Development features of calcareous nannoplankton in southern and eastern Ukraine in cretaceous period

Abstract

For the Cretaceous sections in eastern and southern Ukraine some changes in taxonomic composition and quantitative ratios between certain species of calcareous nannofossils have been found. The sections: north-west of Donbass, Dnieper-Donets Basin, Black Sea Depression, the plain and the mountain Crimea and Black Sea shelf were studied. A completely unique stage of nannoplankton development has been noted in Cretaceous period. At that time this group reached both qualitative and quantitative maximum development. During the period, basic taxonomic core of its evolutionary development was formed, which almost completely extincted at the end of the period. Development of the Cretaceous nannofossils suggests three major stages: Berriasian - Aptian; Albian - early Turonian; Turonian - Maastrichtian, degree of conversion complexes on the boundaries of these stages are much lower than at the boundaries of the Cretaceous period.

The boundary of Jurassic and Cretaceous is conducted with the appearance of Nannoconus steinmannii minor. There were significant changes in the taxonomic composition of cretaceous nannoplankton, as compared to the Jurassic. Moreover, if there is practically no change in the Coccolithales order, then in Nannolithales we note the first order, and then a sharp increase in both the number and variety of species.

During the Berriasian period taxonomic diversity of nannoplankton increases.In Berriasian-early Valanginian time there is flourishing Nannoconus, against which Podorhabdus role gradually increases. Late Valanginian-Barremian is time of nanoflora’s stable development. The complex is dominated by Podorhabdus. In the Aptian nannoplankton gets rock-forming role. The acme embraces all the Lower Cretaceous groups, aside from the Nannoconus. In the Albian-Cenomanian the nannoplankton complexes are structured. The groups that were dominating in the Early Cretaceous lose their importance, they do not become extinct, however. New groups characteristic of the Late Cretaceous replace their predecessors.

In the Turonian-Santonian, the nannoflora was the most stable. At this time there is not any new family, the rate of genera and species appearance is reduced. In the Campanian, family Arkhangelskielaceae appeared and swiftly won a dominating role. During the Campanian-Maastrichtian, this family evolved quickly enough. The evolution ended just at the late Maastrichtian. At the boundary of the Cretaceous and the Paleogene all the Cretaceous groups became practically extinct. We spotted the transition of six species only.

The data on the stratigraphic position of 132 species, 59 genera, 14 families: Chiastozygaceae, Eiffellithaceae, Rhagodiscaceae, Stephanolithiaceae, Axopodorhabdaceae, Biscutaceae, Mazaganellaceae, Prediscosphaeraceae, Watznaueriaceae, Arkhangelskiellaceae, Kamptneriaceae, Microrhabdulaceae, Nannoconaceae, Polycyclolithaceae. Each family is given a brief description of the changes in species composition during the Cretaceous period.