University of Nevada, Reno Paleozoic Stratigraphy and Structure at Iron Point, Humboldt County, North-Central Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology By: Erica Leanne Key Dr. Patricia H. Cashman/Thesis Advisor May, 2015 © by Erica Leanne Key 2015 All Rights Reserved THE GRADUATE SCHOOL We recommend that the thesis prepared under our supervision by ERICA L. KEY Entitled Paleozoic Stratigraphy And Structure At Iron Point, Humboldt County, North-Central Nevada be accepted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Patricia H. Cashman, Ph. D., Advisor James H. Trexler Jr., Ph. D., Committee Member Jill Heaton, Ph. D., Graduate School Representative David W. Zeh, Ph.D., Dean, Graduate School May, 2015 i ABSTRACT Detailed mapping and reconsideration of biostratigraphic data provide new insights into how the Comus Formation at its type locality at Iron Point, Humboldt County, Nevada fits into the regional stratigraphic framework. The age designation of the Comus Formation was reevaluated by this study using the most current understanding of Ordovician graptolite biostratigraphy. Previous studies at Iron Point had determined that the graptolites found in the siltstone units in the Comus Formation were middle Ordovician. This study determined that the species of graptolites found at Iron Point had been reclassified as late Ordovician since the original biostratigraphic study had been performed. A portion of the Vinini Formation at Iron Point was remapped in this study as the uppermost unit of the Comus Formation. Originally, this part of the Vinini Formation was mapped as a thrust “klippe” over the Comus Formation. This area originally mapped as “Vinini” is a very different rock type than the rest of the Vinini mapped at Iron Point. The lithology was much more similar to the underlying Comus Formation. Additionally, the geometry of the contact did not make sense; the thrust contact between the Vinini and the Comus was mapped as subhorizontal, however all of the strata dipped moderately to the west. The boundary between the "Vinini" and Comus Formation in this area was remapped and determined to be depositional, not structural. The Comus Formation in its type locality at Iron Point is not correlative with the "Comus Formation" that hosts Carlin-style gold deposits to the north in the Osgood Mountains. The Comus Formation at Iron Point is a sequence of interbedded carbonate and siliciclastic rocks deposited on the continental slope during the late Ordovician. The ii “Comus Formation” mapped in the Osgood Mountains is a sequence of carbonate, siliciclastic, and mafic volcanic rocks deposited on or near a carbonate seamount from the late Cambrian to late Ordovician (Hotz and Willden, 1964; Breit et al., 2005). The Comus at Iron Point and the “Comus” in the Osgood Mountains are composed of some similar types of Ordovician rocks, but their internal stratigraphy is too different to be classified as the same continuous unit. The Comus Formation at Iron Point is here interpreted to be correlative with the late Ordovician Hanson Creek Formation. These units have similar internal stratigraphy and timing of deposition. The Hanson Creek Formation was deposited on the continental shelf during the late Ordovician, and the Comus Formation is interpreted here to be the continuation of the Hanson Creek Formation onto the continental slope. A new unit composed of conglomerate, breccia, and a mature quartzite was identified at Iron Point underlying the Comus Formation. The quartzite portion of the unit was previously associated with the lower part of the Comus Formation, but the conglomerate and breccia were never recognized. The quartzite is composed entirely of quartz, and the conglomerate and breccia have a quartz sand matrix. The quartzite may be correlative with the middle Ordovician Eureka Quartzite. The Eureka Quartzite is the only widespread siliciclastic deposit on the continental shelf or slope during Paleozoic time. Additionally, the Eureka Quartzite underlies the Hanson Creek Formation and its correlative units in other areas in the Great Basin. Structural analyses using the new detailed mapping yielded evidence of six different deformational events at Iron Point. Their relative ages were determined through cross-cutting relationships and comparison to deformation recorded at Edna Mountain iii less than a kilometer east of Iron Point. The first fold set (F 1) is west-vergent, and likely correlative to mid-Pennsylvanian folds observed at Edna Mountain (Villa, 2007; Cashman et al., 2011). F 1 folds are asymmetric, steeply inclined, and locally overturned to the west. The second fold set (F 2) records north-south contraction and is likely correlative to early Permian