Geological Survey of Canada CURRENT RESEARCH 2001-A10 Discrimination of hot versus cold avalanche deposits: implications for hazards assessment at Mount Meager, British Columbia M.L. Stewart, J.K. Russell, and C.J. Hickson 2001 ©Her Majesty the Queen in Right of Canada, 2001 Catalogue No. M44-2001/A10E ISBN 0-660-18390-0 A copy of this publication is also available for reference by depository libraries across Canada through access to the Depository Services Program's website at http://dsp-psd.pwgsc.gc.ca A free digital download of this publication is available from the Geological Survey of Canada Bookstore web site: http://gsc.nrcan.gc.ca/bookstore/ Click on Free Download. All requests for permission to reproduce this work, in whole or in part, for purposes of commercial use, resale or redistribution shall be addressed to: Earth Sciences Sector Information Division, Room 200, 601 Booth Street, Ottawa, Ontario K1A 0E8. Authors’ addresses M.L. Stewart (mstewart@eos.ubc.ca) J.K. Russell (russell@perseus.geology.ubc.ca) Igneous Petrology Laboratory Earth and Ocean Sciences The University of British Columbia 6339 Stores Road Vancouver, B.C. V6T 1Z4 C.J. Hickson (chickson@NRCan.gc.ca) GSC Pacific, Vancouver 101-605 Robson Street, Vancouver, B.C. V6B 5J3 Discrimination of hot versus cold avalanche deposits: implications for hazards assessment at Mount Meager, British Columbia M.L. Stewart, J.K. Russell, and C.J. Hickson GSC Pacific, Vancouver Stewart, M.L., Russell, J.K., and Hickson, C.J., 2001: Discrimination of hot versus cold avalanche deposits: implications for hazards assessment at Mount Meager, British Columbia; Geological Survey of Canada, Current Research 2001-A10, 10 p. Abstract: Avalanche deposits within the Mount Meager volcanic complex are nearly monolithological and comprise mainly intermediate volcanic rock clasts up to 10.5 m, lack internal structure, and are very poorly sorted. Despite these physical similarities, the deposits originate from two distinct processes, cold rock-avalanche deposits from mass wasting and hot block-and-ash avalanche deposits derived from gravi- tational collapse of active lava domes and flows. Hot block-and-ash deposits are recognized by the presence of features indicative of high emplacement temperatures. Clasts within rock-avalanche deposits preserve predepositional weathering and jointing surfaces as well as fragmentation surfaces from transport. The abil- ity to discriminate between hot and cold avalanche deposits and the recognition of multiple volcanic events are critical to hazard assessment in volcanic terranes. Misidentification can lead to underestimating the risk of one (i.e. volcanic) hazard whilst overestimating the other (i.e. mass wasting). Résumé : Les dépôts d’avalanche dans le complexe volcanique de Mount Meager sont presque tous du même type lithologique et comprennent essentiellement des fragments de roches volcaniques de composi- tion intermédiaire pouvant atteindre 10,5 m de diamètre; ils sont très mal triés et dépourvus de structure interne. Malgré ces similarités physiques, ces dépôts sont issus de deux processus distincts : dépôts d’avalanche de pierres froide provoquée par un mouvement de masse et dépôts d’avalanche de cendres et de blocs chaude provoquée par l’effondrement gravitaire de dômes et de coulées de lave active. Les dépôts de cendres et de blocs chauds se distinguent par des caractéristiques indicatrices de températures de mise en place élevées. Les fragments se trouvant dans les dépôts d’avalanche de pierres préservent les surfaces d’altération et de débit antérieures à leur accumulation ainsi que les surfaces de fragmentation issues du transport. Afin de pouvoir évaluer les risques dans les terranes volcaniques, il est essentiel de distinguer les dépôts d’avalanche chaude des dépôts d’avalanche froide et de reconnaître les événements volcaniques multiples. Si l’identification est inexacte, on risque de sous-estimer la probabilité d’un danger (p. ex. un aléa volcanique) tout en surestimant la venue d’un autre danger (p. ex. le mouvement de masse). 1 Current Research 2001-A10 INTRODUCTION prominent peaks of the volcanic edifice, Meager, Capricorn and Plinth, rise to elevations of 2650 m, 2551 m, and 2677 m, Volcanic landscapes along convergent plate margins typi- respectively. cally feature large volcanic structures situated in regions of The geology of the Mount Meager volcanic complex has extreme topographic relief. They represent one of the most been well described by Anderson (1975), Read (1977, 1978, hazardous natural environments on the planet. Volcanic erup- 1990), Stasiuk and Russell (1989, 1990), Stasiuk et al. tions represent an obvious set of hazards. Additionally, (1996), and Hickson et al. (1999). For the purposes of this stratovolcanoes form relatively