Late Quaternary stratigraphy and geochronology of the western Killpecker Dunes, Wyoming, USA

Quaternary Research 61 (2004) 72 – 84 www.elsevier.com/locate/yqres

Late Quaternary stratigraphy and geochronology of the western Killpecker Dunes, Wyoming, USA James H. Mayer a,* and Shannon A. Mahan b a

b

Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA U.S. Geological Survey, MS 963, P.O. Box 27046, Federal Center, Denver, CO 80225, USA Received 24 October 2002

Abstract New stratigraphic and geochronologic data from the Killpecker Dunes in southwestern Wyoming facilitate a more precise understanding of the dune field’s history. Prior investigations suggested that evidence for late Pleistocene eolian activity in the dune field was lacking. However, luminescence ages from eolian sand of f15,000 yr, as well as Folsom (12,950 – 11,950 cal yr B.P.) and Agate Basin (12,600 – 10,700 cal yr) artifacts overlying eolian sand, indicate the dune field existed at least during the latest Pleistocene, with initial eolian sedimentation probably occurring under a dry periglacial climate. The period between f13,000 and 8900 cal yr B.P. was characterized by relatively slow eolian sedimentation concomitant with soil formation. Erosion occurred between f8182 and 6600 cal yr B.P. on the upwind region of the dune field, followed by relative stability and soil formation between f5900 and 2700 cal yr B.P. The first of at least two latest Holocene episodes of eolian sedimentation occurred between f2000 and 1500 yr, followed by a brief (f500 yr) episode of soil formation; a second episode of sedimentation, occurring by at least f700 yr, may coincide with a hypothesized Medieval warm period. Recent stabilization of the western Killpecker Dunes likely occurred during the Little Ice Age (f350 – 100 yr B.P.). The eolian chronology of the western Killpecker Dunes correlates reasonably well with those of other major dune fields in the Wyoming Basin, suggesting that dune field reactivation resulted primarily due to departures toward aridity during the late Quaternary. Similar to dune fields on the central Great Plains, dune fields in the Wyoming Basin have been active under a periglacial climate during the late Pleistocene, as well as under near-modern conditions during the latest Holocene. D 2003 University of Washington. All rights reserved. Keywords: Killpecker Dunes; Eolian chronology; Wyoming Basin; Paleoenvironments; Late Pleistocene; Holocene

Introduction The Wyoming Basin contains a number of areas characterized by the accumulation of unconsolidated Quaternary eolian sand (Fig. 1). Of these areas, the Killpecker Dune Field is the largest active dune field, where dormant dunes have perhaps the longest history of investigations carried out specifically to understand the eolian history of the dune field (Hack, 1943; Moss, 1951; Ahlbrandt, 1973, 1974a; Ahlbrandt et al., 1983). Ahlbrandt et al. (1983) concluded that the Killpecker Dunes, like most dune fields of the northern Great Plains and Rocky Mountain basins, developed primarily during the Holocene. In addition, they pointed out that evidence for a Pleistocene age of the dune field is ‘‘either circumstantial or indirect’’ (Ahlbrandt et al., * Corresponding author. E-mail address: [email protected] (J.H. Mayer).

1983, p.400). This paper presents new chronometric and stratigraphic data from the western portion of the Killpecker Dunes in order to test the hypothesis that eolian erosion and sedimentation occurred primarily during the Holocene. Since the pioneering study of Ahlbrandt et al. (1983), a relatively large body of literature has emerged supporting the use of eolian chronologies as indicators of periods of aridity in the central United States (e.g., Muhs, 1985; Holliday, 1989, 1997, 2001; Forman and Maat, 1990; Forman et al., 1992, 1995, 2001; Muhs and Maat, 1993; Loope et al., 1995; Madole, 1994, 1995; Arbogast, 1996; Dean et al., 1996; Muhs et al., 1996, 1997a,b, 1999, 2000; Mason et al., 1997; Stokes and Swinehart, 1997; Arbogast and Johnson, 1998; Forman et al., 2001; Muhs and Holliday, 2001; Muhs and Za´rate, 2001; Rawling et al., 2003). Nevertheless, with the exception of the Ferris Dunes (Fig. 1; Gaylord, 1982, 1990; Stokes and Gaylord, 1993), work during the past 20 years oriented specifically toward better defining the

0033-5894/$ - see front matter D 2003 University of Washington. All rights reserved. doi:10.1016/j.yqres.2003.10.003

J.H. Mayer, S.A. Mahan / Quaternary Research 61 (2004) 72–84

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Fig. 1. Maps showing (a) location of Wyoming in North America, (b) major dune fields within the Wyoming Basin (after Fenneman, 1931) as mapped by Kolm (1974; reproduced by permission of the publisher), and (c) location of the Killpecker Dunes in southwestern Wyoming (after Ahlbrandt, 1974a; reproduced by permission of the publisher).

eolian history of dune fields in the Wyoming Basin is lacking. This paper is an attempt to fill this gap, as well as to aid in our understanding of the response(s) of eolian systems to changes in climate.

