Norway Basin Research Articles

Hummock fields in the Norway Basin and Eastern Iceland Plateau: Rayleigh-Taylor instabilities?

Side-scan imagery and 3.5 kHz profiles in the Norway Basin reveal ∼3–4 × 10 4 km 2 of sea floor covered by regularly spaced (500–2000 m), low-relief hummocks previously attributed to bottom currents. Extrapolating from nearby borehole data, I attribute these mound fields to gravitational (Rayleigh-Taylor) instabilities resulting from superposition of 50–100 m of thick Pliocene-Pleistocene, glacigenic (glacier-derived) sandy clays (density 1800 kg/m 3 ) on a several-hundred-metre thickness of Miocene biosiliceous oozes having a density of 1300 kg/m 3 . This hypothesis predicts or explains similar hummock fields in many oceanic regions where such density inversions exist.

The Aegir Ridge: Structure of an extinct spreading axis

In 1988 a reflection seismic survey was carried out by the Alfred‐Wegener‐Institute for Polar and Marine Research to study structure and development of the Aegir Ridge, an extinct spreading axis in the Norway Basin. A number of volcanolike structures were detected in the central ridge. The emerging lava flows dip away from their source in ridge strike. Perpendicular to ridge strike, we observed lava flows dipping towards the ridge center. This pattern might represent a more general flow pattern at mid‐ocean ridges. After spreading ceased, sedimentation resulted in the deposition of 800 ms two‐way time (TWT) to 1300 ms TWT (about 900–1400 m) sediments in the central ridge. Distribution of the sediments indicate subsidence of the central ridge probably due to cooling after termination of spreading. This subsidence started no later than late Oligocene or early Miocene. We detected an area where migration of a fluid phase led to obstruction of the sedimentary layering and the formation of mounds and faults at the ocean bottom. Several sources for the fluid phase are discussed.

M�re Margin: Crustal structure from analysis of Expanded Spread Profiles

Analysis in both the x—t and τ—p domains of high-quality Expanded Spread Profiles across the More Margin show that many arrivals may be enhanced be selective ray tracing and velocity filtering combined with conventional data reduction techniques. In terms of crustal structure the margin can be divided into four main areas: 1) a thicker than normal oceanic crust in the eastern Norway Basin; 2) expanded crust with a Moho depth of 22 km beneath the huge extrusive complex constructed during early Tertiary breakup; 3) the More Basin where up to 13–14 km of sediments overlie a strongly extended outer part with a Moho depth at 20 km west of the Ona High; and 4) a region with a 25–27 km Moho depth between the high and the Norwegian coast. The velocity data restricts the continent-ocean boundary to a 15–30 km wide zone beneath the seaward dipping reflector wedges. The crust west of the landward edge of the inner flow is classified as transitional. This region as well as the adjacent oceanic crust is soled by a 7.2–7.4 km s−1 lower crustal body which may extend beneath the entire region that experienced early Tertiary crustal extension. At the landward end of the transect a 8.5 km s−1 layer near the base of the crust is recognized. A possible relationship with large positive gravity anomalies and early Tertiary alkaline intrusions is noted.

The Storegga slide

One of the World’s largest known submarine slides is found in the Storegga area off the coast of Mid-Norway. The slide area has been investigated by seismic profiling, seabed sampling and long-range (Gloria) and medium-range side-scan sonars. The 290 km long headwall is located at the shelf edge 100 km off the coast. The slide extends down the continental slope and into the abyssal plain to a distance of more than 800 km. The maximum thickness is 450 m and a total of about 5600 km 3 of sediment was involved in the sliding. Three main slide events are distinguished. The First Storegga Slide was the largest (about 3880 km 3 ) and probably was formed 30000—50000 years BP. The two other events seem to have occurred in near succession about 6000—8000 years BP. The Second Slide, which consisted of more consolidated sediments than the First Slide, cut back 6-8 km headwards beyond the First Slide and removed some 450 km 2 of the continental shelf edge. It involved large blocks (olistoliths) of sediments that can be recognized in hummocky slide deposits both within the slide scar and on the abyssal plain. Two huge sediment slabs, 150—200 m thick and up to 10 x 30 km wide, were transported about 200 km down an average slope 0.3°. The Third Storegga Slide was limited to the upper part of the Second Slide scar, and probably occurred as a final, somewhat delayed stage of the Second Slide. In the deepest part of the Norway Basin, more than 750 km from the headwall, a thick (more than 6 m) fine-grained turbidite is related to the Second Storegga Slide. Several other turbidites are found in cores from within the slide scar and on the inner part of the abyssal plain. We believe that earthquake loading and decomposition of gas hydrates caused liquefaction of the sediments and thus triggered the slides.

