New evidence for the timing of aeolian sand supply to the Algodones dunefield and East Mesa area, southeastern California, USA

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63-75

New evidence for the timing of aeolian sand supply to the Algodones dunefield and East Mesa area, southeastern California, USA S. Stokes a, G. Kocurek b, K. Pye °'*, N.R.

Winspear c

" School of Geography, University of Oxford, Mansfield Road, Oxford, OX1 3 TB, UK b Department of Geological Sciences, University of Texas at Austin, Austin, TX 78712, USA Postgraduate Research Institute for Sedimentology, University of Reading, Whiteknights, Reading, RG6 6AB, UK Received 7 December 1995; accepted 15 May 1996

Abstract Results are presented from a preliminary investigation undertaken to provide insight into the timing and causes of aeolian sand supply and dune dynamics within the East Mesa-Algodones dunefield area of southeastern California. Samples for radiocarbon and optically stimulated luminescence (OSL) dating were collected from a number of sites along a west-east transect extending from the 12 m shoreline of palaeolake Cahuilla to the central part of the Algodones dunefield. The western Algodones dunefield is underlain by weathered aeolian sands which yielded an OSL age of c. 31 ka, confirming earlier estimates of the timing of dunefield initiation. These deposits are largely buried by unweathered aeolian sediments which gave OSL ages of c. 3.1 ka and < 0.4 ka. These ages correspond quite closely with radiocarbon and OSL ages obtained from deposits within, and immediately overlying, the former lake shorelines, providing support for the hypothesis that the shorelines acted as an important source of aeolian sediment. Dates from within the Algodones suggest an average long-term eastward movement of the dunefield of c. 16.6 m ka 1. Individual transverse megadunes are indicated to have moved southeastwards at an average rate of 2-5 m a-1 over the last 100-200 yr, an order of magnitude higher than previous estimates based on process studies.

Keywords: aeolian dunes; luminescence; optical dating; Algodones dunefield; California

1. Introduction T h e A l g o d o n e s dunefield, l o c a t e d o n the e a s t e r n side o f the I m p e r i a l Valley, s o u t h e a s t e r n C a l i f o r n i a , is the largest active dunefield in C a l i f o r n i a , h a v i n g a m a x i m u m length o f 75 km, a m a x i m u m w i d t h o f a b o u t 8 km, a n d an a p p r o x i -

* Corresponding author. 0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved

PHSOO31-O182(96)OOO48-X

m a t e l y N W - S E o r i e n t a t i o n ( F i g . 1). T h e centre o f the dunefield consists o f a series o f S W - N E t r e n d ing transverse m e g a d u n e s with s u p e r i m p o s e d b a r c h a n o i d d u n e s ( H a v h o l m a n d K o c u r e k , 1988; Sweet et al., 1988, 1989). L i n e a r d u n e s with a b r o a d l y N W - S E crest o r i e n t a t i o n , t o g e t h e r with a c o a r s e - g r a i n e d s a n d r a m p with zibars, d o m i n a t e the western side o f the dunefield. To the east o f the dunefield lies a series o f alluvial fans which a d j o i n the C h o c o l a t e a n d C a r g o M u c h a c h o

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63-75

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Mountains, while to the west is an extensive area of flat terrain, known as the East Mesa, which is characterized by vegetated sand sheets, sand streaks and low linear dunes. The western margin of East Mesa is marked by the 12 m shoreline of

former Lake Cahuilla, a large palaeo-lake which existed in the Imperial Valley at a number of times during the late Quaternary (Waters, 1983). Previous authors have suggested that the Algodones dunefield is composed largely of sand

