Late Pleistocene Basaltic Ash and Volcanic Eruptions in the Bonneville Basin, Utah

Late Pleistocene basaltic ash and volcanic eruptions in the Bonneville basin, Utah

CHARLES G. OVIATT Department of Geology, Kansas State University, Manhattan, Kansas 66506 WILLIAM P. NASH Department of Geology and Geophysics, University of Utah Salt Lake City, Utah 84112-1183

ABSTRACT

Microprobe analyses of glass and radiocarbon dating of samples stratigraphic correlation tools (Wilcox, 1965; Westgate and Gold, 1974; associated with three basaltic volcanic ashes in the deposits of Lake Self and Sparks, 1981; Izett, 1982; Izett and Wilcox, 1982; Sarna- Bonneville make the ashes useful stratigraphic markers for correlation Wojcicki and others, 1987). In volcanic regions, tephrochronology has within the Bonneville basin. Two of the ashes were derived from tuff been helpful in lacustrine stratigraphic studies because of the large number cones (Pavant Butte and Tabernacle Hill) that erupted into Lake of ash layers available for dating and correlation, and because lakes are Bonneville in what is now the Black Rock Desert of south-central ideal settings for the deposition and preservation of volcanic ash (Davis, Utah. Near Kanosh, Utah, the Pavant Butte ash is interbedded with 1978,1985). In most previous studies employing tephrochronology in the barrier-beach lagoon marl that is slightly less than 16,000 yr old. Five western United States, including those in the Bonneville basin (Eardley radiocarbon dates on carbonate materials stratigraphically associated and Gvosdetsky, 1960; Morrison, 1965a; Mehringer and others, 1971; with the ash elsewhere in the Sevier Desert average 15,300 yr B.P. Eardley and others, 1973; Nash and Smith, 1977; Izett, 1982; Izett and Lake Bonneville was in its transgressive phase, about 15 m below the Wilcox, 1982; Spencer and others, 1984; Machette, 1985), silicic ashes Bonneville Shoreline, when the Pavant Butte eruption began. At the have been utilized because they are far more common and widespread Tabernacle Hill tuff cone, a basalt flow that has pillows, wave- than are basaltic ashes in the stratigraphic record. rounded cobbles, and tufa on its outer margins was erupted into Lake In this paper, we report on three basaltic ashes in the deposits of Lake Bonneville shortly after the tuff cone at or near the Provo Shoreline. Bonneville, a large lake that covered parts of what are now Utah, Nevada, The Tabernacle Hill tuff cone and basalt flow are older than a radio- and Idaho between about 30,000 and 10,000 yr B.P. (Fig. 1). Two of the carbon date of about 14,300 yr B.P. on tufa collected from the outer ashes were derived from hydrovolcanic eruptions into Lake Bonneville, margin of the basalt flow, but younger than the Bonneville Flood at whereas the source for the third has not yet been established. Although the about 14,500 yr B.P. The third ash, informally termed the "Thiokol" basaltic ashes in the Bonneville basin are not areally extensive, they are ash, has been found interbedded with Lake Bonneville deposits in two important stratigraphic markers and provide valuable clues to the history exposures in northern Utah and in sediment cores from the Great Salt of Lake Bonneville. Conversely, an understanding of the stratigraphy of Lake. It is about 25,000 yr old, and may have been erupted from a the lake sediments and the lake's history is essential in determining the volcanic field on the northwest shore of Great Salt Lake or from the eruptive history of the volcanoes. The Lake Bonneville stratigraphic se- Snake River Plain. quence is well known (Gilbert, 1890; Hunt and others, 1953; Eardley and Microprobe analyses of glass from samples of the Black Rock others, 1957; Morrison, 1965a, 1965b, 1966; Scott and others, 1983; Desert ashes show them to have similar chemical compositions, but Spencer and others, 1984; McCoy, 1987; Oviatt, 1987a; Currey and Ovi- glass from Tabernacle Hill can be distinguished from Pavant Butte att, 1985). glass by its higher concentrations of CaO and P2O5. Systematic differ- The results reported in this paper were derived from field work, ences in chemical composition between samples of Pavant Butte glass consisting of stratigraphic studies and mapping of surficial deposits (Oviatt, can be explained by comagmatic processes. "Thiokol" glass has less 1984,1986,1987a, 1987b, 1988), and microprobe analyses of the chemi- Si02, AI2O3, MgO, and Na20 but greater total iron and P2O5 than do cal composition of glass shards in ash samples. the Black Rock Desert glasses. PAVANT BUTTE INTRODUCTION Pavant Butte is a basaltic tuff cone in the Black Rock Desert (Figs. 1 Quaternary stratigraphic studies have been greatly aided by tephro- and 2). The tuff cone was first described by Gilbert (1890, p. 325-329), chronology, the use of volcanic ash1 or tephra layers as precise who suggested that it had erupted subaerially during a lowstand of Lake

'In this paper, we use the term "ash" as a textural term following Schmid unconsolidated pyroclastic deposits, and therefore we use it mostly when referring (1981). We also use it as a stratigraphic term, referring to a layer or bed of to pyroclastic deposits in the immediate vicinity of the source vent where the texture fine-grained pyroclastic sediment interbedded with other kinds of deposits; in this of the deposits ranges widely. This usage differs from that of Thorarinsson (1974) case, lacustrine deposits. Following Schmid (1981), the term "tephra" refers to all but is consistent with that of Izett (1982, p. 3).

Geological Society of America Bulletin, v. 101, p. 292-303,10 figs., 2 tables, February 1989.

