ARTICLE IN PRESS

Quaternary International 166 (2007) 61–78

Late Quaternary stratigraphy and luminescence geochronology of the northeastern Mojave Desert

Shannon A. Mahana,Ã, David M. Millerb, Christopher M. Mengesc, James C. Younta

aUnited States Geological Survey, Box 25046 MS 974, Denver, CO 80225, USA bUnited States Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, CA 94025, USA cUnited States Geological Survey, 520 N. Park Ave., Tucson, AZ 85719-5035, USA

Available online 8 January 2007

Abstract

The chronology of the Holocene and late Pleistocene deposits of the northeastern Mojave Desert have been largely obtained using radiocarbon ages. Our study refines and extends this framework using optically stimulated luminescence (OSL) to date deposits from Valjean Valley, Silurian Lake Playa, Red Pass, and California Valley. Of particular interest are eolian fine silts incorporated in ground- water discharge (GWD) deposits bracketed at 185–140 and 20–50 ka. Alluvial fan deposits proved amenable for OSL by dating both eolian sand lenses and reworked eolian sand in a matrix of gravel that occurs within the fan stratigraphy. Lacustrine sand in spits and bars also yielded acceptable OSL ages. These OSL ages fill gaps in the geochronology of desert deposits, which can provide data relevant to understanding the responses of several depositional systems to regional changes in climate. This study identifies the most promising deposits for future luminescence dating and suggests that for several regions of the Mojave Desert, sediments from previously undated landforms can be more accurately placed within correct geologic map units. Published by Elsevier Ltd.

1. Introduction Previous chronologic studies in the northeastern Mojave Desert area include magneto-stratigraphic studies and Extracting paleoclimatic and paleoenvironmental infor- tephrochronology of the upper Pliocene to middle Quatern- mation from terrestrial deposits is an important area of ary Tecopa beds (Hillhouse, 1987; Sarna-Wojcicki et al., Quaternary geologic research. It is desirable to establish 1987; Morrison, 1999; Cox and Hillhouse, 2003), and both the quality and availability of as many terrestrial, radiocarbon dating of Lake Mojave deposits and associated climatic, and environmental proxies as possible (Andrews, piedmont alluvial units (Reheis et al., 1989; Wells et al., 2003). 2006). Deserts provide challenges for dating deposits. In The Mojave Desert contains several Quaternary eolian the northeastern Mojave Desert, a substantial body of sand fields, some of which include large sand streams work detailing absolute age chronologies, much of it based (Lancaster, 1999; Lancaster and Tchakerian, 2003)(Fig. 1). on radiocarbon ages, exists for part of the area, such as the One of these areas, the Kelso dune field, has been studied lower Colorado River, Silver Lake, Kelso Valley, Pahrump extensively, and several sand ramps comprised of collu- Valley, Las Vegas Valley, and central Death Valley (Reheis vium and eolian sand have been dated using infrared- et al., 1989; Wells et al., 1990; Bull, 1991; McDonald and stimulated luminescence (IRSL) (Edwards, 1993; Clarke, McFadden, 1994; Menges et al., 2001; Lundstrom et al., 1994; Rendell et al., 1994; Wintle et al., 1994; Bedford, 2003; Machette et al., 2003; Page et al., 2005). Using 2003). Elsewhere in the Mojave, eolian deposits have been optically stimulated luminescence (OSL) will help build widely sampled and correlated using luminescence dating correlations between those solidly dated deposits and those (Wintle et al., 1994; Clarke et al., 1996; Rendell and that remain poorly or not dated. Sheffer, 1996; Clarke and Rendell, 1998), including dates for short-duration periods of eolian deposition adjacent to, ÃCorresponding author. and corresponding with, low levels of Lake Mojave E-mail address: [email protected] (S.A. Mahan). (Busacca et al., 2004).

1040-6182/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.quaint.2006.12.010 ARTICLE IN PRESS 62 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78

Pahrump Valley Resting Spri Shoshone

Tecopa lake ng Range beds

Mesquite

Nopah Range Tecopa Valley

California Valley

Kingston Range Sperry Hills Hwy. 127 Mesquite Range

AmargosaDumont River Dunes Death Valley Kingston Wash

Shadow Valjean Valley Salt Creek

Valley Avawatz Mts.

Silurian Shadow Mts. Lake Silurian Hills

Silurian

Valley

Silver Cima volca Lake N Red Pass I-15 Soda Mts. nic field

Baker

051015202.5 Kilometers

Fig. 1. The Landsat 6 image of the northeast Mojave Desert showing locations of the OSL sampling sites (stars) discussed in this paper and principal geographic features. Silver Lake is on the northern edge of pluvial Lake Mojave.

Samples were collected for OSL from six sites in the Pass were found suitable for OSL dating techniques. The greater Silurian Valley area, as well as south and east of California Valley ground-water discharge (GWD) deposits Death Valley (Fig. 1, Table 1, Table 2). At the present time, were used as a test of several OSL parameters; these include this area is characterized by a bimodal wind system and a the utility of polymineral, fine-silt luminescence, and the long-term annual average precipitation of 137 mm/yr, with demonstration for the potential to date deposits at least as much of the moisture during October–April (Hereford old as 200 ka. Some deposits proved to be beyond the range et al., 2004). Within the collection area, lacustrine of OSL dating (Table 1). Fine-grained stream sediment, sediments from Silurian Lake playa, stream deposits from alluvial sediment that incorporates eolian sand, and GWD Salt Creek, and alluvial fans of Valjean Valley and Red that incorporates fine windblown silt all proved amenable ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 63

Table 1 Feldspar IRSL and quartz OSL ages from the Mojave Desert study area

Lab ID Field Dose rate Equivalent dose Age (yr) Dose rate Equivalent dose N Age (yr) Sample information ID, Moisture (Gy/ka) (Gy) (Gy/ka) (Gy) area and genesis (%) (71 s%) (71 s%) IRSLa (71 s%) (71 s%) Quartz OSLb

Valjean Valley alluvial fan M99VJ-990 0.8 4.5370.11 20.770.53 45837288 Alluvial fan unit 20.670.52 45837291

M99VJ-991 0.1 4.7370.18 16.870.59 35847197 Alluvial fan unit 13.870.62 29187243 Kingston Wash M99VJ-992 0.4 5.3570.19 411272.91 420,87271334 Eolian sand in Qia2 alluvium 483.474.71 415,52271036 M99VJ-993 15c 4.9070.11 27776.91 51,68673957 Cemented sand in Qoa alluvium 28279.59 52,81973308 M99VJ-994 0.3 4.9070.11 183721.9 37,36272447 Eolian sand in Qia2 alluvium 187723.3 38,25472453 Salt Creek Wash M99VJ-989 2.4 6.4770.20 31.870.38 49127328 Sand just under debris flow

M01OM-1843 0.3 6.2770.09 33.170.54 52837234 3.7870.06 18.870.42 42 (52) 49827161 Below M99VJ-989, sand

Silurian Lake Playa M99VJ-987 1.3 6.1870.19 41.271.32 66647478 Northern bar 39.370.98 63477451 M99VJ-988 1.2 6.1970.19 5.8970.68 9257229 Southern bar M01OM-1844 0.1 6.3170.10 4.6370.37 7337119 4.2670.06 1.5270.26 32 (47) 3567121 Southern bar (duplicate) 6.0270.66 9537206 California Valley LCW-1a 20c 6.1370.25 4804760.3 4131,080728,574 Not suitable Qia deposit (upper) 4823774.1 4134,219713,468 LCW-1b 15c 4.9770.17 4722746.6 4145,233714,152 Not suitable Qia deposit (lower) 4663746.4 4133,467712,171 LCW-3a 20c 5.1670.12 11471.30 22,16771178 3.6870.08 72.472.43 31(36) 19,68271580 Upper GWD calcareous sand