folds observed at Edna Mountain (Villa, 2007; Cashman et al., 2011). F2 folds are upright, symmetrical, and trend west-southwest. The King fault is a normal fault that strikes north-south and dips east. It post-dates the first two fold sets, and has not been active since the early Permian. The Silver Coin thrust strikes east-west, places the Vinini Formation over the Comus Formation, truncates the King fault, and is not affected by the first two fold sets. The West fault strikes southeast and dips southwest. The West fault truncates the Silver Coin thrust and juxtaposes the Comus and Vinini Formations in the footwall with the Cambrian Preble Formation in the hanging wall. Finally, Iron Point is bounded on the east side by the Pumpernickel fault, a normal fault that strikes north-south and dips east. The Pumpernickel fault Eureka Quartzite and Comus Formation in the footwall and the rock unit in the hanging wall is covered by Quaternary alluvium, so is not exposed. The movement on this structure is likely related to Basin and Range faulting starting in the Miocene. iv ACKNOWLEDGEMENTS I would like to thank all of my committee members: Dr. Pat Cashman, Dr. Jim Trexler, and Dr. Jill Heaton. To my advisor, Pat Cashman: this process has been long , and you’ve stuck with me through all of it. You’ve made me a better writer, and more importantly, a better geologist and overall scientist. Thank you to my sediment guru, Dr. Jim Trexler. Some graduate students never had their advisor visit their study area, but I had you AND Pat accompany me more than once, and you managed to come out on the hottest and coldest field days. Thank you for helping me make sense of the rocks out there. There are also a few key people and groups that contributed greatly to getting this thesis done. Thank you Dr. Wanda Taylor, for making a trip all the way up from Las Vegas, just for a trip to Iron Point to look at some more late Paleozoic structures. And thank you to Dr. Harry Cook, for providing invaluable insight into the rocks at Iron Point, and Nevada, in general. Thank you to Dr. Mike Ressel and Tony Dorff for advocating for me, even though it didn’t work out at that big company where I used to work. A huge thanks goes to the Society of Mining, Metallurgy, and Engineering. Without the Stephen C. Potter and Stewart R. Wallace Scholarships, I wouldn’t have been able to afford to complete my degree. Thank you to my daughter, Harper. This thesis is dedicated to you, kid. You were with me through my last finals, at the end of my fieldwork, and during my defense. I wrote the majority of this while either holding you or acting as your human jungle gym. You gave me the drive to finish this, and now I can devote my time and attention 100% to you. v Finally, and most importantly, thank you to my husband, Thomas. You supported me going back to school not once, but twice. You spent all your free time driving from Elko to Reno, or in the field with me. You were the best field assistant: you carried my samples, reminded me to eat and drink water when I got grumpy, and encouraged me when I would get frustrated. You’re an amazing partner, and I really couldn’t have done any of this without your love and support. Now that this chapter is finally coming to a close, I’m excited to see what we do with all of the free time we’ll have! vi TABLE OF CONTENTS ABSTRACT ........................................................................................................................ i ACKNOWLEDGEMENTS ............................................................................................ iv TABLE OF CONTENTS ................................................................................................ vi LIST OF FIGURES ......................................................................................................... ix INTRODUCTION............................................................................................................. 1 BACKGROUND ............................................................................................................... 4 Regional Paleozoic Tectonic Setting ............................................................................ 4 Regional Ordovician Stratigraphy .............................................................................. 8 Continental Shelf: Ordovician Eureka Quartzite ....................................................... 8 Continental Shelf: Ordovician-Silurian Hanson Creek Formation .......................... 10 Continental Slope: Ordovician Comus Formation ................................................... 12 Deep Ocean Basin: Ordovician Vinini Formation ................................................... 15 Iron Point Stratigraphy