The lower Oligocene Kurgan Beds of West Siberian and trans-Uralian regions

Stratigraphic position and distribution of the lower Oligocene Kurgan Beds in southern West Siberia are considered. Their age is substantiated by the data on malacofauna, ichthyofauna, dinocysts, spores, and pollen. Two scena rios of their formation are probable. According to the first one, the Kurga n Beds were accumulated during the retreat of already brackish Tavda sea fr om the West Siberian plate in the Eocene-Oligocene boundary time. The secon d scenario, which seems more probable, especially for the southern areas of the West Siberian plate, supposes the repeated sea ingression from the wes tern Turan plate via the Turgai depression in the early Oligocene (Ashcheai ryk) time. Similarly to the sea expansion into the Middle Volga and Cis-Ura lian areas along the older river system in the Akchagylian time (Pliocene), the Kurgan sea spread from the south to the north for more than 1000 km al ong the erosional Turgai depression and former Tobol valley with its tribut aries, To the north and east of the town of Kurgan, seawaters penetrated in to the Omsk syncline and negative landforms behind the Middle Urals. An ext ensive deposition of brown iron ores in the Turgai depression (the Lisakovs k basin with estimated reserves of two billion tons) is related to the Kurg an sea ingression. The iron ores were formed at the "geochemical barriers" in the mouths of rivers flowing into the sea strait. A high percentage of s mall pollen grains of Quercus graciformis and other thermophilic plants in the Kurgan Beds is likely a consequence of both the warm seawater ingressio n to the north and partial reworking of underlying Tavda sediments, where t hat pollen is dominant component of palynological spectra.