AN ABSTRACT OF THE THESIS OF John Henry Whitaker for the degree of Master of Science in Geology presented on May 6, 1985 Title: GEOLOGY OF THE WILLOW CREEK AREA, ELKO COUNTY, NEVADA Abstract approved: Redacted for Privacy . G. Joh761 The purpose of this study was to test two hypotheses. The first hypothesis interprets allochthonous rocks in the Willow Creekarea of the Pinyon Range as lying on flat, superficial thrusts that truncate the underlying folded Mississippian rocks of the Antler flyschsequence. The second hypothesis interprets the allochthonous rocksas being folded within the Mississippian sequence as if they had arrivedas part of the Antler allochthon after flysch sedimentation had begun andwere overlapped as sedimentation continued. Evidence presented here supports the second hypothesis. During Early and Middle Devonian time, deposition in the central Pinyon Range occurred in a miogeoclinal shallow-shelf environment characterized by carbonates of the Nevada Group. Onset of the Antler orogeny in Late Devonian time was marked by uplift and erosion or nondeposition in the study area. A west-dipping subduction zone formed to the west in oceanic crust, developing an accretionary wedge which became the Roberts Mountains allochthon. The allochthon was obducted onto the continent as incipient subduction of continental crust occurred. Downbending of the edge of the continent led to the formation of a foreland trough at the leading edge of the allochthon.Telescoping within the allochthon caused it to rise and shed siliciclastic sediments into the trough forming the submarine fan deposits of the Dale Canyon Formation. The allochthon was then emplaced over the Dale Canyon Formation and was subsequently overlapped by deltaic sediments of the Diamond Peak Formation. The Roberts Mountains allochthon in the Willow Creek area consists of the Devonian Woodruff Formation forming its base, the Ordovician Vinini Formation at the top, with a small slice of the Siluro-Devonian Roberts Mountains Formation lying in between.Emplacement of the allochthon is dated as during or before typicus Zone time of the early Osagean on the basis of conodont collections from the overlapping Diamond Peak Formation. Geology of the Willow Creek Area, Elko County, Nevada by John H. Whitaker A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed May 6, 1985 Commencement June 1985 APPROVED: Redacted for Privacy Profess r of Geology charge of major Redacted for Privacy Cha rm n of Department or Geology Redacted for Privacy Dean of Gr t5 ateSchoo(r Date thesis is presented May 6, 1985 Typed by Rebecca Simpkins forJohn H. Whitaker ACKNOWLEDGMENTS Thanks go to many people who aided me on the way to completion of this task. Thanks first to Dr. J. G. Johnson who provided me with this challenging and important project and who was helpful and encouraging throughout. Thanks also to Claudia Regier who picked many of my conodont samples and to GilbertKlapper of the University of Iowa for identification and dating of the conodonts.Also thanks to William B. N. Berry of the University of California at Berkeley who identified my graptolite samples.Thanks to Dr. Robert Yeats and Dr. Cyrus Field for residing on my committee and providing excellent critical comments and questions. Robert Visconti provided many interesting discussions in the field and split ridesinto town with me. Funding was provided in part by a National Science Foundation research grant. Funding was also provided by Superior Oil, Minerals Division of Reno, Nevada. Thanks to Don Cohen of Superior Oil for spending time in the field with me at the study area. Thanks also go to my parents, without whom this would never have been possible, and to my many roommates and friends who put upwith me during the long haul. Special thanks go to Ian Walsh, and Philip and Emily Verplanck who helped me prepare my talk. Special thanks also go to Becky Simpkins for her fast, on-call typingservices. TABLE OF CONTENTS Page INTRODUCTION 1 Purpose 1 Location and Accessibility 2 Previous Work 2 Methods and Terminology 7 GEOLOGIC SETTING 8 VININI FORMATION (ALLOCHTHONOUS) 13 General Statement 13 Age 13 Lithology 13 Structure 14 Depositional Environment 16 ROBERTS MOUNTAINS FORMATION (ALLOCHTHONOUS) 18 General Statement 18 Age 18 Lithology 19 Structure 21 Depositional Environment and Regional Significance 22 WOODRUFF FORMATION (ALLOCHTHONOUS) 24 General Statement 24 Age 24 Lithology 25 Laminated quartz-silty dolomitic mudstone and siltstone 25 Limestone 27 Chert and siliceous mudstone 27 Siliceous dolomite 30 Structure and Contact Relations 34 Depositional Environment and Regional Significance 36 NEVADA GROUP (AUTOCHTHONOUS) 41 Oxyoke Canyon Sandstone 42 Telegraph Canyon Formation 