Setting Eolian sediments of the Killpecker Dunes, both active and dormant, cover an area of over 550 km2, extending from the eastern margin of the Green River Basin, over the continental divide, and continuing into the Great Divide Basin (Fig. 1). Elevations in the western portion of the dune

field range between 2000 and 2050 m. Climate in the Wyoming Basin is classified as a semiarid midlatitude steppe (Strahler and Strahler, 1997). The Eden Valley, located west of the Killpecker Dunes (Fig. 1), has a mean annual temperature of 3.3jC and receives a mean annual precipitation of 190 mm (Ravenholt and Lewis, 1990). The western portion of the dune field is currently dormant and contains primarily dome, parabolic, and blowout dunes, as well as extensive sand sheets. In active areas of the dune field, Ahlbrandt (1973) mapped an ‘‘evolutionary sequence’’ of dome, transverse, barchan, and parabolic dunes occurring in a downwind (easterly) direction. Based on the heavy mineral assemblage, Ahlbrandt (1973, 1974b) deter-

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mined that the Laney Member of the Eocene Green River Formation is the main source of sand for the dune field. Erosion and transport of eolian sediments occur when: (1) wind velocities exceed the threshold of particle transport, (2) there is an adequate sediment supply, and (3) vegetation cover is inadequate to stabilize the land surface (Pye and Tsoar, 1990, p.127 – 145). Studies of dune morphology and orientation in the Killpecker Dunes indicate that paleowind directions were primarily from the west and southwest (Ahlbrandt, 1973, 1974a; Kolm, 1974, 1982) and did not vary from current dominant wind directions in southern Wyoming (Martner and Marwitz, 1982). The southern half of Wyoming, or the ‘‘Wyoming Wind Corridor’’ (Kolm, 1982, p.49), often experiences winds of up to 50 km h 1. The reactivation of currently dormant portions of the Killpecker Dunes, as well as most dune fields in the Wyoming Basin, is hypothesized to be the result of departures to more arid climates during the late Quaternary, causing a decrease in effective moisture available to dunestabilizing vegetation.

Methods The stratigraphy in the western Killpecker Dunes was observed in both natural and artificial exposures. Blowouts provided natural exposures, while backhoe trenches at the Krmpotich and Finley archaeological sites, and manually excavated trenches southwest of the Krmpotich site, provided additional exposures (Fig. 2). Stratigraphic sections were measured, described, and sampled in the field. Descriptions and designations were carried out using standard geologic and pedologic nomenclature (Compton, 1985; Soil Survey Staff, 1993, 1999; Birkeland, 1999). Samples were collected for radiocarbon and luminescence dating. Although radiocarbon dating of organic sediments and buried soils is somewhat problematic (e.g., Scharpenseel, 1979; Matthews, 1985; Abbot and Stafford, 1996; Wang et al., 1996; McGeehin et al., 2001), studies have shown that with proper sampling care and interpretation, these materials can provide good age control, espe-

Fig. 2. Topographic map from the western Killpecker Dunes showing described and sampled localities. FS, Finley site profiles; KD, Killpecker Dunes profiles; KS, Krmpotich site; SW, Southwest trenches. Shaded area shows approximate western limit of late Quaternary eolian sand. See Fig. 1c for location of mapped area in the Killpecker Dunes. Source: USGS Fifteen Mile Springs, WY, 7.5 minute topographical quadrangle map series, 1968. Reproduced by permission of the publisher.

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cially in dry environments (e.g., Haas et al., 1986; Holliday et al., 1994; Martin and Johnson, 1995; Rawling et al., 2003). Ages from organic-rich lenses in interdunal deposits provide good estimates for deposition of interdunal strata, while organic matter from buried soil horizons provides minimum ages for periods of stability and soil formation, as well as maximum ages for overlying deposits. With the exception of one sample (AA40736) processed for a bulk soil age at the NSF – University of Arizona AMS Laboratory, radiocarbon ages were determined at the National Ocean Sciences AMS Facility, Woods Hole Oceanographic Institution, for the base soluble (humic acid) fraction isolated using a standard acid –base –acid pretreatment at the Institute for Arctic and Alpine Research, University of Colorado (Table 1). The optically stimulated luminescence (OSL), infrared stimulated luminescence (IRSL), and thermoluminescence (TL) dating methods have proven to be effective means for determining ages of eolian deposits (e.g., Stokes and Gaylord, 1993; Wintle, 1993; Forman et al., 1995; Stokes and Swinehart, 1997; Personius and Mahan, 2000; Rawling et al., 2003), essentially measuring the last time a mineral grain was exposed to sunlight (i.e., when the mineral grain experienced burial). Buried grains are exposed to a radiation flux (the dose rate) from the decay of radioactive elements and their daughter products, as well as cosmic rays (Aitken, 1998). The mineral grain serves as a natural dosimeter, and the paleodose is measured by heating (TL) or exposure to light (OSL/IRSL). The age (time since burial) is calculated by dividing the paleodose by the dose rate. Samples were processed at the U.S. Geological Survey’s Luminescence Dating Laboratory in Denver, Colorado. Dose rates were taken in situ with an Exploranium g-ray spectrometer using an internal sodium-iodide crystal detector and calculated as outlined by Aitken (1985).

