Chapter 2: A Summary of the Quaternary Geology of the White Sands Area

Sections Introduction Stratigraphy of the Southern Tularosa Basin Exposures of Lake Otero Sediments at the Monument Eolian Terrains of the White Sands Area Geomorphic map of the Monument and vicinity Active eolian terrains Other features of the Geomorphic Map Wind regimes Regional Sand Seas of the High Plains Illustrations

The purpose of this chapter is to summarize what is known about the Quaternary geology of certain regions surrounding White Sands, as well as the Monument itself, and to apply this knowledge to construct a timetable of events for the eolian system. In addition, new stratigraphic data obtained from fieldwork and air photo mapping are summarized here, and a geomorphic map is presented based on a new air photo montage of the entire dune field. We also report new thermoluminescence dates for some of the lacustrine and eolian deposits.

In general, the regions around the Monument have been well studied, with firm chronologies based on soils, radiocarbon dating and other methods to control chronology and stratigraphy. The Monument itself has been the subject of some stratigraphic work, but has, nevertheless, suffered from some lack of dating work, as well as an incomplete lake stratigraphy due to lack of sedimentological cores.

The Quaternary stratigraphy of the area is basically a record of the pluvial events of the glacial periods, with by far the most important, in terms of surface geology, being the latest Pleistocene (Wisconsin) glaciation and de-glaciation. During glacial periods, weather in the Tularosa Basin was cooler and wetter that today, with lower evapotranspiration. This led to higher lake levels for Lake Otero, that were maintained both by greater groundwater infiltration and greater fluvial runoff into the Tularosa Basin. Interglacial periods, especially the most recent about which the most is known, have been drier. As illustrated in further detail below, the dunes at the White Sands National Monument probably were created by the effects of the drying and warming that has occurred since the last glacial retreat about 10,000 years ago, with the most significant events beginning about 7000 y.b.p. (years before present). It was during the last 10,000 years that Lake Otero dried up, precipitating the gypsum that was mobilized during later times to form the present-day dune fields.

The chronological relationships for the Quaternary deposits of the region as described by various authors are summarized in Table 2-1. In this table the dates and stratigraphy of Buck and Blair, based on soil mapping, and Seager et. al (1987), based on traditional geological methods, are compared with each other (they are mainly in agreement) and with typical Pleistocene chronologies and absolute dates. Also included are the Lake Estancia events and glacial states. It can be seen that much of the Quaternary deposition involves the emplacement of great thicknesses of fluvial and lacustrine sediments in both the Tularosa Basin, and Hueco Basin to the south. Indeed, from the time of the Pliocene-Pleistocene Fort Hancock Formation of Strain (1966, 1969) until about 7000 years ago, the history of the Tularosa Basin is mainly one of fill by fluvial sediments on the basin margins and lacustrine sediments (with occasional eolian interludes) deposited in the basin centers (Table 2-1). These include the deposits of the Camp Rice Formation laid down in the Tularosa and Hueco Basins by the Ancestral Rio Grande prior to its entrenchment approximately 225,000 years ago. Since that time the waxing and waning of the glaciers during the Pleistocene has laid down comparable piedmont and valley-floor sequences of sediments in several major horizons within the closed-drainage portion of the Tularosa Basin, as discussed further below.

An additional purpose of this chapter will be to illustrate the relationship of Pleistocene Lake Otero to other Pluvial lakes of the Pleistocene in the Western United States. We also summarize the well-described and dated sequences of deposits at Lakes Trinity and Estancia for comparison with Lake Otero. The White Sands dune field is compared with a map of the dune fields of the Great Plains, to provide a perspective on it’s relationship to aridification which occurred during the Altithermal of 7000 ypb.

Recent experimental thermoluminescence dating by Stephen Stokes and associates at Oxford University, at the invitation of the National Park Service has produced some tentative dates. For quartzose fluvial sediments near the base of the Otero gypsum deposits a date of about 14,000 years was obtained, which places the Otero Evaporite sediments solidly in the late Pleistocene. Additionally, several dates were obtained in the fossil dune terrain just northeast of NE 30 observation station that averaged about 16000 years. A younger date of 6,500 years was obtained for rather similar gypsite-encrusted terrain on the ridge of the long parabolic dune arm that extends northeastward from South Playa Lucero to near Point of Sands. This would seem to indicate two episodes of dune movement for the parabolic dunes of the “fossil dune field” area. The latter date fits well with the Altithermal of Antevs (1955). The much older date is a little surprising, given that the culminating evaporative events at Lake Estancia are much later—indeed, the 16,000 year date coincides more with glacial maxima than any arid episode that is known to have occurred locally. Nevertheless, the Tularosa Basin is much, much larger than the Estancia Basin, and moreover, there is a major evaporative event recorded at Estancia at roughly 16000 y.b.p. which corresponds to the end of glacial highstand. Given the lower elevation and more southerly location of Lake Otero, it seems at least possible that some precipitation of gypsum occurred immediately following the end of the glacial pluvials.

