Late Quaternary sedimentology and geochronology of small playas on the Southern High Plains, Texas and New Mexico, U.S.A.

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Quaternary Research 70 (2008) 11 – 25 www.elsevier.com/locate/yqres

Late Quaternary sedimentology and geochronology of small playas on the Southern High Plains, Texas and New Mexico, U.S.A. Vance T. Holliday a,⁎, James H. Mayer b , Glen G. Fredlund c a

Departments of Anthropology and Geosciences, University of Arizona, Tucson, AZ 85721, USA b Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Department of Geography, University of Wisconsin – Milwaukee, Milwaukee, WI 53201, USA Received 13 April 2007 Available online 2 May 2008

Abstract Playas are small, circular basins forming a ubiquitous component of the southern High Plains landscape. They are filled with carbonaceous mud deposited since the terminal Pleistocene. The stratigraphy and geochronology of 30 playas was investigated to better understand the paleoenvironmental record of basin filling. At the base of the fill in some playas is a well sorted eolian sand dated between ~ 13,000 and ~ 11,000 14 C yr BP. The beginning of mud deposition, representing aggradation of eolian dust on a moist, vegetated playa floor was largely between ~ 12,000 and ~ 10,500 14C yr BP. Playa filling slowed ~ 9000 to ~ 4000 14C yr BP, probably due to dry conditions, increased ~4000 to ~ 2000 14C yr BP, then slowed again. Eolian sand and loam, likely representing regional aridity, accumulated in some basins episodically just prior to ~ 10,700 14 C yr BP, between ~ 8600 and ~ 4700 14C yr BP, and at ~ 1300 14C yr BP. Stable C isotopes from one basin indicate that the playa was inundated only seasonally throughout the record beginning ~11,500 14C yr BP. The phytolith record in that basin indicates an abrupt shift toward cooling ~ 11,400 to ~ 11,200 14C yr BP and then increasing importance of xeric-adapted C4 grasses through the Holocene. © 2008 University of Washington. All rights reserved. Keywords: Playas; Southern High Plains; Younger Dryas; Altithermal; Stable isotopes; Phytoliths; C3 and C4 plants

Introduction Playas on the Great Plains of the central United States are circular to irregularly shaped basins that hold water seasonally (Fig. 1). They contain terminal Pleistocene and Holocene fill and, therefore, have the potential for containing clues to regional landscape evolution and environmental changes. Most playas on the southern High Plains (SHP) have a relatively homogenous fill of black to dark gray clay dateable by radiocarbon, commonly a few meters thick, that accumulated more or less continuously over the past 12,000 to 15,000 radiocarbon yr (Holliday et al., 1996). These characteristics of playas contrast with the other principal settings of late Quaternary sedimentation, dunes and dry valleys, which have more discontinuous and sedimentologically variable stratigraphic records commonly low in dateable material. Playas fills, therefore, provide the best opportunity ⁎ Corresponding author. E-mail address: [email protected] (V.T. Holliday).

for reconstructing environmental evolution of the region because variables that might affect paleoenvironmental proxies are minimal and because their stratigraphic record is the most continuous and most easily dated one in the region. This paper presents the results of further systematic sampling of playas, focusing on the history of basin filling and paleohydrology, and their paleoenvironmental implications, building on the work of Holliday et al. (1996). Thirty playas from throughout the SHP, including a few of the twenty-three discussed by Holliday et al. (1996), were sampled for additional stratigraphic and sedimentological information, radiocarbon dating, and paleovegetation indicators. Setting and background The SHP is an extensive semi-arid plateau (~80,000 km2) in northwest Texas and eastern New Mexico (Fig. 2). There are ~25,000 small playa basins (b5 km2) dotting the landscape (Fig. 1) (Sabin and Holliday, 1995), providing minimal topographic relief

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on the plateau (Reeves, 1972, 1990; Holliday et al., 1996; Holliday, 1997a). The playa basins are inset against the Blackwater Draw Formation and locally the Ogallala Formation. The Ogallala Formation (Miocene–Pliocene) is alluvial and eolian sediment with a thick, highly resistant, pedogenic calcrete at the top (Gustavson and Finley, 1985; Gustavson and Winkler, 1988; Reeves and Reeves, 1996). The Blackwater Draw Formation is a widespread, loamy eolian deposit comprising the major surficial layer of the region and burying the Ogallala (Reeves, 1976; Holliday, 1989a). The origins of the High Plains playa basins have been debated for decades (e.g., Gilbert, 1895; Johnson, 1901; Judson, 1950; Osterkamp and Wood, 1987; Wood and Osterkamp, 1987; Gustavson et al., 1995). Processes most commonly cited for playa development include deflation, subsidence related to either salt dissolution or dissolution of calcic soils and calcretes, and animal activity (Gustavson et al., 1995). Several lines of evidence indicate that many if not most basins resulted from erosive processes, however, including centripetal fluvial erosion and deflation (Gustavson et al., 1995; Sabin and Holliday, 1995). This evidence includes: disconformable contacts between basin fill and the Blackwater Draw Formation, cross-cutting relationships between the basins and the Blackwater Draw Formation, and variation in playa size and shape as a function of variation in sediment texture of the Blackwater Draw Formation. None of the study basins were produced by extraterrestrial processes, except for the Odessa Meteor Crater, despite claims to the contrary by Firestone et al. (2006, p. 216–217). Playa basins vary considerably in size. The smallest are barely perceptible depressions a few meters in diameter. Larger playas are up to 5 km2 (2.5 km diameter), although a few are much larger (Holliday et al., 1996). Most are b1.5 km2, however, and about half are b 0.1 km2 (Fig. 1) (Sabin and Holliday, 1995). Present-day depths range from basins that are completely filled with lacustrine sediment and have no topographic expression to basins N 14 m deep. As discussed by Holliday et al. (1996) and

below, the modern basin floors are underlain by fill that varies in thickness from b 1 m to N 10 m. In this study, representing playas from throughout the SHP, the basins vary in size from 50 m diameter (2000 m2) to 840 m diameter (0.55 km2) (Table 1). Methods Most field data came from cores recovered with a Giddings hydraulic soil coring machine in continuous 120-cm sections, 5 cm or 7.5 cm in diameter. Recovery in most playas was 90– 100% with little compaction. Very few natural or artificial exposures are available. An exception is the San Jon playa complex, where erosion exposed the large main playa basin (Hill et al., 1995; Holliday et al., 1996; Holliday, 1997a) and an adjacent small, filled playa identified as the “sub-basin.” All cores and sections were measured and described in the field for sedimentary structures, color, and pedologic horizonation. Samples were subjected to a variety of analyses for further assessment of sedimentologic and pedologic characteristics, although the field characteristics and descriptions were the most informative data (e.g., Holliday, 1985a,b,c, 1989a; Holliday et al., 1996). Laboratory analyses included particle-size distribution (sand–silt–clay content), carbonate content, and organic carbon content, following methods described in Singer and Janitzky (1986). Samples from most cores were dated by radiocarbon. Both conventional and AMS radiocarbon methods were used, depending on the carbon yield of samples (Table 2). Approximately half of the cores and sections yielded multiple dates spanning the Holocene, providing a well-dated stratigraphic sequence (Fig. 3). Radiocarbon ages were determined on total carbon or humic acids from bulk decalcified samples of soil organic mater (SOM) from the muddy sediment that composes most of the fill at most sites investigated (Table 2). As discussed elsewhere (Hill et al., 1995; Holliday et al., 1996; Holliday, 1997a) in this semi-arid setting, playa fills provided reasonable

Figure 1. Aerial view of playas filled with Spring–Summer storm runoff near Lubbock, Texas. These are typical of medium-sized basins, here formed in the sandy clay loam facies of the Blackwater Draw Formation (photo by Loren Smith, with permission).

