Mudrock facies analysis and their geochemical significance in paleoenvironmental reconstruction of Proterozoic Vempalle Formation, Cuddapah Basin, India

The geochemical characterization of shale may imply the average crustal composition of provenance more reliably than any other detrital sediments. Shales of Proterozoic Vempalle Formation at different stratigraphic levels are categorized into four types based on texture, CaO contents, concentration of quartz-clay, grain size and sedimentary structures. From top to bottom in order of appearance, these are Calcareous Siliceous Mudstone, Calcareous Siliceous Laminated Mudstone, Calcareous Massive Mudstone (CMM) and Grey Siliceous Laminated Mudstone. Lithofacies analysis in the field, followed by petrography and whole rock geochemistry data, connotes an intertidal to shallow marine shelf environment of deposition. The observed felsic provenance and passive margin tectonic setup are also well recognizable from various plots. In this depositional model, all data fits appropriately to describe a comprehensive and coherent mechanism of shale precipitation with subsequent influences of diagenesis, paleo-weathering, alteration and erosion effects. In Proterozoic time higher total organic carbon (TOC) content is mostly found in black shale. However, high TOC content in ferruginous Vempalle shale is given special attention. The possibility of diagenetic migration of fluid and dissolved organic carbon (DOC) during diagenetic compaction from uraniferous dolostone to overlying CMM is found as a suitable explanation for the high TOC content in this red shale in which TOC contents decrease upwards. Further, the fewer uranium contents in grey shales compared to purple/red colored shales indicate the presence of cyanobacteria and associated photosynthesis, which produced oxygen to favor hematite precipitation. In this manner, organic matter played a dual role by adsorbing uranium and oxidizing iron.