Abstract

Cheekye fan, a large (∼25 km2) fan located at the head of Howe Sound, southwestern British Columbia, has its origins in the collapse of the western flank of Mount Garibaldi onto a waning Late Pleistocene glacier, followed by post-glacial redistribution of colluvial–glacial sediments. Reconstruction of internal fan architecture allowed characterization of the rapid decline in sediment delivery to the lower fan through the paraglacial period, a result holding important implications for hazard assessment. A ground penetrating radar profile on lower Cheekye fan has reflectors with a steep (25°), westerly dip lying between 48 and 26 m above sea level, which are interpreted as foreset beds. Radiocarbon ages from marine deltaic deposits at the head of Howe Sound indicate that the sea stood at about 45 m above sea level at 10 200 BP. Together, the data indicate that the head of Howe Sound was deglaciated by 10 200 BP, that the fan had prograded 2.5 km into the fjord by this time, and continued to prograde as relative sea level fell. Reflectors at –10 m above sea level with a steep southerly dip along the southern edge of the lower fan, and radiocarbon ages in this vicinity, indicate it had reached close to its modern extent by 8000–9000 BP. Calculation of the volume of sediment stored in the lower fan indicates that at least 90% of the material was deposited before 6000 BP. This indicates that the Cheekye fan is largely a relict feature, putting its large size into context for future hazard assessments.

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