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Luminescence dating of sand ramps in the Eastern Mojave Desert
GHHORPHOIOGY ELSEVIER
Geomorphology 17 (1996) 187-197
Luminescence dating of sand ramps in the Eastern Mojave Desert H.M. Rendell, N.L. Sheffer Geography Laboratory, University of Sussex, Falmer, Brighton BN1 9QN, UK
Received 28 March 1994; revised 4 January 1995; accepted 4 January 1995
Abstract Until recently the timing of the movement of sand and accumulation of sand by aeolian processes in the Eastern Mojave Desert has remained the subject of speculation. The results of a luminescence dating program involving 78 samples of material from nine sand ramp complexes have enabled recognition of regional and local patterns of sand accumulation in the Eastern Mojave. The st~Ldy area extends from Lake Manix, in the west, to the Colorado River, in the east. The periods of accumulation have been identified on the basis of quartz TL dating, and potassium feldspar TL and IRSL dating. At a regional scale, two major depositional phases can be identified: (1) Late Pleistocene: 20-30 ka, with sequences at Hank's Mountain, Balch, Dale Lake and the Big Marias; and (2) Late Pleistocene-Early Holocene: 15-7 ka with sequences at Cat Dune, Dale Lake, Iron Mountain, Big Marias (above palaeosol) and, at the western end of the study area, at Soldier Mountain (20-7 ka). Late Holocene sequences appear to be much more localized, and are confined to the West Cronese, Old Dad Mountain and Balch sand ramps. The earliest phases of sand accumulation on the ramps studied immediately predate the formation of pluvial Lake Mojave (ca. 24.5 ka). The later main phases of accumulation coincide with the existence of Lake Mojave and the end of accumulation on the majority of the sand ramps immediately post-dates the final end of Intermittent Lake Mojave III (9.7 ka). The continuity of sand supply to potential deflation areas is seen as a critical condition for accumulation of material on the sand ramps during the Late Quaternary in the Eastern Mojave.
1. Introduction Luminescence techniques, either thermoluminescence (TL) or optic.ally stimulated luminescence (OSL), have the potential to date the last exposure of sediment grains to light. The present study involves the application of the~;e techniques to the quartz and feldspar fractions of :sand samples from nine major sand ramps in the Eastern Mojave Desert. In contrast to luminescence dating research which often uses a single mineral fraction and a single luminescence technique, the approach adopted in the present study enables any systematic biases with regard to particular methods or samples to be identified. Thus, data
are presented for quartz TL, feldspar TL, and infrared stimulated luminescence (IRSL) from feldspar.
2. Field area Nine sand ramps (or complex climbing or falling dunes) were sampled in the Eastern Mojave Desert, from Soldier Mountain in the west to the Big Marias sand ramp, near to the Colorado River in the east. The sampling sites and relationships to playa lakes, dune fields, and ephemeral flow channels are shown in Fig. 1. The thickness and complexity of the sequences sampled vary from sand ramp to sand ramp. The
0169-555X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0169-555X(95)00102-6
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
188
mering plastic tubes into freshly cleaned vertical sections.
Hank's Mountain (HM) and Big Marias (BM) sections are both relatively short, 4.4 m and 6.0 m thick, respectively, with straightforward stratigraphies. Soldier Mountain (SM) represents a much thicker (19.0 m) depositional sequence that is comparable in thickness with the 17.6 m composite section at Dale Lake, whereas the West Cronese (WC) and Balch (BA) sand ramps are more complex composite sequences. Initial work concentrated on the sand ramps at Dale Lake (Tchakerian, 1991) and the Cronese Cat Dune (Evans, 1961). Considerable problems were encountered in trying to date horizons comprising aeolian and colluvial mixtures from these two sand ramps (Rendell et al., 1994). Sampling of material from the other seven sand ramps was undertaken after the initial results from the Dale Lake sequence had been processed. Every attempt was made to take samples of sand from horizons free of colluvial clasts. Samples were collected by pushing or ham-
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3. Luminescence analysis Samples were prepared for analysis by pretreatment to remove any carbonates, sieving to separate out a dominant size fraction (normally 250-180 p~m or 180-125 txm), and density separation followed by etching, to isolate quartz and feldspar fractions. The density separation involved the use of a 2.58 g cm -3 sodium polytungstate solution, to isolate the potassium-rich feldspar fraction from the quartz. Quartz fractions were etched for 40 min or 60 min in 40% hydrofluoric acid (see Rendell and Wood, 1994, for discussion), whereas potassium-rich feldspar separates were etched for 40 rain, but in 10% hydrofluoric acid.
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Key: SM Soldier Mountain WC West Cronese ~ CD Cat Dune HM Hank's Mountain BA Baloh ~'OZZ/o4, ODM Old Dad Mountain DL Dale Lake -, ', % IM Iron Mountain "-~'~\1 BM Big Marias Mountain ~,
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~ Kelso Dunes
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Fig. 1. Location of sample sites in the Eastern Mojave Desert.
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
With the exception of a small number of samples from the Cat Dune and Dale Lake sand ramps, all of the luminescence measurements were made using multiple aliquots (subsamples). Each aliquot was prepared by using silicone spray to fix a single layer of grains on to the surface of a 10 mm diameter aluminium disc, or a rhodium-plated copper disc. Potassium-rich feldspar separates from selected samples from the Cat Dune and Dale Lake sand ramps were analysed using 'single aliquot' IRSL methods developed by Duller (1991). These measurements were done using a Rise automated reader in the Aberystwyth laboratory. Sample discs were preheated at 220°C for 10 min prior to each measurement and emissions were monitored via a Schott BG-39, Coming 7-59 and a neutral density filter by an EMI 9635QB PM tube. Short (0.5 s) IRSL measurements were made using IR-emitting diodes (TEMT-484), with peak emission at 880 nm, while holding the sample discs at a constant temperature
(50oc). TL analysis of quartz and feldspar fractions from Hanks Mountain, West Cronese and Balch sand ramps was undertaken using a manual TL reader. Samples were heated in an argon atmosphere at 150°C min -1, and TL emissions were monitored via Coming 5-58 (blue) and Chance-Pilkington HA3 filters by an EMI9636Q photomultiplier tube; no preheats were employed prior to measurement. The quartz and feldspar fi:actions from the Soldier Mountain, Big Marias, Old Dad Mountain and Iron Mountain sand ramps were measured on an automated Rise TL-OSL reader. Emissions were monitored via a Hoya U-340 filter and an EMI9635QB photomultiplier tube. Samples were heated in a nitrogen atmosphere at a rate of 2.81°C s -1. All of the multiple aliquot IRSL measurements were made on the Rise automated system using the filter-photomultiplier tube combination specified above. A preheat tre,atment of 1 min at 220°C was used prior to all IRSL measurements. Empirical data indicate that a short preheat such as 1 min is sufficient to remove the unstable component of the IRSL signal (Duller, 1993, fig. 4.7.3b). IRSL measurements were made by exposing sample discs to 30 s of IR while holding ~Ihem at 50°C. Depending on the technique used, disc-to-disc normalization was by weight, second glow or 'short shine'. In the latter, all
189
discs are exposed to IR light for a brief period (0.1 s) prior to all other treatments. Laboratory irradiations were performed using a 9 ° S t / 9 0 y beta source. Laboratory bleaching was undertaken using a 300-watt Wotan sunlamp at a distance of 30 cm from the sample discs. A sheet of plate glass was interposed between the lamp and the sample discs to remove the UV component of the light. A bleaching time of 4 h was routinely employed.
4. Dosimetry measurements The environmental gamma dose was measured in the field using a portable gamma spectrometer, with an external pitchblende-resin source to check for instrument drift. Bulk beta activity was measured in the laboratory using thick-source beta counting. The internal dose for potassium feldspar-rich fractions was calculated using measurements of the potassium content of each fraction. Potassium contents were obtained via acid digestion, using 40% hydrofluoric acid, and flame photometry.
