Evidence for mid- to late-Holocene palaeotsunami deposits,Kakawis Lake,Vancouver Island,British Columbia |
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| Authors: | Gloria I López |
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| Institution: | (1) School of Geography and Earth Sciences, McMaster University, 1280 Main Street West, Hamilton, ON, L8Z 4K1, Canada;(2) School of Earth and Ocean Sciences, University of Victoria, Bob Wright Centre A405, PO Box 3065 STN CSC, Victoria, BC, V8W 3V6, Canada |
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| Abstract: | 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. |
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