MIDDLE-TO-LATE QUATERNARY VOLCANIC ACTIVITY REVEALED BY TEPHRA-LOESS SUCCESSION OF THE DEPOSITIONAL BASIN: AN EXAMPLE FROM OSORE-ZAN VOLCANO AND TANABU LOWLAND, SHIMOKITA PENINSULA, NORTHEAST JAPAN ARC

The Quaternary volcano of Mt. Osore-zan of the northeast Japan arc exceeds 800 m in maximum height and is accompanied by the Usori Caldera of 5 km in diameter.  Several previous works clarified the stratigraphic succession of the volcanic body, but these studies obtained no chronological data on the explosive volcanism.  Using the tephrochronology, we therefore identify the explosive eruptions of this volcano and show their ages, behavior, and volumes as below.

The Tanabu Lowland of the northeast Japan arc is a tectonic basin on the east (the downwind side) of the Osore-zan Volcano.  Upon the marine and fluvial terraces of this lowland, many tephra layers, which are traced to the Usori Caldera as source vent, and several paleosol layers, which were formed in loess, are accumulating.  Five middle-to-late Quaternary eruption sequences named the Tanabu-D to Tanabu-A and Ohatagawa from older to younger ones are identified about the Osore-zan Volcano, through the recognitions of the loess as the sequence boundaries in the above tephra-loess succession.

During the Tanabu-D eruption cycle, one pumice-flow deposit erupted to eastern foot of the Osore-zan Volcano before MIS (marine isotope stage) 10.

For the eruption cycles of the Tanabu-C to Tanabu-A, the plinian eruptions supplied five pumice-fall deposits to the Tanabu Lowland and three pumice-flow deposits to the valleys 45 m deep at northern to eastern foots of the Osore-zan Volcano at MIS 8.  The Usori Caldera had already been occurred before these eruption cycles, and is thus covered with these pumice-flow deposits.  Also, seven stratified ash-fall deposits erupted to this lowland area through the phreatomagmatic eruptions.  The eruption volumes of these sequences are estimated to be 1.9, 0.3, and 1.5 km³, respectively.

The Ohatagawa eruption cycle provided northern foot of the Osore-zan Volcano with one lithic tephra layer of 0.1-km³ volume after MIS 5.

The Osore-zan Volcano had repeated many explosive eruptions and supplied volcanoclastics of 3 to 4 km³ in volume during MIS 8, but there are few explosive eruptions at this volcano after MIS 8.