GEOLOGY of the UPPER CRETACEOUS HORNBROOK FORMATION, OREGON and CALIFORNIA Editor

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GEOLOGY OF THE UPPER CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA

Editor TOR H. NILSEN U.S. Geological Survey Menlo Park, California

September 28-30, 1984

Published by The Pacific Section Society of Economic Paleontologists and Mineralogists Los Angeles, California U.S.A. Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California, 1984 Copyright © 2012 Pacific Section, SEPM (Society for Sedimentary Geology) For copies of this volume, write to: Treasurer, Pacific Section S.E.P.M. P.O. Box 10359 Bakersfield, CA 93389

Copyright© 1984 by the Pacific Section, Society of Economic Paleontologists and Mineralogists COPYRIGHT The papers in this volume were prepared for presentation at the 1984 fieldtrip of the Pacific Section of the Society of Economic Paleontologists and Mineralogists, held in Ashland, Oregon, September 28-30, 1984. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior writtten permission of the copyright owner.

Printed by Comet Reproduction Service Santa Fe Springs, CA 90670 Pacific Section Society of Economic Paleontologists and Mineralogists

Field Trip Leaders TOR H. NILSEN U.S. Geological Survey Menlo Park, California MONTY A. ELLIOTT Southern Oregon State College Ashland, Oregon

Pacific Section SEPM Officers - 1984 PRESIDENT ROBERT E. GARRISON University of California, Santa Cruz, California VICE PRESIDENT JEFFREY MOUNT University of California, Davis, California SECRETARY CHRISTINE CARLSON U.S. Geological Survey, Menlo Park, California TREASURER PAUL F. BERTUCCI Chevron, U.S.A., Concord, California PAST PRESIDENT KENNETH A. PISCIOTTO SOHIO Petroleum, San Francisco, California PRESIDENT ELECT J. ALAN BARTOW U.S. Geological Survey, Menlo Park, California MANAGING EDITOR REINHARD SUCHSLAND DEPCO, Inc., Bakersfield, California PREFACE AND ACKNOWLEDGEMENTS

This field trip guidebook to the Upper Cretaceous Hornbrook Formation of north-central California and southwestern Oregon has been prepared for the Annual Fall Field Trip of the Pacific Section of the Society of Economic Paleontologists and Mineralogists (SEPM), to be held in Ashland, Oregon, September 28-30, 1984. This field trip is the northernmost annual field trip held by the Pacific Section—almost all previous trips have been in the areas of either central or southern California. I am thankful to the executive council of the Pacific Section SEPM for their support, encouragement, and willingness to extend the geographic limits of their annual field trips and, in particular, for their attempts to include the geologic community of the Pacific Northwest in the planning of the trip. I am especially indebted to Kenneth Pisciotto, Steven Graham, Robert Garrison, Virgil Frizzell, Alan Bartow, Paul Bertucci, Jeffrey Mount, Reinhard Suchsland, and Christine Carlson of the Pacific Section for their help and support in organizing the field trip and preparing the guidebook.

The field trip co-leader, Monty Elliott of Southern Oregon State College (SOSC), also undertook the responsibility of initially organizing lodging, meals, transportation, tickets to the Shakespeare Festival, and lecture halls, and is due many thanks. Ralph Golia and Greg Barats helped during various stages of the field trip organization, and Jan Zigler has carefully edited most of the papers and provided invaluable assistance in all stages of the guidebook preparation.

The Ashland area is certainly a beautiful one that provides many exciting opportunities for both cultural, educational, and outdoor activities. I hope that, within the framework of the field trip, the exciting geology of the area will add to your enjoyment of it.

"I have thrust myself into this maze, Haply to wive and thrive, as best I may." The Taming of the Shrew, Wm. Shakespeare

Tor H. Nilsen Contents

Page INTRODUCTION TO FIELD TRIP Tor H. Nilsen 1 DESCRIPTION OF FIELD TRIP STOPS AND ROADLOG, UPPER CRETACEOUS HORNBROOK FORMATION, SOUTHERN OREGON AND NORTHERN CALIFORNIA Tor H. Nilsen 9 GEOLOGIC AND GEOGRAPHIC SETTING OF THE HORNBROOK FORMATION, OREGON AND CALIFORNIA Monty Elliott 43 STRATIGRAPHY, SEDIMENTOLOGY, AND TECTONIC FRAMEWORK OF THE UPPER CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA .... Tor H. Nilsen 51 AGE AND CORRELATION OF THE CRETACEOUS HORNBROOK FORMATION, CALIFORNIA AND OREGON W.V. Sliter, D.L. Jones, and C.K. Throckmorton 89 SANDSTONE PETROGRAPHY OF THE UPPER CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA Ralph T. Golia and TorH. Nilsen 99 CONGLOMERATE CLAST COMPOSITION OF THE UPPER CRETACEOUS HORNBROOK FORMATION, OREGON AND CALIFORNIA Greg M. Barats, TorH. Nilsen and Ralph T. Golia 111 GEOCHEMISTRY OF COAL FROM THE DITCH CREEK SILTSTONE MEMBER OF THE HORNBROOK FORMATION, NORTHERN CALIFORNIA Jan L. Zigler and TorH. Nilsen 123 POROSITY, PERMEABILITY, AND DIAGENESIS OF SURFACE SAMPLES OF SANDSTONE FROM THE HORNBROOK FORMATION C.W. Kreighin andB.E. Law 129 THE PETROLEUM SOURCE-ROCK POTENTIAL OF THE UPPER CRETACEOUS HORNBROOK FORMATION, NORTH-CENTRAL CALIFORNIA AND SOUTHWESTERN OREGON B.E. Law, D.E. Anders and T.H. Nilsen 133 STRATIGRAPHY AND SEDIMENTOLOGY OF THE OSBURGER GULCH SANDSTONE MEMBER OF THE UPPER CRETACEOUS HORNBROOK FORMATION, NORTHERN CALIFORNIA AND SOUTHERN OREGON M.T. Gaona 141 LATE CRETACEOUS TRANSGRESSIVE SEDIMENTATION: A COMPARISON OF THE BASAL HORNBROOK FORMATION AND THE CAPE SEBASTIAN SANDSTONE, NORTHERN CALIFORNIA AND SOUTHWESTERN OREGON Joanne Bourgeois 149 TECTONOSTRATIGRAPHIC TERRANES IN SOUTHWESTERN OREGON M.C. Blake, Jr. 159 GEOLOGY OF THE KLAMATH MOUNTAINS NEAR YREKA, CALIFORNIA N. Mortimer 167 A NEOGENE STRUCTURAL DOME IN THE KLAMATH MOUNTAINS, CALIFORNIA AND OREGON N. Mortimer and R.G. Coleman 179 STRATIGRAPHY AND SEDIMENTOLOGY OF THE PAYNE CLIFFS FORMATION, SOUTHWESTERN OREGON Brian K. McKnight 187 THE LOWER WESTERN CASCADE VOLCANIC GROUP IN NORTHERN CALIFORNIA Joseph A. Vance 195 CATASTROPHIC DEBRIS AVALANCHE FROM AN ANCESTRAL MOUNT SHASTA VOLCANO, CALIFORNIA .. D.R. Crandell, C.D. Miller, R.L. Christiansen, H.X. Glicken and C.G. Newhall 197 A GEOLOGIC CROSS SECTION OF NORTHEASTERN CALIFORNIA FROM SEISMIC REFRACTION RESULTS G.S. Fuis and J.J. Zucca 203 A POTENTIAL-FIELD INTERPRETATION OF THE STRUCTURAL EDGE OF THE CRETACEOUS HORNBROOK BASIN IN NORTHERN CALIFORNIA M.C. Erskine, Jr., J. A. Wolleben and D.L. Lawler 211 IMPLICATIONS OF PALEOMAGNETISM FOR THE TECTONIC HISTORY OF THE EASTERN KLAMATH AND RELATED TERRANES IN CALIFORNIA AND OREGON Edward A. Mankinen, William P. Irwin and C. Sherman Gromme 221 PALEOMAGNETIC CONSTRAINTS ON THE INTERPRETATION OF EARLY CENOZOIC PACIFIC NORTHWEST PALEOGEOGRAPHY RayE. Wells 231 A PALEOGEOGRAPHIC REINTERPRETATION OF SOME MIDDLE CRETACEOUS UNITS, NORTH-CENTRAL OREGON: EVIDENCE FOR A SUBMARINE TURBIDITE SYSTEM Lewis C. Kleinhans, Elizabeth A. Balcells-Baldwin and Richard E. Jones 239 INTRODUCTION TO FIELD TRIP

Tor H. Nilsen U.S. Geological Survey 345 Middlefield Road Menlo Park, California 94025

GENERAL SETTING The Hornbrook Formation has attracted the attention of the petroleum industry for many years, The Upper Cretaceous Hornbrook Formation crops partly because natural gas has been found in out to the northwest of Mount Shasta in north- numerous water wells in the area. The potential for central California and southwestern Oregon (Fig. finding both petroleum source and reservoir rocks in 1). It forms a northwest-striking and northeast- the Hornbrook Formation to the east of the outcrop dipping homoclinal sequence that underlies a series belt, beneath the western Cascade Range, has of narrow, discontinuous, northwest-trending valleys resulted in the leasing of large areas, especially between highlands of the Klamath Mountains and the in the Modoc Plateau region (Alldredge and Meigs, Cascade Range (Fig. 2). The Hornbrook Formation 1984). An exploration well drilled by Klamath rests unconformably on highly deformed Paleozoic, Exploration in 1983-84 about 21 km northeast of Triassic, and Jurassic metamorphic rocks and on Yreka, on the south flank of Bogus Mountain (Fig. Triassic and Jurassic plutons of the Klamath 2), penetrated about 640 m of Cretaceous sedimentary Mountains. The Hornbrook is overlain unconformably rocks, with a show of wet oil or gas (Alldredge and to the northeast by lower Tertiary nonmarine Meigs, 1984). sedimentary and volcanic rocks of the western Cascade Range. The location of all relevant U.S. The Cretaceous Ochoco basin in the Mitchell Geological Survey quadrangles in southern Oregon and area of central Oregon (Fig. 1) has been an area of northern California is shown in Figure 3. active petroleum exploration because it contains more than 4,300 m of Cretaceous marine sedimentary The Hornbrook Formation is one of a large rocks. If the Hornbrook Formation, Ochoco basin, number of widely scattered Cretaceous sedimentary and Great Valley sequence originally formed part of units that crop out in various parts of Oregon and the same large depositional basin, and are possibly northern California (Fig. 1). These discontinuous interconnected today beneath the cover of volcanic remnants of Cretaceous strata crop out in the rocks, then a large petroleum province could Mitchell area and other parts of the Blue Mountains, potentially exist in southern Oregon and in the Grave Creek and O'Brien areas of the Klamath northeastern California, - and possibly farther north Mountains, on the northern and western flanks of the as well. Klamath Mountains in the Days Creek and Gold Beach areas, and on the northern flanks of the Great Because of this diverse and regional interest Valley of California (Fig. 1). in the Hornbrook Formation and related strata, this guidebook contains a number of papers and Paleogeographic reconstructions of the contributions that describe the geology of the Hornbrook Formation and related Cretaceous strata basement rocks of the Klamath Mountains, the have proven difficult because of (1) the great volcanic and sedimentary rocks of the Cascade Range, separation of the outcrop areas, (2) the extensive the Cretaceous sedimentary sequence of the Mitchell cover of Tertiary volcanic and sedimentary rocks in area and the related subsurface Ochoco basin, and the Cascade Range, Modoc Plateau, and volcanic several papers covering the regional geophysics and plateaus of eastern Oregon and Washington, (3) the implications of paleomagnetlc data. Future few exploration wells that have been drilled to subsurface exploration should eventually shed more determine the subsurface extent of Cretaceous light on the regional tectonic setting and framework strata, and (4) the extensive Tertiary deformation, of Cretaceous sedimentation in northern California including the rotation and translation of major and Oregon. blocks in Oregon and Washington, east-west extension in the Basin and Range province, and major right- HORNBROOK FORMATION lateral displacements along coastal parts of northwestern California and southwestern Oregon. The Hornbrook Formation was subdivided by Nilsen (1984) into five members, listed in ascending Nevertheless, there is continuing great order: (1) the Klamath River Conglomerate Member; interest in the Hornbrook Formation and its regional (2) the Osburger Gulch Sandstone Member; (3) the setting. It has been mined extensively for gold in Ditch Creek Siltstone Member; (4) the Rocky Gulch the past century, and several firms are presently Sandstone Member; (5) and the Blue Gulch Mudstone engaged in extensive leasing activities and sampling Member, containing the Rancheria Gulch Sandstone programs. Geothermal water has been derived from Beds and the Hilt Bed. The Hornbrook Formation has wells drilled to the basal contact of the Hornbrook a total composite thickness of 1,213 m and ranges in Formation and this water has been used in the age from Cenoraanian to Maestrichtian. Various time Ashland area by a mineral springs industry and for scales for the Late Cretaceous have been proposed, hot-house production of plants and flowers (Black and some of these are shown in Figure 4 to help and others, 1983). The Hornbrook Formation has also field trip participants grasp the significance of been a major source for ground water throughout the the various age assignments for members of the region. Hornbrook Formation. As can be seen from the newer

in Nilsen, Tor H„ ed., 1984, Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California: Pacific Section S.E.P.M., Vol. 42, p. 1-7 1 Outcrops of Cretaceous rocks adjacent to the Hornbrook Formation

