Pericratonic Basins: The Mesozoic Scenario

Abstract

The northward drifting India experienced crustal extension during the Late Triassic to Early Cretaceous period due to India moving and rotating away from Australia and Antarctica and attendant transformation along the Coromandal coast.

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

15.1 Crustal Extension in Continental Margins

The northward drifting India experienced crustal extension during the Late Triassic to Early Cretaceous period due to India moving and rotating away from Australia and Antarctica and attendant transformation along the Coromandal coast. The leading edge of the Indian continent subsided deeper into the NeoTethys Sea in the north and its western and eastern passive margins started sinking intermittently, giving rise to pericratonic basins. Embracing the whole of the Himalaya domain, the NeoTethys Ocean spread all over the subsiding part of the continental margin in western Rajasthan and Kachchh. The Indian Ocean likewise encroached on the sinking parts of the Kaveri–Palar, the Krishna–Godavari, the Mahanadi, and the Bengal–Meghalaya sectors of the passive continental margin (Fig. 15.1). The sagging of the continental crust took place along faults and lineaments of the Precambrian antiquity. This gave rise to rift-related depressions and ridges in the Late Triassic in the Jaisalmer sector, during the Middle Jurassic in Kachchh, and in the Upper Jurassic in the Coromandal Belt. There was also a seaward tilting of rift basins, leading to formation of embayments and gulfs in the Coromandal Belt and in Kachchh. During the Late Cretaceous, an arm of the sea extended deep into the land along the Narmada rift zone.

Fig. 15.1

The NeoTethys Ocean in the north and north-west, and the Indian Ocean in the south-east and east overspread the subsiding pericratonic margin of India affected by crustal extension

The pericratonic basins, it may be recapitulated, formed due to rifting of the continental margin and evolved in stages on the passive margins. The reactivation of subparallel faults and lineaments of considerable antiquity was responsible for their evolution. The sediments that accumulated in these basins bear strong stamp of the oceanic environments with which they were associated. The basins of Kachchh, Jaisalmer and Himalaya belong to the NeoTethys faunal province and the Bengal–Meghalaya, the Krishna–Godavari and the Kaveri–Palar basins had affinity with the Australasian domain of the Indian Ocean. There is preponderance of lamellibranchs, relative poverty of gastropods and corals, richness in the assemblage of ammonites, and notable presence of Mediterranean Hippurites in the Kachchh, Jaisalmer and Himalaya sectors (Krishna 1987). The Coromandal domain in the south-east is characterized by dominance of cephalopods, notable presence of gastropods and warm-water reef-building corals, and absence of Hippurites. Evidently, the NeoTethys Ocean and the Indian Ocean were not mutually connected in the Mesozoic.

15.2 North-eastern Continental Margin

15.2.1 Bengal Basin

During the Mesozoic era on the passive continental margin in the northern part of Bengal and Bangladesh, there was a 100-km-wide shelf that deepened eastwards into the ocean basin (Johnson and Alam 1991). It began to subside in the Lower Cretaceous (at 127 Ma) when the Indian plate drifted away from Antarctica. The shelf is characterized by transverse Gondwana grabens filled with Late Permian sediments of the Kuchma and Paharpur formations in the Bangladesh grabens and their equivalents in the Gondwana domain of Jharkhand and West Bengal. Unconformably overlying is a pile of volcanic rocks with intertrappean beds constituting the Rajmahal Formation of the earliest Cretaceous age. The Late Cretaceous subsurface succession comprises Sibganj, Dhananjaypur and Bolpur formations (Roybarman 1992; Khan et al. 1994, 1996; Alam 1989; Alam et al. 2003). All these formations lie under the thick pile of Indo-Gangetic sediments. The Early Cretaceous Sibganj Formation is an accumulation of trapwash with ferruginous red sandstone and claystone. It grades into the Dhananjaypur/Bolapur successions towards the west. Consisting of dark grey shale, the Dhananjaypur Formation represents the earliest marine transgression in this basin. The overlying Bolpur Formation comprises a fluvial facies made up of arkosic sandstone.

15.2.2 Meghalaya Region

In the Jaintia and Khasi Hills in southern Meghalaya, the 150- to 200-m Cretaceous succession is exposed as thin strips of outcrops bordering the southern faulted margin of the Meghalaya massif (Figs. 15.2 and 15.3). The rocks dip gently southwards, and in Bangladesh are concealed under an alluvial expanse. Resting unconformably on partly Sylhet Volcanics and partly Precambrian gneisses, the Jadukata Formation along the Dauki Fault zone is a Lower Cretaceous fluvial deposits made up of conglomerate–pebbly sandstone alternation. The 150- to 180-m-thick succession of conglomerates intercalated with subarkose and volcanifeldspathic wackes is known as the Mahadek Formation (Fig. 15.3). The Mahadek onlaps both the Sylhet Volcanics and the Precambrian gneisses. A characteristic element of the Mahadek in many places is the localized occurrence of pyroclastic beds (Kak and Subrahmanyam 2002). Above a layer of laterite, the Upper Mahadek comprises marine sediments containing Companian–Maastrichtian nannofossils (Jafar 1996) and foraminifers Globotruncana linnaena and Globotruncana ventricosa (Raju and Mishra 1996; Raju et al. 2002). The homotaxial Sibganj Formation is characterized by pollen Aquinopollenites indica—assemblage of the Upper Cretaceous (Reimann 1993). Near Singrimani in the Garo Hills Gondwana, equivalent rocks are present.

