LANDSCAPE DEVELOPMENT IN THE WESTERN ...

LANDSCAPE DEVELOPMENT IN THE WESTERN TRANSVERSE RANGES, CALIFORNIA: INSIGHTS FROM MAPPING, GEOCHRONOLOGY, AND MODELING

by Stephen Berend DeLong

______________________

A Dissertation Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY In the Graduate College THE UNIVERSITY OF ARIZONA

2006

THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE As members of the Dissertation Committee, we certify that we have read the dissertation prepared by Stephen B DeLong entitled “Landscape Development in the Western Transverse Ranges, California: Insights from Mapping, Geochronology, and Modeling” and recommend that it be accepted as fulfilling the dissertation requirement for the Degree of Doctor of Philosophy _______________________________________________________________________

Date: 4/03/06

Jon D Pelletier _______________________________________________________________________

Date: 4/03/06

Jay Quade _______________________________________________________________________

Date: 4/03/06

Clem Chase _______________________________________________________________________

Date: 4/03/06

Phil Pearthree

Final approval and acceptance of this dissertation is contingent upon the candidate’s submission of the final copies of the dissertation to the Graduate College. I hereby certify that I have read this dissertation prepared under my direction and recommend that it be accepted as fulfilling the dissertation requirement.

________________________________________________ Date: 4/03/06 Dissertation Director: Jon D Pelletier

STATEMENT BY AUTHOR This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgement of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his or her judgment the proposed use of the material is in the interests of scholarship. In all other instances, however; permission must be obtained from the author. SIGNED: Stephen B DeLong

TABLE OF CONTENTS ABSTRACT……………………………………………………………..………….5 INTRODUCTION……………………………………………………………….…7 PRESENT STUDY……………………………………………………….……….13 REFERENCES………………………………………………………………….…17 APPENDIX A: DATING ALLUVIAL DEPOSITS WITH OPTICALLLY STIMULATED LUMINESCENCE, AMS 14C AND COSMOGENIC TECHNIQUES, WESTERN TRANSVERSE RANGES, CALIFORNIA, USA………………………………………………………………………….…….20 APPENDIX B: COUPLED ALLUVIAL FAN AND AXIAL CHANNEL DEVELOPMENT IN CUYAMA VALLEY, CALIFORNIA…………………….50 APPENDIX C: GEOMORPHIC FATE OF LATE CENOZOIC BASINS IN SOUTHERN CALIFORNIA: AN EXAMPLE FROM THE UPPER CUYAMA VALLEY……………………………………………………….……..66 APPENDIX D: BEDROCK LANDSCAPE DEVELOPMENT MODELING: CALIBRATION USING FIELD STUDY, GEOCHRONOLOGY AND DEM ANALYSIS……………………………………………………………………..…96

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ABSTRACT Understanding how climate and tectonics have interacted to shape current landscape configuration requires application of the latest geomorphological techniques. This dissertation presents results from a combination of field mapping, geochronology, and numerical landscape development modeling. The papers contained here focus on studies from Cuyama Valley, California, at the junction of the Coast Ranges and Western Transverse Ranges in southern California. Combining field observation with three geochronological techniques has led to a detailed understanding of the late Quaternary alluvial history of Cuyama Valley. The alluvial history, in turn, allows for a better understanding of important events in the history of landscape development. In the western Cuyama Valley, the timing and morphology of alluvial fans record both climatic forcing in the form of variable sediment supply from drainage basins, and tectonic forcing from ongoing tectonically driven incision of the axial Cuyama River. Fan-terrace surfaces are subparallel (older surfaces are slightly steeper) and offset systematically in relation to their ages, suggesting response to ongoing base-level incision and fluctuation in sediment supply. The fans aggraded during relatively cool and wet climate of the last glacial period, which is out-ofphase with the regional model developed in nearby desert regions. In the upper Cuyama Valley, deposition in the Cuyama sedimentary basin ceased in the mid-Pleistocene, after which basin fill was uplifted, deformed, and beveled, forming a low-relief erosion surface on which the alluvium of San Emigdio Mesa was

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deposited. Subsequent fluvial drainage network development formed the Cuyama badlands by incising into the deformed Cuyama basin sediments. The history of the upper Cuyama Valley was used to calibrate a numerical landscape development model. Uplift rate U, bedrock erodibility K, and landslide threshold-slope Sc are related to steady-state relief, hypsometry, and drainage density for a wide range of synthetic topographies produced by a stream-power-based landscape development model. A combination of fluvial channels and threshold-slopes occurs for only a relatively narrow range of U/K between 10 and 5000 m·kyr/kyr. Using measured values for hypsometric integral, drainage density and relief, the U/K value can be further constrained, enabling K to be determined if U is known.