5. Results The dosimetry data and the results of the dating determinations are summarised in Table 1, 2 and 3. The TL Equivalent Dose (ED, see Table 2) and age determinations were in all cases obtained using the regeneration method (Wintle and Prozynska, 1983), because of the relatively high degree of scatter exhibited by the additive dose growth curves. In contrast, the IRSL EDs were all obtained using additive dose techniques. The disadvantage of using additive techniques, as with TL, is that some samples exhibited a high degree of scatter amongst aliquots that resisted attempts at normalisation. In some cases, scatter was not a major problem, as shown by the additive growth curves for multiple aliquot IRSL determinations for potassium-rich feldspar separates for samples from the Big Marias and Soldier Mountain sections (Fig. 2). One critical question involves the nature of agreement or disagreement between the luminescence ages estimated using the three different techniques. Table 3 reveals that the sand ramps (e.g. Old Dad Moun-
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
190 Table 1 Mojave sand ramps: dosimetry data Sample code
Height below top of section (m)
External [3 dose (mGy/a)
External "y dose (mGy/a)
Cosmic dose (mGy/a)
Internal [3 dose (mGy/a)
HM01 HM02 HM03 HM04 WC01 WC02 WC03 WC06 WC07 WC08 WC09 WC10 WC12 WC13 WC14 BA01 BA02 BA03 BA04 BA05 BA06 BM01 BM02 BM03 BM04 SM01 SM02 SM03 SM04 SM05 SM06 SM07 SM08 SM09 SM10 SMll SM12 SMI3 SM14 ODM01 ODM02 ODM03 ODM04 ODM05 ODM06 IM01 IM02 IM03 IM04
4.40 4.00 1.00 0.60 0.60 8.50 9.90 18.20 20.20 0.50 1.30 2.20 1.50 3.10 3.60 0.25 2.00 3.00 11.20 10.50 1.50 0.50 2.14 2.70 6.00 7.00 7.50 7.70 1.00 2.40 9.10 10.20 10.70 12.20 14.30 15.10 16.50 18.50 19.00 0.50 3.30 0.61 2.30 5.17 6.90 1.84 2.50 4.35 4.95
2.91 2.96 2.99 3.32 2.80 2.89 3.01 2.78 3.05 2.98 3.10 2.99 3.15 3.09 2.99 3.12 3.13 3.12 3.24 3.13 3.13 3.14 3.05 3.27 3.34 3.06 3.02 3.21 3.39 2.20 2.02 3.57 3.48 3.29 3.67 3.52 1.23 3.85 3.18 3.10 3.01 3.10 3.15 2.96 2.98 2.89 2.74 2.84 2.91
1.31 _+0.02 1.41 + 0.01 1.22 _+ 0.01 1.24 ± 0.01 1.07 _+ 0.01 1.04 _+ 0.01 1.05 _+ 0.01 1.11 ± 0.01 1.05 + 0.02 1.04 _+ 0.01 1.03 _+ 0.01 1.03 ± 0.01 1.15 ± 0.01 1.19 _+ 0.01 1.25 _+ 0.02 1.14 _+ 0.01 1.09 _+ 0.01 1.11 _+ 0.02 1.18 _+ 0.01 1.25 _+ 0.01 1.16 _+ 0.01 1.04 ± 0.01 0.95 ± 0.01 0.95 ± 0.01 0.94 _+ 0.01 1.17 ± 0.01 1.09 _+ 0.01 1.10 + 0.01 1.12 + 0.01 1.07 - 0.01 0.94 + 0.01 1.13 + 0.01 1.12 ___0.01 1.39 + 0.01 1.27 + 0.01 1.22 + 0.01 1.23 + 0.01 1.19 ___0.01 1.18 + 0.01 1.11 + 0.01 0.94 + 0.01 1.01 + 0.01 1.09 _+0.01 0.98 + 0.02 0.99 + 0.01 0.99 + 0.01 1.06 + 0.01 1.05 + 0.01 1.08 + 0.01
0.11 _+ 0.01 0.12 _+ 0.01 0.18 _+ 0.01 0.19 _+ 0.01 0.19 _+ 0.01 0.07 _+ 0.004 0.05 _+ 0.003 0.03 _+ 0.002 0.025 _+ 0.001 0.19 ± 0.01 0.17 _+ 0.01 0.15 + 0.01 0.17 _+ 0.01 0.14 ± 0.01 0.13 ± 0.01 0.19 +_ 0.01 0.16 ± 0.01 0.14 ± 0.01 0.05 _+ 0.003 0.06 _+ 0.003 0.17 _+ 0.01 0.19 ± 0.01 0.15 + 0.01 0.14 _+ 0.01 0.09 _+ 0.005 0.08 + 0.004 0.08 _+ 0.004 0.08 + 0.004 0.18 + 0.01 0.15 + 0.01 0.07 _+0.003 0.06 + 0.003 0.06 + 0.003 0.05 + 0.002 0.04 + 0.002 0.04 + 0.002 0.03 + 0.002 0.03 + 0.002 0.03 + 0.001 0.19 + 0.01 0.13 + 0.01 0.19 + 0.01 0.15 + 0.01 0.10 + 0.01 0.09 + 0.004 0.16 + 0.01 0.15 _ 0.01 0.11 ___0.01 0.11 + 0.01
0.33 0.37 0.34 0.27 0.39 0.35 0.33 0.32 0.23 0.27 0.18 0.35 0.24 0.29 0.27 0.34 0.43 0.51 0.29 0.28 0.27 0.25 0.24 0.26 0.23 0.25 0.36 0.29 0.22 0.30 0.39 0.34 0.28 0.29 0.30 0.33 0.38 0.46 0.25 0.19 0.27 0.22 0.22 0.24 0.23 0.26 0.19 0.18 0.14
± 0.07 _+ 0.07 _+ 0.10 _+ 0.05 _+ 0.07 ± 0.05 _+ 0.08 _+ 0.06 _+ 0.08 _+0.09 _+ 0.11 + 0.07 _+ 0.06 _+ 0.07 _+ 0.04 _+ 0.07 ± 0.06 ± 0.07 _+ 0.07 _+ 0.06 _+ 0.07 ± 0.06 ± 0.08 ± 0.10 _+ 0.05 ___0.11 _+ 0.10 + 0.11 + 0.05 + 0.07 + 0.05 + 0.08 + 0.14 + 0.08 + 0.21 + 0.19 + 0.13 _+ 0.13 + 0.09 ___0.08 -- 0.15 + 0.11 + 0.10 + 0.11 __+0.11 + 0.11 + 0.09 ± 0.09 _ 0.07
_+ 0.02 _+ 0.02 _+ 0.02 _+ 0.01 _+ 0.02 _+ 0.02 _+ 0.02 ± 0.02 _+ 0.01 _+ 0.01 _+ 0.01 ± 0.01 ± 0.01 +_ 0.01 _+ 0.01 _+ 0.02 _+ 0.02 _+ 0.03 _+ 0.01 _+ 0.01 ± 0.01 ± 0.01 _ 0.01 ± 0.01 ± 0.01 + 0.01 _+ 0.02 + 0.01 ___0.01 + 0.02 + 0.02 + 0.02 + 0.01 _+ 0.02 + 0.02 ± 0.02 + 0.02 + 0.02 + 0.01 + 0.01 + 0.01 ___0.01 + 0.01 ___0.01 + 0.02 ± 0.01 ± 0.01 ± 0.01 ± 0.01
Notes: external beta doses assessed using thick source beta counting; external gamma doses assessed in the field using a portable gamma spectrometer; cosmic doses calculated using the formula given by Prescott and Hutton (1988); internal beta dose assessment based on potassium determinations on potassium feldspar-rich separates using acid digestion and flame photometry.
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
191
Table 2 M o j a v e sand ramps: equiv~flent dose estimates Sample
Q u a r t z T L ED ( G y )
HM01 HM02 HM03 HM04 WC01 WC02 WC03 WC06 WC07 WC08 WC09 WC10 WC12 WC13 WC14 BA01 BA02 BA03 BA04 BA05 BA06 BM01 BM02 BM03 BM04 SM01 SM02 SM03 SM04 SM05 SM06 SM07 SM08 SM09 SMI0 SMll SM12 SM13 SM14 ODM01 ODM02 ODM03 ODM04 ODM05 ODM06 IM01 IM02 IM03 IM04
94.60 115.20 122.60 102.04 35.90 59.03 29.20 28.60 * 13.23 * * 30.06 23.40 30.40 23.72 31.53 17.75 108.31 91.85 90.61 105.60 168.30 189.70 226.20 42.40 77.81 31.10 51.00 64.50 * 70.50 * 93.70 * * 89.20 89.60 84.62 18.19 26.39 19.53 * * * 39.88 * 74.24
+ 4.24 _ 3.07 + 3.51 + 5.40 _+ 0.17 ___6.20 + 3.18 + 0.27 + 1.64
+ 0.67 + 0.27 + 0.60 + 0.39 _ 0.87 + 1.26 + 16.70 + 1.06 + 1.24 _ 1.10 + 2.70 + 9.20 ___ 14.00 + 1.07 _+ 2.95 + 2.30 _ 4.70 __. 1.54 + 5.90 + 8.80
+ 3.39 + 10.70 + 2.03 ___0.47 + 4.99 +__0.67
+ 4.78 + 0.66
Feldspar T L E D ( G y ) 128.13 -t- 9.37 113.55 _ 11.30 120.30 + 5.19 118.00 + 2.04 24.13 + 0.50 20.50 + 2.10 2 7 . 5 0 + 5.20 6 2 . 1 4 + 1.20 35.37 + 8.10 8.06 + 2.70 36.28 ___0.41 127.20 + 6.06 19.43 + 0.09 25.25 + 0.76 25.60 + 1.78 26.43 + 2.77 38.43 + 3.33 12.91 __. 0.99 117.50 + 2.12 * 86.02 + 2.75 31.30 + 3.39 34.4 + 4.06 65.80 + 2.21 141.90 + 2.20 * * 33.50 -+- 1.40 36.10 + 6.49 54.70 + 1.31 * 4 8 . 0 0 + 3.43 * * 116.70 _ 4.90 * 100.10 + 6.60 136.90 + 9.20 * 15.15 + 2.10 24.20 + 1.93 15.15 + 5.10 30.95 +_ 0.25 58.32 + 0.55 40.65 + 0.45 56.25 + 1.89 85.66 + 9.02 89.66 + 1.81
Feldspar I R S L E D ( G y ) 136.40 91.88 94.46 6.09 9.43 22.05 28.14 6.84 50.35 53.24 7.60
_+ 11.40 + 22.50 + 3.80 + _ + _ + + + +
0.57 0.27 1.08 1.23 0.30 0.70 2.20 0.30
10.95 + 0.24 9.43 + 0.25 * 68.76 * 70.13 28.01 32.50 48.04 97.90 69.12 87.80 50.70 31.50 39.60 65.60 60.40 65.94 93.10 65.40 69.70 102.00 * 93.11 34.43 13.02 11.68 45.50 40.20 40.20 62.79 78.90
+ 4.13 + 1.92 + 0.63 + 2.17 + 1.50 + 1.34 + 4.50 + 4.98 + 1.64 + 1.48 + 1.78 + 3.39 + 2.76 + 3.21 ___5.37 + 2.79 + 4.84 + 1.46 + 4.51 + 1.66 ___ 1.29 + 1.51 + 1.33 + + + +
2.70 1.36 2.84 4.49
Notes: asterisk denotes h i g h degree o f scatter in data points; short d a s h denotes missing data. All T L E D ' s determined b y the regeneration method, all feldspar I R S L dates determined b y multiple aliquot additive dose.
