Origin of late Quaternary dune ﬁelds on the Southern High ......MUHS and HOLLIDAY Figure 1. Map...

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GSA Bulletin; January 2001; v. 113; no. 1; p. 75–87; 19 figures; Data Repository item 2001013.

Origin of late Quaternary dune fields on the Southern High Plains ofTexas and New Mexico

Daniel R. Muhs*U.S. Geological Survey, M.S. 980, Box 25046, Federal Center, Denver, Colorado 80225, USA

Vance T. HollidayDepartment of Geography, 384 Science Hall, University of Wisconsin, 550 North Park Street, Madison, Wisconsin53706-1491, USA

ABSTRACT

Mostly stabilized late Holocene eoliansands on the Southern High Plains of theUnited States were studied to determinetheir origins and to assess whether presentdune stability depends more strongly onsediment supply, sediment availability, ortransport limitations. Geomorphic, sedi-mentological, and geochemical trends indi-cate that late Holocene dunes formed underwesterly paleowinds, broadly similar tothose of today. Mineralogical and geochem-ical data indicate that the most likely sourcefor the sands is not the Pecos River valley,but the Pleistocene Blackwater Draw For-mation, an older, extensive eolian deposit inthe region. These observations suggest thatnew sand is supplied whenever vegetationcover is diminished to the extent that theBlackwater Draw Formation can be erod-ed, in agreement with modern observationsof wind erosion in the region. We conclude,therefore, that Southern High Plains dunesare stabilized primarily due to a vegetationcover. The dunes are thus sediment-avail-ability limited. This conclusion is consistentwith the observation that, in the warmest,driest part of the region (where vegetationcover is minimal), dunes are currently ac-tive over a large area. Geochemical data in-dicate that Southern High Plains dunes arethe most mineralogically mature (quartzrich) sands yet studied in the Great Plains,which suggests a long history of eolian ac-tivity, either in the dune fields or duringdeposition of the Blackwater DrawFormation.

Keywords: Blackwater Draw Formation,

*E-mail: [email protected].

dunes, geochemistry, New Mexico, PecosRiver, provenance, Texas.

INTRODUCTION

Stabilized eolian sand is extensive on theGreat Plains of North America (Fig. 1). Inter-est in eolian sand in this region has increasedin recent years, based upon the recognition oflate Holocene and historic activity of thesedune fields (Ahlbrandt et al., 1983; Swinehart,1990; Swinehart and Diffendal, 1990; Formanand Maat, 1990; Forman et al., 1995; Madole,1994, 1995; Loope et al., 1995; Holliday,1995a, 1997a, 2000; Wolfe et al., 1995, 2000;Arbogast, 1996; Muhs and Holliday, 1995;Muhs et al., 1996, 1997a, 1997b; Stokes andSwinehart, 1997). This recent activity is sig-nificant, because it means that prospects arehigh for reactivation of many dunes due tofuture climate or land-use changes (Muhs andMaat, 1993), with possibly severe regionaleconomic impacts. Critical to any forecast offuture eolian activity, however, is understand-ing whether a dune field is stable because itis limited by (1) sediment supply, (2) sedimentavailability, or (3) transport capacity. Kocurekand Lancaster (1999) defined sediment supplyas the sediment of suitable grain size that isthe source material for eolian sand. The sourcematerial can be external to a dune field, suchas sand derived from a coastal or fluvial sys-tem, or internal, derived from interdune sedi-ments or cannibalization of previously stabi-lized dunes. Sediment availability, in contrast,is the susceptibility of surface grains to en-trainment by the wind. Kocurek and Lancaster(1999) pointed out that sediment availabilitycan be restricted by raising the threshold value(by addition of moisture, surface binding byclays, or cementation by carbonate or silica),

or increasing the surface roughness (by veg-etation cover or a cover of coarse clasts).Transport capacity of the wind is defined asits sediment-carrying capacity, which varies asa cubic function of friction velocity (Kocurekand Lancaster, 1999). Given these three pos-sible limitations on eolian activity, assessingthe potential for reactivation of a dune fieldrequires identification of source sediments.The diversity of source sediments for GreatPlains dunes has only recently emerged, in-cluding playa sediments (Holliday, 1997a),fluvial sediments (Muhs et al., 1996), glacio-lacustrine and glaciofluvial deposits (Wolfe etal., 1995; Muhs et al., 1997b), and possiblyresiduum from bedrock (Muhs et al., 1996,1997a).