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INTRODUCTION This dissertation addresses fundamental questions that relate climate and tectonics to landscape development. Though the dissertation is a compilation of manuscripts with coauthors, the research design and execution is largely that of the first author. Appendix A presents results of an intercomparison of geochronological techniques as applied to alluvial deposits. 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 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. I present 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 late-Pleistocene 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 is not intend to be a comprehensive review of details of each dating technique; for a complete review of methodology and application of each

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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. Appendix B presents geological interpretations made from the some of the results presented in Appendix A as well as additional field mapping and topographic analysis of alluvial fans in Cuyama Valley, California. Suites of inset alluvial fan surfaces found on mountain piedmonts in arid regions record episodic alluvial episodes caused by changes in upstream sediment and water flux. These episodes of increased piedmont sedimentation (and intervening times of fluvial entrenchment and lateral erosion) are widely thought of as caused by cyclic climate change. In regions where past climates have acted on distinct catchments in similar ways, these flights of terraces are often assumed to be age-equivalent. The causes of these alluvial episodes can be diverse. In desert regions (Wells, et al., 1987; Bull, 1991; Reheis et al., 1996; Harvey, et al., 1999; Ritter, et al., 2000; McDonald et al., 2003) and in at least one semi-arid to subhumid region (Weldon, 1986), changing hillslope vegetation and precipitation regime (increased storm intensity) during and after cool/wet to warm/dry climate transitions are most often cited as causing alluviation. In glaciated regions, drainage basin erosion by valley glaciers led to piedmont alluviation during glacial advances (Ritter et al., 1995; Gillespie et al., 1994, Harvey, 2002). Also sometimes cited is alluviation from unglaciated or minimally glaciated drainage basins during cool/wet climates due to increased effective

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precipitation, fluvial transport, and possibly increased freeze-thaw and periglacial processes on high-elevation hillslopes. This is cited in the northern Basin and Range from catchments with small glaciers (Pierce and Scott, 1982), in southern Spain in what is now Mediterranean climate (Harvey, 2002), and from small, high-elevation catchments in southern California (Bull, 1991). Our limited understanding of the spatial distribution of these (and possibly other) seemingly conflicting causes of piedmont alluviation has limited our ability to put forth predictive conceptual models for the timing of alluvial episodes in diverse settings. Studies outside of desert regions, with a few exceptions are particularly lacking, which leads to the possibility of application of conceptual models of desert alluvial fan development to diverse settings in which they may not be appropriate. Downstream base-level changes should lead to distinct topographic signatures that may replace or superpose climatic signatures in areas where axial-fluvial, marine or lacustrine systems interact with piedmonts, or regional or fault-specific uplift is occurring (Harvey and Wells, 2002). In order to (1) better understand the causes of episodic alluviation beyond desert regions, and (2) to expand our understanding of how regional uplift and an incising axial fan-toe channel affect the topographic configuration of alluvial fan terraces, we mapped, described, and dated the Quaternary deposits of Cuyama Valley, California. Appendix C presents results from geochronology and field observation from the Upper Cuyama Valley. The transition from landscape dominated by long-lived regionalscale late Cenozoic depositional basins to the formation of smaller complex structural and topographic basins occurred relatively recently in parts of southern California (Kellogg

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and Minor, 2005; Page et al., 1998). There is no generally accepted model for the geomorphic fate of these young basins, many of which stopped receiving sediment as recently as the Pleistocene. Furthermore, the details of the complex events that have created the dramatic landscapes of southern California are difficult to constrain due to the challenge of geochronology over the relevant timescales of 103 to 105 yrs. These events can include the transition from basin filling to incision; episodic alluviation occurring at different positions in the landscape; increasing tectonic deformation, often accommodated on an increasingly complex structural array; and progressive regional uplift that is spatially variable across multiple structural boundaries. An appropriate case study in our efforts to better understand late Cenozoic landscape development in southern California is the upper Cuyama Valley, located at the junction between the southern Coast Ranges and western Transverse Ranges, where changes in tectonic regime over the last few million years include increased transpression in the Big Bend region of the San Andreas fault, expressed by complex contractional faulting and folding (Kellogg and Minor, 2005; Atwater and Stock, 1998; Page et al. 1998; Ellis, et al. 1993; White, 1992). A striking landscape feature in this area are the Cuyama Badlands, characterized by deeply incised valleys and gullied slopes cut into Neogene sedimentary strata. Associated with these incised basin strata are spatially variable tectonic deformation, erosional unconformities covered by Quaternary alluvium, and the regionally significant Big Pine fault. Our objectives in this study were to determine the history and timing of landscape development in the upper Cuyama Valley region from the late stages of deposition in the

