GLORIA Mosaic of West Coast U.S. Exclusive Economic Zone, Southern Sector

James V. Gardner, Brian D. Edwards, Michael E. Field, David S. McCulloch, Neil H. Kenyon, Douglas G. Masson

The long-range side-scan sonar system GLORIA was used to produce digitally enhanced mosaic of the sea floor of the entire U.S. Exclusive Economic Zone. The data resolution, about 50 × 50 m, provides a mesoscale reconnaissance that reveals the continuity and extent of bottom features, some of which were previously unrecognized. The transform continental margin from the Mendocino Escarpment to the United States-Mexican border is cut by numerous submarine canyons and gullies of varied size and complexity. The number, size, and extent of gullies appear directly related to the underlying bed-rock geology. Surprisingly, relatively few slumps and slump scarps are apparent. Submarine fans characterize the basins adjacent to the margin in this sector. The fans vary in size an complexity: relatively small, immature fans of the borderland basins, such as Redondo and Hueneme; fans intermediate in size and age, such as Arguello and Farallon; and large, relatively mature fans, such as Monterey and Delgada. Most fans have well-defined depositional lobes at the distal reach of a single channel. Distributary channels are not apparent on all fans, and on some (e.g., Monterey Fan), the single channel can be seen in seismic reflection profiles to have originated on or close to the basement, directly below its present position. The older depositional lobes that have been identified on the fan systems are adjacent to the present main channel, which implies that channel avulsion is not always a process that accompanies fan growth.

Seamounts are prominent features in the region, ranging in number from hundreds in the Baja Seamount province to tens in the region west of San Francisco. The gradient of increasing numbers of exposed seamounts and volcanic ridges from north to south is a direct result of decreasing sediment supply from the continent to the south. The seamounts vary in morphology from sediment-draped features with subdued topography to fresh-appearing seamounts with cratered summits and distinct flows on the flanks. Several young-appearing seamounts reside on Oligocene crust. Their youthful appearance suggests that middle plate volcanism may have occurred in this region. The seamounts range in size from less than 1 km in diameter and only a few hundred meters high, to more than 8 km in diameter at the summit and more than 2 km above the surrounding sea floor.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.