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Figure 2. The southern High Plains showing selected physiographic features, cities, and field sites mentioned in the text (numbered to correspond to sites listed in Table 1). The Odessa Meteor Crater, just southwest of the city of Odessa, is not on Table 1. Inset shows the southern High Plains in relation to Texas and New Mexico.

radiocarbon age control in the absence of charcoal, wood, or well-preserved bone. Shrinking and swelling, and cracking of smectitic clays in most playa fills raises the possibility of mixing older and younger carbon, but intact, unmixed archaeological features in some playa fills and cross-checks between archaeological time-diagnostic artifacts and charcoal dates indicate that mixing is not a significant problem. Nevertheless, field samples were carefully examined for evidence of younger sediment filling cracks. Some mixing is inevitable given the likelihood of some bio- and argilliturbation so the radiocarbon

determinations do not have the precision of samples collected from more discrete stratigraphic provenience. Radiocarbon ages are presented in uncalibrated radiocarbon years to facilitate comparisons with other dated sections in the region and throughout the Great Plains. Calibrations are presented in Table 2 and are used in the section on the San Jon site and in the Discussion and Conclusions section. Proxy indicators of paleoenvironments in the playa fill were difficult to recover. Pollen and diatoms are typically very poorly preserved, probably due to the fluctuating wet–dry conditions

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Table 1 Morphometry and lithology of study playas a Playa d

Bailey 1 Baker 2 Barnes 3 Bilbrey Homestead 4 Bilbrey SW 4 Black 5 Blackburn 6 Buckeye #1 7 Buckeye #2 7 Bural 8 Bushland 9 Cage 10 Colston 11 Day Spray 12 Fitzgerald 13 Francis 14 Gandy SW 15 Hogue 16 Kizer 17 McFarland 18 Radcliffe 19 Scharbauer 20 San Jon h 21 San Jon, sub-playa 21 Tillman 22

Morphometry b 2

Fill thickness m

Substrate e

GC GC GC Marl Marl GC ? Marl g GC g ? GC Marl/GC GC BWD SC Marl C GC Marl ? GC Marl BWD BWD BWD

D km

A km

Rm

0.39 0.30 0.31 0.15 1.2 0.29 0.40 0.38 0.39 0.68 0.84 0.07 0.46 0.07 0.16 0.90 0.07 0.17 1.10 0.05 0.20 0.05 1.0 0.07 0.29

0.12 0.07 0.08 0.02 1.13 0.07 0.13 0.11 0.12 0.36 0.55 0.004 0.17 0.004 0.02 0.64 0.004 0.02 0.95 0.002 0.03 0.002 0.8 0.004 0.07

3 3 3 2 3 3 7 4 6 4 10 b1 7 1 3 4 2 2 6 2 5 1

1.5 2.4 5.7 1.0 0.4 3.6 N4.8 1.0 3.0 N4.0 3.9 1.2 3.6 3.0 2.6 1.6 0.8 1.5 0.4 2.7 2.4 3.4

0i 4

4.0 5.2

Marl bench

Facies c Black/gray mud

•

• •

• • • • • • • • • • • • • • • • • • • • • •

Reddened mud

Delta

Sand lenses

•f

• •

• •

•

• • •

• •

•

• •

•

•

• • •

Eolian loam

• •

• ?

• • • •f • • ?

a

Numbers after playa names correspond to numbers on Figure 2. Not included in the table is the playa inset against the fill of the Odessa Meteorite Crater, described and discussed by Holliday et al. (2005). b Morphometry: D = diameter; A = area, R = relief (approximations based on field estimates and 7.5′ topographic maps). c The terms qmudq and qloam,q though not standard in both geology and pedology, are used here for convenience and brevity to describe poorly sorted mixtures of fine-grained sediments, common in Quaternary sections of the region. Mud contains subequal amounts of silt and clay (after Jackson, 1997). Loam contains more or less equal amounts of sand, silt, and clay (Soil Survey Division Staff, 1993). d Numbers after each site correspond to numbers on Figure 2. e Substrate: GC = gleyed clay (“Tahoka Fm”); BWD = Blackwater Draw Formation; S = sand lens; SC = bedded sand & clay; C = carbonate; ? = core or exposure did not penetrate below basin fill. f Eolian loam at Bailey and Radcliffe are on marl bench. g Uplands around Buckeye #1 and #2 are underlain by Ogallala Caprock Caliche within 1–2 m (or less) of the surface. h San Jon main basin statistics are estimates. i San Jon subbasin is completely filled.

in the playas, their well-drained character when the fill is dry and cracked, and the calcareous nature of the fill. Phytoliths, however, are well preserved and pervasive, and stable-carbon isotopes are readily recoverable from the SOM in the playa fill. Samples from the San Jon sub-basin were processed for stable C isotopes (following methods in Holliday et al., 2006) and phytoliths (following methods in Fredlund, 1986). Carbon isotope ratios of SOM are a proxy for relative contributions of C3 and C4 vegetation (see summary in Nordt, 2001). δ13C values of C4 plants range between about − 19 and − 10‰, with a mean around − 13‰, while δ13C values of C3 plants range between − 30 and − 20‰, with a mean around − 27‰ (Smith and Epstein, 1971). In the central U.S., the distribution of C4 grass cover is best explained by temperature (Teeri and Stowe, 1976; Ehleringer et al., 1997), a pattern that appears to extend to δ13C values of SOM (Fredlund and Tieszen, 1997; Tieszen et al., 1997). δ13C values from SOM over timescales of thousands to tens of thousands of yr (Kelly et al., 1993) likely yield fairly accurate records of vegetation change. We used the δ13C

values of SOM to estimate the proportion of C3 and C4 plant biomass contributing to the total organic matter pool using the formula (after Ludlow et al., 1976): %C4 component ¼ ððδ13 CSOM −δ13 C100%C3 Þ=ðδ13 C100%C4 −δ13 C100%C3 ÞÞ100 Based on samples of modern playa vegetation (n = 26), we assume values of − 26.7 and − 12.6 for 100% C3 and C4 endmembers, respectively. Opal phytoliths have proven useful for reconstructing grassland plant communities on the Great Plains, in particular when analyzed in tandem with stable-C isotopes (Fredlund and Tieszen, 1994, 1997). This study focuses on epidermal short-cell phytoliths of grasses, classification of which is taxonomically significant at the grass sub-family or tribal level (Table 3) (Twiss et al., 1969; Brown, 1984; Mulholland, 1989). Although the correspondence of this classification and grass taxonomy is not perfect (Brown, 1984; Mulholand and Rapp, 1992), this methodology

V.T. Holliday et al. / Quaternary Research 70 (2008) 11–25

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Table 2 Selected radiocarbon dates from playa fills Playa

Core/profile

Depth (cm)

