Oligocene environmental changes in the Central Paratethys: Geochemical and palynofacial record from the north-western Transylvanian Basin (Romania)

Abstract

The Oligocene succession exposed at Fântânele village in the north-western Transylvanian Basin (Romania), composed of the Ileanda and Vima formations, was deposited in a semi-closed Central Paratethys domain. Foraminiferal associations and lithology revealed the Late Rupelian (NP 24) and Late Rupelian-Chattian (NP 24/25) ages for these formations.

The analysed organic matter, particularly abundant in the black shales of the Ileanda Formation, is of low thermal maturity (T_max ∼ 420 °C, R_o ∼ 0.4 %). Thus, it has not reached the hydrocarbon generation stage. The Ileanda Formation contains mainly Type II kerogen, reflecting a period of increased marine phytoplankton production driven primarily by diatoms (high C₂₈ steranes and HBIs) and dinoflagellates (4-methyldiasterenes and dinoflagellate cysts), with bacteria playing a lesser role. The Vima Formation shows a more significant influence of terrestrial inputs, evidenced by Type II/III and III kerogen, a high share of phytoclasts, and substantial amounts of angiosperm- and conifer-derived biomarkers.

High primary bioproductivity combined with basin deepening in the Rupelian resulted in water column stratification and subsequent enhanced organic matter preservation imprinted by high TOC, hydrogen index values, and steranes/hopanes ratios in the Ileanda Formation. Low Pr/Ph ratio (mostly <1), higher contents of aryl isoprenoids, the occurrence of sulfurised highly branched isoprenoids, and a high proportion (86–96 %) of tiny pyrite framboids point to permanent anoxia/euxinia in the sea floor expanding higher, at least intermittently, into the photic zone. At the Late Rupelian/Chattian transition, the depositional setting changed to an outer shelf characterised by normal-marine conditions with oxygen-deficient bottom water. Later in the Chattian, shorter episodes of anoxia/euxinia related to freshwater incursions and subsequent salinity stratification are evidenced by pronounced input of higher plant-derived organic matter recorded in the uppermost part of the Vima Formation.

The findings highlight the role of regional tectonic activity in the Transylvanian Basin and its implications for understanding the hydrocarbon potential of similar basins in the region.