The document summarizes the stratigraphy of the Mesozoic sedimentary basins of Kutch and Cretaceous sedimentary succession in Ariyalur district, India. It describes the depositional environments, lithology, thickness, and fossil content of the different formations in these basins. The Kutch basin formed due to rifting in the Mesozoic and contains over 3000m of sediments ranging from Triassic to Cretaceous in age. The Cretaceous succession in Ariyalur district represents a marine transgression and contains rich fossil records across formations totaling over 2300m of sediments deposited over 35 million years.

1. Presented by :
AMBER BHAUMIK

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

13. 
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.