The Southern High Plains, or Llano Esta-cado, has some of the most extensive dunefields in the Great Plains (Figs. 1 and 2). Eo-lian sand thicker than 2 m in the region maycover more than 10 000 km2. Both well-formed dunes and low-angle eolian sandsheets (as defined by Fryberger et al., 1979)are present. Three major east-west–trendingdune belts (Muleshoe, Lea-Yoakum, and Mon-ahans-Andrews) and one north-south–trendingdune belt (Mescalero) cover large areas of theLlano Estacado and parts of the Pecos Rivervalley to the west (Figs. 3, 4, and 5). Muhsand Holliday (1995) pointed out that dunes onthe Southern High Plains may be near the crit-ical climatic limits of stability, because thenorthern dune fields (Muleshoe, Mescalero,and Lea-Yoakum) are largely stable, whereas;300 km2 of the southernmost dune field(Monahans) is active. Therefore, if the dunesare transport limited or sediment-availabilitylimited, only a small shift in moisture balance,wind strength, or intensity of land use mightbe sufficient to activate them. However, if the

76 Geological Society of America Bulletin, January 2001

MUHS and HOLLIDAY

Figure 1. Map showing the location of eolian sand in the central United States, locationof the Southern High Plains study area, and other dune fields referred to in the text andfigures. Eolian sand distribution is mostly from compilation by Muhs and Holliday (1995),except that shown in eastern states, which is from Thorp and Smith (1952).

dunes are supply limited, then future climatechanges may be of lesser consequence to duneactivation.

Although several investigators have specu-lated on the origin of the dunes, there has beenno systematic effort to link the composition ofSouthern High Plains dunes to potentialsource sediments. In a summary of manyyears of Great Plains eolian research, Lugn(1968) stated categorically that the Pliocene–Miocene Ogallala Formation was the mainsource of Pleistocene and Holocene eoliansand for the entire Great Plains region, fromNebraska to Texas. Although the Ogallala For-mation is extensive under the southern GreatPlains, it has a well-cemented pedogenic cal-crete in its upper part and is usually overlainby the Pleistocene Blackwater Draw Forma-tion (Holliday, 1990; Gustavson and Holliday,1999). Thus, we do not consider the OgallalaFormation to be a very likely source forSouthern High Plains dune sands. Green(1961), Hawley et al. (1976), and Carlisle andMarrs (1982) suggested that the dune fields ofthe Southern High Plains owed their originsto funneling of winds through reentrants in theHigh Plains escarpment. Although these work-ers did not explicitly state it, this model im-plies that sediments of the Pecos River valley

were the sources of eolian sand. Conversely,Hefley and Sidwell (1945) proposed that eo-lian sands in the Muleshoe dune field werederived from underlying Tertiary sands, whichwe interpret to mean the Blackwater DrawFormation. The two possible sources havevastly different implications with regard to po-tential future dune reactivation. If the PecosRiver valley is the source of eolian sands, thendune mobility may be linked to changes in thefluvial regime as they apply to sediment sup-ply (e.g., following the model of Muhs andHolliday, 1995). However, if the BlackwaterDraw Formation is the source, then climate orland-use effects (see effects of agriculture inFig. 6) alone, through a decrease in vegetationcover (and therefore increase in sedimentavailability), could bring about dune mobility.In this paper we present sedimentological,mineralogical, and geochemical data that shednew light on the origin of these dune fields.

METHODS

Samples were collected mostly from sanddunes and sand sheets that are stabilized byvegetation and are estimated to be no olderthan late Holocene. This age estimate is basedon the presence of A-AC-C soil profiles that

are characteristic of eolian sand of late Holo-cene age (see Holliday, 1995a, 2000). The ex-ceptions to this sampling scheme are in theMonahans dune field, where many dunes areactive today. Thus, herein, ‘‘eolian sand’’ re-fers to either stabilized but young eolian sandor active eolian sand. Samples were collectedwell below the zone of pedogenesis in orderto avoid effects of weathering and soil for-mation at the site of collection. Particle-sizedistribution of selected eolian sands was de-termined by sieving (for the sand-sized frac-tions) at 0.5 phi intervals after destruction oforganic matter with H2O2 and dispersion withNa-pyrophosphate. Silt and clay contents weredetermined by the sedimentation and pipettemethod. Graphic mean and standard deviation(as defined by Folk, 1974) were calculated forall samples. Concentrations of certain majorand trace elements were measured in the eo-lian sands and potential source sediments us-ing energy-dispersive X-ray fluorescence.Semiquantitative mineralogical analyses weremade using X-ray diffraction methods. Forgeochemistry and mineralogy, samples of eo-lian deposits analyzed included all size frac-tions. Valid mineralogical and geochemicalcomparisons to eolian sands can only bemade, however, on source sediments that areof the same size as those in eolian sands. Forgeochemical and mineralogical analyses ofpotential source sediments (the Pecos Riverand the Blackwater Draw Formation), only thevery fine, fine, and most of the medium sandfractions (53–425 mm) were analyzed. Thesesize fractions cover the bulk of sizes found ineolian sands. Gravels were removed by drysieving. Carbonates were removed with anNa-acetate buffer (pH 5 5), after which thesamples were treated with an Na-pyrophos-phate dispersant. Sands were separated fromsilts and clays by wet sieving; sand fractionswere then concentrated by dry sieving.