292

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/2/292/3380667/i0016-7606-101-2-292.pdf by guest on 29 September 2021 Figure 1. Location map showing the out- line of Lake Bonneville at its highest stage (after Currey, 1982). "Thiokol" ash collection localities (T-l and T-2), location of core C (C; Spencer, 1982), Kelton (K), Pavant Butte (PB), and Tabernacle Hill (TH) are shown.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/2/292/3380667/i0016-7606-101-2-292.pdf by guest on 29 September 2021 Figure 2. Maps of Pavant Butte. A. Topographic map. B. Geologic map. All volcanic map units (e.g., vt) and lacustrine map units (e.g., Ig) are late Pleistocene in age. af = alluvial fan deposits (Holocene); va = unconsolidated basaltic ash and lapilli; vt = basaltic tuff; vpl, vp2, vp3 = palagonitized basaltic tuff; of the three overlapping units, vpl is the oldest; la = basaltic ash and lapilli reworked by waves in Lake Bonneville at and below the Provo Shoreline; lg = lacustrine gravel and sand. Strike-and-dip symbols indicate primary attitude of tuff and ash units. The Bonneville Shoreline is shown as a heavy dashed line. PBN-2 = location of ash sample (Table 1). Cross section A-A' is shown in Figure 3. The Bonneville and Provo Shorelines have lower altitudes at Pavant Butte than in surrounding areas (Currey, 1982) because of (1) post-eruptive subsidence (Currey, 1982), or (2) isostatic loading by volcanic rocks at the surface of the crust (Bills and May, 1987).

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Figure 3. Schematic cross section through Pavant Butte. See Figure 2 for location and explanation of symbols. B = Bonneville Shoreline; P = Provo Shoreline.

Bonneville prior to the lake's transgression to the highest shoreline. Condie the pedestal, where the ash has been reworked by waves in Lake Bonne- and Barsky (1972) and Hoover (1974) reinterpreted the eruptive history of ville, however, the layers of ash dip as much as 16° (Fig. 2). Cross- Pavant Butte in relation to Lake Bonneville history as interpreted by bedding, thin planar beds, disconformities, and massive lens-shaped beds Morrison (1966) and Bissell (1968). More recently, Currey (1982) refined (Wohletz and Sheridan, 1983, p. 402) are present throughout the ash Hoover's interpretations of eruptive chronology, and Wohletz and Sheri- unit and are well exposed in the gullies on the west and east sides of the dan (1983) have described the Pavant Butte eruption as a typical hydro- tuff cone. volcanic eruption. We present here new data on the geology of the tuff The upper half of the tuff cone consists of thick massive layers of cone and its eruptive history, and on the ash derived from the eruption. palagonitized tuff (Wohletz and Sheridan, 1983; vp in Figs. 2 and 3), which are inclined at primary dips of up to 36°. Three units of palago- Pavant Butte Tuff Cone nitized tuff are mapped in Figure 2, from oldest (vpl) to youngest (vp3). Successive palagonite units overlap with angular discordance, but there is Pavant Butte stands 275 m above a platform of basalt flows (Pavant no evidence of weathering, erosion, or other indicators of significant time Ridge) of pre-Bonneville age (Hoover, 1974). The lower half of the tuff lapse between units. The angular discordance of the palagonite units is cone consists of a pedestal of unpalagonitized tuff and unconsolidated ash most likely caused by minor faulting and slumping during the deposition and lapilli (Wohletz and Sheridan, 1983, p. 402-403; va and vt in Figs. 2 of the tuff and by rapid accumulation of new tuff layers over the exposed and 3), which are largely horizontally bedded. Around the outer edges of fault or slump scarps (Wohletz and Sheridan, 1983).

« -o

00 m

Figure 4. Schematic cross section showing stratigraphic relationships inferred from gravel-pit exposures near Eightmile Point, about 10 km southwest of Kanosh, Utah (K-5A in Fig. 5). A = pre-Bonneville buried soil; B = lagoon-facies marl (thickness exaggerated); C = radiocarbon dates of 15,900 ± 290 and 14,130 ± 100 yr B.P. on charcoal mixed with sediment (see text); D - highest exposure of Pavant Butte ash (K-5A) in lagoon marl; E = barrier-beach gravel; F = hand-leveled vertical distance (14 m) from the highest exposure of Pavant Butte ash to the Bonneville Shoreline. The crest of the gravel barrier, behind which marl and ash were deposited, was probably at, or slightly higher than, this altitude (1,550 m). The gravel barrier has been completely removed, but the buried soil, the lagoon marl and ash, and the lower portion of the gravel sequence are still present on the floor of the pit.

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Figure 5. Map of the Sevier Desert arm of Lake Bonneville showing localities where Pavant Butte or Tabernacle Hill ash are exposed (shorelines after Currey, 1982; and Oviatt, 1987b). The area south of Pavant Butte is referred to as the Black Rock Des- ert. Sample localities are labeled as in Figure 6 and Table 1. Numbers next to the localities represent ash-bed thickness in centimeters. At the three localities where both ashes are present, the upper number represents Tabernacle Hill ash thickness and the lower number represents Pavant Butte ash thickness. PB = Pavant Butte tuff cone; TH = Tabernacle Hill tuff cone; B = Bonneville Shoreline; P = Provo Shoreline; ORB = Old River Bed; SR = Sevier River; BR = Beaver River; SL = Sevier Lake.

