Formation, Deformation, and Incision of Colorado River Terraces
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FORMATION, DEFORMATION, AND INCISION OF COLORADO RIVER TERRACES UPSTREAM OF MOAB, UTAH by Andrew P. Jochems A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in Geology Approved: _____________________ _____________________ Dr. Joel L. Pederson Dr. Patrick Belmont Major Professor Committee Member _____________________ _____________________ Dr. Tammy M. Rittenour Dr. Mark R. McLellan Committee Member Vice President for Research and Dean of the School of Graduate Studies UTAH STATE UNIVERSITY Logan, Utah 2013 ii Copyright © Andrew P. Jochems 2013 All Rights Reserved iii ABSTRACT Formation, Deformation, and Incision of Colorado River Terraces Upstream of Moab, Utah by Andrew P. Jochems, Master of Science Utah State University, 2013 Major Professor: Dr. Joel L. Pederson Department: Geology Fluvial terraces contain information about incision, deformation, and climate change. In this study, a chronostratigraphic record of Colorado River terraces is constructed from optically stimulated luminescence (OSL) dating of Pleistocene alluvium and real-time kinematic (RTK) GPS surveys of terrace form. This record is analyzed to relate terrace formation to late Pleistocene climate fluctuations, and terrain analyses and longitudinal profile patterns reveal recent salt-related activity in the northern Paradox Basin as well as patterns in Colorado Plateau incision. A well-preserved, correlative suite of mainstem (M) fluvial deposits exists along the Colorado River upstream of Moab, Utah. Absolute dates indicate sedimentation >70 ka (M7, M6/M5), 70-50 ka (M4), 50-40 ka (M3), and 35-25 ka (M2). The M4 and M2 formed during the crescendo to glacial maxima, but the M7, M6/M5, and M3 were deposited during variable climate of marine isotope stages (MIS) 5 and 3. Deposits include thin (<7 m) strath terraces and thick (10-20 m) fill terraces. iv Our results suggest that terrace sedimentation is linked to enhanced sediment flux during glaciations in Rocky Mountain headwaters (M4 and M2), but major deposits also formed during dryland tributary sediment loading with markedly different timing (M6/M5 and M3). Conversely, incision may be driven by higher deglacial flows. Clast provenance data demonstrate greater percentages of locally-sourced sediment in M6/M5 and M3 deposits. Valley-bottom geometry and neotectonics control terrace form, with strath terraces found in bedrock-restricted reaches and fill terraces in wider valleys. Previously speculated salt deformation in this area is confirmed by localized collapse preserved in M4 stratigraphy in the Cache Valley graben and ~15 m of broader subsidence upstream. Concavity and knickzone distributions in tributary profiles are discordant and represent subtle expressions of salt-tectonic activity. Finally, a surprisingly rapid incision rate of ~900 m/Ma over the past ~70 ka suggests that the Colorado River may be responding to flexural rebound in the central plateau, but is faster than that predicted by the debated bull’s-eye pattern of regional incision. This locally high rate may also reflect a transient wave of incision, as suggested by increased Pleistocene rates interpreted by studies in Glen and Grand canyons. (193 pages) v PUBLIC ABSTRACT FORMATION, DEFORMATION, AND INCISION OF COLORADO RIVER TERRACES UPSTREAM OF MOAB, UTAH The history of rivers is laid down as sediment in all landscapes, typically as a function of climate, geologic structures, and/or changes in sea level. When a river abandons its floodplain, this sediment collectively constitutes a landform called a fluvial terrace. Terraces are used to unlock prior characteristics of a river flowing through a given area at both local and regional scales. Dating terrace sediment allows comparison to known changes in climate and geologic deformation, two significant controls on the hydraulics of rivers and the deposition of their sediment loads. The importance of terraces lies in their utility as markers of climate change and geologic deformation. By constructing a record of terrace formation by large rivers such as the Colorado, we develop the capacity to predict how a river system will respond to changing climate or geologic events such as the rupturing of faults or the uplift of the Earth’s surface. These capabilities permit informed decisions wherever humans live along or otherwise depend on rivers, including water management and floodplain zoning. We describe the history of the Colorado River upstream of Moab, Utah over the last 100,000 years. By dating terrace sediment and taking topographic surveys of terrace shape and form, we identify patterns in climate change and geologic deformation related to subsurface salt deposits. Our findings imply that the Colorado River is sensitive to both of these controls, especially in terms of where it deposits its sediment load, how much sediment is deposited, and the timing of floodplain abandonment. Andrew P. Jochems vi ACKNOWLEDGMENTS I wish to thank my committee members, Dr. Tammy Rittenour and Dr. Patrick Belmont, for their valuable advice and guidance throughout the execution of this research. I greatly appreciate the help and company of many field hands: Natalie Bursztyn, Santiago Flores, Faye Geiger, Troy Siddoway, and Cianna Wyshnytzky. Michelle Nelson in the Utah State Luminescence Laboratory was integral to the processing and analysis of luminescence results. I am grateful to the USU Department of Geology for the Peter R. McKillop Memorial Scholarship, and for financial support in the form of research and teaching assistantships. The support of my family and my significant other, Amanda Gallegos, was instrumental during my time at Utah State. My grandfather, Willis Nigh, graciously lent his geologic expertise and inexhaustible encouragement. Finally, I must express extreme gratitude to my advisor, Dr. Joel Pederson, for his patience and scientific insight, which will continue to inspire my development as both a geoscientist and human being long after I depart Logan. Andy Jochems vii CONTENTS Page ABSTRACT ....................................................................................................................... iii PUBLIC ABSTRACT .........................................................................................................v ACKNOWLEDGMENTS ................................................................................................. vi LIST OF TABLES ...............................................................................................................x LIST OF FIGURES ......................................................................................................... xiii CHAPTER 1. INTRODUCTION .......................................................................................1 2. CONTROLS ON THE FORMATION OF COLORADO RIVER TERRACES UPSTREAM OF MOAB, UTAH ........................................3 ABSTRACT .....................................................................................3 INTRODUCTION ...........................................................................4 BACKGROUND .............................................................................7 Regional setting ...................................................................7 Local setting .........................................................................9 Paleoclimate of the Rocky Mountains ...............................13 Conceptual models of terrace formation ............................17 METHODS ....................................................................................21 Geochronology ...................................................................21 Survey data.........................................................................24 Spatial analyses ..................................................................25 Bedload provenance ...........................................................26 RESULTS ......................................................................................27 Chronostratigraphy ............................................................27 Dewey Bridge reach ...............................................27 Professor Valley reach ...........................................32 Cache Valley graben reach ....................................38 viii Ida Gulch/Castle Valley reach ...............................41 Regional correlations .........................................................44 Bedload provenance ...........................................................47 Spatial variations in terrace form .......................................51 DISCUSSION ................................................................................55 Timing and climate controls of terrace sedimentation versus incision ..................................................................55 Salt tectonism as an alternative control on terrace formation ..........................................................................59 Reach changes in terrace form ...........................................60 Implications........................................................................62 REFERENCES CITED ..................................................................63 3. QUATERNARY SALT DEFORMATION AND INCISION ALONG THE COLORADO RIVER NEAR MOAB, UTAH ...............................73 ABSTRACT ...................................................................................73 INTRODUCTION .........................................................................74 BACKGROUND ...........................................................................76