Deep-sea trace fossils controlled by palaeo-oxygenation and deposition: an example from the Lower Cretaceous dark flysch deposits of the Silesian Unit, Carpathians, Poland

摘要

Trace fossil associations and ichnofabrics have been studied in three lithostratigraphic units of the Lower Cretaceous dark flysch deposits, i.e. within the Upper Cieszyn Beds (Valanginian- Hauterivian), the Verovice Shale (Barremian-lowermost Albian) and the Lgota Beds (Albian- Cenomanian). The associations differ from those of the Upper Cretaceous and Tertiary flysch deposits. The trace fossil association of the Upper Cieszyn Shale belongs to the Nereites ichnofacies, but the associations of the other two units do not; therein the trace fossil diversity is distinctly lower, and graphoglyptids or horizontal meandering pascichnia are absent or very rare. These trace fossil associations supposedly changed as a result of general evolutionary processes that influenced deep-sea trace makers after the Early Cretaceous, and which intensified the aforementioned differences. The composition of trace fossil associations and the vertical extent of the bioturbated zone in turbidite-hemipelagite beds were strongly affected by changing oxygenation on the deep-sea floor during the global Early Cretaceous anoxic events. The Verovice Shale is dominated by anoxic sediments interlayered with rare, thin, bioturbated horizons. In the Upper Cieszyn Shale and in the Lgota Beds, most tops of turbidite-hemipelagite rhythms are bioturbated. Non-bioturbated rhythms record anoxia, but their occurrence is influenced by the frequency of turbiditic deposition. Protovirgularia obliterata and Protovirgularia pennata occur in the Verovice Shale in the deepest tier below Chondrites. These trace fossils were probably produced below the redox boundary by chemosymbiotic bivalves. The studied associations show that the deep-sea environment is influenced by many factors that change with time, which records the large-scale dynamics of deep-sea ecological processes.