Petrology and Sedimentation of Cretaceous and Eocene Rocks in the Medford-Ashland Region, Southwestern Oregon
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AN ABSTRACT OF THE THESIS OF Brian Keith McKnight of the Doctor of Philosophy (Name) (Degree) in Geology presented on ,u 1 ,IT76 (Major) (Date) Title: Petrology and Sedimentation of Cretaceous and Eocene Rocks in the Medford-Ashland Region. Southwestern Oregon Abstract approved: Redacted for privacy Harold E. Enlows Late Cretaceous and late Eocene rocks over 12,000feet thick are exposed in Bear Valley between the Klamath and Cascade Mountains.Detailed field and laboratory examinations of these rocks were undertaken todeter- mine the conditions under which they were formed. Late Cretaceous rocks of the predominantly marineHornbrook Formation consist of 1,000 feet of sandstone overlain by 3,000 feetof mudstone. Fossils indicate a Cenomanian to late Turonian orpossibly Maestrichian age. The Cretaceous sandstones are arkosic to feldspathicarenities and wackes. The basal sandstones strongly reflect the local bedrocklithology. Higher in the formation, the local bedrock has nocontrol over the mineral composition. A gradual change from a plutonic source to ametasedimentary and metavolcanic source is suggested by the verticalchange in mineralogy of the sandstones. Animal borings, ripple marks, cross-bedding,and other sedimentary structures as well as the shallow waterfauna suggest that these sandstones were deposited on the continentalshelf. Above the 300 foot thick basal sandstone unit, shale and thin coal seamssuggest temporary paralic sedimentation after the initial transgression. A deepening of the sedimentary basin or an increase in the distancefrom the source of the sediment is indicated by the overlying 3,000feet of mudstone. The mudstone has numerous sandstone interbeds and is locally richly fossiliferous. Sedimentary structures and a progressive decrease in the age of the basal sandstone unit from north to south suggestthat the Hornbrook Formation was deposited in a sea which trangressedtoward the southwest. Overlying the Cretaceous Hornbrook Formation are nearly8,500 feet of late Eocene sedimentary rocks referred to in this reportinformally as the Payne Cliffs Formation. The Payne Cliffs Formation is composed primarily of sandstone with lesser amounts of conglomerate,shale, tuffaceous sedimentary rock, and coal. A progressive change in the com- position of the sandstone from the base to the top of theformation sug- gests a change from the metamorphic and plutonic sourcerocks of the Klamath Mountains to volcanic source recies of the earlyCascade Range. Most of the Payne Cliffs Formation, with itsebundant cress-bedding, cut-and-fill structures, and silicified logs, is clearly theresult of sedimentation by northward flowing streams. The elastic material was deposited in the Medford-Ashland area, at that time anextensive lowland situated between the rising highlands of the Klamathand Cascade provinces and draining toward the Pacific coastal plain 60 to 100miles to the north and northwest. Small lakes and swamps locally received organicand tuffaceous sediments. A subtropical flora flourished leaving manyplant remains, especially in the tuffaceous beds. PETROLOGY AND SEDIMENTATION OF CRETACEOUSAND EOCENE ROCKS IN THE MEDFORD-ASHLAND REGION, SOUTHWESTERN OREGON by BRIAN KEITH MCKNIGHT A THESIS submitted to OREGON STATE UNIVERSITY in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY June, 1971 APPROVED: Redacted for privacy MajcfProfessor and Head of Department of Geology in charge of major Redacted for privacy Dean of Graduate School Date thesis is presented Lam. ()) 70 Typed by Pamela Spaulding ACKNOWLEDGEMENTS The writer wishes to express his appreciation to the many individuals that have made a contribution to this study. To Dr. Harold E. Enlows, my major professor, a very special thank you is extended. I also wish to acknowledge Drs. John V. Byrne, David A. Bostwick, and Keith F. Oles, who reviewed the manuscript and added constructive criticism. Dr. Paul T. Robinson aided in interpreting the x-ray data. His helpful dis- cussions during the early part of the investigation are appreciated. Dr. David L. Jones, of the United States Geological Survey, identified fossil material collected during the field investigation. Financial support for laboratory expenses was provided through a grant from The Society of the Sigma Xi. The geology departments at Oregon State University and Wisconsin State University-Oshkosh assisted me in many ways. The able secretarial assistance of Pamela Spaulding has been invaluable. I would also like to thank my wife for her help during the preparation of this manuscript and for her encouragement, understanding, and patience. TABLE OF CONTENTS I. Introduction 1 Regional Geology 1 Previous Workers 6 II. Hornbrook Formation 8 Stratigraphy and Age Relationships 8 Sedimentary Structures 19 Cross-bedding 19 Ripple Marks 21 Sole Markings 23 Parting Lineation 25 Shale Clasts 25 Cut-and-Fill 27 Miscellaneous Structures 29 Directional Studies 32 Petrology 37 Sandstones 39 Siltstones, Mudstones, and