A drying-upward aeolian system of the Bohdašı́n Formation (Early Triassic), Sudetes of NE Czech Republic: record of seasonality and long-term palaeoclimate change

Abstract

The record of aeolian processes was studied in Early Triassic sandstones of the upper part of the Bohdašı́n Formation, NE Bohemia, an isolated erosional relict of a Triassic fluvial–aeolian succession in the Sudetes block of the Bohemian Massif (central Europe). The aeolian system exposed in the Devět Křı́žů quarry near Červený Kostelec provides a significant addition to the generally poorly preserved record of Early Triassic palaeoclimate and palaeowind regime in central Europe. Two mutually interbedded facies occur in the aeolian succession: (1) dune facies comprising cross-strata that show interdune, reactivation and superimposition bounding surfaces, and (2) interdune facies, formed by horizontally stratified, commonly mud-draped sandstones, which display a number of indicators of damp to wet conditions and fluctuations in water table. The succession of the Devět Křı́žů Sandstones is interpreted as a record of migration and climb of dunes with intervening interdune flats, which replaced an underlying fluvial system. It is likely that a coeval fluvial system existed near the Devět Křı́žů dune field, providing available sand in dry periods and flooding in wet periods. Dominant palaeowind directions are southwestward and northward in present-day coordinates. Repeated reversals of palaeowind direction, interpreted as seasonal, are recorded in changes in the internal structure of the cross-strata (reworking and reactivation) in the dune migration direction. At least some episodes of dune reworking occurred simultaneously with episodes of water-table rise. A drying-upward trend in the aeolian succession is interpreted from evidence for increased availability of sand through time. The upward transition from a braided fluvial to a wet, drying-upward aeolian system within the Bohdašı́n Formation represents a shift toward maximum aridity of the palaeoenvironment, which marks also the peak arid conditions in the long-term palaeoclimatic evolution of the Carboniferous–Triassic succession of the Sudetes region, and correlates with the establishment of aeolian settings in other regions of central Europe during the Early Triassic. The record of changing palaeowind directions, documented in several cases to have been coupled with episodes of rapid rise in water table, is taken as evidence for pronounced climatic seasonality and brings additional support to the models predicting strong monsoonal circulation during the Early Triassic, affecting particularly the eastern subequatorial coast of Pangaea and adjoining regions.