Geology of the Central Pueblo Mountains Harney County, Oregon
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AN ABSTRACT OF THE THESIS OF DENNIS BRIAN TOWER for the MASTER OF SCIENCE (Name) (Degree) in GEOLOGY presented on (Major) (Date) Title: GEOLOGY OF THE CENTRAL PUEBLO MOUNTAINS HARNEY COUNTY, OREGON Redacted for Privacy Abstract approved: Dr. Harold E. En lows The thesis area consists of 44 square miles in southeast Harney County, Oregon. The Pueblo Mountains are located in the northwest part of the Basin and Range structural province, and are characterized by normal faults of large displacement that produced the tilted fault block mountain range. The oldest rocks exposed are mafic lava flows, and marine sedimentary rocks, which were subjected to regional metamorphism probably during Jurassic time. A thin sequence of late Oligocene to early Miocene silicic volcanic rocks, arkosic sedimentary rocks and andesite unconformably overlies the metamorphic sequence.These rocks are disconformably overlain by a thick sequence of Miocene basalt flows.Included in the basalt sequence is a unit of silicic vol- canic rocks. Overlying the basalt sequence with slight angular un- conformity is a sequence of tuffaceous sedimentary rocks,tsillari, and welded tuff.A fan.glomerate unit cropsout in the northeast corner of the areaQuaternary depositsinclude lacustrine, alluvial fan, and dune deposits. Major normal faulting,which began in late Miocenetime, has continued into the present. $ Geology of the Central PuebloMountains, Harney County, Oregon by Dennis Brian Tower A THESIS submitted to Oregon State University in partial fulfillment of the requirements forthe degree of Master of Science June 1972 APPROVED: Redacted for Privacy Associate Professor of Geology in charge of major Redacted for Privacy Acting Chairman of theDepartment of Geology Redacted for Privacy Dean of Graduate School Date thesis is presented Typed by Donna L. Olsonfor Dennis Brian Tower ACKNOWLEDGEMENTS The writer extends special thanks toDr. Harold E. En lows for suggesting the thesis problem, forassistance throughout preparation of the thesis, and for criticallyreading the manuscript.Appreciation is extended to Dr. Cyrus W.Field and Dr. William H. Taubeneckfor the many helpful suggestions theyoffered, and for critically reading the manuscript.Appreciation is also extended to Dr.Edward M. Taylor for providing chemical analyses. Special thanks is extended to mywife, Sue, for her assistance in typing the manuscript, andfor her moral support during the preparation of the thesis. The writer extends thanks tohis parents, Mr. and Mrs. W. R. Tower, for the loan of theirpickup during the summer of1970 while the writer conducted the fieldinvestigation. Thanks must also be extended toDon and Esther Peters of Denio, Nevada, for their hospitalityduring the summer of 1970. TABLE OF CONTENTS Page 1 INTRODUCTION Location and Accessibility 1 1 Topography and Drainage 4 Climate and Vegetation Investigation 5 Purpose and Methods of 6 Previous Investigations 7 Terminology 7 Geologic Setting 8 STRATIGR APHY Rocks 11 Pre-Tertiary Metamorphic 11 Field Relationship 14 Lithology and Petrography 14 Unit One 19 Unit Two 20 Unit Three 21 Classification and Origin 21 Age and Correlation 23 Late Tertiary Volcanicand Sedimentary Rocks ( ?) 23 Pike Creek Formation 23 Field Relationship 27 Lithology and Petrography 27 Basal Conglomerate 28 Arkosic Wacke 28 Ash Flow Tuff 29 Tuff Breccia 29 Andesite 30 Origin 31 Correlation and Age 34 Steens Basalt 34 Field Relationship 39 Lithology and Petrography 39 Porphyritic Basalt 43 Non-Porphyritic Basalt 43 Basalt Dikes 44 Ash Flow Tuff 45 Ash Fall Tuff 45 Origin Page Correlation and Age 45 Rhyolite Unit 46 Field Relationship 46 Litho logy and Petrography 51 Rhyolite and Vitrophyre 51 Tuffaceous Sedimentary Rocks 52 Origin 53 Correlation and Age 53 Sedimentary-Pyroclastic Unit 57 Field Relationship 57 Litho logy and Petrography 58 Tuffaceous Sedimentary Rocks 58 Sillar Sequence 59 Welded Tuff Sequence 60 Origin 62 Correlation and Age 62 Rhyolite Tuff Sequence at RedPoint 63 Field Relationship 63 Litho logy and Petrography 66 Air Fall Tuff 66 Ash Flow Tuff (Unit One) 66 Ash Flow Tuff (Unit Two) 68 Origin 71 Correlation and Age 71 Turn-Turn Conglomerate 72 Field Relationship 72 Litho logy 73 Origin 75 Correlation and Age 75 Quaternary Deposits 76 77 STRUCTURE 79 GEOMORPHOLOGY 82 ECONOMIC GEOLOGY 83 GEOLOGIC HISTORY 85 BIBLIOGRAPHY 89 APPENDIX LIST OF FIGURES Page Figure 2 1. Regional location of thesis area. 2. View south along the eastridge from top of Pueblo Mountain, with eastdipping pre- Tertiary metamorphicrocks exposed along ridge crest. 13 3. Photomicrograph showingoriginal andesine phenocryst containingunaltered andesine surrounded by sericitizedand epidotized 16 albite. 