Chapter 7: Sediments of Lake Otero,
Playa Lucero and Alluvial Systems


Sections

Introduction
Sediments of Pleistocene Lake Otero
Sediments of Holocene Playa Lucero
Fluvial Sedimentation
Illustrations


Introduction

The purpose of this chapter is to describe and briefly illustrate the sediments of Pleistocene Lake Otero, Holocene Playa Lucero and, in addition, the fluvial sediments of the west side of the basin. There has been little previous work on the sedimentology of these deposits and, thus, much of what is described here is relatively new. This study was only reconnaissance in nature. The lake, playa and lateral fan sediments cover extensive areas, and further studies will be required to make facies maps based on detailed mapping of outcrops. Nevertheless, the facies described here are distinctive and may perhaps be of assistance to other workers.

Sediments of Pleistocene Lake Otero

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The history of Lake Otero and a discussion of the distribution of its sediments below the approximately 4000 foot (above sea level) high stand shoreline (Figure 2-17A) has been summarized in Chapter 2. We did not undertake a mapping project, but have seen a number of distinctive facies in our reconnaissance work around the margins of this Pleistocene lake.

The outcrops of Otero sediments appear on photo montages such as Figure 2-17B. Figure 7-1 is an enlargement of a portion of Figure 2-17B that shows playa Lucero in flood (blue areas). The 4000 foot contour on Figure 7-1, below which (toward the lake) are old outcrops of the Pleistocene evaporites, roughly follows the white areas visible along the west edge of the lake. Most test holes drilled around the margin of Playa Lucero have encountered 10 to 25 feet of evaporitic Otero Lake beds, below which lie typical non-evaporite lacustrine lithologies of sand, silt and clay (Almendinger, 1971). It is quite possible that the base level to which scour occurred in the gypsum sediments of Lake Otero was determined in part by scour to underlying clays, or simply to the water table. Both the groundwater table and lacustrine clays currently lie at shallow depths below the surface of Playa Lucero (Almendinger, 1971).

As noted by Almendinger (1971), there are significant facies changes within the Otero Lake deposits. On the eastern side of south Playa Lucero are outcrops of Otero sediments that consist of fine gypsum sand, with some cemented horizons that hold up the outcrops, and a few horizons of coarse-grained gypsum crystals (Figure 7-2A). Sedimentary structures within these beds include salt ridges and irregular beds suggesting deposition in shallow water that occasionally evaporated completely—in manner similar to formation of gypsum crystals in very shallow brine ponds in the MacLeod Basin, Australia (Logan, 1987). In contrast, Otero outcrops on the western shore of Playa Lucero have much more coarse-grained material including numerous horizons of twinned gypsum crystals up to several centimeters in size (Figures 7-3, 7-4).

The Otero outcrops along much of the western margin of the alkali flat exist as a series of broad, gently basinward-sloping terraces, commonly developed on selenite horizons or some other wind-resistant layer (Figures 7-3, 7-5). These terraces may have mixed origins related both to lake level stands and resistance to wind scour. Figure 7-5 shows an outcrop of the Otero sediments where a flat bed of selenite crystals several feet thick forms the surface above a gully, protecting the bank from collapse. These large selenite crystals, which are contemporaneous with the Otero sediments, appear to have formed mainly at the western boundary of Lake Otero (Figure 7-6). They may have formed due to proximity to the evaporitic center of the basin, or perhaps due to proximity to the mixing zone of fresh and evaporitic waters. In general, the rate of weathering of the large selenite crystals is so slow that, rather than contributing sand to the modern dunes, they appear instead to have a major geomorphic role in maintaining extensive terraces on the west side of Playa Lucero, protecting lake beds underneath from further wind erosion, (Figure 7-8).

