Eocene dinoflagellate cyst biostratigraphy of research borehole 011-BP, Omsk Region, southwestern Siberia

Abstract

A shallow marine Eocene section recovered in the cored borehole 011-BP, southwestern Siberia, was analyzed palynologically. Age-diagnostic dinoflagellate cyst events including the first occurrences of Charlesdowniea coleothrypta, Dracodinium politum, Ochetodinium romanum, Samlandia chlamydophora, Areosphaeridium diktyoplokum, Hystrichosphaeropsis costae, Wetzeliella eocaenica and Duosphaeridium nudum are recognized at successive levels in the Upper Lulinvor Formation (498.0–459.0 m), indicating a Middle-latest Ypresian age. A hiatus spanning the Early Lutetian is present at an unconformity at 459.0 m which underlies sediments of the uppermost Lulinvor Formation (459.0–456.0 m), which are referred to the Middle Lutetian based on the presence of Costacysta bucina, Cordosphaeridium cantharellus and Wilsonidium echinosuturatum. The overlying interval from 456.0 m to 265.0 m (Tavda Formation) is referred to the latest Lutetian to Priabonian primarily based on a combination of published paleomagnetic signals and the presence of Rhombodinium draco, Membranosphaeridium aspinatum, Svalbardella sp., Thalassiphora fenestrata, Thalassiphora reticulata, Rhombodinium perforatum and Rhombodinium longimanum. According to the combined dinoflagellate cyst data and normal magnetic polarity in the uppermost part of the Tavda Formation, marine sedimentation was interrupted in southwestern Siberia during the Late Priabonian (∼34.8 Ma). The dinoflagellate cyst assemblages are illustrated, the new species Thalassiphora dominiquei is formally described and a neotype for Wetzeliella coronata (Vozzhennikova 1967) Lentin & Williams 1976 is designated. The morphology and taxonomy of several taxa are discussed.