LITHOFACIES ARCHITECTURE OF PERMIAN COAL-BEARiNG UPPER BARAKAR SUCCESSION, WEST BOKARO COALFIELD, INDIA

Abstract

The Permian coal-bearing Upper Barakar Formation of West Bokaro coalfield is reassessed through detailed sedimentological and sequence stratigraphic analysis. The analysis allows the recognition of 11 facies types from a —150m thick coal-bearing sedimentary succession. These facies are grouped together in three facies associations, namely, FA-1, FA- 2 and FA-3. The facies association I (FA-1) is a coarsening upward sequence comprising of coal, carbonaceous shale and sandstone facies, overlain by the facies association 2 (FA-2), a sequence of pebbly to medium grained fluvial sediments; followed by the facies association 3 (FA-3), which is a fining upward sequence comprising of sandstone, sandstone -mudstone heterolith and coal facies. This cycle of facies associations is reoccurring several times in the vertical sedimentary succession with some variations in thickness. Mainly, facies of FA-I and FA-3 show the signatures of tide-wave influence and marine invertebrate bioturbation. FA-2 is unbioturbated and characterized by the more prevalence of fluvial influence over the tide-wave influences. Detailed facies analysis, delineation of the facies associations and their interpretation in terms of the sequence stratigraphic parameters helped to identify several transgressive-regressive cycles. The prograding sequence constituting the tide-wave influenced sediments of the facies association 1 (FA1) represents highstand system tract (HST). The aggrading sequence of the facies association 2 (FA2) with signatures of intermittent fluctuations is deposited under lowstand system tract (LST), and represent a phase of transition from regressive to transgressive depositional system. The onlapping, retrograding succession of the facies association 3 (FA3), representing sustained phases of sea level rise, is interpreted as transgressive system tract (TST). Stacking of several transgressive-regressive cycles in the studied Barakar sedimentary succession is a manifestation of relative change of accommodation space and the rate of sedimentation in a fluvial - marine interactive setting during the early Permian time. The overall lithofacies distribution of the Upper Barakar succession and its sequence stratigraphic architecture suggest a changeover from basal fluvial to an upper fluvio-marine estuarine depositional system in the West Bokaro coalfield.