Late Cenozoic evolution of sedimentation and slope stability along the U.S. Atlantic margin.

摘要

The U.S. Mid-Atlantic margin,from the Norfolk Canyon south to Cape Hatteras, exhibits three major seafloor morphologies: elongated shelf-edge blowouts, the Currituck submarine landslide, and a large slope-sourced canyon failure. The late Cenozoic evolution of slope stability on the Middle Atlantic margin can be linked to the framework geology as well as spatial and temporal variations in sedimentation patterns across the margin. The framework geology impacts sediment pathways, while the shape of the margin influences the position of sediment depocenters across the margin. Changes in sedimentation rates and sediment source dictate the amount of sediment being delivered to each region and can vary through time. High sediment supply can lead to progradation of deposits across the shelf and slope that can increase the probability of larger sediment failure. Lower sediment supply tends to be associated with sediment deposition on the outer shelf and shelf-edge, which leads to oversteepening the upper slope and increases the probability of progressive slides. Here, I used a dense grid of industry multichannel seismic data across the outer shelf and slope of the U.S. Mid-Atlantic margin to investigate the evolution of geologic framework evolution and spatial variability in sediment depocenters over time. The Currituck region of the margin has a sigmoidal morphology, characterized by a rounded shelf-edge with a relatively gentle slope gradient (< 6°). This morphology, combined with rapid sedimentation across the middle slope appears to have preconditioned the margin for retrogressive slope failure. In contrast, sediment buildup on the shelf-edge at the blowout region to the north resulted in the development of an angular shelf-edge and a steep upper slope gradient, > 11°. This led to oversteepening and downslope sediment bypass, resulting in the heavily canyonized slope morphology observed on the slope today. The large slope sourced canyon failure region exhibits an intermediate margin shape of the Currituck and blowout regions, where sediment progradation on the shelf-edge and upper slope led to a large canyon-confined failure that on the upper slope. Across the study area, it appears that much of the modern morphology has been inherited from the spatial distribution of depocenters emplaced as much as 11 mya. These results highlight the role of framework geology and sediment distribution in preconditioning the margin for slope failure.