Journal of Volcanology and Geothermal Research 180 (2009) 203–224

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Journal of Volcanology and Geothermal Research

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The architecture, eruptive history, and evolution of the Table Rock Complex, : From a Surtseyan to an energetic eruption

Brittany D. Brand ⁎, Amanda B. Clarke 1

School of Earth and Space Exploration, Arizona State University, Box 871404, Tempe, AZ, 85287-1404, of America article info abstract

Available online 28 October 2008 The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. rings; examples from the Fort Rock-Christmas valley basin, south-. J. Geophy. Res. Keywords: 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake phreatomagmatic Valley Basin, south-central Oregon. It spans an area of approximately 40 km2, and consists of a large tuff cone in Surtseyan the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters maar were formed along the flanks of the complex in the time between the two main eruptions. The first period of base surge activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are Table Rock Complex dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions. © 2008 Elsevier B.V. All rights reserved.

1. Introduction has been experimentally and theoretically determined to be a function of melt composition, flux, water–melt mass ratio, confining Hydromagmatic eruptions occur when rising magma violently pressure, magma viscosity, and the degree of turbulent mixing of fragments after intersecting and mixing with shallow surface water or magma with water, steam, or water sprays (Sheridan and Wohletz, groundwater (Sheridan and Wohletz, 1983). Fragmentation in this 1983; Wohletz and McQueen, 1984; Büttner and Zimanowski, 1998; style of is driven principally by the energetic interaction Zimanowski et al., 1991; Mastin, 2007). between magma and external water (Houghton and Wilson, 1989), In the last couple of decades there have been many advances in although expansion of magmatic volatiles can occur depending on the understanding how the hydromagmatic deposits of tuff cones, tuff rings, volatile content of the magma, and may provide a secondary and relate to the eruptive dynamics and depositional mechanisms mechanism of fragmentation (i.e., Houghton and Wilson, 1989; that produced them (e.g., Fisher and Waters, 1970; Crowe and Fisher, Houghton et al., 1999; Cole et al., 2001; Brand et al., in press). The 1973; Lorenz, 1974; Sheridan and Wohletz, 1983; Kokelaar, 1983; Fisher degree of fragmentation associated with magma–water interaction and Schmincke, 1984; Houghton and Hackett, 1984; Kokelaar, 1986; Sohn and Chough, 1989; Dellino et al., 1990; White, 1996; Houghton et al., 1999; White, 2001; Nemeth et al., 2001; Cole et al., 2001; Mastin ⁎ Corresponding author. et al., 2004; Brand and White, 2007; Brand et al., in press). To assess the E-mail addresses: [email protected] (B.D. Brand), [email protected] fl – (A.B. Clarke). relative in uence of external water (water magma ratio) on overall 1 Tel.: +1 480 965 6590; fax: +1 480 965 8102. eruption dynamics, researchers look at deposits for evidence for liquid

0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.10.011 204 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 water at the time of deposition. Wet conditions are typically indicated by Towards the center of the basin, the basaltic features are dominantly a combination of features such as accretionary lapilli, fine-grained tuff cones and tuff rings; whereas, towards the boundaries the vesiculated tuff, pervasive soft-sediment deformation, surge cross-strata features are dominated by maars and cinder cones (Heiken, 1971). with stoss-side accretion and evidence for plastering, mud cracks, The oldest formation identified in the area is the Picture Rock debris-flow filled erosion channels, and lahar deposits (Waters and , a 230 m thick sequence of 10 m thick basalt flows interbedded Fisher, 1971; Lorenz, 1974; Wohletz and Sheridan, 1983; Sohn and with various sandstones, conglomerates, and tuffaceous mudstones, Chough,1989; Dellino et al.,1990; Nemeth et al., 2001; White, 2001; Cole which are interpreted as flood plain and/or shallow lake deposits et al., 2001; Brand and White, 2007; Brand et al., in press). Wet- (Walker et al., 1967). The Silver Lake graben, located immediately phreatomagmatic conditions are interpreted to occur when external south of TRC, is the beginning of a 25 km wide structural arch that water at the zone of magma–water interaction is not efficiently forms the southwest boundary of the lake basin (Heiken, 1971). converted to steam (Sheridan and Wohletz, 1983; Wohletz and Driller's logs show that ∼220 m of flat-lying lacustrine sediments and McQueen, 1984) and abundant liquid water is retained in the eruption interbedded tuffs overlie the Picture Rock basalt formation at the column and density currents. center of the basin, and that these sediments thin to 0 m towards the Dry conditions are distinguished by their lack of features indicative basin boundary (Hampton, 1964). At the center of the basin (beneath of liquid water at the time of pyroclastic deposition. These deposits TRC), these sediments consist primarily of diatomites, whereas closer contain no evidence of accretionary lapilli, little to no soft-sediment to the basin margins they consist of coarse clastic sediments, lava deformation, low-angle cross strata with dominantly lee-side accre- flows, and volcanic breccias (Heiken, 1971). tion (although antidunes are also common), and no evidence for The underlying stratigraphy exposed on the west side of TRC muddy deposits plastering against obstacles or on the stoss side of consists of a 5–10 m thick basaltic lava flow, followed by ∼8mof dunes (e.g., Fisher and Waters, 1970; Sheridan and Wohletz, 1983; interbedded volcanic litharenites, lithic arkoses, and diatomaceous Sohn and Chough, 1989; Chough and Sohn, 1990; Doubik and Hill, siltstone and mudstones (Heiken, 1971). In contrast, the non-volcanic 1999; Nemeth et al., 2001; Brand and White, 2007). Dry-phreatomag- stratigraphy beneath the eastern portion of TRC is dominated by well- matic conditions represent efficient conversion of external water to bedded diatomites (Heiken, 1971). The interbedded, non-volcanic steam at the site of magma–water interaction, or subsequently in the units exposed on the west side of the edifice are interpreted as eruption plume. As a consequence, little or no liquid water is retained outwash apron deposits from the Connley Hills to the northwest, a in the eruption column or proximal density currents, although steam 6.4 km wide by 19-km-long volcanic feature consisting of a basaltic may condense in density currents and rising plumes with distance shield and intermediate to silicic domes, which likely formed an island from the source (e.g., Sheridan and Wohletz, 1983; Wohletz and in the Pliocene–Pleistocene basin lake (Heiken, 1971). McQueen, 1984). These conditions are thought to represent highly efficient conversion of thermal energy to kinetic energy, yielding 1.2. Table Rock Complex the highest degree of melt fragmentation and vapor production, and therefore the highest explosivity (Wohletz and McQueen, 1984; TRC is located ∼14 km east of Silver Lake, Oregon, on the shore of Büttner and Zimanowski, 1998). present day Silver Lake, which is likely the small remnant of the much Many moderately-to well-exposed remnants of tuff cones, tuff larger lake that occupied the Fort Rock–Christmas Valley Basin in rings, and maars exist in south-central Oregon (Peterson and Groh, Pleistocene time (Heiken, 1971). TRC has an elongated-oval shape, 1961, 1963; Heiken, 1971; Heiken et al., 1981). The Table Rock Complex trends NNW (along strike with the other phreatomagmatic complexes (TRC; Pliocene–Pleistocene), one of the best exposed examples of in the Fort Rock–Christmas Valley region), and covers an area hydrovolcanism in the Fort Rock-Christmas Lake Valley Basin, was first approximately 8 km by 5 km (Heiken, 1971). Two large phreatomag- mapped and described by Heiken (1971). We have revisited TRC with matic edifices; a large southern tuff cone with a capping solidified lava the goal of studying the stratigraphy in detail in order to reconstruct lake at 395 m above the basin floor (TRC1), and a low, broad tuff ring in the volcanic evolution of the complex. Our results provide evidence the northeast (TRC2; Fig. 2), make up the complex. Additionally, seven for two major eruptions, TRC1 and TRC2, for which we constrain the smaller tuff rings and vents were identified along the flanks of the temporal evolution, dominant depositional mechanisms, the influ- complex, yielding a complicated network of tuff ring–tuff cone ence of liquid water on deposit characteristics, and relative eruption deposits. For the purposes of this paper, only the two largest and energy. The objectives of our work are to build on the existing most significant eruptions of TRC1 and TRC2 will be discussed in framework for hydromagmatic pyroclastic deposits by continuing to detail. The flank vents will be discussed briefly in terms of size, identify relationships between deposit characteristics and eruptive location, and cross-cutting relationships with other erupted tuffs. dynamics of mafic hydromagmatic eruptions. The main topics of this paper include (1) subaqueously emplaced 2. Data density currents of a large Surtseyan-style eruption early in the complex's history (TRC1); and (2) The formation of multiple, large- Detailed geologic mapping and twenty-three stratigraphic sections scale base surge deposits produced during a later, highly energetic were completed to determine the eruptive history of TRC (Fig. 2), maar-forming eruption (TRC2). Finally, a more general, broad-scale resulting in the identification of 42 lithofacies based on variations in reason for studying tuff cones, tuff rings, and maars is that they offer grain size, composition, and sedimentary structures (following Fisher, an opportunity to observe, describe, and study the deposits of many 1961; Schmid, 1981; Table 1). Detailed descriptions of each lithofacies pyroclastic processes on easily accessible vertical and lateral scales. are available as online supplementary data. These lithofacies were grouped into six Facies Associations (FA) according to common 1.1. Geologic setting bedding styles, juvenile fragment morphology, and the percentage and type of accidental components (Table 2). Five of these FAs The Fort Rock–Christmas Valley basin is 64 km long by 40 km wide, correspond to the TRC1 eruption, and one corresponds to the TRC2 and was the location of an extensive, ancient Pliocene to Pleistocene eruption. Inferred eruptive conditions and depositional mechanism lake (lake boundary dashed in Fig. 1; Heiken, 1971). The basin is are discussed below in the text. occupied by a variety of basaltic eruptive features which trend Grain percentage was estimated in the field and supported by northwest to southeast. The ages of these features are poorly subsequent petrographic thin section analysis of each FA from constrained, but due to the obvious interaction with external water, multiple locations around the complex. In the field, juvenile clasts they likely formed during the time of the extensive basin lake. were distinguished from accidental basalt by their fresh appearance, B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 205