Other interesting dates obtained by Stokes, et. al. (2000) include an age of approximately 2,100 y.b.p. for dune sediments underlying the Alkali Flat near the terminus of the Alkali Flat Trail. Dates in a modern dune and a pedestal dune yielded comfortably modern 820 and 440 y.b.p. dates. The 820 y.b.p. date on the modern dune is probably too old, given the rapidity of advance of these bedforms. Nevertheless, these first experiments, on obtaining dates from the very quartz-poor gypsum dunes of White Sands, have provided a tantalizing glance at a possible time structure for the eolian events of the Monument.

The work of Buck (1996) and Seager, et. al. (1987) provides a good framework for viewing the stratigraphy at the Monument.

Buck (1996) studied the Quaternary geology of the Fort Bliss/Missile Range area from just south of the Monument and dated a number of significant events in the valley, which she tied to the soil stratigraphy of Giles (1993) and Monger (1995). Figure 2-1 summarizes the soil stratigraphy of the alluvial fans recognized by Giles and Monger that could be correlated throughout the Hueco and Tularosa Basins, as well as the basin of the Rio Grande to the west near Las Cruces. It is an interesting and powerful concept that recognizes the relative entrenchment of younger fans versus the older fans, as well as the fact that the stratigraphically older fans (such as the Jornada 1 in Figure 2-1) are exposed highest on the mountain front. Indeed, the youngest Organ deposits exist only as stream valley fills except in the basin centers. The ages of the various horizons shown in Figure 2-1 are displayed on Table 2-1. The Organ deposits, mentioned above, date from 7000 y.b.p., and are thus of greatest interest to us, corresponding to the possible time of formation of the White Sands dunes. The older deposits, such as the Isaack’s Ranch (7500–15000 y.b.p.) and Jornada I and II (50,000–250,000 y.b.p.) reflect earlier events of the Pleistocene.

A summary of Buck’s findings for the southern Tularosa Basin is shown in Figure 2-2. The Organ alluvium (and related basin-floor deposits) are subdivided into the Organ I , II and III (Gile et.al, 1981); with the Organ II not present in the valley floor, generally due to erosion and/or non-deposition. The highest layer in Buck's stratigraphy is a modern blowsand horizon about 150 years old, consisting of widespread coppice dunes and some sand sheet deposits. The formation of these is attributed to overgrazing during the 19th century. Beneath the historic blowsand is the Organ III unit dating from about 150 to 1000 y.b.p., which has an “A” soil horizon with no carbonate. Locally, eolian deflation in the modern interdunes has cut into the Organ III, removing the “A” horizon and leaving an interdunal lag of materials from the Organ III at the present day scour surface. Beneath the Organ III is the Organ I, then the Isaack’s Ranch unit (with a second paleolag) and the La Mesa unit.

Figure 2-3 Summarizes one of Buck’s study sites, which includes data from trenches cut by front end loaders, very close to the southern boundary of the Monument. At this site, 6A south, are found the Historic Blowsand, the Organ I and the Lake Tank/Petts Tank sediments, lacustrine equivalents of the Isaack’s Ranch and Jornada Units of Gile and Hawley along the mountain fronts (alluvial fans). The Organ I at this site is eolian. The dates in Figure 2-3B, as well as the stable isotope data in Figure 2-3C, suggest that the late Pleistocene Playa of the Petts Tank/Lake Tank was infilled by eolian sand approximately 5000 to 6000 years ago. This may be the first major eolian event that can be tied to the present dune field at White Sands.

