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 shorefaceforeshore-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 1 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.

Shales of Palaeo-Mesoproterozoic Vindhyan Basin, central India: insight into sedimentation dynamics of Proterozoic shelf

“…Schieber, 1998;Chakraborty & Sarkar, 2005). In the absence of any sand interbed or any kind of wave-or current-related feature, this argillaceous interval suggests deposition in a prodeltaic outer-shelf setting near storm wave base (Banerjee, 2000;Bose et al 2001;Samanta et al 2016). The presence of wrinkle structures on bedding planes is probably due to gentle wave agitation on microbially mediated cohesive fine sand and/or silt surfaces during storm events (cf.…”

“…The occurrence of basin instability structures such as slumps, small-scale listric faults, conjugate faults, contorted beds and joints within the Koldaha shale are indicative of horizontal-extension-related regional tectonics (Sarkar et al 1995;Bose et al 1997;AK Singh, unpub. PhD thesis, University of Delhi, 2015;Samanta et al 2016). The small-scale deformation structures observed at various stratigraphic levels of the Kajrahat Formation and Koldaha shale bear indication of a SW-wards regional slope, whereas large-scale normal faults with conglomeratic fan delta sediments at their toes point to a persistent NE-wards dipping slope.…”

“…1b), and ranges in thickness from 350 to 600 m (Gupta et al 2003). As a variegated shale unit, the Koldaha shale gradationally overlies the Porcellanite Formation, comprising dark-grey to greenish-grey splintery shale with sand and/or silt interbeds, and conformably passes into the overlying Churhat sandstone (Bose et al 2001;Samanta et al 2016). Rasmussen et al (2002) constrained the age of Koldaha sediments as 1.60-1.63 Ga using U-Pb SHRIMP of zircon grains obtained from a tuffaceous interbed within this shale interval (Fig.…”

“…The Churhat sandstone of the Kheinjua Formation that underlies the Rampur shale comprises fine-grained marine siliciclastics (Bose et al 1997) and is modelled as a product of sea-level highstand (Bose et al 2001;Sarkar et al 2002a;Samanta et al 2016). The initiation of shelf sedimentation of the Rampur shale is considered as a signature of transgression in the basin and defined as the product of a transgressive systems tract (Bose et al 2001).…”

“…Leeder & Gawthorpe, 1987;Lambiase, 1990). The emplacement of fluvial systems on the Koldaha shelf at two stratigraphic levels indicates a relative fall in sea level and its emergence and subsequent development of intraformational unconformity (Bose et al 1997;Samanta et al 2016). Unlike Koldaha shale, Rampur shale shows a uniform fining-and thinning-upwards character, indicating an overall transgressive depositional motif, as it was deposited on a quiet shelf ranging in bathymetry from inner to outer shelf.…”