ALBERT

Description: Correlation of the logs from the Izu-Bonin forearc sedimentary sections at Sites 787, 792, and 793 with the core data from Holes 787A-787B, 792A-792E, and 793A-793B allows the development of a more detailed lithostratigraphic model for those sites, and a more precise correlation of lithologic boundaries to basin-wide seismic reflections. Early Oligocene arc volcanics form the basement strata (Unit 5) sampled at Sites 792 and 793. Downdropped and rotated blocks of Eocene forearc may form a sub-basement beneath these flows in the central forearc basin; mid-Eocene basement was recovered at Sites 782 and 786 on the outer-arc high during Leg 125. Basement at Site 792 was defined using the vertical seismic profile (VSP) and logging data. Deep reflectors observed on the vertical seismic profile may originate in the Eocene sub-basement. Thick sequences of coarse-grained volcaniclastic and hemipelagic sediment fill the 70- to 140-km-wide forearc sedimentary basin. Unrecovered (early Oligocene) strata beneath an unconformity, imaged by the multichannel seismic (MCS) line passing over Site 792, fill the deepest grabens of the central forearc and constitute Unit 4. The rapid deposition of volcaniclastics (Unit 3) during a dominant eruptive phase spanning much of the Oligocene, together with erosion of the basement highs bounding the basin, contributed to rapid subsidence and infilling. An inspection of cored materials from Unit 3 and logging data from Sites 792 and 793 reveals microfaults and other structural evidence for extension; on a much larger scale, MCS data show large normal faults near the frontal-arc high and outer-arc high that downfault the sediment section towards the central basin. Much of the largely pelagic or hemipelagic early Miocene section (Unit 2) has been removed by submarine valley formation and erosion, as at Sites 787 and 792. Middle Miocene to Holocene volcaniclastics and hemipelagics (Unit 1) top the forearc sedimentary section.