Early Palaeogene siliceous microfossils of the Middle Volga Region: stratigraphy and palaeogeography

The Sengiley section (Middle Volga Region, Russia) provides one of rhe most complete late Palaeocene sedimentary sequence with well-preserved diatoms, silicoflagellates, and radiolarians. Three zones of regional zonal scheme (Kozlova 1994) based on radiolaria were distinguished in the sediments: Buryella tetradica, Tripodiscinus sengilensis, Petalospyris foveolata zones. Based on diatom regional scheme (Strelnikova 1992) Trinacria ventriculosa and Hemiaulus peripterus zones were recognised. Although assemblages of siliceous microfossils strongly differ from the oceanic coeval associations, the precise age of the boreal zones was determined on the basis of direct correlation with standard zonal scales of diatoms, silicoflagellates and radiolarians. For example, from sediments of Petalospyris foveolata zone, several species described by Nishimura (1992) from the upper part of the Bekoma campechensis standard radiolarian zone of the North-West Atlantic were found and allowed us to correlate these two zones. Two zones of the standard oceanic diatom scheme (Barron & Baldauf 1995) (Hemiaulus peripterus and Hemiaulus incurvus zones) and standard silicoflagellate Naviculopsis constricta zone were distinguished in the Sengiley section. Siliceous-terrigenous Palaeogene sediments of the Middle Volga can be considered as typical sediments of the marginal epicontinental basin. Siliceous assemblages of the Sengiley section are very close to assemblages from Lulinvort and Serov formations of the West Siberia and the eastem Urals slope, Fur Formation and Sambian Formation of North-East Europe, although the geometry of connections between these basins during fate Palaeocene is still not clear.