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
192
Table 3 Mojave sand ramps: annual dose rates and age estimates Sample code
Depth (m)
Annual dose to quartz grains (mGy/a)
Quartz TL age estimates (ka)
Annual dose to feldspar grains (mGy/a)
Feldspar TL age estimates (ka)
Feldspar IRSL age estimates (ka)
4.40 4.00 1.00 0.60
4.23 4.31 4.21 4.51
_+ 0.21 _+ 0.22 _+ 0.21 _+ 0.23
22.93 26.72 29.13 22.60
_+ 2.52 _+ 2.93 _+ 3.20 _+ 2.48
4.45 4.69 4.55 4.79
_+ 0.22 _+ 0.23 __+0.23 __+0.24
28.77 24.23 26.47 23.05
_+ 3.88 _+ 3.27 -+ 3.57 __+3.11
30.63 _+ 4.13 20.22 -+ 2.73 19.74 _+ 2.66
0.60 8.50 9.90 18.20 20.20
3.86 3.80 3.91 3.73 3.92
__+0.19 + 0.19 __+0.20 +__0.19 + 0.20
9.30 14.53 7.47 7.67 *
__+1.02 __+1.60 __+0.82 + 0.27
4.24 4.15 4.24 4.05 4.15
+ 0.21 __+0.21 + 0.21 __+0.20 __+0.21
5.68 4.94 6.48 15.34 8.53
__+0.77 __+0.67 __+1.22 __+2.07 __+ 1.95
Hanks Mtn. HM01 HM02 HM03 HM04
West Cronese (a) WC01 WC02 WC03 WC06 WC07
1.47 2.23 5.44 6.78
__+0.19 + 0.30 + 0.74 __+0.92
West Cronese (b) WC08 WC09 WCIO W e s t Crouese (e) WC12 WC13 WC14
0.50 1.30 2.20
4.00 _+ 0.20 4.10 _+ 0.20 3.99 -+ 0.20
3.31 _+ 0.36 * *
4.28 _+ 0.21 4.26 + 0.21 4.34 + 0.22
1.88 _+ 0.63 8.51 + 1.15 29.32 + 3.95
1.60 + 0.22 11.81 _____1.59 12.27 _+ 1.65
1.50 3.10 3.60
4.23 _+ 0.21 4.20 -+ 0.21 4.16 -+ 0.21
7.03 __+0.95 5.57 _+ 0.75 7.31 _____0.99
4.52 -+ 0.23 4.49 _+ 0.22 4.42 -+ 0.22
4.29 -+ 0.58 5.79 _+ 0.78
1.68 + 0.22 5.62 + 0.76 2.48 _+ 0.33
0.25 2.00 3.00
4.24 ± 0.21 4.16 +__0.21 4.15 + 0.21
5.60 _+ 0.76 7.58 + 1.02 4.27 + 0.58
4.58 + 0.23 4.58 __+0.23 4.66 -+ 0.23
5.77 __+0.78 8.38 __+1.13 2.77 + 0.37
2.06 + 0.28 -
11.20 10.50 1.50
4.25 -+ 0.21 4.21 ± 0.21 4.24 _+ 0.21
24.48 __+3.44 21.83 __+2.95 21.30 _+ 2.88
4.54 -+ 0.23 4.49 + 0.22 4.50 _+ 0.23
25.89 + 3.49 * 19.10 __+2.57
15.15 __+2.04 * 15.57 -+ 2.10
0.50 2.14 2.70 6.00
4.15 __+0.21 3.95+0.20 4.14+__0.21 4.15 __+0.21
24.43 + 3.43 42.92+5.79 45.81 ± 8 . 1 8 54.51 + 7.35
4.41 + 0.22 4.19+0.21 4.40__+0.22 4.38 __+0.22
7.10 + 0.96 8.21-+1.11 14.94-+2.02 32.39 -+ 4.37
6.36 _+ 0.86 7.76_+1.04 10.91 -+ 1.47 22.35 __+3.02
7.00 7.50 7.70 1.00 2.40 9.10 10.20 10.70 12.20 14.30 15.10 16.50 18.50 19.00
4.10__+0.21 3.98 __+0.20 4.17 __+0.21 4.45 __+0.22 3.26 __+0.26 2.88+0.14 4.52 __+0.23 4.42 + 0.22 4.50 __+0.23 4.73 _+ 0.24 4.54 __+0.23 4.13 ± 0.21 4.81 -+ 0.24 4.17 __+0.21
10.34+1.39 19.55 + 2.64 7.46 + 1.01 11.45 + 1.55 19.80 5= 2.67 * 15.60 + 2.10 * 20.82 ± 2.81 * * 21.50 + 2.92 18.63 + 2.51 20.29 5= 2.74
4.35__+0.22 4.34 __+0.22 4.45 + 0.22 4.67 __+0.23 3.56 __+0.18 3.27__+0.16 4.85 __+0.24 4.71 + 0.24 4.79 __+0.24 5.03 __+0.25 4.87 __+0.24 4.51 + 0.23 5.27 5= 0.26 4.42 __+0.22
* * 7.52 7.72 15.38 * 9.89 * * 23.20 * 23.08 25.98 *
15.88__+2.14 20.24 __+2.73 11.39 __+1.53 6.74 __+0.91 11.14 __+1.50 20.06_+2.70 12.45 __+ 1.68 14.01 __+1.89 19.43 _+ 2.62 13.00 -+ 1.75 14.31 __+ 1.93 22.62 + 3.05 * 21.05 __+2.84
Baleh (lower) BA01 BA02 BA03
Balch (upper) BA04 BA05 BA06
Big Marias BM01 BM02 BM03 BM04
Soldier Mtn. SM01 SM02 SM03 SM04 SM05 SM06 SM07 SM08 SM09 SM10 SM11 SM12 SM13 SMI4
__+ 1.02 + 1.38 + 2.07 __+ 1.33
__+3.13 -+ 3.12 _+ 3.50
ll.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
193
Table 3 (continued) Sample code
Depth (m)
Annual dose to quartz grains (mGy/a)
Quartz TL age estimates (ka)
Annual dose to feldspar grains (mGy/a)
0.50 3.30
4.21 + 0.21 3.93 + 0.20
4.32 + 0.35 6.71 -I- 1.10
4.40 + 0.22 4.21 +_ 0.21
0.61 2.30 5.17 6.90
4.11 4.20 3.86 3.88
+ 0.21 + 0.21 ___0.19 + 0.19
4.75 + 0.38 * * -
4.33 4.42 4.11 4.10
1.84 2.50 4.35 4.95
3.87 3.78 3.83 3.92
+ _ + +
* 10.55 + 1.16 * 18.92 _+ 1.51
4.12 3.97 4.01 4.06
Feldspar TL age estimates (ka)
Feldspar IRSL age estimates (ka)
Old Dad Mtn.
Upper ODM01 ODM02 O l d D a d Mtn. Lower ODM03 ODM04 ODM05 ODM06 Iron Mtn. IM01 IM02 IM03 IM04
0.19 0.19 0.19 0.20
3.44 +_ 0.41 5.75 ___0.52
* 8.19 + 0.73
___0.22 + 0.22 + 0.21 + 0.21
3.49 + 1.14 7.01 + 0.84 14.17 + 1.27 -
3.00 + 0.24 2.60 + 0.34 11.05 + 0.88 -
_ + + +
9.85 14.16 21.37 22.09
9.75 10.12 15.67 19.44
0.21 0.20 0.20 0.20
+ + _ +
1.08 1.13 2.35 1.77
___ 1.07 _ 0.91 + 1.72 + 1.75
Notes: dose rates adjusted Io take account of (a) water content of 2% + 2% and (b) attenuation of external beta dose within large grains.
tain (ODM), Iron Mountain (IM), Balch (BA), Hanks Mountain (HM)) haw ~. good agreement between all three techniques. In other cases, a reasonable agreement exists between two of the techniques (e.g. Big Marias (BM)). Details of the sampled sections at Iron Mountain, Big Marias and Balch are shown in Fig. 3. A comparison of all available results for quartz TL vs. feldspar TL age determinations shows no relative bias between the two techniques. In the case of the Big Maria,s and the West Cronese ramps, however, the quartz TL dates are significantly older than those for feldspar. In the case of feldspar TL vs. feldspar IRSL, a systematic bias shows up in some of the older samples, with the feldspar TL ages tending to be slightly older than those for IRSL. The slight tendency for overestimation of the TL ages, as compared to IRSL ages, might be explained by a consistent underestimation of the level of the residual (unbleachable) TL component. Despite some discrepancies, these results show broad agreement between the three luminescence techniques and do not show the strong systeraatic overestimation of feldspar TL ages compared with quartz TL ages which was observed in the result~ for the lower part of the Dale Lake sand ramp (Rendell et al., 1994).
6. Discussion A correlation diagram, showing the time range(s) of deposition on each sand ramp is useful for assessing the sequence of depositional events in a regional context (Fig. 4). Prominent palaeosols are present in the Balch, Dale Lake, Iron Mountain and Big Marias sequences and appear to coincide with a break in the deposition within these sand ramps between 15 ka and 20-25 ka. The ages of these palaeosols, interpolated from the luminescence age estimates, are also shown in Fig. 4. The Iron Mountain sequence appears more complex than some of the other sequences. Depending on whether the IRSL or feldspar TL chronology is selected, the break in deposition, marked by a major palaeosol horizon, is either between 14 ka and 21 ka or 10 ka and 16 ka. Less well-developed palaeosol horizons appear to be present at Soldier Mountain (?ca. 11 ka) and Hanks Mountain (?ca. 25 ka) (see Fig. 4). Deposition and preservation of sand on the majority of the ramps studied appears to have terminated by ca. 7 ka. Only in the case of the West Cronese, Old Dad Mountain and Balch sand ramps does clear evidence occur of
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
194
Late Holocene deposition and preservation of sand on the ramps. The pattern of accumulation and preservation of sand on the ramps investigated appears to show regional and local effects. At a regional scale two major depositional phases can be identified: (1) Late Pleistocene (20-30 ka): this phase is identified in sequences at Hanks Mountain and, all below a prominent palaeosol, Balch, Dale Lake (a), Iron Mountain and the Big Mar±as.
