On the origin of large shelf embayments on glaciated margins—effects of lateral ice flux variations and glacio-dynamics west of Svalbard

Glaciated continental shelves are characterised by large amphitheatre-like embayments between prominent cross-shelf troughs. The integration of swath bathymetry and high-resolution seismic data (3D, 2D) collected across the western Svalbard continental margin indicates how such embayments form. Although their bathymetric expression resembles headwall scarps of submarine slope failures, the shelf embayments are the result of the interplay between sediment dynamics and transport underneath fast-moving ice streams in the cross-shelf troughs and the slower-moving parts of the ice sheets on the adjacent shallower shelf banks during full glacial conditions. This is supported by (1) the absence of major landslide deposits at their toe, (2) continuous prograding shelf deposition and (3) absence of landslide-related faulting. Instead, the seismic data suggest a depositional origin of the shelf embayments that is characterised by continuous sediment input at lower rates off a slow-moving ice mass in the centre of the embayment which is fringed by the lateral ice-stream ridges. These findings put into perspective the importance of submarine slope failure on glaciated margins

A modern canyon-fed sandy turbidite system of the Norwegian continental margin

The sedimentary processes and evolution of a presumably old but still relatively recent active canyon-fed sandy turbidite system on the Norwegian continental margin, the Andoya Canyon - Lofoten Basin Channel system, were studied using high-resolution seismic, deep-towed side-scan sonar records and cores (gravity and vibro cores). The canyon has a length of about 40 km and represents a continental slope incision of up to 1100 m. From the canyon mouth, a deep-sea channel up to 30 m deep and 3 km wide continues for approximately 200 km into the deepest part of the Lofoten Basin. The most recent activity of this system was deposition of coarse-grained sediments from turbidity currents as indicated by sediment waves on the canyon floor, thin sand layers within the levees and a sandy lobe at the channel mouth. From this modern system we suggest that there are a number of features that could be of relevance for hydrocarbon exploration of deep-water turbidite systems: a) this canyon-fed system probably received sand from piracy of shelf sediments and/or canyon wall erosion, b) the sandy deposits are located in the deepest part of the basin, about 200 km outside the canyon mouth, c) the sandy lobe is connected to the source area by a straight channel with poorly developed mud-rich levees, d) the sandy deposits have a sheet-like or tabular geometry, e) post-depositional deformation of the sand could be widespread suggesting rapid sand deposition. Thus, modern, canyon-fed sandy turbidite systems provide additional data relevant for future hydrocarbon prospecting in deep-water areas

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 Andøya Slide and Andøya Canyon, north-eastern Norwegian–Greenland Sea were mapped and interpreted. The Andøya 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 Andøya Slide implies that three giant slides (the Storegga, Trænadjupet and Andøya Slides) have occurred along the continental slope of Norway during the last 10,000 years. A large canyon, the Andøya Canyon, is located immediately south of the Andøya 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 Andøya Canyon represents the upper part of the Lofoten Basin Channel. Based on analogy with other deep-sea canyon/channel systems, the Andøya 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 Andøya 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

Episodic Cenozoic tectonism and the development of the NW European 'passive' continental margin

The North Atlantic margins are archetypally passive, yet they have experienced post-rift vertical movements of up to kilometre scale. The Cenozoic history of such movements along the NW European margin, from Ireland to mid-Norway, is examined by integrating published analyses of uplift and subsidence with higher resolution tectono-stratigraphic indicators of relative movements (including results from the STRATAGEM project). Three episodes of epeirogenic movement are identified, in the early, mid- and late Cenozoic, distinct from at least one phase of compressive tectonism. Two forms of epeirogenic movement are recognised, referred to as tilting (coeval subsidence and uplift, rotations <1° over distances of 100s of Kilometres) and sagging (strongly differential subsidence, rotations up to 4° over distances <100 km). Each epeirogenic episode involved relatively rapid (<10 Ma) km-scale tectonic movements that drove major changes in patterns of sedimentation to find expression in regional unconformity-bounded stratigraphic units. Early Cenozoic tilting (late Paleocene to early Eocene, c. 60–50 Ma) caused the basinward progradation of shelf-slope wedges from elongate uplifts along the inner continental margin and from offshore highs. Mid-Cenozoic sagging (late Eocene to early Oligocene, c. 35–25 Ma) ended wedge progradation and caused the onset of contourite deposition in deep-water basins. Late Cenozoic tilting (early Pliocene to present, <4±0.5 Ma) again caused the basinward progradation of shelf-slope wedges, from uplifts along the inner margin (including broad dome-like features) and from offshore highs. The early, mid- and late Cenozoic epeirogenic episodes coincided with Atlantic plate reorganisations, but the observed km-scale tectonic movements are too large to be accounted for as flexural deflections due to intra-plate stress variations. Mantle–lithosphere interactions are implied, but the succession of epeirogenic episodes, of differing form, are difficult to reconcile with the various syn-to post-rift mechanisms of permanent and/or transient movements proposed in the hypothetical context of a plume beneath Iceland. The epeirogenic movements can be explained as dynamic topographic responses to changing forms of small-scale convective flow in the upper mantle: tilting as coeval upwelling and downwelling above an edge-driven convection cell, sagging as a loss of dynamic support above a former upwelling. The inferred Cenozoic succession of epeirogenic tilting, sagging and tilting is proposed to record the episodic evolution of upper mantle convection during ocean opening, a process that may also be the underlying cause of plate reorganisations. The postulated episodes of flow reorganisation in the NE Atlantic region have testable implications for epeirogenic movements along the adjacent oceanic spreading ridge and conjugate continental margin, as well as on other Atlantic-type ‘passive’ margin

