A Lower Palaeozoic shallow water sequence in the eastern European Variscides (SW Poland): provenance and depositional history |
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| Authors: | Kryza Ryszard Turniak Krzysztof Muszyński Andrzej Zalasiewicz Jan A |
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| Institution: | (1) Institute of Geological Sciences, Wroc\/law University, ul. Cybulskiego 30, 50-205 Wroc\/law, Poland;(2) Department of Geology, A. Mickiewicz University, Maków Polnych 16, 61-606 Pozna , Poland;(3) British Geological Survey, Keyworth, NG125GG Nottingham, UK |
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| Abstract: | The Gackowa Formation of inferred Cambrian —Ordovician age is part of the metamorphosed pre-Variscan basement of the southern Kaczawa Mountains of the Sudetes region, south-west Poland. Previously variously interpreted as lavas, tuffs and sandstones, it is shown to consist of about 200 m of originally well-sorted siliclastic sedimentary rocks within a sequence of predominantly mafic volcanic rocks. Four facies have been distinguished based on relict sedimentary structures and textures and these suggest that deposition took place on a storm-dominated shelf, mostly above the wave base. The petrography, detrital zircon morphologies and geochemistry suggest affinity with, and derivation in large part from, acid volcanic rocks at a comparable stratigraphic position in a separate tectonostratigraphic unit to the south. The Gackowa Formation and its acid volcanic source rocks have a clear continental geochemical signature, in contrast with the mantle-derived basic/bimodal volcanic rocks below and above; it is suggested that the parent acid magma originated by crustal melting during the ascent of these mantle-derived magmas. A continental crust setting is inferred for the deposition of the Gackowa Formation sandstones, thus suggesting a continental setting for the associated within-plate basalts. The entire volcanic/sedimentary succession resembles elements of the Lower Palaeozoic of Germany, and all may have formed in an initial rift setting during Late Precambrian to Ordovician times. |
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| Keywords: | Storm-dominated shelf sandstones Prove nance Zircon morphology Geochemistry Lower Palaeozoic Variscides Sudetes |
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