Although slumps and associated deposits appear to be widespread beneath parts of the continental slope, recent studies of sedimentation and near-bottom processes on the continental rise of eastern South America indicate that mass wasting (slumps, slides, debris flows, etc) is of only limited or local extent. This conclusion is based on detailed examination of moderate- to close-spaced 3.5-kHz echograms and seismic reflection profiles plus examination of several hundred piston cores. Zones identified as mass-wasting deposits are usually of small regional extent (< 50 km) and are confined to the upper rise or to regions adjacent to many of the large seamount chains (e.g., North Brazilian, Fernando de Noronha, and Columbia-Trindade Ridges) which cross the rise. Thus, on t e basis of available data, the continental rise of eastern South America does not appear to have large, widespread slump or debris-flow complexes that cover thousands of square kilometers and extend hundreds of kilometers downslope as they do on other parts of the Atlantic continental rise (e.g., northwest Africa and eastern United States). However, the present data spacing on the South American Rise may preclude recognition and delineation of the regional extent of many mass-wasting deposits.

An exception appears to be the Amazon Cone, a large deep-sea fan that crosses the continental rise off the Amazon River. Recent studies have delineated two major zones of failure and associated debris-flow complexes which extend 300 km downslope and are up to 100 km wide. The morphology of these features is complex, and recognition is complicated by the fan channels and their associated levees plus the high (> 50 cm/10³ year) Quaternary sedimentation rates. The eastern slide or debris-flow complex heads at about 2,500-m depth (middle fan) and appears to terminate downslope against the Ceara Rise at a depth of about 4,200 m. The associated debris flow near the Ceara Rise is particularly well defined, and scarps are recognizable at the head of the complex. Three cores from the region of the flow indicate that the age of the flow is older than late Wisconsin. The slide or debris-flow complex on the western side of the cone appears to head near a depth of 500 m in a narrow (~ 25 km) scar. The associated debris flow extends to at least 3,750 m. A core indicates a late Wisconsin age for the flow. The occurrence of these two large slide or debris flows emphasizes the possible importance of mass-wasting processes to the formation and growth of large deep-sea fans even though such processes have generally been disregarded in most deep-sea fan models.

End_of_Article - Last_Page 438------------