On the Taxonomic Composition of Mollusks from the Tavda Formation of Western Siberia

The taxonomic composition of bivalves and gastropods from the clays of the Tavda Formation was studied from the Kyshtyrla Quarry, 23 km south of the city of Tyumen and from the drill core of Boreholes 8 and 11 south of the city of Omsk. The Kyshtyrla malacofauna is represented by siderite molds and imprints, rarely entire shells, sometimes bivalved specimens, among which 19 taxa were identified. The borehole material contained eight species of eight genera. These assemblages show striking resemblance to the Chegan Fauna of North Ustyurt, Aral and Turgay, suggesting their similar age and direct paleogeographic connections between these basins. Both assemblages indicate marine, probably slightly brackish-water environments and relatively shallow-water deposition below the surf zone in conditions varying from normal aeration (favorable for benthic organisms) to dysoxic.