PREHISTORIC LANDSLIDES IN GLACIOMARINE CLAYS REVEALED BY ONSHORE MAPPING AND MARINE SEISMIC INVESTIGATION AT LEIRFJORD, NORTH NORWAY

Quaternary mapping and marine seismic investigation in the Leirfjord area, north Norway, reveals its glacial history and landscape development including large landslide events. The latter involved glaciomarine clays that emerged to dry land due to glacioisostatic rebound following the last ice age. The marine limit is c. 118 m a.s.l. Sensitive clay (and quick clay) developed locally due to leaching of salt from the deposit. Beach deposits and peat commonly drape the marine sediments. A highly irregular, metamorphic bedrock surface is exposed locally. The terrain displays various erosional features. These are escarpments formed by fluvial incision, marine abrasion, landslide activity or a combination, which is not always easy to interpret. For example, landslide scars are difficult to identify since marine erosion has potentially smoothed out landslide debris near the shoreline. For this reason, the area’s prehistoric landslide activity could easily be underestimated. However, marine seismic profiles in the fjord reveal at least one major, prehistoric landslide event in the area. A rough estimate of a minimum landslide volume gives 1.6x10⁶ m³. Age estimation is possible through a comparison of a relative sea level curve for the area and the level of reworking of landslide debris. This gives a maximum age of 2000 yrs before present. In addition, the combined onshore mapping and marine seismic data confirm that the final retreating ice-margin had a prolonged still-stand south of the area, which helps to explain the distribution and varied characteristics of onshore, glaciomarine sediments. Slopes that were affected by the former glacier tongue are relatively compact because they have been subjected to glaciotectonic deformation and mixing with glacial sediments. These slopes, now incised by major ravines, are interpreted as less susceptible to major failure events compared with slopes in the area consisting of more undisturbed glaciomarine clays and silts.