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Dating alluvial deposits with optically stimulated luminescence, AMS 14C and cosmogenic techniques, western Transverse Ranges, California, USA
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Quaternary Geochronology 2 (2007) 129–136 www.elsevier.com/locate/quageo
Research paper
Dating alluvial deposits with optically stimulated luminescence, AMS 14C and cosmogenic techniques, western Transverse Ranges, California, USA S.B. DeLonga,, L.J. Arnoldb a
Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson AZ 85721, USA Oxford Luminescence Research Group, School of Geography and the Environment, University of Oxford, Mansfield Rd, Oxford OX1 3TB, UK
b
Received 12 October 2005; accepted 27 March 2006 Available online 23 May 2006
Abstract In an effort to better understand chronology of alluvial episodes in Cuyama Valley in the western Transverse Ranges of California, USA, we employed optically stimulated luminescence, radiocarbon and cosmogenic radionuclide surface exposure dating methods. Twenty-one optical dates ranging from 0.01 to 27 ka were obtained from exposures of late-Holocene axial-fluvial deposits, Pleistocene–Holocene alluvial-fan deposits, and axial-fluvial sands interbedded within a late Pleistocene alluvial fan. These were crosschecked with 37 AMS radiocarbon dates from charcoal and wood from within a and five 10Be surface exposure dates from boulders on alluvial-fan surfaces. The OSL results show generally good stratigraphic consistency, logical comparison with the radiocarbon and cosmogenic data, and appear to be the best method for accurate dating within deposits of this nature because suitable material is fairly easy to find in these environments. The radiocarbon data contained numerous ‘‘detrital ages’’, but well-bedded lenses of apparently in situ or minimally transported charcoal provide reliable age estimates for the associated alluvium. Radiocarbon dating of detrital charcoal in the older alluvial fan deposits was problematic. Our cosmogenic surface-exposure dating was consistent stratigraphically and with our other data, but we were unable to determine its accuracy due to the limited number of samples and the possibility of inherited radionuclides and post-depositional erosion. In light of our results, we suggest that OSL dating using the latest analytical techniques combined with rigorous methods for estimation of paleodase is reliable and of increasing utility in otherwise difficult-to-date coarse alluvial environments in the southwestern United States and elsewhere. r 2006 Elsevier Ltd. All rights reserved. Keywords: Alluvia fans; Fluvial sediment; Optically stimulated luminescence dating; Radiocarbon dating; Cosmogenic surface-exposure dating; Cuyama valley, CA
1. Introduction Accurate age-determination of alluvial deposits in arid and semi-arid climates is possible using a number of techniques; each with its own limitations. Most widely applied are radiocarbon dating, cosmogenic radionuclide surface-exposure dating, and optically stimulated luminescence dating. Radiocarbon (14C) dating relies on the presence of organic material in an interpretable context within the alluvial deposit, which is rare in dry environments. Cosmogenic radionuclide (CRN) techniques require Corresponding author. Tel.: +1 520 621 6003.
E-mail address: [email protected] (S.B. DeLong). 1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.03.012
determination of the effects of pre-depositionally inherited radionuclides and post-depositional erosion of the target deposit, and proper calibration of isotope production rates. Optically stimulated luminescence (OSL) dating shows great promise, but is still regarded as developmental in its application to fluvial deposits. This paper presents results of a ‘‘blind’’ comparison of all three techniques (with emphasis on direct comparison of radiocarbon and OSL dating by two independent laboratories) applied to latePleistocene to late-Holocene axial-fluvial and alluvial-fan deposits in Cuyama Valley, in the western Transverse Ranges, California, USA. This study serves to highlight both limitations and successful applications of these techniques within a detailed case-study. This paper is not
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intended to be a comprehensive review of details of each dating technique; for a complete review of methodology and application of each technique the reader is directed to publications such as Gosse and Phillips (2001) for CRN techniques, Wallinga (2002) and Aitken (1998) for OSL dating, and Faure (1986) for radiocarbon dating. This paper also does not detail the geologic interpretations made within the wider scope of the study, but instead focuses on the comparison of the geochronologic data. 2. Geological setting and description of lithologic units Cuyama Valley is located at the western end of the western Transverse Ranges where they meet the southern Coast Ranges in southern California, USA (Fig. 1). The modern climate is semi-arid (MAP ¼ 15–25 cm, Mediterranean regime) and hot (MAT ¼ 10–15C). Cuyama Valley is a relatively young structural valley, formed by transpression associated with the San Andreas Fault Zone which has increased since ca. 3 Ma (Ellis et al., 1993). The valley is bounded by the Caliente Mountains to the north and the larger Sierra Madre Mountains to the south (Fig. 2). The Sierra Madre piedmont is mosaic of deformed and eroded late-Cenozoic basin-fill units capped in places by lateQuaternary alluvial fans and their well-preserved planar geomorphic surfaces. The axial Cuyama River is a meandering ephemeral channel that is incised up to 12 m below late-Holocene axial fluvial-terrace surfaces for over 50 km. The focus of this study was on age-determination of the late-Pleistocene alluvial-fan deposits on the Sierra Madre Mountain piedmont and the suite of axial-fluvial deposits along the Cuyama River. On the Sierra Madre piedmont, there are five extensive and well-preserved alluvial fan units preserved as a sequence of planar depositional geomorphic surfaces. We classified these as Qaf1–Qaf5 from oldest to youngest. Deposits capped by these geomorphic surfaces tend to be coarse-grained, clast-supported and -bedded, indicative of fluvial processes. Sandy beds suitable for OSL dating were rare, though a large road cut through a Qaf4 unit revealed
Fig. 2. Landsat7 image of study area showing physiography, and exposure locations discussed in text.
both sandy lenses in alluvium and interbedded sandy axial material. Unit Qaf5 was markedly different than the other units, as it was apparently sourced locally from reworking of a large exposure of Pleistocene lacustrine or shallowwater deposits, giving it a finer sandy texture. Organic material suitable for radiocarbon dating of these deposits is rare. The units oriented along the axis of the valley are dominated by silty and sandy bedded sediment. Exposures of axial material are widespread over 450 km of the Cuyama River. The stratigraphy has been simplified for this paper, and we present data from four sedimentary units, Qa1–Qa4, which represent the majority of exposed sediments preserved as axial terraces along the Cuyama River. This paper presents geochronologic comparisons from three axial-fluvial exposures and two alluvial-fan exposures. The stratigraphic interpretations at the sites used in this paper rely on a larger data set of radiocarbon ages and several more described stratigraphic sections from elsewhere in the study area. Detailed description of these are beyond the scope of this paper and will be presented elsewhere. 3. Methods 3.1. OSL dating
Fig. 1. Location of study area in Southern California.
