle and upper part of the type section. In the middle part, trough cross stratified bioclastic calcarites of shoreface setting with westward longshore paleo current are observed. A few layers show distinct tempestites with storm generated pebbly layers in an overall transgressive shelf setting. In the upper part, Turritella bed is a marker horizon. The uppermost beds are highly bioturbated indicating sediment starvation. Characteristic foraminifera include Miogypsina tani, M. Dehaarti, Nepherolepidina and Austrotrillina. Plant fossils are common in the lower part. Good leaf impressions are often seen in the shaly beds. Turritella, Ostrea and echinoides are common mega fossils. The lower part of the formation is poorly fossiliferous. It is assigned Aquitanian (Early Miocene) age. Tidal flat, littoral to shallow marine environment of deposition is inferred for the succession. 35
Chhasra Formation (Early Miocene, Burdigalian) This formation is named after the village of Chhasra. The type section is exposed along the Khari Nadi from the top of Khari Nadi Formation to the south of Chhasra village. A good reference section is encountered in the Kankawati river. The lower contact of the formation is generally conformable with the Khari Nadi Formation. The upper contact with the Sandhan Formation is marked by a prominent disconformity seen in all the major river sections in the southern slope of Kachchh mainland, including Chhasra. This formation consists of two distinct members. The lower Claystone Member consists of grey and khaki coloured, laminated to splintery, gypseous shales and claystone alternating with thin, hard, yellowish,highly fossiliferous argillaceous limestone. The upper Siltstone Member is well exposed along the Kankawati river. The member consists of alternating micaceous siltstone and laminated silty shales of monotonous khaki colour. The formation is very richly fossiliferous with variety of gastropods, lamellibranchs, echinoides, corals and bryozoa. Trails and burrows are also common. Foraminifera are dominated by Miogypsina, with three distinct zones: Miogypsina (Lepidosemicyclina) excentrica zone, Miogypsina (Lepidosemicyclina) droogeri zone and Miogypsina (Miogypsina) globulina-thecideaeformis zone. Miogypsina (Miogypsinoides) dehaarti and Miogypsina (Miogypsinoides)indica also occur in the lower zone. Among others Austrotrillina howchini and Archaias malabaricus are common.The foraminiferal assemblage suggests an Early Miocene (Burdigalian) age. The faunal assemblage is suggestive of fluctuating marginal marine to shallow inner shelf depositional environment. 36
Sandhan Formation (Middle Miocene-Pliocene) Overlying the Chhasra Formation, a dominantly sandstone formation has been named as Sandhan Formation after Sandhan village. The type section is exposed along the Kankawati river south of Vinjhan village,where it is overlain by Quaternary and Recent deposits.It has a lower disconformable relation with Chhasra Formation. The upper contact is not distinct. It grades into Quaternary and Recent beds with increasing calcareous nodules forming ‘kankars’.In the present study the formation has been studied from the Chhasra cliff, where it overlies the uppermost beds of Chhasra Formation. The lower part of the formation consists of well sorted, medium to coarse grained, massive, micaceous, quartzose sandstone overlain by clayey, laminated siltstone and topped by yellow fossiliferous limestone beds. The middle part comprises of conglomerate and coarse sandstone. The upper part consists mainly of hard, calcareous grits, overlain by pink and grey mottled silty sandstone with calcareous nodules. Foraminifera recorded from the formation include Ammonia, Pararotalia, Elphidium and miliolids. Fossil wood is common in the lower conglomerates. Lenticular pockets of oyster debris and thin oyster bands are common in the upper conglomeratic sandstone. Several vertebrate fossils have also been reported. The formation has been assigned a Middle Miocene to Pliocene age. The sandstone appears to be littoral sands of the advancing sea after a major regression. The depositional environment thus appears to be supra littoral to deltaic or a foreshore. 37
