Pyroclastic Passage Zones in Glaciovolcanic Sequences
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ARTICLE Received 4 Oct 2012 | Accepted 29 Mar 2013 | Published 30 Apr 2013 DOI: 10.1038/ncomms2829 Pyroclastic passage zones in glaciovolcanic sequences James K. Russell1, Benjamin R. Edwards2 & Lucy A. Porritt1,3 Volcanoes are increasingly recognized as agents and recorders of global climate variability, although deciphering the linkages between planetary climate and volcanism is still in its infancy. The growth and emergence of subaqueous volcanoes produce passage zones, which are stratigraphic surfaces marking major transitions in depositional environments. In glacio- volcanic settings, they record the elevations of syn-eruptive englacial lakes. Thus, they allow for forensic recovery of minimum ice thicknesses. Here we present the first description of a passage zone preserved entirely within pyroclastic deposits, marking the growth of a tephra cone above the englacial lake level. Our discovery requires extension of the passage-zone concept to accommodate explosive volcanism and guides future studies of hundreds of glaciovolcanic edifices on Earth and Mars. Our recognition of pyroclastic passage zones increases the potential for recovering transient paleolake levels, improving estimates of paleo- ice thicknesses and providing new constraints on paleoclimate models that consider the extents and timing of planetary glaciations. 1 Volcanology and Petrology Laboratory, Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4. 2 Department of Earth Sciences, Dickinson College, Carlisle, Pennsylvania 17013, USA. 3 School of Earth Sciences, University of Bristol, Bristol BS8 1RJ, UK. Correspondence and requests for materials should be addressed to J.K.R. (email: [email protected]). NATURE COMMUNICATIONS | 4:1788 | DOI: 10.1038/ncomms2829 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2829 laciovolcanism encompasses all forms of interaction occurs at B1,850 m.a.s.l. and defines a passage zone within between volcanism and ice. The resulting volcanic pyroclastic deposits. Subsequent effusive eruptions formed an Gdeposits provide evidence for the timing and nature of onlapping lava-fed delta25 comprising steeply dipping beds of glaciation and provide a record of paleoclimate and paleoenvir- pillow lava, pillow breccias and pillow-lava-derived hyaloclastite onmental conditions and, thus, supply important constraints on capped by subaerial lava sheets. This assemblage creates a minimum planetary climate models. For example, glaciovolcanic edifices of two effusive passage zones situated at lower (1,630 and and their deposits are used forensically to recover properties 1,695 m.a.s.l.) elevations (Fig. 1) and records the dynamic of prehistoric ice sheets on Earth and Mars, including: interplay between changes in the intensity of the effusive phase of thicknesses1–5, hydrology4–7, timing1,6,8 and distributions9–11. eruption and changes in lake levels. Hence, Kima’Kho is a subglacial Tools for interpreting the geological record of glaciovolcanism volcano that preserves passage-zone geometries in both explosive are, therefore, crucial to improving our abilities to test and refine and effusive sequences20,22,24. The evidence for, and implications of, rapidly evolving paleoclimate reconstruction models12,13. a passage zone within a pyroclastic succession are developed below. Passage zones are diachronous surfaces marking transitions between subaqueous and subaerial depositional environments Subaqueous pyroclastic deposits. The lower two-thirds of the during volcanic eruptions; they form in a variety of littoral exposed tephra cone comprises highly palagonitized, massive to settings14–17. In glaciovolcanic settings, the elevation of the crudely bedded (decimetre scale) lapilli tuffs (Fig. 2a,b). The passage-zone surface unequivocally records the height and deposits are moderately sorted, ranging from matrix to clast- depth of the paleo-englacial lake at a specific point in time and supported, and locally show normal grading. The deposits com- space16–18. This elevation fixes the minimum thickness of the prise angular to subrounded, lapilli-sized and dense to vesicular enclosing ice sheet and has been used as a paleoclimate proxy by basaltic pyroclasts within a palagonitized matrix of vitric ash constraining glaciations in both southern and northern particles. Grading and moderate sorting along with weak indi- hemispheres, including Antarctica5,8,16, Iceland7,19 and British cators of reworking (subrounded grains) suggest these deposits Columbia1,20–23. However, previous workers have only identified were subaqueously deposited. Relative to the volcaniclastic glaciovolcanic passage zones16,17,22 within effusive volcanic