Modern turbidity-current deposits are well known from the deep sea but, in most areas, core control is insufficient to establish correlation between individual graded beds. Color, texture, and composition of beds

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in 10 cores, 3-10 m. thick, from the floor of the Puerto Rico trench form the basis for correlation of graded beds from several millimeters to 7 m. thick over distances of 200 miles. The largest unit is 2-7 m. thick and covers an area of approximately 4,000 square miles. Variations in bottom topography and sediment properties show that the layers were deposited by turbidity currents originating near the Puerto Rico-Virgin Islands shelf. These currents flowed through numerous canyons northwestward down a high-level abyssal plain and into the lower main trench plain where they spread laterally. In at least two places the turbidity currents were powerful enough to deposit 20-50 cm. of fine sand 60 miles away from and 30 feet higher than the place near which they entered the main trench f oor. There is a general decrease in the grain-size of the coarsest detritus as well as the thickness of the basal sandy section in each graded bed down the high-level abyssal plain, into and along the trench floor. Thick beds of homogeneous clay, commonly burrowed in the uppermost 10-20 cm. of the beds, make up most of the graded beds in the deepest part of the lower trench floor. The basal sections of most graded units consist of layers of graded or laminated sand whose modal grain-size decreases upward within each graded bed. Cross-stratified fine sand occurs mainly at the base of the graded beds at the distal margins of the lower trench floor.

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