Figure 1. Index map of the western United States showing location of outcrops of the Hornbrook Formation and adjacent Cretaceous strata, and principal geographic and tectonic features (from Nilsen, 1984, Fig. 1). 3

to Crater Lake EXPLANATION 3 Field trip stop

20 km j

122°15' —|— 42°15'

123°00' 42°00' -(="

Klamath National Forest

to Fort Jones

to Weed

Figure 2. Index map to the outcrop area of the Hornbrook Formation, showing location of field trip stops. 4

124° 123° 122° 43°- I CRATER LAKE| NATIONAL PARK AND VICINITY (S) DUTCHMAN ABBOTT 18 88-1956 ROWERS BONE MTN BUTTE (S) CANYONVILLE DAYS CREEK 1 BUTTE PROSPECT 1944 1956*1 'i95< 1954 ; 1954 1954

PELICAN GALICE BUTTE CHILOQUIN 1946 1954 1957 M E D F 0 R D

COLLIER PEARSOLL MT i LAKE Of LAKECREEK MC LOUGHLjN THE WOODS MODOC POINT BUTTE PEAK SELMA MEDFORD 1957 1954 - ^ 1954 1954 1954 1955

-GRANTS PASS-

CAVE OREGON HYATT ' SURVEYOR KLAMATH MT EMILY TALENT ASHLAND RESERVOIR MTN FALLS CHETCO PEAK JUNCTION CAVES RUCH 1957 ma 19b4 , 1954 1954 1954 1954 1954 1955 ' 1955

42°

GASQUET PRESTON PEAK HAPPY CAMP SEIAD VALLEY CONDREY MTN HORNBROOK COPCO DORRIS 1955 1954 1950 1942-51 1956 195T 1955 1955 .

THE. LAKE BRAY (S) DILLON MTN UfcONOM LAKE FORT JONES SHASTINA WHALEBACK 1950 1955 1955 1955 1954 1954 WEED 41°30'

FORKS.OF M CHINA MTN SHASTA TECTAH CREEk 0#L€ANS . "SALMON _ 1954 1943 52 , 1952 c, 1955 1955 - . jSTtS- V

Figure 3. Index map to northern California and southern Oregon showing location of U.S. Geological Survey 15-minute quadrangles.

time scales of Harland and others (1982) and Palmer Hollow area adjacent to Jacksonville, Oregon in the (1983)I the Turonian, Coniaci an, and Santonian north (Fig. 2); all stops will be within Jackson stages represent a time interval of only about 6 County, Oregon, and Siskiyou County, California. m.y. Thi3 narrow range has some significance for interpretations of possible unconformities within The purpose of the field trip is to examine in the Hornbrook Formation. detail the stratigraphy, sedimentology, and regional setting of the Upper Cretaceous Hornbrook ORGANIZATION AND OBJECTIVES OF THE FIELD TRIP Formation. On the first day, we will make seven stops south and east of Ashland to examine the This guidebook to the geology of the Hornbrook various members and other stratigraphic units of the Formation has been prepared for a one and one-half Hornbrook Formation, in roughly ascending day trip that begins and ends in Ashland, Oregon, stratigraphic order. On the second day, we will with overnight accommodations also in Ashland. The make four stops in the morning to examine other good trip has eleven stops that range geographically from exposures of the Hornbrook Formation in and north of the Yreka, California area in the south to the Dark Ashland. Age Harland et al van Hinte Harland et al Palmer Age (m.y.b.p.) (1964) (1976) (1982) (1983) (m.y.b.p.) 65- -65 -

Maestrichtian Maestrichtian Maestrichtian 70- Maestrichtian -70

Campanian Campanian

Campanian Santonian Campanian 80- Santonian -80

Coniacian Coniacian Santonian Santonian

Coniacian Coniacian Turonian 90- Turonia'n Turonian -90 Turonian

Cenomanian Cenomanian Cenomanian Cenomanian

-100- -100-

Figure 4. Subdivisions of Late Cretaceous time proposed by various workers (from Nilsen, 1984, Fig. 3). 6

GENERAL ITINERARY ORGANIZATION OF GUIDEBOOK

Friday, September 28: This field trip guidebook contains a variety of different papers and contributions that have been AM: Arrive on United Airlines at organized so that field trip participants can obtain Medford Airport. a better understanding of both the detailed geology Shuttle van meets plane. of the Hornbrook Formation and its regional Shuttle to SOSC (18 m). setting. The description of the field trip stops Register at Housing Office in and roadlog by Nilsen, Elliott, and Purdom that Siskiyou Hall, leave luggage in follows this introductory paper provides detailed dormitory room. information about individual stops and the route. Place Guest Parking Permit on In the envelope at the back of the guidebook is a car dash. detailed geologic map of the outcrop area of the Locate Geology Department on Hornbrook Formation at 1:62,500 scale by Nilsen and west side of campus in the Barats that also shows the location of each field basement of the Science stop. Building. 6:00 PM: Deluxe buffet, Stevenson Union. The paper by Elliott provides background 8:00 PM: Evening Program, Lecture Hall, information on the geography, geomorphology, and SC 118. geology of the area, as well as the general stratigraphy and structure of both the basement Saturday, September 29: rocks of the Klamath Mountains and the overlying sedimentary and volcanic rocks of the western 7:00 AM: Breakfast, Cascade Dining Cascade Range. Nilsen1s paper on the stratigraphy, Center. Pick up sack lunch. sedimentology, and tectonic framework of the 8:00 AM: Buses leave from front of Hornbrook Formation provides most of the basic Geology Building. detailed information about the Hornbrook 8:30 AM: Stop 1: Siskiyou Summit, Formation. The papers that follow provide Osburger Gulch Sandstone Member. additional information on the Hornbrook Formation, 10:00 AM: Stop 2: Blue Gravel Mine, the basement rocks, and the overlying Tertiary fanglomerate facies of the rocks, as well as data about surrounding areas, the Klamath River Conglomerate regional tectonic framework, and regional Member. geophysical profiles. 11:15AM: Stop 3: Oberlin Road, fluvial facies of the Klamath River Sliter, Jones, and Throckmorton synthesize all Conglomerate Member. available microfossil and megafossil data from the 12:00 PM: Lunch Stop: Collier Rest Area. Hornbrook Formation and outline how these data 1:00 PM: Stop 4: Interstate Highway 5 constrain age assignments within the Hornbrook roadcuts, Ditch Creek Siltstone Formation. They also provide a general model for Member and Rocky Gulch Sandstone an eastward-migrating Cretaceous transgression Member. across the Klamath Mountains. The petrology of 2:30 PM: Stop 5: Henley, Rancheria Gulch framework grains of sandstone and the compositional Sandstone Beds of the Blue Gulch distribution of conglomerate clasts in the Hornbrook Mudstone Member. Formation are summarized by Golia and Nilsen, and 3:45 PM: Stop 6: Hilt, Hilt Bed of the Barats, Nilsen, and Golia, respectively. The Blue Gulch Mudstone Member. geochemistry ®f a poorly exposed and unusual coal 5:00 PM: Stop 7: Ashland, Blue Gulch from within the Hornbrook Formation in the Shasta Mudstone Member. Valley area is discussed by Zigler and Nilsen. 6:00 PM: Barbeque dinner, Cascade Dining Keighin and Law present the results of analyses of Center. porosity and permeability of surface samples of 8:00 PM: Shakespearean production, sandstone from the Hornbrook Formation. Law, downtown Ashland. Anders, and Nilsen summarize the organic geochemistry of surface samples of shale from the Sunday, September 30: Hornbrook Formation. Gaona summarizes the stratigraphy and sedimentology of the Osburger Gulch 7:00 AM: Breakfast, Cascade Dining Sandstone Member, the basal, transgressive shallow- Center. Pick up sack lunch. marine unit of the Hornbrook Formation. Bourgeois 8:00 AM: Buses leave from front of compares the sedimentology of the basal shallow- Geology Building. marine units with similar Upper Cretaceous storm- 8:15 AM: Stop 8: Ashland, Tertiary dominated shallow-marine deposits in coastal south- nonmarine conglomerate. western Oregon. 9:00 AM: Stop 9: Ashland, Rocky Gulch Sandstone Member. 10:00 AM: Stop 10: Interstate Highway 5 roadcuts near Medford, Rocky Gulch Sandstone Member. 11:00 AM: Stops 11 A and B: Dark Hollow area, Blue Gulch Mudstone Member. 12:00 PM: Lunch Stop: Old Court House, Jacksonville. 2:00 PM: Return to Ashland. 7

deposition of the Hornbrook Formation controlled by Cretaceous paleogeography. Mankinen and Irwin an eastward-migrating Cretaceous transgression summarize paleomagnetic data from the Hornbrook across the Klamath Mountains. The petrology of Formation and the Klamath Mountains and show that framework grains of sandstone and the compositional there is no paleomagnetic evidence for post- distribution of conglomerate clasts in the Hornbrook Cretaceous movement of this area. Wells discusses Formation are summarized by Golia and Nilsen, and the regional implications of paleomagnetic data from Barats, Nilsen, and Golia, respectively. The various Cenozoic rock units in the Pacific Northwest geochemistry of a poorly exposed and unusual coal for Cretaceous paleotectonic reconstructions. from within the Hornbrook Formation in the Shasta Valley area is discussed by Zigler and Nilsen. Finally, one paper is included that summarizes Keighin and Law present the results of analyses of the subsurface geology of the Mitchell area of porosity and permeability of surface samples of central Oregon. Because some Cretaceous sandstone from the Hornbrook Formation. Law, paleogeographic reconstructions of northern Anders, and Nilsen summarize the organic California and Oregon (Nilsen, 1984) suggest that geochemistry of surface samples of shale from the strata in this Mitchell area were originally Hornbrook Formation. Gaona summarizes the deposited on the northeastern margin of the stratigraphy and sedimentology of the Osburger Gulch Hornbrook basin, the stratigraphy and sedimentology Sandstone Member, the basal, transgressive shallow- of this area are critical to fuller understanding of marine unit of the Hornbrook Formation. Bourgeois the Hornbrook Formation. Kleinhans, Balcells- compares the sedimentology of the basal shallow- Baldwin, and Jones summarize the outcrop geology of marine units with similar Upper Cretaceous storm- Cretaceous rocks in the Mitchell area. dominated shallow-marine deposits in coastal southwestern Oregon. REFERENCES CITED

Blake presents a regional synthesis of the Alldredge, M. H., and Meigs, J. V., 1981), N. E. character and distribution of tectonostratigraphic California area drawing interest: Oil and Gas terranes in southwestern Oregon and northern Journal, v. 82, no. 23, p. 83-87. California, and discusses how the accretion of these Black, G. L., Elliott, Monty, D'Allura, Jad, and terranes has influenced the tectonic framework of Purdom, Bill, 1983, Results of a geothermal deposition of the Hornbrook Formation. Mortimer resource assessment of Ashland, Oregon, area, summarizes the general geology and character of Jackson County: Oregon Geology, v. 45, no. 5, tectonostratigraphic terranes in the Klamath p. 51-55. Mountains of northwestern California near Yreka, Harland, W. B., Cox, A. V., Llewellyn, P. G.( California. Tertiary uplift in the Condrey Mountain Pickton, C. A. G., Smith, A. G., and Walters, area of the Klamath Mountains (Figs. 2 and 3) and R., 1982, A geologic time scale: Cambridge, its influence on the structural framework of the England, Cambridge University Press, 128 p. Hornbrook Formation are discussed by Mortimer and Harland, W. B., Smith, A. G., and Wilcock, B., eds., Coleman. 1964, The Phanerozoic time-scale: Geological Society of London Quarterly Journal, v. 120, The geology of the Tertiary and Quaternary 458 p. rocks of the western Cascade Range and Shasta Valley Nilsen, T. H., 1984, Tectonics and sedimentation of is discussed in three papers. McKnight discusses the Upper Cretaceous Hornbrook Formation, the stratigraphy, sedimentation, and paleogeography Oregon and California, _in Crouch, J. K., and of the lower Tertiary Payne Cliffs Formation in Bachman, S. B., eds., Tectonics and southern Oregon, a unit consisting of fluvial Sedimentation Along the California Margin: conglomerate and sandstone. Vance summarizes the Society of Economic Paleontologists and geology of the volcanic rocks of the western Cascav Mineralogists, Pacific Section, v. 38, p. 101- Range in northern California. Possibly the world's 118. largest landslide deposit fills a large part of the Palmer, A. R., compiler, 1983, The Decade of North Shasta Valley, covering the southern limit of American Geology 1983 geologic time scale: outcrops of the Hornbrook Formation; this Geology, v. 11, p. 503-504. volcaniclastic debris avalanche was derived from an van Hinte, J. E., 1976, A Cretaceous time scale: ancestral Mount Shasta, and its morphology and American Association of Petroleum Geologists deposition are summarized by Crandell and others. Bulletin, v. 60, p. 498-516.

