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Bailey Mark Hopkins 1980 AN ABSTRACT OF THE THESIS OF MARK H. BAILEY for the degree of Master of Science in Geology presented on June 7, 1979 Title: BEDROCK GEOLOGY OF WESTERN TAYLOR PARK, GUNNISON COUNTY, COLORADO Signature redacted for privacy. Abstract approved: I ._&V_j - Dr. Robert D. Lawrence The western Taylor Park area covers about 65 square miles on the western flank of the Sawatch Range. It is underlain chiefly by Precambrian rocks, partiallycovered on the east by glacial debris andoverlapped on the west and south by Paleozoic sedimentary rocks. The Precambrian rocks include a group ofmetasedimen- tary rocks of an undetermined age greaterthan 1700 m.y. The metasedimentary rocks include five mineralogicallydis- tinct units, quartzite, quartz-muscovite schist, quartz- epidote schist, quartz-plagioclase-biotite schist,and quartz-biotite-microcline-sillimaflite schist. The higher grade sillimanite-bearing biotite schists arerestricted to the western half of the map area and closelyassociated with the Forest Hill granitic rocks. The metasedimentary rocks were intruded twiceduring Precambrian time. The oldest intrusive event resulted in syntectonic emplacement of granodiorite and biotite tona- lite rocks. These rocks correlate with the 1700 m.y.old Denny Creek granodiorite gneiss and Kroenkegranodiorite which are mapped and informally named in the adjoining Mount Harvard quadrangle. The second, post-tectonic, igneous eventproduced granitic rocks that are dated at 1030 m.y. byRb-Sr meth- ods. These rocks include two comagmatic graniticphases, the Forest Hill porphyritic granite and the ForestHill cataclastic granite. A third phase, the Forest Hill horn- blende granodiorite, intrudes the porphyritic graniteand, therefore, is younger than the granitic phases. The 1030 m.y. age is very significant because theonly other 1000 m.y. old intrusive rocks in Colorado(the Pikes Peak batholith) are restricted to thethe FrontFront Range.Range. Paleozoic sedimentary rocks are exposed along the western and southern margins of the map area. They range in age fromfrom CairbrianCambrian throughthrough Permian.Permian. Tertiary igneous rocks, occurring as plugs, dikes andand flows, crop out locally. Two major episodes of Tertiary activity occurred in the region, one during the Laramide orogenic event and the other during Oligocene time. The Tertiary rocks in the map area were formed during the later igneous event. The principle structural features of the area include: northeast-trending Precambrian shear zones and faults, northwest-trending folding and faulting, (3) east-west-east-west trending tear faults in the Paleozoic rocks along the wes- tern margin of the map area. Although the northeast and northwest-trending structural trends originated during the Precambrian, reactivation along these zones of weakness probably occurred into mid-Tertiary time. Bedrock Geology of Western Taylor Park, Gunnison County, Colorado by Mark Hopkins Bailey A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed June 7, 1979 Commencement June 1980 ACKOWLDG{ENTS The writer wishes to express a sincere thanks to Exxon Company U.S.A. for the material and financial support re- ceived during this investigation. I would like to give special thanks to Art Pansze, Bob Woodfill and ri.Gassaway Brown who were instrumentalInstrumental in initiating and supporting tijSt1115 study.study. I thank my advisor Dr. iobert D. Lawrence and faculty raembersraernbers Dr. Dr. Cyrus Cyrus W. W. Field, Field, Dr Dr. HaroldHarold E.E. Enlows,Enlows, and Dr. idwarddward ii.11.Taylor for their constructive criticism during the final preparation of this report. I would also like to thank my family and friends for tkleirtheir iaoraliaoral supportsupport througtioutthrougnout this endeavor. TABLE OF CONTENTS Page INTRODUCTION 1 Location and Access 1 Purpose and Methods of Study 2 Previous Work 6 Geologic Setting 8 PRECAMBRIAN ROCKS AND THEIR NOMENCLATURE 12 METASEDIMENTARY ROCKS OLDER THAN 1700 M. Y. 15 General Statement 15 Quartzite 15 Field Description 15 Petrography 16 Quartz-Muscovite Schist 17 Field Description 17 Petrography 17 Quartz-Epidote Schist 21 Field Description 21 Petrography 22 Biotite Schist 24 Quartz-Plagioclase-BiotiteQuar tz-Plagioclase-Biotite SchisSchist t 25 Field Description 25 Petrography 25 Quartz_Biotite-Microcline-SillimafliteQuartz_Blotite-Microcline-Sillimanite Schist.Schist. .. 