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Cuyama basin to the present time, and to use the age of offset fluvial terraces to evaluate the strain rate of the eastern Big Pine fault along the base of Pine Mountain. To do this, we synthesized the current understanding of the post-Miocene history of the area and applied cosmogenic radionuclide (CRN) surface-exposure dating and optically stimulated luminescence (OSL) burial dating techniques to several late Quaternary alluvial deposits that are useful geologic recorders of events leading up to the current landscape configuration. Appendix D presents the results of an effort to use the history of the upper Cuyama Valley to calibrate a numerical bedrock landscape development model. The stream-power-law (or similar shear-stress-based methods) forms the foundation for many bedrock landscape development models (e.g., Howard, 1994; Whipple and Tucker, 1999). When stream-power-based bedrock channel development models are coupled with hillslope process models that include threshold-landsliding and/or hillslope diffusion components, three-dimensional landscape development modeling is possible (e.g., Tucker et al., 2001; Howard, 1994). We were motivated by the need to understand how each parameter in bedrock landscape development models affect model topography, and the need to develop general techniques for calibrating landscape development models using geologic and morphometric analyses. This motivation led us to apply a landscape development model with a minimum of free parameters in an effort at calibration using geologic and morphologic data from a field site in southern California. Wide-ranging estimates for model parameters are often used in bedrock landscape development models. Stock and Montgomery (1999) proposed a range in stream-power

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law erodibility coefficient K over five orders of magnitude for varying rock types, and this wide range is often used in other modeling studies. Because the stream-power law is very sensitive to the value of K and because Snyder et al. (2000) proposed a linkage between uplift rate and K, we were interested in creating a more specific calibration technique for the stream-power law that relies on geologic constraints of uplift rate, and morphometric landscape analyses to calibrate K. Snyder et al. (2000) also provided insight regarding use of landscape morphometry to constrain the values of stream-power law exponents m and n. By integrating these studies’ findings into a fully-coupled landscape modeling environment, we hoped to further refine our understanding of the effect of model parameters as a step towards improving our ability to calibrate even more sophisticated landscape development models.

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PRESENT STUDY The methods, results, and conclusions of this study are presented in the papers appended to this dissertation. The following is a summary of the most important findings in this document. Our multi-technique geochronological 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. Though late Quaternary alluvial-fan development in desert regions of the southwestern U.S. is largely well understood and similar on a regional scale, the coupled piedmont-axial system in Cuyama Valley, CA is striking in its spatial and temporal characteristics. The preservation of at least five latest-Quaternary alluvial surfaces suggest that either drainage basins in the Sierra Madre range were particularly sensitive to cyclic climate fluctuation, or cut-and-fill cycles on the piedmont were driven by both upstream sediment supply and downstream incision driven by the axial system. Fans in Cuyama Valley aggraded substantially during the last glacial period. A possible cause of

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this was increased saturation-driven hillslope failure and sediment transfer to the piedmont during a cool and wet climate. The late Pleistocene and Holocene has been a time of relative stability of the north-facing, chaparral-covered slopes. The PleistoceneHolocene transition did appear to lead to local fan aggradation where material is sourced from the now-unvegetated slopes of unconsolidated, possibly lacustrine, Morales Formation on the Sierra Madre piedmont. These findings suggest further studies of piedmonts in diverse climatic and tectonic zones in the southwestern U.S. are warranted, and caution must be used when applying widely accepted models of alluvial-fan evolution beyond the regions in which they have been thoroughly tested. Our new understanding of the upper Cuyama River geomorphic system provides a working model for the evolution of other late Cenozoic basins in coastal California. These structural basins (such as the Salinas, Lockwood Valley, Ventura, Carrizo Plain, Ridge, etc.) are often long-lived, but have been profoundly affected by late Cenozoic tectonic and climatic changes. Kellogg and Minor (2005) highlight the tectonic changes in adjacent Lockwood Valley, primarily using observations of Pliocene and earlier structural geology and stratigraphy. Similar traditional mapping-based approaches can be coupled with our increasing ability to establish timing of Quaternary events to lead to a rich understanding of the interactions of tectonics, erosion, deposition and climate over multiple timescales. While timing, environments of deposition, and physiography may differ greatly between basins, it seems likely that most tectonically active sedimentary basins in southern California record: 1) increased clastic deposition as contractional deformation increased beginning in the Pliocene; 2) increased tectonic uplift and

