Geology of the Blue Mountains Region 

of Oregon, Idaho, and Washington:

Cenozoic Geology of the 

Blue Mountains Region

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Geology of the Blue Mountains Region 

of Oregon, Idaho, and Washington:

Cenozoic Geology of the 

Blue Mountains Region

GEORGE W. WALKER, editor

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1437

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1990
DEPARTMENT OF THE INTERIOR 

MANUAL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this

publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

Cenozoic geology of the Blue Mountains Region / George W. Walker, editor

p. cm. (Geology of the Blue Mountains Region of Oregon, Idaho, and Washington) (U.S. Geological Survey 

professional paper; 1437)

Includes bibliographical references. 

Supt. of Docs, no.: I 19.16:1437

1. Geology, Stratigraphic Cenozoic. 2. Geology Blue Mountains Region (Or. and Wash.) 3. Geology Idaho. 

I. Walker, George Walton, 1921- . II. Series. III. Series: U.S. Geological Survey professional paper ; 1437. 

QE690.C424 1990 551.7'8'097957-dc 20 89-600366

CIP

For sale by the Books and Open-File Reports Section, U.S. Geological Survey 

Federal Center, Box 25425, Denver, CO 80225
PREFACE

This U.S. Geological Survey Professional Paper is one volume of a series that 

focuses on the geology, paleontology, and mineral resources of eastern Oregon, 

western Idaho, and southeastern Washington. The purpose of this series is to 

familiarize readers with the work that has been completed in the Blue Mountains 

region and to emphasize this region's importance for understanding island-arc 

processes, the accretion of an allochthonous terrane, and postaccretion magmatism and volcanism. These professional papers provide current interpretations of 

a complex island-arc terrane that was accreted to ancient North America during 

the late Mesozoic, of a large batholith that intruded after accretion had occurred, 

and of overlying Cenozoic volcanic and sedimentary rocks that were subsequently 

uplifted and partly stripped off the older rocks by erosion.

Most of the Cenozoic volcanogenic rocks were locally derived, although the 

source vents for some large volumes of rock remain unknown; other rocks formed 

from material erupted in areas west of the Blue Mountains and were transported 

eastward into the region. Volcanism within this large region appears to have been 

more or less continuous throughout the Cenozoic, but the kinds of volcanic 

products varied both geographically and over time. Earth scientists who have 

worked on the Cenozoic volcanic rocks of the Blue Mountains and adjacent areas 

have attributed the volcanism partly to several different volcanic arcs and partly 

to extensional volcanism in regions behind arc axes.

Most earth scientists who have worked in the Blue Mountains region agree that 

the pre-Tertiary rocks there 'form one or more allochthonous terranes. The 

importance of such terranes in the evolution of circum-Pacific continental margins 

has been recognized for about a decade, but many complex questions remain. For 

example, how, when, and where did most of the circum-Pacific allochthonous 

terranes form? How did they accrete to continents, and how have they affected 

and been affected by subsequent processes? What are the mechanisms of 

amalgamation during terrane formation and transport? And, perhaps most 

importantly, what are the effects of these processes on mineral and hydrocarbon 

resources? Although the volumes in this series provide some answers, the data 

and interpretations contained in them will no doubt raise new and equally 

intriguing questions for future generations of earth scientists.

m

CONTENTS

{Numbers indicate chapters]

Page

1. Overview of the Cenozoic geology of the Blue Mountains region

Georjre AV. AValker ______________________________________________ i

2. Paleocene(?>, Eocene, and Oligocene(?) rocks of the Blue Mountains region

George W. Walker and Paul T. Robinson 13

3. Eocene(?), Oligocene, and lower Miocene rocks of the Blue Mountains region

Paul T. Robinson, George W. Walker, and Edwin H. McKee 29

4. The Columbia River Basalt Group and associated volcanic rocks of the Blue Mountains 

province 

Peter R. Hooper and Donald A. Swanson 63

5. Miocene and younger rocks of the Blue Mountains region, exclusive of the Columbia River Basalt Group 

and associated mafic lava flows 

Georce AV. AValker ______________________________________________ 101

6. Cenozoic tectonism and volcanism of the Blue Mountains region

George W. Walker and Paul T. Robinson 119

1. OVERVIEW OF THE CENOZOIC GEOLOGY OF 

THE BLUE MOUNTAINS REGION

By GEORGE W. WALKER

CONTENTS

Page

Abstract 1

Introduction 1

Acknowledgments 3

Previous work 3

Purpose and scope 6

Regional stratigraphic relations 7

References cited 10

ABSTRACT

This volume, besides this brief overview, presents five papers that 

deal with the Cenozoic volcanic and sedimentary rocks of the Blue 

Mountains region. Most of the descriptive material concerning these 

rocks is organized into chapters that discuss stratigraphic relations, 

isotopic and paleontologic ages, and petrographic and petrologic data by 

age groups. The final chapter discusses some implications of Cenozoic 

tectonism and volcanism of this large region.

INTRODUCTION

The Cenozoic geologic history of the Blue Mountains, 

northeastern Oregon and adjacent southeastern Washington, and of contiguous areas in Idaho and central 

Oregon is dominated by major episodes of volcanism 

interspersed with periods of sedimentation that took 

place mostly in local tectonic or volcanotectonic depressions. The episodes of andesitic, rhyolitic to rhyodacitic, 

and basaltic volcanism, which are related to widely 

dispersed vent systems of many different kinds, exhibit 

both temporal and geographic differences; during some 

periods, diverse chemical types of rock were being 

erupted in large volumes more or less synchronously.

Regional correlations and extrapolations are only approximate in this area dominated by complex mixtures of 

intrusive, volcanic, and volcaniclastic rocks and by numerous separate and ephemeral basins of deposition. 

Early workers relied mainly on general lithology and

Manuscript approved for publication, July 11, 1988.

sparse paleontologic evidence to extrapolate and extend 

stratigraphic units over large parts of the region, particularly for Miocene and older Cenozoic rocks. Dependence 

on these lithologic characteristics has resulted in establishing and mapping units of questionable time-stratigraphic significance. Many of the resulting correlations 

were approximate at best, and some were incorrect. 

Since about 1965, however, geologic mapping supported 

by extensive use of radiometric dating, precise majorand minor-element chemistry, magnetic-polarity determinations, reevaluation of old fossil collections, and 

evaluation of new ones has provided a reasonably consistent regional time-stratigraphic framework suitable for 

reconstructing the Cenozoic geology of the region.

Although widely recognized and accepted stratigraphic 

nomenclature has been developed for important parts of 

the Cenozoic section, the terminology and definition of 

units for some parts are still in an elementary stage of 

development. Many problems of terminology and regional correlation remain for Eocene, Oligocene, and lower 

Miocene volcanic and volcaniclastic rocks of the Blue 

Mountains region. Difficulties in regional correlation 

result partly from the mixed and recurring lithologic 

assemblages, their different degrees of alteration, and, 

most importantly, from absence of detailed mapping and 

adequate age data. However, recent intensive study of 

the Miocene basalts of the region by numerous individuals representing many diverse interests has led to a fairly 

precise stratigraphic nomenclature for these rocks over 

major parts of the region, supported by extensive chemical, radiometric age, and magnetostratigraphic data. 

Miocene and younger volcaniclastic and sedimentary 

rocks are mostly restricted to separate local basins of 

deposition and, in general, have not received the same 

attention as the Miocene basalts.

For the purposes of this volume, the geographic area of 

coverage is arbitrarily restricted to an area of approximately 140,000 km2, including parts of southeastern 

Washington, northeastern Oregon, and west-central Idaho. This area includes the Blue Mountains province, parts 

of Columbia Basin, the Columbia Intermontane province, 

parts of the Deschutes-Umatilla Plateau, the Joseph
CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Upland, and small parts of the High Lava Plains and 

Owyhee Upland provinces of southeastern Oregon (fig. 

1.1). Excluded from consideration are areas dominated 

by volcanic rocks of the Cascade Range, which lie far to 

the west of the Blue Mountains and also west of the 

Deschutes River and areas to the south and southeast, 

within the northern Basin and Range, although these 

areas contain sequences of volcanic and volcaniclastic 

rocks correlative, in part, with rocks of the Blue Mountains. Some of those rock units are mentioned, however, 

in the following pages to obtain a better regional understanding of the temporal and spatial distribution of 

different kinds of volcanism within the Cenozoic. In

addition, some volcanic rocks erupted in these peripheral 

areas extend long distances into parts of the Blue 

Mountains province.

A diverse assemblage of Cenozoic terrestrial volcanic 

and sedimentary rocks occupies most of the surface area 

within this 140,000-km2 area; pre-Cenozoic rocks represent perhaps only 10 to 15 percent of the total outcrop 

area (fig. 1.2). In the Oregon and Washington parts of the 

region, inliers of Paleozoic and Mesozoic metasedimentary and metavolcanic rocks, partly representing a tectonically mixed assemblage of accreted ophiolite and islandarc terranes, are intruded by several varieties and ages 

of plutonic rocks. In western Idaho, the Cenozoic cover

Approximate boundary 

of study area

HIGH LAVA PLAINS

50 100 KILOMETERS

INDEX TO LOG ALTTIES

BB, Boise Basin pAB, Pascoe Basin 

HH, Horse Heaven mining district PB, Paulina Basin 

J, John Day and John Day Basin PG, Picture Gorge 

JB, Juntura Basin si, Suplee-Izee area 

M, Mitchell WW, Walla Walla Basin

FIGURE 1.1. -Index map of the Blue Mountains region (screened area), showing major physiographic provinces and localities mentioned in text.
OVERVIEW OF THE CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

rests on similar Paleozoic and Mesozoic rocks and on 

high-grade metamorphic rocks of Precambrian age.

Within the Blue Mountains province, the outcrop 

distribution of pre-Cenozoic and lower Cenozoic rocks is 

related partly to burial by subsequent volcanism from 

vents marginal to and within the province and partly to 

differential erosion of a structurally and physiographically high region. A plexus of folds and tilted fault blocks 

make up this large, structurally uplifted block, which 

extends for about 360 km across northeastern Oregon 

into western Idaho and the southeast corner of Washington. The width of the elevated block is difficult to 

determine but is generally about 80 km or more. Distribution of Cenozoic units indicates that deformation was 

initiated during latest Eocene or Oligocene time and has 

persisted into late Miocene and probably into Quaternary 

time. The deformation has been inferred by some geologists to be related to northwest-southeast regional compressional stresses and by others to be related in part to 

deformation of brittle surface and near-surface rocks as a 

result of tangential stresses from transcurrent movement 

between crustal blocks. Different workers in the region 

have labeled this elevated crustal block an anticline, 

anticlinorium, antiform, or uplift; we prefer the nonspecific term "uplift," inasmuch as the origin of the structure 

is only poorly known.

Before the early 1970's, investigators assigned stratigraphic units to geologic epochs, the boundary ages of 

which differed considerably from those acceptable in the 

1980's. This practice led to some confusion, particularly 

where formations originally assigned to the Miocene or 

Pliocene are now referred to, respectively, as Oligocene 

or Miocene in age. Throughout this volume, authors of 

chapters use the epoch-boundary ages of Palmer (1983), 

which are younger in years than those used in many 

previous reports on the geology of the Blue Mountains 

and adjacent areas. Every attempt has been made in the 

figures and text to integrate these different assigned 

ages.

Most of the K-Ar ages referred to in this and subsequent chapters are from published reports. Because some 

of these published ages were originally calculated using 

decay constants that are no longer acceptable, all ages for 

which analytical data are available have been recalculated using decay constants acceptable in 1983 (see Fiebelkorn and others, 1983). The recalculated ages using 

the new constants generally are older by a few percent 

than the original reported ages that were calculate using 

the old constants.

ACKNOWLEDGMENTS

Preparation of this series of volumes on the geology of 

the Blue Mountains region was initiated by T.L. Vallier

of the U.S. Geological Survey and H.C. Brooks of the 

Oregon Department of Geology and Mineral Industries, 

both of whom have devoted large segments of their 

careers to the geology of this region. Because they 

wished to concentrate their efforts on their main interests in the pre-Cenozoic geology of the Blue Mountains, 

they asked me to oversee the volume on the Cenozoic 

geology of the region: to identify potential authors of 

chapters, to assist these authors in establishing appropriate coverage, and to act in an informal editorial 

capacity.

This summary volume is based on results from the 

work of nearly all geologists who have worked in the area 

and on discussions over the years with many of them. 

Their work is acknowledged mostly through references 

to their published papers and, in a few places, to either 

written or oral communications.

The authors thank T.L. Vallier and H.C. Brooks for 

generating interest in this endeavor, and W.H. Taubeneck, T.L. Robyn, T.P. Thayer, E.M. Taylor, and A.C. 

Waters for many interesting discussions over the years 

concerning their investigations of the Cenozoic geology of 

the region. The critical reviews and helpful suggestions 

of N.S. MacLeod and T.L. Vallier substantially improved 

the separate manuscripts that make up this volume.

PREVIOUS WORK

Previous summary reports dealing with aspects of the 

geology of the Blue Mountains and contiguous areas are 

mainly topical discussions of selected parts of the stratigraphic column or of special features in restricted geographic areas; several areally extensive geologic maps 

also have been prepared for different parts of the region.

Early discussions of John Day Basin in Oregon, located 

mainly at the west end of the Blue Mountains province 

and including the southwestern part of the DeschutesUmatilla Plateau, include papers on the general geology 

(Merriam, 1901), the petrography of the Eocene and 

Oligocene(?) Clarno Formation (Calkins, 1902), and both 

fossil floras (Knowlton, 1902; Chaney, 1924) and faunas 

(Merriam and Sinclair, 1907). Since these early studies, 

many additional papers have been published on details or 

reviews of Clarno Formation floras (Arnold, 1952; Scott, 

1954; Hegert, 1961; Elemenderf and Fisk, 1978; 

Manchester, 1981) and Clarno Formation faunas (Stirton, 

1944; Hanson, 1973). In addition, many topical papers 

have been published concerning the distribution, lithology, structure, and alteration of both the Clarno and John 

Day Formations of central Oregon; foremost among these 

papers are those by Waters and others (1951), Hay 

(1962), Hogenson (1964), Fisher (1967), Fisher and Rensberger (1972), and Noblett (1981).
CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

121° 120° 119° 118°

SiH I N G T O N

45'

44° KBase modified from U.S. Geological Survey 

1:1,000,000 Idaho and Washington, 1976; 

Oregon, 1986

40 60 80 KILOMETERS

FIGURE 1.2. Generalized geologic map of northeastern
118°

OVERVIEW OF THE CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

117° 116°

45°

44°

EXPLANATION

CENOZOIC ROCKS

Miocene and younger volcanic and sedimentary rocks

Oligocene and lower Miocene volcanic and sedimentary 

rocks

Eocene volcanic and sedimentary rocks

PRE-CENOZOIC ROCKS

Mesozoic intrusive rocks

Precambrian, Paleozoic, and Mesozoic sedimentary and 

volcanic rocks, locally metamorphosed. Includes 

ophiohte, island-arc, and accreted terranes of the 

Blue Mountains region

Geology modified from Wells and Peck (1961). 

Waker (19771 and Bond (1978)

Oregon and adjacent areas in Washington and Idaho.
CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Brief mention was made by Anderson (1930, p. 25), 

Bond (1963) and, more recently, Jones (1982, p. 43-52) of 

andesitic and latitic volcanic rocks in western Idaho that 

are probably equivalent to the Challis Volcanics and 

correlative with the Clarno Formation; some of these 

volcanic rocks exposed in western Idaho were thought to 

be as old as Permian(?) (Tullis, 1944, p. 40).

The Miocene basaltic rocks, mostly making up what is 

now known as the Columbia River Basalt Group but 

including some petrographically and chemically distinct 

mafic flow units, have been treated in many publications; 

earlier ones are by Mackin (1961) and Waters (1961), and 

more recent ones are by Bond (1963), Thayer and Brown 

(1966), Schmincke (1967), Wright and others (1973), 

Hooper (1974), and Nathan and Fruchter (1974). Proliferation of formal and informal names for basaltic flows 

within this group led to a review by Swanson and others 

(1979) in which a revised formal stratigraphic nomenclature was established and the petrologic, magnetic polarity, and chemical characteristics were described. That 

report also summarized the geographic distribution of 

units and K-Ar ages of selected flows. Other formal 

publications and numerous dissertations concerning 

these rocks are referenced throughout this volume.

Several early publications dealt with both lava flows 

and terrestrial sedimentary rocks of Miocene and younger age in and adjacent to Boise Basin. For example, 

studies by Lindgren (1898), Russell (1902), Merriam 

(1918), and Buwalda (1924) established early stratigraphic concepts for many of the units exposed in the basin. 

Regional stratigraphic relations and nomenclature, as 

well as precise age assignments, of these diverse rocks 

recently have been reconsidered and revised, largely 

through the work of Malde and Powers (1962), Armstrong (1975), Armstrong and others (1975), and other 

workers.

Many reports are available on the stratigraphy and 

structure of Pasco Basin in southeastern Washington, 

most of them related to investigations of ground water 

and radioactive-waste disposal at and near Hanford. 

Principal among these reports are those by Strand and 

Hough (1952), Newcomb (1958), Brown and McConiga 

(1960), Newcomb and others (1972), and Gustafson 

(1978). Newcomb (1965) described the geology and water 

resources of Walla Walla River basin, which lies about 60 

km southeast of Pasco Basin.

Regional geologic maps and compilations that have 

contributed to our understanding of the distribution of 

Cenozoic volcanic and sedimentary units include principally those by Gaston and Bennett (1979), Mitchell and 

Bennett (1979), Rember and Bennett (1979), and Swanson and others (1981) for Idaho; Brown and Thayer 

(1966), Walker and others (1967), Swanson (1969), 

Greene and others (1972), Walker (1973,1979), Robinson

(1975), and Brooks and others (1976) for northeastern 

Oregon; and Swanson and others (1980) for southeastern 

Washington. Smaller-scale maps of Idaho (Bond, 1978), 

Washington (Huntting and others, 1961), eastern Oregon 

(Walker, 1977), and Oregon and Washington (Luedke and 

Smith, 1982) also provide background information on the 

regional distribution of volcanic and sedimentary sequences. Each of these regional maps provides an index 

to more detailed work in specific small areas; much of this 

more detailed work is in unpublished theses.

PURPOSE AND SCOPE

The chapters in this volume describe the volcanic and 

sedimentary rocks of the Blue Mountains region, their 

distribution by time-lithologic units, and their relations 

to vents; the Cenozoic structural development of the Blue 

Mountains and of several subsidiary depositional basins; 

and, finally, the interrelations of tectonism and volcanism. The volume will be a companion to several other 

volumes published (Vallier and Brooks, 1986, 1987) or in 

preparation that discuss the pre-Cenozoic geology of the 

same major parts of eastern Oregon, southeastern Washington, and western Idaho, including the Idaho batholith 

and its wallrocks.

The large size of the area being considered in this 

report and the diversity of the Cenozoic geology required 

the talents of several authors whose interests dealt with 

specific parts of the stratigraphic column or with single 

basins of deposition or related groups of basins. There is 

little uniformity in the amount and kind of data available 

for the different subjects that are considered. Intensive 

studies of some parts of the stratigraphic column or of 

small geographic areas have provided much data on 

thicknesses of section, lithology, distribution of lithologic 

units, chemistry, isotopic ages, and magnetic-polarity 

characteristics. Other areas and other parts of the 

stratigraphic column have been studied by only the 

broadest reconnaissance methods, and the resulting data 

are sparse and generally provisional.

A final summary chapter deals with the Cenozoic 

tectonic and volcanic history of the region. It attempts to 

bring data presented in the other sections into focus, 

particularly in relation to structural development and 

volcanism and to concepts of magma generation and 

crustal development.

In 1981,1 initially provided each author with a general 

outline of the entire volume, suggesting general topics to 

be covered in each chapter and giving very general 

guidelines as to scope and philosophy of approach. Within 

these guidelines, authors independently prepared their 

reports dealing with particular parts of the Cenozoic 

section. Because of different approaches to the subject by
OVERVIEW OF THE CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

the authors and the vast differences in availability of 

data, the chapters are not uniform in either style or 

coverage. Furthermore, several chapters prepared in 

1982 and 1983 represent the status of knowledge at that 

time and have not been substantially updated as new 

work was completed and published.

In this region, as in most volcanic regions, generalized 

stratigraphic columns are of limited usefulness, and 

regional correlations are only approximate. In and marginal to the Blue Mountains, numerous separate ephemeral and mostly structurally controlled depositional 

basins have received various volcanic, volcaniclastic, and 

sedimentary materials from many source areas. Before 

radiometric dating and the development of precise 

geochemical and magnetostratigraphic techniques, many 

correlations of lithologic units among the separate basins 

were somewhat haphazard; only in recent years have 

time-stratigraphic relations reached a stage where regional correlation charts are meaningful and reasonably 

consistent.

Correlations that are well supported by numerous 

radiometric ages, by clearly defined rock chemistry, or 

by systematic magnetostratigraphy are restricted largely to the Oligocene and Miocene part of the section and 

concern primarily the John Day Formation of central 

Oregon and the overlying flows of the Columbia River 

Basalt Group.

REGIONAL STRATIGRAPHIC RELATIONS

One purpose of this introductory chapter is to provide 

a regional stratigraphic framework for the individual 

chapters that deal with specific part of the stratigraphic 

column. Those chapters present detailed descriptions of 

the physical and chemical characteristics of the different 

lithologic units and the relations within the described 

sequences of rocks.

Composite and highly generalized stratigraphic columns have been prepared for four separate but interrelated parts of the region (fig. 1.3). These four columns 

represent (1) the Blue Mountains province and southwestern part of the Deschutes-Umatilla Plateau; (2) the 

southern part of Columbia Basin and acjjacent parts of 

Idaho, including the Clearwater embayment; (3) the 

southern margin of the Blue Mountains province and 

contiguous parts of the High Lava Plains; and (4) the 

northern Owyhee Upland and northwestern part of the 

King Hill section of the Columbia Intermontane province, 

forming the northwestern and western part of Boise 

Basin. The columns are arranged in a correlation chart 

(fig. 1.3) and referenced to lines representing ages of 20, 

15, 10, and 5 Ma. Stratigraphic units common to more 

than one column are indicated, and age spans of isotopically dated units are shown in parentheses. Gaps in the

columns, in part shown as regional unconformities, indicate that volcanism in this region was episodic. However, 

as new radiometric ages have been determined from 

various parts of the region, some of the apparent gaps in 

the volcanic history seem to be diminishing in duration or 

disappearing. Where interfingering relations are known 

or inferred, they are shown by jagged lines (fig. 1.3); 

unconformities, which are present in many parts of the 

section, are shown only if they are of regional extent or, 

if of local extent, are important in understanding relations in this complex volcanic area.

The Cenozoic rocks of the region are divisible into 

several major geologic units, some with wide geographic 

distribution and others restricted to small ephemeral 

basins of deposition or to geographically restricted volcanic piles. Even though some of these units are recognizable over large parts of the the region, the Cenozoic 

column is so segmented by regional and local unconformities and by faults that estimates of composite thickness 

are almost meaningless. Nowhere are all the diverse 

units displayed in a continuous sequence, and in most 

parts of the region only small segments of the column are 

present. One of the thickest sequences of Eocene and 

Oligocene(?) rocks, representing the Clarno Formation, 

is that described by Waters and others (1951) for the 

Horse Heaven mining district of Oregon, where the total 

thickness is about 1,800 m. Oles and Enlows (1971) 

described a section of similar thickness in the Mitchell 

quadrangle; they divided this section into upper and 

lower parts and identified the total section as the Clarno 

Group. The thickest described section of uppermost 

Eocene(?), Oligocene, and lower Miocene rocks, representing the John Day Formation, is that estimated by 

Peck (1964) to be about 1,200 m thick near the west limits 

of the formation. The John Day Formation has a composite thickness of about 820 m near Mitchell (Hay, 1963) 

and 760 m near Picture Gorge (Fisher and Rensberger, 

1972). Miocene basaltic rocks and the partly coeval 

Strawberry Volcanics vary extremely in thickness. Maximum total thickness of this part of the section was 

estimated at about 2,450 m by Thayer and Brown (1966), 

although Swanson and others (1979) indicated that this 

thick section of basaltic flows may be partly duplicated by 

faulting. In the deep canyons adjacent to the Snake and 

Grande Ronde Rivers, the total exposed thickness is 

about 800 m, and this value represents only a part of the 

Miocene basalt column. The Mascall Formation of middle 

Miocene age, which locally overlies the basalt flows and 

elsewhere appears to interfinger with them, was reported (Thayer and Brown, 1966) to be locally as thick as 

1,830 to 2,135 m.

Measured thicknesses of the Miocene silicic volcanic 

and volcaniclastic rocks and of the upper Miocene and 

overlying volcaniclastic and sedimentary rocks vary so
CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

much from one depositional basin to another and within 

basins that few generalizations are possible. In basins

marginal to the Blue Mountains, volcaniclastic and tuffaceous sedimentary deposits of middle Miocene (BarAGE 1 SOUTHERN MARGIN OF BLUE MOUNTAINS AND 

CONTIGUOUS PARTS OF HIGH LAVA PLAINS

COMPOSITE SECTION OF BLUE MOUNTAINS

PROVINCE AND SOUTHWEST PART OF

DESCHUTES-UMATILLA PLATEAU

Holocene

and 

Pleistocene

Surficial deposits 

(alluvium, colluvium, ash beds, terrace gravels, fanglomerate)

Otivine-beanng basalt

Surdcial deposits

(alluvium, colluvium, fanglomerate, terrace gravels, loess, pumiceous ash beds, glacial 

deposits)___

5 Ma 

-5.3 Ma Tuffaceous sedimentary rocks 

(local fanglomerate)

<^Diktytaxitic divine basalt I

Fanglomerate

(local fanglomerate) Fanglomerate

S Drinkwater Basalt (-7 Ma)

Drewsey Formation / Tuffaceous sedimentary rocks

(8.7 Ma)

\ giving basalt flows (- 4.9 Ma)\

basalt flows ( 5.8 Ma) "^

Deschutes Formation z

and Dalles Formation

(including Tygh Valley and

Chenoweth Formations2 )

\ Olivine basalt flow >

10 Ma

ruffaceous sadimantary rocks

ruffaceous sedimentary rocks 

UNCONFORMITY

Mascall Formation

Dooley Rhyolite 

Breccia (14.7 Ma) and 

associated ash-flow 

s and tuff breccias

Strawberry Volcanics 

(~ 10 - 20 Ma) Wanapum Basalt (part) 

Roza Member

Frenchman Springs Member 

Eckler Mountain Member

Flows of (basalt of) 

neville chemical 

type o

Dinner Creek Welded Tu

Grande Ronde 

Basalt ~ Ash-now turn-IBM

Picture Gorge Basalt 

(-15 - 16 4 Ma)

Strawberry Volcanics COLUMBIA RIVER BASALT GROUP (PART)

Hornblende-, 

pyroxene-, and 

plagioclase- phyric 

flows and breccias.

15 M,

>

Flows, breccias, and pepente: 

of Columbia River Basalt 

Group and of (basalt of) 

Prineville chemical type

23 7 Ma

36.6 Ma

57.8 Ma

John Day Formation4 

(-20 - 37 Ma)

UNCONFORMITY

Clarno Formation5

(many local unconformities)

UNCONFORMITY

mostly unaltered (19.5 

Ma; 28.8 Ma)

John Day Formation4 

(-20 - 37 Ma)

^ Picture Gorge igmmbme6 (26 9 Ma) ;,

Welded tuff unit I 3 (27.7

CONFORMITY - 

Clarno Formation5 

(many local unconformities) 

(-40 - 54 Ma)

Paleocene(?) arkosic sandstone, siltstone, and coaly shale 

UNCONFORMITY

Paleozoic and Mesozoic sedimentary, metasedimentary, 

metavolcanic. metamorphic, and intrusive rocks

1 Time scale of Palmer (1983)

2 Of Farooqui and others (1981)

3 Peck (1964)

Unit age is latest Eocene (?) to early Miocene 

 ' Unit age is Eocene and earliesl Oligocene (') 

6 Informal unit of Fisher (1966)

Of the Ellensburg Formation

FIGURE 1.3. Generalized stratigraphic columns
OVERVIEW OF THE CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

stovian) and younger age are locally as much as 650 m 

thick, but they generally thin rapidly and pinch out on

older units. Bowen and others (1-963) reported measured 

thickness of approximately 270 m for middle Miocene

PART OF COLUMBIA PLATEAUS, INCLUDING

CLEARWATER EMBAYMENT. ADJACENT TO

BLUE MOUNTAINS PROVINCE

NORTHERN OWYHEE UPLAND AND NORTHWESTERN

PART OF KING HILL SECTION OF COLUMBIA

INTERMONTANE PROVINCE

Surf icial deposits 

(alluvium, colluvium, fanglomerate, terrace gravels, loess)

Surficial deposits 

(alluvium, colluvium, tangtomerate. terrace gravels, glacial, and glaciofluvial deposits)

UNCONFORMITY

/ McKay and Alkali Canyon

^~*~ UNCONFORMITY j~-~~> / pomelo '.undivided (partly

/ equivalent to Snutlw Formation 

/ ol Hodge. 1932)

Saddle Mountains Basalt <\

Bully Creek Formation ol Kittlei 

Lower Monumental Member (~6 Ma) andothe

Ice Harbor Member (8.5 Ma)

Chalk Butte Formation of Corcora 

and others (1962) and related 

sedimentary rocks, undivided

Grassy Mountain Formation of Kittleman and 

others (I965I (7 6 Ma on basalt) Fluvial and lacustrine 

deposits

Idaho Group - 

ofMaMeand 

Powers (1962)

Butord Member

Elephant Mountain Member |10 5 Mai

Pomona Member (12 0 Ma I 

Esquatzel Member 

Weissenlels Ridge Member 

Asotin Member 

Wilbur Creek Member 

Umatilla Member

"Wanapum Basalt ~ 

Pnesl Rapids Member 

Roza Member

Frenchman Springs Member 

___Ecfcler Mountain Member o__9?

Ellensburg Formation

Grande Ronde Basalt (14 0 to 16 5 Ma)

Deer Butte Formation of 

Corcoran and others (1962)

and correlative siltstone

sandstone, and tuffaceous

sedimentary rocks

7

 UNCONFORMITY - UNCONFORMITY.

Miocene basalts rest unconformably on pre-Cenozoic 

rocks, Clamo Formation, John Day Formation, and in 

western Idaho on Kamiah Volcanics and related rocks, 

probably part of Challis Volcanics. Pre-Cenozoic 

rocks include metasedimentary, metavolcanic, and 

crystalline intrusive rocks

Flows of Columbia River Basalt Group rest 

unconformably on rocks of Idaho batholith 

and Tnassic and Jurassic sedimentary, 

volcanic, and volcaniclastic rocks that 

are variously metamorphosed

WASHINGTON |

T

"\

OREGON j IDAHO

LOCATIONS OF STRATIGRAPHIC COLUMNS

of Blue Mountains region and adjacent areas.
10 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

(Barstovian) clastic rocks, about 130 m for overlying 

upper Miocene (Clarendonian), and 250 m for uppermost 

Miocene and Pliocene(?) (Hemphillian) rocks, for a total 

section thickness of nearly 640 m in Juntura Basin. 

Similar thicknesses of correlative tuff and sedimentary 

rocks are present in areas to the west of the Blue 

Mountains, where parts of the sections are exposed in 

deep canyons of the Deschutes River and its tributaries. 

Rock sequences several hundred meters thick are 

present in several basins marginal to the Blue Mountains 

on the southeast in the north end of the Owyhee Upland 

province and in the Columbia Intermontane province. 

Within the Blue Mountains, local sections of these middle 

Miocene and younger clastic rocks are generally much 

thinner; the thickest sections may be those that are about 

300 m thick in Paulina Basin (Davenport, 1971) and to the 

east in the Suplee-Izee area (Dickinson and Vigrass, 

1965).

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Department of Geology and Mineral Industries Bulletin 58,109 p.

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Smith, J.G., 1983, Index to K-Ar determinations for the State of 

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American Association of Petroleum Geologists Bulletin, v. 51, no. 

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Fisher, R.V., and Rensberger, J.M., 1972, Physical stratigraphy of the 

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Gustafson, E.P., 1978, The vertebrate faunas of the Pliocene Ringold 

Formation, south-central Washington: Eugene, University of 

Oregon, Museum of Natural History Bulletin 23, 62 p.

Hanson, C.B., 1973, Geology and vertebrate faunas in the type area of 

the Clarno Formation, Oregon [abs.]: Geological Society of America Abstracts with Programs, v. 5, no. 1, p. 50.

Hay, R.L., 1962, Origin and diagenetic alteration of the lower part of 

the John Day Formation near Mitchell, Oregon, in Engel, A.E.J., 

James, H.L., and Leonard, B.F., eds., Petrologic studies A 

volume in honor of A.F. Buddington: New York, Geological 

Society of America, p. 191-216.

 1963, Stratigraphy and zeolitic diagenesis of the John Day

Formation of Oregon: University of California Publications in

Geological Sciences, v. 42, no. 5, p. 199-262. 

Hergert, H.L., 1961, Plant fossils in the Clarno Formation, Oregon:

Ore Bin, v. 23, no. 6, p. 55-62. 

Hodge, E.T., 1932, Geologic map of north-central Oregon: University of

Oregon Geology Publication Series, v. 1, no. 5, 7 p., scale

1:250,000. 

Hogenson, G.M., 1964, Geology and ground water of the Umatilla River

basin, Oregon: U.S. Geological Survey Water-Supply Paper 1620,

162 p. 

Hooper, P.R., 1974, Petrology and chemistry of the Rock Creek flow,

Columbia River Basalt, Idaho: Geological Society of America

Bulletin, v. 85, no. 1, p. 15-26. 

Huntting, M.T., Bennett, W.A.G., Livingston, V.E., Jr., and Moen,

W.S., comps., 1961, Geologic map of Washington: Olympia,

Washington Division of Mines and Geology, scale 1:500,000. 

Jones, R.W., 1982, Early Tertiary-age Kamiah Volcanics, in Bonnichsen, Bill, and Breckenridge, R.M., eds., Cenozoic geology of

Idaho: Moscow, Idaho Bureau of Mines and Geology, p. 43-52. 

Kittleman, L.R., Green, A.R., Hagood, A.R., Johnson, A.M., McMurray, J.M., Russell, R.G., and Weeden, D.A., 1965, Cenozoic

stratigraphy of the Owyhee region, southeastern Oregon: Eugene,

University of Oregon, Museum of Natural History Bulletin 1,45 p. 

Knowlton, F.H., 1902, Fossil flora of the John Day Basin, Oregon: U.S.

Geological Survey Bulletin 204, 153 p. 

Lindgren, Waldemar, 1898, The mining districts of the Idaho Basin and
OVERVIEW OF THE CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION 11

the Boise Ridge, Idaho: U.S. Geological Survey Annual Report 18, 

pt. 3, p. 617-719.

Luedke, R.G., and Smith, R.L., 1982, Map showing distribution, 

composition, and age of late Cenozoic volcanic centers in Oregon 

and Washington: U.S. Geological Survey Miscellaneous Investigations Series Map I-1091-D, scale 1:1,000,000.

Mackin, J.H., 1961, A stratigraphic section in the Yakima Basalt and 

JEllensburg Formation in south-central Washington: Washington 

Division of Mines and Geology Report of Investigations 19, 45 p.

Malde, H.E., and Powers, H.A., 1962, Upper Cenozoic stratigraphy of 

western Snake River Plain, Idaho: Geological Society of America 

Bulletin, v. 73, no. 10, p. 1197-1219.

Manchester, S.R., 1981, Fossil plants of the Eocene Clarno Nut Beds: 

Oregon Geology, v. 43, no. 6, p. 75-81.

Merriam, J.C., 1901, A contribution to the geology of the John Day 

Basin [Oreg.]: University of California Publications, Department 

of Geology Bulletin, v. 2, no. 9, p. 269-314.

 1918, New mammalia from the Idaho Formation: University of

California Publications, Department of Geology Bulletin, v. 10, no.

26, p. 523-530. 

Merriam, J.C., and Sinclair, W.J., 1907, Tertiary faunas of the John

Day region: University of California Publications, Department of

Geology Bulletin, v. 5, no. 11, p. 171-205. 

Mitchell, V.E., and Bennett, E.H., 1979, Geologic map of the Baker

quadrangle, Idaho: Idaho Bureau of Mines and Geology Geologic

Map Series, scale 1:250,000. 

Nathan, Simon, and Fruchter, J.S., 1974, Geochemical and paleomagnetic stratigraphy of the Picture Gorge and Yakima Basalts

(Columbia River Group) in central Oregon: Geological Society of

America Bulletin, v. 85, no. 1, p. 63-76. 

Newcomb, R.C., 1958, Ringold Formation of Pleistocene age in type

locality, the White Bluffs, Washington: American Journal of

Science, v. 256, no. 5, p. 328-340.

 1965, Geology and ground-water resources of the Walla Walla 

River basin, Washington-Oregon: Washington Division of Water 

Resources Water-Supply Bulletin 21, 151 p.

Newcomb, R.C., Strand, J.R., and Frank, F.J., 1972, Geology and 

ground-water characteristics of the Hanford Reservation of the 

U.S. Atomic Energy Commission, Washington: U.S. Geological 

Survey Professional Paper 717, 78 p.

Noblett, J.B., 1981, Subduction-related origin of the volcanic rocks of 

the Eocene Clarno Formation near Cherry Creek, Oregon: Oregon 

Geology, v. 43, no. 7, p. 91-99.

Oles, K.F., and Enlows, H.E., 1971, Bedrock geology of the Mitchell 

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Peck, D.L., 1964, Geologic reconnaissance of the Antelope-Ashwood 

area, north-central Oregon, with emphasis on the John Day 

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Rember, W.C., and Bennett, E.H., 1979, Geologic map of the Pullman 

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Formation in the southwestern part of the Blue Mountains and 

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Miscellaneous Investigations Series Map 1-872, scale 1:125,000.

Russell, I.C., 1902, Geology and water resources of the Snake River 

plains of Idaho: U.S. Geological Survey Bulletin 199, 192 p.

Schmincke, H.-U., 1967, Stratigraphy and petrography of four upper 

Yakima Basalt flows in south-central Washington: Geological 

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Stirton, R.A., 1944, A rhinoceros tooth from the Clarno Eocene of 

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Swanson, D.A., Anderson, J.L., Camp, V.E., Hooper, P.R., Taubeneck, W.H., and Wright, T.L., 1981, Reconnaissance geologic map 

of the Columbia River Basalt Group, northern Oregon and western 

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scale 1:250,000.

Swanson, D.A., Wright, T.L., Camp, V.E., Gardner, J.N., Helz, R.T., 

Price, S.P., Riedel, S.P., and Ross, M.E., 1980, Reconnaissance 

geologic map of the Columbia River Basalt Group, Pullman and 

Walla Walla quadrangles, southeast Washington and adjacent 

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Map 1-1139, scale 1:250,000.

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Revisions in stratigraphic nomenclature of the Columbia River 

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late Tertiary silicic volcanism in the Canyon City quadrangle, 

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Geological Society of America Bulletin, v. 55, no. 2, p. 131-164.

Vallier, T.L., and Brooks, H.C., eds., 1986, Geology of the Blue 

Mountains region of Oregon, Idaho, and Washington Geologic 

implications of Paleozoic and Mesozoic paleontology and biostratigraphy, Blue Mountains province, Oregon and Idaho: U.S. Geological Survey Professional Paper 1435, 93 p.

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and Washington The Idaho batholith and its border zone: U.S. 

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Walker, G.W., 1973, Reconnaissance geologic map of the Pendleton 

quadrangle, Oregon and Washington: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-727, scale 1:250,000.

 1977, Geologic map of Oregon east of the 121st meridian: U.S. 

Geological Survey Miscellaneous Investigations Series Map 1-902, 

scale 1:500,000. 

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Grangeville quadrangle, Baker, Union, Umatilla, and Wallowa 

Counties, Oregon: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1116, scale 1:250,000.

Walker, G.W., Peterson, N.V., and Greene, R.C., 1967, Reconnaissance geologic map of the east half of the Crescent quadrangle, 

Lake, Deschutes, and Crook Counties, Oregon: U.S. Geological 

Survey Miscellaneous Geologic Investigations Map 1-493, scale 

1:250,000.

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p. 583-611.

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G.W., and Williams, Howel, 1951, Quicksilver deposits of the 

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Bulletin 969-E, p. 105-149.

Wells, F.G., and Peck, D.L., 1961, Geologic map of Oregon west of the 

121st meridian: U.S. Geological Survey Miscellaneous Geologic 

Investigations Map 1-325, scale 1:500,000.

Wright, T.L., Grolier, M.J., and Swanson, D.A., 1973, Chemical 

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Geological Society of America Bulletin, v. 84, no. 2, p. 371-385.

2. PALEOCENE(P), EOCENE, AND OLIGOCENE(P) ROCKS OF 

THE BLUE MOUNTAINS REGION

By GEORGE W. WALKER and PAUL T. ROBINSON l

CONTENTS

Page

Abstract 13 

Introduction 13 

Distribution 16 

Stratigraphy 16 

Age - 22

Structure 23

Composition 23

Source vents 26

References cited 26

ABSTRACT

Lower Cenozoic terrestrial rocks of the Blue Mountains region are 

separable into a lower, possibly Paleocene, sequence of quartzose and 

feldspathic sandstone and siltstone overlain by a thick sequence of 

largely volcanogenic deposits, most commonly referred to the Clarno 

Formation in Oregon and the Challis Volcanics in western Idaho.

The volcanogenic sequence is dominated by several kinds of andesite 

flows, mudflows, breccia, and moderate to large volumes of andesitic to 

rhyolitic tuff and tuffaceous sedimentary rocks. Isotopic ages on 

volcanogenic rocks shown on published maps as part of the Clarno 

Formation in the Blue Mountains range in age from about 54 Ma to 19 

Ma. Assignment of rock units to the Clarno Formation that have been 

dated at younger than about 37 to 35 Ma are suspect; reexamination of 

outcrops of those rocks younger than about 37 Ma indicates that 

although they are petrographically and, presumably, chemically much 

like rocks of the Clarno Formation they are invariably less altered. 

Much evidence indicates that volcanism similar to that of the Clarno 

Formation extended well into the Oligocene and, possibly, into earliest 

Miocene time and is, therefore, partly coeval with volcanism that 

produced the John Day Formation.

INTRODUCTION

A heterogeneous assemblage of terrestrial volcanic, 

volcaniclastic, and epiclastic rocks primarily of Paleocene(?) and Eocene age crop out in widely separated 

areas in the Blue Mountains of Oregon and in parts of

1Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada.

several adjoining provinces. The assemblage typically 

comprises a basal sequence of terrestrial quartzose and 

feldspathic sandstone and siltstone overlain by a thick 

sequence of largely volcanogenic deposits, dominantly of 

andesitic composition but including some basaltic and 

rhyolitic rocks. Although these rocks vary lithologically, 

many of them are similar in kinds and degree of alteration 

and in structural deformation. These similarities, along 

with erratically distributed fossil floras and faunas, have 

been used by various investigators to correlate detached 

parts of the lower Cenozoic section over many tens or 

even hundreds of kilometers, with various degrees of 

precision.

In northeastern Oregon nearly all of these lower 

Cenozoic volcanogenic rocks have been assigned to the 

Clarno Formation by most investigators, none of whom 

has considered this complex assemblage on a regional 

basis. Although Mendenhall (1909) and Hogenson (1964) 

included the epiclastic sedimentary rocks in the Clarno 

Formation, Pigg (1961) placed them in an older, unnamed 

unit. Shorey (1976) also considered these arkosic sedimentary rocks to be older than the Clarno Formation and 

referred them to his Herren unit. Because these rocks 

are distinct in composition from the bulk of the Clarno 

Formation and appear to be older (Trauba, 1975; Shorey, 

1976; Elemendorf and Fisk, 1978), we also treat them 

herein as a separate unit.

On the basis of an angular unconformity in the Mitchell 

area (fig. 2.1), Oles and Enlows (1971) divided the 

volcanogenic sequence into what they termed the "Upper 

Clarno" and "Lower Clarno" Formations and identified 

the aggregate sequence as the Clarno Group. Other 

investigators (Waters and others, 1951; Noblett, 1981) 

have also recognized unconformities at various localities 

within the sequence separating sections with slightly 

different lithologies, types of alteration, or degrees of 

deformation. However, these unconformities appear to 

be local features (Swanson and Robinson, 1968), and 

there is no evidence of a regional unconformity. Therefore, we disagree with this terminology and instead 

follow the usage of Merriam (1901) in referring to this 

volcanogenic sequence as the Clarno Formation.

13
14 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

In adjacent parts of western Idaho, two small inliers 

(kipukas) of volcanic flow, volcaniclastic, and epiclastic(?) 

rocks surrounded by flows of the Columbia River Basalt 

Group presumably correlate with parts of the Clarno 

Formation and, most recently, have been correlated with 

the Eocene Challis Volcanics of eastern Idaho (Jones, 

1982).

Most radiometric ages on samples that have been 

assigned to the Clarno Formation on the basis of lithology 

range from 55 to 40 Ma. As discussed below, some ages 

younger than about 35 Ma suggest possible temporal

overlap with parts of the John Day Formation (table 2.1), 

but many of those ages are suspect. Thus, in this report 

we consider the Clarno Formation to be chiefly Eocene in 

age (we actually consider the overall age range to be 

Eocene and earliest Oligocene?), but if some of these 

young ages can be substantiated, the unit as presently 

mapped will have a greater timespan.

A few of these young Clarno-like rocks are less altered 

and less deformed than those in the type locality of the 

Clarno Formation, which most workers consider to be 

near Clarno (or Clarno's Ferry), thus introducing the

12V 120 116°

45° -

44° -

	INDEX TO LOCALITIES 

C Clamo (Clamo Ferry) 

CB Cherry Creek. Bridge Creek area 

EB East Birch Creek 

H Heppner 

HC Hells Canyon 

HH Horse Heaven mining district

KB Kamiah Butte

M Mitchell

P Prineville

PH Potato Hill

PR Pilot Rock

UB Unity Basin and Reservoir

FIGURE 2.1. Index map of Blue Mountains region, showing study area and localities mentioned in text.
PALEOCENE(?), EOCENE, AND OLIGOCENE(?) ROCKS OF THE BLUE MOUNTAINS REGION 15

TABLE 2.1. Potassium-argon ages for rocks mapped as the Clarno Formation

[References: 1, Fiebelkom and others (1983); 2, Enlows and Parker (1972); 3, Swansea and Robinson (1968); 4, Evernden and others 

(1964); 5, Walker (1973); 6, Brooks and others (1976); 7, Brown and Thayer (1966). do., ditto]

Sample General location Rode type Material References

648-628

E-16-77

648-695

W-3-58

KFO-1702B

KFO-1112

648-657

DAS-67-80

DX-1

B-l

KA818

KA824A

M-859

KA1204

KFO-901

10-6-78-2

EMT-11

SC-1-70

UB-2-70K

EastofPrineville

Mitchell

North of Mitchell

South of Mitchell

Mitchell

do.