Fig. 1. Study location (modified from Heiken, 1971). Upper right image is the state of Oregon (USA), and the gray shaded area is enlarged as the lower left image. The enlarged image shows the location of the Table Rock Complex, as well as many of the other hydromagmatic edifices and cinder cones within the basin. irregular shapes, or twisted-fluidal shapes, greater vesicularity, and 2.1.1. PH1: massive to stratified tuff and lapilli tuffs quenched glassy rinds with vesicular cores. Thirty thin sections, PH1 consists of alternating, palagonitized lithofacies, T1, LT1, and LT2 sampled from multiple lithofacies of the TRC1, TRC2, and flank vent (Tables 1 and 2; Fig. 3). Early beds alternate from grain supported, eruptions, were analyzed and point counted to better constrain the graded, and generally stratified lapilli tuff (LT1, 0.1–1 m thick), and a composition and percentage of accidental versus juvenile clasts, massive, matrix supported (95–100% fine-medium grained ash) deposit especially in the matrix. For each thin section, 300 points were with occasional imbricated pebble stringers of mudstone and rare counted with ∼1 mm spacing between traverses and between points. accidental basalt fragments (T1, 0.5 to 2 m in thickness; Fig. 3a, c). Details of these analyses are available as online supplementary data, Stratigraphically higher beds are well-stratified, wavy-planar bedded, and a summary is presented below in Sections 2.2 and 2.4. and show reverse grading from coarse ash up to coarse lapilli-to-blocks (32 cm down to b6 cm thick with distance from source). The coarse 2.1. TRC1 Facies Associations: descriptions and interpretations lapilli to block-sized juvenile grains consist of scoriaceous, twisted, fluidal, and often flattened juvenile grains with quenched rinds (found The TRC1 deposits originated from the eruption that formed the both in LT2, and at random, concentrated horizons within LT1). These large tuff cone with a capping solidified in the south-central juvenile spatter-bomb clasts are not found above the PH1 horizon. Soft part of the complex (Fig. 2). This was the first eruption in the sequence sediment deformation is prevalent throughout PH1. of tuffs, as it directly overlies the pre-existing lake and lacustrine sediments described above. The deposits are well exposed along the 2.1.2. PH1 interpretation south, east, and north sides of the edifice, and intermittently exposed T1 and LT1 are likely a combination of fallout and on the west side as they were eroded away or covered by later subaqueous sediment gravity flows. The angular grains and moderate eruptions. The sequence is consistent around the vent with minor sorting within LT1 may be indicative of fallout, possibly through variations in depositional characteristics such as grain size and a density current (Valentine and Giannetti, 1995), whereas the bedding thickness due to distance from source. lenticular interbeds, erosive contacts, and reverse grading often The grains within TRC1 consist dominantly of juvenile basalt, found within LT1 are consistent with a deposition by lateral move- accidental basalt, and white mudstone–siltstone lake sediments, ment of grains (White, 1996, 2000). The fine-grained, massive, poorly which will be referred to as mudstone from here onward. Most of sorted, and non-graded deposits of T1 suggest deposition from a the deposits also contain abundant fine-to-medium ash matrix. concentrated suspension density current with little tractional trans- Petrographic analysis of the matrix reveals that ∼98% of the matrix port, which inhibits the development of grading (Sparks, 1976; Sparks and grains within the thin sections are of juvenile origin (a detailed et al., 1978; Chough and Sohn, 1990; Freundt and Bursik, 1998). The thin section analysis is presented in Section 2.3 below). reverse-graded, coarse ash to fine block deposits of LT2 are interpreted 206 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Fig. 2. Geologic map (modified from Heiken, 1971) overlain on topographic map of TRC. B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 207

Table 1 Lithofacies of both the TRC1 and TRC2 eruptive products, as well as the non-volcanic products (NV), classified on the basis of grain size/abundance, sedimentary features, and dominance of juvenile fragments [table modeled after Nemeth et al. (2001) and Nemeth and White (2003)]. Detailed descriptions of individual lithofacies are available as online supplementary data

Lithofacies Tuff Breccia (TB) Lapilli Tuff (LT) Tuff (T) Magmatic (M) Non-volcanic (NV) Pre-TRC1 eruption NV1

TRC1 eruption Clast supported Massive LT4, LT6 Massive-to-diffuse stratification LT3, LT5 Stratified LT1 Stratified, reverse grading LT2 Scour-fill massive TB1 Scour-fill bedded LT8, LT9 Cross-stratified LT18 Matrix supported Massive-to-diffuse stratification T1 Diffusely stratified T18 Stratified, laminated T2, T3 Scour-fill bedded LT7 Planar-to-wavy beds T17 Strombolian deposits, lava lake, and radial dikes M1

Post-TRC1 eruption NV2

TRC2 eruption Clast supported Massive, non-graded TB2 Massive, reverse grading LT17 Massive-to-diffusely stratified bed in a well-stratified deposit sequence LT10 T13, T15 Stratified, reverse grading LT14, LT15, LT16 T21 Scour-fill massive LT12 Planar-to-wavy stratification, occasional lenticular deposits LT11 Wavy-to-cross-stratified T20 Matrix supported Massive T10 Massive-to-diffusely stratified bed in a well-stratified deposit sequence T9, T16 Stratified — pinch and swell T4, T5 Stratified T12 Stratified, laminated T11, T14, Scour-fill bedded T6, T7, T8 Wavy-to-cross stratified T19 as a combination of fallout and density current deposits. The coarse LT5), and is capped by a planar-to-cross laminated, medium-to-coarse ash deposits of LT2 are interpreted as having an initial fallout origin that, (grains subangular to subrounded) bed that commonly displays soft after landing, transitioned into thin, laterally moving, traction carpet sediment deformation (20–40 cm thick, T2, T3; Fig. 4). Individual dominated, subaqueous sediment gravity flows. The finely-laminated, lithofacies pinch and swell laterally, but the sequences overall are laterally well-sorted, normal-graded ash beds that overlie the coarser deposits continuous for 10's up to 100's of meters away from source. are interpreted to have been deposited by ash-fallout settling through Field and petrographic analysis show that the grains in all of these a column of water. These beds are typically highly deformed under the deposits are dominantly juvenile. Accidental grains of mudstone and weight of the overlying material, further suggesting that they were dense, angular basalt compose b5% of the deposits. However, many of unconsolidated and water-saturated during emplacement of subse- the block-sized ballistic clasts with underlying sags are composed of quent units (Nichols et al., 1994; White, 2000). The field and thin accidental basalt. section analyses show that N98% of the grains are juvenile and typically angular to subrounded, indicating minor to no abrasion (see 2.1.4. PH2 interpretation Section 2.2 below; also see online supplementary data). The repeated, fining upward packages of PH2 are interpreted to The angular, blocky glass shards indicate magma–water interaction correspond to the traction carpet and suspension sedimentation stages as the dominant fragmentation mechanism. However, the fluidal, of high-density, turbulent, subaqueous sediment gravity flows (Nemec scoriaceous juvenile coarse lapilli-to-fine blocks suggest that, in the et al.,1980; Lowe, 1982; Postma, 1986). Lithofacies LT6–LT5 and LT4–LT3 early stages of the eruption, both hydromagmatic and magmatic are thick, diffusely stratified deposits with alternating coarsening- and processes were occurring simultaneously (Houghton and Wilson, fining-upward sequences. They have no distinct bedding surfaces, and 1989; Cole, 1991; Houghton et al., 1999). have an overall fining upward trend. Changes in grain size throughout a given deposit reflect the grain size variation of supplied sediment with 2.1.3. PH2: normal graded, massive to diffusely stratified sequences time, and/or intermittent waxing and waning flow conditions (Sohn, The PH2 facies association consist of repeating packages of lithofacies 1997). The overall fining upward sequence is interpreted to be a con- (i.e., LT6–LT5–T3 and LT4–LT3–T2; Tables 1 and 2). Each package begins sequence of waning flow conditions and loss of sediment supply over with a coarse-grained (angular to subangular grains of fine-to-coarse the duration of the current. The fine-grained, stratified lithofacies (T2, lapilli), massive to diffusely-stratified, poorly-to-moderately well sorted, T3), which overlie the coarser, thicker deposits, represent the last stage grain-supported deposit with variable thickness (0.25 up to 1.5 m thick; of sedimentation from the collapsing turbulent, suspended-load region. LT6, LT4). Both normal and reverse grading is common within the same As with the deposits of PH1, grains from PH2 are subangular to bed (Fig. 4), however the package as a whole gradually fines upwards (LT3, subrounded, and consist dominantly of juvenile volcanic clasts. This 208 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Table 2 The lithofacies have been grouped into Facies Associations (FA) according to common occurrences, depositional features, and compositional similarities. These are discussed in detail in the text. PH=Phreatomagmatic and M=Magmatic

Facies Associations description (TRC1) Facies Associations description (TRC2) PH1: Massive to stratified tuff and lapilli tuffs PH5: Strata within large dune-forms.