Bedded waterlain sediments were sampled using opaque ABS pipe without exposing the sediment to light during sampling. Laboratory analysis was carried out at Oxford University by the second author. Refinement of pure coarse-grained quartz separates was undertaken using the standard laboratory preparation procedures outlined in Aitken (1998). Individual equivalent dose (De) estimates were measured using small aliquots (100–300 grains/disc) for all samples except 070402.01 and single grains (SG) for
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all samples except OSL20-22. All De measurements were made using Risor TL-DA-15 readers. The OSL signals were detected using a blue-sensitive EMI9235QA photomultiplier tube fitted with two U-340 filters. SG and single-aliquot (SA) De estimates were both calculated using the SAR protocol developed by Murray and Wintle (2000). The SAR measurement conditions adopted in this research follow those used by Arnold et al. (this issue). SA De estimates were accepted for further analysis if they displayed (i) recycling ratios within 10% of unity, (ii) OSL-IR depletion ratios 40.9 (Duller et al., 2003), (iii) thermal transfer o5% of the natural signal. SG De’s were only accepted where (i) the recycling ratio Lx =T x points were consistent with each other within their 1 s errors, (ii) OSL-IR depletion ratios were 40.9, (iii) thermal transfer was o10% of the natural signal, (iv) the error on the natural test dose signal was o20%, (v) calculated De uncertainty was o30%, (vi) the natural signal intensity was 43 times the standard deviation of the late-light background signal. The number of grains/aliquots measured and accepted can be found in Table S1. Sample bleaching characteristics were assessed from the accepted De populations using the decision procedures approach proposed recently by Bailey and Arnold (in press). For each sample, the final burial dose estimate was calculated using the age model deemed most suitable according to this statistical decision procedure. Environmental dose rates were calculated using a combination of field gamma spectrometry (FGS) and inductively coupled mass spectrometry (ICP-MS). External g-dose rates were calculated using FGS for all samples and ICP-MS for all samples except OSL18-OSL22. External bdose rate contributions were calculated using ICP-MS measurements for all samples. Cosmic-ray dose rate contributions were determined using the calculations of Prescott and Hutton (1994). Present-day water content values were assumed to be representative of those pertaining to the full burial period, and were assigned relative uncertainties of 750%.
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traction and graphite target preparation under vacuum at the University of Arizona Desert Laboratory. AMS analysis of graphite targets were performed at the NSF/ Arizona AMS Facility. Sample radiocarbon ages o21 ka were calibrated to 2-sigma calendar years using Reimer et al. (2004) and older samples were calibrated according to Fairbanks et al. (2005). While proposed calibrated ages have been included in figures without error values, wide ranges in calibrated ages exist for most samples (as reported in Table S3), and caution must be used when using calibrated ages. 3.3. CRN surface exposure dating Coarse-grained, extremely well-indurated sandstone boulders partially embedded in the alluvial surfaces of the Sierra Madre piedmont were sampled for 10Be surface exposure dating. These were selected if they showed no obvious signs of spallation, weathering, or past burial and excavation. Isotopic analysis of 10Be abundance in quartz was carried out at Purdue University’s PRIME Laboratory. These data were corrected for sample thickness and topographic shielding, and were then corrected for latitude, longitude, elevation, and past geomagnetic effects following Pigati and Lifton (2004). In order to use the most accurate cosmogenic production rate for 10Be, we also corrected the raw data of Stone’s et al. (1998) Younger Dryas-aged samples from Scotland using the techniques in Pigati and Lifton (2004). From this we took the long-term integrated high-latitude sea-level 10Be production rate to be 4.35 atoms/g/yr. Following Partridge et al. (2003) we use the meanlife of 10Be to be 1.9370.10 Ma; for discussion of ambiguity related to this value see note 34 therein. No corrections were made for either sample erosion or predepositionally inherited radionuclides. This leads to increased uncertainty in the relationship of exposure age to the age of the geomorphic surface, and the need for independent verification. 4. Results
3.2. Radiocarbon dating Charcoal fragments from alluvial deposits were sampled for radiocarbon dating. Preference was given to charcoal that either appeared to have burned in situ, or deposited as a large concentration, indicating minimal fluvial reworking, as these were most likely to have radiocarbon ages that represent the age of the surrounding deposit. These are referred to as ‘‘charcoal layers’’ in Table S3. ‘‘Detrital’’ charcoal, which was usually isolated fragments of charcoal within fluvial material, was also dated, although this type of material has the potential to have ages much older than the surrounding sedimentary deposit. In one case wood was dated, and one sample of base-soluble humates from charcoal was dated. Samples were subjected to physical removal of visible contaminants, a standard ABA (acid–base–acid) pretreatment, bulk combustion and CO2 ex-
Table S2 contains all OSL results, Table S3 contains radiocarbon results, and Table S4 contains CRN results. The data are described at each exposure in the following sections and shown visually in the figures. In the figures, SG OSL ages are presented when available, though both SG and SA ages are included in Table S2 when available. Comparison of radiocarbon ages to OSL ages is somewhat problematic. Table S3 contains radiocarbon results including ages in 14C years, 2-sigma calibration ranges, the median probability age in calendar years B.P. (before 1950), and also our proposed age before 2002 which is simply the median probability age plus 50 years rounded to the nearest 10 years. On figures, the ‘‘proposed age before 2002’’ is given in calendar years without error estimates. OSL and CRN dates are presented as ka with analytical error.
