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

Kowalski Aleksander

Annales Societatis Geologorum Poloniae

2020

Vol. 90, No. 2

125--148

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.

Late Quaternary evolution of the Czyżynka river valley, Wałbrzych Upland, Middle Sudetes Mts, southwestern Poland

Krzyszkowski D.

Geologia Sudetica

1998

Vol. 31, nr 2

259-288

The lower part of the Czyżynka river valley (Wałbrzych Upland, Sudetes) was formed entirely after the early Saalian (Odranian) stage. The subsequent, 60-80 m deep, valley was incised into the mountain plateau. The valley is generally narrow with an alternating wide and narrow segments, and characterized by steep slopes, including common subvertical rock walls. Three terraces, formed most probably during the Wartanian/Eemian, Middle Weichselian and Late Weichselian represent the main stages of fluvial activity. Fluvial sequences show that sinuous to meandering rivers were active throughout the late Quaternary. Two phases of loess deposition are inferred (Wartanian and Middle Weichselian), as the deposits of the older terraces contain large admixtures of loess-like deposits. Moreover, two phases of large angular debris production and formation of extensive slope covers and fans have been recognized. The older occurred during the Upper Pleniglacial of the Weichselian stage. A continuum of slope colluvium was formed at that time, from fine-grained sheet wash deposits (including redeposited loess) to coarse grained deposits formed by cohesive debris flows, noncohesive debris flows and grain flows, and open-work loose material formed by debris avalanches. Frost activity and permanently moist conditions have been reported for that time, most probably with permafrost. Also, all these slope deposits contain an admixture of loess, suggesting loess deposition that time, too. The second phase of coarse debris production occurred, most probably, during the the Younger Dryas forming less extensive slope covers.