REGIONAL CLIMATIC SETTING

The Southern High Plains region has a con-tinental climate and is semiarid. Mean annualprecipitation and mean annual temperaturesboth show northeast-southwest gradients. Thenorthern part of the region is both cooler andwetter; mean annual precipitation at Amarillois almost 500 mm and mean July and January(1961–1990) temperatures are 26 8C and 1.78C, respectively. In contrast, Pecos receivesonly ;280 mm of precipitation per year andmean July and January temperatures are 29 8Cand 6.4 8C, respectively. As a consequence ofthese moisture and temperature gradients, ra-tios of precipitation to potential evaporation

Geological Society of America Bulletin, January 2001 77

ORIGIN OF LATE QUATERNARY DUNE FIELDS

Figure 2. Map showing the location of eolian sand in the Southern High Plains and ad-jacent areas, redrawn from compilation by Muhs and Holliday (1995). Also shown areannual resultant drift potentials (RDP) and resultant drift directions (RDD) that showpotential net sand migration directions. For clarity, we omit the multidirectional armsthat are often shown in sand roses and present only RDP (length of the arrow) and RDD(direction of the arrow).

Figure 3. Map of the Clovis 18 3 28 quadrangle and adjacent areas showing the extent of the Muleshoe dune field, sample localities,and late Holocene paleowinds based on parabolic dune orientations.

range from ;0.60 in the northern part of theregion to ;0.30 in the southern part. Muhsand Holliday (1995) pointed out that climati-cally, the southernmost part of the GreatPlains (including the Monahans dune field) ispart of the Chihuahuan Desert. Over the entireregion, maximum precipitation occurs be-tween May and September.

Wind regimes, as they apply to the forma-tion of sand dunes and sand sheets, can becharacterized in terms of sand-moving poten-tial. Fryberger and Dean (1979) developed amethod for graphic representation of sand-moving potential called sand roses, which arecircular histograms showing the weightedmagnitudes and directional variability ofwinds for a given station. The arms in a sandrose are weighted sums of the amount of timethat the wind is above the threshold velocityfor sand from a given direction. Higher veloc-ity winds are weighted higher in the summa-tion because the sand-moving capacity ofwind is a function of the cube of wind speed.Fryberger and Dean (1979) defined several pa-rameters from sand rose data: drift potential,which is the scalar sum of all sand-movingwinds, regardless of direction; resultant driftpotential, which is the vector sum of all sand-moving winds and will always be less than orequal to drift potential; and resultant drift di-rection, which is the net direction of sandmovement. Good agreement between resultant


MUHS and HOLLIDAY

Figure 4. Map of the Brownfield 18 3 28 quadrangle and adjacent areas showing theextent of the Lea-Yoakum dune field, part of the Mescalero dune field, adjacent sheetsands, and sample localities.

drift direction values and dune orientationsfrom many parts of the world suggest that itis a valuable parameter in studying duneforms and their relations to winds (Frybergerand Dean, 1979; Breed et al., 1979; Ahlbrandtand Fryberger, 1980; Muhs, 1985; Lancaster,1989; Wells et al., 1990; Muhs et al., 1996,1997a, 1997b, 2000).

We were able to obtain climate records ofsufficient detail to construct sand roses fornine localities in the Southern High Plains(Fig. 2). Although Lubbock shows a resultantdrift direction from west to east and Midlandshows a resultant drift direction from south tonorth, the annual resultant drift direction forthe region is dominantly from southwest tonortheast. The Southern High Plains have re-sultant drift potential values that range from47 to 318, with a mean value of about 155 (n5 9). Resultant drift potential values are gen-erally higher (155–318) in the New Mexicoportion of the region. Elsewhere in the GreatPlains, resultant drift potential values aresomewhat higher, such as in southeastern Col-orado and southwestern Kansas (range of 39–762, with a mean of 241) and Nebraska (rangeof 97–284, with a mean of 211) (Muhs et al.,2000, and unpublished data; Arbogast andMuhs, 2000). However, Southern High Plainsresultant drift potential values are as high asthose found in many of the world’s desert re-gions where there are large sand seas (cf. Fry-berger and Dean, 1979; Breed et al., 1979).Relatively high resultant drift potential values

for the Southern High Plains are significant inassessing possible dune reactivation because itmeans that wind is not a limiting factor. Re-activation of dunes is likely to be more sen-sitive to changes in sediment supply, sedimentavailability, degree of vegetation cover, andland use.

GEOMORPHOLOGY ANDSEDIMENTOLOGY

Geomorphology

As discussed in Holliday (2000), latestPleistocene sheet sands and late Holocenedune sands (which overlie the older sheetsands) are the most extensive deposits inSouthern High Plains dune fields. Similar tothe central Great Plains of Colorado and Ne-braska (Swinehart and Diffendal, 1990; Ma-dole, 1994, 1995; Muhs et al., 1996, 1997a),there are very few mid-Holocene eolian sandspreserved in the Southern High Plains otherthan those deposited in draws and lunettes(Holliday, 1995b, 1997a, 1997b). Most of thefollowing discussion regarding the geomor-phology of the dune fields refers to late Ho-locene dunes, as described by Holliday(2000).

