Bonnie A. McGregor

Wilmington Submarine Canyon: a marine fluvial-like system.

Midrange sidescan sonar data (swath width = 5 km) show that a system of gullies and small channels feeds into large submarine canyons on the Middle Atlantic Continental Slope of the United States. The surveyed canyons all have relatively flat floors, but they have different channel morphologies. Wilmington Canyon has a meandering channel that extends down the Continental Slope and across the Continental Rise, whereas two canyons south of Wilmington Canyon have straight channels that trend directly downslope onto the rise. The morphology of these submarine canyon systems is remarkably similar to that of terrestrial fluvial systems.

Reconnaissance in DSRV Alvin of a “fluvial-like” meander system in Wilmington Canyon and slump features in South Wilmington Canyon

Three dives in DSRV Alvin on the Atlantic Continental Rise in Wilmington and South Wilmington Canyons, off the east coast of the United States, allowed examination and sampling of morphological features, in water depths of 2,300 to 2,400 m, that were observed in midrange sidescan sonar data. In Wilmington Canyon, a “fluvial-like” meandering system was confirmed. The meandering channel had steep undercut outer banks and gently sloping inner banks. Localized slumping is inferred from many steplike depressions on the steep outer banks. Although the meander system is as much as 500 m wide and has many characteristics typical of a fluvial system, extreme depth and evidence of episodic ongoing sedimentary processes preclude a true fluvial origin. Currents of unknown origin—that is, downcanyon turbidity flows—appear to be the only agent capable of sculpturing the observed features. A meander system was not observed in South Wilmington Canyon. Channel-floor features, including deformed and displaced sediment, support a previous suggestion of large-scale slumping in the area. These sediments include upturned clay beds, disaggregated gravels, loosely bound gravel conglomerates in a reddish-brown matrix, and a tubular structure, resembling a tree-root cast, within a thinly bedded, reddish-brown sandstone.

Sedimentary features of the south Texas continental slope as revealed by side-scan sonar and high-resolution seismic data

Sedimentary provinces on the south Texas slope have been identified by their acoustic character on long-range side-scan sonar records and high-resolution seismic profiles. Probable lithofacies within these provinces have been identified by core data and by analogy with previously cored acoustic facies. In the northern part of the study area, the East Breaks Slide is a prominent mass-transport feature. Revised bathymetry shows that the slide originated on the upper slope (200-1000 m), in front of a sandy late Wisconsinan shelf-margin delta, where the gradient is up to 3 degrees. It was deposited in a middle slope position (1000-1500 m) where the gradient is about 0.5 degrees. Side-scan sonar data indicates that the slide is a strongly backscattering feature extending more han 110 km downslope from the shelf edge. It consists of two lobes that are separated by a diapiric high. Diapiric highs on the middle slope have blocked most of the flow. Borehole data shows that the slide deposit contains intercalated sands and contorted bedding. The slide is therefore attributed to failure of sandy deltaic material deposited close to the shelf edge during the last period of low sealevel (late Wisconsinan, circa 11-29 Ka). Core data suggests that the weakly backscattering acoustic facies adjacent to the slide are fine-grained sediments (mudturbidites and hemipelagites) of a slope mud drape. The middle slope in front of the sandy late Wisconsinan shelf-margin delta of the Rio Grande has an intermediate level of backscattering with numerous channels leading to the Sigsbe Deep. These channels are believed to be part of the northern lobe of the Rio Grande Fan. Acoustic facies mapping using long-range side-scan sonar matches well with acoustic facies mapping using 3.5-kHz high-resolution seismic profiles. Within this study area higher levels of GLORIA backscatter are returned from sea floors containing shallow subbottom sand than from muddy sea floors.

Smooth seaward-dipping horizons - An important factor in sea-floor stability?

