The nature of the Late Oligocene Turtas lake-sea in West Siberia

Deposits of the regional Upper Oligocene Turtas Horizon are widespread in W est Siberia. They are exposed in numerous boreholes and natural outcrops al ong the Ob', Irtysh, and their tributaries in the center of the West-Siberi an Plain (Khanty-Mansiisk depression), in the south of the plain (Novosibir sk and Omsk Regions), and in the Ob' region near Tomsk. Most scientists consider the deposits of the Turtas Horizon to be lacustrin e. Shatsky believed that the Turtas lake basin was subsaline and close in a rea to the Paleocene sea. It was assumed that the Turtas lake-sea was conne cted with the Baigubek sea by the Turgai channel in the south and with nort hern seas by a system of channels in the north. However, there is still no unambiguous opinion on the origin and nature of the vast Late Oligocene wat er basin. This prompted us to carry out closer microfossil and paleomagneti c investigations. The study concerns main sections of the Turtas Horizon and their paleontolo gical and paleomagnetic description. Dinoflagellates, abundant in Upper Eoc ene sediments, were absent from the deposits of the Turtas Formation. An as semblage of green algae - conjugates - was found and described for the firs t time In main natural outcrops and boreholes of the Turtas Horizon. It was found that the conjugates contain no typically marine species. The genus O voidites demonstrates a great range of species. The diatoms are also repres ented mainly by fresh-water forms inhabiting cold lakes. Paleontological data and the history of the Oligocene relief in West Siberi a led us to the conclusion that in the Late Oligocene the Turtas lake-sea, named for its vast size, was not a sea and was not connected with southern and northern seas. Preliminary paleomagnetic data indicate that the paleoma gnetic section of the Turtas Formation in the south of the West-Siberian Pl ain consists of four major magnetozones in the rank of orthozones, of norma l and reverse polarities. Only one magnetozone of normal polarity was recor ded In the east of the plain, in the Ob' region near Tomsk. This indicates that the section of the formation on the eastern margin of the plain is inc omplete.