Evidence for mid- to late-Holocene palaeotsunami deposits,Kakawis Lake,Vancouver Island,British Columbia

Kakawis Lake situated four metres above sea level on western Vancouver Island, British Columbia, Canada, was the target of a palaeotsunami investigation. Six percussion cores recovered from this lake contain six anomalous deposits interbedded within the unconsolidated lacustrine sediments. Detailed sedimentological, geophysical and macro-fraction analyses were performed. The methods new to palaeoseismic approaches proved to be successful tools to characterize the anomalously coarse layers enriched in terrestrial plant detritus and marine shells. Based on at least eight types of evidence, six tsunami inundations are suggested as mechanisms responsible for the anomalous deposition, spanning from 3,634 to 2,534 cal yrs BP. Each tsunami event consists of a combination of different lithological facies resulting from different stages of tsunami inundation and settling of the material in the lake basin (pulses and inter-pulses). Tsunami deposits in lakes are shown to be less vulnerable to erosional and bioturbation processes than those found in marshes or beaches as well as underwater marine environments. However, few palaeoseismic studies have been carried out in low-elevation lakes along the Cascadia Subduction Zone region. The three last tsunami events known to have inundated areas along the Pacific shores of southern British Columbia, Canada and northern USA are not present at Kakawis Lake, establishing a current <4 m above mean sea level vertical limit as possible maximum tsunami height for areas located away from fjord heads on Vancouver Island. The anomalous deposits found in Kakawis Lake may be the oldest geological evidence of inferred tsunami on Vancouver Island, providing a possible recurrence interval between 200 and 400 years.