Impact of a Quaternary Volcano on Holocene Sedimentation in Lillooet River Valley, British Columbia
Sedimentary Geology 176 (2005) 305–322 www.elsevier.com/locate/sedgeo Impact of a Quaternary volcano on Holocene sedimentation in Lillooet River valley, British Columbia P.A. Frielea,T, J.J. Clagueb, K. Simpsonc, M. Stasiukc aCordilleran Geoscience, 1021, Raven Drive, P.O. Box 612, Squamish, BC, Canada V0N 3G0 bDepartment of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada V5A 1S6; Emeritus Scientist, Geological Survey of Canada, 101-605 Robson Street, Vancouver, BC, Canada V6B 5J3 cGeological Survey of Canada, 101-605 Robson Street, Vancouver, BC, Canada V6B 5J3 Received 3 May 2004; received in revised form 15 December 2004; accepted 19 January 2005 Abstract Lillooet River drains 3850 km2 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 aÀ1 and the average delta progradation rate is 6.0 m aÀ1. 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 aÀ1 over 50-yr intervals.
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