Organic-walled microfossils of doubtful origin in Permian and Triassic sequences on peninsular India

Abstract

The Permian and Triassic succession of the Indian Gondwana Sequence, with the exception of Lower Permian Talchir Formation, has been considered to be deposited in fluviatile-lacustrine environment-Palynological investigations of these deposits have revealed the presence of rich assemblages of spores, pollen and other organic-walled microfossils of doubtful origin (OMIDO) belonging to the group Acritarcha in its broader sense. Recent discoveries of marine signatures from these deposits depicted by sedimentological, biotic and chemical features strongly prompt for a detailed investigation of OMIDO’s for their authentic application in determining the palaeoenvironment. Sporadic or consistent occurrence of OMIDO’s has been recorded from Talchir to Panchet formations at various time intervals. The increase in the brackish water regime on to the Indian Peninsula near the deltaic sea-shore regions could have provided suitable environment from time to time for growth of OMIDO’s. This could have occurred due to the well-known global transgressions during Permian and Triassic times. It is, therefore, important that the non-marine nature of Indian Gondwana should be skeptically viewed in order to find possible marine signatures in this sequence. The present study reveals that there had been three major diversity acme phases of OMIDO’s during Permian, viz., (i) Talchir/Karharbari, (ii) Upper Barakar, and (iii) Upper Raniganj Formations. They broadly coincide with the onset of regression. Although the data is meager, a similar trend in occurrences of OMIDO’s has been observed in the Triassic.