Gilbert (1890, p. 328) interpreted south-dipping tuff layers on the of basalt, granitic rocks, and sandstone, as well as lapilli, all of which were north flank of the tuff cone as representing remnants of an older tuff cone, derived from wave erosion of the tuff cone. Lacustrine gravel is found which was located north of the present cone. Anomalous dips southeast of within a vertical interval 15 m below the crests of the spits on the east side the main cone could also be interpreted as indicating the remnants of an of the tuff cone (Fig. 2). There is no other direct evidence of wave action older tuff cone. Alternatively, some of the anomalous dips may represent on the flanks of the cone between the Bonneville Shoreline and the Provo layers within slump blocks that rotated off the steep flanks of the cone Shoreline (Figs. 2 and 3). during the eruption of Pavant Butte and may not represent older eruptions. A deep gully on the northwest side of the cone exposes the contact Lacustrine gravel (lg, Fig. 2), which was deposited in the shorezone of between the ash (va) and the overlying palagonite (vpl) at about the same Lake Bonneville, caps the pedestal of ash and lapilli on the flanks of altitude as the lacustrine gravel-capped bench around the south and east Pavant Butte (Fig. 2). The gravel consists of rounded cobbles and pebbles sides of the cone. There is a probable genetic significance to the similarity

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TH-3 M P L-2,6 K"5A EXPLANATION 1600 — B — urn Tabernacle Hill ash — B — — B — — B — — B — — B — B AAAA 1550 • AAAA Pavant Butte ash

A A A A local altitude of Bonneville 1500 Shoreline A A A A — P — p local altitude of Provo — P — — P — — P — _p _ P — P — Ü 1450 Shoreline A A A A AAAA AAA A H~H marl, deep-water facies 1400 H§§ marl, lagoon facies

1350 A [^pj si I ty marl

calcareous silty clay & silt o 5 m'.-:- o [MJJ SQ nd i^n EIE 15 m En: < 6 m. pebbly sand e^E AAAA :o :• .-.o1 13 T~T 70 ÚÚ&A gravel 0.1 0.3 til» T~T" AAAA «0 o rzc o EZE °0°o° pre-Bonneville buried soil OOOo O Oo rn Ci rzi o°°° [X3 basalt flow UZZE o„° Oof oOoo' |—| conformable contact

disconf or ma ble contact

Figure 6. Measured stratigraphie sections at selected Tabernacle Hill and Pavant Butte ash collection localities (see Figs. 2,5, and 7). In A, the altitudes of the ash exposures are shown relative to the local altitudes of the Bonneville and Provo Shorelines. In B, measured sections show the ashes in both shorezone (L-2, -6, and K-5A), and deep-water facies (ORB, TH-13, B-1A, TH-3, and MP). The numbers adjacent to the sections represent ash-bed thickness in centimeters.

of the altitudes of the lake level during and after the eruption, and the Bonneville deposits in many localities at lower altitudes in this area, but it ash/palagonite contact. Wohletz and Sheridan (1983, p. 406) suggested is not present in what would have been favorable depositional settings at that palagonitization of the upper, thickly bedded deposits of tuff cones higher altitudes, such as in the lagoon marl at C in Figure 4. was caused by hot water trapped within the deposits as they were laid The minimum age of the Pavant Butte eruption is constrained by down. The upper thickly bedded tuff of Pavant Butte was deposited sub- three radiocarbon dates on gastropods and two on ostracodes that are aerially, and waters that were erupted with the tuff could have remained stratigraphically associated with the ash and that average about 15,300 yr hot and capable of chemically altering the glass due to the insulating effect B.P. (Broecker and Kaufman, 1965; Currey and Oviatt, 1985). The max- of the surrounding tuff. In contrast, the ash (va) of the pedestal is fresh and imum possible age of the Pavant Butte eruption is constrained by strati- unpalagonitized, possibly because it was deposited in Lake Bonneville and graphic and geomorphic relationships at Pavant Butte and K-5A and by cooled quickly by circulating lake water. This hypothesis is consistent with radiocarbon dates of 15,900 ± 290 yr B.P. (Beta-22044) and 14,130 ± 100 observations of palagonitized pillow lavas and tuff in Iceland (Sigvaldason, (Beta-25233; Fig. 4).2 Therefore, the eruption occurred between about 1968) and may be a viable explanation for the vertical stratigraphic se- 16,000 and 15,300 yr B.P. Lake Bonneville was still in its transgressive quence at Pavant Butte, but it may not apply to all tuff cones (K. Wohletz, phase at the time of the eruption and had not yet begun to overflow into 1987, personal commun.). the Snake River drainage in southern Idaho. On the basis of geomorphic and stratigraphic interpretations of gravel-pit exposures of shorezone facies 40 km south of Pavant Butte and close to the Bonneville Shoreline (K-5A, Figs. 4 and 5), the eruption that 2The dates are on bulk-soil and sediment samples containing charcoal collected produced Pavant Butte began in water about 85 m deep when Lake from the upper part of the buried pre-Bonneville soil and the lower part of the Bonneville was within 15 m of its highest level, the Bonneville Shoreline. lagoon marl, 3 m higher than the highest exposure of ash at K-5A (Fig. 4). The At locality K-5A, the Pavant Butte ash is interbedded with a thin (20 cm) charcoal and other organic materials in the samples are older than the lagoon marl marl unit and with sand and fine gravel, which were deposited in a shallow at locality K-5A, but there is some possibility of contamination from young organic lagoon behind a gravel barrier-beach (Fig. 4). The upper altitudinal limit matter, such as decomposed root hairs of Holocene age. This contamination is most likely to be a problem in Beta-25233, because the date is too young when compared of ash in the lagoon marl is about 1,550 m, which is about 14 m below the with other radiocarbon dates in the Bonneville basin (compare Currey and Oviatt, local Bonneville Shoreline. Pavant Butte ash is interbedded with Lake 1985).

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Figure 7. Geologic map of the Tabernacle Hill basaltic lava flow and tuff cone. The dashed line around the margin of the flow is the 1,445-m topographic contour. Ash collection sites TH-3 and TH-6, and tufa-collection site (TH-5), are shown. Faults, with bar and ball on the downthrown side, and fractures having minor offset, are shown as heavy lines, le = lacustrine and eolian deposits; vb = basalt of the Tabernacle Hill flow; vt = basaltic tuff; vc = scoriaceous cinders of post-vt cinder cones. Hachured line outlines the volcanic crater. Strike and dip symbol indicates primary dip of tuff layers.

