The sedimentary succession of intraoceanic forearc basins is poorly known. Boreholes from ODP Leg 126 rectify this by providing long sections through a volcaniclastic, basin plain, predominantly turbidite succession 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 currents of various sizes. Turbidites and related debris flow deposits range in thickness from approximately the 2.5 cm resolution of the Formation MicroScanner tool to 10-15 m. Bed thicknesses are distributed according to a power law with an exponent of about 1.0. Upwards thickening or thinning sequences are absent. Groups of thick and very thick beds may reflect global sea-level lowstands, particularly at 30 Ma, periods of increased tectonic uplift, or periods of more intense volcanism. The very thickest beds, with maximum recurrence intervals of 0.3-1 million years, may have been deposited from flows triggered by powerful subduction zone earthquakes, or may simply be the result of the failure of unusually large accumulations of volcaniclastic sand and ash on the flanks of arc volcanoes.
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