Morphological signatures of mass wasting and delta processes in a fjord-lake system

Abstract

High-resolution bathymetrical data from the Lovatnet lake, western Norway, reveal a diverse morphology including traces from historic and prehistoric landslides. Onshore LiDAR and selected seismic profiles support the interpretation. Our data from the lake show significant debris lobes below Ramnefjellet that was the source of the well-known rock avalanche and tsunami disasters in 1905 and 1936 which caused a total of 134 casualties. The debris is divided into three main lobes reaching a total length of 1.8 km along the lake bottom from the impact site, and diffuse lobe features below the Bødalsdelta. Frontal deformations are minor. Lobes on the basin plain have a volume of more than 1.2 × 10⁶ m³ including debris entrained from the surrounding slopes and lake floor. The entire lake volume was set in motion during the catastrophic events. Returning bottom flows transported debris into the lake where lobes and minor, distinct mounds were deposited up to c. 1.5 km from the lake’s outlet. The mounds could be anthropogenic, and possibly include a vessel that disappeared during the event. Erosion and smaller failures occurred elsewhere in the lake basin. Fan-delta morphologies are varied, with ridges and protuberances, reflecting deltaic sedimentation interrupted by major mass-wasting or other events in the deltas and/or their catchments. This is especially clear for the Raudi, Hellsete and Bødalen deltas, but minor protuberances are also found in other deltas. The data also reveal traces from large and previously undocumented landslide events. Gravitational deposits generally dominate the steep slopes and glacial landforms are scarce. The study demonstrates the importance of linking onshore and offshore morphological datasets in order to obtain an overview over process variability in fjord-type lakes of importance for further investigation and hazard assessment.