| Summary: | Side-scan sonar imagery, 40 in$ sp3$ sleeve gun and 3.5 kHz high-resolution profiles reveal a dichotomy of the upper Labrador Slope into (i) a low-relief sector off Hudson Strait that is dominated by mass-transport deposits resulting from the direct input of subglacially derived debris, and (ii) a high-relief sector to the south resulting mainly from deposition out of turbid surface plumes, and headward canyon erosion. This dichotomy continues in the Labrador Basin, where the leveed Northwest Atlantic Mid-Ocean Channel (NAMOC) is flanked by a sandy submarine braidplain in the east, which is the basinward extension of the differences on the upper slope. The floodplain west of NAMOC contains extensive debris-flow deposits generated on the Labrador Slope. The meandering low-sinuosity, low-gradient NAMOC contains three segments: In the upper equilibrium channel, channel morphology is in equilibrium with spill-over from low-velocity, low-density upper portions of turbidity currents flowing in the channel; in the middle modified equilibrium channel, equilibrium morphology is altered by the confluence with tributary channels, and in the lower segment, channel morphology and position is controlled by basement topography (oceanic fracture zones and seamounts). The NAMOC levees interfinger with the braidplain deposits to the east and prograde southeastward in 7-8 packages. Most packages show a strong cross-channel decrease in thickness, and a strong asymmetry between the left and the right levee, as a Coriolis effect. The braidplain east of the NAMOC, which developed simultaneously with and prior to the NAMOC, shows a north to south gradient from laterally very extensive to highly channelized deposits. In the channelized part, channel positions change rapidly and deposits vary from sandy to gravelly. Extensive sheet-like turbidity currents on the braidplain may have been caused by catastrophic outbursts of subglacial lakes during times of ice-stream surges. Quantitative morphological analysis shows that the NAMOC is characterized by particularly low sinuosities due to very low channel-gradients compared to other submarine channels. The channel displays several features not recognized before in the deep-sea, or not in the same detail; including submarine hanging valleys and chute pools, submarine point-bars within the channel, coarse-grained wash-over fans on the lee-side of the levees, and terraces within the channel. The talweg is variable in depth and meanders within the channel. An up to 80 m deep talweg is present in the distal NAMOC resulting from flow restrictions due to seamounts. Channel morphology as well as grain size of spill-over deposits suggest that the flow tops of turbidity currents in the NAMOC are slow ($<$0.7 m/s) and dilute ($<$12 kg/m$ sp3$). Gravel deposits in the channel require velocities of 6.5-8 m/s for suspension transport, suggesting a strong vertical velocity and density gradient.
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