Geologic effects of ocean bottom currents : western North Atlantic

Abstract

Bottom photographs, sediment cores and high resolution echograms from the Atlantic continental margin of North America reveal distinctive features created by the Western Boundary Undercurrent, a deep current associated with the thermohaline circulation of the Atlantic. Current lineations observed in oriented bottom photographs taken on the continental rise show features indicative of significant sediment transport and deposition parallel to bathymetric contours. Bottom current directions inferred from the orientation of current lincations correspond to the flow direction of the Western Boundary Undercurrent predicted long ago by the pioneering work of Georg Wüst. Sediment cores reveal a distribution of distinctive brick-red clay that has apparently been transported from the Cabot Strait southwesterly to. the Blake-Bahama Outer Ridge. Massive muddy sands are typically found on abyssal plains. The graywacke-type sands from the abyssal plains may exceed 50 cm in thickness, and as a rule, rarely exceed 50 per 10 m of core. The continental rise is, on the other hand, underlain by sediment containing as many as 500, thin (<I cm), well-sorted silt lamina per 10 m. These silts are relatively clay-free and usually contain cross-beds accentuated by heavy mineral placers. The continental rise appears to have been constructed by: I) material injected laterally into low velocity ( < 30 cm/sec) contour-following bottom currents by high velocity (> IOOcm/sec) turbidity currents flowing downslope; and 2) continental material transported in suspension through the water column and subsequently mixed with pelagic components to form hemipelagic sediment. The very largest and most competent turbidity currents reach the abyssal plains where they deposit relatively thick beds of muddy, graded coarse sand, silt and clay known as Turbidites. The well-sorted fine sand, silt and clay laminations deposited by contour currents flowing on the continental rise are known as Contourites.