Pavant Butte Ash data on the geology and age of the eruptions in the Tabernacle Hill volcanic field and on the Tabernacle Hill ash. Pavant Butte ash is widely distributed in the Sevier Desert arm of The Tabernacle Hill volcanic field consists of an approximately circu- Lake Bonneville, where it is interbedded with lake deposits (Fig. 5). It is a lar basalt flow and a central crater that is encircled by an asymmetrical tuff prominent black marker bed in the white deep-water marl facies of Lake cone and smaller spatter or cinder cones (Fig. 7). A small tuff cone on the Bonneville. It occurs in the white marl both above and below the Provo northern margin of the basalt flow was erupted along a major fault that Shoreline and in shorezone facies close to the Bonneville Shoreline (Ovi- cuts the basalt flow. The Tabernacle Hill ash that is interbedded with the att, 1984; Fig. 5). About 100 km north-northwest of Pavant Butte, at marl of Lake Bonneville was most likely derived from the larger central locality ORB (Figs. 5 and 6), the ash is only about one grain-diameter tuff cone. thick; at B-1A, 30 km southwest of the tuff cone, it has a maximum Several features suggest that the Tabernacle Hill basalt flow was thickness of 0.3 cm. Southeast and northeast of the tuff cone, however, the erupted into Lake Bonneville. The outer rim of the basalt flow has a Pavant Butte ash is much thicker, suggesting that during the eruption the uniform altitude of 1,445 m (Fig. 7). Rounded and irregular pillows wind was predominantly westerly. having a glassy texture on their outer surfaces and coarser-grained interiors Previous authors who recognized the ash, which we regard as Pavant are common on the steep flanks of the edge of the flow. In addition, tufa Butte, include Maxey (1946), Varnes and Van Horn (1961, 1984), encrustations (Gilbert, 1890, p. 330) and wave-rounded cobbles and Broecker and Kaufman (1965), and Hoover (1974). boulders are found as high as 1,454 m in altitude. At points a few kilome- ters from the Tabernacle Hill flow, the Provo Shoreline has an altitude of TABERNACLE HILL 1,457 m, which is 3 m higher than the highest evidence of wave activity on the outer edges of the Tabernacle Hill flow. If the basalt flow was erupted Tabernacle Hill Lava Flow and Tuff Cone into the lake at the level of the Provo Shoreline, as Gilbert (1890, p. 330) suggested, the shoreline is slightly lower on the flow than in surrounding Gilbert (1890, p. 329-332) first described the Tabernacle Hill lava areas, either because of incomplete isostatic rebound due to the new load flow and tuff cone, and subsequent work by Condie and Barsky (1972) of basalt at the surface of the crust or because of magma-chamber subsi- and Hoover (1974) has added details to his analysis. We present here new dence directly below the eruptive center. Alternatively, the basalt flow

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TABLE I. MICROPROBE ANALYSES OF GLASS FROM BASALTIC ASH SAMPLES AND WHOLE-ROCK ANALYSES OF BLACK ROCK DESERT LAVAS

Pavant Butte Tabernacle Hill "Thiokol" Black Rock Desert lavas*

PBN-2(A) o PBN-2(B) K-5A B-1A TH-I3A 80-15 PBN-SW M-6 ORB MP L-2 L-6 TH-6 TH-3A TH-13B T-l T-2 BRD-6 BRD-15

Si02 50.8 (0.21) 50.9 50.8 50.5 51.4 51.1 50.9 50.3 50.8 51.2 51.0 51.4 50.6 50.6 50.5 46.8 47.1 48.97 49.04

Ti02 1.78 (0.02) 1.81 1.81 1.60 1.57 1.66 1.77 1.76 1.61 1.65 1.73 1.65 1.88 1.82 1.83 3.63 3.83 1.51 1.55

A1203 16.1 (0.13) 16.0 16.2 16.1 16.0 15.8 15.6 15.5 15.7 15.5 15.7 15.5 15.6 15.7 15.5 13.1 13.2 16.80 16.88 Feot 11.4 (0.20) 11.2 11.6 11.6 12.0 11.8 11.4 11.4 12.1 12.3 12.1 12.9 11.8 11.7 11.5 14.7 14.5 9.41 10.56 MnO 0.18 (0.01) 0.17 0.18 0.19 0.18 0.18 0.16 0.17 0.20 0.19 0.19 0.22 0.18 0.18 0.18 0.20 0.21 0.17 0.19 MgO 5.49 (0.15) 5.62 5.28 5.09 5.14 5.31 5.33 5.31 4.89 4.95 4.96 4.40 5.45 5.34 5.42 5.17 5.09 7.88 7.46 CaO 9.17 (0.15) 9.11 8.88 8.64 8.52 8.91 9.15 8.84 8.57 8.51 8.48 8.20 9.83 9.63 9.60 9.98 9.83 10.61 9.30

Na20 3.20 (0.03) 3.24 3.31 3.33 3.38 3.36 3.29 3.30 3.34 3.37 3.30 3.35 3.01 3.32 3.29 2.81 2.83 2.85 3.13 KjO 1.27 (0.03) 1.22 1.27 1.27 1.30 1.29 1.26 1.28 1.34 1.33 1.33 1.49 1.23 1.28 1.28 1.26 1.20 0.71 0.89

P2Os 0.44 (0.03) 0.48 0.43 0.37 0.38 0.34 0.35 0.38 0.35 0.32 0.34 0.38 0.51 0.49 0.46 1.05 1.04 0.38 0.41 CL§ 0.02 (0.01) 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.03 0.02 0.02 0.02 0.01 0.01 F§ 0.06 (0.01) 0.06 0.05 0.07 0.07 0.05 0.06 0.07 0.05 0.07 0.08 0.07 0.06 0.09 0.10 0.10 0.10 TOTAL 99.9 99.8 99.8 98.7 100.0 99.8 99.3 98.3 99.0 99.4 99.2 99.6 100.2 100.2 99.7 98.8 98.9 99.43 99.48

Note, see Figures 1,2, 5. and 7 for locations of ash samples, a = standard deviation of three or more repetitive analyses on PBN-2(A). •BRD-6 = Pavant Ridge lava; BRD-15 = Tabernacle Hill lava. tTotal iron. §Detection limits for CI and F are 0.01 and 0.04%, respectively.

