Impact of a Quaternary volcano on Holocene sedimentation in Lillooet River valley, British Columbia

Abstract

Lillooet River drains 3850 km 2 of the rugged Coast Mountains in southwestern British Columbia, including the slopes of a dormant Quaternary volcano at Mount Meager. A drilling program was conducted 32–65 km downstream from the volcano to search for evidence of anomalous sedimentation caused by volcanism or large landslides at Mount Meager. Drilling revealed an alluvial sequence consisting of river channel, bar, and overbank sediments interlayered with volcaniclastic units deposited by debris flows and hyperconcentrated flows. The sediments constitute the upper part of a prograded delta that filled a late Pleistocene lake. Calibrated radiocarbon ages obtained from drill core at 13 sites show that the average long-term floodplain aggradation rate is 4.4 mm a −1 and the average delta progradation rate is 6.0 m a −1. Aggradation and progradation rates, however, varied markedly over time. Large volumes of sediment were deposited in the valley following edifice collapse events and the eruption of Mount Meager volcano about 2360 years ago, causing pulses in delta progradation, with estimated rates to 150 m a −1 over 50-yr intervals. Two of the volcaniclastic units identified in drill core correlate with previously documented strong acoustic reflectors in Lillooet Lake at the downstream end of the basin. The Mount Meager massif constitutes only 2% of the Lillooet River drainage, but lithology counts of Lillooet River channel gravels indicate that a disproportionate percentage of the sediment is derived from the volcano. The data indicate that deposits of large debris flows are important elements of the sedimentary sequence and that Mount Meager dominates the sediment supply to Lillooet River.