Despite the stability provided to the landscape by the selenite horizons, gullying is very common along the outcrop belt (see also Figure 7-1). Here, selenite crystals have been unable to prevent headward erosion of gullies feeding Playa Lucero, although they do protect the tops of the steep-sided gullies from collapse, forming a rim-rock. These gully outcrops are instructive, as they reveal the intertonguing of Otero sediments and alluvial sediments brought down by streams during deposition (Figure 7-7).

Sediments of Holocene Playa Lucero

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The deposits of Playa Lucero occupy an extensive, active deflation basin carved from the Otero evaporite-phase deposits. Total sediment accumulation by Playa Lucero is small, at most only a few feet. This occurs for several reasons. Stream flows are only intermittently active, mainly during summer flood season. (Figures 7-9A, B, C). These flash floods bring gravel and sand to the western margin of Playa Lucero. Silts and mud also are brought to the playa and redistributed by both water and wind processes, but the rate of sedimentation is small. There is some wind erosion of the playa surface during dry periods, however, the groundwater table is shallow in this area and the sediment is kept moist and cemented by evaporites for much of the year. Thus, the contribution of Playa Lucero to the sediment budget of the White Sands is very minimal. Most of the terrains downwind of Playa Lucero are areas not of sand deposition, but of wind scour—plainly demonstrating that Playa Lucero is not presently a significant source of gypsum, nor is it an appropriate model for the origin of White Sands as a whole. More likely is that the sediments of Lake Otero accumulated to a complete thickness, then were removed by wind erosion less than 6000 years ago to form the present modern dune field,—a down cutting perhaps regulated by a falling water table, but not dependent upon a Playa Lucero model for the primary generation of gypsum. Indeed, as will be shown below, most of the sediments of Playa Lucero are quite different from those of upper (evaporitic) Lake Otero. They tend to be sandy or muddy. They also are dark and very organic-rich due to constant moisture, high salinity, and the growth of algae and sulphate-reducing bacteria.

The sediments of Playa Lucero can be divided into fine silts and clays, sands and gravels, and evaporites. The coarsest material is found on the west side of the playa and reflects input by ephemeral streams. Silts and clays are widespread and are interbedded with the fluvial sands. Figure 7-10 illustrates two classes of fines. Gypsiferous silts and muds are widespread on the playa, perhaps reflecting settling of suspended, recycled gypsum from the Otero formation as well as primary evaporites (Figure 7-10A). Quartzose muddy sediments of chocolate brown color are common in association with sheetflood fans on the west side (Figure 7-10B). Figure 7-11A-C illustrates some of these silty to sandy facies of the playa. These sediments show a more sabkha-like aspect, with abundant irregular salt ridge structures indicating surface exposure of the sediments. Figure 7-12 illustrates some unusual facies seen in our trenches.

Fluvial Sedimentation

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Although not proceeding rapidly in historical terms, the fluvial contribution to sedimentation at White Sands is geologically significant. Supplies of sediment do not enter the playa or Alkali Flats from the east because streams flowing from the Sacramento Mountains are intercepted by the dune field. In many places along the west side, however, fluvial sediments are creating sheetflood fans that are spreading onto the playa (Figure 7-14). There are also internal drainages from one basin to another within the Alkali Flat, such as that shown on Figure 7-14, that runs southward from Andrecito Creek into Playa Lucero.

There are several ways in which fluvial activity takes place along Playa Lucero and the Alkali Flats. One way is by sheetfloods across the alluvial fans or the broad terraces of the Otero deposits (Figure 7-15). These floods deposit thin layers of mud and sand in broad sheets. A second way is by channelized flow. For example, the drainage of Andrecito Creek (Figure 7-13) has incised into Pleistocene sediments, depositing Holocene sands at stratigraphic levels well below Pleistocene deposits. In places the unconformity between Holocene and Pleistocene is easily seen (Figure 7-16).

The third way in which fluvial processes take place in this area is through sheetflood fans on the playa (See Figure 7-14). This is the primary means currently by which new quartzose materials are added to the basin. The plume extending downwind of the Andrecito fan may reflect either wind redistribution of clastics from the fan, or windblown sheet floods across the playa (Figure 7-14).