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4.1. Axial exposure A1 Exposure A1 is a 12-m arroyo-wall exposure of bedded sands, silts, and minor gravel with no apparent soil development at ‘km 5’ (as measured from the upstream extent of incised arroyo channel) (Fig. 3). This exposure has a disconformity halfway down which is evidenced by a weak paleosol, and a change in sediment character from silty, moderately consolidated material (Qa2) to overlying looser sandy material (Qa3). Age control on this section is from three AMS radiocarbon dates and one OSL date near the top of the section. The two younger radiocarbon dates
calibrate from anywhere between 0 and 350 calendar years B.P., so OSL appears to be more useable in this age range. However, with this caveat, the OSL and radiocarbon ages are consistent. 4.2. Axial exposure A2 Exposure A2 is at ‘km 15’, and is a wide exposure of 10-mthick bedded silts and sands capped by a weakly developed fluvent soil (Fig. 4). At this location, we made considerable effort to compare OSL and 14C results. The unit is made up of variable thicknesses of Qa2 and Qa1 that overly each other across a long-wavelength, angular unconformity marked by truncated bedding, paleo-hillslope deposits, a distinguishing paleosol, and onlapping overlying strata. Age control on this section comes from 15 AMS radiocarbon dates and seven OSL dates as illustrated in Fig. 4. 4.3. Axial exposure A3
Fig. 3. Exposure A1. Radiocarbon dates shown with sample numbers as median probability calendar years before 2002. See Table S4 for actual calibration range. OSL dates shown in ka with sample numbers and analytical error in parentheses.
Exposure A3 is a 12-m-thick conformable sequence of bedded silts, sands, gravel and cobbles capped by a weakly developed fluvent soil at ‘km 25’. The entire exposure is unit Qa2. Age control is provided by four AMS radiocarbon dates and a single OSL sample as shown in Fig. 5. The SG OSL date does not overlap within error with the radiocarbon dates—it is 2–300 years older. This could be from a failure of the age model determination criteria (the CAM model produces the oldest age estimate of the six age models presented in Bailey and Arnold (in press). A historical flood deposit adjacent to this exposure gave a SG OSL age of 0.01 ka and a radiocarbon age (AA53752) of 450 calendar
Fig. 4. Exposure A2. Vertical exaggeration is 2X (note horizontal and vertical scales. Sample locations are surveyed with total station and projected into 2-D. Radiocarbon samples AA63784 and AA63783 are from extensive layers of charcoal and should be particularly reliable.
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Fig. 5. Exposure A3. Radiocarbon dates shown with sample numbers as median probability calendar years before 2002. See Table S4 for actual calibration range. OSL dates shown with sample numbers and error in parentheses.
years B.P. This radiocarbon age is certainly detrital, but could be as young as 55 calendar years B.P. due to the shape of the calibration curve in this time-range. Though we cannot be certain, we think that this deposit is the record of the significant El Nino flood event of 1998 (Bowers, 2000). We base this conclusion on analysis of repeat aerial photograph analysis, streamflow data, and the youthful expression of the deposits in the field. This interpretation supports the utility of SG OSL for dating very young fluvial deposits. 4.4. Alluvial-fan exposure AF1
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Fig. 6. Exposure AF1. Radiocarbon dates shown with sample numbers as median probability calendar years before 2002. OSL dates shown with sample numbers and error in parentheses. Field assistant for scale in left photo, 30 cm hammer in right photo.
alluvial-fan material and sandy axial material underlying an extensive and very well-preserved alluvial-fan surface. The fan and axial materials are easily distinguished by reddish-brown sandstone-dominated nature of the fan material and the polymict, granite-bearing nature of the finer axial material. Age control is from six OSL dates, three radiocarbon dates and two CRN surface-exposure dates. While the OSL dates show excellent stratigraphic coherence, the radiocarbon and CRN dates show some scatter, as discussed in Section 5. 4.6. Additional alluvial-fan CRN surface exposure sample sites
Exposure AF1 is a gully formed in a broad swale of unit Qaf5 and possibly underlying Qaf4 (Fig. 6). This is the lowest alluvial-fan surface on the Sierra Madre piedmont. Soil formation is noticeable in this dominantly fine-grained deposit, and has disrupted much of the original fluvial bedding in upper portions of the exposure. We sampled below the primary pedogenic zone from bedded sands. Exposure of sediments is generally poor, making stratigraphic interpretation difficult. Age control is provided by eight detrital radiocarbon samples which show little coherence, and four OSL dates which fall into two groups. Another OSL (070402.01) of 22.2972.12 ka from an exposure of Qaf5 or Qaf4 5 km upstream from a sandy lens in a bouldery deposit is comparable to the oldest OSL date from the gully (OSL19) of 26.0471.97 ka. Both are found below local unconformities in distinctively coarser material, and are likely strath remnants of Qaf4-aged material. Further discussion of the other scatter in these data can be found in Section 5.
Two CRN ages (C011204.01 and C11204.03) were obtained from Qaf1, which is the highest and oldest extensive planar alluvial-fan surface in our study area. The ages were 6972 and 9174 ka, indicating that either the older sample preserved inherited CRNs or the younger sample was eroded or shielded since deposition, or some combination of both. Since both appear extremely erosion resistant and well embedded in the alluvial surface, we favor the former. One CRN age from a Qaf3 surface (C070403.05) gave an age of 29.372.7 ka which is stratigraphically consistent. Our best estimates of ages of all alluvial-fan surfaces fit a linear, survey-derived, ageelevation relationship determined by projecting surface slope to their axial–valley intersection point. Assuming a relatively constant axial-incision-rate supports the integrity of our chronology.
4.5. Alluvial-fan exposure AF2
Overall, agreement between the dating methods is good. The OSL dates appear to have more stratigraphic consistency than the radiocarbon dates, especially for older materials. This is unsurprising, given that radiocarbon
Exposure AF2 is through unit Qaf4 (Fig. 7). These exposures are of interbedded tributary-derived boulder
5. Discussion
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Fig. 7. Exposure AF2. Radiocarbon dates given as calibrated years before 2002. CRN and OSL ages shown as ka with analytical error.
