Hydrocarbon signatures as a tool for unraveling the stratigraphic problem for Upper Cretaceous–Paleogene sediments from Internal Dinarides, Serbia

Inorganic and organic geochemical analyses, assisted by micropaleontological investigations, were performed on the Paskovac sediments to differentiate the Cretaceous and Paleogene deposits in Internal Dinarides region. Owning to the presence of fossil assemblages in the in Upper Cretaceous sediments and their scarcity in the Paleogene siliciclastic sediments, the entire Paskovac area was fundamentally incorrectly classified as the Campanian–Maastrichtian. Besides, the Paskovac sedimentary sequence was formed as a consequence of the uppermost Cretaceous–Paleogene tectonic shortening of the Dinarides, during which the Maastrichtian sediments were re-deposited within the Paleogene sequence from the underlying Maastrichtian sediments. Therefore, this study employed thin-section microscopy, atomic absorption spectrometry (AAS), and gas chromatography–mass spectrometry (GC–MS) techniques to closely inspect 27 samples from the Paskovac drillhole. The results revealed a clear division of the samples into two groups based on their geochemical similarities. One group marked as Paleogene clastic sediments characterizes the highest content of terrigenous elements, namely SiO₂ (49.49 %–62.22 %), Al₂O₃ (16.18 %–21.75 %), Fe₂O₃ (3.41 %–6.67 %), TiO₂ (0.63 %–0.80 %), along with sulfur content (2.33 %–2.80 %). The organic matter is rich in odd long-chain n-alkanes, C₂₉ regular sterane (>60 %), benzohopanes, retene, cadalene, and benzo[b]naphthofurans, suggesting predominately terrestrial biomass deposited under suboxic–oxic conditions. Another group, represented by carbonates (CaO up to 51.02 %), contains Maastrichtian assemblages (e.g., Siderolites calcitrapoides), indicating shallow-marine depositional settings. Short-chain even n-alkanes, branched alkanes (e.g., 5,5-diethylalkanes), and C₂₇ regular sterane suggest the presence of biodegraded mixed algal/microbial biomass.