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

Abstract

The Tara-Irtysh region, located in the central part of West Siberia, is of interest because it represents the reference stratotype region for the elaboration of the Neogene stratigraphic scale. The stratigraphic research of continental Oligocene-Miocene deposits in the region is difficult because fragmentary sections here recorded a complex combination of lateral and vertical stratification. Previously, peculiar lithostratigraphic units (formations, beds) often bearing distinct paleofloral assemblages were traced in many borehole sections. These assemblages form a succession of floral levels, three of which - the Lyamin, Ekaterininskoe, and Vasyugan - probably correspond to the early Miocene. New multipurpose studies allowed paleobotanicalal data to be correlated with paleomagnetic data. Three magnetic polarity zones were distinguished in the combined section. The lower zone of reverse polarity is characterized by the Lyamin flora, the middle, zone of normal polarity, by the Ekaterininskoe flora, and the upper zone, of reverse polarity by the Vasyugan flora. Based on the correlation performed, we draw the conclusion that the magnetic polarity zones are of a regional extension and their geochronologic equivalents are correlative with epochs of the standard magnetochronologic scale. Thus, beds with the Ekaterininskoe flora that were correlated via auxiliary sections with the upper Baigubek Horizon of the Ustyurt-Aral region were accepted to be correlative with Epoch 21 and assigned to the Aquitanian Stage. The overlying beds with the Vasyugan flora are probably correlative with Epoch 20 (the lower Burdigalian), whereas the lowermost peculiar member of the section with the Lyamin flora can correspond to the lowermost Aquitanian-upper Chattian interval (Epochs 22 and, probably, 23).