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63 75

blown from the former shorelines of Lake Cahuilla. Brown (1923), Norris and Norris (1961) and McCoy et al. (1967) suggested that most of the sand was blown from the 12 m N.A.D. (North American Datum) shoreline whereas Thomas (1963), Loeltz et al. (1975), Sharp (1979) and Sweet et al. (1988) thought shoreline sources at 20-50 m N.A.D. more likely. Sharp (1979) identified possible beach ridges less than 1.6 km west of the western Algodones dunes on East Mesa, and Loeltz et al. (1975) mapped a shoreline along the base of the Algodones which they correlated with the 46 m N.A.D. shoreline on the western side of the basin, from which an associated tufa deposit yielded a radiocarbon age of 37.4_+2.0 14C yr B.P. (LJ-959; Hubbs et al., 1965). This shoreline reportedly occurs at an altitude of 49 m N.A.D. at the southern end of the Algodones and dips towards the northwest, where it is truncated north of Niland by the 10-12 m shoreline (Loeltz et al., 1975). Other radiocarbon dates have shown that Lake Cahuilla reached the level of the 10-12 m shoreline on several occasions during the late Holocene (Waters, 1983), raising the possibility that a large proportion of the Algodones and East Mesa sand could have been supplied quite recently. McCoy et al. (1967) suggested that much of the sand deflated from lake shoreline sources was introduced to the northern end of the Imperial Valley by the Whitewater River and neighbouring channels. Based on an assumption that the late Pleistocene wind regime was similar to the present, they suggested net southerly longshore drift of sediment along the eastern shoreline of the lake. A comparison of dunefield volume and longshore drift rates under an assumed wave regime led these authors to estimate a minimum age of 160 ka for the Algodones dunefield. However, on the basis of mineralogical and foraminiferal evidence, several other authors have concluded that the dune sands in the southeastern Imperial Valley are derived mainly from the Colorado River, which lies to the south (Merriam and Bandy, 1965; Muffler and Doe, 1968; Merriam, 1969; Van de Kamp, 1973; Muhs et al., 1995). Recent work by Winspear and Pye (1995) has also supported a Colorado River origin for the majority of the Algodones sand, although contributions from sources around

65

Mammoth Wash and the alluvial fans to the east are locally significant. This paper presents the results of a preliminary investigation, combining the use of both radiocarbon and optically stimulated luminescence (OSL), or optical, dating methods, aimed at providing additional chronological evidence to test the hypothesis that there is a relationship between lake shorelines and aeolian accumulation episodes, and to provide further information about the dynamics of the Algodones dunefield. In particular, the study utilises the experimental method of optical dating of quartz sand grains to directly date aeolian sediments. Since the optical dating method was first developed (Huntley et al., 1985), the technique has been used successfully in several studies to provide a chronostratigraphic framework in continental and coastal dunefields (e.g. Stokes and Gaylord, 1992; Stokes and Breed, 1993; Pye et al., 1995).

2. Geological background The Imperial Valley forms part of the Salton Trough, a tectonically-active fault-bounded structure which contains a thickness of over 6000 m of mainly non-marine Cenozoic sediments (Wilson and Wood, 1980). Numerous freshwater lakes have occupied parts of the Imperial Valley during the late Quaternary, and relict shorelines are prominent around the basin margins (Stanley, 1962; Wilson and Wood, 1980). Loeltz et al. (1975), Sharp (1979) and Waters (1983) concluded that high stands of these lakes, collectively known as Lake Cahuilla, were caused by periodic westward avulsion of the Colorado River into the Salton Trough, triggered either by large floods or tectonic activity. Waters (1980) recognized the tectonically deformed remnants of six late Pleistocene shorelines at altitudes of between 52 m and 31 m N.A.D. on the western side of the basin, but they are poorly preserved or absent on the eastern side. Waters (1983) also identified a number of essentially undeformed Holocene shorelines, and suggested that the basin has been filled to the level of the 12 m N.A.D. shoreline at least four times during the last 2000 yr, most recently 400-550 yr

66

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63 75

ago. The 12 m level represents the height at which lake waters were able to overflow across the lowest point of the Colorado delta, located at Cerro Prieto (Fig. 1), towards the Gulf of California during the late Holocene. The decreasing altitide of the Lake Cahuilla shorelines over time is likely to have been a result of continuing tectonic subsidence in the Cerro Prieto area (cf. Silver and Vallette-Silver, 1987). When filled to the level of the 12 m shoreline, Lake Cahuilla had a surface area of more than 5700 km 2 and a maximum depth of 95 m (Waters, 1983). Wilke (1978) calculated that 12-20 yr of Colorado River discharge would be required to fill the lake to this level, and that approximately 60 yr would be required to desiccate the lake completely with an average rate of lake level fall of 1.8 m yr -1 when isolated from Colorado River discharge. Formation of the modern Salton Sea occurred during the period 1905-1906, when winter flood waters of the Colorado and Gila rivers entered the irrigation system leading to the Imperial Valley (Sykes, 1937; Stephen and Gorsline, 1975; Waters, 1980). Since that time the level of the Salton Sea has fallen again and its surface area has decreased accordingly.