TECTONICS The Kachchh basin is a pericratonic rift basin, formed during the breakup of Gondwanaland in the Late Triassic. The basin is situated between the subsurface Nagar Parkar uplift in north, Radhanpur- Barmer arch in east and Kathiawar uplift in south. It evolved through three tectonic phases: Rift Phase, Late rift divergent wrench phase and post rift divergent wrench phase. These correspond to breakup, drifting and collision of Indian plate respectively. The rift was aborted in Late Cretaceous, prior to the collision with Asian plate. Post collision compressive regime is responsible for present neo-tectonic movements. Rifting of Gondwanaland in the early Jurassic or late Triassic involved reactivation of Precambrian structures in the eastern as well as western parts of India, and formation of several rift basins like Kachchh, Cambay and Narmada. Geologically, the Tertiary/Quarternary sediments, Deccan volcanic and Middle Jurassic clastics resting on the Archean basement mainly characterise the Kachchh region. The Mesozoic rift related extensional structures of the basin got reactivated as strike-slip or reverse faults as a result of regional compressive stresses due to the collision of Indian and Eurasian plates since Neogene times. Focal mechanisms of the some earthquakes indicate reverse faulting suggesting ongoing inversion tectonics in the Kachchh basin. Major structural features of the Kachchh basin include several E-W trending faults/folds. The rift zone is bounded by a north dipping 38
Nagar Parkar fault in the north and a south dipping Kathiawar fault in the south. Other major faults in the region are the E-W trending Allah Bandh fault, Island belt fault, Kachchh mainland fault and Katrol hill fault. In addition several NE and NW trending small faults/lineaments are observed. Seismic,gravity and magnetotelluric surveys indicate undulated basement with 2-5 km deep sediments and MOHO depth at 35-45 km in the southern Kachchh region. TECTONIC FRAME WORK AND STRUCTURAL STYLE The Kachchh pericratonic rift is situated at the southern edge of the Indus shelf at right angle to the Indus fossil rift. Rifting took place within the Mid-Proterozoic mobile belt, which is the basement of the basin. The rifting was controlled by NE-SW trend of Mid-Proterozoic Delhi fold belt that swings to E-W in Kachchh region. The E-W striking master faults are resurgent basement faults. These faults controlled the structural style of the basin. Seismic data indicate the basement at 1.5- 2km. Depth in Banni area and that the faults are steep, sub vertical. Apparently, the tilted basement domino blocks bounded by planer faults formed the rift set up, which was covered by a thin layer (1500-2000m) of sediments. During rift inversion, the sediment layer was drape folded over the tilted up edges of the upthrusting domino blocks forming marginal flexures. E-W faults, related uplifts and drape fault design the structural style. The footwall uplift along master faults formed the highlands and intervening half-grabens formed the plain lands. The uplifts show typical geometry of tilted basement blocks at depths below the sediment cover 39
– long gently sloping back limbs and short steep and locally overturned forelimbs of folded sediment layers. The marginal hill ranges of the highlands with escarpments facing the plains of the sub-basins are marginal flexures or monoclines draping over the footwalls of the master faults. The steep forelimbs draping over the footwalls are locally over folded as fault propagated folds and are locally stretched and sheared by faults branching off from the main faults at depth below the sediment cover as a result of up thrust. However, the marginal flexures are not simple drape folds. They are in fact narrow deformation zones at the edge of the uplifts consisting of a chain of closed structures along the marginal master faults viz., domes, branchy anticlines and doubly plunging elongates anticlines of varying dimensions and shapes. The uplifts are oriented E-W along five parallel master faults (from north to south, Figs.5, 6): (1) Nagar Parkar Fault (NPF), (2) Island Belt Fault (IBF), (3) South Wagad Fault (SWF), (4) Kachchh Mainland Fault (KMF) and (5) North Kathiawar Fault (NKF). Post-rift uplift gave rise to four sub-parallel tilted upthrust blocks bordered by linear ridges: Nagar Parkar Uplift (NPU), Island Belt Uplift (IBU), Wagad Uplift (WU), Kachchh Mainland Uplift (KMU) and Kathiawar Uplift (KU). The Kathiawar Uplift (Saurashtra horst) along NKF is a quadrangular block bounded by faults on all sides. The IBU is broken into four individual uplifts presumably by unexposed transverse (NNE-SSW) wrench faults as evidenced by relative displacements and orientations: Pachcham (PU), Khadir (KU), Bela (BU) and Chorar (CU). Standing amidst the plains, these uplifts appear as chain of islands and hence collectively called Island belt. Several small fault related uplifts occur in line with the 40
