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. 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. D 2005 Elsevier B.V. All rights reserved. Keywords: Valley fill; Stratigraphy; Debris flow; Hyperconcentrated flow; Fiord-lake; Holocene; Lillooet River; Mount Meager; British Columbia 1. Introduction T Corresponding author. Fax: +1 604 898 4742. Delta geomorphology, architecture, and sedimen- E-mail address: [email protected] (P.A. Friele). tary processes are governed by a host of factors, 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2005.01.011 306 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 including the character of the receiving basin (bathy- Slaymaker (1972, 1977) summarized historic sedi- metry, water circulation, wave and current regimes, ment yield in the valley, whereas Jordan and Slay- and thermal and density stratification) and changes in maker (1991) and Slaymaker (1993) estimated the base level, sediment supply, and climate (Kostaschuk, long-term yield using a sediment budget approach. 1987; Smith, 1991; Kazuaki et al., 2001). In this Jordan and Slaymaker (1991) noted a 50% discrep- paper, we examine the contribution of a Quaternary ancy between the budgeted and historic sediment volcano to a deltaic valley fill in the Lillooet River yields and surmised that it might be due to a change in valley in southwestern British Columbia. We demon- sediment delivery to Lillooet River following the strate that instability on the volcano during the Little Ice Age or to underestimation of the frequency Holocene strongly influenced the evolution of the or magnitude of landslides at Mount Meager. delta. Jordan and Slaymaker (1991) further proposed a The sediment fill in the Lillooet River valley modification of the paraglacial sediment concept. records postglacial infilling of a 75-km-long fjord lake Paraglacial sedimentation involves large transfers of downstream from Mount Meager volcano (Fig. 1). sediment from uplands to river valleys during and Fig. 1. A) Map of southern British Columbia showing the location of the Mount Meager volcanic complex (MMVC). B) Southwestern British Columbia, showing Highway 99 from Vancouver to Pemberton, and the Mount Meager volcanic complex. C) The study area showing the Lillooet River valley, and drill sites DHPV01-12 and SH1. The white bars delineate the four river reaches: 1-Meager Creek to Railroad Creek; 2- Railroad Creek to Ryan River; 3-Ryan River to Green River; 4 Green River to Lillooet Lake. Human settlement extends from the lower end of reach 1 (at DHPV09) to Lillooet Lake. P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 307 immediately after deglaciation, causing rapid growth km2) is rugged, with up to 2800 m of local relief and of fans at tributary mouths (Ryder, 1971)and peaks up to 3000 m in elevation. About 15% of the subsequent aggradation of floodplains (Church and basin is glaciated. Most of the basin is underlain by Slaymaker, 1989). Jordan and Slaymaker (1991) resistant plutonic rocks (Woodsworth, 1977), but the suggested that Holocene sediment yield in the Mount Meager volcanic complex (Fig. 2), a dormant Lillooet River basin is episodic, with numerous Quaternary volcano, underlies about 2% in the basin pulses induced by volcanism, landslides, Neoglacia- headwaters. tion, and land-use change. Large parts of the Mount Meager volcanic complex Desloges and Gilbert (1994) studied sedimentation are hydrothermally altered (Read, 1979). The alter- rates in Lillooet Lake and suggested that fines (b63 ation, together with the steep slopes, result in high Am), which make up the lacustrine sediment pile, are rates of mass wasting and landslides. Four landslides produced most abundantly by glacial comminution of in excess of 1Â106 m3 occurred in the last century rock debris and that landslides are a relatively minor alone-in 1931, 1947, 1975, and 1998 (Bovis and source of fine sediment. They thus stress the Jakob, 2000; Carter, 1932; Croft, 1983; Evans, 1987; dominance of the paraglacial sediment source to Mokievsky-Zubok, 1977). The 1931 failure produced Lillooet Lake. a secondary debris flow that reached the mouth of Based on the history of other Cascade volcanoes, Meager Creek (Fig. 2) and caused flood surges along episodic sedimentation should be recorded as a large Lillooet River (Decker et al., 1977, p. 161). Friele and number of discrete thick beds within the Holocene Clague (2004) documented flank collapses 8700 and sediment fill in Lillooet River valley. Radiocarbon 4400 years ago, involving at least 6Â108 m3 of ages from landslide debris exposed along Meager material on the south side of the volcanic complex. Creek and upper Lillooet River (Friele and Clague, Debris flows from these flank collapses traveled the 2004; Jordan, 1994; McNeely and McCuaig, 1991) length of Meager Creek into Lillooet River valley. In suggest that at least 12 large prehistoric landslides addition, Plinth Peak (Fig. 2) erupted explosively occurred in these valleys. In addition, an outburst about 2360 years ago (Clague et al., 1995; Nasmith et flood occurred shortly after the last eruption at al., 1967), producing volcanic and landslide deposits Mount Meager (Stasiuk et al., 1996). There probably that dammed upper Lillooet River (Stasiuk et al., have been other large landslides, but they have not 1996; Stewart, 2002). The dam failed, causing an been identified due to burial or erosion, while outburst flood that swept down the valley (Evans, smaller debris flows occur almost annually (Jakob, 1992; Stasiuk et al., 1996). This instability is well 1996). documented in the valleys adjacent to Mount Meager, By drilling the upper sediments of the Lillooet but its effects in the Lillooet valley to the south are valley fill, we test the episodic sediment yield model unknown because Lillooet River aggraded throughout proposed by Jordan and Slaymaker (1991). In this the Holocene (Jordan and Slaymaker, 1991), burying paper, we describe the architecture of the upper valley the evidence. fill and calculate rates of floodplain aggradation and Lillooet River is divisible into four reaches below delta-front progradation using radiocarbon ages on the mouth of Meager Creek (Fig. 1; after Jordan and fossil plant material recovered from drill cores. Slaymaker, 1991). The first, most northerly reach Finally, we compile the evidence to counter the notion extends 25 km downvalley from Meager Creek to that the pararglacial sediment supply dominates in the Railroad Creek. The river in this reach is braided Lillooet River basin. (Fig. 2), has a cobble bed (Fig. 3), and a gradient of 0.006. Gravelly colluvial fans from tributary valleys and rockslide deposits extend onto the active flood- 2. Setting plain, which is up to 1 km wide. The second reach extends 10 km downvalley from Railroad Creek to Lillooet River flows in a glacially modified river the Ryan River fan (Fig. 4). In this reach, the valley in the southern Coast Mountains of British channel is wandering (Desloges and Church, 1987) Columbia (Fig. 1). The Lillooet River basin (3850 to meandering and has a pebble-cobble bed (Fig. 3) 308 P.A. Friele et al. / Sedimentary Geology 176 (2005) 305–322 Fig. 2. View northwest up Lillooet River to the Mount Meager volcanic complex (Province of British Columbia airphoto BC563: 113; taken September 29, 1949). In the foreground is a 12-km length of reach 1, showing the braided channel of Lillooet River and confining debris fans. DGS indicates the fresh track of the 1931 debris flow that traveled the length of Meager Creek. The basin on the west flank of Pylon Peak is the source of two large landslides mentioned in the text (8700 and 4400 years ago).

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