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structural relief across increasingly abundant and concentrated structures within and bordering the depositional basins, leading to basin “extinction” as basin-filling was replaced by incision, and 3) significant climatically-controlled late Pleistocene alluviation over a wide variety of erosional surfaces, allowing for establishment of timing in these landscapes. Additionally, we propose a latest-Quaternary fault-slip estimate on the Big Pine fault of 0.7 m/kyr. This serves as a reminder of both the ongoing nature of tectonic landscape development and the seismic potential of historically aseismic reverse faults throughout southern California. Three-dimensional modeling that utilizes the stream-power law for fluvial erosion and threshold-landsliding for hillslope development allows for careful analysis of how model parameters such as uplift rate, bedrock erosivity, threshold-slope, channel concavity and time effect landscape development. We suggest that by careful comparison of (1) actual landscape morphology via field and DEM analysis, and (2) actual landscape development process-rates from geochronology to synthetic topography derived from a numerical model with carefully controlled parameters, we can calibrate modeling efforts, and in particular, narrow our range of estimates for K. We suggest that characterization of m/n, landslide threshold-slope, mean elevation, topographic relief, drainage density and hypsometric integral are necessary for comparison of actual topography to synthetic topography. In our study area three late-Cenozoic sedimentary units are estimated to have K values on the order of 0.3 to 0.09 m0.2-0.4kyr-1. We address possible complications from temporal and spatial scaling, and suggest that even complex

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and/or non-steady-state real topography can be compared to idealized synthetic topography with some measure of success. Work on widely different rock types and spatial scales will be necessary to further validate our results.

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REFERENCES Atwater, T., and Stock, J., 1998. Pacific-North America plate tectonics of the Neogene southwestern United States; an update, in Ernst, W.G. and Nelson, C.A., eds, Integrated earth and environmental evolution of the southwestern United States; The Clarence A. Hall, Jr. Volume: Columbia, Maryland, Bellwether Publishing, 393-420. Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press. Oxford. Bull, W.B., 1991 Geomorphic responses to climatic change. Oxford, Oxford University Press, London. 326 p. Ellis, B.J., Levi, S., and Yeats, R.S., 1993. Magnetic stratigraphy of the Morales Formation: Late Neogene clockwise rotation and compression in the Cuyama basin, California: Tectonics, v. 11, 1170-1179. Faure G., 1986. Principles of Isotope Geology. NewYork:. Wiley. Gillespie, A.R., Burke, R.M. and Harden, J.W., 1994, Timing and regional paleoclimatic significance of alluvial fan deposition, western Great Basin. Geological Society of America Abstracts with Programs, 26, 6, A150–A151. Gosse, J.C. and Phillips, F.M., 2001. Terrestrial in situ cosmogenic nuclides: theory and application. Quaternary Science Reviews, 20(14): 1475-1560 Harvey, A.M., Wigand, P.E., and Wells, S.G., 1999, Response of alluvial fan system to the late Pleistocene to Holocene climatic transition: Contrasts between the margins of pluvial Lakes Lahontan and Mojave, Nevada and California, USA: Catena v. 36, p. 255281. Harvey, A.M., 2002, The role of base-level change in the dissection of alluvial fans: case studies from southwest Spain and Nevada. Geomorphology, v. 45, p. 67-87. Harvey, A.M. and Wells, S.G., 2003, Late Quaternary variations in alluvial fan sedmentologic and geomorphic processes, Soda Lake basin, eastern Mojave Desert, California, in Enzel, Y., Wells, S.G., and Lancaster, N., eds. Paleoenvironments and paleohydrology of the Mojave and southern Great Basin Deserts: Boulder, Colorado, Geological Society of America Special Paper 368, p. 207-230. Howard, A.D., 1994, A detachment-limited model of drainage basin evolution: Water Resources Research, v. 30, no. 7, p. 2261-2285 Kellogg, K.S., and Minor, S.A., 2005. Pliocene transpressional modification of depositional basins by convergent thrusting adjacent to the “Big Bend” of the San

