A long-range side-scan sonar (GLORIA) survey of the upper Laurentian Fan shows, for the first time, the regional geomorphology of this large and complex sedimentary body. The channels of the upper continental slope coalesce on the upper fan to form four major fan valleys. The largest of these, Eastern Valley, is U-shaped in cross section and up to 25 km (15 mi) wide with a large levee on the western flank that reaches a maximum height of more than 1 km (3,300 ft) above the valley floor. The remaining valleys, Western, Central, and Grand Banks, are typically more V-shaped in cross section and are up to 5 km (3 mi) wide. Extensive gullying is observed on the walls of all the channels. Sonographs of the floor of Eastern Valley show a strong, linear fabric elongated parallel with the valley walls that appears to be related to mesoscale relief on the valley floor. At water depths between 3,500 and 4,100 m (11,550 and 13,530 ft), two major fan valleys are created by the merging of the four major valleys of the upper fan. Both fan valleys are associated with large, asymmetric levee complexes that reach heights of more than 500 m (1,650 ft) above the valley floors.
The GLORIA data show evidence for several amphitheaterlike slump scars in the region of the 1929 earthquake epicenter. However, we see no evidence for movement of large coherent sediment blocks as postulated by earlier workers. Therefore, we suggest that the turbidity currents that occurred after the 1929 earthquake may have formed by the coalescence of many small slumps rather than from a single large slump.