U.S. EEZ Atlantic Continental Margin GLORIA
GLORIA Geology Interpretation
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The GLORIA survey of the U.S. Atlantic EEZ covers the deepwater section of the EEZ that extends from approximately the shelf edge to 200 nautical miles offshore. The survey area extends from the U.S.-Canadian international boundary southward to 28oN. This area has been divided into five regions: the Georges Bank and southern New England region (from the U.S.-Canadian border to Hudson Canyon off New York City), the Middle Atlantic region (from Hudson Canyon to Norfolk Canyon off Chesapeake Bay), the Carolina region (offshore of North Carolina), the Blake Plateau region (offshore of the southern States and in water depths less than 1,000 m), and the Blake Escarpment (seaward of the Blake Plateau).
North of Cape Hatteras, the EEZ is divided into three physiographic provinces. The continental shelf has a very gentle gradient and extends from the shoreline seaward to the shelf edge at about 200 m water depth. The continental slope has a steeper gradient and extends from the shelf edge to depths of 1,800-2,500 m, and the continental rise, which has gentler gradients, extends beyond the seaward edge of the EEZ. South of Cape Hatteras, the continental slope is interrupted by the Blake Plateau in water 800-1,200 m deep; the plateau separates the Florida Slope from the Blake Escarpment. The Blake Escarpment is a precipitous cliff that plunges from the seaward edge of the Blake Plateau 4,000 m to the abyssal plain.3, 7 and 10). The seamounts are partly buried; adjacent seafloor sediments lap smoothly onto the seamount flanks. Only Buell Seamount appears to have a slight moat, presumably caused by erosion.
East of 70oW, the Neogene(?) and Pleistocene strata of the upper rise and lower slope are extensively eroded and are incised by canyons (mosaics 1, 2, 3, 6 and 7). The larger canyons on the middle to lower slope attain erosional relief as great as 400 meters and widths as great as 5 km. West of 70oW, most of the canyons converge or die out on the lower slope; only six of eleven shelf-indenting canyons extend across the continental rise to the border of the EEZ.
Mass movement was the dominant sediment distribution process on the New England continental rise in late Pleistocene to Holocene time. Most large mass movements or debris flows originated on the lower slope at water depths greater than 1,000 m and have involved stacks of strata as much as 300 meters thick. During movement, the displaced masses broke apart and were delivered to the rise as debris flows. Debris-flow deposits show up in the GLORIA images as relatively bright, reflective areas of sea floor, but individual flows are difficult to distinguish because most lack well-defined edges (mosaics 5 and 7). Individual deposits include large, flared sheets having wispy distal borders and elongate, linguloid bodies resembling mudflows. The debris deposits are generally less than 30 m thick; they typically thin to a feathered edge or to a convex margin less than a meter thick.
Mass movement is centered in two main areas of recurrent activity. The larger area is located west of long 69oW. Here, mass wasting has scarred much of the slope at depths deeper than 750 meters, and most of the debris is spread across the upper rise, mixed with sediment funneled by the canyons. Several large slides coalesce to form a complex 220 km wide that extends at least 200 km downslope. The larger continental-slope canyons in this area, such as Babylon, Atlantis, and Alvin, are barely detectable on the rise; they are either flooded by debris or are chutes that contributed debris to the total mass on the rise without forming distinct depositional borders (mosaics 5).
Considerable debris from this area moved downslope, intercepting and partly filling Hudson Canyon and Veatch Canyon. Debris crossed Veatch Canyon from the north, partly filling it. The GLORIA image shows that debris was not sluiced to the lower rise through the canyon and that the channel was not re-incised following debris emplacement. Although debris flows were mobile enough to have made appreciable progress across unobstructed sea floor toward the lower rise before stabilizing, little, if any, debris seems to have moved down the intercepted canyons to the lower rise. In fact, there is evidence that later down-channel flow in Hudson Canyon cut through the infilling debris without removing much of it. It would seem, then, that most of the sediment that debouched from the canyons to the lower rise had been eroded from the middle to upper slope through the canyons before the major lower slope slides occurred.4 and 5). It connects to the Hudson River via the Hudson Shelf Valley across the continental shelf. Seaward of the continental slope it continues across the continental rise as Hudson Valley. Sediment moves through the Hudson system to the Hudson Fan, which is on the lower rise, seaward of the area shown by the mosaic. The GLORIA mosaic and seismic profiles across the continental rise to either side of the Hudson Valley show a smooth surface covered by hemipelagic drape.
