NEW DATA ON BIOSTRATIGRAPHY OF THE VOLGIAN STAGE LECTOSTRATOTYPE NEAR THE GORODISHCHE VILLAGE (MIDDLE VOLGA REGION)

Abstract

A thorough revision of the lectostratotype of the Volgian Stage near the Gorodishche Village (Ul'yanovsk region) and neighboring section of the upper Kimmeridgian-lower Volgian (Dubki Village) revealed the radiolarian assemblage with Parvicingula jonesi corresponding to the cymodoce and exodoxus ammonite zones. The radiolarian assemblage from the lower Volgian klimovi ammonite zone is dominated by Paivicingnla blowi. Representatives of the genus Parvicingula prevail also in the sokolovi and pseudoscythica zones. The uppermost middle Volgian Zarajskites zarajskensis Zone and the Dorsoplanites panderi Zone contain the diverse radiolarian assemblage with Parvicingula haeckeli. The late Volgian radiolarian assemblage from the subditus ammonite zone includes abundant Stichocapsa devorata and is close to that from coeval sediments of the North Sea. In contrast to Kimmeridgian-Tithonian radiolarian assemblages of the Caucasus, the studied assemblages are of the boreal type. The Virgatites gerassimovi Zone is established and the scope of the Virgatites virgatus and Epivirgatites nikitini zones of the middle Volgian Substage is specified. The upper Volgian Kachpurites fulgens Zone is subdivided into subzones. The thorough study of the phosphorite horizon at the boundary between Volgian and Hauterivian stages did not confirm the presence of the upper Berriasian and lower Valanginian fossils even in the redeposited state.