Figure 4. Summary of geological processes on continental margins
During the Neogene, the Atlantic margin of NW Europe developed through the interaction of a number of sedimentary processes active on the shelf, slope and basin floor. For the purposes of this summary, these processes can be assigned into three main groups with the key distinction being that between downslope, alongslope and hemipelagic (vertical flux) processes (Stow
et al. 1996). It should be fully appreciated that continental margin systems may include all of these components, albeit in differing proportions in space and time. It is also important to recognise their variation in magnitude, and their frequency. Continuous background processes, unlike larger catastrophic events, may be more difficult to recognise in the geological record, but they remain fundamental to our understanding of the complete system.
Figure 4 schematically illustrates the interaction of the key processes in the context of the NW European glaciated margin. The characteristics of the main sedimentary processes and their interaction are described in the following text:
Downslope processes are synonymous with resedimentation by gravity mass-transport, whereby sediment moves downslope over the sea floor into deeper-water areas driven by gravitational forces. They range in scale from minor creep to major translational slides, and occur over a range of timescales. Sediment creep is a semi-continuous process resulting from load-induced stress, whereas discrete debris flows, slumps and slides may occur with a duration of minutes to hours, and turbidity currents may flow for up to a few hours (high-density flows) or a few days (low-density flows) (Nardin et al. 1979). During a single downslope event these various processes may operate together or in temporal sequence. Continental margins undergoing high-frequency downslope activity, comprising low- to high-magnitude events, are commonly characterised by slope aprons and submarine fans. Slumps, slides, debris flows and turbidites are common constituents of both these environments.
On the NW European margin, downslope processes were particularly active during the Plio-Pleistocene interval associated with shelf-margin progradation. Such progradation was particularly enhanced during shelf glaciation in the last 0.5Ma, when ice streams extended out to the shelfbreak and fed sediment directly onto the slope e.g. North Sea Fan, Barra/Donegal Fan
(Fig. 3) (King
et al. 1996, 1998; Vorren & Laberg 1997; Vorren et al. 1998; Armishaw
et al. 2000; Sejrup et al. 2000). Thick packages of debris flows commonly comprise the slope apron, which has locally undergone episodic failure, e.g. Storegga Slide
(Fig. 3) (Britsurvey 1999; Haflidason
et al. 2000, 2001).
Bottom currents driven by thermohaline circulation greatly influence both sediment accumulation and erosion on continental margins. The effects of thermohaline circulation are particularly marked on slopes and basin plains adjacent to continental margins, where the varied sea-floor bathymetry may locally intensify and focus bottom-current activity. Sediment drifts and associated bedforms, such as sediment waves, are a major depositional product of bottom currents, and commonly accumulate where there is a change in the gradient of the sea bed, such as at the base of continental slopes or within other depressions including slide scars (Laberg
et al. 2002). The deposits of bottom currents are collectively referred to as contourites. Where particularly strong, bottom currents may cause erosion of the sea bed and the formation of channels and moats.
On the NW European margin, bottom currents have been influencing sedimentation throughout the Neogene interval, and contourites represent a significant part of the sediment record. Away from downslope depocentres, contourite deposition is commonly the dominant component of the lower slope and basin floor deposits. Contourites characteristically form plastered sediment drifts on the slopes, with upslope accretion locally contributing to the shelf-margin construction. These are common in the Rockall Trough, Faroe-Shetland Channel, Vøring Plateau and Lofoten Basin (Stoker
et al. 1998; Laberg et al. 2002). Mounded and sheeted drifts accumulating on the adjacent basin floor locally develop into giant, elongate drifts, such as the Feni Ridge in the southern Rockall Trough
(Fig. 3) (Stoker
et al. 2001). By way of contrast, active sea-bed erosion has prevailed locally throughout the late Neogene interval, e.g. NW Rockall Trough (Howe
et al. 2001)
Hemipelagic processes (vertical flux)
Hemipelagic sedimentation is the primary 'background' process on continental margins, and results in fine-grained, biogenic (carbonate and silica) and non-biogenic, including ice-rafted, sediments formed by a combination of vertical settling and slow lateral advection. Under relatively tranquil depositional conditions they characteristically form a sediment drape, which is indicative of a reduction in the sediment supply from the shelf. On the NW European margin, this style of sedimentation has remained relatively constant throughout the Neogene, although its expression is commonly obscured by the volumetrically greater accumulations of downslope and alongslope deposits.
No part of the continental margin receives sediment from one process in isolation. Oceanographic circulation, albeit of varying intensity, has remained a constant factor and sediment supplied from downslope or hemipelagic processes will to some extent have been entrained in contour-following currents and deposited within this current regime. Fine-grained sediments are deflected from downslope depocentres to form contourite accumulations elsewhere in the basin. Where alongslope sediments interdigitate or overlap with downslope deposits, a mixed sediment drift–slope-apron depositional system is developed, e.g. NE slope of Rockall Trough (Armishaw
et al. 2000; Stoker et al. 2001). Bottom-current reworking of the present-day sea bed, may result in the remobilisation and deposition of thin sandy or muddy contourite sheets, and the partial/complete burial of the prograding-wedge deposits, e.g. lower West Shetland Slope (Leslie & Long 2001). Alternatively, winnowing of fines may create a residual, gravel-lag contourite (Howe
et al. 2001). These thin-bedded contourites commonly form part of the clinoform-reflecting units that separate discrete packages of stacked debris-flows in the prograding wedges (Stoker 1995).
These surfaces may form detachment layers for slides during episodic
mass-failure of the continental margin.