3. Aeolian processes and geomorphology The Imperial Valley presently experiences an arid climate, with a mean annual precipitation in the range 53-63 mm yr -1. The total wind energy can be classified as ranging from low to intermediate using the calculation method and terminology of Fryberger and Dean (1979). At Indio, located at the northern end of the Imperial Valley, winds blow predominantly from the northwest throughout the year but the annual drift potential (DP) is only 114 vector units (Muhs et al., 1995). At E1 Centro, located to the west of the Algodones, there are more important westerly and southwesterly components, and the annual resultant drift direction ( R D D ) is almost west to east. The annual drift potential at this station was calculated by Muhs et al. (1995) to be 392 vector units. At Yuma, which lies to the east of the southern limit of the Algodones, the wind regime is multi-direc-

tional; northerly winds dominate between October and February, westerly winds are most common during March, April and May, and southerly and southeasterly winds dominate between June and September. The annual resultant drift direction is from northwest to southeast but the annual drift potential is low (87 vector units). Sweet et al. (1988) calculated the resultant drift direction at a station (Drop One) located 3 km from the southwestern margin of the Algodones to be $24°E. The crest orientations and internal structures of the transverse megadunes in this area also suggest dominant movement to the SSE. However, Havholm and Kocurek (1988) and Sweet et al. (1988) recognized a significant component of ENE sand flow along the lee faces of the megadunes, which they attributed to secondary airflow generated by the interaction of the bedforms with the regional winds, and suggested that the resultant sand flow direction at present is approximately easterly. Spring is the windiest season when sand-moving winds are frequently associated with the passage of fronts (Havholm and Kocurek, 1988; Sweet and Kocurek, 1990). Based on an examination of sediments and internal structures, Sweet et al. (1988) suggested that the Algodones can be divided into three distinct regions, corresponding to the back-erg, central-erg and fore-erg regions of Porter (1986), each of which is characterized by distinct aeolian bedforms and sediments. The western margin of the dunefield, representing the back-erg environment, is formed by a sand ramp upon which zibar, inter-zibar and coarse-grained sand sheet deposits are superimposed (Nielson and Kocurek, 1986; Fig. 2). The ramp has a slope of 2 5 ° and rises to a height of approximately 15 m above the adjacent vegetated flats. It is underlain by aeolian sands which display high-angle cross-bedding, indicating that dunes formerly occurred further to the west than at present (Sweet et al., 1988). The area immediately east of the sand ramp is capped by N W - S E trending linear dunes. The central part of the dunefield is dominated by W S W - E N E trending transverse and barchanoid megadunes which are separated by flat interdune areas up to 500 m wide. The eastern part of the Algodones (fore-erg) is formed by small crescentic dunes, parabolic

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology128 (1997) 63 75

Fig. 2. Oblique air photograph looking southeast along the western sand ramp. Coachellacanal and East Mesa to the right. dunes, shrub-coppice dunes and wind-rippled sand sheets. The lower areas in this region are periodically flooded by waters from the alluvial fans to the east. Deposits characteristic of the fore-erg environment are exposed in interdune areas within the central erg region, indicating eastward migration of the central-erg dunes (Sweet et al., 1988). The surficial sand deposits of East Mesa, lying between the western margin of the Algodones and the 10-12 m shoreline, are up to 6 m thick. Aeolian sand forms thin sand sheets, mounds, streaks and poorly developed linear dunes up to 4 m high (Loeltz et al., 1975). Much of the area is now vegetated by the creosote bush (Larrea divaricata), although localised blowouts and areas of shifting sand occur. Along Interstate 8 much of the surface below the aeolian sands is armoured by gravel and granules, while along State Highway 78 the surface is extensively capped by a silty, calcareous crust. The long axes of the sand streaks and poorly developed linear dunes are broadly aligned N W - S E or W N W - E S E , striking the western margin of the Algodones at an oblique angle (Fig. 3).

4. Sampling rationale and analytical methods 4.1. Research questions

A suite of sediment samples was collected for OSL dating along a transect between the 12 m

67

Fig. 3. Oblique air photograph looking westwards across East Mesa towards the 12 m shoreline. The linear sand streaks, low linear dunes and largely vegetatednature of the terrain can be clearly seen. shoreline and the western part of the Algodones dunefield, and from a site within the central part of the dunefield close to Highway 78 (Fig. 4). Where possible, organic materials were also collected for radiocarbon dating, both to contribute to the chronological framework and to allow comparison with the OSL dates. In selecting sampling locations, a number of specific questions were borne in mind with a view to providing a more detailed picture of the evolution of the dunefield. These included: 1. What is the age of the sediments comprising the western sand ramp? According to the model proposed by Sweet et al. (1988), the western sand ramp should contain the oldest sediments in the dune field, and hence dates obtained from this area should provide an insight into the timing of dunefield initiation. 2. Are there significant gaps in time between the emplacement of the various aeolian bedforms and other morphological units present in the area? In particular, was the formation of major aeolian depositional units approximately coeval with periods of lake shoreline formation? 3. Is it possible to gauge the migration rate and turnover time of the megadunes by sampling crossbedded lee face deposits which are now exposed on the upwind sides of the bedforms? 4. What are the ages of the linear dunes which flank, and occur to the west of, the dunefield?

