BIOSTRATIGRAPHY AND AGE OF THE LOWER VINDHYAN (SEMRI GROUP) OF EASTERN VINDHYAN BASIN, INDIA

Abstract

In the Indian Peninsula, Vindhyan Basin is one of the largest sedimentary basins. Since Riphean time-frame, • the Vindhyan Supergroup (VSG) became so deep rooted that even some glaring fossil records (e.g. Shukla and Sharma, 1990; Venkatachala et al.. 1996) indicating younger ages (Vendian / Ediacarian - early Cambrian) were generally ignored. While Seilacher et al. (1998), believing in the generally accepted Mesoproterozoic age of the Lower Vindhyan, claimed to have found the evidence of exceptionally deep origin of multicellular animals (>1100 Ma) in the form of triploblastic animal traces in the Chorhat Sandstone (Kheinjua Subgroup, Lower Vindhyan), Azmi (1998) on the basis of the discovery of Lower Cambrian small shelly fossils (SSFs, PC-C boundary markers -544 Ma) from the Rohtasgarh Limestone (top of the Lower Vindhyan) argued for the need of a major upward age revision of the VSG (Vendian-early Paleozoic). Contrary to it, however, recent radiometric ages from the Lower Vindhyan have even suggested that the base of the Lower Vindhyan would go >1700 Ma (Hasmussen et al., 2002; Ray et al., 2002, 2003). A rich collection of fossils from the Lower Vindhyan comprising small shelly fossils (SSFs), calcareous algae, sponge spicules, acanthomorphic acritarchs. scolecodonts, metaphytes (megascopic carbonaceous compressions) and trace fossils are being systematically described, totaling >39 species belonging to 35 genera which also includes a new genus and species, Vindhyanitubulus semriensis. The stratigraphic distribution of large number of fossil collections have resulted in providing for the first time firm biostratigraphic constraints to the Lower Vindhyan (Semri Group) sediments of central India. The diagnostic small acanthomorphic acritarchs occur in the middle part of the Rohtasgarh Limestone which are also known to evolve near the transition of Vendian to Early Cambrian. Megascopic metazoan burrowing trace fossils are generally recorded worldwide from the rocks of < 600 Ma old (Vendian / Ediacaran age). In Lower Vindhyan trace fossils appear in the Chorhat Sandstone (Sarkar et al., 1996; Seilacher et al.. 1998) of Khenjua Subgroup. From the sequential appearance of different fossil groups in the Semri Group it is absolutely clear that it corresponds with the well established global evolutionary scenario of life during the Vendian - Early Cambrian period. On the basis of Precambrian - Cambrian boundary SSFs and calcareous algae, small acanthomorphic acritarchs, Vendian metaphytes (carbonaceous compressions), scolecodonts. horizontal trace fossils as well as the global Vendian marker tillites at the base of the Lower Vindhyan succession, it is clear that the Lower Vindhyan (Semri Group) is now correlatable with the well established Vendian - Early Cambrian Mussoorie Group (Blaini - Krol - Tal succession) and also with the Tejam Group of the Outer ard Inner Lesser Himalaya, respectively. Global correlation of the Semri Group has also been possible because of the characteristic assemblage of SSFs from the upper Rohtasgarh Limestone and Bhagwar shale (Rohtash Subgroup), which corresponds with the earliest Cambrian Meishucunian Zone I (Anabahtes - Protohertzina - Hyolithellus Assemblage Zone) and lower part of the Meishucunian Zone II (Paragloborilus - Siphogonuchites -Lapworthella Assemblage Zone) of South China Platform. The metaphyte assemblage from the middle part of the Rohtasgarh Limestone is also in correspondence with the Late Sinian Doushantuo Formation of South China. The scolecodonts bearing Koldaha (Olive) Shale is comparable with the Middle Vendian Ridkinia spinosa bearing Ridkino beds ofMoscow Basin of the East European Platform. The above mentioned Vendian - Early Cambrian successions invariably begin with tillite beds of Varangerian age (~ 600 Ma). It can easily be judged from the database that while the radiometric dates are quite inconsistent or to say self-contradictory, the paleobiological records in the Lower Vindhyan are in accordance with the global evolutionary records, consistently indicating Vendian to Early Cambrian age for the Lower Vindhyan. In view of the ongoing international debate on the age of the Vindhyan Supergroup of central India, present record of large number ofVendian - Early Cambrian fossils from the Lower Vindhyan is of utmost significance. Biostratigraphically. the age ofthe Lower Vindhyan (Semri Group) is, thus, fairly well-constrained within the Vendian - Early Cambrian period, where the PC-C boundary would certainly lie in the upper part of the Rohtasgarh Limestone. Consequently, the Upper Vindhyan succession (Kaimur. Rewa and Bhander Groups) has to embrace the Lower Paleozoic period, but its precise upper age limit is presently uncertain. Further, it is construed that the northern extension of the Vindhyan Basin is represented in the Lesser Himalayan zone, which was part of a large Vendian- Cambrian shallow carbonate platform covering the northern part of the Indian shield.