EastofPrineville

Horse Heaven

East of John Day

Northeast of 

John Day.

Mitchell

North of Mitchell

Mitchell

Northwest of 

Mitchell.

Mitchell

Smith Rock

Mitchell

Upper Grande 

Ronde.

Unity

Andesite?

Andesite

do.

do.

Basalt

Andesite

?

Rhyolite

Basalt

Andesite

do.

Bentonite

Diabase

1W

Andesite

Basalt

Diabase

Andesite

Dacite (clast)

Plagioclase

Hornblende

do.

Hornblende 

Whole rock

Whole rock

Whole rock

Hornblende

Sanidine

?

Whole rock

do.

Sanidine

Whole rock

Plagioclase

Whole rock

do.

do.

Plagioclase

Hornblende 

Plagioclase

53.7±1.0

48.9±5.2

47.0±9.5

46.1±0.4 

41.6±0.9

46.1±0.1

44.010.6

42.915.4

42.110.8

41.716.0

41.210.4

37.5

36.510.9

34.310.9

34.0

33.713.1

30.8+0.5

29.310.4

28.810.8

19.618.0 

19.510.6

1

1

1

2

2

2

1

3

1

1

4

4

2

4

2

1

2

5,1

6,7,1

fundamental problem of what should be included in the 

formation. Should it include all those rocks assigned by 

different workers to the Clarno Formation in northeastern Oregon and the questionable age-equivalent rocks in 

western Idaho assigned to the Chalk's Volcanics? Should 

it include only those rocks clearly dated by whatever 

means as Paleocene and Eocene? Having no direct 

answers to these questions, we herein arbitrarily include 

in the Clarno Formation only those bodies of volcanic, 

volcaniclastic, and sedimentary rocks within the region 

that meet one or more of the following criteria: (1) They 

represent rocks of the type locality of the Clarno Formation (Merriam, 1901) or are directly traceable into those 

rocks(2) they are well established as either Eocene or 

earliest Oligocene(?) in age, either by diagnostic fossils or 

by isotopic ages; and (3) they lie either conformably or

unconformably beneath rocks clearly identifiable with the 

uppermost Eocene(?) to lower Miocene John Day Formation and overlie pre-Cenozoic rocks. All other bodies of 

rock with "typical" Clarno Formation characteristics that 

do not meet these criteria are looked at with some 

skepticism; isolated bodies that are near the type locality 

at and near the hamlet of Clarno (originally Clarno's 

Ferry; fig. 2.1), evidently are time-stratigraphic correlatives of the Clarno Formation, whereas others farther 

east and west of the type locality are suspect. Some of 

these Clarno-like rocks have been radiometrically dated 

as Oligocene and even early Miocene in age but show 

relations with the John Day Formation or with other 

units that make them suspect; others are considerably 

less altered than typical Clarno Formation rocks (see 

chap. 3).
16 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

DISTRIBUTION

The epiclastic sedimentary rocks of Paleocene(?) and 

early Eocene age are exposed over an area of approximately 80 km2 at two localities south of Heppner, 

Oreg., and along East Birch Creek southeast of Pilot 

Rock (fig. 2.2). South of Heppner, the rocks form a 

northeast-trending body exposed along the crest of the 

Blue Mountains uplift and are conformably(?) overlain by 

volcanogenic rocks of the Clarno Formation.

The largest exposures of the Clarno Formation occupy 

an arcuate area of slightly more than 4,000 km2 mostly 

along and to the south of the axis of the Blue Mountains 

uplift at and near the western margin of the Blue 

Mountains province in Crook, Jefferson, and Wheeler 

Counties, Oreg. (fig. 2.2). A smaller northeast-trending 

body of about 400-km2 area lies along the axis of the Blue 

Mountains uplift in southern Morrow County. Smaller 

inliers of Clarno rocks are surrounded by younger flows 

and sedimentary rocks in areas that are deeply dissected 

by the Deschutes River and its tributaries in the southwesternmost tip of the Deschutes-Umatilla Plateau province (see fig. 1.2). Detached bodies of correlative rocks 

are found sporadically southeast of the axis of the Blue 

Mountains uplift, particularly in areas of considerable 

structural relief. The largest of these detached bodies, in 

central Grant County, covers about 600 km2, and another 

northwest-trending elongate body that crosses the 

Grant-Baker County line covers about 500 km2. The 

isolated small inliers (kipukas) at Potato Hill, Latah 

County, and Kamiah Butte, Idaho County, Idaho, represent erosional highs of volcanic, volcaniclastic, and 

epiclastic(?) rocks of early Cenozoic(?) age surrounded by 

flows of the Columbia River Basalt Group. Neither inlier 

is more than a few tens of square kilometers in maximum 

extent, and neither has been studied in detail.

Some of these rocks in isolated bodies away from the 

type localities of the Clarno Formation in Oregon and the 

Challis Volcanics in Idaho have been assigned ages either 

by fossils or by isotopic dating, whereas others have not. 

Isotopic dating of some of these isolated bodies that are 

considered to be correlative with either the Clarno 

Formation or the Challis Volcanics indicates that they 

may be related to later periods of volcanism. Only 

additional isotopic dating and mapping can resolve these 

uncertainties.

Distribution of the Paleocene(?) to lowermost Oligocene(?) rocks in the subsurface beneath the very 

widespread Miocene and younger flows and sedimentary 

rocks of the region is virtually unknown. Lipman and 

others (1972, fig. 2a) interpreted the distribution to be 

extensive and relatively continuous throughout the Pacific Northwest, as inferred from widely separated outcrop areas. However, great variations in thickness and

rapid thinning away from volcanic centers indicates that 

these rocks are probably erratically distributed and may 

be quite discontinuously distributed on pre-Cenozoic 

terranes. Rocks that are lithologically similar and appear 

to be partly the same age as the Clarno Formation are 

exposed about 100 km south of the Blue Mountains near 

Paisley, Oreg. (Muntzert, 1969; Walker, 1980, p. 4), and 

a little farther south in the Warner Range of northeastern California. Also, the older part of the Fisher Formation of the western Cascade volcanic sequence (Peck and 

others, 1964) shows some lithologic similarities to, as well 

as differences from, rocks of the Clarno Formation; and 

K-Ar ages indicate some apparent temporal overlap 

(Lux, 1982; Fiebelkorn and others, 1983) with younger 

parts of the Clarno Formation.

STRATIGRAPHY

Paleocene(?) to lowermost Oligocene(?) rock assemblages in northeastern Oregon form a thick sequence of 

largely volcanogenic deposits mostly of andesitic composition, which locally includes a basal section of terrestrial, 

epiclastic quartzose and feldspathic fine-grained sedimentary rocks (fig. 2.3). In the Blue Mountains province 

(see fig. 1.2), this sequence rests unconformably on many 

different types of rocks, including Cretaceous marine 

sedimentary rocks, Triassic and Jurassic sedimentary 

and volcanic rocks, ophiolite, Paleozoic(?) and Mesozoic 

granitic to gabbroic intrusive rocks, and Paleozoic sedimentary, metasedimentary, and metamorphic rocks. The 

sequence is overlain by Eocene(?), Oligocene, and Miocene rocks of the John Day Formation or, in some 

places, by flows of the Columbia River Basalt Group. 

Contact relations with the overlying John Day Formation 

are not fully resolved (see chap. 3). In most places the 

contact is clearly unconformable (Waters and others, 

1951; Swanson and Robinson, 1968; Oles and Enlows, 

1971; Fisher and Rensberger, 1972; Robinson, 1975), but 

in a few localities the Clarno and John Day apparently 

exhibit some temporal overlap, in which the youngest 

isotopic ages of rocks assigned to the Clarno Formation 

postdate the oldest ages determined on rocks assigned to 

the John Day Formation. Misidentification of the isotopically dated units is the most likely explanation for this 

apparent overlap, inasmuch as no evidence has been 

found to suggest interfingering of the Clarno and John 

Day Formations.

The terrestrial, epiclastic sedimentary rocks exposed 

along the axis of the Blue Mountains uplift south of 

Heppner are probably the oldest part of the Cenozoic 

section, as suggested by stratigraphic relations and 

available age data. However, the absence of regionally 

extensive marker horizons in the lower part of the 

section, repetition of some lithologic types in different 

parts of the section, and considerable relief on the
PALEOCENE(?), EOCENE, AND OLIGOCENE(?) ROCKS OF THE BLUE MOUNTAINS REGION 17

underlying unconformity surface (Fisher, 1967; Oles and 

Enlows, 1971, p. 13; Thayer and Brown, 1973) make it 

nearly impossible to firmly establish which part of the 

assemblage is the oldest. In the Heppner area, the rocks 

consist of bedded and folded, well-sorted, fine- to medium-grained, quartzose and feldspathic, micaceous sandstone, as well as micaceous and carbonaceous siltstone 

and shale. The absence or near-absence of volcanic debris 

in the fine- to medium-grained, well-sorted sedimentary 

rocks indicates that they were derived from granitic and, 

possibly, sedimentary, metasedimentary, or metamorphic source rocks exposed in a terrane with little to 

moderate relief. South of Heppner they are conformably 

overlain by andesitic volcanic and volcaniclastic rocks 

typical of the Clarno Formation as exposed in adjoining 

areas.

The total thickness of the sedimentary rocks is not 

known, but it is estimated to range from about 365 to 455 

m for the section near Willow Creek 25 to 30 km 

southeast of Heppner (Pigg, 1961). Shorey (1976) estimated the thickness of these sedimentary rocks, which he 

identified as his Herren unit, at "slightly less than 2,000 

feet" or about 600 m. Whether significant thicknesses of 

these lithologically distinct rocks are present in areas far 

removed from Willow Creek is not known. Pigg (1961, p. 

58) indicated that pre-Clarno post-Cretaceous sedimentary rocks were encountered in deep oil test holes, one in 

an area about 50 to 60 km west of the Willow Creek area 

and another as much as 170 km south of the area. His 

description of the volcanic and sedimentary rocks penetrated in these test holes indicates that they differ 

significantly from the quartzose and feldspathic sedimentary rocks south of Heppner and are nearly identical to 

tuff, claystone, and mudstone typical of the Clarno 

Formation. None of the sedimentary rocks in these test 

holes apparently has been dated; thus, if they predate 

rocks typical of the Clarno Formation and are of Paleocene age, they represent a significantly different 

facies from the facies represented by the sedimentary 

rocks southeast of Heppner.

The Clarno Formation is a very thick sequence that 

comprises extremely diverse volcanic, volcaniclastic, and 

sedimentary rocks and that apparently overlies the 

Paleocene(?) and lower Eocene epiclastic sedimentary 

rocks. The sequence is dominated by several kinds of 

andesite flows, mudflows, and breccia; it also contains 

moderate to large volumes of andesitic to rhyolitic 

volcaniclastic rocks and tuffaceous sedimentary rocks. 

The aggregate thickness is more than 1,825 m, although 

in no one area is the total section displayed.

Two thick sections of the Clarno Formation are exposed near the axis of the Blue Mountains uplift, one in 

the Horse Heaven mining district of Jefferson County, 

Oreg., and the other a few kilometers to the east between

Cherry and Bridge Creeks astride the Jefferson-Wheeler 

County line. The section near Horse Heaven (Waters and 

others, 1951) is more than 1,768 m thick. Presumably, the 

section is stratigraphically above the terrestrial sedimentary rocks exposed south of Heppner, although this 

relation has not been demonstrated in the field. Waters 

and others (1951, p. 112-114) subdivided this thick 

section into four units, as follows (fig. 2.3):

Unit 1. A basal unit, 183 m thick, of platy andesite 

flows interbedded with layers of varicolored clay. The 

base of this unit is not exposed.

Unit 2. An overlying unit, 411 m thick, of tuff, 

tuffaceous claystone, volcanic mudflow deposits, and a 

few thin lava flows, all of andesitic composition.

Unit 3. About 533 m of tuffaceous claystone containing a few beds of coarse-grained tuff and a few andesite 

flows.

Unit 4. White tuff, largely rhyolitic in composition, at 

least 640 m thick, with some interlayered andesite flows. 

Quartz-bearing basaltic andesite flows are present in this 

part of the section and apparently extend eastward into 

adjoining areas (see unit 5 below).

The section between Cherry Creek and Bridge Creek, 

which is about 1,646m thick, was subdivided by Noblett 

(1981), first, into a lower sequence more than 1,463 m 

thick and an upper sequence locally from 152 to 182 m 

thick and, second, into smaller units dominated either by 

flows or by volcaniclastic and epiclastic deposits. The 

lower and upper sequences are separated by an angular 

unconformity and by a saprolite 3 to 6 m thick developed 

in the lower unit. Noblett's secondary divisions, from 

bottom to top and somewhat generalized, are:

Unit 1. A 42-m-thick unit composed of mudflow 

(lahar) deposits and conglomeratic sandstone containing 

andesite fragments. The base of this unit is not exposed.

Unit 2. About 457 m of dominantly plagioclase-, 

augite-, and hypersthene-phyric andesite flows.

Unit 3. An overlying 30-m-thick unit of bouldery 

andesitic mudflow.

Unit 4. More than 610 m of tuffaceous fluvial and 

lacustrine sedimentary rocks, locally containing fossil 

plants, and a few local thin andesite flows.

Unit 5. A sequence of aphyric olivine-, augite-, and 

hypersthene-bearing andesite flows with an aggregate 

thickness of more than 305 m. In the middle part of this 

unit are distinctive basaltic andesite flows containing 

quartz xenocrysts probably correlative with those in unit 

4 of the Horse Heaven mining district.

Unit 6. About 100 m of tuffaceous sandstone and 

plagioclase-, augite-, and hypersthene-phyric andesite 

and basaltic andesite flows.

These lower units were broadly(?) folded, and a 6- 

m-thick saprolite was developed before deposition of the 

overlying units.
18 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

121° 120° 119° 118°

WALLA WALLA |_ 

/X_

S H Walla Walla I N G T O N

MORROW

OREGON

) D E S C H U

20 40 60 80 100 KILOMETERS

FIGURE 2.2. Approximate outcrop area of Paleocene(?), Eocene, and
PALEOCENE(?), EOCENE, AND OLIGOCENE(?) ROCKS OF THE BLUE MOUNTAINS REGION 

117° 116°

19

EXPLANATION

* 

Major vents for Eocene volcanic rocks

46° Outcrop area Queried where uncertain

Anticline (or uplift) Dashed where approximately 

located. Arrow on axis shows direction of 

plunge

45°

44°

Distribution of outcrops from Brown and Thayer, 1966; 

Waters. 1968a,b; Swanson, 1969; Walker, 1973; 

Bond. 1978.

lowermost Oligocene(?) rocks in Blue Mountains region and adjacent areas.
20 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Unit 7. A fresh pilotaxitic basaltic andesite, 30 m 

thick, overlying the unconformity.

Unit 8. A hornblende andesite in the form of a 

bulbous dome more than 122 m thick but with restricted 

lateral extent.

Unit 9. An uppermost aphyric flow, 18 m thick, of 

basaltic andesite.

Another section, about 1,800 m thick, has been described in the Mitchell area, where Oles and Enlows 

(1971) subdivided what they called the Clarno Group into 

a lower unit dominated by thick andesite flows interlayered with varicolored tuffaceous sedimentary rocks and 

an unconformably overlying unit dominated by andesite 

flows, mudflows, and lacustrine and fluvial tuffaceous

Section 2 Section 3 Section 4

John Day 

Formation

John Day 

Formation;

locally 

Miocene basalt

John Day 

Formation

1000 METERS

Section 1

John Day

Formation and

Columbia River

Basalt Group,

undivided

EXPLANATION

Saprolite

Aphyric andesite flows

Mudflows (lahars)

<D

Horse Heaven _ 

mining district (4)

Tuffaceous sedimentary rocks

INDEX MAP SHOWING LOCATION OF SECTIONS

Porphyritic andesite and 

basaltic andesite

Quartzose and feldspathic 

sedimentary rocks

FIGURE 2.3. Generalized sections of Paleocene(?), Eocene, and lowermost Oligocene(?) rocks. Section 1 from Willow 

Creek area southeast of Heppner (modified from Pigg, 1961); section 2 from Horse Heaven mining district (Waters 

and others, 1951); section 3 from Cherry Creek-Bridge Creek area (Noblett, 1981); section 4 from Mitchell area 

(modified from Taylor, 1960, and Oles and Enlows, 1971).
PALEOCENEC?), EOCENE, AND OLIGOCENEC?) ROCKS OF THE BLUE MOUNTAINS REGION 21

sedimentary rocks. According to Oles and Enlows (1971, 

p. 14), rocks of the lower unit were affected by deformation that folded underlying Cretaceous rocks, whereas 

those of the upper unit were not.

In the northwest corner of the Mitchell quadrangle, 

near the hamlet of Clarno, Taylor (1960) found the Clarno 

Formation to be considerably thinner, about 500 to 600 

m, and to contain a much higher percentage of volcaniclastic sedimentary rocks. A lower unit there is composed 

of interlayered andesitic laharic deposits, cliff-forming 

volcanic conglomerate, and sparse andesitic and basaltic 

lava; it is conformably overlain by a sequence of tuffaceous sedimentary rocks, air-fall tuff, welded ash-flow 

tuff, and interlayered basalt and andesite flows. Taylor 

(1960) recognized basalt and basaltic andesite flows 

containing quartz xenocrysts in both his lower and upper 

units, in contrast to the sections at Horse Heaven and in 

the Cherry Creek-Bridge Creek area, where quartzbearing mafic flows were reported only in the upper part 

of the section.

The measured sections one near Horse Heaven, another between Cherry and Bridge Creeks, and a third 

near Mitchell exhibit some of the many lithologic variations in the Clarno Formation, but they are individually 

and even collectively unrepresentative of the entire 

sequence. Probably the most characteristic aspect of the 

volcanogenic part of the formation is its variation from 

place to place, with coarse-grained, near-vent facies, 

including vent and flow breccia, volcanic mudflows (lahars), and volcanic conglomerate, grading outward from 

the many separate volcanic centers into finer grained 

facies. Intermingled with the andesitic flows and clastic 

rocks are rhyolite and dacite domes, flows and flow 

breccia, small intrusions, and beds of coarse-grained 

near-vent pumiceous ejecta. Most of the rhyolites and 

dacites are flow-banded and aphyric, but locally they 

contain phenocrysts of sanidine, quartz, and altered 

oligoclase. In several widely separated areas, including 

southwest of Clarno near Current Creek and east of 

Prineville, Swanson (1969) showed that the Clarno Formation also contains many layers of pumice lapilli tuff, 

most of which he attributed to an ash-flow origin. The 

ash-flow tuff layers are dacitic and rhyolitic in composition and contain plagioclase and sanidine in an originally 

glassy matrix, now mostly devitrified and altered to clay 

minerals and feldspar.

Complexly intermixed with the volcaniclastic rocks are 

numerous, commonly local, small-volume flows and flow 

breccias of plagioclase-, clinopyroxene-, orthopyroxene-, 

and some olivine-phyric, pilotaxitic to hyalopilitic basaltic 

andesite. Some hornblende-phyric andesite flows and 

breccias are locally present.

Many of the flows and breccias are separated by thin, 

discontinuous layers of saprolite, and more continuous

saprolite layers are present locally in the uppermost part 

of the formation; however, in some places the overlying 

John Day Formation rests disconformably(?) on unweathered rocks of the Clarno Formation. Many of the different 

lithologic phases of the Clarno Formation are complexly 

intermingled and commonly separated by local unconformities. Rarely can individual units be traced for more 

than 1 or 2 km.

Most of these rocks have been diagenetically and 

deuterically altered and, locally, subjected to low-grade 

metamorphism from deep burial in a region of slightly 

elevated heat flow (Riccio, 1978) and from thermal effects 

adjacent to the numerous domes, plugs, and small 

intrusions. The original volcanic glass in these rocks is 

mostly devitrified and further crystallized into various 

secondary minerals, and most of the primary minerals 

are variously altered to smectite, zeolites, calcite, secondary silicic minerals, and, locally, chlorite. Iron-rich 

hornblende commonly has been completely altered to 

hematite and an unidentified opaque mineral, and hematite fills fractures and pores in much of the andesite.

Although these alteration features are not ubiquitous, 

they are characteristic of most of the formation. Where 

the rocks are unaltered or only slightly altered, they are 

difficult, if not impossible, to distinguish from similar 

plagioclase-, pyroxene-, hornblende-, and olivine-phyric 

flows and breccias of younger age. Therefore, radiometric ages are probably the most reliable means of identifying rocks of the Clarno Formation in areas widely 

separated from the type locality near Clarno.

Rocks exposed in the inliers at Potato Hill and Kamiah 

Butte in western Idaho also present problems in regional 

correlation. The inlier at Kamiah Butte consists of 

comparatively fresh, commonly trachytic augite and 

olivine(?) andesite flows and flow breccias, both porphyritic and aphyric, whereas the Potato Hill inlier largely 

comprises well-indurated and altered breccia and conglomerate^) deposits that consist of poorly sorted angular fragments of fine-grained sedimentary rocks, gneiss, 

sheared quartzite, and welded tuff, as well as angular to 

subangular quartz and feldspar. Thin veinlets of quartz 

cut the clastic rocks at Potato Hill, and minor epidote and 

chlorite replace original constituents. The lithology and 

comparative freshness of the andesite at Kamiah Butte, 

and its stratigraphic position beneath flows of the Columbia River Basalt Group, certainly permit correlation 

with similar rocks in the Clarno Formation and the 

Challis Volcanics. Whether the clastic rocks at Potato 

Hill are correlative with rocks of either of those formations is questionable because even though they are 

lapped by flows of the Columbia River Basalt Group, 

their induration, alteration, and quartz veining are similar to those of the Mesozoic volcaniclastic and sedimentary rocks exposed in Hells Canyon, Idaho, nearly 150 km
22 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

to the southeast. However, angular fragments of welded 

tuff contained in these rocks suggest a Cenozoic rather 

than Mesozoic age. None of the rocks in either inlier has 

been dated radiometrically.

AGE

The Paleocene(?) to Oligocene(?) rocks rest unconformably on various Mesozoic or older rocks and are overlain 

by mainly Oligocene and younger rocks representing 

several formations. Relations between these lower Cenozoic rocks and overlying rocks, composing mainly the 

John Day Formation, are not fully resolved, but in most 

areas the formations are separated by a fairly widespread 

unconformity and a saprolite zone (regolith) developed on 

the Clarno Formation (Waters and others, 1951; Waters, 

1954; Peck, 1964; Swanson and Robinson, 1968; Oles and 

Enlows, 1971).

Knowlton (1902) originally assigned a late Eocene age 

to fossil plants from some of the arkosic sandstone, 

carbonaceous siltstone, and shale beds south of Heppner 

and near Pilot Rock on East Birch Creek. However, 

R.W. Brown (oral commun., 1955; see Hogenson, 1964) 

considered the fossil plants in these beds to be Eocene or, 

possibly, Paleocene in age. A reevaluation of the East 

Birch Creek fossil flora collected from sandstone and 

shale beds indicated to Elemendorf and Fisk (1978) an 

age older than typical Clarno Formation floras, that is, 

Paleocene or early Eocene. Floras collected from the 

Clarno Formation at localities on Cherry, Current, and 

Bridge Creeks near and northwest of Mitchell (fig. 2.1), 

as well as near Clarno, apparently are somewhat younger 

and have been assigned an Eocene or early Oligocene age 

(Knowlton, 1902; p. 102-103; Chaney, 1938; Arnold, 1952, 

p. 68-72; Scott, 1954; Hergert, 1961; McKee, 1970). 

Apparently, the only isotopic ages on these sedimentary 

rocks were obtained on white detrital mica separated 

from the arkosic sedimentary rocks. Two samples from 

the Herren unit yielded white mica with ages of 85.7 and 

84.1 Ma (Heller and others, 1985).

Isotopic ages obtained on rocks of the Clarno Formation range mostly from 55 to 40 Ma, whereas rocks of the 

John Day Formation range in age from about 37 to 21 Ma. 

Eight K-Ar ages on rocks that have been assigned to the 

Clarno Formation by several investigators are younger 

than 37 Ma (table 2.1), the approximate age of the lowest 

ash-flow sheet of the John Day Formation2 (Swanson and 

Robinson, 1968; Robinson, 1975). In addition, several 

undated bodies of Clarno-like rocks south and southeast

aflie original published age was 36.4 Ma; the 37.4-Ma age was obtained by using the original 

analytical data and applying abundance and decay constants acceptable in 1982.

of Prineville (fig. 2.1) appear to postdate the Clarno 

Formation; these rocks are briefly described in chapter 3. 

Two of the anomalously young K-Ar ages are on dikes in 

the Mitchell area that are compositionally similar to 

basalts of the John Day Formation (Taylor, 1981), and 

one is so young that the rocks involved clearly have been 

misidentified as the Clarno (table 2.1). This young age 

(19.5±6 Ma) is from a comparatively fresh hornblendeporphyritic andesite or mafic dacite exposed in roadcuts 

just east of the Unity Reservoir (fig. 2.1), Baker County, 

Oreg. Although this young unit is superficially similar in 

lithology to some Clarno-age rocks, it shows none of the 

alteration that has produced secondary clays, zeolites, 

silica minerals, and calcite in the Clarno Formation. 

Thus, this unit should not be considered part of the 

Clarno Formation, nor even correlative with the youngest part of the John Day Formation.

Most of the other young Clarno ages are also suspect 

because they are on whole-rock specimens of somewhat 

altered lava (Enlows and Parker, 1972) or on feldspar 

separates from highly weathered bentonite (Evernden 

and others, 1964). The effect of such alteration on 

whole-rock ages is shown by sample W-3-58 (table 2.1). 

A hornblende separate from this rock yielded a K-Ar age 

of 46.1 ±4 Ma, whereas the whole-rock sample yielded an 

age nearly 5 m.y. younger.

Not only are these ages suspect because of the materials dated, but several conflict with ages on fossil floras 

and faunas. As pointed out by Manchester (1981, p. 77), 

a K-Ar age of 34.0 Ma obtained on tuff from the Clarno 

"Nut Beds" of local usage conflicts with vertebrate fossil 

dating by Stirton (1944) and Hanson (1973) of about 

middle Eocene age, or about 48 Ma. According to Hanson 

(see Manchester, 1981, p. 77), the "Nut Beds" may 

represent a part of the lower part of the Clarno Formation that was uplifted by folding before deposition of 

mudflows and tuff of the upper part of the Clarno.

Thus, some and probably all of the ages in the range of 

35 to 28 Ma for rocks that have been identified as the 

Clarno on the basis of lithology should be considered 

suspect and reevaluated as additional mapping and age 

data are obtained. It seems reasonable, however, to 

suppose that Clarno-type volcanism may have persisted 

locally within the Blue Mountains province until after 

deposition of the John Day Formation had already begun. 

Rocks of the Clarno Formation were all erupted locally, 

whereas those of the John Day Formation were derived 

mostly from vents west of the present-day outcrops 

(Robinson and Brem, 1981).

The isotopic ages further imply the possibility of two 

major hiatuses, one at about 40 to 38 Ma and another, 

best demonstrated in John Day Formation ages, at about 

35 Ma (fig. 2.4). Whether these apparent breaks in the 

geologic record have any significance in terms of regional
PALEOCENE(?), EOCENE, AND OLIGOCENE(?) ROCKS OF THE BLUE MOUNTAINS REGION 23

volcanism and resulting stratigraphy is not known. The 

older break may represent a decline of calc-alkaline 

volcanic activity near the end of Clarno time, or it may 

result from insufficient data. The younger break probably reflects incomplete age dating of the lower part of the 

John Day Formation.

Future investigators would be well advised to question 

and reevaluate bodies of rocks mapped as the Clarno 

Formation that are far removed from the type locality 

and that are not characterized by secondary minerals 

denoting low-grade metamorphism. Many of the bodies of 

rock assigned to the Clarno Formation that lie east of 

U.S. Highway 395 are of particular concern in this 

regard.

STRUCTURE

Structural and stratigraphic relations among the diverse Paleocene(?) to lowermost Oligocene(?) rock types, 

as well as distributional relations with overlying Oligocene and Miocene rocks, demonstrate that early Cenozoic structural trends developed before deposition of

Base of John Day ^ 

Formation (Swanson 

and Robinson, 1968}

Clarno Formation

Previously mapped 

as Clarno Formation

1 1 Uppermost

"Lower Clarno" Formation 

(Enlows and Parker, 1972)

15 20 30 40 

MILLIONS OF YEARS

50 60

FIGURE 2.4. K-Ar ages (dots, including analytical errors, where 

available, shown by horizontal bars) for the Clarno and John Day 

Formations. Within each formation, dated samples are not necessarily arranged according to stratigraphic position. Age data from 

Fiebelkorn and others (1983). Vertical heavy-dashed lines indicate 

apparent age gaps.

these younger rocks and that subsequent deformation 

followed these earlier structural trends.

The distribution of rocks younger than the Clarno 

Formation indicates that during the early Tertiary a 

series of northeast-trending structural highs formed 

essentially along the axis of what is now the Blue 

Mountains uplift (Rogers, 1966; Fisher, 1967; Taylor, 

1977). This early Tertiary uplift (or antiform) includes 

northeast-trending fold elements that appear to have 

resulted from northwest-southeast compressions! forces 

(Taylor, 1977, 1981), but it also includes some faults and 

is marked by many coalescing constructional volcanic 

edifices. Many of the initial dips from these coalescing 

constructional highs are to the northwest and southeast, 

tending to emphasize the antiformal nature of the northeast-trending structural high. How much of the structural high is tectonic and related to arching of the Blue 

Mountains uplift (or antiform) and how much to the 

coalescing of numerous volcanic piles along the axis of the 

arch is not known. However, a significant tectonic uplift 

in early Oligocene time can be demonstrated from the 

distribution of and stratigraphic relations within the John 

Day Formation (Robinson, 1975; Woodburne and Robinson, 1977). A few folds east of Mitchell trend westnorthwest at a considerable angle (50°-60°) to the 

dominant northeastward trend, indicating either local 

divergence in the regional stress regime or possible 

differences in the age of development of these divergent 

structures. The distribution of rock units of different 

ages associated with both the northeast- and westnorthwest-trending folds indicated to Fisher (1967, p. 

120) that the folds predate the beginning of John Day 

deposition either pre-Oligocene or earliest Oligocene 

and probably developed after the Cretaceous. A more 

precise lower age limit may be indicated by the characteristics of the Paleocene(?) and lower Eocene sedimentary rocks south of Heppner. The fairly uniform and 

locally thin bedding of these rocks, moderate to good 

sorting, and presence of carbonaceous shale and coaly 

lenses, as well as an absence of volcanic debris, indicate 

a terrain of little physiographic or structural relief. Thus, 

these structures are apparently of Eocene or earliest 

Oligocene age.

COMPOSITION

Considerable chemical data are available on the volcanic and volcaniclastic rocks of the Clarno Formation. Some 

chemical data also are available on intrusions that represent sources of some of the volcanic rocks. Little is 

known, however, of the chemistry of the terrestrial 

quartzose and feldspathic sedimentary rocks, which apparently represent the earliest part of the Cenozoic 

section.
24 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

From the mineralogic composition of their coarsegrained facies, the terrestrial sedimentary rocks appear 

to represent detritus derived largely from a fairly homogeneous granitic to quartz dioritic or, possibly, metasedimentary or metamorphic source area in which quartz, 

feldspar, and white mica were major constituents. Minor 

constituents include apatite, zircon, garnet, sillimanite, 

staurolite, hornblende, and andalusite (Pigg, 1961; Trauba, 1975). The sandstone, composed mostly of angular to 

partly subrounded grains, is moderately well indurated 

and cemented by calcium carbonate. Finer grained beds 

contain large amounts of carbonaceous material, some 

altered to lignite and bituminous coal; the coal beds are 

invariably thin and contain large amounts of clay-size 

detritus. Sedimentary features indicate that deposition 

was at least partly in a deltaic environment.

Rocks of the Clarno Formation vary noticeably in 

composition and form a diverse assemblage that compositionally ranges from basalt to rhyolite (fig. 2.5), with a 

great preponderance of andesite and basaltic andesite. 

Some parts of the section are characterized by plagioclase-, augite-, hypersthene-, or hornblende-phyric 

flows, breccia, lahars, and some tuff, and other parts by 

abundant aphyric flows. Some of the lava flows are 

characterized by sparse quartz xenocrysts (Calkins, 1902; 

Waters and others, 1951; Noblett, 1981). Still other 

parts of the section are dominated by dacitic, rhyodacitic, 

and rhyolitic ash-flow tuff, pumice lapilli tuff, and mudstone, siltstone, sandstone, and conglomerate, mostly 

composed of volcanic debris. A few conglomerates in the 

eastern part of the Blue Mountains province that have 

been assigned to the Clarno Formation contain rounded 

pebbles and cobbles of metasedimentary rocks, mostly 

quartzite, as well as some metamorphic and igneous rock 

types.

Inasmuch as the chemistry of these volcanic rocks has 

been published elsewhere (Calkins, 1902; Oles and Enlows, 1971; Rogers and Novitsky-Evans, 1977; Noblett, 

1981) or released in dissertations (for example, Wilson, 

1973), only a brief summary will be reported here. 

Conspicuous chemical characteristics of the Clarno Formation are: (1) Basaltic andesite and andesite (Si02, 

~54-62 weight percent) are volumetrically predominant 

and accompanied by lesser amounts of basalt (Si02, 

<~52 weight percent) and rhyolite or rhyodacite (Si02, 

~70-76 weight percent); (2) the assemblage is mostly 

.calc-alkaline, although some rocks appear to be transitional to tholeiite (Rogers and Novitsky-Evans, 1977); 

and (3) there appears to be little relation between 

variations in rock chemistry and different stratigraphic 

units within the assemblage, although rhyolitic and 

rhyodacitic volcaniclastic and tuffaceous sedimentary 

rocks are more abundant high in the section. A histogram 

presented by Rogers and Novitsky-Evans (1977) shows

that 54 percent of the rocks they analyzed contain from 56 

to 63 weight percent Si02; this range may indicate actual 

volumes of rocks with different Si02 contents or simply 

reflect sampling bias.

Trace-element investigations of rocks of the Clarno 

Formation are of limited extent and pertain principally to 

abundances of lithophile elements (Rogers and Ragland, 

1980; Noblett, 1981) in both porphyritic and nonporphyritic intermediate to mafic flows. Rogers and Ragland 

(1980) have examined abundances of Rb, Sr, Y, Zr, Nb, 

Ba, V, Cr, and Ni, and Noblett (1981) examined 

abundances of Rb, Sr, Y, Zr, Nb, and Ni. Abundances of 

K, Rb, and Ba in andesite of the Clarno Formation are 

generally greater than those in andesite from continental 

margins, and Rogers and Ragland (1980) inferred that 

these rocks may have formed on a crust that was 

intermediate or transitional between oceanic and continental. They considered, further, that this assemblage of 

calc-alkaline rocks of the Clarno Formation most likely 

formed by partial melting of mantle or modified mantle 

material. Noblett (1981), however, considered the porphyritic lava to have formed as hydrous melt that rose 

from a subducting slab and that interacted with the 

overlying mantle. The nonporphyritic lava presumably 

formed by partial melting of anhydrous quartz eclogite 

with little or no subsequent crystal fractionation.

The chemistry and terrestrial character of rocks of the 

Clarno Formation suggested to Rogers and NovitskyEvans (1977, p. 63) that the sequence represents a 

subduction-zone assemblage erupted on a continental 

margin, rather than in an island arc. On the basis of 

trace-element chemistry, Rogers and Ragland (1980) 

inferred that the eruptions took place in an area of thin 

crust, transitional in character between oceanic and 

continental. Noblett (1981, p. 995) also considers these 

rocks to be related to subduction-induced volcanism along 

the continental margin and to be a late phase of Challis 

arc volcanism. This interpretation is supported by the 

abundances of lithophile elements in Clarno lavas relative 

to those elements in island arcs and continental margins, 

by the composition of pre-Cenozoic bedrock of the region, 

in which ophiolite, Paleozoic and Mesozoic island-arc, and 

continental crustal elements are tectonically intermixed, 

and by geophysical data (Thiruvathukal and others, 1970) 

that can be interpreted as indicating a transition zone 

between continent and ocean basin.

The geochemical characteristics of these rocks, their 

distribution in the Pacific Northwest, and their geometric relations to postulated crustal plates and their convergence directions suggest that the calc-alkaline 

volcanogenic assemblage of the Clarno Formation and 

coeval volcanic rocks near Paisley and Lakeview, Oreg., 

along the California-Oregon State line, and those in the 

Washington Cascades are underlain largely by dismem-
PALEOCENE(?), EOCENE, AND OLIGOCENE(?) ROCKS OF THE BLUE MOUNTAINS REGION 25

bered oceanic crust and accreted terranes and, most 

likely, are related to a north-south-trending, possibly

somewhat sinuous pre-Cascade. volcanic arc. These data 

further suggest that the somewhat similar and partly

13

12 -

10 -

uj 0

i e

Z

19 r

17

15

13

11

CaO MgO

 * 

+ *- 

/V. .

 x- ? : ' * < 

o ..t

D

x

Na,0

1 *

 / .i.° '.^.' . ' '

K0

%V

Ti0

'x- On

J .. '. A 

 .: *? *..

. ** "' ' :'^** *".

50 60 70 80 50 60 70 80 

SiO, CONTENT, IN WEIGHT PERCENT

EXPLANATION

X Average of 3 analyses, "Upper Clarno" Formation of Oles and Enlows (1971) 

~f- Average of 4 analyses, "Lower Clarno" Formation of Oles and Enlows (1971) 

O Average of 6 analyses of porphyritic andesite (Nobfett, 1981) 

A Average of 6 analyses of nonporphyritic andesites (Noblett, 1981) 

D One analysis of hornblende andesite (Noblett, 1981) 

 One analysis of "quartz basalt"-(Calkins, 1902) 

	Analyses from Rogers and Novitsky-Evans (1977)

50 60 70 80 

Si02 CONTENT, IN WEIGHT PERCENT

FIGURE 2.5. Barker diagrams of major oxides in rocks of the Clarno Formation. From Rogers and Novitsky-Evans (1977), with 

additions from Calkins (1902), Oles and Enlows (1971), and Noblett (1981).
26 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

coeval Challis and Absaroka Volcanics, which are underlain by cratonic rocks, are separate and genetically 

unrelated to this early Cenozoic arc and, thus, to the 

Clarno Formation. Further discussion of the relations of 

early Cenozoic volcanism to tectonism is presented in 

chapter 6.

SOURCE VENTS

Most of the eruptive centers that have been recognized 

for Clarno Formation rocks are centered on or near the 

axis of the Blue Mountains uplift (Swanson, 1969) and to 

the south of this axis within the large Clarno outcrop area 

in Crook County (fig. 2.2). Some are located in areas to 

the west near the Deschutes River (Waters, 1968a, b), 

and others to the southeast of the axis of the Blue 

Mountains uplift in Grant County (Brown and Thayer, 

1966). Taylor (1977, p. 768) indicated that many of the 

inferred vents are elongate intrusions oriented in a 

northeast-southwest direction, "coincident with axes of 

recurrent early Tertiary folding." Many vents are exposed in the area near Mitchell, Oreg., where they are 

commonly represented by isolated intrusions (fig. 2.6) 

that penetrate and disrupt Cretaceous sedimentary rocks 

(Oles and Enlows, 1971). Several vent areas also have 

been mapped in deep canyons of the Deschutes River and 

its tributaries (Waters, 1968a, b) and in widely scattered 

areas southeast of the axis of the Blue Mountains uplift in 

Grant County (Brown and Thayer, 1966).

Most of the vent complexes are deeply eroded and now 

consist largely of exhumed dikes, plugs, sills, and irregular intrusions of basaltic andesite or hornblende or 

pyroxene andesite identical to the lava flows, flow

FIGURE 2.6. Clarno-age conical andesitic plugs (White and Black 

Buttes on horizon) in rounded and subdued topography eroded from 

altered volcanic and volcaniclastic rocks of the Clarno Formation. 

View westward down Bridge Creek toward Mitchell, Wheeler 

County, Oreg. Elevation of White Butte 0eft) is 5,665 ft, and of Black 

Butte (right) 5,080 ft.

breccias, and lahars that characterize most of the Clarno 

Formation. A few vent complexes are less eroded and 

consist of cinders, agglomerate, thin flows, and lahars 

intruded by thin dikes and sills.

Domal masses of rhyolite and dacite, as well as related 

near-vent silicic breccias, deposits of pumice blocks, and 

coarse pumice lapilli tuff, have been recognized in several 

areas near the axis of the Blue Mountains uplift.

REFERENCES CITED

Arnold, C.A., 1952, Fossil Osmudaceae from the Eocene of Oregon: 

Palaeontographica, v. 92, pt. B, p. 63-78.

Bond, J.G., 1978, Geologic map of Idaho: Moscow, Idaho Bureau of 

Mines and Geology, scale 1:500,000.

Brooks, H.C., Mclntyre, J.R., and Walker, G.W., 1976, Geology of the 

Oregon part of the Baker 1° by 2° quadrangle: Oregon Department 

of Geology and Mineral Industries Geologic Map Series Map 

GMS-7, 25 p., scale 1:250,000.

Brown, C.E., and Thayer, T.P., 1966, Geologic map of the Canyon City 

quadrangle, northeastern Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1^447, scale 1:250,000.

Calkins, F.C., 1902, A contribution to the petrography of the John Day 

Basin: University of California, Department of Geology Bulletin, 

v. 3, p. 109-172.

Chaney, R.W., 1938, Ancient forests of Oregon A study of earth 

history in western America, in Cooperation in research: Carnegie 

Institution of Washington Publication 501, p. 631-648.

Elemendorf, Jean, and Fisk, L.H., 1978, The age of the "Pilot Rock 

flora," Umatilla County, Oregon [abs.]: Geological Society of 

America Abstracts with Programs, v. 10, no. 5, p. 215.

Enlows, H.E., and Parker, D.J., 1972, Geochronology of the Clarno 

igneous activity in the Mitchell quadrangle, Wheeler County, 

Oregon: Ore Bin, v. 34, no. 6, p. 104-110.

Evernden, J.F., Savage, D.E., Curtis, G.H., and James, G.T., 1964, 

Potassium-argon dates and the Cenozoic mammalian chronology of 

North America: American Journal of Science, v. 262, no. 2, p. 

145-198.

Fiebelkorn, R.B., Walker, G.W., MacLeod, N.S., McKee, E.H., and 

Smith, J.G., 1983, Index to K-Ar age determinations for the State 

of Oregon: Isochron/West, no. 37, 60 p.

Fisher, R.V., 1967, Early Tertiary deformation in north-central Oregon: American Association of Petroleum Geologists Bulletin, v. 51, 

no. 1, p. 111-123.

Fisher, R.V., and Rensberger, J.M., 1972, Physical stratigraphy of the 

John Day Formation, central Oregon: University of California 

Publications in Geological Sciences, v. 101, 45 p.

Hanson, C.B., 1973, Geology and vertebrate faunas in the type area of 

the Clarno Formation, Oregon [abs.]: Geological Society of America Abstracts with Programs, v. 5, no. 1, p. 50.

Heller, P.L., Peterman, Z.E., O'Neil, J.R., and Shafiqullah, Muhammad, 1985, Isotopic provenance of sandstones from the Eocene 

Tyee Formation, Oregon Coast Range: Geological Society of 

America Bulletin, v. 96, no. 6, p. 770-780.

Hergert, H.L., 1961, Plant fossils in the Clarno Formation, Oregon: 

Ore Bin, v. 23, no. 6, p. 55-62.

Hogenson, G.M., 1964, Geology and ground water of the Umatilla River 

basin, Oregon: U.S. Geological Survey Water-Supply Paper 1620, 

162 p.

Jones, R.W., 1982, Early Tertiary-age Kamiah Volcanics, north-central 

Idaho, in Bonnichsen, Bill, and Breckenridge, R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 

26, p. 43-52.
PALEOCENE(?), EOCENE, AND OLIGOCENE(?) ROCKS OF THE BLUE MOUNTAINS REGION 27

Knowlton, F.H., 1902, Fossil flora of the John Day Basin, Oregon: U.S. 

Geological Survey Bulletin 204, 153 p.

Lipman, P.W., Prostka, H.J., and Christiansen, R.L., 1972, Cenozoic 

volcanism and plate tectonic evolution of the Western United 

States Pt. I, Early and middle Cenozoic: Royal Society of 

London Philosophical Transactions, ser. A, v. 271, no. 1213, p. 

217-248.

Lux, D.R., 1982, K-Ar and ^Ar-^Ar ages of mid-Tertiary volcanic 

rocks from the western Cascade Range, Oregon: Isochron/West, 

no. 33, p. 27-32.

Manchester, S.R., 1981, Fossil plants of the Eocene Clarno Nut Beds: 

Oregon Geology, v. 43, no. 6, p. 75-81.

McKee, T.M., 1970, Preliminary report on fossil fruits and seeds from 

the mammal quarry of the Clarno Formation, Oregon: Ore Bin, v. 

32, no. 7, p. 117-132.

Mendenhall, W.C., 1909, A coal prospect on Willow Creek, Morrow 

County, Oregon, in Contributions to economic geology, 1907 Pt. 

II, Coal and lignite: U.S. Geological Survey Bulletin 341, p. 

406-408.

Merriam, J.C., 1901, A contribution to the geology of the John Day 

Basin [Oreg.]: University of California, Department of Geology 

Bulletin, v. 2, p. 269-314.

Muntzert, J.K., 1969, Geology and mineral deposits of the Brattain 

district, Lake County, Oregon: Corvallis, Oregon State University, M.S. thesis, 70 p.

Noblett, J.B., 1981, Subduction-related origin of the volcanic rocks of 

the Eocene Clarno Formation near Cherry Creek, Oregon: Oregon 

Geology, v. 43, no. 7, p. 91-99.

Oles, K.F., and Enlows, H.E., 1971, Bedrock geology of the Mitchell 

quadrangle, Wheeler County, Oregon: Oregon Department of 

Geology and Mineral Industries Bulletin 72, 62 p.

Peck, D.L., 1964, Geologic reconnaissance of the Antelope-Ashwood 

area, north-central Oregon: U.S. Geological Survey Bulletin 

1161-D, 26 p.