dating in the fluvial environment can be subject to a number of problems (Gillespie et al., 1992). The coupled OSL-radiocarbon approach allows for cross-checking of each dating method, and given enough sample density, conclusions can be made about the accuracy of particular dates. Exposure A2 in particular is an example of how both dating a large number of radiocarbon samples and crosschecking with OSL and reliable ‘‘charcoal layers’’ can identify ‘‘detrital’’ radiocarbon ages, leading to an increased understanding of actual stratigraphic ages. This could also be accomplished by employing more careful radiocarbon sampling criteria than we did; however, this is difficult in fluvial environments, where isolated detrital charcoal is common. Use of isolated and often small charcoal fragments to date older alluvial-fan deposits was problematic. Both AF1 and AF2 exposures would be less interpretable without the complementary OSL dates. More invasive radiocarbon preparation such as ABOX-SC (Bird et al., 1999) perhaps could have improved the accuracy of our older radiocarbon dates from the Pleistocene alluvial-fans by better removing contaminants. The five CRN surface-exposure ages from boulders on alluvial surfaces would also be difficult to rely on independently of other data, but in the context of our larger data set, we conclude that only samples C070503.01 and C11204.03 were subject to significant inherited CRN contamination. This suggests that moderately large data sets (perhaps 3–4 per surface) of cosmogenic ages from isolated boulders on alluvial-fan surfaces can be utilized to get a sense of age succession, perhaps without the much more labor-intensive depthprofile approach. Taken with the strongly coherent OSL dates from Qaf4, we can confidently conclude that the stratigraphic age of unit Qaf4 is between 29 ka at 410 m depth to 20–22 ka at the surface. This sort of age precision is rarely possible with radiocarbon dating in these environments without serendipitous location of significant organic deposits having clear relation to the stratigraphy. The coupled OSL–radiocarbon–cosmogenic dating approach shows great promise for these types of dryland alluvial deposits. Dating results from exposure AF1 are more difficult to interpret. Both radiocarbon and OSL ages show significant
scatter (Fig. 4). The radiocarbon ages range from 13 to 30 calendar years B.P. over less than 1 m vertically and several laterally. This is indicative of reworked detrital charcoal contaminating younger sediments, which is reasonable given that the alluvium was sourced from organic-rich Pleistocene lacustrine or shallow-water deposits preserved farther upslope on the piedmont. The OSL dates also show scatter between 6 and 28 ka. A subtle unconformity separates the three younger OSL ages (6, 7 and 16.5 ka) from the two oldest ages (22 and 26 ka), which is perhaps a contact between Qaf5 and older underlying Qaf4. Likely, the complicated geochronological results are a combination of geological and methodological factors. Deposition was likely slow, spatially complex, and difficult to interpret due to poor exposure. The data scatter also likely indicates methodological problems; however, at this time we are unable to diagnose any specific causes of this. From this, we urge caution when attempting to date deposits that are not clearly interpretable in the field, as adding problematic chorological data to difficult-to-interpret geology compounds challenge. That said, we can place this exposure within the context of our understanding of the local stratigraphy with some confidence based on soil development, topographic position and the chronological data. Our OSL data set (Table S2) contains both SG and SA ages for most samples. It is widely accepted that SG analyses are favorable to SA analyses in fluvial environments (e.g., Olley et al., 2004). Comparison of the two in the context of our interpretations of the local stratigraphy provides significant support for this idea, but also some comparisons between SG and SA ages that are somewhat equivocal. The best example of clear superiority of a SG age comes from the dating of a very young (likely deposited in 1998) flood deposit (Qa4, OSL sample 070202.02). The OSL age of 10 years would not have been possible to determine without SG techniques; the SA age was 1450 years, and the radiocarbon date from that deposit was a clearly detrital 343 14C years B.P. SG OSL is particularly useful when dating sediments that fall within the difficult-to calibrate, less than 350 14C years B.P. range. Several radiocarbon dates in this study fall into that range, and having OSL data to compare increases the precision of dating efforts.
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A closer look at other SG–SA age comparisons also support SG ages as superior. In particular, the SA OSL from exposure A1 (Fig. 3) appears to overestimate the age of the deposit by a factor of two based on our understanding of the stratigraphy, lack of soil development, and two radiocarbon dates from the same section. Data from the well-dated exposure A2 (Fig. 4) also supports SG use. The SG ages are both stratigraphically consistent and very similar to the chronology suggested by the radiocarbon data set, whereas the SA ages contain a stratigraphic reversal (samples 070302.01 and 070302.02), and occasionally appear too old when compared to the radiocarbon dates. Somewhat less conclusive is the comparison between SA and SG ages for OSL samples 070302.06 and 070302.07. The SG ages are possibly too young based on the 2nearby radiocarbon dates (see Fig. 4), but since the radiocarbon dates could include some detrital age, the SA ages may well be too old. The one case where we can say with a bit of confidence that the SA age appears to better represent the age of the deposit is exposure A3 (Fig. 5). There, the SG age appears too old when compared to the radiocarbon ages, and the SA age more closely matches. The comparison between the SG and SA ages from the early Holocene and late-Pleistocene alluvial-fan deposits suggests both techniques worked similarly. All of the SG and SA ages from the alluvial-fan exposures overlap within error. 6. Conclusions Though our discussion tends to highlight the challenges in our study, overall our multi-technique approach led to a detailed understanding of the alluvial chronology in Cuyama Valley, CA. In particular, OSL showed great utility in dating samples of all ages in this study. OSL is useful in difficult-to-calibrate radiocarbon age ranges, and in environments where detrital-aged charcoal is common or no reliable charcoal can be found. CRN techniques were moderately successful, but given our sampling strategy and limited number of samples, it was difficult to assess accuracy. Radiocarbon dating continues to show its effectiveness at providing alluvial stratigraphic ages, and though not perfect, single-grain OSL dating should now be thought of as a routine method for age-estimation of dryland alluvial-fan and axial-fluvial deposits if latest methods are employed carefully. Acknowledgments This study was performed with financial support from NSF EAR-0309518 and a USGS EDMAP grant to J. Pelletier (University of Arizona), and generous support from the USGS Southern California Areal Mapping Project and scholarships from the University of Arizona, Department of Geosciences to SD. 10Be analyses were performed at PRIME Lab under a ‘seed analysis’ grant to JP. OSL analyses were supported by grants from Chevron-
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Texaco and the Arizona Geological Society to SD. Thanks to M. Grace and A. Moore for field assistance. Thanks to J. Quade, J. Pigati and for guidance regarding radiocarbon methods and for lab access. Thanks to T. Fischer for radiocarbon sample prep assistance. AMS analyses were provided by A.T. Jull and the NSF-Arizona AMS Facility as a student assistance grant. Thanks to J. Pigati for guidance with cosmogenic sample preparation and data analysis. Thanks to S. Mahan, USGS, for assistance with FGS OSL dosimetry on samples OSL18-22. Thanks to the land owners and managers whose cooperation was essential. Editorial handling by: R. Gru¨n Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at: doi:10.1016/j.quageo. 2006.03.012.
