Geotechnical data from Plio-Pleistocene sediments on the Hebrides Slope provide the basis for an analysis of potential factors controlling their acoustic character as observed on high-resolution seismic reflection profiles. Differences in acoustic signature within the sedimentary succession are related to the void index and thus to soll structure and, in turn, to the depositional history of the sediments. In this example, a correlation can be demonstrated between the seismic and geotechnical stratigraphies. Introdu¢tionThe geotechnical properties of Plio-Pleistocene sediments from the upper Hebrides Slope west of Scotland (Fig. 1) have been studied in British Geological Survey borehole 88/7, 7A. The stratigraphical and sedimentological setting of this borehole have been described elsewhere (Leslie 1992(Leslie , 1993Stoker et al. 1994). Here we report the geotechnical profile of the uppermost 90 m of sediment and discuss its relationship to the depositional histories and seismic signatures of the various units penetrated.Previous geotechnical work on the Hebrides Slope using shallow (6-m) vibrocores (Talbot 1992;Paul et al. 1993) has demonstrated a correlation between acoustic and geotechnical characteristics in the uppermost seismic units. In this study we present vertical profiles of water content and associated indices, acoustic velocity, undrained shear strength, and preconsolidation pressure to a depth of 90 m and relate these to the depositional history of the sequence. We find that the geotechnical profiles contain a useful record of the history of accumulation, compression, and

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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.