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(2) Late Pleistocene-Early Holocene (15-7ka): this phase is found in sequences at the Cat Dune (a and b), Dale Lake (a and b), Iron Mountain (above palaeosol), Big Mar±as (above palaeosol) and, at the western end of the study area, Soldier Mountain (20-7 ka).
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
Intermittent Lake I began ca. 22 ~4C kyr BP and Intermittent Lake III finally dried up 8.7 14Ckyr BP. All of these uncalibrated radiocarbon dates are based on the Libby ]4C half-life of 5570 ___30 yr. Calibrated dates, calculated by Clarke et al. (1996) using the age program of Stuiver and Reimer (1993), put the start of Intermittent Lake I back to 24.5 ka, the
Late Holocene sequences are identified locally at West Cronese, Balch and Old Dad Mountain ramps. Independent palaeoenvironmental evidence is available from the detailed reconstruction of the Quaternary history of Lake Mojave (Brown, 1989; Brown et al., 1990). These studies revealed high lake stands at 20-17 tac kyr BP and 15-12.5 ~4C kyr,
E A S T E R N MOJAVE D E S E R T S A N D R A M P S
range of luminescence age estimates (ka) 45
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195
RECORD OF PLUVIAL LAKE MOJAVE
Fig. 4. Luminescence age range correlation diagram including data for the timing of pluvial Lake Mojave.
196
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187 197
high stands of Lake Mojave I at 20.9-19.6 ka and Lake Mojave II at 16.5-13.4 ka, with the end of Intermittent Lake III at 9.7 ka. The comparison between the timing of the various phases of pluvial Lake Mojave, using the calibrated dates given above, and the timing of periods of active accumulation of sand on the ramps studied (see Fig. 4) raises a series of interesting issues. Although some of the sand accumulation on the ramps at the Cat Dune, Hanks Mountain, Dale Lake and, possibly, the Big Marias predates the start of Intermittent Lake Mojave I, the main phases of deposition and preservation of sand on the ramps broadly coincide with the main pluvial phases. Although the accuracy and the precision of the luminescence dates preclude detailed correlation of periods of soil formation with high stands of the lake, all of the palaeosols identified on the ramps studied fall within the timing of the various lake phases. The increasing aridity in the Early Holocene, associated with the demise of Intermittent Lake III, appears to coincide broadly with the cessation of accumulation on all sand ramps except those close to the point where the Mojave River flows out of the Afton Canyon (Balch) and those close to the West Cronese Playa (West Cronese sand ramps). The picture that emerges from the ramp data involves the interplay of a series of key factors namely: the continuity of sand supply to deflation areas, the (seasonal) occurrence of strong turbulent winds and the nature and abundance of the vegetation cover present in the East Mojave area during the Late Quaternary. Although reactivation of sand movement in areas of sand dunes may still be interpreted as linked to increased aridity, the coincidence of active accumulation on the sand ramps with a major pluvial phase, moves the emphasis in the Eastern Mojave at least on to the key role of sand supply. It is not unreasonable to suppose that sand movement was episodic and seasonal, with sand being supplied to washes and playas during periods of ephemeral flow ready for subsequent deflation. The lack of Late Holocene accumulation on the majority of the ramps studied is, therefore, interpreted as reflecting problems with sand supply, as increasing aridity reduced the frequency of ephemeral flow events in the fiver washes and playas. Late Holocene accumulation on the West Cronese and
Balch ramps but not the adjacent Cat Dune or Hank's Mountain ramps perhaps indicates that the orientation of the ramps relative to the dominant wind direction is also an important factor. As far as sediment sources are concerned, the likely source of material for the sand ramps at Balch and Old Dad Mountain was, and remains, the area of the Mojave River wash (Fig. 1). Even under presentday conditions, large amounts of sand can be brought into the wash area by occasional winter or spring floods. The most recent phase of sampling, in March/April 1993, coincided with the second wet (El Nifio) winter/spring season in the Mojave in successive years. The 'dry' playas of Silver Lake and the East Cronese basin were flooded. Moreover, the Mojave River had deposited substantial amounts of sand within, and at the exit from, the Afton Canyon. In the case of some of the other ramps such as Dale Lake or Iron Mountain, the local source of sediment would appear to be a playa.
7. Conclusions The luminescence dating of material from nine of the major sand ramps of the Eastern Mojave has raised questions about the application of these techniques in areas where independent dating control is largely absent and about fundamental issues of sediment supply and transport during the Late Quaternary. This approach has involved the application of three different techniques to the same suite of samples in order to look for concordance between the results. The results from quartz TL, feldspar TL and feldspar IRSL measurements are in broad agreement, and have provided a basis for the reconstruction of periods of sand accumulation and preservation on the sand ramps, at regional and local scales, during the Late Quaternary. The picture that emerges is one of episodic sand movement, with accumulation of sand on the ramps studied continuing throughout most of the Late Pleistocene to Early Holocene pluvial period in the Eastern Mojave. Increased aridity, since ca. 9 ka, appears to have been associated with only a limited amount of continued deposition on a few of the ramps. The results indicate that sediment transport by fluvial activity to deflation areas is the critical pro-
ll.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197 cess that ensured the continuity o f sand supply to the ramps. Such a finding is in contrast to m o r e traditional v i e w s that hawe equated sand m o v e m e n t with aridity. In the Eastern M o j a v e the m a j o r controls appear to be source(s) of material and the (seasonal) o c c u r r e n c e o f strong turbulent winds.
Acknowledgements The financial support o f Natural E n v i r o n m e n t Research C o u n c i l grants G R / 9 / 4 4 5 and G R / 9 / 7 8 0 and o f N A T O C o o p e r a t i v e R e s e a r c h P r o g r a m s Grant 900151 is gratefully a c k n o w l e d g e d , as is the assistance o f Dr M.L. Clarke, D r G.A.T. Duller, M r S. Edwards, Dr N. Lancaster, D r C.A. Richardson, Dr V.P. Tchakerian, and D r A.G. Wintle.
References Brown, W.J., 1989. The late Quaternary stratigraphy, palaeohydrology, and geomorrhology of pluvial Lake Mojave, Silver Lake and Soda Lake basins, CA. Unpublished M.S. thesis, University of New Mexico, Albuquerque, 275 pp. Brown, W.J., Wells, S.G., Enzel, Y., Anderson, R.Y. and McFadden, L.D., 1990. The late Quaternary history of pluvial Lake Mojave-Silver Lake and Soda lake Basins, California. In: R.E. Reynolds, S.G. Wells and R.H.I. Brady (Editors), At the
197
End of the Mojave: Quaternary Studies in the Eastern Mojave Desert. Special Publication of the San Bernardino County Museum Association, San Bernardino, Calif., pp. 55-72. Clarke, M.L., Wintle, A.G. and Lancaster, N., 1996. Infra-red stimulated luminescence dating of sands from the Cronese Basins, Mojave Desert. In: N. Lancaster (Editor), Response of Aeolian Processes to Global Change. Geomorphology, 17: 199-205, this issue. Duller, G.A.T., 1991. Equivalent dose determinations using single aliquots. Nucl. Tracks Radiat. Measurements, 18: 371-378. Duller, G.A.T., 1993. Luminescence chronology of raised marine terraces, south-west North Island, New Zealand. Unpublished Ph.D. dissertation, University of Wales. Evans, J.R., 1961. Falling and climbing sand dunes in the Cronese ('Cat') Mountain area, San Bemadino County, California. J. Geol., 70:107-113. Prescott, J.R. and Hutton, J.H., 1988. Cosmic ray and gamma ray dosimetry for TL and ESR. Nucl. Tracks Radiat. Measurements, 14: 223-227. Rendell, H.M. and Wood, R.A., 1994. Quartz sample pretreatments for TL/OSL dating: studies of TL emission spectra. Radiat. Measurements, 23: 575-580. Rendell, H.M., Lancaster, N.L. and Tchakerian, V., 1994. Luminescence dating of Late Quaternary aeolian deposits at Dale Lake and Cronese Mountains, Mojave Desert, California. Quat. Geochronol., 13: 417-422. Stuiver, M. and Reimer,P.J., 1993. Extended L4C data base and revised CALIB 3.0 14C age program. Radiocarbon, 35: 215230. Tchakerian, V.P., 1991. Late Quaternary aeolian geomorphology of the Dale Lake sand sheet, southern Mojave Desert, California. Phys. Geogr., 12: 347-369. Wintle, A.G. and Prozynska, H., 1983. TL dating of loess in Germany and Poland. PACT, 9: 547-554.
Geomorphology 17 (1996) 187-197
Luminescence dating of sand ramps in the Eastern Mojave Desert H.M. Rendell, N.L. Sheffer Geography Laboratory, University of Sussex, Falmer, Brighton BN1 9QN, UK
Received 28 March 1994; revised 4 January 1995; accepted 4 January 1995
Abstract Until recently the timing of the movement of sand and accumulation of sand by aeolian processes in the Eastern Mojave Desert has remained the subject of speculation. The results of a luminescence dating program involving 78 samples of material from nine sand ramp complexes have enabled recognition of regional and local patterns of sand accumulation in the Eastern Mojave. The st~Ldy area extends from Lake Manix, in the west, to the Colorado River, in the east. The periods of accumulation have been identified on the basis of quartz TL dating, and potassium feldspar TL and IRSL dating. At a regional scale, two major depositional phases can be identified: (1) Late Pleistocene: 20-30 ka, with sequences at Hank's Mountain, Balch, Dale Lake and the Big Marias; and (2) Late Pleistocene-Early Holocene: 15-7 ka with sequences at Cat Dune, Dale Lake, Iron Mountain, Big Marias (above palaeosol) and, at the western end of the study area, at Soldier Mountain (20-7 ka). Late Holocene sequences appear to be much more localized, and are confined to the West Cronese, Old Dad Mountain and Balch sand ramps. The earliest phases of sand accumulation on the ramps studied immediately predate the formation of pluvial Lake Mojave (ca. 24.5 ka). The later main phases of accumulation coincide with the existence of Lake Mojave and the end of accumulation on the majority of the sand ramps immediately post-dates the final end of Intermittent Lake Mojave III (9.7 ka). The continuity of sand supply to potential deflation areas is seen as a critical condition for accumulation of material on the sand ramps during the Late Quaternary in the Eastern Mojave.
