A series of sediment cores ranging in length from

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several meters to more than 18 meters was raised from the ocean floor between Panama and Antofagasta, Chile. Sediment types represented in the cores are silty and sandy lutite of terrigenous origin, globigerina ooze, and red clay. A variety of bottom environments was sampled, including those from the continental slope and rise, trenches and marginal basins, and the Carnegie and Nasca ridges.

Shear strength was measured aboard ship with a Swedish Fall Cone Penetrometer at 20-cm. intervals down the length of the core immediately after extrusion from the core pipe. Bulk and dry density, moisture content, porosity, and particle-size distribution were determined for each lithologic unit represented in the core. These data are correlated with bottom and sub-bottom reflecting horizons identified on the records of the precision echo sounder.

The data indicate that strong acoustic reflectivity is obtained at the boundary between a lithologic unit of low shear strength and an underlying layer of relatively high shear strength. Zones of high shear strength are characterized by lower moisture content and porosity, and by an increase in the coarse size fraction. Layers of volcanic ash and layers of manganese oxide were identified as strong reflecting horizons. At least one reflecting layer identified in the cores could be identified for several hundred miles, and other reflecting horizons for shorter distances. A layer of volcanic ash just a few centimeters thick was sufficient to provide a strong reflecting horizon.

Although the porosity of the sediment differed from core to core, there was no significant decrease in the porosity or moisture content with increasing depth below the bottom in a single core. This suggests that little if any compaction of the sediment has occurred within the zone that was sampled. However, in several cores there was a gradual increase in shear strength with depth.

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