Seager, et.al, produced a geologic map of the east half of Las Cruces and northeast El Paso 1 and 2 degree sheets at 1: 125,000 scale. This is a thorough map that provides excellent detail on the surficial geology of the monument as well as surrounding bedrock geology and structure. It is incorporated here in part, in summary fashion, as Figure 2-4, with the time scheme assigned to various units mapped as shown on Table 2-1. Deposits in color on Figure 2-4 are the youngest mappable geologic units of Seager, et. al.

A number of important features are visible on the map. First, the southward extension of Lake Otero lacustrine sediments (blue color), well beyond the present Playa Lucero basin is evident. Note also the numerous young alluvial fans and streams (Organ equivalent) that contribute quartzose fluvial sediments to the lake system on both the east and west sides of White Sands. Seager, et. al. also assigned some older, fixed (inactive) sand dunes surrounding NE 30 to the Otero and younger sediments although they are of course clearly older than the present active dunes. The younger deposits of the White Sands dunefield area also include certain ephemeral and perennial valley floor lakes extending from northwest to southeast just south of the visitor center. The system of younger deposits shown on Figure 2-4 is bounded by the younger and older alluvial fans on the western side of the Monument, and, ultimately, by the main basin bounding faults which have down-thrown the Tertiary against Precambrian of the San Andres Mountains.

Among the more interesting areas mapped by Seager, et. al as “older” piedmont slope and basin fill are the dry lakes and lunettes that lie mostly south of Highway 70/82. These are visible in the air photomontage and map of Figure 2-17A, and 2-17b, as irregular ridges mostly on the east and northeast side of shallow lake basins. Similar deposits lie at the upwind margin of Great Sand Dunes, Colorado, where the water table is close to the land surface.

As recognized originally by Herrick (1904), the Tularosa Basin has been the site of large pluvial Pleistocene lakes, of which the latest is known as Lake Otero. Lake Otero was one of many lakes fed by increased precipitation and lower evapotranspiration around the western United States during Wisconsin times (Figure 2-5). This Pleistocene lake occupied the portion of the Tularosa Basin within a highstand shoreline at approximately 3950 feet above sea level. This contour is marked on the geologic and geomorphic maps (Figures 2-4 and 2-16). Much of the lacustrine sediment of this youngest of the Pleistocene lakes consists of green or black marls and shales, based on drilling records at HELSTF and other sites near the Monument or scattered around the valley (Orr and Myers, 1986). During the culminating phase of evaporation, which began about 18 to 20 thousand years ago and has continued with several interruptions down to the present, the water evaporated and gypsum was precipitated within the shrinking shorelines of the lake. It is difficult to know the original extent of the gypsum deposits, or their thickness, since much of this material has been blown away, or has been covered by migrating dunes (Figure 2-6). Nevertheless, it is apparent from the outcrops, shown in Figure 2-8, and those elsewhere that enormous amounts of gypsum were deposited on the floor of Lake Otero during the last evaporative phase, and that much of it was in the form of small sand or gravel size (often twinned) gypsum crystals perfectly suitable for uptake by wind, especially after breakdown of the small 1–2” twins (Figure 2-7). The author has seen similar gypsum muds and crystals in some modern gypsiferous lakes, in South Australia.

Figure 2-8 Illustrates some outcrops of Lake Otero sediments that have resisted erosion by wind or water. These outcrops are all more or less flat-bedded, consisting of alternating layers of gypsum crystals and mud. These deposits are quite thick along the southwest shoreline of Playa Lucero (Figs. 2-8A and B) but thin northward away from the deepest part of the old lake basin. In places, for example near the locality shown in Figure 2-8D south of the visitors trail to Playa Lucero, the gypsiferous beds of Lake Otero are interbedded with alluvial sediments deposited while the lake was contracting.

There are no firm dates we are aware of on the sediments of Lake Otero within the Monument. The rough timing of the history of the lake however may fit that of basin center lakes and other deposits studied by Buck just south of the Monument, although lacustrine deposits studied by her were much less extensive than those of Lake Otero.

Pleistocene Lake Estancia, that is located in Torrance County about 100 miles north of White Sands is a possible analog for the course of events envisioned for Playa Lucero (Figure 2-9). Lake Estancia occupied a small basin about 60 x 100 km in size. Allen (1991) and Allen and Anderson (1993) studied these lake deposits in detail, documenting at least two highstands during the last glacial maximum, followed by gradual evaporation of the lake as the climate became warmer and drier. Allen also documented the deflationary episode that occurred at Lake Estancia beginning about 8000 years ago. This deflationary episode scoured to a depth of over 30 feet below the original top of the lake sequence, creating gypsum dunes and beach ridges. This process is probably a good analog for the process that created the enormous deflation basin at White Sands.