Fuis and Zucca summarize the results of north- south- and east-west-trending seismic refraction lines in northern California and their implications for (1) the nature of the crust beneath the Klamath Mountains, Cascade Range, and Modoc Plateau, (2) the nature of boundaries or sutures between these areas, and (3) the possible subsurface extent of the Hornbrook Formation. Erskine, Wolleben, and Lawler have interpreted the eastward subsurface extent of the Hornbrook basin in northeastern California from combined gravity and magnetic data.

Paleomagnetic studies of the rotational and translational history of major crustal blocks in southern Oregon and northern California, as well as Washington, Idaho, and northwestern Nevada, have major implications for reconstructions of the DESCRIPTION OF FIELD TRIP STOPS AND ROADLOG, UPPER CRETACEOUS HORNBROOK FORMATION, SOUTHERN OREGON AND NORTHERN CALIFORNIA

Tor H. Nilsen Monty A. Elliott William B. Purdom U. S. Geological Survey Department of Geology Department of Geology 345 Middlefield Road Southern Oregon State College Southern Oregon State College Menlo Park, CA 94025 Ashland, Oregon 97520 Ashland, Oregon 97520

INTRODUCTION

The field trip is divided into two days. On the first day, which consists of seven stops located along and adjacent to Interstate Highway 5 between Ashland, Oregon, and Yreka, California, each member of the Hornbrook Formation is seen in outcrop. On the second day, which consists of four stops in the Ashland-Medford area, local variations in the lithology of the Rocky Gulch Sandstone and Blue Gulch Mudstone Members of the Hornbrook Formation will be observed, as well as an outcrop of the overlying Payne Cliffs Formation. The loca- tions of the field trip stops are indicated on Plates 1 and 2 and Figure 1 of the Introduction. In this section, the descriptions of the field trips stops were prepared by Nilsen and the road log chiefly by Elliott and Purdom.

FIRST DAY Cumulative Mileage Mileage

0.0 . SOSC Qeology Department parking lot: proceed west on Ashland Street to junction with . 0.0 . . Mountain Street, turn right.

0.3 . Junction with Siskiyou Boulevard: turn right and proceed southeast along Siskiyou . 0.3 . . Boulevard (U.S. Highway 99).

Ashland: founded in 1852 and originally called Ashland Mills by Abel Hellman, who built a grist mill and a sawmill on Ashland Creek to supply miners in Jacksonville.

1.7 2.0 Pompadour Bluff: cuesta to left on east side of valley near mouth of Walker Creek, underlain by resistant pebbly sandstone of the Eocene Payne Cliffs Formation. The eroded bluff resembles, in a general way, the style of haircut made famous by the Marquis de Pompadour and by Jim Corbett, and was probably named about the time Jim Corbett was world heavyweight boxing champion.

1.0 3.0 Beacon Hill: cuesta to left in middle of valley held up by a gabbro sill emplaced in the base of the Payne Cliffs Formation.

1.3 4.3 Junction of U.S. Highway 99 and Interstate Highway 5; continue south on Interstate Highway 5.

1.0 5.3 Roadcut on right exposing pebbly sandstone and pebble conglomerate of the Rocky Gulch Sandstone Member.

0.9 6.2 Hydrothermally altered unconformity between the Hornbrook Formation and the Late Jurassic Mt. Ashland pluton.

0.7 6.9 Medium-grained diorite of the Mt. Ashland pluton.

0.4 7.3 Fine-grained biotite diorite of the Mt. Ashland pluton, which is apparently an early phase of emplacement.

0.4 7.7 Discontinuous outcrops of metasedimentary rocks that are probably roof pendants of the Applegate Group and include thin-bedded, ptygmatically folded quartz-biotite gneiss, biotite schist, and marble.

1.3 9.0 Southern Pacific Railroad overpass, the steepest railroad grade on the west coast. Rails reached Ashland in 1884 but the link with California was not made until 1887 in what was then hailed as a major engineering feat.

0.5 9.5 Roadcuts on right of Ditch Creek Siltstone Member downfaulted against the Mt. Ashland pluton.

0.7 10.2 STOP 1. Siskiyou Summit area, roadcuts along west side of south-bound lane of Inter- state Highway 5, a few kilometers south of Steinman (NW 1/4 sec. 21, T. 40 S., R. 2 E., Ashland 15-minute quadrangle). This roadcut exposes the Mt. Ashland pluton, the Klamath River Conglomerate Member, the Osburger Gulch Sandstone Member, and the lowest part of the Ditch Creek Siltstone Member of the Hornbrook Formation.

in Nilsen, Tor H., ed., 1984, Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California: Pacific Section S.E.P.M., Vol. 42, p. 9-41 9 10

At this stop a complete section of the Osburger Gulch Sandstone Member, 127.5 m thick, can be observed (Fig. 1). Both the basal unconformity (Fig. 2A) and underlying granitic rocks of the Mt. Ashland pluton are well exposed, as is the upper transition into the overlying Ditch Creek Siltstone Member. The lower 5 m of the section, which appears to be of nonmarine origin, is included in the Klamath River Conglomerate Member but is too thin to map in this area.

The Osburger Gulch Sandstone Member here consists dominantly of fine- to medium-grained sandstone. Conglomerate is most abundant in the basal 10 m, but is also common at several higher stratigraphic levels. Siltstone is interbedded in the sandstone in the upper part of the member, mostly in the upward transition to the overlying Ditch Creek Siltstone Member.

The Osburger Gulch Sandstone Member generally ranges in thickness from 75-150 m but can locally be much thicker and thinner. At its type section near Hornbrook, it is 116.5 m thick (Nilsen, 1984). The section here is covered adjacent to three gulleys which cut through the exposures, but is about 85 percent exposed.

The Osburger Gulch Sandstone Member contains abundant molluscan megafossils and trace fossils; the megafossils yield an age of Cenomanian to early Coniacian (Sliter, Jones and Throckmorton, this volume). It appears to have been deposited as a result of southward or southeastward transgression of the Late Cretaceous sea over the Klamath Mountains. Sedimentary characteristics of the member generally indicate an upward change from very high-energy, nearshore shelf conditions to lower energy, offshore shelf conditions. However, the abundance of hummocky-stratified sandstone throughout most of the middle and upper parts of the member suggests that the shelf was commonly subjected to storm reworking, probably in a generally high-energy setting (see also Gaona, this volume, and Bourgeois, this volume).

EXPLANATION OF SYMBOLS ON STRATIGRAPHIC SECTIONS

Igneous and metamorphic rocks Trough cross strata

Sedimentary breccia Tabular cross strata

Conglomerate HCS Hummocky stratification

Sandstone Parallel stratification

Siltstone Contorted stratification

I -=-—I Shale and mudstone Ripple markings

IMl Volcanic breccia Convolute lamination

Intrusive rock Sedimentary flaps

Unconformity Rip-up clasts

^•iCr ffW Paleosol ot^- Dish structure

® ® A Foraminiferal fossils ® Carbonate concretions

GRAIN SIZE vc 32 50 sh ss mm cm I—r HCS Base of Ditch Creek Siltstone Member

50- 120-

40- © 25 ft HCS ^ ft

30-' 100-, HCS xlfi HCS —2b ft ft HCS

-aagjj<*a i \ \ ^'o'o'o'o] (gj 10-" 80

HCS Klamath River *: * c,*' Conglomerate Member a- — — 0m - Basementy / _ N Complex \

Figure 1. Measured section of the Osburger Gulch Sandstone Member of the Hornbrook Formation at Field Trip Stop 1 along Interstate Highway 5 near Siskiyou Summit. The 1 ower 5 m of this section is assigned to the Klamath River Conglomerate Member of the Hornbrook Formation. Vertical scale is in meters; arrows indicate paleocurrent directions; HCS = hummocky-stratified beds. Photographs of the Klamath River Conglomerate Member, Osburger Gulch Sandstone Member, and Ditch Creek Siltstone Member of the Hornbrook Formation at Field Trip Stop 1 along Interstate Highway 5 near Siskiyou Summit. A, Basal contact of Klamath River Conglomerate Member with the Mt. Ashland pluton; geologist is pointing to the contact, which is outlined by dashed line. B, Basal contact of grus of the Klamath River Conglomerate Member on the Mt. Ashland pluton; blade tip of hammer rests on unconformity.

The base of the section consists of about 4 m of granitic grus that is massive to poorly stratified (Fig. 2B). The grus contains some scattered angular to well-rounded metavolcanic, quartzite, and chert clasts as large as 10 cm, but only rare angular clasts of granitic rock. This suggests that the plutonic rocks of the Mt. Ashland pluton weathered almost completely in situ to sand and granule-sized material before the marine transgression.

The basal grus and the overlying one meter of reverse-graded conglomerate beds, which are unfossiliferous, are probably of nonmarine origin and are included in the Klamath River Conglomerate Member. The reverse-graded conglomerate beds contain better rounded clasts as large as 10 cm scattered in a sandy matrix. The lower 5 m thus probably represents weathered residuum on the underlying basement and two thin subaerial debris flows. m

Overlying these basal units is about 15 m of conglomeratic sandstone that is dominated by trough cross-stratification. These beds contain abundant marine trace fossils, both subhorizontal types exposed on bedding surfaces (Fig. 2C) and larger vertical burrows of Ophiomorpha type. The conglomerate generally forms discontinuous fining-upward lenses or is scattered within the trough cross-strata (Fig. 2D). The clasts are generally better rounded, better sorted, and less matrix-supported than those in the underlying basal conglomerate. Clasts are as large as 21 cm and include some of granitic composition. Measurement of trough axes and some tabular cross strata suggests sediment transport generally toward the northwest in the lower part of the member (Fig. 1).

Overlying the lower conglomeratic units is a thick section of hummocky-stratlfied, fine- to medium-grained sandstone (Fig. 2E, F, G) with a few scattered conglomerate beds (Fig. 2H). The lower part of the sequence is dominated by beds as thick as 2.5 m of fine-grained or fine- to medium-grained sandstone that are massive, crudely parallel-stratified, hummocky.-stratified, or thoroughly bioturbated. These beds rest with erosive contact on parallel-stratified siltstone, very fine grained sandstone, and fine-grained sandstone that contains abundant burrows and plant fossils (Fig. 2F, G). These fining-upward couplets of sandstone to siltstone with erosive bases probably represent storm deposition by periodic reworking of shelf sands. Some of the parallel- stratified beds of sandstone in this interval also contains heavy mineral concentra- tions along bedding surfaces. A few measurements of primary current lineation from the parallel-stratified beds of sandstone yield northeast-southwest trends that are plotted on Figure 1 as indicating sediment transport toward the northeast.

Megafossil-rich bed of sandstone and conglomerate are common in the section from about 40 m above the base to the top (Fig. 2H). The megafossils are mostly confined to coquina-like layers or lenses that appear to be storm lag deposits and also contain scattered pebbles. These layers typically form the base of the fining-upward couplets. However, pelecypods and other megafossils are also scattered in growth position in the bioturbated beds of fine-grained sandstone. 13

Figure 2 (continued). C, Marine burrows exposed on bedding surface. D, Trough cross strata. E, F, and G, Hummocky-stratified beds of sandstone resting with erosive contact on bioturbated siltstone. Hammer is circled in E for scale and staff in F is 1.5 m long. H, Conglomerate lens with abundant transported mollusk shells. I, Top of Osburger Gulch Sandstone Member. J, Ditch Creek Siltstone Member at top of section. 14

The section gradually fines upward and consists of alternating thin beds of very fine to fine-grained sandstone and siltstone at the top. Most of the beds are hummocky- stratified to parallel-stratified and laminated; partial to thorough bioturbation and fossils in growth position and also typical of this part of the section. Thin coquina- like megafossil-rich beds continue to be present upsection and fossil shrimp in their burrows have been collected from here.

