Carbon isotopic profile across the Bilara Group rocks of trans-Aravalli Marwar Supergroup in western India: Implications for Neoproterozoic-Cambrian transition

The rocks of Marwar Supergroup in the trans-Aravalli sector in western Indi a are presumed to span the time interval between Neoproterozoic and early C ambrian. This, predominantly unfossiliferous, marine sedimentary sequence i s characterized by a tower arenaceous facies (Jodhpur Group), middle carbon ate facies (Bilara Group) and upper argillaceous- arenaceous facies (Nagaur Group) rocks. The sedimentation has been essentially in a shallow basin, d escribed either as the fore-land slope of the rising Aravalli mountains or a sag-basin which developed and evolved due to subsidence of the updomed cr ust during Neoproterozoic Malani magmatism that failed to open rifts. The c arbon isotopic profile for the Bilara Group carbonate rocks in the lower pa rt shows marked oscillations and broadly negative delta C-13 character with negative anomalies as low as <-4.3<parts per thousand>(PDB), observed near the base of Dhanapa Formation (lower unit) and <-6.5<parts per thousand>(P DB) in the overlying Gotan Formation (middle unit). The upper part of the p rofile shows a gradual positive shift. The carbon isotopic signatures of th e Bilara Group rocks can be correlated with the end-Neoproterozoic - early Cambrian (Vendian - Tommotian) carbon isotopic evolution curve. Extremely l ow delta C-13 values indicate the glaciation related cold climatic postulat es of the end-Neoproterozoic, followed by the warmer climatic conditions as indicated by the positive shift. The carbon isotopic data for Gotan Format ion carbonates, at variance with the globally observed delta C-13 trends fo r early Tertiary do not support the recently proposed Tertiary age for the Bilara Group.