Tabernacle Hill Ash Figure 8. Plot of CaO versus FeO* (total Fe) for TABERNACLE HILL Tabernacle Hill ash is not as widespread as Pavant Butte ash, partly glasses from Tabernacle Hill because the hydrovolcanic eruption was smaller, and partly because the O (crosses) and Pavant Butte (solid to _ ash fell into a lake that was much less extensive, and therefore preservation o •! squares). Error limits for micro- , PAVANT BUTTE in the lacustrine stratigraphic sequence was less likely. Only a few localities probe analyses are shown on the have been found where both the Pavant Butte and Tabernacle Hill ashes Tabernacle data points. are present in the same exposure, and all localities are below the Provo Shoreline (Figs. 5 and 6). At these localities, the two ash layers are sepa- rated by 5 to 10 cm of marl. At locality TH-3, 2 km south of the Tabernacle Hill tuff cone, and 50 m south of the southern margin of the basalt flow (Figs. 5,6, and 7), the Tabernacle Hill ash is 70 cm thick and may have been erupted into the lake at a stage slightly lower and younger overlies an erosional unconformity that may have been created by a dis- than the Provo Shoreline. In any case, it is clear that the Tabernacle Hill turbance of the lake bottom during the nearby volcanic eruption. The eruptions are younger than the Bonneville Flood, which caused the cata- deep-water marl underlying the ash is thinner than would be expected at strophic drop in lake level from the Bonneville Shoreline to the Provo this locality, and the Pavant Butte ash is not present. Shoreline (Malde, 1968; Currey, 1982; Jarrett and Malde, 1987). A radiocarbon date of 14,320 ± 90 yr B.P. (Beta-23803)3 on tufa "THIOKOL" ASH collected from an altitude of 1,445 m on the eastern edge of the lava flow (TH-5, Fig. 7) suggests that the tufa was deposited during the time of Spencer (1982, p. 110-111, Appendix 2) encountered five late Pleis- development of the Provo Shoreline. The sample was collected from a tocene volcanic ashes at depths of 4 to 5 m in sediment cores taken from shallow overhang where the tufa was protected from direct exposure to the bottom of Great Salt Lake, one of which was a basaltic ash. He noted meteoric water. In addition, the sample was processed in the laboratory to that the ash appeared to be reworked and that it was chemically similar to reduce the possibility of contamination by diagenetic calcium carbonate in basalts from the Snake River Plain, but he did not identify its source pores. The sample was crushed, sieved to retain sand-sized fragments, and volcano (Spencer, 1982, p. 110). At two exposures in northern Utah then treated with dilute hydrochloric acid to remove approximately half of (Fig. 1), a basaltic ash layer has been found that, in each case, has a the remaining sample. Therefore, contamination with young carbon in the stratigraphic position similar to that of the ash reported by Spencer (1982). sample should be relatively minor compared with the potential contamina- At one of these exposures (T-l), a radiocarbon date on wood of tion in tufa samples exposed to weathering at the surface. Contamination 19,580 ± 290 yr B.P. (Beta-8093; Oviatt, 1986) and ostracode identifica- with old carbon is unlikely because the basalt substrate contains no car- tions and correlations by R. M. Forester (1984, personal commun.) pro- bon. The carbon-isotopic composition of the water from which the tufa vide strong evidence that the ash at T-l is in the same stratigraphic position precipitated is unknown, but any correction for this is likely to be less than as the ash encountered by Spencer (1982) in the cores from the Great Salt 500 yr (Broecker and Kaufman, 1965), and probably less than 200 yr Lake. The ash is only a few millimetres thick and is difficult to find at T-l; (Benson, 1978). The age of the Tabernacle Hill eruptions is, therefore, it is about 2.6 m stratigraphically lower than Beta-8093 at an altitude of between about 14,500 yr B.P., which is the approximate age of the 1,290 m. Bonneville Flood (Currey and Oviatt, 1985 and unpub. data), and At locality T-2 (Fig. 1), in an exposure along Blue Springs Creek near 14,300 yr B.P. the Morton Thiokol Plant, a 0.5-cm-thick layer of basaltic ash occurs near the base of a thick sequence of fine-grained Lake Bonneville deposits at an altitude of 1,330 m. 3C-13 adjusted age; the 13C/12C content of the tufa is +4.00/oo (PDB), and the Although our geochemical correlations are not conclusive (see unadjusted age is 13,840 ± 90 yr B.P. below), we prefer to regard the basaltic ash from T-l, T-2, and the Great

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Si/K

(a) (b)

8.5 y = 2.57 + 0.46x R = 0.91 /

(0 8.0- Z C+O An 82 / a // • O * CM a 7.5 QT LL. CzM + An 63 / / O)

