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Sedimentation and echo characteristics in the abyssal hills of the west-central North Atlantic

¹ Geophysics Laboratory, Marine Science Institute, University of Texas, Galveston, Texas 77550

The abyssal hills province of the western North Atlantic 600 km southeast of Bermuda contains irregular hills and basins 10 to 20 km wide, with about 400 m of relief probably generated by ridge-flank volcanism and faulting. Conical(?) abyssal hills 5 to 10 km wide with about 300 m of relief are thought to represent a later stage of midplate volcanism, perhaps related to the post-Eocene uplift of the Bermuda Rise.

The 3.5-kHz echo characteristics of the surficial sediments were combined with piston core data to produce a geologic map with six main units. Pelagic brown-clay sedimentation has been fairly constant at 1 to 3 m/m.y. since mid-Cretaceous time. In some of the high areas and on the Bermuda Rise at depths less than 5,500 m, a 30-m opaque unit records carbonate sedimentation that began sometime in Pliocene time with lowering of the calcium carbonate compensation depth. Three distinct echo characters are observed from the various stratified sediment bodies. One is owing to near-outcrop of a deep reflector unit. Another unit is composed of discrete, closely spaced reflectors of the Nares Abyssal Plain and parts of the southern fracture valley. A third unit contains widely spaced (20 m) reflectors restricted to the more northerly fracture valley and certain cross valleys. The stratified sediments in both the Nares Abyssal Plain and the fracture valleys consist of brown clays at the surface and gray clays at depth. The gray clays are interpreted as distal turbidites, having 12% more silt than the pelagic brown clays and always occurring in water depths greater than 5,700 m. The turbidity currents crossed the Nares Abyssal Plain into the southern valley and over low sills into the northerly fracture valley. The veneer of brown clay indicates that turbidity currents probably have not been active in this area for at least 300,000 yr.

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