A wide variety of eolian landforms existson the Southern High Plains (see Holliday,2000, for detailed descriptions). In the Mule-shoe dune field (Fig. 3), low-relief eolian sandsheets and both simple and compound para-

bolic dunes occur (Fig. 6). The simple para-bolic dunes are commonly 200–400 m long,similar to parabolic dunes found in the centralGreat Plains region of Colorado (Muhs, 1985;Madole, 1994, 1995; Muhs et al., 1996). Dunemorphology is less well expressed in the Lea-Yoakum dunes (Fig. 4), but highly degradedcompound parabolic dunes to ;2–3 km longcan be found in places, adjacent to very lowrelief sand sheets and abundant shinnery oak–(Quercus harvardii)–covered coppice dunes.The Lea-Yoakum dune field also has abundantfence-line dunes, some to 3–4 m high, thatdeveloped in historic time. In the Mescalerodune field (Figs. 4 and 5), there are smallfields of stabilized parabolic dunes, ;400–800 m long, as well as one small area of par-tially active northwest-trending barchanoidridges ;200–800 m long. In other areas of theMescalero dune field, we observed mostlysand sheets and low (1–3 m high) coppicedunes. The Monahans dune field, and a north-east-trending extension of it called the An-drews dune field (Fig. 5), have the greatestvariety of eolian bedforms on the SouthernHigh Plains, and also show the greatest degreeof activity. Fully active dunes are found inmany parts of the Monahans dune field, andinclude both barchanoid ridges as long as1000 m and parabolic dunes (Fig. 7). Thewestern portion of the Monahans dune fieldhas stable or partially active parabolic dunesand sand sheets.

In the areas surrounding the Monahansdune field, and to the south of the Lea-Yoak-um dune field, there are extensive areas thatwere mapped previously as eolian sand (Eifler,1976; Eifler and Reeves, 1974, 1976), but thatdo not show distinctive dune morphology onaerial photographs. We examined these depos-its and found that they include small coppicedunes, thin continuous sand sheets, and thindiscontinuous sand sheets, almost all of whichare underlain by calcrete of the Ogallala For-mation. We map these deposits as eolian sheetsand in Figures 4 and 5.

Dune orientations were measured in detailin the Muleshoe dune field and on a recon-naissance basis in the other dune fields. In theMuleshoe dune field, measurement of the longaxes (based on dune arms) and nose orienta-tions of ;180 stabilized parabolic dunesshows a relatively narrow range, with nosespointing from N798E to E48S and an averageof almost due east, indicating paleowindsdominantly from the west (Fig. 3). At Clovis,the nearest locality for which we have winddata, annual resultant drift direction is fromthe southwest, but winter resultant drift direc-tions are from the northwest and summer re-



Figure 5. Map of parts of the Hobbs and Pecos 18 3 28 sheets showing the Monahans-Andrews dune fields, part of the Mescalero dune field, adjacent sheet sands, and samplelocalities.

sultant drift directions are from the southeast.The parabolic dune orientations in the Mu-leshoe field show the best agreement with thewinter and spring resultant drift directions,which is also the time when vegetation covermay be at a minimum. It is possible, however,that the dunes may have been modified slight-ly by summer winds when the dunes were ac-tive. Alternatively, late Holocene paleowindannual resultant drift directions may havebeen more consistently from the west at thetimes of dune formation. In the Lea-Yoakumand Mescalero fields, there are fewer parabolicdunes and many dunes are not well expressedgeomorphically. Interpretation of aerial pho-tographs shows, however, that most stabilizeddunes are oriented similarly to those in theMuleshoe field, with arms pointing to the westand noses pointing to the east, indicating dom-inantly westerly winds.

The active dunes of the Monahans fieldshow the most complex orientations of the re-gion. The geomorphology of the dunes in thisregion reflects seasonally variable wind direc-tions. On the basis of climate data for Mid-land, Wink, and Hobbs, winds are from thesouthwest, west, or northwest from Novemberthrough April, and from the south or southeastfrom May through October. On the basis ofexamination of aerial photographs taken inNovember 1984 (Fig. 7), active barchanoidridges have slip faces that dip to the north-west, which reflect the summer wind pattern.In contrast, active and stabilized parabolicdunes have arms that point to the northwest,indicating formation under predominantlynorthwesterly, winter winds. Machenberg(1984) pointed out that the bimodal wind di-rections in the Monahans dune field result inlittle annual net migration in the active portionof the dunes.

Sedimentology

Sediments of the Muleshoe dune field haveconsiderable spatial variability of color, car-bonate content, mean particle size (in the sandfraction), and clay content. Dune color varieswithin the Muleshoe field in two ways. Somedunes have predominantly reddish (5YR and7.5YR) hues and others have mostly light grayor light brown colors (e.g., 10YR 6/2, 10YR6/3, 10YR 6/4, and 10YR 7/2, all dry colors).For convenience, we refer to these as pink andwhite dunes, respectively. Mineralogical andchemical data (discussed in the following sec-tions) indicate that the white dunes containvarying amounts of carbonates, whereas thepink dunes are largely carbonate free. A moresystematic spatial variation in dune color, seen

within the pink dunes, is decreased redness ina downwind direction (Fig. 8). In the westernpart of the field, dunes have mostly 5YR hues,whereas dunes farther downwind have 7.5YRhues and those on the periphery of the easternhalf of the dune field have 10YR hues. Thistrend is contrary to observations made in otherregions that dunes become redder in a down-wind direction (Walker, 1979; Gardner andPye, 1981; Lancaster, 1989; Pye and Tsoar,1990). However, particle size data shed lighton this apparent contradiction, which we dis-cuss later.

