A molecular organic geochemical study of black shales associated with diatomites from the Oligocene Meinilite Shale (Flysch Carpathians, SE Poland)

Abstract

Black shales of the Menilite Formation, the source rock for oils in the Carpathian overthrust belt, display a large variability in their bulk and molecular geochemical parameters. Biomarker and stable carbon isotope analyses indicate a variable contribution from different algae (particularly dinoflagellates and diatoms) and cyanobacteria. This is reflected by specific, predominantly sulfurised biomarkers (e.g. C35 homohopanoids, C25 highly branched isoprenoids and marine n-alkanes) and by particular distributions of steranes and 4-methylsteranes comprising 24-nor- and 24-methyl-27-norcholestanes, and related, novel steranes with a methylation at C-23. The presence of hopanoids of methanotrophic origin (13C up to −57 ) implies a temporarily enhanced full methane cycle in a marine environment which affected the isotopic composition of organisms dwelling in the upper photic zone. The presence of isorenieratene derivatives indicate periods of euxinic conditions within the photic zone in all investigated sub-basins.