Fig. 15.2

Tectonic framework of the Meghalaya plateau showing occurrence of ultramafic–alkaline–carbonatite complexes in the context of the Sylhet Volcanics and the Cretaceous—Tertiary sedimentary succession (after Srivastava et al. 2004a)

Fig. 15.3

Extent of the exposed Mesozoic, including the Cretaceous formations in eastern continental margin (modified after Mishra and Sen 2004)

15.2.3 Ultrabasic–Alkaline–Carbonatite Complex

Related to a N/NE–S/SW trending shear zone and to the Sylhet Volcanics (Fig. 15.2) is a 7- to 8-km-wide dyke swarm, made up of peridotite, pyroxenite and meliltolite lamprophyre, constituting the Jasra Complex (Mamallan et al. 1994; Srivastava and Sinha 2004a, b). It is intruded by minor veins of ijolite, tinguite and nepheline syenite. There are two intrusive bodies of carbonatite in the Karbi Along district. Post-dating the main event of the Sylhet flood basalts by about 10 million years, there was emplacement of the carbonatite–alkaline-rock complex in the Sung Valley Jaintia Hills, Samchampi and Barbang (Mikir Hills). Geochemical and isotopic data indicate that the Sung Valley carbonatite was derived from and interacted with an isotopically heterogeneous mantle and that the carbonatites show εNd values of +0.7 to +1.8, εSr values of +4.7 to +7.0—within the range exhibited by the ocean island basalts of the Kerguelen province in the Indian Ocean, implying linkage of the two (Srivastava et al. 2005). The weighted mean ⁴⁰Ar–³⁹Ar age of the Sung Valley Complex is 107.2 ± 0.8 Ma (Ray and Pande 2001).

15.2.4 Mineral Deposits

In West Khasi district about 140-km SW of Shillong at Domiasiat (Fig. 15.3) and at Phlamgdiloin, rich uranium deposits are associated with sedimentary fills of a braided river characterized by plant litter and biogenic pyrite (Sengupta et al. 1991; Singh 1992; Maithani et al. 1995). Occurring in the Lower Mahadek, the Domiasiat deposit of pitchblende and coffinite is world’s largest deposit of its kind.

15.3 South-eastern Continental Margin

The Coromandal coast embraces the Eastern Ghat terrane and the deltaic expanses of the Krishna–Godavari and the Kaveri–Palar rivers and their offshore equivalents (Sastry et al. 1973). The Mesozoic formations are exposed few and far between in the coastal stretch, but their larger parts lie concealed under younger deltaic and marine sediments of coastal and offshore shelf (Figs. 15.4, 15.5, 15.6 and 15.11; Table 15.1). The sediments here were deposited in the grabens. The East Coast, stretching from Athagarh in Odisha to beyond Ramanathapuram in Tamil Nadu, came under the sway of marine waters towards the end of the Jurassic period. Demarcated by an east–west trending boundary fault, the 700-m-thick Athgarh Sandstone represents an extensive alluvial fan, the distal part of which grades into the lacustrine deposits in the centre of the basin (Mishra et al. 2004) and into deltaic deposits towards the shelf. The Athagarh succession in the Mahanadi domain comprises sandstone with thin intercalations of claystone and carbonaceous shale. The shale is characterized by Early Cretaceous flora of the Upper Gondwana Rajmahal affinity (Patra 1973). Evidently, there was interfingering of land-derived sediments (with their plant contents) and the marine sediments containing invertebrate fossils, indicating a delta-front environment.

Fig. 15.4

a Tectonic setting of the Krishna–Godavari domain (after Majumdar et al. 1995). b Exposed Mesozoic formations in the coastal stretch of Andhra Pradesh (after Prasad and Pundir 1999)

Fig. 15.5

Structural architecture and lithostratigraphic pattern of the Krishna–Godavari Basin. Sections show the sources and the nature of the traps of the hydrocarbon deposits (modified after Majumdar et al. 1995)

Fig. 15.6

a Graben-and-horst structure developed in the Coromandal Coast, on the passive continental margin of India (modified after Sastry et al. 1977). b Exposed Jurassic and Cretaceous formations in the Kaveri–Palar Basin in Tamil Nadu (after Pandey and Dave 1998)

Table 15.1 Cretaceous lithostratigraphy of the Krishna–Godavari Basin (based on Biswas 2003)