The upper contact with the Ditch Creek Siltstone Member is placed at the transition upward to thicker beds of massive, concretionary siltstone that contain abundant ammon- ites (Fig. ?I) The lower part of the Ditch Creek Siltstone Member here is very fossil- iferous and consists of thoroughly bioturbated sandy siltstone (Fig. 2J).

Continue south on Interstate Highway 5.

0.5 .... 10.7 . . Siskiyou Summit fault, volcaniclastic rocks of the Colestin Formation downfaulted against Jurassic diorite. Using the base of the Hornbrook Formation and assuming purely dip slip movement along it, this fault may have as much as 5,500 m of displace- ment.

1.1 . . . . 11.8 . . Siskiyou Pass (elevation 1,290 m): gently dipping volcaniclastic rocks of the upper part of the Colestin Formation exposed in roadcuts. This is the highest pass on Inter- state Highway 5 between Canada and Mexico. The last stage coach went over the pass on December 17, 1887.

Siskiyou: Name given in 1828 by members of Hudson Bay Company expedition. Chief factor Archibald McLeod lost a pack horse during a snow storm on the pass. French- Canadian followers applied the name "Pass of the Siskiyou" based on the Cree Indian word "siskiyawatim", meaning spotted horse or pack horse.

0.5 .... 12.3 • • Colestin basin and Mt. Ashland (elevation 2,294 m). The Colestin basin is the type locality of the Colestin Formation (Wells, 1956). Mt. Ashland is underlain by Jurassic porphyritic granodiorite and is the highest point in the Klamath Mountains of Oregon.

Colestin: named in 1885 after the Rufus Cole family, which owned much land here and the nearby Colestin mineral springs.

0.4 .... 12.7 • . Contact between lava beds of the Roxy Formation and red volcanic claystone of the Colestin Formation; note the basaltic dike.

0.7 .... 13.4 • • Upper part of the Colestin Formation, tuffs and volcaniclastic rocks; rim-forming lava flows mark base of the Roxy Formation, dated at 30 m.y. (middle Oligocene).

0.8 . . . . 14.2 . . Tuffaceous sedimentary rocks of the middle part of the Colestin Formation exposed in numerous roadcuts to the left and right.

1.8 . . . . 16.0 . . "Stateline" fault, upper part of the Blue Gulch Mudstone Member downfaulted on south against lower middle part of the Colestin Formation, with a basic dike intruded along fault, which is the southern bounding fault of the Colestin graben.

0.2 .... 16.2 . . Oregon-California border. Roadcut exposing thin-bedded basin-plain turbidites of the Blue Gulch Mudstone Member on the right.

0.8 .... 17.0 . . Hilt Overpass.

Hilt; named after John Hilt, an early miner and settler in the area.

0.9 .... 17-9 . . Bailey Hill to the left, a prominent rim formed by a basaltic sill emplaced in the upper part of the Blue Gulch Mudstone Member; the Hilt Bed is exposed halfway up the slope.

0.1 ... . 18.0 . . Conformable contact of the Blue Gulch Mudstone Member on the Rocky Gulch Sandstone Member exposed in roadcuts to the right along the west side of the highway.

0.5 .... 18.5 . . Southern Pacific Railroad overpass; a cross fault exposes the Blue Gulch Mudstone Member.

0.8 . . . . 19-3 . . Contact of the Colestin Formation unconformably overlying the Hornbrook Formation along the eastern valley wall.

0.7 .... 20.0 . . Bailey Hill Overpass; numerous sills and dikes form hills on the valley floor east of the highway. 15

1.8 .... 21.8 . . Bridge over Cottonwood Creek; basalt plug of Blaok Mountain can be seen at the southeast end of the valley.

0.9 .... 22.7 . • Ditch Creek Overpass; extensive outcrops of the Blue Gulch Mudstone Member.

1.1 .... 23.8 . . Placer mining scars to the west. Quarry to the west exposes the Rancheria Gulch Sand- stone Beds of the Blue Gulch Mudstone Member.

0.3 .... 24.1 . . Old mining camp of Cottonwood (now Henley) to the east.

Cottonwood: established in 1851 to supply the rich placer mines of Cottonwood Mining District. The name was changed to Henley in 1856 when the Post Office was established.

0.1 . . . . 24.2 . . Hornbrook-Henley Underpass; the valley of the Klamath River is visible to the east, and the highly eroded hills and ridges in the middle distance are the Western Cascades, which are overlapped by the broad basalt shield of the Eagle Rock-Black Rock volcano and the andesitic Willow Creek Mountain volcano (elevation 2,384 m).

0.6 . . . .24.8 . . Contact of the Blue Gulch Mudstone Member on the Rocky Gulch Sandstone Member.

0.8 . . . . 25.6 . . The Ditch Creek Siltstone Member underlies the swale to the west.

0.1 . . . . 25.7 . • Top of the Osburger Gulch Sandstone Member. Across the Klamath River to the east is the type section of the Osburger Gulch Sandstone Member.

0.4 .... 26.1 . . STOP 2. Blue Gravel mine section, area of artificial fill within old mine workings west of Interstate Highway 5 in the Osburger Gulch area, near the Klamath River south of Hornbrook (S 1/2 SE 1/4 Sec. 32, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle). This outcrop contains the type secton of the Klamath River Conglomerate Member.

The member generally varies from 0 to about 90 m in thickness and is 36.5 m thick here (Fig. 3). The basal contact of the conglomerate on Triassic metavolcanic rocks can be observed in the quarry as well as the upper contact with the overlying marine Osburger Gulch Sandstone Member (Fig. 4A). The Klamath River Conglomerate Member here consists chiefly of debris-flow deposits inferred to have been deposited on a small alluvial fan.

The basal part of the section consists of sedimentary breccia that averages about 0.5 m thick but is locally thicker over topographic lows on the basement surface (Fig. 3). The breccia consists of about 95 percent angular fragments of metavolcanic rocks derived from the underlying basement complex and 5 percent subrounded clasts of quartzite, vein quartz, and chert (Barats and others, this volume; Fig. 3C). The maximum clast size observed is 62 cm. This basal breccia appears to be mostly residual in character because the clasts are both angular and poorly sorted and the breccia appears to be completely unstratified. No conspicuous paleosol on the basement complex can be seen in this area, however.

Most of the remaining section consists of alternating beds and lenses of reverse-graded conglomerate and pebbly mudstone (Fig. 4B-D). The beds are typically laterally discontinuous and commonly have erosive, partly channeled contacts. The maximum amount of downcutting observed in the outcrops, however, is only about 40 cm.

The conglomerate beds are reverse-graded and range 7-135 cm in thickness. The clasts which are as long as 22 cm and consist overwhelmingly of metavolcanic rock fragments, are subangular to subrounded. The conglomerate beds are both clast-supported and matrix supported. The larger clasts are systematically concentrated in the upper parts of the beds. The tops of these beds appear to be generally planar and the bases either planar or irregularly erosive into the underlying beds of pebbly mudstone. The clast size gradually decreases upward from about 20 cm near the base to about 8 cm near the top.

The pebbly mudstone beds are massive, ungraded, and contain scattered angular to subrounded clasts scattered in a silty mudstone matrix (Fig. 4E, F). These beds are matrix- supported, very poorly sorted, and most clasts are randomly oriented and not organized into strata. The maximum clast size is generally 3 cm. The pebbly mudstone beds appear to cap the underlying conglomerate and are commonly eroded into by the overlying conglomerate. Plant fragments are common in the pebbly mudstone beds near the top of the section.

The conglomerate beds are dark gray on fresh surfaces and yellowish brown on weathered surfaces; the pebbly mudstone beds are grayish red and grayish brown on fresh surfaces and dusky red to reddish brown on weathered surfaces. As a result of these color differences, the sequence appears to be banded from a distance. 16

west

vc 32 50 ss mm cm r

^r^asal breccia paleosol 0 m complex / \ /

Figure 3. Measured sections of the Klamath River Conglorr .'ate Member of the Hornbrook Formation at and adjacent to Field Trip Stop 2 along Interstate Highway 5 near the Klamath River. The section to west is the type section in the Blue Gravel mine, and the section to the east is along the northbound lane of Interstate Highway 5. Vertical scale is in meters; arrow indicates paleocurrent direction.

The sequence is overlain by a poorly exposed, nonresistant, massive to laminated, bioturbated brownish siltstone and fine-grained sandstone that contains abundant plant fragments and, locally, molluscan fossils. This siltstone is assigned to the Osburger Gulch Sandstone Member, and was deposited by the transgressing Cretaceous sea on the underlying nonmarine strata. The transition is typically abrupt and marked by a grass- covered bench.

The conglomerate and pebbly mudstone beds are interpreted to be subaerial debris-flow and mudflow deposits transported eastward or southestward from local sources in the Klmath Mountains. One bed of conglomerate had poorly developed imbrication suggestive of sediment transport toward the southeast (Fig. 3). The sediments were probably deposited on a small debris-flow-dominated alluvial fan. Along the northbound lane of Interstate Highway 5 about 0.5 km to the east, the same debris-flow-dominated deposits are only about 11 m thick, are thinner bedded and contain smaller clasts (Fig. 3), supporting the idea of eastward transport.

Continue south on Interstate Highway 5. 17

Figure 4. Photographs of the Klamath River Conglomerate Member of the Hornbrook Formation at Field Trip Stop 2 along Interstate Highway 5 near the Klamath River. A, View northwestward of the Blue Gravel mine, showing northeast-dipping strata; massive bluffs above and to the right are outcrops of the overlying Osburger Gulch Sandstone Member of the Hornbrook Formation. B-D, Interbedded light-colored, more resistant conglomerate and dark-colored, less resistant pebbly mudstone. E and F, Matrix- supported angular clasts of metavolcanic rocks in pebbly mudstone. 18

26.3 • • Basement of sheared and contorted greenstone, argillite, and chert of the Western Paleozoic and Triassic belt of Irwin (1960).

26.6 . . Radiolarian chert of Late Triassic and Jurassic age exposed along the east side of Interstate Highway 5.

27.ft . . Klamath River Bridge and Collier Rest Area.

28.0 . . Sheared contact between argillite and greenstone on east side of highway. Klamath River to the west.

28.6 1 . Tubular pillows preserved in greenstone on both sides of the highway, with argillite curtains between some pillows.

29.7 . . Youthful topography of the Klamath River canyon to the right.

34.0 . . Shasta Valley Overlook, sited on contorted, thin-bedded quartzite and schist of the Stuart Fork Formation. View to southeast of large volcanogenic debris avalanche deposit (Crandell and others, this volume).

35.2 . . Stuart Fork Formation exposed along the roadcuts; blueschist blocks visible on grassy hills.

36.2 . . Osburger Gulch Sandstone Member caps ridge to the east.

37.8 . . Yreka, north exit.

Yreka: in March, 1850, Abraham Thompson and partners struck it rich on Yreka Flats, after which 2,000 miners arrived in six weeks time; originally called Thompson's Dry Diggings and then Wyreka in 1852.

0.5 38.3 . . Exit into Yreka; turn left onto South Main Street and proceed south.

1.4 39.7 . , Junction with Oberlin Road, turn left and proceed east.

1.3 41.0 . . STOP 3. Oberlin Road, roadcut along Oberlin Road in northern Shasta Valley, 3 km southeast of Yreka (NE 1/4 NE 1/4 sec. 35, T. 45 N., R. 7 W., Yreka 15-minute quadrangle). This roadcut exposes an excellent section of the Klamath River Conglomerate Member.

At this stop, the reference section for the Klamath River Conglomerate Member, about 50 m of the unit are exposed (Fig. 5). Because the basal contact is covered, there may be as much as 35 m of additional section of the Klamath River Conglomerate Member here. The upper contact with the overlying marine Osburger Gulch Sandstone Member is, however, well exposed at the top of the northern roadcut (Fig. 6A).

