Neil H. Kenyon

THE NORWEGIAN–GREENLAND SEA CONTINENTAL MARGINS: MORPHOLOGY AND LATE QUATERNARY SEDIMENTARY PROCESSES AND ENVIRONMENT

The continental margins surrounding the Norwegian–Greenland Sea are to a large degree shaped by processes during the late Quaternary. The paper gives an overview of the morphology and the processes responsible for the formation of three main groups of morphological features: slides, trough mouth fans and channels. Several large late Quaternary slides have been identified on the eastern Norwegian–Greenland Sea continental margin. The origin of the slides may be due to high sedimentation rates leading to a build-up of excess pore water pressure, perhaps with additional pressure caused by gas bubbles. Triggering might have been prompted by earthquakes or by decomposition of gas hydrates. Trough mouth fans (TMF) are fans at the mouths of transverse troughs on presently or formerly glaciated continental shelves. In the Norwegian–Greenland Sea, seven TMFs have been identified varying in area from 2700 km2 to 215 000 km2. The Trough Mouth Fans are depocentres of sediments which have accumulated in front of ice streams draining the large Northwest European ice sheets. The sediments deposited at the shelf break/upper slope by the ice stream were remobilized and transported downslope, mostly as debris flows. The Trough Mouth Fans hold the potential for giving information about the various ice streams feeding them with regard to velocity and ice discharge. Two large deep-sea channel systems have been observed along the Norwegian continental margin, the Lofoten Basin Channel and the Inbis Channel. Along the East Greenland margin, several channel systems have been identified. The deep-sea channels may have been formed by dense water originating from cooling, sea-ice formation and brine rejection close to the glacier margin or they may originate from small slides on the upper slope transforming into debris flows and turbidity currents.

Large-scale sedimentation on the glacier-influenced polar North Atlantic Margins: Long-range side-scan sonar evidence

Long-range side-scan sonar (GLORIA) imagery of over 600,000 km² of the Polar North Atlantic provides a large-scale view of sedimentation patterns on this glacier-influenced continental margin. High-latitude margins are influenced strongly by glacial history and ice dynamics and, linked to this, the rate of sediment supply. Extensive glacial fans (up to 350,000 km³) were built up from stacked series of large debris flows transferring sediment down the continental slope. The fans were linked with high debris inputs from Quaternary glaciers at the mouths of cross-shelf troughs and deep fjords. Where ice was slower-moving, but still extended to the shelf break, large-scale slide deposits are observed. Where ice failed to cross the continental shelf during full glacials, the continental slope was sediment starved and submarine channels and smaller slides developed. A simple model for large-scale sedimentation on the glaciated continental margins of the Polar North Atlantic is presented.

Sand ribbons of European tidal seas

Abstract Ribbons of sediment, up to 15 km long and up to 200 m wide, extend parallel to the bed load transport paths of the tidal currents. The ribbons have been found by the side-scan Asdic method in most of those areas of the sea floor around the British Isles where tidal currents have maximum near surface velocities of 2 knots or more. The four types of ribbons, distinguished by their form, are related to tidal current velocity and supply of sand. Underwater photographs taken in a sand ribbon zone reveal bodies of shelly sand usually too thin to mask completely the underlying coarse material, but sometimes thick enough for the formation of small-scale bedforms including ripple marks and sand shadows. There is a variety of underlying coarse material and locally there are accumulations between the sand ribbons of the more durable shells that may be more readily preserved in ribbon form than the sand itself.

