UDAIPUR EPICONTINENTAL SEA: A PALEOPROTEROZOIC PARADOX?

The Aravalli Supergroup sediments of northwestern India, exhibit contrasting lithofacies associations: shallow-marine carbonate-rich facies and the carbonate-free deep-sea facies. The Aravalli Sea has four distinctive microenvironmental settings namely, Udaipur Epicontinental Sea (UES), Jhamarkotra Embayment, Umra Sub-basin (USB) and Ghasiar-Karouli Bank shelf (GKBS). The first two settings are grouped together as the PDD (Phosphatic dolomitic domain) which hosts the stromatolitic rock-phosphate deposits. The last two are called the NPDD (Non-phosphatic dolomitic domain), which is devoid of rock-phosphate and hosts significant uranium minerals (in black shale lithofacies). PDD is characterized by close to zero 13C values while NPDD is characterized by moderate to high 13C values. Though the sedimentation in both PDD and NPDD was from a uniform provenance, as indicated by low pH and high PCO2, the small-scale environmental conditions varied considerably. Hypersalinity has been envisaged for the USB of NPDD in particular, suggestive of precipitation of carbonate in non-equilibrium conditions. By contrast, methanogenesis (possibly simultaneously with sulphate reduction, under hyposaline conditions) might have been responsible for the high 13C enrichment in GKBS. The methane generated from this process acted as a carrier of the uranyl ions and led to secondary enrichment in isolated pockets. Presumably the microenvironment favouring development of cyanobacteria (leading to formation of stromatolites) in UES of PDD in particular prevented growth of methanogenetic archaea under anoxic environment. Carbon, sulphur and oxygen isotope studies and geochemical characterization are suggestive of domination of local paleo-seawater signals signifying microenvironmental variations overprinting the global secular changes. Studies of diagenetic alterations strongly indicate hyposaline diagenetic dolomitization. The variations observed in the microenvironmental conditions of the Udaipur Epicontinental Sea are paradoxical but probably helpful for understanding the mode of operation of local biogeochemical cycles occurring globally, in modern seas also.