bigger ones e.g., Kuar Bet, NW of PU and Khadir, Gorabir and Gangta Bets (small uplands), which occur between KU and WU. Fault bounded domes and/or faulted anticlines define these small uplifts. The positive Bouguer anomalies along the lineaments formed by these ridges indicate that these are basement highs. The E-W trend of the positive gravity lineaments changes to NE-SW across the Cambay Basin following the trend of Delhi fold belt. This confirms the control of the Pre Cambrian trends in rifting. The NE-SW orientation of CU is significant in this context.Both KMU and IBU are tilted blocks with upthrown northern edge draped by the marginal deformation zone. From north to south, the Great Rann sub-basin is a narrow graben (GRG) between NBF and IBF, the Banni half graben (BHG) is formed by southward tilting of the IB block along KMF. Similar tilting of the KMU block along NKF formed the Gulf of Kachchh half graben (GOKHG). The southern parts of the PU and KMU are effected by secondary faults parallel to the master ones along their northern margins - Katrol Hill fault (KHF) in KMU and Gordongar fault (GDU) in PU. These southern fault blocks are also tilted north side up producing steep folds in the same style as that along the marginal master faults. Each of these uplifts is thus styled by a pair of north side up step faulted blocks accompanied by flextures with chains of closed structures and inverting half-grabens forming synclinal structures: Bhuj syncline (BS) in KMU and Dhorawar syncline (DS) in PU. The WU, placed en echelon to KMU is bounded by SWF along its southern margin and is tilted down to the north. The SWF is a fault 41
zone with complicated pattern of faults and associated folds. WU is tilted down to the north along Gedi fault forming Rapar half graben (RHG) against the Bela horst. Thus, the western part of the rift is styled by south tilted step faults whereas in the eastern part the blocks are tilted down to the north. The KU is a south tilted block. Its northern fault and flexure have been eroded away and covered by Rann sediments. In PU a part of the northern flexure is exposed. The BU is a horst bounded by E-W faults with accompanying flexures and a central syncline. Most parts of the CU flexures have also been eroded away and covered by sediments excepting the westernmost part. The MH stretches NNE-SSW across GOKHG, KMU, BHG and PU and beyond. It divides the KMU and BHG almost symmetrically. No uplift is seen NW of this orthogonal high. West of the high is the featureless residual depression of the Great Rann. IGNEOUS ACTIVITY Mesozoic sediments are affected by intensive igneous activity. Igneous intrusions are fairly common in all the uplift areas, both the major and minor uplifts. All known forms of intrusive bodies are present and are mainly concentrated in the narrow deformation zones accompanying the master faults. The intrusive bodies are associated with folds and faults as dykes, sills, laccoliths and plugs. The maximum intensity of igneous activity is seen in the southwestern part of KMU, west of Median High (MH) and in the northern part of PU (Kaladongar Hill) along the marginal faults, i.e., KMF and IBF. A series of igneous plugs occur along an E-W belt in the central region of KMU, sub parallel 42
to the rift axis and close to the KHU. Some of these plugs are connected to the outliers of Deccan Trap flows capping the hills. Evidently, these plugs are the main feeder of Deccan Trap flows now exposed by erosion. The main trap province of Kachchh is about 10-20kms to the south bordering the coastal plains. The trap flows drape over the tilted eroded surfaces of Mesozoic flows and dip parallel to the overlying Tertiary beds. The plugs consist of alkaline basalts with xenoliths of spinel and olivine nodules, indicating that they are derived from upper sub-crustal lithospheric mantle at a depth of ~ 40 km. The intrusive bodies occurring as laccoliths, master dykes, massive and extensive discordant plutonic bodies, large sills as well as smaller dykes and sills associated with marginal deformation zone in the western KMU are composed of gabbroic rocks, The massive plugs and dykes swarms at the core of the bordering Kaladongar anticline and large sills associated with the folds in the eastern part of the Goradongar flexure of PU are picrites associated with lamprophyre and diorite. The trap flows are predominantly tholeiitic basalt. The association of different groups of mafic and ultramafic rocks are suggestive of different phases of igneous activity viz., synrift stage, post rift thermo-tectonic stage and inversion stage. The common occurrence of intrusive bodies with folds and faults of different generations suggests syn tectonic igneous activity at different stages of deformation. It has been suggested that several phases of thermo magmatic episodes related to the passage of Reunion plume during Late Cretaceous Deccan volcanic event and related under plating in the upper crust. 43
