First Dinoflagellate Finds in Upper Oligocene Continental Sediments of the Zhuravka Formation (Western Siberia)

摘要

The Turtas Formation and its facies analogue represented by the Zhuravka Formation are widespread over the West Siberian Lowland. They have been recovered by numerous boreholes in the central part of the lowland along with the Novosibirsk, Omsk, and Tomsk Ob River regions. They are also exposed in natural outcrops along the Irtysh and Ob rivers and in lower reaches of the Ishim, Tuya, and Ui rivers. During the Late Oligocene, the vast territory of western Siberia accumulated clayey mud and silt with the varved horizontal lamination. South- and eastward of the basin, they are replaced by coarser-grained sediments. The thickness of the formations ranges from 70-100 m in the central part of the lowland to 130 m in the Omsk Irtysh River region and then gradually decreases eastward. The sediments of the Turtas and Zhuravka formations rest with erosional contact on the Lower Oligocene Novomikhailovka Formation and is conformably (or with partial hiatus) overlain by the Lower Miocene Abrosimovka Formation. The Late Oligocene (Chattian) age of the formations is substantiated by gastropods and diverse myospore assemblages with Fagus grandifoliiformis Pan.