Facies a

Lab number b

Material c

14

Bailey Terrace Baker

C99-3 C-99-3a C 00-2

Barnes

C 99-1

Bilbrey Blackburn

C 99-1 C 00-3

125–145 240–258 20–25 50–55 90–100 140–150 190–200 230–240 50–60 100–110 150–160 200–210 250–260 300–310 350–360 400–410 450–460 500–510 550–560 65–69 100–110 150–160 220–230 360–400 400–480 100–112 50–55 75–85 115–125 170–180 220–230 250–255 275–285 310–325 50–55 100–105 150–155 200–205 245–250 300–305 390–395 395–400 50–55 100–105 150–155 200–205 250–255 300–310 350–355 400–410 505–515 545–555 700–705 53–63 63–75 87–100 100–120 55–60 105–110 155–160 205–210 255–260 305–310

C/Ab MC M M M rM rM M/gC M rM rM rM rM rM M M M M M/S Btb M rM rM M M S/Ab M M M rM rM rM M gC M M rM rM rM rM M M M M M M M M M MS gC gC gC M M M M M M M rM gM gM

NRSL NRSL AA AA CURL CURL CURL CURL NRSL NRSL NRSL NRSL NRSL NRSL NRSL NRSL NRSL NRSL NRSL NRSL CURL CURL CURL CURL CURL CURL AA CURL CURL CURL CURL AA CURL CURL AA AA AA CURL CURL AA CURL CURL AA AA AA AA AA AA AA AA CURL CURL CURL CURL CURL CURL CURL AA AA AA AA AA AA

HA HA Bulk Bulk HA HA Bulk HA HA HA HA Bulk HA HA Bulk Bulk Bulk Bulk Bulk HA HA HA HA Bulk Bulk HA Bulk HA HA HA HA Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk HA Bulk HA HA HA Bulk HA Bulk Bulk Bulk Bulk Bulk Bulk

8590 19850 1700 3161 3320 4170 7450 8640 2640 2890 4160 5540 8360 8890 9480 9990 10660 11300 12440 5890 2670 2950 4750 7630 8690 6200 2665 2800 3770 4830 6300 8544 11800 15500 2010 2427 5846 5890 9360 9615 10600 10800 1590 3890 6030 6210 8690 10295 11610 12730 18450 20400 21600 4710 6340 8370 10790 2190 2965 3380 4790 8160 9890

C 00-1 Buckeye #1 Buckeye #2

C 00-2 C 00-1

Bural

C 01-1

Bushland

C 01-1

Cage #1

C 00-3

Colston

C 01-1

10994 10995 56663 56664 5876 5878 5418 5420 11280 11281 11282 10985 11283 10986 11284 10987 11285 10988 11286 10996 5870 5872 5874 5415 5416 5401 56674 5825 5827 5829 5831 56675 5402 5405 56676 56677 56678 5776 5777 56679 5779 5781 56680 44154 44155 44156 44157 44158 44159 56682 5770 5771 5773 5862 5864 5411 5949 44178 44179 44180 44181 44182 44183

Cal yr BP (1 sigma) d

C yr BP ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

70 160 29 50 35 35 45 50 55 50 30 50 35 65 35 70 40 65 140 95 30 35 35 50 50 45 41 35 60 55 65 47 60 80 40 40 43 40 50 48 50 70 41 79 57 91 70 49 88 55 85 130 100 35 50 55 60 45 51 52 72 100 83

9625–9496 24008–23544 1622–1557 3444–3354 3544–3480 4762–4691 8265–8204 9630–9540 2794–2725 3080–2949 4734–4689 6350–6293 9455–9398 10104–9913 10772–10661 11508–11308 12793–12703 13239–13126 14757–14190 6802–6618 2788–2750 3167–3068 5582–5504 8456–8380 9684–9555 7128–7012 2793–2746 2947–2862 4238–4080 5538–5477 7294–7164 9543–9499 13752–13597 18877–18750 1998–1921 2490–2358 6733–6633 6745–6665 10606–10513 10960–10862 12773–12629 12866–12794 1470–1416 4422–4230 6944–6796 7180–7002 9708–9546 12162–11980 13569–13350 15148–14921 22198–21945 24592–24205 5384–5327 7321–7242 9469–9397 12857–12796 2307–2227 3222–3066 3692–3564 5601–5465 9272–9003 11404–11202

δ13C (‰)

(−25) − 14.1 − 15.9 − 15.9 − 17 − 17.3 − 17.9 − 17.4 − 17.6 − 18.5 − 17.9 − 17.1 − 17.6 − 17.1 − 18.1 − 18.4 − 20.2 − 18.6 − 19.6 − 19.6 − 19.9 − 18.6 − 18.7 − 15.7 − 20.6 − 18.1 − 17.8 − 17.9 − 17.8 (−21) − 20.9 − 22.2 − 23.9 − 23.4 − 21.4 − 20.1 − 19.6 − 21.3 − 20.9 − 20.3 − 24.6 − 24.9 − 26.4 − 25.4 − 25.1 − 22.3 − 25.1 − 22.4 − 22 est − 20.4 − 18.9 − 16.6 − 17.1 − 12 − 20 est − 21.2 − 21 − 21.6 − 21.2 − 21.6 − 20.8

(continued on next page)

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V.T. Holliday et al. / Quaternary Research 70 (2008) 11–25

Table 2 (continued ) Playa

Core/profile

Day Spray

Fitzgerald

C 00-1 C 00-1 C 00-1 Tr 2 C 00-1 Tr 2 C 00-1

Francis

C 01-1

Gandy SW

C 99-2

McFarland

C 99-1

Odessa Meteor Crater

C 01-1

Radcliffe

C 99-1

San Jon main basin

C 04-3

C 04-4

San Jon sub-playa

Pr 99-1

Scharbauer

C 00-1

Depth (cm)

Facies a

Lab number b

Material c

14

405–410 43–53 100–110 150–160 170–180 200–210 290–300 25–30 45–55 90–100 140–150 190–200 250–260 80–90 90–100 150–160 80–85 95–100 240–255 30–35 65–70 110–115 160–165 180–185 210–215 265–270 122–152 244–274 366–396 90–95 100–105 115–120 295–340 50–55 90–95 140–145 195–200 220–225 285–290 320–325 375–380 420–430 565–575 620–630 710–720 760–770 805–815 70–75 100–158 200–205 290–295 310–315 320–325 343–348 50–55 90–100 140–150 190–200 240–250 290–300

gC M M M M rM M/gC M M M rM M MS M M MC MC C MC M M rM M M M M/S M M M M M M gC ML M M ML ML M ML MbS S/M M M S/M M M M M M M M M M M rM rM rM rM M

AA CURL CURL AA A AA A AA CURL CURL CURL CURL CURL CURL CURL CURL NSRL NSRL NSRL AA NSRL NSRL NSRL AA NSRL NSRL AA AA AA NSRL NSRL NSRL NSRL AA AA AA AA AA AA AA AA AA AA A A A A NSRL NSRL NSRL NSRL NSRL NSRL NSRL AA CURL CURL CURL CURL CURL

Bulk HA HA Bulk HA Bulk HA Bulk HA HA HA HA Bulk HA HA Bulk HA HA HA Bulk HA HA HA Bulk HA Bulk Bulk Bulk Bulk HA HA HA HA Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk Bulk HA HA Bulk Bulk Bulk Bulk Bulk HA HA HA HA HA HA