1. Introduction Luminescence techniques, either thermoluminescence (TL) or optic.ally stimulated luminescence (OSL), have the potential to date the last exposure of sediment grains to light. The present study involves the application of the~;e techniques to the quartz and feldspar fractions of :sand samples from nine major sand ramps in the Eastern Mojave Desert. In contrast to luminescence dating research which often uses a single mineral fraction and a single luminescence technique, the approach adopted in the present study enables any systematic biases with regard to particular methods or samples to be identified. Thus, data
are presented for quartz TL, feldspar TL, and infrared stimulated luminescence (IRSL) from feldspar.
2. Field area Nine sand ramps (or complex climbing or falling dunes) were sampled in the Eastern Mojave Desert, from Soldier Mountain in the west to the Big Marias sand ramp, near to the Colorado River in the east. The sampling sites and relationships to playa lakes, dune fields, and ephemeral flow channels are shown in Fig. 1. The thickness and complexity of the sequences sampled vary from sand ramp to sand ramp. The
0169-555X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI 0169-555X(95)00102-6
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
188
mering plastic tubes into freshly cleaned vertical sections.
Hank's Mountain (HM) and Big Marias (BM) sections are both relatively short, 4.4 m and 6.0 m thick, respectively, with straightforward stratigraphies. Soldier Mountain (SM) represents a much thicker (19.0 m) depositional sequence that is comparable in thickness with the 17.6 m composite section at Dale Lake, whereas the West Cronese (WC) and Balch (BA) sand ramps are more complex composite sequences. Initial work concentrated on the sand ramps at Dale Lake (Tchakerian, 1991) and the Cronese Cat Dune (Evans, 1961). Considerable problems were encountered in trying to date horizons comprising aeolian and colluvial mixtures from these two sand ramps (Rendell et al., 1994). Sampling of material from the other seven sand ramps was undertaken after the initial results from the Dale Lake sequence had been processed. Every attempt was made to take samples of sand from horizons free of colluvial clasts. Samples were collected by pushing or ham-
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3. Luminescence analysis Samples were prepared for analysis by pretreatment to remove any carbonates, sieving to separate out a dominant size fraction (normally 250-180 p~m or 180-125 txm), and density separation followed by etching, to isolate quartz and feldspar fractions. The density separation involved the use of a 2.58 g cm -3 sodium polytungstate solution, to isolate the potassium-rich feldspar fraction from the quartz. Quartz fractions were etched for 40 min or 60 min in 40% hydrofluoric acid (see Rendell and Wood, 1994, for discussion), whereas potassium-rich feldspar separates were etched for 40 rain, but in 10% hydrofluoric acid.
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H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
With the exception of a small number of samples from the Cat Dune and Dale Lake sand ramps, all of the luminescence measurements were made using multiple aliquots (subsamples). Each aliquot was prepared by using silicone spray to fix a single layer of grains on to the surface of a 10 mm diameter aluminium disc, or a rhodium-plated copper disc. Potassium-rich feldspar separates from selected samples from the Cat Dune and Dale Lake sand ramps were analysed using 'single aliquot' IRSL methods developed by Duller (1991). These measurements were done using a Rise automated reader in the Aberystwyth laboratory. Sample discs were preheated at 220°C for 10 min prior to each measurement and emissions were monitored via a Schott BG-39, Coming 7-59 and a neutral density filter by an EMI 9635QB PM tube. Short (0.5 s) IRSL measurements were made using IR-emitting diodes (TEMT-484), with peak emission at 880 nm, while holding the sample discs at a constant temperature
(50oc). TL analysis of quartz and feldspar fractions from Hanks Mountain, West Cronese and Balch sand ramps was undertaken using a manual TL reader. Samples were heated in an argon atmosphere at 150°C min -1, and TL emissions were monitored via Coming 5-58 (blue) and Chance-Pilkington HA3 filters by an EMI9636Q photomultiplier tube; no preheats were employed prior to measurement. The quartz and feldspar fi:actions from the Soldier Mountain, Big Marias, Old Dad Mountain and Iron Mountain sand ramps were measured on an automated Rise TL-OSL reader. Emissions were monitored via a Hoya U-340 filter and an EMI9635QB photomultiplier tube. Samples were heated in a nitrogen atmosphere at a rate of 2.81°C s -1. All of the multiple aliquot IRSL measurements were made on the Rise automated system using the filter-photomultiplier tube combination specified above. A preheat tre,atment of 1 min at 220°C was used prior to all IRSL measurements. Empirical data indicate that a short preheat such as 1 min is sufficient to remove the unstable component of the IRSL signal (Duller, 1993, fig. 4.7.3b). IRSL measurements were made by exposing sample discs to 30 s of IR while holding ~Ihem at 50°C. Depending on the technique used, disc-to-disc normalization was by weight, second glow or 'short shine'. In the latter, all
189
discs are exposed to IR light for a brief period (0.1 s) prior to all other treatments. Laboratory irradiations were performed using a 9 ° S t / 9 0 y beta source. Laboratory bleaching was undertaken using a 300-watt Wotan sunlamp at a distance of 30 cm from the sample discs. A sheet of plate glass was interposed between the lamp and the sample discs to remove the UV component of the light. A bleaching time of 4 h was routinely employed.
4. Dosimetry measurements The environmental gamma dose was measured in the field using a portable gamma spectrometer, with an external pitchblende-resin source to check for instrument drift. Bulk beta activity was measured in the laboratory using thick-source beta counting. The internal dose for potassium feldspar-rich fractions was calculated using measurements of the potassium content of each fraction. Potassium contents were obtained via acid digestion, using 40% hydrofluoric acid, and flame photometry.