15.3.1 Krishna–Godavari Basin

The coastward extension of the Gondwanic Pranhita–Godavari graben is terminated by the NE–SW trending Barpatla Ridge along the Eastern Ghat characterized by ridges and grabens (Manmohan et al. 2003) that became sites of sedimentation (Figs. 15.4, 15.5 and 15.11). The tilting of later basins south-eastwards resulted in drowning of the coastal land. In the lower reaches of the Pranhita–Godavari Valley, towards the delta region, the Gondwana sediments rest on the Archaean basement. The Early Permian Draksharama Formation is made up of grey black shale alternating with claystone, sandstone and coal seams. Overlying is the Mandapeta Sandstone, comprising intercalations of shale that have yielded Triassic assemblage of palynomorphs. Unconformably overlying is the Bapatla Sandstone, characterized by Upper Jurassic assemblage of dinoflagellates and acritarchs (Mehrotra et al. 2002). The Cretaceous starts with the Gollapalli/Budavada Sandstone that rests upon the Gondwanic Mandapeta Sandstone. In the marine shelf, carbonates and clastic sediments formed a fan-shaped delta along the ridges; and turbidites covered the slopes and the graben floors. By the Campanian epoch, a large delta had emerged and was prograding rapidly (Prabakaran and Ramesh 1995). The Gollapalli/Budavada comprises micaceous ferruginous gritty sandstone and shale containing abundant plant fossils of the Upper Gondwana affinity such as Williamsonia, Taeneopteris, Cladophlebis, Otozamites, Ptilophyllum, Pterophyllum and Elatocladus of the Neocomanian age. The plant remains imply considerable contribution of terrestrial detritus and plants by rivers draining upland and coastal region. As everywhere in the pericratonic basins of India, the plant fossils along with detritus were washed down into the sea, resulting in the admixture of marine and terrestrial materials. It could also be a large prograding delta lobe over the coastal region where fluvial and marine sediments interfingered (Biswas 2003). The Raghavapuram Shale represents the first marine transgression. It comprises a succession of brittle shale and soft claystone, the latter characterized by a rich assemblage of ammonites, lamellibranchs, brachiopods and foraminifers. Dinoflagellete cysts, ammonites and palynofossils indicate Barremian to Early Aptian age of the Raghavapuram Shale (Prasad and Pundir 1999). The Tirupati Sandstone is a formation of coarse- to medium-grained white grey sandstone containing pollens of the Cenomanian angiosperms and gymnosperm plants (Kapoor et al. 1999).

The Tirupati Sandstone and its offshore equivalent to the Chintalapudi Sandstone are conformably overlain by the Rajahmundry Volcanics/Razole Volcanics of the Palaeocene age.

15.3.2 Hydrocarbon Deposits of Krishna–Godavari Basin

Rich in organic material, the Raghavapuram Shale is the source rock of oil and gas; and the associated sandstone serves as the reservoir rock in the offshore zone. The deposits (Fig. 15.5) are confined to the lower part of the Raghavapuram where high-gamma and high-resistivity anomalies have been detected (Manmohan et al. 2003). The gas-bearing sandstone straddles across the unconformity on top of the high-gamma and high-resistivity horizon at the base of the formation. According to some workers, this part of the Raghavapuram Shale represents the deposit of an estuarine valley fill, deposited in lowstand tract. An incised valley carved out earlier in the underlying unit was filled back by sediments during the Cenomanian–Turonian transgression (Chitra Rao and Asthana 2000). This is the feature which had a significant bearing in the formation of the hydrocarbon deposits. Over 325-m-deep wells (4600–5200 m) have identified more than 160 structures of hydrocarbon deposition, out of which 48 have been proved hydrocarbon-bearing. The Mandapeta gas field is related to the Permian sandstone, and the Lingala oil field to the Lower Cretaceous Raghavapuram Shale (Rao 2001, 2002). It may be mentioned that besides the Mesozoic deposits of hydrocarbon, there are Tatipaka–Pasarlapudi gas field and Mori oil field related to Lower Eocene and Miocene-connected Ravva oil field (Chandra et al. 1994; Rao 2002) (Table 15.2).

Table 15.2 Jurassic–Cretaceous lithostratigraphy of the Kaveri–Palar Basin in Tamil Nadu (after ONGC 1993)

15.3.3 Kaveri–Palar Basin

Like the Krishna–Godavari Basin in Andhra Pradesh, the Palar–Kaveri Basin in Tamil Nadu coast is characterized by horst-and-graben structures beneath a thick cover of Tertiary–Quaternary sediment (Figs. 15.6 and 15.11; Table 15.2). The evolution of the structural depression commenced in the Late Jurassic and continued all through the Cretaceous and the Cenozoic (Banerji 1972; Sastry et al. 1968; Sastry et al. 1977). In the Santonian interval 87.5–84 million years ago, strong faulting caused subsidence of high grounds, leading to development of steeply sloping river valleys, acceleration of terrestrial erosion, transgression of the sea, and high influx of detrital sediments into the basin (Ramkumar et al. 2005). The basin extends over offshore with an easterly tilt, as is evident from progressive eastward-thickening of the sedimentary succession. The larger basin comprises five subbasins, each representing a graben oriented in the NE–SW direction parallel to the coast (Fig. 15.6a).

