pure.aber.ac.uk€¦ · web viewidentification of a kulshan caldera correlative tephra in the...

http://dx.doi.org/10.1016/j.yqres.2016.06.004

Identification of a Kulshan caldera correlative tephra in the Palouse loess of Washington State, northwest USA

Georgina E. King1*, Nicholas J.G. Pearce1, Helen M. Roberts1, Victoria C. Smith2, John A. Westgate3, David R. Gaylord4 and Mark Sweeney5

1Department of Geography and Earth Sciences, Aberystwyth University, Wales, UK2Research Laboratory for Archaeology and the History of Art, University of Oxford, UK 3Department of Earth Sciences, University of Toronto, Canada4School of the Environment, Washington State University, USA5Department of Earth Sciences, University of South Dakota, USA

*[email protected] Present address: Institute of Geography, University of Cologne, Cologne, Germany

Key words: Kulshan caldera, Lake Tapps tephra, Palouse loess, trace element geochemistry

AbstractThe Kulshan caldera formed at ~1.15 Ma on the present-day site of Mt. Baker, Washington

State, northwest USA and erupted a compositionally zoned (dacite-rhyolite) magma and a

correlative eruptive, the Lake Tapps tephra. This tephra has previously been described, but

only from the Puget Lowland of NW Washington. Here an occurrence of a Kulshan caldera

correlative tephra is described from the Quaternary Palouse loess at the Washtucna site

(WA-3). Site WA-3 is located in east-central Washington, ~340 km southeast of the Kulshan

caldera and ~300 km east-southeast of the Lake Tapps occurrence in the Puget Lowland.

Major- and trace element chemistry and location of the deposit at Washtucna within reversed

polarity sediments indicates that it is not correlative with the Mesa Falls, Rockland, Bishop

Ash, Lava Creek B or Huckleberry Ridge tephras. Instead the Washtucna deposit is related

to the Lake Tapps tephra by fractional crystallisation, but is chemically distinct, a

consequence of its eruption from a compositionally zoned magma chamber. The correlation

of the Washtucna occurrence to the Kulshan caldera-forming eruption indicates that it had

an eruptive volume exceeding 100 km3, and that its tephra could provide a valuable early-

Pleistocene chronostratigraphic marker in the U.S. Pacific Northwest.

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1. Introduction The Kulshan caldera is located adjacent to Mt. Baker, Washington State, northwest

USA (Fig. 1) and is one of only three calderas identified within the Cascade volcanic range

(Hildreth, 1996). Despite comprising a similar scale to the eruption of Mount Mazama

(Hildreth, 1996) dated to 7,627 ± 150 cal yr BP (ice-core derived calendrical age from

Zdanowicz et al., 1999), which produced a voluminous and regionally extensive blanket of

tephra, the Kulshan caldera-forming eruption correlative Lake Tapps tephra has only been

found in lacustrine/glaciolacustrine sediments in the Puget Sound area of Washington State

(Easterbrook et al., 1981; Westgate et al., 1987; Hildreth, 1996). Westgate et al. (1987)

identified the Lake Tapps tephra at five locations in this area (Fig. 1). They dated sample

UT462 to 1.06 ± 0.11 Ma using fission track dating with correction for partial track fading and

the isothermal plateau technique (Westgate et al., 1987; Westgate, 1989), before Hildreth

(1996) identified the correlative eruption. Hildreth (1996) made the correlation of the Lake

Tapps tephra to the Kulshan caldera-forming eruption on the basis of several factors,

including: a similar nine-phase mineral assemblage between the distal tephra and proximal

ignimbrite, similar rhyolitic glass composition (data unpublished), the intercalation of the

Lake Tapps tephra within magnetically reversed lacustrine silt (Westgate et al., 1987), and

similarity between the tephra fission track age and Kulshan caldera 40Ar/39Ar ages.

Plagioclase from a Kulshan intracaldera pumice block (sample MB188) yields an 40Ar/39Ar

plateau age of 1.149 ± 0.010 Ma (Hildreth and Lanphere, 1994). The robust chronology and

source correlation of the Lake Tapps tephra means that it potentially forms an important

early Pleistocene temporal marker, complementing other widespread markers including the

Rockland tephra (~0.4-0.6 Ma; Lanphere et al., 1999, Sarna-Wojcicki, 2000), Lava Creek B

(~0.63 Ma; Matthews et al., 2015), Bishop Ash (0.765 ± 0.008 Ma; Zeeden et al., 2014),

Mesa Falls (1.293 ± 0.012 Ma; Gansecki et al., 1998) and the Huckleberry Ridge (2.003 ±

0.014 Ma; Gansecki et al., 1998) deposits. Tephra horizons offer a valuable means of

constraining early Quaternary environmental changes, which are beyond the scope of other

Quaternary dating methods such as luminescence dating. However, beyond the initial

identification of the Lake Tapps tephra in Puget Lowland lacustrine deposits (Westgate et al.,

1987) no further deposits have been reported, and it has been suggested that preservation

of this important marker horizon may have been limited due to glaciation of this region

throughout the late Pleistocene (Hildreth, 1996).

This study reports the identification of a second occurrence of the Lake Tapps tephra

(sample 93/WA3-1502). This tephra, collected from the Palouse loess near Washtucna, WA,

is correlated with the Kulshan caldera-forming eruption on the basis of major and trace

element glass chemistry, and geochronological constraints. As explained in this paper,

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however, this distal horizon differs chemically from the Lake Tapps tephra. We interpret this

as a consequence of the zoned nature of the magma chamber and the degree of

compositional evolution of the melt. In addition to forming an important early-Pleistocene

stratigraphic marker horizon within the thick loess deposits of the Columbia plateau

(McDonald and Busacca, 1988; Bjornstad et al., 2001; Pluhar et al., 2006), this second

occurrence provides further insights into the scale of the Kulshan caldera-forming eruption.