Late Quaternary palaeoenvironment and chronology in the Traenadjupet Slide area offshore Norway

The northern mid-Norwegian continental slope was studied based on high-resolution side-scan sonar data, multibeam bathymetry, high-resolution and multichannel seismics together with gravity cores. Sedimentary provinces identified include a partly buried slide on the eastern, inner Voring Plateau, an area dominated by glacigenic debris flows south-west of the Traenadjupet Slide, the Traenadjupet Slide, and an area of glacimarine sedimentation and a slide scar north-east of the Traenadjupet Slide. The Traenadjupet Slide affected an area of about 14100 km2 and mobilised about 900 km3 of sediments. Little is known about the areal extent and volume of the older events. The glacigenic debris flows and glacimarine sediments were deposited while the Fennoscandian ice sheet was at the shelf break during the late Weichselian glacial maximum (prior to 13.2 14C kyr BP). Hemipelagic and/or contouritic sedimentation prevailed during the Holocene period. Two large slide scars were probably formed sometime prior to or during the late Weichselian glacial maximum (inner Voring Plateau and north-east of the Traenadjupet Slide) and another during the mid-Holocene interglacial period immediately prior to 4000 14C kyr BP (the Traenadjupet Slide). The two older scars may represent one event or two separate events. Deposition of poorly permeable glacigenic sediments over high-water-content fine-grained hemipelagic and/or contourites may have prevented water escape and increased failure potential Thus continental slope areas of episodically high sediment input of glacigenic sediments are prone to failure as illustrated by this study, which has identified at least two large slope failures. Failures have occurred both during glacial maxima, periods of climate deterioration and low global eustatic sea level, and during interglacials as today with improved climatic conditions and a high global eustatic sea leve

Seep mounds on the Southern Wiring Plateau (offshore Norway)

Multidisciplinary study of seep-related structures on Southern Voring Plateau has been performed during several UNESCO/IOC TTR cruises on R/V Professor Logachev. High-resolution sidescan sonar and subbottom profiler data suggest that most of the studied fluid discharge structures have a positive relief at their central part surrounded by depression. Our data shows that the present day fluid activity is concentrated on the top of these "seep mounds". Number of high hydrocarbon (HC) gas saturated sediment cores and 5 cores with gas hydrate presence have been recovered from these structures. delta C-13 of methane (between -68 and -94.6 parts per thousand VPDB) and dry composition of the gas points to its biogenic origin. The sulfate depletion generally occurs within the upper 30-200 cm bsf and usually coincides with an increase of methane concentration. Pore water delta O-18 ranges from 0.29 to 1.14 parts per thousand showing an overall gradual increase from bottom water values (delta O-18 similar to 0.35 parts per thousand). Although no obvious evidence of fluid seepage was observed during the TV surveys, coring data revealed a broad distribution of living Pogonophora and bacterial colonies on sea bottom inside seep structures. These evidences point to ongoing fluid activity (continuous seepage of methane) through these structures. From other side, considerable number and variety of chemosynthetic macro fauna with complete absence of living species suggest that present day level of fluid activity is significantly lower than it was in past. Dead and subfossil fauna recovered from various seep sites consist of solemyid (Acharax sp.), thyasirid and vesicomyid (cf. Calyptogena sp.) bivalves belonging to chemosymbiotic families. Significant variations in delta C-13 (-31.6 parts per thousand to -59.2 parts per thousand) and delta O-18 (0.42 parts per thousand and 6.4 parts per thousand) of methane-derived carbonates collected from these structures most probably related to changes in gas composition and bottom water temperature between periods of their precipitation. This led us to ideas that: (1) seep activity on the Southern Voring Plateau was started with large input of the deep thermogenic gas and gradually decries in time with increasing of biogenic constituent; (2) authigenic carbonate precipitation started at the near normal deep sea environments with bottom water temperature around +5 degrees C and continues with gradual cooling up to negative temperatures recording at present time. (C) 2009 Elsevier Ltd. All rights reserved