TECTONIC EVOLUTION The style and the nature of the structures suggest repeated tectonic movements involving both vertical and horizontal stress along primordial faults. The knee and ankle geometry of the marginal flexures draping over the edges of uplifts seems to be forced folds developed by vertical forces. Up thrust of basement blocks (footwall uplift) along quasi-vertical faults with normal separation caused drape folding of blanketing sediment layer. The subsequent convergence caused strike- slip movements along steep faults, which deformed the marginal flexures into strings of folds of varying geometry. The marginal fold belts are narrow, 5km wide, along the uplift edges forming hill ranges. The individual folds are separated by well-developed saddles, which are effected by fault conjugate to the movements along the main faults. Along the marginal faults, steeply upturned Tertiary beds (deposited in the adjacent half-grabens) are seen against the folded Mesozoic suggesting post –Tertiary tectonic movement. The KMF and SWF appear to be the parts of the same fault, KMF extending eastward with left side–stepping as SWF. This suggests that KMF/SWF is a major strike slip fault. The left stepping KMF/SWF seems to be the principal strike slip fault and main architect of the structural style. It divides the basin into two main domains of sedimentation, the BHG and GOKHG. WU block is apparently shifted dextrally with respect to KMU block. In fact, all the uplift blocks are located in the eastern part of the basin east of MH and their occurrence shows en echelon arrangement with respect to KMU and right lateral 44
shift along respective faults. The change of up thrown side in case of right lateral KMF left stepping as SWF suggest convergent wrenching with transpression in the step over zone. In the north, the uplift of BU against down tilted WU (RHG) along south dipping GF also suggests convergent wrench movement. Further, breaking up of the marginal flexures into faulted closed anticlines and domes, appear to be caused by transpressional tectonics, which modified the original flexures during the late stage compressive episode. Some of these folds are highly faulted e.g., Jhurio Dome at the northern edge of KMU. These structures appear to be flower structures related to strike-slip movements. The rifting was initiated during Late Triassic break up of Eastern Gonwanaland by crustal stretching. The rift expanded from north to south by extensional activation of E-W striking primordial faults. The RG formed first and remained shallow with initial fan conglomerate and continental sediments of Rhaetian age. Successive southward younging half grabens formed with increasing thickness of sediments towards the master fault, NKF. This is evident from outcropping of the oldest sediments (Late Triassic /early Middle Jurassic) in the northernmost IBU and successive younger rocks in WU and KMU. The Banni and Gulf of Kachchh half-grabens were the main depositional domains separated by KMF, which acted as the principal intra-rift fault along the rift axis. The latter, GOKHG subsided most along the master fault NKF accommodating thickest sediment fill. Evidently, the basin evolved in two stages - 1) an extensional rift phase, when Mesozoic sedimentation took place, and 2) a compressional inversion stage in Late Cretaceous, when the present 45
structural style evolved. Due to near vertical fault planes bounding basement blocks and drift motion of plates, strike slip movements dominated the inversion stage. Same set of faults were first reactivated as normal fault during rift phase and later as transpressional strike slip faults during inversion stage. Evidences suggest that there was a transitional phase between the two, when vertical stress was dominating. The rift was aborted during this phase when footwall uplifts were enhanced by upthrust producing marginal flexures. Later, horizontal stress developed and strike-slip movements took place along steep faults. As indicated by block orientation and fold geometry, in the initial stage of horizontal stress transtensional strike slip movement prevailed followed by transpressional tectonic which is active till today and is responsible for causing earthquakes. The inversion stage, therefore, had two phases of wrenching – a divergent followed by a convergent phase. The tectonic phase