LCW-3b 15c 4.9270.07 Saturated 3.3670.05 Saturated Yellow sand

LCW-3c 15c 6.0170.08 30979.03 51,51573338 4.0870.06 239711.3 44 (48) 58,65274049 Lower GWD calcareous sand M01OM-1842a 15c 4.4870.07 4440724.8 498,231711,509 top GWD gravelly silt 4607743.5 4153,368722,524 M01OM-1842b 15c 10.970.38 Disequilibrium in dose 6.3670.22 Lower sand Red Pass M01OM-1849a 1.2 5.3170.08 64.371.22 12,1127588 3.8870.06 44.870.67 46 (48) 11,5377234 Upper sand, Qya4 67.970.93 12,7887524 M01OM-1849b 1.4 4.8570.07 15071.70 30,89471108 3.8570.06 14272.55 31(40) 36,88171290 Lower sand, Qia2? 10471.87 21,4197986

aData from Daybreak Reader, fine-grained silt (4–11 mm). See Table 3 for specific parameters. bData from Riso Reader, fine-grained sand (90–105, 105–125, 125–150, 150–180 fractions). See Table 4 for specific parameters. N ¼ Number of replicated equivalent dose (De) estimates used to calculate the mean. Figures in parentheses indicate total number of measurements made including failed runs with unusable data. cAverage water content had to be assumed as the sample was indurated, collected as a solid block and required HCl treatment for disaggregation. ARTICLE IN PRESS 64 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78

Table 2 Sample location data

Sample No. UTM E (m) UTM N (m) Date Deposit Feature sampled

M99VJ-990 579355 3938206 10/17/99 Qya3 Eolian sand lens M99VJ-991 580056 3929066 10/17/99 Qya3 Sand-rich alluvium M99VJ-992 586066 3942713 10/17/99 Qia2 Eolian sand lens M99VJ-993 585942 3942763 10/17/99 Qoa Eolian sand lens M99VJ-994 586756 3942774 10/17/99 Qia2? Eolian sand lens M99VJ-989 571220 3938243 10/17/99 Wash Sand bed M01OM-1843 571213 3938241 12/04/01 Wash Sand bed Mud block 571220 3938243 10/17/99 Active channel Mud-cracked block M99VJ-987 574130 3933553 10/17/99 Lake bar Playa bed M99VJ-988 576194 3930412 10/17/99 Lake bar Sand bed M01OM-1844 576202 3930409 12/04/01 Lake bar Sand bed LCW-1a 586095 3963330 12/03/01 4Qia2; pQia3 Ground-water enhanced sand- silt bed LCW-1b 586116 3963318 12/03/01 4Qia2; pQia3 Ground-water enhanced sand- silt bed LCW-3a 585881 3963746 12/03/01 GWD Sandy mud LCW-3b 585881 3963746 12/03/01 GWD Sand bed LCW-3c 585881 3963746 12/03/01 GWD Sandy mud M01OM-1842a 590542 3978404 12/04/01 Tertiary? Muddy sand M01OM-1842b 590542 3978404 12/04/01 Tertiary? Muddy sand M01SM-1849a 564267 3909254 12/05/01 Qia2 Sand bed M01SM-1849b 564267 3909254 12/05/01 Qia2 Sand bed

Note: All UTM coordinates are from Zone 11 using NAD83 projection.

to OSL dating by using adaptations of existing OSL Mojave) and the pluvial lakes (Mojave, Dumont, and techniques (Murray and Wintle, 2000; Banerjee et al., 2001; Manly) was closely linked to, and driven by, climatic Wallinga, 2002). changes (Wells et al., 2003) and pre-existing drainage patterns. Regional aquifer hydrology influences the springs 2. Regional geologic setting and wetlands in Tecopa Basin and California Valley (Quade et al., 1995, 1998, 2003). In contrast, local aquifer Silurian Valley, in the northeastern Mojave Desert, levels probably controlled the timing of wetland discharge extends from Baker and the Soda Mountains in the south in and near Silurian Valley. Piedmont systems underwent to Death Valley in the north (Fig. 1). The valley floor widespread deposition during the Pleistocene–Holocene altitude ranges from about 100 to 300 m, whereas fringing transition (McDonald et al., 2003), with later Holocene piedmonts, mountains, and upland valleys extend to depositional events being more restricted in both volume 2600 m. Salt Creek and the Amargosa River, the two main and area. Eolian sand and dust deposition in the greater drainages in the area, merge near the junction of Death Silurian Valley area is widespread, with prolific sources Valley and Silurian Valley. Between about 20 and 10 ka from the many playas, as well as distal alluvial fan systems time the south end of Silurian Valley was occupied by (Figs. 1 and 2). pluvial Lake Mojave (Wells et al., 2003). Overflow from Much of the stratigraphic and geologic control for our Lake Mojave traversed northern Silurian Valley to join the study is from recent refinement of the geologic mapping. Amargosa River, which emptied into Lake Manly in Death The surficial geology of the greater Silurian Valley area was Valley (Anderson and Wells, 2003a). mapped at a scale of 1:100,000 (Schmidt and McMackin, A smaller lake, Lake Dumont, existed where Valjean 2006; Menges and Miller, written communication) and Valley meets Silurian Valley, at Salt Spring Hills. Lake locally in more detail (Yount et al., 1994) using standard Dumont existed before Lake Mojave and started to fill field methods. During these field studies, locations of before 26 ka (radiocarbon years) radiocarbon (Anderson potentially datable deposits were noted, and selected for and Wells, 2003b). Recently, studies have suggested that the following reasons: applicability to regionally extensive some or all of the Lake Dumont deposits are wetland units and aspects of particular local problems. At these deposits (Reynolds et al., 2003; Bright and Anderson, locations, stratigraphic sections were measured and de- 2006), rather than widespread lacustrine deposits. scribed, and sediments were sampled. Most of the samples The Pleistocene to Holocene transition in the Silurian were from well-studied and mapped sites (Fig. 1 and Valley area is marked by hydrologic responses of several Table 2). Related splits for particle size analysis, paleontol- kinds. The history of the desert rivers (Amargosa and ogy, or other purposes were taken as needed. ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 65

0 1 2 4 6 kilometers M99VJ-993 M99VJ-994 M99VJ-992

Wash Kingston

M99VJ-989 M99VJ-990 M01OM-1843 Salt Creek Wash

M99VJ-987

GEOLOGIC UNITS FLUVIAL SYSTEM Holocene braidplain ¯ Holocene playa Principal wash Silurian Hills Holocene lake gravel Principal alluvial Silurian channel Lake Holocene alluvial fan M99VJ-987 Holocene and Pleistocene M01OM-1844 eolian sand and eolian-rich fan SAMPLE SITE Pleistocene alluvial fan M99VJ-992 Pre-Quaternary deposits M99VJ-991

Fig. 2. Map of sample sites and principal geologic features in Valjean Valley, Salt Creek, and Silurian Lake. The massive, widespread debris flow of Kingston Wash is indicated by a red color. This map is adapted from detailed geologic mapping of Miller and Yount (unpublished data).