T1, LT1, LT2 PH5a — Repeating and interbedded lithofacies found just above the contact with the TRC1 deposits on the north side of the complex. PH2: Normal graded, massive-to-diffusely stratified sequences with occasional zones of inverse grading T4, T5, LT10, LT11

LT4–LT3–T2, LT6–LT5–T3 PH5b — Repeating lithofacies found on the east side of the crater.

PH3: Scour and fill deposits T9, T10, T11, T12, T13, LT13

TB1, LT7, LT8, LT9 PH5c — Interbedded, planar-to-cross stratified lithofacies that dip into the TRC2 crater on east side. PH4: Cross-bedded deposits T14, T15, LT14, LT15, LT16 T17, T18, LT18 PH5d — These are interbedded and alternating beds found in the southern part of M1: Spatter, lava lake deposit, and radially intruding dikes within tuff the complex. cone walls of TRC1. T19, T20, T21, T22 M1 PH5e — Found filling in large scours— likely closely related to first 20 m of the PH5a deposits.

LT12, T6, T7, T8

PH5f — Massive tuffs, lapilli tuffs, and tuff breccias found within the large dune forms throughout the east and southeast quadrants

TB2, T16, LT17 observation, combined with the lack of highly abraded and inter- that fill the large scours suggest that the sediment gravity flows bedded non-volcanic clasts and sediments, suggests that the density were saturated, and therefore likely emplaced subaqueously and currents were eruption-fed, and that grain size and morphology of the deformed by the weight of the subsequently deposited overlying clasts reflect eruptive conditions at the vent (White, 2000). Additional material. The sediment gravity flows could have originated from coarse tephra dispersed through the water column, either by fallout one of two sources: (1) remobilization of tephra avalanching down from tephra jets or from dilute density currents traveling across the the steepening slopes of the outer tuff cone as large quantities of surface of the water, may have caused ash and lapilli to be new sediment were added to the system from the erupting vent; continuously rained into the subaqueous flows (White, 2000). This or (2) high-concentration, eruption-fed density currents derived process may have suppressed the formation of distinct thin layering from column collapse. The fine strata that fill the scour features are (Lowe, 1988; Arnott and Hand, 1989; White, 2000), and could further likely the deposits of the waning sediment gravity flows, or fill from explain the thick massive nature of LT6–LT5 and LT4–LT3 (Fig. 4). subsequent currents and fallout. In several areas N1 km from the vent, up to 50 cm of smectitic lake sediments overlie the deposits of PH2 (representing the end of the 2.1.7. PH4: cross-bedded deposits TRC1 eruptive sequence distal from source). This observation, PH4 was found only proximal to the TRC1 tuff cone, above 1451 m. combined with the lack of subaerial features such as base surge PH4 deposits more distal from the tuff cone are not exposed, and deposits and well-sorted strata from fallout, further supports the either were eroded, or never deposited. PH4 deposits are dominated subaqueous depositional environment, and suggests the lake existed by wavy- to cross-stratified tuffs and lapilli-tuff beds (T17, T18, and well after the eruption ceased. LT18; Tables 1 and 2; also see online supplementary data). Dunes are symmetrical, have wavelengths from 10–20 m parallel to the direction 2.1.5. PH3: scour and fill deposits of flow, and amplitudes of 0.5–1 m. Individual beds within a single PH3 consists of channel-shaped, scour-and-fill deposits within dune consist of one of the aforementioned lithofacies (e.g., a dune the FAs of PH1 and PH2. The lower contact is invariably erosive with a 1 m amplitude may be composed of multiple, 10–20 cm layers into underlying substrate, and is filled with a variety of lithofacies of T17). The PH4 FA beds dip inwards towards the vent at roughly 5–8°, including massive, poorly sorted tuff breccias (TB1), and various similar to the dip of the underlying PH2 and PH1 FAs. The contact lapilli tuffs that fill in large scour features (LT7, LT8, and LT9; see between PH2 and PH4 is not exposed, but is constrained to within 10 Table 1 for description of these lithofacies; also see online sup- vertical meters. plementary data). Pervasive soft sediment deformation such as sags and flame structures are common, and fine-grained beds on the 2.1.8. PH4 interpretation channel walls are often deformed into small convolute folds towards Based on the cross-stratified nature of these deposits, PH4 is the axis of the channel. interpreted to be the result of dilute density currents flowing across a subaerial, gently-sloping platform. These were determined to origi- 2.1.6. PH3 interpretation nate from the TRC1 eruption based on location and dip inwards The scour and fill features of PH3 are interpreted to be the result towards the TRC1 vent, and consistency in dip with the underlying of erosive subaqueous sediment gravity flows, and are primarily TRC1 deposits. The presence of preserved base surge deposits suggests found N1 km from the vent on the flanks of the growing tuff that in this part of the stratigraphy, tephra was being deposited above cone platform. The highly deformed and occasionally folded beds the level of the lake. The stratigraphy therefore indicates that the level B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 209

Fig. 3. a) Generalized stratigraphic column representing PH1 (composed of lithofacies T1, LT1, and LT2). The dark blobs represent the spatter-bomb clasts which range from medium lapilli to fine blocks depending on distance from source (as described in text); b) LT1; c) T1 with lenses of LT1; d) LT2 (photographs a–c taken from section W4, 2.4 km from source, pencil for scale); e) LT2 (photograph taken in section SW1, 0.6 km from source).

of the lake at the time of the TRC1 eruption was approximately 1450 m 2.1.10. M1 interpretation above sea level. Over the duration of the TRC1 eruption a large, symmetrical tuff cone was built above the surface of the lake water. This cone 2.1.9. M1: spatter deposits and solidified lava lake as seen today is ∼1530 m in diameter at the base, and ∼360 m in The last FA identified in this sequence is that of black and red diameter at the top. M1 is interpreted to represent a final, scoriaceous-to-spatter deposits overlain by a cap of flat-lying, gray, magmatic stage of the TRC1 eruption, in which the vent of the aphanitic, high-alumina basalt (Heiken, 1971). M1 (M for magmatic) is growing tuff cone was gradually isolated from interaction with found at the highest point in the complex, at 395 m above the basin external water, resulting in a fire-fountaining, Strombolian stage (Fig. 2). Two dikes, one trending north and one south–southeast, (cinders and spatter deposits), and a final effusive stage which extend from this point. formed a crater filling lava lake. The dikes appear to radiate out 210 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Fig. 4. (a) Generalized stratigraphic column representing PH2; (b) LT4–LT3–T2 sequence, ∼2.8 km from source; (c) LT6–LT5–T3 sequence, 3.6 km from source (section N4, person 1.5 m for scale); (d) Closer view of LT6–LT5–T3 sequence, 1.6 km from source (section W4, hammer for scale).

from the lake and likely intruded the unconsolidated tuff cone sionally abraded within the fine ash matrix. The altered, glassy, fine- walls, which have now been eroded away. ash matrix contains features similar to mud cracks (on the micron scale). 2.2. TRC1 thin section Accidental material consists of (1) subrounded to rounded, crystal- line basalt lava flow clasts with a plagioclase-rich groundmass in which Petrographic analyses of samples from the TRC1 eruption (includ- the elongated microlites and phenocrysts are aligned; (2) rounded, ing all FAs) show that the grains are composed dominantly of highly weathered mudstone clasts, which in some locations contain silt- palagonitized glass shards of varying sizes. Some glass has elongated, sized particles of altered glass in the matrix; and (3) silicic pumice with swirly, and fluidal textures, but most are angular, fractured, and blocky elongated, stretched, thin vesicles walls. The silicic pumice are identical with little evidence of rounding and abrasion. Many of the larger in composition and micro-scale texture to samples of the NV1 deposits glass shards (N300 µm) have tiny, round, thick-walled vesicles, which just below PH1 of TRC1 (Table 1). On average, samples from TRC1 contain are on average 20 to 80 µm in diameter, but can be found up to less than 5% accidental clasts (detailed petrographic analysis available as 240 µm in diameter. These vesicles compose 1% to 26% of the glass online supplementary data). The subaerial deposits (PH4) contain 1–4% grain; however, coalesced bubbles are rare. Subhedral to euhedral armored and accretionary lapilli, but overall the components do not vary phenocrysts of plagioclase, orthopyroxene, and olivine are common significantly from the beginning to the end of the pyroclastic sequence. within the glass fragments, and are also found broken and occa- Details of the spatter and lava flow deposit can be found as online B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 211

