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ABSTRACT: Deep-Water Large-Scale Slides & Debris Avalanches: Common Occurrences Rarely Resolved

William R. Normark

Large-scale mass failures on sedimented continental margins are generally identified on conventional seismic-reflection and 3.5-kHz profiles by their distinct acoustic/morphometric characteristics. Most well-studied mass failures are from shallow-water deltaic, fjord, and outer-shelf/upper-slope settings in areas of high sedimentation rates. In midslope or deeper water settings, however, resolution of the configuration of slide margins, internal structure, and distribution of debris components is rarely accomplished. Using only conventional profiling equipment, nonsediment failures, e.g., volcano slopes, in deep water produce deposits that cannot be distinguished from constructional volcanic forms. Long-range side-looking sonar imagery and deep-tow geophysical survey syst ms not only allow observation of the deposits in three dimensions but provide sufficient resolution of the debris components and slide-scar zone to permit interpretation of some aspects of failure/transport processes. Application of these techniques is exemplified by three slide deposits from the Californian and Hawaiian slopes. (1) The Sur slide off central California is a deep-water (>3000 m) deposit shed from an active transform margin in an area of relatively low sedimentation rates. Several failure episodes have resulted in a small (35 km3) deposit that covers 1000 km2. (2) Deep-tow records from a smaller (12 km3) slide deposit in a similar margin setting off Baja California, Mexico, document the transition of large (>107 m3) blocks to small debris less than a few meters across during transport over distances of tens of kilometers. (3) Hawaiian Ridge slopes are characterized by numerous mass failures, some of which are among the largest landslides on Earth. The full extent of these deposits, which individually can cover more than 20,000 km2, are unequivocally defined by GLORIA long-range side-scan data.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990