Triassic palaeogeography of NE Bohemian Massif based on sedimentological record in the Wleń Graben and the Krzeszów Brachysyncline (SW Poland)

Abstract

This paper presents a new sedimentological and palaeogeographic interpretation of the Early to Middle(?) Triassic continental deposits in the Wleń Graben and the Krzeszów Brachysyncline (Sudetes, SW Poland). These two tectonic subunits, located in the peripheral segments of the North Sudetic and Intra-Sudetic synclinoriums, respectively, represent the crucial elements for deciphering the post-Variscan palaeogeographic evolution of the NE termination of the Bohemian Massif. Sedimentological studies and facies analysis show that the Early Triassic siliciclastic deposition in both areas was dominated by fluvial sedimentation of the typical Buntsandstein facies within a gently sloping alluvial plain, locally followed by ephemeral lake environments in the late Early or Middle(?) Triassic. The lithofacies and the measured palaeocurrent directions indicate that the area was drained by braided streams flowing towards the northwest and north. Individual fluvial channels had depths not exceeding 1 m and widths of up to a few metres. Overbank deposits are poorly preserved due to the lateral shifting of channels. Based on available borehole data, an analysis of isopach maps was performed for the first time in the study areas. The study areas are presently separated by crystalline units devoid of sedimentary rocks, but the analysis indicate that they might have been in constant or periodical connection during the Triassic. It is concluded that the present-day extent of Triassic deposits is a result of the primary basin configuration combined with the Middle Triassic to Late Cretaceous erosion and post-Cretaceous tectonic uplift. The paper summarizes the present state of research on the continental Triassic preserved in the terminal parts of the North Sudetic and Intra-Sudetic synclinoriums, presents the first detailed sedimentological logs, and proposes new palaeogeographic interpretation. New findings include bioturbation structures, such as plant-root traces or tunnels formed by invertebrates and possible tetrapod footprints, which shed new light on the sedimentological interpretation of the continental Mesozoic deposits in the NE part of the Bohemian Massif.