Raymond S. Eilertsen

The paleo–ice stream in Vestfjorden, north Norway, over the last 35 k.y.: Glacial erosion and sediment yield

The basal erosion rate and sediment yield of the Vestfjorden paleo–ice stream, a marine-based part of the northwestern Fennoscandian Ice Sheet, were elucidated during a complete advance-retreat cycle using high-resolution seismic data and multi-beam bathymetry. In most of the area, there is a semitransparent, continuous upper seismic unit showing a southwestward (down-fjord) progradational pattern with megascale glacial lineations on top, interpreted to represent a subglacial deformable till. The till formed a continuous bed beneath most of the Vestfjorden paleo–ice stream, except for the upstream parts, where ice streaming is inferred to have occurred above bedrock. The till thickness is up to an order of magnitude larger than those reported from modern ice streams due to a more efficient transfer of material from the source area to the ice streambed. The till is draped by glaciomarine and marine sediments. The average sedimentation rate in the paleo–ice stream drainage route during the period from ca. 35 to 11 ka was ~2.6 m/k.y. This equals an average sediment discharge of 35 × 106 t/yr and an average sediment yield of 6.4 × 103 t km−2 yr−1. From this, we estimate an average erosion depth of ~40 m or rate of ~1.7 mm/yr in the sediment source area, the encircling fjord, and valley landscape. These values are in agreement with the results reported from similar settings in southern Norway, but they are nearly twice the figures reported for arctic ice-stream erosion. They are also significantly higher than the rates estimated from the British Ice Sheet. In conclusion, glacial erosion beneath paleo–ice streams seems to have been more efficient in subarctic regions.

The 1978 Quick Clay Landslide at Rissa, Mid Norway: Subaqueous Morphology and Tsunami Simulations

The 1978 landslide at Rissa is the largest to have struck Norway during the last century and is world-famous because it was filmed. Swath bathymetry data and seismic reflection profiles reveal detailed information about the subaqueous morphology of the mass-transport deposits (MTD). Results show that the landslide affected nearly 20% of the lake floor and that it exhibits a complex morphology including distinct lobes, transverse ridges, longitudinal ridges, flow structures and rafted blocks. The rafted blocks found at the outer-rim of the MTD travelled a distance of over 1,000 m in the early stage of the landslide on an almost flat basin floor. Simulation of sediment dynamics and tsunami modelling show that the rafted blocks most likely triggered the flood wave with a recorded maximum surface elevation of 6.8 m.

Landslides Along Norwegian Fjords: Causes and Hazard Assessment

A collection of 28 well-known historical and near-shore landslide data is analysed in order to better understand the key factors governing mass-wasting processes along Norwegian fjords. The distribution of near-shore slope failures in Norway is linked to the occurrence of thick marine deposits. Compared to those found along deltas and on the steep side-walls of fjords, slope failures in bays and inlets have more often endangered coastal populations and infrastructures due to their extensive retrogression. Factors such as the presence of a weak layer, unfavourable groundwater conditions and stream erosion are found to often contribute to the failure of slopes. However, the dataset shows that more than 60% of historical failures along Norwegian fjords are related to human activity. This enhances the need for a methodology integrating both on- and off-shore data for mapping the hazard and risks associated to such natural processes in Norway.

A Younger Dryas moraine ridge and fjord delta in Valldal, Norddalsfjorden, Møre og Romsdal, Norway

The fjords and valleys of Norway have been shaped during several glacial cycles of the Quaternary period. Subaquatic ridges that formed at or close to the grounding lines of glacier termini during stillstands or readvances within the last deglaciation are commonly found within the fjords. An example is a prominent moraine ridge representing a readvance of the Fennoscandian Ice Sheet during the Younger Dryas ( c. 12.9–11.5 ka BP) in Norddalsfjorden, More og Romsdal, Norway. Fjord deltas may form in the same setting at the mouths of tributary valleys or at fjord heads following deglaciation. They may grow to relatively large dimensions and have morphological features that reflect the dynamic environment that exists across the fluvial–marine transition. One such delta is located close to, and partly drapes, the proximal slope of the moraine ridge at Norddalsfjorden. Norddalsfjorden is a narrow (0.8–3.5 km wide) and relatively deep (up to about 500 m deep) fjord situated between high mountain peaks and intermittent valleys (Fig. 1). A transverse, subaquatic ridge is situated on a bedrock sill just SW of the Valldal tributary valley. High-resolution multibeam data clearly depict the ridge as a well-defined topographic high with an arcuate crest that is convex down-fjord (Fig. 1c). The crest of the ridge is c. …

