Microfossil assemblages (diatoms, calcareous nannofossils, and silicoflagellates), paleoenvironment, and hydrocarbon source rock potential of the Oligocene Ruslar Formation at Karadere, Bulgaria

Abstract

The Oligocene Ruslar Formation, an equivalent of the Maykop Suite, is a potential hydrocarbon source rock in the western Black Sea Basin. In contrast to the offshore areas, the depositional environment and hydrocarbon source rock potential of onshore Bulgaria 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 diatomrich mudstones with frequent thin sandstone beds and a prominent concretion horizon are exposed. Furthermore, the fossil diatom assemblages provide a key component to understand the paleoenvironment. Overall, Paleogene diatoms are understudied in the Black Sea Basin and therefore only a genus-level study is undertaken here. The studied Ruslar Formation contain remarkably diverse and wellpreserved diatom assemblage with 23 different genera. The most frequent genera are Paralia, Distephanosira, and Stephanopyxis. Common genera include Coscinodiscus, Hemiaulus, Pseudopodosira, Rouxia, and Xanthiopyxis. Rare taxa include Actinoptychus,

Rupelian Kazakhstan floras in the context of early Oligocene climate and vegetation in Central Asia

“…Diatoms (Actynoptychus sp. ; two samples) known from the marine sediments of the Lower Oligocene (Tulan et al, 2020) were also found at Toguzken. This genus is characteristic of coastal zones with low salinity and its presence serves as a marker of a decrease in sea-level (Kovaleva et al, 2015).…”

Fossil Diatom Study Reveals Significant Freshwater Input in Miocene Coastal Marine Environments of the Southwestern Atlantic (Patagonia, Argentina)

“…Pseudopodosira is a diatom genus containing species that have been mentioned as tychoplanktonic. Although it was originally defined as a marine genus, several works mentioned Pseudopodosira species in coastal marine environments; it was also used to infer past sea-level changes (e.g., Witkowski et al, 2011;Sar et al, 2017;Tulan et al, 2020;Fayó et al, 2022).…”