2. Sample siteThe silt- and clay-dominated Palouse loess blankets ~50,000 km2 of south-eastern

Washington, northern Oregon and northern Idaho comprising up to 75 m depth of windblown

sediments (Ringe, 1970; Busacca, 1991; Sweeney et al., 2007). The loess deposits range in

age and depth across the Palouse region, but potentially span the full Quaternary period in

some areas (Busacca, 1991), and include numerous palaeosols and numerous tephra

lenses and beds. The loess is primarily derived from remobilisation of glacial outburst

megaflood deposits that accumulated in Columbia Plateau depocentres throughout the

Quaternary (Busacca, 1991; Sweeney et al., 2005; Sweeney et al., 2007). These

depocentres were concentrated in south-central Washington and north-central Oregon in

areas prone to slackwater deposition associated with the megafloods that produced the

Channeled Scabland (Bretz, 1923; Bretz et al., 1956; Waitt, 1985; Baker et al., 1991), a

region of steep-walled basalt-floored coulees, dry falls, and loess islands that separate the

main channelways.

The loess deposits at Washtucna site 3 (WA-3, 46o46’05N, 118o20’46W)

accumulated in a loess island located between basalt-floored coulees and have been the

subject of several previous studies (e.g. Blinnikov et al., 2002; Busacca et al., 1992;

McDonald and Busacca, 1992). Sample 93/WA3-1502 was collected from this road-cut

exposure in August 2012 at a depth of 15.02 m; a tephra from the same site and depth has

been described previously by Busacca et al. (1992; their sample WA-3B). The tephra

horizon is laterally continuous as a distinctly white streak within the loess over a distance of

~60 m and was clearly visible following removal of vegetation; it is approximately 15 cm thick

at the sampling location, with evidence of down-working through bioturbation of up to 10 cm

below the unit sampled. Site WA-3 is located 340 km southeast of the Kulshan caldera

(Hildreth, 1996) and 300 km east-southeast of Lake Tapps (Westgate et al., 1987), ≥300 km

east of the other Cascade volcanoes, 660 km west-northwest of the Yellowstone caldera and

1,000 km north of the Long Valley Caldera (Fig. 1).

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The magnetostratigraphy published previously for site WA-3 by Busacca (1989)

indicates that the sampling location of 93/WA3-1502 is below the Brunhes-Matuyama

boundary, which Busacca (1989) identified at a depth of 11 m beneath the modern surface at

this site (Fig. 1b). Using the stratigraphy published by Busacca (1989) and an age of 0.770 ±

0.007 Ma for the Brunhes-Matuyama boundary (Suganuma et al., 2015) the age of sample

93/WA3-1502 is estimated at ~1.05 Ma if a constant sediment accumulation rate is assumed

throughout the profile. This age is consistent with the fission track and 40Ar/39Ar ages

previously determined for the Lake Tapps tephra and formation of the Kulshan caldera

(Westgate et al., 1987; Westgate, 1989; Hildreth, 1996), but could also be associated with

the Yellowstone, Mesa Falls caldera-forming eruption, which has been dated to 1.293 ±

0.012 Ma (Gansecki et al., 1998). The chemistry of sample 93/WA3-1502 was also

compared to published data for the Bishop Ash, Lava Creek B, Huckleberry Ridge (Pearce et

al., 2004) and Rockland tephra deposits (Sarna-Wojcicki et al., 1985), which despite their

different ages are nevertheless major North American eruptions that could have produced

tephra falls in the WA-3 area.

3. Sample preparationSample 93/WA3-1502 was prepared for major and trace element analyses using

standard methods. Shards were wet sieved to isolate the 180-212 µm grain size fraction and

then density separated at 2.5 g cm-3 to concentrate the glass fraction. Shards were mounted

in resin and polished using increasingly fine (down to 0.3 µm) polishing mats and were

finished using colloidal silica (0.02 µm). Samples MB188 and MB189 (Table 1) are glass

extracts from pumice clasts of proximal Kulshan caldera ignimbrites, the bulk chemistry of

which is reported in Hildreth et al. (2004). The major element chemistry of individual shards

was measured using wavelength-dispersive electron probe micro-analysis (EPMA) at the

Research Laboratory for Archaeology and the History of Art at the University of Oxford, UK.

A JEOL 8600 electron microprobe was used to determine oxide values for ten major and

minor elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn and Fe). Samples MB188 and MB189 were

measured in a separate analytical run and Cl was also analysed in these samples. An

accelerating voltage of 15 kV, a beam current of 6 nA and a defocused 10 µm beam were

used for glass shard analyses to minimise Na migration. The instrument was calibrated

using a suite of mineral standards prior to analysis and secondary standards of ATHO-G,

StHs6/80-G and GOR132-G (Jochum et al., 2006) were measured during each analytical run

to monitor system reproducibility and the precision and accuracy of glass analyses

(secondary standard data are listed in the Supplementary Data). All major element

concentrations have been normalised to an anhydrous basis, i.e. 100% total oxides. Where

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comparisons are made against the published literature that have reported fewer elements,

the data presented here have been normalised to an anhydrous basis using only the same

suite of elements to ensure direct comparison.

Trace element analyses were measured for the same glass shards using laser

ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) at the Department of

Geography and Earth Sciences, Aberystwyth University, UK. The system comprises a

Coherent Geolas 193 nm Excimer laser with a Thermo Finnigan Element 2 high-resolution

sector field mass spectrometer (Pearce et al., 2007; 2011). A 20 µm diameter beam was

used to ablate the majority of shards, however a small number of vesicular shards were

analysed using a 10 µm beam (see Supplementary Information). The data from both beam

diameters are comparable, and therefore have been interpreted together. Laser energy was

10 J cm-2 pulsed at 5 Hz with an acquisition time of 24 s. Trace element concentrations were

calculated using the method of Perkins and Pearce (1995) and Pearce et al. (2007). NIST

SRM 612 silicate glass was used as the calibration standard relative to concentrations from

Pearce et al. (1997) and 29Si was used as an internal standard, determined from the same

shards using EPMA and normalised to an anhydrous basis. ATHO-G (Jochum et al., 2005)

was used as a secondary standard and was measured at approximate two-hour intervals

throughout analysis. Element fractionation, related to laser interaction with the sample, was

applied to all analyses (unknowns and reference materials) based on the methods

established in Pearce et al. (2011).