Peck, D.L., Griggs, A.B., Schlicker, H.G., Wells, F.G., and Dole, 

H.M., 1964, Geology of the central and northern parts of the 

western Cascade Range in Oregon: U.S. Geological Survey 

Professional Paper 449, 56 p.

Pigg, J. H., Jr., 1961, The lower Tertiary sedimentary rocks in the Pilot 

Rock and Heppner areas, Oregon: Eugene, University of Oregon, 

M.S. thesis, 67 p.

Riccio, J.F., 1978, Preliminary geothermal resource map of Oregon: 

Oregon Department of Geology and Mineral Industries Geologic 

Map Series Map GMS-11, scale 1:50,000.

Robinson, P.T., 1975, Reconnaissance geologic map of the John Day 

Formation in the southwestern part of the Blue Mountains and 

adjacent areas, north-central Oregon: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-872, scale 1:125,000.

Robinson, P.T., and Brem, G.F., 1981, Guide to geologic field trip 

between Kimberly and Bend, Oregon, with emphasis on the John 

Day Formation, in Johnston, D.A., and Donnelly-Nolan, Julie, 

eds., Guides to some volcanic terranes in Washington, Idaho, 

Oregon, and northern California: U.S. Geological Survey Circular 

838, p. 29-40.

Rogers, J.J.W., 1966, Coincidence of structural and topographic highs 

during post-Clarno time in north-central Oregon: American Association of Petroleum Geologists Bulletin, v. 50, no. 2, p. 390-396.

Rogers, J.J.W., and Novitsky-Evans, J.M., 1977, The Clarno Formation of central Oregon, U.S.A. Volcanism on a thin continental 

margin: Earth and Planetary Science Letters, v. 34, no. 1, p. 

56-66.

Rogers, J.J.W., and Ragland, P.C., 1980, Trace elements in continental-margin magmatism Pt. I, Trace elements in the Clarno 

Formation of central Oregon and the nature of the continental

margin on which eruption occurred: Geological Society of America 

Bulletin, v. 91, pt. 2, no. 4, p. 1217-1292.

Scott, R.A., 1954, Fossil fruits and seeds from the Eocene Clarno 

Formation of Oregon: Palaeontographica, v. 96, pt. B, no. 3-6, p. 

66-97.

Shorey, E.F., 1976, Geology of part of southern Morrow County, 

northeast Oregon: Corvallis, Oregon State University, M.S. 

thesis, 131 p.

Stirton, R.A., 1944, A rhinoceros tooth from the Clarno Eocene of 

Oregon: Journal of Paleontology, v. 18, no. 3, p. 265-267.

Swanson, D.A., 1969, Reconnaissance geologic map of the east half of 

the Bend quadrangle, Crook, Wheeler, Jefferson, Wasco, and 

Deschutes Counties, Oregon, in Geological Survey Research, 

1968: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-568, scale 1:250,000.

Swanson, D.A., and Robinson, P.T., 1968, Base of the John Day 

Formation in and near the Horse Heaven mining district, northcentral Oregon, in Geological Survey Research, 1968: U.S. Geological Survey Professional Paper 600-D, p. 154-161.

Taylor, E.M., 1960, Geology of the Clarno Basin, Mitchell quadrangle, 

Oregon: Corvallis, Oregon State University, M.S. thesis, 173 p.

 1977, The Clarno Formation A record of early Tertiary volcanism in central Oregon [abs.]: Geological Society of America 

Abstracts with Programs, v. 9, no. 6, p. 768. 

-1981, A mafic-dike system in the vicinity of Mitchell, Oregon, and

its bearing on the timing of Clarno-John Day volcanism and early 

Oligocene deformation in central Oregon: Oregon Geology, v. 43, 

no. 8, p. 107-112.

Thayer, T.P., and Brown, C.E., 1973, Ironside Mountain, Oregon A 

late Tertiary volcanic and structural enigma: Geological Society of 

America Bulletin, v. 84, no. 2, p. 489-498.

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Regional gravity of Oregon: Geological Society of America Bulletin, v. 81, no. 3, p. 725-738.

Trauba, W.C., 1975, Petrography of pre-Tertiary rocks of the Blue 

Mountains, Umatilla County, northeast Oregon: Corvallis, Oregon 

State University, M.S. thesis, 171 p.

Walker, G.W., 1973, Reconnaissance geologic map of the Pendelton 

quadrangle, Oregon and Washington: U.S. Geological Survey 

Miscellaneous Investigations Series Map 1-727, scale 1:250,000.

 1980, Preliminary report on the geology of the Lakeview 

uranium area, Lake County, Oregon: U.S. Geological Survey 

Open-File Report 80-532, 33 p.

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Geological Society of America Bulletin, v. 65, no. 12, p. 120.

 1968a, Reconnaissance geologic map of the Dufur quadrangle, 

Hood River, Sherman, and Wasco Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-555, 

scale 1:125,000. 

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Jefferson and Wasco Counties, Oregon: U.S. Geological Survey

Miscellaneous Geologic Investigations Map 1-556, scale 1:125,000. 

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G.W., and Williams, Howel, 1951, Quicksilver deposits of the

Horse Heaven mining district, Oregon: U.S. Geological Survey

Bulletin 969-E, p. 105-149. 

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basal Clarno Formation, central Oregon: Houston, Tex., Rice

University, M.A. thesis, 47 p. 

Woodburne, M.O., and Robinson, P.T., 1977, A new late Hemingfordian mammal fauna from the John Day Formation, Oregon, and its

stratigraphic implications: Journal of Paleontology, v. 51, no. 4, p.

750-757.

3. EOCENE(P), OLIGOCENE, AND LOWER MIOCENE ROCKS OF 

THE BLUE MOUNTAINS REGION

By PAUL T. ROBINSON 1 , GEORGE W. WALKER, and EDWIN H. McKEE

CONTENTS

	Page

Abstract 29

Introduction 29

John Day Formation 30

Definition 30

Base 30

Stratigraphy 34

Eastern fades 34

Lower member 34

Middle member 38

Upper member 38

Southern facies 39

Western facies 40

Member A 40

Member B 41

Member C 43

Member D 44

Member E 44

Member F 44

Member G 45

Member H 47

Member I 47

Northern subfacies 48

Mutton Mountains subfacies 51

Isolated rhyolite domes and flows 52

Composition 53

Age 54

Structure 56

Origin 58

Unusual rocks of John Day age 59

References cited 60

ABSTRACT

A distinctive assemblage of tuffaceous sedimentary rocks, air-fall and 

ash-flow tuffs, and sparse lava flows crops out widely in north-central 

Oregon. The sequence is mainly Oligocene and early Miocene in age and 

lies between the Clarno Formation below and the Columbia River 

Basalt Group above. Most of the rocks in this sequence have been 

assigned to the John Day Formation, but a few are of uncertain age or 

affinity.

1Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada.

Three lithologic facies are recognized in the John Day Formation. An 

eastern facies that crops out east of the Blue Mountains uplift consists 

of tuffaceous claystone, fine-grained air-fall tuff, and a single ash-flow 

sheet in the middle part of the section. The western facies, which occurs 

between the Blue Mountains uplift and the Cascade Range, consists of 

tuffaceous claystone, coarse-grained tuff, and numerous interlayered 

ash-flow sheets and lava flows. A southern facies, lithologically similar 

to the eastern facies, crops out south of the Ochoco Mountains. 

Correlations between these facies are based on radiometric ages and 

vertebrate faunas.

The John Day Formation is compositionally heterogeneous. Most of 

the tuffaceous claystone and air-fall tuffs have pyrogenetic mineralogies 

indicating an original andesitic or dacitic composition. This air-fall 

material is believed to have been erupted from vents in what is now the 

western Cascade Range and thus provides a record of early Cascade 

volcanism. The interlayered ash-flow sheets are all rhyolitic and are 

believed to have been erupted from vents east of the present-day 

Cascade Range, such as those in the Mutton Mountains. Lava flows in 

the John Day Formation range in composition from alkali olivine basalt 

to rhyolite and are clearly derived from local vents scattered throughout the outcrop area.

Well-documented radiometric ages for the John Day Formation range 

from 37.4± 1.1 to 22.7±2.7 Ma, in good agreement with the faunal ages.

A few unusual sequences in north-central Oregon have ages equivalent to the John Day Formation but andesitic lithologies more typical of 

the Clarno Formation, suggesting some overlap in these two different 

styles of volcanism.

INTRODUCTION

In northern and northeastern Oregon, rocks mainly of 

Oligocene and early Miocene age form a distinctive 

sequence lying between the Eocene and lowermost 

Oligocene(?) Clarno Formation and the lower Miocene 

and younger Columbia River Basalt Group. In a few 

places, where flows of the Columbia River Basalt Group 

pinch out on the mainly Oligocene and lower Miocene 

rocks, the sequence is overlain unconformably by middle 

Miocene tuffs and tuffaceous sedimentary rocks. Rocks of 

the sequence crop out chiefly on the two flanks of the Blue 

Mountains uplift and in an area south of the Ochoco 

Mountains. There are major differences in lithology and 

stratigraphy among these outcrop areas, but the different sections can be correlated on the basis of distinctive 

fossil flora and fauna and numerous isotopic ages.

The mainly Oligocene and lower Miocene sequence 

consists chiefly of intermediate and silicic air-fall tuffs

29
30 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

and tuffaceous sedimentary rocks, locally accompanied 

by abundant ash-flow tuffs, mafic lava flows, and silicic 

flows and domes. Most of these rocks have been assigned 

to the John Day Formation, which is distinguished from 

the underlying Clarno Formation largely on the basis of 

lithology, composition, and age (see chap. 2). In addition, 

the John Day Formation consists of well-layered and 

laterally continuous units, whereas the Clarno Formation 

has no throughgoing stratigraphy. However, there is 

some overlap between the two units in lithology and 

possibly in age, and the boundary is not everywhere 

obvious.

Because the bulk of the Clarno Formation is andesitic 

in composition, isolated rhyolite flows and domes that 

overlie or intrude Clarno lavas have generally been 

assigned to the John Day Formation (Waters and others, 

1951; Waters and Vaughan, 1968a, b). However, a few 

isotopic ages suggest that most of these bodies are 

Eocene in age and should be included in the Clarno 

Formation. In a few localities, such as the Horse Heaven 

mining district, these rhyolitic rocks are overlain unconformably by the oldest ash-flow tuffs of the John Day 

Formation (Swanson and Robinson, 1968); in others, they 

are commonly mantled with 1 to 2 m of red saprolite, such 

as is found locally at the top of the Clarno Formation.

More puzzling are isolated sequences of andesitic lava 

flows of typical Clarno lithology and interlayered rhyolitic volcaniclastic rocks whose isotopic ages are significantly younger than the oldest ages obtained thus far for 

the John Day Formation. Some of these enigmatic 

sequences (for example, that at Smith Rock, which has 

been dated at 30.8 ±0.5 Ma) may be unconformably 

overlain by rocks of the upper part of the John Day; 

others are isolated masses whose structural and stratigraphic relations are unknown. If the young ages can be 

substantiated, they imply some temporal overlap of 

Clarno-type volcanism and John Day volcanism. Although physical interfingering of the two units has yet to 

be demonstrated, such overlap is considered possible 

because the Clarno Formation was erupted from vents 

within its outcrop area, whereas the bulk of the John Day 

Formation was probably derived from vents farther 

west.

JOHN DAY FORMATION

DEFINITION

The name "John Day Formation" was first used by 

Marsh (1875) for a sequence of fossiliferous, supposedly 

lacustrine sedimentary rocks exposed along the banks of 

the John Day River (Condon, 1871). Merriam (1901) 

defined the formation as a sequence of varicolored tuffs 

and tuffaceous sedimentary rocks, approximately 350 m

thick, lying between the older Clarno Formation and the 

younger, then-named Columbia River Basalt. Merriam 

did not designate a type section, but most of the outcrops 

he described lie along the John Day River between 

Kimberly and Picture Gorge, the area generally considered to be the "type" area2 of the formation (Fisher and 

Rensberger, 1972).

Rocks of John Day lithology and age are widespread in 

north-central Oregon, in three principal areas: (1) an 

eastern area, including the "type" area, that extends 

along the east flank of the Blue Mountains uplift from the 

vicinity of Mitchell nearly to La Grande; (2) a western 

area lying between the Blue Mountains uplift and the 

Cascade Range; and (3) a southern area lying along the 

south flank of the Ochoco Mountains, generally between 

Prineville and Paulina (figs. 3.1, 3.2). The rocks in these 

three areas can be considered different facies of the 

formation.

In the eastern area, the formation has a maximum 

aggregate thickness of approximately 750 m and consists 

chiefly of fine-grained air-fall tuff and tuffaceous claystone (Calkins, 1902; Coleman, 1949; Fisher and Wilcox, 

1960; Hay, 1962a, 1963; Fisher and Rensberger, 1972). A 

conspicuous rhyolitic ash-flow sheet, the informally designated Picture Gorge ignimbrite of Fisher (1966a), lies 

near the middle of the formation. Rocks in the southern 

area are similar to those of the eastern facies and consist 

chiefly of fine-grained, volcaniclastic sedimentary rocks, 

with one conspicuous ash-flow sheet near the middle of 

the sequence. In the area south of Prineville is a second 

ash-flow sheet, which lies near the base of the formation 

(Waters and Vaughan, 1968a, b). In the western area, 

however, the rocks of the formation are much coarser 

grained and consist chiefly of lapilli tuff and tuffaceous 

sedimentary rocks, with numerous interbedded ash-flow 

sheets and lava flows of rhyolitic, basaltic, and trachyandesitic composition. The aggregate thickness in the 

western area is nearly 1,200 m, but individual sections 

rarely exceed 500 m in thickness.

BASE

In gross lithology the Clarno and John Day Formations 

are quite distinct; the Clarno consists chiefly of flows and 

breccias of pyroxene andesite and basalt (see chap. 2), 

and the John Day of dacitic to rhyolitic tuffs, tuffaceous 

sedimentary rocks, and flows. In detail, however, there 

is considerable overlap in composition, and it is commonly 

difficult to distinguish the two formations on the basis of

No formal type section or type locality has been established for the John Day Formation. 

Most investigators generally consider the section exposed between Picture Gorge and Kimberly 

as typical of the formation.
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 31

lithology alone. For example, trachyandesite and alkali 

olivine basalt flows are common in both the lower part of

the John Daiy Formation (Peck, 1961, 1964; Robinson, 

1969, 1975) and the upper part of the Clarno Formation

Approximate boundary 

of study area

44° _

A, Antelope and Antelope Valley

C, Clarno (Clarno Ferry)

CB, Cline Butte

D, Dant

DB, Deschutes Basin

DN, Donnely Basin

F, Fossil

FH, Forked Horn Butte

G, Grizzly

H, Heppner

HC, Hay Creek and Hay Creek Ranch

HH, Horse Heaven mining district

J, John Day

JB, Juniper Butte

K, Kinzua

KM, Kimberly

KS, Kahneeta Hot Springs

LG, La Grande

LR, Lonerock

M, Mitchell

MD, Madras

MM, Maury Mountains

INDEX TO LOCAUT1ES

MN,

MR,

MT.

NJ,

OM,

P,

PB,

PC,

PG,

PH,

PL,

RC,

SB,

SC,

SP,

SR,

T,

TC,

UB,

W,

WC,

WS,

Monument

Muddy Ranch

Mutton Mountains

North Junction

Oehoco Mountains

Prineville and Prineville Reservoir

Powell Buttes

Pine Creek

Picture Gorge

Painted Hills

Paulina

Rowe Creek

Snowboard Ridge

Service Creek

Spray

Smith Rock

Twickenham

Trout Creek

Unity Basin and Unity Reservoir

Willowdale

Wilson Creek

Warm Springs

FIGURE 3.1. Index map of Blue Mountains region, showing study area, and localities mentioned in text.
32 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

(Robinson, 1969). Silicic flows and volcaniclastic rocks 

similar to those of the John Day Formation are locally 

abundant in the Clarno Formation, particularly in the 

Monument quadrangle (Wilcox and Fisher, 1966) and in 

the Painted Hills area northwest of Mitchell. Tuffs and 

tuffaceous sedimentary rocks are abundant in the Clarno 

Formation in the Horse Heaven mining district (Waters 

and others, 1951) and in the vicinity of Muddy Ranch, and 

small outcrops of ash-flow tuff also are locally present 

(Oles and Enlows, 1971; Taylor, 1960).

After deposition of the Clarno Formation, an episode of 

weathering and erosion resulted in a moderately hilly 

terrain with as much as 120 m of relief (Waters, 1954). 

This surface became locally mantled with a red lateriticsoil zone (saprolite) ranging from a few centimeters to as 

much as 150 m in thickness (Waters and others, 1951; 

Hay, 1962a, 1963; Fisher, 1964). In many parts of the 

eastern outcrop area, this soil zone is a distinct lithologic 

unit that marks the contact between the Clarno and John 

Day Formations. However, this unit must be interpreted 

with care because similar soil zones occur at several 

levels within the Clarno Formation (Waters and others,

1951; Swanson and Robinson, 1968), and in numerous 

localities the deep-red claystones of the basal part of the 

John Day Formation rest directly on the post-Clarno soil 

zone, making it difficult to pinpoint the contact. Hay 

(1962a) suggested placing the contact in such areas just 

below the first appearance of pyrogenetic minerals.

In the western fades, the soil zone at the top of the 

Clarno is thin or missing altogether. There, the base of 

the John Day Formation is located at the base of an 

extensive rhyolitic ash-flow sheet that unconformably 

overlies andesitic and basaltic flows and breccias of 

typical Clarno lithology (Peck, 1961, 1964; Swanson and 

Robinson, 1968; Robinson, 1975). Two K-Ar ages of 37.4 

and 37.1 Ma indicate that this ash-flow sheet is latest 

Eocene (Palmer, 1983) in age (table 3.1); the 1-Ma error 

bars for these ages straddle the Eocene-Oligocene boundary at 36.6 Ma.

The position of the contact between the Clarno and 

John Day Formations in the Blue Mountains uplift 

between the eastern and western facies is not so clearly 

established. Waters and others (1951), working in the 

Horse Heaven mining district, designated a saprolite122° 121° 120° 119°

PORTLAND

45°

C( c) s>\

MUTTON _ 

.MOUNTAINS /;L\v.'-.V:

Ant0lm*u> .^^ " L"»". * " j

^<^:~'tl T^.TTr^rr-v-'-v-v//:..:.:;^ ^\ -

 ^. . .^. . , »^i t*>^."- ; .V^rf»* _ .,. -*^.*T^ ^* »*

: ."". " '^ fc^rtl tntain<; . -^"V^"; "*".'.'." "/:*."^ Dale

VMOUNTAINS yi;:v._\:: :: > lii-Xi^-.*-"" ^, -r-.-r^-v

/? ?$ Antelope^,^'. :l:- :.:-.>'xi' Mountains ^^.\?;.." " '. .' :'.'::. 

,A -'.: -: :-\ x--^ ' ' ' ' .' ' ^ >> ^ Vr^^.'^ - '::' ' .V- :;.v. . ? .' ' > 

RAic:CTFi3Ki: '-.' : '-V>i*" \ ^.' ' . . '/ : >; .'-.M-.'- Monument;/ 

y WEbTERN-.-x' p J/:.-vv^^T>S.-'- ' ' " / 

 ^FAnFS':--/Ashwood \^}Xl^'.-::-:--'^'EASTERN JS'E'ASfERN':. ':/ ::y 

/. /.;.-VPaihted Hilts'.. ;!; '/[.'FACIES^ FACIES,

Mitcheii ^"'"sjX; ;{;:>:/ &: '£*

S^.-;.:. ;.^;;5CRcture 

OCHOCO !O MOUNTAINS ^^

/ '*"">> WFL (m

Gorge

John Day

-___..._.. IN '.' .'i 

XJT; FACIES-VvirV

^iif v^i^-.-y

AREA OF MAP

25 50 KILOMETERS

FIGURE 3.2. Approximate outcrop extent of the John Day Formation in north-central Oregon. Dashed hnes enclose areas of known outcrops; 

dotted Hnes enclose areas where rocks of probable John Day affiliation are present. Formation also extends an unknown distance to the north 

beneath the Columbia River Basalt Group.
EOCENE(?), OLIGOGENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION

TABLE 3.1. Potassium-argon ages for the John Day Formation, Oregon 

[do. and Do., ditto]

33

Sample Unit Rock type Material Age (Ma) Reference

Eastern facies

KA1384 Lower member Claystone Adularia 21.6 Evemden and others (1964). 

KA845 do. Basalt Whole rock 31.5 Do. 

KA489 do. Tuff Sanidine 31.1 Do. 

Unknown do. do. do. 26.7 Curtis and others (1961). 

25.7 

KA647A Unknown do. do. 23.3 Evemden and others (1964). 

648-456 Lower member do. do. 26.810.4 This chapter. 

PTR-71-11 Picture Gorge Ash-How toff Plagioclase 26.9±2.3 Do. 

ignimbrite. 

KA648 do. Obsidian Whole rock 25.5 Evemden and others (1964). 

KA649A Unknown Unknown Albite 24.9 Do.

Southern facies

648-625A Lower member Ash-flow toff Plagioclase 32.110.7 This report.

Western facies

KA1384 Lower member Bentonite Sanidine 32.0 Evemden and others (1964). 

DAS-66-208 Member A Ash-flow toff do. 37.411.1 Swanson and Robinson (1968). 

DAS-66-195 do. do. do. 37.111.0 Fiebelkorn and others (1983). 

PTR-71-10 Member G do. do. 30.510.4 This chapter. 

648-584 do. do. do. 28.610.5 Do. 

648-34B do. do. do. 28.110.5 Do. 

648-552 do. do. do. 27.010.3 Do. 

PTR-71-5A Member H do. Plagioclase 30.910.7 Do. 

PTR-71-6 Member I do. do. 27.710.3 Do. 

RobTufS do. Tuff do. 23.413.3 Do. 

RobTufl do. do. do. 22.712.7 Do.

Miscellaneous outcrops

PTR-71-9A Snowboard Ash-flow tuff Plagioclase 57.0 This chapter. 

2/UT225 PowellButtes Rhyolite Whole rock 30.111.1 Evans and Brown (1981). 

PB-5/AH-34 do. do. Anorthoclase 28.311.0 Do. 

648-623B do. do. Sanidine 25.810.2 This chapter. 

CC-1 Unknown do. Anorthoclase 28.115.0 Walker (1973). 

GWW-56-69 do. Vitrophyre Biotite 27.710.8 Do.

mantled unconformity as the top of the Clarno Formation 

and referred to younger andesitic and rhyolitic flows as 

"post-Clarno," possibly equivalent to John Day rocks 

elsewhere. However, Swanson and Robinson (1968) 

found that the basal ash-flow sheet of the western facies 

of the John Day, which is latest Eocene(?) and earliest 

Oligocene in age, unconfbrmably overlies the post-Clarno 

rocks of the Horse Heaven mining district. In addition, 

one of the post-Clarno rhyolites has a K-Ar age of 

42.1 ±0.8 Ma® (Swanson and Robinson, 1968), well within

the age limits of the Clarno Formation. Because of these 

relations, Swanson and Robinson (1968) included the 

post-Clarno rocks of the Horse Heaven district in the 

Clarno Formation and suggested that the saprolite zone 

in this area marks a local unconformity within that unit.

^Their original published age was somewhat less than the value given here; this older age was 

obtained by applying abundance and decay constants acceptable in 1982 (Steiger and Jfiger, 

1977) to the original analytical data.
34 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Such local unconformities are common in the Clarno 

Formation, and no means has yet been found to distinguish individual saprolites. Thus, outside the "type" area 

of the John Day Formation in the eastern facies, the 

presence or absence of a red saprolite is not a reliable 

guide to the position of the ClamoJohn Day contact.

STRATIGRAPHY

EASTERN FACIES

Merriam (1901) originally divided what he called the 

John Day Series (or Formation) in the study area into 

three units based primarily on color: (1) a lower unit 

consisting chiefly of deep-red clays and tuffs, (2) a middle 

unit composed of bluish-green tuffs that crop out in 

rounded hills or pinnacled cliffs, and (3) an upper unit of 

buff-colored material containing abundant sand and gravel.

Fisher and Rensberger (1972) proposed a somewhat 

similar fourfold subdivision of the John Day Formation 

(fig. 3.3), from oldest to youngest: (1) the Big Basin 

Member, composed of deep-red claystones and approximately comparable to Merriam's lower unit; (2) the 

Turtle Cove Member, composed chiefly of green, buff, or 

red zeolitized tuff; (3) the Kimberly Member, consisting 

of light gray to buff unzeolitized tuff; and (4) the 

Haystack Valley Member, a sequence of fluviatile, lacustrine, and air-fall gray unzeolitized tuff. Both of these 

subdivisions are based on diagenetic features, chiefly 

color and the presence or absence of zeolites, and thus 

have limited stratigraphic significance. For example, 

Hay (1962a) showed that the red claystones in the lower 

part of the John Day Formation were in part derived 

from the underlying saprolite locally present at the 

ClarnonJohn Day contact. Likewise, Hay (1963) and 

Fisher and Rensberger (1972) showed that the boundary 

between the zeolitized and fresh-glass facies in the 

formation is an irregular surface that cuts across stratigraphic boundaries. For these reasons, we prefer the 

threefold subdivision of Hay (1962a, 1963) for the eastern 

facies of the John Day Formation, which is based on the 

presence of a widespread ash-flow sheet (Picture Gorge 

ignimbrite) near the middle of the formation (fig. 3.4). 

The lower member, approximately 335 m thick, consists 

chiefly of tuffaceous claystone and tuff, with sparse, 

small, interbedded flows of olivine basalt. The (informal) 

Picture Gorge ignimbrite is a compound cooling unit of 

rhyolitic tuff, ranging from about 3 to 85 m in thickness. 

The upper member consists of about 400 to 500 m of 

yellowish-gray to olive-gray tuffaceous claystone and 

vitric tuff, with local accumulations of reworked tuff and 

conglomerate near the top.

Detailed stratigraphic descriptions of the eastern facies were presented by Hay (1962a, 1963) and by Fisher 

and Rensberger (1972), and so only a brief discussion is 

given below.

LOWER MEMBER

The lower member of the John Day Formation consists 

chiefly of thick-bedded, massive tuffaceous claystone, 

with thin interlayered beds of air-fall tuff and small flows 

of alkali-olivine basalt. At the base of the formation along 

Rowe Creek, just north of Twickenham, a 15-m-thick 

ash-flow tuff overlies a red saprolite developed on andesitic lava flows of Clarno lithology. This white to lightgray, strongly welded eutaxitic tuff contains 5 to 10 

percent crystals, chiefly sanidine, quartz, and oligoclase. 

On the basis of its mineralogy (table 3.2) and stratigraphic position, this tuff was tentatively correlated with the 

basal ash-flow sheet of member A of the western facies by 

Hay (1962a, 1963), Swanson and Robinson (1968), and 

Robinson (1975).

Most claystone in the lower member is pale brown to 

reddish brown; some is green, yellowish green, or olive 

gray. Brick-red claystone forms a distinctive basal layer, 

which sometimes extends through the lower half of the 

member. Hay (1962a) showed that some red claystone 

consists of mixtures of weathered John Day ash and red 

kaolinitic clay derived from the saprolite at the top of the 

Clarno Formation; Fisher (1964), however, argued that 

some of the red claystone formed in place by weathering 

and diagenetic alteration.

In the overlying claystone, brown and yellow varieties 

consist chiefly of montmorillonite, whereas the green 

varieties contain abundant celadonite. All of the claystone is tuffaceous, originally containing about 85 to 90 

percent vitric material and 3 to 13 percent sand-size 

crystals and silicic rock fragments (Hay, 1962a, 1963; 

Fisher, 1968). The abundant andesine and sanidine crystals in the claystone suggest an original dacitic composition for much of the pyroclastic material. The original 

pumice fragments and shards have been extensively 

altered to clay minerals and zeolites. Hay (1962a) suggested that most of the lower member consists of vitric 

material which accumulated slowly at the surface and 

weathered to montmorillonite before burial. The zeolites 

and other diagenetic minerals formed after burial.

Air-fall tuff interbedded with the claystone forms 

light-gray, well-indurated layers, 2 cm to 3 m thick, that 

crop out as slightly resistant ledges. Most of the tuff is 

fine to medium grained and vitric and contains sparse 

pumice lapilli as much as 5 mm across. Sanidine, sodic 

plagioclase, and clinopyroxene make up 3 to 5 percent of 

most of the tuff, and quartz is locally present. Silicic rock 

fragments also are typically present in no more than 5 

percent by volume.
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 35

UNIT

Picture Gorge 

Basalt (part)

i

 fs 

a)

E

£

to

o

c

-C

 io

Haystack

Valley

Member '

Kimberly

Member 1

Turtle

Cove

Member 1

Big Basin 

Member '

Clarno

Formation

(part)

THICKNESS 

(m)

700 - 

650-

600-

500-

450- 

400-

350-

300-

icn 250 

200-

150

100-

50

0

LITHOLOGY

-VWV

^-^r

~_n_~

77?.*-:-:

DESCRIPTION

| Dark-gray, medium- to coarse-grained, commonly porphyritic, columnar jointed flows

Light-gray, well-bedded, commonly crossbedded, unzeolitized, tuffaceous conglomerate.

>_L '( sandstone, and lacustrine tuff containing abundant layers of light-gray, massive air -fall tuff.

 j Thick conglomerate layers common in lower third of member

-ffi'3

> > > 

~li.~

1"^.-

 .--

 . i

Light-gray, buff, or pale-greenish-gray, fine- to medium-grained, massive, unzeolitized

tuff. Rocks crop out in smooth grass-covered slopes

 [ Greenish-gray, fine- to medium-grained, massive zeolitized tuff and tuffaceous 

' 7 sedimentary rocks containing thin resistant layers of white to light-gray tuff

~ \

~ ~>~\ / Cream-colored, ledge-forming air-fall tuff composed of fresh or altered glass shards.

xv.v.v/) unit referred to as the Deep Creek Tuff by Fisher (1962. 1963)

 - __ (\ Greenish-gray, fine- to medium-grained, massive zeolitized tuff and tuffaceous claystone

' ~ '_] rocks containing thin resistant layers of white to light-gray tuff

 >r3

~^Tr^\ Two cooling units of varicolored, medium-grained welded tuff and lapilli tuff. Gray

H!P j vitrophyres locally present at base of each cooling unit Upper parts altered to

-*" -H

^tP HH

' ^~

__ <

> -

 <

 < 

> 

~ light-brownish-gray claystone. Contains 2-10 percent pumice lapilli, 1-2 percent

rock fragments, and 1-2 percent andesine crystals. Unit referred to as the

(informal) Picture Gorge ianimbrite bv Fisher (1966a)

Dominantly green, fine- to medium-grained, massive, diagenetically altered tuff and

tuffaceous claystone containing thin ledges of white air-fall tuff. Crops out in

pinnacled and badlands topography. Contains rich vertebrate fauna

Deep-red to yellowish- brown, diagenetically altered, fine- to medium-grained tuffaceous

claystone containing a few thin interbedded layers of gray to white tuff. A 5- to 

7-m-thick layer of thin-bedded lacustrine tuff is locally present, and thin, discontinuous olivine basalt flows may be present in the lower 25-30 m. Outcrops are

jrS^Y low, rounded hills mantled with clay soil having 'popcorn' texture

Gray to reddish- or brownish-gray, commonly porphyritic andesite and basalt flows

and breccias with interlayered tuffaceous sedimentary rocks. Typically contains

red saprolite at top

Unit of Fisher and Rensberger (1972). EXPLANATION

Mafic lava flows

Silicic ash-flow sheet

Tuff and tuffaceous 

sedimentary rocks

Reworked tuff and 

tuffaceous siltstone

Tuffaceous conglomerate

Covered areas

FIGURE 3.3. Generalized composite stratigraphic column for eastern facies of the John Day Formation in Kimberly, Oreg., area

(modified from Fisher and Rensberger, 1972).
36 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

UNIT

Picture Gorge

Basalt (part)

c

 4=

CO

E

 £so

c 

-c

o-J

Upper

member

Middle

member

Lower

member

Clarno Formation

(part)

THICKNESS 

(m)

800 -

750 -

700 -

650 -

600 -

550 -

500 -

jrn450 -

400 -

350 -

300 -

250 -

200 -

150 -

100 -

50 -

0 -

LITHOLOGY

rlriTLrL

s£ricrl!

;:r7?

_ _ _

;'. . '

 -

 <

) _ . _

....

:J?H

DESCRIPTION

Dark-gray, medium- to coarse-grained, commonly porphyritic, columnar jointed flows

Yellowish-gray to olive gray, poorly sorted, fine- to medium-grained tuffaceous claystone and vitric

tuff. Contains 2-3 percent sodic plagioclase crystals and 3-5 percent small rock fragments.

Includes rare layers of rhyolitic tuff and tuffaceous claystone, and numerous beds of wellsorted, crossbedded tuff and tuffaceous conglomerate, particularly near the top. Most glassy

material altered to clay minerals and zeolites. Rocks weather to smooth slopes or pinnacles

Corresponds to Haystack Valley Member and upper part of Kimberly Member of Fisher and

Rensberger (1972) of John Day Formation

^3

? *" ~\ / Coarse-grained rhyolitic air-fall tuff containing abundant black obsidian shards, as much as 5 mm

 v-'-^te-i lon9- Probat>lY correlative with Deep Creek Tuff of Fisher (1962; 1963)

 < -I Light-gray to greenish-gray, massive, poorly sorted, fine- to medium-grained tuffaceous claystone

^ ~3 with some layers of white tuff. Weathers to pinnacles. Corresponds to part of Turtle Cove

 < ~\ Member of Fisher and Rensberger (1972) of John Day Formation

j_ _-) Brownish-gray, coarse-grained, weakly to densely welded, rhyolitic ash-flow sheet with well1_~ ~\ developed eutaxitic texture. Two cooling units present, locally marked by basal vitro phyres.

_ _- *-'

HP HJHP

Contains abundant pumice blocks as large as 25 cm across, blocks of rhyolitic glass, sparse

rock fragments, and 1-2 percent sodic plagioclase crystals. Ash-flow sheet typically underlain

by thin layer of air -fall tuff. Unit referred to as (informal) Picture Gorge ignimbrite by Fisher

^L-^tS (igeea)

_ _ _ I Yellowish-gray, massive, poorly bedded, tuffaceous claystone with numerous resistant ledges of

 :"/*/VV-vA) white tuff. Glassy material mostly zeolitized. Contains 1-2 percent plagioclase crystals and

 < ~| sparse rock fragments

 :' ; ^'-:^V / Thin air-fall tuff with abundant soda sanidine phenocrysts, as large as 4 mm across. Probably

ji'i'ii;- / / correlates with member G of western facies of John Day Formation

- ( / Chiefly yellowish-gray, massive, poorly bedded, zeolitized, tuffaceous claystone with some layers of

~ ~ ~

>~ '

Tl"~

" : 'A

 - 

white air-fall tuff. Contains sparse plagioclase crystals and small rock fragments. Corresponds

to lower part of Turtle Cove Member of Fisher and Rensberger (1972) of John Day Formation

\ Red or yellowish- brown, massive, poorly bedded tuffaceous claystone with thin layers of air-fall

 -j tuff. Includes thin-bedded, reworked tuffs, carbonaceous shales, lignites, and sparse basalt

~ .*!*< flows in lower parts. Contains well-known Bridge Creek flora (Chaney, 1927). Corresponds to

_ * ' j Big Basin Member of Fisher and Rensberger (1972) of John Day Formation "i/i^ '{

...:..". S / Cream-colored, highly altered, moderately welded ash -flow tuff. Contains 5-15 percent crystals of

-~- TaC / plagioclase, sanidine, and quartz. Tentatively correlated with member A of western facies of

> /*£) ' John Day Formation

jXplyJp^ Gray to reddish- or brownish-gray, commonly porphyritic andesite and basalt flows and breccias

f-?\ l l lp) with interlayered tuffaceous sedimentary rocks. Typically contains red saprolite at top

EXPLANATION

^>

'^rel

Mafic lava flows

Silicic ash-flow sheet

Tuff and tuffaceous 

sedimentary rocks

 :v.'::--&'f-'S~.

 ^^

".'0'.°: °

A A

Reworked tuff and 

tuffaceous siltstone

Tuffaceous conglomerate 

Lapilli tuff

Air-fafl tuff

FIGURE 3.4. Generalized composite stratigraphic column for eastern facies of the John Day Formation in Painted Hills, Oreg., area (modified

from Hay, 1962a; 1963).
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 37

TABLE 3.2. Compositions of feldspars in the lower ash-flow sheet of member A, John Day Formation

[Oxide values in weight percent; tr, trace; ad., not determined. Values for end-member components (An, anorfhite; Ab, albite; Or, orthoclase; Cn, celsian) in

molecular percent]

Plagioclase Sanidine

No. of grains

648-1A

2

648-1B 

1

648-1B 

1

71-8 

1

71-8 

2

648-1A 

3

648-1B 

3

71-8 

4

Total 100.22

Major oxides

ow^2~ SiO>

Aurv, __ _ /M2V3 

Ca0 ________

Na2O 

KoO - --- '

MgO - -- 

FeO _ _ -

TifV»

jj aO 

ai ai

22.37 

4.12 

8.12 

1 54

tr 

.13 

.03

.24

£.1 11

23.89 

6.14 

7.52 

.90

n.d. 

.20 

n.d.

.10

at. at.

22.21 

3.99 

7.89 

1 J. **J 59j

tr 

.08 

.11

.06

£1 SS

23.51 

5.45 

6.56 

i 04

tr 

.31 

.03

.28

c.1 11

22.58 

4.18 

6.74 

1 54

tr 

.17 

.03

.14

65.00

19.31 

.40 

4.61 

9.03

tr 

.05 

n.d.

2.01

65.00

18.68 

.37 

4.35 

9.62

n.d. 

.05 

tr

1.45

6443

19.38 

.40 

4.35 

9.15

n.d. 

.07 

tr

1.99

99.98 99.59 99.07 98.59 100.41 99.52 99.77

End-member components

A-n ____ . 90 d.

Ab _________ 71.1

Cn A

10 51

65.4 

5.1

9

0

70.8 

9.4 

.1

10 3 t*y m*J

64.0 

6.7 

.5

23 3

66.9 

9.8 

3

2.0

42.9 

55.1 

32*J*£*

1.9

40.2

57.9

2

a

2.1

41.0 

56.9 

3.7

A distinctive, coarse-grained, sanidine-rich, air-fall 

tuff crops out widely in the upper third of the lower 

member in the Painted Hills area. This unit is generally 

0.5 to 1 m thick, but where it has been extensively 

reworked it may be as much as 10 m thick. It typically lies 

150 to 250 m above the base of the formation and 60 to 120 

m below the Picture Gorge ignimbrite. The feldspar 

crystals, which range from 1 to 4 mm in diameter and are 

commonly mantled with myrmekitic overgrowths, are 

soda sanidine with an average composition of about 

Or^Ab^Ang (table 3.3; Hay, 1962b). On the basis of 

textural and compositional similarities (table 3.3), this 

tuff is correlated with the basal ash-flow sheet of member 

G of the western facies (Peck, 1964). This correlation is 

strengthened by the fact that from southwest to northeast the ash-flow sheet of member G grades from a 

welded tuff, through an unwelded ash-flow tuff, to an 

air-fall tuff (Robinson, 1975).

The alkali-olivine basalt flows in the lower member are 

generally 10 to 15 m thick and rarely extend more than a 

few hundred meters along strike. Pahoehoe structures on

some flow surfaces (Hay, 1962a) and local accumulations 

of agglomerate and breccia suggest subaerial emplacement.

A 120-m-thick sequence of basalt flows capped by the 

Picture Gorge ignimbrite is exposed in Donnely Basin 

southeast of Service Creek. These flows were originally 

assigned to the Clarno Formation by Lindsley (1961), but 

Hay (1963) included them in the lower part of the John 

Day Formation because of the freshness of the flows, the 

similarity of the tuffs between the flows to the overlying 

John Day Formation, and the chemical similarity of the 

basalts to known John Day lavas. Although basalts of 

similar composition are present in the Clarno Formation 

elsewhere (Robinson, 1969), they are much more abundant in the John Day Formation.

The basalts are typically fine or medium grained and 

aphyric, with intersertal to subophitic or, rarely, intergranular textures. They consist chiefly of about equal 

amounts of labradorite and titaniferous augite, 10 to 15 

percent olivine, and 5 to 10 percent ilmenite and magnetite. Common interstitial alkali feldspar and biotite attest
38 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

TABLE 3.3. Composition of feldspars in the basal ash-flow sheet of member G, western fades, John Day Formation, and its

air-fall equivalent in the eastern fades

[Oxide values in weight percent; tr, trace; n.d., not determined; , not found. Values for end-member components (An, anorthite; Ab,

albite; Or, orthoclase; Cn, celsian) in molecular percent]

Sample 

No. of grains -

Ash-flow sheet

648-34B 

4

Sanidine

648-552 

3

648-456 

3

Air-fall tuff

Sanidine

648-456 

3

Plagioclase

648-456 

2

Major oxides

Si02-»» ----- -

A1203 - 

CaO 

Na2o - 

K2O 

VToO

FJO

TO»

TJof) .ucivy __ -- - _______ - 

Total 

67.45

18.94

.50

6.05

7 3

tr

.15

.07

 2S***s

100.73

67.14

19.17

.40

5.92

1 <C

n.d.

1 9.

.04

.20

100.70

67.04

18.39

.44

6.05

7.50

n.d.

.16

04

.41

100.03

65.21

20.12

1.92

7.15

4.47

tr

i ft

04

100.35

59.01

25.51

7.98

6.24

2fi

04

37

tr

tr

99.41

End-member components

^-\1 An g __ _ -»_H

AJj _____________

Qr ___________

pn __

2.5

54.3

43.2

.5

2.0

53.0

45.0

.3

2.2

53.8

44.0

9.5

64.1

26.4

22£**£*

40.7

57.7

1.6

to an alkalic composition. In most specimens, the olivine 

is partly to completely replaced by clay minerals, and clay 

minerals are also common in the groundmass. Chemically, these flows are high-titania, alkali olivine basalt 

similar to widespread flows in the western facies (Lindsley, 1961; Hay, 1962a; Robinson, 1969).

MIDDLE MEMBER

The Picture Gorge ignimbrite, which forms the middle 

member of the formation, is a compound ash-flow sheet 

composed of two distinct cooling units and a thin basal 

layer of air-fall tuff with an aggregate thickness ranging 

from approximately 3 to 85 m. Fisher (1966a) estimated 

that the ignimbrite layer originally covered an area of 

approximately 5,000 km2, was thickest in the MitchellSpray-Service Creek area, and thinned progressively to 

the northeast. K-Ar ages on the unit are 26.9±3 and 25.5 

Ma, or late Oligocene in age (table 3.1).

The two cooling units are texturally and mineralogically similar, with well-defined zonal structures (fig. 3.5; 

Hay, 1963; Fisher, 1966a). Both contain abundant pumice 

blocks, as much as 10 cm in diameter, and irregular 

fragments of black glass, as much as 5 cm across. In 

densely welded zones the pumice fragments are 

stretched and as large as 25 cm, imparting a welldeveloped eutaxitic texture.

Crystals and silicic rock fragments make up about 3 

percent of typical specimens (table 3.4). The crystals are 

chiefly sodic plagioclase (table 3.5), with lesser amounts 

of sanidine, quartz, and green clinopyroxene. Hay (1963) 

reported a few pseudomorphs after fayalitic olivine.

UPPER MEMBER

The upper member varies greatly in preserved thickness, depending on the depth of erosion before eruption 

of the overlying Columbia River Basalt Group. A maxi-
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 39

mum thickness between 400 and 520 m was recorded by 

Hay (1963), but the member is thin or absent where 

deeply eroded. This member is lithologically similar to 

the upper part of the lower member and is composed 

chiefly of tuffaceous claystone and vitric tuff, with small 

amounts of sandstone and pebble to cobble conglomerate 

near the top. The principal differences between the two 

members are diagenetic. The upper member is lighter in 

color and generally less altered than the lower. The 

unzeolitized tuff of the upper member corresponds to the 

Kimberly Member as defined by Fisher and Rensberger 

(1972).

The claystone is poorly bedded, poorly sorted, and 

massive and is generally light yellowish gray to light olive 

gray in color. Crystals make up about 5 percent of typical 

specimens and consist chiefly of andesine, with trace 

amounts of clinopyroxene, magnetite, ilmenite, hornblende, and biotite. This mineral assemblage implies a 

dacitic to rhyodacitic composition for the original pyroclastic material.

Air-fall tuff is commonly interbedded with the claystone, particularly in the lower 175 m of the member. The 

tuff is typically lighter gray and coarser grained than the 

claystone and may contain abundant oligoclase. One

ZONE THICKNESS (m) DESCRIPTION

CM

hZ

o <

§

J

,_

b

z->

CJ

z

_l0

A

6

5

4

3

2

1

IR FALL

0-1.5

0-25

p=a =^

r/W^^v.

) / : '

0-15

\

5-50

3-6.5

0-16

'N. / \

^ / " '

-\_ N / 

^ \

-v ' ^

 

i ^

X N

^X/V^V.

\

/

^

I/////////

^~-

^-

~ ~

0-3 \ E^-_>

0-3 \=^i

^ AIR-FALL TUFF

UNWELDED LAPILLI TUFF

PARTIALLY WELDED

LAPILLI TUFF

UNWELDED LAPILLI TUFF

VAPOR-PHASE ZONE

DENSELY WELDED TUFF

UNWELDED TUFF

AIR-FALL TUFF

FIGURE 3.5. Generalized columnar section of (informal) Picture Gorge 

ignimbrite of Fisher (1966a, fig. 2).

distinctive air-fall layer (Deep Creek Tuff of Fisher, 

1962, 1963) in the upper member crops out discontinuously. It lies 3 to 60 m above the Picture Gorge ignimbrite 

and is as much as 10 m thick. It forms cream-colored 

outcrops with internal lamination and platy parting and is 

composed largely of glass shards easily visible in hand 

specimen. In most outcrops the pyroclastic material is 

altered to clinoptilolite, but fresh black glass is locally 

preserved (Fisher and Rensberger, 1972).

Lenticular beds of reworked tuff and conglomerate are 

interbedded with the claystone at several levels. These 

layers are typically moderately to well sorted and commonly thin bedded and crossbedded. Conglomerate is 

most abundant near the top of the formation and correlates approximately with the Haystack Valley Member of 

Fisher and Rensberger (1972). Clasts are chiefly of tuff 

eroded from the upper member, but at a few localities 

clasts of welded tuff and andesite also are present.