Table 3 This table shows the range of dune wavelengths (λ) and average maximum dips of PH5 both parallel and perpendicular to the direction of flow around the TRC2 tuff ring. The maximum dip refers to measurements taken just below the crest of a dune

W–NW N–NE E SE Orientation relative to flow direction λ (m) Average dip (°) λ (m) Average dip (°) λ (m) Average dip (°) λ (m) Average dip (°) Perpendicular 80–300 m 19° 80–200 m 10° 300–500 m 19° − – Parallel 80–150 m 30° 80–120 m 33° 20–200 m 27° 20–100 m 28° supplementary data, and compositional data can be found in Heiken contains 26 lithofacies (Tables 1 and 2; also see online supplementary et al. (1981). data). There are two scales on which the deposits of TRC2 must be described. The first focuses on the large features (tens to hundreds of 2.3. TRC2 Facies Associations: descriptions and interpretations meters), and the second focuses on individual strata (centimeters to decimeters). 2.3.1. PH5: long-wavelength dune bedforms On the large scale, the TRC2 deposits consist of long-wavelength, The TRC2 deposits originated from the eruption that formed the three-dimensional, symmetrical dune structures with wavelengths large depression and tuff ring in the north–northeast part of the that vary from 20 to 200 m in the direction parallel to flow and from complex (Fig. 2). The TRC2 eruption has only one FA, PH5, which 80 to 500 m perpendicular to flow. Table 3 presents the range of dune

Fig. 5. (a) TRC2 dune form above section W4 in the western TRC. Flow direction is oblique to the plane of the photograph; (b) Large lobate feature in the TRC2 surge deposits. The photo was taken from an adjacent ridge to the south, and the flow direction was from right to left. Note that the strata dip to the west (left), shallow in a trough, and then dip to the east (right). Person 1.5 m circled for scale in (a) and (b); (c) TRC2 dune in the eastern sector of TRC. The crest of this dune is on the right side of the photograph. To the right and left of the crest, the strata dip away from the crest. This dune is parallel to flow direction (flow direction from left to right), and is close to 200 m in length; (d) trace of dune in (c). 212 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Fig. 6. Small scale PH5 features from the TRC2 eruption (a) massive sandy beds with gas (steam) escape pipes; (b) alternating lapilli and tuff beds, dark clasts are juvenile; (c) antidune; (d) laterally continuous beds that slightly pinch and swell; (e) laterally continuous beds with some cross stratification at base; (f) low angled, low amplitude dune bed. Hammer for scale in (a)–(c), (e), (f); 10 cm tall notebook for scale in (d). wavelengths and average dips just below the crest for four regions and El Chichon dunes is that the TRC2 dunes are symmetric with dips on around the TRC2 tuff ring (west–northwest, north–northeast, east, the lee and stoss sides of 27–33°, whereas the El Chichon dunes are and southeast). Wavelengths in the east tend to exceed those in other asymmetrical with steep stoss and gently sloping (20° dip) lee sides. regions. Dips parallel to flow tend to exceed dips perpendicular to flow The deposits of PH5 also include features such as (1) large (30° vs. 10°–20°), with similar values in all regions. Dune wavelengths U-shaped channels (10–32 m deep; Heiken, 1971; Heiken et al., in the direction perpendicular to flow exceed wavelengths in the 1981) where the steeply dipping deposits along the channel walls are direction parallel to flow, consistent with the dip data. The amplitude plastically deformed, slumped and folded towards the channel axis of the dunes are consistently approximately 1/10th of the wavelength (PH5e; Tables 1 and 2, also see online supplementary data); (2) Large- parallel to the direction of flow (Fig. 5). scale chute-and-pool features (up to 13 m tall); and (3) deposits The dips shallow towards the middle and tops of the waveforms to plastered up and around pre-existing obstacles. Also, each set of dunes b1°, often forming topographic saddles and flat crests. Similar features truncate pre-existing deposits and/or are truncated by later deposits, were noted at El Chichon and described as transverse, sinuous ridges which results in large-scale hummocky cross-stratification. These up to 100 m in length with horizontal form index (breadth/wavelength, features will be discussed in more detail in Section 3.3 below. or in our case perpendicular wavelength/parallel wavelength) ranging On the small scale, each dune consists of a range of lithofacies, the from1to10(Sigurdsson et al., 1987). The horizontal form index for the most dominant being centimeter to decimeter thick, laterally TRC2 dunes ranges from 1 to 5, with the longest perpendicular flow continuous, wavy-planar strata that are internally massive, poorly to direction wavelength equal to 500 m. The difference between the TRC2 moderately well-sorted tuff and lapilli tuff beds. The large dunes also B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 213

Fig. 7. Measured stratigraphic sections, from right (proximal to vent) to left (distal from vent), along with FAs interpreted from the section. commonly contain smaller, isolated, sporadic, meter-scale antidunes 1987; Valentine, 1987; Druitt, 1992; Wohletz, 1998). The variation in (Fig. 6). depositional characteristics from one bed to the next within the large In general PH5 deposits are dominated by fine- to medium-grained dune features (i.e., fines rich, fines poor, massive, stratified, inverse ash, and contain abundant accretionary and armored lapilli, vesicu- graded, non-graded), and lack of distinct or discernible vertical bedding lated tuff, and pervasive soft sediment deformation. The grains consist patterns between various internal strata, reflect spatial and temporal dominantly of juvenile scoria, but also contain a higher percentage of variations in bed load flow dynamics as the deposits aggraded vertically. accidental clasts than those of TRC1 (35–55%). Accidental lithics are Deposits consisting of laterally continuous beds with grain align- dominated by platy, aphanitic basalt, and to a lesser degree by angular, ment likely reflect laminar flow conditions in the depositional region of vesicular basalt and mudstone clasts (same as found in deposits of the current (Druitt, 1992), whereas deposits consisting of massive TRC1). Grain types includes 10–40% accidental basalt, 5–15% mud- deposits indicate inertial grain flow (Wohletz and Sheridan, 1979). stone, and 45–85% juvenile clasts and matrix. Deposits consisting of thin inversely graded strata suggest traction Where exposed, the base of PH5 has a sharp and erosive contact carpet transport in the depositing layer of the bed load (Lowe, 1982; with underlying TRC1 deposits. However, in a few areas up to 50 cm of Sohn, 1997), whereas cross-stratified dunes and antidunes indicate diatomitic lake sediments exist between TRC1 and TRC2 deposits, at turbulent flow during sedimentation (Valentine, 1987; Druitt, 1992), or the contact between FAs PH2 and PH5. Facies Association PH5 consists energetic, turbulent sweeps through a basal granular fluid, as suggested of several subgroups (PH5a–PH5f; Tables 1 and 2, online supplemen- by Brown et al. (2007). Therefore, variations in small-scale bedform tary data). features are interpreted as a consequence of temporal unsteadiness in the flow, similar to the interpretation for isolated dune features within 2.3.2. PH5 interpretation the distal Mt St Helens blast deposits (Druitt, 1992). PH5 is interpreted to be the result of multiple, high velocity, column Where present, the small-scale dune features within the larger collapse-induced, dilute pyroclastic density currents (e.g., Fisher and TRC2 bedforms are dominated by antidunes, which suggest that the Waters, 1970; Schmincke et al., 1973). These currents, also known as flow velocity exceeded internal wave speed in the bed load at the time pyroclastic base surges, are unsteady, density-stratified gas-particulate of deposition, and that standing internal waves developed within the currents where turbulence is the dominant particle transport mechan- stratified flow (Crowe and Fisher, 1973; Hand, 1974; Allen, 1982; ism (Fisher and Waters, 1970; Crowe and Fisher, 1973; Sigurdsson et al., Valentine, 1987). The same argument could be made for the larger, 214 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Fig. 8. (a) Deformed clastic and lacustrine lake sediments (NV1) beneath the TRC1 pyroclastic deposits; (b) NV1 (base) and overlying, massive, sandy T1 deposits of TRC1. Note the rectangular rip-up clasts of lacustrine substrate in the tuff deposits (NV1; Brunton for scale); (c) Large clastic dike intruding the TRC1 deposits. Person 1.5 m for scale (a) and (c).

