Ammonites and infrazonal stratigraphy of the lower bathonian besnosovi zone of the Russian plate

Abstract

Ammonites and deposits of the Lower Bathonian Besnosovi Zone of the central and southern regions of the Russian Plate are studied. The paper discusses the infrazonal stratigraphy and correlation of two reference sections: the Pletnyovka Quarry in the north of the Penza oblast and the Sokur Quarry near the city of Saratov. The successive biohorizons Oraniceras issae, O. calvum, and O. besnosovi are recognized on the basis of ammonites of the genus Oraniceras (family Parkinsoniidae) in the Besnosovi Zone. Provisional O. mojarowskii and O. scythicum biohorizons are proposed below the above-mentioned biohorizons, whereas the O. cf./aff. besnosovi Biohorizon is proposed above them. The Besnosovi Zone corresponds to the middle part of the Western European Zigzag Zone. The Michalskii Zone underlining the Besnosovi Zone apparently corresponds to the Bajocian–Bathonian boundary interval. It is suggested that only one A. excentricum migrational biohorizon, corresponding to the upper part of the O. besnosovi Biohorizon and correlating with the lowermost part of the Ishmae Zone of the Boreal scale, can be established on the basis of Boreal ammonites of the genus Arcticoceras in the Sokur Section. Central Russian representatives of the genera Oraniceras [M] and Sokurella [m] are assigned here to the subfamily Pseudocosmoceratinae that is eudemic to the Central Russian late Bajocian–early Bathonian sea. It is suggested that the above-mentioned genera evolved from Pseudocosmoceras [M and m] that is cryptogenic neoendemic of the East European Peritethyan marine basin. New species are described: Oraniceras issae sp. nov. [M], O. calvum sp. nov. [M], Sokurella pletniovkae sp. nov. [m], and S. elshankae sp. nov. [m].