11570 980 5210 6465 7625 8410 11375 1325 2090 3530 5010 6990 12050 2770 3540 3610 3490 5060 18230 895 4660 6080 7780 7864 9990 11660 7975 8890 11285 3460 4210 4250 21360 1375 2980 4215 6395 6910 7625 8135 8695 9650 10690 10255 11140 11580 11715 6180 8130 10800 11210 11360 11400 11500 2120 3440 5300 7770 8330 11280

44185 5422 5424 44174 9268.1 44175 9269.1 56687 5833 5835 5857 5860 5407 5785 5811 5812 11322 11323 11324 56667 11320 10991 11321 56668 10992 10993 44164 44165 44166 11309 11310 11311 11312 61233 61234 61235 61236 61237 61238 61239 61240 77813 77814 14644 14655 14646 14647 11292 11293 11294 11296 11297 10983 10984 60883 5816 5811 5823 5326 5398

C yr BP ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

± ± ± ± ± ± ± ± ± ± ± ± ±

68 40 40 61 +695/− 640 76 +685/− 630 40 40 50 30 45 75 40 35 35 60 40 100 32 45 55 80 44 70 75 57 61 71 50 80 80 110 39 43 47 49 69 53 53 67 50 55 +235/− 225 +295/− 285 +305/− 290 +215/− 210 75 90 150 130 55 65 65 36 35 40 45 50 620

Cal yr BP (1 sigma) d

δ13C (‰)

13471–13309 934–901 5991–5923 7428–7323 9299–7845 9503–9399 14134–12552 1296–1255 2115–2034 3798–3722 5751–5708 7868–7785 13983–13819 2891–2839 3885–3824 3933–3874 3839–3691 5892–5842 21964–21505 902–863 5464–5372 7008–6857 8779–8398 8719–8592 11508–11308 13611–13416 8988–8768 10101–9914 13234–13114 3733–3685 4764–4622 4763–4625 25922–25613 1319–1274 3218–3078 4759–4700 7332–7271 7797–7676 8456–8377 9127–9008 9710–9548 11179–11070 12812–12713 12396–11603 13308–12832 13752–13180 13770–13353 7169–6973 9256–8991 12927–12666 13211–12982 13283–13184 13318–13240 13400–13280 2147–2042 3723–3639 6119–6039 8596–8515 9472–9236 13944–12563

− 20.7 − 14.25 − 15.61 − 17.9 − 20 − 18.7 − 17.7 − 15.85 − 15.8 − 15.79 − 16.64 − 17.41 − 21.44 − 20.23 − 21.91 − 14.3 − 14.5 − 15.8 − 16 − 15.4 − 16.4 − 16.8 − 18.7 − 17 − 17.8 − 6.12 − 6.02 − 5.27 − 16.2 − 16.7 − 18.3 − 17.7 − 16.9 − 18.4 − 18 − 18.3 − 18.7 − 18.8 − 19.3 − 14.6 − 19.4 − 19.4 − 21.2 − 22.9 − 22 − 22.2 − 14 − 17.2 − 18.9 − 21.2 − 19.3 − 18.5 − 18 − 17.2 −16.18 − 15.41 − 15.67 − 16.27 − 22.1

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Table 2 (continued ) Playa

Core/profile

Depth (cm)

Facies a

Lab number b

Material c

14

Walker

C 01-1

100–110 200–210

rM M/cr

AA AA

Bulk Bulk

2540 8485

44171 44172

Cal yr BP (1 sigma) d

C yr BP ± ±

45 55

δ13C (‰)

2742–2697 9529–9474

a Key to facies: gM = gleyed mud; gC = gleyed clay; M = mud; rM = rubefied mud; M/gC= base of mud just above gleyed clay; MS= mud in sand lens; M/S = base of mud resting on sand lens; MbS = mud below sand lens; M/cr = base of mud resting on caprock; MC = mud in carbonate (marl); M&C = mud and carbonate (marl); M/C= base of mud resting on carbonate; C/Ab = A horizon buried by carbonate; S/Ab = A horizon buried by sand; S/M = mud buried by sand; Btb = organic matter in buried Bt horizon; MbS = mud in bedded sand (delta deposit). b Lab numbers: NSRL CURL = University of Colorado INSTAAR - Laboratory for AMS Radiocarbon Preparation and Research; A = University of Arizona conventional radiocarbon lab; AA = University of Arizona AMS facility. c Material: Bulk = decalcified bulk sediment; HA = humic acid. d Calibrations follow Stuiver and Reimer (1993).

provides a working means for documenting relative change in grassland composition and climate (Fredlund and Tieszen, 1994). Lithostratigraphy and geochronology Six distinctive lithofacies compose the playa-basin fills (Table 1) (Holliday et al., 1996): mud; marl; bedded, poorlysorted sand and gravel; well-sorted sand and silt; poorly-sorted loam; and the loamy Blackwater Draw Formation, strongly modified by pedogenesis. Dark gray to black smectitic mud deposited under ponded or heavily vegetated subaerial conditions on the floors of the basins is the most common facies and is the surficial deposit on the floors of most playas, typically forming Vertisols. This sediment is informally referred to as “Randall clay” based on the soil series mapped on the floors of most playas in the region, but it is a mix of silt and clay and is better considered a mud (Jackson, 1997). The base of the gray/ black mud is typically unconformable with the underlying substrate. Carbonate precipitated under lacustrine or palustrine conditions and preserved as a bench around the margins of some basins, or under the Randall clay in a few basins, is another common facies and surface deposit. This carbonate or marl is informally termed the “Arch loam” on the basis of soil surveys. Lacustrine delta deposits, derived from eroded Blackwater Draw Formation, are common near the basin margins. They were subjected to subaerial exposure and pedogenesis before burial by lacustrine or paludal sediments. Well-sorted layers of eolian sand and silt underlie the Randall clay in some sections. Poorly sorted eolian loam, deflated from the High Plains surface, occur locally above, within or below the lacustrine and paludal sediments. The well-sorted deposits probably were buried rapidly as water levels rose, but the loams were subjected to bioturbation and pedogenesis before burial. In larger basins with thicker fills, generally coincident with thicker Blackwater Draw Formation, the formation interfingers with the lacustrine fill. In the current study, the Blackwater Draw Formation was encountered only as “bedrock” below the basin fill. Radiocarbon ages are available for all facies of basin fill (Table 2), but most of the age control is for the dark gray mud because of its relatively organic-rich character (Fig. 3). The emphasis in the dating program was on both stratigraphic boundaries and the rate of accumulation of the dark gray mud. Beneath the lacustrine/paludal deposits in about half of the playas are clayey, sometimes calcareous pale olive gray or pale olive green sediments. These deposits were referred to as the

