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Geology and Eruptive History of the Late Oligocene Nathrop Volcanics, Central

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Geology and Eruptive History of the Late Oligocene Nathrop Volcanics, Central GEOLOGY AND ERUPTIVE HISTORY OF THE LATE OLIGOCENE NATHROP VOLCANICS, CENTRAL COLORADO VOLCANIC FIELD William D. Emery A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of Master of Science May 2011 Committee: Dr. Kurt Panter, Advisor Dr. Charles Onasch Dr. Jeff Snyder ii ABSTRACT Dr. Kurt Panter, Advisor The Nathrop Volcanics consist of rhyolite lava and pyroclastic deposits located on the eastern shoulder of the upper Arkansas Graben in south-central Colorado and are part of the extensive late Eocene-Oligocene Central Colorado Volcanic Field. Deposits of the Nathrop Volcanics at Ruby Mountain consist of a lower lithic-rich lapilli tuff (ca. 3 m thick) with multiple layers that are reversely graded with respect to pumice clasts and are overlain by an approximately 30 m thick lithic-poor tuff breccia containing pumice blocks up to 1 m in diameter. The upper portion of the tuff breccia transitions up into a 5 m thick, moderately to densely welded tuff (vitrophyre), which in turn is overlain by a 20 m thick flow-banded rhyolite. A similar stratigraphic sequence is found at Sugarloaf Mountain (<1 km to the NNE), and also portions of the sequence crop out as faulted and eroded blocks in the valley between the two mountains. These deposits have been interpreted as being formed by exogenic lava dome growth; pyroclastic facies (fall overlain by flow) followed by lava extrusion. This study considers three possible scenarios to explain the origin and geometry of these deposits. Pivotal to these scenarios is the explanation for the cause of welding and stratigraphic position of the vitrophyre. The three models are: 1) the flow-banded rhyolite was erupted as a lava immediately after the pyroclastic flow (tuff breccia) which caused the welding; 2) the tuff breccia and flow-banded rhyolite are not from the same eruptive episode and welding occurred in a thick pyroclastic flow that subsequently was eroded down to the level of the more resistant vitrophyre followed by eruption of the rhyolite as a lava flow; and 3) the whole sequence represents a single short-lived eruptive event in which the pyroclasts accumulated rapidly iii enough to weld and flow rheomorphically. This study evaluates all three models based on field relationships. iv ACKNOWLEDGEMENTS I would like to acknowledge the following people who made this thesis possible. First and foremost, I’d like to thank my parents, Cheryl and Bill Emery, for their support and encouragement not just over the last 2 years of graduate school, but over the last 25 years of my life. I would like to thank Dr. Kurt Panter, for his patience and understanding during the investigation and writing of my thesis. This is also extended to Dr. Jeff Snyder, and Dr. Charles Onasch, for serving on my committee and offering many years worth of valuable insight into the Nathrop Volcanics. Finally I would like to thank my peers at BGSU, especially Laura Webb, Megan Castles, Colleen O’Shea, and Asako Kawatsura for the support they gave me during the course of my graduate work. v TABLE OF CONTENTS INTRODUCTION ............................................................................................................................................. 1 GEOLOGIC BACKGROUND ............................................................................................................................. 2 Rio Grande Rift .......................................................................................................................................... 2 The Nathrop Volcanics .............................................................................................................................. 4 Background on volcanic domes and their deposits ................................................................................... 5 METHODS .................................................................................................................................................... 10 RESULTS: DESCRIPTION OF VOLCANIC DEPOSITS ....................................................................................... 12 Tnt – Tertiary Tuff ................................................................................................................................... 12 Tntf - Lapilli tuff ................................................................................................................................... 12 Tntb – Tuff breccia ............................................................................................................................... 14 Tntv – Vitrophyre ................................................................................................................................. 16 Tntvp – Perlitized vitrophyre ................................................................................................................ 17 Tnts – Volcanic sandstone ................................................................................................................... 17 Tnr – Tertiary Rhyolite lava ..................................................................................................................... 19 DISCUSSION ................................................................................................................................................. 22 Lapilli tuff (Tntf) – Pyroclastic fall facies of Tnt. ...................................................................................... 22 Tuff breccia (Tntb) – Pyroclastic flow facies of Tnt .................................................................................. 23 Vitrophyre (Tntv) – welded facies of Tntb ................................................................................................ 26 Perlitized vitrophyre (Tntvp) altered vitrophyre facies of Tntv ................................................................. 31 Volcanic sandstone (Tnts) – sedimentary facies of Tnt ........................................................................... 31 Tertiary Rhyolite (Tnr) – lava flows ......................................................................................................... 32 MODEL RECONSTRUCTIONS ....................................................................................................................... 34 Model A: Lava-induced welding of pyroclastics ...................................................................................... 34 Model B: Welding in a thick pyroclastic flow .......................................................................................... 36 Model C: Welding by Rheomorphic Flow ................................................................................................ 37 CONCLUSIONS ............................................................................................................................................. 39 vi REFERENCES ................................................................................................................................................ 41 TABLES ......................................................................................................................................................... 51 FIGURES ....................................................................................................................................................... 53 APPENDIX .................................................................................................................................................... 88 vii LIST OF TABLES Table 1: Previously postulated ages for the Rio Grande Rift ...................................................................... 51 Table 2: Modified whole-rock geochemistry analysis by Honea (1955) ..................................................... 51 Table 3: Modified bulk composition chemistry by Schooler (1982) ........................................................... 52 Table 4: (Appendix) Details of samples collected in this study ................................................................... 88 viii LIST OF FIGURES Figure 1: Basins of the Rio Grande Rift ....................................................................................................... 53 Figure 2: Basic volcanic dome shapes ......................................................................................................... 54 Figure 3: Particle relationships between pyroclastic surges/flows/falls. ................................................... 55 Figure 4: The Nathrop Volcanics and sample locations .............................................................................. 56 Figure 5: Geologic Map of the Nathrop Volcanics ...................................................................................... 57 Figure 6: Stratigraphic section of Ruby Mountain ...................................................................................... 58 Figure 7: Northeast Ruby Mountain area. .................................................................................................. 59 Figure 8: Three subfacies of tuff ................................................................................................................. 60 Figure 9: Six subfacies of tuff ...................................................................................................................... 61 Figure 10: Coarsening-up sequence of tuff ................................................................................................. 62 Figure 11: Microscopic
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