The eolian deflation basin at Lake Estancia is currently occupied by a playa complex in a manner similar to the way modern Playa Lucero occupies the lowest part of the ancient eolian deflation basin cut into Lake Otero sediments. Since many of the Pleistocene lakes of the western United States have histories roughly similar to Lake Estancia, it may be assumed that the timing of events at nearby Lake Otero are also similar, and that the deflation events at White Sands probably began about 7500 years ago as at Lake Estancia (e.g.; during the Altithermal, as documented by Buck in her area). The time-structure of these events, however, is very imperfectly known. Allen (1991) noted that many of the depositional events at Lake Estancia occurred as centennial and decadal oscillations (Figure 2-11), so it is possible that erosional events at these places followed multiple scales of cyclicity, if they were cyclic at all. The other possibility is that the erosive events at White Sands that led to the formation of the dunefield were dependent upon some threshold, perhaps the breakdown of a gypsite crust which, once having occurred, liberated the gypsum sand in vast quantities.

Another Pleistocene lake very near to White Sands is Lake Trinity (Neal, et. al., 1983) located near the Trinity atomic bomb test site (See Figure 1-7 for location). Lake Trinity shows a similar history of more or less freshwater/evaporitic deposition followed by a culminating evaporative event that precipitated over 25 feet of massive sulphates (mainly gypsum) in the center of the small lake basin (Figure 2-13). These events, however, were followed not by massive erosion, but by stabilization of the lake surface and partial transgression by younger quartz dunes.

One of the main purposes of this chapter is to present the geomorphic map of the Monument shown as Figure 2-17, to which the reader may wish to refer for geographical references in the following discussion. It is first necessary, however, to provide a brief overview of the typical eolian terrains of White Sands. This brief discussion will explain and illustrate the basic terrain types and their origins, including the Alkali Flat, the main dune field, the fossil dunes and the parabolic dunes. Later chapters will provide more detailed discussions of the sedimentology of the various eolian facies present at the Monument.

Broadly speaking there are only four eolian terrain types, or types of eolian facies known. These are dunes, interdunes, sand sheets and sabkhas. Most eolian landscapes, however they may differ in detail, have these facies present in one form or another. These terrain types are illustrated in Figure 2-14A-D and Figure 2-14E-H. At the downwind edge of the White Sands dune field lies a great mass of parabolic dunes, bedforms with elongate arms that extend upwind. In Figure 2-14A shows the entrance of Lost River into the dune field near Holloman Air Force base. Note the nearly flat, vegetated sand sheets surrounding the parabolic dune buildups. Figure 2-14B shows a portion of the main dune field at White Sands. Shown in the picture are barchanoid dunes, a form that tends to be dominated by a single slipface. In the view shown in Figure 2-14B, the dunes are migrating toward the viewer from the southwest. In the foreground is an eolian sand sheet. Note also that the interdune areas immediately behind the leading dunes also resemble sand sheets. It is not uncommon for the surface appearance of one eolian facies to resemble that of another if the environmental conditions are similar.

The broad Alkali Flat portion of the Monument (which includes much of the lake basin and scour platform areas of the geomorphic map) has many examples of eolian sabkhas. In Figure 2-14C, an air photo taken from a helicopter, shows a large area of the Alkali Flat just upwind of the dunes. This broad, flat expanse of sand consists mainly of the truncated remnants of the former dunefield, as evidenced by the curving remnants of dune bodies etched out in relief on the surface of the sabkha. In this case very little deposition is occurring on the sabkha, it is mostly an area of scour. Figure 2-14D shows a portion of the Alkali flat closer to Playa Lucero, which has dunes migrating across it. There is little permanent deposition in this area, although locally flood events during the rainy season (usually winter) add some new sediment or redistribute available sediment. As discussed further in Chapter 6, despite a superficial similarity of surface sediments, there is great variety of sedimentary structures within the eolian sabkha of the Alkali Flat.