Trace element data were collected for samples 93/WA3-1502, MB188 and MB189 as

well as for a suite of reference samples from the University of Toronto collection including

seven of the samples originally reported by Westgate et al. (1987): UT-55, UT-57, UT-58,

UT-400, UT-52, UT-56, and UT-462. The published average Si concentrations of Westgate

et al (1987) were used to provide an internal standard for samples UT-55, UT-57, UT-58 and

UT-400. The average SiO2 content of the published Lake Tapps tephra horizons from

Westgate et al. (1987), 78.3 wt%, was used as the internal standard for samples UT-52, UT-

56 and UT-462.

4. Results

4.1 MorphologyThe morphology of the shards from sample 93/WA3-1502 can be differentiated into

two distinct populations: angular with low vesicularity and highly vesicular fragments (Fig. 2),

with approximately 25% of shards classified as pumiceous. Westgate et al. (1987) described

the shard morphology of the Lake Tapps samples, and also recorded two populations of

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which 63% were pumiceous. The morphology of sample 93/WA3-1502 is therefore similar to

that reported for the Lake Tapps tephra horizons, in that it comprises two populations.

4.2 Geochemistry: Major and minor-elementsThe four Lake Tapps samples for which Westgate et al. (1987) report major and

minor element chemistry (UT-55, UT-57, UT-58 and UT-400) are rhyolitic and have a SiO2

content of ~78 wt%, which is consistent with sample MB188 which has a SiO2 content of

77.9 ± 0.2 wt% (n = 28, data from this study, Table 2; Fig. 3). These horizons have been

linked previously by Hildreth (1996) and have ages which are indistinguishable within

uncertainties (Westgate et al., 1987, 1989; Hildreth et al., 2003). Sample MB189 is also

rhyolitic but has a lower SiO2 content than sample MB188 and has two glass populations

with SiO2 of 76.3 ± 0.5 wt% (n = 7) and 73.1 ± 0.6 wt% (n=2). This sample is more similar to

horizon 93/WA3-1502 which also has two populations: a major population (Pop. 1) with SiO2

content of 76.1 ± 0.2 wt% (n=23) and a minor population (Pop. 2) with SiO2 content of 71.7 ±

0.2 wt% (n=3) (Figs. 4 and 5). At least two of the three shards from the low SiO 2 population

of sample 93/WA3-1502 are highly vesicular (no image was available for the final shard of

this chemical population).

The Mesa Falls tephra (UT860, Pearce et al., 2004) is compositionally distinct from

both the Lake Tapps tephra and sample 93/WA3-1502 (Figs. 3 and 4, Table 2). The Bishop

Ash, Lava Creek B and Huckleberry Ridge horizons are also compositionally different

(Pearce et al., 2004) from the Lake Tapps and 93/WA3-1502 tephra horizons on the basis of

glass major element chemistry. In contrast, the Rockland tephra (Sarna-Wojcicki et al.,

1985) is compositionally similar to both the Lake Tapps and 93/WA3-1502 horizons (Figs. 3

and 4, Table 2), but is significantly younger (Sarna-Wojcicki, 2000).

4.3 Geochemistry: Trace element chemistryComparing the average rare earth element (REE) data shows that all Kulshan-

sourced samples are similar (Fig. 5; Table 3); sample UT-462 has the most negative Eu

anomaly and sample MB189 has the least negative Eu anomaly. Plotting Ca against Sr for

the different samples, shows that all of the samples lie on what appears to be a fractionation

trend, with Ca and Sr decreasing sympathetically as a result of feldspar fractionation (Fig.

6a). However when Zr is plotted against Nb, sample MB188 is indistinguishable from the

Lake Tapps tephra horizons, but samples MB189 and 93/WA3-1502 (Pop. 1) are

compositionally different, although possessing the same Nb/Zr ratio (Fig. 6b). MB189 and

93/WA3-1502 are also distinct from the Lake Tapps horizons when Y and Zr are contrasted,

but again have the same Y:Zr ratio (Fig. 6c). No successful trace element analyses were

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obtained for the second population of sample 93/WA3-1502 (microlite contaminated

analyses), and no trace element data are available for the Rockland tephra.

5. DiscussionThe morphology of the glass shards of sample 93/WA3-1502, coupled with the

thickness of the deposit (>15 cm) and its distance from any nearby volcano (>300 km)

indicate that 93/WA3-1502 is most likely sourced from a major eruptive event. Based on the

magnetostratigraphy of the sample, two known caldera-forming eruptions in this region of the

Pacific Northwest provide potential correlatives: Mesa Falls eruption from Yellowstone and

the Kulshan caldera-forming eruption. A further possibility is that this horizon may be related

to an earlier, previously unidentified eruption from a Cascades volcano (cf. Dethier et al.,

2008). The major element data presented here confirm that sample 93/WA3-1502 does not

correlate with the Mesa Falls tephra (Figs 3 and 4); this assessment is also supported by a

~0.25 Ma difference in age suggested by the magnetostratigraphy at site WA-3 (Busacca,

1989). Furthermore, although the major element chemistry of the Rockland tephra is similar

to sample 93/WA3-1502, the identification of the Rockland tephra in normal polarity

sediments (Sarna-Wojcicki, 2000), and the sampling of 93/WA3-1502 from reversed polarity

sediments (Busacca, 1989), means that they cannot be correlative.

The geochemical data suggest that sample 93/WA3-1502 is linked to the Kulshan

caldera-forming eruption, but that Kulshan caldera had a complex magmatic evolutionary

history, similar to that reported for other Cascade volcanoes. Compositional zoning is typical

for Cascade Range calderas e.g. Crater Lake (Bacon and Druitt, 1988; Bacon, 1983), and

investigations into the chemistry of the Kulshan ignimbrite deposits have shown that the

caldera-forming eruption tapped much of the most fractionated volume of a compositionally

zoned reservoir (Hildreth et al., 2004). Contrasting the major and trace-element composition

of the Lake Tapps tephra horizons with proximal samples MB188 and MB189 shows

chemical differences. However these different samples lie on a trend consistent with the

evolution of the glass composition from rhyo-dacite to rhyolite (Fig. 4) by fractional

crystallisation of a feldspar-dominated assemblage. This is in agreement with the

observations of Hildreth et al. (2004) that Kulshan ignimbrite whole-rock chemistry ranges

from andesite to rhyolitic, and is caused by the extraction of minerals dominated by

plagioclase as well as hypersthene, hornblende, biotite and Fe-Ti oxides as well as traces of

apatite, zircon and minor quartz (Hildreth et al., 2003). This is clearly shown by the reduction

in CaO, FeO and Al2O3 between samples MB189 and MB188 (Fig. 4). Concentrations of K

increase as evolution progresses, reflecting the fact that plagioclase, rather than K-feldspar

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is crystallising (Fig. 4). An increase in SiO2 (Fig. 4c) may also be associated with

equilibration at shallower depths within the magma chamber (Gualda and Ghiorso, 2013).