SOUTHERN FACIES

Little detailed work has been done in the southern 

facies, which lies in a belt along the south side of the 

Ochoco Mountains from Prineville to Paulina. Reconnaissance studies indicate that this section is most like the 

eastern facies and consists chiefly of fine-grained tuff and 

tuffaceous claystone (Swinney and others, 1968; Waters, 

1968c; Waters and Vaughan, 1968a, b; Fisher and Rensberger, 1972). Two ash-flow sheets are interlayered with 

the tuff in the vicinity of the Prineville Reservoir, but 

farther east only one is present. Fossil faunas in the 

southern facies are similar to those in the eastern facies 

(Fisher and Rensberger, 1972).

Although the southern facies has not been formally 

divided into members, the stratigraphy is similar to that 

of the eastern facies. At the base of the section is a thin, 

discontinuous saprolite locally overlain by red claystone 

that corresponds approximately to the Big Basin Member 

of Fisher and Rensberger (1972). The overlying tuff and 

tuffaceous claystone can be divided into zeolitized and 

unzeolitized zones that are similar to the Turtle Cove and 

Kimberly Members, respectively.

The lower welded tuff south of Prineville lies near the 

base of the formation and either rests directly on Clarno 

lavas or is separated from them by 50 to 100 m of altered 

dacitic tuff. The ash-flow tuff is a light-gray to reddishgray, coarse-grained, stony rhyolite with a good eutaxitic 

texture defined by flattened pumice fragments, as long as 

10 cm. Crystals make up 1 to 2 percent of the tuff and are 

chiefly plagioclase with traces of magnetite and altered 

biotite (table 3.4). The matrix is a microcrystalline 

mixture of quartz and feldspar in which the original 

vitroclastic texture has been largely obliterated. Many of 

the larger pumice fragments have been zeolitized.
40 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

TABLE 3.4. Average modal compositions of ash-flaw tuff sheets in the John Day Formation

[Component contents in volume percent An content of plagioclase in molecular percent; tr, trace; , not found]

Rock unit 1 8 10

Quartz 

Hornblende 

Biotite 

Opaque minerals 

Zircon 

94.9 97.8 

1.4

1.1

3.2 .1 

.8 .7

tr 

tr 

tr 

22 7

96.4 

1.6 

tr 

.6 

1.2 

.2

tr 

tr 

tr

9

84.4 98.6 

3.6 .7

10 9 lv.^

1.6 - 

- .7

.2 tr 

tr tr 

tr 

9 12

96.2 

2.1

1.4 

.2

.1 

tr 

tr

7

98.4 

.5

tr 

1.0 

tr 

tr

.1 

tr 

tr

3

97.3 

1.7

.9 

.1

tr

4

93.8 

5.4

.8 

tr

tr

6

99.1 99.3 

.1 .7

.8 - 

.1

tr tr

 

1 2

Unit

1. Basal ash-flow sheet of member A, western facies (Swanson and Robinson, 1968).

2. Upper ash-flow sheet of member A, western facies (Swanson and Robinson, 1968).

3. Ash-flow tuff sheets of member E, western facies.

4. Basal ash-flow sheet of member O, western facies (Robinson and Brem, 1981).

5. Basal ash-flow sheet of member H, western facies.

6. Basal ash-flow sheet of member I, western facies.

7. Picture Gorge ignimbrite, lower cooling unit, eastern facies (Robinson and Brem, 1981).

8. Picture Gorge ignimbrite, upper cooling unit, eastern facies.

9. Ash-Sow tuff north of Fossil, Oreg., northern subfacies.

10. Basal ash-flow tuff, southern facies.

11. Upper ash-flow tuff, southern facies.

The upper ash-flow sheet is about 10 m thick and 

consists of light-gray to reddish-gray, densely welded, 

fine-grained, generally stony tuff. Small pumice fragments, about 1 cm long, give the rock a crude eutaxitic 

texture. Crystals are absent, but most specimens contain 

1 to 2 percent small andesitic rock fragments (table 3.4).

Neither of these ash-flow sheets can be correlated with 

welded tuffs in the eastern or western facies; these sheets 

are believed to have originated from separate vents south 

or southwest of the Ochoco Mountains.

WESTERN FACIES

The western facies crops out in a broad arc between the 

Cascade Range and the Blue Mountains uplift (fig. 3.2; 

see fig. 1.2). Only the upper part of the unit consists of 

light-colored, massive tuff and tuffaceous sedimentary 

rocks similar to those exposed between Kimberly and 

Rcture Gorge. The lower half consists chiefly of ash-flow 

tuff, silicic and basaltic lava flows, and pumice lapilli tuff, 

all of which were originally included in the Clarno 

Formation (Hodge, 1932a, b). Peck (1961, 1964) recognized that these lower rocks rested unconformably on 

andesitic flows of typical Clarno lithology and found a 

fossil flora near the base of the sequence that indicated an

Oligocene age. For this reason, he included the lower 

sequence in the John Day and subdivided the formation in 

the Antelope-Ashwood area into nine members (A-H), 

largely on the basis of the presence of ash-flow sheets and 

lava flows (fig. 3.6). These members can be recognized 

throughout the western facies (Robinson, 1975), except 

in the Mutton Mountains and in the area between Fossil 

and Lonerock. Outcrops in these areas are separated 

from those in the Antelope-Ashwood area, and the 

stratigraphy is quite different.

MEMBER A

Member A consists of an upper and a lower ash-flow 

sheet separated by about 30 m of tuff and pumice lapilli 

tuff. The basal ash-flow sheet is areally extensive and 

well exposed (Swanson and Robinson, 1968), but the 

upper sheet is thin and discontinuous. Nearly complete 

sections of member A crop out along the county road 

about 3 km west of Ashwood, near the Horse Heaven 

mining district, and near Hay Creek; elsewhere, only the 

ash-flow sheets are well exposed.

In most outcrops, the basal ash-flow sheet forms a 

prominent ledge composed of moderately to strongly 

welded tuff with a well-developed eutaxitic structure.
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 41

TABLE 3.5. Compositions of feldspars in ash-flow tuff sheets of the John Day Formation

[Oxide values in weight percent; tr, trace. Values for end-member components (An, anorthite; Ab, albite; Or, orthoclase; Cn,

celsian) in molecular percent]

No. of grains

Picture Gorge Ignimbrite

Member 

H

Member 

I

Tuff near 

Fossil, Oreg.

Total - 100.69

Major oxides

<\ifV»

MO

00 

V K2O f\

 

B2 =

66.39

9O SI

2.33

8 QA

1.65

.24

tr 

.31

65.61

21 ^ J. 12L£*

2.65

8

0C

1 42

.19

tr 

.28

f\\ 99

2^ ^,J»\J^ 02

4.32 

814

/;i .62

.30

tr

.25

£.~1 \ C O/.15

19.02

.26

6

/V7 .07

^ ji *> 6.43

.22

.06 

.30

65.94

2092 *AJmS*4

2.41 

9.22

I

CO

.22

.04 

.32

66.96

10 ^8

.47 

6.55

6

1 7

.13

.05 

.37

100.12 99.97 100.24

End-member components

An __ .

Ah- -

Or - 

rv, __ .

.. _ M ^

.. . ._ 70 1

- 9.6 

. .. .6

13.1

79. ^

8.4

.5

21 7 ^ X* /

74.6

3.7

.5

1.4

^8 1

40.4 

.6

11.5

79.5

9.0 

.6

23

60.0

37.7 

.7

Along the county road west of Ashwood, two cooling 

units are clearly recognizable, each with a basal vitrophyre; elsewhere, separate cooling units cannot be distinguished. At one locality southeast of Antelope, the tuff 

is red to purple and almost completely unwelded, but it 

still is highly indurated.

The tuff contains about 10 percent phenocrysts, averaging 1 to 2 mm in diameter (table 3.4). Although the 

modal composition of the tuff varies considerably (Swanson and Robinson, 1968), there are no apparent systematic mineralogic or chemical differences between the two 

cooling units. Corroded quartz bipyramids, from 0.5 to 3 

mm in diameter, are the most common crystals, making 

up 1 to 6 percent of the rock. Feldspar phenocrysts 

average about 2 percent and include both sanidine and 

sodic plagioclase. The sanidine crystals are euhedral to 

slightly rounded prisms, ranging from 0.5 to 1 mm in 

diameter. The sanidine is a soda-bearing variety containing 1 to 2 weight percent BaO (table 3.2). Plagioclase 

occurs as subhedral, moderately zoned crystals of oligoclase that average about 1 mm in diameter.

Overlying the basal ash-flow sheet in many areas is a 

1-m-thick layer of red tuff overlain by about 20 m of

poorly indurated, poorly bedded, white pumice lapilli 

tuff, overlain, in turn, by about 15 to 20 m of very poorly 

exposed red tuff and tuffaceous siltstone. Just below the 

upper ash-flow sheet is a 1-m-thick layer of white, very 

thin bedded,, fine-grained tuff containing numerous leaf 

impressions. In a few localities, this tuff is overlain by 2 

to 3 m of thick-bedded tuff containing fossilized tree limbs 

and trunks.

The upper ash-flow sheet is a light-brown to pinkishgray, fine-grained, densely welded tuff containing 1 to 2 

percent plagioclase crystals and 1 to 2 percent small rock 

fragments (table 3.4). A light-gray vitrophyre, as much 

as 2 m thick, occurs locally at the base of the sheet, but 

most of the unit is stony and contains very sparse 

lithophysae. This ash-flow sheet has a maximum thickness of about 50 m in exposures west of Ashwood. 

Elsewhere, it rarely exceeds 10 m in thickness, and it is 

much less continuous in outcrop than the lower sheet.

MEMBER B

A series of dark-gray trachyandesite flows, which are 

present in several separate outcrop areas, compose
42 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

UNIT DESCRIPTION

Grande Ronde 

Basalt (part)

Member

Member 

H

Member 

G

Member 

F

Member 

E

Member 0

Member 

C

Member 

B

Member 

A

1000

950

900-

850-

800-

750-

700 -I

650-

600

350-

200-

150

100

50 -

Clarno Formation 

(part)

Black, fine-grained, aphyric, columnar-jointed basalt flows

Uniform, thick-bedded to massive, tan to light-gray, moderately indurated tuff and

tuffaceous siltstone containing 1-2 percent small pumice fragments, 1-2 percent sodic 

plagioclase crystals, and 2-3 percent small rock fragments. Glassy material mostly altered 

to clay minerals and zeolites. Layer of bedded sandstone found locally at the base. Fossil 

memmals are present in some sections

Light-gray to brownish-gray, coarse-grained, weakly to moderately indurated, rhyolitic ash-flow sheet Contains abundant large pumice fragments, smaller chips of glass, 1-2 percent sodic plagioclase crystals, 

and 1-10 percent rock fragments. Locally contains two cooling units, each with basal vitrophyre of 

black glass. Sparsely lithophysal in densely welded sections. Weakly welded sections typically glassy

Light-gray to light-brown, poorly bedded to massive, moderately indurated, fine-grained tuff and tuffaceous 

siltstone with several layers of coarse-grained pumice lapilli tuff. Typically contains 2-10 percent pumice fragments,!-2 percent sodic plagioclase crystals, and as much as 5 percent small rock fragments. Glassy 

material completely altered to zeolites and clay minerals

Light-gray, orange-brown-weathering,fine-grained rhyolitic ash-flow sheet containing sparse pumice lapilli. 

Locally contains abundant lithophysae; some sections have basal vitrophyre of light-gray glass. Contains 

1-2 percent sodic plagioclase crystals

Tan to light-brown or grayish-brown, poorly bedded, moderately indurated tuff, sandy tuff, and coarse-grained 

lithic lapilli tuff. Locally has thin flows of aphyric vesicular basalt and layers of scoriaceous material

Pink to reddish-brown, medium-grained, porphyritic, generally stony, ash-flow sheet. Some sections contain 

gray basal vitrophyre, others contain sparse lithophysae. Contains 10-15 percent large, sodic-rich 

sanidine and quartz crystals, 10-15 percent rock fragments, and sparse pumice lapilli

Dark-gray, medium-grained, aphyric alkalic basalt flows containing interbads of white or red, commonly baked 

pumice lapilli tuff and tuffaceous sedimentary rocks

Chiefly varicolored, poorly bedded, poorly indurated tuffaceous siltstone and claystone containing sparse plagioclase crystals. Weathers to 'popcom-textured* surface. Locally includes thin-bedded to laminated vitric tuff, 

tuffaceous shale, white shaly tuff, and rare layers of coarse-grained, lithic lapilli tuff. Glassy material is 

completely altered to clay minerals and zeolites

Reddish-brown, fine-grained, aphyric, strongly welded, generally stony, rhyolitic ash-flow sheet containing abundant lithophysae, as much as 5 cm across. Locally has gray to yellowish-gray basal vitrophyre. Contains about 

1 percent plagioclase and quartz phenocrysts. Contains rare interlayered flows of dark-gray, medium-grained 

alkalic baselt

Discontinuous layers of light-gray to grayish-brown, poorly exposed, friable vitric tuff, lapilli tuff, and minor tuffaceous siltstone. Locally includes unwelded ash-flow tuff

Light-gray to pinkish-gray, massive to flow-bended rhyolite flows. Flows contain as much as 2 percent sodic 

plagioclase phenocrysts, which weather to give rock pitted appeerance

Black, fine- to medium-grained, aphyric trachyandesrte flows separated by thin layers of tuffaceous siltstone and 

sandstone. Rows contain blocky and platy jointing and break into angular fragments 5-10 cm across. Tops of 

some flows are vesicular or scoriaceous

Light-brown, fine-grained, densely welded, rhyolitic ash-flow sheet with weak eutaxitic texture. Gray vitrophyre 

locally present at base. Contains 1-2 percent quartz and feldspar crystals and 1 percent small rock fragments

Very poorly exposed, cream-colored to light-gray, thick-bedded tuff, coarse lapilli tuff, and tuffaceous siltstone. 

Contains sparse crystals and rock fragments. Locally contains shaly beds with leaf impressions near top

Light-gray, brownish-weathering, densely welded, coarse-grained rhyolitic ash-flow sheet with well-developed 

eutaxitic texture. Two cooling units locally present, each containing light-gray basal vitrophyre. Contains 5-10 

percent sodic plagioclase, sanidine, and quartz crystals, and 2-3 percent small rock fragments

Gray to reddish-gray porphyritic andesite and basalt flows and breccias, with some beds of tuff and tuffaceous 

claystone

FIGURE 3.6. Generalized composite stratigraphic column for western fades of the John Day Formation in Antelope-Ashwood-Willowdale, Oreg. 

area (modified from Peck, 1964, and Robinson, 1975). See figure 3.4 for explanation of lithologic symbols.
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 43

TABLE 3.6. Average modal compositions of lava types in the John Day Formation 

[Values in volume percent; tr, trace; ., not found]

Plflcrinrl

Clinopyroxene 

Opaque minerals 

Glass and interstitial

material.

Apatite - -

Zircon 

Othgr _________

Groundmass 

Rhyolite1

2 2 -W«-W

.1

.2

tr

tr

97.5

3

Quartz 

latite

35 1

6.1

3 3

3.2 

52.2

.1

tr

tr

3

2*<<

21.7

2.7

1

Trachy-

 5/r f\

22.8 

1 1

8.7 

29.2

.3

1.8

7

Basalt

49 5 ~«/ fc*'

17.4 

134

7.7 

11.1

.6

.3

22

Member C, western fades r\ 

Includes 3.1 volume percent microphenocrysts

member B. The largest mass extends for about 25 km 

along the western margin of the Blue Mountains uplift 

from Hay Creek on the south to the Ashwood-Willowdale 

highway on the north. Smaller outcrops are found in the 

east end of Antelope Valley and northeast of the hamlet 

of Clarno. At most localities, the trachyandesite flows 

rest on member A and are overlain by rocks of members 

C, D, E, or F.

In the thickest and most complete section, about 4 km 

southwest of Ashwood, the unit is approximately 500 m 

thick and consists of at least three flows or sequences of 

flows separated by thin layers of tuff. The flows form low, 

rounded outcrops mantled with fist-size fragments that 

commonly form stone rings and stripes. Typical specimens are dark gray, very fine grained, nearly aphyric, 

and sparsely vesicular. One flow near Eagle Creek in 

Antelope Valley is relatively coarse grained, with 

groundmass crystals averaging 2 to 3 mm across. Sparse 

microphenocrysts of plagioclase, clinopyroxene, and rare 

olivine are set in a felted matrix of plagioclase laths, 

granular clinopyroxene, iron oxides, and glassy interstitial material (table 3.6; Peck, 1964; Robinson, 1969). The 

groundmass plagioclase averages about An45, but some 

specimens contain sparse, highly corroded xenocrysts of 

calcic labradorite. In most specimens the clinopyroxene is 

light-brown pigeonite accompanied by minor low-calcium 

augite.

No source vents were located for the trachyandesite 

flows, but the outcrop pattern and thickness variations 

indicate that they were erupted locally. A possible vent

identified by Peck (1964) is here considered to be a flow 

remnant.

The interlayered sedimentary rocks, which range from 

about 0.5 to 5 m in thickness, are light-brown, poorly 

bedded, sandy pumice lapilli tuff, tuffaceous siltstone, 

and pebble conglomerate. These rocks are friable and 

poorly exposed, and the only known outcrops are along 

the county road between Ashwood and Madras about 8 

km southwest of Ashwood.

MEMBER C

A thick sequence of rhyolite flows and associated 

domes that crops out in the Antelope-Ashwood area 

makes up member C. The flows are best exposed along 

the Trout Creek and Wilson Creek drainages, where they 

have an aggregate thickness of about 125 m. Thinner and 

less extensive outcrops are also present along Antelope 

Creek farther north. At most localities the flows rest 

with structural conformity on tuffs of member A and 

occupy the same stratigraphic position as the trachyandesite flows of member B. Locally, however, the rhyolite 

flows clearly rest on the trachyandesite flows of member 

B.

The flows crop out in low, rounded hills and consist of 

light-gray to reddish-gray, massive or flow-banded rhyolite. The flows grade into a domelike mass of rhyolite 

that cuts the Clarno and lower part of the John Day 

Formation about 6 km north of Ashwood. Here, the 

rhyolite is exposed in crudely columnar-jointed cliffs that
44 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

rise more than 400 m above the surrounding valley. A 

breccia composed of rhyolite fragments, as much as 1 m 

in diameter, in a tuffaceous matrix is present along the 

southwest side of the domal complex. Inasmuch as the 

contacts are not well exposed, this rhyolite mass could be 

a very thick flow that ponded in an old topographic low. 

However, the form of the body indicates that it crosscuts 

the older rocks, and it is tentatively interpreted as an 

exogenous dome (Peck, 1964). The flows surround this 

mass and thin rapidly to the northeast and southwest.

The flows and the probable dome are compositionally 

similar, consisting of fine-grained, very sparsely porphyritic rhyolite. Phenocrysts are sodic andesine and trace 

amounts of altered pyroxene(?) and opaque minerals 

(table 3.6). Weathering of the phenocrysts commonly 

gives the rock a pitted appearance. The groundmass is a 

mosaic of very fine grained anhedral quartz and alkali 

feldspar.

MEMBER D

A thin sequence of white to yellowish-gray tuff and 

tuffaceous sedimentary rocks makes up member D. The 

member crops out discontinuously from Wilson Creek on 

the south to Antelope Valley on the north, where it 

separates the rhyolite flows of member C from the 

ash-flow tuffs of member E. At most localities it is less 

than 5 m thick, but along Trout Creek, between Ashwood 

and Willowdale, it is more than 30 m thick. The unit 

consists chiefly of poorly bedded, poorly indurated tuff 

and minor lapUli tuff, some of which contains small blebs 

of black glass. Locally, it appears to grade upward into 

the weakly welded base of member E. Outcrops north 

and northeast of Trout Creek also contain a light-gray, 

unwelded lapilli ash-flow tuff, as much as 3 m thick. At 

one locality the unit consists of angular blocks of gray to 

brownish-gray glass in a white tuffaceous matrix.

This unit is interpreted as an accumulation of air-fall 

and minor ash-flow material erupted before the major 

explosions that produced the thick welded tuffs of member E. The tuff of member D is generally aphyric, but a 

few layers contain trace amounts of megascopic quartz 

and feldspar.

MEMBER E

Member E is a light-brown, aphyric ash-flow sheet 

characterized by large and abundant lithophysae. Known 

outcrops extend in a broad arc from Hay Creek on the 

south to the east end of Antelope Valley on the north, a 

distance of at least 40 km. The unit is missing from 

sections east of the John Day River and in the Mutton 

Mountains. It attains a maximum thickness of about 120

m between Willowdale and Ashwood, where it consists of 

a series of ledges, each 2 to 5 m thick, that may be 

individual flow units. From this locality the unit thins 

rapidly to the north, south, and east; to the west it is 

overlain by higher members of the formation. At many 

localities the ash-flow sheet is overlain by tuffs and 

tuffaceous sedimentary rocks of member F; elsewhere, 

these friable rocks have been stripped by erosion, leaving 

a series of tablelike outcrops.

The base of the unit is commonly marked by a 

light-gray to yellowish-gray vitrophyre, as much as 4 m 

thick, which is overlain by light-pink to reddish-gray, 

densely welded, stony tuff containing layers of welldeveloped lithophysae and locally exhibiting crude columnar jointing. In a few localities, vitrophyres also are 

present within the sequence. The lithophysae are open, 

elongate cavities, as large as 10 cm in maximum dimension. Most are in distinct layers, a few centimeters to 2 m 

thick, separated by zones of massive material. Such 

alternations of lithophysae-rich and lithophysae-free 

zones are particularly pronounced in exposures along the 

highway between Ashwood and Willowdale. These zones 

may define separate flow units within the sheet, but 

there are no other lithologic breaks between layers, and 

in some localities the lithophysae are concentrated in 

pipelike masses that cut across the layers. At several 

localities (for example, the Friday Agate deposit near 

Willowdale) this ash-flow sheet contains abundant silicafilled spherulites or thunder eggs (Ross, 1941; Renton, 

1951; Brown, 1957) that appear to have formed by 

secondary silica filling cavities within spherulites or 

lithophysae.

The ash-flow tuff is very sparsely porphyritic and 

contains a few crystals of calcic oligoclase, 1 to 3 mm long, 

and traces of quartz and altered pyroxene; a few small 

dark rock fragments also are present (table 3.4). The 

groundmass in glassy specimens consists of densely 

welded glass shards, averaging about 0.3 mm in length, 

accompanied by rare, flattened pumice fragments. In the 

stony layers it consists of microcrystaUine quartz and 

feldspar.

MEMBER F

A heterogeneous sequence, composed chiefly of diagenetically altered tuff and tuffaceous sedimentary rocks, 

lies between ash-flow sheets of members E and G. Thin 

basalt flows are locally interlayered with the sedimentary 

rocks in the uppermost parts of the unit. Rocks of 

member F can be traced in outcrop from just north of 

Grizzly on the southwest to just west of Fossil on the 

northeast, a distance of nearly 75 km. Because the unit 

consists chiefly of nonresistant tuff and sedimentary 

rocks, it is poorly exposed, commonly mantled with talus
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 45

from overlying ash-flow tuffs and basalts or completely 

covered by landslide material. Where exposed, outcrops 

typically form low, rounded hills with a crackly or 

popcorn-textured surface.

The most complete sections are found in a triangular 

area, bounded by Hay Creek, Willowdale, and Ashwood, 

in which the member is more than 150 m thick. From this 

area the unit thins to the northeast and southwest, 

eventually pinching out as it overlaps older units and 

rests directly on the underlying Clarno Formation. In the 

vicinity of Hay Creek, the base of the member is marked 

by about 5 m of white, medium-bedded, coarse-grained 

pumice lapilli tuff. At other localities the lowest beds 

consist of light-gray to greenish-gray, poorly bedded, 

moderately indurated vitric tuff, commonly containing 3 

to 4 percent pumice lapilli. The rest of the unit consists 

largely of grayish-brown or grayish-green, poorly bedded, finable tuffaceous claystone and siltstone. Beds are 

lenticular, however, and the unit varies widely in thickness from place to place. For example, in many areas, 

particularly where members B, C, D, and E are missing 

and member F rests directly on member A, the lower 40 

to 50 m of the unit are brick red in color. Elsewhere, 

layers of red or pink claystone and tuff are interbedded 

with more typical gray to greenish-gray rocks. Other 

local variations include layers of coarse-grained pumice 

lapilli tuff containing fragments of perlitic glass, wellbedded and crossbedded sandstone and siltstone, white 

platy shale with abundant leaf impressions and petrified 

wood, and coarse-grained tuffaceous conglomerate. In 

the area between Clarno and Fossil, member F consists 

chiefly of green and buff tuffaceous claystone, with 

interbeds of green sandy tuff and white vitric tuff. A thin 

layer near the top of this sequence contains fossil snails 

and fish-bone fragments. As elsewhere in the formation, 

these color variations appear to reflect different degrees 

of diagenetic alteration. The green claystone is rich in 

montmorillonite, celadonite, and clinoptilolite, whereas 

the buff and gray sequences contain chiefly montmorillonite and minor amounts of fresh glass.

In Antelope Valley and in the area north and northeast 

of Clarno, several extensive flows of olivine basalt are 

interlayered with the uppermost tuffaceous sedimentary 

rocks of member F. These flows extend discontinuously 

from Trout Creek near Willowdale on the west nearly to 

Fossil on the east, a distance of approximately 50 km. As 

many as three separate flows, each 10 to 15 m thick, are 

present in some outcrops; elsewhere, multiple flows have 

not been recognized. Locally, the flows are associated 

with beds of brownish-gray, moderately bedded, basaltic 

lapilli tuff and agglomerate. Some of the tuffs immediately overlying the lava flows contain fragments of 

vesicular basalt, as large as 10 cm across, indicating 

erosion before burial.

The lava flows are dark-gray, aphyric, medium- to 

coarse-grained alkali olivine basalt (table 3.7) (Robinson, 

1969). A few exhibit poorly developed columnar jointing, 

and many have vesicular zones in the upper few meters. 

Most are highly altered and weathered, but some fresh, 

even glassy, specimens are present. As seen in thin 

section, the basalt is a medium-grained rock with intersertal to subophitic textures composed of plagioclase, 

titaniferous clinopyroxene, ilmenite, and 15 to 20 percent 

olivine, most of which is altered to orange smectite. A 

few outcrops contain pegmatitic segregations composed 

of ilmenite, titanaugite, and plagioclase crystals, as much 

as 3 cm across.

MEMBER G

A distinctive, crystal-rich ash-flow sheet and an overlying thick sequence of air-fall tuff and tuffaceous sedimentary rocks make up member G. It is one of the most 

widespread and easily recognized units in the western 

facies, extending from the Mutton Mountains on the west 

to beyond the John Day River on the east and from Smith 

Rock northward to Antelope Valley.

The basal ash-flow sheet consists of reddish- to yellowish-gray, densely welded, crystal-rich tuff, commonly 

with abundant lithophysae in its upper few meters. It 

varies systematically in thickness, degree of welding, and 

crystallization over its outcrop area. In the vicinity of 

Hay Creek Ranch, east of Madras, the tuff is as much as 

30 m thick and is densely welded, completely devitrified, 

and moderately lithophysal in the upper few meters. In 

an outcrop about 20 km north of Hay Creek Ranch along 

Oregon Highway 206 between Willowdale and Antelope, 

the tuff is 5 to 6 m thick, only weakly welded, and 

completely g;lassy. Still farther to the northeast, between 

Antelope and Clarno, an air-fall tuff containing crystals of 

sanidine similar to those in the welded tuff is present in 

the same stratigraphic position. This air-fall tuff averages about 1 m thick, but where it is extensively reworked 

it is 5 m or more thick. This tuff has been correlated on 

the basis of mineralogy with a similar air-fall tuff in the 

eastern facies in the Painted Hills area north of Mitchell 

(Hay, 1963; Woodburne and Robinson, 1977; Robinson 

and Brem, 1981).

Where it is devitrified, the ash-flow sheet is typically 

dark reddish brown, less commonly pink or purple. In the 

Mutton Mountains it is light yellowish to reddish brown 

and is characterized by pronounced color banding. Locally, a light-gray vitrophyre as much as 3 m thick is present 

at the base of the sheet.

Crystals average about 12 percent of this tuff and 

consist chiefly of sanidine and quartz accompanied by 

trace amounts of iron oxide and rare altered pyroxene 

(table 3.4). The sanidine crystals are as large as 4 mm
46 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

TABLE 3.7. Average chemical compositions of basalt and trachyandesite units in the John Day

Formation

[Values in weight percent; n.d., not determined]

Si02 ------ 

A1203 ------

2£=

K2O 

JJoO"*" _______

P2o5 . -

MnO 

COi \*S\J } "-J-- - - ««-

1

44.98 

15.47 

6.75 

7.98 

4.97 

7.61 

3.42 

1.23 

1.76 

1.74 

3.25 

.62

.17 

.12

2

45.57 

14.56 

4.50 

10.84 

4.97 

8.42 

3.12 

.84 

1.76 

.86 

3.82 

.65

.19 

n.d.

3

45.76 

14.66 

5.44 

9.68 

5.20 

8.37 

2.88 

.82

1.63 

1.28 

3.51 

.50

.19 

n.d.

4

46.17 

16.20 

7.68 

5.42 

4.75 

9.66 

3.04

.55 

1.83 

1.19 

2.72 

.49

.23 

n.d.

5

55.00 

13.55 

2.90 

9.70 

2.09 

6.14 

3.25 

1.72 

.93 

1.27 

2.10 

.59

.21

.73

6

51.17 

14.75 

1.15 

3.76 

2.63 

6.45 

3.60 

1.64

1.60 

1.17 

2.93 

.10

.09 

n.d.

Total- 100.07 100.10 99.92 99.93 100.18 100.04

No. of samples 

Unit

1. Alkalic basalt of lower member, eastern facies (data from Lindsley, 1961; Hay, 1962a; Robinson, 1969).

2. Alkalic basalt of member E, western facies (Robinson, 1969).

3. Alkalic basalt of member F, western facies (Robinson, 1969).

4. Alkalic basalt of sequence north of Fossil, Oreg., northern subfacies (Robinson, 1969).

5. Trachyandesite of member B, western facies (Peck, 1964; Robinson, 1969).

6. Trachyandesite of sequence northeast of Fossil, Oreg., northern subfacies (Robinson, 1969).

across, typically euhedral, and commonly rimmed with 

myrmekitic intergrowths of quartz and feldspar. These 

crystals have a distinctive soda-rich composition averaging about Or^Ab^Ai^ (table 3.3). This unique composition, along with the textural characteristics of the grains, 

has been used to correlate the air-fall tuff with the 

ash-flow sheet. The quartz crystals are corroded bipyramids as large as 1 mm across, and the opaque minerals 

are small laths and irregular grains, generally less than 

0.2 mm in diameter. Most specimens also contain 2 to 5 

percent silicic to andesitic rock fragments. The groundmass consists chiefly of devitrified shards, commonly 

with axiolitic textures, and some microcrystalline quartz. 

In the basal vitrophyre the shards consist of light-brown 

glass, some of which is bleached and corroded.

The tuffs and tuffaceous sedimentary rocks above the 

basal ash-flow sheet are typically light gray to light 

brownish gray, poorly bedded, and moderately to weakly 

indurated. In the Willowdale-Antelope-Ashwood area

these rocks are chiefly lapilli tuff and tuffaceous siltstone, 

but they become finer grained to the east and grade into 

green to brownish-gray tuff and tuffaceous claystone.

A fairly well exposed section, near Wilson Creek, 

consists of about 7 m of gray pumice-lapilli tuff overlain 

by 35 m of light-brownish-gray, fine-grained tuff and 

tuffaceous siltstone, 4 m of light-gray, coarse-grained 

lapilli tuff, and, finally, 4 m of orangish-brown, massive, 

fine-grained tuff containing 10 to 15 percent pumice 

lapilli. These tuffs and tuffaceous sedimentary rocks 

typically contain 1 to 2 percent crystals, chiefly plagioclase, and sparse silicic rock fragments (table 3.8).

Thin, discontinuous basalt flows are interbedded with 

the tuffs near Wilson Creek and in Antelope Valley 

between Willowdale and Antelope. In the Wilson Creek 

area the thickest flow is about 4 m thick and consists of 

green to black, highly altered amygdaloidal basalt. In 

Antelope Valley the flows are thin and consist of highly 

porous, altered basalt with associated tuff, agglomerate,
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 47

TABLE 3.8. Modal compositions oftuffaceous sedimentary rocks in the John Day Formation 

{Values in volume percent; tr, trace amount; , not found]

Unit

Groundmass 

T?rv"*lr 'FrcmmAT'ttc««______..__

CiniHinA ....,,_. -,,--.,.... O&UIJ.kilil^ «« - 

QugrtZ ___________________

PlagioclaseBiotite - - - -- -

f\ntmn& Tin'in^t*olo

Oth<»r vyuiwL -- «- - - «- __ 

Member G

S7-6

97.6

.7

.3 

1.4

SI-15

92.6 

3.1

3.9

.2 

tr 

tr

Member H

207 S2-9 S2-11

93.1 93.8 91.3

4.7 2.5 4.8 

i

tr 1 U » J2.1 3.1 1.8 

tr 

.1 .5

IT _

Member I

S2-14A

95.8 

2.8

.8 

.5

and breccia. The agglomerate and breccia are coarsegrained, tan to light-gray or reddish-gray, moderately 

well bedded rocks containing scoria and basalt fragments, as much as 40 cm across, which crop out in 

resistant ledges extending for 130 to 150 m along strike. 

Several accumulations of cinders and scoria as much as 35 

m across with rather steep, quaquaversal dips are 

interpreted as near-vent deposits.

MEMBER H

A basal, fine-grained ash-flow sheet and about 35 m of 

overlying fine- to medium-grained tuff and tuffaceous 

sedimentary rocks make up member H. This unit can be 

traced in nearly continuous outcrop from the vicinity of 

Smith Rock on the south almost to Fossil on the northeast. The tuffs and tuffaceous sedimentary rocks of 

member H are similar to those of the overlying and 

underlying members and cannot be distinguished except 

where the ash-flow sheet is present. For this reason, the 

unit cannot be recognized in outcrops west of Willowdale.

The ash-flow sheet ranges from about 2 to 30 m in 

thickness and consists of a single cooling unit of finegrained, moderately to densely welded tuff containing 

sparse lithophysae. In outcrops east of Smith Rock, the 

ash-flow sheet rests on about 1 m of white, medium- to 

fine-grained, thin-bedded tuff that appears to be an 

air-fall precursor of the ash-flow sheet. Commonly 

present at the base of the sheet is a layer, 0.5 to 2 m thick, 

of light-gray to black, fine-grained vitrophyre with platy 

jointing. This layer is overlain by 3 to 30 m of light-gray 

or brownish-gray stony tuff, which weathers a distinctive 

orangish brown. In a few outcrops, such as those near

Willowdale, the stony tuff is brick red in color and is 

quarried for building stone. Many outcrops have welldeveloped platy jointing; others have crude columnar 

joints. In outcrops east of Madras, where the stony 

section is thickest, lithophysae as large as 3 cm across are 

common, particularly in the upper parts of the sheet.

The ash-flow sheet is composed chiefly of shards, and 

only in a few places does it contain small pumice lapilli or 

rock fragments. Crystals of zoned oligoclase (table 3.5) 

average 1 to 2 percent of the rock and are commonly 

accompanied by traces of quartz (table 3.4).

The overlying tuffs and tuffaceous sedimentary rocks 

are poorly exposed in low, talus-mantled outcrops. They 

consist chiefly of poorly bedded, fine- to medium-grained 

tuff and minor cross-bedded, tuffaceous siltstone and 

sandstone. Thin layers of light-gray air-fall tuff, 10 to 50 

cm thick, form resistant ledges in many sections. Most of 

the tuffaceous sedimentary rocks have a speckled appearance due to the presence of 5 to 10 percent white pumice 

lapilli, as much as 5 mm across. Crystals of sodic andesine 

make up 1 to 2 percent of most of the tuffs, and small 

lithic fragments are locally present (table 3.8). Some of 

the tuffs also contain small fragments of black glass, but 

most glassy material in these rocks has been diagenetically altered to clay minerals and zeolites, chiefly clinoptilolite.

MEMBER I

A distinctive, coarse-grained ash-flow sheet marks the 

base of member I. This member is overlain by about 5 to 

10 m of thin-bedded tuffaceous sedimentary rock, followed by as much as 120 m of white to light-gray massive
48 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

tuff and tuffaceous claystone. This member crops out 

intermittently in a triangular area extending from Smith 

Rock on the south to Willowdale on the northwest and 

nearly to Fossil on the northeast. Light-colored massive 

tuffs and tuffaceous sedimentary rocks similar to those in 

the upper parts of member I also crop out extensively in 

the Mutton Mountains and in the area north and northeast of Fossil. In the absence of the basal ash-flow sheet, 

the exact stratigraphic position of these rocks is uncertain, but they occur near the top of the formation and 

probably correlate with tuffs of both members H and I.

Member I is thickest in the Willowdale area and 

generally thins to the northeast and south. In Antelope 

Valley, part or all of the member was eroded before 

eruption of the Columbia River Basalt Group, and so 

locally the basalt flows of that group rest directly on 

rocks of member H or older units. Between Madras and 

Smith Rock the Columbia River Basalt Group is absent, 

and member I is unconformably overlain by the Madras 

Formation (Deschutes Formation of Farooqui and others, 1981).

The basal ash-flow sheet is easily distinguished from all 

other tuffs of the John Day Formation by its very coarse 

grain size. It contains 5 to 30 percent pumice fragments, 

as large as 10 cm across, and 5 to 10 percent rock 

fragments. Densely welded sections have a pronounced 

eutaxitic texture, whereas less welded sections are open 

and porous. Both the thickness and degree of welding in 

the sheet decrease from the southwest to the northeast 

over the outcrop area. South of Hay Creek Ranch the 

sheet is a compound cooling unit, 35 to 40 m thick, 

composed of two thick vitrophyres, each overlain by 10 to 

25 m of densely to moderately welded, stony tuff containing sparse lithophysae. About 25 km to the north, in 

the vicinity of Willowdale, the sheet is approximately 35 

m thick and consists of a single cooling unit of unwelded, 

glassy, pumice lapilli tuff. Between Clarno and Fossil, 

about 50 km northeast of Willowdale, the sheet is 15 to 20 

m thick and consists of unwelded, fine-grained vitric tuff 

(fig. 3.7).

The tuff consists of angular to flattened pumice fragments, rock fragments, and sparse crystals in a matrix of 

glass shards and dust. In addition, most specimens 

contain 2 to 5 percent structureless glass fragments, 

which may be either fresh or devitrified. Rock fragments 

make up 1 to 10 percent of the tuff and are as much as 1 

cm in diameter. Most are composed of silicic, crystalline 

material, but a few are fine-grained, welded vitric tuff. 

Crystals make up 1 to 2 percent of the rock and consist 

chiefly of sodic andesine (table 3.5) accompanied by trace 

amounts of iron oxides, altered pyroxene, and zircon 

(table 3.4).

The overlying tuffs and tuffaceous sedimentary rocks 

are light gray to yellowish or brownish gray, poorly

bedded to massive, and medium to coarse grained. 

Outcrops have steep, even slopes, typically mantled with 

talus from the overlying Columbia River Basalt Group. 

Large landslide masses composed of basalt fragments in 

a tuff matrix are common along the major river canyons.

Near the base of the sequence, the rocks are typically 

thin-bedded and crossbedded tuffaceous sandstone and 

siltstone composed chiefly of pumice and small lithic 

fragments. They also contain 1 to 2 percent andesine 

crystals, sparse lithic fragments, and 2 to 5 percent small, 

white, pumice lapilli that give the rocks a speckled 

appearance. The matrix consists of montmorillonite and 

zeolites, chiefly clinoptilolite, derived from altered glass. 

Some of the least altered rocks contain small fragments of 

black vesicular glass, 1 to 3 mm across.

A rich and well-preserved vertebrate fauna is present 

in the upper parts of member I, particularly in the Warm 

Springs area (Woodburne and Robinson, 1977; Dingus, 

1979).

NORTHERN SUBFACIES

The ash-flow tuffs and lava flows that are used to 

define members of the John Day Formation in the 

western facies thin from southwest to northeast and 

pinch out against an old Clarno high between Clarno and 

Fossil (fig. 3.8). Northeast of this high, in the area 

between Fossil and Lonerock, the stratigraphy of the 

formation differs markedly from that of the normal 

western facies. Several ash-flow tuffs and lava flows are 

present, but these cannot be correlated directly with 

those of the Antelope-Ashwood sequence.

This lithologically different sequence is thickest and 

best exposed in the area immediately north of Fossil. 

Here, the top of the Clarno Formation is marked by a red 

saprolite, as much as 3 m thick, developed on a series of 

porphyritic andesite flows. The overlying John Day 

Formation is about 500 m thick and consists of a lower 

unwelded ash-flow sheet and an upper sequence of tuff 

and tuffaceous sedimentary rocks, with several interlayered lava flows.

The basal ash-flow sheet is about 245 m thick and 

consists of two cooling units separated by a lava flow of 

quartz latite. The sheet can be traced as far west as the 

head of Pine Creek east of Clarno, where it appears to 

interfinger with the upper part of member G of the 

western facies. To the northeast, it can be traced to the 

vicinity of Lonerock, where it is considerably thinner and 

better bedded than in the Fossil area. Except in the 

vicinity of Fossil, exposures are poor, and outcrops are 

commonly mantled with talus from the Columbia River 

Basalt Group or from lava flows within the John Day 

Formation.
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 49

H?

55

38

27

EXPLANATION

^ Mafic lava flows

-/^! Intermediate to silicic 

"I lava flows

C| Rhyolitic lava flows

I Silicic ash-flow

^lJ sheet

I Basal vitrophyre of

I ash-flow sheet

_j-| Tuff and tuffaceous

~c l sedimentary rocks

tuff

1 3-= 3 Reworked tuff and 

I -1 tuffaceous siltstone

[t"V.t'| Tuffaceous conglomerate 

Covered areas

EXPLANATION r

^3 Post-John Day rocks 

I I John Day Formation 

| | Pre-John Day rocks

FIGURE 3.7. Measured sections showing stratigraphic variations in western fjicies of the John Day Formation in AshwoodWillowdale-Fossil, Oreg., area. Letters indicate members of formation. Sections measured in meters. Index map shows locations 

of sections. Horizontal lines connecting sections represent basal ash-flow sheet of indicated members.
50 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

In the Fossil area the lower cooling unit is about 235 m 

thick. At the base it consists of coarse-grained, poorly 

sorted tuff containing as much as 15 percent angular 

pumice lapilli and 10 percent dark rock fragments. The 

unit grades upward into moderately well sorted vitric 

tuff containing very sparse pumice and rock fragments. 

The upper cooling unit is 9 to 10 m thick and consists of 

coarse-grained tuff similar to that in the lower unit. Both 

cooling units are well indurated but completely unwelded.

Crystals average 1 to 2 percent of each cooling unit and 

are mostly small prisms of anorthoclase (table 3.5), 

accompanied by trace amounts of plagioclase, quartz, 

green pyroxene(?), and zircon (table 3.4). The sparse rock 

fragments are chiefly andesitic and silicic volcanic material. The original glass has been partly devitrified and 

partly altered to clinoptilolite.

The ash-flow sheet is overlain by about 25 m of poorly 

exposed, white to red, medium-bedded, fine- to mediumgrained tuff. Locally, a thin amygdaloidal basalt flow is 

interbedded with the tuff. Overlying the white tuff is a 

230-m-thick section of uniform, light-brown to gray, 

massive to thick-bedded, moderately indurated tuff containing 1 to 2 percent small pumice fragments, which give 

the rock a slightly speckled appearance. Crystals of 

andesine feldspar also make up 1 to 2 percent and are 

accompanied by trace amounts of iron oxide and altered 

pyroxene (table 3.4). Most of the glass is zeolitized, but 

angular fragments of fresh black glass occur in the 

uppermost parts of the section.

Intel-layered lava flows are present near the middle of 

the unit and range from about 2 to 25 m in thickness. The 

thickest flow is a dark-reddish-brown, flow-banded dacite 

or quartz latite that crops out discontinuously in a

LAND

MAMMALIAN

STAGE

BARSTOVIAN

HEMINGFORDIAN

ARIKAREEAN

WHITNEYANORELLAN

CHAORONIAN

DUCHESNIAN

MUTTON MTS ANTELOPE-ASHWOOD MITCHELL-PAINTEO HILLS KIMBERLY-PICTURE GORGE

COLUMBIA RIVER BASALT GROUP 

(PART)

MASCAL FM

'flflt --* - £ £ -^- >-^_^

TUFF AND

TUFFACEOUS

SEDIMENTARY ROCKS

MEMBER I

MEMBER H

MEMBER G

UPPER MEMBER

HAYSTACK VALLEY 

MEMBER '

KIMBERLY MEMBER 1

TURTLE COVE MEMBER '

PICTURE GORGE IGNIMBRITE'

BASAL ASH-FLOW SHEET OF MEMBER G AIR-FALL EQUIVALENT OF ASH-FLOW TUFF

CLAYSTONE

RHYOLITE DOMES

MEMBER F

TURTLE COVE MEMBER '

? ? LOWER MEMBER BIG BASIN MEMBER

CLARNO FORMATION AND LOCAL OLDER ROCKS

1 Unit of Fisher and Rensberger (1972).

2 Informal unit of Fisher (1966a).

FIGURE 3.8. Chronostratigraphic relations within the John Day Formation, based on interpretations of existing age data (modified from

Woodburne and Robinson, 1977). Time scale from Palmer (1983).
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 51

12-km-long band north and west of Fossil (Robinson, 

1975). It is a fine-grained, hypocrystalline rock containing sparse microphenocrysts of plagioclase, clinopyroxene, and altered olivine in a hyalopilitic groundmass of 

plagioclase laths, clinopyroxene granules, magnetite, and 

abundant brown glass (table 3.6).

Farther east, north of Kinzua, the quartz latite is 

missing, but a basalt flow occupies approximately the 

same stratigraphic position (Robinson, 1975). This flow is 

generally less than 10 m thick and extends for about 8 km 

along strike, pinching out both to the east and to the 

west. It consists of light-gray to brick-red, fine- to 

medium-grained, commonly vesicular and amygdaloidal, 

alkali olivine basalt, similar to that of member F of the 

western facies (Robinson, 1969). Locally, the basalt 

contains megacrysts of alkali feldspar, as large as 2 cm 

across, probably xenocrysts derived from the pre-Clarno 

basement.

A thin flow of andesite locally overlies the basalt. This 

flow is a dark-gray, fine-grained rock containing 2 to 3 

percent small plagioclase and altered olivine microphenocrysts in a pilotaxitic groundmass of plagioclase, clinopyroxene, iron oxides, and sparse olivine (table 3.6).

