Integrated ichnology and ichthyology of the Oligocene Menilite Formation, Skole and Subsilesian nappes, Polish Carpathians: A proxy to oxygenation history

The anoxic, mostly black or brown fine-grained sediments of the Menilite Formation (Oligocene–Early Miocene) in the Skole and Subsilesian nappes contain thin layers of bioturbated green or grey-green mudstones, some of which contain the trace fossils Halimedides annulata, Multina isp., Palaeophycus ?tubularis, ?Planolites isp., Rhizocorallium isp., Trichichnus isp. and Zoophycos isp. The Trichichnus–Palaeophycus–(Multina, Halimedides)–Rhizocorallium suite indicates an increase in oxygenation of sediments. The contribution of different ecological groups of fishes, including epipelagic, bathypelagic, benthopelagic, neritic and reef, and demersal taxa changes significantly through the Menilite Formation. The absence or reduction of bathypelagic fishes points to anoxia in the water column. The combination of ichnological and ichthyological data and incorporation of data on benthic foraminifers allowed a reconstruction of oxygenation changes in the sediment and water column during deposition of the Menilite Formation. Total anoxia at the sea floor and in the water column, attributed to a combination of thermo-stratification and extremely high organic productivity, occurred only during the period reflected by ichthyofaunal Zone IPM2 (middle part of the NP23 Zone). Anoxia restricted to the basin floor or upper slope, related in part to upwelling, occurred during sedimentation of the upper part of the Menilite Formation.