Abstract Mass movement has influenced in varying degrees the morphology of the United States east coast continental margin seaward of the Baltimore Canyon trough as revealed by detailed geophysical studies using high-resolution 3.5-kHz, and seismic reflection data. Each of three areas studied is along the slope within a distance of 225 km, and is seaward of a nonglaciated shelf but near major land drainage systems. Thick sequences of material believed to be Pleistocene were deposited on the slope in all three areas. Sediment failure in the form of large block movement involving block thicknesses of more than 100 m, however, has taken place in only two of the areas. A factor common to the two areas where failure took place, but absent in the area where no failure took place, is smooth seaward-dipping sub-bottom horizons. Whatever the triggering mechanism, a smooth slip surface that has a seward slope may contribute to mass movement by reducing the internal friction. This may be one of several factors that should be considered in assessing slope stability.

Ancestral head of Wilmington Canyon

A 12-channel seismic-reflection survey over parts of the head of Wilmington Canyon and the surrounding shelf revealed four major buried valleys and many small channels. The data indicate that the ancestral Delaware River valley was the most recent of a series of valleys to enter the canyon from the west. The head of Wilmington Canyon probably was eroded and then filled at various times from late Miocene to late Pleistocene time. Thick sedimentary sequences indicate a sediment supply from both the north and west. A large buried valley entering the canyon from the north was not mapped with the data presented here, but it is possible that such a valley exists. This is one of the first documentations of the long history of erosion of the United States east coast canyons.

Small‐scale mapping of the exclusive economic zone using wide‐swath side‐scan sonar

Abstract The U.S. Geological Survey (USGS) started a program in April 1984 to map the deep‐water (greater than 200 m) areas of the U.S. Exclusive Economic Zone (EEZ) at a reconnaissance scale as a first effort to develop a geologic understanding of the new national offshore territory. This effort was in response to President Reagans EEZ proclamation in March 1983, which extended U.S. jurisdiction over an area extending 200 nautical miles offshore of the United States and U.S. trust territories. This USGS mapping effort is a cooperative effort with the Institute of Oceanographic Sciences (IOS) of the United Kingdom and uses a unique side‐scan sonar system named GLORIA (Geological Long‐Range Inclined Asdic) developed by IOS. To date, over 3,500,000 km2 have been mapped off the west coast, the east coast, the Island of Hawaii, Puerto Rico, and the U.S. Virgin Islands and in the Gulf of Mexico and the Bering Sea. All of these surveys have been highlighted by discoveries of major geologic features that contri...

Role of submarine canyons in shaping the rise between Lydonia and Oceanographer canyons, Georges Bank

Abstract Three large submarine canyons, Oceanographer, Gilbert, and Lydonia, indent the U.S. Atlantic continental shelf and, with four additional canyons, dissect the continental slope in the vicinity of Georges Bank. On the upper rise, these canyons merge at a water depth of approximately 3100 m to form only two valleys. Differences in channel morphology of the canyons on the upper rise imply differences in relative activity, which is inconsistent with observations in the canyon heads. At present, Lydonia Canyon incises the upper rise more deeply than do the other canyons: however, seismic-reflection profiles show buried channels beneath the rise, which suggests that these other six canyons were periodically active during the Neogene. The rise morphology and the thickness of inferred Neogene- and Quaternary-age sediments on the rise are attributed to the presence and activity of the canyons. The erosional and depositional processes and the morphology of these canyons are remarkably similar to those of fluvial systems. Bear Seamount, which has approximately 2000 m of relief on the rise, has acted as a barrier to downslope sediment transport since the Late Cretaceous. Sediment has piled up on the upslope side, whereas much less sediment has accumulated in the “lee shadow” on the downslope side. Seismic-reflection profile data show that Lydonia Canyon has not eroded down to the volcanic rock of Bear Seamount.

Sea Floor Mapping Using Gloria Digital Techniques

GLORIA imaging sidescan sonar data were collected in 1982 and 1985 over the northern part of the Gulf of Mexico seaward of Texas, Louisiana, and Florida. They were digitally processed using the USGS-developed Mini Image Processing System (MIPS). The radiometrically and geometrically corrected 1982 image strips were film-mosaicked to enhance the interpretation of the sea floor morphology. Digital mosaicking is presently being utilized on all data to produce a marine geology atlas of the Gulf.