Resting on the Archaean gneisses–khondalites or locally on patches of the inland Gondwana sedimentary rocks, the Mesozoic succession begins with the Pre-Albian Sivaganga Formation near Ariyalur and its near temporal equivalent the Therani Formation near Thanjavur. Characterized by calcareous concretions in feldspathic sandstone and conglomerate at the base, the Sivaganga Formation forms isolated hillocks in Ramanathapuram. It comprises fluvial, lacustrine and paludal sediments and is characterized by plant fossils (filicales, cycades, gingkos and conifers) of the Upper Jurassic to lowermost Cretaceous age. The plant assemblages represent terrestrial material washed down by streams into a tide-swept estuary. Or possibly there was interfingering of fluvial and marine environments of deposition. The claystone in the upper part of the Therani Formation contains ammonites and arenaceous foraminifers of Neocomanian–Aptian age (Mamgain et al. 1973; Banerjee 1982). Palynormorph assemblage together with plant Ptilophyllum corroborates the Early Cretaceous time span of the Therani–Sivaganga succession (Venkatachala 1977). In the northern Palar Basin, the Satyavedu Formation in the north, the Avadi Formation in the central part and the Sriperumbadur Formation in the south represent the Neocomanian–Aptian succession (Kumaraguru and Trivikram Rao 1994).

The overlying marine formation Dalmiapuram comprises coralline-algal reef limestone with black shale. Besides ammonites, there are foraminifers and ostracodes of the Aptian to Albian age (Jain 1968; Bhatia and Jain 1969; Banerji 1970; Banerji et al. 1996). The Dalmiapuram Limestone is intensively used for manufacture of cement.

The Uttatur Group comprises reefal limestone, greenish grey gypseous shale and black carbonaceous shale. The bioherms developed during the Middle Aptian to Cenomanian time, along an edge of the block associated with an active fault parallel to the coast, as borne out by breccias and conglomerates intimately associated with the reefs. The bioherm, that grew as high as 150 m, is composed of sponges and a variety of scleractinid corals and algal crust (Steinhoff and Bandel 2000). The Karai claystone with gypseous shales and phosphatic nodules in the Uttatur Group is very rich in planktonic and benthic foraminifers, the δ¹⁸O values of which indicate near-surface sea water temperature of 29 °C and seafloor temperature of 21 °C (Dasgupta et al. 2006). The Late Cretaceous Trichinopoly Group is made up of Garudamangalam and Paravay formations, comprising calcareous gritty and conglomeratic sandstone with bands of sandy limestone and gypseous shale and marlite. The Ariyalur Group comprises cross-bedded green sandstone (locally conglomeratic) interbedded with grey and purple shale. The biohermal limestone within the succession is known as the Kallankurichchi Limestone (Sundaram et al. 2001). Phylloceratina, Lytoceratina and Ammonitina found near Pondicherry comprise most diverse Upper Maastrichtian ammonite assemblage anywhere known (Kennedy and Henderson 1992). The Kallakkudi Limestone (Albian–Cenomanian) and the Kallankuruchchi Limestone (Maastrichtian) are composed of packstone and greenstone, representing carbonate shoals. These two horizons have yielded condensate with 49° API gravity and hydrocarbons (Yadagiri and Govindan 2000; Govindan et al. 2000). The Early Cretaceous shale deposits that accumulated in depressions under anoxic conditions are the sources of hydrocarbons (Fig. 15.7) in the Kaveri–Palar Basin (Raju and Mishra 1996; Raju et al. 2002). The petroleum fields of the offshore zone tap these pay horizons. The top part of the Creataceous succession of the Kaveri Basin records gradual change of climate and sea level as reflected in the change in isotope geochemistry of sediments, which received increased influx of detrital material from land (Ramkumar et al. 2004).

Fig. 15.7

Location of oil and gas fields in the Kaveri–Palar Basin (after Govindan et al. 2000)

The names Uttaltur, Trichonopoly and Ariyalur were given by H.F. Blanford in 1862. The subsurface Cretaceous succession exhibiting considerable lateral facies variation in the onland part of the basin (Fig. 15.8) extends eastwards over the continental slope. The offshore extension of the Uttattur Group is represented by the Andimadam Conglomerate and the Sattapadi Shale ranging in age from Pre-Albian to Cenomanian. The Sattapadi Shale is overlain by the Bhuvanagiri Formation made up of predominant sandstone with minor shale. The Bhavanagiri contains Cenomanian to Conacian species of Rotalipora and Marginotruncana. The offshore facies of the Ariyalur Group are described as the Palk Bay Formation of calcareous sandstones, the Kudavasal Shale, the Nannilam Shale, the Porto Novo Shale and the Komarakshi Shale containing Campanian to Maastrichtian species of foraminifers including Rosita, Globotruncana and Abathomphalus (Raju and Mishra 1996). The faunas indicate the depositional environment varying from outer shelf to bathyal zone.

