Abstract

Reflectivity patterns in echograms recorded from relatively smooth continental slope and rise off Nova Scotia can be grouped into four depositional associations. The shallowest association corresponds to gravelly sand on the outer shelf that spills over the shelf edge and mixes with mud down to 500-m water depth. A sharp boundary near 62°30′ W, extending from 500 m down to the abyssal plain, separates associations farther seaward. Sediments transported downslope by near-bottom gravity processes accumulated west of the boundary; hemipelagic sediments accumulated east of the boundary from 500 to 4,300 m. Below 4,300 m, sediment waves are common in a contonrite-fan association.

These associations indicate that during the late Pleistocene radically different sedimentation processes were juxtaposed across a sharp boundary down the slope and rise. West of 62°30′W, small-scale slope failure occurred over a wide area due to relatively rapid accumulation of fine-grained glacial debris. The slope progiraded seaward; the lower rise received distal turbidity-current and debris-flows deposits. East of 62°30′W, relatively less failure occurred in hemipelagic sediment. Lower on the rise seaward of the hemipelagics, turbidity currents and contour currents carried sediment (originating in deep canyons to the east of 61°30′W) along the lower rise toward the south and west. In the west, the primary depocenter was the slope and upper rise; in the east, the depocenter was on the lower rise. A glacial-sediment source on land and relict shelf morphology probably controlled sedimentation processes and, thus, the location of depocenters seaward of the shelf edge.

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