Geotechnical and acoustic characteristics of Plio-Pleistocene sediments from the Hebrides Slope

Abstract

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

Debris flows on the Sula Sgeir Fan, NW of Scotland

“…The sediment supply may initially have been boosted by intense weathering and erosion under warm, humid climatic conditions which characterized the mid-Pliocene (Dowsett et al 1992), but the bulk of the fan has been built during the mid-to late Quaternary by a large supply of glacially transported sediment. Sula Sgeir Fan build-up probably ended in the early Devensian, as the regional seismostratigraphy indicates that the extent of the last Scottish ice sheet (late Devensian) was restricted to the inner shelf (Stoker et al 1994).…”

“…These transparent packages are commonly separated by thin, continuous reflectors that probably represent intervals of reduced sediment supply to the slope. This may have been due to an interstadial or interglacial rise in sea level, or to more localized conditions within a glacial phase, such as less extensive shelf glaciation (Stoker et al 1994). The size of the glacigenic debris flows varies from 0.5 to 3-5 km in length, and they have a thickness of 5-35 m. Accumulation of these mass-flow packages was episodic and related to specific rapid phases of downslope sedimentation concomitant with maximum glaciations and glacio-eustatic lowstands.…”

“…Even in the 'between-fan' area, the sediment accumulation rates increased significantly during glacial intervals (Stoker et al 1994). This is reflected by sediment accumulation rates, which increased from 25 mm/ka during the early Pleistocene to a maximum of 560 mm/ka during the mid-to late Pleistocene (Talbot et al 1994).…”

“…At their maximum extent, these ice sheets extended, at least locally, over the shelf edge, and delivered ice-marginal sediments directly to the slope (Stoker 1995). Even in the 'between-fan' area, the sediment accumulation rates increased significantly during glacial intervals (Stoker et al 1994). This is reflected by sediment accumulation rates, which increased from 25 mm/ka during the early Pleistocene to a maximum of 560 mm/ka during the mid-to late Pleistocene (Talbot et al 1994).…”

“…This fan, previously described as an accumulation of debris-flow deposits with intervening hemipelagic sediments (Stoker 1995), is here recognized as comprising two kinds of debris flows, termed 'classic' and glacigenic debris flows. This study is based on sparker and deep-tow-boomer seismic profiles collected as part of the British Geological Survey (BGS) offshore regional survey of the United Kingdom, and on geotechnical analysis of gravity cores of maximum length 2 m and vibrocores of maximum length 6 m collected between 200 m and 1200 m water depth as part of the same survey (Strachan & Evans 1991;Talbot et al 1994).…”