Development of Indian Coast

Presented by :AMBER BHAUMIK

Among the different basins in the western sector, the Mesozoic basins of Kutch and Jaisalmer range from Triassic well into the Upper Cretaceous with some break in the sedimentation. These enclose sediments of 1500-3000 m thickness in Kutch and about 800 m in Jaisalmer, covering 43000 sq. km including about 15000 sq. km and 800 sq. km respectively of exposed sedimentary areas.

The basins of Kutch and Jaisalmer owe their formation to rifting along the major Precambrian lineaments in Western India- East Africa divergent plate region, prior to the fragmentation of Gondwana superplate.

The successions in both these basins are thick, well developed and organically rich, exhibiting a fair degree of stratigraphic simiarity. The sediments, except for the Middle Jurassic carbonates, are dominantly siliciclastic. A marine facies is advocated for the successions, including the plant- fossil bearing part of the succession, traditionally held to be non-marine.

Mesozoic rocks ranging in age from Middle Jurassic to lower Cretaceous are exposed in the following six disconnected areas: (i) Kutch Mainland (ii)Patcham “Island”(iii) Khadir “Island”(iv) Bela “Island” (v) Chorar “Island” (vi) Wagad

Wynne mapped Mesozoic rocks of Kutch. Stoliczka and Waagen studied the fossils and it is the latter who suggested the well known subdivisions of Patcham ,Chari ,Katrol and Umia in the ascending order.

Sedimentation in Kutch basin

Mesozoic sedimentation in Kutch basin, in general, commenced with the deposition of retrogradational and aggradational succession (RS & AS) in the lower part followed by the progradational succession(PS)in the upper part. About 200 m of Aalenian sediments are nonmarine and transgression starts with. RS and AS deposited during Aalenian-Tithonian, exibit overlapping, thick highly fossiliferous limestone/sandstones. PS deposited during Neocomian (?)- Turonian – Coniacian represents sandy lithologies of coarsening upward, offlapping sandstones, shales and thin limestones.

Qualitative and quantitative analyses of heavy minerals, quartz grains and lithic fragments suggest at least two lithologically different Precambrian provenances, during Mesozoic, namely (i) the Aravalli Range of Rajasthan towards north and east and (ii) the Nagar- Parkar massif (granites, syenites) toward north and northwest, partly in Pakistan.

STRATIGRAPHY:

The Umia Formation is diffrentiated into members in northwest Kutch only, but elsewhere in the mainland, it is undiffentiated above Katrol Formation.

The Dhosa Oolite Member occurs at the top of the Chari Formation all over Kutch Mainland.

Patcham Formation is present only in Patcham “island” and Jumara Dome.

Structure The western margin of Indian Peninsular Shield is affected by rifting

along major Proterozoic orogenic trends viz., Aravalli-Delhi and Satpura mobile belts. Kutch rift is the northern most pericontinental embayed basin situated between the subsurface Nagar Parkar Uplift(NPU) in the north, Radhanpur- Barmer arch in the east and Kathiawar Uplift in the south. The two uplifts are the rift shoulders respectively along Nagar Parkar Fault(NPF) and North Kathiawar Fault(NKF) and the Radhanpur- Barmer arch is the rift and hinterland. This arch is the western shoulder of the Cambay rift, which terminates the west to east Kutch rift. Besides horsts(H), grabens (G) and half- grabens(HG), there are anticlines and domes of varying sizes and shapes.

The Kutch basin evolved in two stages : (i) Rift stage: basin subsidence along faults and (ii) Inverse stage: uplifts along the same faults by reverse movements. The east-west oriented, fault bounded, ridges and depressions influnced the thickness and facies distribution of the formation here. The westerly plunge of the basin is reflected in the general westward thickening of the sedimentary units deposited during the two Mesozoic transgressive and regressive cycles,and their relatively deeper- shelf facies prograding towards west.

Fauna and Flora The rich ammonoid faunas of Kutch Jurassic together with those of

the Tethys Himalaya have a significant bearing on the nature and the extent of the faunal provinces within the Tethyan realm as also on the marine seaways connecting them. Biogeographical reconstruction made with the available data invariably present a direct and often persistent marine connection at least from late Early Jurassic onwards from East Africa, Madagascar, Kutch, Jaisalmer, Salt Range, Spiti, Nepal, south Tibet and also to Sula Islands to New Guinea.

The Callovian stage in Kutch is onsidered to be the one best developed in the world, while Kutch Jurassic sequence as a whole is considered of primary reference in the entire East Tethyan region.