4. Light gray quartziteinterbedded with green- stones on the eastside of Pueblo Mountain. 17 the ridge crest north 5. An outcrop of schist on 22 of Van Horn Canyon. exposed on north side 6. Basal conglomerate 26 of Cottonwood Creek,north of thesis area. 7. View across Van HornCreek showing the Pike Creek Formation(?). 32 showing thick 8. View up Van Horn Basin 35 sequence of SteensBasalt flows. 330 W., 9. Prominent basalt dikewhich trends N. for 3.5 miles acrossthe thesis area. 38 10. Porphyritic Steens Basaltcontaining pheno- crysts of plagioclase upto 6 cm in length. 40 forming 11. View toward thenorth showing cliff rhyolite in background,thin basalt flows light colored rhyolite exposed in cirque, and 48 in foreground. lithophysae up to 15 cm 12, Vitrophyre exhibiting 50 in diameter. Figure Page 13. Crudely stratified tuffaceoussedimentary rocks cropping out along the top ofthe west ridge. 54 14. View toward the northeast showingreddish- brown ash flow tuff at Red Point, 65 15. Photomicrograph of upper ash flow tuffunit showing cavity in elongate pumicefragment lined with vapor phase minerals,probably potassium feldspar and cristobalite,and light brown opal, 69 16. Photomicrographof upper welded tuff unit. 70 17, Typical outcrop of Turn-TurnConglomerate showing its chaotic, unsorted nature. 74 18. View toward the east showingsmall cirque eroded in the Steens Basalt sequence onthe dip slope of the west ridge. 80 LIST OF TABLES Page Table 9 1. Rock units found in thesis area. 2. Chemical analyses of samplesfrom volcanic sequence. 41 3. C.I. P. W. norms calculatedfrom the chemical analyses in Table 2. 42 Modal analyses of samplesfrom the volcanic 4. 42 sequence. Modal analyses of samplesfrom the rhyolite 5. 52 unit. 6. Comparison of chemicalanalyses of rhyolites from Virgin Valley,Nevada, the southernPueblo Mountains, and the centralPueblo Mountains. 56 7. C . I. P. W. normscalculated from chemical analyses in Table 6. 56 8. Modal analyses of samplesfrom welded tuff sequence on the westside of the Pueblo 61 Mountains. Modal analyses of weldedtuffs from Red 9. 67 Point. LIST OF PLATES Page Plate 10 1 Correlation chart. 2 Geologic map of the centralPueblo Mountains, Harney County, Oregon. Folder GEOLOGY OF THE CENTRALPUEBLO MOUNTAINS, HARNEY COUNTY, OREGON INTRODUCTION Location and Accessibility The Pueblo Mountains arein the extreme north-centralpart of Humboldt County Nevada, andin south-central HarneyCounty, Oregon. The area discussed in thisreport includes approximately44 square miles in T. 40 S., R.34 E., and R. 35 E. of Oregon(Figure 1).The town of Denio, Nevadais 3.5 miles south of thethesis area. The area may be reachedby a graded gravel roadparalleling the eastern front of thePueblo Mountains.The central part is acces- sible only by off-highwayvehicles along an unimproveddirt road lead- ing up Arizona Creek intoVan Horn Basin. Anunimproved road up Stonehouse Creek from theOregon End Ranch in BogHot Valley, pro- vides the only access to the westernpart of the area. Topography and Drainage The Pueblo Mountains are asouthern extension ofSteens Moun- the east tain to the north.The mountains areabruptly terminated on by a steep fault scarp,and to the west long dipslopes formed on 2 ,--------51 OREGON J Harney County Thesis Area Humboldt County H, CALIFORNIA NEVADA Figure 1.Location of the thesis area. 3 resistant basalt flows give the mountains anasymmetrical profile. The mountains have a maximum east-westwidth of nine miles in the thesis area and narrow rapidly southof the area. The maximum relief is approximately4, 525 feet.Pueblo Mountain is the highest point with anelevation of 8, 725 feet.Pueblo Valley has an average elevation ofabout 4, 200 feet. The Pueblo Mountains consist of two majorridges separated by a high median valley.The west ridge trends north-south andhas an average elevation ofapproximately 8, 000 feet.The highest point on the west ridge is 8, 545 feet.The east ridge also trends north-south but has a much more rugged,variable terrain.It is dominated by the bulky mass of Pueblo Mountain. The drainage system is characterizedby steep, short, inter- mittent streams that flow only duringspring run-off or during summer thundershowers.The drainages terminate in alluvialfans at the foot of the mountains. The larger intermittent streamsof the western Pueblo Moun- tains are tributaries of RinconCreek, which drains south intoContin- ental Lake, an alkaline playa atthe southern tip of the Pueblo Range. The only perennial stream drainingthe west ridge is Stonehouse Creek. The eastern Pueblo Mountains aredrained by many steep, short streams which empty intothe structural basin occupiedby Pueblo 4 Valley, and ultimately into AlvordLake, and alkaline playa 20 miles to the north. Van Horn Creek, which drainsVan Horn Basin, is the most prominent perennial stream in the easternPueblo Mountains.The