The Ediacaran Period: It's lower and upper boundaries in India

Abstract

The newly established Ediacaran Period (terminal Proterozoic) 'begins with termination of the last great global glaciation (Marinoan) of the Neoproterozoic Era' and ends with appearance of biologically distinct world characterised by an assemblage of diverse skeletal fossils of bilaterian animals of the Cambrian Period. Significant depletion in C-isotope values is also recorded at both the boundaries. In India, Ediacaran sequences form part of a continuous sequence that rests unconformably over the oldest platform sequences (Meso-Cryogenian) in many parts of the Lesser and Tethys/Higher Himalaya, and grade into Cambrian. Of the various sections studied, the Maldeota section of the Baliana-Krol-Tal succession, Krol Belt, Lesser Himalaya is found to be the best for the study of both the boundaries. The Lower boundary of the Period is marked at the base of red-green shale and pinkish lenticular dolomite (Member G) - the cap carbonate, forming topmost bed of the Blaini Formation, Baliana Group. There is marked change in assemblage of acritarch and cyanobacteria within upper part of the Baliana Group with appearance and extinction of Ediacaran fauna in the overlying Krol. This change is also accompanied with a significant depletion in C-isotope values in the 'cap carbonate'. The upper boundary could not be precisely demarcated in the absence of boundary diagnostic trace fossils. However, a significant depletion in C-isotope values is recorded in the upper part of the Krol Group with appearance of spiny and processed acritarch (acanthomorphs), scaphomorphs and hercomorphs, small shelly fossils, trace fossils and trilobites of Early Cambrian age in the overlying Tal Group, which is considered to mark the boundary. A review of biota recorded from older sequences viz. Vindhyan Supergroup, Bhima, Kurnool, Chhattisgarh, and Indravati Groups, etc. of the peninsular India, suggest them to be of Mesoproterozoic-Cryogenian (pre-Sturtian glaciation) age. The paper discusses in detail both lower and upper boundaries of the Ediacaran Period and attempts at the correlation with GSSP.