Post-glacial sedimentary processes and slope instabilities off Nordnesfjellet, Lyngenfjorden, northern Norway

Abstract

The Lyngen- and Storfjorden system, Troms County, northern Norway, is surrounded by an alpine landscape, partly with steep mountain sides. Parts of the ~700 m high mountain Nordnesfjellet, on the eastern side of the fjord, are currently moving downslope, and could in the event of sudden failure generate tsunami waves which would affect low-lying areas within a larger region. The main objective of this study is to integrate multi-proxy analyses of five sediment cores, swath bathymetry and high-resolution seismic data from one NNE-SSW oriented basin of Lyngenfjorden, off Nordnesfjellet, with the purpose of reconstructing mass-wasting activity in the past. The fjord sides in the study area are up to >30o steep and irregular with depressions, escarpments and furrows of various sizes and ‘freshness’. These features are interpreted to be slide scars and gullies, respectively, related to slope failures. The central parts of the up to 270 m deep basin are generally smooth. However, an up to 8 m high step, crossing almost the entire fjord width, defines the northern limit of a relatively irregular seafloor in the southern part of the study area. Furthermore, sediment-lobe areas are visible as undulating seafloor on the western and eastern sides of the basin. The high-resolution seismic data reveal that the uppermost <12 ms two-way-travel time (~9 m) are composed of acoustically transparent sediments with few, discontinuous reflections. Deposits causing a chaotic reflection pattern underlie this package within the areas of irregular and undulating seafloor, where also several acoustically well stratified sediment blocks (up to ~300 x 300 x 10 m3) are identified. The acoustically chaotic deposits are e.g. debris flows or turbidites, whereas the blocks are either run-out blocks embedded in the debris flows or slide blocks. The mass-transport deposits are stacked together in several complexes which cover and partly erode into glaciomarine sediments. It is assumed that these complexes were deposited during a relatively short period of frequent mass-wasting directly following the last deglaciation of the study area (~10,800 cal. years BP). Multiple acoustically transparent wedges thinning from the fjord sides towards the fjord axis are identified. These are most probably debris flows or turbidites. One prominent high-amplitude reflection can be traced over an area of > 7.4 x 1.3 km2. It has varying thickness, on-laps the eastern fjord flank, while it thins towards west. The cored sediment is mostly massive, olive grey mud. However, coarser, partly well sorted, intervals (up to 13 cm) with sharp and partly erosive lower boundaries occur. Well-sorted intervals are suggested to be turbidites. One turbidite was deposited from a high-density turbidity flow between 3026 and 2930 cal. years BP. It correlates with the strongest reflection within the acoustically transparent interval, thus, indicating that it extends over more than 7.4 x 1.3 km2 of the fjord floor. Wood fragments and moss found within the turbidite suggest that the origin of the failure was onshore. Mass-transport activity was high shortly after the deglaciation, decreased during the mid-Holocene and increased again during the late Holocene. The late Holocene increase in mass wasting might be related to a climatic deterioration.