Tectonic and volcanic events at the Jan Mayen Ridge microcontinent

Summary Two main tectonic phases were responsible for the formation of the Jan Mayen Ridge microcontinent: (1) the opening of the Norway Basin in late Palaeocene/early Eocene times, and (2) subsequent rifting within the Greenland margin by which complete separation was achieved in early Miocene times. During the first phase the eastern ridge flank developed as a volcanic passive margin. The initial break-up was associated with flexuring and the formation of sequences of eastward-dipping basalt flows, which are considered equivalent to similar features beneath the Vøring and Faeroe-Shetland marginal highs off Norway. Rifting along the Greenland margin during the second phase was accompanied by uplift, listric normal faulting and the formation of large extensional fault blocks. To the W and S of the ridge a flat volcanic marker of probable earliest Miocene age covers the subsided rift and masks the ocean-continent transition. It was formed by a volcanic event of large magnitude, either as submarine lava flows or as a sill complex.

The distribution of living benthic foraminifera on the continental slope and rise off southwest Norway

Surface sediment samples taken by ☐ corer from 45 stations on the Norwegian continental margin and in the Norway Basin have been investigated for their benthic foraminiferal content. Unlike previous studies, the living benthic foraminiferal fauna was differentiated from empty tests comprising the foraminiferal death assemblage. Factor analysis of both the living and dead faunal data reveals six living species assemblages and five corresponding dead assemblages. The additional living assemblage is characterized by the arenaceous species Cribrostomoides subglobosum that dominates between 1400 and 2000 m water depth, but is rare in the dead faunal data. Trifarina angulosa and, to a lesser extent, Cibicides lobatulus characterize the shallowest foraminiferal assemblage from 200 to 600 m water depth, where it is associated with strong bottom currents and warm, saline Atlantic water of the North Atlantic Drift. On the slope between 600 and 1200 m water depth, the Melonis zaandami Species Assemblage dominates, particularly in areas characterized today by rapid sedimentation of terrigeneous material. Between 1000 and 1400 m depth, where the slope is covered by fine grained, organic-rich, terrigeneous mud, the living foraminiferal assemblage is characterized by Cassidulina teretis and Pullenia bulloides. Below 1400 m, three foraminiferal assemblages are found: C. subglobosum is found from 1400 to 2000 m, Cibicidoides wuellerstorfi and Epistominella exigua predominantly live from 2000 to 3000 m water depth, and below 3000 m, Oridorsalis umbonatus and Triloculina frigida dominate the fauna. All of the Elphidium excavatum tests found in this study and the Cassidulina reniforme tests found above 500 m water depth were found to be reworked. Analysis of the sediment grain-size distribution and the organic carbon content in surface samples from the deepest stations suggest that the abundance of C. wuellerstorfi and E. exigua is positively correlated to relatively coarse (caused by planktic foraminifera) and organic-rich sediments, whereas high frequencies of O. umbonatus and T. frigida coincide with low organic carbon content. We suggest that C. wuellerstorfi is adapted to deep-sea environments with relatively high food supply, tolerating relatively low interstitial water oxygen content, whereas O. umbonatus may tolerate lower food supply prefering well-oxygenated interstitial waters.

Bacteria in Cream

Abstract We present a revised tectonostratigraphy of the Jan Mayen microcontinent (JMMC) and its southern extent, with the focus on its relationship to the Greenland–Iceland–Faroe Ridge area and the Faroe–Iceland Fracture Zone. The microcontinent9s Cenozoic evolution consists of six main phases corresponding to regional stratigraphic unconformities. Emplacement of Early Eocene plateau basalts at pre-break-up time (56–55 Ma), preceded the continental break-up (55 Ma) and the formation of seawards-dipping reflectors (SDRs) along the eastern and SE flanks of the JMMC. Simultaneously with SDR formation, orthogonal seafloor spreading initiated along the AEgir Ridge (Norway Basin) during the Early Eocene (C24n2r, 53.36 Ma to C22n, 49.3 Ma). Changes in plate motions at C21n (47.33 Ma) led to oblique seafloor spreading offset by transform faults and uplift along the microcontinent9s southern flank. At C13n (33.2 Ma), spreading rates along the AEgir Ridge started to decrease, first south and then in the north. This was probably complemented by intra-continental extension within the JMMC, as indicated by the opening of the Jan Mayen Basin – a series of small pull-apart basins along the microcontinent9s NW flank. JMMC was completely isolated when the mid-oceanic Kolbeinsey Ridge became fully established and the AEgir Ridge was abandoned between C7 and C6b (24–21.56 Ma).