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Mapping the EEZ: The First Million Square Miles
By Bonnie A. McGregor
The USGS effort to map the seafloor of the U.S. Exclusive Economic Zone (EEZ) reached a major milestone in 1987. The GLORIA (Geologic Long-Range Inclined Asdic) sidescan sonar system has now mapped over a million square nautical miles of seafloor in 3 years. One-third of the EEZ has been mapped completely, including the seafloor adjacent to the lower 48 States, to Puerto Rico and the U.S. Virgin Islands, and to Alaska in the Bering Sea region (fig. 5). The mapping is continuing on the Research Vessel M/V Farnella. The EEZ south of Alaska and around the Hawaiian Islands will be completed next, which will culminate the EEZ mapping for the 50 States by 1990. Efforts will then focus on the U.S. territories and possessions in the Pacific.
The M/V Farnella began the mapping of the Bering Sea in summer 1986 and completed it in July 1987. As in earlier EEZ mapping ventures, the mosaics are showing scientists geologic features that they had not imagined. The sonar mosaic of the floor of the Bering Sea shows extensive dissection of the continental margin and the northern slope of the Aleutian Ridge by some of the largest submarine canyon systems ever identified. Large, well-defined channel systems that emanate from submarine canyons in the southeastern corner of the Bering Sea extend for several hundred miles across the abyssal plain. Flow patterns on the seafloor appear to have formed by sheet flow as sediment is transported across the Aleutian Basin. The dominant erosional and transport processes active along the margins of the Bering Sea are a variety of types of mass movement ranging from small debris flows to massive submarine landslides and slumps of blocks of sediment measuring miles in diameter and thickness. Within the Bering Sea, Bowers Ridge is a crescent-shaped ridge, which was believed to be inactive because of its lack of seismicity and its flat crest. GLORIA images provided a new perspective of
Figure 5. Status of the GLORIA mapping program and location of CONMAP parcels.
Completed GLORIA surveys
Continental Margin Map
(CONMAP) Series panels Published GLORIA atlas, and
BC Baltimore Canyon number in USGS Miscellaneous
800 NAUTICAL MILES
GB Georges Bank Investigations Map Series
----Exclusive Economic Zone (EEZ)
the ridge. Large (6-mile-wide) volcanoes, and what are interpreted as numerous smaller volcanic features, were identified as being associated with Bowers Ridge and its extension. Recent uplift is also occurring along the ridge.
Besides providing new insights, the GLORIA data occasionally provide geologists with inexplicable puzzles. One of these is an interference pattern that resembles the characteristic pattern light makes on taffeta cloth. Speculation on the origin of the interference ranges from density differences in the water column to correlation with subseafloor features; the latter seems the most likely. An area was resurveyed 1 year later, and almost the identical pattern was again present on the image, implying that the interference is not transient.
In February through May 1987, the East Coast GLORIA survey was completed except for a shallow-water southern portion of the Blake Plateau. Striking regional differences were identified all along the East Coast. The area south of Cape Hatteras on the Blake Plateau, which is composed of limestone rocks, shows a seafloor that has been eroded by the northflowing Gulf Stream. Differences in the reflectivity can be correlated with the varied rock units that crop out on the seafloor. The seaward edge of the plateau is marked by an escarpment having over 13,200 feet of relief that has been carved by ocean currents aided by biochemical erosion North of Cape Hatteras to Long Island, the continental margin is cut by several large submarine canyons and channels that cross the EEZ for over 100 miles. Seaward of New Jersey, a vast dendritic pattern of canyons merge on the continental rise to form a single large channel that extends across the EEZ, and immense areas are marked by bottom failure and landslide deposits. A field of "mud waves” was mapped near the seaward edge of the EEZ, off North and South Carolina. The presence of these massive ripplelike features, 250 feet high with a spacing of 3 miles, suggests that some sediments in the deep water of the continental rise are being moved toward the northeast. Off Georges Bank, the GLORIA mosaic shows an extensively dissected continental slope with most canyons terminating in sheets of debris. Groups of canyons converge to form single channels; however, only six of the canyons that indent the shelf extend from
the upper slope to the outer margin of the EEZ. Mass movement of sediment has obliterated some channels, burying them in debris. Five of the New England seamounts show up strikingly, and it appears that they influence the paths of submarine channels and the distribution of sediments on the continental rise.
Image processing of the Bering Sea and East Coast GLORIA data is already underway. The data are being digitally mosaicked to produce the image maps for the respective atlases that will be published for each area mapped. The second atlas of the EEZ was published in October 1987 as USGS Miscellaneous Investigations Map 1–1864-A and 1–1864-B. This atlas covers the EEZ in the Gulf of Mexico and around Puerto Rico and the U.S. Virgin Islands. The atlas presents sonar-imagery mosaics (25 maps, at 1:500,000 scale) of the EEZ seafloor, along with generalized geologic interpretations and bathymetry, seismic-reflection-profile data collected during the surveys, and bathymetry and residual-magnetic-anomaly profiles in the survey areas.
The long-range sidescan sonar (GLORIA) images provide a striking view of the seafloor in the deep water of the Gulf of Mexico and in the Puerto Rico-U.S. Virgin Islands area. A better understanding of the morphology, surficial geology, and sedimentary processes is important for developing energy and mineral resources (fig. 6) and for siting seafloor structures. The GLORIA data provide information on depositional environments and geologic processes that is important in developing depositional models useful as analogs in understanding the rock record and resource distribution.
In the western Gulf of Mexico the most striking feature on the imagery is the