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Andreas fault, An example from Lockwood Valley, southern California; Tectonics, v. 24, 1-12. McDonald, E.V., McFadden, L.D., and Wells, S.G., 2003, Regional response of alluvial fans to the Pleistocene-Holocene climatic transition, Mojave Desert, California, in Enzel, Y., Wells, S.G. and Lancaster, N., eds., Paleoenvironments and paleohydrology of the Mojave and southern Great Basin Deserts: Boulder, Colorado, Geological Society of America Special Paper 368, p. 19-205. Page, B.M., Coleman, R.G., Thompson, G.A., 1998, OVERVIEW: Late Cenozoic tectonics of the central and southern Coast Ranges of California. Geological Society of America Bulletin 110: 846-876 Pierce, K.L., and Scott, W.E., 1982, Pleistocene episodes of alluvial-gravel deposition, southeastern Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin v. 26, p. 685-702. Reheis, M.C., Slate, J.L., Throckmorton, C.K., McGeehin, J.P., Sarna-Wojcicki, A.M., and Dengler, L., 1996, Late Quaternary sedimentation on the Leidy Creek fan, NevadaCalifornia: Geomorphic responses to climate change: Basin Research, v. 12, p. 279-299. Ritter, J.B., Miller, J.R., and Husek-Wulforst, J., 2000, Environmental controls on the evolution of alluvial fans in Buena Vista Valley, North Central Nevada, during late Quaternary time. Geomorphology, v.36, p. 63-87. Ritter, J.B., Miller, J.R., Enzel Y., and Wells, S.G., 1995, Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution. Geology, v. 23, p. 245-248. Tucker, G.E., Lancaster, S.T., Gasparini, N. M., and Bras, R. L., 2001, The ChannelHillslope Integrated Landscape Development (CHILD) model: in Landscape Erosion and Evolution Modeling, edited by Harmon R. S. and Doe III W. W., pp. 349–388. Snyder, N.P., Whipple, K.X., Tucker, G.E., and Merritts, D.J., 2000, Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California: Geological Society of America Bulletin, v. 112, no. 8, p. 1250-1263. Stock, J.D., and Montgomery, D.R., 1999, Geologic constraints on bedrock river incision using the stream power law: J. Geophys. Res., v. 104, no. B3, p. 4983-4993. Wallinga, J. 2002. Optically stimulated luminescence dating of fluvial deposits: a review. Boreas, 31, pp. 303–322.

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Weldon, R.J., 1986, Late Cenozoic geology of Cajon Pass; implications for tectonics and sedimentation along the San Andreas fault. Ph.D. thesis. California Institute of Technology. Wells, S.G., McFadden, L.D., and Dohrenwend, J.C., 1987, Influence of late Quaternary climatic change on geomorphic and pedogenic processes on a desert piedmont, eastern Mojave Desert, California: Quaternary Research, v. 27, p. 130-146. Whipple, K.X., and Tucker, G.E., 1999, Dynamics of the stream-power river incision model: Implications for height limits of mountain ranges, landscape response timescales, and research needs: J. Geophys. Res., v. 104, no. B8, p. 17661 - 17674. White, L.A., 1992. Thermal and unroofing history of the western Transverse Ranges, California: Results from apatite fission track thermochronology. Ph.D. Thesis, University of Texas, Austin.

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APPENDIX A Submitted to Quaternary Geochronology, 2005 DATING ALLUVIAL DEPOSITS WITH OPTICALLLY-STIMULATED LUMINESCENCE, AMS 14C AND COSMOGENIC TECHNIQUES, WESTERN TRANSVERSE RANGES, CALIFORNIA, USA. DeLong, Stephen B.1 Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson AZ 85721, USA Arnold, Lee, J. Oxford Luminescence Research Group, School of Geography and the Environment, University of Oxford, Mansfield Rd, Oxford OX1 3TB, UK 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 cross-checked with thirty-seven AMS radiocarbon dates from charcoal and wood from within fluvial material and five

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Be 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

1

Corresponding author: [email protected] tel. 520-621-6003

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charcoal provide reliable age estimates for the fluvial material. 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 analyzing paleodose estimates is reliable and of increasing utility in otherwise difficult-to-date coarse alluvial environments in the southwestern United States and elsewhere.

Keywords: Alluvial 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 determination of the effects of pre-depositionally inherited radionuclides and postdepositional erosion of the target deposit, and proper calibration of isotope production

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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 late-Pleistocene 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 does not intend 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 = 15 to 25 cm, Mediterranean regime) and hot (MAT = 10 to 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

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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 late-Quaternary alluvial fans and their well-preserved planar geomorphic surfaces. The axial Cuyama River is a meandering ephemeral channel that is incised up to 12 meters below lateHolocene 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 alluvialfan units preserved as a sequence of planar depositional geomorphic surfaces. classified these as Qaf1-Qaf5 from oldest to youngest.

We

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 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 >50km of the Cuyama River. The stratigraphy has been simplified for this paper, and we present data from four

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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 dataset 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

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 for all samples except OSL20-22. All De measurements were made using Riso© TL-DA-15 readers. The OSL signals were detected using a blue-sensitive EMI9235QA photomultiplier tube fitted with two U-340 filters.

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Single grain and single aliquot 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). Single aliquot De estimates were accepted for further analysis if they displayed (i) recycling ratios within 10% of unity, (ii) OSL-IR depletion ratios >0.9 (Duller et al, 2003), (iii) thermal transfer 0.9, (iii) thermal transfer was 45? 60-120? #

* Wide scatter in geochronological data makes age interpretation of Qf5 difficult, but landscape position, and soil development suggest younger part of age range may be better estimate. † Age estimate from AMS radiocarbon dating. ‡ Age estimate from optically-stimulated luminescence dating. # Age estimate from 10Be CRN surface exposure dating Note: See DeLong and Arnold (in press, 2007) for data, techniques, and discussion of dating methods.