3. Luminescence methods o1.10 is not considered to represent significant radon escape under laboratory conditions. These count rates are 3.1. Sampling and dosimetry accurate for calculating dose rates (Aitken, 1985, 1998; Snyder and Duval, 2003). For all but one sample, the range Samples were collected by auguring into a freshly of ratio change was 0.99–1.12 (Table 1). Alpha and beta cleaned face (natural exposure or man-made pit), and contributions to the dose rate were corrected for grain-size driving a polyvinyl chloride (PVC) tube into the sediment. attenuation (Aitken, 1985). Cosmic-ray dose rate was The ends of the tube were capped, and shielded from estimated for each sample as a function of depth, elevation sunlight. OSL samples were also collected as a large intact above sea level, and geomagnetic latitude (Prescott and block if the sediment was indurated. Block samples were Hutton, 1988). wrapped in aluminum foil and placed in a photography Measured elemental concentrations, associated dose bag. Samples were also collected for water content and rates, and cosmic ray contributions for both gamma-ray dose-rate measurements. Dosimetry measurements of spectrometry and instrumental neutron activation analyses potassium (K), uranium (U), and thorium (Th) were taken (INAA) were tabulated (Table 3). INAA is a non- in situ using a portable Exploranium gamma-ray spectro- destructive, highly precise, and accurate analytical techni- meter for samples collected in 1999. The gamma-ray que capable of determining up to 48 elements in most types spectrometer provides the isotopic discrimination of of samples. The INAA procedure involves irradiating the gamma rays; correspondingly, beta and alpha dose rates sample and appropriate standard reference material, with may be estimated. At each OSL sample location there were neutrons in the USGS TRIGA reactor to produce unstable at least two 1000 s counts recorded (about 16.5 min) from radioactive nuclides. Many of these radionuclides emit the gamma-ray spectrometer. gamma rays with characteristic energies that can be For lab dosimetry, bulk samples were dried, homo- measured using high-resolution semiconductor detectors. genized by gentle disaggregation, weighed, sealed in plastic The rate that the gamma rays are emitted from an element planchets having a diameter of 15.2 cm by 3.8 cm (some in the sample is directly proportional to its concentration. modification from Murray et al., 1987), then immediately The agreement between in situ and laboratory gamma-ray placed in a gamma-ray spectrometer for about 8.5 h. measurement as well as INAA results suggests negligible Samples were then stored for a minimum of 21 days to or no disequilibrium in the uranium and thorium dose- allow radon to achieve radioactive equilibrium, and the rate components, with the exception of some GWD measurements were repeated. The fraction of radon deposits. For the GWD deposits we followed dosimetric emanation was estimated from the difference of these two approximations over time using formulas developed by spectrometer measurements. A sealed/unsealed ratio of Rich et al. (2003). ARTICLE IN PRESS 66 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78

Table 3 Concentration of potassium, uranium, thorium and their contributions to the dose rate

Sample ID K (%) U (ppm) Th (ppm) Elevation Depth Dose ratec K U Th Rb Cosmic ray (m) (cm) (Total Gy/ka) (Gy/ka) (Gy/ka) (Gy/ka) (Gy/ka) (Gy/ka)

Valjean Valley alluvial fan M99VJ-990 2.03 2.24 6.00 309 25 4.53 2.146 1.189 0.911 0.040 0.242 M99VJ-991 2.01 2.53 6.04 298 20 4.73 2.148 1.359 0.928 0.040 0.252 Kingston Wash M99VJ-992a 2.14 2.20 10.70 462 125 5.35 2.284 1.180 1.642 0.043 0.196 M99VJ-993b 2.07 2.34 10.65 462 125 5.34 2.209 1.256 1.635 0.041 0.196 M99VJ-993 2.19 2.31 11.01 462 125 5.51 2.338 1.239 1.690 0.044 0.196 M99VJ-994a 2.46 3.16 8.00 485 125 4.90 2.224 1.413 1.026 0.041 0.197 Salt Creek Wash M99VJ-989a 2.70 3.01 10.31 188 25 6.23 2.820 1.577 1.546 0.053 0.236 M99VJ-989b 2.64 2.90 12.70 188 25 6.47 2.757 1.519 1.904 0.052 0.236 M99VJ-989 2.65 2.85 11.10 188 25 6.21 2.768 1.493 1.664 0.052 0.236 M01OM-1843 2.72 2.58 11.42 188 130 6.27 2.900 1.382 1.751 0.054 0.183 Silurian Lake Playa M99VJ-987a 2.41 3.51 9.94 208 40 6.18 2.547 1.863 1.510 0.048 0.214 M99VJ-988a 2.66 3.35 8.60 197 25 6.19 2.812 1.778 1.306 0.053 0.236 M01OM-1844 3.18 2.03 10.06 197 40 6.31 3.398 1.091 1.546 0.064 0.213 California Valley LCW-1a 2.71 3.89 13.14 2334 350 6.13 2.450 1.740 1.686 0.046 0.205 LCW-1b 2.61 2.37 10.78 2334 770 4.97 2.359 1.060 1.383 0.044 0.125 LCW-3a 2.51 3.20 11.54 2340 350 5.16 2.157 1.355 1.403 0.040 0.205 LCW-3b 2.53 2.33 10.63 2340 435 4.92 2.287 1.042 1.364 0.043 0.184 LCW-3c 2.67 3.75 13.31 2340 520 6.01 2.413 1.677 1.708 0.045 0.166 M01ML-1842a 2.11 2.98 6.05 2323 5 4.48 1.907 1.333 0.776 0.036 0.424 M01ML-1842b 2.74 16.32 7.42 2323 610 10.92 2.477 7.299 0.952 0.046 0.149 Red Pass M01ML-1849a 2.84 2.32 10.65 2331 75 5.31 2.567 1.038 1.366 0.048 0.287 M01ML-1849b 2.61 2.22 9.84 2331 395 4.85 2.359 0.993 1.262 0.044 0.193

Also listed are rubidium contribution and the cosmic dose rate. These contributions show alpha, beta and gamma components. Dose rates for blue-light OSL do not have an alpha component and only 90% of the beta component but are not shown. aCounted in situ with portable gamma spectrometer (see text for discussion). bCounted using instrumental neutron activation analyses (INAA) (see text for discussion). cDose-rate figures have been rounded to three significant figures, but the total dose rates and contributions have been calculated using values before rounding. Dose rates were calculated assuming a field water content (expressed as % dry mass) between 1–1075% and 15–2075% (indurated block values), using a value of 0.04070.002 (Rees-Jones, 1995). Central values are given for dose rates, errors are incorporated into that given for the total dose rate.

3.2. Sample preparation and determination of the equivalent stimulate feldspars) was performed on a polymineralic dose (no mineral separation) fine silt fraction (4–11 mm). All sand-sized quartz samples were analyzed by single-aliquot Under subdued orange light in our laboratory, possible regeneration (SAR) procedures (Murray and Wintle, 2000; light-exposed end material from each block or tube Banerjee et al., 2001) with blue-light excitation. Dose (3 cm) was discarded and samples prepared for dating recovery and preheat plateau tests were performed to using standard procedures (Millard and Maat, 1994; ensure that the sediments were responsive to optical Roberts and Wintle, 2001; Singhvi et al., 2001; Sohn techniques and that the proper preheat temperatures were et al., 2007) with appropriate modifications. Generally, the used in producing the equivalent dose (De) values. Run predominantly preferred sand size was determined to be parameters for the IRSL data are given in Table 4, and run 90–125 mm, although sometimes the 125–150 mm and the parameters for blue-light OSL are given in Table 5. 180–250 mm sizes were also utilized for OSL. The fine-grained (4–11 mm) polymineral extracts from all Two types of luminescence dating were performed on samples were dated using the total-bleach multiple-aliquot different grain sizes and mineral fractions. Blue-light OSL additive-dose (MAAD) method (Singhvi et al., 1982; Lang, (that OSL which uses a blue wavelength to stimulate 1994; Richardson et al., 1997; Forman and Pierson, 2002). quartz) was performed on fine-sand-sized quartz separates. At least two attempts were made per IRSL sample to IRSL (the OSL which uses infrared wavelengths to determine MAAD ages. Anomalous fading tests on the ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 67

Table 4 Single aliquot quartz OSL measurement parameters

Instrument: Riso TL/OSL-DA-15A/B Stimulation source: six clusters LED, emission centered 470 nm Power delivered to aliquot: 22 mW/cm2 Duration of stimulation:40s PMT: EMI 9236Q Aliquot temperature: 125 1C Detection filters: two Hoya U-340 Preheat: 220 1C–260 1C/10 s with cut heat of 160 1C/10 s Analytical procedures: IRSL 100 s ‘‘wash’’, Botter-Jensen et al., 2000; Duller, 2001 and Minisys 14.

