DIATOM TURNOVER IN THE EARLY PALEOGENE DIATOMITE OF THE SENGILEY SECTION, MIDDLE POVOLZHIE, RUSSIA: A RESPONSE TO THE INITIAL EOCENE THERMAL MAXIMUM?

Abstract

The lower Paleogene sequence in the Middle Povolzhie (Ulyanovsk-Saratov syncline) of Russia is dominated by a terrigenous-rich siliceous facies with abundant and well-preserved siliceous plankton. Calcareous microfossils are virtually absent. The diatom successions of the Sengiley diatomite are correlated to diatom assemblages of the Fur Formation (Denmark) and the Polosataya Formation (Kazakhstan), and are calibrated to calcareous nannofossil and dinocyst zonations. The Trinacria ventriculosa Zone, which is found in the lower part of the Sengiley diatomite (Kamyshin Formation), is age equivalent to the upper part of calcareous nannofossil zones NP8 through NP9a. The upper part of the Sengiley diatomite (Kamyshin Formation; Hemiaulus proteus Zone) can be correlated to the uppermost part of the Apectodinium augustum dinocyst Zone, which corresponds to the NP9b calcareous nannofossil Subzone, in which the globally observed negative carbon isotope excursion occurs. A major floral turnover in the upper third of the Sengiley diatomite is characterized by a succession of bioevents, including first appearances at generic and specific levels. Quantitative diatom analyses show pronounced compositional changes: (1) the decline of the meroplanktonic Pyxidicula group, typical of neritic conditions (2) the increase of the Paralia group, a marker for the proximity of the shoreline, and (3) a relatively high abundance of pelagic cosmopolitan species. Grain size analyses show an increase in terrigenous input in the upper part of Sengiley diatomite. The floristic and lithologic shifts indicate changes from stable, highly productive neritic conditions during a sea-level lowstand (Phase 1) to the onset of a transgression (Phase 2), which is followed by a highstand (Phase 3). The highstand phase coincides with enhanced exchange between the Tethys and the adjacent epicontinental seas. The diatom bioevents in Phases 2 and 3 in the Sengiley section are most probably coeval to the welldocumented biotic changes of the Initial Eocene Thermal Maximum.