Fig. 15.8

Cross section shows subsurface structure and lithostratigraphy of the Kaveri–Palar Basin (Sastry et al. 1997)

15.4 North-western Continental Margin

15.4.1 Kachchh Basin

The north-western part of the Indian continental margin experienced lithospheric stretching and rifting in the Late Triassic (Biswas 1982, 1987, 2003, 2005). The resulting depression (rift valleys) is oriented east–west (Figs. 15.9 and 15.11; Table 15.3). The depositional sites were of the nature of embayment between the uplifted Tharad–Nagarparkar Ridge in the north, the raised-up Saurashtra High in the south and the Radhanpur–Barmer High in the east. A NNE–SSW trending high divides the basin into two parts—the western part exhibiting progressive thickening and facies variation of sediments, and the eastern part having shallow-water sediments (Biswas and Deshpande 1983). The sediments, laid down on the Archaean and Proterozoic basements, provide a coherent record of stratigraphy of the Mesozoic (Agrawal 1957, 1977, 1981; Krishna 1987). The lithostratigraphic nomenclature was proposed by W. Waagen in 1873.

Fig. 15.9

Rifting of the north-western continental margin gave rise to the Kachchh Basin, comprising a series of ridge- and half-graben-oriented east–west (after Biswas 2005)

The sedimentation in the Kachchh Basin (Figs. 15.9, 15.10 and 15.11; Table 15.3) started in the Middle Jurassic time. The Patcham Formation (also described as Jhurio) comprises Bajocian–Early Callovian carbonate–shale association. The bioclastic limestone of the upper part contains rich assemblage of corals, molluscs, brachiopods, ammonites and bryozoans and foraminifers. In the lower part, the shale is interbedded with thick limesone with golden oolites. The overlying Chari Formation (Jumara) comprises laminated shales with intercalation of siltstone, marl and bioclastic limestone of deltaic environment. The limestone is oolitic; the characteristically golden coloured oolitic limestone is known as the Dhosa Oolite. The sedimentary rocks making the Jhumra, Juma and Habo domes in western Kachchh contain ostracode assemblages indicating their Bathonian to Callovian (Middle and Upper Jurassic) ages, and also pointing to the similar assemblages in Laurasian as well as Indo-East African provinces along the Tethys. Across a paraconformity, the thick coursing upwards succession of cross-bedded sandstone and shale characterized by ferruginous concretions of the Katrol Formation (Jhuran) belong to the Upper Jurassic to very Early Cretaceous (Kimmeridgian to Neocomanian) period. It consists of yellow brown cross-bedded feldspathic sandstone with cyclic interbeds of yellow-laminated sandstone, grey carbonaceous shale and locally kaolinitic white shale and dark brown to purple bioturbate siltstone. Even as littoral zone expanded and the delta complex grew into the open sea to the west, the fluvial processes had become active (Krishna et al. 1998).

Fig. 15.10

Sketch map showing the Jurassic–Cretaceous formations in the Kachchh Basin (after Biswas 1992)

Fig. 15.11

Lithological logs of the Mesozoic formations of the Kachchh and the Jaisalmer basins

Table 15.3 Mesozoic lithostratigraphy of the Kachchh and Jaisalmer basins in north-western continental margin of India

The Jurassic succession is overlain probably with break by the Cretaceous rocks, 1000–1200 m in thickness (Table 15.3) and ranges in age from Berriasian–Valanginian to Late Turonian–Coniacian (Krishna 1991). Fission-track dating of glauconite from the uppermost horizon (Bhuj Formation) placing it between 112 and 89 Ma (Biswas 2005) confirms the early Upper Cretaceous age of the youngest horizon of the Mesozoic of Kachchh. The Bhuj Formation (Table 15.3), made up of Ghuneri, Ukra and Upper Unit, comprises grey silty shale with limonitic parting and locally carbonaceous shale. As elsewhere, there is a rich assemblage of Ptilophyllum flora, understandably derived from the land through rivers and streams. The bioturbated sandstone characterized by iron oxide enrichment and leaf impressions contains Middle Albian nannofossils. The Ghuneri Member is made up of rhythmic alternation of red and yellow sandstone and shale and with bands of ironstone or ferruginous sandstone and lensoid bodies of coal formed in a deltaic environment. The Ukra Member consists of glauconitic sandstone, shale and oolitic limestone and marl that contain abundant ammonites and pelecypods as well as fossil wood. Occurring in the westernmost part of the basin, this horizon represents a short transgressive episode in the delta-front during sea-level highstand in the Aptian time. Plant fossils Ptylophyllum, Otozamites, Cladophlebis and Williamsonia recall upper Gondwana flora. It is overlain by Upper Member comprising cross-bedded coarse-grained sandstone with ferruginous bands, limestone and white kaolinitic clays. The hummocky cross-bedding indicates interference of storm waves with ebb-tide currents along a prograding shoreline (Bose et al. 1986). Such features such as wavy lamination, ripple drift lamination and flaser bedding indicate that the sediments of the terminal horizon of the Kachchh Cretaceous were deposited in tidal channels and tide-affected shelf embracing tidal flats and lagoons (Krishna et al. 1983).

