Inge Aarseth

Quaternary seismic stratigraphy of the North Sea Fan: glacially-fed gravity flow aprons, hemipelagic sediments, and large submarine slides

Abstract Approximately 1000 km of high resolution sleeve-gun array transects on the North Sea Fan, located at the mouth of the Norwegian Channel, reveal three dominant styles of sedimentation within a thick (> 900 m) Quaternary sediment wedge comprising numerous sequences. These are interpreted as: terrigenous hemipelagic sedimentation, large scale translational slides, and aprons of glaciogenic debris flow deposits contributing to considerable fan construction. Four large, buried translational slides involved sediment volumes upwards of 3000 km 3 each and preceded the similarly dimensioned “first” Storegga Slide on the NE fan flank. Several thick (> 100 m) terrigenous hemipelagic deposits apparently represent long-lived (150–200 kyr) periods of sedimentation whose distribution indicates fan input via the Norwegian Channel. The upper sequences are each made upper sequences are each made up of one or several thick (> 100 m) aprons comprising stacked lensoid and/or lobate forms which range from 2 to 40 km in width and 15 to 60 m in thickness. They characterize debris flows attributed to periodic input from several phases of a Norwegian Channel ice stream reaching the shelf edge. Subsidence in the outer Norwegian Channel allowed preservation of several glaciation cycles represented by sheet erosion-bounded tills and progradational units. Much of the shelf/slope transition has been preserved, allowing a preliminary chronology of the fan sequences through correlation with borehole sediments in the Norwegian Channel. Debris flows, which signal the initial shelf-edge glaciation, are not recognized from the initial glaciation in the Channel (> 1.1 Myr) but are associated with a Middle Pleistocene and all following glacial erosion surfaces (GES) in the outer Norwegian Channel. This was followed by six further sequences, probably totalling over 13,000 km 3 of sediment. At least four of these were shelf-edge ice-maximum events the last of which was Late Weichselian age ( 14 C AMS). Considering earlier glaciation-related hemipelagic sedimentation, material since removed by the large slides, and extensive unmapped areas, total Quaternary fan sedimentation was in the vicinity of 20,000 km 3 .

The Quaternary succession in the northern North Sea

Abstract Studies of core material show that the upper 200 m of sediments deposited in central parts of the North Sea span the last ca. 1 Ma. In the Norwegian Trench, the upper 120 m span the last ca. 500 ka. Sedimentation in both areas was characterized by rapid rates over short intervals separated by long hiatuses. In the central area, fine-grained shallow-water sediments dominate the Pleistocene sequence. These sediments were redeposited/transported by rivers flowing northward from the British Isles and the continent during periods of low sea level through the Quaternary. The extreme low stands of sea level are also indicated by river channels cut in the sequence in the northern North Sea. In the Norwegian Trench, thick glacial and glacial marine diamictons make up ca. 80% of the record in the Middle and Late Pleistocene sequence. The northern North Sea was glaciated as early as 850 ka. Evidence of possibly the most extensive glaciation of the North Sea Basin has been dated to between 200 and 450 ka in cores from The Fladen area. The British and Scandinavian Ice Sheets probably did not coalesce in the North Sea during the late Weichselian maximum.

Western Norwegian fjord sediments: age, volume, stratigraphy, and role as temporary depository during glacial cycles

Abstract The sediments in the western Norwegian fjords are described chronologically, qualitatively and quantitatively by means of seismic profiling and correlation with dated terrestrial sediments along the fjords. Five chronostratigraphic units (I–V) are proposed for the fjord sediments using the well established Quaternary stratigraphy in the area. Sediment volumes of the main fjords are calculated using a dense seismic grid combined with map analyses. Approximately 150 km 3 of sediments are present in the fjords between 59–63 °N. Ninety percent of this material is considered to be deposited during deglaciation of the last (Weichselian) ice sheet. Less than ten percent of the fjord sediments in this area is older than the last glacial (Weichselian) maximum. Both glacial and glaciofluvial processes are active in emptying the fjord basins and transporting the sediments to the continental shelf and slope during glaciations.

Quaternary stratigraphy of the Draugen area, Mid-Norwegian Shelf

Abstract Lithostratigraphic, biostratigraphic and chronostratigraphic studies on a 130 m long core from Haltenbanken, Mid-Norwegian Shelf (DR 88 30 ) shows sequences of short episodic glacigenic sedimentation and periods of more continuous marine sedimentation. The sediment sequence is dated, by palaeomagnetism as well as the amino-acid and the strontium isotope methods, back to ca. 1.1 Ma. Previous seismostratigraphic interpretations placed the regional Pliocene/Pleistocene boundary at ca. 90 m depth at the core site; this level was dated to be of Brunhes/Matuyama age. The study area has been covered by grounded glaciers at least five times during the last 1.1 Ma. The first expansion of the Scandinavian ice sheet was at ca. 1.1 Ma, depositing a diamicton. These sediments are overlain by a long sequence of normal marine sediments deposited during both glacial and interglacial conditions. Two interglacial periods from this sequence have been given an informal interglacial notation. Except for the glaciation at ca. 1.1 Ma the glacials during the Early Pleistocene, which are identified in the core, were relatively limited. A transition to more extensive glaciations occurs in the early Middle Pleistocene. During these more extensive glacial phases, major erosion took place along the inner continental shelf. This erosional phase resulted in overdeepening of the inner shelf, which during the Middle and Late Pleistocene influenced the depositional dynamics in the central and the outer part of the shelf. In the study area the Middle and Late Pleistocene periods are dominated by glacigenic sediments, with only very thin interglacial sediments occasionally preserved. One interglacial period from the Middle Pleistocene has been given an informal interglacial notation. The core has been correlated to cores from the North Sea Basin on the basis of amino-acid stratigraphy and palaeomagnetism. The lithological and the foraminiferal stratigraphy indicates a synchronous appearance of the major lithological units and of the fauna assemblages in these areas.

