On the Stratigraphy of the Bajocian-Bathonian Boundary Beds (Middle Jurassic) of the Izhma River Basin, Northern European Russia

Data on fossils and stratigraphy of the Bajocian and Bathonian boundary deposits in the north of European Russia recently published by Ippolitov, Zakharov, Kiselev, et al. are discussed. Their previously proposed correlation of the Middle Jurassic reference sections of the Pechora North region on the Dreshchanka River (a tributary of the Izhma River), as a result of which the visible thickness of deposits is doubled, is critically revised. New finds of the ammonite Arctocephalites arcticus (Newton) confirm the biostratigraphic data previously obtained by Mitta et al. The interval with Mezounio zwerkovi and Retroceramus lungershauseni, previously recognized by Zakharov et al., occurs at the visible base of the Sysola Formation immediately below the interval of finds of Arctocephalites arcticus and most likely should be correlated with the uppermost Bajocian, rather than with the lower Bajocian. Meledina's proposal to transfer the Borealis, Indistinctus, and Pompeckji zones of the Boreal scale from the upper Bajocian to the lower Bajocian is considered to be unsubstantiated.