The distributaries on the fans are sometimes curving, sometimes straight, and are subject to infill by windblown sand or other (later) floods. Grain size of the non-gypsiferous sediments becomes rapidly finer toward the center of the Playa Lucero and the Alkali Flat (Figure 7-18).





Illustrations

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Figure 7-1—An air photograph of Holocene Playa Lucero during summer flooding in June, 1996. The white outcrops immediately to the west of the lake and around the southern rim are exposures of the sediments of ancient Lake Otero. The highest topographic limit of these sediments is approximately 4000 feet above sea level.

Figure 7-2—Outcrops of Pleistocene Lake Otero Sediments. (A) Along the eastern shore of south Playa Lucero. At this locality Otero sediments consist of fine-grained gypsum sand with rare interbedded layers of coarse crystals. Resistant cap rock of gypsite is underlain by powdery gypsum. Outcrops extend around the lake basin margin to the horizon. The environment of deposition suggested by the sedimentary structures in the outcrops is that of an evaporitic sabkha. (B) Lake Otero sediments outcropping along the western shore of Playa Lucero. These outcrops contain more coarsely crystalline material than those in (A) above, as discussed further below, and in text.

Figure 7-3—Outcrops of the sediments of Lake Otero along Andrecito Creek. (A) The view basinward toward the yardang in (B), showing the gully of Andrecito creek (with trees), and Playa Lucero (dry) in the distance. The flat mesa on the right is a resistant horizon in the uppermost Otero beds. Picture was taken while standing on uppermost Otero beds. (B) view toward the San Andres Mountains and Andrecito Creek. The creek has incised into Lake Otero sediments, which appear below the surface of the yardang in the foreground, and as tan bluffs in the middle distance. This has resulted in the deposition of Holocene sands and gravels at elevations below that of Pleistocene Lake Otero sediments, and the re-cycling of some Otero sediments onto the playa.

Figure 7-4—Close-up views of remnants of the lake beds that have supplied most of the sand for the White Sands. (A) Photograph of a thin-bedded outcrop of Otero sediments at the west shore of Playa Lucero about ½ mile north of the visitor's trail. This outcrop consists of fine gypsum sand and silt, with numerous layers of coarse gypsum crystals. (B) A detail view of the coarse layer visible in the center of (A), showing gypsum crystals weathering from the outcrop.

Figure 7-5—Selenite horizons in the Otero beds. (A) A horizon of variously oriented selenite crystals in gypsum sand and mud overlying fine gypsum mud in a gully about 1 mile south of the visitor's trail on the eastern shore of Playa Lucero. This rather flat horizon has resisted both wind and fluvial erosion, and now underlies the rather extensive topographic bench in the vicinity. (B) A closer view of the selenite crystals in gypsum mud.

Figure 7-6—The extremely large selenite crystals observed along the western shoreline formed at the transition zone between the Otero beds and alluvial fans encroaching from the west. (after Almendinger, 1971). Similar large crystals have not been found along the eastern margins of the Otero sediments, although this may be due to lack of exposures as well as non-occurrence.

Figure 7-7—Outcrops of the Otero sediments about ½ mile west of the shoreline of Playa Lucero. (A) Red fluvial sands and gravels intertongue with white, gypsiferous Lake Otero sediments along a highly irregular contact. View is toward the southeast, with white surface of Playa Lucero in the distance. (B) Detail view of the interbedding. Note the rounded boulders in the fluvial sediments, and the mixing of gypsiferous and quartzose lithologies at places on the outcrop.

Figure 7-8—The gypsum crystals that break down to supply sand to the dunefield. (A) swallowtail twins weathered from Otero sediments on the Alkali Flat, partially dissolved by fresh water. (B) Larger selenite crystals that weather along crystal planes, opening book-like into plates. These plates then break into flattish sand grains that can easily be entrained by wind.