Late Holocene sands in the Muleshoe dune

field are mostly fine sands and are generallymoderately well sorted (Fig. 9). In this regard,Muleshoe sands are close to average valuesfor eolian sands for many of the world’s dunefields (Ahlbrandt, 1979). Compared to dunefields elsewhere in the Great Plains, the Mu-leshoe sands are similar to those in the Ne-braska Sand Hills, but finer and better sortedthan sands in northeastern Colorado (Fig. 9).Mean particle size decreases with distance eastof the Pecos River, which, based on modernwinds and dune orientations, is the downwinddirection. The eastward decrease in mean par-ticle size is a function of expected winnowing


MUHS and HOLLIDAY

Figure 6. Aerial photograph of a portion of the Muleshoe dune field, showing morphologyof parabolic dunes and sand sheets.

of coarse particles downwind, although thisexplains only about half of the variation (Fig.10). Downwind distance, however, explains66% of the variation in clay content, whichalso decreases in an eastward direction (Fig.10). Both a downwind decrease in mean par-ticle size of sand and clay content would seeminitially to be contradictory trends. However,Gillette and Walker (1977) showed that eoliansands in the Southern High Plains have abun-dant clay mineral coatings. These coatings arevery fragile and are easily abraded away dur-ing eolian transport. Therefore, a greater dis-tance of travel would result in more abrasionof the sand-sized particles as well as a loss inclay. This trend may also explain the eastwarddecrease in sediment reddening in the Mu-leshoe dune field, in a process described byGillette and Walker (1977) and Walker (1979).The 5YR hues in sands in the western part ofthe dune field may be due to a greater amountof iron oxides and clay coatings, and thedownwind decreased in redness reflects theabrasion of this pigment. These observationssuggest that the source for the Muleshoedunes is a sediment that is high in sand butcontains at least some clay.

MINERALOGY

Bulk mineralogical analyses indicate thateolian sands in the Muleshoe dune field aredominated by quartz (Fig. 11). Both K-feld-spar and plagioclase were detected in somesamples, but the quantities of these mineralsare extremely low. These observations agreewith those of Gile (1985), who reported thatsoils developed in eolian sands in the Mulesh-oe dune field are dominated by quartz in theirsand fractions. The Muleshoe dunes may havesome of the highest quartz contents (relativeto feldspars) of any Great Plains dune fields.Quartz contents are much higher in the Mu-leshoe dunes than in dunes of northeasternColorado and even those of the NebraskaSand Hills region, which has been regarded asa mineralogically mature dune field (Muhs etal., 1997a).

The relative amount of quartz vs. feldsparin the Muleshoe dunes is similar to the twomost likely source sediments (Fig. 11). PecosRiver sediments are relatively high in quartz,but have slightly more plagioclase and K-feld-spar than the Muleshoe dunes. However, thereis virtually no difference in the relative abun-dances of quartz, K-feldspar, and plagioclasein Muleshoe sands and sands derived from theBlackwater Draw Formation.

GEOCHEMISTRY

Bulk geochemical analyses1 reflect the min-eralogical differences of the pink and whitesands in the Muleshoe dune field. The maindifference between these two eolian sands isthe slightly higher amount of carbonate in thewhite dunes, and this difference is reflected invariable, but generally higher, Ca and Sr con-tents (Fig. 12). The relative amounts of quartz,K-feldspar, and plagioclase in the two dunetypes are similar, and this is reflected in thesimilar abundances of K and Rb (found in K-feldspar). The K/Rb values in the two dunetypes do not show a significant difference,which suggests that the noncarbonate mineralsin the two dune types have a common origin.This interpretation is supported by the similarconcentrations of Ti and Zr in both dunetypes, which reflect the abundances of heavyminerals such as ilmenite, anatase, rutile, ti-tanomagnetite, and sphene (Ti) and zircon(Zr). The Ti/Zr value for both groups is sim-ilar, suggesting a common source for theheavy mineral fraction.

Heavy mineral fractionation during eoliantransport is reflected in the declining down-

1GSA Data Repository item 2001013, geochem-ical data for eolian sands and source sediments, isavailable on the Web at http://www.geosociety.org/pubs/ft2001.htm. Requests may also be sent to Doc-uments Secretary, GSA, P.O. Box 9140, Boulder,CO 80301; e-mail: [email protected].

wind concentrations of Fe, Ti, and Zr (Fig.13). Plots shown are for pink dunes only andprobably reflect winnowing of heavy mineralsin the same way that coarse particles are win-nowed with distance from a source. These ob-servations reinforce the interpretation that thedominant dune-forming winds have been fromthe west, at least during the late Holocene.