Table 5 Multiple aliquot feldspar IRSL measurement parameters

Instrument: Daybreak 1100 Automated TL Systems Stimulation source: 30 IR diodes, emission centered on 880 nm Power delivered to aliquot: 19 mW/cm2 Duration of stimulation: 100–30 s PMT: Thorn-EMI 9635Q Aliquot temperature:301C Detection filters: Schott BG-39 & Kopp 7-59 Preheat: 124 1C/64 h or 140 1C/6 h Analytical procedures: TLApplic 4.26 software

stability of the IRSL luminescence signal indicated little to no signal instability (recording ratios of 0.89–1.07). These values are a ratio of luminescence emission after storage of 60–30 days divided by the immediate measurement; a ratio of 1.0 indicates stable luminescence, and our ages seem to Fig. 3. Aerial photograph showing sample sites and lacustrine beach bars. obviate the need for an extensive correction factor Smaller arrows indicate flow direction through Silurian Lake playa. (Huntley and Lamothe, 2001). Although we acknowledge that the feldspars do show some fading, we did not perform tests on differing aliquots of the same sample to measure Wash is composed of muddy sand for the most part, the range of fading within a given sample, and thus were although gravel beds are present where it flows into and not able to calculate ‘‘corrected’’ ages for applicable fade exits Silurian Lake playa. ratios (Huntley and Lian, 2006). At Salt Creek Wash, thin-bedded, well-sorted sand of the distal fan underlies a debris-flow deposit. The fine-grained 4. Results and discussion unit within the debris flow was sampled for OSL dating because of the likelihood of deposition by shallow flow. 4.1. Valjean Valley geologic setting This shallow flow would have lasted from hours to days in streams. We collected two samples in stratigraphic order: Valjean Valley meets Silurian Valley approximately (1) M99VJ-989, 15–25 cm below the debris-flow base, and where Kingston Wash joins Salt Creek Wash and contains (2) M01OM-1843, 105 cm below M99VJ-989 (Fig. 4). The an array of deposits that were luminescence dating targets samples were analyzed about two years apart, and the (Fig. 2). Valjean Valley is a gently undulating broad plain deeper sample (M01OM-1843) was analyzed using both of alluvial sediment derived from three trunk streams that IRSL and blue-light OSL. The upper sample (M99VJ-989) drain the uplands of Squaw Mountain and nearby land, yielded an age of 4.9 ka and the deeper sample yielded ages and Kingston Wash. Kingston Wash drains Shadow Valley of 5.3 ka (IRSL) and 4.9 ka years (quartz from blue-light and consequently is much larger than Salt Creek Wash. OSL). Both ages for the lower bed are within overlapping Eolian sand sheets distal to Dumont Dunes overlie and are error and indicate an age of 5 ka. The debris flow is interbedded with alluvium in much of the valley north of therefore younger than the upper dated bed, or less than Kingston Wash. South of the wash, eolian sand deposits 4.9 ka. are present only near the east end of Silurian Lake playa, We also targeted eolian sand lenses in Pleistocene but eolian sand is an abundant constituent of the alluvium alluvium exposed in the cut bank of Kingston Wash and much farther up the fans (Fig. 3). The bed of Salt Creek at Riggs Quarry (Fig. 2). These eolian sand lenses are ARTICLE IN PRESS 68 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78

and strongly developed soils. At both sample sites, the Debris flow bed upper soil horizons, above the calcic horizon, were removed, and the calcic horizon now overlain by a thin Position of sample sheet of gravelly reworked eolian sand. OSL sample M99VJ-989 (sample M99VJ-992 was taken from 1.0 m below the calcic horizon taken 4 m laterally along bed) dating (Fig. 5b). The muddy sand bed contains matrix- supported cobbles at the base, possibly a debris-flow deposit, similar to sample site M99VJ-993. The calculated IRSL age is a minimum value 421 ka because of saturation in the IRSL. The sample also showed blue-light OSL saturation, at high dose levels, indicating that any luminescence age is a minimum and thus the deposit could not be more precisely dated. It is also possible that the grains were poorly exposed to sunlight during deposition, but this could not be resolved from the data. Site of sample Adjacent to the deposit, minimally dated by sample M01OM-1843 M99VJ-992, is an inset terrace cut into the fan. The sample was collected 65 cm below the base of the weakly developed stage II calcic horizon (Fig. 5c) from a fluvially reworked eolian sand bed. This sample (M99VJ-994) yielded IRSL ages of 38 ka (Table 1). The two samples, from older deposits, therefore yielded results that only modestly constrain the possible depositional ages at 421 and Fig. 4. Photograph of the sampled section at Salt Creek. M99VJ-989 was sampled from the indicated bed but out of the field of view. Note boulders 52 ka, and the third sample indicated a depositional age and cobbles of granite in the overlying debris-flow bed, in contrast to sand of 38 ka. and finer grain sizes of lower, thin beds of distal alluvial fan origin. The The deposit sampled at Riggs Quarry, southwest of the total section thickness is about 1.35 m. Silurian Hills, is part of a widespread mid-Holocene unit (Qya3) that is younger than pluvial lake stands formed interbedded with alluvium that shows no apparent break in around 10 ka (Reheis et al., 1989). The Qya3 deposit deposition, evidenced by a soil or angular unconformity. consists of thin- to medium-bedded sheetflow and channel At Kingston Wash, sample M99VJ-993 was taken from a deposits. Sample M99VJ-991 (Fig. 2) was taken from a gravelly sand lens that contains fluvially reworked eolian sand lens 60 cm below the surface, and yielded IRSL ages sand. The sampled horizon is high in an old alluvial unit of 2.9 and 3.6 ka. These ages correlate with a known short (Qoa) (Yount et al., 1994). Qoa units have well-developed wet period around 3.6 ka (Wells et al., 2003). calcic soils, are strongly stripped by erosion and form Eolian sand is present throughout Valjean Valley, but balenas. The sample was taken 170 cm below the eroded not as discrete eolian depositional forms. The eolian sand crest of the deposit as an intact block because the material fraction is reworked into the alluvial fan deposit by fluvial was indurated and tube sampling was impossible. processes forming sand-rich deposits, but these eolian The base of the calcic horizon extends into the upper grains may be identified by their very fine size, grain part of a sand lens, above the sampled interval. The frosting, and sorting. These deposits should be good targets resulting 52 ka IRSL age (Table 1) is much younger than for OSL dating. To test this hypothesis, we sampled a we expected for this deposit. Other mapped old alluvial sandy mid- Holocene fan deposit near the townsite of units with this degree of soil development and landform Valjean. The unit is characterized by a remnant bar-and- modifications contain the 774 ka Bishop Ash. We are swale topography, a very weak pavement, weak reddening, uncertain how to interpret the results, although it is almost and stage I calcic horizon development. We sampled the Bk certain that the early to mid-Pleistocene age of this deposit horizon and were unable to avoid illuvial fine grains is simply beyond IRSL and TL dating limits. Factors that (sample M99VJ-990, Table 1). Replicate IRSL ages on the are associated with the young IRSL age are possible same sample are 4.6 and 4.5 ka and closely match ages infiltration of fines associated with soil development after obtained from Salt Creek Wash. stripping to form the ballena, sampling problems that have Distal Kingston Wash forms a broad fan that is created an artificially young age, or that the previously correlative to the mid-Holocene Qya3 deposit, based on uncalibrated soil surface paradigms are in need of an soil development and surface characteristics (Fig. 2). This absolute age calibration. deposit contains abundant boulders and was emplaced in a Kingston Wash samples M99VJ-992 and M99VJ-994 single catastrophic event, as a debris flow, or a hyper- were taken from eolian sand lenses in intermediate age concentrated sediment flood deposit. The debris flow alluvial units (Qia2). This mapping unit is characterized by extends 10.3 km along Kingston Wash and is 7 km wide a nearly flat surface, moderate varnish on pavement clasts, along Salt Creek, with many boulders that are 1 m in ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 69

on the current thickness, the volume of the deposit is greater than 36 106 m3. Because the debris flow may have been deposited under cloudy or turbid flow conditions, there is some probability of inadequate grain bleaching during deposition. In order to mitigate the likelihood of insufficient light exposure before burial, we searched for interbedded fine-grained beds or fine-grained beds just below the debris flow.