Figure 7-9—Playa Lucero in various stages of flood. (A) Playa Lucero at flood stage, view southward along the west shore. (B) Air view of Playa Lucero when flood waters are receding, with a dark, muddy rim bordering receding water. (C) The dry surface condition that commonly prevails on the playa. This view from the mouth of Andrecito Creek southeastward across the playa.

Figure 7-10—Trenches in muddy sediments of Playa Lucero. (A) Banded light gypsiferous muds overlie organic rich clays, on the west margin of the playa. This gypsum may be recycled from the Otero beds. (B) Detrital quartzose muds brought into the western shoreline area by surface runoff. These have been deposited on the playa surface at the mouth of Andrecito Creek. Note desiccation cracks at the playa surface. Underlying brown sandy/muddy unit shows extreme haloturbation. Light sediments below are probably Otero gypsum deposits. (That is, the bounding surface between the brown silts and the white gypsum in this photo may be an unconformity between Pleistocene and Holocene sediments.)

Figure 7-11—Trenches in Playa Lucero. All trenches were dug along the western shore of the playa near the visitor trail. (A) A thin layer of dark quartzose mud overlies irregularly bedded gypsum sand and silt that has salt-ridge structures. More massive gypsum begins about base of sign. This light-colored gypsum is a remnant of Lake Otero sediments. (B) Dark gypsum sand and mud deposits, laminated and bedded, haloturbated. This style of sedimentation is common worldwide on evaporite sabkhas. (C) Faintly bedded to massive gypsiferous sands with vertical bioturbation traces beneath thin clay-rich layers. Most of the sediments in this trench have salt ridge structures. White salt crust at top.

Figure 7-12—Playa Lucero sediments. (A) Algae sometimes bind fine sediments at the surface to create mats that are then torn and folded backward. In this instance, a bed of coarse selenite crystals has been revealed. West shore of Playa Lucero. (B) A channel-like form encountered in a trench ½ mile onto Playa Lucero from the shoreline that has placed brown, quartzose sand within white, bedded gypsum. Origin of the feature is not known. Surface expression (visible in upper right of photo) is circular.

Figure 7-13—Photograph showing low-relief alluvial fan/sheet flood deposits at the mouth of an distributary on the north side of Andrecito Creek. (A). Note the tongues of brown, quartzose sand extending outward toward the playa. The light layer is a very clay-rich mud, probably an overbank deposit from earlier flooding by the main branch of Andrecito Creek, which is behind (south) of the viewer. (B). A view of the same channel and coarse sands, slightly upstream and to the left of (A). Images in (A) and (B) overlap.

Figure 7-14—An air photograph of the Andrecito fan on the west side of the Alkali Flat. Note the brown plume of quartzose material extending downwind over a kilometer from the fan. Prevailing wind is from the southwest (lower left to upper right).

Figure 7-15—Sheetflood sediments on terraces above Andrecito Creek. (A) Surface of the sheet flood with mud cracks and light vegetation compared to surrounding areas. (B) Trench in sheet flood sediments showing banded sand and mud, with horizons of preserved mud cracks such as those presently on the surface.

Figure 7-16—The unconformity between brown fluvial quartzose sands and gravels above and gypsiferous fine grained deposits of Pleistocene Lake Otero below.

Figure 7-17—Distributary channels on Playa Lucero. (A) A sharply curving distributary channel cut into Playa Lucero sediments during floods that has later been draped by settling muds that formed mud cracks. (B) A straight sided distributary channel that changes from scour habit to depositional habit in the middle distance, losing cut banks in the process. Photographs are from the sheetflood fan at Andrecito Creek.

Figure 7-18—Andrecito Creek sheetflood fan deposits. (A) Gravel and sand below white evaporite efflorescence. (B) Laminated sand and brown silty mud interbedded beneath a distributary channel. Some of the sands and muds have been contorted by salt ridges.