Some of this variety is related to the effects of water on the sediments. As noted above, the sediments nearest the main dune field are also topographically the highest and thus, in most places, are drier than those closer to the lake. Nearer the lake, the effects of increasing regular flooding become more and more apparent. Some of these effects are shown in Figures 2-14D, F, G and H. Figure 2-14E shows an irregular pattern, perhaps related to desiccation polygons. The surface of Playa Lucero, the modern playa, is white from evaporitic salts during the dry times such as that shown in the photograph. Although groundwater table is very shallow in the environs of Playa Lucero, much of the flooding, visible when the lake fills, comes down alluvial fans such as those shown in Figure 2-14G and H. These fans bring water to the basin, which then evaporates, leaving behind more sodium chloride and gypsum, which is either incorporated into the sediments of the playa or removed by wind.

There are several dune types that are common at the Monument. These are described here, although further discussions of dune types and their origins will follow in later chapters. The dune types of greatest importance at White Sands are illustrated in Figure 2-15. Dome dunes are low forms without slipfaces that are commonly found on the upwind margins of the main dune field at White Sands (Figure 2-15A). McKee (1966) made the observation that these dunes migrate downwind and eventually become barchans and barchanoid ridge dunes, such as those shown on Figure 2-15C. In this photograph the large, gently curving slipface typical of the barchanoid type is plainly visible. Figure 2-15C provides a good further illustration of the parabolic dune type, whose shape mimics that of the mathematical curve of the same name. The trailing arms of each dune are plainly visible, along with the more active, advancing centers of these dunes.

There are some terrains at White Sands that have formed by wind scour of previously deposited eolian sands. These terrains are important to understand in order to correctly interpret the history of White Sands.

Fossil dune fields are an important part of the landscape at White Sands (Figure 2-16A). There are extensive areas of fossil dunes downwind of the Playa Lucero (See Figure 2-16). Many of these old dunes have surfaces of case-hardened gypsum (gypsite) and thus resist wind erosion. In many places around the Monument however, the wind is eroding these old dunes and recycling their sand into the present system (Figure 2-16A).

Some terrains consist of wind-eroded lake and sabkha deposits. When the conditions are correct, features known as yardangs are formed. These resemble inverted boat hulls. Yardangs are often all that remains following the erosion of considerable thicknesses of sediment (Figure 2-16D).

Other wind-eroded terrains are more subtle. For example, the area immediately downwind of Playa Lucero is a sand sheet which is covered by a thin lag of pebbles just above a widespread scour surface. With the exception of some sand trapped behind bushes (coppice dunes), there is very little deposition in this area, evidence that not much sand is being presently supplied to the dune field from Playa Lucero.

One of the principal sources of new information about the Monument that has come from this study is the first complete air photo mosaic of the White Sands Dune Field. It includes not only the Monument area, but also the portions of the dune field to the north on the Missile Range as well. From the air photo color composite image prepared by the University of New Mexico, Albuquerque, a geomorphic map has been prepared which shows the types of dunes and other terrain within the image (Figure 2-17). Along with this map we have included sand rose diagrams showing potential sand drift across the region based on wind records at nearby Holloman Air Force Base.

The map is more or less self-explanatory, but major features are worth discussion here. It is clear from the map that there are four active terrains associated with the present geomorphology of the dune field, including (1) the western lake basins and circular depressions, (2) the Alkali Flats, which are essentially a scour platform, (3) the modern barchanoid dune field and (4) the marginal downwind and crosswind parabolic dunefields.

On the upwind, southwest side of the dunefield lies a string of small, oval or circular ephemeral lake basins occupying a small portion of a much more extensive wind-scoured basin. The lakes are in the lowest part of a basin from which wind removed gypsiferous lacustrine sediments below about 3950 feet prior to formation of the present lakes. This scour event is part of the reason for the existence of cliffs and escarpments along the western and southwestern shoreline of present day Playa Lucero. The present or recently occupied lakes (such as Playa Lucero) give way upwind to a modern scour platform, over much of which ancient beds of Lake Otero are exposed to the wind, and are in the process of being stripped off to supply fresh sand to the dune field (Figure 2-17). There are, however, portions of the eastern scour platform (Alkali Flat) in which the wind is scouring truncated dunes. Further upwind, the dunefield itself is reached, followed ultimately by the transformation of the system to parabolic dunefields. The parabolic dunes are found on all but the upwind (source areas) of the dunefield. Their boundary with the active dunes of the barchanoid dune field corresponds roughly to the point at which interdunes are damp due to proximity to the water table. Evaporation of this slightly saline water during summer produces short term hypersalinity, limiting vegetation to only the most salt-tolerant varieties in the damp/evaporitic interdunes.