The member can be divided into at least 7 cycles characterized by fining-upward and thinning-upward trends (Fig. 5). These cycles begin with an erosional and locally channelized contact between underlying shale, mudstone or siltstone and overlying conglomerate (Fig. 6A-D). The maximum clast size and thickness of bedding generally decrease upward within each cycle, which finally terminates with massive mudstone* The 7 cycles average about 7 m in thickness, although there is great variability in both thickness and internal organization between the cycles. At the base of the section is interstratified very fine-grained sandstone and siltstone with plant fragments that may define the top of yet another underlying cycle. The overlying cycle contains in its lower part interstratified lens-shaped bodies of conglomerate, conglomeratic sandstone, and very coarse grained sandstone. The conglomerate beds and lenses typically have erosive and slightly channeled bases and crude parallel layering to show no internal stratification. The clasts are well rounded, framework-supported, and imbricated. The longest clast observed is about 6 cm long. The conglomerate units typically fine upward into conglomeratic sandstone or sandstone that is crudely parallel-stratified, massive, or cross-stratified. Eastward transport of sandstone was determined from one cross-stratified sandstone bed. The top of this cycle is defined by a crudely parallel-stratified bed of medium-grained sandstone rather than shale or mudstone.

The overlying two cycles, which are both about 10 m thick, are more complexly organized and are separated by a major angular, channelized contact rather than an intervening shale unit (Fig. 6B). These cycles contain coarser conglomerate that has a maximum clast size of 10 cm; however, the bedding types in the conglomerate and sandstone units are similar (Fig. 6E) to those below except for the presence of some slumping, contorted bedding, liquefaction and water-escape features, large loadcasts, and abundant shale rip-up clasts in some beds. Carbonaceous partings and plant fragments are also common. Paleocurrent measurements from cross strata indicate sediment transport toward the northwest rather than toward the east. 19

base of Osburger Gulch Sandstone Member

GRAIN SIZE vc 32 50 sh ss mm cm I I I

Figure 5. Measured section of the Klamath River Conglomerate Member of the Hornbrook Formation at Field Trip Stop 3 along Oberlin Road. Vertical scale is in meters; arrows indicate paleocurrent directions; inclined bars indicate fining-upward cycles. 20

Figure 6. Photographs of the Klamath River Conglomerate Member of the Hornbrook Formation at Field Trip Stop 3 along Oberlin Road. A, View northward of the section, showing fining-upward cycles of conglomerate, sandstone, and mudstone; massive, resistant sandstone at the top of the bluff is the basal marine sandstone of the Osburger Gulch Sandstone Member of the Hornbrook Formation. B, Angular discordance and channeling in lower part of section. C, Detailed view of fining-upward cycle showing erosive scouring at base of cycle and lens-shaped bodies of conglomerate and sandstone within the cycle. D, Lower part of fining-upward cycle showing basal scoured surface of conglomerate on mudstone and interbedded conglomerate and sandstone; staff is 1.5 m long. E, Interbedded, cross- straitified and eroded lenses of conglomerate and sandstone in middle of fining-upward cycle. F, Mudstone at top of fining-upward cycle with paleosol in lower part and plant roots in growth position in middle part of photograph. 21

The top of the third cycle is formed of about 1.5 m of massive, red and yellow, mica- ceous siltstone and mudstone that has a minor paleosol developed on its top.

The upper four fining-upward cycles generally are thinner, finer, and contain a greater thickness of mudstone and shale in the upper parts of the cycles (Fig. 5; Fig. 6C). Maximum clast sizes of 12, 11, 8, and 9 cm are present in the upper four cycles, respectively. The cycles are more clearly fining-upward and trough cross-bedding is characteristic of the conglomeratic sandstone (Fig. 6D) and current ripple markings characteristic of thinner beds of very fine-grained sandstone and siltstone. The mudstone and shale at the tops of the cycles are varicolored and include green, yellow, yellow-green, red, and black intervals. Thin coal seams as well as in situ roots and trunks of trees are preserved in these fine-grained units (Fig. 6F).

These cycles in the Klamath River Conglomerate Member are of fluvial origin and represent the cutting and migration of channels across associated flood plains. The archi- tecture of the channels is difficult to ascertain from the limited exposures, but the upper channels and associated floodplain deposits appear to be laterally continuous for the extent of the outcrop. The general upward decrease in the thickness of the cycles, size of the clasts, and thickness of the coarse-grained parts of the cycles suggests an upward gradation from less sinuous, probably braided streams to more sinuous, probably meandering streams with larger surrounding flood plain areas.

The overlying more resistant conglomeratic sandstone of the Osburger Gulch Sandstone Member (unit "s" of Nilsen and others, 1983) rests abruptly on mudstone of the uppermost cycle of the Klamath River Conglomerate Member (Figs. 5, 6A). This unit consists of trough-cross-stratified conglomeratic sandstone with a maximum clast size of about 3 cm. This sandstone is medium- to coarse-grained and much better sorted than the underlying fluvial sandstones. Although megafossils were not observed in the lower part of the Osburger Gulch Sandstone Member at this locality, they are present farther north along the same ridge crest and marine trace fossils are moderately abundant.

* Return west on Oberlin Road to Yreka.

2.4 43.4 . . Junction of Oberlin Road and South Main Street: turn right, proceed north.

1.0 44.4 . . Junction with Interstate Highway 5; turn right and enter northbound ramp.

3.8 48.2 . . Bridge over the Shasta River, an underfit stream.

3.3 51.5 . . Riverview Mountain west of the highway, near the confluence of Shasta Canyon and Klamath Canyon.

3.9 55.4 . . Lunch stop: exit at Collier Rest Area.

Continue north on Interstate Highway 5.

2.1 57.5 . . Osburger Gulch on east side of Klamath River.

0.9 58.4 . . STOP 4. Klamath River, roadcuts along north-bound lane of Interstate Highway 5 adjacent to and west of the Klamath River, approximately 3 km south of the town of Hornbrook (NE 1/4 NE 1/4 sec. 32, T. 47 N., R. 6 W., Hornbrook 15-minute quadrangle). These roadcuts expose the type sections of the Ditch Creek Siltstone Member and overlying Rocky Gulch Sandstone Member of the Hornbrook Formation.

At this stop, we will examine the upper part of the type section of the Ditch Creek Siltstone Member (Fig. 7A) and the lower part of type section of the Rocky Gulch Sand- stone Member (Fig. 7B). The Ditch Creek Siltstone Member is 61.7 m thick here and the Rocky Gulch Sandstone Member is 171 m thick. The erosive contact between the two units is very well exposed and clearly shows the lithologic differences between the two members (Fig. 8A, B).

The Ditch Creek Siltstone Member grades upward from the top of the Osburger Gulch Sand- stone Member in a manner that is similar to that observed at Field Trip Stop 1. The Osburger Gulch Sandstone Member contains beds of megafossiliferous sandstone here as thick as 1.5 m in its upper part, whereas the lower part of the overlying Ditch Creek Siltstone Member consists mostly of massive thoroughly bioturbated sandy siltstone and silty sandstone.

In this section, the Ditch Creek Siltstone Member is fairly uniform in character, consisting of interbedded very fine grained sandstone and siltstone (Fig. 8C). The sandstone beds are commonly 10-30 cm thick and have a crudely developed parallel stratification; seme beds appear to be graded, but extensive bioturbation typically has de^ stroyed most of the internal sedimentary structures. The siltstone beds are as thick as 6 m and are typically massive as a result of thorough bioturbation. Detrial plant 22

GRAIN SIZE B vc 32 50 sh ss mm cm 1—I 1 1 1

base of Rocky Gulch I base of Blue Gulch Sandstone Member IMudstone Member n 60 ® 80 170 &ft

70 160 50 \ 60 150

40 50 140

40 130+5 Dft 30 Z3 30 120

=imfi

20 20 110-

ft ft 10 100 10 n n imH top of Ditch Creek a GRAIN SIZE Siltstone Member vc 32 50 ft —lsh s1s m1m 1cm

Om top of Osburger Gulch Sandstone Member

Figure 7. A, Ditch Creek Siltstone Member. Figure 7 (continued). B, Rocky Gulch Sandstone Vertical scale is in meters; arrows Member. Vertical scale is in meters; indicate paleocurrent directions. arrows indicate paleocurrent directions. 23

Figure 8. Photographs of the Ditch Creek Siltstone and the Rocky Gulch Sandstone Members of the Hornbrook Formation at Field Trip Stop 4 along Interstate Highway 5 near the Klamath River. A, View westward across the Klamath River showing the contact, marked by dashed line, between the Ditch Creek Silt- stone Member to the left and the Rocky Gulch Sandstone Member to the right. B and C, Contact between the two members, showing the irregular and erosive nature of the contact, concretionary nature of the Ditch Creek Siltstone Member, and massive character of the Rocky Gulch Sandstone Member. D, Abundant concretions at top of Ditch Creek Siltstone Member; intervals marked on staff are 10 cm long. E, Massive beds of sandstone in the lower part of the Rocky Gulch Sandstone Member cut by a vertical fault (dashed line) and with prominent shale interval. F, Thin, discontinuous shale interbed in the lower part of the Rocky Gulch Sandstone Member. G, Conglomerate composed of shale rip-up clasts. fragments and carbonaceous layers, calcareous concretions, and megafossils, including pelecypods, gastropods, and ammonities, are common throughout the section. Trace fossils include Ophiomorpha and a myridad of other small and large forms.

In the upper part of the member, calcareous concretions are especially abundant, both as irregular nodules, larger "cannonballs,11 and discontinuous layers that are parallel to bedding (Fig. 8C, D). Paleocurrents are generally impossible to obtain from the sequence because almost all primary sedimentary structures and bedding surfaces are destroyed by burrowing; however, primary current lineation was observed on the surface of one bed of very fine to fine-grained sandstone about 15 m above the base of the section and yielded an orientation of 105°/285°, which is plotted on Figure 7A as an easterly direction of flow.

The Ditch Creek Siltstone Member appears to have been deposited here in low-energy, outer-shelf conditions, probably below storm wave base. The thin sandstone interbeds may represent storm deposits transported offshore and subsequently strongly modified by burrowing. No well-defined turbidites are present in this section, in contrast to parts of the Ditch Creek Siltstone Member in the northern Cottonwood Creek Valley and Dark Hollow areas, nor are there nonmarine coal beds such as those in the northern Shasta Valley (Zigler and Nilsen, this volume).

The contact between the Ditch Creek Siltstone Member and the Rocky Gulch Sandstone Member is well exposed and clearly erosional (Fig. 8 A-D). The uppermost part of the Ditch Creek Siltstone Member contains a great concentration of irregular nodular calcareous concretions just below the contact (Fig. 8D), and some of these concretions are found in the basal beds of the Rocky Gulch Sandstone Member as clasts and eroded fragments.

Each of the lower beds of conglomeratic sandstone in the Rocky Gulch Sandstone Member fines gradually upward into massive medium- to coarse-grained sandstone that forms beds almost as thick as 10 m (Fig. 8E, F). The conglomerate beds contain clasts as large as 15 cm and also scattered rip-up clasts of shale as large as 50 cm. The base of the fourth conglomeratic bed contains very abundant large rip-up clasts as long as 1 m (Fig. 8G); this rip-up layer grades laterally eastward into a 4-m-thick shale bed overlain by a thin conglomerate (Fig. 8E, F). These relations are indicative of moderate amounts of scour and erosion at the base of the thick beds of conglomerate and sandstone. These beds do not contain any primary sedimentary structrues except some crudely developed swirly to subparallel stratification and sole marks, although grading is very well developed. No megafossils are found in growth position and only one abra- ded mollusk fragment was found in one bed of conglomerate.

Stratigraphically higher beds contain more complete Bouma sequences; some of these beds are more than 13 m thick. The beds weather in a pattern reminiscent of irregular cross bedding (Fig. 8H), but are not cross bedded. The beds, which are conglomeratic at their base, grade sucessively upward into massive medium-grained sandstone of the _a division, parallel-stratified fine- to medium-grained sandstone of the _b division, ripple-marked very fine to fine-grained sandstone of the c^ division, parallel laminated siltstone to very fine grained sandstone of the _d division. The shale intervals have been sampled for microfossils here but have been barren. The shale and, in many cases, the thinner upper parts of the Bouma sequence have been eroded off by the overlying bed to produce amalgamated beds of sandstone.

The thickness and coarseness of beds appear to gradually decrease upward through the section, although there are a number covered intervals (Fig. 7B). Some thin, coarse- grained, and laterally discontinuous beds of Mutti and Ricci Lucchi (1972, 1975) facies E sandstone where thin-bedded turbidites are preserved. Rip-up clasts of shale remain abundant throughout the section at the bases of the coarser grained beds, and marine burrows have been noted in a few shale interbeds above about 100 m above the base of the section.

The general character of the beds that make up the Rocky Gulch Sandstone Member suggests deposition by sediment gravity flows rather than by wind-wave-, storm-, or tide- generated currents. One paleocurrent measurement from a flute cast in the lower part of the section indicates southeastward transport (Fig. 7A) and the regional paleocurrent pattern suggests sediment transport toward the northeast (Nilsen, this volume). The section is generally organized in its lower part into very thick beds that fine upward and in its upper part into cycles that thin and fine upward and average about 10 m in thickness. The lack of major channeling and the lateral continuity of the beds suggests deposition as a turbidite apron on a sandy submarine slope (Nilsen, this volume).

