AN ABSTRACT OF THE THESIS OF STEVEN JOHN SKURLA for the MASTER OF SCIENCE (Name) (Degree) in GEOLOGY presented on April 8, 1974 (Major) (Date) Title: THE GEOLOGY OF THE STURGILL PEA REA, WASHINGTON COUNT Y,AHO Abstract approved: Redacted for Privacy Cyrus W. Field The oldest rocks of the Sturgill Peak area are herein named the grassy slope phyllites,and are of Permian (?) age.They are divided into a phyllitic member, consisting of complexly folded and highly sheared phyllites and phyllitic cherts, and a limestone member, containing sheared limestone, silicified limestone, and chert.The grassy slope phyllites are in fault contact with the underlying sedimentary rocks of Jurassic age.The grassy slope phyllites may correlate with the Burnt River Schist of northeast Oregon. Andesitic to dacitic welded tuffs and volcanic flows of the Upper Triassic Seven Devils Volcanics have a minimum thickness of 3,200 feet in the Sturgill Peak area.Rare interbeds of metaconglomerate, volcaniclastic wackes, and metachert are present. The Jurassic Sturgill Peak rhyolite welded tuff was deposited unconformably on the Seven Devils Volcanics,The welded ash flow origin of the unit is documented by relict eutaxitic textures.The unit is up to 700 feet thick, but is characterized by highly variable thick, nesses that are attributed to an interval of erosion that followed deposition of the welded tuff. The Mann Creek slates, an estimated 3, 000 feet in thickness, consist predominantly of slates, with lithic and feldspathic wackes, and limestone.They tectonically overlie the rhyolite welded tuff and Seven Devils Volcanics.The contact is marked, in part, by the conglomeratic schist lithology that was formed by shearing.The conglomeratic schist is correlated with the Red and Green Conglom- erate (Brooks, 1967) and shearing is attributed to faulting along the Bayhorse Thrust (Livingston, 1932).The Mann Creek slates are correlated with the Lucile Series of the Cuddy Mountains (Slater, 1969) on the basis of similar lithology. Between Early to Middle Jurassic and Early Cretaceous time, a small serpentinite body was emplaced along the thrust fault that separates the Mann Creek slates and the grassy slope phyllites.The occurrence of ultramafic rocks suggests that this fault may be of regional importance. The Sturgill Peak intrusive complex is believed to have been emplaced in Early Cretaceous time.Members of the complex, in order of emplacement, are hybrid gabbro (containing secondary quartz), quartz diorite, granodiorite, and mafic and felsic dikes and quartz veins.The Early Cretaceous age is based on two 120 million year K-Ar age dates from the granodioritic phase, asreported in Henricksen, Skurla, and Field (1972),Because of problems involving the dated rock, the complex could be older.Intrusion of the complex resulted in contact metamorphism of the country rocks to the hornblende-hornfels facies, and the formation of minor hydrothermal mineral deposits containing small amounts of copper, silver, and gold. Uplift and erosion during Cretaceous and Tertiary time exposed the intrusive complex.In Early to Middle Miocene time (Waters, 1961) flows of Picture Gorge Basalt completely buried the basement rocks of the Sturgill Peak area.In Middle to Late Miocene to Early Pliocene time (Waters, 1961) flows of Yakima Basalt were deposited, with slight angular unconformity, on the underlying Picture Gorge Formation. Pre-Tertiary rocks were uplifted above the Columbia River Basalts by major northwest and northeast-trending normal faults in late Tertiary and Quaternary time. In terms of plate tectonic concepts, the Seven Devils Volcanics, Mann Creek slates, and grassy slope phyllites are considered to be an island arc, arc-trench gap sediments, and deep sea sediments, respectively.Late Permian to Middle (?) Jurassic deformation is related to the separation of North American and African plates. Assuming the 120 million year age is correct, the Sturgill Peak intrusive complex may be related to subduction along the western margin of the North American plate in Early Cretaceous time, per- haps predating the separation of the European and North American plates. Geology of the Sturgill Peak Area, Washington County, Idaho by Steven John Skurla A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science June 1974 APPROVED: Redacted for Privacy AssocC._ to Professor of Geology in charge of major Redacted for Privacy Head /of Department of Geology Redacted for Privacy Dean of Graduate School Date thesis is presented April 8, 1974 Typed by Mary Jo Stratton forSteven John Skurla ACKNOWLEDGEMENTS The writer wishes to express his sincere gratitude to his major professor, Cyrus W. Field, for suggesting the problem, for critically reading the