unstable structures and when paper we review the general distribution and nature of two situated in a high-relief landscape are optimal sources of rock formations, namely the Plinth Formation and the Pebble avalanches. The steep slopes also have an impact on the Creek Formation. The Plinth Formation is 90 000 to 110 000 nature of volcanic hazards associated with an eruption. Spe- years old and comprises volcanic deposits and subvolcanic cifically, steep slopes are an efficient mechanism for chang- stocks and feeders (Read, 1978). The Pebble Creek Forma- ing the style of volcanism. For example, lava flows and tion represents the youngest known volcanism at Mount Mea- domes that would, in other environments, represent lower ger and has recently been redated at 2360 BP (Clague et al., risk events can be quickly transformed into explosive 1995; Leonard, 1995). This formation includes Plinian- and pyroclastic avalanches when reaching steep slopes. For these Merapi-style deposits of intermediate volcanic rocks (Stasiuk reasons, in the stratigraphic successions that underlie these and Russell, 1989; Stasiuk et al., 1996; Hickson et al., 1999). landscapes, avalanche deposits derived from both volcanic The latter Merapi-style avalanche or pyroclastic and mass-wasting events are common. block-and-ash-flow style deposits are distributed as a broad, The Mount Meager volcanic complex is a deeply eroded stratovolcano in the Coast Mountains that has produced numerous rock avalanches and, most recently, has erupted in 2360 BP. Our objectives in this paper are twofold. Firstly, we Quaternary volcanic rocks use our revised geological map to document the distribution Late Tertiary intrusive rocks and characteristics of two distinct types of avalanche depos- Prehistoric avalanche deposits Mount its. These deposits are superficially similar in character, but 1986 1986 Historic debris avalanches derive from mass wasting (‘cold’ avalanches) and volcanic N Meager 1931 Lillooet eruption (‘hot’ avalanches). Secondly, we discuss the impli- 1947 02040 1975 GARIBALDI cations of recognizing the origins and timings of these two River types of avalanche deposits. The discrimination of these two kilometres different deposits is essential to identify natural hazards prop- Pemberton erly (e.g. Hickson, 1994). Furthermore, proper identification 122o 12¢ of these hazards is critical when it comes to quantitatively 50o 18¢ assessing the risk of specific hazards (Morgan and Henrion, Mt. Cayley 1990). Whistler 1963 1984 Jervis 1855-56 GEOLOGICAL SETTING Inlet Mt. Garibaldi The Mount Meager volcanic complex is the most recently active volcano on the Canadian side of the Cascade magmatic BELT arc within the Coast Mountains of southwestern British Squamish Columbia (Fig. 1). The Quaternary stratovolcanoes of south- western British Columbia, including Mount Meager, Mount Cayley, and Mount Garibaldi, make up the Garibaldi volcanic belt (Mathews, 1958). Mount Meager comprises a number of Pitt volcanic centres that have been active during the past 2.2 Ma Lake Strait of Stave (Read, 1978). The volcanic deposits overlie basement rocks Lake of the southern Coast Belt including greenschist- to amphibo- Georgia lite-facies supracrustal rocks of the Cadwallader Terrane, and Vancouver Tertiary monzonite intrusions of the Coast plutonic suite Figure 1. Location of the Mount Meager volcanic complex (Read, 1978; Gabrielse et al., 1991). This region of the Coast Mountains has been characterized by rapid rates of uplift over with respect to other Quaternary volcanic edifices in the the past 4 Ma (K.A. Farley, M.E. Rusmore, and S.W. Bogue, Garibaldi Volcanic Belt of southwestern British Columbia pers. com., 2000), which has led to relatively high rates of (after Hickson, 1994). Also shown are the locations of large erosion. Thus, the Mount Meager volcanic complex is highly accumulations of prehistoric avalanche deposits and dissected and is presently perched at 1100 to 1200 m eleva- historical rock avalanches associated with the same volcanic tion, well above the present day erosion surface marked by terranes (after Evans, 1992). the Lillooet River (400–500 m elevation). The three most 2 M.L. Stewart et al. valley-filling apron extending downslope below an inferred All three deposits are very poorly sorted, clast-rich, but vent crater seated on the northeastern flank of Plinth Peak. matrix supported, and feature a fine- to medium-grained We describe and contrast avalanche deposits that derive sand-sized matrix. Internal structures are rare in these depos- from, or are part of, these two stratigraphic formations. its though some reverse grading and concentrations of coarse blocks are visible in the upper layers. All the avalanche deposits discussed
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