was well correlatable with the major movements of the Indian plate. The Kachchh basin originated as pericratonic rift during break up of Africa-India in Late Triassic. Normal faulting along primordial trend formed a series of half –grabens successively from north to south. Rift fill sedimentation continued till Early Cretaceous. In Late Cretaceous, the trailing edge of the Indian plate uplifted as the leading edge, was pulled down towards the Tethyan trench. The rifting at the trailing edge of the plate was aborted at this time and the region was uplifted. Reversed movement of blocks along steep faults (normal faults during rifting) took place. The uplifts came into existence by upthrust of footwall blocks. This was the time of 46
rift-drift transition. During the drift stage, the horizontal stress increased and wrench related structures developed. This tectonic episode reached its acme in Late Cretaceous when the plate motion was accelerated considerably prior to its collision with Tibetan plate. At this stage divergent wrench movements set up the framework of the present structural style. Post collision stage was the period of compression and wrench movements were mainly convergent forming compressional structures. This tectonic movement continued episodically till present day as evident from neo tectonic movements. First order topography, local thrusts on the back limbs of uplifts, local occurrence of fault propagated folds, over turned limbs of flexures, linking of folds by lateral migration along master faults are some of the evidences of neotectonic activities. Absence of younger sediments in the Mainland half-graben (BS) against KHF in KMU and Kaladongar half-graben (DS) against GDF tend to suggest that these faults KHF and GDF are late generation faults originated by thrusting during the tectonic inversion cycle. This is supported by southward dip of these faults and reverse separation. The tectonic episodes were accompanied by deep crustal magmatic activities. At least two phases of magmatic activities are evident. One during extensional stage when gabbroic rocks intruded into the older Jurassic sediments and the other during Late Cretaceous post-rift uplift stage when plume related alkali and tholeiitic basalts were intruded into the younger Early Cretaceous sediments. The basin fill comprises two super sedimentation cycles – Late Triassic – Cretaceous Mesozoic cycle and Late Paleocene –Pliocene – Quaternary cycle separated by late tectonic igneous activity and volcanic flows across 47
K/T boundary (Deccan Trap). The Mesozoic sediments were deposited in two mega cycles – a Late Triassic –Late Jurassic synrift marine transgression cycle and Late Jurassic – Early Cretaceous post rift regressive deltaic cycle. The rift sedimentation was terminated by Late Cretaceous tectonic episode. The Mesozoic mega-cycle include many sub-cycles in response to tectonic controlled sea level changes. The subsequent Tertiary sedimentation was restricted to peripheral slopes of uplifted Mesozoic terrain controlled mainly by sea level changes. These sediments are tectonically less disturbed since they post-date the main Late Cretaceous tectonic episode and are affected only by the low intensity reactivation of principal faults during Tertiary. 48
REFERENCES Biswas, S.K., 1977. Notes on the geology of Kutch, Quart. Jour. Min. Met Soc., India, 44, 223-236 Biswas, S.K., 1982.Rift basin in western margin of India with special reference to Kutch Basin and its hydrocarbon prospects, Bull. Amer. Assoc. Petr. Geol., 66 (10), 1497-1513 Biswas, S.K., 1992.Tertiary Stratigraphy of Kutch, J. Paleont. Soc.India, 37, 1-29 Biswas, S.K., and Raju D. S. N., 1971. Note on rock- stratigraphic classification of tertiary sediments of Kutch. Quart. Jour. Min. Met Soc., India, 1 Biswas, S.K., and Raju D. S. N., 1973. The rock-stratigraphic classification of the Tertiary sediments of Kutch. Bull. ONGC, 10 (172), 37-46 Catuneanu Octavian and Dave Alok, 2017. Cenozoic sequence stratigraphy of Kachchh Basin, India, Marine and Petroleum Geology 86 (1106-1132) Pandey,J. and Dave Alok,.1993. Studies in Mesozoic foraminifera and Chronostratigraphy of Western Kutch, Gujarat, Paleontographica Indica –1, KDMIPE, ONGC publication, Dehradun Pandey,J. and Dave Alok,1998. Stratigraphy of Petroliferous Basins of India, Presidential Address, XVI Indian Colloquium on Micropaleontology and Stratigraphy, Goa Zutshi, P.L.Mittal. S.K., Shah,L.,1993. Lithostratigraphy of Indian Petroliferous basins _ Kutch –Saurashrta basin. KDMIPE, ONGC, Dehradun publication, 1-49 49
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