Shales 56 Conglomerate 58 X-Ray Analyses 60 Provenance 64 Probable Transport Direction 64 Mineralogy of Source Area 66 Mineralogical Maturity 70 Environment of Deposition 71 Tectonics, Relief and Climate 76 78 III. Payne Cliffs Formation Stratigraphy and Age Relationships 78 Sedimentary Structures 87 Bedding 87 Cut-and-Fill 88 Graded Bedding 90 Imbrication 91 TABLE OF CONTENTS (Continued) Directional Studies 91 Petrology 94 Sandstones 95 Conglomerate 117 Tuff 120 X-Ray Analyses 121 Transport Direction 124 Composition of Source Rocks 125 Mineralogical Maturity 128 Environment of Deposition 129 Tectonics, Relief and Climate 133 IV. Summary of Geologic History 136 V. Bibliography 141 VI. Appendices 148 Appendix A 148 Appendix B 157 Appendix C 163 Appendix D 173 LIST OF ILLUSTRATIONS Figure Page 1 Index map 2 2 View of general topography 3 3 Stratigraphic column of Hornbrook Formation 9 4 Basal sandstone of Hornbrook Formation overlying Ashland granite 15 5 Pinch-out of shale lens 15 6 Abundant pelecypod shells in sandstone 17 7 Interbedded shale and mudstone 17 8 Hornbrook Formation mudstone 20 9 Interbedded shale and mudstone 20 10 Cross-bedded sandstone 22 11 Current ripple cross lamination 22 12 Current ripple marks 24 13 Load casts from Hornbrook Formation 24 14 Parting lineation in sandstone 26 15 Rounded shale clast in sandstone 26 16 Abundant shale clasts in sandstone 28 17 Burrowing in Cretaceous sand 28 18 Animal trails on bedding plane 30 19 Flame structures 30 20 Sandstone dike 33 21 Current rose of foreset beds 35 LIST OF ILLUSTRATIONS (Continued) Figure Page 22 Fabric diagram of cross-bedding 36 23 Modal analyses of Cretaceous arenites 47 24 Modal analyses of Cretaceous wackes 48 25 Photomicrograph showing biotite crinkled by compaction 51 26 Photomicrograph showing biotite expanded by calcite crystallization 53 27 Photomicrograph showing replacement of plagioclase by calcite 53 28 Photomicrograph showing siderite- ankerite rimming calcite 57 29 Vertical variation of six minerals in Hornbrook Formation rocks 69 30 Sketch of Payne Cliffs Formation roadcut 80 31 Stratigraphic column of a conglomerate unit 81 32 Interfingering sandstone and conglomerate 84 33 Large boulder in conglomerate bed 84 34 View of the Payne Cliffs 85 35 View of the Van Dike Cliffs 85 36 Honeycomb weathering pattern in sandstone 86 37 Fossil plant fragments in sandstone 86 38 Cross-bedding in Payne Cliffs Formation 89 LIST OF ILLUSTRATIONS (Continued) Figure Page 39 Cross-bedding in Payne Cliffs Formation 89 40 Current rose of foreset beds 93 41 Photomicrograph of sandstone 98 42 Photomicrograph of volcanic grains 98 43 Photomicrograph of volcanic grains 103 44 Photomicrograph of feldspar partially replaced by calcite 103 45 Modal analyses of Late Eocene arenites 107 46 Modal analyses of Late Eocene wackes 108 47 Modal analyses of Late Eocene sandstones 109 48 Photomicrograph of biotite in sandstone 111 49 Photomicrograph of grains replaced by biotite 114 50 Photomicrograph of grains replaced by chlorite 114 51 Photomicrograph of authigenic chlorite 116 52 Photomicrograph of authigenic chlorite and a zeolite 116 53 Vertical variation of six minerals in Payne Cliffs Formation rocks 127 LIST OF ILLUSTRATIONS(Continued) Page Table 11 1 Cretaceous fossils in auartz 41 2 Extinction and inclusions rocks 42 3 Plagioclase in Cretaceous 49 4 Textural features ofCretaceous rocks Pebble composition ofCretaceous 5 59 rocks 61 6 X-ray data of Cretaceousrocks 97 7 Extinction and inclusionsin quartz Textural features of LateEocene 8 110 rocks . Pebble composition ofLate Eocene 9 118 rocks 122 10 X-ray data of Late Eocenerocks types .... 128 11 Vertical change of sandstone Plate vicinity .. In Pocket 1 Geologic map of Medford and PETROLOGY AND SEDIMENTATION OF CRETACEOUS AND EOCENE ROCKS IN THE MEDFORD-ASHLAND REGION, SOUTHWESTERN OREGON INTRODUCTION Regional Geology The primary study area is within a broad valley situated between the Klamath Mountains to the west and the Cascade Mountains to the east (Figure 1); the towns of Medford and Ashland, Jackson County, Oregon, are in this valley. It is within the U. S. Geological Survey 15-minute Medford, Ashland, Lakecreek, and Talent topographic quadrangles. Additional studies were made to the north in the 15-minute Trail, Gold Hill, and Wimer topographic quadrangles. The rocks under investigation rise approximately one to two hundred feet up the west flank of the valley and several hundred feet up the east flank (Figure 2). They are relatively unresistant marine Cretaceous and nonmarine Eocene strata. These rocks lie between two physiographic and structural provinces, the Mesozoic Klamath Mts. and the Cenozoic Cascade Mts. (Figure 1). These provinces are separated in age by at least 75 million years. Rocks immediately underlying those investigated and extending farther to the west as part of the Klamath esaia =MP CO / I- / 1 I ).- 1 W I- / I W / I CDi I 1 Z --/ // i 2 I c7J/ CO / Q 1 -J Q/ Z I I I -J>/ I CC 1 / l / v I.- 1 I / I I i Z I I I \ 1- I / ) / / 0 / CO / I, Z r II 1 r Or to 1 /1 C.) I/11 0 / If I Q / /(/ / /z. CO v. I i IL 1 1 , I ...