Abstract
The Lakelse Lake area in northwestern British Columbia, Canada, has a long history, and prehistory, of rapid sensitive clay landslides moving on very low gradients. However, until now, many landslides have gone undetected. We use an array of modern tools to identify hitherto unknown or poorly known landslide deposits, including acoustic subbottom profiles, multibeam sonar, and LiDAR. The combination of these methods reveals not only landslide deposits, but also geomorphic and sedimentologic structures that give clues about landslide type and mode of emplacement. LiDAR and bathymetric data reveal the areal extent of landslide deposits as well as the orientation of ridges that differentiate between spreading and flowing kinematics. The subbottom profiles show two-dimensional structures of disturbed landslide deposits, including horst and grabens indicative of landslides classified as spreads. A preliminary computer tomography (CT) scan of a sediment core confirms the structures of one subbottom profile. We also use archival data from the Ministry of Transportation and Infrastructure and resident interviews to better characterize historic landslides.
Highlights
Sensitive clays are largely restricted to uplifted glaciomarine sediments in a few areas of the globe, but they are most common in Canada, Alaska, and Norway (Torrance, 1983)
Stories of vertically oriented trees moving into waterbodies have been recounted in oral traditions of local First Nations and provide context and evidence of translational landslides in general (Geertsema et al, 2017)
Unpublished archival data provided information for the 1962 Lakelse landslides, whereas the nearby Mink Creek landslide was previously described by Geertsema et al (2006b and references therein), but a brief summary is given below
Summary
Sensitive clays are largely restricted to uplifted glaciomarine sediments in a few areas of the globe, but they are most common in Canada, Alaska, and Norway (Torrance, 1983). Clays may be described as sensitive when the undisturbed strength is greater than the disturbed (remoulded) strength, where the remoulded material can behave as a fluid (Geertsema and Torrance, 2005). Landslides involving sensitive clays typically occur as spreads or flows (flowslides) depending in part on the degree of remoulding of the displaced material (Demers et al, 2014; Geertsema et al, 2017). Spreads display both extensional ridges and depressions, typically referred to as horsts and grabens, oriented perpendicular to movement direction (Fig. 1)
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