Early Oligocene environment of the Central Paratethys revealed by biomarkers and pyrite framboids from the Tarcău and Vrancea Nappes (Eastern Outer Carpathians, Romania)

Abstract

The Menilite facies, representing the most prolific hydrocarbon source rocks in the Carpathian fold and thrust belt, has been studied in two outcrop sections of the Vrancea and Tarcău Nappes (Eastern Romania) with the aim of determining the depositional environment during the Early Oligocene in the Vrancea and Tarcău sub-basins of the Central Paratethys. The biomarker composition indicates spatial variability in organic matter sources with mainly algal (particularly diatoms and dinoflagellates) and bacterial contributions in both sub-basins. This is reflected by the occurrence of such biomarkers as triaromatic dinosteroids, C25 highly branched isoprenoid thiophenes, marine n-alkanes, hopanoids and monomethyl alkanes. Solely in the Vrancea domain, the input of terrigenous organic matter of higher plant origin can be anticipated from the presence of abietane-class biomarkers (conifer-derived) and oleanane (angiosperm-derived). Distinct nutrient availability related to different positions of individual sub-basins (i.e. shallower, temporarily eutrophic Vrancea sub-basin vs. open-marine mesotrophic Tarcău domain) is reflected by different paleoproductivity indicators, such as the total organic carbon content, hydrogen index and 17α-hopanes to steranes ratio.Water column stratification with bottom water anoxia enhanced by bacterial sulfate reduction stretching into the chemocline, at least intermittently, is expressed by a characteristic tiny pyrite framboid distribution and the lack of bioturbation. However, molecular indicators (Pr/Ph ratio, C35 homohopane index) imply fluctuations of redox conditions with the dominance of dysoxic conditions, which may suggest the occasional oxidation of bottom waters linked to local upwelling. Early diagenetic organic matter transformations such as photodegradation taking place in the oxidised upper part of the water column could also have affected the biomarker distributions. The presence of aryl isoprenoids and Me,i-Bu-maleimides indicates periods of euxinic conditions within the photic zone only in the Vrancea sub-basin during the deposition of the Lower Menilite Member. This is probably related to freshwater incursions from the adjacent, at least partly emerged forebulge, as suggested by a high MTTC ratio and the occurrence of higher plant-derived organic matter.