THE LOWER OLIGOCENE KURGAN BEDS OF WEST SIBERIAN AND TRANS-URALIAN REGIONS

Abstract

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 scenarios of their formation are probable. According to the first one, the Kurgan Beds were accumulated during the retreat of already brackish Tavda sea from the West Siberian plate in the Eocene-Oligocene boundary time. The second scenario, which seems more probable, especially for the southern areas of the West Siberian plate, supposes the repeated sea ingression from the western Turan plate via the Turgai depression in the early Oligocene (Ashcheairyk) time. Similarly to the sea expansion into the Middle Volga and Cis-Uralian 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 along the erosional Turgai depression and former Tobol valley with its tributaries. To the north and east of the town of Kurgan, seawaters penetrated into the Omsk syncline and negative landforms behind the Middle Urals. An extensive deposition of brown iron ores in the Turgai depression (the Lisakovsk basin with estimated reserves of two billion tons) is related to the Kurgan 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 small pollen grains of Quercus graciformis and other thermophilic plants in the Kurgan Beds is likely a consequence of both the warm seawater ingression to the north and partial reworking of underlying Tavda sediments, where that pollen is dominant component of palynological spectra.