NEW STRATIGRAPHIC SCHEME OF THE LOWER APTIAN IN THE VOLGA RIVER MIDDLE COURSES

Abstract

Aptian deposits in middle courses of the Volga River have been under investigation for more than a century. Although stratigraphy was elaborated in detail for that region, new data allowed revision and further development of available stratigraphic schemes. Previously, the age of lower Aptian deposits was traditionally established based on changing ammonite assemblages of the family Deshayesitidae. The studied diverse assemblage of heteromorphic Ancyloceratidae, the habitants of relatively deep basin parts, made it possible to propose a new scheme of ammonoid zonation in the lower Aptian epipelagic deposits of the Russian plate. Many of identified ancyloceratids were established here for the first time. The analysis of coexisting deshayesitids and heteromorphs enabled a correlation of stratigraphic schemes for the monomorphic Deshayesitidae and heteromorphic Ancyloceratidae. The described generic taxa and species are Volgoceratoides I. Michailova et Baraboshkin, gen. nov., V. schilovkensis I. Michailova et Baraboshkin, sp. nov., Koeneniceras I. Michailova et Baraboshkin, gen. nov., K. tenuiplicatum (von Koenen, 1902), K. rareplicatum I. Michailova et Baraboshkin, sp. nov.