20–200 m wavelength dune forms, as their symmetrical nature also these currents contained a significant amount of liquid water during implies standing waves within the broader context of the overall bed transport and deposition (Wohletz and Sheridan, 1979; Allen, 1982; load, and therefore possibly supercritical flow conditions. In this Wohletz, 1998). regard, we interpret the larger dune forms (20–200 m wavelengths) in the TRC2 deposits to reflect internal waves in the thick bed load region 2.4. TRC2 thin section analysis of the greater flow, and the smaller-scale features as the result of layer- by-layer deposition and variations in internal waves and flow Twelve samples were taken from various strata throughout the dynamics within the lowermost depositional bed load region. PH5 sequences (detailed descriptions of these samples are available The average clast size most proximal to the vent (b1 km) is less than as online supplementary data). Petrographic analysis shows that 0.5 cm. However, the particle sizes increase to an average grain size of grain morphology within the matrix is consistent in each of the 1–5 cm with distance from source, suggesting maintained high flow samples, and varies only in grain size depending on the coarseness of velocities and current competence with distance from source up to 1.5 km, the deposits they were collected from. Grains consist of subangular and thereafter declining current competence up to 3 km distance where to rounded, clear-brown to dark and sub-opaque (altered) brown theaveragegrainsizeagaindecreasestob1cm.Abraded,block-sized glass with 2–25% rounded vesicles. The glass shards are blocky and clasts are found intermittently at bedding horizons, and are interpreted as have fine fractures running across their surfaces. Vesicles within the ballistic clasts that were entrained into the flow after impact, as it is glass shards are rounded, have thick bubble walls, and range from unlikely that block-sized clasts remained in suspension over the distance 20–100 µm in size. Plagioclase, orthopyroxene, and olivine are also of the current. Their abraded nature also suggests that the block-sized present as individual grains in the matrix. They are also angular to clasts were tumbled or bounced along via saltation at the base of the flow. subrounded, except when found as phenocrysts within the glass Other than the large-scale dune forms, additional features which grains where they are subhedral to euhedral. The only new grain attest to the high velocity of the base surges include deep U-shaped within these deposits (i.e., not found in the TRC1 samples) is a dark, channels in the northeastern part of the complex, the 13 m tall chute- dense, most commonly irregularly shaped, but occasionally sub- and-pool feature observed in TRC2, and evidence for currents rounded juvenile grain. It differs from the glass grains in that it surmounting and depositing across 21 to 45 m tall obstacles N2km contains 10–18% plagioclase needles that are dispersed throughout from source (discussed further in Section 3.3). The presence of the sample, rather than found as radiating clusters. The dense, accretionary lapilli, vesiculated tuff, steam-escape structures, plas- altered matrix of the new juvenile grain, which contains 2–5%, and tered beds, and pervasive soft sediment deformation suggests that rarely up to 25% rounded vesicles, composes the rest of the juvenile B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 215 clast. Accidental material within PH5 consists of the same sub- 3. Distribution of Facies Associations rounded to rounded crystalline basalt, rounded, highly-weathered mudstone, and silicic pumice that were found in the TRC1 deposits. The geologic map (Fig. 2) shows the distribution of the TRC1, TRC2, Most grains, especially those larger than 300 µm, have a thin fine ash and flank vent deposits around the complex. The dashed circles with coating. speckled fill represent the approximate locations of the various vents. The grains within these samples have very similar textures and These are surrounded by dotted lines which represent the approximate morphology to the TRC1 eruptive products; therefore, it is not possible location of the inferred crater rims. Numerous stratigraphic sections to tell if the glass shards in these samples were derived from juvenile were measured in four sectors around the vent (West, North, East, and material from the TRC2 eruption, or entrained from the deposits of Southern arm) in order to reconstruct the temporal evolution of the TRC1. eruption as well as visualize facies variations with distance from source.

Fig. 9. Measured stratigraphic sections of N4 and NE1 (Fig. 1 and 10) and associated FAs. N4, the western-most measured section is on the right, and the eastern-most on the left. 216 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

3.1. Western TRC exposures ∼250 m in diameter; and the crater of FV 2 is ∼190 m in diameter with a surrounding tuff ring ∼440 m in diameter. The lateral tuff ring Four vertical sections were measured in the western sector of the deposits have been eroded away, and the exposed outer tuff ring ; SW1 at 0.6 km from TRC1 source, W4 at 2.4 km from TRC1 deposits are highly weathered and form inaccessible cliffs making source, W8 at 2.6 km from TRC1 source, and W10 at 3.4 km from TRC1 detailed stratigraphic sections impossible. FV 3, previously named source (Fig. 7d, c, b, and a, respectively). The sections reveal a vent 8 by Heiken (1971), is also located in the northern section. It is progression from less than 10 m of PH1 (first set of eruption-fed a small flank vent, only 100 m at the base and 200 m in diameter at turbidity current deposits) to a few tens of meters of PH2 (second set the top, and is well exposed in the cliffs of the northern face. of eruption-fed turbidity current deposits). PH3 (TRC1 scour and fill Three large U-shaped channels were identified in the northeastern deposits) appears in only one section, near the base of section W8, but side of the complex (location shown by the three arrows in Fig. 2;also was traced to more distal locations in the field. PH4 (TRC1 surge first recognized by Heiken, 1971). The channels, which scour the TRC1 deposits) is found only proximal to source, and was likely eroded away deposits, are located close to the measured section of NE1. The upstream in other locations, replaced by the later deposits of TRC2-PH5, or never side of the westernmost channel is 7.5 m tall, 6.75 m wide at the base, deposited in the first place. The stratigraphic sequence (other than in and 13 m wide at the top (Fig. 11a, b). The downstream side, which is areas proximal to the TRC1 tuff cone) is overlain by up to 45 m of roughly 30 m further from source, is 13 m tall,10 m wide at the base, and PH5 (TRC2 surge deposits). Given the location of the subaqueous– 35 m wide at the top (Fig. 11d). Similar increases in channel dimensions subaerial depositional transition within the TRC1 eruptive products, with distance from source have been noted at Koko crater, HI (Fisher, and the basal contact of the TRC1 pyroclastic deposits and older 1977) and Barcena Volcano, Mexico (Richards, 1959). Near the west lake sediments, the paleo-lake at the site of the TRC1 vent was roughly channel, parallel to the flow direction the strata are observed to plaster 60–70 m deep. up and over the pre-existing TRC1 deposits (Fig. 11c), and drape the The contact between the lake sediments and pyroclastic deposits is obstruction perpendicular to direction of flow (Fig. 11a). exposed in sections W8 and W10 (Fig. 7a, b). The sediments dip inwards beneath the pyroclastic section at 30–40° (Fig. 8a), and form clastic dikes 3.3. Eastern TRC complex-PH5 and flame structures into the overlying pyroclastic sediments of TRC1 (Fig. 8c). Thin (b3 cm), interbedded, lenses of NV1 lake sediments or The eastern flank of the complex is dominated by the surge large (up to 40 cm long), intact pieces of the NV1 substrate (Fig. 8b) are deposits of TRC2. The TRC1 deposits are poorly exposed in one small commonly found at the base of the pyroclastic section. area in the east, and another in the southeast (Fig. 2). TRC1 deposits were either eroded by the base surges of the TRC2 eruption, or were 3.2. Northern TRC exposures eroded by non-volcanic processes prior to the TRC2 eruption. The hummocky topography created by the PH5 deposits is most Two sections, N4, 3.6 km from the TRC1 source, and NE1, 3.5 km obvious in the eastern side of the complex. A well exposed outcrop in from the TRC1 source (Figs. 2 and 9), were measured on the northern the east, beginning with the diamond labeled E4 on Fig. 2, and side of the complex. Both sections begin much lower in elevation than extending 500 m to the south, exposes ∼40 vertical meters of section the western sections, suggesting a deepening of the lake to the north. in the direction perpendicular to flow (Fig. 12a). The corresponding The two most interesting features are the increased number of 500 m long dune truncates pre-existing TRC2 deposits on the north ballistics in the N4 section, and the finer grain size (average medium- (right) side (Fig. 12c); and is truncated by another large dune on the to-coarse ash) in the NE1 section. The ballistic clasts in the N4 section south (left) side (Fig. 12b). Fig. 12d represents the direction parallel to are on average 0.3–0.5 m in diameter, but occasionally up to ∼1 m, and flow, and shows two 40–50 m long dunes that are part of the larger all have deep bomb sags (e.g., at base of the stratigraphic column, a feature. These dunes dip ∼13° to the southeast, consistent with the dip 97×66 cm accidental clast of basalt deforms the underlying strata shown in Fig. 12c (crest of dune on right side of photograph). These N1.5 m, Fig. 9b). Ballistic clasts comprise 5–7% of clasts, are found photographs illustrate the three-dimensionality of the dune forms. randomly scattered throughout the deposits and are present in much What is interpreted as a large chute-and-pool structure, first higher proportions than in more proximal sections. Dewatering mentioned in Section 2.3.1 above, is found at location E5 (Fig. 2). This features such as squeeze-up and flame structures (0.1–1 m) and exposure is 1.6 km from the source, extends 40 m parallel to the discontinuous offsets and faults (usually N1 m in length) are also direction of flow, and is 13 m tall (Fig. 13). The deposits parallel to the common in this section. A 40 cm thickness of NV2, the non-volcanic flow direction contain wavy-planar and horizontal strata for the lithofacies between the TRC1 and TRC2 deposits, was found about first ∼35 m of the outcrop. These strata range from 10–30 cm thick, 80 m southwest of the N4 outcrop. and are composed of alternating beds of matrix supported (up to Fig. 10 is a schematic fence diagram for the north face of the complex 100% fine ash beds) and poorly sorted, grain supported beds. The over a cross section shown in Fig. 2. Stars indicate where the cross strata begin to bend upwards at angles of 33 to 44°, where the beds section bends (Fig. 10). The fence diagram extends from east (left) to abruptly thin and fine to an average 2–7 cm thick (Fig. 13). The strata west (right; Fig. 10a). On the east side of the complex, the contact continue to steepen and thin towards the downstream flow direction between NV1 and PH1 is exposed at lower elevations (approximately to the full outcrop height of 13 m before they begin bending back 1326 m a.s.l.), suggesting that the lake deepened to the east. towards horizontal (Fig.13). The rest of the downstream side of this The TRC1 deposits are grouped in Fig. 10 to illustrate their lateral feature has been eroded away. The exposed flat-lying strata are 4 m extent and relationship with the overlying TRC2 deposits. The contact thick on the left side of the feature (but are probably closer to 6 m in between the TRC2 and TRC1 deposits is irregular and varies in elevation, thickness given the exposed strata a few meters further to the east), which we attribute mostly to scouring by TRC2 surges. However, NV2 and are overlain by slightly thicker and somewhat more diffusely lake sediments were found in several locations in the west (40 cm thick stratified deposits which also cover the steeply dipping strata to the near section N4), and in the east (east of section NE1), indicating that right. Although poorly exposed, the upward bending strata can be the TRC1–TRC2 contact is not erosive in all locations. traced N100 m north of this feature, at the same elevation and Three smaller flank vents, FV 1, FV 2, and FV 3 (Fig. 10b and c; first distance from vent. recognized by Heiken, 1971), cut through the TRC1 deposits but are Chute-and-pool features are commonly observed in base-surge overlain by the deposits of TRC2. This indicates that the flank deposits, and representpffiffiffiffiffiffi a hydraulic jump where supercritical flow eruptions occurred after the TRC1 event, but before the TRC2 event. (chute, Fr N1,Fr = V= gh, where V=velocity, g=gravitational accelera- The crater of FV 1 is ∼180 m in diameter with a surrounding tuff ring tion, and h=depth of flow) abruptly changes to subcritical flow ..Bad ..Cak ora fVlaooyadGohra eerh10(09 203 (2009) 180 Research Geothermal and Volcanology of Journal / Clarke A.B. Brand, B.D.