5. Results The dosimetry data and the results of the dating determinations are summarised in Table 1, 2 and 3. The TL Equivalent Dose (ED, see Table 2) and age determinations were in all cases obtained using the regeneration method (Wintle and Prozynska, 1983), because of the relatively high degree of scatter exhibited by the additive dose growth curves. In contrast, the IRSL EDs were all obtained using additive dose techniques. The disadvantage of using additive techniques, as with TL, is that some samples exhibited a high degree of scatter amongst aliquots that resisted attempts at normalisation. In some cases, scatter was not a major problem, as shown by the additive growth curves for multiple aliquot IRSL determinations for potassium-rich feldspar separates for samples from the Big Marias and Soldier Mountain sections (Fig. 2). One critical question involves the nature of agreement or disagreement between the luminescence ages estimated using the three different techniques. Table 3 reveals that the sand ramps (e.g. Old Dad Moun-
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
190 Table 1 Mojave sand ramps: dosimetry data Sample code
Height below top of section (m)
External [3 dose (mGy/a)
External "y dose (mGy/a)
Cosmic dose (mGy/a)
Internal [3 dose (mGy/a)
HM01 HM02 HM03 HM04 WC01 WC02 WC03 WC06 WC07 WC08 WC09 WC10 WC12 WC13 WC14 BA01 BA02 BA03 BA04 BA05 BA06 BM01 BM02 BM03 BM04 SM01 SM02 SM03 SM04 SM05 SM06 SM07 SM08 SM09 SM10 SMll SM12 SMI3 SM14 ODM01 ODM02 ODM03 ODM04 ODM05 ODM06 IM01 IM02 IM03 IM04
4.40 4.00 1.00 0.60 0.60 8.50 9.90 18.20 20.20 0.50 1.30 2.20 1.50 3.10 3.60 0.25 2.00 3.00 11.20 10.50 1.50 0.50 2.14 2.70 6.00 7.00 7.50 7.70 1.00 2.40 9.10 10.20 10.70 12.20 14.30 15.10 16.50 18.50 19.00 0.50 3.30 0.61 2.30 5.17 6.90 1.84 2.50 4.35 4.95
2.91 2.96 2.99 3.32 2.80 2.89 3.01 2.78 3.05 2.98 3.10 2.99 3.15 3.09 2.99 3.12 3.13 3.12 3.24 3.13 3.13 3.14 3.05 3.27 3.34 3.06 3.02 3.21 3.39 2.20 2.02 3.57 3.48 3.29 3.67 3.52 1.23 3.85 3.18 3.10 3.01 3.10 3.15 2.96 2.98 2.89 2.74 2.84 2.91
1.31 _+0.02 1.41 + 0.01 1.22 _+ 0.01 1.24 ± 0.01 1.07 _+ 0.01 1.04 _+ 0.01 1.05 _+ 0.01 1.11 ± 0.01 1.05 + 0.02 1.04 _+ 0.01 1.03 _+ 0.01 1.03 ± 0.01 1.15 ± 0.01 1.19 _+ 0.01 1.25 _+ 0.02 1.14 _+ 0.01 1.09 _+ 0.01 1.11 _+ 0.02 1.18 _+ 0.01 1.25 _+ 0.01 1.16 _+ 0.01 1.04 ± 0.01 0.95 ± 0.01 0.95 ± 0.01 0.94 _+ 0.01 1.17 ± 0.01 1.09 _+ 0.01 1.10 + 0.01 1.12 + 0.01 1.07 - 0.01 0.94 + 0.01 1.13 + 0.01 1.12 ___0.01 1.39 + 0.01 1.27 + 0.01 1.22 + 0.01 1.23 + 0.01 1.19 ___0.01 1.18 + 0.01 1.11 + 0.01 0.94 + 0.01 1.01 + 0.01 1.09 _+0.01 0.98 + 0.02 0.99 + 0.01 0.99 + 0.01 1.06 + 0.01 1.05 + 0.01 1.08 + 0.01
0.11 _+ 0.01 0.12 _+ 0.01 0.18 _+ 0.01 0.19 _+ 0.01 0.19 _+ 0.01 0.07 _+ 0.004 0.05 _+ 0.003 0.03 _+ 0.002 0.025 _+ 0.001 0.19 ± 0.01 0.17 _+ 0.01 0.15 + 0.01 0.17 _+ 0.01 0.14 ± 0.01 0.13 ± 0.01 0.19 +_ 0.01 0.16 ± 0.01 0.14 ± 0.01 0.05 _+ 0.003 0.06 _+ 0.003 0.17 _+ 0.01 0.19 ± 0.01 0.15 + 0.01 0.14 _+ 0.01 0.09 _+ 0.005 0.08 + 0.004 0.08 _+ 0.004 0.08 + 0.004 0.18 + 0.01 0.15 + 0.01 0.07 _+0.003 0.06 + 0.003 0.06 + 0.003 0.05 + 0.002 0.04 + 0.002 0.04 + 0.002 0.03 + 0.002 0.03 + 0.002 0.03 + 0.001 0.19 + 0.01 0.13 + 0.01 0.19 + 0.01 0.15 + 0.01 0.10 + 0.01 0.09 + 0.004 0.16 + 0.01 0.15 _ 0.01 0.11 ___0.01 0.11 + 0.01
0.33 0.37 0.34 0.27 0.39 0.35 0.33 0.32 0.23 0.27 0.18 0.35 0.24 0.29 0.27 0.34 0.43 0.51 0.29 0.28 0.27 0.25 0.24 0.26 0.23 0.25 0.36 0.29 0.22 0.30 0.39 0.34 0.28 0.29 0.30 0.33 0.38 0.46 0.25 0.19 0.27 0.22 0.22 0.24 0.23 0.26 0.19 0.18 0.14
± 0.07 _+ 0.07 _+ 0.10 _+ 0.05 _+ 0.07 ± 0.05 _+ 0.08 _+ 0.06 _+ 0.08 _+0.09 _+ 0.11 + 0.07 _+ 0.06 _+ 0.07 _+ 0.04 _+ 0.07 ± 0.06 ± 0.07 _+ 0.07 _+ 0.06 _+ 0.07 ± 0.06 ± 0.08 ± 0.10 _+ 0.05 ___0.11 _+ 0.10 + 0.11 + 0.05 + 0.07 + 0.05 + 0.08 + 0.14 + 0.08 + 0.21 + 0.19 + 0.13 _+ 0.13 + 0.09 ___0.08 -- 0.15 + 0.11 + 0.10 + 0.11 __+0.11 + 0.11 + 0.09 ± 0.09 _ 0.07
_+ 0.02 _+ 0.02 _+ 0.02 _+ 0.01 _+ 0.02 _+ 0.02 _+ 0.02 ± 0.02 _+ 0.01 _+ 0.01 _+ 0.01 ± 0.01 ± 0.01 +_ 0.01 _+ 0.01 _+ 0.02 _+ 0.02 _+ 0.03 _+ 0.01 _+ 0.01 ± 0.01 ± 0.01 _ 0.01 ± 0.01 ± 0.01 + 0.01 _+ 0.02 + 0.01 ___0.01 + 0.02 + 0.02 + 0.02 + 0.01 _+ 0.02 + 0.02 ± 0.02 + 0.02 + 0.02 + 0.01 + 0.01 + 0.01 ___0.01 + 0.01 ___0.01 + 0.02 ± 0.01 ± 0.01 ± 0.01 ± 0.01
Notes: external beta doses assessed using thick source beta counting; external gamma doses assessed in the field using a portable gamma spectrometer; cosmic doses calculated using the formula given by Prescott and Hutton (1988); internal beta dose assessment based on potassium determinations on potassium feldspar-rich separates using acid digestion and flame photometry.
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
191
Table 2 M o j a v e sand ramps: equiv~flent dose estimates Sample
Q u a r t z T L ED ( G y )
HM01 HM02 HM03 HM04 WC01 WC02 WC03 WC06 WC07 WC08 WC09 WC10 WC12 WC13 WC14 BA01 BA02 BA03 BA04 BA05 BA06 BM01 BM02 BM03 BM04 SM01 SM02 SM03 SM04 SM05 SM06 SM07 SM08 SM09 SMI0 SMll SM12 SM13 SM14 ODM01 ODM02 ODM03 ODM04 ODM05 ODM06 IM01 IM02 IM03 IM04
94.60 115.20 122.60 102.04 35.90 59.03 29.20 28.60 * 13.23 * * 30.06 23.40 30.40 23.72 31.53 17.75 108.31 91.85 90.61 105.60 168.30 189.70 226.20 42.40 77.81 31.10 51.00 64.50 * 70.50 * 93.70 * * 89.20 89.60 84.62 18.19 26.39 19.53 * * * 39.88 * 74.24
+ 4.24 _ 3.07 + 3.51 + 5.40 _+ 0.17 ___6.20 + 3.18 + 0.27 + 1.64
+ 0.67 + 0.27 + 0.60 + 0.39 _ 0.87 + 1.26 + 16.70 + 1.06 + 1.24 _ 1.10 + 2.70 + 9.20 ___ 14.00 + 1.07 _+ 2.95 + 2.30 _ 4.70 __. 1.54 + 5.90 + 8.80
+ 3.39 + 10.70 + 2.03 ___0.47 + 4.99 +__0.67
+ 4.78 + 0.66
Feldspar T L E D ( G y ) 128.13 -t- 9.37 113.55 _ 11.30 120.30 + 5.19 118.00 + 2.04 24.13 + 0.50 20.50 + 2.10 2 7 . 5 0 + 5.20 6 2 . 1 4 + 1.20 35.37 + 8.10 8.06 + 2.70 36.28 ___0.41 127.20 + 6.06 19.43 + 0.09 25.25 + 0.76 25.60 + 1.78 26.43 + 2.77 38.43 + 3.33 12.91 __. 0.99 117.50 + 2.12 * 86.02 + 2.75 31.30 + 3.39 34.4 + 4.06 65.80 + 2.21 141.90 + 2.20 * * 33.50 -+- 1.40 36.10 + 6.49 54.70 + 1.31 * 4 8 . 0 0 + 3.43 * * 116.70 _ 4.90 * 100.10 + 6.60 136.90 + 9.20 * 15.15 + 2.10 24.20 + 1.93 15.15 + 5.10 30.95 +_ 0.25 58.32 + 0.55 40.65 + 0.45 56.25 + 1.89 85.66 + 9.02 89.66 + 1.81
Feldspar I R S L E D ( G y ) 136.40 91.88 94.46 6.09 9.43 22.05 28.14 6.84 50.35 53.24 7.60
_+ 11.40 + 22.50 + 3.80 + _ + _ + + + +
0.57 0.27 1.08 1.23 0.30 0.70 2.20 0.30
10.95 + 0.24 9.43 + 0.25 * 68.76 * 70.13 28.01 32.50 48.04 97.90 69.12 87.80 50.70 31.50 39.60 65.60 60.40 65.94 93.10 65.40 69.70 102.00 * 93.11 34.43 13.02 11.68 45.50 40.20 40.20 62.79 78.90
+ 4.13 + 1.92 + 0.63 + 2.17 + 1.50 + 1.34 + 4.50 + 4.98 + 1.64 + 1.48 + 1.78 + 3.39 + 2.76 + 3.21 ___5.37 + 2.79 + 4.84 + 1.46 + 4.51 + 1.66 ___ 1.29 + 1.51 + 1.33 + + + +
2.70 1.36 2.84 4.49
Notes: asterisk denotes h i g h degree o f scatter in data points; short d a s h denotes missing data. All T L E D ' s determined b y the regeneration method, all feldspar I R S L dates determined b y multiple aliquot additive dose.
