Analysis of sedimentary facies and depositional environments of the Permian Gondwana sequence in borehole GDH-45, Khalaspir Basin, Bangladesh

Abstract

Lithofacies analysis of the Permian Gondwana sequence in borehole GDH-45 of the Khalaspir Basin was performed with a view to deduce the nature of depositional environments. On the basis of dominant lithofacies association, the sequence is divided into six lithostratigraphic units (units A to F). Five lithofacies (conglomerate, sandstone, siltstone, mudstone/shale and coal) are identified within these units. Several sub-lithofacies, such as massive, crudely stratified, cross-stratified, ripple and parallel laminated sandstones are also identified within these lithofacies. The sequence forms a fining-upward trend with a rare coarsening-upward unit. The generalised Gondwana sequence is characterised mainly by channel lags, pebbly massive to crudely cross-stratified sandstone, trough and planar cross-stratified sandstone, ripple laminated sandstone/siltstone, massive to parallel laminated siltstone, mudstone/shale and coal in ascending order. The facies associations represent several repeated fining-upward units and cycles, indicating various sub-environments (channel, floodplain, flood basin/backswamp) in fluvial regime. The conglomerates might have been deposited as debris flow or channel lag deposits. The sandstones were deposited mainly as multistoried channel and lateral bars in moderately braided and sinuous streams. The siltstone and mudstone lithofacies indicate bar top, natural levee or floodplain to flood basin environments. The coal lithofacies suggests deposition in low-lying, short to long persistent, moderately to well drained and sparse to densely vegetated backswamps in fluvial channel-flood-plain complex. The overall succession of the Gondwana borehole sediments suggests that the depositional basin became, with time, gentler in slope gradient, resulting in a more sinuous stream setting.