Fig. 10. (a) North fence diagram. The location of sections N4 and NE1 are designated by long rectangles; (b) Flank vent 3. The vent margins are outlined with a thick white line, the strata with thin white lines. The top of FV 3 is 200 m across, and the base 100 m; (c) Flanks vents 1 and 2. Again, the margins are outlined with a thick white line. The remnant tuff ring around FV 1 is ∼250 m in diameter, and the remnant tuff ring around FV 2 is ∼440 m. – 224 217 218 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Fig. 11. (a) Western channel from the south side of the outcrop (most proximal to vent); (b) Closer view of the western channel, person 1.5 m for scale(flow direction into page for a and b); (c) Plastered TRC2 surge deposits riding up and over a pre-existing obstacle (TRC1 tuffs, flow direction from right to left); (d) North side of outcrop (more distal from vent). Person on right ∼2 m tall for scale (flow direction out of page).

(pool, Fr b1; e.g., Schmincke et al., 1973). However, they are typically there is no evidence of any obstruction in this region (i.e., no remnant much smaller in scale (meters rather than 10s of meters). Similar tuff rings or older deposits). Furthermore, given that the morphology of large chute-and-pool features were identified in the 1991 Pinatubo the feature is the same as previously identified chute-and-pool deposits deposits and at the base of Mt St Helens in the 1980 pyroclastic (Jopling and Richardson,1966; Fisher and Waters,1970; Schmincke et al., flow deposits, but have since been eroded away (Steve Self, personal 1973) and that the ratio of deposit thickness (downstream to upstream communication). However, structures as large as this have never side of the jump, 2.2) is similar to other well documented chute-and- before been identified in basaltic hydromagmatic eruptions. pool features (Schmincke et al.,1973; Weirich,1988), the chute-and-pool An alternative interpretation for this feature is a surge current that interpretation is preferred. This indicates supercritical flow conditions surmounted a pre-existing obstacle that is no longer exposed, although up to 1.6 km from source. B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 219

Fig. 12. (a) 500-m-long dune form, exposed in the direction perpendicular to flow; (b) and (c) Closer view of the crests of the feature with the strata outlined; (d) Direction parallel to flow, located on the north side of the larger feature (see c for location marked (d) illustrating the three-dimensionality of this dune form; (e), (f), and (g) Closer views of the strata at the crest of the dune in (d) (location E4 in Fig. 2).

3.4. Southern arm of TRC complex Strike and dip data suggest that another sizeable crater may have existed in the southern part of the complex (FV 6; Fig. 2), but the The long arm that extends towards the south–southeast of the deposits have since been eroded away beyond confident recognition. complex is composed primarily of bedded from one or more of Based on distinct differences in accidental components and deposi- the surrounding flank vents, and is overlain by the deposits of PH5. At tional characteristics, the first 15 m of strata along the southern arm least four obvious flank vents exist in this area (FV 4–7, Fig. 2). Flank were determined to have originated from vents other than TRC1 or vents 4 and 5 are located in the west–southwest part of the complex TRC2, and are likely the products of either FV 6 or 7. The upper ∼60 m (Fig. 2) and represent two highly eroded inner craters, which are of bedded tephra deposits in the southern arm consist of strata from partially nested within each other. FA PH5 of the TRC2 eruption, and contains dunes 20 m to 80 m in 220 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224

Fig. 13. This series of photographs were taken from location E5 (Fig. 2). (a) Large chute-and-pool feature. Outcrop is 40 m in length, and 13 m tall (person for scale); (b) closer view of the strata steeply bending upwards; (c) Feature with strata outlined. The six-meter height within the chute regions was extrapolated from deposits exposed to the east (right). Everything further east of the outcrop has been eroded away. wavelength parallel to the direction of flow. At the southern tip of the the time of the TRC1 eruption, and deformed under the weight of the arm, the surge deposits ramp up and over a pre-existing high in the rapidly accumulating pyroclastic deposits (Heiken, 1971). The clastic topography (likely the remnant of an older crater), and have nearly dikes likely represent dewatering features that occurred both due to vertical dips as they plaster against the 21 m high obstacle. the overlying load and volcanic seismicity (Nichols et al., 1994).

4. Discussion Table 4 4.1. TRC1 Lithofacies thickness at various distances from the vent (LT1 is not included as it was rarely completely exposed, and accurate thicknesses were not obtained)

TRC1, the first eruption to take place at the Table Rock Complex, Facies Lithofacies 0.6 km 2.4 km 2.7 km 3.5 km initiated with a Surtseyan-style eruption though a 60–70 m deep, Association (SWI) max (W4) max (WI0) max (NEI, N4) max thickness thickness thickness thickness fresh water lake, as suggested by the distance between the freshwater (cm) (cm) (cm) (cm) lake sediments and the subaqueous–subaerial pyroclastic contact. PHI LT2 32 10 10 6 Where exposed, the NV1 lake sediments dip toward the vent beneath PH2 LT4 25 70 40 50 the pyroclastic section at 10–30°, soft sediment deformation struc- PH2 LT3 25 50 30 50 tures are pervasive, and clastic dikes varying from decimeters up to PH2 T2 10 10 10 20 6 m in length and 1–100 cm in width are common. These features PH2 LT6 0 100 80 0 PH2 LT5 0 80 40 0 suggest that the NV1 sediments were unconsolidated and saturated at B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 221