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Quaternary Geochronology 2 (2007) 129–136 www.elsevier.com/locate/quageo
Research paper
Dating alluvial deposits with optically stimulated luminescence, AMS 14C and cosmogenic techniques, western Transverse Ranges, California, USA S.B. DeLonga,, L.J. Arnoldb a
Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson AZ 85721, USA Oxford Luminescence Research Group, School of Geography and the Environment, University of Oxford, Mansfield Rd, Oxford OX1 3TB, UK
b
Received 12 October 2005; accepted 27 March 2006 Available online 23 May 2006
Abstract In an effort to better understand chronology of alluvial episodes in Cuyama Valley in the western Transverse Ranges of California, USA, we employed optically stimulated luminescence, radiocarbon and cosmogenic radionuclide surface exposure dating methods. Twenty-one optical dates ranging from 0.01 to 27 ka were obtained from exposures of late-Holocene axial-fluvial deposits, Pleistocene–Holocene alluvial-fan deposits, and axial-fluvial sands interbedded within a late Pleistocene alluvial fan. These were crosschecked with 37 AMS radiocarbon dates from charcoal and wood from within a and five 10Be surface exposure dates from boulders on alluvial-fan surfaces. The OSL results show generally good stratigraphic consistency, logical comparison with the radiocarbon and cosmogenic data, and appear to be the best method for accurate dating within deposits of this nature because suitable material is fairly easy to find in these environments. The radiocarbon data contained numerous ‘‘detrital ages’’, but well-bedded lenses of apparently in situ or minimally transported charcoal provide reliable age estimates for the associated alluvium. Radiocarbon dating of detrital charcoal in the older alluvial fan deposits was problematic. Our cosmogenic surface-exposure dating was consistent stratigraphically and with our other data, but we were unable to determine its accuracy due to the limited number of samples and the possibility of inherited radionuclides and post-depositional erosion. In light of our results, we suggest that OSL dating using the latest analytical techniques combined with rigorous methods for estimation of paleodase is reliable and of increasing utility in otherwise difficult-to-date coarse alluvial environments in the southwestern United States and elsewhere. r 2006 Elsevier Ltd. All rights reserved. Keywords: Alluvia fans; Fluvial sediment; Optically stimulated luminescence dating; Radiocarbon dating; Cosmogenic surface-exposure dating; Cuyama valley, CA
1. Introduction Accurate age-determination of alluvial deposits in arid and semi-arid climates is possible using a number of techniques; each with its own limitations. Most widely applied are radiocarbon dating, cosmogenic radionuclide surface-exposure dating, and optically stimulated luminescence dating. Radiocarbon (14C) dating relies on the presence of organic material in an interpretable context within the alluvial deposit, which is rare in dry environments. Cosmogenic radionuclide (CRN) techniques require Corresponding author. Tel.: +1 520 621 6003.
E-mail address: [email protected] (S.B. DeLong). 1871-1014/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.quageo.2006.03.012
determination of the effects of pre-depositionally inherited radionuclides and post-depositional erosion of the target deposit, and proper calibration of isotope production rates. Optically stimulated luminescence (OSL) dating shows great promise, but is still regarded as developmental in its application to fluvial deposits. This paper presents results of a ‘‘blind’’ comparison of all three techniques (with emphasis on direct comparison of radiocarbon and OSL dating by two independent laboratories) applied to latePleistocene to late-Holocene axial-fluvial and alluvial-fan deposits in Cuyama Valley, in the western Transverse Ranges, California, USA. This study serves to highlight both limitations and successful applications of these techniques within a detailed case-study. This paper is not
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intended to be a comprehensive review of details of each dating technique; for a complete review of methodology and application of each technique the reader is directed to publications such as Gosse and Phillips (2001) for CRN techniques, Wallinga (2002) and Aitken (1998) for OSL dating, and Faure (1986) for radiocarbon dating. This paper also does not detail the geologic interpretations made within the wider scope of the study, but instead focuses on the comparison of the geochronologic data. 2. Geological setting and description of lithologic units Cuyama Valley is located at the western end of the western Transverse Ranges where they meet the southern Coast Ranges in southern California, USA (Fig. 1). The modern climate is semi-arid (MAP ¼ 15–25 cm, Mediterranean regime) and hot (MAT ¼ 10–15C). Cuyama Valley is a relatively young structural valley, formed by transpression associated with the San Andreas Fault Zone which has increased since ca. 3 Ma (Ellis et al., 1993). The valley is bounded by the Caliente Mountains to the north and the larger Sierra Madre Mountains to the south (Fig. 2). The Sierra Madre piedmont is mosaic of deformed and eroded late-Cenozoic basin-fill units capped in places by lateQuaternary alluvial fans and their well-preserved planar geomorphic surfaces. The axial Cuyama River is a meandering ephemeral channel that is incised up to 12 m below late-Holocene axial fluvial-terrace surfaces for over 50 km. The focus of this study was on age-determination of the late-Pleistocene alluvial-fan deposits on the Sierra Madre Mountain piedmont and the suite of axial-fluvial deposits along the Cuyama River. On the Sierra Madre piedmont, there are five extensive and well-preserved alluvial fan units preserved as a sequence of planar depositional geomorphic surfaces. We classified these as Qaf1–Qaf5 from oldest to youngest. Deposits capped by these geomorphic surfaces tend to be coarse-grained, clast-supported and -bedded, indicative of fluvial processes. Sandy beds suitable for OSL dating were rare, though a large road cut through a Qaf4 unit revealed
Fig. 2. Landsat7 image of study area showing physiography, and exposure locations discussed in text.
both sandy lenses in alluvium and interbedded sandy axial material. Unit Qaf5 was markedly different than the other units, as it was apparently sourced locally from reworking of a large exposure of Pleistocene lacustrine or shallowwater deposits, giving it a finer sandy texture. Organic material suitable for radiocarbon dating of these deposits is rare. The units oriented along the axis of the valley are dominated by silty and sandy bedded sediment. Exposures of axial material are widespread over 450 km of the Cuyama River. The stratigraphy has been simplified for this paper, and we present data from four sedimentary units, Qa1–Qa4, which represent the majority of exposed sediments preserved as axial terraces along the Cuyama River. This paper presents geochronologic comparisons from three axial-fluvial exposures and two alluvial-fan exposures. The stratigraphic interpretations at the sites used in this paper rely on a larger data set of radiocarbon ages and several more described stratigraphic sections from elsewhere in the study area. Detailed description of these are beyond the scope of this paper and will be presented elsewhere. 3. Methods 3.1. OSL dating
Fig. 1. Location of study area in Southern California.
