ABSTRACT

Carbonate rocks of late Neoproterozoic-early Cambrian (?) age occur in two extremities of the Nagaur-Ganganagar basin in western India: in east as part of Bilara hills (type locality) and in west as part of Baghewala oil field. These rocks have close stratigraphic correlation with the Ara Formation (Huqf Group, south Oman salt basin) which transgresses the late Neoproterozoic-early Cambrian boundary. Sedimentological and stable isotopic studies of these rocks show prevalence of arid and evaporitic conditions during their depostion. Close resemblance of C-isotopic profiles of Hanseran evaporites and Bilara carbonates allows intra-basinal correlation and suggests that they are coeval facies variants. There are several characteristic carbon isotopic excursions one of which can be tentatively correlated with the globally recorded excursion close to the late Neoproterozoic-early Cambrian boundary. In contrast other peaks are possibly of regional significance and can be attributed to fluctuations in bioproductivity, correlatable with variation in nutrient supply on a basinal scale. Highly depleted carbon isotopic composition of the organic matter in subsurface organic-rich carbonates (Upper carbonate Formation, Baghewala-II core) has been ascribed to incorporation of biomass synthesized by chemoautotrophic bacteria under anoxic or eutrophic basinal condition caused by salinity stratification.