initially high magma mass flow rate at the onset of the eruption that could have isolated some of the erupting magma from the external water, and led to less efficient mixing (Wohletz and McQueen, 1984; Büttner and Zimanowski, 1998; Mastin, 2007). The lack of welding textures in the matrix surrounding the fluidal juvenile clasts suggests that the flows which produced these deposits contained water as the continuous intergranular phase rather than hot gas, which is consistent for deposits of Surtseyan-style explosive eruptions (White, 1996; Sohn, 1997; White, 2000). In contrast, hot gas is speculated to be the interstitial fluid in high-temperature pyroclastic flows into subaqueous environments when the flows are derived from sustained explosive eruptions with well-developed gas thrust regions (Sparks et al., 1980; Cas and Wright, 1991; Kokelaar and Busby, 1992; Schneider et al., 1992; White and McPhie, 1997; White, Fig. 14. Wavelength versus distance from source for several well-documented surge 2000). Therefore, PH1 deposits likely originated from eruption-fed deposits (Mt St Helens Blast, Druitt, 1992; El Chichon, Sigurdsson et al., 1987; Ubehebe, aqueous density currents including combinations of low- and high- Crowe and Fisher, 1973; Taal, Moore, 1967; Sinker Butte, Brand and White, 2007; Narbona Pass, Brand et al., in press). Note: Only the measured wavelengths with concentration turbidity currents (Lowe, 1982; Postma et al., 1988; distance from source are shown on this plot, thus the total runout distance is not Kneller and Branney, 1995), cohesionless sediment gravity flows represented for each example. or grain flows (Lowe, 1976; Postma, 1986; Nemec, 1990), and a con- current fallout component. As only the coarsest juvenile clasts show a The beginning of the pyroclastic section is dominated by eruption- fluidal nature, we suggest that the larger clasts remained insulated by fed aqueous density currents, some of which probably transformed self-generated steam jackets within a flow in which water was the after initial fallout. These deposits were interpreted to have been interstitial fluid phase (White, 2000). eruption-fed rather than reworked due to the dominance of juvenile The PH2 sequence is interpreted to be the result of multiple, high- material (N95%), general consistency in the bedding style, and lack of density, eruption-fed turbidity currents, where the overall fining significant grain abrasion and rounding. Furthermore, the repeating upward nature and progressive development of different bedding packages of lithofacies (i.e., LT1–T1, LT2, LT4–LT3–T2) indicate that this structures (i.e., normal and reverse grading, diffuse stratification, fine- was a pulsating eruption, and each repeated lithofacies or set of grained interbeds) in the deposits represents waxing and waning flow lithofacies (i.e., LT6–LT5–T3) represents one explosive pulse. Indivi- conditions over the duration of deposition (Bouma, 1962; Nemec et al., dual, fine-ash glass shards within the matrix are fractured, blocky, and 1980; Lowe, 1982; Postma, 1986). This repeating set of lithofacies angular, indicating that the dominant mechanism of fragmentation elucidates the pulsating nature of the eruption, and the variation in was magma–water interaction (Heiken and Wohletz, 1985). grain size and depositional characteristics from one package to White (2000) argues that although subaqueous eruption-fed another represents variations in initial particle concentration, grain flows commonly involve water-supported transport, the transport size, and velocity of each flow. and depositional processes are controlled by the nature of the The subaqueous deposits thicken away from vent, consistent with eruption and its interaction with the surrounding water. The contrast a growing platform with shallow slopes (1–7°) into deeper water. in the bedding style and thickness, the juvenile grain morphology Table 4 shows the variation of individual lithofacies thickness with between individual lithofacies, and the considerable differences distance from vent. The coarsest deposits, LT6 and LT5, are not between FAs PH1 and PH2 demonstrate this well; both FAs were recognized in either proximal or distal locations from source. It is determined to have been emplaced subaqueously, however, we interpreted that the initial high-density turbidity currents carrying attribute the differences in deposit characteristics to differences in the coarser-grained loads of LT6–LT5 had a higher initial capacity and eruptive style. competence to carry the coarse clasts N0.6 km away from the vent PH1 begins with thick, alternating sequences of tuff and lapilli tuff before sedimentation occurred. The lack of these lithofacies at the that often contain either lenses of the underlying lake sediment, or in most distal exposures suggests that the coarsest grains settled out at some cases large, elongated, and intact slabs of the substrate (section medial locations. This also suggests that, while we have distinguished W10; Fig. 8b), suggesting that the first deposits were dominated by LT6–LT5–T3 and LT4–LT3–T2 stratigraphically, the deposits of LT4–LT3 lateral erosive transport. The sharp transition to LT2 deposits, which in the distal regions may represent a lateral facies change from the are repeating 10–30 cm thick beds of reverse-graded coarse ash to coarser-grained LT6–LT5 currents. lapilli, followed by finely laminated fine ash, are interpreted as a It is also interesting to note that the deposits of LT4–LT3 thicken combination of fallout-initiated, thin density currents, and water- with distance from source, with the thickest deposits occurring at a settled, fine ash tuff. While the initial 5–8 m of PH1 deposits represent distance of ∼2.4 km. These deposits subsequently thin to a distance of thick, concentrated flows, the more thinly bedded deposits of LT2 2.7 km. This likely represents a low initial rate of deposition, followed suggest that the eruption eventually attained a higher frequency of by an increasing rate of sedimentation with distance from source, jetting or explosivity. which finally tapers off with decreasing current load at distal PH1 contains coarse lapilli to block-sized juvenile scoria with locations. Note also that the most distal deposits of LT4–LT3 thicken quenched rinds. These coarse juvenile grains are found either again, and then follow the thinning trend of LT6–LT5 (Table 4), which randomly dispersed throughout the T1 and LT1 lithofacies and also supports a lateral facies change from LT6–LT5 to LT4–LT3. supported in a fine-grained matrix, or in concentrated horizons in Significant soft sediment deformation (SSD) was found throughout the reverse-graded strata of lithofacies LT2. While it has been PH1 and PH2 deposits. Some SSD occurs in the steep walls of scour and determined based on ash morphology and texture that magma– fill channels, where it is interpreted that thinly bedded, fine-grained fill water interaction was the dominant fragmentation mechanism, the deposits slipped and formed convolute folds towards the axis of the fluidally-shaped, scoriaceous juvenile coarse lapilli-to-fine blocks channel. Other SSD in the form of flame features and localized folded suggest a concurrent fire-fountaining stage at the beginning of the strata appear to be due to the weight of overlying and likely quickly eruption (Houghton and Schmincke, 1986; Mueller and White, 1992). deposited pyroclastic deposits. SSD is also found beneath ballistic blocks. As all evidence points to magma interacting with an abundant source All of these SSD features attest to the subaqueous and saturated of lake water, the early phase of fire-fountaining may suggest an conditions at the time of deposition. In several distal locations around 222 B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 the TRC1 remnants, lake sediments of NV2 were identified at the contact underlying country rock. Based on the accidental clast composition, of the TRC1 and TRC2 deposits, suggesting that the lake level was well and the level of the TRC2 crater floor, we hypothesize that the above the height of the distal TRC1 deposits long after the end of the magma–water interaction occurred dominantly within the TRC1 TRC1 eruption, and that much of the distal deposits may have been deposits, and that the source of the water was related to a shallow eroded below the wave base. Additionally, this suggests a significant playa lake, saturated, unconsolidated or very poorly lithified TRC1 repose period between the TRC1 and TRC2 eruptions. deposits, and/or near-surface, water-saturated playa-lake sediments. Given the distance from the subaqueous–subaerial contact to the Therefore the TRC2 feature could be considered to be a shallow maar solidified lava lake, the subaerial portion of the TRC1 tuff cone grew at in the sense that magma–water interaction occurred below the pre- least 285 m above the lake level. The most proximal, final pyroclastic existing surface, and accidental clasts derived from the underlying deposits recognized before the transition to spatter at the top of TRC1 country rock are common, although subordinate to the juvenile clasts. contain similar accidental clasts as in the lower tuff cone, and also The rest of the TRC2 discussion will focus on the large scale surge have similar juvenile glass morphologies including bubble number bedforms that comprise the tuff ring, and the insight they provide into density and size. This suggests that as the eruption evolved, the eruption dynamics. The pyroclastic surge deposits radiate axi- location of magma–water interaction did not excavate into the symmetrically from the center of the crater. Vertical cross-sections subsurface to entrain deeper, or a higher percentage of, country rock through the TRC2 dune features reveal large-scale hummocky cross- fragments. Additionally, these deposits are highly palagonitized and stratification. The fact that each set of dunes truncate the previous contain a high proportion of accretionary and armored lapilli, which ones, and form the same three-dimensional, hummocky features supports the interpretation that abundant liquid water was present at suggests that the deposits were not simply mantling a pre-existing, the time of deposition. This implies that a relatively high water– hummocky topography. Rather the currents were carving, depositing, magma ratio existed just before the magmatic stage. and re-creating the hummocky topography observed around the The transition from phreatomagmatic to Strombolian deposits is edifice with the passing of each surge current. These features demon- evidenced by a rapid increase in progressively coarser-grained scoria strate the pulsating nature of the . (1–3 cm) over two vertical meters just below the Strombolian-spatter The dune forms at TRC2 are more than an order of magnitude larger deposits. This observation suggests that the vent was quickly sealed than dune features recognized in other basaltic hydrovolcanic deposits off from the influence of external water to produce the Strombolian- (e.g., Fisher and Waters,1970; Waters and Fisher,1971; Crowe and Fisher, spatter and a ponded lava lake at the close of the eruption. 