Time Pulses of Cenozoic Explosive Phreatic Eruptions in Southwestern Primorye: Correlation of the Results of Isotopic and Phytostratigraphic Age Dating

The Cenozoic tephroid deposits as products of explosive phreatic eruptions of maar volcanoes in the southwestern part of the Primorye region are studied. These deposits represent rhyolitic ash and pumice pyroclastic layers with a high terrigenous component, including tephroid pseudoconglomerates. Isotopic dating of tephroid layers has made it possible to identify two time pulses of explosive volcanism: 30–34 and 23–24 Mya. The first time pulse coincides with the beginning of the formation of marginal seas and continental coal basins. It corresponds to the most productive coal accumulation stage, accelerated wood waste burial and coalification, and the development of a high-temperature geothermal field, and it can be compared with the well-known catastrophic eruption of Mount St. Helens in the United States of America. The second time pulse of explosive volcanism is regional in nature. 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. The volcanic activity is accompanied by the accelerated sinking of the bottom of the Sea of Japan in response to the active rising of asthenospheric diapirs. The estimated isotopic ages do not conform to those determined for fossil leaves and pollen from the deposits likely due to the climate-forming type of this explosive process.