Bedded waterlain sediments were sampled using opaque ABS pipe without exposing the sediment to light during sampling. Laboratory analysis was carried out at Oxford University by the second author. Refinement of pure coarse-grained quartz separates was undertaken using the standard laboratory preparation procedures outlined in Aitken (1998). Individual equivalent dose (De) estimates were measured using small aliquots (100–300 grains/disc) for all samples except 070402.01 and single grains (SG) for
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all samples except OSL20-22. All De measurements were made using Risor TL-DA-15 readers. The OSL signals were detected using a blue-sensitive EMI9235QA photomultiplier tube fitted with two U-340 filters. SG and single-aliquot (SA) De estimates were both calculated using the SAR protocol developed by Murray and Wintle (2000). The SAR measurement conditions adopted in this research follow those used by Arnold et al. (this issue). SA De estimates were accepted for further analysis if they displayed (i) recycling ratios within 10% of unity, (ii) OSL-IR depletion ratios 40.9 (Duller et al., 2003), (iii) thermal transfer o5% of the natural signal. SG De’s were only accepted where (i) the recycling ratio Lx =T x points were consistent with each other within their 1 s errors, (ii) OSL-IR depletion ratios were 40.9, (iii) thermal transfer was o10% of the natural signal, (iv) the error on the natural test dose signal was o20%, (v) calculated De uncertainty was o30%, (vi) the natural signal intensity was 43 times the standard deviation of the late-light background signal. The number of grains/aliquots measured and accepted can be found in Table S1. Sample bleaching characteristics were assessed from the accepted De populations using the decision procedures approach proposed recently by Bailey and Arnold (in press). For each sample, the final burial dose estimate was calculated using the age model deemed most suitable according to this statistical decision procedure. Environmental dose rates were calculated using a combination of field gamma spectrometry (FGS) and inductively coupled mass spectrometry (ICP-MS). External g-dose rates were calculated using FGS for all samples and ICP-MS for all samples except OSL18-OSL22. External bdose rate contributions were calculated using ICP-MS measurements for all samples. Cosmic-ray dose rate contributions were determined using the calculations of Prescott and Hutton (1994). Present-day water content values were assumed to be representative of those pertaining to the full burial period, and were assigned relative uncertainties of 750%.
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traction and graphite target preparation under vacuum at the University of Arizona Desert Laboratory. AMS analysis of graphite targets were performed at the NSF/ Arizona AMS Facility. Sample radiocarbon ages o21 ka were calibrated to 2-sigma calendar years using Reimer et al. (2004) and older samples were calibrated according to Fairbanks et al. (2005). While proposed calibrated ages have been included in figures without error values, wide ranges in calibrated ages exist for most samples (as reported in Table S3), and caution must be used when using calibrated ages. 3.3. CRN surface exposure dating Coarse-grained, extremely well-indurated sandstone boulders partially embedded in the alluvial surfaces of the Sierra Madre piedmont were sampled for 10Be surface exposure dating. These were selected if they showed no obvious signs of spallation, weathering, or past burial and excavation. Isotopic analysis of 10Be abundance in quartz was carried out at Purdue University’s PRIME Laboratory. These data were corrected for sample thickness and topographic shielding, and were then corrected for latitude, longitude, elevation, and past geomagnetic effects following Pigati and Lifton (2004). In order to use the most accurate cosmogenic production rate for 10Be, we also corrected the raw data of Stone’s et al. (1998) Younger Dryas-aged samples from Scotland using the techniques in Pigati and Lifton (2004). From this we took the long-term integrated high-latitude sea-level 10Be production rate to be 4.35 atoms/g/yr. Following Partridge et al. (2003) we use the meanlife of 10Be to be 1.9370.10 Ma; for discussion of ambiguity related to this value see note 34 therein. No corrections were made for either sample erosion or predepositionally inherited radionuclides. This leads to increased uncertainty in the relationship of exposure age to the age of the geomorphic surface, and the need for independent verification. 4. Results
3.2. Radiocarbon dating Charcoal fragments from alluvial deposits were sampled for radiocarbon dating. Preference was given to charcoal that either appeared to have burned in situ, or deposited as a large concentration, indicating minimal fluvial reworking, as these were most likely to have radiocarbon ages that represent the age of the surrounding deposit. These are referred to as ‘‘charcoal layers’’ in Table S3. ‘‘Detrital’’ charcoal, which was usually isolated fragments of charcoal within fluvial material, was also dated, although this type of material has the potential to have ages much older than the surrounding sedimentary deposit. In one case wood was dated, and one sample of base-soluble humates from charcoal was dated. Samples were subjected to physical removal of visible contaminants, a standard ABA (acid–base–acid) pretreatment, bulk combustion and CO2 ex-
Table S2 contains all OSL results, Table S3 contains radiocarbon results, and Table S4 contains CRN results. The data are described at each exposure in the following sections and shown visually in the figures. In the figures, SG OSL ages are presented when available, though both SG and SA ages are included in Table S2 when available. Comparison of radiocarbon ages to OSL ages is somewhat problematic. Table S3 contains radiocarbon results including ages in 14C years, 2-sigma calibration ranges, the median probability age in calendar years B.P. (before 1950), and also our proposed age before 2002 which is simply the median probability age plus 50 years rounded to the nearest 10 years. On figures, the ‘‘proposed age before 2002’’ is given in calendar years without error estimates. OSL and CRN dates are presented as ka with analytical error.
