Jan Sverre Laberg

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.

Trough mouth fans — palaeoclimate and ice-sheet monitors

Abstract Trough mouth fans are fans at the mouth of transverse troughs/channels on glaciated continental shelves. On the northwest European glaciated continental margin, eight trough mouth fans, varying in size between 2700 and 215,000 km 2 have been identified. The trough mouth fans are depocentres dominated by debris flows accumulated in front of ice streams draining the former large northwest European ice sheets. The debris flow units are separated by hemmipelagic interglacial/interstadial sediments. it is inferred that the number of debris-flow units record the number of shelfbreak-positions of the ice sheet margins: the number found varies between three in the south and eight in the north. Typical trough mouth fans and related debris-flow units seem to have been formed later than the early mid-Pleistocene, thus the north European ice sheets did not form ice streams extending to the shelfbreak in any appreciable length before the mid-Pleistocene. Besides being loci for sediment deposition, the trough mouth fans were also the main sites of fresh water supply to the ocean (in the form of icebergs) during the mid/late Pleistocene ice ages.

Late Weichselian submarine debris flow deposits on the Bear Island Trough Mouth Fan

Sedimentary processes on the Bear Island Trough Mouth Fan during the last glacial were studied using high resolution reflection seismics and gravity cores. The fan succession is dominated by large debris flow deposits of up to 24 km in width and 50 m thick. Debris lobes can be followed over 100 km downslope and the largest cover an area of 1880 km2. The large debris flows were generated when the Barents Sea Ice Sheet reached the shelf break. Glacigenic sediments transported to the grounding-line were temporarily stored on the upper slope. Due to the high sedimentation rate, the sediments were unstable and earthquakes, oversteepening and/or build up of excess pore pressure triggered sediment release generating large debris flows. Sediments were also eroded and incorporated during the downslope flow. Debris flows were the most important sediment distributing process due to the relatively high shear strength, low flow mobility of the glacigenic diamicton, and the low fan gradient. The Weichselian evolution of the fan was characterized by a relatively low sedimentation rate during the Early and Mid Weichselian. The Barents Sea Ice Sheet might have reached the shelf break twice during the Late Weichselian. On average one large debris flow, comprising 18 km3 of sediments, was released between every 35 to 75 years during the last glacial maximum.

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.

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.

Seismic analyses of Cenozoic contourite drift development in the Northern Norwegian Sea

Four drift accumulations have been identified on the continental margin of northern Norway; the Lofoten Drift, the Vesterålen Drift, the Nyk Drift and the Sklinnadjupet Drift. Based on seismic character these drifts were found to belong to two main groups; (1) mounded, elongated, upslope accretion drifts (Lofoten Drift, Vesterålen Drift and Nyk Drift), and (2) infilling drifts (Sklinnadjupet Drift). The drifts are located on the continental slope. Mainly surface and intermediate water circulation, contrary to many North Atlantic and Antarctic drifts that are related to bottom water circulation, and sediment availability have controlled their growth. Sediments were derived both from winnowing of the shelf and upper slope and from ice sheets when present on the shelf. The main source area was the Vøring margin. This explains the high maximum average sedimentation rate of the nearby Nyk (1.2 m/ka) and Sklinnadjupet (0.5 m/ka) Drifts compared with the distal Lofoten (0.036 m/ka) and Vesterålen (0.060 m/ka) Drifts. The high sedimentation rate of the Nyk Drift, deposited during the period between the late Saalian and the late Weichselian is of the same order of magnitude as previously reported for glacigenic slope sediments deposited during glacial maximum periods only. The Sklinnadjupet Drift is infilling a paleo-slide scar. The development of the infilling drift was possible due to the available accommodation space, a slide scar acting as a sediment trap. Based on the formation of diapirs originating from the Sklinnadjupet Drift sediments we infer these sediments to have a muddy composition with relatively high water content and low density, more easily liquefied and mobilised compared with the glacigenic diamictons.

Neogene evolution of the Atlantic continental margin of NW Europe (Lofoten Islands to SW Ireland): anything but passive

A regional stratigraphic framework for the Neogene succession along and across the NW European margin is presented, based on a regional seismic and sample database. The stratigraphy provides constraints on the timing and nature of the mid- to late Cenozoic differential tectonic movements that have drivenmajor changes in sediment supply, oceanographic circulation and climate (culminating in continental glaciation). The overall context for Neogene deposition on the margin was established in the mid-Cenozoic, when rapid, km-scale differential subsidence (sagging) created the present-day deep-water basins. The Neogene is subdivided into lower (Miocene–lower Pliocene) and upper (lower Pliocene–Holocene) intervals. The lower Neogene contains evidence of early to mid-Miocene compressive tectonism, including inversion anticlines and multiple unconformities that record uplift and erosion of basin margins, as well as changes in deep-water currents. These movements culminated in a major expansion of contourite drifts in the mid-Miocene, argued to reflect enhanced deep-water exchange across the Wyville-Thomson Ridge Complex, via the Faroe Conduit. The distribution and amplitude of the intra-Miocene movements are consistent with deformation and basin margin flexure in response to enhanced intra-plate compressive stresses during a local plate reorganization (transfer of the Jan Mayen Ridge from Greenland to Europe). The upper Neogene records a seaward tilting (

