Regressive depositional architecture on a Mesoproterozoic siliciclastic ramp: Sequence stratigraphic and Nd isotopic evidences from Bhalukona Formation, Singhora Group, Chhattisgarh Supergroup, central India

Abstract

A paleo-environmental and sequence stratigraphic study of the Mesoproterozoic Bhalukona Formation, Singhora Group, central India provides new interpretations applicable to understanding sedimentation and stratigraphic architecture in the Proterozoic siliciclastic ramp settings under falling and lowstand sea level conditions. Beside delineation of two diachronous surfaces, process-based facies analysis identified fourteen facies types that are grouped under five different facies associations. Paleo-environments range among continental fluvial, beach-foreshore, upper shoreface, lower shoreface and wave-dominated delta front. From the paleocurrent measurement within the fluvial channel sandstones and measurement of crest line orientations of wave generated swash bedforms, it is inferred that the Bhalukona Sea had NNE-SSW shoreline trajectory and the south-easterly flowing river system carried sediment on the shoreline from a source in the west-northwest. Such inference though in clear discordance with the earlier proposed south-southeastern sediment source for the Singhora basin, finds support in the shift in εNdt (t=1.42Ga) values (from −3.5±3.3 to −9.3±2.2) indicating change in sediment provenance at the early Bhalukona sedimentation history; sediments in the Bhalukona Formation derived from more evolved or older continental crustal sources in comparison to those of the underlying Saraipalli Formation. A tectonic forcing behind the shift in sediment provenance and fall in relative sea level is inferred that established the forced regressive and lowstand shoreline in the Singhora basin during the Bhalukona time.Abrupt basin-ward shift of facies tract and incision on shelf is exhibited by the occurrence of poorly sorted, coarse granular Bhalukona fluvial system (carrying rip-up mud clasts) directly above the argillaceous highstand Saraipalli shelf with ∼10m incision and is inferred as the signal for forced regression and development of Type-I sequence boundary. In low-gradient Proterozoic ramp settings without shelf-slope break, we interpret that the Bhalukona fluvial system incised the coastal prism developed on the Saraipalli highstand coastline. The low-gradient of the ramp, however, prompted long distance (∼15km) regression represented by the offlapped and detached delta front lobe away from the shoreline. The slow, steady rise in sea level, onset of lowstand and establishment of a wave-dominated coastline caused reworking of fluvial sediments in the basinal part (within the wave base) and resulted development of ravinement deposit. Basin-ward, the surface grades into correlative conformity. With aggradational and weak retrogradational stacking the beach-foreshore, upper- and lower-shoreface, in order of superposition, record the lowstand depositional history. The basin-scale transgression is witnessed with formation of Transgressive surface of erosion (TSE) and establishment of the Chuipalli shelf system, dominantly beyond storm wave base. Taking into consideration ∼23m preserved shoreface succession, 1m per year eustatic rise consistent with present day rate and average rate of shoreface retreat 0.5m per year, ∼11.5km retreat for the Bhalukona shoreline is estimated in its lowstand history.