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Figure 1. Location and generalized surficial geology of study area in southern California. Qa – active channels; Qyp – palustrine deposits; Qya – late Holocene deposits; Qf5-Qf1 – see Table 1; Qof – Quaternary alluvium, undifferentiated, lacking depositional surfaces; Qyls – recent landslides in QTmol; QTmol – fine-grained (lacustrine?) Morales Formation; QTm – coarse-grained Morales Formation; Tr – Pre-Pliocene bedrock, undifferentiated.

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Figure 2. Schematic representations of fan terrace profiles in response to differing baselevel and sediment-flux/texture scenarios. End-member hypothesis #1 is a generalization of terrace suites common in tectonically inactive regions where alluvial fan development is largely a response to fluctuating upstream sediment flux and texture. End-member hypothesis #2 follows Mackin’s (1948) concept of the graded stream response to downcutting.

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Figure 3. Alluvial fan profiles generated using field Total Station survey and crosschecked with 10-meter DEM. Horizontal axis is distance from centerline of modern Cuyama River channel in direction of maximum fan slope. Vertical axis is distance above elevation of Cuyama River channel at same location. All profiles except Qf5 are from terrace suite flanking Aliso Canyon. Qf5 profile is from northwestern portion of map area, and relatively high slope likely related to the observation that the Qf5 alluviation is sourced from erosion of piedmont exposure of readily erodible Morales Formation. The arrangement of fan terrace profiles is suggestive of our “mixed” hypothesis from Figure 2, with vertically offset profiles at the fan-toes, and steeper older surfaces.

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APPENDIX C Submitted to Geomorphology, 2006 Geomorphic fate of late Cenozoic basins in southern California: An example from the upper Cuyama Valley Stephen B. DeLong Department of Geosciences, University of Arizona, 1040 E 4th Street, Tucson AZ 85721, USA Scott A. Minor United States Geological Survey, Denver CO 80225, USA Lee J. Arnold Oxford Luminescence Research Group, School of Geography and the Environment, University of Oxford, Mansfield Rd, Oxford OX1 3TB, UK

Keywords: landscape development; neotectonics; surface exposure dating; optical dating; Cuyama Badlands; Big Pine fault

ABSTRACT Many long-lived Cenozoic depositional basins in southern California have been affected by increased movement on local faults and increased regional-scale uplift and transpression associated with the San Andreas fault since the Pliocene. Stratigraphic, structural and geomorphic evidence for how one of these basins responded to tectonic and climatic forcing is particularly well expressed in the upper Cuyama Valley in the western Transverse Ranges. Plio-Pleistocene terrestrial deposition in the Cuyama sedimentary basin continued through at least ~0.76 Ma, after which basin fill was uplifted, deformed, and beveled, forming a low-relief erosion surface on which the alluvium of San Emigdio Mesa was deposited between >70 ka and ~15 ka (with major alluviation

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ending by ~28 ka). Subsequent fluvial drainage network development formed the Cuyama badlands by incising into the deformed Cuyama basin sediments. Localized deposition of alluvium sourced from the Pine Mountain massif occurred at the southern end of the basin near the elevation of the Cuyama River between 25 and 14 ka. This alluvium was subsequently offset ~10 m vertically by the Big Pine fault, providing a latest Quaternary vertical slip-rate estimate of ~0.7 m/ky for the Big Pine fault in the upper Cuyama Valley. The Big Pine fault has no confirmed record of historic rupture; however based on our results, we suggest the likelihood of multiple reverse-slip rupture events since ~14 ka. Our results allow us to propose a general model for late Cenozoic landscape development in structural basins o coastal California. Though timing may vary, thick sequences of terrestrial PlioPleistocene basin fill deposits have been deformed, incised, alluviated, and offset by an increasingly dense fault array. Combined analysis of stratigraphy, structure, and alluvial deposits that mantle a range of paleosurfaces allow for establishing the geomorphic history of these complex landscapes.