In the area between Fossil and Lonerock, several 

small, isolated masses of welded tuff crop out in the 

sequence. These are reddish- to yellowish-gray, densely 

welded rocks with poorly developed eutaxitic textures 

and platy jointing. A light-gray basal vitrophyre, about 5 

m thick, is preserved in one outcrop. Crystals and rock 

fragments are sparse, and these rocks appear to be most 

similar to parts of the basal ash-flow sheet of member H 

in the western facies. However, because the outcrops are 

isolated and the tuff has no distinctive features, a definite 

correlation is not possible.

MUTTON MOUNTAINS SUBFACIES

Rocks of the John Day Formation crop out over an area 

of about 700 km2 in the Mutton Mountains area. Most 

ash-flow sheets of the western facies in the AntelopeAshwood-Willowdale area are missing in the Mutton 

Mountains, and the sequence consists chiefly of tuffs and 

tuffaceous sedimentary rocks, with numerous domes and 

flows of rhyolite. The Mutton Mountains themselves 

consist of a series of large rhyolite domes that locally 

interfinger with the tuffs. Although outcrops in the 

Mutton Mountains area are not continuous with those of 

the western facies, correlations are possible on the basis 

of age and lithology.

In the vicinity of Kahneeta Hot Springs and along the 

Deschutes River approximately between North Junction 

and Dant, the John Day Formation rests unconformably 

on andesitic and dacitic lavas and breccias assigned to the 

Clarno Formation on the basis of lithology (Waters,

1968a, b). Near Kahneeta Hot Springs, the contact 

between the two units is marked by a bright-red saprolite, some of which was reworked into the lower part of 

the John Day Formation. Farther north, along the 

Deschutes River, no saprolite is preserved, but there is 

an angular discordance between the two formations.

On the north, east, and southeast flanks of the Mutton 

Mountains, the John Day Formation is unconformably 

overlain by flows of the Columbia River Basalt Group. 

These basalt flows thin and pinch out toward the core of 

the Mutton Mountains, indicating that the area was a 

topographic high in middle Miocene time. On the west, 

between the Mutton Mountains and the Cascade Range, 

the John Day Formation is overlain directly by the 

Miocene and lower Pliocene Madras Formation (Deschutes Formation of Farooqui and others, 1981) and 

younger basalt flows.

At the base of the John Day Formation in the Mutton 

Mountains is a 20- to 30-m-thick sequence of red tuffaceous claystone, which overlies Clarno lavas near Kahneeta Hot Springs. This sequence is overlain by about 30 

m of white to light-brownish-gray, thin- to mediumbedded tuff and lapilli tuff. The thin-bedded zones consist 

of coarse-grained ash, whereas the thicker bedded zones 

contain abundant pumice lapilli and lithic fragments, as 

much as 2 cm across. These weakly to moderately 

indurated tuffs are completely argillized and zeolitized, 

and some contain narrow bands and small spots of red 

material, probably hematite.

The lower tuffs and tuffaceous claystones are overlain 

by a distinctive, crystal-rich ash-flow tuff that is correlated with the basal ash-flow sheet of member G of the 

western facies. Locally, the underlying tuffs and claystones pinch out, and the ash-flow sheet rests directly on 

flow-banded rhyolite of the Mutton Mountains.

The ash-flow tuff, which is about 5 to 10 m thick, is 

yellowish to reddish brown, moderately welded, and 

moderately indurated. It contains 7 to 8 percent crystals 

and 1 to 2 percent rock fragments in a fine-grained matrix 

composed of shards and glass dust. Locally, sparse 

pumice fragments impart a poorly developed eutaxitic 

texture to the rock. The crystals are mostly 1 to 3 mm 

across and consist chiefly of sanidine and quartz. The 

sanidine is compositionally and texturally similar to that 

in the basal ash-flow sheet of member G near Willowdale 

(table 3.3). A single K-Ar analysis from the tuff in the 

Mutton Mountains yielded an age of 27.0±3 Ma (table 

3.1), or about the same age as member G.

Overlying the ash-flow tuff is about 100 to 200 m of 

white to light-gray, poorly bedded tuff and lapilli tuff 

similar to those of member I near Willowdale. These 

rocks typically contain 1 to 2 percent plagioclase crystals 

and sparse lithic fragments. Pumice fragments are common in the coarser grained layers and are as much as
52 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

about 5 mm across. The original glassy material is almost 

completely replaced by montmorillonite and clinoptilolite. Locally, these beds contain well-preserved vertebrate fossils, particularly near the top of the sequence 

(Woodburne and Robinson, 1977; Dingus, 1979).

The main mass of the Mutton Mountains consists of 

light-gray to reddish-gray, fine-grained, generally aphyric, flow-banded rhyolite. Thicknesses are estimated to 

be in excess of 500 m, and the flow banding is commonly 

steep, possibly indicating a domal structure. Along the 

eastern margin of the Mutton Mountains, individual flows 

can be recognized where they are interbedded with 

layers of white pumice-lapilli tuff. The exact stratigraphic position of these flows and domes is uncertain, but 

because they underlie the welded tuff dated at 27 Ma and 

unconformably overlie rocks of Clarno lithology, they are 

presumed to be equivalent in age to the middle and lower 

parts of the John Day Formation.

A separate sheetlike mass of rhyolite that overlies 

John Day tuffs southwest of Kahneeta Hot Springs 

appears to be a lava flow, locally as much as 200 m thick. 

The pluglike mass of rhyolite that forms Eagle Butte,

about 4 km south of Kahneeta Hot Springs, is believed to 

mark the vent from which this flow erupted.

ISOLATED RHYOLITE DOMES AND FLOWS

Many isolated masses of flow-banded lava have been 

mapped as part of the John Day Formation by various 

workers. Some of these rocks, such as the ones in the 

Antelope-Ashwood area and the Mutton Mountains, are 

interlayered with John Day tuffs, and their age assignments are undoubtedly correct. Others along the crest of 

the Blue Mountains uplift overlie or cut andesitic rocks of 

typical Clarno lithology but have no field relation to 

younger rocks. Where age relations have been established for these bodies, they appear to be primarily of 

Clarno age (Swanson and Robinson, 1968). Still others, 

such as Cline Buttes, Powell Buttes, Forked Horn Butte, 

and several unnamed bodies, are completely surrounded 

by basalt flows and sedimentary rocks of the Madras 

Formation (Williams, 1957). Only Powell Buttes has been 

dated by the K-Ar method, and it has a definite John Day 

age (table 3.1). It is a porphyritic rhyolite (table 3.9)

TABLE 3.9. Average chemical compositions of silicic lava flows and domes of the John Day Formation 

[Oxide values in weight percent; * total iron as Fe2O-j; n.d., not determined; LOI, loss on ignition]

Rock unit 1

c;ru. _ __ ..

Ary^} AI^TV, __ _ _.

Fe2O3*- 

MgO 

QlQ.. . _ .

NfloO 1^1*1 jvy

K^O 

Tiru _ __ _

PoOc- 1-2^5 

MnO- - -

Totali rvi

 76 3

10 fi

- 2.2

.06

 »J5

^ 9

- 5.1 

.29

09

.06

100 ^

1.1

70.5

n

o

6.1 

.10 

1.6

 3 0

3.6

.53

.07

.03

100.1

1.0

81 8

9.7

1.5 

.01 

.15

2.1

4.3 

.14

.01

.02

99.7

.9

78 7

11.7

1.3

.01 

 20ZA/

3 5

4.4 

.16

n.d.

.01

100.0

.6

76.9

12.4

2.1 

.02

r)f .2o

3

/j

.O

4.4 

 27Ar 1

02

.01

100.0

.9

759

12 3

1.6

.14

/A/\ .90

3.1

5.2 

.15

03

99 3

1.1

Unit

1. Rhyolite flows of member C, western facies.

2. Quartz latite flows north of Fossil, Oreg., northern subfacies (Robinson, 1969).

3. Rhyolite of Jumper Butte, western facies.

4. Rhyolite of Powell Buttes, western facies.

5. Rhyolite flows of Smith Rock, western facies.

6. Rhyolite flows and domes of the Mutton Mountains.
EOCENE(?>, OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 53

containing about 10 percent sanidine, quartz, and sparse 

amphibole phenocrysts in a microcrystalline groundmass. 

Several other bodies, including Cline Buttes, consist of 

pyroxene andesite and are most likely part of the Clarno 

Formation.

COMPOSITION

The John Day Formation consists of three main components: air-fall tuff and tuffaceous sedimentary rocks, 

which form the bulk of the unit, welded and unwelded 

ash-flow tuff, which is abundant in the western facies, 

and lava flows and domes.

Little direct compositional information is available for 

the air-fall tuffs and tuffaceous sedimentary rocks because most of the original glass has been replaced by clay 

minerals and zeolites, either during weathering before 

burial or by later diagenetic alteration. Chemical analyses of altered rocks indicate extreme hydration accompanied by significant changes in Si02, MgO, CaO, Na^O, 

and K20 contents (Hay, 1963). Even in the fresh-glass 

facies in the upper part of the formation the glass is 

bleached, etched, and commonly partly replaced by clay 

minerals. Thus, the initial composition of these rocks can 

only be deduced from their pyrogenetic mineralogy.

Crystals rarely make up more than 2 to 3 percent of the 

tuffaceous sedimentary rocks and are chiefly andesine 

feldspar, accompanied by trace amounts of labradorite, 

oligoclase, clinopyroxene, iron oxides, biotite, and rare 

quartz (table 3.8; Hay, 1962a; Fisher, 1968). Amphibole 

and sanidine are rare accessory minerals. Lithic components in these rocks are chiefly andesitic and dacitic in 

composition, more rarely rhyolitic. Glass shards have 

refractive indices ranging from 1.496 to 1.502 (Hay, 

1963), implying compositions in the range of about 70 to 

74 weight percent Si02. These data indicate a dacitic to 

rhyodacitic composition for most of the original pyroclastic material in the sedimentary rocks.

The interbedded vitric tuffs are commonly somewhat 

coarser grained than the sedimentary rocks, and many 

contain small pumice lapilli. Sanidine is present in many 

of these rocks and equals or exceeds plagioclase in about 

half of the specimens (Hay, 1963). On average, the 

plagioclase is somewhat more sodic than in the claystones, being chiefly oligoclase and sodic andesine (Hay, 

1962a). Quartz is common in a few tuff layers, particularly those rich in sanidine. Mafic minerals are sparse and 

consist chiefly of clinopyroxene and iron oxides. On the 

basis of their mineralogy, the quartz- and sanidine-rich 

tuffs are inferred to have been originally rhyolitic in 

composition, whereas the others are inferred to have 

been dacitic or rhyodacitic.

Both primary mineralogy and whole-rock chemical 

compositions indicate that the ash-flow tuffs are rhyolitic

(table 3.10). Phenocrysts are primarily oligoclase, sanidine, anorthoclase, and quartz, with only traces of such 

mafic minerals as hornblende, clinopyroxene, biotite, and 

iron oxides (table 3.4). Quartz and sanidine dominate in 

the tuffs of members A and G and are present in small 

amounts in the tuff of member E (table 3.4). Plagioclase 

is present in all of the ash-flow tuffs except that in 

member G, but it varies considerably in composition 

(tables 3.2, 3.5).

Although the tuffs are all rhyolitic in composition, 

whole-rock compositional data show wide variations even 

within the same unit (Swanson and Robinson, 1968; Hay, 

1963). These variations are believed to be due to differences in devitrification, vapor-phase crystallization, and 

diagenetic alteration rather than to primary compositional differences. Densely welded, devitrified zones with low 

porosity are believed to represent the least altered parts 

of the sheets (Robinson and Brem, 1981). Within these 

zones, analyses are consistent, K20 and Na^O contents 

are subequal, water content is low, and there is no 

petrographic evidence of diagenesis or vapor-phase crystallization. Porous glassy material from near the base of 

individual sheets has been largely altered to clay minerals and zeolites, whereas less porous vitrophyres have 

been hydrated. Compositions in the upper parts of the 

sheets appear to have been affected by diagenetic alteration rather than vapor-phase crystallization. The soft, 

unconsolidated tuff at the top of the Picture Gorge 

ignimbrite has been completely altered to claystone 

(Hay, 1963).

Ash-flow tuffs containing abundant quartz and sanidine 

phenocrysts are undoubtedly rhyolitic, whereas those 

containing only plagioclase as a phenocryst phase may be 

either rhyolitic or rhyodacitic. Minor chemical variations 

must have existed originally within individual ash-flow 

sheets, but these variations have been obscured by later 

alteration and hydration. Some original compositional 

differences between sheets are implied by different 

phenocryst assemblages. In general, the ash-flow tuffs 

are slightly alkalic rhyolite containing about 75 to 76 

weight percent Si02 (table 3.10).

Mafic lavas of the formation range in composition from 

alkali olivine basalt to trachyandesite. Most basalts 

contain 15 to 20 percent modal olivine and are undersaturated with respect to silica if the Fe203/FeO ratios are 

adjusted to take into account the effects of deuteric 

alteration and oxidation (Robinson, 1969). The trachyandesites are fine-grained, fresh lavas that contain 

sparse microphenocrysts of olivine, clinopyroxene, and 

plagioclase. They are all silica saturated but, like the 

basalts, are fairly rich in Ti02 and alkalis (table 3.7). 

These flows are closely associated in space and time to 

the basalt flows, but the chemical relation between the 

two, if any, is unclear (fig. 3.9).
54 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

TABLE 3.10. Average chemical compositions of ash-flow tuff sheets in the John Day Formation 

[* Total iron as F&^^', n.d., not determined; LOI, loss on ignition]

Rock unit 

SiO2~ 

A12O3 --- 

Fe203 * -

MgO ~

CaQ- -- 

Na2O-- 

K2O - 

Ti02~-~- ------

P O

MnO-- -

TV\tal - _ ... ...

T J^A_U OT _________________ ""~ .... 

Unit

1

76.2

12.5

1.6

12

7

2.6

5.6

32

.06

.05

99.89

7

10

2

73.0

12.8

3.0

.20

1.0

2.0

6.6

.73

.23

.14

9.7

1.4

4

3

76.2

12.6

1.5

.07

.7

2.5

6.2

.19

.01

.06

100.0

1.5

5

4

78.9

10.7

2.0

.05

.5

2.6

4.6

.20

0

.02

99.6

1.5

4

5

76.4

12.0

2.1

.11

1.0

2.3

5.1

.21

 \}£i 02

.05

99.3

3.8

4

6

77.2

11.6

2.0

.10

.4

2.8

5.5

.29

.05

.02

100.0

.8

10

7

77.3

12.3

1.5

.10

.4

3.7

4.1

.19

.04

.03

99.7

.9

9

8

76.4

12.4

2.3

.24

.7

2.8

4.7

.27

O4

.05

99.9

2.3

6

9

74.9

12.9

2.6

.17

1.0

3.6

4.5

.30

.11

.03

100.1

n.d.

7

10

82.2

9.2

1.0

.04

.6

2.4

4.0

.24

n.d

.03

99.7

.9

3

11

76.4

13.5

1.8

1.0

2.9

.8

3.9

.13

0

0

100.4

5.7

2

1. Basal ash-flow sheet of member A, lower cooling unit, western fades.

2. Basal ash-flow sheet of member A, upper cooling unit, western fades.

3. Upper ash-flow sheet of member A, western fades.

4. Lithoidal welded tuff of member E, western facies.

5. Vitrophyric welded tuff of member E, western fades.

6. Basal ash-flow sheet of member G, western facies.

7. Basal ash-flow sheet of member H, western facies.

8. Basal ash-flow sheet of member I, western fades.

9. Picture Gorge ignimbrite, lower cooling unit, eastern facies.

10. Picture Gorge ignimbrite, upper cooling unit, eastern facies.

11. Basal ash-flow sheet north of Fossil, Oreg., northern subfacies.

The silicic lava flows are primarily rhyolite, but one 

flow of quartz latite is present in the area north of Fossil. 

The rhyolites are microcrystalline rocks that show little 

evidence of alteration except for dissolution of rare 

feldspar phenocrysts. Most specimens are compositionally similar to the ash-flow tuffs, consisting of slightly 

alkalic rhyolite containing 76 to 82 weight percent SiO2 

and with an NagO/E^O ratio ranging from 0.60 to 0.82. 

The most siliceous specimens are from domes containing 

vapor-phase tridymite. The quartz latite flow is significantly less silicic than the rhyolite flows and has an 

Na2O/K20 ratio greater than 1.

AGE

From studies of stratigraphic relations and extensive 

faunal collections, the John Day Formation was clearly

identified as Oligocene and Miocene in age many decades 

ago. Recent radiometric dating has substantiated this 

general age assignment, has added greater precision to 

the age of selected units within the formation, and has 

provided accurate lower and upper age limits to the 

entire formation. All of the available K-Ar ages for the 

formation are listed in table 3.1. Several of the ages listed 

by Fiebelkorn and others (1983) and identified as from 

the John Day Formation are repetitions of ages published 

both by Hay (1962a) and by Evernden and others (1964), 

and two ages (samples 648-657, 648-695)'identified as 

being from the John Day Formation should be ascribed to 

the Clarno Formation. The interpreted age relations in 

the John Day Formation are summarized in figure 3.8. 

In the western facies, Peck (1961, 1964), Swanson and 

Robinson (1968), and Robinson (1975) designated the 

base of the formation as the base of a distinctive ash-flow 

sheet that rests unconformably on rocks of Clarno
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 55

15

10 AI20,

10

I- 5

UJ 

O

tr ui Q.

g

UJ

. CaO

40 60

* +

4T*

+

4*

*-

+

z

Ul

H 4

Q

O

UJ 

Q

X 2

O

0

8

6

4

2

o

1 + 1

~ *.* ~

o ** + +*» . ^o 4-k +

o * + 

- <Po 5wtx

V ^&* «?*+

* + +*

x

Na2O *

I |

1 ' 1

 Mi-

+ V| + -

+*\x+x "

+**?*? * ~

4-

- 4- -

K2O +

~ ° ~

0 .

o

i , 1

EXPLANATION

o Basalt

 Trachyandesite

* Quartz latite

+ Rhyolite ash flow

x Rhyolite lava flow 

and dome

15

10

01

o

UJ

UJ

O

O 

Ul Q

X

O

FeO

TiO2

o 

MgO

80 40 60

SiOa CONTENT, IN WEIGHT PERCENT

4^^^ff^>'

FIGURE 3.9. Oxide versus Si02 content, showing compositional ranges of major rock types within the John

Day Formation.
56 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

lithology and is separated from them locally by a thin 

layer of red saprolite. The appearance of this ash-flow 

sheet marks a change in the style of volcanism from local 

emplacement of predominately andesitic lavas flows and 

breccias to regional deposition of predominately silicic 

pyroclastic material. Two radiometrically dated samples 

from this sheet (DAS-66-208, DAS-66-195) yielded ages 

of 37.4 and 37.1 Ma, respectively, indicating an age near 

the Eocene-Oligocene boundary. This ash-flow sheet can 

be traced eastward from the Antelope-Ashwood area to 

the Horse Heaven mining district on the crest of the Blue 

Mountains uplift (Swanson and Robinson, 1968) and 

probably as far as Rowe Creek, where an isolated outcrop 

is found at the base of the eastern facies (Hay, 1962a; 

Robinson, 1975; Woodburne and Robinson, 1977; Robinson and Brem, 1981). If this correlation is correct, it 

implies that the base of the formation in the eastern facies 

is also about 37 Ma.

Several ages have been reported from lavas of the 

upper part of the Clarno Formation in the Mitchell area 

that are younger than 37 Ma (Evernden and others, 1964; 

Enlows and Parker, 1972). However, most of these are 

whole-rock ages of somewhat altered lava and dike rocks, 

and the reported ages are suspect. Others are on feldspar 

separates from bentonite that had been subjected to 

intense weathering during alteration of the enclosing ash. 

The feldspars could have lost argon during this process, 

thus giving rise to younger apparent ages. Taylor (1981) 

concluded that the anomalously young ages for the 

Clarno Formation in the Mitchell area are incorrect on 

geologic grounds and that the age of the base of the John 

Day Formation is approximately 37 Ma.

The next younger radiometrically dated unit in the 

western section is the basal ash-flow sheet of member G. 

Four ages on sanidine separates range from 30.5 ±0.4 to 

27.0 ±0.3 Ma and average about 28.5 Ma. The range in 

ages may be due to incorporation of small but varying 

amounts of accidental or accessory feldspar in the ashflow sheet, or to slight alteration. On the basis of its 

unusual feldspar composition, this sheet has been correlated with an air-fall tuff in the eastern facies that lies 

about 150 to 250 m above the base of the formation (Hay, 

1962a, 1963; Woodburne and Robinson, 1977). An early 

determination (sample KA647A) by Evernden and others 

(1964) indicated a probable age of 23.3 Ma for this tuff, 

but a more recent determination (sample 648-456) yielded an age of 26.8±0.4 Ma (table 3.1), which is within the 

range of ages on the ash-flow sheet of member G.

Several other determinations from the lower member 

of the eastern facies yielded ages ranging from 31.5 to 

26.7 Ma (table 3.1). One age (sample KA1384) of 21.6 Ma 

from a claystone near the base of the formation was 

obtained from authigenic feldspar and reflects the age of 

diagenesis rather than deposition (Hay, 1962a).

K-Ar ages from the basal ash-flow sheets of members 

H and I of the western facies are 30.9±0.7 and 27.7±0.3 

Ma, respectively. Both of these ages are slightly older 

than the Picture Gorge ignimbrite of the eastern facies, 

for which two dates are available, 26.9±2.3 and 25.5 Ma 

(table 3.1). Because the ash-flow tuff of member H is 

stratigraphically higher than that of member G, its 

reported age of 30.9 Ma is too old. This tuff is very 

sparsely porphyritic in plagioclase and may have been 

contaminated with some slightly older material. The age 

from the basal ash-flow sheet of member I is probably 

somewhat in error for the same reason.

The youngest rocks in the formation dated by the K-Ar 

method are two air-fall tuff beds near the top of the 

formation in the Mutton Mountains area. These tuff 

samples have ages of 23.4±3.3 and 22.7±2.7 Ma, and 

they are associated with a fossil fauna believed to be 

Hemingfordian in age, approximately 20 to 18 Ma (Woodburne and Robinson, 1977; Dingus, 1979).

Several ages are also available from isolated outcrops 

of silicic rocks believed to be correlative with the John 

Day Formation. The oldest of these samples (PTR71-9A) is from a welded tuff that rests unconformably on 

rocks of Clarno lithology on Snowboard Ridge about 7 km 

southeast of Kinzua. This outcrop was tentatively correlated with the basal ash-flow tuff of member A of the 

western facies (Robinson, 1975), but its reported age of 

57.0 Ma is much too old (table 3.1). Either the correlation 

is wrong and this is a Clarno tuff, or the date is incorrect, 

possibly because the unit has been contaminated with 

older material or altered in some way. Powell Buttes, a 

rhyolite domal complex in Crook County, has three age 

determinations ranging from 30.1±1.1 to 25.8±0.2 Ma, 

indicating a definite John Day age. Thus, in summary, in 

this report we consider the John Day Formation to range 

in age from latest Eocene(?) to early Miocene.

STRUCTURE

Rocks of the John Day Formation are generally little 

deformed and typically dip less than 15°. Most structures 

within the formation are subparallel to the major tectonic 

features of north-central Oregon (fig. 3.10). Small northeast-trending folds are present on both flanks of the Blue 

Mountains uplift, and a few east-trending structures 

parallel the Ochoco uplift (Swanson, 1969). Several small, 

tight folds are present in the western facies adjacent to 

faults or small intrusions (Peck, 1964; Robinson, 1975). 

Northeast-trending faults are common on the west flank 

of the Blue Mountains uplift, where they offset all rocks 

older than the Madras Formation of Miocene and early 

Pliocene age. These are chiefly normal faults, with 

throws in the range 2 to 100 m, but a few exhibit minor 

strike-slip displacement. Along one rather large fault
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 57

system, between Willowdale and Grizzly, the John Day 

rocks are exposed in a west-tilted block with dips 

between 15° and 30°.

The distribution and stratigraphy of the Tertiary units 

in north-central Oregon indicate that a topographic high 

has persisted along the Blue Mountains axis from at least

early Oligocene time (Rogers, 1966; Fisher, 1967; Robinson, 1975; Taylor, 1977). In Eocene time, numerous 

Clarno vents were active along this trend (Swanson, 

1969), indicating that it was a volcanotectonic feature. By 

earliest John Day time, however, erosion had produced a 

terrain of little relief, and the earliest ash-flow sheet of

118°

0 10 20 30 40 50 KILOMETERS WASHINGTON

EXPLANATION

Anticline 

Syncline

Fault Dashed where approximately located; 

hachures on downthrown side

Rhyolite dome and vent coeval with John Day 

Formation

Axis of broad uplift Several broad uplifts (or 

anticlinoria) are characterized by local 

changes in dip due partly to gentle folds and 

partly to titled fault blocks. Some opposing 

dips in basalt flows may be initial and 

related to original topography or vent 

areas

OREGON

AREA OF MAP

FIGURE 3.10. North-central Oregon, showing major tectonic features (modified from Walker, 1977).
58 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

the John Day Formation (member A of the western 

facies) was deposited across this axis (Swanson and 

Robinson, 1968). After deposition of member A, uplift 

along a northeastward trend produced a topographic high 

that acted as a barrier between the eastern and western 

facies. The ash-flow sheets of members E, G, H, and I of 

the western facies thin to the east and pinch out against 

this Clarno high, as do the intervening tuffs and tuffaceous sedimentary rocks (Robinson, 1975; Woodburne 

and Robinson, 1977). The Picture Gorge ignimbrite, 

which was erupted east of the Blue Mountains uplift, did 

not extend into the western facies. Air-fall tuff, mostly 

derived from vents in the present-day Cascade Range, 

was deposited across the entire area.

Continued uplift along the Blue Mountains axis is 

indicated by the angular discordance between John Day 

tuffs and lava flows of the overlying Columbia River 

Basalt Group. In the area northeast of Kinzua, the John 

Day Formation pinches out entirely, and the Columbia 

River Basalt Group there rests directly on rocks of the 

Clarno Formation. Deformation since the middle Miocene 

is indicated by broad, open folds and local faults in the 

Columbia River Basalt Group. However, most tectonic 

activity ceased before deposition of younger parts of the 

Madras Formation (Deschutes Formation of Farooqui 

and others, 1981), probably about 10 to 8 Ma. Rocks of 

this unit are nearly flat lying and only rarely cut by faults.

ORIGIN

The John Day Formation is a compositionally heterogeneous unit that consists of air fall and ash-flow tuff, 

tuffaceous sedimentary rocks, and various lava flows and 

domes. The thickness of the unit varies extremely, 

ranging as high as about 750 m in a given section. The 

maximum age span of the formation is about 37 to 19 Ma. 

The bulk of the unit in the eastern, southern, and western 

facies consists of tuffaceous sedimentary rocks, chiefly 

claystones and siltstones with relict shard textures. Most 

of these rocks are believed to have formed from material 

deposited as thin ash falls on the land surface (Hay, 

1962a, 1963; Fisher, 1966b). Such structures as root 

cavities and animal burrows in the sedimentary rocks and 

pahoehoe surfaces on some of the lava flows provide 

direct evidence of subaerial deposition. Reworking of the 

ash by roots, burrowing organisms, and sheetwash 

eliminated the initial stratification and produced the 

massive bedding that is so characteristic of these rocks 

(Hay, 1962a, 1963). Some reworked tuffs in the lower 

part of the unit forms very thin bedded lacustrine 

deposits, whereas those in the upper part of the unit are 

fluviatile.

On the basis of age data then available, Hay (1963) 

calculated an average sedimentation rate for the lower

member of the eastern facies of about 36 mm per 1,000 yr. 

More recent K-Ar ages indicate that the 335-m-thick 

lower member spans a period of about 12 m.y. rather 

than the 7 m.y. estimated by Hay, implying an average 

sedimentation rate of about 28 mm per 1,000 yr. The 

upper member, which is about 550 m thick, was deposited 

over a period of about 5 m.y., indicating an average rate 

of accumulation of about 110 mm per 1,000 yr.

The tuffaceous sedimentary rocks become progressively thicker and coarser grained from east to west (Waters, 

1954; Peck, 1964; Robinson, 1975), indicating a probable 

source for these materials west of the present-day 

outcrops. The pyrogenetic mineralogy of these rocks 

implies a generally dacitic composition similar to that of 

the pyroclastic rocks of the western Cascades (Peck and 

others, 1964). K-Ar ages from western Cascade rocks 

indicate extensive volcanism coeval with deposition of the 

John Day Formation in latest Eocene(?), Oligocene, and 

early Miocene time (Fiebelkorn and others, 1983). The 

sorting characteristics of the pyroclastic material in the 

eastern facies are compatible with transportation for 

distances of 50 to 150 km (Hay, 1963; Fisher, 1966b). 

These relations indicate that that the bulk of the pyroclastic material in the John Day Formation was derived 

from volcanos in the western Cascades. Hay (1962a, 

1963) suggested that the claystone in the lower part of 

the formation formed by weathering at the land surface, 

whereas that of the Picture Gorge ignimbrite presumably 

formed after burial. Probably both weathering and diagenesis were responsible for formation of the claystone in 

the upper member.

The air-fall tuffs interlayered with the tuffaceous 

claystones and siltstones differ from the sedimentary 

rocks in being commonly coarser grained and in having 

somewhat more silicic compositions. They may have been 

formed by more violent eruptions from the same volcanos 

that produced the pyroclastic material in the sedimentary 

rocks or may, in part, have come from vents east of the 

Cascades, such as those now marked by rhyolite domes in 

the western facies. The fact that the tuffs retain their 

original bedding characteristics implies burial before 

extensive weathering and reworking had taken place.

Most of the ash-flow sheets in the western facies 

become thicker, coarser grained, more intensely welded, 

and richer in lithophysae from east and northeast to west 

and southwest, implying sources in that direction. These 

sheets cannot be traced to any known source and 

presumably were erupted from vents west of the 

present-day outcrops. Because the tuffs are coarse 

grained and apparently more silicic than coeval deposits 

in the western Cascades, they are believed to have been 

derived from vents west of long 121° W. but east of the 

Cascade Range. A separate source for these rocks is also 

indicated by their rare-earth geochemistry (I.G. Gibson
EOCENE(?), OLIGOCENE, AND LOWER MIOCENE ROCKS OF THE BLUE MOUNTAINS REGION 59

and P.T. Robinson, unpub. data, 1985), which differs 

significantly from that of Cascade rocks (Lux, 1981). The 

ash-flow tuff sheets of member E in the western facies lie 

in an old topographic depression between Ashwood and 

Willowdale and pinch out in all directions from this area. 

This unit is closely related in space, time, and composition to the rhyolite flows of member C and may have been 

derived from the same vent, believed to be marked by a 

rhyolite dome about 6 km north of Ashwood.

The ash-flow tuff sheets vary sufficiently in age and 

composition to indicate that they did not originate from a 

single source. Most have volumes in the range 10 to 20 

km , larger than expected for the many small vents in the 

western facies marked by rhyolite domes, such as Juniper Butte or Powell Buttes. However, eruption of these 

volumes of material would probably not produce collapse 

calderas. The large rhyolite domes in the Mutton Mountains were originally considered as possible sources for 

some of the ash-flow sheets, but only the tuff of member 

G can be traced into that area, and it is thinner, finer 

grained, and less densely welded there than in outcrops 

to the east and south. In summary, most of the ash-flow 

sheets appear to have originated from vents west of long 

121° W. but east of the Cascades. This area is now largely 

covered by the Madras (or Deschutes) Formation, and 

only a few isolated outcrops of older rock are present.

The Picture Gorge ignimbrite crops out only in the 

eastern facies. It was originally thought to correlate with 

member H of the western facies (Robinson, 1966), but its 

lithology and distribution indicate that it is probably a 

separate unit (Fisher, 1966a). It has an estimated volume 

of about 40 km3, suggesting a fairly large vent system. 

From the variations in thickness, crystal content, and 

size of lithic fragments, Fisher (1966a) inferred a possible 

source somewhere in the Ochoco Mountains southwest of 

Mitchell; however, no vent has yet been found in that 

area.

Both the mafic and silicic lava flows in the John Day 

Formation were erupted from local vents, many of which 

are still preserved. Feeder dikes for the mafic flows have 

been found in the vicinity of Antelope, Mitchell, and 

Service Creek (Lindsley, 1961; Hay, 1963; Robinson, 

1975), and deposits of basaltic agglomerate are common 

locally. Peck (1964) suggested that the voluminous trachyandesite flows of member B of the western facies 

were erupted from a vent in sec. 14, T. 11 S., R. 16 E., 

Foley Butte (7.5-minute) quadrangle, but this suggestion 

has not been confirmed. However, even where known 

vents are missing, the distribution and thickness variations of individual flows leave no doubt that their origin 

was local.

Most of the silicic lava flows within the formation can 

be related to vents now marked by rhyolite domes or 

plugs. The largest rhyolite flow (member C of the

western facies) is believed to have been erupted from a 

vent about 6 km north of Ashwood. Other small flows are 

associated with Juniper Butte and with domes in the 

Mutton Mountains. A source has not been located for the 

quartz latite flow north of Fossil, but it probably lies 

north of the present-day outcrops and in an area buried 

beneath flows of the Columbia River Basalt Group.

UNUSUAL ROCKS OF JOHN DAY AGE

Within the Blue Mountains province, isolated sequences of comparatively fresh lava flows and pyroclastic rocks 

have been recognized that are lithologically similar to 

rocks of the Clarno Formation but whose stratigraphic 

relations and age indicate that they are at least in part 

coeval with the John Day Formation. Little is known of 

these basaltic to silicic volcanic rocks in terms of their 

regional distribution, volume, petrology, and origin; 

almost invariably, they have been mapped either as part 

of the Clarno Formation or, in a few places, as anomalous 

parts of the John Day Formation. Examples of these 

enigmatic sequences are found at Smith Rock in the 

northeast corner of Deschutes County (Peterson and 

others, 1976; Robinson and Stensland, 1979), southwest 

of the Maury Mountains in southern Crook County 

(Walker and others, 1967), in the headwaters of the 

Grande Ronde River in southern Union County (Walker, 

1973), and in several areas marginal to Unity Basin in 

western Baker County (Brown and Thayer, 1966; Brooks 

and others, 1976). Other sequences of lithologically 

similar but undated rocks east of the main outcrop area of 

the Miocene and Pliocene Strawberry Volcanics (Brown 

and Thayer, 1966) are suspect and may be coeval with 

either the upper part of the John Day Formation or the 

lower part of the Strawberry Volcanics, which have been 

radiometrically dated at about 20 Ma (Robyn, 1977).

Although in the field most of these rocks look superficially like rocks of the Clarno Formation and consist 

commonly of hornblende-, pyroxene-, and plagioclaseporphyritic andesite and dacite lava and breccia, aphyric 

platy andesite and basaltic andesite, and rhyolitic to 

dacitic tuff and lapilli tuff, they generally differ in degree 

and type of alteration from most rocks of the Clarno 

Formation. Rocks of the Clarno Formation typically 

show some evidence of deuteric and low-temperature 

hydrothermal alteration to montmorillonite, secondary 

silica and carbonate minerals, zeolites, hematite, and, 

locally, chlorite. Areas of most intense alteration are 

commonly adjacent to small intrusions of andesite, dacite, or rhyolite. The lithologically similar but younger 

rocks are typically much less altered, although in a few 

places reddish-brown iron oxide, probably hematite, 

occurs on fractures and in vesicles. Hornblende is commonly replaced by magnetite and other unidentified
60 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

alteration products of late deuteric or diagenetic processes.

At Smith Rock a sequence of andesitic and basaltic 

lavas, laharic breccias, and tuffs, at least 1,000 m thick, 

is overlain conformably by a succession of rhyolite flows 

and tuffs, which, in turn, are succeeded by white tuffs of 

typical John Day lithology. The tuffs of typical John Day 

lithology are capped by flows of the Columbia River 

Basalt Group (Robinson and Stensland, 1979). The lower 

andesitic sequence is similar lithologically to rocks of the 

Clarno Formation and locally is overlain unconformably 

by ash-flow tuffs of the upper part of the John Day 

Formation (Robinson and Stensland, 1979). Williams 

(1957) originally included the entire sequence in the John 

Day Formation, but Robinson and Stensland (1979) 

mapped the lower andesitic sequence as part of the 

Clarno Formation. However, a single K-Ar analysis 

recently obtained from the upper part of this sequence 

indicates an age of 30.8 ±0.5 Ma, well within the recognized age span of the John Day Formation (Fiebelkorn 

and others, 1983). More recent work suggests that most 

of this sequence is equivalent in age to the John Day 

Formation or even younger (Walter Obermiller, oral 

commun., 1987).

Southwest of the Maury Mountains, largely unaltered, 

platy, plagioclase-, pyroxene-, and hornblende-pyroxeneporphyritic andesite flows rest with discordance on 

deeply weathered and altered flows and breccias typical 

of the Clarno Formation; neither sequence has been 

isotopically dated, but apparent interfingering of the 

overlying, unaltered unit with tuffaceous sedimentary 

rocks that in nearby areas contain a vertebrate fauna 

typical of the John Day Formation indicates that it may 

be coeval with some part of that formation.

In the upper drainage basin of the Grande Ronde 

River, unaltered to moderately altered platy, aphyric, 

commonly trachytic andesite flows and associated plagioclase-, pyroxene-, and hornblende-porphyritic andesite 

flows and monolithologic flow breccias rest unconformably on pre-Cenozoic rocks and are overlain (discordantly?) by dacitic to rhyolitic volcaniclastic rocks, flows, 

perlite, and coarse breccias, which, in turn, are overlain 

by flows of the Columbia River Basalt Group. The 

andesitic flows and breccias were originally mapped as 

part of the Clarno Formation, but more recently they 

have been considered coeval with part of the John Day 

Formation. Hornblende in the andesite flows commonly 

is baked and largely converted to opaque iron oxides and 

other unidentified alteration products, and fractures in a 

few places contain zeolites presumably derived from 

groundmass feldspar. A K-Ar analysis on fresh plagioclase phenocrysts from a porphyritic hornblende andesite 

(Walker, 1973) indicates an age of 28.8±0.8 Ma (Fiebelkorn and others, 1983), approximately in the midrange

of radiometric ages on the John Day Formation. A K-Ar 

age on biotite from the (discordantly?) overlying silicic 

assemblage is 27.7±0.8 Ma. Whether all of the andesitic 

rocks in this area are of late Oligocene age is not known.

In the area southwest of Lonerock and along Oregon 

Highway 207 about 15 km to the east, fairly fresh basalt 

crops out along the crest of the Blue Mountains uplift. 

These rocks underlie sequences of white tuffaceous 

claystone of typical John Day lithology and appear to rest 

on more altered lava flows of the Clarno Formation. 

Exposures are poor, however, and the structural relations of these lavas are unclear. On the basis of their 

stratigraphic position, Robinson (1975) included these 

rocks in the Clarno Formation, whereas Walker (1977) 

mapped them as part of the John Day Formation because 

they are fresher than most Clarno lavas. They are 

medium-grained aphyric rocks with pilotaxitic to subophitic textures composed of plagioclase laths, clinopyroxene, iron oxides, and about 5 percent olivine. 

Petrographically, they are more similar to the alkaliolivine basalt flows of the John Day Formation than to 

the calc-alkalic lavas of the Clarno Formation. However, 

alkalic rocks are present in both units (Robinson, 1969), 

and composition is not a reliable guide to stratigraphic 

position. Resolution of this problem will have to await 

more detailed studies, including radiometric-age determinations.

Exposed in areas mostly northeast, east, and southeast 

of the Unity Reservoir are rocks that have been mapped 

as the Clarno Formation. These rocks consist of a diverse 

assemblage of hornblende- and plagioclase-porphyritic 

andesitic to dacitic flows and flow breccias, tuffs, a few 

thin basalt or basaltic andesite flows, and intrusions 

related to local eruptive centers. In the lava flows 

containing both plagioclase and quartz crystals, some of 

the plagioclase and all of the quartz show resorption 

features implying that they are xenocrysts. These rocks 

are mostly unaltered, and none shows evidence of the 

type of alteration that characterizes the Clarno Formation. K-Ar ages on hornblende and plagioclase from a 

monolithologic andesitic to dacitic breccia originally 

mapped as the Clarno Formation are 19.6 ±0.8 and 

19.5 ±0.6 Ma, respectively. The breccia is approximately 

coeval with the youngest part of the John Day Formation, as represented by rhyolitic tuffs and tuffaceous 

sedimentary rocks of late Hemingfordian (early Miocene) 

age exposed near Warm Springs along the Deschutes 

River (Woodburne and Robinson, 1977), and, possibly, 

also with the oldest part of the Strawberry Volcanics.

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Peck, D.L., Griggs, A.B., Schlicker, H.G., Wells, F.G., and Dole, 

H.M., 1964, Geology of the central and northern parts of the 

western Cascade Range in Oregon: U.S. Geological Survey 

Professional Paper 449, 56 p.

Peterson, N.V., Groh, E.A., Taylor, E.M., and Stensland, D.E., 1976, 

Geology and mineral resources of Deschutes County, Oregon: 

Oregon Department of Geology and Mineral Industries Bulletin 

89, 66 p.

Renton, J.L., 1951, Some notes on thunder eggs: Mineralogist, v. 19, 

no. 4, p. 171-177.

Robinson, P.T., 1966, Facies changes in John Day Formation, Oregon 

tabs.]: Geological Society of America, Rocky Mountain Section 

Annual Meeting, Las Vegas, Nev., 1966, Program, p. 49.

 1969, High-titania alkali-olivine basalts of north-central Oregon, 

U.S.A.: Contributions to Mineralogy and Petrology, v. 22, no. 4, 

p. 349-360. 

-1975, Reconnaissance geologic map of the John Day Formation in

the southwestern part of the Blue Mountains and adjacent areas, 

north-central Oregon: U.S. Geological Survey Miscellaneous Investigations Series Map 1-872, scale 1:125,000. 

Robinson, P.T., and Brem, G.F., 1981, Guide to geologic field trip 

between Kimberly and Bend, Oregon, with emphasis on the John 

Day Formation, in Johnston, D.A., and Donnelly-Nolan, Julie, 

eds., Guides to some volcanic terranes in Washington, Idaho,
62 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Oregon, and northern California: U.S. Geological Survey Circular 

838, p. 29-40.

Robinson, P.T., and Stensland, D.H., 1979, Geologic map of the Smith 

Rock area, Jefferson, Deschutes, and Crook Counties, Oregon: 

U.S. Geological Survey Miscellaneous Investigations Series Map 

1-1142, scale 1:48,000.

Robyn, T.L., 1977, Geology and petrology of the Strawberry Volcanics, 

NE Oregon: Eugene, University of Oregon, Ph.D. dissertation, 

224 p.

Rogers, J.J.W., 1966, Coincidence of structural and topographic highs 

during post-Clarno time in north-central Oregon: American Association of Petroleum Geologists Bulletin, v. 50, no. 2, p. 390-396.

Ross, C.S., 1941, Origin and geometric form of chalcedony-filled 

spherulites from Oregon: American Mineralogist, v. 26, no. 12, p. 

727-732.

Steiger, R.H., and Jager, Emilie, 1977, Subcommission on geochronology Convention on the use of decay constants in geo- and 

cosmochronology: Earth and Planetary Science Letters, v. 36, no. 

3, p. 359-362.

Swanson, D.A., 1969, Reconnaissance geologic map of the east half of 

Bend quadrangle, Crook, Wheeler, Jefferson, Wasco, and Deschutes Counties, Oregon, in Geological Survey Research, 1968: 

U.S. Geological Survey Miscellaneous Geologic Investigations 

Map 1^568, scale 1:250,000.

Swanson, D.A., and Robinson, P.T., 1968, Base of the John Day 

Formation in and near the Horse Heaven mining district, northcentral Oregon, in Geological Survey Research, 1968: U.S. Geological Survey Professional Paper 600-D, p. 154-161.

Swinney, C.M., Waters, A.C., and Miller, C.P., 1968, Reconnaissance 

geologic map of the Lookout Mountain quadrangle, Crook and 

Wheeler Counties, Oregon: U.S. Geological Survey Miscellaneous 

Geologic Investigations Map 1-543, scale 1:62,500.

Taylor, E.M., 1960, Geology of the Clarno Basin, Mitchell quadrangle, 

Oregon: Corvallis, Oregon State University, M.S. thesis, 173 p.

 1977, The Clarno Formation A record of early Tertiary volcanism in central Oregon [abs.]: Geological Society of America 

Abstracts with Programs, v. 9, no. 6, p. 768. 

-1981, A mafic-dike system in the vicinity of Mitchell, Oregon, and

its bearing on the timing of Clarno-John Day volcanism and early 

Oligocene deformation in central Oregon: Oregon Geology, v. 43, 

no. 8, p. 107-112.

Walker, G.W., 1973, Reconnaissance geologic map of the Pendleton 

quadrangle, Oregon and Washington: U.S. Geological Survey

Miscellaneous Investigations Series Map 1-727, scale 1:250,000. 

-1977, Geologic map of Oregon east of the 121st meridian: U.S.

Geological Survey Miscellaneous Investigations Series Map 1-902, 

scale 1:500,000.

Walker, G.W., Peterson, N.V., and Greene, R.C., 1967, Reconnaissance geologic map of the east half of the Crescent quadrangle, 

Lake, Deschutes, and Crook Counties, Oregon: U.S. Geological 

Survey Miscellaneous Geologic Investigations Map 1-493, scale 

1:250,000.

Waters, A.C., 1954, John Day Formation west of its type locality [abs.]: 

Geological Society of America Bulletin, v. 65, no. 12, pt. 2, p. 1320.

 1968a, Reconnaissance geologic map of the Dufur quadrangle, 

Hood River, Sherman, and Wasco Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-555, 

scale 1:125,000.

 1968b, Reconnaissance geologic map of the Madras quadrangle, 

Jefferson and Wasco Counties, Oregon: U.S. Geological Survey 

Miscellaneous Geologic Investigations Map 1-556, scale 1:125,000. 

-1968c, Reconnaissance geologic map of the Post quadrangle,

Crook County, Oregon: U.S. Geological Survey Miscellaneous 

Geologic Investigations Map 1-542, scale 1:62,500.

Waters, A.C., and Vaughan, R.H., 1968a, Reconnaissance geologic 

map of the Eagle quadrangle, Crook County, Oregon: U.S. 

Geological Survey Miscellaneous Geologic Investigations Map 

1-540, scale 1:62,500.

 1968b, Reconnaissance geologic map of the Ochoco Reservoir 

quadrangle, Crook County, Oregon: U.S. Geological Survey 

Miscellaneous Geologic Investigations Map 1-541, scale 1:62,500.

