STATE OF OREGON DEPARTMENT OF GEOLOGY AND MI N ERAL INDUSTRIES 1069 State Office Building Portland, Oregon 97201 BULLETIN 72 BEDROCK GEOLOGY OF THE MITCHELL QUADRANGLE, WHEELER COUNTY, OREGON Keith F. Oles and Harold E. En lows Department of Geology, Oregon State University Corvallis, Oregon 1971 GOVERNING BOA RD Fayette I. Bristol, Rogue River, Chairman R. W. deWeese, Portland William E. Miller, Bend STATE GEOLOGIST R. E. Corcoran FOREWORD The Mitchell area of centra l Oregon has been well known, both to geologists and pa leontologists, for more than l 00 years . Dr . Thomas Condon, the first state geologist, coll ected fossi I rema ins from the Cretaceous marine sedimentary rocks back in the 1860's. In more recent years students and professors from Oregon State University have used th is region as an oui·door classroom in geology because of the wide variety of geologic features that can be studied here at first hand. This report is the culmination of more than ten years field work by the authors while instructing at the summer geology field camp. The information contained in the bulletin will be of particular interest to the petroleum industry because of the excellent descriptions of the Cretaceous sediments that form a broad northeast-trendi ng belt through the center of the quadrangle. The late Mesozoic sedimentary section of centra l Oregon should be a major target for oil exp loration because of the environmenta l conditions under which it was deposited . This part of Oregon also has great scenic beauty, due in no small part to the varicolored volcanic formations that cover much of the land . Pa inted Hills Sta te Park lies along Bridge Creek in the western edge of the Mitchell quadrangle, and the famous John Day fossi I beds are a few miles beyond the eastern border . The professional geologist or pal eontologist will find this comprehensive report is a valuable source of basicin formation on the stratigraphy and structure of centra l Oregon. The interested layma n and recreationist will be able to learn more about the earth history of this region and the geologic develop­ ment of the present topographic features . R • E. Corcoran Oregon State Geologist ii CONT ENTS Foreword . ii List of illustrations v Introduction . Location, genera l discussion and significance of the area Previous work . 4 Methods of study 4 Acknowledgments and dedication 4 Stratigra phy . 5 Permian metasediments (Pms) 5 Cretaceous roc ks (K) . 5 Hudspeth and Gable Creek Formations (Kh and Kgc) 5 Outcrop area • 8 Petrography. 11 Congl omerates 11 Sandstones 11 Mudstones . 11 Age of Cretaceous rocks 11 Tertiary rocks (T) • 13 Clarno Grou p (T c) 13 Lower Clarno Formation (T cvb, T cf1 , T ct1 ) 13 Lower Clarno flows (Tcf1 ) 13 Lower Clarno tuffaceous sediments (Tct1 ) 14 Lower Clarno volcanic breccia (Tcvb) • 16 Upper Clarno Formation (Tcva, Tcf2, Tcmf, Tct2). 17 Upper Clarno mudflows (T emf) . 17 Upper Clarno tuffaceous sediments (Tct2) 21 Upper Clarno flows (Tcf2) 23 Upper Clarno vent aggl omerate (Tcva) . 25 Petrography of the Clarno flows 25 Classification . 25 Texture and mineralogy • 25 Petrography of the Clarno ignimbrite 28 John Day Formation (Tjd) • 34 Columbia River Group (T cr) 40 Petrography. 43 Rattlesna ke ignimbrite (Tr) • 43 Petrography . 44 Petrography of major intrusions 45 White Butte andesite (Tia) 48 Andesite dikes, sills and irregular intrusions (Tia) 48 iii Stratigraphy -Continued Tertiary rocks (T) - Continued Petrography of major intrusions - Continued Tote-Goat Hill andesite (Tia). 48 Basalt or diabase dikes (li b) . 48 Marshall Butte melabasa lt (Tib) 51 Tony Butte dacite (Tid). 51 Sargent Butte leucorhyolite (Tir) 51 Structure . 54 Introduction 54 Mitchell fa ult. 54 Geologic history . 56 Introduction 56 0 I dest orogeny 56 Cretaceous deposition 57 Lower Clarno events . 58 White Butte intrusion 58 Laramide orogeny . 58 Keyes Mountain volcanic episode 59 Mitchell fau lt inception 59 La tera l movement along the Mitchell fault 59 Renewed orogeny 60 Columbia River Group 60 Renewed fo lding 60 Recent events 61 References cited . 61 iv ILLUS TRATIONS Plates I. Geologic ma p and sections of the Mitchell quadrangle Pocket Figures 1. Panorama of Picture Gorge basa lts of Sutton Mountain with pediments devel oped on the John Day tuffaceous sediments . 2 2. Panorama of Keyes Creek Va lley and the summits of Keyes Mountain, an Upper Clarno volcano 3 3. Typica l Hudspeth mudstones interbedded with thin, resistant sandstones 7 4. Cliff-forming member of Gable Creek tongue 3 at Mitchell 7 5. Schematic diagram of intertonguing Hudspeth and Gable Creek Formations 8 6. Panorama looking southeast across Gable Creek Va lley at staircase of southeast dipping Gable Creek cuestas and intervening Hudspeth va lleys . 9 7. Sharp contact between Gable Creek pebble conglomerate and sandstone 10 8. Typica l Gable Creek conglomerate and pebbly sa ndstone 10 9. Poor sorting in Gable Creek conglomerate unit 11 10. Mazama ash layer exposed in dissected older alluvium of Meyers Canyon 1 2 11. Varicolored Lower Clarno tuffaceous sediments capped by Lower Clarno lava flows. North rim, Meyers Ca nyon . 