31. THE SEISMIC STRATIGRAPHY AND SEDIMENTARY HISTORY OF THE EAST MARIANA AND PIGAFETTA BASINS OF THE WESTERN PACIFIC1

The successful completion of Leg 129, resulting in the first and only holes to penetrate igneous basement (not necessarily layer 2) in the East Mariana and Pigafetta basins, now allows the calibration of our regional multichannel seismic site surveys and the extrapolation of drilling results throughout these oldest Pacific basins. Our study indicates that mid-Cretaceous flows/sills overlie Jurassic/Lower Cretaceous sediments and oceanic crust throughout the East Mariana Basin and the southeast Pigafetta Basin. Jurassic-age oceanic crust and overlying Upper Jurassic-Lower Cretaceous sediments unquestionably exist at Site 801 and extend semicontinuously between Sites 801 and 800. Turbidite sequences of varying thicknesses and ages are ubiquitous features of both basins. Cretaceous turbidite sequences were derived from Magellan and Marcus-Wake seamounts and Seamount chains of Aptian age or younger. The seamounts/atolls of the Caroline Ridge more than 300 km to the south of Site 802 were the source of extensive Miocene-age volcanogenic turbidites which are restricted to the south central East Mariana Basin, while the carbonate caps that developed on Ita Mai Tai Guyot and other edifices of the Magellan chain were the source for the redeposited shallow-water carbonate sequences recovered along the eastern margin of the East Mariana Basin. The Ogasawara Fracture Zone, Magellan Seamounts, and associated flexural moat separate the Pigafetta Basin from the East Mariana Basin and influence the source and distribution of redeposited material. A distinct basinwide reflection that is correlated with the shallowest chert/por-cellanite/clay and chert/chalk sequences in the Pigafetta Basin and East Mariana Basin, respectively, is a time-transgressive horizon (Eocene-Campanian) resulting from the passage of the Pacific Plate beneath the equatorial zone of high productivity