Microstructure and physical properties of Amazon Fan Sites 940 and 946 from wireline, laboratory, and sedimentologic data

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The Amazon Fan began to form in the early Miocene and is characterized by a highly meandering channel-levee system. This paper concerns the physical properties of sediments deposited during fourth- (10 3 to 10 4 yr) and fifth-order (instantaneous) events. Core and downhole wireline log data were used to characterize lateral variations from a middle fan site down to a midlower fan site. Scanning electron microscope backscatter (BSE) images were generated from prepared samples obtained from Holes 940A and 946A and analyzed to quantify relative percentages of grains, matrix, and visible pores in sediments, as well as to observe changes in microfabric associated with compaction at each site and from one site to another. Although there is no significant downhole trend in the relative percentages of grains and visible porosity, quantitative analysis of images suggests that there is a progressive reorientation of grains downsection. There appears to be a transitional boundary at -150 meters below seafloor (mbsf) at both sites. Above 150 mbsf, variability in relative percentages of grains at both sites is more a function of lithologic variability controlled by changing sedimentary dynamics rather than being strictly a result of increased overburden. At Site 940, index and wireline log porosity values agree well, and the difference between index and matrix porosities is -7%-8%. Matrix porosity values generally approach those predicted in situ porosity values. At Site 946, laboratory index and wireline log porosity values generally agree, but much higher variability is observed in the log data, which suggests the presence of sand-rich intervals, many of which were not recovered in cores. The difference between laboratory index and matrix porosity values at Site 946 was -5%. Hole 940A compressional wave velocities (wireline log in situ values) closely match with calculated instantaneous compressional wave velocity values derived for terrigenous sediments (silt-clays, turbidites, mudstone-shales) and suggest that the sediments are relatively homogeneous with respect to texture and composition.