In the Wagad–Rapar–Adesar area in north-eastern Kachchh, the feldspathic, locally ferruginous and conglomeratic sandstone with minor shale contain both marine fossils and plant remains of the Lower Cretaceous age. These sediments represent nearshore deposits in a tidal environment where the distributary channels had NW–SE direction (Bandyopadhyay 2004).

It may be stressed that the Jurassic rocks of Kachchh have acquired global importance for their exceptionally rich ammonite assemblages, particularly of the Callovian–Tithonian (Upper Jurassic) time span (Krishna 1987, 1991; Krishna and Cariou 1986; Krishna et al. 1998). The ammonite assemblage, including Gregoryceras fouguei, Euaspidosa, Perisphinctes and Epimayaites, implies connection of the Kachchh fauna with that of Indo-African faunal province. The horizon of the Dhosa Oolite, an important stratigraphic marker, is characterized by a faunal assemblage that contains elements of both the Indo-African (Mayaitinae) and the Tethyan (Perisphinctinae) faunal provinces.

In north-eastern Saurashtra, there is a 500- to 600-m-thick succession of cross-bedded feldspathic and ferruginous sandstones with intercalations of shale containing leaf impressions of the Gondwana affinity and local layers of conglomerate above the Early Proterozoic basement. The succession is known as the Dhrangadhra Formation (named by F. Fedden in 1884). This succession (Fig. 15.12) is time transgressive of the Wadhwan Beds in the Surendranagar district. The two formations are regarded as the inland facies of the marine Cretaceous. The upward-coarsening lower part was formed by the distributaries of a delta in the proximity of the shoreline, while the upper part is made up of deposits of subtidal to tidal and beach zones (Casshyap and Aslam 1992; Aslam 1992). At the top is a horizon of fossiliferous limestone interbedded with pebbly sandstone and mudstone deposited in embayments and estuaries. The calcareous upper part interbedded with lava flows of the Wadhwan contains, besides the plant Cladophlebis, remains of a fish of the Upper Tithonian to Albian age (Borkar 1973).

Fig. 15.12

Pattern and environment of deposition of the sediments of the Dhrangadhra succession in north-eastern Saurashtra (after Casshyap and Aslam 1992)

The Sadara Sill in the Pachchham area in Kachchh having composition transitional between basalt and basanite, and characterized by Sr, B, Pb and LREE enrichment and Nb, Cr, Y, Cs, Lu depletion, shows magnetic pole in the pre-Deccan Volcanism period in the 85- to 91-Ma interval (Ray et al. 2006). It must have been emplaced along faults resulting from the rifting of the crust in Kachchh.

15.4.2 Jaisalmer Basin

The Jaisalmer Basin is one of the four basins developed in the Rajasthan continental margin—the Bikaner–Nagaur Basin, the Barmer Basin, the Sancher Basin and the Jaisalmer Basin, the last one being pericratonic (Datta 1983). The Bikaner–Nagaur Basin is a Precambrian–Palaeozoic Basin, and the Barmer and Sancher basins evolved in the Middle Jurassic. In the Jaisalmer Basin (Fig. 15.13; Table 15.3), sedimentation commenced in the Permian time and continued throughout the Mesozoic and the Cenozoic. It may be pointed out that while the Barmer Basin in the south received its sediments dominated by heavy mineral hornblende from this Mewar–Marwar Ridge, the Jaisalmer Basin was fed by rivers draining the Jaipur–Ajabgarh–Alwar region of the central Aravali Range, as testified by staurolite, kyanite and garnet (Siddique 1963). The 1200-m-thick Jaisalmer succession on the continental shelf is characterized by several cycles of carbonate sedimentation.

Fig. 15.13

Sketch map showing the Mesozoic formations of the Jaisalmer Basin and adjoining areas in western Rajasthan. Important structural lines delineate the tectonic boundaries (based on Singh 1996 and Sinha-Roy 1984)

The Barmer Basin (Fig. 15.14) is a narrow N–S-oriented fault-lined depression. In the Barmer Basin, the Lower Cretaceous Sarnu Formation is exposed along the western-faulted margin. The horizon consists of dominant fine-grained to pebbly feldspathic sandstone with plant fossils (the Sarnu is unconformably overlain by Palaeocene Fatehgarh Formation). The intertidal to neritic shelf deposit at the base gives way upwards to a fining-upward succession of sandstone, locally ferruginous and phosphatic and containing fossil wood. Its upper part is made up of tidal-flat mud (Sisodia and Singh 2000).