4.2. Silurian Lake playa

Silurian Lake is an unusual playa, one that we term a ‘‘through-flow playa’’, because Salt Creek Wash enters it on the south, at about elevation 205 m, and exits on the north at about elevation 204 m. The playa is not a true closed basin, but rather a stretch in the wash that is broad, nearly horizontal, and experiences both bed load and suspended load deposition. During deep flow events, much of the water and sediment goes downstream, and beds are typically thin and the sediment is poorly sorted, ranging in size from sand to clay. If the playa was in through-flow configuration during the mid-Holocene, as suggested by wash deposits above and below the playa that range widely in age, it might be a sensitive recorder of stream dynamics, including damming by the Kingston Wash debris flow. Subtle lacustrine bars are present on the north and south sides of the playa (Figs. 2 and 3) and imply shallow lakes either from sustained high-volume stream flow or from temporary damming of Salt Creek. To explore the possibility that the debris flow from lower Kingston Wash dammed Salt Creek, we conducted reconnaissance lumi- nescence sampling of bars in Silurian Lake, reasoning that thinly bedded fine sand and silt beds were likely exposed to sunlight in the shallow waters. The bar at the north end of Silurian Lake is around 205 m in elevation and is about 20 cm higher than the playa floor. The bar crest is very rounded and capped by fine gravel that is moderately well sorted. We dug a pit through the bar, which consists of 12 cm of lacustrine sand and gravel on 7 cm of sandy playa mud. Underlying playa deposits, which are poorly sorted and less distinctly bedded than the overlying bar deposits, are about 10 cm thick. Under this interval is ripple-laminated sand. Because the lacustrine beds of the bar were so thin and contained abundant silt that was probably reworked, we sampled underlying playa beds that contained abundant sand and Fig. 5. Photographs of the south bank of Kingston Wash, showing sampled sand beds and wash sediments. Tape is 2 m long in C. (A) Qoa silt. The resulting age for M99VJ-987 at 6.5 ka (Table 1) site for sample M99VJ-993. Our OSL sample is taken just to right of the indicates that the bar formed sometime after 6.5 ka, and photographed interval. The section is approximately 3 m high. Note the probably significantly before the debris-flow emplacement boulders in the lower part of sand bed. (B) Qia2 site for sample M99VJ- around 4.6–5.2 ka. 992. OSL sampled as a block from the irregular-shaped sand lens near the The bar at the south end of Silurian Lake is a long, top of the tape (whitish color). (C) Qia2 site for sample M99VJ-994. The sampled bed shows recessive weathering, near the center of the tape; the broad feature that shows little sign of erosion; a few small OSL sample site was later mammal modified. channels are cut through it and carry flow from Salt Creek to the playa (Fig. 6a). However, the channel cut banks are diameter, and up to 2 m in diameter near the Tonopah and sharp and little rounding of the crest of the cut-bank Tidewater railroad grade (Fig. 2). The deposit is now less surface is apparent. It appears morphologically younger than 1 m thick; and thins to 40 cm at the distal toe. Based than the bar at the north end of the playa. This southern ARTICLE IN PRESS 70 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78

B

A Surface of lacustrine bar

Site of sample M01OM-1844 Site of sample M99VJ-988

Sample M01OM-1844

Fig. 6. Photographs of the sampled sections of a lacustrine beach bar at Silurian Lake. (A) View of two sample sites looking eastward along the crest of the bar. Note the vegetation-choked channel cutting through the bar behind samplers Mahan and Menges. The photo was taken on 12/4/01. (B) Sampling pit for M01OM-1844. Note the concentration of gravel at the surface, but the lack of an Av horizon, as most of the pit wall is composed of thin beds of moderately sorted sand and fine gravel. The sample was taken at the bottom of the pit, which was near tape measure 0.8, and was 40 cm deep.

bar is composed of thin bedded to laminated fine gravel and sand in moderately- to well-sorted parallel beds. Little evidence of illuvial silt or sand is present, so we sampled and dated a silty sand bed that was 38 cm below the surface. Sample M99VJ-988 yielded an IRSL age of 9507230 years, surprisingly young. To test the validity of the young date we re-sampled 5 m away in a gravelly sand bed beneath the bed previously sampled, at a depth of LCW-3a-b 40 cm. Sample M01VJ-1844 yielded similarly young ages: IRSL ages of 7307120 and 9507210 years, and a blue- light OSL age on quartz of 3607120 years (Fig. 6b). The uncertainty between IRSL and OSL methods is much greater than the analytical uncertainty. The deposit dated between 500 and 1100 years old and suggests that the LCW-1a LCW-1b bar is 800–850 years old. This southern bar is much younger than the bar at the northern end of Silurian Lake.

