Cheakamus basalt lavas, British Columbia: a Pleistocene record of rapid, continuous eruption within a mountainous drainage system

The Cheakamus basalts are a voluminous (1.65 km3) set of Late Pleistocene, valley-filling lavas erupted from a vent situated near present-day Conflict Lake, in the alpine Callaghan Valley near Whistler, British Columbia. Geochemical and petrographic properties suggest these olivine-plagioclase porphyritic basalt lavas derive from a single batch of magma affected by minor sorting of phenocrysts and xenocrystic plagioclase. Thirty-four sites sampled for paleomagnetic directions record a mean pole direction of 345.2 degrees /73.0 degrees (alpha 95 = 1.3 degrees) and show no statistical variation nor drift with stratigraphic position. These data suggest that the Cheakamus basalt lavas were emplaced in a single paleomagnetic moment--a period of time significantly less than 2000 years. 40Ar/39Ar geochronometry on three lava samples returns a weighted mean age estimate of 15.95 +/- 7.9 ka (2 sigma) and field evidence, including well-glaciated lava flow surfaces overlain by till, indicate the eruption coincided with the early stages of the Fraser glaciation (similar to 20-18 ka). The lavas preserve features indicative of a landscape hosting diverse and dynamic paleoenvironments. Subaerial eruption of basalt lava filled an ice-free Callaghan Creek drainage system before inundating and damming of the paleo-Cheakamus River creating an upstream rising body of water. Periodic overtopping of the lava dam resulted in syn-eruptive intermittent flooding and overtopping of lavas expressed by discontinuous lenses of interflow sediment. Rare instances of enigmatic cooling columns may also indicate localized ice contact with glaciers that partially filled the Cheakamus Valley. The displacement of the modern Cheakamus River and the long-term damming and formation of Callaghan and Conflict lakes remain direct indicators of the control and impact these basaltic eruptions have had on the geomorphology of the present-day Callaghan and Cheakamus valleys.