Late Mesoproterozoic iron and manganese deposition on a deep-water carbonate platform (Penganga Group), Adilabad, India: Constraints from C, O and Sr isotopes

Abstract

Abstract The Penganga Group in the Pranhita-Godavari Rift Valley of central India includes a rather unique example of Late Mesoproterozoic iron and manganese formations deposited in a deep-water below storm wave base distal carbonate platform environment. The carbonate rocks of the Penganga Group are mostly lime-mudstones that have been classified into a number of sedimentary cycles with intervals mostly distinguished by color, ranging from brown/pink and siliceous gray to steel-gray and black. At least two about one-meter-thick Mn-Fe formations occur in siliceous gray limestone of the distally steepened part of the Penganga carbonate platform. A number of northwest-southeast striking thrust faults repeat the succession in down dip sections bringing up distal profiles of the Penganga Group against proximal counterparts at the present exposure level. This contribution provides the first comprehensive record of stable C and O and radiogenic Sr isotopes for proximal and distal profiles, allowing for direct comparison and thus, providing context for the environmental conditions of Fe and Mn deposition in a Mesoproterozoic platform environment. Carbon and oxygen isotope compositions determined from the proximal and distal profiles of the platform yield trends that are attributed to variations in circulation pattern and cycles of warm and cold climatic stages across the platform. Secular trends of Sr-isotope compositions suggest a variation in continental input between cold (0.7065) and warm climatic stages (0.7149), similar to that observed for modern marine sediments. The sedimentation of siliceous deposits and ferromanganese deposits in the distal platform environment was likely favored by the onset of upwelling circulation during cold climatic stages. The moderately negative δ13CPDB values (-5.34‰ to -6.34‰) of the Mn-carbonates and variation in δ13Corg values 31.7‰ to -21.7‰ obtained for early diagenetic Mn-carbonate ovoids indicate oxidation of organic matter in the ferromanganese deposits by Mn-oxides during early diagenesis. We conclude that upwelling and platform-wide deposition of ferromanganese deposits on the Penganga carbonate platform may mark the transition from the stagnant Mid-Proterozoic ‘sulphidic’ ocean to a ‘respiring’ open ocean during the Late Mesoproterozoic Era.