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Continental Margin Sedimentation, with Special Reference to the North-East Atlantic Margin

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Continental Margin Sedimentation, with Special Reference to the North-East Atlantic Margin Sedimentology (2000) 47 (Suppl. 1), 239±256 Continental margin sedimentation, with special reference to the north-east Atlantic margin PHILIP P. E. WEAVER, RUSSELL B. WYNN, NEIL H. KENYON andJEREMY EVANS Southampton Oceanography Centre, Empress Dock, Southampton SO14 3ZH, UK (E-mail: [email protected]) ABSTRACT The north-east Atlantic continental margin displays a wide range of sediment transport systems with both along-slope and down-slope processes. Off most of the north-west African margin, south of 26°N, upwelling produces elevated accumulation rates, although there is little ¯uvial input. This area is subject to infrequent but large-scale mass movements, giving rise to debris ¯ows and turbidity currents. The turbidity currents traverse the slope and deposit thick layers on the abyssal plains, while debris ¯ows deposit on the continental slope and rise. From the Atlas Mountains northwards to 56°N, the margin is less prone to mass movements, but is cut by a large number of canyons, which also funnel turbidity currents to the abyssal plains. The presence of a lithospheric plate boundary off SW Iberia is believed to have led to high rates of sediment transport to the deep sea. Even larger quantities of coarse sediments have fed the canyons and abyssalplainsin the Bay of Biscay as a result of drainage from melting icecaps. Bottom currents have built sediment waves off the African and Iberian margins, and created erosionalfurrows south of the Canaries. The Mediterranean out¯ow is a particularly strong bottom current near the Straits of Gibraltar, depositing sand waves and mud waves in the Gulf of Cadiz. North of 56°N, the margin is heavily in¯uenced by glacial and glaciomarine processes active during glacial times, which built glacial trough-mouth fans, such as the North Sea Fan, and left iceberg scour marks on the upper slope and shelf. Over a long period, especially during interglacials, this part of the margin has been greatly affected by along-slope currents, with less effect by turbidity currents than on the lower latitude margins. Large-scale mass movements are again a prominent feature, particularly off Norway and the Faeroes. Some of these mass movements have occurred during the Holocene, although high glacial sedimentation rates may have contributed to the instability. Keywords: Continentalmargin, sediments, slopeprocesses, debris ¯ow, turbidity current, bottom current INTRODUCTION low gradient continental slopes, and areas with steep continentalslopes. In this paper, we describe the sedimentary Continentalmargins are important areas for processes, and map the major sedimentary fea- investigation, since they form a `new frontier' for tures, on the north-east Atlantic continental exploitation by the hydrocarbons industry, and margin from north-west Africa to northern increasing numbers of telecommunication cables Norway (Fig. 1). This region includes areas of are laid across them. Nevertheless, many of the passive margin with low sediment input, areas sedimentary processes displayed here remain with high glacial sedimentation, areas incised by poorly understood. Major advances in our knowl- canyons, areas of fan sedimentation, areas with edge of continental margins have followed the # 2000 InternationalAssociation of Sedimentologists 239 240 P. P. E. Weaver et al. development and deployment of long-range side- occurs along one or several surfaces (see Hampton scan sonars, such as GLORIA, which can image et al., 1996, for a thorough review). Many sub- large areas in relatively short time intervals. marine slides disintegrate down-slope into debris Further advances have been achieved following ¯ows which are described as the movement of the development of much higher-resolution deep- granular solids, sometimes mixed with minor towed side-scan, and of swath bathymetry. There amounts of entrained water on a low slope. The has been some ocean drilling of passive con- typicalend product is a mass of displacedblocks tinental margins by Deep Sea Drilling Project of sediment or rock embedded in a more highly (DSDP) and the Ocean Drilling Program (ODP), disrupted matrix. Debris avalanches are used here especially of modern fans (Legs 96, 116, 155), to describe submarine slides where little or no sediment drifts (e.g. Legs 94, 172, 181), and water entrainment occurs, and the resulting run- abyssal plains (Legs 149, 157). Down-slope out distances are less. High-latitude slopes com- processes have not been speci®cally targetted monly consist of stacked debris ¯ows (Vorren except in Legs 150 and 174 A, both on the New et al., 1998). Such ¯ows have been studied in the Jersey margin, where a down-slope transect of ®eld and experimentally (e.g. Mohrig et al., 1999), cores was drilled. and are believed to ¯ow on a lubricating layer of In this paper, we have limited our discussion to water between the ¯ow and the underlying sedimentary processes operating from the shelf materialÐa process referred to as hydroplaning. edge to the abyssal