Using LiDAR data to characterize and distinguish among different types of raised terraces in a fjord-valley setting

Abstract Depositional terraces of diverse origin are common in Norwegian fjord valleys. They differ subtly morphologically and may be difficult to distinguish from one another without detailed field and stratigraphic investigation. We used LiDAR elevational data to describe previously identified glaciofluvial, glaciodeltaic, fluvial, and fluviodeltaic terraces in the Målselv valley, northern Norway, and show how they differ morphologically. Fluvial terraces are divided into two subtypes based on their difference in morphology. One type shows an undulating surface, often with a ridge-and-swale topography (scroll bars) associated with lateral accretion of moderate-to-high-sinuosity (meandering) channels, while the other shows remains of channels and braid bars associated with low-sinuosity channels. Fluviodeltaic terraces are generally flat and featureless, probably as a result of wave and tidal reworking of the terrace surface as it was raised above sea level. Glaciofluvial terraces show braided channel forms and occasional eolian dunes, while glaciodeltaic terraces are generally flat and featureless, in some cases having a moraine ridge on top and small beach ridges on the proximal, ice-contact slope. Terraces also show a distinct distribution pattern, with glaciofluvial and glaciodeltaic terraces graded to the marine limit occurring at higher elevations than fluviodeltaic and fluvial terraces. Based on the results presented here we show that a distinction can be made between different terraces based on morphological criteria revealed by LiDAR data. Such data should be acquired where possible to improve the quality of geomorphological mapping.

Kettle holes, ‘dead-ice’ topography and eskers on a lake floor in Telemark, southern Norway

Hummocky terrain in deglaciated areas is often attributed to ‘dead-ice’ activity, that is, a depositional environment produced by the presence and melt-out of large blocks of stagnant ice (Gravenor & Kupsch 1959; Eyles et al. 1999). Morphologically similar dead-ice terrain is found on the floor of Lake Bandak in southern Norway, a former fjord now isolated from the sea, where a stagnant glacier terminated during the Late Weichselian deglaciation. Lake Bandak is an east–west-orientated lake with a length of c. 26 km, an average width of c. 1 km, and an area of 26.4 km2 (Fig. 1). It is situated 72 m above sea level some 85 km inland from the outer coast (Fig. 1b). The marine limit is 120–130 m above present sea level due to postglacial isostatic rebound (Bergstrom 1999), implying that the glacier which filled the valley at the Last Glacial Maximum terminated in a fjord during deglaciation. Both the glacial ice divide and present watershed lie west of the lake, indicating flow from west–east. The lake is deepest (200–300 m) and relatively flat from Lardal and eastwards with steep lateral slopes (Fig. 1a), whereas the western part is shallower (typically <100 m deep). Seismic and backscatter data suggest that …

Mapping of Subaqueous Landforms for Near-Shore Landslide Susceptibility Assessment Along Norwegian Fjords

Landslides in unconsolidated sediments along the shoreline of Norwegian fjords are recurrent phenomena and can have large consequences. With high-resolution bathymetric mapping it is possible to get a quick overview over regional patterns of subaqueous, near- shore conditions. Test programs at the Geological Survey of Norway have mapped submarine near shore areas along selected Norwegian fjords. Subaqueous features were identified and classified to obtain a comprehensive overview over near-shore processes and deposits. The focus was identification of mass-wasting processes and features of relevance for the stability of fjord sediments. The mapping shows that traces of, for example, mass-wasting processes are more widespread than previously known. The maps can be combined with terrestrial data and used for near-shore landslide susceptibility assessment and hazard mapping, but are also considered as relevant for other purposes.