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63-75

68

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4.2. Sampling and laboratory methods Nine samples for OSL dating were collected by hammering light-proof PVC cylinders of known

volume (c. 500 cm a) horizontally into the vertical walls of freshly cleaned exposures prepared at each site. The ends of the cylinders were sealed with black tape and placed in black polythene bags for

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63-75

transportation to the dating laboratory in Oxford. In the laboratory, all samples were processed under subdued red light. A portion of each sample was wet-sieved to separate the 90-125 mm size fraction and immersed for two days in 1N HC1 to remove carbonate, followed to by two days immersion in H 2 0 2 to remove organic matter. Heavy minerals (density >2.72 g cm 3) were removed from the treated sample fraction by magnetic and heavy liquid (sodium polytungstate) separations. The samples were then treated with 48% HF for 60 min and 40% H2SiF 6 for 2 days in order to further concentrate quartz grains. A further heavy liquid separation and a second stage of dry sieving through a 75 mm sieve was undertaken to obtain the final quartz concentrate used for dating. At each stage of the separation procedure samples were generously rinsed in distilled water. The quartz separates were then mounted as monolayers (approximately 5 mg per disc) onto 10 mm diameter stainless steel discs using a silicone spray adhesive (Silkospray). A number of the prepared discs were tested for contaminant grains by infra-red light exposure (Stokes, 1992). Paleodoses were calculated using the multiple aliquot dose method (Aitken, 1992). Aliquots were exposed to an argon ion laser (Coherent 2W), operated at an emission wavelength of 514.5 nm and at a power output level at the sample of 40 mW cm-/). The resulting sample OSL emissions were detected using a photomultiplier filtered by BG-39 and Corning 7-51 glass filters. Prior to OSL measurements, aliquots were pre-heated to remove geologically unstable charge populations created during laboratory irradiation procedures. The preheat procedure involved heating the discs at 160°C for 16 hours. Normalisation of growth curve data was undertaken either by sampling small portions of natural OSL prior to dosing and pre-heating (so-called natural normalisation), or by measuring the OSL responses to a test dose following a series of bleaching and pre-heating procedures (so-called equal total dose normalisation). Details of these methods are given in Stokes (1994). There was insufficient measurable natural OSL in sample 908/2 to perform natural normalisation. Instead, a dose normalisation procedure was employed

69

(Stokes, 1994). In all cases the resulting palaeodoses exhibit good agreement at one standard deviation errors, independent of the normalisation procedure used (see Table 1 and below). OSL growth curves were generated for progressive durations of laser exposure and palaeodoses were extrapolated for a range of exposure periods. Palaeodose estimates quoted in Table 1 are based on the light from integrated laser exposure at c. 100 mJ cm -2. For the purposes of discussion in this paper, all ages are quoted based on equal total dose normalisation. Sample splits for dose rate determinations were crushed and homogenized by ring milling for 1 hour. K 2 0 , U and Th concentrations were determined by neutron activation analysis. Conversion from concentrations to dose rate followed the procedures outlined in Aitken (1985). The cosmic ray dose rate contribution was estimated using the method described by Prescott and Hutton (1988). The OSL dates calculated are based on equal total dose normalisation, incorporate both random and systematic errors (Aitken and Alldred, 1976), and are quoted to _+1 standard deviation. At each sampling locality details of the morphostratigraphy were recorded in the field and samples collected for laboratory analysis of sedimentological characteristics. In a number of instances where suitable organic materials were found, samples were also collected for conventional radiocarbon dating at the University of Texas. Dates are reported as RCYBP (radiocarbon years before present, "present"= 1950 A.D.). By international convention, the modern reference standard used was 95% of the a4C content of the National Bureau of Standards Oxalic Acid and calculated using the Libby half-life (5568 yr). Quoted errors represent 1 standard deviation statistics (68% probability) and are based on combined measurements of the sample, background, and modern reference standards. Reported 613C values, based on measured 13C/11C ratios, were calculated relative to the PDB-1 international standard and the RCYBP ages were normalised to - 2 5 per mil PDB. Calibration of the conventional radiocarbon ages to calendar years was undertaken using the CALIB programme of Stuiver and Reimer (1993).

