TY - JOUR

T1 - Microfossil assemblages (Diatoms, calcareous nannofossils, and silicoflagellates), paleoenvironment, and hydrocarbon source rock potential of the oligocene ruslar formation at karadere, Bulgaria

AU - Tulan, Emilia

AU - Sachsenhofer, Reinhard F.

AU - Witkowski, Jakub

AU - Tari, Gabor

AU - Ćorić, Stjepan

AU - Bechtel, Achim

PY - 2020/1/2

Y1 - 2020/1/2

N2 - The Oligocene Ruslar Formation, an equivalent of the Maykop Suite, is a potential hydrocarbon source rock in the westernBlack Sea Basin. In contrast to the offshore areas, the depositional environment and hydrocarbon source rock potential of onshoreBulgaria sediments are largely unknown. Hence, a 14-m-thick section of the Ruslar Formation, exposed near Karadere (Black Cape)along the Black Sea coast, provides an excellent opportunity to study the upper part of the Ruslar Formation. Here, laminated diatomrichmudstones with frequent thin sandstone beds and a prominent concretion horizon are exposed. Furthermore, the fossil diatomassemblages provide a key component to understand the paleoenvironment. Overall, Paleogene diatoms are understudied in the BlackSea Basin and therefore only a genus-level study is undertaken here. The studied Ruslar Formation contain remarkably diverse and wellpreserveddiatom assemblage with 23 different genera. The most frequent genera are Paralia, Distephanosira, and Stephanopyxis. Commongenera include Coscinodiscus, Hemiaulus, Pseudopodosira, Rouxia, and Xanthiopyxis. Rare taxa include Actinoptychus, Asterolampra,Azpeitia, Delphineis, Distephanosira, Diploneis, Eunotogramma, Eurossia, Lyrella, Liradiscus, Plagiogramma, Radialiplicata, Rutilaria,Saeptifera, and Triceratium. The diatom assemblages together with calcareous nannoplankton, silicoflagellates, and the presence of rareforaminifera indicate a fully marine neritic environment without major salinity variations. The calcareous nannoplankton investigatedcan be assigned to biozone NP23 (Early Oligocene). The exposed fragment of the Ruslar Formation was deposited after the low salinity“Solenovian event”, which represents the maximum isolation of the Paratethys present in the lower part of the NP23. Bulk geochemicalparameters from 35 samples (avg. TOC: 1.80% wt.; avg. HI: 226 mg HC/g TOC) show that the exposed part of the Ruslar Formationcontains type II-III kerogen and a fair to good hydrocarbon potential. The Ruslar Formation is immature (avg. Tmax: 424 °C), but maygenerate about 0.5 tons of hydrocarbons per square meter if mature. Biomarker proxies support the low maturity and are characterizedby diatom-related biomarkers (24-norcholestane; C25HBI alkanes and thiophenes). Land-plant-derived biomarkers suggest a significantinput of angiosperms. Based on biomarker ratios, the depositional environment was oxygen-depleted but probably not strictly anoxic.Reworking of biomass by chemoautotrophic bacteria is suggested by the presence of 28,30-bisnorhopane.

AB - The Oligocene Ruslar Formation, an equivalent of the Maykop Suite, is a potential hydrocarbon source rock in the westernBlack Sea Basin. In contrast to the offshore areas, the depositional environment and hydrocarbon source rock potential of onshoreBulgaria sediments are largely unknown. Hence, a 14-m-thick section of the Ruslar Formation, exposed near Karadere (Black Cape)along the Black Sea coast, provides an excellent opportunity to study the upper part of the Ruslar Formation. Here, laminated diatomrichmudstones with frequent thin sandstone beds and a prominent concretion horizon are exposed. Furthermore, the fossil diatomassemblages provide a key component to understand the paleoenvironment. Overall, Paleogene diatoms are understudied in the BlackSea Basin and therefore only a genus-level study is undertaken here. The studied Ruslar Formation contain remarkably diverse and wellpreserveddiatom assemblage with 23 different genera. The most frequent genera are Paralia, Distephanosira, and Stephanopyxis. Commongenera include Coscinodiscus, Hemiaulus, Pseudopodosira, Rouxia, and Xanthiopyxis. Rare taxa include Actinoptychus, Asterolampra,Azpeitia, Delphineis, Distephanosira, Diploneis, Eunotogramma, Eurossia, Lyrella, Liradiscus, Plagiogramma, Radialiplicata, Rutilaria,Saeptifera, and Triceratium. The diatom assemblages together with calcareous nannoplankton, silicoflagellates, and the presence of rareforaminifera indicate a fully marine neritic environment without major salinity variations. The calcareous nannoplankton investigatedcan be assigned to biozone NP23 (Early Oligocene). The exposed fragment of the Ruslar Formation was deposited after the low salinity“Solenovian event”, which represents the maximum isolation of the Paratethys present in the lower part of the NP23. Bulk geochemicalparameters from 35 samples (avg. TOC: 1.80% wt.; avg. HI: 226 mg HC/g TOC) show that the exposed part of the Ruslar Formationcontains type II-III kerogen and a fair to good hydrocarbon potential. The Ruslar Formation is immature (avg. Tmax: 424 °C), but maygenerate about 0.5 tons of hydrocarbons per square meter if mature. Biomarker proxies support the low maturity and are characterizedby diatom-related biomarkers (24-norcholestane; C25HBI alkanes and thiophenes). Land-plant-derived biomarkers suggest a significantinput of angiosperms. Based on biomarker ratios, the depositional environment was oxygen-depleted but probably not strictly anoxic.Reworking of biomass by chemoautotrophic bacteria is suggested by the presence of 28,30-bisnorhopane.

KW - Black Sea

KW - Calcareous nannofossils

KW - Diatom assemblages

KW - Hydrocarbon source rock

KW - Oligocene

KW - Ruslar Formation

UR - http://www.scopus.com/inward/record.url?scp=85079893498&partnerID=8YFLogxK

U2 - 10.3906/yer-1907-9

DO - 10.3906/yer-1907-9

M3 - Article

AN - SCOPUS:85079893498

VL - 29.2020

SP - 154-169+Anhang

JO - Turkish Journal of Earth Sciences

JF - Turkish Journal of Earth Sciences

SN - 1300-0985

IS - 1

ER -