Journal of Volcanology and Geothermal Research 185 (2009) 251–275 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Evolution of an englacial volcanic ridge: Pillow Ridge tindar, Mount Edziza volcanic complex, NCVP, British Columbia, Canada Benjamin R. Edwards a,⁎, Ian P. Skilling b, Barry Cameron c, Courtney Haynes a, Alex Lloyd a, Jefferson H.D. Hungerford b a Department of Geology, Dickinson College, 5 N. Orange Street, Carlisle, PA, 17013, USA b Department of Geology and Planetary Science, 200 SRCC Building, University of Pittsburgh, Pittsburgh, PA, 15260, USA c Department of Geology, University of Wisconsin-Milkwaukee, WI, USA article info abstract Article history: Glaciovolcanic deposits are critical for documenting the presence and thickness of terrestrial ice-sheets, and Received 24 April 2008 for testing hypotheses about inferred terrestrial ice volumes based on the marine record. Deposits formed by Accepted 9 November 2008 the coincidence of volcanism and ice at the Mount Edziza volcanic complex (MEVC) in northern British Available online 24 November 2008 Columbia, Canada, preserve an important record for documenting local and possibly regional ice dynamics. Pillow Ridge, located at the northwestern end of the MEVC, formed by ice-confined, fissure-fed eruptions. It Keywords: comprises predominantly pillow lavas and volcanic breccias of alkaline basalt composition, with subordinate tindar fi ∼ ∼ glaciovolcanism ner-grained volcaniclastic deposits and dykes. The ridge is presently 4 km long, 1000 m in maximum volcano–ice interaction width, and ∼600 m high. Fifteen syn- and post-eruptive lithofacies are recognized in excellent exposures Mount Edziza volcanic complex along the glacially dissected western side of the ridge. We recognize five lithofacies associations: (1) poorly pillow lava sorted tuff breccia and dykes, (2) proximal pillow lava, dykes and tuff breccia, (3) distal pillow lava, poorly sorted conglomerate and well-sorted volcanic sandstone, (4) interbedded tuff, lapilli tuff, and tuff breccia units, and (5) heterolithic volcanogenic conglomerate and sandstone. Given the abundance of pillow lavas and the lack of surrounding topographic barriers capable of impounding water, we agree with Souther [Souther, J.G., 1992. The late Cenozoic Mount Edziza volcanic complex. Geol. Soc. Can. Mem., vol. 420. 320 pp] that the bulk of the edifice formed while confined by ice, but have found evidence for a more complex and variable eruption history than that which he proposed. Preliminary estimates of water-ice depths derived from FTIR analyses of H2O give ranges of 300 to 680 m assuming 0 ppm CO2, and 857 to 1297 m assuming 25 ppm CO2. Variations in depth estimates among samples may indicate that water/ice depths changed during the evolution of the ridge, which is consistent with our interpretations for the origins of different lithofacies associations. Given that the age of the units are likely to be ca. 0.9 Ma [Souther, J.G., 1992. The late Cenozoic Mount Edziza volcanic complex. Geol. Soc. Can. Mem., vol. 420. 320 pp], Pillow Ridge may be the best documentation of a regional high stand of the Cordilleran Ice Sheet (CIS) in the middle Pleistocene, and an excellent example of the lithofacies and stratigraphic complexities produced by variations in water levels during a prolonged glaciovolcanic eruption. © 2008 Elsevier B.V. All rights reserved. 1. Introduction retreats for the North American Ice Sheet (NAIS) are well constrained from studies of end-moraines, geomorphology and isostatic effects One of the outstanding problems in studies of Earth's paleo- (Kutzbach, 1987; James et al., 2000; Clague and James, 2002; Fulton climate is the comparison of very detailed records of global et al., 2003), but the terrestrial record of pre-Last Glacial Maximum temperature from Pleistocene marine and ice core proxies to the (LGM; e.g., Illinoian, Kansan and Nebraskan) Pleistocene ice is much relatively sparse records on the extents and thicknesses of terrestrial less detailed, since much of the evidence was destroyed during the ice-sheets, particularly in the Northern Hemisphere. Data from marine LGM (Fulton, 1992; Jackson et al., 1996; Barendegt and Irving, 1998). cores and lacustrine sediments suggest multiple episodes of glacia- The products of volcano–ice interactions can provide critical tions during the Pleistocene (Raymo, 1992; Benson et al., 1998; constraints for the presence and characteristics of pre-LGM ice, as Marshall et al., 2002). The pattern and timing of the most recent they can be more resistant to erosion than other types of glaciogenic deposits (e.g. till). Two volcanic landforms, both capable of surviving ⁎ Corresponding author. Tel.: +1 717 254 8934; fax: +1 717 245 1971. multiple episodes of ice burial and erosion, are broadly recognized as E-mail address: [email protected] (B.R. Edwards). forming only in ice-confined environments: tuyas (Mathews, 1947) 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.11.015 252 B.R. Edwards et al. / Journal of Volcanology and Geothermal Research 185 (2009) 251–275 and tindars (Jones, 1969). Tuyas are flat-topped volcanoes that tephra, and capped by subaerial lava-fed deltas (e.g., Mathews, 1947; comprise a distinctive subaqueous to emergent sequence of basal Jones,1969; Skilling, 2002; Smellie, 2006). Tindars are distinctly linear lavas and volcanic breccias, overlain by Surtseyan phreatomagmatic volcanic landforms that comprise lithofacies similar to those found at tuyas except that they generally lack extensive lava-fed deltas (Jones, 1969). They are thought to form during ice-confined, fissure-fed eruptions. Jones (1969) first proposed that the word ‘tindar’, which is an Icelandic word meaning ‘row of peaks’, be used as a general term to describe linear volcanic landforms in west central Iceland formed by eruptions beneath ice. Although strictly speaking tindar is the plural form of tindur, we follow Jones (1969) usage of ‘tindar’ as singular and ‘tindars’ as plural. Jakobsson and Guðmundsson (2008) have recently suggested that tindars be distinguished from tuyas by having at least a 2:1 length to width ratio. Linear glaciovolcanic landforms have also been referred to as ‘hyaloclastite ridges’ (e.g., Schopka et al., 2006) and ‘pillow ridges’ (e.g., Höskuldsson et al., 2006). Neither of these terms is satisfactory as a general term because the majority of the fragmental material found at tindars is not hyaloclastite sensu stricto (i.e. vitric fragments formed by quench fragmentation and mechanical ‘spalling’; Rittman, 1962; Honnorez and Kirst, 1975; McPhie et al., 1993), and the volumetric proportions of pillow lavas at tindars can vary from nearly 100% to almost 0%. The word ‘tindar’ has also been used more generally to denote the explosion-dominated stage of iced-confined basaltic eruptions regardless of vent geometry (Smellie and Skilling, 1994; Smellie, 2000). Published studies of tindars indicate that their lithostratigraphy is highly variable between two extremes, one dominated by extensive magmatic fragmentation, leading to a predominance of fragmental lithofacies (Guðmundsson et al., 2002a,b; Schopka et al., 2006) and the other dominated by pillow lavas (Höskuldsson et al., 2006). It is very likely that the variations between the two end members are controlled by variations in ice thickness, sub-ice hydrology at the time of the eruptions, eruption rates, eruption duration and volume, and possibly pre-eruption volatile contents. Tindars with significant proportions of fragmental and coherent lithofacies have been described in Iceland (Kalfstindar; Jones, 1969, 1970) and in northern British Columbia, Canada (Pillow Ridge; Souther, 1992). This paper focuses on the physical evolution of Pillow Ridge, a tindar at the northwestern end of the Mount Edziza volcanic complex (Fig. 1). Our goals are to: (1) give detailed descriptions of the lithofacies and their associations, (2) present viable interpretations for their origins, (3) evaluate Souther's (1992) model for the evolution of the ridge, and (4) explore the implications of tindar formation for local and regional ice/meltwater dynamics at ∼1 Ma in northern British Columbia. 2. Geological setting Pillow Ridge is part of the northern Cordilleran volcanic province (NCVP), which comprises isolated cones, lava flow remnants, and a few larger volcanic complexes located in northwestern British Columbia (BC), the western Yukon Territory, and east-central Alaska (Fig. 1A; Edwards and Russell, 1999, 2000). The NCVP formed mainly during the Neogene and Holocene in response to extensional tectonic stresses spread across the Canadian Cordillera (Souther, 1992; Edwards and Russell, 1999). Much of the eruption activity in the NCVP occurred during the past 2 Ma (cf. Edwards and Russell, 2000), when western Canada was periodically inundated by the Cordilleran Fig. 1. Maps showing the location of the Mount Edziza volcanic complex (MEVC) and Pillow Ridge. A) Map of the Canadian Cordillera showing the location of the northern Cordilleran volcanic province (NCVP) and the MEVC. Hillshade is derived from 30 m Digital Elevation Models (DEM) available from the National Topographic Atlas. B) Map of the MEVC showing underlying topography, general outline of the distribution of formations within the MEVC (light grey), and locations of remnant ice still extant (after Souther, 1992, Fig. 4). The general locations