70

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63 75

Table 1 Summary of optical dating results, nn = natural normalisation, etd = equal total dose normalisation, dn = dose normalisation, WF = saturation water content (W) multiplied by the estimated average sample moisture content over the burial period (F), H20% = measured water content. Samples OSL 904/1,906/1 and 907/1 were checked by field gamma spectrometry. Full details of analytical procedures are given in Stokes (1994). Data for sample 908/2 relate to a surface sample undergoing active aeolian transport Sample (OXOD)

Normalisation method

903/1

nn etd nn dn etd etd nn etd dn etd etd dn etd nn etd

904/1 904/2 905/1 906/1 907/l 908/1 908/2 909/1

Palaeodose (Gy) 0.89_+0.07 0.80_+0.10 72+11/-9 0.5_+0.2 0.5_+0.2 6.1+_l.2 0.30_+0.09 0.28 _+0.07 0.78_+0.18 0.50_+ 0.17 0.22_+0.23 0.04_+0.12 0.01 _+0.11 0.46_+0.09 0.13 _+0.08

WF H20 (%)

K20

0.03 2.9

2.05+_0.27 3.10_+0.18 1.00_+0.14 2.25_+0.31 0.39_+0.06 0.36_+0.13 1.69_+0.23 3.20_+0.18 1.00_+0.15 2.35+0.11 30.58+6.9/-6.3 1.45_+0.24 3.70_+0.t8 1.20_+0.15 1.87+0.27 0.27_+0.13 0.27_+0.13 2.17_+0.27 2.90-+0.15 1.00_+0.14 2.32-+0.31 3.17_+1.1 1.93_+0.27 3.20_+0.18 0.90_+0.14 2.30_+0.11 0.13_+0.05 0.12 _+0.04 1.93_+0.27 3.00_+0.16 1.00_+0.14 2.33_+0.12 0.33_+0.09 0.23 _+0.08 1.69_+0.28 3.30-+0.16 0.80+_0.14 1.93-+0.32 0.11_+0.13 NA NA NA 2.10_+0.33 0.02_+0.06 -0.004_+0.07 1.57_+0.25 3.60_+0.18 1.20-+0.16 1.95_+0.28 0.23_+0.06 0.09_+0.05

0.02 1.5 0.01 0.7 0.03 2.9 0.01 1.3 0.03 0.3 0.03 3.2 NA NA 0.01 1.2

(%)

5. Results S a m p l i n g site 903 was l o c a t e d in a f o r m e r gravel q u a r r y w h i c h e x p o s e d the s t r a t i g r a p h y o f the 10-12 m shoreline which at this l o c a t i o n c o m p r i s e d a series o f N - S striking b e a c h ridges, c o m p o s e d m a i n l y o f gravel a n d m e d i u m to c o a r s e sand, which d i s p l a y w e s t w a r d - d i p p i n g a c c r e t i o n surfaces ( F i g . 5). T h e u p p e r shoreface a n d lower b e a c h crest d e p o s i t s c o m p r i s i n g the ridges are d o m i n a t e d

Fig. 5. Section through the 12 m shoreline exposed in a gravel quarry, site 903.

Th (ppm)

U (ppm)

Dose rate Age (Gy/ka) (ka)

by i m b r i c a t e d gravel clasts up to 12 cm in diameter, b u t the lower shoreface sediments are p r e d o m i n a n t l y sandy. The p r o p o r t i o n o f gravel in the b a r r i e r decreases s o u t h w a r d s , a n d to the east lie s a n d y l a g o o n a l deposits ( V a n de K a m p , 1973). The g r a v e l - d o m i n a t e d u p p e r b e a c h ridge units have a c l a s t - s u p p o r t e d fabric a n d i m b r i c a t i o n o f the larger, flattened clasts t o w a r d s the east is c o m m o n , i n d i c a t i n g d e p o s i t i o n b y p e r i o d i c washover. A h o r i z o n t a l flattened clast fabric is develo p e d on the crest o f the barrier, which is overlain locally b y b l o w n sand a n d small s h r u b - c o p p i c e dunes. T h e crest o f the b a r r i e r has an elevation o f a p p r o x i m a t e l y 10-12 m N . A . D . , while the u p p e r surface o f the s u b - b a r r i e r d e p o s i t s lies at a p p r o x i m a t e l y 7 - 8 m N . A . D . T h e > 5 m m clast fraction is c o m p o s e d m a i n l y o f p o r p h y r i t i c felsic a n d mafic volcanics, granite, chert, m e t a q u a r t z i t e , calcrete f r a g m e n t s with rare b i o t i t e - m i c a schist a n d amphibolite. In the sequence s a m p l e d for O S L dating, m o r e t h a n 3 m o f i m b r i c a t e d gravels a n d coarse to m e d i u m sands are overlain by thin ( < 10 cm) b u t laterally c o n t i n u o u s shelly m a r l a n d o r g a n i c m a t t e r - r i c h ( p e a t y ) horizons. These deposits, in turn, are o v e r l a i n by > 1 m o f c r o s s - b e d d e d