Submarine slides in glaciomarine sediments in some western Norwegian fjords

Abstract Submarine slides from four western Norwegian fjords, Fjaerlandsfjord and Ardalsfjord in the Sognefjord, and Nordfjord and Romsdalsfjord are studied. Sparker and high-resolution seismic profiles are used to illustrate the thickness, morphology and internal structure of the primary material as well as of the accumulated slides. Sediment cores are studied to calibrate the acoustic signals. Acoustic lamination is thought to represent sequences of variable density or some of the zones of fine sand and silt laminae which have acted as sliding planes. These rapidly deposited layers constitute underconsolidated zones that are susceptible to liquefaction, causing sliding of the overlying material along bedding planes or faults. The morphology and structure of the slides are in some cases very similar to that of the mass movements described from continental slopes. A 25 m deep and 1 km 2 wide “plunge pool” is formed in the Sognefjord at the bottom of a 800 m high and 25° steep slide path down from the Fjaerlandsfjord. Most of the slides are ancient, but one slide, increasing the fjord depth in an area of the inner Nordfjord by 55 m, most probably took place in 1967. The volumes of the slides are in the order of 3–30 × 10 6 m 3 .

Late-Holocene changes in ocean circulation and climate: foraminiferal and isotopic evidence from Sulafjord, western Norway

A 278 cm long sediment core spanning the last 5500 years was collected from 440 m water depth in Sulafjorden, western Norway. Detailed analyses of benthonic foraminfera, stable isotopes and lithology have been performed on the core, and a chronology based on five AMS dates on benthonic foraminifera has been established. The foraminiferal assemblages consist of species that are common in the North Sea region today. The benthonic fauna seems to respond to changes in oxygenation of the bottom water in the fjord indicating relatively low dissolved oxygen contents prior toc. 2000 cal. bc followed by a gradual increase untilc. 700 cal. bc. The oxygen levels in the water masses are assumed to reflect the ingress of oceanic water from the shelf. Downcore variations in the oxygen isotope ratios in the benthonic speciesUvigerina mediterranea in Sulafjorden are interpreted to reflect temperature variations with a minimum amplitude of 2°C. The isotopic data indicate five cold periods atc. 2150–1800 cal. bc,c. 850–600 cal. bc, 150 cal. bc to cal. ad 150,c. cal. ad 500–650, and one in the top of the core corresponding to the ‘Little Ice Age’ (c. cal. ad 1625). These periods are characterized by 1.5–2°C reduction in the bottom-water temperatures in Sulafjorden. Some of the cold periods in the fjord record are contemporaneous with the Holocene ice-rafting events in the North Atlantic and glacier fluctuations in western Norway and Northern Scandinavia. This implies that late-Holocene climate fluctuations in Scandinavia are caused by circulation changes in the North Atlantic region.

Late Quaternary sediments from Korsfjorden, western Norway

Abstract A 555 cm long undisturbed piston core was taken from the bottom of Korsfjorden, south-west of Bergen, at a depth of 603 m. The core, containing olive-grey, fine-grained, and very homogeneous sediments, has been studied for textural and lithological variation and distribution of macro- and microfossils, i.e. molluscs, foraminifers, sponge spicules, radiolarians, and diatoms. The data obtained indicate that the core represents Holocene sediments, and that the average rate of sedimentation is about 40 cm/1000 years since the end of the Preboreal Chronozone (9000 Y.B.P.).

A Holocene lacustrine rock platform around Storavatnet, Osterøy, western Norway

A recently discovered lacustrine rock platform at low altitude exists along a 1Okm2 lake on the Osterøy Island near Bergen, western Norway. The lake was converted to a reservoir in 1920 and therefore is subject to frequent changes in level above and below the previous natural level of 151.5 m a.s.l. The rock platform, up to 20 m wide, is developed in strongly foliated metamorphic Proterozoic and Palaeozoic berock. Overhanging notches and small caves are developed along bedrock fractures in some places along the platform, which typically tilts 5 to 10 degrees towards the lake and is veneered with angular debris from the cliffs. The study area was deglaciated at the Pleistocene/Holocene transition 10 000 14C yr BP, and the tilting of the glacial rebound was complete about 6000 14C yr BP. The west-coast climate in Norway duing the latter part of the Holocene, with high precipitation and frequent freeze-thaw cycles during winter, resulted in highly fluctuating lake levels. These conditions are consistent with the conclusion that the plaform was formed by frost weathering. Mass movement from the steep slopes, together with ice-push duing ice breakup, was responsible for transportation of the debris. Because the platform around the lake is essentially at the same level, it must have been formed between the mid-Holocene and AD 1920.

Younger Dryas Moraines in the Nordfjord, the Norddalsfjord and the Dalsfjord, western Norway

The Younger Dryas (Y.D.) moraine, (c. 10,500 B.P.) can be traced more or less continuous along the western Norwegian coast, crossing many fjords, Fig. 1 [Aarseth et al. 1996].

Glacial Erosion of Sediments in the Ålfjord, western Norway

In western Norway pre Late Weichselian sediments (older than 15,000 B.P.) are limited except at a few locations. Alfjord (Fig. 1) is a coast parallel fjord with up to 250 ms TWT thick sediment sequences. Ice movement was towards the Norwegian Channel [Holtedahl 1975] and the area has been ice free during Mid Weichselian [Sejrup & Larsen 1991].