Waters, A.C., Brown, R.E., Compton, R.R., Staples, L.W., Walker, 

G.W., and Williams, Howel, 1951, Quicksilver deposits of the 

Horse Heaven mining district, Oregon: U.S. Geological Survey 

Bulletin 969-E, p. 105-149.

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quadrangle, Grant County, Oregon: U.S. Geological Survey Geologic Quadrangle Map GQ-541, scale 1:62,500.

Williams, Howel, 1957, A geologic map of the Bend quadrangle, 

Oregon, and a reconnaissance geologic map of the central portion 

of the High Cascade Mountains: Portland, Oregon Department of 

Geology and Mineral Industries, scales 1:125,000 and 1:250,000.

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stratigraphic implications: Journal of Paleontology, v. 51, no. 4, p. 

750-757.
4. THE COLUMBIA RIVER BASALT GROUP AND ASSOCIATED 

VOLCANIC ROCKS OF THE BLUE MOUNTAINS PROVINCE

By PETER R. HOOPER1 and DONALD A. SWANSON

CONTENTS

Page

Abstract 63

Introduction 64

Main series 66

Imnaha Basalt 66

Grande Ronde Basalt 69

Eckler Mountain Member of the Wanapum Basalt 74

Main part of the Wanapum Basalt 76

Saddle Mountains Basalt 78

Lewiston Basin 81

Troy Basin 82

Composition of the Saddle Mountains Basalt in Lewiston and

Troy Basins 83

Other flows of the Saddle Mountains Basalt 83

Origin of the Saddle Mountains Basalt 84

Picture Gorge Basalt 86

Basalt of Prineville 88

Basalts of Powder River and Weiser and associated volcanic

rocks 88

Powder River 88

Weiser 90

Discussion 90

Tectonic setting of the basaltic volcanism 92

Genesis of the Columbia River Basalt Group 94

Physical constraints 94

Chemical constraints 95

References cited 96

ABSTRACT

Between about 17.5 and 6.0 Ma, flows of the Columbia River Basalt 

Group were erupted from north-northwest-oriented fissures concentrated in the accreted terrane(s) of the Blue Mountains province. The 

flows spread northward over the cratonic crust of eastern Washington 

and westward into Pasco Basin, where they accumulated as a pile as 

much as 4.5 km thick.

In this chapter, the stratigraphic succession is briefly described and 

the characteristic features of the flows within each unit are given. 

Feeder systems and the areal extents of each unit are illustrated.

The Columbia River Basalt Group and associated volcanic rocks are 

subdivided on the basis of their isotopic compositions. Each of the six 

subdivisions is distinct in its time-space relations, and most possess a

'Department of Geology, Washington State University, Pullman, Wash.

unique range of composition. The first subdivision, the main series of 

the Columbia River Basalt Group consisting of the Imnaha, Grande 

Ronde, and Wanapum Basalts, contains over 90 percent (by volume) of 

the group and was erupted from the Chief Joseph dike swarm in the 

eastern part of the Blue Mountains province. This subdivision displays 

a small stepped increase in the ^Sr/^Sr ratio (and a corresponding 

decrease in the 143Nd/144Nd ratio) over time and may record varying 

degrees of mixing between at least two source compositions. The 

subdivision represented by the Saddle Mountains Basalt was also 

erupted from the Chief Joseph dike swarm and represents the last 

minor episodes of Columbia River Basalt Group volcanism. The Saddle 

Mountains flows have varying compositions and high but varying values 

of the ratio ^Sr/^Sr, they lie unconformably on earlier flows, and there 

is evidence of prolonged periods of deformation and erosion between 

eruptions.

The Picture Gorge Basalt and the other three remaining subdivisions 

are restricted to the southern margin of the Columbia Plateau, each 

with its own feeder system. The Picture Gorge was erupted from the 

Monument dike swarm in John Day Basin and is coeval with part of the 

eruption of the Grande Ronde Basalt farther northeast. The isotopic 

signature of the Picture Gorge Basalt implies an origin from a depleted 

mantle source. The basalt of Prineville was erupted from unknown 

vents close to the east edge of the Cascade Range. It is remarkably 

homogeneous and was erupted in late Grande Ronde time. The basalts 

of Powder River and Weiser were erupted during Wanapum and Saddle 

Mountains time; compositionally, they resemble the Slide Creek 

Member and other lower units of the Strawberry Volcanics.

Deformation of the Columbia River Basalt Group was apparently 

continuous throughout the approximately 11 m.y. of eruption and 

continues today. Structural elements include: (1) southeastward to 

northwestward tilting due to isostatic rise of the Idaho batholith and 

other plutons to the east, together with depression of Pasco Basin to the 

west; (2) older structural discontinuities, including the suture zone 

between the thicker cratonic crust and the thinner accreted terranes, as 

well as a pre-Miocene northwest-southeast and northeast-southwest 

fracture pattern; and (3) a regional stress system, including a horizontal 

north-northwest/south-southeast direction of maximum stress and a 

horizontal west-southwest/east-northeast direction of minimum stress, 

which resulted in potential north-northwest/south-southeast extensional fissures at depth that became feeders for the basalt flows, approximately northwest-southeast (right-lateral) and northeast-southwest 

Qeft-lateral) strike-slip faults at more moderate depths, and approximately east-west anticlinal folds associated with reverse faults near the 

surface where the vertical direction became the direction of minimum 

stress. In addition, clockwise rotation of individual blocks has occurred, 

and preliminary work indicates that such rotation may be concentrated 

adjacent to the suture zone and the southeastward extension of the 

Olympic-Wallowa lineament.

Genesis of the Columbia River Basalt Group and related volcanic 

rocks was complex. Multiple sources are required by isotopic data, and 

crystal fractionation of plagioclase, olivine, and some pyroxene of as

63
64 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

much as 50 percent may have occurred between flows within the same 

isotopic sequence. Magma mixing has also occurred and may have been 

widespread in the Grande Ronde Basalt. Of the many unanswered 

questions, some of the most profound are why the Columbia River 

Basalt Group erupted when and where it did, what were the depth and 

compositions of the various mantle sources, and what was the relative 

significance of crustal contamination in their genesis. It is particularly 

difficult to distinguish among crustal contamination of a mantle source, 

a mantle-enrichment process, and contamination by crustal assimilation 

as the magma made its way to the surface.

INTRODUCTION

The Blue Mountains province of northeastern Oregon 

and adjacent parts of Washington and Idaho is defined by 

pre-Tertiary rocks that form a complex of one or more 

allochthonous terranes accreted to the west side of the 

North American craton during the Mesozoic. These 

accreted terranes are separated from the craton by a 

suture zone running northward along the west side of the 

Idaho batholith and eastward through southeastern 

Washington, defining a 90° reentrant angle (fig. 4.1; 

Hamilton, 1962; Armstrong and others, 1977; Fleck and 

Criss, 1985; Mohl and Thiessen, in press).

The northwestern margin of the Blue Mountains province is marked by the Klamath-Blue Mountains lineament 

(KBML; Riddihough and others, 1986), a sharp geophysical discontinuity corresponding to the northwestward 

limit of accreted-terrane rocks and trending parallel to 

their regional foliation. The west edge of the cratonic 

crust under the Columbia River Basalt Group in eastern 

Washington is not well defined but almost certainly 

trends northward along the east side of Pasco Basin (fig. 

4.1). The composition of the crust beneath the basalt in 

the triangular area between the Cascades in the west, the 

KBML, and the cratonic margin east of Pasco Basin is not 

known. Available geophysical data are consistent with 

4.5 km of the Columbia River Basalt Group overlying a 

wedge of sedimentary rocks resting directly on oceanic 

crust.

In contrast to the thick Precambrian and lower Paleozoic continental crust of the craton, the accreted terrane(s) is of oceanic origin a complex of Devonian to 

Jurassic island-arc volcanic rocks, oceanic lithosphere, 

and fore-arc graywacke turbidites, capped by carbonate 

and associated metasedimentary units and welded together by Jurassic and younger granitoids (Brooks and 

Vallier, 1978; Dickinson and Thayer, 1978; Vallier, 1985). 

The units have a pronounced northeastward structural 

trend at all scales and are wedged into the craton's 

reentrant angle as the southern limb of a structural bend 

that has been called the Columbia arc (Carey, 1958; Wise, 

1963).

The accreted terrane is largely covered by Eocene to 

Pliocene volcanic rocks. Of these rocks, the lower to

upper Miocene Columbia River Basalt Group is dominant 

in the northern part of the province, and lava flows 

spread northward and westward from there to form the 

Columbia Plateau (fig. 4.1).

The Columbia River Basalt Group was erupted between about 17.5 and 6.0 Ma (Watkins and Baksi, 1974; 

McKee and others, 1977, 1981; Swanson, Wright, and 

others, 1979; Long and Duncan, 1982) from northnorthwest-trending fissures (Taubeneck, 1970; Swanson 

and others, 1975; Swanson, Wright, and others, 1979) 

concentrated in the southeastern part of the Columbia 

Plateau and almost wholly within the Blue Mountains 

province. Most eruptions took place within the province, 

although much lava flowed long distances downslope and 

now is found north and west of the province. A few dikes, 

however, crossed the east-west suture zone to feed 

eruptions on the craton in southeastern Washington (fig. 

4.1; also, see figs. 4.4,4.8), and the feeder dikes of the Ice 

Harbor Member of the Saddle Mountains Basalt (Swanson and others, 1975; Swanson, Wright, and others, 1979) 

lie entirely outside the Blue Mountains province. Also, 

Taubeneck (1970) reported dikes within the Idaho batholith that resemble dikes of the Columbia River Basalt 

Group.

A clear consensus on the origin and evolution of the 

Columbia River Basalt Group has not yet been reached, 

but field, petrographic, chemical, and isotopic data clearly indicate that different flow sequences were derived 

from different sources by a wide variety of processes 

(Carlson, 1984; Hooper, 1984a, 1985). The probability of 

multiple sources was recognized in early studies of the 

Columbia River Basalt Group (Waters, 1961; Wright and 

others, 1973).

With these various sources and processes in mind, we 

divide the Columbia River Basalt Group and associated 

volcanic rocks into six sequences: (1) the main series of 

the Columbia River Basalt Group (Imnaha, Grande 

Ronde, and Wanapum Basalts), (2) the Saddle Mountains 

Basalt of the Columbia River Basalt Group, (3) the 

Picture Gorge Basalt of the Columbia River Basalt 

Group, (4) the basalt of Prineville, (5) the basalt of 

Powder River and associated volcanic rocks in Oregon, 

and (6) the basalt of Weiser and associated volcanic rocks 

in Idaho.

The Strawberry Volcanics, of middle to late Miocene 

age (Robyn, 1977, 1979), are discussed in chapter 5. As 

noted below, the Slide Creek Member, the basal unit of 

the Strawberry Volcanics, is probably correlative with 

both the basalts of Powder River and Weiser, and the 

flows of these two sequences are thus best considered 

part of the Strawberry Volcanics. The basalt of Bear 

Creek occurs farther west and may be of similar age; this 

unit was described by Goles (1986) and is not reviewed 

here.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 65

The rocks of all six sequences have been included in the 

Columbia River Basalt Group by various authors (Waters, 1961; Uppuluri, 1974; Swanson, Wright, and others, 

1979; Carlson and others, 1981; Swanson and others, 

1981; Fitzgerald, 1984). Their relative ages, where 

known, are shown in figure 4.2. Their approximate 

extents are shown in figures 4.1, 4.4,4.5, and 4.8 to 4.10. 

Each sequence is distinctive in its time-space relations

and, with the possible exceptions of the Powder River 

and Weiser rocks, can be recognized by a distinctive 

range of chemical and isotopic compositions.

Acknowledgments. Much of the work discussed here 

was carried out in cooperation with T.L. Wright, W.D. 

Kleck, S.P. Reidel, V.E. Camp, M.E. Ross, and many 

others, to all of whom we are grateful. We thank both 

Gary Smith for permission to use his analyses of the

123° 121 C 119° 117° 115°

49°

48°

47°

46°

45°

440

MONTANA

75 150 KILOMETERS

/ / 

ssss

EXPLANATION

Contact-Dashed where approximately located 

f Structural trend of Tertiary rocks

Structural trend of pre-Tertiary accreted 

terranes

Columbia River Basalt Group and associated 

volcanic rocks

Cascade volcano

FIGURE 4.1. Northwestern United States, showing distribution of the Columbia River Basalt Group and associated volcanic rocks. Blue 

Mountains province lies between cratonic margin on east and Klamath-Blue Mountains line (KBML) in west. OWL, Olympic-Wallowa 

lineament; PG, Picture Gorge Basalt; PR, basalt of Powder River; PRI, basalt of Prineville; W, basalt of Weiser.
66 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

basalt of Prineville and Chris Hawkesworth for permission to use his ^Sr/^Sr and 143Nd/144Nd values, and we 

are indebted to G.G. Goles for his careful review of the 

manuscript.

MAIN SERIES

The Imnaha, Grande Ronde, and Wanapum Basalt 

constitute at least 90 percent (probably more than 95 

percent) of the volume of the whole Columbia River 

Basalt Group, including the associated volcanic sequences shown in figure 4.2. These three units were all erupted 

from the broad Chief Joseph dike swarm in the southeastern part of the Columbia Plateau (Taubeneck, 1970).

Four petrographically distinct units (Imnaha Basalt, 

Grande Ronde Basalt, Eckler Mountain Member of the 

Wanapum Basalt, and main part of the Wanapum Basalt) 

can be recognized on the basis of chemical and isotopic 

composition (fig. 4.3; Waters, 1961; Swanson, Wright, 

and others, 1979; Carlson, 1984; Hooper, 1984a). The 

differences among these units indicate that they could not 

have originated from a single primary magma derived by 

partial melting of a homogeneous source.

IMNAHA BASALT

Early eruptions of the Columbia River Basalt Group 

produced coarsely porphyritic (plagioclase+olivine+

Columbia River Basalt Group

Saddle 

Mountains 

Basalt

Wanapum Basalt

/ / / / / Hiatus ///////

Grande Picture 

Ronde Gorge

Basalt Basalt

Imnaha Basalt

Magnetic 

polarity

Various

R

T 

N

/////////

N2

R2

_ ___ N1

R1

NO

R 0

Age (Ma)

6.0

11 R

///// 14.5 /////

4C R

17.5

Associated volcanic rocks

? ? 

Basalt of ? Basalt of

Powder River Weiser

 ? __?__

/////////////// Hiatus /////////

Basalt of 

Prineville

FIGURE 4.2. K-Ar ages (Watkins and Baksi, 1974; McKee and others, 

1977, 1981). Age of the Vantage Member of the Ellensburg Formation, between the Grande Ronde and Wanapum Basalts, is 15.6 Ma, 

determined by ^Ar/^Ar methods (Long and Duncan, 1982). Magnetic polarities: N, normal; R, reversed; T, transitional. Polarity 

intervals numbered from oldest to youngest (for example, N0 to N3) 

where sequential.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 67

augite) flows about 17.5 to 16.5 Ma (McKee and others, 

1981; Long and Duncan, 1982). These flows make up the 

Imnaha Basalt and cover at least 30,000 km2 in western 

Idaho and adjacent parts of Oregon and Washington (fig. 

4.4; Camp and Hooper, 1981; Fitzgerald, 1984; Hooper 

and others, 1984). Their north-northwest-trending feeder dikes and vent complexes are located in Oregon close 

to the Idaho-Oregon State line, largely within Waters' 

(1961) Cornucopia dike swarm (fig. 4.4). The fissures 

crossed the major east-west topographic divide defined 

by the Wallowa and Seven Devils Mountains. Flows of 

similar composition filled deep canyons north and south of

0.713

0.711

< 0.709 

cc

(55

0.707

0.705

0.703

A LM

ALA

(2448)

IH IH

f\

A'

0.713

200 400 600 8002400 2600 

Ba/Ti RATIO

0.711

0.709

0.707

0.705

0.703

J.M

A WS

A WL

200 300 400

Sr CONTENT, IN PARTS PER MILLION

500

FIGURE 4.3. Plots of ^Sr/^Sr ratios against Ba/Ti ratios and Sr 

contents for flows of the Columbia River Basalt Group. °, Imnaha 

Basalt (N0 flows); , Imnaha Basalt (R0 flows); o, Picture Gorge 

Basalt; , Grande Ronde Basalt; +, Eckler Mountain Member (of the 

Wanapum Basalt); o, main part of the Wanapum Basalt (PO, 

Powatka flow); A , Saddle Mountains Basalt (A, Asotin; B, Buford; E, 

Esquatzel; EM, Elephant Mountain; IH, Ice Harbor; LA, Lapwai; 

LM, Lower Monumental; P, Pomona; U, Umatilla; W, Wilbur Creek; 

WL, Lewiston Orchard flow; WS, Slippery Creek flows). Data from 

McDougall (1976), Carlsonand others (1981), Carlson (1984), and C.J.

Hawkesworth (unpub. data, 1984). Magnetic polarities: N, normal; R, 

reversed. Polarity intervals numbered from oldest to youngest (for 

example, N0 to N3) where sequential. A, ^Sr/^Sr ratio against Ba/Ti 

ratio. B, ^Sr/^Sr ratio against Sr content. AB and RC, American Bar 

and Rock Creek subunits, respectively, of the Imnaha Basalt with 

reverse magnetic polarity. Analyses from McDougall (1976) are 

omitted because they are consistently biased to lower Sr contents 

relative to those of Carlson and others (1981) and analyses in this 

study (table 4.2) by approximately 40 ppm.
68 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

118° 117° 116?

\ 

Pullman V<jMo

4 _/- -"oj* \ xx Orofino 

£^ \ X*

WASHINGTON

,,-v'\WAULOWA'):.

40 KILOMETERS

Southeast limit of flow

 Imnaha (1) and Grande Ronde

(Rj flows) Basalts 

Grande Ronde Basalt 

I i J N| flows

 - Rg flows

 M, flows

EXPLANATION

Limekiln fault Dashed where approximately 

located. Bar and ball on downthrown side; 

arrow indicates dip of monoclinal limb at 

both ends of Limekiln fault

Grande Ronde vent Without" associated flows 

^r^l Imnaha dike and vent H^V^

/I Imnaha dikes

FIGURE 4.4. Blue Mountains region, showing distribution of flows, dikes, and vents of the 

Imnaha and Grande Ronde Basalts. Grande Ronde dikes and vents are concentrated in two 

main swarms (shaded), the Grande Ronde dike swarm in the north and the Cornucopia dike 

swarm in the south. Known dikes of the Imnaha Basalt are marked separately. Magnetic 

polarities: N, normal; R, reversed. Polarity intervals numbered from oldest to youngest (for 

example, Nj to N2) where sequential.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE

the divide and covered all but the highest peaks of the 

two ranges. As the canyons filled and the basalt plain 

expanded laterally, however, successive flows became 

thinner though covering larger areas. This change permitted the continuing gradual uplift of the predominantly 

granitic mountain blocks to overtake the rising tide of 

basalt. As a result, subsequent flows of the Grande 

Ronde Basalt sequentially offlap the rising mountains 

(Camp and Hooper, 1981), even though eruptions were 

more frequent and possibly more voluminous than during 

Imnaha time (Shaw and Swanson, 1970; Hooper, 1981, 

fig. 3).

In the Imnaha Basalt, the lowest flows exposed at 

Squaw Butte (fig. 4.4) and on China Cap Ridge on the 

southwest flank of the Wallowa Mountains have reversed 

magnetic polarity (fig. 4.2; Swanson, Wright, and others, 

1979; Fitzgerald, 1984; Martin, 1984), but nearly all flows 

have normal magnetic polarity, with transitional to 

reversed polarity in the youngest flows (Hooper and 

others, 1979).

Hooper and others (1984) provided stratigraphic, petrologic, and chemical data for the Imnaha Basalt north of 

the Wallowa-Seven Devils divide. Two basalt types are 

distinguished within the Imnaha Basalt (Kleck, 1976): 

The American Bar (AB) type dominates the lower part of 

the stratigraphic section, and the Rock Creek (RC) type 

dominates the upper part, although the two types interfinger throughout the section. The AB type is recognized 

in the field by its fine-grained groundmass, 

plagioclase+olivine+augite phenocryst assemblage, and 

minor amounts of olivine. The RC type has a coarser 

grained groundmass, much more abundant and conspicuous olivine, and no augite phenocrysts.

Representative major- and trace-element analyses and 

isotopic data for the Imnaha Basalt are presented in 

tables 4.1 and 4.2. The two Imnaha types show a 

considerable overlap in the concentrations of most elements, but AB flows tend to have higher A1203 and MgO 

contents and lower Si02 contents than RC flows. The two 

flow types are clearly distinguished by their different 

concentrations of Sc, V, Ni, and Sr. Fractionation 

dominated by plagioclase appears to have been a major 

process relating individual flows within each type. Majorelement modeling demonstrates that the fractionated 

assemblage is similar to the observed phenocryst assemblage for each type (Hooper, 1984a; Hooper and others, 

1984). However, large-ion-lithophile (LIL)-element contents increase at a greater rate than do high-fieldstrength (HFS)-incompatible elements. Such decoupling 

might result from an open magma-chamber system, in 

which crystal fractionation was accompanied by crustal 

assimilation and possible recharge by primitive magma 

(DePaolo, 1981; O'Hara and Mathews, 1981). As yet, 

however, no quantitative model has been devised that is

able to account for the chemical variations within each 

type and the differences between them. In particular, 

such simple models are quite unable to account for the 

differences between the two flow types (AB and RC). It 

is significant that the low values of isotopic ratio 87Sr/86Sr 

for the Imnaha Basalt provide little evidence of crustal 

contamination (fig. 4.3) and, instead, indicate magmas 

derived from different mantle sources (Carlson, 1984).

GRANDE RONDE BASALT

The mostly aphyric flows of Grande Ronde Basalt may 

form as much as 85 percent of the Columbia River Basalt 

Group (Reidel and others, 1982). These flows conformably overlie the porphyritic flows of the Imnaha Basalt 

wherever the latter are present in the southeastern part 

of the Columbia Plateau, but they spread much farther to 

the north and west (fig. 4.4). Some of the younger flows 

reached the Pacific Ocean (Anderson and others, 1987). 

All known feeder dikes are within the Blue Mountains 

province and are concentrated in the Grande Ronde and 

Cornucopia dike swarms (fig. 4.4).

By detailed geochemical studies, previous workers 

have succeeded in distinguishing individual flows or 

limited sets of flows in the Grande Ronde Basalt over 

substantial parts of the Columbia Plateau (Reidel, 1983; 

Mangan and others, 1986), but the fine-grained aphyric 

texture and comparatively homogeneous chemical composition of the flows have inhibited the mapping of 

individual flows except in limited areas. However, the 

Grande Ronde Basalt has been subdivided and mapped 

regionally as four informal, successively younger magnetostratigraphic units: Rlf Nx , R2 , and N2 (fig. 4.2; 

Swanson, Wright, and others, 1979).

As much as 200 m of unit Rx of the Grande Ronde 

Basalt overlies the Imnaha Basalt in the Weiser River 

embayment (fig. 4.4; Fitzgerald, 1984); this thickness 

increases northwestward and reaches a maximum of 

approximately 400 m in the Imnaha River valley (Camp 

and Hooper, 1981). Flows of unit Nl form a thin veneer 

over R! flows in the northern part of the Weiser River 

embayment, and unit Nx thickens progressively into 

southeastern Washington. Unit R2 thickens northwestward but pinches out southeastward against the northwest flanks of the Wallowa and Seven Devils Mountains 

and is not found southeast of the divide. Plateau-forming 

flows of unit N2 are limited to the northwest side of the 

Limekiln fault (fig. 4.4), with only local exceptions (Camp 

and Hooper, 1981).

The apparent northwestward trend (fig. 4.4) of progressively younger stratigraphic units has been ascribed 

to continued regional tilting of the ground surface, the 

southeastern part of the area rising relative to the 

northwestern part during Grande Ronde time. The
70 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

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THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 71

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72 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

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THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 73

TABLE 4.2. Selected isotopic data for samples of the Columbia River Basalt Group and associated volcanic rocks

[Data sources: H (samples listed in table 4.1), Sr and Nd isotope analyses by C J. Hawkesworth, unpub. data (Open University, England); LT, 518O values, P.B. Larson and 

HP. Taylor, unpub. data; C. Carlson and others (1981) and Carlson (1984); MC. Sr from McDougall (1976) and Pb from Church (1985). Many of the 87Sr / ^Sr 

values are firom McDougall (1976) and Pb ratios, firom Church (1985). Plow types as for table 4.1 wim these addidons: symbols N and R refer to normal and reversed 

magnetic polarity (flg. 4.2); Huntz., Huntzinger flow; Ihd. Mt., basalt of Indian Mountain, which, along wim basalts of Basin City, Martindale, and Goose Island 

(Goose Is.), is part of the Ice Harbor Member. ?, uncertain; , no data; do, ditto]

Sample

Dam 

somoes

How »*,«» 143Nd/ M*Nd 2"Spb/ZMpb -h,-S "*,-*. 8"0

Inmaha Basalt

SQB100

Cl

BUK-1

BUK-5

W-25

BUK-16

BUK-21

M-l

M-13

H.LT

C

H.LT

H.LT

ILLT

H.LT

H.LT

C

C

RC-0

RC-0

AB-2

AB4

AB-7

RC-1

LogCr.

?

7

0.70363

.70349

.70415

.70397

.70398

.70383

.70383

.70373

.70395

0.51290

.51302

.51281

.51284

.51286

.51285

.51289

.512938

.512995

_

18.769

19.086

19.040

15.511

15.646

15.610

38.28

38.73

38.68

6.50

5.60

6.40

6.00

5.90

6.10

6.09

6.48

Picture Gorge Basalt

PG-1

PG-14

C

C

- _ 0.70340

.70351

0.513033

.513017

18.837

19.088

15.555

15.646

38.38

38.73

5.64

5.64

Grande Ronde Basalt

SW73-355

SW75-219

SW77-127

SW78-258

SW78-221

L-ll

L-41

YB-1

BCR-1

C

C

C

C

C

C

C

C

C

_

Ni

R2

N2

Rl

NI?

R2

0.70500

.70489

.70444

.70507

.70502

.70428

.70462

.70445

.70493

0.51265

.51268

.51284

.51269

.512736

.51284

.512808

.51274

.512654

_

18.810

18.978

18.785

18.969

18.918

18.925

18.801

_

15.564

15.640

15.601

15.603

15.607

15.611

15.609

_

38.66

38.81

38.76

38.67

38.74

38.74

38.66

7.13

6.62

6.01

6.77

6.27

6.91

6.33

Wanapum Basalt

SW73-322

EP-2

SW73-296

SW73-152

SW78-408

CR-19

K-9040

SW71-42

SW77-321

RS-1A

C

H

C

C

C

MC

H

C

C

H

Rb. ML

da

Dodge

Sh. Cr.

FSP

FSP

Roza

Lolo

Lookingglass

Powatka

0.70438

.70432

.70435

.70481

.70517

.7053

.70517

.70506

.70546

.70475

0.51277

.51273

.51274

.51267

.51263

_

.51259

.51266

.51266

19.009

18.818

18.881

_

18.84

18.729

18.773

15.584

15.588

15.612

15.59

15.558

15.625

38.58

38.54

38.75

_

38.67

38.70

38.77

6.60

6.69

6.82

6.60

6.47

6.36

Saddle Mountains Basalt

WU-1

SW72-213

HS1-51

HAS-56

SW72-31

20-6.63.1

HAS-51

HAS-74

WSM-2

WPOM-1

WEM-2

SW75-46

SW75-114

SW75-35

SW75-36

HWE-03

SW73-361

C

C

H

H

C

C

H

H

C

C

C

C

C

C

C

K

C

Umatilla

Wilbur

da

Lapwai

Asotin

Huntz.

Lew. Or.

Slipp. Cr.

Esquatzel

Pomona

m. Mt.

Basin City

Martindale

Goose Is.

Ind. ML

Buford

L. Mon.

0.70892

.71047

.71055

.70922

.70755

.70884

.70593

.71013

.71317

.70750

.70762

.70782

.70764

.70780

.70723

.71087

0.51227

.51177

.51173

.51234

.51199

.51206

.51245

.51243

.51231

.51233

.51234

.51233

_

.51213

18.148

17.736

_

18.085

17.819

19.751

18.367

19.109

18.626

18.656

_

_

_

18.464

15.629

15.589

_

15.627

15.587

15.952

15.656

15.777

15.653

15.652T

_

15.654

39.36

38.81

39.17

38.89

40.13

39.74

39.91

39.45

39.44

_

38.76

6.85

8.03

6.37

6.73

6.09

6.09

6.34

5.81

7.41

Basalt of Powder River

F-2

D-6

7-50

C

C

C

Olivine basalt

Andesite

Nepheline basalt

0.70353

.70354

.70343

0.512931

.512932

.513008

_ _ _ _
74 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

tilting is attributed to the continued, possibly isostatic, 

rise of the Idaho batholith and other associated granitecored mountain blocks (Hooper and Camp, 1981). The 

tilting was augmented by normal faulting, such as that 

displayed by the northeast-trending Limekiln fault at the 

end of unit R2 (Grande Ronde) time (fig. 4.4).

The fissure system and, presumably, the magma 

source may have remained in much the same position 

while the magma erupted progressively farther north, 

where the fissures intersected a lower ground surface. 

This apparent stability is supported by the presence of 

low shields of unit N2 of the Grande Ronde Basalt (Joseph 

Volcanics unit of Kleck, 1976; see Walker, 1979) directly 

along the axis of the Grande Ronde dike swarm on the 

uplifted side of the Limekiln fault (fig. 4.4). The shields 

demonstrate the presence of unit N2 feeder dikes farther 

south than the main eruption site of unit N2, yet they 

form only isolated units resting on the unit R2 plateau 

surface. The large volumes of unit N2 magma, which 

covered the greater part of the Columbia Plateau and 

reached the Pacific Ocean, were erupted from those parts 

of the fissures that intersected the lower topographic 

surface on the northwestern, downthrown side of the 

Limekiln fault. A similar relation can be observed for the 

Roza Member (of the Wanapum Basalt) across the same 

fault.

Detailed descriptions of the stratigraphic and petrologic features and the chemical composition of the Grande 

Ronde Basalt in the Blue Mountains province were 

provided by Reidel (1983), and isotopic data were given 

by McDougall (1976), Carlson and others (1981), Nelson 

(1983), and Carlson (1984). Major-element and traceelement analyses and isotopic data for two representative 

samples of the Grande Ronde Basalt are listed in tables 

4.1 and 4.2.

The Grande Ronde Basalt is consistently richer in Si02 , 

K20, and LIL-incompatible trace elements (such as Ba 

and Rb) than is the Imnaha Basalt (Hooper and others, 

1984), but the two formations contain similar concentrations of most other major and trace elements. Gradual 

fluctuations of incompatible elements through the Grande 

Ronde stratigraphy can be partly explained by crystal 

fractionation, in which removal of plagioclase and pyroxene played a dominant role (Reidel, 1983). The scarcity of 

well-developed phenocrysts of either of these phases is 

anomalous. However, rare orthopyroxene phenocrysts 

mantled by clinopyroxene are present (Reidel, 1983).

A small stepped-upward increase in the 87Sr/86Sr ratio 

(with an upward decrease in the 143Nd/144Nd ratio) is 

evident in the Grande Ronde Basalt (Hooper, 1984a, fig. 

2). These changes, together with Pb isotopic data, led 

Carlson (1984) to conclude that progressive crustal contamination, in addition to crystal fractionation, played a 

role in the evolution of the magmas. Hooper (1984a)

suggested, however, that the absence of physical and 

chemical evidence for crustal assimilation points more 

obviously to mixing of two mantle sources.

ECKLER MOUNTAIN MEMBER OF THE WANAPUM BASALT

A break in volcanic activity separates the eruptions of 

the Grande Ronde Basalt from those of the Wanapum 

Basalt across the whole Columbia Plateau (fig. 4.2; 

Beeson and others, 1985). During this break, sediment 

was deposited in many areas of the western plateau, 

giving rise to the Vantage Member of the Ellensburg 

Formation.

In the Blue Mountains area a residual soil developed, 

and sediment was trapped in local basins. The oldest 

flows of the Wanapum Basalt are of relatively small 

volume, diverse chemical compositions, and normal magnetic polarity and consist of the basalts of Robinette 

Mountain, Dodge, Shumaker Creek, and Lookingglass, 

which together form the Eckler Mountain Member 

(Swanson and others, 1979b, 1981). Despite their chemical diversity, these flows have similar isotopic signatures, in that the 87Sr/86Sr ratio increases with the Ba/Ti 

ratio (fig. 4.3A), a relation suggesting mixing between 

two sources. These sources are clearly different from 

those that fed the large eruptions of the main part of the 

Wanapum Basalt which followed. A comparison of erupted-magma volume with approximate K-Ar ages implies a 

substantial decrease in the rate of magma supply relative 

to the Grande Ronde (Swanson and Wright, 1981; Hooper, 1981, 1982). The areal extents of individual flows of 

the Eckler Mountain Member and of their feeder dikes 

are shown in figure 4.5. Major-element and trace-element 

analyses with isotopic data are listed in tables 4.1 and 4.2.

Five chemically distinguishable flows or sets of flows 

can be recognized in the Eckler Mountain Member 

(Swanson, Bentley, and others, 1979; Swanson, Wright, 

and others, 1979; Swanson and Wright, 1983). The oldest 

of these, the Robinette Mountain flow, is present in an 

area elongate north-northwest area flanking its feeder 

dike, which crosses the Blue Mountains uplift (fig. 4.5). It 

is a coarse-grained diktytaxitic basalt containing olivine 

phenocrysts that is chemically the most primitive flow in 

the Columbia River Basalt Group. Two or more Dodge 

flows (Swanson, Wright, and others, 1979) have been 

reported to lie above the Robinette Mountain flow (Ross, 

1978; Shubat, 1979; Wright and others, 1982). Southwest 

of Troy Basin, north of Elgin (fig. 4.5), the lowest of these 

flows is compositionally similar, but not identical, to the 

Robinette Mountain flow (figs. 4.6, 4.7; Shubat, 1979). 

This flow (Robinette Mountain II) is coarse grained and 

aphyric. Elsewhere, however, as on the northwest side of 

Troy Basin in the Wenaha Tucannon Wilderness (Swanson and Wright, 1983), as many as three plagioclase-
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 

118° 117°

75

Pullman, Q Moscow;

o

10 20 30 40 50 KILOMETERS

FIGURE 4.5. Blue Mountains region, showing distribution of flows of (Lg). Vertical line pattern, Shumaker Creek flow; SC, associated 

the Eckler Mountain Member (of the Wanapum Basalt). Diagonal dike. Limekiln fault, dashed where approximately located, bar and 

northeast-southwest line pattern, Robinette Mountain flow; RM, ball on downthrown side; arrow indicates dip of monoclinal limb at 

associated dike. Diagonal northwest-southeast line pattern, Dodge both ends of fault, 

flow; D, associated dikes. Shaded area (no dikes), Lookingglass flow
76 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

porphyritic Dodge flows are present in individual 

sections, with only the Robinette Mountain I flow at the 

base.

The undisputed Dodge has a much wider distribution 

than do the two Robinette Mountain flows discussed 

above (fig. 4.5). The Dodge is a coarse-grained basalt 

containing plagioclase and olivine phenocrysts and has a 

characteristically rounded, yellow- to brown-weathering, 

commonly grusy outcrop appearance that makes it an 

excellent marker unit. Two main feeder-dike systems 

were mapped by Ross (1978) and Swanson and Wright 

(1983), but other dikes of similar composition are found to 

the east (fig. 4.5). The wide distribution of Dodge feeder 

dikes is unusual. Another uncommon feature of the 

Dodge is the presence of a second, somewhat less 

porphyritic flow of otherwise identical texture and mineralogic and chemical composition and of indistinguishable age, far to the northeast in the St. Maries River 

drainage of northern Idaho (Swanson, Bentley, and 

others, 1979).

The Shumaker Creek and Lookingglass flows both lie 

directly above the Dodge and below flows of the main 

part of the Wanapum. Their relative ages are not known 

because of their geographic separation (fig. 4.5). A dike 

of Shumaker Creek composition is present in upper 

Joseph Creek (fig. 4.5). Both flows have relatively 

evolved chemical compositions and are aphyric, with 

chemical compositions, including P205/Ti02 ratios and 

incompatible-element concentrations, similar to but less 

extreme than those of the Umatilla Member (of the 

Saddle Mountains Basalt). Their Ba contents, only onethird that of the Umatilla, are greater than that of any

Saddle Mountains Basalt

Wanapum Basalt

Q.

C

«

Priest Rapids 

Member

Roza Member

Frenchman Springs 

Member

Eckler Mountain 

Member

Lolo flow 

Rosalia flow

? ? 

Onaway, Potlatch, and 

Feary Creek flows 

? ?

Two (identical) 

flows

Powatka flow

At least six flows 

(including Rondowa flow)

Shumaker Creek and Lookingglass 

flows 

Dodge flow 

Robinette Mountain II flow 

Robinette Mountain I flow

Grande Ronde Basalt

FIGURE 4.6. Stratigraphic succession of the Wanapum Basalt. Queried where relation uncertain.

other flow in the Columbia River Basalt Group and are 

equivalent in this respect to the basalt of Prineville.

The similar P205/Ti02 ratios of the more primitive 

flows of the Eckler Mountain Member (see fig. 4.7), 

together with the increase in such ratios as Ba/Ti coupled 

with an equivalent increase in the ratio 87Sr/86Sr (fig. 

4.3), suggest crystal fractionation accompanied by crustal 

assimilation in an open reservoir system (O'Hara and 

Mathews, 1981).

MAIN PART OF THE WANAPUM BASALT

Three voluminous multiple-flow members (Frenchman 

Springs, Roza, and Priest Rapids Members) and the 

single less extensive Powatka flow constitute the main

0.9

0.7

LU

o 

cc

LU 

Q.

HI

o 

o

0.5

0.3

Shumaker 

Creek flow

Lookingglass 

flow

EXPLANATION

 Dodge flow

o Robinette Mountain

land IIflows 

x Dikes

0.1 

0.5 1.5 2.5 

TiO2 CONTENT, IN WEIGHT PERCENT

FIGURE 4.7. Ti02 versus P205 content for flows of the Eckler 

Mountain Member (of the Wanapum Basalt). Encircled patterned 

areas enclose plots of the Shumaker Creek and Lookingglass flows. 

Also encircled are plots of Robinette Mountain flow I (lowest P205 

and Ti02 contents), Robinette Mountain flow II, and Dodge flow.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 77

part of the Wanapum Basalt. The three members and, to 

some degree, the individual flows within each member 

are best distinguished in the field by the different sizes 

and proportions of plagioclase and olivine phenocrysts 

and by their magnetic polarity (normal for the Frenchman Springs, transitional for the Roza, reverse for the 

Priest Rapids). Chemical abundances overlap and are 

difficult to use as discriminants within the Roza and 

Frenchman Springs (see Beeson and others, 1985), but 

they are distinctive for the Lolo and Rosalia flows of the 

Priest Rapids Member.

Six units of the Frenchman Springs Member, some 

containing several flows, have been recognized over

considerable distances on the north side of the Blue 

Mountains uplift (Beeson and others, 1985). All have 

normal polarity, but they show subtle chemical differences. Feeder dikes are found in a wide zone across the crest 

and north flank of the uplift (fig. 4.8), but they have not 

yet been correlated with individual flows. One Frenchman Springs flow is present near Rondowa (Shubat, 

1979) and locally in the Wenaha Tucannon Wilderness 

(Swanson and Wright, 1983), but other flows of this 

member appear to have flowed northward and westward, 

a pattern suggesting that the northeastward extension of 

the Blue Mountains uplift had become active by that 

time.

116°

r, . , Pullrrtan j o Moscow

46°-

O 10 20 30 40 50 KILOMETERS 

i II ill

FIGURE 4.8. Blue Mountains region, showing distribution of R, associated dikes and vents (X). Solid line withhachures, Priest 

flows of main part of the Wanapum Basalt. Broken line with Rapids flows; PR, associated dikes. Limekiln fault, dashed 

three dashes, southeast limit of Frenchman Springs (FSP) where approximately located, bar and ball on downthrown side; 

flows; F, associated dikes. Shaded area (no dikes), Powatka flow arrow indicates dip of monoclinal limb at both ends of fault. 

(PO). Broken line with single dash, southeast limit of Roza flow;
78 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

The Roza may be the best documented member on the 

Columbia Plateau (Shaw and Swanson, 1970; Swanson 

and others, 1975; Swanson, Wright, and others, 1979). 

Two chemically identical flows are commonly recognized, 

but more flows occur locally near the vent system. At 

least 20 vents and many dikes (fig. 4.8) define a linear 

vent system along the axis of the Chief Joseph dike 

swarm and extend from 30 km north of Enterprise, 

Oreg., to 10 km northwest of Winona, Wash. Calculations 

of the rate of discharge from this 150- by 5-km fissurevent system (Shaw and Swanson, 1970; Swanson and 

others, 1975) indicate that a flood of basaltic lava covered 

a large part of the Columbia Plateau in a matter of days. 

The flows produced by two major eruptions, each with a 

volume of about 700 km3 , are mineralogically and chemically indistinguishable; a typical chemical analysis of the 

Roza Member is listed in table 4.1.

The length of this and other fissure systems, and the 

homogeneity of such large volumes of magma, have 

profound implications for the magma plumbing system. 

The magma reservoir must have had a volume many, 

possibly 10, times that of the magma erupted. The 

reservoir for the Roza magma must therefore have been 

very large (approx. 14,000 km3) and must have been 

located at least as deep as the crust-mantle boundary, 

which is approximately 30 km (Hill, 1978) below the 

surface of the accreted terrane(s). The combination of 

magma homogeneity and high eruption rate make significant assimilation of crustal rock during passage between 

reservoir and surface unlikely. To the extent that crustal 

assimilation took place, it must have occurred around the 

top of the magma reservoir before eruption and must 

have been accompanied by very efficient magma mixing 

within the reservoir.

The outcrop area of the Roza Member (fig. 4.8) reflects 

the continued rise of the southeast quadrant of the 

Columbia Plateau relative to Pasco Basin area and the 

initiation of significant uplift of the north end of the Blue 

Mountains.

The Priest Rapids Member includes two compositionally and stratigraphically distinct sets of flows, the 

Rosalia and the younger Lolo (fig. 4.6). Both sets appear 

to have been erupted across the northeast corner of the 

Blue Mountains province (fig. 4.8). Feeder dikes to the 

Priest Rapids flows are present near Orofino along the 

eastern margin of the Columbia Plateau. The Priest 

Rapids fissure system, like that of the Roza, must have 

extended north-northwestward into the craton (fig. 4.1; 

Camp, 1981). The rising south and east sides of the 

Columbia Plateau and the northeast end of the Blue 

Mountains uplift confined the flows to areas north of the 

Blue Mountains province once they had escaped from the 

embayment of the Clearwater River drainage (fig. 4.8).

The Powatkaflow (Ross, 1978; Hooper, 1981; Swanson 

and others, 1981) poses a problem in classification. In 

field appearance (medium to fine grained aphyrie) and 

magnetic polarity it resembles the upper (N2) flows of the 

Grande Ronde Basalt. Its exceptionally high P205 content, however, makes its composition distinctive (table 

4.1). It overlies the Rondowa flow north of Elgin (fig. 

4.8), which has a Frenchman Springs composition (Swanson and others, 1981), and so its stratigraphic position 

appears to be within the main part of the Wanapum 

Basalt. Its isotopic signature, however, more closely 

resembles those of the Grande Ronde Basalt and Eckler 

Mountain Member than that of the main part of the 

Wanapum Basalt (fig. 4.3). The Powatka flow was the 

first to be confined by the topography of Troy Basin (fig. 

4.8).

The similarity in chemical and isotopic composition of 

the three voluminous members of the Wanapum Basalt is 

noteworthy. The Frenchman Springs was erupted 

through the accreted oceanic terrane of the Blue Mountains province or through the apparently thinner crust 

northwest of the KBML (fig. 4.1). Half of the Roza 

fissure-vent system lies within the Blue Mountains province, and half within the old craton farther north. The 

Priest Rapids also appears to have been erupted through 

both cratonic and accreted oceanic rocks (fig. 4.8).

The crustal rocks of the craton and of the accreted 

terranes have diverse chemical and isotopic compositions. These differences are known to have profoundly 

affected the composition and isotopic signature of the 

granitic rocks that intruded or were generated within 

them (Armstrong and others, 1977; Fleck and Criss, 

1985). The flows of the main part of the Wanapum show 

no variation in the ratio 87Sr/86Sr comparable to that in 

the different types of crustal rocks through which they 

passed. Nor do they show significant variation in such 

trace-element ratios as Ba/Ti (except for the anomalous 

Powatka flow) that might be expected from varying 

degrees of crustal contamination. Again, we must conclude that little crustal assimilation occurred between 

magma reservoir and surface and that a mantle source of 

varying composition, created either by subduction of 

sediment and ocean crust or by metasomatic processes 

in the mantle, would better explain the rather high 

87gr/86gr ratios Of these flows than does assimilation of 

crust.

SADDLE MOUNTAINS BASALT

Flows of the Saddle Mountains Basalt (figs. 4.9, 4.10) 

are the youngest on the main part of the Columbia 

Plateau (Swanson, Wright, and others, 1979). They rest 

unconformably on the Wanapum Basalt and on each 

other. They probably represent less than 1 percent of the
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 79

total volume of the Columbia River Basalt Group but 

were erupted over an extended period (approx. 13.5-6.0 

Ma) of waning volcanic activity. Like those in the Eckler 

Mountain Member, flows in the Saddle Mountains Basalt 

vary as widely in chemical composition as the entire 

Columbia River Basalt Group. They are distinguished

119

isotopically from all other flows on the Columbia Plateau 

by their consistently large, if varying, ^Sr/^Sr ratio (fig. 

4.3; table 4.2), which is apparently unrelated to their age 

or their chemical composition (Hooper, 1984a).

The unconformities between most flows in the Saddle 

Mountains Basalt result directly from the long periods

116°

EXPLANATION

 Umatilla flow 

Vent 

Dike

Wilbur Creek flow 

Asotin flow

- Weissenfels Ridge Member

Dike 

P Pomona flow (includes basalt of Weippe

of Swanson and others, 1981)

Dike 

Elephant Mountain flow

Dike

Eden flow 

Dike

Limekiln fault Dashed where approximately 

located; bar and ball on downthrown side. 