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
192
Table 3 Mojave sand ramps: annual dose rates and age estimates Sample code
Depth (m)
Annual dose to quartz grains (mGy/a)
Quartz TL age estimates (ka)
Annual dose to feldspar grains (mGy/a)
Feldspar TL age estimates (ka)
Feldspar IRSL age estimates (ka)
4.40 4.00 1.00 0.60
4.23 4.31 4.21 4.51
_+ 0.21 _+ 0.22 _+ 0.21 _+ 0.23
22.93 26.72 29.13 22.60
_+ 2.52 _+ 2.93 _+ 3.20 _+ 2.48
4.45 4.69 4.55 4.79
_+ 0.22 _+ 0.23 __+0.23 __+0.24
28.77 24.23 26.47 23.05
_+ 3.88 _+ 3.27 -+ 3.57 __+3.11
30.63 _+ 4.13 20.22 -+ 2.73 19.74 _+ 2.66
0.60 8.50 9.90 18.20 20.20
3.86 3.80 3.91 3.73 3.92
__+0.19 + 0.19 __+0.20 +__0.19 + 0.20
9.30 14.53 7.47 7.67 *
__+1.02 __+1.60 __+0.82 + 0.27
4.24 4.15 4.24 4.05 4.15
+ 0.21 __+0.21 + 0.21 __+0.20 __+0.21
5.68 4.94 6.48 15.34 8.53
__+0.77 __+0.67 __+1.22 __+2.07 __+ 1.95
Hanks Mtn. HM01 HM02 HM03 HM04
West Cronese (a) WC01 WC02 WC03 WC06 WC07
1.47 2.23 5.44 6.78
__+0.19 + 0.30 + 0.74 __+0.92
West Cronese (b) WC08 WC09 WCIO W e s t Crouese (e) WC12 WC13 WC14
0.50 1.30 2.20
4.00 _+ 0.20 4.10 _+ 0.20 3.99 -+ 0.20
3.31 _+ 0.36 * *
4.28 _+ 0.21 4.26 + 0.21 4.34 + 0.22
1.88 _+ 0.63 8.51 + 1.15 29.32 + 3.95
1.60 + 0.22 11.81 _____1.59 12.27 _+ 1.65
1.50 3.10 3.60
4.23 _+ 0.21 4.20 -+ 0.21 4.16 -+ 0.21
7.03 __+0.95 5.57 _+ 0.75 7.31 _____0.99
4.52 -+ 0.23 4.49 _+ 0.22 4.42 -+ 0.22
4.29 -+ 0.58 5.79 _+ 0.78
1.68 + 0.22 5.62 + 0.76 2.48 _+ 0.33
0.25 2.00 3.00
4.24 ± 0.21 4.16 +__0.21 4.15 + 0.21
5.60 _+ 0.76 7.58 + 1.02 4.27 + 0.58
4.58 + 0.23 4.58 __+0.23 4.66 -+ 0.23
5.77 __+0.78 8.38 __+1.13 2.77 + 0.37
2.06 + 0.28 -
11.20 10.50 1.50
4.25 -+ 0.21 4.21 ± 0.21 4.24 _+ 0.21
24.48 __+3.44 21.83 __+2.95 21.30 _+ 2.88
4.54 -+ 0.23 4.49 + 0.22 4.50 _+ 0.23
25.89 + 3.49 * 19.10 __+2.57
15.15 __+2.04 * 15.57 -+ 2.10
0.50 2.14 2.70 6.00
4.15 __+0.21 3.95+0.20 4.14+__0.21 4.15 __+0.21
24.43 + 3.43 42.92+5.79 45.81 ± 8 . 1 8 54.51 + 7.35
4.41 + 0.22 4.19+0.21 4.40__+0.22 4.38 __+0.22
7.10 + 0.96 8.21-+1.11 14.94-+2.02 32.39 -+ 4.37
6.36 _+ 0.86 7.76_+1.04 10.91 -+ 1.47 22.35 __+3.02
7.00 7.50 7.70 1.00 2.40 9.10 10.20 10.70 12.20 14.30 15.10 16.50 18.50 19.00
4.10__+0.21 3.98 __+0.20 4.17 __+0.21 4.45 __+0.22 3.26 __+0.26 2.88+0.14 4.52 __+0.23 4.42 + 0.22 4.50 __+0.23 4.73 _+ 0.24 4.54 __+0.23 4.13 ± 0.21 4.81 -+ 0.24 4.17 __+0.21
10.34+1.39 19.55 + 2.64 7.46 + 1.01 11.45 + 1.55 19.80 5= 2.67 * 15.60 + 2.10 * 20.82 ± 2.81 * * 21.50 + 2.92 18.63 + 2.51 20.29 5= 2.74
4.35__+0.22 4.34 __+0.22 4.45 + 0.22 4.67 __+0.23 3.56 __+0.18 3.27__+0.16 4.85 __+0.24 4.71 + 0.24 4.79 __+0.24 5.03 __+0.25 4.87 __+0.24 4.51 + 0.23 5.27 5= 0.26 4.42 __+0.22
* * 7.52 7.72 15.38 * 9.89 * * 23.20 * 23.08 25.98 *
15.88__+2.14 20.24 __+2.73 11.39 __+1.53 6.74 __+0.91 11.14 __+1.50 20.06_+2.70 12.45 __+ 1.68 14.01 __+1.89 19.43 _+ 2.62 13.00 -+ 1.75 14.31 __+ 1.93 22.62 + 3.05 * 21.05 __+2.84
Baleh (lower) BA01 BA02 BA03
Balch (upper) BA04 BA05 BA06
Big Marias BM01 BM02 BM03 BM04
Soldier Mtn. SM01 SM02 SM03 SM04 SM05 SM06 SM07 SM08 SM09 SM10 SM11 SM12 SM13 SMI4
__+ 1.02 + 1.38 + 2.07 __+ 1.33
__+3.13 -+ 3.12 _+ 3.50
ll.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
193
Table 3 (continued) Sample code
Depth (m)
Annual dose to quartz grains (mGy/a)
Quartz TL age estimates (ka)
Annual dose to feldspar grains (mGy/a)
0.50 3.30
4.21 + 0.21 3.93 + 0.20
4.32 + 0.35 6.71 -I- 1.10
4.40 + 0.22 4.21 +_ 0.21
0.61 2.30 5.17 6.90
4.11 4.20 3.86 3.88
+ 0.21 + 0.21 ___0.19 + 0.19
4.75 + 0.38 * * -
4.33 4.42 4.11 4.10
1.84 2.50 4.35 4.95
3.87 3.78 3.83 3.92
+ _ + +
* 10.55 + 1.16 * 18.92 _+ 1.51
4.12 3.97 4.01 4.06
Feldspar TL age estimates (ka)
Feldspar IRSL age estimates (ka)
Old Dad Mtn.
Upper ODM01 ODM02 O l d D a d Mtn. Lower ODM03 ODM04 ODM05 ODM06 Iron Mtn. IM01 IM02 IM03 IM04
0.19 0.19 0.19 0.20
3.44 +_ 0.41 5.75 ___0.52
* 8.19 + 0.73
___0.22 + 0.22 + 0.21 + 0.21
3.49 + 1.14 7.01 + 0.84 14.17 + 1.27 -
3.00 + 0.24 2.60 + 0.34 11.05 + 0.88 -
_ + + +
9.85 14.16 21.37 22.09
9.75 10.12 15.67 19.44
0.21 0.20 0.20 0.20
+ + _ +
1.08 1.13 2.35 1.77
___ 1.07 _ 0.91 + 1.72 + 1.75
Notes: dose rates adjusted Io take account of (a) water content of 2% + 2% and (b) attenuation of external beta dose within large grains.
tain (ODM), Iron Mountain (IM), Balch (BA), Hanks Mountain (HM)) haw ~. good agreement between all three techniques. In other cases, a reasonable agreement exists between two of the techniques (e.g. Big Marias (BM)). Details of the sampled sections at Iron Mountain, Big Marias and Balch are shown in Fig. 3. A comparison of all available results for quartz TL vs. feldspar TL age determinations shows no relative bias between the two techniques. In the case of the Big Maria,s and the West Cronese ramps, however, the quartz TL dates are significantly older than those for feldspar. In the case of feldspar TL vs. feldspar IRSL, a systematic bias shows up in some of the older samples, with the feldspar TL ages tending to be slightly older than those for IRSL. The slight tendency for overestimation of the TL ages, as compared to IRSL ages, might be explained by a consistent underestimation of the level of the residual (unbleachable) TL component. Despite some discrepancies, these results show broad agreement between the three luminescence techniques and do not show the strong systeraatic overestimation of feldspar TL ages compared with quartz TL ages which was observed in the result~ for the lower part of the Dale Lake sand ramp (Rendell et al., 1994).
6. Discussion A correlation diagram, showing the time range(s) of deposition on each sand ramp is useful for assessing the sequence of depositional events in a regional context (Fig. 4). Prominent palaeosols are present in the Balch, Dale Lake, Iron Mountain and Big Marias sequences and appear to coincide with a break in the deposition within these sand ramps between 15 ka and 20-25 ka. The ages of these palaeosols, interpolated from the luminescence age estimates, are also shown in Fig. 4. The Iron Mountain sequence appears more complex than some of the other sequences. Depending on whether the IRSL or feldspar TL chronology is selected, the break in deposition, marked by a major palaeosol horizon, is either between 14 ka and 21 ka or 10 ka and 16 ka. Less well-developed palaeosol horizons appear to be present at Soldier Mountain (?ca. 11 ka) and Hanks Mountain (?ca. 25 ka) (see Fig. 4). Deposition and preservation of sand on the majority of the ramps studied appears to have terminated by ca. 7 ka. Only in the case of the West Cronese, Old Dad Mountain and Balch sand ramps does clear evidence occur of
H.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197
194
Late Holocene deposition and preservation of sand on the ramps. The pattern of accumulation and preservation of sand on the ramps investigated appears to show regional and local effects. At a regional scale two major depositional phases can be identified: (1) Late Pleistocene (20-30 ka): this phase is identified in sequences at Hanks Mountain and, all below a prominent palaeosol, Balch, Dale Lake (a), Iron Mountain and the Big Mar±as.
a
BIG MARIAS
IRON
MOUNTAIN
7"
{ {: i
:..o . • . .i ~'~ 7.10-+0.96 FTL .~ . ~ . 6.36:1:0.86 IRSL .~
".
•
."
"
•
~
9.85±1.08 FTL 9.75:1:1.07 IRSL
• . . . ". "<'--8.21:1:1.11 FTL ' - . 7.76:1:1.04 IRSL
.: :"
. . ' '~ • ,•- . . ."
~4
14.94-+2.02 FTL 10.914"1.47 IRSL
i~ o.. ~ " ~,
~3
:.2i)i .-.
. . ,/-/.
BMOIL
•
./.;r 2
1
. . ,
21.37:1:2.35 FTL 15,67+1.721RSL
~:~,~.~I:;;~,'~,
. /./Z: •
• " i " '" ~
.
• . " . • 'c--'32.39±4.37 FTL • . ' . " 22.35:1:3,02 I R S L
-: O-
J
b
3'0
15
Gy
14
BALCH UPPER SECTION 7//'//7////, " . ' . .
• • " •
IP--21.30:t2.88 QTL
13
SMO4L
BALCH LOWER SECTION
12 11 10 -
== ~ 8. c J=
4.27:1:0.58 QTL 7. ,o.oo -......
o_ =
.o.~op
f
5-
9b Gy
" . • ".' • ,,., "~-~"
~-- 21,83:1:2.95 QTL -24,48:1:3,44 QTL
section obscured
SM14L Fig. 3. (a) Sand ramp sections at the Big Mar±as and Iron Mountain. (b) Sand ramp sections at Balch.
I
I
I
I
0
30
60
I
90 Gy
Fig. 2. Examples of additive dose growth curves for multiple aliquot feldspar IRSL measurements.
