Causes of compressional‐wave anisotropy in carbonate‐bearing, deep‐sea sediments

Abstract

Forty indurated sediment samples from DSDP site 516 were studied with the principle objective of determining which of several proposed mechanisms is the cause of acoustic anisotropy in carbonate‐bearing deep‐sea sediments. Recovered from sub‐bottom depths between 388 and 1222 m, the samples have properties exhibiting the following ranges: wet‐bulk density, 1.90 to [Formula: see text]; fractional porosity, 0.46 to 0.14; carbonate content, 34 to 88 percent; compressional‐wave velocity (at 0.1 kbar), 1.87 to 4.87 km/sec; anisotropy, 1 to 13 percent. Velocities were measured in three mutually perpendicular directions through the same specimen in 29 of the 40 samples studied. Calcite fabric has been estimated by x‐ray pole figure goniometry. The major findings of this study are. (1) Carbonate‐bearing deep‐sea sediments may be regarded as transversely isotropic media with symmetry axes normal to bedding. (2) Calcite c‐axes are weakly concentrated in a direction perpendicular to bedding, but the preferred orientation of calcite does not contribute significantly to velocity anisotropy. (3) The properties of bedded and unbedded samples are distinctly different. Unbedded sediments exhibit low degrees of acoustic anisotropy (1 to 5 percent). By contrast, bedded samples show higher degrees of anisotropy (to 13 percent), and anisotropy increases markedly with depth of burial. Thus, bedding must be regarded as the principal cause of acoustic anisotropy in calcareous, deep‐sea sediments.