Sedimentology, sequence stratigraphy and geochemical reappraisal of the Mesoproterozoic Rampur Shale, Vindhyan Basin, India: Implications for paleoenvironment and basin evolution

This paper investigates the sedimentology and geochemistry of the black shale deposit of the Rampur Shale Member and interprets the depositional environment, the provenance of the sediments, and the paleoclimatic and paleo-redox conditions to elucidate the evolution history of the Vindhyan Basin in central India during the Mesoproterozoic time. The Rampur Shale Member consists of five facies, representing a fining-upward succession dominated by a coarse-grained sandstone-siltstone alternation at the basal part with an increasing shale content towards the top, culminating into a black shale deposit. Process-based facies analysis documents the shift in the depositional environment from the lower shoreface to the outer part of the continental shelf, suggesting a basin-wide marine transgression which resulted in the deposition of the black shale within the maximum flooding zone (MFZ). Abundant organic matter, with δ 13 C org values ranging between -25‰ and -34.30‰, preserved in the form of the crinkly laminated, lensoidal, and spheroidal-shaped microbial mat growth on the sediment surface contributes to the characteristic dark hue of the black shale, which is further supplemented by the high TOC (Total Organic Carbon) content of the black shale. Several geochemical proxies suggest that the sediments were sourced predominantly from felsic igneous provinces; however, increased contributions from mafic sources were recorded during the later stages of deposition. The weathering intensity was high, and the black shale was deposited under humid climatic conditions. The paleoredox proxies suggest predominantly anoxic depositional conditions, most likely forming via organic matter degradation. The paleosalinity remained low due to the epicontinental nature of the basin, possibly as a consequence of dilution by freshwater influx from the basin margin. The trace metal proxies suggest the prevalence of upwelling conditions, perhaps resulting in the increased bio-productivity during the black shale deposition. Numerous slide planes at successive intervals bear the signature of basin subsidence and deepening, facilitating basin-wide black shale formation. • Detailed facies analysis and sequence stratigraphy of the Mesoproterozoic Rampur Shale Member identifies coarsening-up parasequences juxtaposed within a fining upward TST sequence. • Abundant slide planes within the Rampur Shale Member indicate basin subsidence and deepening resulting in black shale formation. • Different geochemical proxies suggest high intensity weathering of the hinterland under warm-humid paleoclimatic condition. The source rock was predominantly felsic, promoting metal depletion within the black shale. • The degradation of organic matter, represented by the growth of microbial mats possibly resulted into pore-water anoxic to dysoxic condition.