THE LOWER MIOCENE OF THE TARA-IRTYSH REGION - STRATIGRAPHY, PALEOBOTANY, PALEOMAGNETISM

The Tara-Irtysh region, located in the central part of West Siberia, i s of interest because it represents the reference stratotype region fo r the elaboration of the Neogene stratigraphic scale. The stratigraphi c research of continental Oligocene-Miocene deposits in the region is difficult because fragmentary sections here recorded a complex combina tion of lateral and vertical stratification. Previously, peculiar lith ostratigraphic units (formations, beds) often bearing distinct paleofl oral assemblages were traced in many borehole sections. These assembla ges form a succession of floral levels, three of which-the Lyamin, Eka terininskoe, and Vasyugan-probably correspond to the early Miocene. Ne w multipurpose studies allowed paleobotanicalal data to be correlated with paleomagnetic data. Three magnetic polarity zones were distinguis hed in the combined section. The lower zone of reverse polarity is cha racterized by the Lyamin flora, the middle, zone of normal polarity, b y the Ekaterininskoe flora, and the upper zone, of reverse polarity by the Vasyugan flora. Based on the correlation performed, we draw the c onclusion that the magnetic polarity zones are of a regional extension and their geochronologic equivalents are correlative with epochs of t he standard magnetochronologic scale. Thus, beds with the Ekaterininsk oe flora that were correlated via auxiliary sections with the upper Ba igubek Horizon of the Ustyurt-Aral region were accepted to be correlat ive with Epoch 21 and assigned to the Aquitanian Stage. The overlying beds with the Vasyugan flora are probably correlative with Epoch 20 (t he lower Burdigalian), whereas the lowermost peculiar member of the se ction with the Lyamin flora can correspond to the lowermost Aquitanian -upper Chattian interval (Epochs 22 and, probably, 23).