The Ammonite Catacadoceras Barnstoni Beds and the Problem of Separation of the Middle and Upper Substages of the Bathonian Stage in Northern Siberia

Abstract

The paper presents the results of studying the taxonomic composition of ammonites of the family Cardioceratidae in the Bathonian of northern Siberia in the range of distribution of the last representatives of the genus Arcticoceras and the first representatives of the genus Catacadoceras to solve the problem of distinguishing between the Middle and Upper Bathonian. The incorrectness of all previous identifications of the Middle Bathonian species Arcticoceras cranocephaloide Callomon et Birkelund, 1985 based on the northern Siberian material has been established. These determinations were mainly based on Late Bathonian ammonites belonging to the species Catacadoceras barnstoni (Meek, 1859) or the form characterized by a narrow umbilicus and described herein as Cat. aff. barnstoni. An updated zonal scale of the Middle–Upper Bathonian of Siberia based on ammonites is proposed, which is recommended for use both in the regional stratigraphic schemes of the Jurassic deposits of Eastern (Middle) Siberia and in the Boreal (Siberian) zonal standard of the Bathonian Stage. The extreme importance of preserving an auxiliary biostratigraphic subdivision in this scale, the Cat. barnstoni Beds, is noted, which is due to the wide distribution and abundance of the index species in Siberian sections, as well as its significance for the correlation of the lower part of the Upper Bathonian of northern Canada, northern Siberia, Franz Josef Land and East Greenland. In addition, two intervals, namely interval with Cat. aff. barnstoni and interval with Cat. perrarum, are fixed as benchmarks, which are useful for the rapid age estimation and correlation of the Upper Bathonian deposits in Arctic sections within the upper part of the Cat. barnstoni Beds.