In evaluating potential source sediments foreolian sands, it is important, when using bi-variate element plots, to use elements that notonly have different concentrations in the com-peting sources, but also have different ele-mental ratios. This approach helps avoid aninterpretation of apparently different sourcesthat are due simply to dilution by quartz. Weevaluated four complementary element pairsin this manner for Pecos River sediments andthe Blackwater Draw Formation (Figs. 14–17). Much of the difference in element con-centrations in the two sediment groups is afunction of the amount of quartz, which hashigh concentrations in the Blackwater DrawFormation (Holliday, 1989). Both Ca and Sr,found in Ca-bearing minerals, have muchhigher concentrations and different Ca/Sr val-ues in Pecos River sediments than in theBlackwater Draw Formation. Therefore, thesetwo elements can be considered to be gooddiscriminators for the two source sediments.Note that because the sample processing re-quired removal of carbonates, the Ca and Srshown in Figures 14–17 are found in Ca-bear-



Figure 7. Aerial photograph of a portion of the Monahans dune field. Arrows point torepresentative active barchanoid ridges and parabolic dunes. Note that dune orientationsreflect seasonally variable winds, which come from the west in winter and from the southor southeast in summer (see discussion in text).

Figure 8. Map of the Muleshoe dune field, showing dominant Munsell hues in late Holocene eolian sands.

ing noncarbonate minerals such as calcic pla-gioclase. Although Zr shows some overlap be-tween the two sources, Ti/Zr values are quitedifferent, and therefore Ti/Zr can be consid-ered to be a reasonable discriminator. Fe andMn have greater abundances in Pecos River

sediments, but the two sediment groups havevery similar Fe/Mn values. K-feldspar abun-dances are represented by Ba and Rb, whichsubstitute for K. Compared with the Black-water Draw Formation, Pecos River sedimentsare high in Ba and have high Ba/Rb values.

Overall, therefore, we consider Ca, Sr, Ti, Ba,and Rb concentrations and Ca/Sr, Ti/Zr, andBa/Rb values to be the best sand sourcediscriminators.

Eolian sands in the Muleshoe dune fieldhave compositions that most closely resemblesediments of the Blackwater Draw Formation(Fig. 14). Because many eolian sands in theMuleshoe dune field are calcareous, we ana-lyzed only sediments from the pink dunes, andtook the added precaution of pretreating thesesamples with Na-acetate in order to removeany trace amounts of carbonates that might bepresent. Ca and Sr concentrations (in noncar-bonate minerals) in Muleshoe sediments over-lap those of the Blackwater Draw Formationand are significantly lower than in Pecos Riversediments. Moreover, Ca/Sr values in mostMuleshoe dunes are closest to those in theBlackwater Draw Formation. Concentrationsof Ti and Zr are lower in the Muleshoe dunesthan in either potential source sediment, butTi/Zr values are very close to those in theBlackwater Draw Formation and are muchlower than in Pecos River sediments. Thesame is true for Ba and Rb concentrations andBa/Rb values; all are lower in Muleshoe dunesand Blackwater Draw Formation sedimentscompared to Pecos River sediments. Thus, el-ements that represent low-density (light) min-erals, such as feldspars (Ca, Sr, Ba, Rb) andthose that represent high-density (heavy) min-erals (Ti and Zr) have values in Muleshoedunes that are closest to sediments of theBlackwater Draw Formation.

Eolian sands from the other dune fieldswere not pretreated for carbonate removal pri-or to analysis because field examination ofthese sands suggested that little or no carbon-ate was present. This assumption proved to bemostly, but not completely, correct based ongeochemical results. In sediments of the Lea-Yoakum dune field, all samples have Sr con-


MUHS and HOLLIDAY

Figure 9. Textural parameters of eolian sands in the Muleshoe dune field compared todunes in Nebraska and Colorado. Nebraska data are from Ahlbrandt and Fryberger(1980); Colorado data are from Muhs et al. (1996).

Figure 10. Variation of mean particle size and clay content in the Muleshoe dune field asa function of distance from the Pecos River.

centrations that overlap with the BlackwaterDraw Formation, but some samples haveslightly higher Ca concentrations, probablydue to trace amounts of carbonate minerals(Fig. 15). Most samples of the Lea-Yoakumdunes have Ca/Sr values that overlap with theBlackwater Draw Formation and are lowerthan Pecos River sediments. Concentrations ofTi and Zr and Ti/Zr values in Lea-Yoakumdunes overlap both sediment groups. Howev-er, concentrations of Ba and Rb, as well as Ba/Rb values, are much closer to the BlackwaterDraw Formation than Pecos River sediments.The same interpretations can be made forsands from the Mescalero and Monahans dunefields for most element pairs, with two excep-tions. A few samples have very high Ca andSr abundances, probably due to the presenceof carbonate minerals (Figs. 16 and 17). Inaddition, the Ti and Zr data for the Mescalerodunes suggest the possibility of a mixture ofPecos River and Blackwater Draw Formationsediments as a source.

Mineralogical maturity in sandstones orsandy sediments derived from crystallinerocks is characterized by an abundance ofquartz and depletion of feldspars (Blatt et al.,1972). Because of their dominance by quartz,many of the world’s largest dune fields aretherefore mineralogically mature (Cooke etal., 1993; McKee, 1983; Pye and Tsoar, 1990).In some cases, mineralogical maturity cansimply be inherited from a quartz-rich parentmaterial, as is the case with the Algodonesdunes in California (Muhs et al., 1995). Muhset al. (1997a) showed, however, that differentGreat Plains dune fields show different de-grees of mineralogical maturity, even thoughall have relatively high-feldspar source sedi-ments. The Nebraska Sand Hills region hasmuch higher quartz contents than dunes ofnortheastern Colorado. Muhs et al. (1997a) in-terpreted this difference to reflect K-feldspardepletion over long periods of activity due toballistic (presumably grain to grain or grain tosurface) impacts (Dutta et al., 1993). The de-gree of mineralogical maturity in the SouthernHigh Plains dune fields is important becausepotentially it has significance for the long-term evolution of the dune fields.

