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GCR Display Cores

Section 126-788D-1R-1

Location

Pacific Ocean, Southeastern Japan. Izu-Bonin intraoceanic island arc.

Leg objectives

Leg 126 focused on three aspects of this arc system:

• the origin and evolution of the forearc;
• the process and products of arc rifting;
• Recycling of subducted lithosphere and evolution of the mantle.

Site objectives

Site 788 is located at 1102 mbsl on the arc margin of the rift. Site 788 objectives were to determine (1) the vertical-motion history of the rift margin, (2) the time of initial rifting, and (3) the nature and history of volcanism and sedimentation between the major arc volcanoes.

Lithology

Pumiceous conglomerate

Age

Pliocene to Quaternary

Interpretation

The pumice deposits were either produced by individual events of explosive silicic volcanism, or they were deposited during extended cycles of eruption. If site 788 stood above the surrounding submarine terrain during the accumulation of the pumice conglomerate and gravel, there are two plausible mechanisms that would have brought pumice clasts to the site and deposited them as blankets. The pumice could have arrived more or less directly as air fall, or, alternatively, it may have rafted (or floated in the water column) to the site. Very well rounded pumice clasts in some intervals implies that they were subjected to extensive abrasion, perhaps from wave activity before or during their journey to the seafloor. This rounding supports the rafting transport mechanism rather than the direct air-fall one.