Forced regressive wedge in the Mesoproterozoic Koldaha Shale, Vindhyan basin, Son Valley, central India

Abstract

The present paper highlights the sequence development within the Mesoproterozoic Koldaha Shale Member of the Kheinjua Formation, Vindhyan Supergroup which records the occurrence of a forced regressive wedge and associated discontinuity surfaces at the base of the wedge. Nine lithofacies have been identified within the study area that are grouped into three lithofacies associations varying in depositional setting from outer shelf, through shoreface-foreshore-beach to continental braidplain. The outer shelf sediments are aggradational to slightly progradational representing highstand systems tract. The rapidly progradational, wedge-shaped shoreface to foreshore-beach succession occurs sharply or erosively above the outer shelf sediments and is bounded by a regressive surface of marine erosion (RSME) at the base and by a subaerial unconformity at the top. This, along with its downstepping trajectory, supports deposition of this sedimentary wedge during falling sea level. A laterally extensive soft sediment deformation zone occurs at the base of the wedge. The forced regressive wedge is incised by fluvial braidplain deposits that rest on an erosive surface representing a sequence boundary. The thin braidplain deposits are the product of aggradation during a subsequent early rise in relative sea level, and thus, they are inferred to represent a lowstand systems tract. The constituent architectural elements that characterize the braidplain deposits are downstream accretion elements and small channel elements. Further landward, the base and top of the shoreface wedge merge to form an unconformity across deposits that rest directly on the outer shelf sediments. The identification of forced regressive wedges has significant economic importance in view of the potential occurrence of hydrocarbons within the Proterozoic formations.