U) 7.0 H // An 58 o

6.5 10 11 12 13

Si/K Al/K

Figure 9. Pearce element ratios for Pavant Butte glasses. The probable error for each ratio, calculated from the standard deviations in Table 1, is shown for each ratio as an error bar. (a) 0.5 (Mg + Fe)/K versus Si/K; the calculated slope is less than unity, which indicates that the glasses cannot be related through the removal of olivine alone. Arrows in the inset illustrate the compositional effect (slope) due to the fractionation of individual mineral species, (b) [0.5 (Mg + Fe) + 2Na + Al]/K versus Si/K, which illustrates that the chemical diversity can be primarily the result of removal of olivine and plagioclase, with clinopyroxene playing a very minor role, (c) [0.5 (Mg + Fe) + 2Ca + 3Na]/K versus Si/K; a theoretical slope of 1.0 is consistent with fractionation of plagioclase and olivine. The inset shows that the residual glasses are related to four Pavant lavas (open squares) by the same slope of 1.1 (R = 1.0). (d) 2Na/K versus Al/K; the central line is the statistical fit to all

data (R = 0.91) and yields an average plagioclase composition of An63. The other two lines bracket the data at Ang2 and An5g, consistent with the observed range of composition of plagioclase in the Pavant Butte tephra.

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Salt Lake cores as a single stratigraphic unit. We refer to it as the "Thio- TABLE 2. NORMALIZED ANALYSIS OF GLASS FROM NORTHERN kol" ash because it is well exposed at locality T-2. The age of the "Thio- BONNEVILLE BASIN BASALTIC ASHES kol" ash can be inferred from its stratigraphic context. In core C of Spencer "Thiokol" Spencer (1982) Kelton* (1982, Fig. 50) the ash that we correlate with the "Thiokol" ash occurs a few centimeters below the Wono ash, which has been dated at about T-l T-2 C-5 (A) C-5 (B) C-5 (C)

24,800 yr B.P. (Davis, 1978,1985). The "Thiokol" ash is therefore inter- Si02 47.42 47.66 47.19 47.82 48.14 47.41 4.36 preted as about 25,000 yr old. Ti02 3.68 3.88 4.47 4.51 3.53 AI203 13.27 13.36 14.16 14.01 14.25 14.48 FeO 14.89 14.67 15.16 14.86 14.27 14,66 MnO 0.20 0.21 nd nd nd 0.23 COMPOSITION OF VOLCANIC ASH MgO 5.24 5.14 5.88 6.00 5.92 4.03 CaO 10.11 9.95 9.30 9.14 9.15 8.77 Na20 2.85 2.86 (2.67) (2.55) (2.73) 3.10 1.21 1.11 1.19 1.32 The correlation of tephra-bearing stratigraphic units is based upon the K20 1.28 1.16 H2O* nd nd nd nd nd 0.71 chemical composition of volcanic glass as determined by analysis with the H2O nd nd nd nd nd 0.21 p2°5 1.06 1.05 nd nd nd 1.15 electron microprobe. Glass compositions are preferable to bulk-tephra CI 0.01 0.01 nd nd nd nd analyses, which may possess heterogeneities due to selective winnowing of F 0.10 0.10 nd nd nd nd TOTAL 100.00 100.00 100.00 100.00 100.00 99.91 phenocryst phases during transport or the admixture of foreign material

during deposition. Nonetheless, glass from an individual volcano may not •Lava from northwest margin of Great Salt Lake; analysis by D. H. Fiesinger (1988, personal commun.); Fe203 = be homogeneous, leading to possible ambiguities in correlation as evalu- 3.14, FeO= 11.83. ated in detail below.

Black Rock Desert Ashes these elements are reduced in amount in residual glasses due to crystallization of Fe-Ti oxides and apatite. Basaltic glasses from Pavant Butte and Tabernacle Hill are grossly In order to illustrate the comagmatic processes responsible for the similar in composition (Table 1) and bear strong affinities with lavas chemical diversity, glass analyses have been plotted in Figure 9a in terms erupted either prior to (Pavant Butte), or contemporaneously with (Taber- of the Pearce element ratios 0.5 (Mg + Fe)/K versus Si/K. The possible nacle Hill), the tephra. Representative analyses of lavas are given in Table comagmatic character of the glasses is consistent with the high correlation 1. The lavas and glasses are hypersthene-normative, tholeiitic basalts. In coefficient (R = 0.99). On this diagram, processes related solely by the detail, glass from Tabernacle Hill can be distinguished from Pavant Butte addition or subtraction of olivine will have a slope of 1.0. The involvement glass by higher contents of P2O5 and CaO (Fig. 8). The three analyzed of other phases will cause the slope to deviate from 1.0 as shown in Figure glasses from Tabernacle Hill are identical in composition; the standard 9a. It is evident from the slope being substantially less than one that olivine deviation of the analyses is less than the analytical precision for all ele- crystallization alone cannot account for the diversity of the Pavant Butte ments except Na20. In contrast, glasses from Pavant Butte display substan- residual liquids. Pavant lavas contain phenocrysts of plagioclase (Ang0_55) tial, but systematic, heterogeneities in composition. In order to validate the and olivine (Fo8g_72). If both plagioclase and olivine are involved in the stratigraphic correlations in this study, it is necessary to demonstrate that differentiation process, then the combined index of [0.5 (Mg + Fe) + 2Ca + the Pavant Butte glasses represent comagmatic residual liquids whose 3Na]/K versus Si/K should have a slope of 1.0. The data in Figure 9c compositional diversity can be accounted for by reasonable magmatic have a slope of 1.1, consistent with these glasses being comagmatic and processes. The alternative hypothesis is that these glasses are the products related to each other by the removal of phenocrysts of plagioclase and of more than one volcanic episode and that the stratigraphic correlations olivine. Furthermore, as shown in the inset (Fig. 9c), the compositions of are spurious. the glasses are consistent with residual liquids derived by the same process A convenient method to assess whether or not diverse volcanic prod- from magmas represented by Pavant lavas erupted prior to formation of ucts are comagmatic and, if so, to identify the differentiation process, is the Pavant Butte tuff cone. The dominant role of olivine and plagioclase is through the use of Pearce element ratios (Pearce, 1968,1970; Russell and further demonstrated by a plot of [0.5 (Mg + Fe) + 2Na + Al]/K versus Nicholls, 1988; Nicholls, 1988). Pearce element ratios can be used to Si/K (Fig. 9b). In this plot, fractionation of olivine and/or plagioclase represent chemical data in variation diagrams which reveal information on produces a slope of 1.0, whereas clinopyroxene alone produces a slope of the possible roles of distinct and competing magmatic processes. 0.25. The Pavant Butte ashes yield a slope of 0.9, indicating that clinopyroxene (which is not an abundant phenocryst phase) does not play a Following the notation of Russell and Nicholls (1988), Pearce ele- significant role in the chemical evolution of the glasses. ment ratios are calculated by first converting weight percent oxides to their element fractions. A further test of the comagmatic hypothesis is to evaluate whether or not the composition of plagioclase predicted by the variation in glass