28 Field Description 28 Petrography 30 MIGMATITES 37 INTRUSIVE ROCKS 39 Denny Creek Granodiorite GneissGneiss 40 Field Description 40 Petrography 40 Table of Contents (Cont'd.) Page Kroenke Granodiorite 48 Age and Correlation 49 Biotite Tonalite 50 Field Description 50. Petrography 50 Age and Correlation 56 Forest Hill Granitic Rocks 57 Forest Hill Porphyritic GraniteGranite 61 Field Description 61 Petrography 66 Forest Hill Cataclastic Granite 75 Field Description 75 Petrography 77 Forest Hill Hornblende GranodioriteGranodiorite 84 Field Description 84 Petrography 84 PEGMATITES 89 CHEMISTRY 91 PALEOZOIC STRATIGRAPHY 99 General Statement 99 Cambrian 99 Sawatch Quartzite 99 Ordovician 100 Manitou Dolomite and Fremont Limestone . 100 Devonian 101 Chaffee Formation 101 Mississippian 101 Leadville Limestone 101 Pennsylvanian System 102 Table of Contents (Cont'd.) Page TERTIARY IGNEOUS ROCKS 1U3 General Statement 103 Local Tertiary Rocks 103 STRUCTURE 105 Regional Structure 105 Local Structure 106 Folding 109 Faulting 109 Foliation 110 PRECAMBRIAN GEOLOGIC HISTORY 112 REFERENCES 114 APPENDIX I 120 APPENDIX II 122 APPENDIX III 123 ILLUSTRATIONSILLUSTRAT IONS Figures Page Index Map of Colorado 3 Panorama of the eastern half of the thesis area 4 Western half of the thesisthesis areaarea 5 Prominent geomorphic features of Colorado . 9 Photomicrograph of quartz-muscovite schist. 18 Photomicrograph of quartz-muscovite schist. 19 Photomicrograph of quartz-epidote schist 23 Photomicrograph of quartz-plagioclase-biotitequartz-plagioclase-biOtite schist 27 Xenoliths of biotite schist in Forest Hill granite 29 Photomicrograph of quartz-biotite-microcline-quartz-biotite-microcline sillimanite schist 31 Photomicrograph of quartz-biotite-sillimanite schist 32 Photomicrograph of contact between quartz-biotite schist and quartz-biotite-sillimanite schist. 35 Outcrop of Denny Creek granodiorite gneiss and migmatite 38 Q-A-P diagram for Denny Creek granodiorite gneiss 42 Outcrop of Denny Creek granodiorite gneiss. 44 Hand specimen of Denny Creek granodiorite gneiss.gneisS. 44 Photomicrograph of highly foliated Denny Creek gneiss 45 Photomicrograph of moderately foliated Denny Creek gneiss 46 Photomicrograph of weakly foliated Denny Creek gneiss 46 Q-A-P diagram for Biotite Tonalite 52 Photomicrograph of typical biotite tonalite . 54 Rb-Sr isochron plot of Forest Hill granitic rocksrocks 60 Outcrop of Forest Hill porphyritic granite. 62 Close up of Forest Hill porphyritic granite . 62 Roof pendant ofof quartz-biotite-sillimanitequartz-biotitesillimanite schist overlying Forest Hill porphyritic granite . 64 Illustrations (Cont'd.) Figures Page Jointing in Forest Hill porphyritic granite . 65 Q-A-P diagram for ForestForest HillHill porphyriticporphyritic granite 68 Photomicrograph of ForestHillporphyritic granite 70 PhotomicrographPhotoinicrograph ofof ForestForestHillprophyritic granite 72 Photomicrograph of ForestHillporphyritic granite 72 3la. ForestHillcataclastic granite dike intrudingIntruding ForestHillporphyritic granite 76 3lb. Close up of contact betweenbetween ForestForestHillcataclas- tic granite dike and ForestHillporphyritic granite 76 Q-A-P diagram for Forest HillHill cataclasticcataclastic granitegranite 80 Photomicrograph showing cataclastic texture of Forest Hill cataclasticcataclastic granitegranite 83 Photomicrograph of ForestHillcataclastic gran- ite 83 Photomicrograph of Forest Hill hornblende grano- diorite 86 Pegmatite outcrop in road cut 90 Close up of pegmatite outcropoutcrop 90 A-F-M plot of recalculated major oxides for ig- neous rocks 95 Harker variation diagram of major oxides versus silica 97 Regional structure 107 Looking north along Taylor Park graben l0ilOis Sheared ForestHillporphyritic granite inin majormajor shear zone 111 Close up of sheared granite 111 Tables Classification of Precambrian rocks 14 Mineralogy of Metasedimentary rocks 36 Modal analyses of Denny Creek granodloritegranodiorite gneiss 41 Illustrations (Cont'd.) Tables Page Modal analyses of Biotite Tonalite 51 Rb-Sr isotope data for Forest Hill granitic rocks 59 Modal analyses of Forest Hill porphyritic granite 6757 Modal analyses of Forest Hill cataclastic granite 78 Major element analyses 92 Major element analyses 93 Plates Geologic map, with cross section, of the thesis in area pocket Sample location map in pocket Bedrock Geology of Western Taylor Park, Gunnison County, Colorado INTRODUCTION Taylor Park is a wide, open, and comparatively level valley situated on the western flank of the Sawatch Range. The Sawatch Range, a north northwest-trending anticlinor- ium 90 miles long and 40 miles wide, is the westernmost range of the four main north-south trendingmountain ranges that compose the Rocky Mountains of Colorado. The irregu- lar boundaries of the Sawatch Range are determined by the contact of Paleozoic sedimentarysedimentary rocksrocks againstagainst thethe Precam- brian core along the west side, by the Arkansas Valley (northern extension ofof thethe RioRio GrandeGrande Rift)Rift) onon thethe easteast side and topographic termination of the northernand
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