Cheakamus Basalt Lavas, British Columbia: A Pleistocene record of rapid, continuous eruption within a mountainous drainage system

The Cheakamus basalts are voluminous (1.65 km3) Late Pleistocene, valley-filling lavas erupted from a vent situated near Conflict Lake, in the alpine Callaghan Valley near Whistler, British Columbia. Geochemical and petrographic properties suggest these olivine-plagioclase porphyritic basalts derive from a single magma batch affected by minor sorting of phenocrysts and xenocrystic plagioclase. Thirty-four sites sampled for paleomagnetic direction measurement record a mean pole direction of 345.2 ̊ / 73.0 ̊ (α95 = 1.3°), showing no statistical variation nor drift with stratigraphic position. These data suggest the lavas were emplaced in a single paleomagnetic moment – a period of time significantly less than 2,000 years. 40Ar/39Ar geochronometry on three samples return a weighted mean age estimate of 15.95 ± 7.9 ka (2s) and field evidence, including till-covered, well-glaciated lava flow surfaces, indicate the eruption coincided with the early stages of the Fraser glaciation (~18-20 ka). The lavas preserve features indicative of a landscape hosting diverse and dynamic paleoenvironments. Subaerial eruption of basalt filled an ice-free Callaghan Creek drainage before inundating and damming the paleo-Cheakamus River, creating an upstream rising body of water. Periodic lava-dam overtopping resulted in syn-eruptive intermittent flooding of lava surfaces, expressed by discontinuous interflow sediment lenses. 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 formation of Callaghan and Conflict Lakes via long-term lava-damming remain direct indicators of the impact basaltic eruptions have on the geomorphology of valley systems.