42 DEVILS GATE LIMESTONE (AUTOCHTHONOUS) 44 Silicification 45 WEBB FORMATION (AUTOCHTHONOUS) 50 Page DALE CANYON FORMATION (AUTOCHTHONOUS) 52 General Statement 52 53 Age Contact Relations and Thickness 53 Facies Interpretation 54 Basin slope facies 55 Inner fan facies 57 Lithology 62 Chert-arenite 62 Pebbly chert-wacke 63 Fine-grained rocks 65 Regional Significance 68 DIAMOND PEAK FORMATION (AUTOCHTHONOUS) 70 General Statement 70 Age 71 Contact Relations and Thickness 73 Facies Interpretation 74 79 Lithology 79 Chert-arenite Chert-conglomerate 81 Clay- and silt-shale 81 Thin-bedded argillaceous biomicrite 81 Regional Significance 85 87 MOLEEN FORMATION UPPER PENNSYLVANIAN AND PERMIAN ROCKSUNDIVIDED 90 93 NEWARK CANYON FORMATION 95 IGNEOUS ROCKS 96 STRUCTURE 96 Thrust Faulting 97 Folding Normal Faulting 102 103 GEOLOGIC HISTORY 106 SUMMARY 109 REFERENCES CITED 115 APPENDIX LIST OF FIGURES Figure Page 1 Index map of Nevada. 3 2 Index map of Northeast Nevada. 4 3 Summary stratigraphic section for the study area. 5 4 Hypothetical depiction of facies and structural 11 relations during early or middle Famennian time, the time of deposition of the true Woodruff Formation and time of probable initial development of the Roberts Mountains allochthon. 5 Hypothetical depiction of facies and structural 12 relations during late Kinderhookian or early Osagean time, immediately after emplacement of the Roberts Mountains allochthon and before overlap deposition of the Diamond Peak Formation. 6 Photomicrograph of bedded chert, Vinini Formation. 15 7 Platy calcareous and carbonaceous shale, Roberts 20 Mountains Formation. 8 Laminated quartz-silty dolomitic siltstone, 26 Woodruff Formation. 9 Thin-bedded limestone of Woodruff Formation. 28 10 Photomicrograph of limestone from Woodruff 29 Formation. 11 Photomicrograph of bedded chert, Woodruff 31 Formation. 12 Siliceous dolomite from the Woodruff Formation. 32 13 Photomicrograph of siliceous dolomite, Woodruff 33 Formation. 14 Tight small to medium scale folding in bedded 35 chert of the Woodruff Formation. 15 Outcrop exposure of depositional contact between 37 the Devonian Woodruff Formation of the allochthon (left of hammer) and the Mississippian Diamond Peak Formation of the overlapping flysch sequence (right of hammer). Page Figure 38 16 Close-up of depositional contact betweenDevonian Woodruff Formation (right) andMississippain Diamond Peak Formation (left) shown in Figure15. 47 17 Silicified ridge of the Devonian DevilsGate Limestone. 48 18 Close-up of brecciated and partiallysilicified Devonian Devils Gate Limestone. 56 19 Bedded shaly mudstone and sandstone ofbasin slope facies of lower member of theMississippian Dale Canyon Formation. 58 20 Deformed mudstone clasts in massivepebbly mudstone of the Mississippian Dale CanyonFormation. 59 21 Deformed flute casts at the base ofmassive sandstone bed, Dale Canyon Formation. 60 22 Massive sandstone beds separated by shalypartings, Dale Canyon Formation. 61 23 Amalgamated platy sandstone beds, Dale Canyon Formation. 64 24 Photomicrograph of chert-arenite, Dale Canyon Formation. Canyon 66 25 Photomicrograph of pebbly chert-wacke, Dale Formation. 67 26 Photomicrograph of argillaceousbiomicrite, Dale Canyon Formation. Peak Formation. 76 27 Amalgamated sandstone packets, Diamond 78 28 Graded bedding in coarse unit ofMississippian Diamond Peak Formation. Peak 80 29 Photomicrograph of chert arenite, Diamond Formation. Diamond 82 30 Photomicrograph of chert-conglomerate, Peak Formation. lens near base 83 31 Thin-bedded argillaceous biomicrite of Mississippian Diamond PeakFormation. Page Figure 85 32 Photomicrograph of argillaceous biomicrite,Diamond Peak Formation. 88 33 Thick-bedded limestone of the Pennsylvanian Moleen Formation. 91 34 Bedded calcareous sandstone of thePennsylvanian and Permian Undivided Sandstone. 94 35 Thin-bedded limestone of the Cretaceous Newark Canyon Formation. 36 Structural map of the study area showingprincipal 98 folds and faults. 101 37 Pi-diagrams of poles plotted to bedding planes of rocks of the Roberts Mountains allochthonand Mississippian rocks of the Antler flysch sequence enclosing the allochthon. 107 38 Cross sections showing the interpretationof Smith and Ketner (1975, 1978) and theinterpretation of this report. LIST OF PLATES Page Plate pocket 1 Geologic Map of the Willow Creek Area, Elko County, Nevada pocket 2 Geologic Cross Sections GEOLOGY OF THE WILLOW CREEK AREA, ELKO COUNTY, NEVADA INTRODUCTION Purpose This study involved detailed mapping and structuralinterpretation of early and middle Paleozoic sedimentary rock formations atthe toe of the Roberts Mountains thrust fault in theWillow Creek area of the Pinyon Range, north-central Nevada. Smith and Ketner