oa Environmental Changes During Oligocene Inferred from Biomarker and Palynofacial Record from the NW Transylvanian Basin, Central Paratethys

The presented study investigates the paleoenvironmental conditions during the Oligocene in one of the Paratethys sub-basins, namely the Transylvanian Basin. The research uses a comprehensive approach, integrating palynofacial and organic geochemical analyses to reconstruct the organic matter sources and depositional environment of the Oligocene succession exposed at Fântânele village, Romania. Additionally, pyrite framboid distribution was performed as a proxy for redox conditions. The sampled outcrop succession consists of the Rupelian Ileanda Formation, represented by organic-rich black shales (TOC ∼6%), and the Late Rupelian/Chattian Vima Formation, represented by mudstones and marly shales. The results reveal substantial changes in organic matter sources and environmental conditions, including depositional setting and water-column structure. The Ileanda Formation records sedimentation in the deeper basin setting, characterized by increased marine phytoplankton production. In contrast, the Vima Formation shows a shallowing trend towards the outer shelf, with a greater influence of terrestrial inputs, reflected in abundant phytoclast fragments, as well as gymnosperm- and angiosperm-derived biomarkers. Our data reveal permanent euxinia stretching periodically into the photic zone during the Rupelian, responsible for an enhanced organic matter preservation potential. During the Late Rupelian and Chattian, redox conditions were variable but predominantly oxic, with episodes of bottom-water euxinia/anoxia in the Chattian.