Fig. 15.14

Structure and lithostratigraphy of the Barmer Basin illustrates the pattern and sequence of the Mesozoic sedimentation in Rajasthan (after Sisodia and Singh 2000)

In the Jaisalmer Basin (Fig. 15.13 and Table 15.3) overlying the Permian Karampura Formation, the Shumarwali Formation of the Triassic time comprises a sequence of fluvial to deltaic and shallow epineritic clastic sediments. The Early Jurassic Lathi Formation represents the continuation of the Triassic sedimentation nearly the same environment on an intertidal to neritic shelf. The sediments of the overlying Jaisalmer Formation were laid down on the extensive stable shelf, in which several cycles of carbonate depositions took place. The situation then changed, and sandstone–shale alternation of the Baisakhi and Bhadasar formations was laid down in the Upper Jurassic time. The Cretaceous period commences with the deposition of yellow brown siltstone–sandstone alternation, calcareous feldspathic sandstone and claystone (with a fossil of tree trunk) making up the Pariwar Formation. Palynomorph assemblage and arenaceous foraminifers indicate Neocomanian age of the deltaic sediments of the Pariwar Formation (Das Gupta 1975; Dave and Chatterjee 1996; Mehrotra et al. 2002). The overlying Habur Formation comprises alternation of cross-bedded calcareous sandstones and coquinoidal limestone, foraminiferal limestone and shale. This nearshore sequence is characterized by the Lower Aptian to Middle Albian ammonites and foraminifers (Krishna 1987). The succeeding Goru Formation consists of greenish grey locally pyritous shale, and calcareous siltstone with marl. The glauconite grains give Rb–Sr age of 103.3 ± 3.0 Ma (Vijan et al. 2000) confirming the Albian age. The thick succession of argillaceous limestone and marl exposed in the Parh Hill in Sindh (Pakistan) and its subsurface easterly extension in western Rajasthan is known as the Parh Formation. The Parh contains Coniacian fossils (Mukherjee et al. 1995). At the top of the Cretaceous formation, the horizon described as “Siliceous Earth” contains volcanic ash made up of glass shards, agglutinates and hollow spheroids at Matti-ka-god and Ni-rich vesicular glass, sanidine spherules, magnesioferrite crystals with soot at Bariyara, implying an event that possibly could be related to the K–T boundary (Sisodia et al. 2005).

15.5 Northern Continental Margin

Even as a large part of the subcontinent including Saurashtra, Kachchh, Marwar, Meghalaya–Bengal and Coromandal coast came under the sway of sea water, the leading edge of the northward-moving India (which had been the site of sedimentation all through the Proterozoic and Palaeozoic) started to subside during the Cretaceous. The sea continued to deepen progressively along the northern periphery. In the deep sea were deposited siliceous and calcareous oozes and pelagic clays, giving rise to chert beds and limestones and shales. Phosphatic nodules and glauconite, occurring locally, indicate deep and cold water that was deficient in oxygen. Simultaneous with the subsidence of the northern edge of India, an oceanic trench started forming along the edge of the Tibetan landmass, and by the end of the Cretaceous the trench had become more than 2000 m deep.

The sinking of the seafloor was followed by its fissuring. Through the fissures came out lavas on a grand scale all along the tract. A chain of volcanic sea mounts and islands developed in front of the sinking continental margin of India. The arc-shape chain of volcanic island stretched from Kohistan in the west to Shigatse in the east. By then, the northward-moving Indian plate had come close to the Tibetan landmass of the Asian plate. In between lay the arcuate chain of volcanic islands and a trench filled with sediments.

This aspect is dealt with in Chaps. 16 and 17.

15.6 An Arm of Sea Inside Central India

Related to the regional Narmada graben, there were a number of linear depressions of the nature of half-graben in central India (Tripathi and Lahiri 2000). These were sites of fluvial sedimentation. Reactivation of faults caused down-faulting in the zone of the Narmada graben, inviting sea water from the west. In the Narmada Valley, the Upper Cretaceous is thus represented by the fluvial horizon succeeded by a marine formation.

Outliers of conglomerate and gritty calcareous sandstones measuring a few tens to a few hundred metres in thickness were first described as the Nimar Sandstone by P.N. Bose in 1884 in the area of the famous Bagh Caves in the Narmada Valley. In and around Chakrud, polymictic conglomerate, strongly graded pebbly sandstone that is characterized by hummocky cross-bedding, and ripple-marked mudstone with limestone, bear testimony to the influence of alternating fair weather and storm conditions in the site of the Nimar sedimentation (Bose and Das 1986). Possibly tides were also active (Ahmad and Akhtar 1990; Akhtar and Ahmad 1991). The upward-fining sequence of the Nimar was laid down in freshwater, as carbon- and oxygen-isotopic composition of limestone indicates (Bhattacharya et al. 1997a, b). The Nimar Sandstone is succeeded by the Karondia Limestone representing a platform deposit formed on the ramp of the shelf dotted with islands in the subtidal zone (Akhtar and Khan 1997). The upper part of the sequence has yielded Cretaceous benthic foraminifers (Rajashekhar 1991) and nannoplankton species Marthastenites furcatus of the Turonian age (Jafar 1982).

The marine incursion in the Late Cretaceous is represented by small isolated outcrops of the Bagh Beds in the Dhar district, Madhya Pradesh (Fig. 15.15). W.T. Blanford gave the name in 1869. The group extends eastwards as far as Barwaha. The Bagh Beds unit comprises three formations (Fig. 15.15). The Nodular Limestone in the lower part is a clastic carbonate characterized by ripple marks formed in shallow-water environment (Singh and Srivastava 1981; Akhtar and Khan 1997), containing rich assemblages of pelecypods and gastropods (Chiplonkar et al. 1977; Chiplonkar 1959, 1987) and benthic foraminifers (Rajashekhar 1991). In the upper part near Kukshi, the Late Turonian marker ammonite Prionocyclus germari occurs in the Hathni Valley (Kennedy et al. 2003). Essentially, a bryozoan carbonate horizon, the Coralline Limestone is characterized by the dominant bryozoan Chiplonkarina dimorphora of “Lower Cenomanian–Turonian age” (Taylor and Badve 1995).