manuscript, and for his encouragement, assistance, and patience throughout the investigation.Thanks are also extended to Professors William H. Taubeneck and Harold E. En lows for criti- cally reading the manuscript and offering many helpful suggestions. Bear Creek Mining Company is acknowledged for providing analyses of stream sediment samples.Professor Richard L. Armstrong of Yale University and Donald J. Parker of Oregon State University are extended thanks for performing a potassium-argon age determination. The assistance and interest of fellow graduate students, par- ticularly Thomas A. Henricksen, David M, Rohr, and Robert A. Schmuck, during field work and the preparation of this thesis is appreciated.Special thanks are extended to my brother, Michael, for his encouragement and companionship during the 1972 field season, TABLE OF CONTENTS Pale I. INTRODUCTION 1 Location and Accessibility 1 Topography, Climate, and Vegetation 1 Previous Studies Purposes of the Study 4 Methods 4 II. REGIONAL SETTING 6 III. ROCK TYPES 9 Grassy Slope Phyllites 9 Field Description 9 Lithology and Petrography 11 Origin and Depositional Environment 13 Age and Correlation 15 Seven Devils Volcanics 16 Field Description 16 Lithology and Petrography 17 Origin and Depositional Environment 19 Age and Correlation 19 Sturgill Peak Rhyolite Welded Tuff 20 Field Description 21 Lithology and Petrography 22 Chemistry 25 Origin and Depositional Environment 25 Age and Correlation 27 Mann Creek Slates 27 Field Description 28 Lithology and Petrography 29 Origin and Depositional Environment 33 Age and Correlation 34 Serpentinite 34 Field Description 35 Lithology and Petrography 35 Origin 36 Age and Correlation 36 Page Sturgill Peak Intrusive Complex 37 Hybrid Gabbro 38 Field Description 38 Litho logy and Petrography 39 Chemistry 42 Quartz Diorite 44 Field Description 44 Litho logy and Petrography 45 Chemistry 47 Granodiorite 49 Field Description 49 Litho logy and Petrography 50 Chemistry 52 Dikes 56 Age and Mode of Emplacement 58 Columbia River Basalt 60 Picture Gorge Basalt 60 Field Description 60 Litho logy and Petrography 61 Yakima Basalt 63 Field Description 63 Litho logy and Petrography 63 Dikes 64 Origin 65 Age and Correlation 65 Quarternary Unconsolidated Deposits 66 IV,STRUCTURE 67 Regional Structure 67 Local Structure 69 V.METAMORPHISM 76 Regional Metamorphism 76 Contact Metamorphism 77 Dislocation Metamorphism 79 VI,ECONOMIC GEOLOGY 80 Gold Prospect 80 Skarn Prospect 81 Rhyolite Prospect 81 Serpentinite "Prospect" 82 Page East Fork of Mann Creek Prospect 83 Gabbr o Prospect 84 Economic Potential 84 VII,GEOMORPHOLOGY 86 VIII PLATE TECTONICS 88 IX, GEOLOGIC SUMMARY 91 BIBLIOGRAPHY 94 LIST OF FIGURES Figure Page 1 Index map of the thesis area. 3 2 Geologic column of the Sturgill Peak area. 10 3 View of the grassy slope phyllites. 12 4 Hand specimen of a typical phyllite of the phyllitic member. 12 5 Outcrop of the limestone member of the grassy slope phyllites. 14 6 Photomicrograph of a microperthitic ortho- clase crystal. 24 7 Hand specimen of the rhyolite welded tuff. 24 8 Outcrop of the conglomeratic schist. 31 9 Photomicrograph of a volcanic rock fragment in the conglomeratic schist. 31 10 Hand specimen of the conglomeratic schist. 32 11 Photomicrograph of the conglomeratic schist. 32 12 Photomicrograph of the hybrid gabbro. 40 13 Comparison of altered and unaltered granodiorite samples. 53 14 Columnar jointing in Picture Gorge Basalt. 62 15 Reconnaissance geologic sketch map of the northern Hitt Mountains. 68 16 Picture Gorge Basalt overlapping the quartz diorite. 74 Figure Page. 17 Photomicrograph of the cummingtonite- cordierite-biotite hornfels. 78 18 Epidote-orthoclase veins cutting the hybrid gabbro. 85 LIST OF TABLES Table ?age 1 Modal, chemical, and trace element analyses of rhyolite welded tuff sample SS-74-A. 26 2 Modal, chemical, and trace element analyses of hybrid gabbro sample SS- 1. 43 3 Modal, chemical, and trace element analyses of quartz diorite sample SS-13. 48 4 Modal analyses of unaltered, and altered granodiorite samples. 54 5 Modal, chemical, and trace element analyses of granodiorite sample SS-60-A. 55 6 Trace element analyses for copper of stream sediment samples collected on the East Fork of Mann Creek. 83 LIST OF PLATES Plate Page 1 Geologic map of the Sturgill Peak area, Washington County, Idaho. in pocket GEOLOGY OF THE STURGILL PEAK AREA, WASHINGTON COUNTY, IDAHO L INTRODUCTION Location and Accessibility The thesis area comprises 16square miles in the Hitt Mountains, Washington County, Idaho.The four corners of the areaare sec.14, 17, 32, and 35 of T. 15 N., R. 5 W., Sturgill Peak Quadrangle, Idaho.The Cuddy Mountains and Seven Devils Mountainsare approxi- mately 7 and 40 miles