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Abstract

The Greater Antilles Outer Ridge, located north and northwest of the Puerto Rico Trench, is a deep (>5100 m), distal sediment drift more than 900 km long and up to 1 km thick. It has been isolated from sources of downslope sedimentation throughout its history and is formed of clay- to fine silt-size terrigenous sediments that have been deposited from suspended load carried in the Western Boundary Undercurrent, together with 0-30% pelagic foraminiferal carbonate. Because of the fine, relatively uniform grain size of the sediments, the outer ridge consists of sediments that are seismically transparent in low-frequency reflection profiles. Sediment tracers (chlorite in sediments and suspended particulate matter, reddish clays in cores) indicate that at least a portion of the ridge sediments has been transported more than 2000 km from the eastern margin of North America north of 40°N. The outer ridge began to develop as early as the beginning of Oligocene time when strong, deep thermohaline circulation developed in the North Atlantic and the trough initiating the present Puerto Rico Trench had cut off downslope sedimentation from the Greater Antilles. The fastest growth of the outer ridge probably occurred beginning in the early Miocene, about the same time that large drifts such as the Blake Outer Ridge were initiated along the North American margin. Since that time, the most rapid sedimentation has been along the crest of the northwestern outer ridge where suspended load is deposited in a shear zone between opposing currents on the two ridge flanks

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