Abstract

Holocene, Pleistocene, Tertiary and Upper Cretaceous sediments occur in the Puerto Rico Trench area. The Holocene sediments include mainly brown pteropod and foraminiferal oozes on the slope and brown abyssal clays in the hilly parts of the lower Trench and on the Outer. Ridge. The south wall scarp consists mostly of gray-green Miocene days and scree slopes containing serpentine and limestone rubble. Soft sediments overlie Upper Cretaceous andesitic volcanics, radiolarian cherts and foraminiferal cherty marls on the faulted North Wall. Color, texture, and composition of beds in ten cores 3 to 10 m long from the floor of the Puerto Rico Trench form the basis for correlation of graded beds several millimeters to 7 m thick over distances of 200 km. There is a general decrease in grain size of the coarsest detritus and the thickness of the basal sandy section in each graded bed down a high level abyssal plain, into and along the abyssal plain of the lower Trench floor. Thick beds of megascopically homogeneous silty clay, commonly burrowed in their uppermost 10 to 20 cm, make up most of the graded beds in the deepest part of the lower Trench floor. Variation in bottom topography and sediment properties show that the layers were deposited by turbidity currents originating near the Puerto Rico-Virgin Islands shelf and flowing through numerous canyons northwestward down a high-level abyssal plain and into the lower main trench plain on which they spread laterally. In at least two instances the turbidity currents were powerful enough to deposit 20 to 50 cm of fine sand 100 km away from and 10 m above the point which they entered the main trench floor.

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