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The AAPG/Datapages Combined Publications Database
The slopes of the California borderland are an important pathway for sediments transported from the mainland shelf, bank tops, and island platforms to the adjacent basin floor. Sedimentologic conditions on the slope are governed by a complex interplay of depositional and mass movement processes which are controlled by driving forces and stabilizing factors. Driving forces are predominantly external to the sediment mass and include oceanographic, biologic, and tectonic factors. Sedimentologic and geotechnical properties of sediments, as well as certain environmental parameters such as declivity, may provide a stabilizing influence to the sediments. These same factors, however, may be an influence in decreasing the stability and enhancing the influence of the driving mechan sms. Declivity, an environmental property commonly considered a major factor controlling failure, is less important than either proximity to active sediment sources or the influence of external driving forces.
We have conducted field studies at different scales of examination including high resolution acoustic profiling, sediment sampling, bottom photography, and observations from manned submersibles. These data show that failures themselves are greatly varied in scale, ranging from large features kilometers on a side and approximately 50 m thick, through smaller scale failures tens to hundreds of meters on a side, and about 1 m thick, to very small displacements composed of locally contorted and deformed sediment layers only a few centimeters thick.
We regard these small displacements as being more important in the basin filling process than has been previously recognized. In a detailed study of an acoustically defined failure zone on the mainland slope off San Mateo Point, we observed the zone itself to be composed of numerous narrow slumps. Cores from these deposits contain combinations of hemipelagic sediments and small-scale mass flow deposits exhibiting internal plastic deformation and basal scour. Vertical stacking of these sediment packets show that small-scale displacements can occur repeatedly at a given locality. Continued, episodic loading of the sediments through time produces locally inhomogeneous, weakened sediment masses which, in conjunction with driving forces, may contribute to the generation of large-scale fail res. Such large-scale features are those commonly identified by conventional acoustic techniques.
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