When a thick enough sand layer has accumulated to form a sand sheet, it is possible for damp, fresh, rainwater to be retained in the pore spaces of the sand, and less salt-tolerant flora can survive, thus leading to the formation of the parabolic dunes.

Not all of the circular depressions on the west side of the eolian basin are occupied by lakes as active as Playa Lucero. Frequency of flooding is related to elevation, with higher lakes further east flooded less and less, until no traces of flooding are visible on the imagery. Most of the lakes, even if they are dry most of the time, have upwind rims of dunes, sand sheets or coppice dunes that geomorphologically are incipient lunettes. These probably have analogy with similar older lunettes south of the Monument and U.S. Highway 70/82.

In the main dunefield there is a central ridge of sand of unknown origin, as shown on Figure 2-17. It is also of interest that a major indentation in the main dunefield on the western side is opposite the entrance of Lost River to the dunefield on the eastern side. Perhaps the fresh water inflow of the creek has somehow led to this situation. There are similar, but much more subtle associations with stream entry points to the north, as well, that are visible on the image in Figure 2-17.

It is intriguing that the most active portions of the present day dune field lie opposite the driest portions of the modern day eolian basin. This may reflect the common fact that playas tend to hold sediment in place through the effects of dampness on loose sands. Slightly drier areas, without this effect, may more easily contribute sand to the wind, especially if the source is old lake beds with sediments dominantly of sand size, as is the case with ancient Lake Otero.

There exist quite extensive terrains of inactive or “fossil” dunes (clearly visible on the imagery) which merit discussion. Near the NE 30 locality is a subcircular area of eroding fossil dunes associated with a dome centered on the NE 30 (Figure 2-17). Along the west and southwest side of this structure lie bands of sediment that may consist in part of old shorelines representing former lake stands, since they are at the right topographic elevation. In the field, however, these light and dark bands on the imagery consist of terraces and blowouts that appear to be the product of wind scour of slightly cemented horizons within dune deposits. We found no unequivocal shoreline deposits preserved in outcrop. This may be due to removal of shoreline dune and other sands by wind scour or the fact that this “shoreline” is the edge of the wind scour basin, but not necessarily the precise edge the former Lake Otero, at least as expressed in typical shoreline sedimentary structures.

The origin of the domal structure at NE 30 is not known, although one might speculate, because of its round shape, that it is caused by an intrusive body. Extrusive volcanic cones of the Three Sisters lie a few miles to the southeast.

South of the main dunefield lies the region of former small playa-like lakes very similar to the “dry lakes” of Great Sand Dunes, Colorado. These are lunette deposits typically developed on the downwind sides of small ponds (See Chapter 4 for a modern example). In some places, traces of old parabolic dunes blown off the lunettes are visible, for example near the HELSTF site on the lower left portion of the image.

Wind data for the monument, based on records from Holloman Air Force Base are also summarized along the bottom of the Geomorphic Map. These diagrams, known as “sand roses,” reflect the sand-moving power of the wind. They are drawn with arms facing into the wind, proportional in length to the amount of sand that has potentially been blown from that direction. The resultant arrow projecting from the sand rose indicates the combined effects of all winds (Fryberger, 1979).

A glance at the annual and bi-monthly sand roses reveals several significant components of the wind regime. The most significant component is from the southwest during winter-spring, with a peak during March and April, when most sandstorms occur. A secondary, but very important direction is from the northwest and north, which occurs during the same seasons. There is also a component from the southeast during summer. Thus, the wind regime at White Sands, while dominantly from the southwest, has other significant components. These “crosswinds” build secondary slipfaces on the dunes as well as shift sand laterally through interdune corridors along with other effects discussed in more detail in succeeding chapters. All this being noted, however, it is clear that the resultant of all these winds aligns well with the present trend of the parabolic dunes and somewhat less well with the normals to the trends of the barchanoid dunes in the main dune field.

Considerable work has been done recently on dating the activity of various sand seas of the Rocky Mountains and High Plains of the United States. U.S. Geological Survey has studied the high plains dune fields, due to concerns about re-activation of these by global warming (Muhs and Holliday, 1994). The map showing the distribution of stabilized and active dune fields of the high plains (Figure 2-18) illustrates that the eolian system at White Sands is part of a much more extensive system of eolian dune fields located both on the high plains and in the greater Rocky Mountain region. Unfortunately for the Rocky Mountains there exists no map similar to that of the high plains prepared by Muhs and Holliday.