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Each sample location was counted for at least 30 min. Two bulk samples were counted on a low-level resolution laboratory g-ray spectrometer and those readings were within 10% agreement of in situ readings. Samples were washed in 4 N HCl and 35% H2O2 and then washed and wet-sieved in deionized water to separate the <63- and 90- to 125-Am fractions. The <63-Am fraction was transferred to cylinders, and the 4- to 11-Am size was obtained by making use of settling velocities in deionized water. Quartz grains in the 90- to 125-Am fraction were isolated using heavy liquid separation. OSL ages were determined on 90- to 125-Am-size quartz grains using the blue-light OSL method using the single-aliquot regeneration-dose technique (SAR). A Riso TL/OSL reader fitted with three 3-mm-thick Hoya U-340 detection filters was used for all quartz luminescence measurements, and a minimum of 12 aliquots per sample were run. Equivalent doses measured on two samples that had been etched for 45 min with 50% HF were virtually identical to samples not etched. Thus, the majority of samples run for blue-light OSL were not etched in HF, and any potentially contaminating feldspar luminescence in the 90- to 125-Am quartz fractions was mechanically removed by exposure to an infrared wavelength for 100 s prior to running blue-light OSL. IRSL and TL ages were determined on 4- to 11-Am-size polymineral separates. IRSL and TL samples were run as multiple-aliquot, additive-dose (MAAD) using total bleach techniques on a Daybreak TL/OSL reader. IRSL and OSL ages were originally determined and reported for much drier field moisture conditions (Mayer, 2002, 2003). Because eolian sands in the Killpecker Dunes display evidence of past conditions more moist than at present, luminescence ages were also calculated and reported based on a 10% moisture level. These ages are considered a more reasonable estimate and are presented as such here (Table 2).

Table 1 Radiocarbon ages from the western Killpecker Dunes Sample ID Krmpotich site, Profile 00-5 60 – 70 cmbs 75 – 85 cmbs SW trenches, Profile 00-4 75 – 80 cmbs 90 – 95 cmbs Finley site, Profile 00-1: 60 – 85 cmbs Profile 00-3: 135 – 150 cmbs a

y13C (x)

Age (14C yr B.P.)

Calibrated agea (cal yr B.P.)

Lab No.

Material dated

Remarks

Top of Krmpotich soil in stratum 4A Base of Krmpotich soil in stratum 4A

23.4

930 F 50

948 (831) 731

CURL-5151

Soil, humic acids

23.6

1400 F 70

1413 (1298) 1178

CURL-5153

Soil, humic acids

19.5

5160 F 55

6167 (5916) 5748

CURL-5147

Soil, humic acids

20.9

7400 F 50

8343 (8182) 8039

CURL-5149

Organic mud, humic acids

22.4

1990 F 35

2000 (1942) 1869

CURL-5155

Soil, humic acids

25.3

7271 F 84

8284 (8090) 7878

AA40736

Bulk soil

Radiocarbon ages converted to cal yr B.P. using CALIB 4.0 (Stuiver et al., 1998); ranges shown are F2j.

Base of Farson soil in stratum 3 Organic-rich lens in stratum 2 Top of Farson soil in stratum 3 Top of Washington soil; unreliable

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Table 2 Luminescence ages from the western Killpecker Dunes Sample ID

K U Th TL/IRSL Equivalent (%) (ppm) (ppm) dose rate dose (Gy) (Gy/103 yr)a

Krmpotich profile 00-5 KRM-1: 1.73 1.76 stratum 4B, 45 – 50 cm KRM-2: 1.69 1.67 stratum 4A, 79 – 84 cm KRM-3: 1.74 1.66 stratum 1, 115 – 120 cm KRM-3e 1.46 1.26 Finley profile 00-1 FIN-3: 1.71 stratum 4, 50 – 55 cm FIN-2: 1.64 stratum 3, 90 – 95 cm FIN-2e 1.51 FIN-1: 1.73 stratum 1, 175 – 180 cm

IRSL ageb (yr) Equivalent dose (Gy)

TL ageb (yr)

SiO2 Equivalent dose rate dose (Gy)c (Gy/103 yr)a

OSL ageb (yr)