(2) Late Pleistocene-Early Holocene (15-7ka): this phase is found in sequences at the Cat Dune (a and b), Dale Lake (a and b), Iron Mountain (above palaeosol), Big Mar±as (above palaeosol) and, at the western end of the study area, Soldier Mountain (20-7 ka).
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187-197
Intermittent Lake I began ca. 22 ~4C kyr BP and Intermittent Lake III finally dried up 8.7 14Ckyr BP. All of these uncalibrated radiocarbon dates are based on the Libby ]4C half-life of 5570 ___30 yr. Calibrated dates, calculated by Clarke et al. (1996) using the age program of Stuiver and Reimer (1993), put the start of Intermittent Lake I back to 24.5 ka, the
Late Holocene sequences are identified locally at West Cronese, Balch and Old Dad Mountain ramps. Independent palaeoenvironmental evidence is available from the detailed reconstruction of the Quaternary history of Lake Mojave (Brown, 1989; Brown et al., 1990). These studies revealed high lake stands at 20-17 tac kyr BP and 15-12.5 ~4C kyr,
E A S T E R N MOJAVE D E S E R T S A N D R A M P S
range of luminescence age estimates (ka) 45
40
35
30
25
20
15
10
5
0
I
I
I
I
I
I
I
I
I
I
? .......................................... ~ .......................... ~
,: ..-...:
.:.: ........................ .:...~ ~.~
QTL
• :: .~. ~:,.,
FrL
............................................... ~ ........................ Palaeosolhorizons
FIRSL
~? QTL ..............~ FTL .....~.................. FIRSL o
[]
o []
[] []
[]
D B
..................
...
~
......................................
[]
o
o
[] []
4
? :,:::<: : :::::. :l
[]
Soldier Mountain West Cronese R a m p (a)
QTL FTL FIRSL
West Cronese R a m p (b)
QTL FTL FIRSL
West Cronese R a m p (c)
QTL FTL FIRSL
Cat D u n e Ramp (a)
QTL FTL FIRSL
Cat Dune R a m p (b)
QTL FTL FIRSL
Hanks Mountain
QTL FTL FIRSL
Balch
QTL FTL FIRSL
Old Dad Mountain
QTL FTL FIRSL
Dale Lake R a m p (a)
QTL FTL FIRSL
Dale Lake R a m p (b)
QTL FTL FIRSL
Iron Mountain
QTL FTL FIRSL
Big Marias
? D
m
HighLakeStands IntermittentLake
195
RECORD OF PLUVIAL LAKE MOJAVE
Fig. 4. Luminescence age range correlation diagram including data for the timing of pluvial Lake Mojave.
196
H.M. Rendell, N.L. Sheffer/ Geomorphology 17 (1996) 187 197
high stands of Lake Mojave I at 20.9-19.6 ka and Lake Mojave II at 16.5-13.4 ka, with the end of Intermittent Lake III at 9.7 ka. The comparison between the timing of the various phases of pluvial Lake Mojave, using the calibrated dates given above, and the timing of periods of active accumulation of sand on the ramps studied (see Fig. 4) raises a series of interesting issues. Although some of the sand accumulation on the ramps at the Cat Dune, Hanks Mountain, Dale Lake and, possibly, the Big Marias predates the start of Intermittent Lake Mojave I, the main phases of deposition and preservation of sand on the ramps broadly coincide with the main pluvial phases. Although the accuracy and the precision of the luminescence dates preclude detailed correlation of periods of soil formation with high stands of the lake, all of the palaeosols identified on the ramps studied fall within the timing of the various lake phases. The increasing aridity in the Early Holocene, associated with the demise of Intermittent Lake III, appears to coincide broadly with the cessation of accumulation on all sand ramps except those close to the point where the Mojave River flows out of the Afton Canyon (Balch) and those close to the West Cronese Playa (West Cronese sand ramps). The picture that emerges from the ramp data involves the interplay of a series of key factors namely: the continuity of sand supply to deflation areas, the (seasonal) occurrence of strong turbulent winds and the nature and abundance of the vegetation cover present in the East Mojave area during the Late Quaternary. Although reactivation of sand movement in areas of sand dunes may still be interpreted as linked to increased aridity, the coincidence of active accumulation on the sand ramps with a major pluvial phase, moves the emphasis in the Eastern Mojave at least on to the key role of sand supply. It is not unreasonable to suppose that sand movement was episodic and seasonal, with sand being supplied to washes and playas during periods of ephemeral flow ready for subsequent deflation. The lack of Late Holocene accumulation on the majority of the ramps studied is, therefore, interpreted as reflecting problems with sand supply, as increasing aridity reduced the frequency of ephemeral flow events in the fiver washes and playas. Late Holocene accumulation on the West Cronese and
Balch ramps but not the adjacent Cat Dune or Hank's Mountain ramps perhaps indicates that the orientation of the ramps relative to the dominant wind direction is also an important factor. As far as sediment sources are concerned, the likely source of material for the sand ramps at Balch and Old Dad Mountain was, and remains, the area of the Mojave River wash (Fig. 1). Even under presentday conditions, large amounts of sand can be brought into the wash area by occasional winter or spring floods. The most recent phase of sampling, in March/April 1993, coincided with the second wet (El Nifio) winter/spring season in the Mojave in successive years. The 'dry' playas of Silver Lake and the East Cronese basin were flooded. Moreover, the Mojave River had deposited substantial amounts of sand within, and at the exit from, the Afton Canyon. In the case of some of the other ramps such as Dale Lake or Iron Mountain, the local source of sediment would appear to be a playa.
7. Conclusions The luminescence dating of material from nine of the major sand ramps of the Eastern Mojave has raised questions about the application of these techniques in areas where independent dating control is largely absent and about fundamental issues of sediment supply and transport during the Late Quaternary. This approach has involved the application of three different techniques to the same suite of samples in order to look for concordance between the results. The results from quartz TL, feldspar TL and feldspar IRSL measurements are in broad agreement, and have provided a basis for the reconstruction of periods of sand accumulation and preservation on the sand ramps, at regional and local scales, during the Late Quaternary. The picture that emerges is one of episodic sand movement, with accumulation of sand on the ramps studied continuing throughout most of the Late Pleistocene to Early Holocene pluvial period in the Eastern Mojave. Increased aridity, since ca. 9 ka, appears to have been associated with only a limited amount of continued deposition on a few of the ramps. The results indicate that sediment transport by fluvial activity to deflation areas is the critical pro-
ll.M. Rendell, N.L. Sheffer / Geomorphology 17 (1996) 187-197 cess that ensured the continuity o f sand supply to the ramps. Such a finding is in contrast to m o r e traditional v i e w s that hawe equated sand m o v e m e n t with aridity. In the Eastern M o j a v e the m a j o r controls appear to be source(s) of material and the (seasonal) o c c u r r e n c e o f strong turbulent winds.
Acknowledgements The financial support o f Natural E n v i r o n m e n t Research C o u n c i l grants G R / 9 / 4 4 5 and G R / 9 / 7 8 0 and o f N A T O C o o p e r a t i v e R e s e a r c h P r o g r a m s Grant 900151 is gratefully a c k n o w l e d g e d , as is the assistance o f Dr M.L. Clarke, D r G.A.T. Duller, M r S. Edwards, Dr N. Lancaster, D r C.A. Richardson, Dr V.P. Tchakerian, and D r A.G. Wintle.
References Brown, W.J., 1989. The late Quaternary stratigraphy, palaeohydrology, and geomorrhology of pluvial Lake Mojave, Silver Lake and Soda Lake basins, CA. Unpublished M.S. thesis, University of New Mexico, Albuquerque, 275 pp. Brown, W.J., Wells, S.G., Enzel, Y., Anderson, R.Y. and McFadden, L.D., 1990. The late Quaternary history of pluvial Lake Mojave-Silver Lake and Soda lake Basins, California. In: R.E. Reynolds, S.G. Wells and R.H.I. Brady (Editors), At the
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End of the Mojave: Quaternary Studies in the Eastern Mojave Desert. Special Publication of the San Bernardino County Museum Association, San Bernardino, Calif., pp. 55-72. Clarke, M.L., Wintle, A.G. and Lancaster, N., 1996. Infra-red stimulated luminescence dating of sands from the Cronese Basins, Mojave Desert. In: N. Lancaster (Editor), Response of Aeolian Processes to Global Change. Geomorphology, 17: 199-205, this issue. Duller, G.A.T., 1991. Equivalent dose determinations using single aliquots. Nucl. Tracks Radiat. Measurements, 18: 371-378. Duller, G.A.T., 1993. Luminescence chronology of raised marine terraces, south-west North Island, New Zealand. Unpublished Ph.D. dissertation, University of Wales. Evans, J.R., 1961. Falling and climbing sand dunes in the Cronese ('Cat') Mountain area, San Bemadino County, California. J. Geol., 70:107-113. Prescott, J.R. and Hutton, J.H., 1988. Cosmic ray and gamma ray dosimetry for TL and ESR. Nucl. Tracks Radiat. Measurements, 14: 223-227. Rendell, H.M. and Wood, R.A., 1994. Quartz sample pretreatments for TL/OSL dating: studies of TL emission spectra. Radiat. Measurements, 23: 575-580. Rendell, H.M., Lancaster, N.L. and Tchakerian, V., 1994. Luminescence dating of Late Quaternary aeolian deposits at Dale Lake and Cronese Mountains, Mojave Desert, California. Quat. Geochronol., 13: 417-422. Stuiver, M. and Reimer,P.J., 1993. Extended L4C data base and revised CALIB 3.0 14C age program. Radiocarbon, 35: 215230. Tchakerian, V.P., 1991. Late Quaternary aeolian geomorphology of the Dale Lake sand sheet, southern Mojave Desert, California. Phys. Geogr., 12: 347-369. Wintle, A.G. and Prozynska, H., 1983. TL dating of loess in Germany and Poland. PACT, 9: 547-554.