Brigham Young University BYU ScholarsArchive All Theses and Dissertations 2017-11-01 40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah Michael Seth Jensen Brigham Young University Follow this and additional works at: https://scholarsarchive.byu.edu/etd BYU ScholarsArchive Citation Jensen, Michael Seth, "40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah" (2017). All Theses and Dissertations. 7270. https://scholarsarchive.byu.edu/etd/7270 This Thesis is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in All Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. 40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah Michael Seth Jensen A thesis submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of Master of Science Bart J. Kowallis, Chair Eric H. Christiansen Jeffrey D. Keith Department of Geological Sciences Brigham Young University Copyright © 2017 Michael Seth Jensen All Rights Reserved ABSTRACT 40Ar/39Ar Ages, Compositions, and Likely Source of the Eocene Fallout Tuffs in the Duchesne River Formation, Northeastern Utah Michael Seth Jensen Department of Geological Sciences, BYU Master of Science Thin fallout tuffs in the Duchesne River Formation in the Uinta Basin, Utah are evidence that volcanism was active in northern Nevada and Utah in the late Eocene. The Uinta Basin is a sedimentary basin that formed during the Laramide orogeny. Ponded lakes of various salinity filled and emptied and during the late Eocene the northern rim was dominated by a wetland/floodplain depositional setting. Most of the tuffs have rhyolitic mineral assemblages including quartz, biotite, sanidine, and allanite. Rhyolitic glass shards were also found in one of the ash beds. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of biotite compositions suggest 3 or 4 volcanic events. Sanidine compositions from five samples grouped at Or73 and Or79. Only one sample had plagioclase with compositions ranging between An22 - An49. Some beds also contained accessory phases of titanite, apatite, and zircon. Whole rock compositions of the altered volcanic ash beds indicate these tuffs underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. Immobile element ratios and abundances, such as Zr/Nb and Y are typical of a subduction zone tectonic setting and rhyolitic composition. 40Ar/39Ar ages constrain the timing of volcanism. One plagioclase and one sanidine separate from two different tuff beds yielded ages of 39.47 ± 0.16 Ma and 39.36± 0.15 Ma respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field. These new ages, along with previously published ages in the Bishop Conglomerate which unconformably overlies the Duchesne River Formation, constrain the timing of two uplift periods of the Uinta Mountains at 39 Ma and 34 Ma. Finally, the ages also date the fauna of the Duchesnean Land Mammal Age to be about 39.4 Ma as opposed to less precise earlier estimates that placed it between 42 and 33 Ma. Keywords: fallout tuffs, 40Ar/39Ar ages, laramide orogeny, vernal northwest quadrangle, northeast Nevada volcanic field, Duchesnean Land Mammal Age ACKOWLEDGEMENTS I would first like to acknowledge Bart J. Kowallis, the Chair of my graduate committee, for all his help and time spent discussing ideas and interpreting data. Without his guidance this project would not have been a success. I also thank and acknowledge Eric H. Christiansen for sharing his expertise about the volcanic history of western North America and for providing Excel spreadsheets to help analyze and interpret data about the mineral and whole rock compositions. I also acknowledge the assistance of Michael J. Dorais, who helped calibrate and repair the microprobe at BYU so that I could obtain reliable data from mineral grains that were sometimes weathered. Doug Sprinkel, Senior Geologist from the Utah Geological Survey, introduced me to the field area, provided background information, and was always available to discuss ideas. Brian Jicha at the Geochronology lab of the University of Wisconsin-Madison performed the 40Ar/39Ar dating and provided probability-density plots of the age data. I also acknowledge fellow graduate student Casey Webb, who assisted with the sample preparation of the volcanic ash, helped with sample collection, and spent many hours with me in the field. Finally, I thank my wife, Madeline. She has supported and encouraged me throughout this entire process and always pushed me to do my best work. Funding for the project was provided by Brigham Young University. TABLE OF CONTENTS TABLE OF CONTENTS ............................................................................................................... iv LIST OF FIGURES ........................................................................................................................ v LIST OF TABLES ......................................................................................................................... vi INTRODUCTION .......................................................................................................................... 1 GEOLOGIC SETTING .................................................................................................................. 3 Laramide Orogeny .................................................................................................................................... 3 Duchesne River Formation ....................................................................................................................... 5 SAMPLING AND ANALYTICAL METHODS ........................................................................... 7 ISOTOPIC AGES ........................................................................................................................... 9 MINERAL ASSEMBLAGES AND COMPOSITIONS .............................................................. 10 Quartz ..................................................................................................................................................... 11 Biotite ..................................................................................................................................................... 11 Feldspar .................................................................................................................................................. 15 Allanite ................................................................................................................................................... 16 Titanite .................................................................................................................................................... 18 Glass Shards ........................................................................................................................................... 19 Apatite .................................................................................................................................................... 19 WHOLE ROCK COMPOSITION ................................................................................................ 19 Immobile Elements ................................................................................................................................. 19 Argillic Alteration................................................................................................................................... 21 DISCUSSION ............................................................................................................................... 22 Eruptive Source of the Duchesne River Formation Tuffs ...................................................................... 22 Timing of Uinta Mountain Uplift ........................................................................................................... 26 Duchesnean Land Mammal Age ............................................................................................................ 27 CONCLUSIONS........................................................................................................................... 28 REFERENCES CITED ................................................................................................................. 30 APPENDIX 1 ................................................................................................................................ 99 APPENDIX 2 .............................................................................................................................. 103 iv LIST OF FIGURES Figure 1: Regional map and simplified geolgioc map of Vernal northwest quadrangle. ............. 38 Figure 2: Stratigraphic column of mapping units. ........................................................................ 39 Figure 3: Startigraphic diagram of Dry Gulch Creek and Lapoint members. .............................. 40 Figure 4: Field photographs of tuffs. ...........................................................................................