The upper 70 m of the section contains numerous beds of siltstone and very fine-grained sandstone that are extensively ripple-marked and wavy-bedded. These beds rest abruptly on massive beds of fine- to medium-grained sandstone with a major break in grain size. These beds appear to represent deposition by currents flowing southward, sub- 25

parallel to the slope, at right angles to the downslope direction of flow of the sediment gravity flows that transported the coarser grained and thicker sand beds. These subsidiary currents, capable only of reworking the tops of the coarser grained sediment gravity flows, may have been contour currents flowing in response to thermohaline cir- culation.

Continue north on Interstate Highway 5.

1.2 . . . . 59.6 . . Exit at Hornbrook-Henley: turn left, proceed west under Interstate Highway 5 and imme- diately turn right.

0.5 .... 60.1 . . STOP 5. Rancheria Gulch, exposures in quarry west of U. S. Forest Service Forestry Camp about 1.5 km southwest of Hornbrook, west of Interstate Highway 5 adjacent to the Henley-Horn brook exit (S 1/2 SW 1/1 sec. 20, T. 17 N.-, R. 6 W., Hornbrook 15-minute quadrangle). This quarry exposes the type section of the Rancheria Gulch Sandstone Beds of the Blue Gulch Mudstone Member of the Hornbrook Formation.

The Rancheria Gulch Sandstone Beds form a lens within the lower part of the Blue Gulch Mudstone Member in the southern Cottonwood Creek Valley area (Nilsen and others, 1983; Nilsen and Barats, this volume). The thickest section of the lens, 86 m, has been measured here (Fig. 9). The lower contact of the lens is well exposed along roadcuts about 150 m to the west of the quarry and the upper contact is exposed in roadcuts about 200 m northwest of the quarry. Although the section here is about 30 percent covered, the lithology does not appear to change significantly upward.

The Rancheria Gulch Sandstone Beds consist mostly of fine-grained sandstone that forms prominent ridges and bluffs in the Cottonwood Creek Valley (Fig. 10A). Here in the quarry they consist chiefly of alternating beds of massive sandstone and hummocky- stratified to parallel-stratified sandstone (Fig. 10B). The massive beds are more thoroughly bioturbated and are slightly less resistant to erosion (Fig. 10C). The hummocky-stratified beds (Fig. 10D) decrease in abundance upward, and are not present in the uppermost 15 m and may possibly be absent from the uppermost 30 m. The beds of massive sandstone range from about 10 cm to 6 m in thickness, and the beds of hummocky- stratified sandstone range from about 10 cm to 1.10 m in thickness. Beds of sandstone are thickest in the middle part of the section and thinner near the base and at the top.

The upper part of the section contains interbeds of bioturbated siltstone and silty mudstone as thick 1.1 m; these interbeds mark the gradational upper contact with the overlying mudstone of the Blue Gulch Mudstone Member. Sandstone of the upper part of the section is generally finer grained, being typically very fine grained sandstone rather than fine-grained sandstone. However, the uppermost bed in the type, section consists of massive, medium-grained sandstone and contains the coarest grain size observed in the entire section. No conglomerate has been observed in the Rancheria Gulch Sandstone Beds.

Burrowing is common in the sandstone as well as the siltstone and silty mudstone. The burrows are typically solitary, but some beds, especially the mudstone and siltstone, are so thoroughly bioturbated that stratification is totally destroyed and individual trace fossils cannot be identified (Fig. 10E, F). The burrows are typically feeding burrows that penetrate subvertically into the beds, but some grazing traces on bedding surfaces are also present.

Megafossils, including pelecypods and gastropods, are found at several levels within the lens. They are present both in growth position within the massive fine sandstone and as reworked fossil debris, commonly near the base of hummocky-stratified beds. The megafossils are generally more abundant in the lower parts of the lens, although the uppermost bed of medium sandstone here contains abundant transported megafossil debris. Scattered plant fossils are present within massive beds of sandstone and also on bedding surfaces of hummocky-stratified sandstone.

The Rancheria Gulch Sandstone Beds were deposited in shallow-marine conditions, probably along a storm-wave dominated coastline. The sandstone-rich section suggest an abundant supply of sand and transport of muddy sediment perhaps further offshore. However, the lack of both paleocurrent measurements and data on the three-dimensional shape of the unit preclude more detailed paleogeographic speculation. The unit clearly records shoaling water depths in the southern Cottonwood Creek Valley area subsequent to and prior to deep-marine turbidite sedimentation in the Rocky Gulch Sandstone Member and upper part of the Blue Gulch Mudstone Member, respectively.

Return to Interstate Highway 5, and head north. 26 base of upper part of Blue Gulch Mudstone Member

©ft ft

80 HCS l^J

HCS 70

l HCS B 40 ft

©%

HCS 30 ft fl

p. ft 20 ©HCS © © H

Figure 10. Photographs of the type section of the Rancheria Gulch Sandstone Beds of the ©HCS Blue Gulch Mudstone Member of the Hornbrook Formation at Field Trip Stop 5 0 m fi near Rancheria Gulch. A, View northwest showing ridge underlain by the top of lower part of the Rancheria Gulch Sandstone Beds and Blue Gulch Mudstone Member massive quarry exposures. B, Hummocky- stratified and parallel-stratified fine- grained sandstone exposed in quarry; Figure 9. Measured type section of the Rancheria beds dips toward the viewer. C, Gulch Sandstone Beds of the Blue Gulch Alternating beds of more resistant, Mudstone Member of the Hornbrook light-colored hummocky-stratified sand- Formation at Field Trip Stop 5 near stone and less resistant dark-colored Rancheria Gulch. Vertical scale is in bioturbated sandstone; pen in meters. bioturbated interval circled for scale. 27

Figure 10 (continued). D, Hummocky-stratified sandstone. E, Hummocky-stratified sandstone that is partly bioturbated, with some stratification destroyed. F, Thoroughly bioturbated bedding surface.

0.7 • . . .60.8 . . Village of Hornbrook on the eastern side of the valley.

Hornbrook: this village grew around the switching yard of the Southern Pacific Rail- road on the south end of the Siskiyou Pass grade. Dave Horn built a hotel-saloon in 1888, shortly after the connection between Hornbrook and Ashland was established in 1887.

0.6 . . . . 61.4 . . Horn Peak, prominent to the east.

6.2 .... 67.6 . . STOP 6. Hilt Station area, roadcut east of Hilt gas station adjacent to the Hilt exit on Interstate Highway 5 (NW 1/4 SW 1/4 sec. 24, T. 48 N., R. 7 W., Hornbrook 15-minute quadrangle). This roadcut exposes the type section of the Hilt Bed of the Blue Gulch Mudstone Member of the Hornbrook Formation.

The Hilt Bed is a thick and laterally continuous bed of turbidite sandstone in the middle part of the Blue Gulch Mudstone Member that appears to extend for the entire outcrop length of the Hornbrook Formation. It ranges in thickness where observed from 1.46 to nearly 5 m. At its type locality here it is 4.08 m thick (Fig. 11).

The Hilt Bed is a compound turbidite bed, organized into a Taba[jabcde Bouma (1962) sequence. The first or basal _a division is typically the thickest part of the bed and here is 285 cm thick (Fig. 12A). The overlying divisions at the top of the bed are much thinner, particularly the middle and upper _a divisions, which are only 8 cm and 10 cm thick, respectively (Fig. 12B). The c division is ripple-marked and grades up into overlying parallel-laminated siltstone of the ji division. Mudstone of the uppermost _e division is several meters thick.

The base of the Hilt Bed here has flute casta that indicate sediment transport toward the northeast. The lower ji division also contains some large rip-up clasts of underlying shale that are concentrated near the base of the bed, although some smaller clasts extend upward into the upper part of the a^ division. 28 A B

Figure 11. Measured sections of Hilt Bed of the Blue Gulch Mudstone Member of the Hornbrook Formation at and adjacent to Field Trip Stop 6 near Hilt. A, Type section east of Hilt overpass (Stop 6). B, Ref- erence section west of Hilt overpass, about 200 m from Stop 6. Vertical scale is in centimeters; arrows indicate paleocurrent directions; lettered symbols to the left of each section indicate Bouma divisions.

Figure 12. Photographs of the Hilt Bed of the Blue Gulch Mudstone Member of the Hornbrook Formation at and adjacent to Field Trip Stop 6 near Hilt. A and B, Type section east of Hilt overpass; note the thin-bedded basin-plain turbidites stratigraphically above and below the Hilt Bed. C and D, Ref- erence section west of Hilt overpass; note the fault truncating the bed at the east (right) side of the photograph, the large slumped blocks in the center of the outcrop, and the layering in the upper part of the bed that outlines the upper Bouma sequences. 29 The Hilt Bed is coarsest in the lower _a division, which consists of fine- to medium- grained sandstone. The succeeding divisions and their grain sizes are the following: b^ fine-grained sandstone; _a, fine-grained sandstone; _b, very fine-grained sandstone; _a, fine-grained sandstone; _b, very fine-grained sandstone; £ and _d, very fine-grained sandstone and siltstone.

Several hundred meters to the west, along the road to Hilt, is another exposure of the Hilt Bed, the reference section. The east end of the bed here is truncated by a northeast-trending fault which has offset the bed from its type section (Fig. 12C, D). At this locality, the Hilt Bed is 4.06 m thick and the basal a^ division is 275 cm thick. This section is very similar to the first, and flute casts at its base also indicate flow toward the northeast. The widespread lateral distribution of the Hilt Bed is clearly indicative of basin- plain sedimentation. It may have been triggered by a sequence of retrogressive land- slides or, alternatively, by three turbidity currents generated almost simultaneously along three different parts of the same slope. The Hilt Bed is compositionally similar to both other beds of turbidite sandstone in the Blue Gulch Mudstone Member "and to beds of sandstone in other members of the Hornbrook Formation (Golia and others, this volume), so that a more exotic source for it is not needed.

Return to Interstate Highway 5 and head north.

0.2 . . . . 67.8 . . Pilot Rock to the east (elevation 1,803 m): originally called Emmons Peak by the Wilkes Expedition of 1841, this volcanic neck received its name because it served as a guide for pioneer travelers over the Siskiyou Mountains.

5.1 . . . .72.9 . . Siskiyou Summit.

2.1 . . . . 75.0 . . Emigrant Creek Reservoir on the valley floor to the right (east).

Emigrant Creek: named after the southern branch (Applegate Trail) of the Oregon Trail, which led into the Bear Creek Valley from the Cascade Mountains.

8.0 . . . . 83.0 . . South Ashland overpass: exit and turn right, proceed east on U.S. Highway 66.

0.5 .... 83.5 . . Junction of U.S. Highway 66 and Dead Indian Road; turn left and proceed north on Dead Indian Road.

0.7 ... . 84.2 . . STOP 7. Road outcrops along Dead Indian Road between the Ashland Airport and Lithia Spring, about 5 km east of Ashland (SWJ sec. 12, T. 39 S., R. 1 E., Ashland 15-minute quadrangle). This roadcut exposes the Blue Gulch Mudstone Member.

At this stop, basin-plain turbidites of the Blue Gulch Mudstone Member are beautifully exposed in a series of roadcuts. About 62 m of section were measured, and every turbidite bed at least 5 cm thick has been plotted on the measured section for this stop (Fig. 13). Although the base and the top of Blue Gulch Mudstone Member in this section are covered by Quaternary alluvium, we know that this section is stratigraphically situated in the lower middle part of the Blue Gulch Mudstone Member because the Hilt Bed crops out in a streamcut about 0.5 km to the east-southeast of this stop. The Hilt Bed there is slightly higher stratigraphically than the top of this section. Benthic foraminifers from this section (U.S.G.S. Mesozoic localities Mf6410 and Mf6579), indicate a Coniacian to Maestrichtian age (W. V. Sliter, written communs., 23 November 1982 and 6 December 1983).

This section consists of rhythmically interbedded turbidite sandstone and mudstone (Fig. 14A, B). Each turbidite bed is graded and organized into the Bouma sequence (Fig. 14C). Thinner beds typically consist of siltstone and are generally massive or laminated, and thicker beds consist of very fine-grained sandstone at the base and grade upward into siltstone. ' The thickest turbidite bed measured is 50 cm and the average bed is probably about 10-15 cm thick. No beds^commence with the basal a^ division; instead, each bed begins at its base with one of the upper Bouma divisions. The grain size of the turbidite beds is very uniform and vertical variations in thickness, except possibly near the base of the section, appear to be random, which is characteristic of basin-plain turbidites. The base of the section may contain some thickening- upward cycles, but these do not appear to be characteristic of the entire section.

