Gran Desierto Dune Field, Sonora, Mexico

Sedimentology (2006) 53, 1391–1409

doi: 10.1111/j.1365-3091.2006.00814.x

Development of spatially diverse and complex dune-field patterns: Gran Desierto Dune Field, Sonora, Mexico CARRIE BEVERIDGE*, GARY KOCUREK*, RYAN C. EWING*, NICHOLAS LANCASTER, P. MORTHEKAI, ASHOK K. SINGHVI and SHANNON A . MAH AN§ *Department of Geological Sciences, Jackson School of Geosciences, University of Texas, Austin, TX 78712, USA (E-mail: [email protected]) Division of Earth and Ecosystem Sciences, Desert Research Institute, Reno, NV 89512, USA Physical Research Laboratory, Planetary and Geosciences Division, Navrangpura, Ahmedabad 380 009, India §US Geological Survey, Denver Federal Center, Lakewood, CO 80225, USA ABSTRACT

The pattern of dunes within the Gran Desierto of Sonora, Mexico, is both spatially diverse and complex. Identification of the pattern components from remote-sensing images, combined with statistical analysis of their measured parameters demonstrate that the composite pattern consists of separate populations of simple dune patterns. Age-bracketing by optically stimulated luminescence (OSL) indicates that the simple patterns represent relatively short-lived aeolian constructional events since 25 ka. The simple dune patterns consist of: (i) late Pleistocene relict linear dunes; (ii) degraded crescentic dunes formed at 12 ka; (iii) early Holocene western crescentic dunes; (iv) eastern crescentic dunes emplaced at 7 ka; and (v) star dunes formed during the last 3 ka. Recognition of the simple patterns and their ages allows for the geomorphic backstripping of the composite pattern. Palaeowind reconstructions, based upon the rule of gross bedform-normal transport, are largely in agreement with regional proxy data. The sediment state over time for the Gran Desierto is one in which the sediment supply for aeolian constructional events is derived from previously stored sediment (Ancestral Colorado River sediment), and contemporaneous influx from the lower Colorado River valley and coastal influx from the Bahia del Adair inlet. Aeolian constructional events are triggered by climatic shifts to greater aridity, changes in the wind regime, and the development of a sediment supply. The rate of geomorphic change within the Gran Desierto is significantly greater than the rate of subsidence and burial of the accumulation surface upon which it rests. Keywords Aeolian, dune patterns, Gran Desierto, Mexico, Quaternary, Sonora Desert.

INTRODUCTION The Gran Desierto Dune Field of Sonora, Mexico, shows a spatially diverse and complex pattern of dunes. The dune pattern is spatially diverse in that different types of dunes occur as clusters over the area (Lancaster et al., 1987; Lancaster, 1995). The field is also complex in the sense of Kocurek & Ewing (2005) in that simple patterns of

dunes occur spatially superimposed (Lancaster, 1992, 1993). At a fundamental level the Gran Desierto dune pattern represents the aeolian system response to the external forcing factors of climate, tectonism and sea level. Located within the oblique rift zone of the Gulf of California (e.g. Axen & Fletcher, 1998), the dune field occupies one of the most tectonically active regions in North America. The

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sedimentary basin of the northern Gulf of California and the adjacent Salton Trough is a major sediment depocentre focused upon the position of the Colorado River and its delta over time. Dates presented in this paper demonstrate that the development of the dune field occurred under the climatic and eustatic changes that occurred during and following the Last Glacial Maximum (20 ka). Given the range of external forcing factors acting upon the evolution of the dune field and the resulting complexity of the dune patterns, the Gran Desierto provides both a significant challenge and an opportunity in understanding aeolian system dynamics at the basinal scale. The purpose of this paper is to build upon the existing body of work on the Gran Desierto by: (i) identifying the separate dune patterns from recent remote-sensing images; (ii) quantifying and statistically analysing the dune morphology to test the hypothesis that each pattern represents a unique population; (iii) bracketing the age of development of the component patterns using optically stimulated luminescence (OSL); (iv) using ‘eomorphic backstripping’ to illustrate the progressive construction of the composite dunefield pattern; (v) reconstructing the most likely wind regimes that gave rise to the individual dune patterns; and (vi) addressing the evolution of the Gran Desierto in terms of its sediment state over time (e.g. Kocurek & Lancaster, 1999).

CONTEXT OF GRAN DESIERTO DUNEFIELD EVOLUTION

side from Yuma to El Doctor. The Mesa Arenosa is a rotated up-faulted block with more than 150 m of uplift during the Quaternary. This uplift, coupled with eustatic changes, has resulted in a series of beach terraces along the coast (Ortlieb, 1991). The Gran Desierto trough passes to the SE into sabkhas and salt marshes of the Bahia del Adair. Parts of the coastal plains are marked by elongate linear arms of parabolic dunes and interdune playas.

