Resumen de Stable isotope ratios and nodular limestone of the Proterozoic Rohtas Limestone: Vindhyan basin, India
Santanu Banerjee, S. Jeevankumar, Prasanta Sanyal
The Proterozoic Rohtas Limestone succession in the Vindhyan basin, central India depicts a transition from outer shelf to inner shelf depositional setting. The succession comprises of seven facies viz., black shale, crinkly-laminated limestone, nodular limestone, thinly-laminated heterolithic, grey shale, plane-laminated limestone and wavy-laminated limestone. From field and petrographic features the nodular limestone is identified as early diagenetic product of crinkly-laminated limestone. d13C and d18O values of the limestone bear essentially the 'primary' seawater composition of late Paleoproterozoic to early Mesoproterozoic time and rules out any postdepositional resetting. d13C and d18O compositions of the nodular and non-nodular portions when analysed separately reveals that the nodular portions are invariably enriched in heavier oxygen and carbon isotopes. Enrichment in oxygen and carbon isotope values in the same nodular limestone, although negligible, suggest preferential cementation within the nodular portion. Nodules were possibly generated by dissolution of carbonate sediments and subsequent reprecipitation of carbonate cements close to the sea floor.