A Late Pleistocene submarine slide on the Bear Island Trough Mouth Fan

A large submarine slide on the southern flank of the Bear Island Trough Mouth Fan, southwestern Barents Sea continental slope, has a run-out distance of about 400 km, a total volume of about 1100 km3, and is younger than 330 ka. Three seismic units, comprising mainly hemipelagic sediments has partly filled the slide scar. An increased sedimentation rate on the Bear Island Trough Mouth Fan from Late Pliocene time, probably in combination with abundant earthquakes, is the most likely cause of the slide. Based on these and previous studies, we suggest that large-scale slides were important sediment transport processes during Plio-Pleistocene.

The glacier-influenced Scoresby Sund Fan, East Greenland continental margin: evidence from GLORIA and 3.5 kHz records

A major submarine fan (gradient about 2 °) offshore of the Scoresby Sund fjord system is indicated by the crescentic shape of the shelf break. GLORIA long-range side-scan sonar imagery was obtained over about 20,000 km2 of the fan along with 1000 km of 3.5 kHz records. Three acoustic facies were defined from GLORIA backscatter signatures and sea-floor morphology and sediment acoustic character on 3.5 kHz records. Facies 1 includes a series of acoustically transparent features (0.5–2 km in width), elongate downslope, with irregular surface topography, which are interpreted as debris flows. This makes up the bulk of relatively recent sedimentation on the upper fan. Diamictic sediments in a core support a debris-flow origin. Facies 2 is featureless on GLORIA images. 3.5 kHz profiles reveal irregular former sea-floor morphology, above which is a draping unit (< 15 m thick). This northern region of the fan is a less active area where hemipelagic sediments and limited ice-rafted debris overlie older material formed by past debris-flow activity. The more distal area of the adjacent ocean basin has a flat floor with parallel sub-bottom reflectors of Facies 3. This facies is probably an area of low-energy hemipelagic sedimentation, punctuated by occasional ice rafting and turbidity current activity. The debris flows interpreted from GLORIA and 3.5 kHz data are basic building blocks in the long-term development of the Scoresby Sund Fan. Glacier-influenced fan volume is about 15,000 ± 5000 km3, based on seismic reflection studies. During full glacials in East Greenland, the inland ice sheet advances to fill the Scoresby Sund fjord system and extends across the shelf to reach the shelf break in some glacial cycles. Debris flows form in areas of most rapid sediment flux. The Scoresby Sund Fan is relatively similar to the Storfjorden Fan on the eastern Polar North Atlantic margin, but differs from the larger Bear Island Fan in having a steeper fan gradient, much smaller debris flows and no large-scale slides.

Late Cenozoic erosion of the high-latitude southwestern Barents Sea shelf revisited

The southwestern Barents Sea has experienced profound erosion during the last ∼2.7 m.y. that has resulted in the development of a characteristic glacial morphology of the continental shelf and deposition of a several-kilometer-thick sediment fan along the western margin prograding into the deep sea. In the period from ca. 2.7 to 1.5 Ma, proglacial processes, including fluvial and glaciofluvial erosion, dominated. For this period, the total average erosion of the shelf was 170–230 m, the average erosion rate was 0.15–0.2 mm/yr, and the average sedimentation rates on the fan were 16–22 cm/k.y. Subglacial erosion affected an area of ∼575,000 km 2 during the period from ca. 1.5 to 0.7 Ma. Total average erosion is estimated at 330–420 m for this interval, and the average erosion rate was 0.4–0.5 mm/yr. Average sedimentation rates were 50–64 cm/k.y. During the last ∼0.7 m.y., glacial erosion mainly has occurred beneath fast-flowing paleo-ice streams topographically confined to troughs (∼200,000 km 2 ). The total average erosion is estimated at 440–530 m, average erosion rate is 0.6–0.8 mm/yr, and average sedimentation rate on the continental slope is 18–22 cm/k.y. The amount of erosion was mainly determined by the duration of the glaciations and the location, velocity, and basal properties of the ice streams. In total, glacial erosion of the troughs has been relatively high throughout the last ∼2.7 m.y. at ∼1000–1100 m. For the banks, erosion is inferred to have increased from ca. 2.7 Ma to a peak between 1.5 and 0.7 Ma. Subsequently, little erosion occurred in these areas, which implies a total of 500–650 m of erosion. Compared with other high-latitude areas, our rates are among the highest so far reported. This comparison also demonstrates that there have been large variations in the rate of sediment delivery to the glaciated continental margins.