1. INTRODUCTION The transition from landscape dominated by long-lived regional-scale late Cenozoic depositional basins to the formation of smaller complex structural and topographic basins occurred relatively recently in parts of southern California (Kellogg and Minor, 2005; Page et al., 1998). There is no generally accepted model for the geomorphic fate of these young basins, many of which stopped receiving sediment as

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recently as the Pleistocene. Furthermore, the details of the complex events that have created the dramatic landscapes of southern California are difficult to constrain due to the challenge of geochronology over the relevant timescales of 103 to 105 yrs. These events can include the transition from basin filling to incision; episodic alluviation occurring at different

positions

in

the

landscape;

increasing

tectonic

deformation,

often

accommodated on an increasingly complex structural array; and progressive regional uplift that is spatially variable across multiple structural boundaries. An appropriate case study in our efforts to better understand late Cenozoic landscape development in southern California is the upper Cuyama Valley, located at the junction between the southern Coast Ranges and western Transverse Ranges, where changes in tectonic regime over the last few million years include increased transpression in the Big Bend region of the San Andreas fault, expressed by complex contractional faulting and folding (Kellogg and Minor, 2005; Atwater and Stock, 1998; Page et al. 1998; Ellis, et al. 1993; White, 1992). A striking landscape feature in this area are the Cuyama Badlands, characterized by deeply incised valleys and gullied slopes cut into Neogene sedimentary strata. Associated with these incised basin strata are spatially variable tectonic deformation, erosional unconformities covered by Quaternary alluvium, and the regionally significant Big Pine fault. Our objectives in this study were to determine the history and timing of landscape development in the upper Cuyama Valley region from the late stages of deposition in the Cuyama basin to the present time, and to use the age of offset fluvial terraces to evaluate the strain rate of the eastern Big Pine fault along the base of Pine Mountain. To do this,

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we synthesized the current understanding of the post-Miocene history of the area and applied cosmogenic radionuclide (CRN) surface-exposure dating and optically-stimulated luminescence (OSL) burial dating techniques to several late Quaternary alluvial deposits that are useful geologic recorders of events leading up to the current landscape configuration.

2. GEOLOGIC SETTING Our study area (Fig. 1) is underlain by deeply-incised and moderately to stronglydeformed upper Cenozoic strata bounded on the south by Pine Mountain, to the north and east by the Mt. Piños-Mt. Abel massif, and to the west by the Cuyama River and Ozena fault. The Big Pine fault is a south-dipping oblique reverse fault (Minor, 2004) along the northern base of Pine Mountain that intersects a flight of fluvial terraces east of the prominent northward bend in the Cuyama River. The history of post-Miocene deposition and landscape development in Cuyama Valley is detailed in Ellis et al. (1993) and Ellis (1994). Those papers built upon contributions by Davis, (1983), Vedder, et al. (1973), and Dibblee (1987a, b) that detailed mapping, stratigraphy, and tectonic reconstructions of the upper Cuyama Valley. During the Miocene, cyclic marine transgression and regression eventually gave way to exclusively terrestrial deposition in the ancestral Cuyama basin. In our study area, the post-Miocene terrestrial deposits dominate the landscape, and consist of two mostly conformable units. The lower unit is the >1000 m-thick Quatal Formation, a clay-rich sandstone and conglomerate, and the upper unit is the >1500 m-thick Morales Formation,

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a coarsening-upward sandstone and conglomerate. The Cuyama Badlands result from incision of a relatively low-relief erosion surface that formed after cessation of terrestrial deposition in the Cuyama basin. This study investigates the geomorphic fate of the basin fill deposits in response to climatic and tectonic changes. Because the late stages of deposition in the Cuyama basin serve as the starting point for topographic development of the Cuyama Badlands, the age of the upper Morales Formation is of particular interest. Blancan faunal remains found by Vedder (1970) in a different part of the basin indicated a Pliocene age for at least part of the Morales Formation. Age correlation over several tens of kilometers across structural boundaries is tenuous so the Morales Formation in our study area might represent a different age. Ellis (1994) applied paleomagnetic techniques to date the Morales Formation in several locations, including the Cuyama Badlands.

Her results did not permit a unique

magnetostratigraphic interpretation, but two normally-polarized samples likely correlated with either the Gauss chron (3.40-2.48 Ma) or the Olduvai subchron (1.87-1.67 Ma), and all other samples above and below had reverse polarity. Ash located in the uppermost Morales Formation by Stone and Cossette (2000) indicate that the upper portion of the basin-fill sequence just below the paleo-erosion surface is between 1.2 and 0.76 Ma based on geochemical correlation to either the Glass Mountain or Bishop Ash (most likely the 0.76 Ma Bishop Ash, A. Sarna-Wojcicki, written commun. 2004). This ash correlation supports the interpretation that the two normally-polarized paleomagnetic samples from the Morales Formation belong to the

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Olduvai subchron, and suggests a significant part of the Morales Formation in the Cuyama Badlands is Pleistocene in age. The Cuyama basin likely stopped receiving sediment soon after deposition of the ash bed, and the deposits were subsequently deformed and beveled to a low-relief surface that truncates bedding, including steeply dipping beds on the northern and eastern margins of the basin. This erosional surface is mantled discontinuously by a relatively thin alluvial deposit, the largest remnant of which is San Emigdio Mesa (Fig. 2). Davis (1983) correlated this deposit with the Riverbank Formation (0.45 – 0.13 Ma) of the northeastern San Joaquin Valley, but did not justify this in detail. This alluvial deposit, which predated formation of the Cuyama Badlands, had not been dated directly prior to this study. More recently, alluvium was deposited across the Big Pine fault along the uppermost Cuyama River, and is now preserved as elevated fluvial terraces.