e; = WjA;/MWj chemistry is consistent with the petrography. The composition of fractionated plagioclase can be estimated from the element ratio plot of 2Na/K

where Wj, Au and MW; are the weight percent, the number of cations in versus Al/K (Fig. 9d). Slopes of the trends in the diagram (M) are related the oxide formula, and the molecular weight of oxide i. The Pearce ele- to the composition of the fractionated plagioclase by (Russell and Nicholls, ment ratio of element i is 1988)

H = e/ez XAb = 2 M/(2 + M).

where z is a conserved element whose amount does not change during the The slope for the regression of the whole data set yields a plagioclase of

magmatic process. For the basaltic system here, the conserved element is An63. Microprobe analyses of plagioclase phenocrysts in Pavant lavas potassium. In some basaltic systems, Ti and P are conserved elements as show them to be zoned, ranging in composition from Ango to An55, well (compare Russell and Nicholls, 1988); however, at Pavant Butte, consistent with the element ratio analysis (see Fig. 9d).

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For these reasons, we conclude that the ashes classed as Pavant Butte of the "Thiokol" glasses, and the analyses are normalized, they compare are comagmatic and hence can be correlated despite measurable, but favorably with the "Thiokol" glass (Table 2), except for discernible differ-

systematic, differences in chemical composition. ences in Ti02, A12C>3, and GaO contents. We are uncertain (1) if these differences are real, or (2) if they are the result of different analytical "Thiokol" Ashes laboratories, (3) or if they are the result of the normalization process.

The high P2Os content of the glass is distinctive, and it suggests that These ashes from the northern Bonneville basin are quite distinct in the source region for the ash may lie on the northwest edge of the Great

their composition from the Black Rock Desert ashes. "Thiokol" glasses Salt Lake in a volcanic field which contains lavas with P205 contents in

have lower concentrations of Si02, AI2O3, MgO, and Na20, and greater excess of 1.0%. An analysis of one such lava (D. Fiesinger, 1988, personal amounts of total iron and P2O5. Basaltic glass in cores from the Great Salt commun.) from the vicinity of the railroad siding of Kelton is presented in Lake (Spencer, 1982) has similar compositional traits in several respects. Table 2 for comparison. Another possible source area for the "Thiokol"

Spencer (1982) did not analyze for Na20 and P2O5; however, if Na20 is ash is the Snake River Plain in Idaho where late Pleistocene hydrovolcanic added to the Great Salt Lake glass in proportion to the Na20/K20 ratio eruptions were common (compare Womer and others, 1980).

DISCUSSION

Figure 10. Summary The named volcanic ashes in lacustrine deposits of the Bonneville diagram showing volcanic basin younger than 30,000 yr are summarized in Figure 10. The Pavant ashes and the range of alti- Butte and Tabernacle Hill ashes are useful stratigraphic markers in the tudes within which they have Sevier and Black Rock Deserts. Of these two, the Pavant Butte ash is more been identified in Lake widespread and useful in stratigraphic studies. It may eventually be en- Bonneville and post-Lake countered in Lake Bonneville deposits in Utah Valley 100 km downward Bonneville lacustrine depos- from the volcano and may also be preserved in alluvial or other facies in its in the Bonneville basin. the Sevier River Valley or other places marginal to the lake. It could thus Silicic ashes are shown as aid in interpretations of nonlacustrine environmental conditions at a time dashed lines; basaltic ashes, when Lake Bonneville was close to its highest level. Likewise, with further as solid lines. Lake Bonne- study, the "Thiokol" ash is likely to become a useful correlation tool in the ville spanned the time from northern Bonneville basin. about 30,000 to 10,000 yr Within the white marl, the Pavant Butte ash occurs a few centimeters Ma z a m a B.P. Bonneville (B) and below an abrupt lithologic contact and a change in lithology that marks Provo (P) Shoreline altitudes the Bonneville Flood (Oviatt, 1987a). Through the aid of the Pavant Butte (Currey, 1982) are shown as ash, the Bonneville Flood marker bed, which differs slightly in character 10 ranges because of differential throughout the basin, can be identified at many localities in the Sevier isostatic rebound. Desert arm of Lake Bonneville. The Pavant Butte ash is exposed in the white marl at a number of localities in the vicinity of the Sevier River delta (Varnes and Van Horn, — Tabernacle Hill 1961,1984; Oviatt, 1984). As a precise correlation tool, it helps to demon- CL strate how clastic sedimentation alternated with deep-water marl precipita- CO tion as Lake Bonneville rose and fell, and the Sevier River shifted its center Pavant Butte of deposition (Oviatt, 1987b). In addition, the Pavant Butte ash is helpful in reinterpreting previous stratigraphic correlations in the Sevier River delta area. For instance, fine-clastic deposits previously mapped as belong- < ing to the Alpine Formation in this area (Varnes and Van Horn, 1984), and thus representative of a pre-Bonneville lake cycle, occur stratigraphi- 20 cally above the Pavant Butte ash and the white marl (Oviatt, 1987b). Therefore, these deposits may be reinterpreted as part of a regressive-phase Sevier River delta that formed during and after the development of the Provo Shoreline. In support of this interpretation, radiocarbon dates and 230Th ages on molluscs and ostracodes reported by Varnes and Van Horn Wono (1984) collected stratigraphically near the ash average about 14,500 yr — "Thiokol' B.P., and amino acid ratios in molluscs (W. D. McCoy, 1987, personal commun.) indicate that the marl containing the ash is Bonneville in age (Oviatt, 1987b). Finally, all three basaltic ashes are proving useful in continuing stud-