Arrow indicates dip of monoclinal limb at 

both ends of fault

OREGON ,' IDAHO

\

AREA OF MAP

FIGURE 4.9. Blue Mountains region, showing distribution of larger flows of the Saddle Mountains Basalt.
80 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

between eruptions. The cumulative evidence points to 

the Columbia Plateau deforming continuously throughout Columbia River Basalt Group time (Hooper and 

Camp, 1981; Reidel, 1984). During Imnaha, Grande 

Ronde, and, to a lesser extent, Wanapum time (17.5-13.5 

Ma), major eruptions occurred much more frequently, 

averaging one every 10,000 to 20,000 years (Swanson and 

others, 1975; Hooper and others, 1984), than during later 

Saddle Mountains time. This frequency was great enough 

to allow little erosion or deformation between eruptions; 

only such cumulative changes as regional tilting (Hooper 

and Camp, 1981) and relatively sudden movement along 

particular faults (such as along the Limekiln fault during 

the interval between units R2 and N2 of the Grande 

Ronde Basalt) can be documented. In contrast, during 

Saddle Mountains time some successive flows differ in 

age by 1 Ma or more, and so evidence of deformation and 

erosion between eruptions becomes obvious. Individual

flows in the Saddle Mountains Basalt fill and clearly 

define developing structural basins and drainage systems. Detailed mapping of these flows provides a fascinating view of the structural and morphologic evolution 

of the Columbia Plateau between about 13.5 and 6.0 Ma. 

In addition to the large Pasco Basin that was developing throughout the eruptions of the Columbia River 

Basalt Group in east-central Washington, four smaller 

structural basins containing flows of the Saddle Mountains Basalt developed within the Blue Mountains province. Two of these, Stites Basin (Camp, 1981) and Riggins 

Basin (Hooper, 1984b), lie along the ancient but still 

active suture zone forming the eastern margin of the Blue 

Mountains province. Throughout Columbia River Basalt 

Group time, this broad zone formed a downwarp, elongate north-south, that was locally filled with the Imnaha 

and lower part (unit Rj) of the Grande Ronde Basalt until 

the whole region was uplifted enough to prevent younger

47°

119° 118° 117° 116°

AAV

WASHINGTON

46°

Umutijja ^ilve'.--Av:idk=tor- ^

 1 '*' * < M ^~-'

\/$&iggins^<

V

ORF^ON

/^ ) ' ~N \ WR

f \

X

\ WR ;

10 20 30 40 50 KILOMETERS

FIGURE 4.10. Blue Mountains region, showing distribution of 

smaller flows of the Saddle Mountains Basalt: B, Buford (with 

possible dikes); CG, Craigmont; FC, Feary Creek (with dike); 

GV, Grangeville (with dike); IF, Icicle Flat (with possible dike); 

IH, Ice Harbor (with dikes); LM, Lower Monumental; PL, 

Potlatch; SC, Swamp Creek; WR, Windy Ridge (with dikes). 

Feary Creek and Potlatch flows may be equivalent in age to either

the Wanapum or Saddle Mountains Basalt (Camp, 1981). A , 

isolated intracanyon outcrops of Esquatzel flow. *, isolated 

intracanyon outcrops of an unnamed flow. X, isolated outcrops of 

Tammany Creek intracanyon flow. Limekiln fault: dashed where 

approximately located, bar and ball on downthrown side; arrow 

indicates dip of monoclinal limb at both ends of fault.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 81

flows entering from the west. The Wanapum and Saddle 

Mountains Basalts occur in these two basins only where 

feeder dikes intersected them (figs. 4.8-4.10; Camp and 

others, 1984).

The other two basins, Troy and Lewiston Basins, 

developed northwest of the Limekiln fault (fig. 4.9). 

These basins, synclines flanking the rising easterly 

extension of the Blue Mountains uplift, first became 

apparent during Wanapum time.

LEWISTON BASIN

No evidence exists of deformation in the Lewiston area 

from Imnaha through Grande Ronde (unit R2) time. 

Flows of these formations, which are now folded and 

faulted, appear to retain their thickness across the area, 

and sedimentary interbeds are absent. Flows of unit N2 

of the Grande Ronde Basalt pinch out on the west edge of 

Lewiston Basin (Swanson and others, 1980) against a 

northwesterly slope formed by drag along the recently 

developed Limekiln fault (fig. 4.4). The Dodge and 

Shumaker Creek flows (of the Eckler Mountain Member), which have local feeder systems, behave similarly 

and indicate no deformation other than the surface rising 

toward the Limekiln fault (fig. 4.5).

Flows of the Frenchman Springs Member, erupted 

from fissures farther west, also pinch out against the 

northwest slope before reaching the Lewiston area (fig. 

4.8). The Roza Member vented along the western margin 

of the present Lewiston Basin yet never reached the 

center of the basin, although it flowed westward for 

hundreds of kilometers. This behavior implies a northwesterly sloping surface between the vent system and 

the Limekiln fault (fig. 4.8). However, there is still no 

evidence of a Lewiston Basin at that time.

The first evidence of such a basin, albeit a very shallow 

one, comes from the Lolo flow of the Priest Rapids 

Member (fig. 4.8). The base of this flow is a pillowpalagonite complex over a wide area in the center of the 

present Lewiston Basin indicating that a shallow lake 

was present when the flow entered the area from the east 

(Camp, 1981). However, ponding of the drainage could 

also have resulted from construction of the Roza vent 

system to form a low topographic barrier across the 

southeast-to-northwest regional slope; thus, the ponding 

does not provide unambiguous evidence of deformation.

Younger flows and interbeds, however, indicate the 

presence of a syncline. The next flow (Umatilla Member 

of the Saddle Mountains Basalt) invaded a thick sedimentary deposit confined to the basin. Subsequent flows of 

the Saddle Mountains Basalt, from the Wilbur Creek 

Member to the four flows of the Weissenfels Ridge 

Member (figs. 4.9, 4.11), either thicken in or are entirely 

limited to Lewiston Basin (Swanson and others, 1980;

Hooper and others, 1985), and many flows are separated 

by thick interbeds. These features could only result from 

contemporaneous subsidence, and they define an eastwest synclinal basin approximately 40 km long.

Two major events modified this slowly developing 

syncline after the eruption of the Weissenfels Ridge 

Member. The Pomona Member (12 Ma; McKee and 

others, 1977) partly filled a deep canyon above and below 

Lewiston similar in size and shape to the current Snake 

River canyon. This deep canyon contrasts with the 

earlier, shallow drainage system that led northward and 

northwestward from the basin and was filled by flows of 

the Wilbur Creek and Asotin Members (of the Saddle 

Mountains Basalt). Clearly, a striking change in the 

drainage pattern occurred at this time.

The most probable explanation for this change in 

drainage pattern is the capture by the poorly developed 

Lewiston Basin drainage of the much larger ancestral 

Salmon River, which had previously flowed westward up 

the present Grande Ronde valley to cross the Blue 

Mountains uplift east of Walla Walla. This old channel is 

now defined by the presence of gravel below the Umatilla 

Member (fig. 4.9; Swanson and Wright, 1983), which 

apparently dammed the westerly flow to form a deep lake 

that eventually spilled northward over a low ridge into 

Lewiston Basin. Once the overflow began, the northflowing stream would have rapidly cut down along the 

Limekiln fault, and the whole ancestral Salmon River 

system (which probably included the ancestral Clearwater River) was diverted through Lewiston Basin in a 

short period of time. The increased volume of water 

rapidly cut deep canyons along the lower Snake River, 

probably guided by an older, shallower drainage pattern. 

The new canyons provided channels for the Pomona and 

Elephant Mountain Members, and many subsequent 

intracanyon flows of the Saddle Mountains Basalt (figs. 

4.9, 4.10).

The second major change in the form of Lewiston Basin 

occurred when the northern limb of the gentle syncline 

broke and was thrust northward and upward to fashion 

the Lewiston structure (Camp, 1976; Camp and Hooper, 

1981; Hooper and others, 1985). This complex brittle 

structure includes a broken east-west anticlinal block 

thrust upward along steep reverse faults. The break 

probably occurred when steady north-northwest/southsoutheast compression, the cause of the original syncline, 

produced a limb angle so steep that further compression 

led to brittle failure.

The result was a much deeper Lewiston Basin. The 

main faulting displaces the Elephant Mountain Member 

(10.5 Ma). Both the Tammany Creek flow and the Lower 

Monumental Member (6.0 Ma) of the Saddle Mountains 

Basalt were apparently erupted into a broad basin similar 

in shape to the present basin, and the main development
82 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

of the Lewiston structure probably preceded their eruption, that is, between 10.5 and 6.0 Ma. Reverse faulting 

associated with the structure continued, however, and 

has offset Pleistocene gravel (Kuhns, 1980; Webster and 

others, 1984).

TROY BASIN

Like Lewiston Basin, Troy Basin developed on the 

downthrown side of the Limekiln fault and its southwesterly monoclinal extension (figs. 4.8,4.9). Unit N2 flows of 

the Grande Ronde pinch out against the northwest slope 

of these structures, as does the Dodge flow, the most 

extensive flow of the Eckler Mountain Member (figs. 4.4, 

4.5). Of the other flows in the Eckler Mountain, the 

oldest the Robinette Mountain flow was erupted west 

of the basin, and its easterly limit apparently reflects its 

limited volume. The younger Shumaker Creek and Lookingglass flows were erupted on the slope and flowed 

northwestward, with no obvious restriction in that direction except that imposed by their own limited volumes 

(fig. 4.5).

In the main part of the Wanapum Basalt, flows of the 

Frenchman Springs Member crossed the Blue Mountains 

uplift east of Walla Walla, but too little is known of the 

areas covered by individual flows to know whether any 

uplift had earlier resulted in a basin between it and the 

Limekiln monocline to the southeast. The Roza flowed 

only northwestward from its vent system (fig. 4.8), 

suggesting that significant movement on the northeasterly end of the Blue Mountains uplift had occurred by 

that time. More clearly, the Lolo flow (of the Priest 

Rapids Member; see fig. 4.2) was held north of high 

ground developed along the Blue Mountains anticlinal 

uplift. The Powatka flow, of Wanapum age, was the first 

flow clearly confined to Troy Basin (fig. 4.8).

The two main flows of the Umatilla Member were 

erupted near the divide between Lewiston and Troy 

Basins, entering both basins and invading basin sediment 

(fig. 4.9; Ross, 1978; Hooper and others, 1985). Their 

greatest volume, however, filled still-shallow Troy Basin 

and advanced westward across the now-rising Blue 

Mountains uplift, probably following the course of the

Lower Snake 

River Canyon

Lower Monumental Mbr.

Harbor IceMember

Based of 

Goose Island 

Basalt of 

Martindale 

Basalt of 

Basin City

Elephant Mountain Membei

Pomona Member

Unnemed flow ^_^_

Esquatzel Member

Priest Rapids Member

Lewiston 

Basin

Lower Monumental Member

? ? 7 .7

Tammany Creek flow

___ 7 ___ 7 _7_ _ 7 ___ .

Buford Member

Elephant Mountain Member

Pomona Member

Unnamed flow (Tic)

Sedimentary rocks

-jjj .2 Slippery Creek flow 

*c 1 Tenmile Creek flow 

g s Lewiston Orcherds flow 

;« J» Cloverland flow 

£

Sedimentary rocks

Asotin Member

Lapwai flow

Wilbur Creek Member

:| J3 Sillusi flow

:§ 5 Umatilla flow

Sedimentary rocks

Priest Rapids Member

Troy Basin

 7 7 7 7 

Buford Member

Elephant Mountain Member

Sedimentary rocks

Eden flow

Sedimentary rocks

Sillusi flow

Umatilla flow

Sedimentary rocks

Powatka flow

Cleerwater and 

Stites Basins

Swamp Creek 

flow

Pomona Member 

(Weippe flow )

Asotin Member

Lepwei flow

Wilbur Creek Member

Priest Rapids Member

Wanapum Basalt

West -central Idaho

Craigmont flow

Grangeville flow

Pomona Member 

(Weippe flow )

 7 ? ? 

Grangeville flow

7. 77 7

Windy Ridge 

flow

Craigmont flow

Icicle Flats 

flow

___ 7 _ _ 7 _ _

Onaway, Potlatch, and Feary

Creek flows

FIGURE 4.11. Stratigraphic succession of the Saddle Mountains Basalt. Relative ages of the Buford, Tammany Creek, and Lower Monumental 

flows are not clearly established. Relative ages of an unnamed intracanyon flow (unit Tic of Swanson and others, 1980) and the Esquatzel flow 

are not known; both are intracanyon and thus post-Weissenfels Ridge Member in age. Boundaries dashed and queried where uncertain.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 83

ancestral Salmon and Grande Ronde Rivers (Swanson 

and Wright, 1983; Hooper and Swanson, 1987). These 

flows may have blocked the drainage for an extended 

period because thick epiclastic sedimentary rocks containing significant lignite beds overlie the Umatilla 

throughout Troy Basin (Ross, 1978; Stoffel, 1984). The 

next younger flow the basalt of Eden (Ross, 1978) was 

erupted from a large dike within the basin and invaded 

the loose sediment.

The dike that supplied the eruption of the Elephant 

Mountain Member also cuts across the center of Troy 

Basin (fig. 4.9). By the time of that eruption, the blocked 

drainage had apparently found an outlet northward 

across the low divide into Lewiston Basin and had cut 

rapidly down the Limekiln fault zone to form the deep 

lower Snake River canyon. The Elephant Mountain filled 

Troy Basin (already almost full of sediment), spilled over 

its northern divide into Lewiston Basin (Swanson and 

others, 1980), and then moved down the new canyon to 

reach beyond Pasco Basin. The younger Buford Member 

of the Saddle Mountains Basalt (fig. 4.10) provides no 

evidence of a basin in the Troy area, and so the present 

topographic basin must have developed since that time.

COMPOSITION OF THE SADDLE MOUNTAINS BASALT IN 

LEWISTON AND TROY BASINS

The flows of the Saddle Mountains Basalt present in 

Lewiston and Troy Basins are shown in figure 4.11. A 

thin ash-flow tuff above the Powatka flow in the southwestern part of Troy Basin is overlain by the Cricket Flat 

flow. This is one of several olivine-rich basalt flows 

mapped southward into the La Grande-Baker graben by 

W.H. Taubeneck (oral commun., 1980; Wright and others, 1980) and farther south to Baker (Swanson and 

others, 1981). Here these flows are regarded as belonging to the basalt of Powder River (Hooper and Swanson, 

1987).

The Umatilla Member (Swanson, Bentley, and others, 

1979) consists of two flows, the Umatilla flow and the 

overlying Sillusi flow, both of which are very fine grained 

and characterized by exceptionally high LIL-element 

contents (for example, more than 3,000 ppm Ba; fig. 4.3; 

table 4.1). Although their ^Sr/^Sr ratio (0.709) is average for the Saddle Mountains Basalt, their large Ba/Ti 

ratio and the presence of silicic xenoliths (S.P. Reidel, 

oral commun., 1985) are good evidence of upper-crustal 

contamination.

The Eden flow (Ross, 1978; Swanson and others, 1981) 

in Troy Basin is a conspicuous black glassy basalt 

containing clear-green olivine phenocrysts. It was fed by 

a single big dike (fig. 4.9). The unusually large P205 

content of the Eden flow (1.2-1.4 weight percent; table 

4.1) is matched only by the basalt of Prineville (Uppuluri,

1974). Samples from a small outcrop directly above the 

dike have even higher P205 and Ti02 contents and 

probably represent a more evolved low-volume variant of 

the same magma near its vent.

In Lewiston Basin, the Umatilla Member is overlain by 

the Wilbur Creek Member, Lapwai flow, and Asotin 

Member. The Wilbur Creek is a dense, aphyric, commonly glassy flow with the highest recorded 8180 value for 

the Columbia Plateau (table 4.2; Carlson, 1984). Like the 

Umatilla Member below, the Wilbur Creek typically 

invades the thick sediment of the basin. The Asotin is a 

dense, fine-grained flow containing abundant small, fresh 

olivine phenocrysts; it is one of the more primitive flows 

on the plateau. These two flows have similar distributions (fig. 4.9; Hooper, 1985), forming thick intracanyon 

flows in the North Fork of the Clearwater River, filling 

Lewiston Basin, and following a shallow ancestral drainage northward from the basin down the present Union 

Flat-Palouse River drainage (fig. 4.9) and thence westward to Pasco Basin north of the Saddle Mountains 

(Swanson, Bentley, and others, 1979; Swanson and 

others, 1980; Anderson and others, 1987). Locally, the 

Lapwai flow lies between the Wilbur Creek and Asotin. 

This flow is intermediate in composition and appears to 

be the result of simple mixing of the magmas forming the 

two larger flows (Hooper, 1985).

The Elephant Mountain Member, present in both Troy 

and Lewiston Basins (Swanson and others, 1980), has one 

of the largest Ti02 contents in the Columbia River Basalt 

Group, but its other chemical and isotopic characteristics 

resemble those of many flows of the Saddle Mountains 

Basalt (fig. 4.3; tables 4.1, 4.2).

In Lewiston Basin the Elephant Mountain is underlain 

by the intracanyon Pomona Member. The Pomona traveled farther than any other known flow, from its feeder 

dike on the east edge of the Clearwater River embayment, Idaho (fig. 4.9), to the Pacific Ocean, 600 km to the 

west (Beeson and others, 1979; Swanson, Wright, and 

others, 1979; Camp, 1981; Hooper, 1982; Magill and 

others, 1982). Despite its rather primitive composition, 

the Pomona flow contains small phenocrysts of olivine, 

plagioclase, and clinopyroxene.

OTHER FLOWS OF THE SADDLE MOUNTAINS BASALT

Numerous other small flows of Saddle Mountains age 

have been recorded on the Columbia Plateau (fig. 4.10). 

Most of these flows are of limited areal extent, and so 

stratigraphic relations are imprecisely known (fig. 4.11). 

Descriptions of these flows may be found in Swanson, 

Wright, and others (1979), Camp (1981), Swanson and 

others (1981), and Hooper and others (1985). The flows of 

the Ice Harbor Member (Swanson and others, 1975) are 

particularly significant because they were fed by a
84 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

fissure-vent system oriented north-northwest close to 

Pasco Basin, apparently entirely outside the Blue Mountains oceanic-crustal terranes. Their wide chemical diversity, in combination with restricted 87Sr/86Sr values 

similar to those of many other flows in the Saddle 

Mountains Basalt (fig. 4.3), make them of special interest 

in discussion of possible crustal contamination.

Most other local flows of the Saddle Mountains Basalt 

are found either in west-central Idaho or in the Snake 

River canyon as intracanyon flows (figs. 4.10, 4.11).

The Buford Member (Walker, 1973) overlies the Elephant Mountain flow on the divide between Lewiston and 

Troy Basins. Erupted locally, the Buford is of limited 

volume and has a texture, mineralogy, and majorelement composition indistinguishable from those of the 

Grande Ronde Basalt, though with a different proportion 

of Sr and a different ^Sr/^Sr ratio (fig. 4.3).

The Lower Monumental Member, probably the youngest flow in the Columbia River Basalt Group (6.0 Ma; 

McKee and others, 1977), is entirely confined to the 

Snake River canyon. Another nearby flow, the Tammany 

Creek flow (Hooper and others, 1985), is of similar age 

but contains augite in addition to olivine phenocrysts and 

is of more local distribution. Compositions of these flows 

are listed in tables 4.1 and 4.2 and shown in figures 4.12 

to 4.16.

A dike, a plug, and a small capping flow in the center 

of a small exposure west of Riggins, Idaho, have a 

composition similar to that of Lewiston Orchards flow (of 

the Weissenfels Ridge Member) in Lewiston Basin (fig. 

4.10; Hooper, 1984b). Small outcrops of a basalt of similar 

composition are present above stream sediment near the 

towns of Colton (between Pullman and Lewiston) and 

Moscow, 30 mi north of Lewiston (fig. 4.10). The basalt of 

Sprague Lake (Swanson, Wright, and others, 1979; 

Wright and others, 1980), found in a small area southwest 

of Spokane (fig. 4.1), has a similar composition (table 4.3). 

All of these outcrops lie along a north-northwest line and 

may represent small eruptions from a single fissure 

system almost 300 km long.

ORIGIN OF THE SADDLE MOUNTAINS BASALT

The Saddle Mountains Basalt displays as much mineralogic and chemical diversity as the entire Columbia 

River Basalt Group. Some flows are aphyric, and others 

are porphyritic, containing phenocrysts of plagioclase, 

olivine, and, more rarely, pyroxene. No correlation 

exists between either abundance or type of phenocryst 

phase and bulk chemical composition. For example, the 

presence of pyroxene as a third phenocryst phase does 

not correlate with a more evolved chemical composition 

as. reflected in the abundance of incompatible trace 

elements; the Pomona Member, containing plagioclase,

olivine, and clinopyroxene microphenocrysts, has smaller 

abundances of incompatible trace elements than do most 

other flows (fig. 4.12). Simple crystal fractionation and 

varying degrees of partial melting are therefore inadequate in themselves to explain the variations between 

flows in the Saddle Mountains Basalt.

The high but varying values of the ratio 87Sr/86Sr (and 

corresponding small 143Nd/144Nd ratios) of these flows 

suggest crustal contamination, although their 6180 values are only moderate (table 4.2). Multiple sources are 

certainly required (Carlson, 1984), and contamination of 

a mantle-derived magma by one or more crustal components may have occurred. This hypothesis is the more 

attractive considering the small volume of most of the 

flows and the long intervals between many eruptions, 

which could imply long residence times within the crust.

However, strong arguments remain against crustal 

contamination as an essential process in the evolution of 

the Saddle Mountains Basalt. The 5180 value is relatively 

small in most flows and does not correlate well with the 

ratio 87Sr/86Sr (Carlson and others, 1981). We have 

already remarked on the evident absence of correlation 

between the isotopic signature of a flow and the composition of the crust penetrated by its feeder system 

(Hooper, 1984a). More critical is the fact that all the Sr 

isotopic data available for rocks from the accreted terFIGURE 4.12. Plots of P205 versus Ti02 contents for the Columbia 

River Basalt Group and associated volcanic rocks. A, Flows of the 

Columbia River Basalt Group and basalt of Powder River and 

associated volcanic rocks. Flows of the Columbia River Basalt Group: 

AB, Imnaha (AB subgroup); GR, Grande Ronde field; PG, Picture 

Gorge field; RC, Imnaha (RC subgroup); Ta, Asotin; Tb, Buford; Ted, 

Dodge; Tek, Lookingglass; Tern, Elephant Mountain; Ter, Robinette 

Mountain; Tes, Shumaker Creek; Tfc, Feary Creek; Tl, Lower 

Monumental; Tp, Pomona; Tpo, Powatka; Tpot, Potlatch; Tprl, Lolo; 

Tprr, Rosalia; Tr, Roza; Tsc, Swamp Creek; Tus, Sillusi; Tuu, 

Umatilla; Tw, Wilbur Creek; Twl, Lewiston Orchards; Tws, Slippery 

Creek. Modified from Hooper (1982). Basalt of Powder River and 

associated volcanic rocks (less than 55 weight percent Si02): ®, 

Cricket Flat area; ®, olivine basalt in La Grande Basin (W.H. 

Taubeneck, in Wright and others, 1980); O, diktytaxitic olivine 

basalt from La Grande Basin (W.H. Taubeneck, in Wright and 

others, 1980); x, east side of Baker Valley; A, Medicine Springs area; 

+, west and south sides of La Grande Basin (Barrash and others, 

1980);^, nepheline basalt from La Grande Basin (Wright and others, 

1980). B, Basalt and basaltic andesite of the basalt of Weiser 

(equivalent to the Weiser Basalt of Fitzgerald, 1984). O, Sugarloaf 

member;X, Star Butte member; A, Cambridge memberjajc, basalt of 

Cuddy Mountain (alkali basalt) member (informal members as 

designated by Fitzgerald, 1984). C, Andesites (more than 55 weight 

percent Si02) of the basalt of Powder River and associated volcanic 

rocks (x) relative to andesitic rocks of the Strawberry Volcanics ( ) 

and the Slide Creek (basaltic) member (®) of the Strawberry 

Volcanics (Robyn, 1977).
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 85

ranes (Armstrong and others, 1977; Fleck and Criss, 

1985) indicate ^Sr/^Sr ratios lower than those of the 

Saddle Mountains Basalt. These are the crustal rocks 

through which almost all of the Saddle Mountains Basalt 

magmas passed on their way to the surface. Clearly,

contamination by such crustal material could not have 

been responsible for the high isotopic ratios (more than 

0.706) observed in these flows. This observation appears 

to leave no alternative but to ascribe the high 

ratios of these basalts to an enriched mantle source.

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

UJ

O

UJ 

Q.

UJ

z 

o 

o

1.0 2.0 3.0 4.0

T\O, CONTENT, IN WEIGHT PERCENT
86 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

PICTURE GORGE BASALT

The Picture Gorge Basalt (Waters, 1961; Swanson, 

Wright, and others, 1979) is confined almost entirely to

John Day Basin south of the Blue Mountains uplift (fig. 

4.1). Flows of the Picture Gorge cross low saddles in the 

uplift and locally interfinger with the Grande Ronde 

Basalt. They cover an area of more than 1,200 km2

UJ

O

UJ 

O.

»-

CD

UJ

UJ

O 

O

TiO2 CONTENT, IN WEIGHT PERCENT

FIGURE 4.12. Continued
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 87

(Bailey, 1985) and were fed by the local Monument dike 

swarm (Fruchter and Baldwin, 1975). Some stratigraphic 

sections are as thick as 800 m (Swanson, Bentley, and 

others, 1979). Many flows near the base and top of the 

formation contain plagioclase phenocrysts. Porphyritic 

flows in the northwest corner of John Day Basin are 

capped by the Grande Ronde Basalt. This relation led 

Waters (1961) to correlate the Picture Gorge with the 

equally porphyritic and chemically similar Imnaha Basalt. Watkins and Baksi (1974) showed that most flows in 

the Picture Gorge had normal magnetic polarity and that 

only a short sequence of flows at the top were reversed. 

They regarded this as confirmation of Waters' correlation.

More recent work has questioned this correlation. 

Nathan and Fruchter (1974) and McDougall (1976) ar0.9

0.8

0.7

0.6

0.5

UJ

o

UJ

ILU

8 0.4

0.3

0.2

0.1

©

© 

© x

1.0 

TiO9> CONTENT, IN WEIGHT PERCENT

FIGURE 4.12. Continued

2.0

gued that the Imnaha and Picture Gorge Basalts have 

distinguishable compositions. Swanson and others (1981) 

traced the Grande Ronde magnetostratigraphic units 

from east to west and showed that the upper, magnetically reversed flows in the Picture Gorge interfinger with 

unit R2 flows in the Grande Ronde. This relation implies 

that the underlying normal flows of Picture Gorge are 

time equivalents of units Nx of the Grande Ronde Basalt 

and not of unit N0 of the Imnaha Basalt (fig. 4.2). More 

detailed comparisons of the Picture Gorge and Imnaha 

Basalts (Osawa and Goles, 1970; Bailey, 1986) make clear 

that the Picture Gorge is more primitive than the Imnaha 

Basalt and can be clearly separated by some trace 

elements, such as Sc versus Zr (fig. 4.17). The most 

primitive flow in the Imnaha (AB-2; see Hooper and 

others, 1984) comes closest to the Picture Gorge in 

composition but is distinguished in a plot of Sc versus Zr 

content (fig. 4.17); flow AB-2 is coarsely porphyritic, but 

the flows of the Picture Gorge Basalt with the most 

nearly equivalent chemical compositions are aphyric 

(M.M. Bailey, oral commun., 1985).

A further distinguishing feature of the Picture Gorge 

Basalt relative to all other units in the Columbia River 

Basalt Group is the rare-earth-element (REE)-abundance pattern (fig. 4.18). The Picture Gorge Basalt has a 

consistently flatter pattern, less fractionated between 

light REE's and heavy REE's, that tends to cross those 

of the more primitive flows of the main series of the 

Columbia River Basalt Group. This REE pattern implies 

a mantle source for the Picture Gorge that was more 

depleted than that for those flows supplied by the Chief 

Joseph dike swarm of the main series.

Bentley and Cockerham (1973) divided the Picture 

Gorge into three stratigraphic units: a coarsely porphyritic lower unit (basalt of Twickenham of Swanson, 

Wright, and others, 1979), an aphyric middle unit (basalt 

of Monument Mountain of Swanson, Wright, and others, 

1979), and a porphyritic upper unit (basalt of Dayville of 

Swanson, Wright, and others, 1979). Using major- and 

trace-element analyses and magnetic polarity, Bailey 

(1985,1986) further subdivided the formation into 15 map 

units, each composed of 1 to as many as 9 separate flows. 

The Twickenham flows are very thick, fill deep canyons, 

and have restricted area! extent. They contain more than 

6.0 weight percent MgO, are magnetically normal, and 

may include very coarse grained segregation veins (Lindsley and others, 1971). The Monument Mountain flows 

are thinner but cover a larger area than the Twickenham 

flows; they also contain more than 6.0 weight percent 

MgO and have normal magnetic polarity, but they are 

aphyric. The Dayville flows form a much more diverse 

group of porphyritic flows; their magnetic polarity is 

normal at the base and reversed at the top of the unit, and 

six of the eight subunits contain less than 6.0 weight
88 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

percent MgO. The chemical variation within the Picture 

Gorge is controlled, at least in part, by gabbroic 

(plagioclase+olivine+clinopyroxene) fractionation (Bailey, 1985, 1986; Goles, 1986).

BASALT OF PRINEVILLE

Uppuluri (1974) described 13 flows of a distinct chemical composition in north-central Oregon (fig. 4.1). 

Nathan and Fruchter (1974) found flows of this composition interfingered with unit N2 flows of the Grande Ronde 

Basalt on the north side of John Day Basin (fig. 4.1), and 

other similar flows have been reported in the western 

Cascade Range (Anderson, 1978; Swanson, Wright, and 

others, 1979).

No feeder dikes have been reported, but the restricted 

extent of these flows requires several local sources 

(Smith and Gushing, 1985), whether of fissure or central 

type, far west of any known Columbia River Basalt 

Group feeder dike. On almost any plot of elemental 

compositions (see figs. 4.13, 4.14), these flows show

900

800

700

oc

LU

o.E<

600

500

400

300

200

100

B 

°+.

EXPLANATION '

 Imnaha Basalt Squaw Butte type (Rg flows) 

D Imnaha Basalt AB and RC types (Ng flows)

 Grande Ronde Basalt

+ Eckler Mountain Member (Wanapum Basalt)

° Wanapum Basalt (main part)

x Saddle Mountains Basalt

o Picture Gorge Basalt

0 Basalt of Prineville

B Basalt of Powder River and associated

volcanic rocks 

V Basalt of Weiser and associated volcanic

rocks

S Slide Creek Member (Strawberry Volcanics) 

PO Powatka flow

«PO

500 1500 2500 

Ba CONTENT, IN PARTS PER MILLION

3500

FIGURE 4.13. Sr versus Ba contents for the Columbia River Basalt 

Group and associated volcanic rocks. Analyses in table 4.1 are 

plotted with analyses of additional samples from the Grande Ronde 

Basalt (X-ray-fluorescence analyses from Reidel, 1983) and analyses of samples from the Slide Creek Member of the Strawberry 

Volcanics (X-ray-fluorescence and instrumental neutron-activation 

analyses from Robyn, 1977). Magnetic polarities: N, normal; R, 

reversed. Polarity intervals numbered from oldest to youngest 

where sequential.

remarkably little variation and possess a distinctive 

combination of high Ba, Sr, and P contents and moderate 

Ti02 and Zr contents. Although these flows are clearly 

contemporaneous with the upper part of the Grande 

Ronde Basalt, their relative geographic isolation, the 

absence of evidence for a fissure eruptive mechanism, 

and their distinctive composition all indicate an origin for 

the Prineville-type flows different from that of the 

Columbia River Basalt Group. New chemical analyses for 

Prineville flows are listed in table 4.1.

BASALTS OF POWDER RIVER AND WEISER 

AND ASSOCIATED VOLCANIC ROCKS

POWDER RIVER

Diverse, relatively local flows in the La Grande-Baker 

Valley of northeastern Oregon (fig. 4.1) are considered 

together under this heading. The older flows lie unconformably on units R! and Nl of the Grande Ronde Basalt 

and consist of olivine-rich basalt. Overlying these flows

500

400

300

200

o 

o

100

.- / *

500 1500 2500 

Ba CONTENT, IN PARTS PER MILLION

3500

FIGURE 4.14. Zr versus Ba contents for the Columbia River Basalt 

Group and associated volcanic rocks. Analyses and symbols same as 

in figure 4.13.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 89

are basaltic andesite, trachybasalt, andesite, and at least 

one flow of nepheline basalt or basanite (W.H. Taubeneck, written communs., 1978,1979; Barrash and others, 

1980; Wright and others, 1980; Swanson and other, 1981; 

Hooper, 1984a; Hooper and Swanson, 1987). At the north 

end of the La Grande graben, one olivine basalt flow, the 

Cricket Flat, rests on the Powatka flow and is overlain by 

the Umatilla Member (figs. 4.2, 4.6, 4.11), and it is 

therefore a late Wanapum age. The Cricket Flat flow is 

locally underlain by a silicic ash-flow tuff. From La 

Grande southward to Baker (fig. 4.1), thick andesite 

flows cap the olivine basalt and spread northward over 

unit N2 of the Grande Ronde Basalt (Swanson and others, 

1981). Measured ages for these andesite flows range from 

7.0 Ma northwest of La Grande (Kienle and others, 1979) 

to 12 to 9 Ma east of La Grande (E.H. McKee and W.H. 

Taubeneck, written commun., 1979). South of Baker, 

olivine basalt flows are interleaved with and overlain by 

flows of basaltic andesite and andesite that are indistinguishable from the Strawberry Volcanics described by 

Robyn (1977, 1979).

Unlike most flows of the Columbia River Basalt Group, 

those of the basalt of Powder River are of small extent, 

lie primarily along the faulted margins of La Grande and 

Baker Basins (fig. 4.19), and form small low-angle cones. 

Both basins are grabens in which the predominant fault 

trend is northwest-southeastward (Walker, 1977). The 

grabens form a series of major basins Elgin, La 

Grande, Baker, and Bowen that lie in a north-south line 

(fig. 4.19). Reconnaissance mapping suggests a direct 

correlation between the development of the grabens and 

the eruption of olivine basalt along their margins. Some 

of the overlying andesite domes and flows have welldeveloped craters (Patterson, 1969; Brooks and others, 

1977). No feeder dikes have been identified.

The basalt flows are commonly diktytaxitic, containing 

small but conspicuous olivine phenocrysts in a groundmass in which isolated crystals of ophitic augite are

FeO

50

MgO

40 30 20 10 \AI203

FIGURE 4.15. Ternary plot of FeO, MgO, and A1203 contents for the Columbia River Basalt Group and associated 

volcanic rocks and for the Slide Creek Member (S) of the Strawberry Volcanics (Robyn, 1977). Symbols same as in 

figure 4.13. Tectonic fields after Pearce and others (1977). Arrows point to pure end members.
90 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

separated by areas of devitrified glass between plagioclase laths. In many flows the olivine is largely altered. 

Pyroxene phenocrysts are rare. Some andesite flows are 

glassy with a dense trachytic texture; others contain 

phenocrysts that may include plagioclase, quartz, hornblende, and, rarely, relics of olivine, which are typically 

rounded by resorption.

Chemically, the Powder River rocks are characterized 

by a higher P205/n02 ratio (0.22) than those of flows of 

the Columbia River Basalt Group (fig. 4.12A). Many 

Powder River flows also have higher A1203 , lower Fe, 

and much higher Sr contents than Columbia River Basalt 

Group flows with the same MgO content (figs. 4.13,4.20). 

Qualifications to each of these statements are required. 

First, the nepheline basalt does not fit this pattern (fig. 

4.12A). Second, some of the most evolved flows of the 

Columbia River Basalt Group (Eden, Powatka, Umatilla, 

Lookingglass, and Wilbur Creek flows) have P205/Ti02 

ratios close to or greater than 0.22. Each of these flows, 

however, is clearly distinguishable from the Powder 

River flows in both mineralogy and chemical composition.

The only Columbia River Basalt Group flows with 

chemical compositions that could be confused with the 

Powder River olivine basalt flows are the Robinette 

Mountain and Dodge flows of the Eckler Mountain 

Member (of the Wanapum Basalt). Of these flows, the 

Dodge has a consistently lower A1203 content than 

Powder River basalt of equivalent MgO content (fig. 

4.20). Both the Dodge and the Robinette Mountain flows 

also have lower K20 and lower Sr contents than otherwise-similar Powder River flows (figs. 4.13, 4.20). The 

Columbia River Basalt Group flow most similar to the 

Powder River rocks, the Robinette Mountain flow, is of 

broadly similar age and was erupted along a northnorthwest fissure close to the northerly extension of the 

Powder River-area graben structures.

WEISER

Fitzgerald (1984) described a sequence of basalt flows 

(which he called the Weiser Basalt) resting on the lower 

part of the Grande Ronde Basalt in the Weiser embayment of west-central Idaho (fig. 4.1). The flows are of 

diverse composition, ranging from olivine alkali basalt 

(basalt of Cuddy Mountain), through various types of 

basalt and basaltic andesite, to typical calc-alkaline 

andesite, dacite, and silicic ash-flow tuff (table 4.1). The 

flows are of limited volume and associated with northwest to north-northwest grabens and horsts.

Comparison of chemical compositions (figs. 4.12^4.16) 

shows that these flows have the same gross chemical 

characteristics as the Powder River flows.

DISCUSSION

Few data are available for the Powder River flows, and 

even fewer for the Weiser flows. Nonetheless, the similar 

tectonic setting, mode of occurrence, and range of gross 

chemical characteristics show that the two units have

Ti/100

Sr/2

FIGURE 4.16. Ternary plot of Ti/100, Zr, and Sr/2 contents for the Columbia River Basalt Group and associated volcanic rocks and for the 

Slide Creek Member (S) of the Strawberry Volcanics (Robyn, 1977). Symbols same as in figure 4.13. Tectonic fields after Pearce and Cann 

(1973). OFB, ocean-floor basalt; IAT, island-arc (low-K) tholeiite; CAB, calc-alkaline basalt.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 91

TABLE 4.3. Comparative compositions of flows and dikes of Lewiston Orchards (Weissenfels Ridge Member) 

chemical type from localities in Washington and Idaho

[Analyses determined at Washington State University's Geoanalytical Laboratory by X-ray-fluorescence analysis. Analyses 

normalized on a volatile-free basis, with iron (Fe*) expressed as 2.00 weight percent Fe2C>3 added to remainder as FeO]

Sample 

S'Ch

Ai cn

T1Q2 ~

FP*

MnO-« --

CaO 

MgO ---

K2O 

Na2O -

P205 

49 ^S*U~T 24

14.40

2.75

13.48

.23

10.05

6.12

.75

2.44

.58

49 83

15 ifc/» 12H*

2.56

12.52

.20

10.60

6.54

.46

1.66

.51

50.30

1 C X"O 15.63

2.61

11.53

.22

11.13

5.93

.31

1.82

.52

49.30

H 23 A *J ***)

2.52

11.74

.20

11.06

6.69

.41

2.34

.52

51.44

H42 ±fc/ tt ^

2.77

12 l^.w 07/

.19

9.14

5.03

.99

2.41

.54

48.86

14.61

2.59

12 26

.20

10.95

6.94

.62

2.41

.55

49.98

14.96

2.37

1220 J. **f^*\J

.20

10.31

6.67

.56

2.19

.48

Sample:

1. Average of four analyses of samples of the basalt of Sprague Lake, Wash. (Wright and others, 1980).

2. Sample HUT-8, flow above interbed near Colton, Wash.

3. Sample HV1-1, thin surface flow northwest of Moscow, Idaho

4. Average of 16 analyses of samples of the Lewiston Orchards flow, Lewiston Basin, Wash, and Idaho (Hooper and 

others, 1985).

5. Average of two analyses of separate dikes (Weissenfels Ridge Member) in the Lewiston Basin, Wash, and Idaho 

(Hooper and others, 1985).

6. Squaw Butte vent, Riggins Basin, Idaho (Hooper, 1984b).

7. Average of two analyses of dike feeding Squaw Butte vent, Riggins Basin, Idaho (Hooper, 1984b).

much in common, despite their geographic separation. 

Original descriptions placed both within the Columbia 

River Basalt Group (Carlson and others, 1981; Swanson 

and Wright, 1981; Fitzgerald, 1984). Because of their 

different tectonic settings and compositions, however, 

this assignment is questionable (Hooper, 1984a), and for 

the purposes of this report we treat them as separate 

informal units (referring to them herein as the basalts of 

Powder River and Weiser, respectively).

Both units correlate more convincingly with the Strawberry Volcanics described by Robyn (1977, 1979; Goles, 

1986) than with the Columbia River Basalt Group. Robyn 

(1977) described a sequence of basalt flows (Slide Creek 

Member) forming the basal part of the Strawberry 

Volcanics that he distinguished from the Picture Gorge 

Basalt by their higher A1203, K20, P205, and Sr contents 

and lower Fe contents. The Powder River and Weiser 

rocks resemble the Slide Creek in these and other 

chemical characteristics (figs. 4.12-4.16). Like the Slide 

Creek, the basalts of Powder River and Weiser are 

closely associated with basaltic andesite, andesite, dacite, and silicic ash-flow tuff of typical calc-alkaline 

affinity. The low Fe content (relative to Mg and Al) and 

the high Sr content (relative to Ti and Zr) two characteristics that distinguish the Slide Creek, Powder River,

50

40

30

8 20

10

EXPLANATION

O Picture Gorge Basalt

 Imnaha (Rg flows) Basalt 

D Imnaha (Ng flows) Basalt

* Basalt of Powder River 

and associated volcanic 

rocks

50 100 150 200

Zr CONTENT, IN PARTS PER MILLION

250

FIGURE 4.17. Sc versus Zr contents for the Picture Gorge Basalt, 

the Imnaha Basalt, and basalt of Powder River and associated 

volcanic rocks. Magnetic polarities: N, normal; R, reversed. Polarity intervals numbered from oldest to youngest (for example, N0 to 

N3) where sequential.
92 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

and Weiser basalts from the Columbia River Basalt 

Group (figs. 4.15,4.16) are typical of subduction-related 

volcanic rocks (Pearce and Cann, 1973; Pearce and 

others, 1977). In addition, both the Powder River and 

Weiser units include flows of obvious alkaline affinity, 

such as the nepheline basalt near La Grande (Wright and 

others, 1980) and the Cuddy Mountain flows of the 

Weiser unit (Fitzgerald, 1984).

Detailed studies of the Powder River and Weiser units 

have not been made. Data on the 87Sr/86Sr ratio for three 

samples of the Powder River unit (an olivine basalt, an 

andesite, and a nepheline basalt) were presented by 

Carlson and others (1981). In contrast to the younger 

Wanapum and Saddle Mountains Basalts of the Columbia 

River Basalt Group north of La Grande, all three Powder

200

100

60

so

40

30

20

I I I

La Ce Pr Nd Sm Eu Qd Tb Dy Ho Er Tm Yd Lu 

RARE - EARTH ELEMENTS

FIGURE 4.18. Chondrite versus normalized rare-earth element abundance pattern for the Picture Gorge Basalt and other units of the 

Columbia River Basalt Group (table 4.1). Long-dashed lines 1, 2, and 

3, Picture Gorge Basalt (1 and 2 represent extreme values for all but 

one flow; 3 is the more evolved exception). Circles, extreme values 

for subgroup AB (of the Imnaha Basalt); triangles, extreme values for 

subgroup RC (of the Imnaha Basalt). Vertical-line pattern represents 

two flows of the Grande Ronde Basalt. Short-dashed lines represent 

extreme values for the Eckler Mountain Member (R, Robinette 

Mountain flow; SH, Shumaker Creek flow of the Wanapum Basalt). 

Dashed lines with three short dashes represent extreme values of the 

Saddle Mountains Basalt (A, Asotin flow; U, Umatilla flow).

River samples retain a so-called primitive Sr isotopic 

ratio (0.7035) similar to that of the older Picture Gorge 

and Imnaha Basalts (Hooper, 1984a).

TECTONIC SETTING OF THE 

BASALTIC VOLCANISM

Delineation of the stratigraphic details of the basaltic 

pile (made possible by rapid analytical and paleomagnetic 

techniques in combination with geologic mapping) provides an unusually clear view of the structural and 

geomorphic evolution of the Blue Mountains province 

during the early to late Miocene. The regional stress 

regime of horizontal compression oriented north-northwest/south-southeast and horizontal extension oriented 

east-northeast/west-southwest remained in effect 

throughout the period of volcanism. However, tectonic 

activity toward the end of Grande Ronde time along a 

broad zone corresponding approximately to the OlympicWallowa Lineament (OWL) resulted in a different struc117"30r

45°30

4SW

Approximate limit of basalt of 

Powder River and associated 

volcanic rocks

X

Elkhorn 

Mountains

20 KILOMETERS

FIGURE 4.19. Structural map of part of northeastern Oregon, showing 

Elgin and La Grande Basins and Baker and Bowen Valleys (stippling), and northwest-trending normal faulting with graben-horst 

development and approximate north limit of the basalt of Powder 

River and associated volcanic rocks. Modified from Brooks and others 

(1977) and Walker (1977,1979). Solid line, normal faults; hachures on 

downthrown side.
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 93

tural expression of that stress regime on either side of the 

OWL, subsequently accompanied by different styles of 

volcanism.

The earliest eruptions (Imnaha Basalt) were concentrated in the southeast corner of the Columbia Plateau 

along the Oregon-Idaho State line (fig. 4.4). The later 

flood of the Grande Ronde Basalt covered areas increasingly farther to the north and west. To a large extent, 

this northward to northwestward migration over time 

was a simple artifact of the progressive uplift of the 

southeast corner of the Columbia Plateau during the 

volcanism (Hooper and Camp, 1981; Hooper, 1984a). 

Magma erupted from the same fissures, but at sites 

progressively farther north as the points of lowest 

intersection of the fissures with the ground surface 

migrated in that direction. We do not know, however, 

that tilting was the only factor involved.

2.0

1.5

1.0

0.5

9.0

7.0

5.0

3.0

\

EXPLANATION

 Dodge flow

A Robinette Mountain flow

O Basalt of Powder River

14.5 15.0 15.5 16.0 16.5 17.0 

AIO CONTENT, IN WEIGHT PERCENT

17.5

FIGURE 4.20. A1203 against KaO and MgO contents, showing 

chemical differences between the Dodge and Robinette Mountain 

flows of the Eckler Mountain Member (of the Wanapum Basalt) 

and the basalt of Powder River and associated volcanic rocks. 