Flute casts and groove casts are present but not common on the bases of the turbidite beds. Most paleocurrent data has been obtained from abundant current ripple marks and convolute lamination in the Bouma (2 divisions (Fig. 14D) an<| primary current lineation in the Bouma Jj divisions. Sediment transport was dominantly toward the northeast in the lower part of the section and toward the north-northwest in the upper part of the section. 30 top not exposed GRAIN SIZE sh ss gravel I—I 1 30 n

D -A f

•• • r> 20 -f •' N -<\ 50 H

3D « Mf &65 10 79 ^CN

ssEzzn

<$> Mf 6410

0 m base not exposed

Figure 13. Measured section of the Blue Gulch Mudstone Member of the Hornbrook Formation at Field Trip Stop 7. along Dead Indian Road in Ashland. Vertical scale is in meters; arrows indicate paleocurrent directions. Figure 14. Photograph of the Blue Guleh Mudstone Member at Field Trip Stop 7 along Dead Indian Road in Ashland. A and B, Rhythmically interbedded sandstone turbidites and mudstone. C, Graded beds of turbidite sandstone having sharp bases with sole markings and gradational tops; hammer circled for scale. D, Turbidite beds organized into Tcde sequences, with lower bed characterized by convolute laminations.

Bioturbation is common throughout the section. It is most abundant in the interbedded mudstone units and is variable in type, including both grazing traces parallel to bedding surfaces, irregular small traces such as Chondrites, and larger vertical and subvertical tubular sandstone-filled burrows that extend into and locally through the beds of sandstone and siltstone.

Two types of interbedded mudstone can be recognized locally in the section. Typically the lower mudstone is gray in color and coarser grained, the upper mudstone is green and finer grained. The lower mudstone is probably of turbidite origin, transported to the basin floor by the same current that carried the coarser silt and sand. The upper mudstone is hemipelagic in origin, probably contains larger amounts of pelagic organ- isms, and was deposited during the much longer interval between turbidite events.

Return to Ashland via Dead Indian Road and U.S. Highway 66.

3.9 .... 88.1 . . SOSC Geology Department parking lot. END OF FIRST DAY. 32

SECOND DAY

0.0 .... 0.0 . . SOSC Geology Department parking lot: proceed west on Ashland Street to junction with Mountain Street, turn right.

0.3 .... 0.3 . . Junction with Siskiyou Boulevard: turn left and proceed northwest to city center, maneuver into right lane.

1.1 ... . 1.4 . . Bridge over Ashland Creek.

0.2 .... 1.6 . . Junction with Laurel Street: turn right (north).

0.6 .... 2.2 . . STOP 8. Ashland, roadcuts northeast of the intersection of Laurel Street and Randy Street (sW/ijsec. 4, T. 39 S., R. 1 E., Ashland 15-minute quadrangle). This roadcut exposes nonmarine conglomerate and arkosic sandstone of the upper Eocene Payne Cliffs Formation.

The Payne Cliffs Formation exposed here is thin and has unconformably overlapped most of the upper part of the Hornbrook Formation to rest unconformably on the Ditch Creek Siltstone Member and Rocky Gulch Sandstone Member (Nilsen and Barats, this volume). As much as 900 m of the upper part of the Hornbrook Formation, including all of the Blue Gulch Mudstone Member, has been stripped off in this area by major local downcutting along the erosional surface between the Hornbrook Formation and the Payne Cliffs Forma- tion.

The purpose of this stop is to familiarize participants with the general character of the overlying Tertiary nonmarine deposits and to contrast their sedimentary features with those of the Hornbrook Formation. This conglomerate is part of the basal conglomerate unit of the Payne Cliffs Formation, which is as thick as 160 m and is thought to have been deposited as the proximal bedload unit of a braided-stream system (McKnight, this volume). The outcrop forms part of a group of outliers of the Payne Cliffs Forma- tion in the northwestern Bear Creek Valley and southeastern Medford Valley (Nilsen and Barats, this volume).

The outcrop consists of alternating lenses of massive conglomerate and sandstone that generally form fining-upward couplets (Fig. 15A). Channeling is difficult to recognize because of the small size of the outcrop, but it appears that the basal surface of the conglomerate beds is always erosive into the sandstone, whereas the sandstone appears only locally to rest erosively on the conglomerate. The sandstone is arkosic, medium- to coarse-grained, commonly pebbly, and appears to be massive to crudely parallel stratified (Fig. 15B).

The conglomerate beds are clast-supported and contain a variety of well-rounded pebbles and cobbles. The conglomerate is generally less resistant and not as well lithified as conglomerate of the Hornbrook Formation. Pebbles here can be removed from the matrix relatively easily.

A pebble count made at this locality by Greg M. Barats yielded a composition of 42 percent quartzite, 26 percent metavolcanic rocks, 19 percent granodiorite and diorite, 8 percent distinctive metamorphic rocks of uncertain origin, 3 percent metasedimentary rocks, and 1 percent each of chert and porphyritic metavolcanic rocks. McKnight (this volume) reports the following clasts, in decreasing order of abundance, in the basal conglomerate unit of the Payne Cliffs Formation: felsic to intermediate metavolcanic rocks, quartzite, mafic volcanic rock fragments, chert, and vein quartz. He also notes the presence of minor amounts of granitic to intermediate plutonic rock fragments, slate, and sandstone. The increased abundance of less resistant plutonic clasts here may reflect the proximity of this locality to the presumed source of plutonic debris, the Mount Ashland pluton to the southwest.

McKnight (this volume) presents paleocurrent and petrographic evidence that the Klamath Mountains were the chief source area for the Payne Cliffs Formation. Braided rivers that flowed toward the north and northeast transported the coarse detritus from the Klamath Mountains, which clearly were subjected to major uplift in early Tertiary time.

The conglomerate of the Payne Cliffs Formation here is strongly imbricated, suggesting sediment transport toward the south (Fig. 15B) This is opposite to McKnight's (this volume) dominant north-northeastward direction of sediment transport for most of the Payne Cliffs Formation. The divergence in transport direction here may be related to the variable topographic slope caused by the extreme amount of downcutting in this area or simply to a local change in flow direction of part of the braided-stream system.

Continue north on Laurel Street. 33

Figure 15. Photograph of the Payne Cliffs Formation at Stop 8 in Ashland. A, Alternating lenses of conglomer ate and sandstone. B, Parallel-stratified sandstone and conglomerate that is well imbricated, suggesting relative transport of clasts from left to right or toward the south.

0.1 2.3 Junction with Nevada Street: turn right.

0.3 2.6 Junction with Oak Street: turn left.

0.4 Cross Bear Creek bridge to junction with Eagle Mill Road: turn left.

0.1 STOP 9. Ashland, roadcuts at the intersecton of Oak Street and Eagle Mill Road, ad jacent to a small dam on the north side of Bear Creek (SW~ sec. 3, T. 38 s., R. 1 E., Ashland 15-minute quadrangle). These roadcuts expose the Rocky Gulch Sandstone Member of the Hornbrook Formation.

About 74 m of the Rocky Gulch Sandstone Member are exposed here, the best exposures of this unit in the Bear Creek Valley (Fig. 16). Unfortunately, however, neither the top nor the bottom contacts of the unit are exposed.

The section can be divided into eighteen thinning- and fining-upward cycles that aver age about 4 m in thickness (Fig. 17A). The overall sandstone-to-shale ratio is about 8-to-1. Sandstone beds are as thick as 3.9 m and are commonly separated by thin shale, silty shale, or siltstone intervals. Amalgamation of sandstone beds is present local ly, but is not as common as in sections of the Rocky Gulch Sandstone Member in the Cottonwood Creek Valley area. The thicker sandstone beds are typically massive and poorly graded whereas the thinner beds are finer, better graded, and contain parts of the Bouma sequence. Little conglomerate is present in this section; only one bed with clasts as large as 0.5 cm was observed. However, rip-up clasts of shale are very com mon, particularly in the lower parts of thicker beds at the base of the thinning-upward cycles. There is noticeable scour at the base of each bed of sandstone, and flute casts are common (Fig. 17B); however, channeling cannot be demonstrated anywhere in the section, and the maximum depth of erosion is less than 30 cm.

Sedimentary flaps that extend from the underlying shale and thin-bedded turbidite de posits into the basal parts of thicker, overlying beds of sandstone are conspicuous in several parts of the section. The flaps are associated with abundant rip-up clasts. Carbonate concretions are also common in many thicker beds. Plant fragments are abund ant in some thinner beds of fine-grained sandstone and siltstone, and locally are pres ent as large fragments in thicker and coarser grained beds.

Reworked tops of many turbidite beds are present in this section. The reworking has formed abundantly ripple-marked, locally convoluted, and wavy-bedded siltstone deposits that are as thick as 30 cm and which rest abruptly with a marked decrease in grain size on coarser grained beds of sandstone (Fig. 17C, D). The reworked siltstone intervals are locally interbedded with shale (Fig. 17E). Other beds of sandstone are abruptly overlain by mudstone without the presence of reworked intervals (Fig. 17F). Paleocur rents determined from current ripple markings in the reworked siltstone suggest sedi ment transport toward the south and southeast, about 90 0 to the dominant northeastward transport direction of the coarser beds of turbidite sandstone. The nature and origin of the currents responsible for this reworking are not known, but thermohaline flow of contour currents along the base or lower part of the northeast-facing submarine slope of the Klamath Mountains seems reasonable.

Continue northwest on Eagle Mill Road. 34

0 ] Tertiary o o sedimentary o ° 0° )rocks

504/-: 10

\fr ~ base not exposed GRAIN SIZE vc 32 50 sh ss mm cm L_l I 1

Figure 16. Measured section of the Rocky Gulch Sandstone Member of the Hornbrook Formation at Field Trip Stop 9 in Ashland. Vertical scale is in meters; arrows indicate paleocurrent directions; inclined lines indicate fining-upward sequences. 35

Figure 17. Phonographs of the Rooky Gulch Sandstone Member of the Hornbrook Formation at Field Stop Stop 9 in Ashland. A, Thinning- and fining-upward cycles (outlined by arrow); geologist circled for scale. B, Top of thinning- and fining-upward cycle; flute casts are present on the sole of the thick sandstone bed forming the base overlying cycle. C and D, Reworked tops of turbidite beds. E, Reworked interval within shale. F, Abrupt top of sandstone bed that is not reworked. 36

1.8 . . . . 4.9 . . Junction with Valley View Road: turn right, proceed north.

0.4 .... 5.3 • • Valley View Road overpass: enter Interstate Highway 5, proceed north toward Medford. Foothills to the right (east) are underlain by fluviatile deposits of the Tertiary Payne Cliffs Formation.

4.1 ... . 9.4 . . Roadcut on right exposes fluviatile conglomerate of the Payne Cliffs Formation.

3.0 .... 12.4 . . STOP 10. Roadcut along Interstate Highway 5 about 5 km southeast of Medford (SW 1/4 sec. 32, T. 37 S., R. 1 W., Medford 15-minute quadrangle). This roadcut exposes turbidites thought to be the Rocky Gulch Sandstone Member of the Hornbrook Formation.

About 73 m of the Rocky Gulch Sandstone Member are exposed along the highway (Fig. 18). The section consists dominantly of thick-bedded sandstone organized into fining- upward cycles (Fig. 19A). Neither the top nor bottom of the unit is exposed. At the southeast end of the roadcut, a prominent northeast-trending fault juxtaposes the marine strata of the Rocky Gulch Sandstone Member with fluvial conglomerate of the Payne Cliffs Formation (Fig. 19B). Wells (1956) and Smith and others (1982) mapped all the rocks at this locality as lower Tertiary, but Nilsen and others (1983) recognized and mapped the sandstone unit as part of the Hornbrook Formation. Foraminifers collected from these strata (U.S.G.S. locality Mf6587) were originally thought to be of Campanian age (W. V. Sliter, written commun., 6 December 1983), but their age has subsequently been revised to middle Turonian (W. V. Sliter, written commun., 27 August 1984).

The exposures are separated from adjacent exposures of the Hornbrook Formation to the southeast by at least several kilometers. Because of this separation and the general stratigraphic position of these rocks above the exposures of the Blue Gulch Mudstone Member in the Dark Hollow area, some workers have suggested that the rocks exposed at Stop 10 could represent a separate sandstone body within the Blue Gulch Mudstone Member, stratigraphically far above the Rocky Gulch Sandstone Member (M. A. Elliott, oral commun., 6 August 1984). The middle Turonian age, however, suggests that this is improbable. The general similarity of these rocks to those of the Rocky Gulch Sand- stone Member in other areas and the presence of northwest-trending normal faults along the southwestern margin of the Medford Valley (Nilsen and others, 1983; Nilsen and Barats, this volume) suggests to me (T.H.N.) that these rocks are a downfaulted piece of the Rocky Gulch Sandstone Member.

