NEW INSIGHTS IN ATLANTIC CONTINENTAL MARGIN PROCESSES

In 2004 and 2005, the University of New Hampshire conducted 5 months mapping the entire U.S. Atlantic margin. The mapping covers ~403.5 km2 between the 1000 and 4600-m isobaths with both bathymetry and acoustic backscatter data using a 12-kHz multibeam system. The new maps image in unprecedented detail many features already known on the margin and also additional, new features that require rethinking the processes that are, or have been, active on the margin.

Mass wasting is pervasive all along the margin, but occurs in several styles. The most common form is unconstrained downslope transport of sediment masses. A second common style is slab detachment where the transport of material has left behind a scarp. A third style is margin collapse where the displaced material formed distinct transport conduits as it traversed downslope. A stunning example of this third style occurs on the margin off New Jersey. A ~250-m thick, ~3500 km2 area of the margin appears to be part of a huge collapse. All of the channels within the collapse coalesce into a single, ~200-m deep, 4-km wide gently curved channel with a 40-m incised thalweg.

Although, submarine channels have been studied on this margin over the past 50 years, the new data show evidence that channels of the northern half of the U.S. margin appear to have changed their equilibrium profiles and are either now, or have recently, undergoing a period of incision. Channels on the southern half of the margin do not show renewed incision. The change in equilibrium profiles could be the result of continued regional uplift on the margin as a result of glacial rebound from the last deglaciation, or alternatively, the northern margin channels might be more active sediment transport conduits in recent time than the southern margin channels. Hudson Canyon/Channel and nearby Johns Valley channel show remarkable changes in trend that correlate with sudden deepening of adjacent channels along the same isobath. High-resolution seismic profiles do not suggest structural control for the changes in channel courses or deepenings. At the distal ends of many of the channel systems, depositional lobes are imaged in remarkable detail. These lobes consistently show an interweaving pattern of high and low backscatter, suggestive of the upper submarine fan deposition.