Trace fossils and bioturbation in the lower part of the Šárka Formation at Praha - Červený vrch Hill (Ordovician, Barrandian area, Czech Republic)


Authors: Mikuláš R

Published in: Bulletin of Geosciences, volume 78, issue 2; pages: 141 - 146; Received 13 December 2002; Accepted in revised form 18 March 2003;

Keywords: Ordovician, Šárka Formation, ichnofossils, Barrandian, Pilichnus, ichnofabrics,

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Sediments of the Šárka Formation are exposed at the Červený vrch Hill in Prague. The lower part of the outcrop shows biogenic structures belonging almost exclusively to the ichnogenus Pilichnus, with the Ichnofabric Index (i.i.) of 1-2. In the upper part, this index reaches the value of 3-5 in thin intervals; however, the 'background' is poorly reworked by infauna (i.i. = 1-2). Most of the distinctive traces (Planolites, Palaeophycus, Nereites) come from an interval rich in body fossils of graptolites and phyllocarids. One horizon, containing siliceous nodules, showed relatively frequent, well-preserved ichnofabric features in the nodules or on their surfaces. The ichnogenera Chondrites, Pilichnus, and Skolithos were recognized here. A taphonomic 'filter' apparently precluded preservation of shallow-tier trace fossils in the strongly bioturbated intervals, therefore, the recognized traces cannot be considered to represent the whole spectrum of benthic activity on/in the sediment. The overall character of the ichnoassemblage indicates ecologic stress, probably a low oxygen level, and only short-time fluctuations in the oxygen content towards the more oxygenated bottom waters and sediment.