Forced regressive wedges on a Neoproterozoic siliciclastic shelf: Chandarpur Group, central India

Abstract

Outcrop-based sequence analysis across the transition from the Chaporadih to the Kansapathar Formation of Chandarpur Group, Chattisgarh Supergroup, central India, resolved a long-standing uncertainty on occurrence of depositional discontinuity through undoubted identification of a forced regressive wedge and associated intraformational unconformity. Process-based facies analysis identified eleven different facies types that are grouped into five different facies associations. Paleoenvironments range from shelf, shoreface, foreshore and beach, tide-influenced estuary and delta. Bounded between the ‘Surface of marine erosion’ at its base and subaerial unconformity (marked by paleosol development) at its top, the coarse grained wedges of shoreface sandstone is interpreted as forced regressive in origin. The basinward downstepping of two such regressive wedges bears evidence for lowering of wavebase in connection with relative fall in sea level. The valley created during the falling sea level was filled by sediments in lowstand, transgressive and highstand depositional motif. The aggradational to weakly progradational bedsets of foreshore/beach sediments record the history of early slow rise in base level. With steepening in the rate of relative sea level rise the coastline transformed from wave-dominated to tide-dominated. Wide variability in the balance between sediment supply and accommodation is recorded in the facies types and stacking motif of transgressive estuarine sections at different studied locations. The highstand tidal delta is a supply driven, normal regressive product at stable or marginally rising base level.