3.82

3.33 F 0.10 2.23 F 0.04d

670 F 45

2.05 F 0.46d

615 F 225

4.16

3.28 F 0.10 5.02 F 0.04d

1530 F 105

5.57 F 0.37d

1700 F 285

4.11

3.30 F 0.10 51.8 F 0.34 15,710 F 960 46.2 F 0.98 14,000 F 1030 2.47 F 0.07 12.1 F 0.05 4910 F 280

4.24

2.86 F 0.06

1.31

2.88

2.92 F 0.08 6.46 F 0.14

2080 F 160

5.95 F 0.27

1920 F 220

2.38 F 0.06 1.66 F 0.02

1.49

3.72

3.08 F 0.09 12.9 F 0.23d

4185 F 285

9.90 F 0.73

3220 F 510

2.33 F 0.06 11.6 F 0.07 4960 F 270

1.32 1.39

4.23 3.50

2.95 F 0.06 3.10 F 0.08 23.6 F 0.22d

8045 F 445

24.7 F 0.52d

8455 F 565

2.26 F 0.05 13.9 F 0.12 6160 F 310

2.49 F 0.06 0.97 F 0.06

390 F 50

2.45 F 0.06 3.56 F 0.18 1450 F 170

700 F 40

a In situ low-level g spectrometry. Dose rates, calculated after Aitken (1985), include contributions of cosmic radiation of 0.27 F 0.12 to 0.30 F 0.12 Gy/103 yr for 2028 m elevation (Prescott and Hutton, 1994) and assume wetter than field moisture as measured, usually 10 F 5%. Gy, Grays. All errors are F1j. b Preferred ages are shown in bold. See text for discussion. c Weighted average of at least 12 aliquots, single-aliquot regeneration; Riso TL-DA-15 reader. d Weighted average of two multiple-aliquot, additive dose runs, if not indicated only one run; Daybreak reader. e Dose rates measured on samples that were in sealed plastic counters using low-level g spectrometry NaI crystals, USGS lab.

Human occupation of the Killpecker Dunes during the latest Pleistocene and early Holocene is indicated by diagnostic Paleoindian projectile points occurring in both surface and buried contexts in the western portion of the dune field (Howard et al., 1941; Howard, 1943; Moss, 1951; Satterthwaite, 1951, 1957; Frison, 1978, 1991; Mayer, 2003). The radiocarbon age ranges of Paleoindian components have been reasonably well constrained at sites across the Great Plains and Rocky Mountain basins (Frison, 1991; Haynes, 1992, 1993; Hofman, 1995; Holliday, 2000), and the occurrence of these diagnostic projectile points in the Killpecker Dunes provides additional age control.

correlate with Pinedale glaciation in the Wind River Mountains (f23,000 –16,000 yr ago; Chadwick et al., 1997). However, in the Krmpotich site area to the south, eolian sands f2– 4 m thick directly overlie bedrock (Fig. 4). Most of the eolian units observed display no primary sedimentary structures, probably the result of bioturbation, common in eolian sands (Ahlbrandt et al., 1978). Cryoturbation has also likely played a role in the obliteration of sedimentary structures, especially under past glacial climates (Ahlbrandt et al., 1978). Sands are poorly to well sorted, subrounded to rounded, and composed primarily of quartz and orthoclase feldspar. The heavy mineral assemblage is dominated by zircon, epidote, and hornblende (Ahlbrandt, 1973, 1974b).

Stratigraphy and radiometric ages Stratum 1 Late Quaternary eolian stratigraphy Four stratigraphic units ranging in age from latest Pleistocene to latest Holocene were recognized in the western portion of the dune field (Figs. 3 and 4), differing slightly from previous investigations (Table 3). At the Finley site, located at the northwestern edge of the dune field, Moss (1951) reported eolian sands f10 to 15 m thick overlying gravels of the Farson Terrace, which he considered to

Stratum 1 is a light gray (f2.5Y 7/2), moderately to well sorted, fine to medium eolian sand ranging from 1 to 7 m in thickness. Upper stratum 1 displays calcified root tubules and iron and manganese oxides in the form of masses, streaks, and nodules. The iron and manganese oxides were considered by Moss (1951, p.40) to indicate podzolization, but are probably redoximorphic features resulting from periodic saturation and drying in the dune field. However,

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Fig. 3. Stratigraphic correlations of investigated sections in the western Killpecker Dunes. Numbers on left sides of sections are reliable radiocarbon ages (from Table 1), and numbers on the right are reliable luminescence ages (from Table 2). See text for discussion of reliable ages. Fig. 2 shows locations of sections in the dune field.

Fig. 4. Schematic geologic cross section through the western Killpecker Dunes illustrating the stratigraphic relationships of late Quaternary eolian units, as well as the Folsom (12,950 – 11,950 cal yr B.P.) and Cody (10,200 – 9500 cal yr B.P.) occupation zones (XXX) at the Krmpotich and Finley sites. Moss (1951) considered Farson gravels to correlate with Pinedale glaciation (f23,000 – 16,000 yr ago; Chadwick et al., 1997), and the Laney Member of the Green River Formation is Eocene in age (Sullivan, 1980).

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Table 3 Correlations of stratigraphic terminology from the western Killpecker Dunes

by: (1) the IRSL and TL ages from Krmpotich, (2) the stratigraphic position of Paleoindian artifacts in the dune field, and (3) evidence for cryoturbation of, and the sand wedge in-filled by, stratum 1 sand.

