The late Triassic development of playa, gilgai floodplain, and fluvial environments from Upper Silesia, southern Poland

Abstract

The numerous discoveries of disintegrated skeletons of large terrestrial vertebrates within several thin levels of the Upper Silesian Keuper initiated broad investigations into the palaeoenvironment and age of the bone-bearing sediments. Despite years of research, the depositional history of the Upper Triassic continental succession and its controlling factors are still poorly recognized. This paper reconstructs the depositional evolution of the Upper Triassic strata in Upper Silesia, Poland, discusses the tectonic and climatic control on deposition, and identifies the sediment provenance. Detailed sedimentological analysis enabled the recognition of three major palaeoenvironments: (1) playa; (2) distal floodplain featured by gilgai micromorphology; (3) fluvial system (sand-dominated meandering, sand- and gravel-dominated braided, and potentially anastomosing river system). The transition from one palaeoenvironment into another reflects climatic changes throughout the late Triassic. The Carnian interval is dominated by gypsum-rich playa mudstones deposited under hot and arid conditions, with only one wet episode recorded as meandering river sandstones (the so-called Carnian Pluvial Event). In contrast, Norian sedimentation was controlled by strong seasonal climatic variations, which is reflected in alternating palaeosol horizons (vertisols and calcisols), thin claystone beds (small water-pond deposits), and conglomerates (rapid flood events). This facies assemblage was formed in a relatively stable floodplain which became the main habitat of numerous vertebrate organisms. The Rhaetian is represented by a gravel-dominated braided river system developed in response to significant climate humidification, with tectonic controls on flow direction. Clast types from Norian and Rhaetian conglomerates revealed that the studied area was fed from the south and south west by the San River and Moesian Massifs. Geochemical analysis of Norian palaeosol horizons suggests mean annual precipitation of ~720 mm/yr in agreement with the palaeoclimatic reconstructions for the area, pointing to seasonal sub-humid to semi-arid climate conditions.