Biostratigraphy and sedimentary settings of the Bajocian-Bathonian beds of the Izhma River basin (European North of Russia)

A comprehensive study of the Bajocian-Bathonian (Middle Jurassic) boundary beds in the basin of the Izhma River led to recognition of five lithological members. The lower sandstone member (0), recognised at the base of the visible section, contains only small bivalves. Based on the occurrence of the index species, Member I is referred to the Arcticus Zone (uppermost Upper Bajocian). Judging from its position in the section, Member II apparently corresponds to the Boreal Greenlandicus ammonite Zone (lowermost Lower Bathonian). Sandstone Member III, well characterized by macrofossils, corresponds to the Ishmae ammonite Zone (uppermost Lower Bathonian) and partly to the Pachyteuthis tschernyschewi belemnite Zone and the Retroceramus bulunensis bivalve Zone. Member IV, which contains only isolated belemnite rostra and microfauna is tentatively assigned to the Middle Bathonian. Based on microfossils, the upper four members belong to the Trochammina aff. praesquamata foraminiferal Zone. The sedimentary settings at the end of the Bajocian and Bathonian in the region studied are reconstructed. The macro- and microfaunal Upper Bajocian-Lower Bathonian assemblages from the basins of the Izhma River and the middle reaches of the Volga River (vicinity of Saratov) are compared.