Paleo-environmental study on Late Quaternary fine-grained volcanic ash beds around Kagoshima University

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Following problems on Late Pleistocene and Holocene tephra layrs were dicussed in the northern part of Osumi Peninsula in southern Kyushu, in which a lot of fine-grained volcanic ash layrs : Late Quaternary tepra stratigraphy, stratigraphic position of fine-grained volcanic ash layrs prtoduced by phreatomagmatic eruptions, and their distribution and source vents. In this region, 17 tephra layrs lie on Ito pyroclastic flow deposit, which is 22,000 yBP in age, 13 tephra layrs of them are derived from Sakurajuma Volcano or Aira caldera. They include 4 fine-grained volcanic ash layrs produced by phreatomagmatic eruptions :(1) Sakurajuma-Takatoge 4 tephra (age : 13,000 yBP),(2) Takano base surge (12,000 yBP),(3) Sakurajima-Satsuma tephra (11,000 yBP),(4) Sakurajima-Takatoge 1 tephra.Takano base surge deposit is not distributed near Wakamiko vent, which has ever been estimated to be the source vent, instead, it is thick in Ushine area north to Sakurajuma Volcano, suggesting that the source vent is situated south to Wakamiko vent.Satsuma tephra is often observed in Holocene marine deposits of Kagoshima lowland by the bore hole analysis. The tephra erupted at the time of low sea-level stand of Younger Dryas stage. The relative sea-level height in this age is 50 m below the present sea level, indicating that phreatomagmatic eruption of Satsuma tephra occured at the depth of more than 50m below the present sea level.