Biostratigraphic Substantiation of the Bajocian-Bathonian Boundary in the Lower Volga Region

摘要

For several decades, the Bajocian-Bathonian boundary in the Russian Platform has remained insufficiently substantiated with respect to biostratigraphy, resulting in uncertainties in the identification of local stratigraphic units (formations). This problem is aggravated by difficulties in comparing ammonite assemblages found in the Volga region with the West European standard scale because of the isolation of Middle Jurassic sea basins and significant faunal provincialism.