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4.1. Axial exposure A1 Exposure A1 is a 12-m arroyo-wall exposure of bedded sands, silts, and minor gravel with no apparent soil development at ‘km 5’ (as measured from the upstream extent of incised arroyo channel) (Fig. 3). This exposure has a disconformity halfway down which is evidenced by a weak paleosol, and a change in sediment character from silty, moderately consolidated material (Qa2) to overlying looser sandy material (Qa3). Age control on this section is from three AMS radiocarbon dates and one OSL date near the top of the section. The two younger radiocarbon dates
calibrate from anywhere between 0 and 350 calendar years B.P., so OSL appears to be more useable in this age range. However, with this caveat, the OSL and radiocarbon ages are consistent. 4.2. Axial exposure A2 Exposure A2 is at ‘km 15’, and is a wide exposure of 10-mthick bedded silts and sands capped by a weakly developed fluvent soil (Fig. 4). At this location, we made considerable effort to compare OSL and 14C results. The unit is made up of variable thicknesses of Qa2 and Qa1 that overly each other across a long-wavelength, angular unconformity marked by truncated bedding, paleo-hillslope deposits, a distinguishing paleosol, and onlapping overlying strata. Age control on this section comes from 15 AMS radiocarbon dates and seven OSL dates as illustrated in Fig. 4. 4.3. Axial exposure A3
Fig. 3. Exposure A1. Radiocarbon dates shown with sample numbers as median probability calendar years before 2002. See Table S4 for actual calibration range. OSL dates shown in ka with sample numbers and analytical error in parentheses.
Exposure A3 is a 12-m-thick conformable sequence of bedded silts, sands, gravel and cobbles capped by a weakly developed fluvent soil at ‘km 25’. The entire exposure is unit Qa2. Age control is provided by four AMS radiocarbon dates and a single OSL sample as shown in Fig. 5. The SG OSL date does not overlap within error with the radiocarbon dates—it is 2–300 years older. This could be from a failure of the age model determination criteria (the CAM model produces the oldest age estimate of the six age models presented in Bailey and Arnold (in press). A historical flood deposit adjacent to this exposure gave a SG OSL age of 0.01 ka and a radiocarbon age (AA53752) of 450 calendar
Fig. 4. Exposure A2. Vertical exaggeration is 2X (note horizontal and vertical scales. Sample locations are surveyed with total station and projected into 2-D. Radiocarbon samples AA63784 and AA63783 are from extensive layers of charcoal and should be particularly reliable.
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Fig. 5. Exposure A3. Radiocarbon dates shown with sample numbers as median probability calendar years before 2002. See Table S4 for actual calibration range. OSL dates shown with sample numbers and error in parentheses.
years B.P. This radiocarbon age is certainly detrital, but could be as young as 55 calendar years B.P. due to the shape of the calibration curve in this time-range. Though we cannot be certain, we think that this deposit is the record of the significant El Nino flood event of 1998 (Bowers, 2000). We base this conclusion on analysis of repeat aerial photograph analysis, streamflow data, and the youthful expression of the deposits in the field. This interpretation supports the utility of SG OSL for dating very young fluvial deposits. 4.4. Alluvial-fan exposure AF1
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Fig. 6. Exposure AF1. Radiocarbon dates shown with sample numbers as median probability calendar years before 2002. OSL dates shown with sample numbers and error in parentheses. Field assistant for scale in left photo, 30 cm hammer in right photo.
alluvial-fan material and sandy axial material underlying an extensive and very well-preserved alluvial-fan surface. The fan and axial materials are easily distinguished by reddish-brown sandstone-dominated nature of the fan material and the polymict, granite-bearing nature of the finer axial material. Age control is from six OSL dates, three radiocarbon dates and two CRN surface-exposure dates. While the OSL dates show excellent stratigraphic coherence, the radiocarbon and CRN dates show some scatter, as discussed in Section 5. 4.6. Additional alluvial-fan CRN surface exposure sample sites
Exposure AF1 is a gully formed in a broad swale of unit Qaf5 and possibly underlying Qaf4 (Fig. 6). This is the lowest alluvial-fan surface on the Sierra Madre piedmont. Soil formation is noticeable in this dominantly fine-grained deposit, and has disrupted much of the original fluvial bedding in upper portions of the exposure. We sampled below the primary pedogenic zone from bedded sands. Exposure of sediments is generally poor, making stratigraphic interpretation difficult. Age control is provided by eight detrital radiocarbon samples which show little coherence, and four OSL dates which fall into two groups. Another OSL (070402.01) of 22.2972.12 ka from an exposure of Qaf5 or Qaf4 5 km upstream from a sandy lens in a bouldery deposit is comparable to the oldest OSL date from the gully (OSL19) of 26.0471.97 ka. Both are found below local unconformities in distinctively coarser material, and are likely strath remnants of Qaf4-aged material. Further discussion of the other scatter in these data can be found in Section 5.
Two CRN ages (C011204.01 and C11204.03) were obtained from Qaf1, which is the highest and oldest extensive planar alluvial-fan surface in our study area. The ages were 6972 and 9174 ka, indicating that either the older sample preserved inherited CRNs or the younger sample was eroded or shielded since deposition, or some combination of both. Since both appear extremely erosion resistant and well embedded in the alluvial surface, we favor the former. One CRN age from a Qaf3 surface (C070403.05) gave an age of 29.372.7 ka which is stratigraphically consistent. Our best estimates of ages of all alluvial-fan surfaces fit a linear, survey-derived, ageelevation relationship determined by projecting surface slope to their axial–valley intersection point. Assuming a relatively constant axial-incision-rate supports the integrity of our chronology.
4.5. Alluvial-fan exposure AF2
Overall, agreement between the dating methods is good. The OSL dates appear to have more stratigraphic consistency than the radiocarbon dates, especially for older materials. This is unsurprising, given that radiocarbon
Exposure AF2 is through unit Qaf4 (Fig. 7). These exposures are of interbedded tributary-derived boulder
5. Discussion
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Fig. 7. Exposure AF2. Radiocarbon dates given as calibrated years before 2002. CRN and OSL ages shown as ka with analytical error.
