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being developed to make digital mosaics of the imagery and display the images on a map at a selected projection. This technique will be used to prepare the 2°X2° imagery maps for the atlas of the Gulf of Mexico and Puerto Rico GLORIA data. This atlas, to be published in early 1987, will be the second in a series that displays the results of the Survey's EEZ mapping program.

Geologic Perspective of the Gulf of Mexico

slope seaward of Texas and Louisiana. Salt tectonics and thick accumulations of sediments in this region along the upper slope have contributed to an offshore oil and gas field called the Flexure Trend. Because this frontier area is located in deep water, advanced exploration and production technologies are required for the development currently underway in the Green Canyon area of the trend.

The complex morphology and dynamic movement of the salt have made the geology and evolution of the Texas-Louisiana slope difficult to unravel. The GLORIA imagery provides a new perspective on this problem because it allows identification of salt pinnacles more than one-half mile across and domes and basins 25 miles across. The seaward edge of the salt front is marked by the Sigsbee Escarpment, which is approximately 2,300 feet high. Piles of talus lying along the base of the escarpment suggest that erosion and deposition are occurring there. Three reentrants present in the salt front mark the locations of canyon systems that provided pathways for sediment transport from the Texas-Louisiana shelf edge to the deep water of the gulf. The sonar image shows one channel meandering on the sea floor from the escarpment southward toward the Sigsbee Abyssal Plains (fig. 9). Sediments that appear to have been reworked by bottom currents in this channel's levees suggest that this channel is no longer actively transporting sediment in the form of density flows. The presence of the channel, however, attests to the geologically recent movement of the salt blocking the path of this channel on the slope. Other remnant pathways that can be identified on the slope provide clues to the distribution of sediment deposition centers. These

The geologic setting of the EEZ in the Gulf of Mexico, a small ocean basin, is different than that of the EEZ off the west coast. The spreading ridge crests and cratered underwater volcanoes, which attest to the dynamics of formation of the Earth's crust and the floor of the Pacific, are not seen in the gulf. Instead, GLORIA imagery shows a sediment-carpeted sea floor that contains a record of the dynamic processes that have laid down and molded that muddy carpet. Features that have more than 650 feet of relief and sea-floor slopes of up to 1,312 feet are present in the gulf. These features are not volcanic, however, but were formed by processes such as erosion of the edge of the carbonate Florida Platform that formed the Florida Escarpment. Also, the weight of thick accumulations of sediment dumped into the gulf by rivers, such as the Rio Grande and the Mississippi, has caused a Jurassic-age layer of salt to flow upward and seaward and form domes and ridges that are flanked by basins, many of which are over 3,200 feet deep and filled with sediments. Movement of the salt controls the complex morphology of the continental

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deposition centers may be the sites of additional oil and gas fields along the flexure trend.

Rivers and submarine canyons are an important clue to understanding the distribution of sediments in the gulf. The Mississippi Canyon and fan are striking features on the GLORIA imagery in the central and eastern gulf. The canyon and the channel that is its seaward extension (fig. 10) can be traced across almost 300 miles toward the southeast to a point due west of southern Florida. Another meandering channel is clearly visible in the northeastern gulf starting seaward of DeSoto Canyon and meandering for about 100 miles to the south parallel to the Florida Escarpment Canyons (fig. 11), whose deepwater meandering channels are important conduits for the transport of sediments by density currents. Like rivers on land, when these density currents are thick enough, they construct levees by overbank flow, depositing mud as they spread out away from the confines of the channel. The channel from DeSoto Canyon has built up its floor and levees above the surrounding sea floor. This type of channelized flow is a major means of distributing sediments in deep water. An understanding of canyonfan systems is valuable for petroleum exploration and production, because many oil fields are associated with ancient submarine fans.

Besides channelized flow, submarine slides and debris flows make a significant contribution to the distribution of sediments in the deep gulf. GLORIA imagery shows that a series of massive debris flows has filled the Mississippi Canyon and buried parts of the Mississippi channel and the channel from the DeSoto Canyon area. These debris-flow deposits cover about 20,000 square miles (about one-third the area of the State of Florida). The very intricate flow patterns on the sea floor pose interesting questions about the dynamics of sediment movement across the sea floor where the gradient is 1° or less. Submarine slide deposits also cover areas ranging from 1,500 to 19,000 square miles in the DeSoto Canyon area and on the slope in the northwestern gulf. Mass wasting is also an important process in distributing sediments in the gulf. The trigger mechanisms for these submarine landslides, their relation to sea-level fluctuations, and whether they are synchronous are all questions to be addressed.

The Florida Escarpment, which flanks the gulf on the east, has approximately 6,500 feet of relief and a slope of about 40° and is composed of outcropping Cretaceous limestones. Because of the steep slope of the escarpment, its morphology could not be well defined by conventional profiling systems towed at the sea surface. For the first time, GLORIA images show the dis

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Figure 10. The Mississippi channel can be seen on the sidescan mosaic meandering across its fan in approximately 8,000 feet of water. The bright areas marked by swirling patterns are the debris flows that also blanket the fan.

sected morphology of the escarpment and how it changes along its length. Massive flat-floored box canyons cut the escarpment toward the south, and smaller dendritic canyons occur to the north. Studying the evolution of this carbonate cliff and the processes that shape it is an integral part of understanding the seascape of the floor of the Gulf of Mexico.

Continuing analysis of the GLORIA data will identify potential sites for detailed study aimed at solving the problems discussed above. The Geological Survey will be conducting GLORIA surveys of the Atlantic coast in fiscal year 1987 and planning additional GLORIA surveys of the Alaskan. and Hawaiian EEZ's.

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WATER RESOURCES

INVESTIGATIONS

Parts of the Southeastern United States were hit by a severe hydrologic drought during the first 6 months of 1986. Indicative of the drought, the water level in the Rose River near Skyline Drive in the Shenandoah National Park, Va., was clearly far below normal on August 3, 1986. (Photographed by A. R. Powers, Water Resources Division, U.S. Geological Survey.)

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Conducts supportive basic and problemoriented research into hydraulics, hy

drology, and related fields of science and

engineering to improve the basis for field investigations and measurement techniques and to understand hydrologic systems in order to predict quantitatively their response to natural or manmade

stress.

• Provides to the public the water resources data and the results of water resources investigations and research through reports, maps, computerized information services, and other forms of public releases.

• Coordinates the activities of Federal agencies in the acquisition of water resources data for streams, lakes, reservoirs, estuaries, and ground water.

• Provides scientific and technical assistance in hydrology to other Federal, State, and local agencies, to licensees of the Federal Energy Regulatory Commission, and, on behalf of the U.S. Department of State, to international agencies.

The Federal program is specifically identified in annual congressional appropriations and provides for the collection of waterresources data, investigations of resources, and research activities in areas where the Federal interest is paramount. These interests include water resources in the public domain, river basins and aquifers that cross State boundaries, and other areas of international or interstate concern.

Federal-State Cooperative Program

The Federal-State cooperative program is based on the concept that Federal, State, and local governments have a mutual interest in evaluating, planning, developing, and managing the Nation's water resources. The Water Resources Division represents national interests, and the cooperating agencies represent State and local interests; the costs are shared 50-50, the Federal share coming from direct congressional appropriations. The Geological Survey has cooperative agreements with all 50 States. Projects generally respond to recognized problems or define a potential one. In emergency situations, such as drought or flood, events are monitored, and the data accumulated under the cooperative program are made available for use by the public.

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