Hack (1943)

Moss (1951)

Ahlbrandt (1973, 1974a)

Mayer (this study)

Yellow sand (nr)

Upper sand Middle sand, upper facies Middle sand, lower facies Lower sand

Upper sand (nr)

Stratum 4 Stratum 3

Stratum 2

Middle sand

Stratum 2

Lower sand

Stratum 1

Stratum 2 is a moderately to moderately well sorted, fine to medium eolian sand 50 to 75 cm thick, displaying light gray (10YR 7/2) to pinkish gray (7.5YR 7/2) color, overlying stratum 1. Evaporite deposits comprising carbonates, gypsum, and other more soluble salts occur in the upper part of stratum 2 throughout the western portion of the dune field and probably correlate with what Ahlbrandt (1973, 1974a) referred to as ‘‘marl’’ in his Middle sand. Stratum 2 typically displays evidence for weathering in the form of a moderately well developed buried soil displaying reddening (7.5YR 7/2), clay films, prismatic structure, and accumulations of sodium and other salts in the upper part of stratum 2. This soil, classified as a Natrargid, crops out in blowouts throughout the western portion of the dune field. Cody Complex (10,200 – 9500 cal yr B.P.) artifacts occur throughout stratum 2 at Finley. Nevertheless, the greatest concentration of artifacts occurs in the soil described above (Moss, 1951, pp.41 – 42; Satterthwaite, 1951, p.103; 1957, p.17), informally referred to as the Finley soil (Eckerle, 1997). A sample from a continuous organic-rich interdunal mud at the top of stratum 2 at the southwest trenches locality yielded a radiocarbon age of 8182 cal yr B.P. (Fig. 3). As mentioned above, Folsom (12,950 –11,950 cal yr B.P.) and Agate Basin (12,600 –10,700 cal yr B.P.) artifacts occur in the dune field. However, they have not been found in situ, but probably derive from this unit (Mayer, 2003). The archaeological data, as well as the single radiocarbon age from the southwest trenches, indicate a latest Pleistocene to early Holocene age for stratum 2.

Buff sand Green sand (nr) not recognized.

a soil displaying a calcic horizon (Calcid) occurs in the upper part of stratum 1, informally referred to here as the Washington soil. At the Krmpotich site, a sand wedge formed in residuum of the Laney Member of the Eocene Green River Formation is filled with stratum 1 sand. Sand wedges and other periglacial features, documented throughout the Wyoming Basin, are considered to date primarily to the last glacial maximum (Mears, 1981, 1997). At the Finley site, an ochric epipedon of the Washington soil is preserved and appears heavily disturbed. This was originally interpreted to indicate soft sediment deformation under saturated conditions during the latest Pleistocene (Mayer, 2003). Alternatively, the soil may display the effects of cryoturbation, consistent with the occurrence of a sand wedge at Krmpotich and elsewhere in the Wyoming Basin. Folsom artifacts (12,950 – 11,950 cal yr B.P.) occur across an unconformity between stratum 1 and younger eolian sand at the Krmpotich site, while Agate Basin artifacts (12,600 – 10,700 cal yr B.P.) were found on a deflated stratum 1 surface south of Krmpotich (Mayer, 2002, 2003). Cody Complex artifacts (10,200 –9500 cal yr B.P.) were recovered from sediments overlying stratum 1 at the Finley site (Figs. 3 and 4; Howard et al., 1941; Howard, 1943; Moss, 1951; Satterthwaite, 1951, 1957). A sample from stratum 1 at the Krmpotich site yielded luminescence ages of 15,710 F 960 (IRSL), 14,000 F 1030 (TL), and 4910 F 280 (OSL) yr. The IRSL and TL ages statistically overlap and are in agreement with the samples’ stratigraphic position below Folsom (12,950 –11,950 cal yr B.P.) artifacts; however, the OSL age is considered unreliable. At Finley, stratum 1 yielded luminescence ages of 8455 F 565 (TL), 8045 F 455 (IRSL), and 6160 F 310 (OSL) yr. A radiocarbon sample from the ochric epipedon of the Washington soil yielded a bulk radiocarbon age of 8090 cal yr B.P. While the radiocarbon, TL, and IRSL ages are in good agreement, all are from samples stratigraphically below the Cody Complex occupation (10,200 – 9500 cal yr B.P.) at Finley. While sampling efforts were made to avoid krotovina common in stratum 1 and overlying sands (Fig. 3), it is likely that the inconsistency of the luminescence and radiocarbon ages with the Cody occupation is the result of intrusive contaminants. Although dating of stratum 1 proved somewhat problematic, a late Pleistocene age is indicated

Stratum 3 Stratum 3 is a brown (10YR 5/3) to pale brown (10YR 6/ 2), moderately to moderately well sorted, fine eolian sand approximately 40 cm thick overlying stratum 2 and probably corresponds to what Moss (1951, p.41) considered a facies of the Middle sand. A buried soil showing variability in morphology occurs in stratum 3 and is informally referred to as the Farson soil (Fig. 3). In former interdunal locations, such as the southwest trenches locality, the Farson soil is characterized by an overthickened A horizon overlying stratum 2 marl. However, at the Finley site, the Farson soil is an ABt horizon welded (after Ruhe and Olson, 1980) on the Natrargid in stratum 2. A sample from the base of stratum 3 at Finley yielded luminescence ages of 4960 F 270 (OSL), 4185 F 285 (IRSL), and 3220 F 510 (TL) yr. Because of the disagreement of the OSL age, it is rejected, and the IRSL and TL ages indicate deposition by f3700 yr. The Farson soil

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yielded radiocarbon ages of 5916 and 1942 cal yr B.P. from lower and upper portions, respectively (Fig. 3), indicating initial sedimentation concomitant with soil formation during the middle Holocene, continuing into the early part of the late Holocene.

