Peat formation dynamics in the meandering fluvial system of the Mudstone Series (Middle Pennsylvanian), Upper Silesian Coal Basin, Poland

Abstract

The Mudstone Series (Langsettian–Duckmantian) in the Upper Silesian Basin of Poland is contemporaneous with the main coal-bearing interval of tropical Pangea. The strata were deposited in a continental setting outside the direct reach of glacioeustatic sea-level oscillations. Facies and architectural analyses of the unit provide evidence that the principal depositional environment of the Mudstone Series was a meandering fluvial system with channel belts that followed the NNE-striking basin axis. Amalgamation of these channel belts resulted in laterally widespread sheet-like sandstone bodies up to several tens of metres thick with internal erosional surfaces, which are sandwiched within dominantly fine-grained floodplain facies bearing coal seams. About 24–71 coal seams of the Mudstone Series are at least locally >1 m thick and economically important. Peat swamps formed by paludification of widespread floodplains that were dissected by active channels and upon which there were shallow lakes. Petrographic analysis indicates that the precursors of most coal beds were wet forest to mixed peat swamps colonised by arborescent – shrubby vegetation, whereas herbaceous peat-forming wetlands were subordinate and concentrated in the eastern, less subsiding part of the basin. Coal seams derived from forest to mixed peat swamps are dominated by bright coal lithotypes composed of vitrinite/vitrite which, together with intercalated clastic bands and an ash yield of 17 wt.%, suggest a high degree of waterlogging and a rheotrophic character for most peat swamps. Local coal bands rich in fusain lenses may record a temporarily lowered water table when the exposed surface was prone to wild fires. Although forest swamps generally occupied proximal parts of the floodplains, mixed peat swamps preferred distal areas that were less affected by active fluvial channels. Coal resulting from forest swamps is characterised by alternating bands of thick telinite and collotelinite derived from arborescent vegetation. Mixed peat swamps generated coal with a higher proportion of collodetrinite and inertodetrinite rich in sporinite derived mostly from shrubby plants. Herbaceous vegetation resulted in dull coal lithotypes only: durain, clarain-durain and durain-clarain, composed of detrital macerals, mainly inertodetrinite, vitrodetrinite and sporinite.