SEDIMENTOLOGICAL EVIDENCE FOR CRETACEOUS REACTIVATION OF THE EASTERN MARGIN OF THE CUDDAPAH BASIN

The east coast Cretaceous Sedimentary basins namely the Krishna-Godava ri Basin (K-G Basin) in north and the Palar Basin in south are located towards the tapering ends of the crescent shaped 'Cuddapah Basin'. Cr etaceous stratigraphic succession in ascending order is Raghavapuram a nd Tirupati formations in the K-G Basin and Sriperambattur and Satyave du formations in the Palar Basin. The Raghavapuram and Sriperambattur formations consist of mainly shale and siltstone deposited in prodelta /shallow marine environments. Sedimentological studies on sandstone an d conglomerate revealed that in the Tirupati Formation coarse elastic detritus was derived from the Archaean crystallines and Proterozoic Cu ddapah and Kurnool Groups of rocks by braided streams flowing towards east. Alluvial fans and the associated braided streams originated in t he southern part of the 'Cuddapah Basin' were fed by mainly 'Nagri Qua rtzite' to deposit the bouldery conglomerate, conglomerate and sandsto ne sequence of the Satyavedu Formation. The Cretaceous sedimentary out crops located east of 'Cuddapah Basin' i.e. Guntur, Ongole and Nellore areas show evidence for deposition in fluvial and fluviomarine settin gs. The Early Cretaceous tectonic disturbances coinciding with the bre ak up of India from east Gondwanaland reactivated the eastern margin o f the 'Cuddapah Basin' thereby enabling it to supply increased amount of detritus to the coastal sedimentary basins. However, the central pa rt of the 'Cuddapah Basin' and the adjacent shield area located betwee n Lat. 14 degrees and 16 degrees acted as a water divide to the Cretac eous drainage.