Sandy inland braidplain deposition with local aeolian sedimentation in the lower and middle parts of the buntsandstein and sandy coastal braidplain deposition in the topmost zechstein in the sudetes (Lower Silesia, Poland)

Abstract

The lower and middle parts of the Buntsandstein between Rot and Zechstein in the Sudetes (Lower Silesia, Poland) crop out in the marginal seams of the North Sudetic Trough and the Intra Sudetic Trough. The continental red beds originate in predominantly sandy braided river systems of an extensive inland alluvial plain in almost arid climate. The sediments are laid down in channels and floodplains of a moderately- to highly-braided, sandy to pebbly stream complex consisting of narrowly- to moderately-spaced low-sinuosity watercourses and narrow to wide overbank plains between the channels. Rapid aggradation and abandonment, quick lateral migration or high avulsion rates of the considerably mobile streams result in effective combing of the interchannel areas. Persistent high-energy overspilling of watercourse banks and invasion of bed-load-saturated flood surges into the overbank areas often lead to primary restriction or even suppression of formation of topstratum suspension fines. Secondarily, the silty-clayey and fine sandy overbank sediments which could occasionally originate in remote or sheltered lakes and ponds are frequently completely reworked by considerable lateral and vertical erosion during sidewards displacement of the rivers. As a result of both primary-depositional restriction and secondary-erosional removal of floodplain fines, the channel sediments are commonly stacked upon each other to multistorey stream sand complexes. Emergence and desiccation of parts of the alluvial plain sometimes give rise to aeolian deflation and accumulation of the winnowed sand to small dunelets and wind ripple trains. The aeolian depositional environment representing a more peripheral erg facies with sheet sand interdune milieu could not be fully ascertained due to poor outcrop conditions, but is likely to occur locally in view of comparative interpretation with other mixed dune and river sand sequences in the Mid-European Buntsandstein. Variations of fluvial style are restricted to a narrow range between the end members of a pebbly to sandy highly-braided stream system representing the coarser facies tracts and a sandy to pebbly moderately-braided river complex representing the finer facies tracts. The evolution in time and space is characterized by mainly random lateral shifting and vertical overlapping of two coexistent fluvial styles within an irregularly zoned braidplain lacking organization of the two facies realms into discrete belts. Palaeogeographically, the two end members of the spectrum of fluvial style may sometimes even be equivalent to a more proximal and a more distal facies belt. In terms of position within the general depositional framework of the whole Mid-European Triassic Basin, the inland braidplain of the Sudetes is situated within the transitional reach from the proximal zone to the medial zone I. Based on comparative interpretation of the topmost Zechstein beds in the region of Chelmsko and Kochanow in the Intra Sudetic Trough, the overall depositional history in parts of the area comprises the passage from marine carbonate sedimentation via a narrow coastal fluvial braidplain with pedogenesis in semi-arid climate at the margin of the Zechstein sea to an extensive inland fluvial braidplain with probably local aeolian deposition in almost arid climate in the Buntsandstein. Comparative evaluation of the marginal Zechstein in the Intra Sudetic Trough in Lower Silesia in Poland and of the marginal Lower Muschelkalk in Luxembourg allows to assess the Buntsandstein succession near the borders of the basin as an overall symmetrical major clastic wedge prograding into the depositional area with retreat of the Zechstein ocean and retrograding from the basin with transgression of the Muschelkalk sea. In view of a general model of continental sedimentation, the alluvial marginal belt of the Zechstein sea in Lower Silesia in Poland represents a predecessor, and the fluvial marginal seam of the Muschelkalk ocean in Luxembourg has the nature of an echo of the extensive braidplain complex of the Buntsandstein.