Tectonic setting The Gran Desierto is structurally positioned within the seismically active, rapidly subsiding Salton Trough, which is the inland extension of the Gulf of California (Fig. 1). The Salton Trough is bounded to the NE by the San AndreasAlgodones-Sand Hills fault zones, and by the San Jacinto-Superstition fault zones to the SW. The trough bifurcates southward into Mexico, with the sub-troughs separated by the Sierra Cocopah Range. The Gran Desierto is situated within the Mexicali Valley sub-trough, which is bounded to the SW by the Cerro Prieto Fault that gives rise to Sonora Mesa and Mesa Arenosa. The structural division between the Salton Trough and the Sonoran sub-province of the Basin and Range is not clear, but an extension of the Algodones-Sand Hills fault zones is reasonable. Although much of the Salton Trough has subsided to below sea level, transgression by the gulf is prevented by deltaic deposits of the Colorado River (Younker et al., 1982).

Sediment depocentre Physiographic setting The Gran Desierto Dune Field is situated on the north-eastern shore of the Gulf of California, east of the Colorado River Delta, and west of the Sierra Pinacate volcanic complex and the Basin and Range Province (Fig. 1). The dune field covers about 5700 km2 and is largely accommodated within a NW-SE trending topographic trough. To the NE, Gran Desierto sands thin against the bajada rimming the Tinajas Altas and Tule Mountains of the Basin and Range Province. To the SE, the dune field is bordered by the Sierra Pinacate, a Pleistocene–Holocene volcanic complex (see Lynch & Gutmann, 1990; Gutmann et al., 2000). To the SW, the Gran Desierto is bordered by Sonora Mesa and Mesa Arenosa. Sonora Mesa is an eastward-sloping platform that forms a prominent escarpment along its western

The Salton Trough forms a major depocentre in which 3–6 km of Miocene and younger sediment overlies oceanic-continental crust (Axen & Fletcher, 1998). Subsidence rates of 1– 1Æ5 mm year)1 have been documented in the western portions of the trough (Lonsdale, 1989), and higher rates of subsidence are associated with movement along specific faults. Initial entry of the Colorado River into the Salton Trough is marked by gravels overlying the Pliocene Bouse Formation and dated at 4 Ma (Johnson et al., 1983; Winkler & Kidwell, 1986). The Colorado River has since taken various paths within the Salton Trough, including periodically flowing inland to give rise to Lake Cahuilla and other lakes (Waters, 1983). The modern Salton Sea formed by accidental diversion of the Colorado River in 1905 (Sykes, 1937).

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Fig. 1. Map showing the physiographic and tectonic setting of the Gran Desierto Dune Field within the Salton Trough. Fault zones include the San Andreas (SA), Algodones-Sand Hills (A-SH), San Jacinto (SJ), Elisinore (E), Superstition Hills (SH), Superstition Mountain (SM), Laguna Salada (LS), Cerro Prieta (CP), as well as the Brawley seismic zone (BS). The Gran Desierto is largely situated within the Mexicali Valley sub-trough bounded by the Cerro Prieta fault to the SW and a possible extension of the Algodones-Sand Hills fault to the NW. The dune field occupies a physiographic trough between Sonora Mesa and Mesa Arenosa, and the ranges of the Basin and Range Province and the volcanic Sierra Pinacate. Note wind roses with resultants at the Algodones Dune Field, Yuma and Puerto Penasco.

Occupation of the Salton Trough by the Colorado River within the immediate area of the Gran Desierto is well documented, but poorly constrained in time. Fluvial sediments of Colorado River origin comprise the Sonora Mesa. These sediments are traceable within interdune hollows deep into the dune field and are thought to underlie most of the field (Blount et al., 1990; Fig. 2). The uplifted Mesa Arenosa exposes about 120 m of fluvial/deltaic deposits dated by a vertebrate fauna as Pleistocene (1Æ8–0Æ5 Ma) (Shaw & McDonald, 1987). Gravity surveys for the region (Sumner, 1972; Kinsland, 1989) show NW-SE trending depressions located E of Yuma, as well as within the Bahia San Jorge and Bahia del Adair (Fig. 2). The trough east of Yuma contains as much as 1000 m of fluvial deposits

including coarse cross-stratified gravels (Olmsted et al., 1973). The trough beneath Bahia del Adair is about 20 km wide and 800 m deep, and is similar in amplitude and orientation to gravity anomalies at the site of the present day Colorado River Delta, leading Kinsland & Lock (2003) to propose an Ancestral Colorado River route between the Basin and Range and Sierra del Rosario through the Bahia del Adair (Fig. 2). Blount & Lancaster (1990) identified three main sand populations within the Gran Desierto, based upon grain texture, mineralogy and spectral signature on Landsat TM images. The populations are derived from: (i) Ancestral Colorado River sediment that underlies the dune field; (ii) the present lower Colorado River valley and local upland sources; and (iii) coastal sediment derived


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from the Bahia del Adair inlet. Although significant dune-field-scale mixing of the sediment populations has occurred (Blount & Lancaster, 1990; Lancaster, 1992), individual aeolian constructional events can be related to specific source areas (Lancaster, 1995).