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63-75

medium to fine sands (Fig. 6). The gravel below the organic layer is ferruginised. The organic layer bifurcates southwards over a lateral distance of 20 m and encloses a 5 15 cm thick marl unit containing gastropods identified as Physa (Physella) humerosa, Fonticella longinqua, and Tryonia protea (G.L. Kennedy, pers. comm.). These deposits are interpreted as having accumulated in an intrabarrier swale environment, which was affected by relatively high groundwater levels, some time after the deposition of the underlying gravel. Gastropods from the marl yielded a conventional age of 1080_+7 14C yr B.P. (TX-7522). The upper part of the organic carbon-rich layer was also sampled for radiocarbon dating and gave a conventional radiocarbon age of 414+50 yr B.P. (TX-7523). Samples for OSL dating (903/1 and 903/2) were collected from positions above and below the radiocarbon dated layers, but only sample 903/1, from the basal part of the crossbedded (aeolian) sand unit, provided sufficient 90-125 gm size quartz to allow palaeodose estimation. The OSL age obtained from sample 903/1 (360_+ 130 yr B.P., Table 1) compares favourably

m

O-

903/1 ) ,.~= (360-+130a) 1 414_+5014C yr BP (TX-7523) 1080-+7014C yr BP (TX-7522}

large scale crossbedded, fining upwards, fine to coarse sands individual beds very well sorted dip decreased down tobose of bed extensive biofurbation at base <..distinct disconformity thin continuous organic layer thin (articulated) shell layer

imbricated, poorly to moderately bedded gravels dip oriented towards palaeoloke

9o ,2 5-l,

gastropod and bivalve bearing, cross-bedded medium to coarse, shorefoce sands

Fig. 6. Schematic representation of the stratigraphy at site 903.The upper part o f the sequence is exposed on the east side of the former gravel quarry; the lower part of the sequence shown was observed in a pit dug on the floor of the quarry.

71

with the radiocarbon age obtained from the underlying organic layer (Table 2). Sediments interpreted as lower beach and nearshore deposits associated with the 10-12 m shoreline were sampled in a 1.5 m deep pit excavated in the floor of the quarry approximately 80 m northwest of the sequence described above. This pit exposed well-stratified sands in which medium and coarse sand units, containing granule stringers, are separated by scour surfaces. Vertical rhizoliths and gastropods occur towards the top of the sequence. The principal gastropods present were identified as the freshwater species Fonticella longinqua and Tryonia protea (G.L. Kennedy, pers. comm.). A conventional radiocarbon age of 3580_+90 yr B.P. (TX-7521) was obtained from gastropods collected from a depth of 0.6 m below the surface (Table 2). In order to obtain an indication of the age of the surficial sand deposits and linear sand streaks on East Mesa, to the east of the 10-12 m shoreline (Fig. 2), a pit was excavated on the crest of a low dune close to State Highway-78 (Site 909, Fig. 4). More than 1 m of ripple laminated, medium to fine sands were observed below a weakly developed soil (Fig. 7). A sample (909/1) collected from a depth of 1 m gave an OSL age estimate of 90_+ 50 yr B.P. (Table 1). At site 904 two pits were dug on the thinly vegetated western sand ramp. The first pit was located near the base of the ramp where 0.2-0.3 m of structureless medium to fine sand overlies 1.3 m of darker brown, indurated, medium to fine sand containing scattered carbonate nodules. A hard, semi-continuous caliche layer occurs at 1.5 m depth, below which the apparent carbonate content decreases again (Fig. 8a). The uppermost sands show evidence of periodic remobilization by the wind, forming small coppice dunes. On the basis of their degree of pedogenesis, the underlying deposits were expected to be considerably older. This was confirmed by a sample from 1.7 m depth which gave an OSL age of 30.6 + 6 . 9 / - 6 . 3 ka. Indurated, reddish brown sediments of similar appearance are locally exposed at the surface over a distance of 300 m upslope. A second pit at locality 904 was dug approximately 100 m upslope from the first. The sediments

72

S. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology 128 (1997) 63-75

Table 2 Summary of radiocarbon dating results Laboratory code no.

Material type

Measured ~4C age

~3C %0

Conventional 14C age

Calibrated age (cal yr)

TX7522 TX7523 TX7521

shell peaty soil shell

690 + 70 410 4- 50 3180 _+90

- 1.06 - 24.70 - 0.70

1080 4- 70 414 4- 50 3580 _+90

A.D. 928 A.D. 1022 A.D. 1438-A.D. 1616 1900 B.C. 2029 B.C.

Calibrated ages determined using the CALIB programme of Stuiver and Reimer (1993).

O_

N

limited soil development medium- fine sands

~J/O'J

cm

~l'J

coarse ripple and planar bedded sands

50some very fine laminations

root traces down to 80cm

909/1 (90+50o)

>

lower contact not observed

Fig. 7. Schematic summary of the stratigraphy at site 909.

