Recent data on Turonian biostratigraphy and foraminifers of Western Siberia
The previous studies on numerous borehole sections of Western Siberia have proved that the Kuznetsovskian Horizon is assigned to the Turonian and encloses two foraminiferal zones: the Gaudryinopsis angustus Zone (the Lower Turonian) and the Pseudoclavulina hastata Zone (the Upper Turonian). The systematic composition and distribution of the Turonian assemblages along the lateral and in the sections of Western Siberia were previously reported by the present author [1, 2, 3, 4, 9]. In the top of the Kuznetsovskian Horizon, numbers of the zonal species Gaudryinopsis angustus Podobina are reduced, while the quantity of the Pseudoclavulina hastata (Cushman) Species increases in the upper zone of the Turonian. Simultaneously, the content of Reophax and Haplophragmiidea in this zone grows, the species Ammoscalaria antis Podobina, Textularia anceps Reuss, Trochammina arguta Podobina appear. In the northern district of Western Siberia (northwards the lateral flow of the Ob River) several holes were first bored within the Van-Eganskaya area. Of them, in sections of boreholes 1002 and 2031 the deposits of the Kuznetsovskian Horizon were studied, in which the clearly defined Late Turonian Pseudoclavulina hastata Zone was established. In the bottom of the horizon the Lower Turonian Gaudryinopsis angustus Zone was established within the section of borehole 2031 (depth interval 936.5-934.9 m). In borehole 1002 (depth interval 942.0-938.5 m) beds with Hedbergella loetterlei, with the planktonic foraminifers dominant in the assemblage, correspond to the above stated zone. The sections of these two boreholes are illustrated in figures 1 and 2, where the data are combined on the lithological structure, zones, beds and characteristic species of the assemblages. The analysis of the systematic composition of the Turonian foraminiferal assemblages of Western Siberia, Northern Alaska and Canada entering the Arctic Paleozoogeographic Realm demonstrated their great similarity not only at the generic level, but also at the specific one. Among the compared assemblages, the common species, geographical subspecies and vicariants were distinguished. The Gaudryina (Gaudryinopsis) irenensis Stelck et Wall Species is the vicariant for the West Siberian zonal species Gaudryinopsis angustus Podobina. In a number of Canadian provinces, the "Pelagic" zone enclosing the planktonic foraminifers seems to correspond to West Siberian Lower Turonian beds with Hedbergella loetterlei. The new factual material on the Turonian foraminifers of the northern district of Western Siberia allowed to compare their systematic composition with that of the coeval species from both the other districts of Western Siberia and more remote regions: Northern Alaska and Canada. Four palaeontological plates illustrate the Turonian characteristic species of the northern district of Western Siberia (boreholes 1002, 2031 of the Van-Eganskaya area).