The geological and hydrographic charactenstics of the continental slope differ m many ways from those of the continental shelf. In this paper we discuss the nature of sedimentary processes and hazards which occur on the continental slope, with specific reference to key issues such as the recognition and causes of slope instability, the effects of alongslope currents, the scale of continental slope processes, and the problems of dating events.
The geological and hydrographic charactenstics of the continental slope differ in many ways from those of the continental shelf, presenting many unique and challenging problems for the site investigation engineer. In this paper, based on the authors' many years experience in deep-sea sedimentology and survey techniques, we present an overview of continental slope sedimentary processes and their possible implications for site investigation.
The important areas for consideration include the identification of potential hazards (e.g. slope instability or strong alongslope sediment transport), determination of the scale of events, and assessment of their frequency (rare, frequent, episodic or continuous). Seafloor instability leading to slope failure is potentially the most important hazard on the continental slope. Single instability events can affect vast areas of continental margin; for example, the slope failure associated with the 1929 Grand Banks earthquake caused major disruption of the seafloor over an area of at least 160,000 km2 (about the size of England). Causes of historical seafloor instability include earthquakes, high rive sediment discharge, loading and over-steepening of slopes, hurricanes and the building of offshore structures themselves. Free gas or gas hydrates in the sediment may contribute to sediment instability, and, on longer timescales, sealevel change may also be important. Most information on historical seafloor instability comes from records of telephone cable damage and breaks. Older events can be recognised and dated in the sediments themselves and their frequencies can be used in risk assessment. Contour following deep water currents occur on many areas of continental slope. These may be continuous, with velocities reaching 1 m/s or more in extreme cases, or episodic, creating so-called 'benthic storms'. Bottom currents are capable of carrying vast volumes of sediment, causing abrasion, burial or undermining of seafloor structures. The pressure and temperature conditions on many continental margins are conducive to the formation of gas hydrates which may pose a risk when drilled. The large scale of continental margin sedimentary processes is a point of fundamental importance. Coupled with our rather sparse knowledge of many continental slope areas, this demands that regional assessments must be carried out prior to local site surveys, in order to place the latter in their correct large-scale context.
Most sedimentary processes are intermittent in their occurrence, and even the more continuous processes are usually best recognised by their integrated effects on sediment deposition or erosion over time. Thus most attempts to understand the sedimentary regime m any slope area rely on deducing what sedimentary processes have influenced the deposition of the immediate sub-seafloor sediments.