The Cretaceous rocks consists of thick succession of marine sequences that ranges in age from Aptian to Maastrichtian. The marine sequences represents at least four phases of marine transgression that were interrupted by brief periods of withdrawal of marine conditions. The marine transgression leads to the deposition of four groups as Dalmiapurum, Uttatur, Trichinopoly and Ariyalur groups.
The late Jurassic – Early Cretaceous rifting along the eastern coast led to the development of a number of NE-SW trending basins on the Precambrian Crystalline basement. Cretaceous rocks occurs as isolated patches of outcrops in the Cauvery Basin. The Cretaceour sequences are divisible into three groups as Uttatur, Trichinopoly and Ariyalur.
The basal units, named as Kovandakurichchi Formation and of the Terani Formation that lie below the Uttatur Group marked the commencement of the deposition of Cretaceous sequence. The basal unit rests unconformably over the basement of Archaean Gneiss.
Kovandankurichchi formation is comprised of conglomerate, and is overlain by the Terani formation which posses Ptilophyllum acutifolium.
- Uttatur Group is consists of the Dalmiapurum formation and Karai formation which has been shown to be time equivalent.
- They have para-conformably contact with the underlaying rocks of Terani formation. Dalmiapurum formation comprise of pyritiferous grey shales and limestone. The shale unit yielded dinoflagellate and benthic foraminifers represented by Lenticulina.
- The overlying Garudambanglam formation also known as Karai formation of the Uttatur group consists of fine silt, calcareous shales and sand clays. The upper part of the sequence contains gypseous and phophatic nodules indicating evaporatic conditions of a regressing sea.
- Trichinopoly Group is comprised of irregularly bedded sandstone, shale, coquina and conglomerate. The succession is divided into two formations named as Kalakkalnapattam Formation. and Aniapdi Formation. The interbedding of neritic and littoral facies indices rapid fluctuation of the sea level during the deposition of the Trichinopoly Group. Trichinopoly Group has yielded a large number of invertebrate fossils and tree trunks.
- Ariyalur Group conformably overlies the Trichinopoly Group. The lower part of the succession contain fossils while the upper part is devoid of fossils showing lacustrine environment. The succession is divisible into three formations, Sillakudi, Kallankurchchi and Kallamedu Formation. The sillakudi formation is overlain by the Kallar conglomerate which shows a possible break in deposition.
K-T Boundary
Ariyalur succession can be subdivided into two parts, the upper one is called as Niniyur formation. Niniyur formation yielded a rich assemblage of lamellibranchia, corals and gastropods. The assemblages includes Cardita Jaquinoti that is identical to the well known Danian index fossil Cardita Beaumonti of the sindh area pakistan. The underlying Ariyalur succession has yielded foraminifera assemblage of Maastrichtian age.
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