Trace element chemistry provides further insights into the chemical evolution of the

Kulshan caldera-forming eruption. MB189 has higher CaO than most of the other glass

samples with the exception of the rhyodacites, and also has higher Sr, indicating it is less

evolved (Fig. 6a), this is consistent with its lower total REEs, Zr and Nb (Fig. 6b) and a

smaller negative Eu anomaly (Fig. 5). The relative CaO, Sr, and Eu concentrations

discussed are consistent with plagioclase-dominated fractionation, as crystallization of

plagioclase extracts these three elements from the magma (and thus results in the

enrichment of those elements not incorporated into plagioclase). In contrast, the Lake

Tapps occurrences have much higher SiO2, lower CaO, lower Sr and also lower Zr and

contain lower concentrations of other zircon-compatible elements (e.g. Y, REE, Th, Hf)

consistent with the onset of zircon crystallisation between 76-77 wt% SiO2 (Fig. 7a).

Comparison of CaO and Sr (Fig. 6a) shows a possible evolutionary trend between MB189

(most primitive), through 93/WA3-1502 to the Lake Tapps tephra samples, and MB188 (most

evolved), consistent with the evolution of the magma by the fractional crystallisation of

plagioclase and ferromagnesian minerals.

Plots of incompatible elements show that ratios such as Zr/Nb (Fig. 6b) and Zr/Y (Fig.

6c) are constant between MB189 and 93/WA3-1502, suggesting an origin of 93/WA3-1502

from the Kulshan caldera. However these ratios differ from the Lake Tapps tephra

occurrences and MB188. This suggests a link by fractional crystallisation between MB189

and 93/WA3-1502, but does not immediately suggest a relationship between 93/WA3-1502,

the Lake Tapps tephra and MB188.

The link between all these compositions is illustrated in Fig. 7, where for clarity

average compositions are presented. As SiO2 increases with continuous magmatic evolution

the Zr content increases between MB189 and 93/WA3-1502, until zircon becomes a liquidus

phase at ~77 wt% SiO2; at this point Zr concentrations start to decrease (from ~220 ppm)

towards the Lake Tapps tephra samples and sample MB188 (with ~130 ppm Zr) (Fig 7a).

Throughout this Rb contents continue to increase with increasing evolution, as Rb remains

strongly incompatible in the crystallising assemblage of plagioclase, ferromagnesian

minerals and zircon (Fig. 7b). Niobium remains mildly incompatible in the overall

assemblage, not being removed from the liquid despite its relative compatibility in zircon (Fig

7c; see Pearce et al., 2014), and thus gradually increases in concentration as crystallisation

proceeds. The reduction of Zr and Y above 77 wt% SiO2 can be accounted for by modest

amounts of zircon extraction (between 0.015 to 0.045%, based on distribution coefficients for

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Y in high-silica rhyolites, calculated from data in Mahood and Hildreth, 1983 and Pearce et

al., 2004). Zircon extraction overcomes the tendency of the continued extraction of

plagioclase and ferromagnesian minerals to cause Zr and Y to increase and reduces the

concentration of these and other zircon-compatible elements in the melt. The heavy rare

earth elements (HREE) and Y (see Fig 7d) increase from MB189 to 93/WA3-1502, and then,

with the onset of zircon crystallisation decrease rapidly, causing the evolutionary trend of the

magma to “double-back” upon itself. The onset of zircon fractionation thus changes the

behaviour of a series of what were hitherto incompatible elements; Zr, Hf, HREE and Y

become compatible, while Nb, Ta and LREE remain largely incompatible, and thus ratios of

these elements change as the magma evolves to >~77% SiO2.

This onset of zircon crystallisation at ~77 wt% SiO2 thus allows some incompatible

elements to continue to increase (Rb) whilst highly zircon compatible elements (e.g. Y,

HREE) decrease with continued fractionation. In this way, the compositions of all of the Lake

Tapps tephra, 93/WA3-1502, MB188 and MB189 samples can be linked by fractional

crystallisation of plagioclase and ferromagnesian minerals (see Hildreth, 1996), with the late

onset of zircon crystallisation controlling many of the high field strength elements (HFSE) in

the evolved, high SiO2 magmas. This is consistent with an evolutionary trend in the major

element composition of all of these samples, which can be linked by fractional crystallisation,

links 93/WA3-1502 to the Kulshan caldera and indicates that it is correlative (i.e. from the

same caldera-forming event) with the Lake Tapps tephra.

Correlating 93/WA3-1502 with the Kulshan caldera enables an improved estimation

of the size of this caldera-forming eruption, which Hildreth et al. (2004) have previously

suggested to have been between 80 and 100 km3 based on the thickness of the Lake Tapps

tephra horizons (Westgate et al., 1987). Horizon 93/WA3-1502 is thinner (~15 cm) than the

20-30 cm thick horizons reported by Westgate et al. (1987), but is ~120 km farther from the

Kulshan caldera. Using the method of Bonadonna and Costa (2012) and estimating an

isopach area of 31,400 km2 from the location of the Lake Tapps tephra (deposit thickness 30

cm) and an area of 90,080 km2 from the location of the 93/WA3-1502 tephra horizon (deposit

thickness of 15 cm) (Fig. 1), results in an estimated volume of ~120 km3 (density 2.3 g cm-3).

