Paraglacial geomorphology of Quaternary volcanic landscapes in the southern Coast Mountains, British Columbia

An important paradigm in geomorphology is paraglacial sedimentation, a phrase first used almost 40 years ago to describe reworking of glacial sediment by mass wasting and streams during and after continental-scale deglaciation. The concept has been extended to include nonglacial landforms and landscapes conditioned by glaciation. In this paper we apply the paraglacial concept to volcanoes in southern British Columbia, Canada, that formed, in part, in contact with glacier ice. The Cheekye River basin, a small watershed on the flank of a volcano that erupted against the decaying Cordilleran ice sheet, has a Holocene history marked by an exponential decay in debris-flow activity and sediment yield. Its history is consistent with the primary exhaustion model of the paraglacial cycle. At larger spatial scales, this primary sediment is reworked by rivers and transported downstream and augmented by stochastic geomorphic events. Repeated large landslides from Mount Meager volcano in southern British Columbia have delivered a disproportionate volume of sediment to the fluvial system: although occupying only 2% of the watershed area, 25–75% of the 10 km of sediment deposited in Lillooet River valley during the Holocene originated from the volcano. In these cases a significant overall reduction in sediment yield must await the removal, by erosion, of volcanic edifices, a process that could take up to millions of years. These examples of paraglacial activity on Quaternary volcanoes are end members in the spectrum of landscape response to Pleistocene deglaciation. The phrase paraglacial sedimentation was coined by Church & Ryder (1972) to describe large-scale reworking of glacial sediment by colluvial and fluvial processes in SW British Columbia, Canada, during and following terminal Pleistocene deglaciation. Since then, the concept has been modified and extended to a wider range of landscapes and has become an important paradigm in geomorphology. Ballantyne (2002, p. 1938) defined paraglacial geomorphology as ‘nonglacial earth-surface processes, sediment accumulations, landforms, landsystems and landscapes that are directly conditioned by glaciation and deglaciation’. Rates of landscape adjustment and the duration of the paraglacial period depend on a variety of factors, principally rock structure and lithology, the distribution and types of surficial materials, relief, storage, climate and spatial scale. In small catchments (,100 km) paraglacial transfer of sediment is dominated by erosion and follows a ‘primary exhaustion’ model (Church & Ryder 1972; Ballantyne 2002). Here, sediment is transferred from slopes to fans as rapidly as the operative processes, including mass movement and fluvial erosion, allow. Eventually, sediment that can readily be moved to lower elevations is exhausted. As scale increases, sediment transferred to valley bottoms becomes available for fluvial reworking. As noted by Church & Slaymaker (1989), sediment yields in larger watersheds in British Columbia typically increase downstream (Fig. 1), reflecting migration of the paraglacial sediment pulse and reworking of sediment from Pleistocene glacial deposits (e.g. Brooks 1994). Dadson & Church (2005) showed that, in order to reproduce the morphology and channel network of a typical post-glacial valley in SW British Columbia, it is necessary to combine: (1) stochastic landsliding, for example large rock avalanches; (2) non-linear diffusion processes, including slopedependent rates of rockfall, soil creep and shallow slope failures; and (3) and fluvial transport. In their model, the pattern of Holocene fluvial sediment yield is complex, commonly multimodal, and varies strongly depending on the rate and timing of landslides and the diffusivity constant. Terrain with the highest landslide and diffusivity rates had the highest late Holocene fluvial sediment yields. We suggest that Quaternary volcanoes represent this end member. Quaternary volcanoes in the area of the former Cordilleran ice sheet in SW British Columbia and From: KNIGHT, J. & HARRISON, S. (eds) Periglacial and Paraglacial Processes and Environments. The Geological Society, London, Special Publications, 320, 219–233. DOI: 10.1144/SP320.14 0305-8719/09/$15.00 # The Geological Society Publishing House 2009. NW Washington (Fig. 2) include Mount Garibaldi, Mount Cayley and Mount Meager in British Columbia, and Mount Baker and Glacier Peak in Washington. They differ from Cascade volcanoes south of the limit of the late Pleistocene ice sheet in that they have formed partly from eruptions into or against former glaciers, and they have a distinctive set of landforms reflecting this history (Mathews 1958; Hickson 2000), such as rapidly quenched, ice-marginal lava flows (tuyas) and subglacial flows similar in form to eskers (Mathews 1952). The volcanoes have been deeply dissected by valley glaciers (see Montgomery 2002) and rivers during the Holocene, leaving steep slopes and local relief in excess of 2000 m. They are the loci of frequent large Holocene landslides (Friele & Clague 2005; Friele et al. 2005, 2006; Simpson et al. 2006). Bedrock structure, relief and geomorphic processes active at these volcanoes have been conditioned by glaciation and, therefore, can be thought of as paraglacial. This paper reviews the late-glacial and postglacial histories of three volcanoes that erupted against Pleistocene glaciers. Our objective is to explore these histories in the context of paraglacial sedimentation and related landscape evolution. The record of sediment delivery from one of the three volcanoes (Mount Garibaldi) broadly fits a sediment exhaustion model, which is the cornerstone of the paraglacial paradigm. However, sediment delivery from all three volcanoes is punctuated by periodic, very large landslides that strongly affect the style and rate of sedimentation far downstream.