William H. Busch2 and Michael R. Brister2


Fine-grained sediments from six sites on the Amazon Fan were examined to determine the relationship between the fabric and physical properties of the sediments and the extent to which the fabric reflects the influence of depositional processes and burial-related change. Secondary and backscattered electron imaging with a scanning electron microscope were used to examine samples selected from proximal levee, bioturbated mud, distal levee, and mass-transport deposits (MTDs). The distal levee sediments display less variable particle orientations than proximal levee deposits and a greater particle reorientation with burial. Fine-grained MTDs are characterized by a variety of fabrics, including randomly oriented clay domains and silt grains, very dense fabrics in displaced sediment blocks, and fabrics reflecting zones of shearing and sediment deformation. Anisotropy of electrical resistivity provides an independent measure of sediment fabric and a means to distinguish levee and MTDs. The correspondence between resistivity measurements and electron microscope fabric observations is weak. In all sediment types, the greatest change in fabric occurs at depths less than 50 m below the seafloor as a result of the collapse of a porous, open fabric to yield a more dense particle arrangement with larger clay domains and reduced porosity. As burial depth increases, the fabric progressively becomes denser, but the transformation occurs at a slower rate than the change at shallow depths. The reduction in bulk porosity matches the rate of fabric change with depth. Because of the variety of fabrics that occur in the mass-transport units, it is difficult to determine what aspect of these deposits is responsible for anomalously high porosity that is common in underlying sediments.

1Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Ed.), 1997. Proc. ODP, Sci. Results, 155: College Station, TX (Ocean Drilling Program).
2Department of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148, U.S.A. busch@geology.uno.edu