4.3. California Valley 00.125 0.25 0.5 0.75 Kilometers At least two generations of GWD deposits are exposed Explanation along incised reaches of the axial channel and adjacent Channel Qigo Bedrock tributaries of the California Wash drainage in the lower Holocene terrace Qigy Sample locations California Valley (Figs. 1 and 7). These GWD deposits Qia2 Qya record significant, local intervals where water intersected the surface at this site. The deposits are situated in a valley Fig. 7. Geologic map of the southern California Valley area, showing sample sites, axial drainage channels, and major surficial deposits that currently lacks any modern spring or seepage activity discussed in the text. These include the intermediate-age alluvial fan and are intricately intermixed with alluvium associated deposit (Qia2), the younger (Qigy), and older (Qigo) ground-water with the incised axial drainage and adjoining dissected discharge deposits, and Holocene terrace and alluvial fan (Qya) deposits. ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 71 piedmont fans. Along a section of exposed valley fill are abundant eolian sediments, secondary carbonate, and fossilized organic remains indicative of deposition within formerly extensive GWD zones. These GWD deposits constrain the ages of alluvial stratigraphy and geomorphic processes in this part of the valley. The entire sequence of GWD forms a series of bluffs that flank anastomosing active channels and related Holocene terraces associated with post-Qigy incision by the axial drainage of the lower California Wash. (Qigy is defined as younger intermediate-age ground-water deposits vs. Qigo or older intermediate-age ground-water deposits). This sequence is extensively exposed throughout the valley axis and thus comprises the primary valley fill, predating the most recent set of axial channels and fill terraces. At several locations, the margin of Qigy deposits are clearly buttressed against a steeply dipping erosional contact cut across the truncated distal margin of adjacent older GWD deposits of Qigo. This erosional contact is interpreted from field relations as the bounding escarpment of an older paleovalley cut into the Qog valley fill and subsequently buried by inset GWD deposits of Qyg. The younger GWD deposit (Qigy) consists of a poorly bedded, 5.5-m thick section of fine-grained sand, silt, and clay (Fig. 8a). This fine-grained section is locally capped by a 1.5 m-thick cemented zone of secondary carbonate that commonly contains nodular and tubular textures sugges- tive of carbonate-replaced root networks. No organic material suitable for radiocarbon dating was found. Several characteristics, including the textures of both sediments and secondary carbonate, strongly suggest a deposition within a ground-water-related wetland environ- Fig. 8. (a) Photograph of younger ground-water discharge deposits (Qigy) ment (Quade et al., 1995), an inference supported by the along the axial drainage of southern California Valley. Sample locations presence in the Qigy of ostracodes with ground-water are described in Table 1 and in the text. The total exposed thickness of the supplied wetland affinities (Forester et al., 2003). section is 5.5 m. The unit has a fine-grained texture, is weakly consolidated, shows a gradational nature in subunits and an absence of The entire Qigy unit, as well as subsequent axial channels well-defined buried paleosols or other stratigraphic discontinuities in the and terrace deposits, are inset within older GWD deposits section. (b) Photograph of older ground-water discharge deposits (Qigo) (Qigo) that are exposed along the walls of incised tributary that are exposed in the wall of a incised tributary canyon. Sample washes to the east of both the modern axial valley and the locations are described in Table 1 and in the text. The section has a total margin of the paleovalley enclosing the inset Qigy deposits exposed thickness of 7.8 m, with the unit truncated at the base (dashed line) of a thin gravel cap of intermediate age alluvial fan gravel (Qia2). The (Fig. 7). These Qigo deposits contain a 7-m-thick section of entire Qigo section is fine grained and weakly to moderately indurated, poorly bedded to massive fine sand and silt with some clay, with dispersed to locally concentrated secondary carbonate. The two rhizoliths, and secondary carbonate dispersed throughout ledges near the sample intervals are related to more indurated zones with the unit (Fig. 8b). The section also contains two prominent enhanced clay and carbonate accumulations interpreted as ground-water 41-m-thick zones, each consisting of a slightly reddened, enhanced paleosols. clay-enriched horizon above an indurated layer of carbo- nate. The carbonate horizon is a buried soil. However, gravel deposit, with surface and soil characteristics similar there are no other sharp stratigraphic discontinuities or to intermediate-age Qia2 alluvial fan deposits. Qia2 paleochannels indicative of either sub aerial exposure or deposits are found in this and other valleys in the region. major period of channel incision in this aggradational U-series methods have been used to date many GWD sequence. Characteristics of the deposit include a fine- deposits or travertine mounds (Grun et al., 1988). These grained texture, an absence of large gravel beds, locally deposits have been problematic to date by using radio- abundant carbonate molded organic material, and a carbon and U-series because of open-system behavior and distribution and texture of secondary carbonate, suggesting contamination effects (Viles and Goudie, 1990; Ford and distal piedmont or basin-interior alluvial deposition with Pedley, 1996), although the work by Paces et al. (1996) is a significant eolian and ground-water influence. These GWD notable exception. Early attempts to date GWD mounds deposits are dissected by a thin (o40–50 cm) capping using luminescence on the secondary carbonates were also ARTICLE IN PRESS 72 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 not successful (Grun et al., 1988; Wieser et al., 1993; stratigraphy and geomorphic processes in the lower Singhvi et al., 1996). However, attempts to date the detrital California Valley. A thin Qia2 layer overlying the older eolian sediment grains instead of the carbonate cement GWD deposits abuts at its distal margin against a terrace were more successful (Paces et al., 1996; Lundstrom et al., with otherwise generally similar Qia2 characteristics, but at 2003; Rich et al., 2003). For our study of the GWD a slightly lower elevation. Both Qia2 fan and terrace units deposits in California Valley we closely followed lumines- are truncated by Qigy and the modern drainage at the cence techniques in both Paces et al. (1996) and Rich et al. eastern margin of the paleovalley. These crosscutting (2003) These papers include previous work in the Mojave relationships bracket the age of the Qia2 units, which Desert and south Texas that showed good concordance represent a major regional depositional interval, between with U-series and 14C on the young GWD episodes. the older and younger ages of Qigo and Qigy, at Another compelling argument for dating these deposits approximately 50 (IRSL on LCW-3c) to 185 ka (using the using luminescence is that modern marsh areas, with TL ages on LCW-1b). emergent plants, are efficient eolian sediment traps, These same relations appear to constrain the timing of thus favoring luminescence ages obtained on the eolian two major intervals of incision along the axial drainage of components. the lower California Valley. The first incision episode Luminescence ages of three samples from a Qigy set occurred in the paleovalley that truncates the western of GWD deposits as described above (LCW-3a, -b, -c; margin of the Qigo and overlying Qia2 deposits. As noted Table 1), ranged from 22.271.2 ka (IRSL) to 19.771.6 ka earlier, this paleovalley encloses, and indeed is largely (blue-light OSL) for the top (LCW-3a), to 58.774.0 ka buried by, the extensive inset GWD deposits of Qigy. These (blue-light OSL) and 51.573.3 (IRSL) ka at depth (LCW- stratigraphic relations broadly bracket the paleovalley 3c). A cross check using TL on two of these samples yielded incision between the same 50 and 185 ka time interval ages of 23.371.9 ka for LCW-3a and 52.2723.3 ka for described above. This appears to be the earliest significant LCW-3c. The TL was run as a check on the dynamics of dissection that occurred along the along the axial drainage the luminescence systematics and as a simple test of the in this part of the valley, based on the primarily bleached conditions of the grains at burial. aggradational character and absence of large paleochan- The upper and lower samples were collected at 2.7 and nels within Qigo deposits predating paleovalley formation. 7.6 m, respectively, below the present-day surface of the A younger interval of incision along the present axial exposed Qigy section. Sample LCW-3b, collected from a channel system, at the Qigy sample sites, is evident. These yellow middle sand zone (4.4 m below the top), had a low channels have dissected 8.2 m below the upper preserved OSL signal, poor dose recovery, and large changes in surface capping the Qigy deposits, and thus establish an sensitivity and thus could not be dated. approximate maximum age of 20–25 ka for the initiation of The older GWD deposits (Qigo) were also sampled for this post-Qigy incision. This incision cuts down 8.2 m luminescence dating (LCW-1a, -b; Table 1; Fig. 8b). Two above the modern channel, the total height of the upper large block samples were taken, one from the top (LCW-1a part of this section, and is dated at approximately at 3.5 m below the present day top) and one from near the 20–25 ka. At least two stages of this incision event are bottom (LCW-1b at 5.2 m) of the exposed section in the further indicated by the 5.5 m height of the Qigy section gully wall. These samples were assumed to be older than above the late Holocene fill terrace and the 2.5 m height of 100 ka due because of two paleosols and the amount of these terraces above the nearest active channel. moderately cemented secondary carbonate in the section. The third GWD mound we sampled for luminescence Because the deposit ages may approach the OSL dating dating did not yield useful results. The mound consists of a limits, feldspar IRSL was attempted rather than quartz sand and mud matrix between abundant angular fragments OSL. Feldspar generally has a higher saturation dose of Tertiary bedrock. Sample M01ML-1842a was collected (Aitken, 1985; Huntley and Lamothe, 2001). The samples near the top of the mound, about 4 m below the present were saturated with respect to equivalent dose levels, and day ground surface. Sample M01ML-1842b was collected only minimum ages were obtained (Table 1). These samples near the bottom of the mound, 6.1 m below the ground were also run for TL, which does not saturate as quickly as surface and above the present-day wash. IRSL dating of OSL and has been utilized for dating samples in excess of the top sample yielded only minimum ages because of 200 ka (Berger, 1988). TL is not generally recommended for signal saturation; the TL signal was saturated as well. The dating sandy water-laid deposits; however, TL has been oldest minimum age is 4174739.8 ka. The bottom sample successful on fine-grained portions of the water-laid was not dated due to the saturation and disequilibrium in sediments (Berger, 1984, 1985, 1987). TL ages obtained the dose rate for the upper sample (Table 3). for LCW-1a and LCW-1b are within error at 158.2718.2 and 162.8722.6 ka. The section must have been accumu- 4.4. Red Pass lating and incorporating silt between 140 and 185 ka ago. The dated eolian sediments captured in the GWD Deposits at Red Pass record the capture of a large deposits, when combined with their stratigraphic context, watershed drainage area within eastern Fort Irwin by Salt provide several important time constraints on alluvial Creek. Red Pass is occupied by an east-flowing wash with ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 73 a narrow, deep gorge. The oldest deposits, adjacent to the The Qya4 contains two reworked eolian sand lenses, in gorge, mantle faulted Miocene strata of the Avawatz an otherwise thin- to thick-bedded fluvial deposit char- Formation, and have the characteristic soils and landform acterized by lateral bed continuity and moderate grain size of Qoa deposits (Qoa has been defined as deposits that and sorting (Fig. 9). An eolian sand lens was sampled have been strongly stripped by erosion and generally (M01SM-1849a), 3 m from the top, on a surface below a considered to be 500–800 ka). This Qoa dips west and has weak stage I calcic horizon and buried Qia2. This upper clasts derived from the Soda Mountains. Inset into the sample yielded an age of 12.570.55 ka for IRSL and eroded Qoa deposits, carrying a different clast composition 11.570.23 ka for blue-light OSL. These two ages overlap derived from the north, are younger alluvial Qia2 deposits. within their error limits and record the deposition of the The base of the Qia2 deposit is several meters above the Qya4 unit at about 12 ka. modern wash in Red Pass, but the surface of the deposits A second sample was obtained 90 cm deeper (sample and sediment structures indicate streams flowed east, M01SM-1849b) within the Qia2 unit. An upper paleosol parallel to the modern drainage. Within the gorge is a developed across the base of this upper sand bed, which compound alluvial unit about 6.5 m thick with an inset: consists of fine sand and contains local ‘‘popcorn’’ texture Qya4 (latest Pleistocene). This Qya4 deposit buries two GWD carbonate. The sample was taken from the sand bed older deposits (Qoa and Qia2), which are separated by below a buried soil and had one IRSL age of 21.471.0 ka, weak paleosols. which was much younger than the other two ages of

