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Owing to recent comprehensive studies of upper Eocene, Oligocene, and lower Miocene deposits of the Sakhalin-Kamchatka region a great volume of available stratigraphic materials was summarized and compilation of paleogeographic maps was started. The studies include detailed investigations of 30 key sections in the region and comparison with the data on sections from Japan and North America. The report mainly discusses distribution of Oligocene molluscs in the North Pacific shelf zones. Characteristic molluscan assemblages are confined to certain regiostages (= horizons): the Amaninskii-Gakkhinskii and Utkholokskii-Viventekskii horizons of West Kamchatka, the Aluginskii Horizon of East Kamchatka, and the Machigarskii Horizon of Sakhalin. They correspond to the Mamijiama Formation of Japan.
Nearly 120 species (including about 50 characteristic and dominating forms) were examined. Their stratigraphic and geographic distribution served as a base for compiling 50 paleobiogeographic maps. The maps are of three types. The first type maps show distribution of shallow- and deep-water individual species. The second type maps reflect areas of distribution of molluscan assemblages. The third type maps demonstrate distribution of genera of different thermotrophy (warm-water and cold-water - boreal).
The maps were made for different time intervals (= levels), i.e., late Eocene, early Oligocene, late Oligocene, and early Miocene.
The maps show that Eocene ? early Oligocene molluscan assemblages were confined to two North Pacific biogeographic provinces, i.e., Japan-Kamchatka and Asia-North America. In transitional paratropical to boreal conditions the Oligocene shelf zones were inhabited by both water-water and cold-water (boreal) genera of molluscs. Significant Oligocene climatic deterioration (evidenced by glendonites, ice-rafted debris, diatom productivity burst, etc.) caused noticeable disjunction and differentiation of the North Pacific molluscan assemblages (diversification in directions to the west-east and the north-south). It was in the Oligocene when the ?boreal type? fauna was formed to be developed in the Neogene and Quaternary.
Analysis of the molluscan assemblages shows that in the Oligocene the North Pacific represented a semiclosed system because the Arctic and North Pacific basins were apparently disconnected.
The results on Oligocene investigations of Kamchatka, Sakhalin, and adjacent regions have been published recently (Gladenkov Yu.B. et al., 2005. The North Pacific Cenozoic Ecosystems: Eocene-Oligocene of West Kamchatka and Adjacent Regions. Moscow: GEOS. 480 pp.).
The work was supported by the Russian Foundation for Basic Research, project no. 08-05-00014.
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