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age is considered unreliable. The absence of a buried soil, greater degree of development in the modern soil (i.e., greater secondary carbonate accumulation and cementation), and luminescence ages suggest that stratum 4 at Finley correlates with stratum 4A at Krmpotich (Figs. 3 and 4).

Stratum 4 Discussion Stratum 4 is a light gray (10YR 7/2) to yellowish gray (10YR 6/2), moderately to moderately well sorted, fine eolian sand capping most dormant dunes and is considered to correlate with the Upper sand of Moss (1951). Stratum 4 typically is f50 –150 cm thick at dune edges to over 3 m thick on dune crests. The modern soil formed in stratum 4 in the western Killpecker Dunes shows weak development, typically displaying A/C or A/AC/C horizonation, and is classified as a Torripsamment (Ravenholt and Lewis, 1990). Buried soils (Cambids and Psamments) occur locally in lower stratum 4. At Finley, stratum 4 yielded luminescence ages of 2080 F 160 (IRSL), 1920 F 220 (TL), and 700 F 40 (OSL) yr, and the OSL age is considered unreliable. A sample from the base of stratum 4 (4A) at the Krmpotich site yielded ages of 1700 F 285 (TL), 1530 F 105 (IRSL), and 1450 F 170 (OSL) yr, and based on statistical overlap, all are considered reliable age estimates. The lower and upper portions of a buried soil occurring in stratum 4 in the Krmpotich site area, informally referred to as the Krmpotich soil (Cambid), yielded radiocarbon ages of 1298 and 831 cal yr B.P., respectively (Fig. 3). A luminescence sample from stratum 4B above the Krmpotich soil yielded ages of 670 F 45 (IRSL), 615 F 225 (TL), and 390 F 50 (OSL) yr. The OSL

Killpecker Dunes eolian chronology and regional correlations The stratigraphic and radiometric data presented above generally support the existing model of Holocene eolian activity in Rocky Mountain basins (Fig. 5), and compare reasonably well with the geochronologies of other Wyoming Basin dune fields (Fig. 6). While Ahlbrandt et al. (1983, p.400) considered evidence for pre-Holocene episodes of eolian activity to be lacking in Rocky Mountain dune fields, the data presented here indicate that significant eolian sedimentation was occurring in the Killpecker Dunes at least during the latest Pleistocene. Stratum 1 is ubiquitous in the western portion of the dune field, ranging in thickness from less than 1 m to more than 7 m, and deposition during the latest Pleistocene was probably in the form of a sand sheet. Hell Gap artifacts (f11,500– 10,200 cal yr B.P.) recovered from eolian sands in the Seminoe and Casper dune fields (Albanese, 1974a,b; Frison, 1974; Miller, 1986) indicate that, like the Killpecker Dunes, these sand bodies date to at least the latest Pleistocene (Fig. 6). Proxy data indicate that the late Pleistocene in the Wyoming Basin was

Fig. 5. Plots of reliable luminescence and radiocarbon ages from the western Killpecker Dunes. Also shown are stratigraphic designations, as well as chronostratigraphic positions of Folsom (12,950 – 11,950 cal yr B.P.), Agate Basin (12,600 – 10,700 cal yr B.P.), and Cody (10,200 – 9500 cal yr B.P.) occupations in the dune field stratigraphy. Vertical dashed line indicates younger limit of Folsom occupation. Lower dashed line represents extension of Ahlbrandt et al.’s (1983) model of eolian activity in the Wyoming Basin based on this study.

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Fig. 6. Stratigraphic correlations showing periods of eolian activity and stability of Wyoming Basin dune fields and Wind River Mountains latest Quaternary glacial chronology. Also shown are the relative chronostratigraphic positions of Folsom (12,950 – 11,950 cal yr B.P.), Agate Basin (12,600 – 10,700 cal yr B.P.), Hell Gap (11,500 – 10,200 cal yr B.P.), and Cody (10,200 – 9500 cal yr B.P.) occupations in Wyoming Basin dune field stratigraphy. Shaded units indicate relatively mesic intervals, i.e., soil formation and organic-rich interdunal sediments, and cross-bedded pattern indicates eolian sedimentation. Pleistocene – Holocene boundary after Hopkins (1975), Eckerle (1989).

