Emplacement and erosion of valley-filling basalt lavas in shifting Quaternary environments of the Garibaldi Volcanic Belt, British Columbia, Canada

The Cheakamus basalts are a set of ~31 km long, 1.65 km³ valley-filling olivine basalts which erupted into the glaciated Callaghan-Cheakamus Valley system of the Garibaldi Volcanic Belt (GVB), British Columbia. Combined paleomagnetic and radiometric (⁴⁰Ar/³⁹Ar) analysis dates the lavas as a short-lived (< 2 ka duration) eruption at 15.95 ± 7.9 ka (2σ); additional field evidence, including well-glaciated lava flow surfaces overlain by till, indicate the eruption coincided with the early stages of the Fraser Glaciation (LGM) at ~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 and well-developed entablatures in the upper portions of lava flows. Rare instances of enigmatic cooling columns indicate localized ice contact with glaciers that partially filled the Cheakamus Valley. Emplacement features and morphologies in the Cheakamus Valley have been heavily altered by the erosional overprinting of a glacial lake outburst flood (GLOF) that preliminary ¹⁰Be analysis dates at the close of the LGM (11-10 ka). Lavas in the Callaghan valley remain untouched by the GLOF. Their aerial extent and distribution, especially at high elevations in tributary valley mouths suggest bottlenecking and backing-up of lavas due to narrowing in valley topography. Current work combines field mapping and analogue modelling and aims to provide insight into the emplacement dynamics of effusive lavas in the steep, confined terrain of the BC Coast Mountains. Despite, and in part because of, their heavily modified morphology, the Cheakamus basalts act as an excellent recorder of both effusive volcanic processes and the paleoenvironments into which they erupted. Their thorough and accurate analysis is especially pertinent temporally, as they erupted during a time of glacial flux and can provide additional evidence for the timing and location of the advancing Cordilleran ice sheet. Spatially, the Cheakamus basalts are proximal to population centers and transport infrastructure and thus have implications for potential volcanic hazards and attendant risks, as any future effusive, valley-filling basaltic eruption from the GVB will likely share similar emplacement characteristics and processes.