The Problems of Radiolarian-Based Dating of Jurassic—CretaceousSiliceous Rocks from Accretionary Complexes, the Russian East

摘要

Radiolarians, which represent the most widespread fossil faunal group in jaspers and cherts, arethe best tool for determining the geological age ofJurassic—Cretaceous volcanogenic—siliceous sequences inthe Pacific margin of Russia, because they meet all the conditions required for orthostratigraphic groups, i.e.,demonstrate rapid evolutionary changes of their assemblages, completeness of their record through the geo-logical section, and a wide lateral distribution. The selection of biostratigraphic scales for determining the ageof radiolarian assemblages is of principal significance. The significant difficulty in correlating Tethyan andPacific assemblages is similar to that appearing in the case of the correlation between the Tethyan and Borealammonite standards. The existing discrepancies are explained by the different life spans of some guide speciesthat determine the stratigraphic range of particular zones in their geographic type areas and may be differentin other paleobiogeographic regions and provinces. The program of scientific studies should include thesearch for and thorough analysis of Jurassic—Cretaceous sections in Russia that contain simultaneously radi-olarians, buchians, and ammonites. Such sections might provide the possibility for developing a single scalefor the transitional sections and outlining ways for correlating the Tethyan and Pacific assemblages.