1973; Schmincke et al.,1973; Sohn and Chough,1989; Dellino et al.,1990; Chough and Sohn, 1990; Brand and White, 2007; Brand et al., in press), 4.2. TRC2 and their unusually long wavelengths extend to great distances away from the vent (e.g., 20-m wavelengths recorded 4.7 km from source). The TRC2 was the last and most energetic eruption to occur in the flows were highly erosive and scoured into the TRC1 deposits on all sides TRC. The crater rim is ∼2.7 km in diameter, and the deposits, which of the tuff ring, truncate earlier surge deposits from the same eruption, form a broad tuff ring surrounding the crater, are dominated by large- and form large U-shaped channels in the north. The channels were scale pyroclastic base surge deposits. The TRC2 eruption occurred a interpreted by Heiken (1971) to have been carved out by an extremely significant time after the TRC1 eruption, as evidenced by the 25–50 cm erosive surge, similar to the U-shaped channels described at Koko Crater, of lake sediments found at the TRC2–TRC1 contact. Additionally, there Hawaii (Fisher, 1977). Most U-shaped channels have been recognized on appears to have been a significant drop in the regional lake level prior the steep slopes of tuff cones rather than the shallow slopes of tuff rings, to the TRC2 eruption. Primary depositional characteristics of surge and it has been suggested that large U-shaped channels similar to those currents are found as low as 1325 m, which suggests the lake, if at TRC2 indicate much larger and faster surges than those required to present at the time of the TRC2 eruption, was no deeper than 15 m. form similar features in steeper tuff cones (Fisher, 1977). Wave cut terraces within the TRC2 deposits are present at 1356 m When we plot the TRC2 data (wavelength vs. distance from source) elevation, which indicates that at some point after the eruption the along with data from the literature, we see that wavelengths of the lake water rose to at least 46 m deep. TRC2 bedforms are much longer than those recorded at other basaltic The TRC2 surge deposits drape features to the east–southeast and hydrovolcanoes, and appear to follow the trends of larger scale northwest of the TRC1 tuff cone, which are likely well below the eruptions such as El Chichon and Mt St Helens (Fig. 14). At El Chichon, maximum height of the original tuff cone walls (Fig. 2). This suggests similar three-dimensional features with shorter wavelengths were either that much of the TRC1 tuff cone was eroded away prior to observed at distances greater than 4 km, and the authors suggest that deposition of the TRC2 deposits, or it was eroded by the TRC2 eruption if the dune wavelengths at greater distances were extrapolated toward itself. the source, they should have wavelengths of 100 to 200 m (Sigurdsson A difficult question to answer is whether or not the TRC2 eruption et al., 1987), consistent with those at TRC2. The TRC2 surges are occurred due to magma–water interaction at depth to create a maar unusual in the sense that wavelengths of this scale are typically and surrounding tuff ring, or if it erupted in a near surface or playa- found on volcanic flanks with much higher aspect ratios and slopes, lake setting to create a tuff ring with the crater floor above the level of consistent with high column collapse or energetic blast origins paleotopography. While the accidental clast component is much (Sigurdsson et al., 1987; Druitt, 1992), or in large ignimbrite ash flow higher in the TRC2 deposits (between 30 and 55%) than in the TRC1 tuffs (e.g., Ohakuri-ignimbrite forming eruption in the central Taupo deposits (b5%), it is not quite as high as proportions recognized for Volcanic Zone; Gravley et al., 2007). other maar deposits where the accidental component composes up to The unusually long wavelengths of the dune features proximal to the 80% of the total tuff deposits (Cas and Wright,1987). However, the true vent are indicative of highly inflated currents at the onset of the flows. accidental component cannot be quantified with certainty as it is The most likely way to obtain highly inflated flows proximal to source is impossible to distinguish between the accidental clasts derived from to entrain air during rise and collapse of an eruption column (Sigurdsson the TRC1 deposits and the juvenile clasts of the TRC2 deposits. Thus, et al., 1987). Therefore, the base surge currents are interpreted to be 30 –55% is a minimum estimate for the total amount of accidental generated by collapse from an unusually high column. This is further material in the TRC2 deposits. supported by the runout distance, which, based on the 20 m The crater floor at 1356 m is well above the floor of the coeval lake wavelengths found at the distal exposures, was likely much greater (1310 m). However, one could argue that the eruption occurred than the exposed 4.7 km. By comparison, eruption column collapses at through the flank of the remnant TRC1 tuff cone, and therefore may Ambae Island, Vanuatu, on the order of a couple hundred meters, have interacted with groundwater within the tuff cone deposits and produced surges to distances of 300 m (Nemeth et al., 2006), and the B.D. Brand, A.B. Clarke / Journal of Volcanology and Geothermal Research 180 (2009) 203–224 223 in Karymskoye Lake, Kamtchaka, collapsed activity subsequently constructed a steep tuff cone with a crater from 1 km and produced surges to distances of b2km(Belousov and floor ∼365 m above the base of the lake. Belousova, 2001). A general relationship can be made from these data The beginning of the eruption is interpreted to have had both a and weapons test-induced surge currents (e.g., Sedan weapons test; hydromagmatic and a simultaneous fire-fountaining stage, however, the Rohrer, 1965); surge runout distance is approximately 1.3 to 1.5 times the eruption changed to an entirely hydromagmatic phase after the initial column collapse height. The TRC2 surge deposits extend at least 4.7 km activity. The rest of the subaqueous deposits contain a mixture of fine from source implying column collapse heights of 3–4km,whichis ash and lapilli, suggesting moderately efficient hydromagmatic mixing. unusual for basaltic hydrovolcanic eruptions. The subaerial deposits of TRC1 are not well preserved due to post- On the fine-scale, the deposits within TRC2 are dominated by eruptive erosion, but where present consist of base surge and fall blocky, angular shards of fine ash, which suggests highly efficient deposits. The focus of explosivity was shallow and did not core into the hydromagmatic fragmentation. The juvenile glass shards have thick- pre-volcanic substrate, and the final eruptive stages are marked by both walled bubbles, and low bubble fractions (0.01 to 0.25), implying that efficient hydromagmatic fragmentation and evidence for liquid water at magmatic volatiles did not play an important role in magma the time of deposition, suggesting relatively high water-to-magma fragmentation. The abundant fine-ash fraction combined with the ratios. The eruption ended with a transition from hydromagmatic to large-scale, long runout surge features, are evidence of efficient Strombolian and effusive activity and produced a crater-filling lava lake conversion of thermal to kinetic energy and a highly energetic that partially intruded the unconsolidated tuff cone walls. eruption, respectively. However, the deposits contain ample evidence Once the eruption ceased, much of the medial to distal tuff cone was for abundant liquid water in the surges during transport and below wave base allowing portions of it to be eroded away, and allowing emplacement, which is contradictory, as these features are often the TRC1 deposits to remain saturated long after the eruption. A long attributed to water–magma ratios above that which flashes all water repose period occurred before later volcanic activity resumed. to steam, and thus are associated with lower explosivity (Sheridan and The seven flank vents around the complex erupted some time Wohletz, 1983; Wohletz and McQueen, 1984). Based on all observed during or after the TRC1 activity, but before the TRC2 eruption. TRC2 is features, it is interpreted that the eruption was highly energetic and represented by a 2.7 km diameter crater in the northeast of the likely had an efficient conversion of thermal to mechanical energy, complex. It was the most energetic eruption in the complex, and likely vaporizing most if not all external water at the site of magma water occurred when magma interacted with a shallow lake, playa-lake, interaction. It is then hypothesized that the water vapor subsequently and/or saturated TRC1 tuff sediments. The TRC2 eruption produced condensed due to entrainment-induced cooling during ascent and multiple, highly inflated, erosive pyroclastic surges that radiated out collapse from the eruption column to produce the “wet” deposits. from the base of a collapsing column. Dunes in the TRC2 surge The small-scale, centimeter to decimeter-thick strata within the deposits have longer wavelengths than other basaltic hydromagmatic larger dune features are interpreted to represent temporal variation in deposits, and are instead more consistent with dune bedforms the bed load of a density stratified current due to flow unsteadiness, produced by larger eruptions such as the 1980 blast at Mt St Helens, resulting in layer-by-layer variations during vertical aggradation and the column collapse surges at El Chichon in 1982. Such large dune (i.e., Druitt, 1992; Brown et al., 2007). The multiple scales of dune wavelengths are rare for low-aspect ratio tuff rings, and suggest highly wavelengths, from the largest that create the hummocky topography inflated flows produced by an unusually high eruption column. around the tuff ring, to the medium dunes (up to 50 m) that appear nested within the larger wavelength dunes, to the smallest antidunes Acknowledgements recorded in the individual 1–2 m thick packages of sediment (3–6min wavelength), support the Valentine (1987) model of deposition within The authors would like to thank Grant Heiken for suggesting this a density stratified current. The various scales of dunes likely field area and spending several days in the field with B.B. We are represent internal gravity waves on different length scales within especially grateful for the assistance of Mike, Jean, and Josh Bandfield, the larger flow, as discussed in Section 2.3.2 above. and Adam Frus in the field during summer 2006. This work greatly Extensive base surge runout distances have been recognized for benefited from the thorough and insightful reviews of John Smellie other phreatomagmatic eruptions. The produced base and Corina Risso, and guest editor Károly Németh. Funding for this surges with a runout of up to 6 km (Moore et al., 1966; Moore, 1967), research was provided by an Arizona NASA Space Grant Consortium the Monte Guardia volcano, Lipari with a runout of up to 7 km Student Fellowship, and the National Science Foundation, USA (EAR (Colella and Hiscott, 1997), and the Glaramara tuff in Scafell caldera, 0538125). English Lake District, UK with runout up to 8 km (Brown et al., 2007). The latter two examples were associated with higher silica , Appendix A. Supplementary data and based on the presence of pumiceous clasts in their deposits, likely had signifi cant influence from magmatic volatiles (Colella and Supplementary data associated with this article can be found, in Hiscott, 1997; Brown et al., 2007). The explosive, basaltic eruption the online version, at doi:10.1016/j.jvolgeores.2008.10.011. at Taal volcano matches TRC2 closest in terms of runout distance, but the bedform wavelengths at Taal were more than an order References of magnitude smaller (Fig. 14; Moore, 1967). Thus, the TRC2 dune wavelengths expand the range possible for basaltic phreatomag- Allen, J.R.L., 1982. Sedimentary Structures: Their Character and Physical Basis. matic eruptions. Development in Sedimentology, vol. 30A & B. Elsevier, Amsterdam. 663 pp. Arnott, R.W.C., Hand, B.M., 1989. Bedforms, primary structures and grain fabric in the – 5. Conclusions presence of suspended sediment rain. J. Sed. Petrol. 59, 61062 61069. Belousov, A., Belousova, M., 2001. 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