“Tahoka Formation” (e.g., Reeves, 1990), one of several Pleistocene lacustrine formations identified in the region (Reeves, 1976, 1990, 1991, modified from the work of Evans and Meade, 1945). These “formations” are very difficult to differentiate from one another on lithologic bases, however (Holliday et al., 1996). Moreover, in the small playa basins they are very difficult to differentiate from the Randall clay gleyed or reduced after burial. The gleyed clayey sediments and the Arch loam or marl beneath the Randall clay are the oldest basin fills encountered in this study, both generally dating to the late Pleistocene. The olive gray to olive green gleyed clayey sediments beneath the Randall were not previously dated but were considered late Pleistocene based on stratigraphic correlation (Holliday et al., 1996). Our new data confirm that interpretation. The upper 2 m of this unit produced a series of ages from ~ 21,600 to ~ 18,450 14 C yr BP at the Bushland playa (Fig. 3; Table 2). In two other sections with a clear stratigraphic break between the green clay and overlying basin fill (either the well-sorted sand or the gray mud), the uppermost gleyed clay dated to ~ 21,360 (Radcliffe) and ~ 15,500 (Buckeye #2) 14C yr BP (Fig. 3; Table 2). Additional clues to the age of the gleyed clay come from dates on sediment immediately above the disconformable contact with the gleyed clay: ~ 12,730 14C yr BP (Bushland) and ~ 12,440 14 C yr BP (Barnes) (Fig. 3; Table 2). Radiocarbon ages were also secured at three sites where the gleyed clay grades upward into the gray mud, indicating that the former is a reduced version of the latter: at Colston playa, the gleyed clay dates ~ 11,570 14C yr BP and the black/gray mud above is ~ 9890 14C yr BP; at Day Spray the gleyed clay dates N11,375 14C yr BP; and at Baker playa the lower-most black/gray mud dates to ~ 8640 14C yr BP (Fig. 3; Table 2). Lenses of well-sorted eolian sand occur either interbedded with the upper olive gray clay or in a single layer at the top of the olive clay on the unconformity at the base of the black/gray mud. Organic-rich mud within the sand lenses dates to ~ 12,730 14 C yr BP at Bushland and ~ 12,050 14C yr BP at Fitzgerald playa (Fig. 3; Table 2). Muds resting on the sand date to ~ 12,440 14C yr BP at Barnes playa, and just over ~ 11,660 14C yr BP at McFarland playa (Fig. 3; Table 2). At Gentry playa (Holliday et al., 1996) the eolian sand dates to ~ 9500 14C yr BP. The marl forms benches or terraces around the margins of some basins. In these playas it also disconformably underlies the Randall clay, deposited in basins inset against the carbonate. Where exposures or cores penetrated the carbonate, olive gray clay was observed beneath it. Deflation of the carbonate

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Figure 3. Radiocarbon dating of playa fills, plotting uncalibrated radiocarbon means by lithology (from Table 2) for all dated sections (located on Fig. 2). Dating for Gentry, Miami, and Elida is reported by Holliday et al. (1996).

resulted in formation of the lunettes that flank some playa basins (Holliday et al., 1996; Holliday, 1997b). In a few sections marl also disconformably underlies the clay but does not form a flanking terrace. Thin lenses of organic-rich mud beneath the surface of a bench (Bailey playa) date to ~ 19,850 14C yr BP (Fig. 4), and mud in marl below the Randall clay dates ~ 18,230 14 C yr BP (Gandy playa) (Fig. 3; Table 2). These ages, limited data from previous playa research (Holliday et al., 1996), and the geochronology of lunettes (Holliday, 1997b) all show that most of the marl deposition occurred in the late Pleistocene. The one exception is a late Holocene age associated with the marl at Francis playa (~ 3610 14C yr BP) (Fig. 3; Table 2). Most of the organic matter dated was from overlying playa mud infilling cracks in the carbonate. The cracks probably resulted from weathering of older carbonate exposed on the playa floor in the middle Holocene. At Gandy playa, the upper marl dates to ~ 5060 14C yr BP, and the base of the overlying playa mud is dated to ~ 3490 14C yr BP (Fig. 4; Table 2). This sequence suggests that at least locally the carbonate was accumulating at that time and that the Randall is only a late Holocene deposit. The black to dark gray Randall clay or mud is the typical playa fill. It was also the principal target of this study because previous work (Holliday et al., 1996) indicated that it is dateable by

radiocarbon and represents more or less continuous sedimentation throughout the Holocene. Where this mud rests on the Blackwater Draw Formation or on the gleyed late Pleistocene clay, the base generally dates to the last millennia of the post-LGM late Pleistocene, ranging from ~12,440 to ~9500 14C yr BP, but mostly between ~12,000 and ~10,500 14C yr BP (Barnes, Buckeye #2, Bural, Cage, Day Spray, Fitzgerald, Gentry, McFarland, Miami, and San Jon; Fig. 3; Table 1). In a few sections where the mud is underlain by marl, it produced late Holocene ages (Francis and Gandy playas, as noted above) (Fig. 3). Where underlain by pedogenic caprock or Cretaceous limestone, the base of the fill dates to ~8485 14C yr BP (Walker) (Table 2) and ~4720 14 C yr BP (Elida; Holliday et al., 1996). Otherwise, the bulk of the sedimentary record spans the Holocene. Most playas exhibit more or less complete Holocene records, but the dating suggests some variation in rates of sedimentation (Fig. 5). Relatively rapid sedimentation in the terminal Pleistocene/earliest Holocene (between ~ 12,000 and ~ 9000 14C yr BP) is apparent at Bural, Barnes, San Jon, and Tillman (Fig. 5). All of these sites are on or near the northern/northwestern edge of the SHP (Fig. 2) and Barnes and Tillman have the highest sedimentation rates throughout the terminal Pleistocene/earliest Holocene. This may be related to the texture of the surrounding

Table 3 Short-cell phytolith classification Photosyntheic pathway

Grass genera

Phytolith morphotypes

Xeric-adapted, warm-season (C4) short grasses (Chloridoid tribe) Mesic-adapted warm-season (C4) tall grasses (Panicoid subfamily) Cool-season (C3) adapted grasses (Pooidiaceae subfamily).

Bouteloua (grama grass), Buchloe (buffalo grass)

Saddle-shaped or Chloridoid-type

Andopogon (big-bluestem), Schizachyrium (little-bluestem), Sorgastrum (Indian grass), Panicum (switch grass) Agropyron (wheat-grass), Koeleria (june grass), Poa (blue-grass), Festuca (festucue)

Lobate, Panicoid-type, and cross-type Trapezoid-crossection (or Pooid) phytolith forms: Keeled, Conical, Pyramidal, and Crenate

V.T. Holliday et al. / Quaternary Research 70 (2008) 11–25

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Figure 4. Stratigraphic sections of dated playa fills with Holocene eolian layers (sand or loam).

