International Geologiical Congress - Oslo 2008


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GSM-01 General contributions to geomorphology


Subaquatic morphology in a perialpine fjord-type lake: Shaped by glaciers, mass movements, delta sedimentation and human impact


Michael J. Hilbe, Swiss Federal Institute of Aquatic Science and Technology (Eawag) (Switzerland)
Flavio S. Anselmetti, Swiss Federal Institute of Aquatic Science and Technology (Eawag) (Switzerland)
Raymond Eilertsen, Geological Survey of Norway (NGU) (Norway)
Louise Hansen, Geological Survey of Norway (NGU) (Norway)


Newly acquired high-resolution swath bathymetry data of Lake Lucerne, a fjord-type lake located at the northern alpine front in Central Switzerland, reveal a variety of features illustrating a complex history of processes shaping the lake floor. Such data was acquired on a Swiss lake for the first time and provides a wealth of information that can be used in the context of glacial morphology, the study of sedimentary processes and natural hazard assessment.
The lake has been formed along tectonically predefined zones that were overdeepened by repeated glacial erosion. Major subaquatic moraine ridges, presumably corresponding to late glacial readvances, divide the lake into individual subbasins. These moraines reach impressive heights of up to 150 m above the basin floor, causing narrow passages with peculiar current-induced erosional and depositional patterns. The flanks of these moraines are furthermore affected by mass wasting processes.
Mass flows also occur persistently on gently dipping (10-20°) lateral slopes draped by several meters of Holocene sediments. Numerous slides were triggered by a historic earthquake in A.D. 1601 (MW ∼ 6.2), leaving behind a scar of typically 4 to 7 m in height. The scar of the largest slide, previously believed to consist of several smaller individual flows, can in fact be traced over a distance of more than 6 km along the lateral margin. Some slides run out for up to 1 km on the basin floor, where they form arched pressure ridges. Combination with subsurface data indicates that these ridges are not surficial features but rather origin from a deeply rooted detachment surface producing a succession of thin overthrusted slices. Next to the soft-sediment flows, debris cones resulting from subaerial rockfalls and few larger rockslides are common below steep cliffs, the largest one covering over one square kilometer.
The prograding slopes of two main deltas in the lake are dominated by a pattern of slope-parallel sediment waves and a network of poorly defined shallow channels rather than major canyons. This morphology suggests that delta-descending underflows are not confined to narrow pathways but form flow sheets occupying the entire delta width, in particular in the lower part of the prograding slope. The upper part is heavily altered by gravel mining and consecutive renaturation through deposition of excavated material causing a massive anthropogenic overprint on the lake floor morphology.


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