(o)

(b) scattered roots no clear evidence of soil development

. . . . . . . .!i!. . .

low angle to horizontally ~arninated sands

fine sand ( 7 5 YR 6 / 6 )

some coarse lenses near top of sequence

distinct contacl (wavyl

structureless, well sorted fine sand, nodular calcite c o m m o n ( 7 . 5 YR 5 / 4 )

904/1 (30.6+6"9 6 $ko )

here consist o f 0.4 m o f structureless, low-angle cross-bedded to horizontally laminated, m e d i u m to fine sands, overlying > 0 . 6 m of l o w angle, m e d i u m to coarse sands (Fig. 8b). The sediments are interpreted as aeolian sand sheet and ripple deposits overlying slighly coarser but otherwise similar sediments containing granule and coarse sand lag horizons. A sample f r o m 0.7 m depth in this profile (sample 904/2) gave an OSL age o f 270_+ 130 yr B.P. Site 905 was sampled to assess the age o f the zibar and underlying sediments which characterize the transition z o n e between the upper part o f the western sand ramp and the m a i n dunefield. A pit in the zibar z o n e exposed small to m e d i u m scale, l o w angle w a v y laminated, coarse to m e d i u m sands underlain by large-scale cross-bedded, m e d i u m to fine sands (Fig. 9a). Because the u p p e r m o s t zibar deposits were not thick enough to provide a suitable sample for OSL dating, a sample was taken f r o m the underlying cross-bedded (dune) strata 1 m below the surface (905/1). This gave an age o f 3 . 1 + 1.1 ka, providing a m a x i m u m age for the zibar deposits at this site ( O S L ages indicate the time elapsed since the sediments were last exposed

904/ ( 270±130a

structureless sand calcite content reducing with depth dOWn proile ( 7 ' 5 YIR 614)

(e)

cm

T...... \ ................. m

Fig. 8. Schematic representation of the stratigraphy and sedimentological features revealed in the trench sections from which samples 904/1 (a) and 904/2 (b) were collected.

gently dipping planer and ripple laminated cross beds some coarse laminations ( volumetrically minor )

distinct contact 50

-

• rn

/

w

c.

(b) Site906

905

wavy laminated sands (zibar-reloted deposits) /

:

Site

]

variably coarse and fine bedded, steeply dipping cross bedded sands

J

1 no distinct stratigraphic boundaries

( 2 0 ° dip - E N E )

905/I---'-> (3.t+-t.l ke)

9 0 6 / 1 ---> (120+-40o)

lower contact not seen

Fig. 9. Schematic representation of the stratigraphy and sedimentological features at sites 905 (a) and 906 (b).

s. Stokes et aL/Palaeogeography, Palaeoclimatology, Palaeoecology128 (1997) 63-75

to light, and the bedforms may be considerably older than the dated deposits). Site 906 was located on the western side of a NW-SE-trending linear dune near the western edge of the dunefield. A 2 m deep pit dug through the plinth of the dune revealed well-sorted, ripplelaminated and planar laminated medium sands (Fig. 9b). A sample from a depth of 1.8 m gave an OSL age of 120_+40 yr B.P. Two localities were sampled within the central erg. Site 907 was selected as it provided an exposure of well-developed interdune sediments comprising a sequence of finely laminated silty sands and silts, overlying 2-10 m thick sets of steeply dipping megadune cross strata (Fig. 10a). It was hypothesised that, if the interdune sediments represent a significant bounding surface, an age from the underlying deposits could provide insight into the timing and rates of megadune migration. The resulting OSL age obtained from sample 907/1 was 330 + 90 a. Site 908 was located on the upwind side of a transverse megadune located south of Highway 78 in the central part of the dunefield. A pit revealed steeply dipping strata, interpreted as former lee slope avalanche deposits, which are now exposed on the upwind side of the megadune as a result of its migration towards the southeast (Fig. 10b). A sample of these sands collected from l m depth gave an OSL age estimate of 110_+130 yr B.P. Considering the average width of the megadune bedforms (c. 500 m), this age suggests an average rate of downwind migration over approximately the last 100-200 yr in the range of 2-5 m a -1.

(o)

.......................................otN ...............

Site

907

(b)

muds with desiccolion crocks

Site 908

distract ~ n t o c t (bOUnding surface

distinct contacl medium- smoll s¢o~e trough Cross bedding, some sedimentary structures lost by pedogenesis neor sur foce

cm

cross-bedded sets of slipfoce deposits { grainfall + avalanche deposits) 50

fine 1o medium Iominoted Sands

~

dip 2 2 - 2 4 °

[

~

10 YR 6 / 6

dipping t O - 1 2 = ESE

}

rare concentrations of

o g a n e ma tt e¢ and root traces

908/1 -~ 30a

O~

lower contact not visible

Fig. 10. Schematicrepresentation of the stratigraphy and sedimentological features at sites 907 (a) and 908 (b).

