Available online at www.sciencedirect.com Precambrian Research 162 (2008) 227–247 Forced regressive wedges on a Neoproterozoic siliciclastic shelf: Chandarpur Group, central India Partha Pratim Chakraborty a,∗ , Soumen Paul b a Department of Applied Geology, Indian School of Mines, Dhanbad 826004, India b Frontier Basins, ONGC Limited, Dehradun 248195, India Received 30 August 2006; received in revised form 7 April 2007; accepted 4 July 2007 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 aggra- dational 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Chaporadih–Kansapathar Formational transition; Chattisgarh supergroup; Forced regression; Base level; Transgression; Highstand; Neoproterozoic 1. Introduction With the growing perception that stratal geometry varies with different stages of shoreline regression (from highstand through falling to early rise; Hunt and Gawthorpe, 2000), it is felt necessary to revisit the shallow marine successions to assess their origin at different stages of base level change. Study of erosive-based shoreface sandstone bodies is crucial in this con- text principally for (i) redefining the placement of unconformity and sequence boundary (at the base; Posamentier et al., 1992 or at the top; Hunt and Tucker, 1992 of these units), and (ii) assessing their potential as hydrocarbon reservoirs. Comparing different geometries of shoreface deposits (accretionary, ravine- ment and sharp based), Proust et al. (2001) suggested enhanced possibility for occurrence of sharp-based units on low-gradient ∗ Corresponding author. Fax: +91 326 2203645. E-mail address: partha geology@yahoo.co.in (P.P. Chakraborty). shallow marine physiography comparable to shelf settings (with mean dip less than 1 m/km; Walker, 1995) in very large platforms or ramp margins. On shelf-type margins (without any major break in depo- sitional slope), relative sea level falls and long-distance regressions result in fluvial lowstand and the deposition of shal- low marine forced regressive wedges (attached or detached) (Plint, 1988; Ainsworth and Pattison, 1994; McMurray and Gawthorpe, 2000). Such forced regressive deposits, if formed, are expected to be thin as they may be eroded in sea level fall and the subsequent sea-level rise (Posamentier and Morris, 2000). The detached and thin character may be missing them in out- crop based sequence analyses, leading to limited description in literature (Plint, 1988; Proust et al., 2001). Slow subsidence and low-gradient depositional slopes in Neoproterozoic epeiric basins (Sarkar et al., 2001; Taylor et al., 2001; Eriksson et al., 2002) make them ideal candidates for understanding the response of low-gradient depositional sub- strates under falling, lowstand and rising sea level. Earlier studies 0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.07.020