f luvial gravel and sand

Site of sample M01ML -1849a

weak soil eolian sand lenses

Site of sample M01ML-1849b

weak soil

Fig. 9. Photograph of fluvial and alluvial fan deposits at Red Pass. The metric tape is 2 m long; although only a portion is shown. The total section is 6.6 m thick. Two sand beds were sampled for OSL; the top is sample M01-OM-1849a and the bottom is sample M01-OM-1849b. ARTICLE IN PRESS 74 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78

30.971.1 ka (IRSL) and 36.971.3 ka (blue-light OSL). Latest Pleistocene to early Holocene fan deposits are There is no simple explanation for the younger measured widespread in the desert southwest (McDonald et al., 2003) IRSL age and we discarded the age. There still exists a and date from 15 to 9 ka across the region (e.g., Reheis discrepancy between the 31,000-year-old-feldspar age et al., 1989; Page et al., 2005). Our OSL date of 12 ka for and the 37,000-year-old-quartz, but these ages probably the upper sand unit Qya4 in Red Pass corresponds well represent the mixture of partially bleached grains within with previous dates. the sample, such that the true age is somewhere between 30 Holocene alluvial fan deposits from Valjean Valley date and 38 ka when we use the error limits. This age at 4.5 to 3.2 ka correlate on the basis of soils and surface corresponds with climatic events at 32–35 ka, including morphology to mid-Holocene deposits at Silver Lake deposition during interstadials 5–7 and associated stadial (Reheis et al., 1989), and fluvial deposits interfingering periods (a period of rapid Dansgaard/Oeschger cycling with these fans yielded ages of 5 ka. Deposits of this age during marine isotope stage 3) (Grootes et al., 1993; Stuiver with similar surface characteristics and soils are widely et al., 1999). recognized in the stratigraphic sequences with independent age control cited above. 5. Correlations with regional and alluvial fan chronology 5.1. Salt creek and silurian lake paleohydrology The stratigraphic and luminescence data from a number of critical sites in the Mojave Desert generally support the Following the last overflow of Lake Mojave at about existing chronologic framework for Quaternary alluvial fan 11.4 ka ago (calibrated 14C yrs) (Wells et al., 2003), Salt activity in the Mojave Desert. The ages also compare with Creek has been fed from a watershed east of the Avawatz additional geochronologic data from the Yucca Mountain Mountain. Local alluvial fan channels within and east of area of southern Nevada. The oldest widespread alluvial Silurian Valley have also contributed. The modern and deposits (Qoa) are 452 ka as dated by luminescence, Holocene hydrologic system within Silurian Valley includes although the degree of original surface modification and three low-gradient stretches: the through-flow playa of strong relict soil development suggest that the deposits are Silurian Lake, a junction with Kingston Wash near the middle Pleistocene and may be significantly older than this Dumont wetland deposits of the Salt Springs area (Bright minimum age (Yount et al., 1994). Qoa deposits exposed at and Anderson, 2006), and at the junction with the Ibex Pass contain an interbedded air-fall ash correlated Amargosa River. We have not dated all elements of this with the Bishop Ash dated at 774 ka (Sarna-Wojcicki et al., system, but offer several observations and dates of 2000) near the base of the unit. The strong development of hydrologic events for pieces of the system. Shallow lakes the relict surface soil and degree of incision and interfluve built gravel bars at Silurian Lake about 6.5 and 0.85 ka rounding of the Qoa is morphologically similar to the ago, indicating obstructed outflow from the playa and Qa1 deposit at Yucca Mountain, which also contains an high-volume sustained fluvial conditions. Timing of the last interbedded ash (Bishop Ash?) near its base (Whitney et al., lake stand is similar to that of a flood event bracketed by 2004). droughts in the record of Walker River (Stine, 1994) and Widespread intermediate-age alluvial deposits (Qia2) Carson Sink (Adams, 2003). yield ages ranging from 33 to 4140 ka. These deposits The primary purpose for dating the debris-flow deposit probably contain two or more distinct alluvial fan of Kingston Wash and the lake-stand deposits of Silurian depositional pulses (Yount et al., 1994). The age range Lake was to examine whether the debris-flow event for Qia2 correlates with ages reported for the Qa4 and all influenced the development of Silurian Lake. This debris but the oldest range of Qa3 deposits at Yucca Mountain flow marks a rare, possibly catastrophic, large-volume (Whitney et al., 2004). There is a similarity in the surface event that deposited material onto a distal piedmont, characteristics and soil properties between Qia2 and Qa3 at including deposition at the axial valley wash of Salt Creek. Yucca Mountain. Qa3 units have eroded surfaces and more Silurian Lake playa is an unusual through-flow playa strongly developed soils than typically are observed on midway along Salt Creek, positioned 2 km south of, and Qia2 fan deposits. The age range of the Qia2 deposits in the only 1–2 m higher than, the toe of the debris flow. Our study area agree with other reported ages of late dating indicates that the debris flow is unlikely to have Pleistocene units in adjacent areas to varying degrees formed the late Pleistocene Silurian Lake playa. The debris (Menges and Anderson, 2005). These include older Qf2 flow was emplaced about 4.4–5.2 ka ago, and the units at Silver Lake (Reheis et al., 1989; Wells et al., 1990), small lacustrine bars at Silurian Lake are about 6.5 and the Q2c unit in the lower Colorado River region (Bull, 0.85 ka old. 1991), the Qf4 and perhaps the younger elements of Qf3 Salt Creek was apparently able to receive and transport alluvium in Kelso Valley near the Providence Mountains the water and sediment load from the debris-flow event (McDonald and McFadden, 1994; Wang et al., 1996), with only slight perturbations to stream course and size. Qai alluvium in central Death Valley (Machette et al., This remarkable resilience of Salt Creek provides evidence 2003), and the Qai alluvium in Pahrump Valley of the ability of arid land fluvial systems to adapt to a wide (Lundstrom et al., 2003). range of runoff conditions. ARTICLE IN PRESS S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 75