73

In addition to the samples collected from buried contexts, a sample of m o d e m saltating sand was collected at site 908 (sample 908/2, Table 1) to establish the magnitude of the residual OSL signal. This sample yielded residual age estimates ranging from - 4 to 20 years, both estimates overlapping zero when one sigma errors are considered. The residual signal at deposition is therefore assumed to be negligible.

6. Discussion 6.1. Timing o f sediment supply to the Algodones

The OSL age of 30.6 ka obtained from the indurated, weathered sediments which underlie the western sand ramp indicates that at least part of the Algodones sand is of late Pleistocene age. Influx of these sands may have been associated with the highstand of Lake Cahuilla at 45-49 m A.H.D., postulated on the basis of limited radiocarbon evidence to have occurred around 37 ka (Loeltz et al., 1975). However, no direct evidence of a causal relationship is yet available. No evidence has been found in this study of extensive aeolian sand accumulation during the mid-Holocene period. The sands which overlie the Pleistocene aeolian sand unit show very limited weathering and pedogenesis. The oldest OSL age estimate obtained (3.1 ka) from the unweathered aeolian sands near the top of the western sand ramp corresponds broadly to the calibrated radiocarbon age of 3.8 ka (TX-7521) obtained from the lower part of 10-12 m shoreline. This is consistent with the hypothesis that lake level regressions following high stands of Lake Cahuilla in the later Holocene provided the source of additional sand which was injected into the Algodones and East Mesa. During periods of falling or oscillating lake level extensive areas of sandy sediment on the lake floor may have been exposed to wind action. Alternatively, beaches formed around the time of lake level maxima could have provided the main source of aeolian sands; at present, this debate cannot be resolved conclusively. Radiocarbon dates obtained in this study support previous conclusions (Waters, 1983) that

74

s. Stokes et al./Palaeogeography, Palaeoclimatology, Palaeoecology128 (1997) 63 75

Lake Cahuilla rose to the level of the l0 12 m A.H.D. shoreline most recently only c. 400-550 yr ago. The optical dates reported here also indicate that the surficial aeolian sediments and bedforms on East Mesa and the western Algodones are very young and post-date this most recent high lake stand. 6.2. Rates o f aeolian bedform and sand sea migration

The OSL dates presented in this paper provide an additional means of estimating rates of dune movement and sand accumulation within the Algodones dunefield. We estimate vertical accumulation rates of 10 20 cm a 1 on the linear dunes along the western margin of the central erg over approximately the last 100 yr. Oblique migration of the dunefield is estimated at approximately 16.6 m ka 1, a rate which is comparable with the previous estimate of 13.5 m ka 1 made by Sweet et al. (1988). The estimated average rate of N W - S E transverse megadune migration over the past 100-200 yr, based on the OSL dating results, lies in the range 2-5 m a-1, an order of magnitude greater than previous estimates based on shortterm measurements of dune movement and sandflow calculations (Sharp, 1979; Havholm and Kocurek, 1988; Sweet et al., 1988). The discrepancy may relate to our analysis of a more rapidly moving, medium-sized bedform, to a period of enhanced aeolian activity during the late nineteenth century compared with the present day, or to the range of error associated with the optical dates.

period of stability and pedogenesis during the very late Pleistocene and/or early to mid Holocene. (3) The available dating evidence is consistent with earlier suggestions that the shorelines of palaeolake Cahuilla provided a major source of the aeolian sand. (4) Many of the surficial sediments and aeolian bedforms are younger than the last recorded high stand of Lake Cahuilla (c. 400-550 yr ago). (5) Estimated average rates of downwind migration of the transverse megadunes over approximately the past 100 200 yr are of the order of 2-5 m a-1. The average rate of oblique migration of the dunefield towards the east over the past 3 ka is estimated to be 16.6 m ka 1, a figure which compares well with previous estimates.

Acknowledgements Financial support for this research was provided partly by grants from the University of Reading Research Endowment Fund ( K P and N R W ) and the Leverhulme Trust (KP). We thank G.L. Kennedy for identification of gastropod species and M. Crabaugh for assistance in sampling. A. Cross drafted the diagrams and earlier drafts of the manuscript were improved by helpful comments from A.S. Goudie, M.J. Aitken, N. Lancaster and D.S.G. Thomas. This paper represents University of Reading PRIS Contribution no. 352.

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