CHRONOLOGY AND BIOCHORES OF CALLOVIAN AMMONITES FROM MANGYSHLAK

Abstract

A comprehensive revision of ammonites, which were collected by the scientific group of M.S. Mesezhnikov in 1985, shows the high taxonomic diversity of these fossils, and affords an opportunity to refine zonal and intrazonal Callovian stratigraphy of Mangyshlak. Most of the species and all distinguished genera from this region display a wide geographic distribution, and are of the subboreal type. The ammonite assemblages of Western Europe and Mangyshlak include many common taxa, which enable a direct correlation with the standard zonation of the Callovian that is useful for the detailed biozonation of local sequences. All standard zones, with the exception of the lowermost one, and some subzones were distinguished in the Callovian deposits of Mangyshlak. All of these units, except for the Macrocephalites herveyi Zone, are marked by index species and other characteristic forms of respective assemblages. Studied features of the very representative ammonite assemblage from the Callovian deposits of Mangyshlak afford an opportunity to better understand its structure and chorology. The comparative analysis of this and other ammonite assemblages from nearby areas reveals the character of chorologic changes and their evolution with time along the profile from the Pechora River basin in the north to the Kugitang Range in the south. The obtained data suggest a new position for the boundary between the Boreal and Tethyan biogeographic provinces.