5.2. Amargosa river drainage history isotope stage 6 (Lowenstein et al., 1999; Machette et al., 2001, 2003). Dates from the carbonates in basin fill at The paleovalley incision of the lower California Valley Brown Spring, the oldest set of palustrine deposits in inferred from the GWD stratigraphy and our new Pahrump Valley, fall within a broadly defined age range of geochronology has important implications to the drainage 200–400 ka (Lundstrom et al., 2003). Related deposits evolution of the Amargosa River and its tributaries. The are either absent or not exposed at the present levels of early-stage axial dissection in California Valley is probably dissection in lower California Wash; however, they related to regional incision along the Amargosa River, the are present as the oldest dated spring mounds master base-level stream in this area (Fig. 1). Dissection by (265–300 ka: average about 283 ka) in Tecopa basin the Amargosa River is related to breaching of a former (Nelson et al., 2001). paleodivide along the Amargosa River paleo-drainage A younger discharge event with ages between about 20 south of Tecopa (Morrison, 1999; Anderson, 2005; Menges and 52 ka is reflected in a fine-grained accumulation of and Anderson, 2005) sometime between 140 and 185 ka. mixed eolian and alluvial sediments (Qigy) within a This breach, in turn, initiated canyon cutting and upstream wetland environment. The young wetland deposits in the dissection along the master stream and its tributaries. The outcrop are homogeneous, having poorly defined grada- paleovalley incision at the sample sites records the arrival tional boundaries and containing no buried soils or of this incision nickpoint in the lower California Valley obvious erosional discontinuities. We did not recognize between 50 and 140 ka. discrete discharge subunits despite the long-time interval The relation of the younger phase of post-Qigy incision recorded in the section. The top of the section has a to other downstream sections of the Amargosa River pronounced relict soil with abundant secondary carbonate drainage is less clear. Certainly all of the dissected reaches and rhizoliths. This soil implies a long period of stabiliza- of the system contain stepped terrace sequences indicating tion of this valley fill within a ground-water environment significant post-Qia2 incision. Some of this incision before initiation of subsequent dissection and excavation correlates with the younger as well as early-stage dissection by the present axial drainage system. intervals in the lower California Wash, but none of the This young (Qigy) GWD event in California Valley dissected reaches elsewhere in the regional drainage contain correlates with recognized ground-water paleodischarge a ground-water-dominated valley-fill sequence similar to events in other basins in southern Nevada and eastern the Qigy unit in California Wash. It is difficult to partition California. This interpretation is based on varying degrees the post-Qia2 incision in other sections of the Amargosa of overlap in the age ranges from GWD deposits in drainage without additional dating on the terraces. California Valley and similar deposits identified and dated However, Anderson (2005) has documented, via radio- by radiocarbon, luminescence, and (or) U-series disequili- carbon-dated terrace alluvium, several pulses of late brium methods in this region. Specific correlative sites are Holocene (o2400 years) to recent channel aggradation present in Las Vegas Valley (Haynes, 1967; Quade, 1986; and downcutting in the Amargosa Canyon. One or more of Bell et al., 1999; Page et al., 2005), Pahrump Valley (Quade these terraces likely correlate in part with the youngest set et al., 1995; Lundstrom et al., 2003; Page et al., 2005), of fill terraces inset into Qigy valley fill along axial channels Tecopa basin, and the northern Amargosa Desert (Paces et of California Wash. al., 1996; Lundstrom et al., 1998). The best age correlation among the basins is the 20–25 ka range or unit D of Haynes 5.3. GWD deposits and later workers (Haynes, 1967; Quade et al., 1995, 1998; Lundstrom et al., 1998). Some basins contain GWD The sedimentology, stratigraphy, and geochronology of deposits with ages in the 50–60 ka range or unit C of GWD deposits within and adjacent to the axial drainage of Haynes and others (Haynes, 1967; Bell et al., 1999; Page the lower California Valley (Table 1) clearly identify two et al., 2005), which match closely with a U-series age of new intervals of water intersecting the surface, whereas 7272 ka for spring-mound carbonate in eastern Tecopa today there is minor spring or seepage activity. The basin (Nelson et al., 2001). minimum age interval for each discharge event is established by luminescence dates from near the top and bottom of the 6. Conclusions exposed GWD sections. An older event, broadly bracketed between 140 and 185 ka ago, is associated with slow Our results indicate that OSL is suitable for dating fine accumulation of distal-piedmont to basin-interior, fine- grains associated with three widespread deposits of the grained alluvial sediment in a wetland environment. There northeastern Mojave Desert: (1) eolian captured sediments is no indication of significant erosional or degradational in GWD deposits, (2) eolian sand reworked into alluvial events during this period, although several possible deposi- fan sediments, and (3) lacustrine sand in beach bar tional hiatuses are suggested by the presence of two weak to deposits. Deposits of these types, when carefully selected moderate buried paleosols within the ground-water deposits. and sampled, greatly expand targets for dating in deserts. The 140–185 ka interval coincides with the last deep Two distinct episodes of GWD deposition are evident stand of Lake Manly in Death Valley during oxygen- from both stratigraphic relations and the luminescence ARTICLE IN PRESS 76 S.A. Mahan et al. / Quaternary International 166 (2007) 61–78 dating in lower California Valley. OSL dates indicate age Andrews, J.E., 2006. Palaeoclimatic records from stable isotopes in ranges for the events of 20 ka–52 ka and 140 ka–185 ka ago. riverine tufas: synthesis and review. Earth-Science Reviews 75, 85–104. A long-lived sand transport system in the Dumont Banerjee, D., Murray, A.S., Boetter-Jensen, L., Lang, A., 2001. Equivalent dose estimation using a single aliquot of polymineral fine Dunes area is indicated by eolian sand interfingering with grains. Radiation Measurements 33 (1), 73–94. alluvial deposits from at least the middle Pleistocene to Bedford, D.R., 2003. Surficial and bedrock geologic map database of the modern time. OSL ages from eolian sand ramps or sheets Kelso 7.50 quadrangle, San Bernardino County, California. US range from 3.2 to 452 ka in various locations. Our Geological Survey Open File Report 03-501 (with accompanying sampling was systematic enough to identify periods of pamphlet of 34p.), scale 1:24000. /http://geopubs.wr.usgs.gov/open-file/ of03-501/S. enhanced eolian activity. Holocene alluvial fan deposits Bell, J.W., Ramelli, A.R., dePolo, C.M., Maldonado, F., Schmidt, D.L., near the fringe of eolian activity appear to be datable 1999. Geologic map of the Corn Creek Springs Quadrangle, Clark by OSL because they incorporate large fractions of eolian County, Nevada. Nevada Bureau of Mines and Geology Map 121, sand. 1:24,000 scale. Lacustrine bars and spits yield accurate OSL dates. The Berger, G.W., 1984. Dating Quaternary deposits by luminescence: recent southern spit of the Silurian Lake playa is about 800–850 advances. Geoscience Canada 13, 15–21. Berger, G.W., 1985. Thermoluminescence dating studies of rapidly years old, which coincides with a regional southwest USA deposited silts from south-central British Columbia. Canadian Journal flood event that is bracketed by two prolonged droughts. of Earth Sciences 22, 704–710. Uncertainties in ages preclude a definite correlation, but Berger, G.W., 1987. Thermoluminescence dating of the Pleistocene Old the temporal coincidence is intriguing. Crow tephra and adjacent loess, near Fairbanks, Alaska. Canadian Luminescence methods yield reliable dates for eolian Journal of Earth Sciences 24, 1975–1984. Berger, G.W., 1988. Dating Quaternary events by luminescence. In: sediments captured in GWD deposits, reworked eolian Easterbrook, D.J. (Ed.), Dating Quaternary Sediments. Geological sand in alluvial fans, axial-valley wash deposits, and playa Society of America Special Paper, vol. 227, pp. 13–51. and lacustrine sands. 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