78 RECONCILING HOLOCENE ALLUVIAL RECORDS IN BUCKSKIN WASH, SOUTHERN UTAH by Jonathan E. Harvey A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology Approved: _____________________ _______________________ Joel L. Pederson, Ph.D. Tammy M. Rittenour, Ph.D. Major Professor Committee Member _____________________ _______________________ John C. Schmidt, Ph.D. Byron R. Burnham, Ed.D. Committee Member Dean of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2009 ii Copyright © Jonathan E. Harvey 2009 All Rights Reserved iii ABSTRACT Reconciling Holocene Alluvial Records in Buckskin Wash, Southern Utah by Jonathan E. Harvey, Master of Science Utah State University, 2009 Major Professor: Dr. Joel L. Pederson Department: Geology Most approaches to interpreting alluvial records in drylands fall into one of two categories: (1) The “arroyo problem,” wherein workers study cycles of streambed aggradation and degradation in broad, unconstricted alluvial valleys; and (2) paleoflood hydrology, where alluvial sequences in constricted bedrock canyons are interpreted as paleoflood deposits from streams with stable channel grade and geometry. Both approaches can be valid in their end-member settings, but there is confusion about how the two record types relate in a single drainage. We address this disconnect in Buckskin Wash, an ephemeral stream that consists of a broad alluvial reach draining into a tightly constricted slot canyon. By employing detailed sedimentology, stratigraphy, and geochronology in both the alluvial and constricted reaches of the watershed, we test the hypothesis that the slot canyon deposits are anticorrelated to valley-fill deposits upstream, implying that arroyo cutting is driven by episodic flooding. Alluvial reach deposits are characterized by stratal packages representing incremental, long-term aggradation bound by erosion surfaces representing channel iv entrenchment. At least four packages younger than ~3 ka are present, the youngest spanning ~0.7 – 0.15 ka. Each is composed of interfingering imbricated gravels, laminated sands, and massive silty clays. Constricted reach deposits consist of five discrete packages, each composed of tabular beds of silty sand that were deposited relatively rapidly. The oldest package dates to ~1.9 – 1.1 ka whereas the rest of the deposits are younger than ~0.15 ka. Traditional paleoflood techniques would suggest that the constricted reach deposits record a ~1000-year absence of paleofloods followed by ~100 years of frequent, high-magnitude flooding that indeed correlate to arroyo cutting upstream. We argue instead that the constricted reach deposits record an episode of higher preservation potential. Transport of sediment from the alluvial reaches during historic arroyo cutting likely led to a pulse of sediment storage and changed stage-discharge relations in the slot canyon downstream, allowing even moderate floods to overtop existing deposits and be preserved. This new interpretation suggests that, because preservation may be a function of episodic sediment loading from upstream, constricted-reach deposits may not accurately record the paleoflood history of a stream. (135 pages) v ACKNOWLEDGMENTS First I would like to thank my advisor, Dr. Joel Pederson, for helping me identify such an interesting problem to tackle with my Masters thesis. His presence in the field was a critical component of my education and has helped me more than he knows. I would also like to thank my committee members: Dr. Tammy Rittenour, for going out of her way to teach me the fundamentals of OSL dating and for generously donating machine time to expedite my results; and Dr. Jack Schmidt, for giving me a rigorous introduction to fluvial geomorphology and its societal applications. I am indebted to the Utah State University Geology Department for all of its support during my graduate work, and for awarding me the J.S. Williams Graduate Scholarship. Thank you to colleagues Erin Tainer and Michelle Summa for tolerating my presence for the past two years, and to my field helpers Todd Parr and Chris Tressler. This thesis was also financially supported in part by a graduate student research grant from the Geological Society of America. I would especially like to thank the Quaternary Geology and Geomorphology division for awarding this project the Arthur D. Howard award. Lastly, I would like to thank my family. Their unwavering support and encouragement over the years as I bounced from subject to subject was critical to my eventual discovery of the Earth sciences. This work is dedicated to them. Jonathan E. Harvey vi CONTENTS Page ABSTRACT .............................................................................................................. iii ACKNOWLEDGMENTS .............................................................................................v LIST OF TABLES ..................................................................................................... viii LIST OF FIGURES ..................................................................................................... ix CHAPTER 1. INTRODUCTION .................................................................................1 2. RECONCILING APPROACHES TO ALLUVIAL RECORDS IN DRYLANDS .................................................................3 INTRODUCTION ...........................................................................3 The Arroyo Problem ..................................................................5 Paleoflood Hydrology of Bedrock Canyons ..............................9 Recognizing the Disconnect Between Paradigms....................11 Reconciliation ..........................................................................16 REFERENCES ..............................................................................18 3. THE ALLUVIAL RECORDS OF BUCKSKIN WASH, COLORADO PLATEAU ......................................................23 INTRODUCTION .........................................................................23 STUDY AREA ..............................................................................25 PREVIOUS WORK .......................................................................31 METHODS ....................................................................................35 Stratigraphy ..............................................................................35 Geochronology .........................................................................36 Surveying .................................................................................40 RESULTS ......................................................................................41 Sedimentology and Stratigraphy ..............................................41 Stratigraphy of Study Sites ......................................................45 vii Summary of Chronostratigraphy..............................................79 DISCUSSION ................................................................................83 Contrast of Alluvial and Constricted Reach Deposits .............83 Chronostratigraphic Interpretation ...........................................86 CONCLUSIONS............................................................................89 REFERENCES ..............................................................................91 IV. SUMMARY AND IMPLICATIONS ..................................................95 APPENDICES ...........................................................................................................100 Appendix A – Optically Stimulated Luminescence Data ..............................101 Appendix B – Facies Designations ................................................................114 Appendix C – Permission Letter ....................................................................134 viii LIST OF TABLES Table Page 3-1 Key to lithologies in the study area..................................................................28 3-2 Facies codes used in text ..................................................................................43 3-3 Depositional facies associations ......................................................................43 3-4 Results of optically stimulated luminescence analyses ...................................53 3-5 Results of radiocarbon analyses .......................................................................53 A-1 Results of optically stimulated luminescence analyses .................................102 B-1 Facies codes and descriptions ........................................................................115 B-2 Facies designations for study site KCW-A ....................................................117 B-3 Facies designations for study site KCW-B ....................................................121 B-4 Facies designations for study site COY-A .....................................................124 B-5 Facies designations for study site BG-A ........................................................127 B-6 Facies designations for study site BG-B ........................................................130 B-7 Facies designations for study site BG-C ........................................................133 ix LIST OF FIGURES Figure Page 2-1 Records for the interior West .............................................................................8 2-2 Cumulative probability distribution functions for radiocarbon- dated deposits in (A) bedrock and (B) alluvial reaches ...................................15