CRETACEOUS SEQUENCE STRATIGRAPHY, TIRUCHIRAPALLI AREA, CAUVERY BASIN, SOUTHERN INDIA

Surface exposures of Albian-Maastrichtian stratigraphic units in the Tiruchirapalli area of the Cauvery basin, southern India, provide a good study area for outcrop-based facies analysis and sequence stratigraphy. The sub-Sivaganga regional nonconformity forms the first sequence boundary (SB1) in the area; and the sub-Dalmiapuram unconformity forms the second sequence boundary (SB2). SB1 may be related to an intra-Albian eustatic sea-level drop, but SB2 likely is related to the middle-late Turonian boundary eustatic sea-level drop (c. 90 Ma), which is a type-1 SB in most places. The sub-Garudamangalam unconformity forms SB3, and the intra-Sillakkudi unconformity is SB4. SB3 is a local tectonically related stratigraphic break that is likely connected with the Marion igneous intrusion and uplift events; SB4 may be related to an early Campanian eustatic sea-level drop (c. 85 Ma). SB5 is a regional unconformity marked by the sub-Kallankurichchi surface and SB6 is an intra-Kallamedu surface. SB5 may be related to a late Campanian eustatic event, or if (as local biostratigraphy suggests) this unconformity is near the Campanian-Maastrictian boundary, then it may be of local origin. SB6 may have formed during a rather significant late Maastrichtian sea-level drop (c. 68 Ma). The Maastrichtian-Danian contact (the sub-Niniyur unconformity) is SB7 in the Cauvery basin. This erosional surface may relate to a late Maastrichtian or an early Danian eustatic sea-level lowering event. Study of the stratigraphy of sequence-bounding stratigraphic breaks in the Cauvery basin affords an opportunity to examine the interplay of tectonic effects and eustatic changes in the development of this Cretaceous basin-filling section on the dynamic sub-continent of India.