Ichnological record of the Frasnian–Famennian boundary interval: two examples from the Holy Cross Mts (Central Poland)

The Frasnian–Famennian (Late Devonian) boundary interval within the carbonate–siliciclastic series in the Kowala and Płucki sections (Holy Cross Mts, Central Poland) has been analysed to evaluate the influence of the Kellwasser event on the macrobenthic tracemaker community. The Upper Kellwasser event has a lithologically variable record, as horizons of flints (Kowala) and as a bed of bituminous, black, cephalopod limestone (Płucki). Both sections show mostly laminated, unbioturbated beds of marlstones or shales just above the Frasnian–Famennian boundary, which point to events of anoxia on the sea floor. However, the first anoxic horizon occurs below the Frasnian–Famennian boundary. The trace fossils and bioturbational structures are uncommon and poorly diversified. Trichichnus and Multina are the only frequent trace fossils in some beds. Moreover, one horizon above the Frasnian–Famennian boundary contains numerous Multina and a single ?Planolites. Such poorly diversified trace fossil assemblage suggests an unfavourable environment for most of burrowing organisms and fluctuations in oxygenation from anoxic, to dysoxic conditions. The occurrence of the trace fossils and bioturbational structures as spotted and mottled ichnofabrics from the 1.3 m above the Frasnian–Famennian boundary is interpreted as an improvement in bottom water oxygen conditions after the Upper Kellwasser event.