Blackwater Draw Formation, which is finer in these areas (Holliday, 1989a) and also somewhat more susceptible to wind erosion (Sabin and Holliday, 1995). In contrast, relatively rapid sedimentation in the latter Holocene (between ~ 4000 and ~ 2000 14 C yr BP) is indicated at Baker, Blackburn, Buckeye #2, Bural, and Colston playas (Fig. 5). For most dated sections, rates of sedimentation since ~ 2000 14C yr BP are similar to the middle Holocene rates (Fig. 5). In the San Jon sub-basin, playa sedimentation ceased after ~ 6180 14C yr BP (Fig. 3) when the small basin was filled with eolian sediment. Roughly half of the sections with mud that span the Holocene exhibit reddening in about the middle of the fill (Table 4). This rubefication is expressed as a shift in both value and chroma in Munsell color, usually by one or two chips, away from the dark gray to black shades of the mud to brighter colors or a shift toward redder hues. There is no relationship between

the redder zone and textural variation in the fill. Moreover, in several playas the reddening is expressed as mottling within the more typical dark gray to black character of the mud. This reddening is attributed to oxidation, probably during a hiatus or slowing in mud accretion. The rubefied zones roughly correspond to relatively low rates of sedimentation (Fig. 5). At most sites with the reddened mud and radiocarbon control, the zone of rubefication was apparently buried between ~ 3500 and ~ 2000 14C yr BP (Baker, Barnes, Blackburn, Bural, Colston, Fitzgerald, Scharbauer, and Walker) (Table 2). At Buckeye #2, McFarland, and Day Spray the reddened zone apparently was buried earlier in the Holocene (~ 4000 14C yr BP, ~ 4600 14C yr BP, and ~ 7600 14C yr BP, respectively) (Table 2). Layers of reddish-brown to brown loamy sand and sandy loam were identified within or on top of the playa mud at several study sites. These deposits were probably derived from the High

Figure 5. Sedimentation rates for playa fills dominated by well-dated muds (“Randall clay”). The plot for San Jon is for the small sub-playa, which ceased filling with mud in the middle Holocene.

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Table 4 Rubefication of playa mud Facies

Playa fill: depth (cm) and color (dry)

Baker Upper playa mud 10–105 10YR 5/1 105–150 10YR 5/2 150–210 10YR 6/2 Lower playa mud 210+ Blackburn Upper playa mud 0–140 10YR 3/2

Radiocarbon means (14C yr BP) Playa fill: depth (cm) and color (dry) Radiocarbon means (14C yr BP) 1700 3320

Rubefied mud

8640

Barnes 13–102 10YR 3/1 2890 102–150 10YR 3/2 150–330 10YR 4/2 330–530 10YR 4/1

9480

2950 Rubefied mud

140–180 10YR 3/2 w/7.5YR 5/6 180–220 10YR 3/2 and 7.5YR 5/6 (50%) 220–359 7.5YR 4/4 7630

Lower playa mud 359–400 7.5YR 3/2 Buckeye #2 Upper playa mud 0–180 10YR 5/1 Rubefied mud 180–275 10YR 6/2 Lower playa mud 275–325 10YR 7/2 McFarland Upper playa mud 30–73 10YR 4/2 Rubefied mud 73–93 10YR 4/3 w/7.5YR 5/6

4830

11,800

4660 6080

Lower playa mud 93–124 10YR 4/2

Plains surface (i.e., the upper Blackwater Draw Formation), mostly via wind erosion, based on the physical similarities between the loam and the upper Blackwater Draw Formation, and the absence of any other source material (Holliday et al., 1996). Multiple layers of these deposits were identified in the main playa at the San Jon site (Holliday et al., 1996) and that chronology is now better constrained (Figs. 3, 6). The oldest eolian layer (stratum 1 s in Fig. 6) is bracketed by ages of ~11,140 and ~10,690 14C yr BP, roughly corresponding to an increase in sand content in the San Jon sub-basin (discussed below) dated between between ~11,200–10,800 14C yr BP. The oldest Holocene eolian layer is underlain by delta deposits with a base date of ~8695 14C yr BP. Above is a mud dated to ~7600 14C yr BP, in turn buried by another eolian loam ~6900–6400 14C yr BP. This eolian deposit was on-lapped by more muds from ~4200 to ~3000 14C yr BP. A surface layer of eolian loam began accumulating ~1700–1300 14 C yr BP. In the sub-basin the Randall clay was buried ~6180 14C yr BP by eolian loam ~60 cm thick, completely filling the subbasin. The loam was then subjected to pedogenesis, probably through most of the rest of the Holocene. At the Bilbrey Homestead, three layers of eolian sand were identified, the oldest resting on an A horizon in marl (Fig. 4). The two older layers of eolian sand each expressed weak Bt horizons, indicating prolonged stability between periods of sedimentation. Organic matter in the deepest Bt horizon dated ~ 5890 14C yr BP, indicating that deposition of the parent material was before that time (middle Holocene or older), and sediments for the overlying Bt horizon were deposited shortly after that time (late middle Holocene or a little younger). The surface layer of sand is unweathered and likely historical. This locality is in the Lea–Yoakum dune field (Holliday, 2001) and the eolian stratigraphy in the playa is probably a record of

Fitzgerald 23–95 10YR 3/1 95–135 10YR 4/2 135–175 7.5YR 5/4 175–205 10YR 4/2 Scharbauer 50–125 10YR 4/3 125–270 10YR 5/4 270–340 10YR 6/2

3530

6990 3440 8330 11,280

middle and late Holocene dune activation. Likewise, the Buckeye #1 playa contains a layer of eolian sand resting on an A horizon (formed in carbonate) dated ~ 6200 14C yr BP. The landscape probably stabilized shortly after deposition, indicated by development of a Bt horizon and then burial by lake mud. The sections at San Jon, Bilbrey Homestead, and Buckeye #1 provide evidence for regional eolian sedimentation ~ 6000 14C

Figure 6. Stratigraphic cross section and radiocarbon dating of the main playabasin fill at the San Jon site, New Mexico (modified from Hill et al., 1995, Fig. 3, and Holliday, 1997a, Fig. 3.51). The column of radiocarbon ages at the left are in Table 2; all other ages on the figure are discussed by Hill et al. (1995) and Holliday (1997a). The “San Jon bone bed” of Roberts (1942) was located at the top of stratum 2 m. The “bedded sand” in stratum 2s is interpreted as a delta deposit.

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yr BP, roughly corresponding to a date of ~ 5730 14C yr BP for a wedge of eolian sediment documented at the Gentry playa (Holliday et al., 1996). Cage playa #1 in the Muleshoe dunes contains a layer of eolian sand dated somewhat later, to just under ~ 4700 14C yr BP (Fig. 4). Stratified Holocene eolian loam was also identified resting on the carbonate terrace at Bailey playa (Fig. 4). Stratigraphy and geochronology very similar to that exposed at Bilbrey Homestead were observed: two layers of eolian sand, the oldest resting on an A horizon in marl buried ~ 8590 14C yr BP. The oldest layer of loam expresses a weak Btk horizon, indicating prolonged stability following deposition in the early middle Holocene, buried by the surface layer of loam also expressing a Bt horizon, indicating deposition in the late-middle or late Holocene. Delta deposits buried within playa-basin fill were examined only along exposures at the San Jon site. Well-bedded gravel, sand, and silt rest on black/gray muds dated to ~ 8600 14C yr BP (Fig. 6). Modern or Historic delta deposits were also recognized on aerial photographs of playas (Gustavson et al., 1995). San Jon sub-basin stable-C isotope and phytolith results We sampled the San Jon sub-basin for stable carbon isotope ratios and phytoliths because the chronology is relatively welldetailed and spans the late Pleistocene through Holocene; and the San Jon site has been subject to intensive archaeological excavations, and the paleoenvironmental measures can provide insights into human use of the area. Phytolith and stable C isotope records from the San Jon subbasin provide complimentary evidence for environmental change in and around the playa since N 11,500 14C yr BP (Fig. 7). The phytolith assemblage from playa sediments probably reflects in situ production and contributions from the basin margin transported via eolian, sheetwash, and possibly herbivory processes (Fredlund and Tieszen, 1994). Organic carbon content of playa soils on the SHP is generally quite low, resulting from relatively

