BASIN PLAIN TURBIDITE SUCCESSION OF THE OLIGOCENE LZU-BONIN INTRAOCEANIC FORE-ARC BASIN

The sedimentary succession of intraoceanic forearc basins is poorly kn own. Boreholes from ODP Leg 126 rectify this by providing long section s through a volcaniclastic, basin plain, predominantly turbidite succe ssion in the Izu-Bonin intraoceanic forearc south of Japan. Cores and continuous Formation MicroScanner (Schlumberger) images form the basis of long bed by bed sections that indicate the style of filling of the forearc basin, the high rate of supply of eruptive products from the Izu-Bonin arc, and the relative frequency of initiation of turbidity c urrents of various sizes. Turbidites and related debris flow deposits range in thickness from approximately the 2.5 cm resolution of the For mation MicroScanner tool to 10-15 m. Bed thicknesses are distributed a ccording to a power law with an exponent of about 1.0. Upwards thicken ing or thinning sequences are absent. Groups of thick and very thick b eds may reflect global sea-level lowstands, particularly at 30 Ma, per iods of increased tectonic uplift, or periods of more intense volcanis m. The very thickest beds, with maximum recurrence intervals of 0.3-1 million years, may have been deposited from flows triggered by powerfu l subduction zone earthquakes, or may simply be the result of the fail ure of unusually large accumulations of volcaniclastic sand and ash on the flanks of arc volcanoes.