Abstract

Side-scan sonar and seismic data from the hitherto little-known middle to lower reaches of the giant, north-south–extending Northwest Atlantic Mid-Ocean Channel (NAMOC) between lat 53° and 44°30′N reveal a low- to medium-sinuosity and partly straight channel, the course of which is increasingly basement-controlled toward south. Flow restriction by seamounts locally narrowing the channel in the south caused talweg incision by as much as 80 m below the channel floor. Asymmetric levees, resulting from southward-flowing turbidity currents spilling over the channel walls by a mechanism of continuous flow stripping, are remarkably consistent; there is a 30 to 60 m higher western levee, reflecting the effectiveness of the Coriolis force at mid-latitudes. The levees rise 10 to 30 m above the adjacent ocean floor. Consistently lower seismic penetration and more closely spaced reflectors on 3.5 kHz profiles and higher reflectivity on sleeve-gun profiles indicate a higher proportion of sand in the eastern levee. Wash-over fans on the levees formed at sharp channel bends, where deeper, coarser portions of the flows spilled out. Major portions of the turbidity currents that mostly originated off Hudson Strait, however, passed this intermediate channel segment without sedimentation. The down-channel decrease in the difference in height between the western and eastern levees, channel cross-sectional area, and levee height indicate a possible influence of the decrease in the effect of the Coriolis force, flow volume, and/or current speed; separation of the effects of individual variables, however, is currently not possible.

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