Additionally, the data of Muhs and Holliday suggests that the White Sands dunefield may have closer affinities, due to current levels of activity, with the greater Basin and Range dunefields, such as those of Arizona, Utah, Nevada and California (for example the Algodones dunes of California), than with those of the high plains (Figure 2-20). Nevertheless, the timing of events of the high plains dune fields is of interest. Muhs (1985) identified a number of major eolian events in the dune fields of the Colorado Front Range and Nebraska. His data suggest dunefield formation at the Altithermal (8000–5000 y.b.p.) and at about 2000 years before present. These events, particularly the Altithermal, fit the dates presently available at White Sands very well. They also support the broad correlation of events suggested earlier for Lakes Estancia, Trinity Lake and Lake Otero and associated dunefields.

The question of the precise age of the current dunefield at White Sands remains open. All evidence, however, points to an age younger than 7000 y.b.p., perhaps considerably younger.

	Table 2-1—Correlation of Quaternary (Pleistocene-Holocene) Sediments, Tularosa Basin
	Figure 2-1—Stratigraphy of alluvial fan deposits along the Organ Mountains. Younger fan sediments are deposited just southwest of White Sands on the distal portion of the older fans. The youngest of the fan deposits intertongue with or overlie the gypsiferous bedded gypsum crystals of the Otero Lake sediments. After Monger, (1993), in Buck, (1996).
	Figure 2-2—Subdivision of the Organ Sands on the valley floor, in schematic cross section. (after Buck, 1996).
	Figure 2-3—Stratigraphy at Buck’s (1996) Basin Floor Fault trench 6A South, near White Sands. (a) soil stratigraphy (b) radiocarbon dates (c) carbon/oxygen isotope data (d) trench location. Location also shown on Figure 2-4.
	Figure 2-4—Geologic Map of the younger deposits near White Sands (After Seager, et. al., 1987). Colored portions of the map emphasize the younger deposits mapped by Seager, et. al. as contemporaneous with Lake Otero and later deposition.
	Figure 2-5—Distribution of Pleistocene lakes in the western United States, after Feth (1961).
	Figure 2-6—Schematic diagram showing deposition of Lake Otero evaporitic deposits and later erosion to form the eolian deflation basin currently occupied by youthful Playa Lucero.
	Figure 2-7—Photograph of wind-erodible gypsiferous sediments of Lake Otero. This is a close up of the outcrop shown in Figure 2-8C. Deposition in the form of such small crystals, sand size or in gravel sized twins is typical of subaqueous gypsum deposition in evaporitic lakes.
	Figure 2-8—Outcrops of Lake Otero Sediments. (A) Air view of cliff scarps on Southwest side of South Playa Lucero that consist of great thicknesses of small gypsum crystals, gypsum sand and mud that have been eroded by wind from background areas of photo. (B) Ground view of these outcrops. Note flat-bedded nature of sediments. View to NW, N. Playa Lucero in distance. (C) In foreground, on right, outcrop of bedded gypsum crystals deposited by Lake Otero. In background, light outcrop at base of alluvial fans marks upper limit of Lake Otero Deposits (arrow). (D) Substantial outcrops of Lake Otero sediments in gullies about ½ km south of the visitor trail on the west side of Playa Lucero. These outcrops contain layers of selenite crystals and are interbedded with alluvial deposits in places, especially near the base of this outcrop.
	Figure 2-9—Index Map of the Estancia Basin and lacustrine deposits on the valley floor.
	Figure 2-10—Schematic stratigraphic column of upper 5 meters of Estancia Lake sediments, showing radiocarbon dates, after Allen (1991).
	Figure 2-11—Lake Estancia sediments. Reconstructed time series from laminated and bioturbated intervals (above) and corresponding power spectra (below). (A) Time series from bioturbated interval showing centennial oscillations in gypsum precipitation. (B) Time-series from laminated interval showing decadal oscillations in thickness of gypsum laminae, See Figure 2-10 for position of time series.
	Figure 2-12—Estancia Valley lake level and water table curve for the last 20,000 years
	Figure 2-13—Stratigraphy of Lake Trinity, near the Trinity Atomic test site. (after Neal et. al. 1983)