Because the Gran Desierto is characterized by both dune-field constructional events and stabilization surfaces (Lancaster, 1992, 1993), it can be inferred that the climate has varied at least marginally from its present configuration. Continent-scale palaeoclimatic models (e.g. Bartlein et al., 1998), regional proxy evidence from pack rat middens (e.g. Van Devender, 1987, 1994; Van Devender et al., 1990), and upwelling patterns in the Gulf of California (Barron et al., 2004) are partly contradictory, but largely suggest enhancement of components of the current wind regime, as well as variations in precipitation over the last 25 ka. Regional patterns of increased precipitation are indicated at the Last Glacial Maximum (greater winter precipitation) and in the early to middle Holocene (enhanced summer monsoons). Increases in precipitation were not significant enough to be reflected in the characteristics of soil profiles located within the Pinacates (Slate et al., 1991), and Cole (1986) found a typical desert scrub vegetation assemblage endured for most of the Holocene, as evidenced by pack rat middens at Picacho Peak, near Yuma. In a review of existing data, Metcalfe et al. (2000) concluded that the Lower Colorado River Valley may have remained a core desert area throughout most of the Quaternary.

DUNE PATTERN GROUPS

Methods Climatic setting The Gran Desierto currently occupies the Lower Colorado Valley subdivision of the Sonoran Desert (Felger, 1980). Regional annual rainfall ranges from 73 mm at Puerto Penasco to 62 mm at Yuma, and occurs as a result of both summer monsoonal convectional storms and winter Pacific fronts (Ezcurra & Rodrigues, 1986). Winds within the Gran Desierto are not well documented but the wind regime within the northern part of the dune field is thought to be similar to that at Yuma, which is characterized by winter winds from the NNWNNE, spring winds from the W-WNW, and summer monsoonal winds from the S-SE (Fig. 1). The S-SE winds become more prevalent southward in the dune field (Lancaster, 1995). To the south at Puerto Penasco, the wind regime is dominated by onshore components, with spring winds from the W, and year-round winds from the S. The present climate supports a relatively dense growth of low shrub and creosote bush, especially over the sand sheets.

The primary morphologic groups of dune types were identified by visual inspection of a Landsat 7 ETM+ satellite image with 30 m per pixel spatial resolution (Fig. 3), and a vertically exaggerated (50·) digital elevation model (DEM) (Fig. 4). A colour composite of bands 7, 4 and 2 provided the best resolution for identifying dune geomorphic features. The DEM was produced by the Shuttle Radar Topography Mission, and has 90 m per pixel horizontal spatial resolution (SRTM90). All resolvable dune crests within representative areas of the primary dune groups were manually traced (Figs 5 and 6), measured using GIS software and grouped by morphologic dune type. Dune crests less than 100 m in length cannot be resolved on images with 30 m spatial resolution, and this sets the lower limit of traced crestlines. Crest length was measured along the entire length of the crestline between terminations. Spacing was measured between crestlines, perpendicular



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to the orientation of the dune. Orientation was measured along a straight line parallel to the overall trend of the crestlines for crescentic dunes, or along a straight line parallel to the actual crestlines for linear dunes. The terminations of crestlines are pattern defects, and defect density (q) is a field-scale parameter that expresses the number of defect pairs (N) per unit length of crestline (L), q ¼ N/L (Werner & Kocurek, 1999). All measurements were recorded in an attribute table (see Appendix I–III in Beveridge, 2004) from which a standard query language macro was used to calculate distance and orientation. The measurements were then exported to a spread sheet

and plotted on cumulative log frequency plots to evaluate the population mean, standard deviation and coefficient of variation for the measured parameters of each dune group. Orientations were plotted on rose diagrams. These statistics were used to identify trends in the patterns following the method described in Ewing et al. (2006). Probability plots of dune crest length and spacing provide a means to statistically identify populations of these pattern parameters within a dune field. Discrete populations are identified as line segments separated by inflection points. Where some degree of overlap occurs between populations a transitional segment occurs, but the


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inflection point is taken as the division between populations.