Bed forms of the Mediterranean undercurrent observed with side-scan sonar

Abstract Details of bed forms beneath the Mediterranean undercurrent in the Gulf of Cadiz are depicted on sonographs (side-scan sonar views to maximum slant ranges of 1 km, 7 km and 14 km). A sequence of bed-form zones, largely comparable with that of strongly tidal continental-shelf seas, is found to depths of about 1400 m. Thus, with decreasing current velocity, measured near the bottom, there are zones of scour hollows, sand ribbons and sand waves, with related deposits. Beyond the sand-wave zone are smooth mud deposits where the undercurrent is in contact with the floor, and large, muddy sediment waves up to 40 m high where it is intermittently in contact with the bottom. These large, muddysediment waves are a bed form not found in the tidal-current environment, and are possibly formed by a combination of fall-out from suspension together with occasional bed movement as the base of the undercurrent fluctuates in depth. Variations in the bedding style in the deposits beneath the mud waves are figured on sub-bottom profiles. A stratigraphic boundary between an acoustically transparent layer, possibly representing pelagic sedimentation, and the overlying bedded sequence may define the age of commencement of the undercurrent. Flow paths, including several downslope trending channels, are influenced by gravity, Coriolis force and topography.

Mass-wasting features on the continental slope of Northwest Europe

Abstract A reconnaissance of the continental slope between the Porcupine Seabight and northern Norway was made using a medium-range sidescan sonar, the GLORIA long-range sidescan sonar and an air-gun seismic profiler. Using these methods the larger features indicative of slope failure have been detected. Submarine canyons and their second- and third-order gullies are inferred to be the site of multiple slope failures. A pair of 200 km long slope-parallel scarps, believed to be due to shallow rotational faulting, and several groups of slope-parallel slump folds have been mapped. Downslope-trending erosional forms range in size from the Storegga Slide, which has a displaced volume of about 5700 km 3 , to submarine canyons, to groups of small slides that have displaced less than 1 km 3 of sediment. Apart from the Storegga Slide, large slides are relatively scarce between 57 and 67°N. The factors which are believed to control the distribution of slope failures in this region include connection with drainage basins, slope gradient (failures are common where the gradient is > 1.5°) and sedimentation rate. Where there is a strong contour current along the upper continental slope, sedimentation rate is low, deeps tend to be rapidly filled in and failure is inhibited. Some understanding of the gravitational processes responsible can be inferred from the superficial morphology but little is known of the age or potential for future failure of these features.

The Andoya Slide and the Andoya Canyon, north-eastern Norwegian-Greenland Sea

Based on GLORIA side-scan sonar imagery, echo sounder records, 3.5 kHz profiles, multichannel seismics and gravity cores the Andoya Slide and Andoya Canyon, north-eastern Norwegian–Greenland Sea were mapped and interpreted. The Andoya Slide covers an area of about 9700 km2 of which the slide scar area comprise ca. 3600 km2. The slide has a total run-out distance of about 190 km. Slope failure is inferred to have occurred during the Holocene because the slide scar has prominent relief on the present sea floor. The area of sediment removal is characterised by an irregular relief were relatively consolidated sediments are exposed at the sea floor. Little or no unconsolidated sediments overlies the slide deposits. Earthquake activity is inferred to have triggered the slide. A Holocene age of the Andoya Slide implies that three giant slides (the Storegga, Traenadjupet and Andoya Slides) have occurred along the continental slope of Norway during the last 10,000 years. A large canyon, the Andoya Canyon, is located immediately south of the Andoya Slide. On the upper slope, the canyon has been incised about 1000 m in the bedrock, and the maximum width at the bottom and between the canyon shoulders is 2 and 12 km, respectively. The Andoya Canyon represents the upper part of the Lofoten Basin Channel. Based on analogy with other deep-sea canyon/channel systems, the Andoya Canyon/Lofoten Basin Channel is possibly of pre-Quaternary age. Holocene sediments recovered from within the canyon, and draping the flanking channel deposits, indicate that the Andoya Canyon is not presently active and has probably not been active during the Holocene. During the Holocene, the canyon acted as a trap for sediments settling from the winnowing Norwegian Current.

Evidence from bedforms for a strong poleward current along the upper continental slope of northwest Europe

Abstract Medium-range sidescan sonographs show a variety of longitudinal and transverse sand bedforms on the upper continental slope west of Scotland and Norway. The asymmetry of 0.5–2 m high barchanoid sand waves, confirms the poleward direction of the current. The style of the bedforms is consistent with peak current speeds measured during the CONSLEX campaign. Comet marks/narrow sand ribbons occur where speeds exceed about 75 cm s−1 and sand waves where speeds exceed about 40 cm s−1. The distribution of bedforms and the position of the “mudline” sand/mud boundary) show that the core of the current reaches down to 400–600 m south of the sill on the Wyville Thomson Ridge and down to 600–800 m in the Norwegian Sea.