characterized by a cold, dry tundra or steppe environment, with mean annual temperatures 10j to 16jC below present (Anderson, 1974; Mears, 1981, 1997; Walker, 1987). It is likely that the inception of most major Wyoming Basin dune fields was initially in the form of sand sheets deposited on vegetated landscapes under a dry, periglacial climate during the last glacial maximum. The occurrence of early and late Paleoindian material (f12,950 –9500 cal yr B.P.) in most major Wyoming Basin dune fields indicates that these settings were not unattractive to humans during the latest Pleistocene and early Holocene. Folsom and Agate Basin material in the Killpecker Dunes, while not found in situ, probably derive from stratum 2 (Mayer, 2003). The Cody Complex material at the Finley site was recovered from the upper portion of stratum 2, apparently associated with the Finley soil. Together, the Paleoindian material and Natrargids in the western portion of the dune field are interpreted as indi-

cating deposition of stratum 2 sand concomitant with soil formation during the latest Pleistocene and early Holocene. Evaporites in upper stratum 2 probably represent the onset of warming and increased rates of evaporation during the early Holocene. How latest Pleistocene and early Holocene episodes of erosion, sedimentation, and soil formation in Wyoming Basin dune fields relate, if at all, to the Temple Lake advance (f13,000 –11,500 cal yr B.P.) in the Wind River Mountains (Zielinski and Davis, 1987; Fall et al., 1995; Gosse et al., 1995; Dahms, 2002) is not entirely clear. However, in the Killpecker Dunes, the onset of the Temple Lake advance appears to coincide with the onset of relatively slow deposition of stratum 2 sand concomitant with soil formation (Fig. 6). Pedological data from throughout Wyoming also indicate relative landscape stability in the latest Pleistocene and early Holocene (Reider, 1990; Eckerle et al., 2000).

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Eolian activity during the middle Holocene, classically referred to as the ‘‘Altithermal’’ of Antevs (1948), is recorded in all Wyoming Basin dune fields (Fig. 6), as well as throughout the central United States (e.g., Holliday, 1989; Dean et al., 1996; Stokes and Swinehart, 1997). Ahlbrandt (1973, 1974a) interpreted an unconformity in the Killpecker Dunes to indicate erosion beginning sometime between 11,500 and 7800 cal yr B.P., continuing until just prior to f6600 cal yr B.P. Radiocarbon ages presented here bracket an unconformity between f8182 and 5916 cal yr B.P., suggesting erosion slightly earlier than the ‘‘phase II’’ middle Holocene eolian activity of Ahlbrandt et al. (1983; Fig. 5). Approximately 3 to 4 m of eolian sand accumulated at the eastern margin of the Casper Dunes after Hell Gap occupation (f11,500 – 10,200 cal yr B.P.), but prior to f5200 cal yr B.P. (Albanese and Frison, 1995). Gaylord (1982, 1990) reports radiocarbon ages of f8400 and 5300 cal yr B.P. from interdunal deposits at the Clear Creek section in the Ferris Dunes that bracket f15 m of eolian sand. Stokes and Gaylord (1993) report optical ages from eolian sand at the Clear Creek section and consider the majority of sedimentation in the Ferris Dunes to have occurred between 8800– 8100 and 4300 – 4000 cal yr. The Killpecker Dunes were apparently dominated by erosion during the middle Holocene, while the Ferris Dunes, located downwind, were dominated by sedimentation (Ahlbrandt, 1974a,b). Eolian activity in Wyoming Basin dune fields during the middle Holocene was followed by an episode of relative stability and soil formation (Fig. 6). The Farson soil indicates stability in the Killpecker Dunes between f5900 and 2000 cal yr B.P., probably representing relatively slow deposition concomitant with soil formation. Haplargid formation between f5300 and 2700 cal yr B.P. occurred in the Casper Dunes (Albanese and Frison, 1995), as well as smaller dune fields in the Casper area (Eckerle, 1997). The onset of soil formation during the early part of the late Holocene coincides with the Alice Lake advance f5500– 4000 yr B.P. in the Wind River Mountains (Dahms, 2002). Mammalian faunal remains from Yellowstone National Park indicate relatively mesic late Holocene conditions at approximately 3200 cal yr B.P. (Hadly, 1996). Alluvial systems in the Yellowstone area experienced overbank sedimentation and terrace formation between f3350 and 2700 cal yr B.P., interpreted to represent cooler, effectively wetter conditions (Meyer et al., 1995). All major dune fields throughout the Wyoming Basin contain evidence for reactivation during the late Holocene (Fig. 6). The Casper (Albanese and Frison, 1995) and Seminoe (Miller, 1986) dune fields contain evidence for eolian sedimentation after f2000 and 5300 cal yr B.P., respectively, and a ‘‘volumetrically minor degree of deposition’’ of eolian sand occurred in the Ferris Dunes sometime after f2200 cal yr B.P. (Stokes and Gaylord, 1993, p.280). Luminescence ages from the Killpecker Dunes

