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ABSTRACT: Pliocene-Pleistocene Massive Pumice Deposits in the Sumisu Rift Area, Izu-Bonin Region, ODP Leg 126

Akira Nishimura, K. Rodolfo, J. B. Gill, K. Fujioka

Unusually thick, multiple deposits of Pliocene-Pleistocene pumice recovered by ODP Leg 126 from the area of the backarc Sumisu Rift, Izu-Bonin Arc, may be diagnostic of submarine basins flanking intraoceanic arcs. Eruptions producing thick pumice layers both predated and accompanied the development of the Sumisu Rift.

Pliocene pre-rift pulses of magmatism within the arc are recorded by several tens of meters thick upward-coarsening sequences of pumice gravel without silt to clay-sized materials.

Pleistocene syn-rift volcanism produced thick pumice units separated by intervals of hemipelagic sediment. Each unit is composed of pumice gravel in the lower part and vitric silt in the rest, and the thickest pumice unit exceeds 100 m in thickness. In particular, the period from about 0.20 Ma to the present appears to have experienced five massive pumice productions. Applying a uniform accumulation rate of muddy sediments throughout this interval, and treating each massive influx of pumice as an instantaneous event, yields dates for the explosive eruptions of 186, 122, 114, 58, and 2 ka. The interval between these calculated ages, assuming that the second and third cycles are part of a single composite eruptive episode, is about 60,000 years. The pumice was delivered to the rift basi by a combination of direct deposition after settling through the water column, resedimentation from basin slopes, and redistribution on the basin floor by bottom currents.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990