This suggests that the Kulshan caldera-forming eruption may have been more than twice as

large as the ~50 km3 eruption which produced the Mazama tephra (Bacon and Lanphere,

2006), although the data presented here are based on only two isopachs, and multiple lobes

of tephra dispersal could imply a smaller volume. Identification of further occurrences of

Kulshan caldera correlative tephra horizons and their thicknesses, which can be influenced

by local geomorphology and post-depositional processes, will improve this estimation.

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ConclusionA tephra horizon at Washtucna site 3 (sample 93/WA3-1502), is correlated on the

basis of single-shard major- and trace element chemistry, and geochronological constraints,

to the Kulshan caldera-forming eruption at 1.15 Ma. Although similar to the previously

described Lake Tapps tephra (Westgate et al., 1987), 93/WA3-1502 differs from the Lake

Tapps occurrences because it was tapped from a different part of a compositionally zoned

magma chamber (Hildreth, 1996). Sample 93/WA3-1502 is linked to the other occurrences

by fractional crystallisation of plagioclase, hypersthene, hornblende, biotite and Fe-Ti oxides

and, at late stages of magmatic evolution, zircon (Hildreth et al., 2003), which is important in

controlling the composition of the evolved, high SiO2 (>77 wt% SiO2), Lake Tapps

occurrences.

The correlation of 93/WA3-1502 to the Kulshan caldera-forming eruption could

provide an important early-Pleistocene stratigraphic marker within the Pacific Northwest,

constraining ages of deposits which are beyond the range of other dating techniques such

as luminescence dating. Furthermore the 93/WA3-1502 horizon extends the known area of

fallout from the Kulshan caldera-forming eruption, which was previously only reported from

the Lake Tapps site ~200 km south of the Kulshan caldera. This increased fallout area

suggests that the caldera-forming eruption may have been larger than initially proposed by

Hildreth (1996), and hence that this early Pleistocene marker may be more widespread and

better preserved than previously identified.

AcknowledgementsGEK was supported at Aberystwyth University by the Climate Change Consortium of Wales

(C3W) whilst undertaking this research. W. Hildreth is thanked for putting us in touch with D.

Tucker (USGS) who was kind enough to provide samples MB188 and MB189. P. Abbott

provided insightful comments on an earlier draft of this manuscript and S. Kuehn and D.

Dethier are thanked for their constructive reviews.

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Figure Captions

Fig. 1: (a) Location of the Lake Tapps and Washtucna (WA-3) sample sites (star shaped symbols) in Washington State (USA) relative to proximal volcanic centres (triangle shaped symbols). Kulshan caldera was located at the present day site of Mt. Baker (Hildreth, 1996), and a correlative tephra has been identified previously at Lake Tapps (Westgate et al., 1987). The isopachs used to estimate the Kulshan caldera-forming eruption tephra volume are shown, but should be regarded as poorly constrained as they are based on only two occurrences of Kulshan-caldera correlative horizons (shown by the star shaped symbols). Inset shows relative locations of the Yellowstone and Long Valley calderas, and the location of Washington State in the USA. (b) Stratigraphic log and magnetostratigraphy of site WA-3 redrawn from Busacca et al. (1989). The Mount St Helens Set C tephra horizon was previously identified at ~3 m depth by Foley (1982). Busacca et al. (1992) reported the tephra chemistry of samples WA3-B, WA3-C and WA3-D but were unable to assign these occurences to a particular eruption. Only sample 93/WA3-1502 was visible when samples were collected for this study, which is from the same horizon as sample WA3-B (Busaccca et al., 1992).

Fig. 2: Images of two shards analysed for major and trace element chemistry from sample 93/WA3-1502 collected in this study. Shard (a) is angular with low vesicularity whereas (b) is a pumiceous fragment. Red circles relate to the 20 μm (a) and 10 μm (b) beam sizes used to ablate each sample in the LA-ICP-MS instrument respectively. The scale bar shown in (a) is also applicable to (b).

Fig. 3: Total alkali silica (TAS) diagram showing data collected in this study with previously published values for horizon 93/WA3-1502 from Busacca et al. (1992; their sample WA3-B) and for the Lake Tapps (Westgate et al., 1987), Mesa Falls (Pearce et al., 2004) and Rockland (Sarna-Wojcicki et al., 1985) tephra horizons. Error bars show the published standard deviations.

Fig. 4: (a-d) Biplots of major element concentrations. The 93/WA3-1502 samples and the Lake Tapps samples sit on a possible fractionation trend, where extraction of plagioclase feldspar and ferromagnesian minerals could link all compositions. The Mesa Falls sample (UT860; Pearce et al., 2004) clearly plots away from the Lake Tapps tephra samples and the 93/WA3-1502 occurrences in (a), (b), (c) and (d), and thus does not correlate with any of these samples.

Fig. 5: Chondrite normalised REE contents of all samples analysed in the current study with the exception of UT860 (Mesa Falls). Chondrite concentrations from Sun and McDonough (1989).

Fig. 6: (a-c) Trace element biplots for the 93/WA3-1502 occurrence and Lake Tapps tephra horizons (Westgate et al., 1987). In (a) samples UT-55, UT-57, UT-58 and UT-400 are plotted relative to published CaO values for the same samples after Westgate et al. (1987); error bars are 1σ uncertainties and are within the symbols for sample UT-400 (Table 2). Note that UT-462 is plotted only in (b) and (c) because there is no major element data for this sample.

15

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504505506507508509510511512513514515516517518

519520521522523

524525526527528

529530531532533534

535536537

538539540541542543


Fig. 7: Biplots of the average concentration of Zr relative to Si (a), Rb (b), Nb (c) and Y (d) for sample 93/WA3-1502, the Lake Tapps tephra horizons (Westgate et al., 1987), and samples MB188 and MB189 which are glass extracted from Kulshan caldera proximal ignimbrites. Error bars are at 1 σ. The differences in trace element composition between samples are explained by fractional crystallisation (f.c.) of feldspars and then zircons; see text for discussion.

Table 1: Sample summary.

Table 2: Glass major and minor element chemical data, normalised to an anhydrous basis.

Table 3: Glass trace element chemical data measured in this study.

