Lost Norian fluvial tracks: Sedimentology and stratigraphy of the Upper Triassic coarse-grained deposits in Kamienica Śląska (Upper Silesia, southern Poland)

Abstract

In the Triassic‒Jurassic boundary profiles of the Upper Silesian region, there are locally developed coarse clastic deposits, commonly known as the Połomia Gravels, Połomia Beds or “Połomia Formation’’ (informal name). Due to the lack of reliable stratigraphic tools, the chronostratigraphic position of these deposits remains one of the most controversial aspects of the Triassic‒Jurassic lithostratigraphy in the region. Sparse biostratigraphic data from the overlying and underlying deposits indicate a wide range of ages, from the Late Triassic to Early Jurassic. This paper presents the results of sedimentological, petrographical and palynological analyses of the coarse-grained deposits from the Kamienica Śląska gravel pit, which is currently one of the biggest facilities of this type in Upper Silesia. The outcrop section, with a total thickness of up to 20 m, is dominated by friable, light grey to beige and locally dark grey, large-scale planar to trough cross-stratified conglomerates and coarse-grained sandstones with subordinate thin interbeds of grey and reddish brown mudstones. Three facies associations have been distinguished, representing a main channel belt, secondary channels and floodplains subenvironments. Facies analysis points to a braided river tract with localised floodplain sediments. The grain composition of the Kamienica Śląska gravel/conglomerate is less diversified than that of the polymictic typical Połomia Beds of the Myszków area and resembles oligomictic conglomerates known from the Grabowa Formation of the Norian age and/or conglomerates of the Gorzów Beds of Rhaetian age, which also occur on the studied region. Palynological analysis of mudstone interbeds within the conglomeratic deposits shows the presence of miospores guiding and characteristic for subzone c of the Corollina meyeriana zone of the late Norian‒early Rhaetian age. The appearance of the coarse-grained deposits in late Norian could be associated with the development of a long-reach braided fluvial tract, draining the S and SE part of the Sudetian-Malopolska-Lublin land (S-M-L land) in response to the tectonic rearrangement in the source area and gradual climatic change from semi-arid to humid in the Rhaetian.