Shallow geotechnical profiles, acoustic character and depositional history in glacially influenced sediments from the Hebrides and West Shetland Slopes

Surficial sediments from the Hebrides and West Shetland Slopes are mostly matrix-supported diamictons or laminated muds with subordinate sand. Those from the outer shelf and upper slope have water content and undrained shear strength profiles that suggest variable light to moderate overconsolidation, whereas those from the lower slope and basin floor have water content and undrained shear strength profiles that suggest normal consolidation. We believe that these geotechnical differences are genetic and reflect the processes operative in former glacial and glaciomarine palaeoenvironments along the continental margin: variably overconsolidated sediments are the products of marginal or proximal glaciomarine environments in which ice loading has occurred, whereas normally consolidated sediments are found in distal glaciomarine to basin plain settings. The depositional setting also controls the void index of the sediment via its sedimentary fabric and we find a sensible relationship between the compression curve of a sediment and its mode of deposition. It is usual to find that individual values lie scattered about this curve as a result of small variations in the packing. These variations correspond to minor sediment layers from which acoustic reflections may interfere constructively to give a multilayered acoustic texture, whose appearance is thus ultimately related to the magnitude of the scatter.