This

alluvium is distinguished by its nearly monomict sandstone-clast composition, derived mainly from the adjacent Pine Mountain massif. An apparent terrace offset, though coincident with the main Big Pine fault trace, had not been previously identified as a tectonic scarp offsetting an equal-aged deposit, so we applied OSL dating to alluvium on both sides of the fault trace in an effort to correlate the two deposits and estimate a late Quaternary fault dip-slip rate.

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3. METHODS

3.1 CRN Surface-Exposure Dating We applied 10Be CRN surface-exposure dating to three boulders on the surface of San Emigdio Mesa. Additionally, we applied OSL burial dating to 15 samples from: 1) bedded sands within alluvium of San Emigdio Mesa; 2) sandy lenses within bouldery alluvium on the terraces offset by the Big Pine fault; 3) bedded sandy axial-fluvial sediment below the bouldery alluvium; and 4) young fluvial terraces preserved along Wagon Wheel Canyon south of the Big Pine fault (Fig 2). For

10

Be surface exposure dating, we sampled material from three flat-topped

granitic boulders partially embedded in the alluvial surface of San Emigdio Mesa. These were selected with the criteria of showing no obvious signs of spallation or weathering such as nearby flakes or ongoing exfoliation, or past burial and excavation as indicated by stable microtopography and intact soil near the sample locations. Isotopic analysis of 10

Be abundance in quartz was carried out at Purdue University’s PRIME Lab. 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 re-corrected the raw data of Stone’s (1998) Younger Dryas-aged samples from Scotland. From this we determined 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

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meanlife of 10Be to be 1.93 ± 0.10 Ma - for discussion of ambiguity related to this value see note 34 therein.

3.2 Optical Dating For OSL dating, bedded sands were sampled below the main pedogenic zone in order to ensure a sample of unmixed primary sediment. Since many of the deposits of interested were bouldery alluvium, we made significant effort to locate bedded and sorted sands that were clearly deposited by subaerial streamflow. These were sampled using opaque ABS pipe without exposing the sediment to light. Dose rate measurements were made directly in the field using a portable 4-channel gamma spectrometer calibrated by personnel at the United States Geological Survey. Environmental dose rate values are calculated using the conversion factors of Adamiec and Aitken (1998) and the grain-size attenuation factors outlined in Aitken (1986). Present-day water content values are assumed to be representative of those pertaining to the full burial period, and have been assigned relative uncertainties of ±50%. Equivalent dose (De) analysis was undertaken at the Oxford University Luminescence Research Group Laboratories. Quartz De measurements were made using the Single Aliquot Regeneration (SAR) protocol developed by Murray and Wintle (2000). Individual De estimates were calculated for 15 aliquots (comprised of 100-300 grains) from each of the samples. Sample bleaching characteristics were then assessed from these populations of individual De estimates using the approach suggested recently by Bailey and Arnold (in press).

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4. RESULTS

4.1 Timing of San Emigdio Mesa Deposition Table 1 shows

10

Be CRN surface-exposure ages for San Emigdio Mesa. In

combination with field observation, we interpret the ages to record two stages of deposition on the San Emigdio Mesa surface; a major one ending near 28 ka, and a very limited alluvial episode around 14 ka that led to localized deposition at the mouths of small, steep drainages that remained graded to the alluvial surface at that time. This interpretation is supported by field observation of distinct, localized depositional landforms of the uppermost portion of San Emigdio Mesa from which the 14.3 ka sample was collected. We realize however that three CRN dates from alluvial boulders can only lead to highly interpreted ages because correction for inherited radionuclides and correction for loss of in situ-produced nuclides due to site erosion are nearly impossible without a larger dataset. To address this, we also employed OSL dating to the same landform. Three OSL ages from San Emigdio Mesa come from bedded sands down-fan and down-section from the CRN sampling sites. These ages and are, as expected, older than the CRN dates (Table 2), suggesting deposition occurred between 51 and 75 ka if analytical uncertainties are taken into account.

Comparison of these data to the

cosmogenic surface exposure data suggests a few possibilities. It is possible that the CRN data accurately indicate that deposition continued fairly slowly after deposition of the OSL-dated sands until at least 28 ka, or erosion of boulder tops has led to an

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underestimation of their true exposure age. In either case, it appears the age of the deposit forming San Emigdio Mesa is