On the basis of abundances of K, Rb, andBa as proxies for K-feldspar contents, dunefields of the Southern High Plains are the mostmineralogically mature dune fields in theGreat Plains (Fig. 18). Concentrations of allthree elements (done on untreated samples ofpink dunes for this purpose) are even signifi-cantly lower than in the Nebraska Sand Hills.The geochemical data are supported by themineralogical analyses, which show high



Figure 11. Ternary diagrams showing relative proportions (from X-ray peak heights) of quartz, plagioclase, and K-feldspar in Muleshoedune sands compared to Pecos River sediments, the Blackwater Draw Formation, and Nebraska and Colorado dune sands (locationsshown in Fig. 1). Relative proportions of minerals are based on the 20.88 (quartz), 27.48 (K-feldspar), and 27.88 (plagioclase) peaks.Nebraska and Colorado data are from Muhs et al. (1996, 1997a).

Figure 12. Comparison of concentrations of Ca, Sr, K, Rb, Ti, and Zr in pink (carbonate-free) and white (calcareous) dunes in the Muleshoe dune field.

quartz and low K-feldspar in Southern HighPlains dunes compared to both Nebraska andColorado (Fig. 11). In addition, Southern HighPlains dune fields show differences: the Mon-ahans dunes are much more K-feldspar de-pleted than the Muleshoe dunes.

DISCUSSION

Dune geomorphology, sedimentology, min-eralogy, and geochemistry allow reconstruc-tion of late Holocene eolian sand movementand the source of sand in the Southern HighPlains. Annual resultant drift potentials sug-gest that there should be a generally southwestto northeast movement of eolian sands overmuch of the region. The resultant drift poten-tials are in partial agreement with dune ori-entations, particle size, and geochemicaltrends, which suggest a more west to eastmovement of sand. Three possibilities mayexplain the differences: (1) much of the sandmovement may take place seasonally, in thewinter and early spring, when resultant drift


MUHS and HOLLIDAY

Figure 13. Variation in concentrations ofFe, Ti, and Zr as a function of distancefrom the Pecos River in the Muleshoe dunefield.

directions are more west to east; (2) paleo-winds may have been more westerly duringthe late Holocene times of dune formation; or(3) as suggested by Green (1961), Hawley etal. (1976), and Carlisle and Marrs (1982), fun-neling of winds through reentrants in the HighPlains escarpment may modify regional south-westerly winds into local westerly winds.None of these explanations are mutually ex-clusive, and based on data in this study, wecannot favor one over the other.

Mineralogical and geochemical analyses in-dicate that two types of dunes in the Muleshoefield—those dominated by pink colors andthose dominated by white-gray colors—aredistinguished on the basis of carbonate con-tents. Abundances of elements that representnoncarbonate minerals in the two dune typesare similar, indicating that the noncarbonatefractions of both dune types have a commonsource. The carbonate fraction may be derivedfrom late Pleistocene lacustrine carbonatesthat are found in middle and upper Blackwater

Draw Formation deposits that underlie signif-icant areas of the Muleshoe dune field (Hol-liday, 1995b). Sediment in the Muleshoe dunefield also changes in degree of redness and inclay content, both diminishing in a downwinddirection. We interpret the decreasing rednessto be caused by increasing downwind abrasionof red-pigmenting clay coatings.

On the basis of both mineralogical and geo-chemical analyses, dunes of the SouthernHigh Plains have compositions that are closerto the Blackwater Draw Formation than to Pe-cos River sediments. Ca/Sr and Ba/Rb (and toa lesser extent, Ti/Zr) values in dunes andBlackwater Draw sediments are similar and allare lower than Pecos River sediments. The red(5YR and 7.5YR) clay coatings on Muleshoedune sands support this interpretation as well:modern surface soils developed in the Black-water Draw Formation commonly have 5YRor 7.5YR hues, and have both high sand andclay contents (Holliday, 1989). Erosion ofsoils of the Blackwater Draw Formation dur-ing historic time has resulted in the formationof low sand dunes or sheets that closely re-semble those of the Muleshoe and other dunefields (see Holliday, 1987; his Fig. 31; repro-duced here as Fig. 19). Furthermore, depositsof the Blackwater Draw Formation are exten-sive over the Southern High Plains and occurupwind of, or underlie, many of the dunefields, particularly the Muleshoe dune fieldand parts of the Lea-Yoakum and Monahans-Andrews dunes.

