Sonar soundings of the Gulf of Mexico: sediment on the move.

Patterns in sediments, swirling like plumes of smoke, mantle the mudflow fanning out from the Mississippi River. This seafloor scene in the Gulf of Mexico is a sample of the latest batch of sonar images taken by GLORIA, the sidescan sonar system towed by the British research ship Farnell. The ship surveyed 14,000 squae nautical miles of the gulf last fall as part of the United States' EEZ-SCAN program--a six-year project to map the U.S. Exclusive Economic Zone (EEZ), which exends 200 nautical miles off U.S. Shores (SN: 9/21/85, p. 191). The new mosaics of sonar images, processed and compiled by the U.S. Geological Survey (USGS), show that there's a lot more activity on the floor of the Gulf of Mexico than previously suspected.

According to Bonnie McGregor, a Reston, Va.-based USGS marine geologist and project chief of the Gulf of Mexico cruises, the swirls of sediments in the Mississippi fan probably resulted from underwater landslides, which she says cover a much larger area than scientists had thought. She suspects that these landslides are generated during times of low sea level, when rivers like the Mississippi deposit piles of mud far out to the edge of the continental shelf. Then the breaking of ocean waves at the shell edge jars the piles, causing them to collapse and slide down the steeper continental slope. McGregor would like to test this idea by seeing if landslides in two coastal gulf areas to the east and west occurred at the same times as thos in the Mississippi fan.

The recent sonar images indicate that river channels are not the only means of carrying sediments to the fan. "Submarine landslides are also an important process in transporting sediments in the deep ocean," says McGregor. Scientists are especially interested in studying these processes on the Mississippi fan, sh add, as part of an effort to make a model for oil exploration in ancient fans on land.

The recent survey of the gulf also produced images of the continental slope off the coasts of Texas and Louisiana. The slope is being extensively deformed by a mass of salt diapirs, or rising domes, which is wedging itself between layers of mud as it flows downslope. At the edge of the salt mass is a 700-meter step called the Sigsbee escarpment. The EEZ-San images reveal that sediment is able to move across this escarpment, forming piles of debris on its seaward side. And engraved in these sediments are wavelike bedforms, indicating the McGregor that "water currents in the gulf are being channeled along the escarpment, reworking the sediments on the seafloor."

On the eastern side of the gulf, the researchers obtained images of the west Florida escarpment, the edge of the carbonate platform that forms Florida. The images reveal an extensive network of channels that have been eroded into the escarpment edge. According to McGregor, these channels vary in shape and depth along the length of the escarpment. Her group is now studying the images in detail to try to understand the processes that form the channels and how these processes differ at different latitudes. Getting these kinds of images with conventional sonar techniques, which look straight down on the ocean bottom, has been difficult because of the steepness of the escarpment, says McGregor. "The sidescan images [which look at a broad swath, 22 km to either side of the ship] for the first time show us clearly what the topography of the steep escarpment is," she observes. "Now we can look at the escarpment as a total unit."

The main advantae of the GLORIA sidescan system is that it can cover large areas very quickly; last summer it surveyed 250,000 square nautical miles off the coasts of Oregon, California and Washington in 100 days and at a cost of about 1^ per acre. According to McGregor, the British are now building another GLORIA system, which the United States will lease or buy. Scientists hope the system will be completed by the time the Farnella, which returned to the United Kingdom for maintenance after surveying the EEZ around Puerto rico, returns early this year to survey the waters around Alaska and Hawaii.