16

544545546547548549

550

551

552


Fig. 1: (a) Location of the Lake Tapps and Washtucna (WA-3) sample sites (star shaped symbols) in Washington State (USA) relative to proximal volcanic centres (triangle shaped symbols). Kulshan caldera was located at the present day site of Mt. Baker (Hildreth, 1996), and a correlative tephra has been identified previously at Lake Tapps (Westgate et al., 1987). The isopachs used to estimate the Kulshan caldera-forming eruption tephra volume are shown, but should be regarded as poorly constrained as they are based on only two occurrences of Kulshan-caldera correlative horizons (shown by the star shaped symbols). Inset shows relative locations of the Yellowstone and Long Valley calderas, and the location of Washington State in the USA. (b) Stratigraphic log and magnetostratigraphy of site WA-3 redrawn from Busacca et al. (1989). The Mount St Helens Set C tephra horizon was previously identified at ~3 m depth by Foley (1982). Busacca et al. (1992) reported the tephra chemistry of samples WA3-B, WA3-C and WA3-D but were unable to assign these occurences to a particular eruption. Only sample 93/WA3-1502 was visible when samples were collected for this study, which is from the same horizon as sample WA3-B (Busaccca et al., 1992).

17

553

554


Fig. 2: Images of two shards analysed for major and trace element chemistry from sample 93/WA3-1502 collected in this study. Shard (a) is angular with low vesicularity whereas (b) is a pumiceous fragment. Red circles relate to the 20 μm (a) and 10 μm (b) beam sizes used to ablate each sample in the LA-ICP-MS instrument respectively. The scale bar shown in (a) is also applicable to (b).

18

555


Fig. 3: Total alkali silica (TAS) diagram showing data collected in this study with previously published values for horizon 93/WA3-1502 from Busacca et al. (1992; their sample WA3-B) and for the Lake Tapps (Westgate et al., 1987), Mesa Falls (Pearce et al., 2004) and Rockland (Sarna-Wojcicki et al., 1985) tephra horizons. Error bars show the published standard deviations.

19

556

557

558


Fig. 4: (a-d) Biplots of major element concentrations. The 93/WA3-1502 samples and the Lake Tapps samples sit on a possible fractionation trend, where extraction of plagioclase feldspar and ferromagnesian minerals could link all compositions. The Mesa Falls sample (UT860; Pearce et al., 2004) clearly plots away from the Lake Tapps tephra samples and the 93/WA3-1502 occurrences in (a), (b), (c) and (d), and thus does not correlate with any of these samples.

20

559

560

561


Fig. 5: Chondrite normalised REE contents of all samples analysed in the current study with the exception of UT860 (Mesa Falls). Chondrite concentrations from Sun and McDonough (1989).

21


Fig. 6: (a-c) Trace element biplots for the 93/WA3-1502 occurrence and Lake Tapps tephra horizons (Westgate et al., 1987). In (a) samples UT-55, UT-57, UT-58 and UT-400 are plotted relative to published CaO values for the same samples after Westgate et al. (1987); error bars are 1σ uncertainties and are within the symbols for sample UT-400 (Table 2). Note that UT-462 is plotted only in (b) and (c) because there is no major element data for this sample.

22

562

563564565566567568


Fig. 7: Biplots of the average concentration of Zr relative to Si (a), Rb (b), Nb (c) and Y (d) for sample 93/WA3-1502, the Lake Tapps tephra horizons (Westgate et al., 1987), and samples MB188 and MB189 which are glass extracted from Kulshan caldera proximal ignimbrites. Error bars are at 1 σ. The differences in trace element composition between samples are explained by fractional crystallisation (f.c.) of feldspars and then zircons; see text for discussion.

23

569

570571572573574575


Tables

Table 1: Sample summary.

Sample Location Lat, Long. ReferenceLake TappsUT-52 Auburn 47°18', 122°15' Westgate et al. (1987)*

UT-55 Frigid Creek (same bed as UT-58) 47°23', 123°15' Westgate et al. (1987)

UT-56 Frigid Creek 47°23', 123°15' Westgate et al. (1987)*

UT-57 Frigid Creek 47°23', 123°15' Westgate et al. (1987)

UT-58 Frigid Creek (same bed as UT-55) 47°23', 123°15' Westgate et al. (1987)

UT-400 Peasley Canyon 47°18', 122°15' Westgate et al. (1987)

UT-462 Algona 47°17', 122°15' Westgate et al. (1987)*Proximal ignimbriteMB188 Mount Baker 48°50'24", 121°42'0" Hildreth et al. (2004)*

MB189 Mount Baker 48°50'24", 121°42'0" Hildreth et al. (2004)*

Washtucna

93/WA3-1502 Washtucna, WA-346°46'05", 118°20'46" This study

WA-3B Washtucna, WA-346°46'05", 118°20'46" Busacca et al. (1992)

*Tephra major-element chemistry not reported in original publication.

24

576

577

578


Table 2: Glass major and minor element chemical data, normalised to an anhydrous basis.

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Cl FO = F,

Cl TotalData collected during this study:

Waschtucna Tephra

93/WA3-1502 Pop. 1 Average 76.09 0.21 13.39 1.11 0.04 0.21 0.96 4.32 3.65 0.02 - - -94.4

6(n = 23) s.d. 0.21 0.03 0.11 0.07 0.03 0.02 0.04 0.17 0.10 0.02 - - - 0.62

93/WA3-1502 - Pop. 2 Average 71.11 0.43 15.40 2.21 0.05 0.62 2.11 5.21 2.76 0.09 - - -94.5

6(n = 3) s.d. 0.16 0.04 0.15 0.07 0.02 0.06 0.05 0.03 0.06 0.00 - - - 0.40

Kulshan Caldera Ignimbrite extracts

MB188 Average 77.88 0.15 12.64 0.77 0.04 0.14 0.72 3.36 4.17 0.020.11 - -

93.84

(n = 28) s.d. 0.20 0.02 0.13 0.08 0.03 0.03 0.03 0.14 0.07 0.020.01 - - 0.61

MB189 - Pop. 1 Average 76.28 0.18 13.69 0.93 0.05 0.22 1.05 3.49 3.97 0.030.11 - -

