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TIME PULSES OF CENOZOIC EXPLOSIVE PHREATIC ERUPTIONS IN SOUTHWESTERN PRIMORYE. CORRELATION OF ISOTOPIC AND PHYTOSTRATIGRAPHIC AGE DATING RESULTS

The Cenozoic tephra deposits - products of explosive phreatic eruptions of maar volcanoes in the southwest of Primorye are studied. The deposits represent rhyolitic ash and pumice pyroclastic beds with a high terrigenous component, including tephroid pseudo-conglomerates. Isotopic dating of tephra beds established two time pulses of explosive volcanism: 30–34 Mya and 23–24 Mya. The first time pulse coincided with the beginning of the formation of marginal seas and continental coal basins. It corresponds to the most productive stage of coal accumulation, the burial of wood wastes and their coalification at a faster rate, and the development of a high-temperature geothermal field and can be compared with the well-known catastrophic eruption of Mount St. Helens in the U.S.A. The second time pulse of explosive volcanism had a regional character of manifestation. It is characterized by the formation of green tuff complexes on submarine elevations of the Sea of Japan as well as along the western and eastern coasts of Japan. Synchronously with the volcanic activity started the acceleration of the sinking rate of the Sea of Japan bottom in response to the active rising of asthenospheric diapirs. The established isotopic ages do not conform to the ages determined for fossil leaves and pollen from the deposits, which may reflect the climate-forming type of such an explosive process.