Problematic age and stratigraphic position of the volcano-sedimentary Słupiec/Broumov formation in the Permian of the Intrasudetic Basin

The Intrasudetic Basin represents one of the larger late- to post-Variscan intramontane troughs of Central Europe. It is situated at the northern margin of the Bohemian Massif. The Basin represents a fault-bounded synclinorial structure and was formed in the late Visean as a depression framed by tectonically active margins. During the Permian, the basin was filled with dominantly fine-grained alluvial to lacustrine deposits, accompanied by volcanic rocks. Volcanic activity evolved with time and comprised emplacement of subvolcanic intrusions, effusion of lava flows as well as deposition of widespread ignimbrites (Awdankiewicz, 1999). These volcano-sedimentary units are known as the Słupiec Formation in the Polish part and the Broumov Formation in the Czech part of the Intrasudetic Basin, respectively. So far, based on generally imprecise biostratigraphic evidence and regional correlations, the Słupiec Formation sedimentary rocks together with the intercalated volcanic rocks were (usually) assigned to the Sakmarian. However, preliminary results of U-Pb SHRIMP zircon dating of the Góry Suche Rhyolitic Tuffs and the Łomnica Rhyolites – a widespread ignimbrite sheet and associated rhyolitic laccoliths intercalated in the Słupiec/Broumov Formation - suggest that these volcanic rocks can be older than supposed by 5-10 My. Such age estimate would assign these ignimbrites and rhyolites to the Asselian, not Sakmarian.

In this contribution the biostratigraphic evidence on the position of the Słupiec/Broumov Formation is re-assessed. The fluvio-lacustrine sedimentary members of these formations accumulated probably in semi-arid palaeoclimatic conditions with seasonally-controlled watertable.Numerous footprints of reptiles and amphibians, aquatic vertebrates: chondrichthyans, actinopterygians and amphibians, also palaeobotanical remains were preserved (e. g.  Jerzykiewicz, 1987; Zajíc, 2000; Stamberg & Zajíc, 2008; Voigt et al., 2012; Opluštil et al., 2016). Unfortunately, they appear only fairly suitable for detailed biostratigraphy as their successions may be environmentally-controlled, and most of them indicate a latest Carboniferous to early Permian age. At this level of knowledge, they are not suitable for detailed biostratigraphy, and thus comprehensive and comparative studies of the Late Carboniferous and Early Permian Central European volcanic-sedimentary basins are necessary to better constrain the stratigraphic position of the Słupiec/Broumov Formation of the Intrasudetic Basin.

This research is funded by the Polish National Science Centre (Grant 017/26/M/ST10/00646).

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