Paleoseismic investigations on the central west coast of Vancouver Island, British Columbia, Canada : pre-historic tsunami deposits in coastal lacustrine environments
Date
2002
Authors
López Cadavid, Gloria Inés
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Abstract
The use of diverse analytical techniques, new to the field of Paleoseismology, proved to be an effective tool to depict and determine geophysical, biochemical and structural parameters of tsunami deposits. Such techniques were applied on seven percussion cores from unconsolidated freshwater lacustrine sediments from Kakawis Lake, central west coast of Vancouver Island, British Columbia. The objective was to identify and characterise sedimentologically tsunami deposits emplaced in low-elevation lakes based on a diversity of parameters, with the exclusion of microfossil identification. Six inferred tsunami events (deposits) were depicted at Kakawis Lake suggesting past inundations by tsunamis of either local or distant sources. Each tsunami deposit was identified based on an association of lithofacies, which are specific to individual stages of any given tsunami inundation in lakes. Two stages were defined (tsunami pulse and tsunami inter-pulse). Based on multiple analyses and observations, more detailed characteristics of lacustrine tsunami deposits are proposed. Theories concerning emplacement of tsunami-transported sediments are reviewed.
Multiple sizes, shapes, species, and types of materials characterise the elastic, carbonate (marine shells) and terrestrial plant detritus fractions common to all Kakawis Lake tsunami deposits. The coarsest tsunamigenic grain-size sedimentation evidenced to date in a lake from Vancouver Island is associated with these paleotsunami inundations. All fractions are from local provenance and two of them are undoubtedly marine in origin.
Kakawis Lake tsunami deposits are believed to be the oldest tsunami events evidenced to date in coastal lakes on western Vancouver Island. From a ~15,500 year old sedimentological record (longest core from Kakawis Lake), tsunamigenic evidence spans from ~4500 to 2384 cal years BP (datum = AD 2000). The three most recent historic tsunami events known to have inundated coastal areas along the Pacific shores of southern British Columbia and northern U.S.A. were not recovered at Kakawis Lake.
Such absence establishes a <4 m above mean sea level (at normal tide) as the newest possible maximum tsunami run-up for areas located on rugged-rocky indented coasts, ~ 9 km inland from open-ocean shores, but away from fjord heads on Vancouver Island. Chrono-stratigraphical correlations between tsunami deposits from other tsunamigenic lakes located on the western shores of Vancouver Island are considered. Other possible counterparts (i.e. tsunan1i sand sheets buried in tidal marshes) are overlooked. The ages of Kakawis Lake tsunami events may correlate with the oldest three mega-thrust earthquake identifiers from the well-established Atwater - Hemphill Haley chronology for the west coast of North America.
Radiocarbon dates obtained from previously studied archaeological sites on western Vancouver Island may constrain the radiocarbon dates obtained from diverse terrestrial plant material mixed within Kakawis Lake pre-historic tsunami deposits. In the Pacific Northwest, there is a substantial lack of both scientific and written historic data related to Cascadia great earthquakes that may help elucidate paleoseismic investigations. Archaeological evidence may help constrain paleoseismic evidence, which in turn may constrain seismic, geodetic and tsunami simulation models suggested for the Cascadia Subduction Zone region. Such information is basic to enhance hazard and prevention/emergency-response plans of any given coastal community neighbouring tectonically active continental margins like Cascadia.