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indicate that initial deposition of stratum 4 was occurring by f2000 yr, continuing until at least f1500 yr. A brief (f500 yr) episode of stability and soil formation in the Killpecker Dunes between f1300 and 830 cal yr B.P. is partially coincident with a moist interval in the Yellowstone area between ca. 1900 and 1100 cal yr B.P. (Meyer et al., 1995; Hadly, 1996). A return to relatively xeric conditions in the Killpecker Dunes sometime after f830 cal yr B.P., continuing until at least f650 yr, coincides with a possible Medieval warm period, ca. 900 –700 yr B.P. (Porter, 1986). Faunal remains between f1000 and 650 cal yr B.P. in age indicate the most xeric conditions in the Yellowstone area in the past 3000 years (Hadly, 1996). At some point during the latest Holocene, major portions of previously active areas of Wyoming Basin dune fields were stabilized, probably coincident with the onset of relatively cooler and/or moister conditions resulting in the Gannet Peak advance in the Wind River Mountains between f350 and 100 yr B.P. (Fig. 6), considered the local manifestation of the Little Ice Age (Dahms, 2002). Implications of late Holocene eolian activity in Wyoming Basin dune fields While no doubt incomplete, the Wyoming Basin eolian stratigraphy correlates relatively well with other late Holocene proxy climate records from the region (e.g., Reider, 1990; Meyer et al., 1995; Hadly, 1996; Dahms, 2002), all indicating episodic departures to warmer and/or drier conditions during the past f2000 years. Multiple proxy records of climate from the Great Plains and western United States indicate numerous departures to relatively arid conditions during the late Holocene (e.g., Laird et al., 1996; Stokes and Swinehart, 1997; Woodhouse and Overpeck, 1998; Forman et al., 2001), many apparently greater in magnitude and duration than the 1930s ‘‘Dust Bowl’’ droughts. A more precise understanding of the timing of late Holocene eolian erosion and sedimentation in other Wyoming Basin dune fields is necessary before correlations can be made with greater confidence. Nevertheless, it is probable that droughts affecting the central and western United States over the past 2000 years are recorded in the stratigraphy of Wyoming Basin dune fields, as well as other dune fields of the North American interior in Colorado (Ahlbrandt et al., 1983; Muhs, 1985; Madole, 1994, 1995; Muhs et al., 1996), Nebraska (Ahlbrandt et al., 1983; Muhs et al., 1997a; Stokes and Swinehart, 1997), Kansas (Arbogast, 1996), and Texas and New Mexico (Holliday, 2001). Luminescence ages from the western Killpecker Dunes This study provides the first optically stimulated and thermoluminescence ages from the Killpecker Dunes. IRSL and TL analyses using MAAD procedures for the Killpecker samples agree well with corresponding radiocarbon and

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artifact ages. That all IRSL and TL ages overlap, as well as agree with other geochronological indicators, suggests that insufficient bleaching or remobilization was not a significant problem in the silt fraction. In contrast, SAR ages on quartz extracts typically produced age shortfalls of 10 to 70% compared to IRSL, TL, radiocarbon, and artifact ages. The SAR technique is fundamentally different from the MAAD technique and is run on only the quartz of a much coarser grain size. Other factors for the younger SAR ages could be the emissions that the blue-light OSL analyses were run against, sensitivity changes that were not accounted for while the sample was being regenerated, or incorrect heating procedures during the analyses. These results are similar to recently published luminescence ages of loess in the Midwestern United States (Forman and Pierson, 2002) and stress the importance of interpreting luminescence ages using not only multiple techniques of luminescence dating, but also additional geochronological indicators, in this case radiocarbon and artifact ages.

Conclusions Since Ahlbrandt et al.’s (1983) synthesis of eolian chronologies of dune fields on the northern Great Plains and in Rocky Mountain basins, which presented unequivocal evidence for significant Holocene eolian activity in both regions, new data regarding the eolian history of the Killpecker Dunes have emerged, both supporting and refining the chronology of Ahlbrandt et al. (1983). Moreover, the general coincidence in the chronologies of dune fields in the Wyoming Basin suggests that the primary controlling factor in region-wide activation and stability is climate. Geochronological evidence indicates that, similar to most major dune fields in the Wyoming Basin, the Killpecker Dunes have existed since at least the late Pleistocene, with inception probably occurring under a dry periglacial climate. In addition, all Wyoming Basin dune fields experienced significant reworking as a result of multiple departures to warmer and/or drier conditions during the Holocene. Very generally, during the Holocene, the onset of relative stability and soil formation in dune fields appears to coincide with glacial advances in the Wind River Mountains, while erosion and sedimentation coincide with glacial recession. Similar to dune fields in eastern Colorado (Forman and Maat, 1990; Forman et al., 1995; Muhs et al., 1996), currently dormant portions of Wyoming Basin dune fields have been active under both cold/arid and warm/arid climatic regimes and are likely near a threshold of reactivation.

Acknowledgments Funding for radiocarbon ages came from the National Science Foundation via Vance Holliday. Mike Daniels kindly processed a radiocarbon sample (AA40736) at the

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