Climatically-driven cyclicity and peat formation in fluvial setting of the Moscovian – Early Kasimovian Cracow Sandstone Series, Upper Silesia (Poland)

Abstract

The approximately 1200 m thick Cracow Sandstone Series (Middle – early Late Pennsylvanian) in the Polish part of the Upper Silesian Coal Basin consists of 25–70 m thick fining up and laterally widespread fluvial cycles. The cycles are marked by alternating sheet-like sandstone bodies overlain by much thinner intervals dominated by mudstone and coal. These two-member cycles record the alternation of a sand-dominated fluvial braidplain and floodplain-dominated fluvial system with channels confined to narrow belts. The lateral persistence, internal architecture, and isochronous alternations suggest an allogenic origin for the cycles, possibly related to climate. This interpretation is supported by a three-stage evolution of pedogenic processes operating on floodplain deposits beginning with the formation of vertisols, followed later by gleying and, finally, termination by a Histosol (peat) formation. Such a succession of paleosols indicates a climatic shift from highly seasonal (Vertisol) to humid climate (coal) with the most seasonal part of the cycle likely represented by the sheet-like sandbody. The mechanistic link between climate and the cyclic pattern of strata is explained as climatically-driven variations in clastic supply, which was highest during periods of prominent seasonality. Short-term climatic oscillations generating cyclic patterns in the Cracow Sandstone Series are superimposed on a long-term climatic shift towards increased seasonality near the Middle – Late Pennsylvanian boundary. This is evidenced by the termination of peat formation in the Upper Silesian Basin as well as in some other coeval basins of the equatorial Pangea and is interpreted to represent a potentially regional climatic event.