Irregular trends of physical properties in homogeneous clay-rich sediments of DSDP Leg 87 Hole 584, midslope terrace in the Japan Trench

Abstract

Compressional velocity ( V p), attenuation (in terms of its inverse, the compressional quality factor Q p), electrical resistivity (σ), bulk ( ρ b) and grain ( ρ g) densities, porosity (ø), and bulk mineralogy were measured on diatomaceous hemipelagic sediments from DSDP Leg 87, Hole 584. Although the sediment lithology and the bulk mineralogy determined by X-ray diffraction are relatively homogeneous, the physical, acoustic and electrical properties of the sediments show irregular trends with depth, with variations in the magnitude and sign of property gradients. Between subbottom depths of 500 and 800 m, V p, ρ b, ρ g, and σ all sharply increase and then sharply decrease, while porosity sharply decreases, and then sharply increases. The changes in gradients are due to abrupt changes in the opal-A content of the sediment, as determined from smear-slide data. Q p decreases with depth to about 650 m and then increases. Such a trend has not been previously documented in laboratory measurements. However, it is assumed that the “attenuation maximum” observed here is due to the increased grain density and decreased porosity caused by the reduction in opal-A content around this depth. The anisotropies for V p and Q p undergo sign inversions with depth. The V p anisotropies change from positive ( V ph > V pv) at the top to negative at the bottom of the hole, whereas Q p anisotropies change from negative ( Q pv > Q ph) to positive. The change in sign with depth is caused by a change in sediment bedding dip from near 0° at the top of the hole to about 65° toward the bottom.