plain, so including the continentalslope,continentalrise and abyssal Turbidity currents plain. In all cases, it is important to include the sediments of the abyssalplainsas part of the Turbidity currents occur on a variety of scales, continentalmargin, because the plainsare formed and are a common feature of many continental by sediments derived from the margin. Within margins. The largest turbidity currents form this area, the dominant sedimentary processes are deposits in the deep ocean, and are termed along-slope sediment transport, which builds megaturbidites; they are often linked to mass sediment drifts and waves, and down-slope movements occurring higher up the slope. These processes involving mass wasting (sediment currents transport tens to hundreds of cubic slides, debris ¯ows and turbidity currents). In kilometres of sediment, and appear to be able to addition, canyons can be incised into the slope, travel immense distances over extremely low- in some cases cutting across the shelf, and large gradient slopes. They dump their sediment load submarine fans may be built off major points of only when they become ponded on the ¯at sediment supply. The scale of some of the abyssal plains at the base of the slope. processes can be very large; for example, the Individualmud-dominated turbidites commonly Storegga slide off Norway has an estimated form layers up to a few metres in thickness over volume of 5700 km3 (Bugge et al., 1988), while a the whole abyssal plain (Pilkey, 1987; Rothwell single turbidite in the Madeira Abyssal Plain has et al., 1992). a volume in excess of 300 km3 (Weaver et al., On the continentalslopeand rise, turbidity 1992, 1995). currents are usually funnelled down-slope by canyons and channels. Canyons are generally V- shaped in pro®le and are often eroded into bedrock. In plan view, they display tributary MAIN SEDIMENTARY PROCESSES canyon systems with a number of orders of tributary, including steep, gullied slopes. They Submarine slides and debris ¯ows may originate offshore of a sediment point source, Sediment instability resulting in submarine slides such as a river, and generally occur on, or just and debris ¯ows is very common on some seaward of the shelf-break. Canyons commonly continentalmargins, and singleevents are fre- pass down-slope into channels, which are shal- quently very large (of the order of hundreds of lower than canyons, more U-shaped in pro®le, square kilometres). Should failure occur, then the and may have depositionalelements such as ocean ¯oor offers unimpeded slopes and ¯at- levees and aggradational ¯oors. Deep-water chan- ¯oored basins hundreds of kilometres in length, nels occur in a variety of forms, from deep, allowing ¯ow over enormous distances. Slides are narrow erosive channels, to broad, shallow dis- de®ned as the downward and outward movement tributary channels. Meandering channels tend to of slope-forming materials, wherein shear failure occur on fans with gentle gradients and a # 2000 InternationalAssociation of Sedimentologists, Sedimentology, 47 (Suppl. 1), 239±256 Continentalslopesedimentation 241 # 2000 InternationalAssociation of Sedimentologists, Sedimentology, 47 (Suppl. 1), 239±256 242 P. P. E. Weaver et al. # 2000 InternationalAssociation of Sedimentologists, Sedimentology, 47 (Suppl. 1), 239±256 Continentalslopesedimentation 243 dominantly ®ne-grained sediment supply (e.g. ¯ows and megaturbidites) (Fig. 2c). This is be- Clark et al., 1992). Both canyon and channel cause of a number of factors, including low ¯oors can have bedforms such as graveland sand terrigenous input and local upwelling. This part waves and erosionalscours. of the margin receives very little ¯uvial sediment, Turbidity currents generally deposit the ma- particularly the margin off the western Sahara. jority of their load on turbidite fans. The rivers However, a number of upwelling cells (Sarnthein running into the north-east Atlantic are rela- et al., 1982) give rise to high accumulation rates tively small and the resultant fans are usually along the upper slope and shelf edge. The built near the foot of the slope, beyond the megaturbidites derived from this margin and canyon mouths. deposited on the Madeira AbyssalPlainare distinguished by their high organic contents (0´5±2%; Rothwell et al., 1992), suggesting that Bottom currents it is the sediments resulting from the upwelling Bottom currents often play a major role in which are unstable, presumably as a result of continentalmargin sedimentation. They can slope oversteepening. erode, mould, transport and redistribute sedi- The upper parts of submarine slides often show ments supplied to the slope and rise by down- spectacular detail when imaged with high-resolu- slope ¯ows and vertical settling. Bottom current tion side-scan sonar (Fig. 3), or swath bathymetry. features include erosional furrows and moats, and The Saharan Slide off west Africa (Simm & Kidd, depositionalbedforms, such as contourite drifts 1984; Masson et al., 1992) included about 600 km3 and sediment waves. of sediment, which was transported up to 700 km Contourite drifts are the primary deposits down-slope, across gradients ranging from 1´5° to of bottom currents, and are widespread through- 0´1°.
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