--Carson Sink ies of lacustrine facies patterns. The precise lake level is known for the 30 Pavant Butte and Tabernacle Hill eruptions, and the approximate lake 1300 1400 1500 1600 level is known for the time of eruption of the "Thiokol" ash (Fig. 10). I—F*—I HB H Studies are underway to map the spatial variations in sedimentology, and

~~ALTITUDE (m) their changes through time, based on correlations of ash beds.~~

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ACKNOWLEDGMENTS Maxey, G. B., 1946, Geology of part of the Pavant Range, Millard County, Utah: American Journal of Science, v. 244, p. 324-356. McCoy, W. D., 1987, Quaternary aminostratigraphy of the Bonneville basin, western United States: Geological Society of This study was partially funded by the Utah Geological and Mineral America Bulletin, v. 98, p. 99-112. Mehringer, P. J., Nash, W. P., and Fuller, R. H., 1971, A Holocene volcanic ash from northwestern Utah: Utah Academy Survey and the U.S. Geological Survey as part of a Cooperative Geologi- of Sciences, Arts, and Letters, Proceedings, v. 48, p. 46-51. Morrison, R. B., 1965a, New evidence on Lake Bonneville stratigraphy and history from southern Promontory Point, cal Mapping (COGEOMAP) project. We thank Ted Burr, Susan Green, Utah: U.S. Geological Survey Professional Paper 525-C, p. CI 10-C119. Dorothy Sack, and Ward Taylor for help in the field. Rick Forester kindly 1965b, Lake Bonneville: Quaternary stratigraphy of eastern Jordan Valley, south of Salt Lake City, Utah: U.S. Geological Survey Professional Paper 477,80 p. identified and interpreted ostracodes, Bill McCoy analyzed amino acids in 1966, Predecessors of Great Salt Lake, in Stokes, W. L., ed., The Great Salt Lake: Utah Geological Society Guidebook to the Geology of Utah, no. 20, p. 77-104. mollusc shells, and Don Fiesinger provided us with unpublished geochem- Nash, W. P., and Smith, R. P., 1977, Pleistocene volcanic ash deposits in Utah: Utah Geology, v. 4, p. 35-42. ical data. We are grateful to Don Currey, Dave Miller, and Kenneth Nicholls, J., 1988, The statistics of Pearce element diagrams and the Chayes closure problem: Contributions to Mineralogy and Petrology, v. 99, p. 11-24. Wohietz for valuable discussions, and to David McConnell, Gerald Os- Oviatt, C. G., 1984, Lake Bonneville stratigraphy at the Old River Bed and Leamington, Utah [Ph.D. thesis]: Salt Lake City, Utah, University of Utah, 122 p. born, and James Russell for constructive review comments. 1986, Geologic map of the Honeyville quadrangle, Box Elder and Cache Counties, Utah: Utah Geological and Mineral Survey Map 88, scale 1:24,000. 1987a, Lake Bonneville stratigraphy at the Old River Bed, Utah: American Journal of Science, v. 287, p. 383-398, 1987b, Quaternary geology of part of the Sevier Desert, Millard County, Utah: Utah Geological and Mineral Survey Open-File Report 106. REFERENCES CITED 1988, Quaternary geology of the Black Rock Desert, Millard County, Utah: Utah Geological and Mineral Survey Open-File Report 128,53 p. Benson, L. V., 1978, Fluctuation in the level of pluvial Lake Lahontan during the last 40,000 years: Quaternary Research, Pearce, T. 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A., 1982, The Bishop ash bed and some older compositionally similar ash beds in California, Nevada, and Utah: American Journal of Science, v. 283, p. 385-413. U.S. Geological Survey Open-File Report 82-582,44 p. Womer, M. B., Greely, R., and King, J. S., 1980, The geology of Split Butte—A maar of the south-central Snake River Izett, G. A., and Wilcox, R. E., 1982, Map showing localities and inferred distributions of the Huckleberry Ridge, Mesa Plain, Idaho: Bulletin Volcanologique, v. 43, p. 453-471. Falls, and Lava Creek ash beds (Pearlette family ash beds) of Pliocene and Pleistocene age in the western United States and southern Canada: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1325. Jarrett, R. D., and Malde, H. E., 1987, Paleodischarge of the late Pleistocene Bonneville Flood, Snake River, Idaho, computed from new evidence: Geological Society of America Bulletin, v. 99, p. 127-134. Machette, M. N., 1985, Late Cenozoic geology of the Beaver Basin, southwestern Utah: Brigham Young University Geology Studies, v. 32, pt. 1, p. 19-37. MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 29,1988 Malde, H. E., 1968, The catastrophic late Pleistocene Bonneville Flood in the Snake River Plain, Idaho: U.S. Geological REVISED MANUSCRIPT RECEIVED MAY 31,1988 Survey Professional Paper 596,52 p. MANUSCRIPT ACCEPTED JUNE 6,1988

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