A Blackwater Draw Formation source forthe dunes would also partially explain themineralogical maturity of Southern HighPlains dune fields, because that formation isalso of eolian origin (Holliday, 1989) andmuch K-feldspar depletion apparently tookplace during its long history. Some of the K-feldspar depletion in dune sand of the South-ern High Plains could have taken place duringextensive mid-Holocene periods of activity,for which there is little stratigraphic record(Holliday, 2000). A Blackwater Draw For-mation source for sand is favored becausemost reaches of the Pecos River are en-trenched and little sediment is at present avail-able for transport to the Llano Estacado.

Parts of some of these dune fields (partic-ularly the Mescalero and Monahans dunes) areupwind of the Southern High Plains, and westof known extents of the Blackwater Draw For-mation, which raises new questions abouttheir origins. Recent field work in these areas(described by Holliday, 2000) shows that aBlackwater Draw Formation equivalent (asandy loam to sandy clay loam with a well-expressed Bt-Bk soil profile called the Judkins

Formation by Huffington and Albritton, 1941)is below the dunes in at least some areas. Thisunit is discontinuous beneath the western partsof the Monahans dunes. Its extent beneath theMescalero dunes could not be determined ow-ing to few exposures, and the overall discon-tinuous nature of the unit may be due to winderosion. As shown by the Ti and Zr data, how-ever, these dunes may contain sediments de-rived from the Pecos River (Fig. 16).

CONCLUSIONS

We conclude from geomorphic, sedimento-logical, mineralogical, and geochemical datathat most late Holocene dunes in the SouthernHigh Plains were formed by dominantly west-erly winds, which may be the result of fun-neling of winds through gaps in the HighPlains escarpment, as suggested by severalprevious workers and Holliday (2001, whichis a companion paper in this issue of the Bul-letin). Mineralogical and geochemical datasuggest that most Southern High Plains dunesprobably have, as their immediate sources, theBlackwater Draw Formation rather than sedi-ments derived from the Pecos River valley. Ifthis is true, it indicates that climate, throughits effects on sediment availability, is the ma-jor control on mobility of dunes in the South-ern High Plains. The Blackwater Draw For-mation is extensive over the region, andhistoric droughts indicate that it can generateeolian sand sheets and dunes that resemble, ona smaller scale, the dune fields of the SouthernHigh Plains. Dune fields in the region (as wellas sand in the Blackwater Draw Formation)therefore appear to be sediment-availability–limited systems, as that term was defined byKocurek and Lancaster (1999). On the basisof geochemical data, dune fields of the South-ern High Plains are the most mineralogicallymature eolian sands yet studied in the GreatPlains, which suggests either inheritance froma quartz-rich source sediment and/or a longhistory of reworking.

The conclusion that Southern High Plainsdunes are sediment-availability–limited sys-tems is consistent with the observation that inthe warmest, driest part of the region (theMonahans dune field) dunes are mostly activetoday. It also leads to implications regardingpossible future dune mobility. Using the Lan-caster (1988) dune mobility index, and 1961–1990 climate data, the area of Midland, Texas(near the Monahans dune field), has a dunemobility index value of 101, which places itin an activity field of fully mobile dunes, ex-cept for interdune areas. In contrast, Clovis(near the Muleshoe dune field) and Hobbs,



Figure 14. Comparison of concentrations of Ca, Sr, Ti, Zr, Ba, andRb in Muleshoe dune sands, Pecos River sediments, and Black-water Draw Formation sediments. Carbonates removed prior toanalysis in all samples.

Figure 15. Comparison of concentrations of Ca, Sr, Ti, Zr, Ba, andRb in Lea-Yoakum dune sands, Pecos River sediments, and Black-water Draw Formation sediments.

Figure 16. Comparison of concentrations of Ca, Sr, Ti, Zr, Ba, andRb in Mescalero dune sands, Pecos River sediments, and Black-water Draw Formation sediments.

Figure 17. Comparison of concentrations of Ca, Sr, Ti, Zr, Ba, andRb in Monahans-Andrews dune sands, Pecos River sediments, andBlackwater Draw Formation sediments.


MUHS and HOLLIDAY

Figure 18. Comparison of concentrations of K, Rb, and Ba in Mu-leshoe and Monahans dune sands and sands from northeasternColorado and Nebraska. Colorado and Nebraska data are fromMuhs et al. (1996, 1997a).

Figure 19. Modern erosion of soils in the Blackwater Draw For-mation near Lubbock, Texas, to form quartz-rich sheet sandsdownwind. Photograph by V.T. Holliday (see Holliday, 1987).

New Mexico (near the Lea-Yoakum dunefield), have dune mobility index values of;80, placing them in an activity field of onlymobile dune crests. Thus, only modest (10%–15%) decreases in precipitation accompaniedby similar increases in evapotranspiration (dueto higher temperatures) might be necessary tomobilize currently stable dunes.

ACKNOWLEDGMENTS

This study was supported by grants from theEarth Surface Dynamics Program of the U.S. Geo-logical Survey through its Southwest LandscapesClimate Impacts project (to Muhs) and the NationalScience Foundation (EAR-9218593 to Holliday).We thank Garry Running, Scott Cowherd, ShannonMahan, and Josh Been for laboratory assistance andPaul Bierman, Alan Busacca, Nick Lancaster, RichReynolds, Al Arbogast, and Todd Hinkley for help-ful comments on an earlier version of the paper.

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