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high rates of decomposition related to wetting and drying cycles (Luo, 1994; Smith, 2003). The δ13C values of SOM from playa basins are likely indicative of the frequency and duration of standing water in the basin. Given the very small, shallow character of the San Jon sub-basin (Table 1), the likelihood of inputs of organic matter from playa margins and uplands is minimal. In uncultivated playas on the SHP, obligate wetland plants such as Eleocharis (spike rush), a C3 plant, are most common on playa floors during wet periods, whereas Bouteloua (grama grasses) and Buchloe (buffalo grass), both C4 grasses, dominate upland settings (Haukos and Smith, 1997; Smith, 2003). Transitional areas along the margin of playa floors show mixing of the upland grasses and rushes. During periods of little standing water in playa basins, upland grasses expand basin-wards, and may comprise a major component of playa floor vegetation. These modern relationships serve as analogues to aid in interpreting the stable C isotope and phytolith records. Phytolith percentages and δ13C values of SOM are in good agreement throughout the entire record (Fig. 7). Mesic-adapted tall grass C4 percentages are all b 15%, and mostly b 10%; our discussion focuses on C3 and xeric-adapted short C4 grasses. Sometime before ~ 11,500 14C yr BP, δ13C values increase from − 23‰ to − 19‰, while C3-type short-cells decrease from 60% to 40%. Between 11,500–10,800 14C yr BP, δ13C values of SOM are between − 20‰ and − 19‰ (~ 50–60% C4 contributions to SOM), while C4-type short-cell phytoliths increase from ~ 40% to over 60%, remarkably similar to C4 contributions to SOM calculated using δ13C values. We interpret the gradual increase in C4-type short-cell phytoliths beginning N11,500 14C yr BP (N13,300 cal yr BP) until just prior to ~ 10,800 14C yr BP (~ 12,800 cal yr BP) to indicate gradual warming coinciding with the later part of the Bølling/Allerød warm period in the North Atlantic (Alley et al., 1993). The δ13C values range from − 20‰ to − 19‰, indicating approximately equal contributions of C3 and C4 plants to SOM and interpreted to reflect only seasonal inundation of the playa. Within the apparent warming trend, however C4-type phytolith content decreases and δ13C

Figure 7. Plot of physical and chemical data and radiocarbon ages for the section in the San Jon sub-playa.

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values decrease (from −19‰ to − 21‰) between ~ 11,400– 11,200 14C yr BP (~ 13,200–13,100 cal yr BP) (Fig. 7), suggesting a brief cool period overlapping with the Inter-Allerod cooling centered on 13,200 cal yr BP (Taylor et al., 1993). What these data suggest about water levels or inundation of the playa floor is unclear, however. Between ~ 11,200 and ~ 10,800 14C yr BP, C4-type phytoliths increase from less than 50% to about 60%, considered to reflect a return to the warming trend recognized prior to 11,200 14C yr BP (~ 13,100 cal yr BP). Between ~ 10,800 and ~ 8130 14C yr BP (~ 12,800–9000 14C yr BP), δ13C values increase from − 20‰ to − 17‰, while C4-type phytoliths gradually increase to about 63%, likely representing a decrease in duration of playa inundation associated with drying during the latest Pleistocene and early Holocene. Together, the phytolith and stable C isotope records indicate relatively unidirectional warming and drying beginning just prior to ~ 8130 14C yr BP (~ 9000 cal yr BP) and continuing until after ~ 6180 14C yr BP (~ 7100 cal yr BP), with δ13C values gradually increasing from − 20‰ to − 14‰. While slightly more conservative, the C4-type short-cell phytoliths also show a general increase throughout most the Holocene to values of ~ 70% at present. The δ13C values, however, decrease abruptly by nearly 5‰ sometime in the latest Holocene, corresponding to a 30% decrease in C4 contributions to SOM. C3-type short-cell phytoliths show no concomitant increase during the late Holocene, suggesting a change in something other than the grass component of the vegetation around the playa basin. One possible explanation for the abrupt decrease in δ13C values is the expansion of juniper (Juniperus spp.). During the late Holocene, and possibly as recently as b1375 14C yr BP (1300 cal yr BP), substantial dissection and canyon-cutting occurred in the San Jon basin as a result of a breach in the High Plains escarpment, ultimately resulting in the extensive modern gully system. Incision and canyon-cutting in the main San Jon basin may have facilitated expansion of juniper from the High Plains Escarpment, where it is common. Today, juniper occurs on steep hillslopes adjacent to gullies in the San Jon basin but is rare on the undissected High Plains surface. The particle-size record of the playa fill shows three noticeable changes in sand content (Fig. 7): prior to ~ 11,500 14C yr BP (~ 13,300 cal yr BP); between ~ 11,200–10,800 14C yr BP (~ 13,100–12,800 cal yr BP); and after ~ 6180 14C yr BP (~ 7100 cal yr BP). The increase in sand prior to ~ 11,500 14C yr BP (~ 13,300 cal yr BP) between about 3.75–4.00 m in depth marks the Blackwater Draw Formation–playa mud contact. Sand percentages of the playa fill are greatest between ~ 11,200 and ~ 10,800 14C yr BP (~ 13,100 and ~ 12,800 cal yr BP; Fig. 7), locally corresponding to eolian sedimentation in the main playa basin (stratum 1s in Fig. 6) bracketed by ages of ~ 11,140 and ~ 10,690 14C yr BP, discussed above, regionally corresponding with the onset of widespread eolian activity on the SHP, interpreted as drying (Holliday, 2000, 2001), and globally overlapping with the abrupt onset of the Younger Dryas in the North Atlantic at 12.9 ka (Alley et al., 1993; Taylor et al., 1993). Rates of playa infilling are also highest during this interval (Fig. 7). The increase in sand in the upper 0.75 m of the section sometime after ~ 6180 14C yr BP (~ 7100 cal yr BP)

probably reflects middle Holocene eolian activity, as noted in our other playa sections, discussed above, and as recognized in other stratigraphic records on the SHP (Holliday, 1989a, 2001). Organic carbon and carbonate content are both relatively low throughout the entire record; however, the increase in carbonate between 1.25–1.50 m is likely reflecting secondary carbonates related to the modern soil (Fig. 7). Discussion and conclusions The stratigraphy and geochronology of playa fills on the SHP provide clues to late Quaternary landscape evolution and environmental change, complementing and building on previously reported studies. The stratigraphy observed in this study essentially reproduced that reported earlier (Holliday et al., 1996) and no sections were observed that contradicted the previous conclusions that most of the basins are erosional features. The playa basins formed in the final millennia of the late Pleistocene, based on dates from the upper gleyed clay and carbonate, and the basal dates from the clean eolian sand and dark gray mud. Precise dates on basin formation are not available, but most apparently formed between 20,000 and 10,000 14 C yr BP. The early stages of basin filling are better dated. Dates from the clean eolian sand beneath the dark gray mud or the base of the mud itself bracket the onset of filling between 14,000 and 10,000 14C yr BP; and between 12,000 and 10,500 14 C yr BP in most dated basins (Table 1). The common occurrence of layers of clean, well-sorted eolian sand at the basal contact provides additional, albeit indirect, evidence of the significance of eolian processes in the development of the basins. These layers are considered eolian because the sand is well sorted and the sand grains are free of coatings, and because it forms a discrete lens across the bottom of many basins. No other geomorphic process can account for these characteristics. Evidence for extensive eolian sand deposits or even minor eolian sands dating to this period are unknown, however (Holliday, 2001). The sand may represent a sort of lag created as the basins were deflated. Wind erosion of the High Plains surface (i.e., the surface of the Blackwater Draw Formation) results in formation of sheets of clean eolian sand as clay and silt coatings on sand grains are abraded from the surface of quartz grains (Gillette and Walker, 1977; Holliday, 1987). In the evolution of a playa basin, perhaps some of the sand remained on the floor as the basin deepened. Wind erosion as a process for basin formation in the region implies regional aridity, loss of vegetation cover, and a lowered water table (below the level of the deepest part of each basin) in the late Pleistocene. Corroborating evidence for such conditions is found in the dated stratigraphic record from lunettes of the region (Holliday, 1997b). From ~ 15,000 to ~ 10,000 14C yr BP the lunettes went through a major phase of construction via deflation of adjacent playa basins. Filling of the basins with the ubiquitous dark gray mud began in the last few millennia of the Pleistocene and continued through the Holocene. The onset of this change in depositional environments (from basin formation via erosion to basin filling)