Earliest known occurances of nannofossils, the tiniest (1- 30 microns) marine calcareous microfossils, are from Early Jurassic. The broad groups that are included in the study of nannofossils are the extant Coccolithophorids and the extinct discoasters and nannoconids. Nannoflora of the Chari Formation has proven to be the best known in the world.

Trichinopoly Area

The great Cenomanian (98 Ma) transgression covered considerable area in the soth eastern part of Tamil Nadu between the Cauvery river and Pondicherry. Ponniar and Vellar rivers separate the areal distribution into three disconnected regions, viz. Pondicherry, Vridhachalam and Ariyalur(eastern part of Trichinopoly district). Of these, the outcrops near Trichinopoly are the largest with an areal extent of about 400 sq. km ranging in age from Aptian/ Albian to early Paleocene with a stratigraphic record of nearly 120- 61 Ma. The edimentation is often interrupted by low angle unconformities.

Stratigraphy

The Archean crystalline gneisses and charnokites with a NE-SW trend constitute the floor for this depocentre. Small hillocks along the Perambalur- Trichinopoly road delimit the western margin. The Boulder Conglomerate is a classic example for fanglomerate, derived from a tectonicaly active source due to faulting, in association with coalescing alluvial fans. The Terani Plant Beds, a unit of kaolinitic clay and ferruginous sand interbeds, are well known for their rich plant fossils. A low energy inland lacustrine environment is ascribed to this unit.

The Cretaceous sediments dip at very low angles, though higher dis have been recorded close to the contact zones of the sediments with the cystalline rocks. The sediments of the Uttatur and Trichinopoly Groups show evidences of folding. The faults traced at the western boundary of the Cretaceous sediments are thought to be reactivated Precambrian faults.

An attempt has been made to infer sedimentation rate, relative sea levels and transgression cycles in the Cretaceous succession in the Ariyalur district based on the study of 2325 m mid- Cretaceous –Paleocene sequence, over an interval of about 35 Ma. The mean rate of sedimentation is estimated to have varied between 15 and 240 mm per thousand years.

Uttatur Group: Basal conglomerate, grey shales , sponge- coral- algal limestone and shale/ clay distinguish this group, resting unconformably over the basement with phosphatic nodules having a maximum thickness of about 600 m.

The Grey Shale, occuring below the Kallakudi Limestone, appears to be extensive unit and it has been logged in the subsurface in different parts of Cauvery basin and identified as primary source of generating hydrocarbons.

The Kallakudi/(Dalmiapuram) Limestone, which is actively mined for cement, is a sponge coral- algal reefal limstone. the age of this unit is Middle Aptian- Middle Albian. The limestone is presumably deposited in an intertidal zone in a protected still- water bay.

Trichinopoly Group: The group unconformably rests over the Uttatur, and it is well evident at places. It is divisible into lower Kulakkalnattam and upper Anaipadi Formations separate from each other by a conglomerate bed. Kulakkalnattam Formation is a calcareous sandstone unit with ferruginous concretionary bodies extensively bioturbated by feeding burrows like Thalassinoides. It is intercalated with dark grey limestone containing thick- shelled bivalves (Oysters), ammonites, rhynconellids, brachiopods and silicified wood. The thickness of the unit is about 60 m.

The succeeding unconformably overlying Anaipadi Formation is essentially made of sandstones, siltstones with concretionary layers rich in fossils.the thickness of this group is about 100 m. A regressive near coastal environment is suggested for the sedimentation of this group.

Ariyalur Group: It lies unconformably over the Trichinopoly Group and consists of two dominantly sadstones bodies seperated by limestones. The lower Sillakudi Formation comprises ferruginised arkosic sandstones and limestone bands. The depositional environment appears to be high- energy transgressive tidal bars or shoals.

The Kallankurichi Formational, resting unconformably over the Sillakudi Formation, is a massive rudist bivalve carbonate shell- bank of about 40 m thickness. This carbonate unit is considered to be a shallow marine carbonate bank/ shoal deposit.

The overlying calcareous sandstone, of about 60 m thickness, mapped as Ottakovil Formation . The age is Maastrichtian. The lithology and faunal remains are suggestive of marginal marine to near shore condition of deposition.

It is overlain by white to grey, cross- bedded thick sandstones intercalated with greenish grey to red clays, which delimit the Cretaceous succession known as Kallamedu Formation in which the presence of reptilian fossils such as bones of Megalosaurus, Titanosaurus have been reported.

Ninyur Formation : The Cretaceous succession is unconformably overlain by a thick carbonate sequence is of about 60 m in thickness. Lithologically this unit is divisible into lower fossiliferous limestone, middle recrystalline shelly limestone and upper argillaceous gritty nodular limestone collectively known as Ninyur Formation. These carbonate beds are too mined for cement industry.