The continental slope between Hendrickson Canyon and Baltimore Canyon is cut by many submarine canyons whose upper reaches on the slope are fed by a complex system of gullies (mosaic 4). These canyons either stop at the base of the slope or extend onto the continental rise as shallow valleys that merge together to make the gather areas shown on the interpretive map. The gather area consists of a system of valleys that coalesce into the large, south-trending Wilmington Valley. Most of the apparent valleys on the rise appear as indistinct, discontinuous, high-backscatter bands on the GLORIA images. High-resolution echo-sounding profiles (3.5 kHz) show that these high-backscatter bands correspond to shallow depressions with poorly defined banks, which are in regions having a hummocky bottom surface. Even the larger valleys that extend across the rise from Wilmington and Baltimore Canyons are shallow, poorly defined, and sediment choked on the upper rise. Thus, many valleys of the upper rise, visible on the GLORIA images, probably represent short-lived features. The concentration of cross-rise valleys in the Hendrickson Canyon-Baltimore Canyon region and the present easterly course of Baltimore and Wilmington Valleys (anomalous with respect to the southeastward regional inclination of the rise) may result from long-term lower deposition rates relative to adjacent regions of the rise that have been supplied with sediments from the Hudson River and Delaware River drainages. The eastward course of the Baltimore and Wilmington Valleys may have been caused by piracy of sediment gravity flows by the lower, valleyed region, or by diversion by massive blocks of displaced continental-slope strata. Southeast-trending valleys that could be former extensions of the Wilmington and Baltimore Canyon drainage are visible in high-resolution bathymetric maps of the Baltimore Canyon-Washington Canyon slope-rise area. These valleys are blocked by topographic highs that may be slide deposits, although the origin of these putative slide blocks is poorly understood.
Southwest of Baltimore Canyon the large Baltimore-Accomac submarine slide and the Albemarle-Currituck submarine slide are large debris-flow complexes that are characterized by high backscatter in the GLORIA images (mosaics 4 and 8). Echo-sounding profiles show hummocky topography and inward-facing scarps that lie along the boundaries of the debris flows in many places. The depression of the slide regions indicates displacement of rise sediments in addition to mass movement of sediments that came from the continental slope. Discontinuous surface channels may have been caused by turbidity currents associated with the mass movements.
The continental slope off North Carolina is generally steeper than the continental slope to the north because it is swept more strongly by the south-flowing Western Boundary Undercurrent. The gradient at the slope averages 7 deg, reaching a maximum of about 16 deg off Cape Hatteras and decreasing to about 3.5 deg where it joins the Blake Ridge at about 32o30'N. This change in gradient is evident in the GLORIA images because sedimentary processes acting on the slope are related to the sea-floor gradient. The continental slope off Virginia and northern North Carolina, where the angle is steep, is highly dissected by canyons reflecting processed dominated by mass wasting (mosaics 8, 11 and 12). The canyons are steep chutes cut into Pleistocene clays and sands, and older rocks of Paleogene or Cretaceous age are exposed on the floors of many of them. East of Cape Hatteras, the steep slope and strong currents also have caused older rocks to crop out on the sea floor between canyons. Both canyon walls and outcropping indurated strata that faced the GLORIA detector produced bright returns on the sonar image. South of 34oN, the continental slope is less steep and it is not cut by canyons. Slope failure is evident here as large debris flows. Farther south, at the Blake Ridge, deposition is the dominant process, and the slope is mantled with a blanket of finely layered Pleistocene and Holocene sediments that produce a monotonous dark return on the GLORIA image (mosaic 15).
Two major canyon systems, Hatteras Canyon and Pamlico Canyon, have cut deep valleys across the upper continental rise that terminate on the lower rise in fans that are characterized by transverse sediment waves. Between the canyons, the bottom sediment consists chiefly of poorly reflective, layered hemipelagic ooze that drapes the bathymetric irregularities and produces a low-backscatter return on the sonar image.
North of Hatteras Canyon, another major canyon system, the Albemarle Transverse Canyon, converges on the rise into a large turbidite fan that fills a broad valley. The fan, in turn, is dissected by flat-floored valleys that lead downslope to the Hatteras Transverse Canyon. Sediment flowing from the canyons and debris flows from adjacent slope areas merge into a turbidite debris apron that flows into and down the southwest-oriented Hatteras Transverse Canyon to terminate in a major fan, the Hatteras Fan, south of the Hatteras Ridge (mosaics 16 and 17).
