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The fluvial Peruc Member (Middle to Upper Cenomanian) of the Bohemian Cretaceous Basin near the town of Benešov nad Ploučnicí (North Bohemia)


Jaroslav Valečka

Geoscience Research Reports 48, 2015 (GRR for 2014), pages 31–36
Map sheets: Děčín (02-23)

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Published online: 12 October 2015

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The deep drilling SK-1t situated near the town of Benešov nad Ploučnicí (North Bohemia, see fig. 1) verified the sedimentary sequence of the Bohemian Cretaceous Basin, 870.15 m thick. Typical fluvial deposits of the Peruc Member defined by Čech et al. (1980) occur at the sequence base. The Peruc Member (Upper to Middle Cenomanian in age) is 22.95 m thick, overlies the Permo-Carboniferous sediments and volcanics, and is covered with marine sandstones of the Korycany Member (Fig. 2). The sedimentological analysis of the Peruc Member was based on the modified method of Miall (1985, 1996). Six facies were distinguished in the Peruc Member section (Fig. 2): 1) mud-supported conglomerates (the only layer on the section base), 2) clast-supported conglomerates, 3) coarse-grained sandstones, 4) silty-clayey, fine-grained sandstones, 5) dark mudstones with sandy laminae and lenses, and 6) dark homogeneous mudstones. In the facies designated as 4, 5 and 6, coalified plants, wood remains and roots are frequent as well as the fusite fragments which are present also in facies 3. Cross-bedded sets occur in facies 2 and 3. The facies are separated by sharp, often erosional surfaces or by gradual transitions. The facies are mostly arranged into upward-fining cyclothems (see Fig. 2). The interpretation of the facies is as follows: facies 1 – plastic debris-flow (avalanche deposit), facies 2 – channel-floor deposits (longitudinal or transverse bars), facies 3 – longitudinal or transverse bars far from the channel floor or point bar deposits, facies 4 – point bar, crevasse splay and flood-plain deposits, facies 5 and 6 – flood-plain deposits. Facies 6 represents also a sedimentation from suspension in the lakes, pools and former channels on the flood plain. One thin layer of facies 6 inserted into very coarse sediments in the lower part of the section was possibly deposited in an ephemeral pool between emerged bars. The wide difference between the lower and upper parts of the Peruc Member section is apparent (see Fig. 2 and Tab. 1). The lower part of the Peruc Member is interpreted to be deposited in a narrow, high-relief valley with a braided stream, the upper part is interpreted as sediments of a meandering river in a low-relief valley. The abrupt change of the braided river to a meandering one is explained in terms of a rapid base-level rise caused by some of the Cenomanian marine transgressions.


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