V.T. Holliday et al. / Quaternary Research 70 (2008) 11–25

apparently was abrupt based on the sharp lower boundary of the dark gray mud with all underlying substrate. The dark gray to black color of the mud suggests that the playa floors had relatively high biomass as they aggraded, a dense vegetation cover acting to trap dust. A return to more moist conditions in the region would raise the water table and produce wetter conditions on the playa floor. The sharp boundary between the eolian sand and overlying gray clay does not necessarily imply an abrupt change in climate, however. Even a gradual rise in water table would abruptly change the floor of a playa from dry to moist conditions, quickly changing the nature of the vegetation cover. Both the rates and nature of sedimentation in the playa basins varied as they filled through the Holocene, at least in part due to changing environmental conditions. In some localities, the dark gray mud accreted more quickly in the early Holocene and in other basins the filling was more rapid in the late Holocene. In yet other basins, slower rates are typical for these periods, but all playas exhibited relatively slow rates of sedimentation in the middle Holocene (Fig. 5). Further, the middle Holocene fill in approximately half of the basins is somewhat oxidized. Oxidation must represent well-drained playa floors with relatively low water tables. A lowered water table in playas during the middle Holocene correlates with an array of other evidence for dry middle Holocene “Altithermal” conditions in the region (Johnson and Holliday, 1986; Holliday, 1989b, 1997b, 1995, 2001; Meltzer, 1991, 1999). Layers of loamy sand deposited in some basins in the middle Holocene provide additional evidence for middle Holocene aridity. These deposits are not present in all or even most playas and the dates range considerably through the middle Holocene, based on the radiocarbon ages of immediately underlying muds. The main basin at San Jon records eolian sedimentation ~ 8700–7600 14C yr BP and b 7600–6400 14C yr BP. Four sites (Buckeye #1, Bilbrey, Gentry, and San Jon sub-basin) have a layer of eolian sandy loam dating around or just after ~ 6000 14C yr BP. Cage, in the Muleshoe Dunes, has an eolian layer dating just under ~ 4700 14C yr BP. Episodes of more rapid sedimentation in the playas (terminal Pleistocene/early Holocene and between ~ 4000 and ~ 2000 14C yr BP) appear to represent wetter playa conditions. This is most obvious in the late Holocene sediments that overly the rubefied zones. The lack of oxidation indicates more poorly drained conditions. Moreover, during moist periods, more dense vegetation on the playa floor probably trapped more eolian sediment. More open vegetation on dry and shallow playa floors would allow sediment falling into the playa (dust) to be deflated out of the basin. These sedimentological indicators of wetter vs. dryer conditions in the playa basins generally correlate well with the sedimentological records from draws and dune fields on the SHP (Holliday, 1995, 2001). Proxy records from the San Jon sub-basin also agree well with regional records of late Quaternary environmental change, which indicate more effective moisture during the latest Pleistocene and gradual warming associated with drying during the Holocene (Johnson, 1986; Johnson and Holliday, 1986, 2004; Holliday, 1995, 2000, 2001). The stable C isotope record indicates that the

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playa basin was probably only seasonally inundated during the latest Pleistocene and Holocene, while the phytolith record indicates increasing importance of xeric-adapted C4 grasses since at least ~ 11,500 14C yr BP (~13,300 cal yr BP). Abrupt shifts in both the phytolith and stable isotope records between ~ 11,500– 10,800 14C yr BP (~13,300–12,800 cal yr BP) are particularly noteworthy because they may reflect changes in the playa environment during such climatic episodes as the Bolling/Allerod (14,700–12,900 cal yr BP), the Inter-Allerod Cold Period (~13,200 cal yr BP), and the Younger Dryas (12,900–11,700 cal yr BP) recognized in North Atlantic records (Alley et al., 1993; Taylor et al., 1993) as well as in various paleoenvironmental records in the Great Plains (e.g., Muhs et al., 1999; Johnson and Willey, 2000; Nordt et al., 2007). The sedimentological record and rates of infilling indicate that environmental changes during at least part of this interval resulted in a relatively unstable landscape around the playa basin. In particular, both the subbasin and the main playa basin at San Jon record eolian sedimentation roughly corresponding to the early Younger Dryas, coincident with regional records of eolian sedimentation and probable aridity (Holliday, 2000). Infilling rates decreased during the early Holocene, apparently associated with unidirectional warming and drying beginning shortly before ~8130 14C yr BP (9000 cal yr BP). This trend culminated in the complete infilling of the basin shortly after ~ 6180 14C yr BP, at which time the playa became inactive and formation of an upland soil in eolian sediments commenced. The sub-basin records correspond well with millennial-scale trends documented in various proxy environmental records across the SHP, but may also record regional changes affecting playa basins on shorter time scales. Comparing the San Jon sub-basin records with those of other playas will be a focus of future work. The playa basins of the SHP have essentially continuous records of sedimentation spanning the past 10,000 to 15,000 14 C yr BP, complementing the more discontinuous stratigraphic records found in the dry valleys and dune systems of the region. The more or less homogeneous nature of the playa fill, however, does not allow the sorts of paleoenvironmental reconstructions possible from, for example, the contrasting alluvial, lacustrine, and eolian sediments in the dry valleys. Our limited paleobiological and isotopic data nevertheless show that the playa basins have excellent potential for insights into regional trends in Holocene and terminal Pleistocene vegetation and climate. Acknowledgments This research was supported by the National Science Foundation: EAR-9807347 (VTH), EAR-9710099 (GGF), an IGERT Fellowship (JHM), and ATM-9809285 (University of Colorado INSTAAR-Laboratory for AMS Radiocarbon Preparation and Research). The Geological Society of America provided support via a Gladys Cole Memorial Research Award (VTH), and a Graduate Student Research Grant and a Farouk El-Baz Student Award for Desert Research (JHM). The University of Arizona Department of Geosciences provided Burt S. Butler and Maxwell N. Short scholarships (JHM). The University of Arizona NSF-AMS Laboratory kindly provided training in sample

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