A-D E-H	Figure 2-14—Air views of terrains at White Sands. (A) Downwind edge of the dunefield near Holloman Air Force Base, where the Lost River enters the dunefield and sinks into the sands. Surrounding landscape consists of vegetated sand sheets and active arms of parabolic dunes. (B) Northeast edge of the main dune field. In the foreground, sand sheets and barchanoid ridge dunes. Barchanoid dunes in background. San Andres Mountains lie to the southwest in distance. (C) The Alkali Flat just upwind of the main dune field; note that scoured remnants of pre-existing dunes form the surface of the Alkali flat at this locality. This surface resists erosion due to early cementation by nearsurface groundwater (Shenk and Fryberger (1988). (D) Low barchan dunes on the Alkali Flat northwest of NE 30, near North Playa Lucero. The dark area of the Alkali Flat between the traces left by the migrating dunes is occasionally flooded. There were rills, shoreline debris, ripples, and other shallow flow structures on the surface at the time of the photograph. (E) The Alkali Flat about ½ km northeast of previous photograph showing the irregular pattern of Alkali Flat. View to southwest. (F) A view of the Alkali Flat comprising a portion of the surface of the active Playa Lucero and the western shoreline of Playa Lucero north of the visitor’s trail. San Andres Mountains in background with basin boundary fault scarp (arrow). (G) Southwest shore of Playa Lucero. In the foreground is a small outwash fan contributing quartz and carbonate detritus to the lake sediments. In the background on the right are escarpments of Lake Otero gypsum crystals and gypsiferous muds shown in Figure 2-5c. (H) The high terrain dividing North Playa Lucero from South Playa Lucero. Note the alluvial fans distributing quartzose sediments that have built the ridge. In the right background is South Playa Lucero. Fossil dune terrain and the highest point in the Monument, where there is an observation tower named “Northeast 30” is in the distance on the left (arrow).
	Figure 2-15—Some important eolian terrains of White Sands. (A) Air view of the upwind edge of the active dune field showing dome dunes in the foreground, and the Alkali Flat and main field of barchanoid dunes in background. (B) The Parabolic dunefield on the northeast side of South Playa Lucero. View to Northeast. (C) Barchanoid ridge and transverse ridge dunes of the main dunefield. Wind is from upper left to lower right. Terminology of dune forms is explained in Chapter 4.
	Figure 2-16—Wind-eroded terrains of White Sands. (A) Fossil dunes near NE 30 (dark vegetated terrain in foreground) being overrun by active parabolic dunes, in part sourced by breakdown of older dune terrain. (B) Wind-scoured terrain on the downwind side of south Playa Lucero, the site of eroded fossil dune fields, lake sediments and yardangs described in chapter 8. (C) Wind-scoured sand sheet and fossil dune terrain downwind of North Playa Lucero. The only significant eolian deposition currently occurring in this area is in the form of coppice dunes in the lee of bushes. This terrain supplies part of the evidence that the current Playa Lucero is supplying little sediment to the present day dune field at White Sands (discussed further in later chapters); (D) Yardangs formed by wind scour of lake sediments, northeast shoreline of South Playa Lucero. Wind from the right side of photo, view to the south.
Figure 2-17A Figure 2-17B	Figure 2-17—Geomorphic map of the White Sands area from air photographs (with sand roses). Sand roses are calculated from wind records. They summarize the ability of the wind, from the various directions of the compass, to move sand, and include a resultant arrow. Methods of calculation are described in Fryberger (1979). Drift potentials express the total sand moving capacity of the wind for a given time interval. Note the concentration of sand moving potential in the spring season.
	Figure 2-18—Map showing the distribution of mostly active (yellow shading) and stabilized (orange shading) eolian dune fields in the Chihuahuan Desert, Southern Great Plains and High Plains, compiled from various sources. (After Muhs and Holliday, 1994).
	Figure 2-19—Correlation of geomorphic events and soils from the Colorado Front Range to the Nebraska Sand Hills.
	Figure 2-20—Comparison of eolian activity on the Great Plains, Chihuahuan Desert and the California-Colorado deserts. Plot shows amount of time that wind is above threshold velocity for medium sand (W) versus ratio of precipitation (P) to potential evapotranspiration (PE) for regions where eolian sand is present. (After Muhs and Holliday, 1994).