Kåre Rokoengen

Surficial sediments in the Norwegian sector of the North Sea between 60°30′ and 62°N

Abstract Based on grain-size distribution and sea floor photographs, the sea bed sediments between Bergen and Maloy have been divided into four groups: (A) sand with coarse material on the northern part of the North Sea Plateau and the Maloy Plateau; (B) very well sorted sand on the southern part of the North Sea Plateau and the upper part of the western slope of the Norwegian Trench; (C) silty, clayey sand in lower part of the slopes; (D) silty, sandy clay in the central part of the Norwegian Trench. Large local variations are found in the group A sediments, on the Maloy Plateau partly caused by relict iceberg plough marks. The net deposition after the transition to boreal conditions with Atlantic water about 10,000 yrs ago is mostly between 0 and 30 cm on the northern part of the North Sea Plateau and exceeds 200 cm in the deepest parts of the Norwegian Trench. The total thickness of sediments above assumed overconsolidated material has been mapped based on sparker records. On the northern part of the North Sea Plateau the sediment cover above a supposed wavecut platform is less than 1 m over large areas, while a Late Weichselian coastal unit fringing the plateau consists of up to 40 m of sand and gravel. On the southern part of the plateau and the upper part of the western slope of the Norwegian Trench up to 10–30 m of sediments are found, probably dominated by sand formed by wave and tidal current action during a period of low sea level. In the central part of the trench a total thickness of up to 40 m of clay deposited during varying environments is found. Most of the investigated area is believed to be underlain by a till deposited by a north-northwestward moving ice sheet.

Glacial processes and large-scale morphology on the mid-norwegian continental shelf

A regional digital bathymetric data set covering most of the mid-Norwegian continental shelf is presented and gives a unique regional view into glacial processes and ice-sheet dynamics on this part of the continental shelf during the Weichselian, indicating that forms and deposits were created by a highly dynamic ice sheet. At times, ice flow was mainly channelised through ice streams located in bathymetric depressions on the shelf areas. Glacial sedimentary processes are discussed with a focus on the marine-based part of the Scandinavian ice sheet during the last glaciation (the Weichselian). Ice sheets that grounded on the shelf edge are thought to have been responsible for depositing complex prograding sequences on the mid-Norwegian shelf during several glaciations from Late Pliocene time, reaching a maximum thickness of 1500 m on the shelf edge. During interglacials, the shelf areas were sediment-starved with little or no clastic sedimentation. On top of these prograding units, several packages of Quaternary sediments (mainly till of Weichselian age) show a more aggradational pattern. Improved knowledge about the deposition and age of the upper Cenozoic sediment wedge has proved vital for revealing the ice-sheet dynamics and may also be important in understanding the maturation and migration of hydrocarbons on the mid-Norwegian shelf.

Description and dating of a submerged beach in the northern North Sea

Abstract An assumed submerged beach, bordering the northern part of the North Sea Plateau towards the Norwegian Trench at 130 to 160 m waterdepth, has previously been described based on interpretation of seismic profiling. The seismic unit is parallel to the edge of the plateau for at least 50 km, with an eastward-dipping bedding. Soil borings and vibrocorer samples reveal that a stiff to hard clay is overlain by a thin lag deposit of coarse material on the plateau, by an up to 40 m-thick coarsening-upwards sequence of sand and gravel on the upper part of the slope and by gradually finer material further eastwards in deeper water. A low sea level is also indicated by the foraminiferal faunas, and abundant littoral and aeolian textures on quartz sand grains. Nine radiocarbon datings of shell material, found in sand and gravel from about 1 to 23 m below the sea bed, gave ages from 10,800 to 12,500 yrs B.P. We thus consider the interpretation of the edge of the plateau and the top of the slope as a submerged beach built out in Late Weichselian time to be confirmed by the sample evidence.

Lower Tertiary volcanic rocks off Kristiansund — Mid Norway

Abstract Grab samples from the Norwegian continental shelf show that diapiric structures occurring within a sequence of Mesozoic and Tertiary sediments represent volcanic rocks consisting of porphyritic olivine-nephelinite. The KAr age of 55.7 ± 0.9 m.y. shows that the area affected by Lower Tertiary magmatism in the North Atlantic region also included the continental shelf off Mid Norway. The highly undersaturated nature of these volcanic rocks, however, indicates that they are unlikely to be closely related to the widespread tuff horizons of the same age known from the North Sea.

Isoleucine epimerization in Quaternary benthonic foraminifera from the Norwegian Continental Shelf: A pilot study

Abstract The extent of epimerization of the protein amino acid L-isoleucine has been measured in Late Quaternary benthonic foraminifera found in cores from two areas of the Norwegian Continental Shelf in order to evaluate the potential of this reaction as a geochronological tool. Measurements were restricted to monospecific assemblages of Cibicides lobatulus and Elphidium excavatum from 14 C-dated and seismostratigraphic units representing a range of ages. By comparing with data on the same species from last-interglacial and late-glacial sites in western Norway (Miller et al., 1983) and with the assumption of similar temperature history for the shelf, and coastal areas, the absolute ages of the samples are discussed. Of the nine samples analysed one was of Holocene, seven of Late to Middle Weichselian and one sample of pre-Weichselian age. From this pilot study we conclude that isoleucine epimerization ratios in benthonic foraminifera provide a reliable index of relative age that can be used to correlate sedimentary units that contain in situ faunas and have similar thermal histories. Absolute ages of the enclosing sediment can be approximated if suitable well-studied calibration sites are available.

Seismic Signature of Glaciomarine Fjord Sediments From Central Norway

The fjord areas in the outer Tr∅ndelag region (Fig. 1) contain large volumes of infilled layered glaciomarine sediments closely related to the deglaciation of the area [Oftedahl,1977; Reite, 1994; Ottesen et al.,1995]. A single-channel seismic profile (Fig. 2) illustrates well the glaciomarine sediments in the area. The profile was shot with a 15 in3 air gun with a shot interval of 2 seconds. The returned acoustic pulses, received through one hydrophone streamer, were filtered between 50 and 600 Hz. The active part of the streamer (7 m) was towed in a subsurface position 30 metres behind the vessel. The data were fully analog and recorded on an EPC printer.