BIOTA IN THE TERMINAL PROTEROZOIC SUCCESSIONS ON THE INDIAN SUBCONTINENT: A REVIEW

Abstract

On the Indian subcontinent, Late Proterozoic rocks form part of a confinuous sequence grading into Cambrian, postdating the Sturtian glaciation (Cryogenian) and terminated by the Xingkaian/Pan African Orogeny (Late Cambrian). These sequences are restricted to the northwestern part of the Indian Shield, are overlain by Cenozoic sediments of the Indo-Gangetic Plain, and rest unconformably on the oldest platform sequences (Meso - Cryogenian) in many parts of the Lesser and Tethys/Higher Himalaya. Of these, the succession in the Krol Belt, Lesser Himalaya, divided into Baliana, Krol and Tal Groups, is fossiliferous. Changes in faunal composition of acritarchs and cyanobacteria are recorded in the upper part of the Baliana Group, as is the appearance and extinction of forms assigned to the Ediacara fauna in the overlying Krol. Such biotic change is accompanied by a significant depletion in δ 13C values in the 'cap carbonates'. Another δ 13C depletion is recorded in the upper part of the Krol Group along with appearance of spiny acanthomorphic acritarchs, scaphomorphs and hercomorphs, small shelly fossils, a variety of Early Cambrian trace fossils and trilobites in the overlying Tal Group. A review of biota reported from the oldest platform sediments (Vindhyan Supergroup, Karnool, Bhima Groups) suggests a Meso-Cryogenian (pre-Sturtian glaciation) age.