Micropaleontological assemblages and biostratigraphy of the Upper Cretaceous - Palaeogene of the southeast Western Siberia
In the south-east of Western Siberia, new sections of boreholes 15, 19, 23 of the Aleyskaya Area of the Gorno- Altaisk Expedition were investigated. Twenty four core samples from these sections were subjected to the micropaleontological analysis. Foraminifera and spore-pollen assemblages appeared the most effective of all detected microfossils. The whole complex of these microfossils enabled the age and forming conditions of the enclosing deposits of these sections to be established. Their definitions resulted in establishing the Upper Cretaceous stages: Campanian- Maastrichtian, as well as the Paleogene series: Paleocene-Eocene. In the Paleocene, the Danian-Selandian stages were outlined, in the Eocene - the Ypresian-Bartonian stages. The accompanying scheme (see Figure) illustrates the borehole 23 section as the most representatively sampled one. The age dating made by foraminifera and sporepollen assemblages are about the same. The aspect of foraminifera, their systematic composition and the roundness of silicified pseudomorphs indicate their dwelling in the shallow coastal conditions of the West-Siberian Basin with frequently changing coastline. The occurrence of the relatively large pseudomorphs of planktonic foraminifera is indicative of their entering via the then-existing straights from southern (Kazakhstan) basins. The spore-pollen assemblages are quite abundant and diverse in systematical composition, thus indicating the vegetation growing on the nearest continent. The following deposits were distinguished: Maastrichtian-Danian (at depth 217.5-213.5 m), Paleocene (180.0 m) in the borehole 15 section; Campanian-Maastrichtian (333.6-291.8 m) in the borehole 19 section; Campanian (348.0 m), Maastrichtian (255.5-229.5 m), Paleocene (206.0 m), Lower Eocene (196.0 m), Middle Eocene (162.4 m) in the borehole 23 section. The Campanian-Maastrichtian samples contain numerous pseudomorphs of foraminifera, among them Rotaliida predominating. The spore-pollen assemblages predominated by the angiosperm pollen of genera Quercus, Castanea, Corylopsis are quite numerous at this stratigraphic level. The tricolpate pollen (e.g. Tricolpites sp.) is of frequent occurrence along with other characteristic forms. All Paleocene foraminifera from the studied sections are poorly preserved. Nevertheless, in the borehole 23 section (206.0 m) the forms are encountered, which allow defining the Ammoscalaria friabilis Assemblage including, along with the index-species, Cyclammina cf. coksuvorovae Uschakova and other species. The Early Eocene Textularia sibirica - Anomalinoides ypresiensis ovatus Assemblage was distinguished in the same section (borehole 23, 196.0 m), which included, along with the second zonal species, other Planorotalites characteristic for the Lower Eocene. The Middle Eocene of the borehole 23 section (162.4 m) is characterized by the presence of species, among which the Gaudryinopsis cf. subbotinae Podobina Species is distinguishable that is zonal species for the Middle Eocene of Western Siberia. But all detected tests are poorly preserved. The authors append the paleontological plates of foraminifera (I-III) and spore-pollen assemblages (IV-VII) detected in the studied sections.