dating in the fluvial environment can be subject to a number of problems (Gillespie et al., 1992). The coupled OSL-radiocarbon approach allows for cross-checking of each dating method, and given enough sample density, conclusions can be made about the accuracy of particular dates. Exposure A2 in particular is an example of how both dating a large number of radiocarbon samples and crosschecking with OSL and reliable ‘‘charcoal layers’’ can identify ‘‘detrital’’ radiocarbon ages, leading to an increased understanding of actual stratigraphic ages. This could also be accomplished by employing more careful radiocarbon sampling criteria than we did; however, this is difficult in fluvial environments, where isolated detrital charcoal is common. Use of isolated and often small charcoal fragments to date older alluvial-fan deposits was problematic. Both AF1 and AF2 exposures would be less interpretable without the complementary OSL dates. More invasive radiocarbon preparation such as ABOX-SC (Bird et al., 1999) perhaps could have improved the accuracy of our older radiocarbon dates from the Pleistocene alluvial-fans by better removing contaminants. The five CRN surface-exposure ages from boulders on alluvial surfaces would also be difficult to rely on independently of other data, but in the context of our larger data set, we conclude that only samples C070503.01 and C11204.03 were subject to significant inherited CRN contamination. This suggests that moderately large data sets (perhaps 3–4 per surface) of cosmogenic ages from isolated boulders on alluvial-fan surfaces can be utilized to get a sense of age succession, perhaps without the much more labor-intensive depthprofile approach. Taken with the strongly coherent OSL dates from Qaf4, we can confidently conclude that the stratigraphic age of unit Qaf4 is between 29 ka at 410 m depth to 20–22 ka at the surface. This sort of age precision is rarely possible with radiocarbon dating in these environments without serendipitous location of significant organic deposits having clear relation to the stratigraphy. The coupled OSL–radiocarbon–cosmogenic dating approach shows great promise for these types of dryland alluvial deposits. Dating results from exposure AF1 are more difficult to interpret. Both radiocarbon and OSL ages show significant
scatter (Fig. 4). The radiocarbon ages range from 13 to 30 calendar years B.P. over less than 1 m vertically and several laterally. This is indicative of reworked detrital charcoal contaminating younger sediments, which is reasonable given that the alluvium was sourced from organic-rich Pleistocene lacustrine or shallow-water deposits preserved farther upslope on the piedmont. The OSL dates also show scatter between 6 and 28 ka. A subtle unconformity separates the three younger OSL ages (6, 7 and 16.5 ka) from the two oldest ages (22 and 26 ka), which is perhaps a contact between Qaf5 and older underlying Qaf4. Likely, the complicated geochronological results are a combination of geological and methodological factors. Deposition was likely slow, spatially complex, and difficult to interpret due to poor exposure. The data scatter also likely indicates methodological problems; however, at this time we are unable to diagnose any specific causes of this. From this, we urge caution when attempting to date deposits that are not clearly interpretable in the field, as adding problematic chorological data to difficult-to-interpret geology compounds challenge. That said, we can place this exposure within the context of our understanding of the local stratigraphy with some confidence based on soil development, topographic position and the chronological data. Our OSL data set (Table S2) contains both SG and SA ages for most samples. It is widely accepted that SG analyses are favorable to SA analyses in fluvial environments (e.g., Olley et al., 2004). Comparison of the two in the context of our interpretations of the local stratigraphy provides significant support for this idea, but also some comparisons between SG and SA ages that are somewhat equivocal. The best example of clear superiority of a SG age comes from the dating of a very young (likely deposited in 1998) flood deposit (Qa4, OSL sample 070202.02). The OSL age of 10 years would not have been possible to determine without SG techniques; the SA age was 1450 years, and the radiocarbon date from that deposit was a clearly detrital 343 14C years B.P. SG OSL is particularly useful when dating sediments that fall within the difficult-to calibrate, less than 350 14C years B.P. range. Several radiocarbon dates in this study fall into that range, and having OSL data to compare increases the precision of dating efforts.
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A closer look at other SG–SA age comparisons also support SG ages as superior. In particular, the SA OSL from exposure A1 (Fig. 3) appears to overestimate the age of the deposit by a factor of two based on our understanding of the stratigraphy, lack of soil development, and two radiocarbon dates from the same section. Data from the well-dated exposure A2 (Fig. 4) also supports SG use. The SG ages are both stratigraphically consistent and very similar to the chronology suggested by the radiocarbon data set, whereas the SA ages contain a stratigraphic reversal (samples 070302.01 and 070302.02), and occasionally appear too old when compared to the radiocarbon dates. Somewhat less conclusive is the comparison between SA and SG ages for OSL samples 070302.06 and 070302.07. The SG ages are possibly too young based on the 2nearby radiocarbon dates (see Fig. 4), but since the radiocarbon dates could include some detrital age, the SA ages may well be too old. The one case where we can say with a bit of confidence that the SA age appears to better represent the age of the deposit is exposure A3 (Fig. 5). There, the SG age appears too old when compared to the radiocarbon ages, and the SA age more closely matches. The comparison between the SG and SA ages from the early Holocene and late-Pleistocene alluvial-fan deposits suggests both techniques worked similarly. All of the SG and SA ages from the alluvial-fan exposures overlap within error. 6. Conclusions Though our discussion tends to highlight the challenges in our study, overall our multi-technique approach led to a detailed understanding of the alluvial chronology in Cuyama Valley, CA. In particular, OSL showed great utility in dating samples of all ages in this study. OSL is useful in difficult-to-calibrate radiocarbon age ranges, and in environments where detrital-aged charcoal is common or no reliable charcoal can be found. CRN techniques were moderately successful, but given our sampling strategy and limited number of samples, it was difficult to assess accuracy. Radiocarbon dating continues to show its effectiveness at providing alluvial stratigraphic ages, and though not perfect, single-grain OSL dating should now be thought of as a routine method for age-estimation of dryland alluvial-fan and axial-fluvial deposits if latest methods are employed carefully. Acknowledgments This study was performed with financial support from NSF EAR-0309518 and a USGS EDMAP grant to J. Pelletier (University of Arizona), and generous support from the USGS Southern California Areal Mapping Project and scholarships from the University of Arizona, Department of Geosciences to SD. 10Be analyses were performed at PRIME Lab under a ‘seed analysis’ grant to JP. OSL analyses were supported by grants from Chevron-
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Texaco and the Arizona Geological Society to SD. Thanks to M. Grace and A. Moore for field assistance. Thanks to J. Quade, J. Pigati and for guidance regarding radiocarbon methods and for lab access. Thanks to T. Fischer for radiocarbon sample prep assistance. AMS analyses were provided by A.T. Jull and the NSF-Arizona AMS Facility as a student assistance grant. Thanks to J. Pigati for guidance with cosmogenic sample preparation and data analysis. Thanks to S. Mahan, USGS, for assistance with FGS OSL dosimetry on samples OSL18-22. Thanks to the land owners and managers whose cooperation was essential. Editorial handling by: R. Gru¨n Appendix A. Supplementary data Supplementary data associated with this article can be found in the online version at: doi:10.1016/j.quageo. 2006.03.012.
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