93.72

(n = 7) s.d. 0.52 0.04 0.33 0.10 0.02 0.09 0.12 0.23 0.17 0.020.02 - - 0.96

MB189 - Pop 2 Average 73.10 0.31 13.35 2.36 0.10 1.67 2.28 3.37 3.33 0.020.10 - -

93.05

(n = 2) s.d. 0.59 0.01 0.44 0.27 0.03 0.73 0.31 0.35 0.00 0.030.01 - - 1.70

Data from literature:Westgate et al. (1987) - Lake Tapps

UT55 Average 78.50 0.18 12.50 0.71 - - 0.80 3.50 3.70 -0.13 - -

93.47

(n=8) s.d. 0.40 0.03 0.20 0.06 - - 0.10 0.40 0.10 -0.03 - - 0.51

UT57 Average 78.20 0.22 12.60 0.87 - - 0.76 3.50 3.70 -0.13 - -

93.07

(n=5) s.d. 0.40 0.04 0.10 0.09 - - 0.15 0.40 0.10 -0.03 - - 1.17

UT58 Average 78.20 0.19 12.80 0.81 - - 0.79 3.30 3.80 -0.15 - -

92.74

(n=5) s.d. 0.50 0.02 0.20 0.05 - - 0.11 0.50 0.10 -0.05 - - 0.51

UT88 Average 78.60 0.20 12.30 0.69 - - 0.81 3.60 3.70 -0.14 - -

93.78

(n=10) s.d. 0.40 0.05 0.20 0.06 - - 0.11 0.40 0.20 -0.03 - - 0.43

UT400 Average 78.10 0.18 12.90 0.73 - - 0.83 3.50 3.70 -0.13 - -

92.76

(n=5) s.d. 0.70 0.02 0.40 0.11 - - 0.03 0.40 0.20 -0.02 - - 0.35

Busacca et al. (1992)

WA-3B Average 71.32 0.46 15.44 2.63 - 0.70 2.32 4.31 2.63 -0.09 - -

95.20

(n=32) s.d. 1.16 0.01 0.32 0.21 - 0.06 0.15 0.32 0.11 -0.02 - - 1.60

Pearce et al. (2004) - Mesa Falls

UT-860 Average 77.11 0.10 12.17 1.33 0.04 0.04 0.53 3.28 5.15 -0.15

0.22 -0.12

94.67

(n=17) s.d. 0.28 0.04 0.12 0.08 0.03 0.02 0.06 0.19 0.18 -0.03

0.11 0.05 0.60

Pearce et al. (2004) - Bishop Ash

UT35 Average 77.98 0.05 12.47 0.65 0.03 0.02 0.43 3.72 4.5 -0.08 0.1 -0.06

93.98

(n=16) s.d. 0.23 0.02 0.11 0.05 0.03 0.02 0.04 0.29 0.39 -0.02 0.1 0.04 0.68

25

579


UT997 Average 77.97 0.06 12.6 0.69 0.04 0.03 0.43 3.81 4.21 -0.08

0.15 -0.08

93.82

(n=17) s.d. 0.23 0.04 0.13 0.05 0.04 0.02 0.04 0.15 0.12 -0.04

0.12 0.05 0.67

Pearce et al. (2004) - Huckleberry Ridge

UA598 Average 76.73 0.14 12.18 1.66 0.04 0.02 0.61 3.44 5.02 -0.14

0.11 -0.07

94.97

(n=15) s.d. 0.36 0.03 0.17 0.19 0.03 0.01 0.08 0.14 0.26 -0.04

0.08 0.03 0.35

Pearce et al. (2004) – Lava Creek B

UT991 >1.6% Fe Average 77.02 0.15 12.13 1.71 0.04 0.02 0.56 3.39 4.86 - 0.10.06 -0.05

95.03

(n=7) s.d. 0.29 0.05 0.15 0.06 0.03 0.02 0.06 0.07 0.13 -0.02 0.1 0.04 0.77

UT991 <1.6% Fe Average 77.28 0.10 12.15 1.39 0.07 0.03 0.52 3.57 4.67 -0.16

0.18 -0.11

95.14

(n=10) s.d. 0.25 0.05 0.14 0.07 0.02 0.02 0.05 0.10 0.09 -0.03

0.13 0.06 0.38

UA256 Average 77.19 0.11 12.19 1.33 0.05 0.02 0.50 3.52 4.86 -0.17

0.14 -0.09

94.52

(n=16) s.d. 0.20 0.05 0.09 0.09 0.04 0.02 0.04 0.14 0.21 -0.04

0.14 0.06 0.54

Sarna-Wojcicki et al., (1985) - Rockland tephra

Average 77.70 0.16 12.60 0.85 0.03 0.17 0.91 3.71 3.68 0.040.11 - -

95.00

(n=4) s.d. 0.40 0.01 0.30 0.03 0.01 0.01 0.02 0.10 0.19 - - - - 0.60

26

580


Table 3: Glass trace element chemical data measured in this study.

Si Int. Std. Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th UWashtucna Tephra

93/WA3-1502 Pop. 1 Average 76.09 74.5 155 18.0 196 10.3 1.8

7 845 28.9 51.2 5.3

619.9 3.62 0.6

0 3.00 0.43 2.91 0.5

41.88 0.30 2.2

6 0.37 6.04 1.08 23.

2 14.1 5.03

(n = 21) s.d. 0.22 1.31

5.16 1.88 16.

8 0.06 0.13 47.3 2.6

9 2.07 0.41

1.08 0.22 0.1

2 0.96 0.08 0.13 0.0

70.06 0.04 0.1

8 0.08 0.94 0.09 0.4

7 1.57 0.48

Kulshan Caldera Ignimbrite extracts

MB188 Average 77.89 76.7 138 15.2 133 11.8 1.5

4 1420 29.9 45.2 4.6

718.0 2.75 0.5

8 2.01 0.34 3.02 0.8

41.80 0.40 2.5

1 0.32 4.94 1.13 22.

