QUATERNARY GEOLOGY AS A BASIS FOR LANDSLIDE SUSCEPTIBILITY ASSESSMENT IN GLACIOMARINE CLAYS, ONSHORE NORWAY

Mapping of Quaternary geology in Norway has for decades been the basis for hazard and risk assessment for landslides in highly sensitive clay (quick clay). Of particular importance is information on the distribution of fine-grained, fjord-marine deposits such as clays. These sediments accumulated in fjords following the final deglaciation of the Fennoscandian Ice sheet. The deposits subsequently emerged and became dry land due to glacioisostatic rebound of the crust. Groundwater leaching gave rise to the formation of quick clay in some areas and numerous quick clay landslides have occurred in the past. The marine limit (ML) defines a natural upper limit of marine influence and for the occurrence of fjord-marine deposits. It follows that any issue concerning marine clays (e.g. quick clay) can be disregarded above ML, which is important information for municipalities during planning work. A database for nationwide ML information was established recently at the Geological Survey of Norway (NGU) and the information is available to the public via web services together with Quaternary maps. The ML varies over the country and reaches up to 220 m a.s.l. Below ML, a filtering of the Quaternary map information is needed to identify areas where clays are potentially present. According to existing models, fine-grained, fjord-marine deposits are more frequently encountered below some deposit types compared to others, and the possibility of encountering marine clays within an area is defined as clay-deposit susceptibility. Stratigraphic information is needed for verification. Further landslide susceptibility assessment requires additional information on topography and ground conditions. A National database for ground investigation is under development and will provide an important input. Other geological information is also important such as the distribution of landslide debris and landslide scars. High-resolution, digital elevation models (e.g. from LiDAR) help to identify such features and are highly beneficial for detailed mapping also below sea level in near-shore areas. Maps with filtered geological information can work as a supplement to maps produced during the ongoing quick-clay mapping program and as a help to prioritize areas requiring further investigation.