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Underway geophysical data (bathymetric, seismic reflection and refraction, gravity, and magnetics) have been examined for approximately 1000 km of track lines within one-degree square near Site 357 on the north flank of the Rio Grande Rise. This examination allows a preliminary interpretation of the physiography, structure, and depositional history for the area around Site 357. The east-west segment of the Rio Grande Rise probably originated approximately 100 m.y. ago as a group of volcanic islands composed of alkalic basalt. A layer, approximately 1 km thick, perhaps composed of extrusive volcanics mixed with sediment, appears to separate the oceanic basement from the sedimentary column overlying it. The deepest reflector noticed throughout the rise does not represent the oceanic basement, but, rather, the top of a marly limestone of Santonian age. This limestone varies in thickness throughout the rise and overlies the extrusive volcanic layer. The intermediate reflectors noticed in the seismic profiles represent diagenetic changes within the sedimentary column. The north flank of the rise is deeply dissected by erosional canyons which become well developed below a depth of approximately 1500 meters. These canyons have in some cases eroded several hundred meters into the carbonate sediments of Cenozoic and possibly Upper Cretaceous age. Seismic profiles and piston coring confirm that pre-Pleistocene strata outcrop on the canyon flanks. Consequently there is a substantial local variation in the completeness of the deposition record at any given location. The origin of the canyons is uncertain, but the piston-core data indicate extensive development only in the late Cenozoic. The contrast in the stratigraphy of Sites 357 and 22, both located on the rise within 30 km of each other, points to the need for caution in interpreting the significance of unconformities at a single drill site. Regional inferences from the presence of an unconformity at a single site must be made in conjunction with the assessment of possible local erosional events.

Regional Setting of Site 357, North Flank of Rio Grande Rise