Ground‐truthing 6.5‐kHz side scan sonographs: What are we really imaging?

A 1000-km2 area on the distal lobe of Monterey Fan shows a digitate pattern of juxtaposed high and low backscatter on GLORIA side scan sonographs. This area was investigated using stereo photography, high-resolution seismic profiles, and measurements of physical properties of cores to quantitatively evaluate the causes of backscatter from the 6.5-kHz side scan sonar. Stereo photography and bottom video were used to determine that the sediment-water interface typically has a bed roughness less than 10 cm over the entire ground truth area; consequently, bed roughness is not a significant contributor to the sonar backscatter. Vertical-incidence 3.5-kHz profiles reveal that high-backscatter areas allow less penetration and have slightly more relief than low-backscatter areas. Closely spaced measurements of p wave velocity, density, and grain size made on transponder-navigated cores are used to investigate the geoacoustic properties of the sediment with the aid of a numerical model. The model results demonstrate that the sediment-water interface is, in most cases, acoustically transparent to the sonar energy and that most or all of the energy is refracted into the sediment to depths of at least a few meters rather than scattered from the surface. The variability of the dominant lithofacies (sands with minor silty clays or silty clay with minor sands) accounts for a regional correlation with backscatter when viewed over hundreds to thousands of square kilometers, but at the detailed scale of hundreds to a few square meters, we have not yet been able to identify a correlation of backscatter intensity with the angle of incidence, surface lithology, measured acoustic impedance, or attenuation. However, there is a qualitative, but unexpected, correlation with lithostratigraphic variability. In this area, thick (up to 50 cm) sand deposits with thin interbeds of silty clay correlate with lower backscatter than do silty clay deposits with thin interbeds of sand. This suggests that volume inhomogeneities and complex constructive and destructive interferences caused by the subsurface volume inhomogeneities within the top few meters of the sediment ultimately modulate the intensity of backscatter. The ground-truthing effort was further complicated by trying to compare the very large sonar footprint of a pixel (∼70,000 m2) relative to the area of an individual core (0.006 m2), bottom photograph (∼5 m2), or seismic profile. Although 6.5-kHz sonographs appear easy to interpret in a conventional and simplistic manner, caution should be used when interpreting lithofacies from backscatter intensities.

Structural grain, mud volcanoes and other features on the Barbados Ridge Complex revealed by Gloria long-range side-scan sonar

Abstract A long-range side-scan sonar survey of the broad band of deformed sediments east of the Antilles Volcanic Arc has revealed the detailed structural trends of folds and reverse faults or thrusts in plan view for the first time. The prevalent trend lies substantially parallel with the arc and is simplest near to the deformation front at the eastern edge of these deformed sediments. These trends are interrupted locally by more or less transverse zones of disturbance associated with ridges in the oceanic basement that are obstructing the relative eastward movement of the deformation front. Local rough-surfaced mounds, interpreted as mud volcanoes underlain by diapirs, are common and occur much further to the north than the mud diapir field already described near Trinidad. Only one large sediment slide has been identified.

Characteristics of a sandy depositional lobe on the outer Mississippi fan from SeaMARC IA sidescan sonar images

SeaMARC IA sidescan sonar images of the distal reaches of a depositional lobe on the Mississippi Fan show that channelized rather than unconfined transport was the dominant transport mechanism for coarse-grained sediment during the formation of this part of the deep-sea fan. Overbank sheet flow of sands was not an important process in the transport and deposition of the sandy and silty sediment found on this fan. The dendritic distributary pattern and the high order of splaying of the channels, only one of which appears to have been active at a time, suggest that coarse-grained deposits on this fan are laterally discontinuous.