The continental slope and upper rise off North Carolina exhibit some of the largest slope failures to be found along the Atlantic margin. The Albemarle-Currituck slide, or debris flow, involved an area of the lower slope 22 km long, 7 to 12 km wide, and 300 meters thick. Debris flows from this failure can be traced from the scar on the slope for 300 km across the rise to where they merge with other debris flows and fan deposits derived from the Hatteras Transverse Canyon. The large debris flow from the Albemarle-Currituck slide had earlier been traced by its seismic acoustic character correlated with bottom cores.
The Cape Fear slide involved slope failure of an area 37 km long, 10 to 12 km wide, and up to 80 m thick. Scarps of sharply truncated bedding on the lower slope are visible on the GLORIA image as dark lines. Failures of discrete layers upslope of this complex are also visible in the GLORIA image but less distinctly. Debris flows from the scar area extend across the rise for over 250 km and provide a high-backscatter return. At the eastern edge of the surveyed area (mosaic 16), debris from the Cape Fear slide overlies earlier debris from the Cape Lookout slide and other sources to the north, indicating that the Cape Fear slide postdates these other slides.
The Cape Lookout slide originated just northwest of 34dN, 75dW. This slide complex was previously unrecognized because only a small scarp occurs on the continental slope above the debris flow and the apparent slide-scar area is cut by canyons. A 280-km-long debris flow, marked by a high-backscatter return, is apparent on the GLORIA image. The debris flow has cut a shallow trough that truncates hemipelagic strata along its edges. The morphology of the Cape Lookout slide suggests that a small upslope failure may have triggered a much larger, 36-km-wide failure on the upper rise.
The Hatteras Ridge, a major transverse barrier that deflects turbidity currents into the Hatteras Transverse Canyon, is a prominent feature along the eastern margin of the surveyed area off North Carolina (mosaics 13 and 17). Oriented in a northeasterly direction, the ridge is more than 500 m high. It parallels the continental slope and is believed to be built of current-drifted sediments, chiefly muds and silts called contourites, that became trapped in the shear zone between contour-following currents, the northeast-flowing Gulf Stream, and southwest-flowing abyssal Western Boundary Undercurrent. Large sediment waves, known as the Lower Rise Hills, characterize the seaward flank of the Hatteras Ridge. These sediment waves are oriented east-west and are clearly seen on the GLORIA image due to the contrasting acoustic return of slopes facing toward and away from the detector.
The Blake Plateau lies off Florida, Georgia, South Carolina and North Carolina (mosaics 14, 15, 18 and 21). The plateau is intermediate in water depth (400 to 1250 meters) between the Florida-Hatteras Shelf (0 to 80 meters in depth) and the upper continental rise (2000 to 4000 meters). It has formed since the early Tertiary when the strong Florida Current first began to flow through the Straits of Florida across what was then a deepwater continental shelf, thus cutting off the supply of continent-derived sediment to the outer shelf. Since that time, sediments deposited landward of the current have formed the present Florida-Hatteras Shelf and Slope, while continued sediment starvation and subsidence of the outer shelf beneath and seaward of the current have formed the Blake Plateau. Although some sediments were deposited on the outer Blake Plateau in the later Tertiary, the combined flow of the Florida Current and the Antilles Current, which merge over the southern Blake Plateau to form the Gulf Stream, has prevented most deposition over the plateau and has deeply scoured the bottom.
The GLORIA image shows that much of the surface of the Blake Plateau has an extremely high backscatter return. This high-backscatter return is from a pavement-capped area and is broken only by linear bright streaks that represent slopes of large scoured valleys and the edges of scour holes or truncated strata that faced the detector during the survey. These valleys are not well mapped on the present bathymetric map of the plateau, and thus they generally do not correspond to depressions shown by contours. A number of deep, circular depressions are visible on the image near 31oN, 78o30'W. These holes are over 150 meters deep and resemble sinkholes, although they are probably related to bottom scour by currents.