Dune pattern groups by visual identification The primary groups of dune types that give rise to the composite Gran Desierto pattern are designated: (i) relict linear dunes; (ii) star dunes; (iii) degraded crescentic dunes; (iv) eastern crescentic dunes; and (v) western crescentic dunes (Figs 3, 5 and 6). In addition, sand sheets characterize the north-western portion of the Gran Desierto, and separate the eastern crescentic dunes from the remainder of the dune field (Fig. 3). This grouping is largely the same as given by Lancaster et al. (1987), Blount & Lancaster (1990) and Lancaster (1992, 1995) with the exception of the relict linear dunes that represent a new interpretation. At a finer scale geographically, yet greater dune-pattern complexity occurs within the Gran Desierto,

especially within the area north of the Sierra Rosario (see Lancaster, 1992). The core of the dune field consists of star dunes that occur in distinct chains that trend 296 (Fig. 3). The star morphology is defined by prominent arms trending 060 and diffuse secondary arms (Fig. 6A). The star dunes range from 80 to 100 m in height, and typically show distinct stoss/lee slopes positioned upon moderate sloping plinths (Lancaster, 1989). The intriguing occurrence of the star dunes in chains along a linear trend that does not coincide with either arm orientation of the star dunes may be best addressed by inspection of the DEM (Fig. 4). Based upon the DEM, at least three lines of evidence suggest that the star dunes occur superimposed upon ridges that are not readily visible on the Landsat image. First, the star dunes are relatively insignificant features upon the much more prominent ridges. Second, ridges



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Fig. 5. Map showing dune groups with lines representing the digitized crestlines for each group from which the statistics regarding their parameters were derived. All visible crests within a dune group or within an area of the dune group were used.

occur that do not host star dunes. Third, some ridges show tuning-fork junctures typical of linear dunes. These elevated ridges are interpreted as relict linear dunes, upon which the star dunes formed. In this interpretation, the constructional event that gave rise to the star dunes partly utilized the sand source represented by the older linear dunes, and thus assumed their distinctive linear-chain configuration. Partly vegetated, degraded crescentic dunes 15– 20 m in height occur to the south of the core of star dunes (Figs 3, 5 and 6B). As recognized by Lancaster (1992), this degraded dune topography extends under the field of star dunes, where it is exposed in some areas between clusters of star dunes. The surface following the modified dune topography (Surface A of Lancaster, 1992) is a pedogenic horizon with abundant rhizoconcretions, incipient cementation and weak caliche development, and is interpreted as representing a period of dune-field stabilization. The degraded crescentic dunes rest upon Surface B of Lancaster (1992), which overlies Colorado River alluvium. To the north and west of the core of star dunes are the western crescentic dunes, which are typically 5–10 m in height, and largely vegetated and stabilized (Lancaster, 1995; Figs 3, 5 and 6C). These dunes have a pedified upper bounding surface and a lower lag bounding surface tentatively correlated by Lancaster (1992) with Surface A and Surface B noted above respectively. This correlation places the degraded crescentic dunes

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south of the core area and the western crescentic dunes between the same bounding surfaces stratigraphically, although they need not be exactly the same in age. The western crescentic dunes yield to a broad area of vegetated sand sheets that separate the dune field from the Colorado River valley (Fig. 3). Lancaster (1993) identified an undulating, pedogenic surface and a planar lag surface within the stratigraphy of the sand sheets, and correlated these to Surface A and Surface B within the dune field (Lancaster, 1992). The eastern crescentic dunes form a distinct cluster separated from the core of the dune field by vegetated sand sheets (Figs 3, 5 and 6D). These dunes are 10–80 m high (Blount & Lancaster, 1990) and range from complex and compound to simple features (Figs 3 and 6D). These dunes appear to rest upon a surface of distal alluvial deposits largely derived from the Pinacate volcanic field. This surface is tentatively correlated with Surface B discussed above. There is no evidence for other surfaces within these dunes.

Statistical analysis of pattern groups Plots of the measured parameters of crest spacing and length as cumulative log frequency plots are shown in Figure 7A and B, calculations of defect density are shown in Fig. 7C, and plots of crest orientations on rose diagrams are shown in Fig. 7D. Calculations of the mean, standard deviation and coefficient of variation (standard deviation/mean) for spacing and crest length are given in Table 1. The plots largely support the visual grouping of dune-pattern types by showing that these groups constitute distinct populations. Consistent with Ewing et al. (2006), dune spacing is the most definitive parameter for recognizing populations within the Gran Desierto. The spacing of the relict linear dunes, the degraded crescentic dunes, the eastern crescentic dunes, the western crescentic dunes, and the primary arms of the star dunes are clearly defined separate, single populations (Fig. 7A). The secondary arms of the star dunes proved too diffuse in orientation for any meaningful measure of spacing. The coefficient of variation is 90,