The exposed section consists dominantly of beds of medium- to coarse-grained sandstone with subordinate amounts of conglomerate, siltstone, and shale. The conglomerate and sandstone-to-siltstone and shale ratio is extremely high, about 90-to-1, and amalgamation of successive beds is common. The beds of sandstone are as thick as 6.1 m and are typically massive and crudely graded. Dish structure, crude subparallel lamination, scattered plant fragments, carbonate concretions, reverse grading in the lower parts of beds, slurried intervals, and flute casts at the bases of beds are all present. The grading is of the coarse-tail type, in which the average or maximum clast size decreases upward. Scattered pebbles with a maximum clast size of about 2 cm are common at the base of many beds, but beds that consist wholly of conglomerate are not present.

Rip-up clasts of shale are also common in some beds, as are sedimentary flaps of thin- bedded turbidites that are upturned and folded up into overlying thick beds of sandstone (Fig. 19C, D). These flaps were originally described and sketched by Boggs and Swanson (1970), who attributed the folding to detachment of the underlying thin-bedded deposits and folding by slumping. These sedimentary flaps are concentrated at the base Boum of a 3.99-cm-thick bed that is beautifully graded and organized into a Tajj0e a sequence. The base of the bed contains the flaps, some of which are completely de- tached, as well as rip-up clasts of shale and fine-grained sandstone, scattered pebbles as large as 0.5 cm, and flute casts which yield a northeastward transport direction. The upper part of the a^ division of the bed contains dish structure; the b^ division, which is 38 cm thick, is parallel-laminated, and the £ division, which is 6 cm thick, is ripple-marked. The overlying bed of sandstone, which is 3.0 m thick, contains smaller flaps and flame structures at its base, and abundant rip-up clasts of shale. Seve- ral other beds contain evidence of syn- and postdepositional disturbance such as slump folding (Fig. 19E), contorted bedding, and fluid-escape structures. Large-scale rip-up clasts (Fig. 19F) and convolute bedding are locally present (Fig. 19G).

Where successive beds are not amalgamated, the graded tops of the beds commonly consist of fine to very fine grained sandstone that grades up into thin shale partings (Fig. 19G). Many of the shale beds are silty and even sandy. Almost no bioturbation can be observed in the section, either in the sandstone or shale. The tops of several beds consist of ripple-marked siltstone and very fine grained sandstone that ap^ars to be reworked, similar to the tops of many beds of sandstone at Field Trip Stop 9. 37

base not exposed Figure 19. Photographs of the Rocky Gulch Sandstone Member of the Hornbrook Formation at Field Trip Stop 10 near Medford. A, GRAIN SIZE Outcrops of thick-bedded sandstone; vc 32 50 arrows indicate vertical extent of sh ss mm cm thinning- and fining-upward cycles. B, I—i 1 1 1 Northeast-trending fault separating the Rocky Gulch Sandstone Member on the left from conglomerate of the Payne Cliffs Figure 18. Measured section of the Rocky Gulch .Formation on the right; dashed line Sandstone Member of the Hornbrook represents main fault surface. C, Formation at Field Trip Stop 10 near Sedimentary flap of thin-bedded Medford. turbidites upturned and folded into overlying thick-bedded sediment-gravity- flow deposit. D, Close-up of fold crest. 38

Figure 19 (continued). E, Slump-folded interval. F, Large folded rip-up clasts; hammer circled for scale. G, Convolute bedding and load casts. H and I, Laterally discontinuous thin-bedded and coarse-grained Facies E beds of Mutti and Ricci Lucchi (1972, 1975) at the top of a thinning- and fining-upward cycle. J, Scoured contact between thick bed of sandstone and shale at base of thinning- and fining-upward cycle. 39

Thin-bedded faeies E turbidites of Mutti and Ricci Lucchi (1972, 1975) are present between some of the thicker beds (Fig. 19H, I). These facies E beds are thin, coarse- grained, and laterally discontinuous. They are cross-stratified or locally massive, and characterized by abrupt lateral changes in thickness, yielding a pinch-and-swell geometry.

The sandstone beds are organized into thinning- and fining-upward cycles that are about 10 m in thickness. Facies E beds are preserved at the tops of many of these cycles and facilitate recognition of the cycle boundaries. The bases of the cycles are typically erosional, but major channels cannot be observed. The maximum amount of downcutting seen in the outcrops is about 0.5 m (Fig. 19J).

Paleocurrents measured from flute casts indicate sediment transport toward the northeast (Fig. 18). Thus, the types of beds, organization of beds, and paleocurrent orien- tations here are similar to other outcrops of the Rocky Gulch Sandstone Member such as those at Field Trip Stops 4 and 9.

Continue north on Interstate Highway 5.

0.9 . . . 13.3 . Exit from Interstate Highway 5 at Exit 27 (Jackson-ville exit).

0.2 . . . 13.5 Junction with Barnett Road: turn left toward Jacksonville and Medford.

0.3 • . . 13.8 Junction with Stewart Avenue: turn left (south).

0.3 • . . 14.1 . Junction with U. S. Highway 99: turn left.

1.7 • . . 15.8 Junction with South Stage Road: turn left.

2.4 . . . 18.2 Junction with Dark Hollow Road: turn left.

0.5 . . . 18.7 . STOP 11A. Dark Hollow area, roadcut along the west side of Dark Hollow Road (SEJ sec. 50, T. 38 S., R. 2 W., Medford 15-minute quadrangle). This roadcut exposes what is thought to be the upper part of the Blue Gulch Mudstone Member of the Hornbrook Forma- tion; however megafossil localities in this area suggest a Turonian age for these rocks (Sliter and others, this volume).

In the Dark Hollow area, conglomeratic beds that form resistant ledges are locally abundant in typical mudstone and thin-bedded turbidites of the upper part of the Blue Gulch Mudstone Member above the Hilt Bed (Fig. 20A). These beds of conglomerate are interbedded with thin beds of normally graded siltstone and very fine-grained sandstone that are dominated by parallel lamination and organized into Bouma T sequences (Fig. 20B). The conglomerate beds contain clasts as large as 3 cm (Fig. 20C) and also contain some retransported broken shelly debris (Fig. 20D). The thin-bedded turbidites yield transport directions from flute casts that are toward the north and northwest. However, locally abundant burrowing and some slumping in the thin-bedded turbidites suggests that deposition might have taken place at shallower depths in this part of the section; this upward shoaling in the Blue Gulch Mudstone Member has also been observed in the type section (Nilsen, 1984).

Some of the conglomerate beds are clearly reverse graded, in contrast to the sandstone and siltstone interbeds. The conglomerate beds are also massive and contain no internal sedimentary structures or divisions of the Bouma sequence. The tops of the beds have a sharp rather than gradational contact with overlying mudstone. The bed thickness and grain size change abruptly over distances of a few meters. Synsedimentary folding is associated with some of the conglomeratic beds. The beds were probably transported as debris-flows rather than turbidites, and may have been derived from the shelf and slope to the southwest as slumps that changed downslope into debris flows. Althogh beds of this type have been observed in a few other localities in the Blue Gulch Mudstone Member, it is not clear why they are so abundant in this area.

Continue south on Dark Hollow Road. 40

Figure 20. Photographs of beds of debris-flow conglomerate interbedded with mudstone of the Blue Gulch Mud- stone Member of the Hornbrook Formation at Field Trip Stop 11A in the Dark Hollow area. A, General view of roadcut showing resistant conglomerate ledges in upper right. B, Beds of fine conglomerate overlying parallel-laminated sandstone and siltstone. C, Bed of fine conglomerate that is massive, reverse graded, and partly matrix-supported. D, Bed of conglomeratic sandstone that contains abundant megafossil debris.

0.4 .... 19.1 . . Stop 11B. Dark Hollow area, roadcut along the east side of Dark Hollow Road (NW4 sec. 12, T. 38 S., R. 2 W., Medford 15-minute quadrangle). This roadcut exposes what is thought to be the middle part of the Blue Gulch Mudstone Member.

At this locality, thin-bedded basin-plain turbidites of the Blue Gulch Mudstone Member are intruded by both basaltic and clastic dikes and offset by faults (Fig. 21A, B). Most of the dikes are oriented vertically; they are locally offset by subhorizontal faults and high-angle reverse faults. Measurements of paleocurrents from flute casts at the bases of the thin-bedded turbidites, which typically form Bouma Tode sequences, yield sediment transport directions toward the north and northeast.

At the southernmost end of the roadcut, the Hilt Bed is exposed adjacent to the road. Here it is only about 2.0 m thick, and primary current lineation in the middle Bouma division indicates sediment transport toward the northwest.

Continue south on Dark Hollow Road.

0.7 .... 19.8 . . Junction with Pioneer Road: turn right.

0.1 . . . . 19.9 . . Right turn on Pioneer Road.

0.7 ... • 20.6 Junction with Griffin Creek Road: turn right. 1.0 . . . . 21.6 Junction with South Stage Road: continue straight ahead. Figure 21. Photographs of the Blue Gulch Mudstone Member of the Hornbrook Formation at Field Trip Stop 11B in the Dark Hollow area. A, Northeast-dipping thin-bedded turbidites intruded by clastic dike at left. B, Several clastic dikes offset by subhorizontal and subvertical faults; inked lines drawn on some of the faults.

0.3 .... 21.9 . • Left turn on South Stage Road.

1.5 . . . . 23.4 . . Bellinger Hill on right, underlain by the Klamath River Conglomerate Member and Osburger Gulch Sandstone Member.

1.3 . . . • 24.7 . . Jacksonville City Limits.

Jacksonville: Gold was discovered on Jackson Creek in January, 1852 by Kluggage and Poole, who were leading a pack train carrying Willamette Valley produce south to miners. Jackson County was named after Andrew Jackson, seventh President of the United States. Jacksonville, at its peak in the 1850's and 1860's, was the most populous town in Oregon with 10,000-12,000 people.

0.4 .... 25.1 • • Junction with Jacksonville Highway (North 5th Street): turn right, proceed one block.

0.1 ... . 25.2 . . Lunch Stop: Jacksonville Museum, old courthouse and county seat of Jackson County. Return to Ashland via South Stage Road and Interstate Highway 5. END OF SECOND DAY

REFERENCES CITED Nilsen, T. H., 1984, Tectonics and sedimentation of Boggs, Sam, and Swanson, F. J., 1970, Unusual slump the Upper Cretaceous Hornbrook Formation, Oregon structure from Cretaceous(?) sandstones, northern and California, _in Crouch, J. K., and Bachman, S. Klamath Mountains region, Oregon: Ore Bin, v. 32, B., Tectonics and Sedimentation along the p. 25-29. California Margin: Society of Economic Paleontol- Bouma. A. H., 1962, Sedimentology of some flysch ogists and Mineralogists, Pacific Section, v. 38, deposits: Amsterdam, Elsevier Publishing Company, p. 101-118. 168 p. Nilsen, T. H., Barats, G. M., Elliott, M. A., and Irwin, W. P., 1960, Geological reconnaissance of the Jones, D. L., 1983, Geologic map of the outcrop northern Coast Ranges and Klamath Mountains, area of the Hornbrook Formation, Oregon and California, with a summary of the mineral re- California: U.S. Geological Survey Open-File sources: California Division of Mines Bulletin Report 83-373, scale 1:62 , 500. 179, 80 p. Smith, J. G., Page, N. J., Johnson, M. G., Moring, B. Mutti, Emiliano, and Ricci Lucchi, Franco, 1972, Le C., and Gray, Floyd, 1982, Preliminary geologic torbiditi dell' Appennino setentrionale— map of the Medford 1° x 2° quadrangle, Oregon and introduzione all' analisi dl facies: Societa California: U.S. Geological Survey Open-File Geologica Italiana Memorie, v. 11, p. 161-199. Report 82-955, scale 1:250,000. , 1975, Turbidite facies and facies associa- Wells, F. G., 1956, Geology of the Medford quadrangle, tions, _in Examples of Turbidite Facies and Facies Oregon-California: U.S. Geological Survey Associations from Selected Formations of the Geologic Quadrangle Map GQ-89, scale 1:96,000. Northern Apennines: Ninth International Congress of Sedimentologyj Nice> Francef Field Trip Guide— book A11, p. 21-36.