There are tonal variations on the GLORIA image over the Blake Plateau that relate to the composition (or induration) of the outcropping bedrock. For instance, pavement and outcropping indurated Paleogene-age limestones produce the brightest returns. Slightly darker returns are produced by outcropping strata of intermediate induration. An example is a band of partly cemented calcareous sands of Miocene age that crosses the plateau in an east-west direction at about 30o30'N (mosaic 18). In the same general area, outcropping Cretaceous-age calcareous siltstone floors a deep valley that crosses the plateau just south of 32oN. The area of Cretaceous outcrop produces a slightly darker tone on the GLORIA image. The darkest areas over the Blake Plateau on the GLORIA image are found where scour is less intense and where unconsolidated post-Miocene calcareous sands and ooze compose the bottom. These depositional areas are found both north and south of the highly scoured area of the Charleston Bump, as well as within several large north-south-oriented scour holes that cross the eastern flank of the bump. Unconsolidated sand is characteristic of the entire bottom north of 33oN, which lies downcurrent and in the lee of the Charleston Bump. Here, reduced bottom-current intensity caused by both frictional loss and deflection of the Gulf Stream by the bump allows some deposition to occur. The dark return areas on the deeper part of the plateau south of the Charleston Bump (mosaics 18 and 21 south of 30o30'N) are also produced by post-Miocene deposition.
Off the southern Blake Plateau, east of Florida, the transition from intermediate depths (1,000-2,000 meters) to oceanic depths (5,000 meters) is unusually abrupt. This rapid change from continental margin to deep-sea basin occurs at the undersea cliff known as the Blake Escarpment. The part of the Blake Escarpment that is within the U.S. EEZ consists of three main morphologic zones. The northern zone, from roughly 29o45'N to 30o15'N, forms the walls of the Blake Spur, a salient of the Blake Plateau that extends eastward of the rest of the plateau (mosaics 19 and 21). The central morphologic zone is the canyoned region that extends from about 29o45'N to about 29o5'N, and the southern zone is a straight section that extends southward to 27o50'N (mosaic 21).
In the Blake Spur zone, the escarpment is steepest, so that profiles commonly show only echo returns generated near the top of the escarpment, which obscure the lower part. Submersible dives at the spur show that the cliff there is near vertical over much of its extent. This extreme steepness is an indication that the cliff has formed by erosion rather than by accumulation, because the front of a carbonate (limestone) platform normally is more gently sloping than this (the Blake Plateau in this region is considered to have been formed as a carbonate platform, analogous to the present Bahamas. Erosion of the base of the escarpment has been at least 10 km. Strong sea-floor currents that flow southward have created a scoured depression off the end of the Blake Spur that extends southward as a moat along the eroded part of the escarpment (mosaic 21).
In the second morphologic zone, south of the Blake Spur, the Blake Escarpment is extensively dissected by canyons and gullies (mosaic 21). These canyons are likely to be relict from a time when the Blake Plateau surface was near sea level and large amounts of carbonate sediment were being created and passed through these chutes to the deep sea floor. This episode of a shallow-water Blake Plateau probably ended about 100 million years ago. Bright reflections in the image, particularly on the lower slope, are characteristic of canyon floors. This brightness could be caused by coarser sediments in the canyon floors, which could suggest that the canyons are still somewhat active. Alternatively, the bright reflections may indicate that bare limestone is exposed in the canyon floors. A band of stronger reflections also extends along the uppermost part of the escarpment in the canyoned zone. This band represents a part of the escarpment that is steeper than that below, and where the limestone strata of the escarpment are exposed. The canyons do not extend into this upper escarpment band, nor is there any channel system on the outer Blake Plateau. If large amounts of sediment once collected on the plateau and spilled over the edge, the system that channeled them is gone, modified by Cenozoic erosion and deposition.
Within the canyoned zone is one large collapse deposit between approximately 29o10'N and 29o25'N (mosaic 21). Canyons seem to wrap around this deposit, which suggests that it is fairly old, although we have no good estimate of the age of the canyons. Most of the escarpment above the slump is poorly reflective, perhaps covered by pelagic deposits, except for one bright area that may mark the site of a more recent slump. The third morphologic zone, south of 29o5'N, is a nearly linear section of the Blake Escarpment that trends slightly west of north and has a stepped character. The stepping is caused by erosional retreat of the escarpment face, modified by alternations of harder and softer limestone beds and by development of blockiness, which is controlled by joints in the rock. The joints probably form as a result of uneven stress relief in unloading of the limestone when the rock to seaward is eroded away. Just seaward of the escarpment, on the deep-sea floor in approximately 5 km of water, is a moat-like depression, which apparently has been eroded by southward-flowing bottom currents that scoured away sediments near the base of the escarpment. Within the moat, differentially eroded outcrops of sea-floor sediment layers give rise to wispy linear reflections, semi-parallel to the escarpment.