Upper Ordovician mudrock facies and trace fossils in the northern Holy Cross Mountains, Poland, and their relation to oxygen- and sea-level dynamics
Abstract The bulk of the Ordovician sedimentary record in the northern Holy Cross Mts., Poland, is composed of dark claystones, which record the long-term dysoxic conditions lasting throughout the entire Caradoc. However, a relatively short-term oxygenation event corresponding to the middle Caradoc sea-level lowstand interrupts this monotonous succession. Termination of the low oxygen environment was triggered by a major change in oceanic circulation due to onset of polar cooling. It resulted in the progressive benthic oxygenation and increase of colonization of the highly enriched sediment by soft-bodied burrowers as can be envisaged from ichnofabric of the lower to middle Ashgill sedimentary record. The rapid but gradual upward progradation of the Upper Ordovician mudrock facies into the coarse-grained deposits appears to be closely related to the late Ashgill glacio-eustatic regressive event.