deposits situated above 1,850 m.a.s.l., these units: (1) have a sequences where the passage zone separates subaqueous lava- higher proportion of subrounded juvenile clasts; (2) feature more fed delta lithofacies from overlying subaerial lavas. layers that are clast-supported or show overall higher levels of We describe results from field mapping of a Pleistocene-aged sorting; and (3) never contain armoured lapilli (see below). These subglacial volcano in northern British Columbia, Canada. Our volcaniclastic deposits are interpreted to be products of subaqu- mapping of a basaltic tephra cone at Kima’Kho Mountain has eous pyroclastic density currents continuously fed by subaqueous identified a unique passage zone that is hosted entirely within tephra jets, with little to minor reworking26,27. proximal volcaniclastic deposits resulting from explosive (versus effusive) eruption. Our discovery of a pyroclastic passage zone at Kima’Kho volcano demands reevaluation of the concept that Subaerial pyroclastic deposits. The subaqueous volcaniclastic most glaciovolcanic tephra cones were fully submerged and deposits are overlain at B1850 m.a.s.l. by a more diverse formed beneath the englacial lake surface9,11,17,18,21,22. Rather, package of pyroclastic units. The majority of these pyroclastic subglacial tephra cones may record and preserve subaqueous to deposits comprise massive to coarsely bedded (metre subaerial transitions in deposition (that is, passage zones), scale), outward dipping (14°–24°), weakly palagonitized lapilli thereby, increasing the opportunities for forensic recovery of tuff. Locally, the lapilli tuff is well bedded (cm-scale), moderately paleolake levels and estimates of paleo-ice thicknesses on Earth sorted, and exhibits lenticular- and cross-bedding defined by and Mars. horizons of coarser pyroclasts associated with minor finely bedded surge-like deposits (Fig. 2c). The majority of these deposits are matrix-supported, with a matrix of palagonitized, Results ash-sized blocky glass particles. The lapilli- to block-sized basaltic Kima’Kho tuya. We have identified a new type of passage zone juvenile pyroclasts within the lapilli tuff are equant in shape, within an entirely explosive glaciovolcanic sequence from a sub- subrounded to angular, dense to vesicular (r40%) and aphanitic glacial volcano (that is, tuya) near Kawdy Mountain in northwestern to glassy. British Columbia, Canada22,24. The Kima’Kho tuya is a highly These deposits are distinct in that they contain armoured lapilli dissected, small volume (B2–3 km3), early Pleistocene (1.82 Ma þ / (Fig. 2d,e), a variety of accretionary lapilli featuring a single, À 40 ka) basaltic volcano24. The edifice forms a high relief structure central nucleus of a juvenile or lithic clast that is concentrically covering 28 km2 and rising to an elevation of 1,946 m.a.s.l. on a coated by fine–to-coarse ash (Fig. 2f)28–32. The armoured lapilli regional, low-elevation plateau situated at B1,470 m.a.s.l. (Fig. 1). are pervasive and distributed exclusively within the upper portion The plateau hosts a total of six other tuyas20,22,24. of the pyroclastic sequence along the highest ridge of the southern The major lithostratigraphic units and their relationships are flank of the volcano (Fig. 1). They are commonly 0.3–2 cm in summarized in a 4-km long cross-section drawn through Kima’Kho diameter, spherical to ovoid in shape, smooth surfaced (Fig. 2d–f) tuya (Fig. 1). The section comprises two S–N linear segments that and occur in two modes: (a) dispersed (o2 volume percent) on bisect the middle of the edifice. The mapped geology is projected the decimetre scale within massive to crudely bedded pyroclastic into the line of section; bedding attitudes are apparent and areas deposits comprising matrix-supported palagonitized lapilli tuff or with poor exposure or talus cover are left blank (Fig. 1a). The (b) concentrated (450 volume %) within discrete, cm-scale, volcano features a 476 m high, B3 km diameter, eroded tephra cone continuous-to-lensoidal beds within the lapilli tuff (Fig. 2d). The (B1.1–1.5 km3) formed during an early explosive phase of eruption cores are scoriaceous fragments (o2 cm) of juvenile, vitric and through an enclosing ice sheet (Fig.1b).Theconecomprisesmostly olivine-phyric basalt, and the interface between the core and vent-proximal (o1 km from vent) volcaniclastic deposits; distal ash-aggregate rim is irregular, cuspate or jagged (Fig. 2f). The deposits are absent or not preserved. The tephra cone deposits were ash-aggregate envelopes are 1–5-mm thick, massive and comprise emplaced subaqueously below the level of the surrounding englacial 10–150 mm blocky–to-cuspate fragments of variably vesicular, lake,aswellassubaeriallyoncetheconebuiltabovelakelevel