Seismic stratigraphic analysis of 40 ins sleeve-gun profiles from the levees of the proximal Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea reveals seven intervals of levee formation that are characterised by packages of low-amplitude reflections separated by a few high-amplitude reflections, except for the second last interval, which shows high-amplitude reflections throughout. The levees are strongly asymmetric with a maximum thickness of 250 m for the western and 150 m for the eastern levee. Levee asymmetry is greatest for the high-amplitude package and nil for the most recent package. The levees prograde south-eastward and overlie a seismic facies of discontinuous high-amplitude reflections that probably represents channel-fill under the channel and submarine braidplain deposits towards the south. The NAMOC levees, which are very stably positioned with <10 km lateral migration, have grown since the mid-Pleistocene by spill-over deposition from turbidity currents under the influence of the Coriolis force. This resulted in increasing levee height and levee asymmetry with time as deposition in the channel raised the channel floor to equilibrium depth. The degree of levee asymmetry changes as a function of inferred flow velocities of turbidity currents within the channel. The latest interval of levee construction has symmetric levees, because increasing levee height through levee growth altered the spill-over mechanism from continuous flow-stripping of the top of turbidity currents to diffusion perpendicular to the channel from sediment clouds that were generated by currents passing through the channel. (C) 1998 Elsevier Science Ltd. All rights reserved.
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