Prehistoric Sensitive Clay Landslides and Paleoseismicity in the Ottawa Valley, Canada

Abstract

The ages of 39 large retrogressive landslides and three areas of disturbed terrain in the Ottawa Valley were compiled using 122 published and unpublished radiocarbon ages. The chronological dataset includes 15 confined-valley landslides, 23 scarp-side landslides, and four special case features (one massive landslide, three disturbed terrain areas). The ages of the features range from ‘modern’ to 8,000 ¹⁴C cal BP. Distinct clusters of 10 and 11 coincidentally-aged landslides at ~1,000 and ~5,150 ¹⁴C cal BP are the groups of landslides previously interpreted to have been triggered by paleoearthquakes. Scarp-side landslides with scars between 0.1 and 10 km² are the dominant failure morphology forming the two age clusters and constitute an important component of the interpreted paleoseismic evidence. Five of the confined-valley landslides are part of the ~1,000 ¹⁴C cal BP cluster, but the other ten failures are of widely varying ages. One of the special case features, a massive landslide originating from a source area of ~20 km², falls within the ~1,000 ¹⁴C cal BP cluster. Radiocarbon ages representing the age of the Treadwell and Wendover disturbed terrains, suggest that these areas are contemporary with the Lefaivre disturbed area at ~7,900 ¹⁴C cal BP, but this is not an unequivocal interpretation. Notwithstanding sampling bias within the dataset, it is inferred from the high proportion of dated landslides falling within the two interpreted paleoearthquake clusters that there is a ‘strong’ paleoseismic signature within the temporal pattern of landsliding within the Ottawa Valley. Based on the Ottawa Valley dataset, scarp-side landslides with preserved debris fields and scars greater than 0.1 km², and landslides in general with scars greater than 1 km², are more promising targets than confined-valley landslides for paleoseismic studies.