Fig. 15.15

a Sketch map showing the spread of the Bagh Beds, a Cretaceous formation in the Narmada Valley (after Tripathi and Lahiri 2000). b Lithology of the Bagh Beds as seen in the man river section near Deola (after Taylor and Badve 1995)

The Bagh Beds give way upwards to feldspathic arenite and carbonates—recognized as the Lameta Formation of lacustrine origin. The Lameta has been discussed in detail in Chap. 14.

15.7 Mesozoic Panorama of Marine Life

15.7.1 Prolific and Diverse Life

As a consequence of Hercynian tectonic upheaval towards the end of the Permian, the sea withdrew from a large part of the continental margin the resultant reduction of environmental niches caused decline in the population of marine invertebrates and extinction of many. The Late Permian was a time of large-scale extinction of marine life all over the world (Kummel 1961). But the return of the sea in the Jurassic and Cretaceous periods over much larger areas of the Indian continental margin, as already elaborated, enlarged the areal extent of the habitats of the marine communities. Availability of diverse niches promoted vigorous growth and proliferation of life in the Mesozoic. By the Middle Triassic, the marine fauna became extremely prolific and diverse, particularly in the Himalayan domain belonging to the Tethyan faunal province. And this trend continued till the end of Cretaceous.

15.7.2 Age of Ammonites

One among the cephalopods, the ammonites dominated marine fauna of the Triassic time. Not only were they abundant and widespread but also very diverse in spite of three extinctions they suffered before their final disappearance at the end of the Cretaceous. Each time, the surviving stock evolved to more diverse and more abundant forms, far grander in scale than that of the previous period (Kummel 1961). The size of ammonites ranged from 2.7 m to a few millimetres, and the shape varied from flat, globular, loosely coiled to tightly coiled shell. Turrilites was an uncoiled ammonite, and Acanthoceras developed spines on their shell for self-defence. The Early Triassic ammonites were unoriented forms with rather simple ceratitic sutures. The Middle Triassic forms were highly ornamented with ribs or nodes or both giving rise to ammonite sutures. They persisted towards the end of the Triassic, giving way to those who had more complex pattern of development—the lytoceratids and phylloceratids peopling the Himalayan domain.

The nautiloids evolved slowly and became highly specialized in ornamentation and pattern of suture during the Late Triassic. Only one group of nautiloids survived extinction, and it produced great number of species belonging to Nautilus in the Jurassic. In the Jurassic and Cretaceous, the belemnoids became abundantly widespread.

15.7.3 Reef Builders and Associated Invertebrates

The pelecypods became very widespread and diverse in the Mesozoic era. The important forms include Monotis, Exogyra, Gryphaea, Mytilus, Hippurites, Trigonia, Inoceraus, Pecten and Ostrea. One group of pelecypods—the rudistids—became attached to the bottom and built reefs in shallow warm waters. Likewise, corals built conspicuous reefs. The reef-building corals belonged to the order Sclerictinia, which first appeared in the Middle Jurassic and became dominant by the Late Jurassic and in the Cretaceous. Echinoderms became very abundant and diverse in the later part of the Mesozoic. Rare in the Triassic, the bryozoans increased in number appreciably during the Jurassic and Cretaceous times. There was explosive growth of planktonic foraminifers during this period. First appearing in the Triassic, the phytoplankton Coccolithophores (made up of calcareous microscopic plates) proliferated greatly and eventually formed the famous chalk beds of the Cretaceous. The siliceous diatoms and the dinoflagellates evolved and increased in number and varieties in cooler waters during the Cretaceous.

15.7.4 Marine Vertebrates

In the Early Permian seas lived small reptiles having slender long body, small neck, teeth-bearing jaws and paddle-like short legs. The Mesosaurus was the oldest aquatic reptile. By the Late Cretaceous, they had grown in size—as much as 25–9 m in length. The Pleisiosurus was 3.5–6 m long. And Ichthyosaurus measured 3 m in length having a streamlined body, flipper-like forelimbs and powerful tail for propulsion.

Among fish, the lung fish crossopterigians were important. By the Late Cretaceous, the advanced group of fish teleosts became dominant. Shark-like fish Otodus, Pycnodus and Ptychodus were common in the Trichinopoly Formation.

Labyrinthodonts, represented by Archegosaurus, possessed crocodile-like long snout, and their eyes were surrounded by rings of long plates, and the body was covered with overlapping scales. Archegosaurus occurs in the Triassic of Kashmir of the Tethyan domain. It was a contemporary of Gondwanasaurus which lived in the Panchet Basin near Pachmarhi in Madhya Pradesh.