Physical properties of sediments from the continental margin of western Africa

Abstract

This study was designed to determine the possibility of utilizing physical properties as a tool to interpret portions of the geologic history of the west African continental margin. The properties measured were: bulk density, porosity, shear strength, water content, Atterberg limits, and consolidation characteristics. Results from Leg 40 of the Deep Sea Drilling Project showed that after initial buildup of the west African continental margin sedimentation has been predominantly biogenic. The physical properties of the sediments from this area are generally related to rock type and follow the general trends of increasing density, increasing sonic velocity and decreasing porosity with depth except where abrupt lithologic changes are encountered. Physical properties data suggested that significant amounts of overburden have been removed from portions of the Cape Basins and the Walvis Ridge. At a site in the Cape Basin results of consolidation testing have suggested that approximately 130 meters of overburden have been removed. Interpretation of physical properties from a sample containing greater than 50 percent carbonate content indicated that the effect of chemical processes are greater than that of gravitational compaction at very shallow depths. This results from cements that formed at grain contacts early in the diagenetic process filling voids and developing a rigid non-elastic structure. Therefore, preconsolidation pressures obtained from laboratory consolidation tests may not represent the maximum effective stress ever exerted on the sample. The fact that gravitational compaction processes are overcome by a chemically precipitated structure yields an explanation for constant porosity intervals observed in many carbonate ooze sections.