Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Preservation of sedimentary structures in greywackes and siltstones of the Barrandian Neoproterozoic


Tomáš Vorel

Geoscience Research Reports 50, 2017, pages 241–245
Map sheets: Zbraslav (12-42)

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Published online: 31 October 2017

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Well-preserved sedimentary structures were newly found in weakly metamorphosed Neoproterozoic rocks of the Teplá-Barrandian Unit at two localities. The first locality lies in a sedimentary complex of the Štěchovice Group near Vrané nad Vltavou, the second one in the Kralupy-Zbraslav Group near Nižbor.
The first site is located about 130 m west of the ferry on the Vltava River, between Strnady and Vrané nad Vltavou. Here, the Štěchovice Group consists of a few hundred meters thick sedimentary complex exhibiting a characteristic rhythmic alternation of siltstones, shales to fine-grained greywackes. Local rocks are affected only by anchimetamorphosis.
In a sequence of dark gray shales and siltstones there were identified whitish weathered, strongly laminated positions showing well-preserved lenticular bedding (Figs 1-3). The lenses are approx. 6-7 cm long and about 6 mm thick. Fine stratification (preserved foresets) can still be observed inside of some lenses so that they may represent original current ripples, and correspond to Tc division of Bouma Sequence (Bouma 1962). The described sedimentary structures well demonstrate distal turbidity sedimentation, that correspond to the overall flysch character of the Štěchovice Group, and to its several-kilometer thick sequence.
About 108 meters west of this locality, there occur some uniquely preserved underwater slump structures (Fig. 4) earlier described by Röhlich (1964).
The second locality lies near Nižbor, in the Neoproterozoic Kralupy-Zbraslav Group. In this area, a sedimentary sequence is developed as a few kilometers thick complex of shales, siltstones and greywackes in which shales and siltstones are slightly more abundant. According to Cháb and Pelc (1968) this sequence belongs to a monotonous facies. Here, the metamorphic grade, compared to the first locality, is slightly higher, with already noticeable phyllitization of shales, and with the development of planar metamorphic foliation, mostly corresponding to the original bedding.
The locality is situated in a railway cut, about 1.17 km east of the train station building of Nižbor, where approx. 0.5 m thick bank of fine greywacke with lenticular and streaky sedimentary structure was found (Figs 5-7), representing again probably the original ripple bedding, which corresponds to Tc division of Bouma sequence. The lenses are up to 3-5 cm long and about 4 mm thick. Similarly to the Štěchovice Group described earlier, this part of the Kralupy-Zbraslav Group also exhibits prevailing turbidity mechanism of sedimentation - probably not so much distal, but still retaining its typical patterns.
This study revealed that even in the central part of the Neoproterozoic Teplá-Barrandian Unit can be found the original sedimentary structures allowing to recognize the original bedding and to study its relationship to subsequent metamorphic foliation or cleavage. The current foliation observed near Nižbor is identical with the original bedding, and the same situation can be seen near Vrané nad Vltavou.