4 13.80 5.02

(n = 27) s.d. 0.20 2.73

8.86 1.02 5.9

3 1.00 0.31 52.5 0.5

4 0.83 0.52

1.44 0.70 0.3

9 0.71 0.07 0.55 0.1

20.30 0.08 0.1

5 0.10 0.99 0.16 3.4

4 0.33 0.49

MB189 - Pop. 1 Average 75.80 62.4 214 13.1 155 8.34 1.0

6 1480 28.2 45.8 4.5

221.0 3.08 0.5

8 1.85 0.64 2.25 0.3

51.38 0.45 2.8

2 0.35 5.57 1.13 20.

6 8.66 3.07

(n = 7) s.d. 1.14 10.8

27.1 1.90 15.

6 0.54 0.23 72.8 2.6

9 7.69 1.17

10.4 2.17 0.1

3 1.38 0.34 0.27 0.1

00.22 0.23 1.7

2 0.41 0.43 0.10 2.9

2 0.17 0.20

MB189 - Pop. 2 Average 73.10 54.5 142 15.5 129 8.43 1.4

1 1260 21.2 26.4 4.2

814.4 3.86 0.9

7 2.23 0.72 2.98 0.0

80.21 0.28 0.6

9 - - - - - -

(n = 2) s.d. 0.59 4.73

8.19 1.72 4.8

2 0.89 0.39 159 1.9

6 0.49 2.37

6.68 1.61 0.1

6 - - - - - - - - - - - - -

Lake Tapps tephra samples

UT52 Average 78.30 86.7 171 15.5 144 11.8 1.6

6 1360 30.2 48.1 4.8

717.0 3.32 0.4

5 3.30 0.41 2.55 0.5

81.84 0.32 2.5

2 0.36 4.62 1.63 82.

7 14.8 5.85

(n = 20) s.d. - 3.03

11.8 1.61 31.

3 0.28 0.17 66.1 2.3

5 1.54 0.42

1.76 0.67 0.1

4 1.52 0.09 0.90 0.1

20.53 0.06 0.3

0 0.06 0.99 0.30 45.

6 1.68 0.61

UT55 Average 78.50 85.2 152 15.4 129 11.8 1.8

7 1340 27.8 45.4 4.7

315.8 2.91 0.3

9 2.31 0.37 2.19 0.5

21.59 0.28 2.2

5 0.31 4.33 1.31 25.

0 13.2 5.36

(n = 20) s.d. - 2.87

9.17 1.04 4.7

3 0.35 0.06 28.4 0.5

3 1.57 0.43

1.08 0.70 0.1

2 0.64 0.13 0.11 0.0

30.17 0.01 0.2

8 0.06 0.83 0.14 4.6

9 0.64 0.16

UT56 Average 78.30 87.1 167 16.4 137 12.4 1.8

7 1420 31.1 48.6 5.3

518.0 3.30 0.5

4 2.34 0.41 2.45 0.5

31.95 0.41 2.5

8 0.43 4.69 1.38 33.

2 14.2 5.48

(n = 19) s.d. - 0.29

26.4 1.51 15.

5 0.51 0.11 96.8 2.8

9 1.64 0.18 1.8 1.04 0.2

2 0.82 0.09 0.26 0.1

10.17 0.10 0.5

6 0.02 1.04 0.21 5.0

3 1.40 0.54

UT57 Average 78.20 81.3 153 15.3 132 11.2 1.8

3 1330 28.4 47.3 4.7

715.6 3.38 0.5

3 2.06 0.40 2.49 0.6

42.10 0.27 2.0

2 0.36 4.03 1.26 68.

8 12.4 5.18

(n=17) s.d. - 5.80

12.7 0.43 3.7

6 0.25 0.14 20.4 0.6

8 1.63 0.21

1.31 0.38 0.2

9 0.38 0.09 0.06 0.0

40.39 0.07 0.2

3 0.09 0.27 0.16 41.

0 0.39 0.20

UT58 Average 78.30 76.3 141 15.5 139 12.0 1.6

2 1470 29.7 48.7 4.8

018.3 3.00 0.5

4 1.77 0.47 2.76 0.5

82.19 0.36 1.9

9 0.34 3.87 1.53 332 14.4 5.4

3

(n=21) s.d. - 1.31

5.52 1.02 5.0

9 0.97 0.22 172 2.6

8 3.85 0.27

1.45 1.45 0.5

2 1.62 0.07 1.43 0.0

70.76 0.11 0.5

0 0.19 0.52 0.05 394 0.58 0.7

3

UT400 Average 78.10 82.1 154 13.9 120 12.0 1.8

0 1270 26.8 46.0 4.9

716.3 2.71 0.5

2 2.51 0.37 1.90 0.4

81.61 0.30 2.1

1 0.31 4.27 1.21 42.

4 12.5 5.50

27


(n=19) s.d. - 1.73

5.12 1.06 3.9

9 0.42 0.03 39.5 0.6

2 2.16 0.52

0.05 0.82 0.2

4 0.50 0.07 0.48 0.0

90.23 0.08 0.4

1 0.03 0.46 0.26 8.9

4 0.71 0.36

UT462 Average 78.30 76.5 156 15.4 129 11.6 1.7

1 1280 28.3 44.0 4.7

916.3 3.19 0.5

1 2.62 0.50 2.43 0.6

11.95 0.35 2.1

4 0.39 4.37 1.38 24.

2 13.4 4.87

(n=27) s.d. - 3.34

15.1 1.58 12.

5 0.72 0.15 98.2 2.3

0 3.74 0.58

2.31 0.65 0.1

5 0.72 0.24 0.47 0.2

00.62 0.13 0.4

6 0.10 0.59 0.18 10.

8 0.94 0.29

Mesa Falls tephra

UT860 Average 77.11 236 11.2 114 257 71.5 3.2

9 131 114 190 21.6

78.2 17.8 0.5

1 17.6 2.82 18.9 3.8

711.5 1.70 10.

8 1.49 10.5 5.85 54.

5 47.9 10.1

(n=14) s.d. - 6.09

0.90 8.04 21.

6 1.31 0.19 5.75 7.6

0 6.43 1.33

6.29 2.20 0.0

6 2.02 0.20 3.26 0.1

50.98 0.35 1.5

7 0.16 1.09 0.20 2.7

2 3.53 0.41

28

581

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