Abstract

Physical properties (sediment texture, water content, wet bulk density, grain specific gravity, porosity, shear strength, and Atterberg limits) were determined for near-surface sediments (0 to 4 m) from the Peru-Chile continental slope and eastern Nazca Plate. These properties are strongly affected by organic matter which is concentrated in the slope deposits as a result of the high productivity in coastal upwelling areas off Peru. Sediments with unique properties occur in a lens of organic-rich mud on a portion of the upper slope (10.5 degrees to 13.6 degrees S) which coincides with an area of intense upwelling. The mud lens sediments are anomalously fine-grained and have the highest water content and plasticity and lowest wet bulk density and grain specific gravity found along the margin. Variation of these properties on the mud lens, and on the remainder of the slope and Nazca Plate, correlates well with changes in organic content. The undrained shear strength of the mud lens sediments is higher than might be expected for their high water content, possibly resulting from the development of clay-organic aggregates. A distinctive feature of the upper slope mud lens is a surface layer (0 to 15 cm) characterized by organic carbon concentrations of up to 20 percent dry weight, extremely high water content (maximum 853% dry weight), and very low wet bulk density (minimum 1.09 Mg/m 3 ). Outside the mud lens organic matter is less abundant and the amount of clay-size material in the sediment exerts a greater influence on the physical properties. Progressive fining of the sediments from south to north along the margin and with increasing distance from shore is accompanied by an increase in water content and plasticity and a decrease in wet bulk density and shear strength. These sediments typically show less change in physical properties with depth of burial than those of the upper slope mud lens. Differences in sediment mineralogy and sedimentation rate also affect the physical properties, but to a lesser extent than the variation of organic content or amount of clay.

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