14 12. Upper Clarno lava flows and mudflows dipping west off Keyes Mountain . 15 13. Huge boulder of Cretaceous conglomerate carried by an Upper Clarno mudflow 16 14. Upper Clarno topography: an andesite flow overlies a mudflow 18 15. Stream-dissected Upper Clarno mudflow 20 16. Textural detail in an Upper Clarno mudflow . 20 17. Photomicrograph of Upper Clarno volcanic wacke 22 18. Photomicrograph of Lower Clarno flow: porphyritic with pi lotaxitic groundmass 22 19a. Photomicrograph of Lower Clarno diabasic flow . 24 b. Same as Figure 19a, nicols crossed 24 20. Photomicrograph of Upper Clarno flow from Keyes Mountain 26 21 . Photomicrograph of Upper Clarno flow from Keyes Mountain with resorbed hornblende 26 22a . Photomicrograph of Upper Clarno pil otaxitic flow from Keyes Mountain 27 b. Same as Figure 22a , nicols crossed . 27 23a . Photomicrograph of a flattened and devitrifi ed pumice fragment in the Clarno ignimbrite 29 b. Same as Figure 23a , nicols crossed . 31 24 . Photomicrograph of phenocrysts, vitroclastic matrix and eutaxitic texture in Clarno ignimbrite . 31 25 . Photomicrograph of sanidine phenocryst partially replaced by calcite in Clarno ignimbrite . 31 26 . Triangular diagram showing quartz, orthoclase and albite for central Oregon ignimbrites 33 27. Triangular diagram showing quartz, albite and anorthite for centra l Oregon ignimbrites . 33 28 . Panorama of Sutton Mountain showing Picture Gorge basa lts overlying John Day tuffaceous sediments . 35 29. John Day tuffaceous claystones, lower red member . 36 30. Mudcrac ked John Day tuffaceous sediments 36 31a. Photomicrograph of Middle John Day ignimbrite showing axiolites and spherulites 38 b. Same as Figure 31a, nicols crossed 38 32a . Photomicrograph of Middle John Day ignimbrite showing axiolites and spherulites 39 b. Same as Figure 32a , nicols crossed 39 33. Looking north down Girds Creek at Picture Gorge basa lt 41 34 . Panorama of Picture Gorge basa lts of Sutton Mou ntain above John Day Forma tion 42 v Illustrations -Continued 35 . Rattlesna ke ignimbrite ledge-former and caprock surmounti ng mesa east of Marshall Creek 44 36 . Photomicrograph of basal, poorly welded lower two feet of Rattlesna ke ignimbrite 46 37 . Photomicrograph of middle, firmly welded Rattlesnake ignimbrite 46 38 . Panorama of Meyers Canyon, White Butte intrusion and Ochoco front 47 39. Clarno andesite dike cutting Hudspeth mudstone and siltstone along the Johnson Creek roa d . 49 40. Contact of Nelson Creek si ll and Hudspeth mudstone • 49 41 . Mitchell Roc k, an irregular intrusion of Clarno andesite into Hudspeth mudstone 50 42. Irregular contact of Clarno andesite intruding Hudspeth mudstone 50 43. Panorama of Keyes Creek Va lley and Marshall Butte 52 44 . Photomicrograph of Marshall Butte melabasa lt . 52 45a . Photomicrograph of Sargent Butte leucorhyol ite 53 b. Same as Figure 45a, nicols crossed . 53 46 . Panorama showing Bai ley Butte sil l, axial region of Mitchell anticline, and Sutton Mountain 55 Tables l. Stratigraphic sequence in the Mitchell quadrangle 6 2. Average modal analyses on Lower and Upper Clarno flows 28 3. Chemical analyses and derived normative-compositions for Clarno flows, Nockold's average andesite and McBirney's average of 29 calc-alkaline andesites from con­ tinental margins 30 4. Chemica l analyses and normative compositions of some central Oregon ignimbrites 32 5. Chemica l analyses and normative compositions of the major intrusions . 45 vi BEDROCK GEOLOGY OF THE MITCHELL QUADRANGLE, WHEELER COUNT� OREGON Keith F. Oles (1) and Harold E. Enlows(1) Department of Geology, Oregon State University IN T RODUCTION LOCATION, GENERAL DISCUSSION AND SIGNIFICANC E OF THE AREA The Mitchell quadrangle of Wheeler County, Oregon lies on th e north flank of the east-west­ trending Ochoco Mountains . Thornbury (1965) classified th is area <;IS lying within the Blue Mountains section, Central H igh lands Subprovince, of th e Columbia Intermontane Province. The quadrangle, com­ prising approximately 215 square miles, is one of great geologic interest, possessing a greater variety of rock types and ages than most areas of similar size within th e state of Oregon . The Permian, Cretaceous Tertiary, and Quaternary systems are represented; marine and continental sedimentary rocks of many types are present; metamorphic rocks crop out in the core of the area; pistons, sills, dikes, and irregular intru­ sions are records of plutonic activity; lava flows and tuffs record volcanic episodes. Major fo lds, faults, and unconformities delineate recurring tectonic events . The quadrangle encompasses a major topograph ic basin draining toward the northwest with most streams flowing into Bridge Creek (Figure 1), a major tri butary of the John Day River, or directly into the John Day River at th e northern border . This basin is flanked by the Ochoco Mountains escarpment on the south , the Keyes Mountain complex on the southeast, and the huge syncline of Sutton and H orse Mountains , which dominates th e topography on the north .