Tore O. Vorren

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.

Glacigenic sediments on a passive continental margin as exemplified by the Barents Sea

Abstract Reflection seismics data, borehole data and gravity cores have been used to map the sedimentary facies of the epicontinental southwestern Barents Sea, its continental margin and adjacent coastal areas. An upper unconformity with a glacially eroded morphology extends over most of the continental shelf. Up to 300 m of stratiform glacigenic sediments overlie this unconformity. These sediments can be grouped into seismic units which are separated by smooth or irregular erosional surfaces. Each sequence may be up to 150 m thick, normally with a lateral extent of 100–300 km. Three dominant seismic signatures may be discerned: (semi)-transparent, stratified and chaotic reflection patterns. These sediments are partly glacifluvial in origin, but on the whole they represent more-or-less tectonized glacimarine deposits. The deeper troughs on the continental shelf contain an undeformed postglacial glacimarine/marine trough-fill. The greatest thickness (up to 1000 m) of glacigenic sediments occurs at the shelf break and below the upper slope. The depocentres are situated at trough-mouth fans. The sediments comprise several prograding sequences with a complex sigmoid-oblique character, indicating alternating upbuilding and depositional bypass/erosion in the topset segment. Small- and large-scale slide scars are observed on the upper slope, and debris lobes on the lower slope. A marked progradation seems to have occurred during the Plio-Pleistocene glacial periods. This progradation also resulted in an unstable slope with various types of sediment gravity flows. Currents flowing along the slope were established during interglacials, and gullying by cold, dense, shelf water and downslope transport occurred. Maximum net sedimentation rates are: ≈0.2 m/ka (continental slope), ≈0.4 m/ka (shelf break), ≈0.1 m/ka (shelf stratiform diamictons), ≈1 m/ka (shelf trough fills) and ≈60 m/ka (fjord trough fills).

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.

FLUCTUATIONS OF THE SVALBARD–BARENTS SEA ICE SHEET DURING THE LAST 150 000 YEARS

Abstract On Spitsbergen, western Svalbard, three major glacial advances have been identified during the Weichselian. All three reached the continental shelf west of the Svalbard archipelago. Radiocarbon, luminescence and amino acid dating of interbedded interstadial and interglacial sediments indicate that these glacial advances have Early (Isotope Stage 5d), Middle (Stage 4), and Late Weichselian ages (Stage 2). An additional, more local, advance has been dated to Isotope Stage 5b. The Late Weichselian ice sheet expanded across the entire Barents Sea. However, in the south-western Barents Sea, the Late Weichselian till is the only till above Eemian sediments, indicating that the Early- and Middle Weichselian ice advances were restricted to the Svalbard archipelago and the northern Barents Sea. A major problem with the onshore sites is the dating of events beyond the range of the radiocarbon method. To overcome this, the onshore record has been correlated with marine cores from the continental slope and the deep-sea west of Svalbard, where a chronology has been established by oxygen isotope stratigraphy. Ice rafted detritus (IRD) was used as the main monitor of glaciation. The IRD record closely mirrors the glaciation history as interpreted from the onshore sections. During the Late Weichselian, the largest IRD peak occurred during deglaciation, a pattern also postulated for the earlier events. Given this, the results from the marine cores indicate that the ages for the first glacial advances during the Weichselian were a few thousand years older than interpreted from the onshore stratigraphy.

Cenozoic erosion and sedimentation in the western Barents Sea

Abstract Seismic reflection data and well logs are used to interpret Cenozoic sedimentary environments in the south-western Barents Sea. Using volumetric calculations and compaction trend analysis, the Cenozoic erosion and sedimentation rates have been quantified. Marine environments, ranging from neritic to bathyal water depths, dominated the southern Barents Sea during the late Palaeocene. At the Palaeocene-Eocene boundary, a southwards progradation commenced along the uplifted and eroded Loppa and Stappen Highs. During the Early Eocene to Mid-Miocene sedimentation continued in the intracratonic basins as well as along the newly established continent-ocean boundary in the embryonic northern Norwegian-Greenland Sea. Substantial erosion occurred on the westernmost margin, particularly on the Stappen High, during this time interval. In the Mid-Miocene, during a period of relative lowstand of sea level, a system of lowstand fans was deposited west of the Stappen High. Later, a pronounced progradation of the shelf edge commenced; during the Mid-Miocene to Late Pliocene the shelf edge prograded between 20 and 40 km. During the Mid-Pliocene to Early Pleistocene the Barents Sea and adjacent land areas were uplifted. A fluvial-glaciofluvial drainage system developed on the continental shelf resulting in pronounced erosion there and very high sedimentation rates on the continental margin. During the last 0.8 Ma several large glaciations on the Barents Sea shelf proper have resulted in overdeepened troughs and high sedimentation rates at the continental shelfbreak and on the continental slope. The overall Cenozoic erosion varies spatially as well as temporally. In the Svalbard area close to 3 km of rocks have been eroded since the Oligocene. The same amount or more was eroded from the Stappen High during the Palaeocene and Early Neogene. In the south-western areas, during the Eocene—Mid-Miocene, erosion occurred along the margin. In addition, the Loppa High and adjacent areas served as sediment source areas. During the Late Pliocene and Pleistoncene the average erosion in the south-western Barents Sea amounted to about 600 m. Erosion was greater in the south-east than to the west and north and was more extensive where the former glacial ice streams had flowed.

The Middle and Late Pleistocence evolution and the Bear Island Trough Mouth Fan

Abstract The evolution of a submarine fan, the Bear Island Trough Mouth Fan, is outlined using high-resolution seismic data. Eight seismic units are identified. The identified units comprise sediments of Middle and Late Pleistocene age. They were probably deposited during eight glacial advances of the Barents Sea Ice Sheet to the shelf break. The units are dominated by a chaotic seismic signature on the upper fan and a mounded seismic facies further downslope. The mounded signature is inferred to reflect large submarine debris flow deposits, probably generated by oversteepening of the upper slope. Unlike many other passive margin fans, glacigenic sediments derived from an ice sheet at the shelf break were the primary sediment input. During interstadials and interglacials the sedimentation rate was reduced markedly. Three large sliding events also influenced the Middle and Late Pleistocene fan growth.

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.

Quaternary sediments and environments on the continental shelf off northern Norway

Abstract Based on lithologic, palaeontologic and chronostratigraphic investigations of close to 200 gravity cores from troughs, deep banks and shallow banks the following late Quaternary environment can be outlined: In the Weichselian, deposition of basal tills was followed by deposition of laminated clay in a sea-ice environment. Later a pebbly pelite was deposited in the troughs at the same time as the banks were iceberg ploughed. Then (13,000 yrs B.P.) a period with incipient winnowing occurred on the deep banks and deposition of sandy pelite took place in the troughs. The Holocene commenced with a marked environmental change due to intrusion of Atlantic water, the fauna changed from arctic to boreal, high-energy winnowing forming a lag deposit took place on the banks, and high accumulation rates in the troughs occurred due to the winnowing and sediment influx from the downwasting continental icesheet. During the later part of the Holocene the winnowing diminished on the deeper banks, on the shallow banks and on the shelf break it still prevails, and in the troughs calcareous sandy mud is being deposited. The surface sediments comprise three main facies, bouldery and pebbly sand, sand, and sandy mud, whose distribution mainly depends on the prevailing bottom current regime. The composition of the older Quaternary sediments is demonstrated by some selected seismic profiles.

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.