Spatial variability in selected geotechnical property measurements from three sedimentary provinces in the Venezuela Basin

Abstract

Relatively undisturbed surficial sediment samples from three sedimentary provinces (pelagic, turbidite dominated and hemipelagic) in the Venezuela Basin were examined for spatial variability in water content and undrained shear strength. A miniature vane shear apparatus boring method which minimized sample disturbance was used to determine variability of sediment shear strength directly within individual box-core samples. The variability of water content and sediment shear strength was also determined from subsamples (61 mm ID cores) collected from replicate box cores at each province. Sediments at the turbidite site yielded a higher average lateral coefficient of variation in water content and sediment shear strength (both methods of measurement) than sediments from the pelagic and hemipelagic sites. Vertical coefficients of variation were consistently higher than lateral coefficients of variations for all measured parameters. Between box-core variability in sediment shear strength was less than within box-core variability reflecting differences in measuring techniques and sample disturbance rather than actual differences. Biological activity, primarily compaction by burrowing polychaetes, maintains steep gradients of decreasing water content and increasing shear strength in the upper 100–200 mm of sediments at all three sites. The biologically controlled gradients and the occasional deposition of turbidites results in much higher vertical compared to lateral variability of some sediment geotechnical properties on scales of tens of millimeters (within box cores) and kilometers (between box cores collected at the same site).