Dashed line separates the basalt of Powder River and associated 

volcanic rocks from all but one of the analyses of the Dodge and 

Robinette Mountain flows (K20 versus A1203 content) and separates the Powder River and associated volcanic rocks from the 

Dodge flow (MgO versus A1203 content). Solid line encloses 

analyses of the Dodge flow (MgO versus A1203 content).

Feeder dikes for the Grande Ronde Basalt are abundant both in the Cornucopia area and in the lower Grande 

Ronde River canyon (fig. 4.4). In contrast, the rarity of 

dikes of the Wanapum and Saddle Mountains Basalts 

south of the Wallowa-Seven Devils divide leaves open the 

possible northward migration of the fissures and the 

magma source over time. Whether or not the source and 

fissures did migrate, the crustal rocks through which 

most of the magma had to pass on its way from the 

reservoir to the surface were similar, consisting of 

accreted island-arc and oceanic terranes.

The northwestward tilting of the eastern part of the 

Blue Mountains province continued throughout Columbia 

River Basalt Group time. The uplift and related tilting 

are most simply seen as consequences of the continued 

rise of the Idaho batholith and, to a lesser degree, of the 

smaller granitic bodies underlying the Seven Devils and 

Wallowa Mountains and the Nez Perce Plateau (fig. 4.4). 

The rise of the batholith is documented throughout the 

Tertiary (Axelrod, 1968) and probably continues to the 

present day; it seems to have little association with the 

origin of the basalt.

The regional stress regime (north-northwest horizontal compression, east-northeast horizontal extension) 

was superimposed on the tilting. A similar stress pattern 

may have been present as early as the Eocene (Hooper 

and Camp, 1981) and is still present today (Rohay and 

Davis, 1983). The regime provides an adequate explanation for most deformation observed on the Columbia 

Plateau if we take account of significant local control by 

the pre-Miocene structural grain.

Hooper and Camp (1981) list the main structural 

elements of the plateau. They are the north-northwest/ 

south-southeast extensional fissures that acted as feeders 

to the basaltic eruptions, approximately east-west buckles accompanied by reverse faulting, and approximately 

northwest-southeast right-lateral and northeast-southwest left-lateral strike-slip faults. At a depth corresponding to the base of the crust, potential extensional fissures 

would result if the vertical stress were large enough to 

equal approximately the maximum north-northwest horizontal stress, with stress along the east-northeast/ 

west-southwest horizontal axis being significantly less 

than the other two. Given large magma reservoirs lying 

near the base of the crust, the magma could use such a 

potential plane of extension to propagate north-northwest/south-southeast fissures as it forced its way to the 

surface.

At higher levels in the crust, the vertical stress would 

be less, and so north-northwest (horizontal) stress would 

exceed vertical stress, which, in turn, would exceed 

east-northeast (horizontal) stress. This is the classic 

condition for development of sets of vertical strike-slip 

faults (west-northwest right lateral, north-northeast left
94 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

lateral). Close to the surface the vertical stress decreases 

to zero, and so north-northwest (horizontal) stress would 

exceed east-northeast (horizontal) stress, which, in turn, 

would exceed vertical stress the conditions under which 

approximately east-west buckling and reverse faulting 

might be expected to develop in brittle basalt flows 

(Price, 1982). An apparent consequence of this model is 

that the approximately east-west folds of the Columbia 

Plateau are near-surface phenomena which probably do 

not continue to great depth.

At least two complications mar this simple model. 

First, the suture zone around the cratonic margin forms 

a major discontinuity that has a different structural 

pattern; second, clockwise rotation for various parts of 

the plateau (Basham and Larson, 1978; Magill and others, 

1981; Martin, 1984) is now well established, notwithstanding recent comments to the contrary (Fox and 

Beck, 1985).

The east-west segment of the suture zone passes 

directly beneath the Lewiston structure (Armstrong and 

others, 1977; Fleck and Criss, 1985; Mohl and Thiessen, 

in press). This ancient discontinuity may have provided a 

weakened zone along which the regional stress found 

relief in the Miocene. Where the suture extends northsouth along the west side of the Idaho batholith (fig. 4.1), 

it underlies an elongate downwarp. This feature has 

continued to develop through the past 20 m.y. as a 

topographic low continually inundated with basalt flows 

whenever they had access (Camp, 1981; Hooper, 1984b). 

The cause for the continuing subsidence parallel to the 

suture zone is unclear but may mildly reflect the same 

dynamic forces that create trenches above subduction 

zones.

Progressive clockwise rotation in the western part of 

the Columbia Plateau is well established (Bentley, 1977, 

1979; Magill and others, 1982). It may be explained by 

movement along northwest-trending right-lateral strikeslip faults or megashears (Bentley, 1977). The most 

obvious rotation associated with the megashears appears 

to die out in the main part of the Columbia Plateau east 

of Pasco Basin (Watkins and Baksi, 1974; Choiniere and 

Swanson, 1979; Hooper and others, 1979). More detailed 

recent studies, however, indicate that local clockwise 

rotation has occurred in the anticlinal crests of the 

Yakima fold belt (Reidel and others, 1984), within the 

suture zone on the east side of the Blue Mountains 

province (Hooper, 1984b), and apparently also on blocks 

within a zone corresponding to the OWL (Martin, 1984).

More work on the size, age, and amount of rotation of 

the crustal blocks is needed, but the general tectonic 

implications are clear. The same forces responsible for 

the northeastward migration and 60° clockwise rotation 

of the accreted terranes on the west side of the North 

American craton continued through the Miocene, albeit

in a very subdued form. The greater extension on the 

south side of the OWL, involving right-lateral motion and 

probable clockwise rotation of small intervening crustal 

blocks, created grabens rather than simple fissures to the 

south of the OWL and may have caused significant 

crustal thinning. The structural geometry of the OWL 

appears to be similar to that recorded on the south side 

of the Lewis and Clark fault zone in northern Idaho and 

Montana by Sheriff and others (1984). The greater 

extension and possible crustal thinning is associated in 

space and time with the change in style of volcanic 

activity from tholeiitic (Columbia River Basalt Group) to 

calc-alkaline (Strawberry Volcanics and basalts of Powder River and Weiser).

GENESIS OF THE COLUMBIA RIVER 

BASALT GROUP

There is as yet no single genetic model for the 

Columbia River Basalt Group, but the detailed field, 

petrographic, chemical, and isotopic data now available 

provide constraints on potential models.

PHYSICAL CONSTRAINTS

(1) The remarkably consistent orientation of feeder dikes 

throughout the approximately 11-m.y. period 

(17.5-6.0 Ma) of volcanism implies a nearly constant 

stress field, not local stress fields related to crustal 

stretching around high-level magma reservoirs.

(2) The great length of many of the fissure-vent systems, 

150 to possibly 300 km, suggests that the fissures 

extend at least to the base of the crust, 30 to 40 km 

down. From this, we infer that eruptions occurred 

when potential fissures resulting from the regional 

stress regime intersected large magma reservoirs, 

probably near the crust-mantle boundary (Cox, 

1980).

(3) Lava erupted from any one of the long fissure 

systems is remarkably homogeneous. This homogeneity is the basis of our ability to define and map a 

detailed flow stratigraphy across the whole plateau 

(Wright and others, 1973; Swanson, Wright, and 

others, 1979; Hooper, 1981; Reidel, 1983; Mangan and 

others, 1986).

(4) The voluminous sheetfloods of basalt resulting from 

individual eruptions imply rapid discharge from the 

fissures (Shaw and Swanson, 1970). 

The large volumes of homogeneous magma require 

large reservoirs in which the magma is thoroughly mixed 

before eruption. It is inconceivable that magma from 

separate reservoirs, or refills of the same reservoir, 

would be as homogeneous as that recorded from individual eruptions of the Columbia River Basalt Group. The
THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 95

rapid discharge, coupled with the homogeneity of the 

lava, makes it improbable that significant contamination 

by the country rock occurred between the reservoir and 

the surface. This inference is supported by the dearth of 

xenoliths in dikes except in one or two localities, such as 

the Wallowa Mountains (W.H. Taubeneck, oral commun., 1985).

CHEMICAL CONSTRAINTS

(1) Variations in isotopic ratios of Sr, Nd, 0, and Pb (table 

4.2; McDougall, 1976; Carlson and others, 1981; 

Carlson, 1984; Church, 1985) separate the Columbia 

River Basalt Group into several subdivisions, in 

which rocks of each subdivision are derived from a 

different source or combination of sources (fig. 4.3). 

Carlson (1984) suggested a combination of four or five 

such sources, including different mantle sources and 

different types of crustal contaminants. We note that 

if the origin of the Columbia River Basalt Group is 

ascribed to multiple sources, then the failure of 

simple partial-melting or crystal-fractionation models 

to fully account for the observed chemical variation 

cannot, of itself, be used as evidence against participation of these processes in the evolution of the 

magmas. Pb isotopic data indicate that the Picture 

Gorge Basalt was probably derived from a typical 

midoceanic-ridge-basalt (MORB)-type depleted mantle (Carlson and others, 1981). The rather high S180 

values and evolved Pb isotopic ratios of the Grande 

Ronde Basalt and younger flows imply a sedimentary 

component, either as a direct crustal contaminant or 

as a contaminant of the mantle source. The light 

REE-enriched patterns in these flow units imply a 

mantle source less depleted than MORE.

(2) No flow composition could have been in equilibrium 

with a Iherzolitic upper mantle. Even the most 

primitive flows of the Picture Gorge Basalt, as well 

as the Robinette Mountain flow and the Asotin 

Member (the most primitive flows in the Wanapum 

and Saddle Mountains Basalts, respectively), have 

Mg' (Mg/(Mg+Fe2)) values, Ni and Cr contents, and 

olivine phenocryst compositions too evolved to represent primary magma derived from a normal Iherzolitic mantle source. Such characteristics require 

either that the mantle source was an iron-rich pyroxenite (Wilkinson and Binns, 1977; Helz, 1978; Wright 

and Helz, 1981; Prestvik and Goles, 1985) or that 

significant crystal fractionation occurred after removal of the magma from its source.

(3) A restricted range in major-element composition and 

mineralogy characterizes most continental flood-basalt provinces, but it is more marked in the Columbia 

River Basalt Group than in either the Karoo or the

Deccan (Krishnamurthy and Cox, 1977; Cox, 1980; 

Beane and others, 1986). Neither primitive picritic 

nor evolved alkalic or silicic varieties are percent. 

The total range of Si02 content for the Columbia 

River Basalt Group is 48 to 57 weight percent (table 

4.1). (Note that, in contrast, the basalts of Powder 

River and Weiser have a much wider variation of 

major elements, more akin to that of calc-alkaline 

rock suites). Again, if a Iherzolitic (as opposed to 

pyroxenitic) mantle source is assumed, then some 

poorly understood, density-controlled filter system, 

permitting extrusion only of basalt with a density 

lowered to some critical level by crystal fractionation 

(Stolper and Walker, 1980), must also be invoked. 

Cox (1980) discussed such a density trap involving a 

large magma reservoir at or near the crust-mantle 

boundary.

(4) Within the subdivisions defined by similar isotopic 

ratios, the flows show wide variations in the abundances of incompatible trace elements and smaller 

variations in major elements. In some samples, the 

abundances of major and many trace elements have 

been adequately modeled by crystal fractionation, in 

which the removal of plagioclase plays a dominant 

role (Reidel, 1983; Hooper and others, 1984). However, increasing fractionation is commonly accompanied by a small systematic increase in the ratios 

between different incompatible trace elements (for 

example, Ba/P205; Hooper, 1984a). Such changes are 

unlikely to result either from simple crystal-fractionation or simple partial-melting processes; however, 

they can occur when these processes are complicated 

by reservoir recharge or crustal assimilation in open 

magma systems. The most obvious situation that 

could cause this interdependence is an open magma 

system in which the degree of crustal assimilation is 

controlled by the heat released by crystal fractionation (DePaolo, 1981; O'Hara and Mathews, 1981). If 

this model is correct, then the crustal contaminant 

had a ^Sr/^Sr ratio nearly identical to that of the 

magma, and their mixing caused little change in that 

value. This constraint suggests that the potential 

contaminant may be recently formed crust, possibly 

underplated during recent subduction to the bottom 

of the crust beneath the Blue Mountains province. 

However, crystal fractionation accompanied by magma recharge (O'Hara and Mathews, 1981) may be 

adequate to account for the observed chemical changes without recourse to significant crustal assimilation 

(P.R. Hooper, unpub. data, 1987).

Conversely, sharp changes in chemical composition 

involving Si02 , K20, and LIL elements between the 

major subdivisions of the Columbia River Basalt 

Group appear to coincide with slight changes in the
96 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

ratio 87Sr/86Sr (fig. 4.3). These changes could reflect 

recharge of the reservoir by magma of varying 

chemical and isotopic composition, which might result 

from various degrees of contamination of the mantle 

source by hydrous emanations from the subducted 

slab.

(5) The high ^Sr/^Sr ratio of all flows of the Saddle 

Mountains Basalt could not have been caused by 

assimilation of the accreted crustal blocks through 

which most of the Saddle Mountains Basalt flows 

were extruded. An enriched subcontinental-mantle 

source appears to be a more probable cause.

(6) Mixing of smaller batches of magma can be demonstrated locally (Hooper, 1985). Mixing between magma batches of similar density, in combination with 

crystal fractionation, could result in a scatter of 

elemental abundances that would camouflage the 

evidence for either process.

Evidently, the origin and evolution of the various 

magmas contributing to the Columbia River Basalt 

Group were complex. The isotopic data require different 

mantle sources, whereas crystal fractionation in the crust 

played a major role. Simple mixing of different magmas 

can be clearly demonstrated in some cases and may have 

been widespread. Assimilation of crustal material has not 

been established unambiguously, but neither has it been 

disproved. We still lack convincing evidence to distinguish between basaltic magma contaminated by crustal 

assimilation, and magma derived either from a mantle 

source contaminated by subducted crust or from a mantle 

source enriched by some metasomatic process.

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calc-alkaline volcanism unrelated to subduction: Journal of Volcanology and Geothermal Research, v. 5, no. 1-2, p. 149-161.

Rohay, A.C., and Davis, J.D., 1983, Contemporary deformation in the 

Pasco Basin area of the central Columbia Plateau, in Caggiano, 

J.A., and Duncan, D.W., eds., Preliminary interpretation of the 

tectonic stability of the reference repository location, Cold Creek 

syncline, Pasco Basin, Richland, Washington: Richland, Wash., 

Rockwell Hanford Operations Report RHO-BW-ST-19P, p. 6-1 - 

6-30.

Ross, M.E., 1978, Stratigraphy, structure, and petrology of Columbia 

River Basalt in a portion of the Grande Ronde-Blue Mountains 

area of Oregon and Washington: Moscow, University of Idaho, 

Ph.D. dissertation, 407 p.

Shaw, H.R., and Swanson, D.A., 1970, Eruption and flow rates of flood 

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6, p. 653-654.

Shubat, M.A., 1979, Stratigraphy, petrochemistry, petrography, and 

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Stoffel, K.L., 1984, Geology of the Grande Ronde lignite field, Asotin 

County, Washington: Washington Division of Geology and Earth 

Resources Report of Investigations 27, 79 p.

Stolper, E.M., and Walker, David, 1980, Melt density and the average 

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v. 74, p. 7-12.

Swanson, D.A., Anderson, J.L., Camp, V.E., Hooper, P.R., Taubeneck, W.H., and Wright, T.L., 1981, Reconnaissance geologic map 

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THE COLUMBIA RIVER BASALT GROUP OF THE BLUE MOUNTAINS PROVINCE 99

Revisions in stratigraphic nomenclature of the Columbia River 

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Columbia River Basalt 1978-1979 sample data and chemical 

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PWright, T.L., Grolier, M.J., and Swanson, D.A., 1973, Chemical 

variation related to the stratigraphy of the Columbia River Basalt: 

Geological Society of America Bulletin, v. 84, no. 2, p. 371-385.

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comparative study of three intraplate provinces, in Subbarao, 

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basalt provinces in other parts of the world: Geological Society of 

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Major-oxide, trace-element, and glass chemistry of Columbia 

River Basalt samples collected between 1971 and 1977: U.S. 

Geological Survey Open-File Report 79-711, 146 p.

5. MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS 

REGION, EXCLUSIVE OF THE COLUMBIA RIVER BASALT GROUP AND

ASSOCIATED MAFIC LAVA FLOWS

By GEORGE W. WALKER

CONTENTS

Page

Abstract 101

Introduction 101

Geographic distribution 102

Stratigraphy and age 102

Basin-fill deposits 103

Deschutes Basin 103

Tygh Basin and The Dalles Basin 103

Agency and Arlington Basins 107

Walla Walla Basin 108

Paulina Basin 109

John Day Valley 110

Fox Basin 111

Juntura Basin 111

Harper Basin 111

Boise Basin 112

Basins north of Harper and Boise Basins 113

Basins north and northwest of Durkee Basin 114

Miocene volcanic accumulations 114

Strawberry Volcanics 114

Other volcanic accumulations 115

References cited 117

ABSTRACT

Diverse volcanic, volcaniclastic, and sedimentary rocks characterize 

the Miocene section of the Blue Mountains region; vast volumes of 

basalt dominate parts of the region, and more restricted andesitic piles 

are present locally. The basalt was erupted mostly from several 

widespread dike complexes, and the andesite from central vents. 

Elsewhere in the region, moderate to large volumes of silicic pyroclastic 

rocks are present, partly erupted as ash flows and pumiceous ash from 

recognized vents within the Blue Mountains and from vents in areas to 

the west, along the Cascade axis, and to the south in the northern Basin 

and Range. Much of the silicic pyroclastic material, locally intermixed 

with much palagonitic tuff and breccia, was deposited in several 

separate and partly ephemeral basins. Some of the widespread Miocene 

ash-flow tuff layers extend into several adjacent basins, whereas most 

sedimentary units are geographically restricted.

INTRODUCTION

During the Miocene, while vast volumes of tholeiitic 

basalt of the Columbia River Basalt Group were being 

erupted from numerous dike complexes in the northern

Blue Mountains, another kind of volcanism was occurring 

immediately to the south and southeast both within and 

marginal to the Blue Mountains province. This volcanism, of calc-alkaline affinity, included moderately large 

volumes of basalt, andesite, and rhyodacite or rhyolite; 

chemically, it was much like the volcanism that occurred 

in the Blue Mountains region during Eocene time. It was 

accompanied by the deposition of large volumes of silicic 

pyroclastic rocks and silicic to mafic tuffaceous sedimentary rocks in several separate basins; some of these 

basins were short lived, whereas others persisted into 

the Pliocene. Some of the persistent basins are grabenlike structures resulting from Miocene and younger 

extensional tectonics that mostly affected areas to the 

south in the Basin and Range province. Other depositional basins, particularly those along the northern and 

western margins of the Blue Mountains, are synclinal 

downwarps in flow sequences of the Columbia River 

Basalt Group. In most of these basins, the sedimentary 

fill appears conformable, or nearly so, with underlying 

Miocene basalts; but in some places along the southern 

margin of the Blue Mountains, volcaniclastic and sedimentary units are interlayered with flows of the Columbia River Basalt Group or with other middle Miocene flow 

sequences.

Eruptive centers for this diverse volcanic activity are 

of several types, including large central-vent complexes, 

large silicic domal masses, and areally restricted plugs 

and dikes. Some of the widespread ash-flow tuff sheets of 

the southern Blue Mountains were erupted from presumed caldera complexes now buried in Harney Basin 

approximately at the boundary between the Blue Mountains and the Basin and Range province; other tuff 

deposits may be related to suspected, but as yet largely 

unsubstantiated, source areas characterized by silicic 

domal complexes or large silicic intrusions; and still other 

deposits were erupted from unknown sources, possibly 

now totally obscured either by the rhyolitic domal 

complexes or by younger flows and sedimentary rocks.

The large volume of basaltic to rhyolitic eruptive 

material of Miocene and younger age, exclusive of flows

101
102 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

of the Columbia River Basalt Group, includes both flows 

and tuffaceous fragmental materials. These sequences, 

which include large volumes of fine-grained rhyolitic, 

andesitic, and basaltic pyroclastic ejecta, are partly found 

in separate depositional basins; volume estimates based 

on these disconnected sequences are approximate at best 

and may even be misleading in making comparisons with 

different types of volcanic activity in adjoining areas. If 

these isolated sequences of Miocene tuff and sedimentary 

rocks are erosional remnants of a widespread and continuous sheet, as postulated by Thayer and Brown (1966) 

for a major part of the section, the volume of rhyolitic and 

andesitic material erupted would be very large, possibly 

nearly comparable to that of temporally equivalent flows 

of the Columbia River Basalt Group. The data indicate, 

however, that many of these basins were never or only 

briefly interconnected and that some evolved during 

deposition; thus, the volume would be greatly diminished. Onlap and offlap relations and the distribution of 

nearshore and offshore facies in many parts of these 

basins indicate that at least some of them were separate 

and resulted from surface deformation related to concurrent tectonism and volcanism.

GEOGRAPHIC DISTRIBUTION

Areas of deposition of the Miocene and younger volcanic and tuffaceous sedimentary rocks are widely distributed, the thicker sections mostly being in basins on the 

west, south, and southeast margins of the Blue Mountains province and in the Strawberry Mountains (fig. 

5.1). Of the many separate basins that have been 

recognized, Deschutes (or Madras) Basin and the smaller 

Tygh and The Dalles Basins dominate on the west. On the 

south, parts of several separate depositional basins 

extend northward into the Blue Mountains province; 

these basins include the northern parts of Harney, 

Juntura, and Harper Basins and the northern part of 

Bully Creek basin (fig. 5.1). Entirely within the Blue 

Mountains province near its southern margin are Paulina 

Basin and Bear and Logan Valleys. All three of these 

basins contain upper Miocene ash-flow tuff sheets, several of which were erupted from buried calderas in 

Harney Basin, and interstratified tuffaceous sedimentary rocks. Both John Day Valley and Fox Basin, well 

within the Blue Mountains province, contain tuff and 

tuffaceous sedimentary rocks, as well as interlayered 

fanglomerate, of middle and late Miocene age.

Extensive andesitic to rhyolitic volcanic rocks of middle and early late Miocene age crop out in a large area 

(more than 4,000 km2) in and adjacent to the Strawberry 

Mountains. These volcanic rocks extend tens of kilometers southward from the Strawberry Mountains to the 

south limits of the Blue Mountains province, eastward

into Mormon and Unity Basins, and northward into the 

headwaters of the Burnt River. On the southeastern and 

eastern margins of the Blue Mountains province and 

extending into the north end of the Owyhee Upland 

province (see fig. 1.2), tuffaceous sedimentary rocks 

representing northward extensions of units exposed in 

Boise Basin are present northwest and west of Ontario, 

Oreg., and Weiser, Idaho (fig. 5.1). Miocene and younger 

ash-flow tuff, tuffaceous sedimentary rocks, and some 

interbedded basalt flows are confined to Mormon, Durkee, and Unity Basins, and somewhat similar units are 

present in several valleys, including Baker Valley and 

Burnt River and lower Powder River valleys. Structurally depressed areas to the north and northeast of the 

Blue Mountains axis, including Arlington and Agency 

Basins, as well as the structurally downdropped block of 

the La Grande Valley, also contain upper Miocene and 

younger epiclastic and volcaniclastic deposits, although 

ash-flow tuff and young basalt flows such as are typical of 

some of the other basins appear to be absent. Much of the 

sedimentary fill in these basins appears to be fanglomerate derived from the erosion of volcanic rocks exposed in 

more elevated parts of the Blue Mountains.

Pasco and Walla Walla Basins lie north of the structurally elevated, northwest-trending block, represented 

by the Horse Heaven Hills in Washington, that extends 

southeastward into Oregon (fig. 5.1). Pasco Basin, on the 

northwest, contains a thick section of terrestrial pyroclastic and sedimentary deposits of Miocene and younger 

age, and Walla Walla Basin to the southeast contains a 

thickness of about 180 m of Pliocene(?) and Pleistocene 

beds of clay, silt, sand, and gravel. Both basins are 

developed in folded and faulted flows of the Columbia 

River Basalt Group.

STRATIGRAPHY AND AGE

The stratigraphic relations of the Miocene and younger 

rocks of the Blue Mountains region, exclusive of the 

Columbia River Basalt Group, are at present too poorly 

known to interpret with confidence. Although the volcanic and sedimentary sequences are approximate age equivalents of major parts of the group, they are present either 

in disconnected depositional basins or in large volcanic 

piles or domal complexes, and few regionally extensive 

marker horizons have been recognized. The absence of 

such marker horizons in most sections makes meaningful 

regional correlations difficult, although recent radiometric dating has provided greater precision in establishing 

the temporal equivalence of separate sequences. In the 

past, correlation of units has relied mostly on the dating 

of scattered floral and faunal collections, with little 

consideration to whether the units in which these collections were made were interconnected. Thus, a single
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 103

formation name has in places been applied to physically 

separated Miocene sequences, and in a few places several 

different formation names have been applied to a single 

widespread tuff layer that transcends divides between 

separate depositional basins.

To clarify some of the problems of stratigraphic nomenclature, the Miocene and younger sedimentary sequences are here discussed first by depositional basin or 

volcanic pile and then by age. Radiometric ages, though 

few and limited in geographic coverage, permit first 

approximations in regional correlation of the numerous 

formation units that have been established and, furthermore, permit age comparisons with major units within 

the Columbia River Basalt Group (fig. 5.2). The stratigraphic nomenclature of each depositional basin has 

evolved through independent mapping, and only in a few 

places do individual units of distinctive ash-flow tuff or 

related air-fall tuff provide physical ties to deposits of 

other basins.

BASIN-FILL DEPOSITS

The Miocene and younger basin-fill deposits consist of 

a great variety of rhyolitic to basaltic volcanic and 

volcaniclastic rocks, complexly interlayered with tuffaceous sedimentary rocks. These deposits occur in several 

separate depositional basins, some of which lie wholly 

within the Blue Mountains region; others, along the 

margins of the Blue Mountains, extend long distances 

outside the region.

DESCHUTES BASIN

Deschutes (or Madras) Basin, which is marginal to the 

Blue Mountains on the west, contains a thick sequence of 

epiclastic and pyroclastic rocks and a few thin interlayered basalt flows, all incorporated into the Deschutes 

Formation of Farooqui, Beaulieu, and others (1981) or 

the Madras Formation (Williams, 1957). The basin fill 

varies considerably in thickness, locally pinching out on 

Eocene and Oligocene rocks at both the northern and 

eastern margins of the basin and apparently resting 

unconformably on the Grande Ronde Basalt of the 

Columbia River Basalt Group in the canyon of the 

Deschutes River about 7 to 8 km northwest of Madras, 

Oreg. (Waters, 1968). The maximum thickness indicated 

for this diverse assemblage is about 450 m for a section in 

the canyon of the Deschutes River south of Warm 

Springs, Oreg. (Hodge, 1940). Much of the section 

consists of poorly to moderately well bedded, fine- to 

coarse-grained clastic sedimentary rocks composed entirely of basaltic, andesitic, and rhyodacite debris (figs. 

5.3, 5.4) derived from volcanic rocks to the south and 

west. Interstratified in the sedimentary sequence are

rare basalt and basaltic andesite flows and several partly 

bedded, pumice-rich layers and elongate lenses of rhyolitic to dacitic ash-flow tuff and laharic tuff. The lithology, 

bedding, and textural features of some of these pumiceous layers and elongate lenses (fig. 5.5) indicate that 

they were initially pumiceous ash flows that became 

mudflows upon encountering and incorporating water 

and, locally, unconsolidated sediment during flow in 

stream channels.

Several K-Ar ages on mafic flows in the Deschutes 

Formation range from 15.9±3.0 Ma to slightly less than 

5 Ma (Armstrong and others, 1975). The older age 

becomes 16.3 ±3.0 Ma when recalculated using the decay 

constants acceptable in 1983 (Fiebelkorn and others, 

1983). More recently, Smith and Snee (1984) obtained an 

40Ar/39Ar age of 7.6 ±0.3 Ma for a basaltic unit identified 

by them as the Pelton Basalt Member of former usage of 

the Deschutes Formation. This apparently is the same 

unit for which Armstrong and others (1975) obtained the 

K-Ar age of 15.9±3.0 Ma. According to Smith and Snee 

(1984), their Pelton Basalt Member of the Deschutes 

Formation represents the basal part of the formation and 

is separated by an angular unconformity from underlying 

clastic rocks that contain middle Miocene fossils. Although earlier workers included the middle Miocene 

rocks in the Deschutes Formation, Smith and Snee 

assigned these fossil-bearing rocks to their Simtustus 

unit, which overlies and locally interfingers with flows of 

the Columbia River Basalt Group. This age range for the 

Deschutes Formation, from either middle or late Miocene 

to earliest Pliocene, indicates some temporal overlap 

with the Columbia River Basalt Group, although the 

youngest part of the formation apparently postdates any 

of the flows of the group (fig. 5.2).

TYGH BASIN AND THE DALLES BASIN

Tygh Basin and The Dalles Basin, both northwest of 

the Blue Mountains, are structural downwarps in flows of 

the Columbia River Basalt Group filled with indurated 

epiclastic and volcaniclastic rocks of Miocene and Pliocene(?) age. Rocks in both basins have been described in 

some detail (Newcomb, 1966; Farooqui, Beaulieu, and 

others, 1981) as debris fans that spread eastward from 

the Cascade Range and include, in addition to erosional 

debris, interbedded lenses and layers of tuff, lahars, and 

minor basalt flows. Various formation names have been 

applied to the rock sequences in both Tygh Basin and The 

Dalles Basin; most recently, they have been named, 

respectively, Tygh Valley Formation and the Chenoweth 

Formation by Farooqui, Beaulieu, and others (1981). 

Both formations are included in their regionally extensive Dalles Group. The two formations vary considerably 

in thickness; maximum thicknesses reported are 550 m
104 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

EXPLANATION

Middle Miocene and younger rocks 

Exclusive of the Columbia River 

Basalt Group

Strawberry Volcanics

 Contact Dashed where approximately 

located

BENTON

WASHINGTON

1 OREGON

20 40 60 80 KILOMETERS

FIGURE 5.1. Blue Mountains region, showing approximate outcrop area
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 105

118° 117° 116°

1 ' }^^~"

WHITMAN LATAH J/ 

_ 0 oMoscow iS tt 

/.'""~s ^ __ ,' \ Pullman }^ n ^

_,-^ \ ,-U-- j^r

GARFIELD ^^^ter^ CLEARWATER

 -. ~ '°Lewiston \~A /

'- \ ( --^-J \\ / 

COLUMBIA \PERCE ] L£WIS \ "^'

ASOTIN ( r^ fL

' r* / 'J'

^~$°^A\ f )

1 / \ / ^ 

, e / \ / \

Wp^ s ) ^ IDAHO -" r^ v^ /

j ( ^ V IDAHO

/ ! >v WALLOWA { \ 'l

/ I ^ \ $ )

f=a) K \ k^" ^^'^n ( x\ / / / * " ^> * \ ~> i / \ -i--.>:-a----\ >_ \ / I / )

fi 

^"Xy 

\ 

\ 

u 

\\ - ] { 

,L_.J\

<£/ \^ \ /I X y "

v^ . _. i / i , y

^ _ __"_ _4-r>±ir:lL_// f- i , 

ll^- ' fe ADAMS /

Index to localities 

BC, Bear Creek 

BCB, Bully Creek Basin 

CC, Camp Creek 

CR, Castle Rock 

CW. Cottonwood 

DM, Dooley Mountain 

FC, Flat Creek 

HBi Harney Basin 

46° HHH, Horse Heaven Hills 

l> Ironside Mountain 

JMi Juniper Mountain 

LG. La Grande Valley 

LMi Lookout Mountain 

MC, McKay Creek 

MGi Malheur Gorge 

MMi Maury Mountains 

O, Ontario 

PG, Picture Gorge 

PVi Powder River valley 

RV, Richland Valley 

S, Strawberry Mountain 

SI, Suplee-lzee area 

TP, The Peninsula 

VF, Virtue Flat 

W, Weiser 

WB, Willow Creek basin 

WG, Wallula Gap 

WS, Warm Springs 

WT, White Bluffs

15°

1 VALLEY

\

BOISE

of middle Miocene and younger rocks, exclusive of the Columbia River Basalt Group.
106 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Unit

Rattlesnake 

Ash -flow Tuff

Prater Creek 

Ash -flow Tuff

DevineCanyon 

Ash -flow Tuff

1Dalles Group

Tygh Valley 

Formation n

Chenoweth 

Formation 1

Deschutes 

Formation 1

Juntura 

Formation

Dinner Creek 

Welded Tuff

Dooley Rhyolite 

Breccia

Mascall Formation

Payette Formation

Strawberry 

Volcanics

 COLUMBIA RIVER BASALT GROUP 

Wanapum Basalt 

?-v-? 

Grande Ronde 

Basalt

? Saddle Mountains ? v 

Basalt Picture Gorge Basalt

i i i i i i i i i i i i i i i i I

_^

 

 , 

.

 

 

* 9

 

,2

?-.-? 

?- -?

1 1 i 1 1 I ._!_ _._! 1 1 1 1 1 1 1 I I 

'Of Farooqui, Beaulieu, and others (1981). 

from Wheeler (1982).

10 15 

MILLIONS OF YEARS

20

FIGURE 5.2. K-Ar ages (overall ranges) for predominantly middle and upper Miocene formations in and adjacent 

to the Blue Mountains. Dots, ages; horizontal bars, analytical error; queried where uncertain, dashed where 

approximate.
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 107

for the volcanic-sedimentary facies in The Dalles Basin 

(Newcomb, 1969) and nearly 200 m for a reference section 

in Tygh Basin (Farooqui, Beaulieu, and others, 1981).

FIGURE 5.3. Miocene strata of the Deschutes Formation of Farooqui, 

Beaulieu, and others (1981) exposed on northeast side of The 

Peninsula, Jefferson County, Oreg.; Lake Billy Chinook in foreground. Section is about 185 m thick and composed mostly of 

pumiceous laharic and ash-flow deposits, bedded volcanic sedimentary rocks, a few interbedded basalt flows, and capping olivine basalt 

or basaltic andesite.

FIGURE 5.4. Deschutes Formation of Farooqui, Beaulieu, and others 

(1981) on west wall of Deschutes River canyon; Lake Billy Chinook in 

foreground. Section is about 230 m thick and consists mostly of fineto coarse-grained, poorly to moderately well bedded clastic rocks. 

Columnar olivine basalt flow, exposed approximately one-third way 

up canyon wall, was isotopically dated at 5.8±1.0 Ma., and capping 

olivine basalt flow at 4.9±0.5 Ma (Armstrong and others, 1975).

K-Ar ages on a basalt flow and a tuff in the Tygh Valley 

Formation are, respectively, 7.5±0.8 and 4.9±0.5 Ma; 

ages for rocks from the Chenoweth Formation are 

7.5±0.7, 5.7±0.6, and 5.1 ±0.5 Ma (Farooqui, Bunker, 

and others, 1981). These ages indicate a late Miocene and, 

possibly, earliest Pliocene age and some apparent temporal overlap with the youngest part of the Columbia 

River Basalt Group (fig. 5.2).

AGENCY AND ARLINGTON BASINS

Extensions of The Dalles Group of Farooqui, Beaulieu, 

and others (1981) are found in Agency and Arlington 

Basins within the Deschutes-Umatilla Plateau at the 

northern margin of the Blue Mountains (see fig. 1.2). 

Agency Basin is a structural depression in flows of the 

Columbia River Basalt Group at the northeast end of the 

Agency syncline. Arlington Basin lies along the axis of 

The Dalles-Umatilla syncline and is partly confined on 

the east by the Service anticline, on the south by the 

Willow Creek monocline, on the west by the Turner 

Butte anticline, and on the north by the Columbia Hills 

anticline. Stratigraphic nomenclature of the sedimentary 

units in these basins has recently been revised, and new 

formation names introduced. Within Agency Basin, indurated gravelly sedimentary rocks informally referred 

to as the McKay beds by Shotwell (1956) and as Pliocene 

fanglomerate by Hogenson (1964) were later called the 

McKay Formation by Farooqui, Beaulieu, and others 

(1981). This formation, which consists of partially cemented basaltic gravel and interbedded tuffaceous sand 

and silt, is conformable on the Frenchman Springs

FIGURE 5.5. Laharic and ash-flow deposits at north end of The 

Peninsula, Jefferson County, Oreg., representing part of the Deschutes Formation of Farooqui, Beaulieu, and others (1981). View 

westward.
108 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Member of the Wanapum Basalt and on the Grande 

Ronde Basalt, both part of the Columbia River Basalt 

Group, along the axis of the syncline; near the margins of 

the downwarp it shows angular relations with some of the 

flows. Sections as much as 77 m thick have been 

penetrated in drill holes in the area between McKay 

Creek and the Blue Mountains to the south (Gonthier and 

Harris, 1977, p. 9). Within Arlington Basin, basaltic 

gravel and tuffaceous sedimentary rocks, as much as 40 

m thick, rest on several different units of the Columbia 

River Basalt Group. These fine- to coarse-grained clastic 

rocks, which were given various formal and informal 

names in the past (Alkali lake beds, Shutler Formation, 

and The Dalles Formation), were later called the Alkali 

Canyon Formation by Farooqui, Beaulieu, and others 

(1981).

Although neither of these new formations has been 

dated radiometrically, vertebrate faunas studied by 

Shotwell (1956) indicate a Hemphillian or, in current 

terminology (Berggren and Van Couvering, 1974; Palmer, 1983), late Miocene and early Pliocene age. Within the 

basins there is evidence of interfingering relations of the 

clastic rocks with flows of the Columbia River Basalt 

Group. Whether any part of these sedimentary formations is as old as the Ice Harbor Member (approx. 8.5 Ma) 

of the Columbia River Basalt Group is not known.

WALLA WALLA BASIN

Walla Walla Basin, about 55 km northeast of Agency 

Basin, contains a section of unconsolidated to partially 

consolidated lacustrine and fluvial sedimentary rocks, 

more than 180 m thick (Newcomb, 1965). As much as 150 

m consists of clay that interfingers with sandy to coarsegrained, bouldery alluvial-fan deposits resting unconformably on flows of the Columbia River Basalt Group. 

Clasts in the fan deposits are exclusively of basaltic 

material; calcareous cement is locally present in these 

deposits. Layers of siltstone and conglomerate that 

overlie the clay and alluvial-fan deposits were tentatively 

correlated by Newcomb (1965, p. 22) with the Pliocene 

Ringold Formation of Pasco Basin, a few tens of kilometers to the northwest. Pebbles and cobbles in these beds 

consist mostly of basalt but include many other rock 

types. These beds are overlain, in turn, by the eolian 

Palouse Formation of Pleistocene age, by clay and silt, 

and by more than 30 m of horizontally bedded silt, fine 

sand, and gravel making up the Touchet Beds of Flint 

(1938), also of Pleistocene age. The gravel of the Touchet 

Beds is distinctive in that the cobbles and boulders are of 

highly diverse lithology and include granitic, metamorphic, metavolcanic, and metasedimentary rock types. 

The rocks immediately overlying the Columbia River 

Basalt Group apparently are not exposed at the surface,

but they have been penetrated in many water wells 

drilled within the basin. On the west, Walla Walla Basin 

is separated from Pasco Basin by a low structural 

threshold a few kilometers east of Wallula Gap. Basin fill 

in Pasco Basin is more than 300 m thick and consists of 

lacustrine and fluvial sedimentary rocks and air-fall silicic 

volcanic ash derived from eruptions in the Oregon and 

Washington Cascades. Approximately half the thickness 

of this section of clastic rocks is exposed at the surface, 

principally in the White Bluffs northwest of Pasco; the 

lower half has been penetrated only in drill holes in 

deeper parts of the basin. The lower part of the basin fill, 

which rests on the Saddle Mountains Basalt (Ledgerwood 

and others, 1978), has been assigned to the Ringold 

Formation, described in some detail by Strand and 

Hough (1952) and Newcomb and others (1972), and the 

overlying rhythmically bedded, pebbly silts of glacialflood origin (Waitt, 1980) have been assigned to the 

Touchet Beds.

According to Strand and Hough (1952), the section of 

the Ringold Formation at White Bluffs consists of a lower 

unit, about 27 m thick, of iron-stained pebble and cobble 

conglomerate and an upper, thicker unit of clay, sand, 

clayey sandstone, laminated siltstone, and some thin 

beds of ash, diatomite, gravel, and well-cemented flaggy 

sandstone. A vertebrate fauna collected from the middle 

part of this section indicated a probable middle to late 

Pleistocene age to Strand and Hough (1952) and Newcomb (1958). From an analysis of the structural deformation of the Ringold Formation and its relation to 

underlying flows of the Columbia River Basalt Group, 

however, Brown and McConiga (1960) inferred a somewhat older age. They indicated that the Ringold sedimentary rocks are locally conformable, or nearly so, with 

the underlying Miocene flows, implying that the oldest 

part of the Ringold Formation may possibly be as old as 

latest Miocene or early Pliocene, rather than Pleistocene. 

This older age is supported by data on the vertebrate 

fauna presented by Gustafson (1978), who dated the 

fossiliferous middle part of the formation as Pliocene and 

the stratigraphically lowest fossils in the formation as 

Hemphillian (late Miocene) or older. He considered the 

younger flows of the Columbia River Basalt Group and 

their sedimentary interbeds, as well as the lowermost 

strata of the Ringold, to be early Pliocene in age, which, 

in current epochal terminology, would be considered 

latest Miocene. Paleomagnetic studies by Packer (1979) 

of the Ringold Formation exposed in White Bluff indicate 

an age older than 0.7 Ma.

The poorly consolidated and disconformably overlying 

glacial-flood deposits and loess of the Touchet Beds of 

Flint (1938) are late Pleistocene in age. Ledgerwood and 

others (1978) indicate that the Touchet Beds are overlain 

by Mount St. Helens, Glacier Peak, and Mazama ash
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 109

beds, all of Holocene age, and Waitt (1980) indicated 

some interbedding of these tephra units.

PAULINA BASIN

Paulina Basin, on the south flanks of the Ochoco 

Mountains uplift in the south-central part of the Blue 

Mountains province, is largely localized by a northeasttrending syncline in Miocene flows of the Columbia River 

Basalt Group that are generally considered to be part of 

the Picture Gorge Basalt (Brown and Thayer, 1966; 

Swanson, 1969). In a study of Paulina Basin with 

emphasis on ash-flow tuffs, Davenport (1971) indicated 

that the basin fill has a maximum thickness of slightly 

more that 300 m and includes part of the middle Miocene 

Mascall Formation, an overlying sequence of upper 

Miocene interlayered clastic sedimentary rocks and ashflow tuff, including the Devine Canyon and Rattlesnake 

Ash-flow Tuffs and, possibly, the Prater Creek Ash-flow 

Tuff (Walker, 1979), and upper Miocene or Pliocene 

rim-forming basalt flows called the Ochoco Basalt by 

Davenport (1971).

The Mascall Formation, which is at least 138 m thick in 

Paulina Basin (Forth, 1965), consists of tuffaceous sandstone, conglomerate, a 45-m-thick vitric ash-flow tuff 

(near the base of the formation), and an interbedded thin 

flow of olivine basalt. The rhyolitic ash-flow tuff (table 

5.1) has yielded a K-Ar age originally reported as 

15.8±1.4 Ma (Enlows and Davenport, 1971) but recalculated with new constants at 16.2 ±1.4 Ma (Fiebelkorn and 

others, 1983). The distribution and volume of the ashflow tuff are not known, nor is it clear whether this tuff 

in Paulina Basin is the same as the Mascall Formation 

ash-flow tuff, dated at 13.8 ±0.8 Ma (Fiebelkorn and 

others, 1983) that crops out southwest of the Maury 

Mountains, a few kilometers south of Camp Creek. Vents 

for these ash flows have not been recognized. Interlayering of clastic rocks of the Mascall Formation and flows 

of the Columbia River Basalt Group was indicated by 

Evernden and James (1964, fig. 5) for a section exposed 

in John Day Basin; a K-Ar age on the interlayered basalt 

is 15.4 Ma. Similar interlayering of Mascall-age clastic 

rocks, Picture Gorge flows, and flows of the basalt of 

Prineville compositional type is present southwest and 

west of Paulina Basin in areas adjacent to the Maury 

Mountains; parts of these sections also include units of 

peperite composed of fragmental basalt and silicic tuffaceous sedimentary material.

In addition to the Mascall-age ash-flow tuff, Davenport 

(1971) recognized three separate and younger ash-flow 

tuff layers that are separated by layers of tuffaceous 

sandstone and pebble conglomerate. He considered these 

ash-flow tuff layers to be of Pliocene age and to form part 

of the Danforth Formation of Piper and others (1939),

TABLE 5.1. Chemical analyses of ash-flow tuff from the Mascall 

Formation

[Samples: 1, Rhyolite tuff from Belshaws Ranch (Calkins, 1902); 2, Rhyolitic 

ash-flow tuff from outcrops at confluence of Deer Creek with South Fork of 

John Day River (Davenport, 1971, table 3). Analysis of sample 1, water 

free. Oxides in weight percent; n.r., not reported (all Fe for sample 2 reported 

as FeO); n.d., not determined; , undetected]

Sample 1

SiOo 

AUfV, -

/M2V3 

Ffv>rv> ^-GFeO 

MoO IVlgU - ----- 

PaO

Na~O

tc^n

TiO-> -----

P05 

£* */

MnO- - - -

74.22 

13.50 

1.09 

1.49 

.19 

1.03 

3.69 

4.33 

.35 

.11 

n.d

74.9 

13.3 

n.r. 

2.6 

.5 

.6 

3.33 

4.30 

.18

comprising sequences of volcanic and sedimentary rocks 

exposed in Harney Basin, 100 km to the southeast. 

Regional mapping has shown that these ash-flow tuffs 

are, indeed, the same as those exposed in Harney Basin, 

which Walker (1979) redefined as separate formations 

and named the Devine Canyon, Prater Creek, and 

Rattlesnake Ash-flow Tuffs. K-Ar ages on these tuff 

units are about 9.2, 8.4, and 6.5 Ma, respectively, all of 

late Miocene age.

Collectively, the ash-flow tuff units discontinuously 

cover a very large area on the south flanks of the Blue 

Mountains and extend for many tens of kilometers to the 

south and southeast into and beyond Harney Basin 

(Walker, 1979). The distinctive, crystal-rich Devine Canyon Ash-flow Tuff can be traced through erosional 

remnants on butte and ridge tops east of Paulina Basin 

into the Suplee-Izee area (Dickinson and Vigrass, 1965) 

and Bear Valley. It has also been recognized in stratigraphic sections many kilometers east and southeast of 

Bear Valley in Juntura Basin, east of Castle Rock, and 

along the northern margin of Harper Basin (Walker, 

1979). Erosional remnants of the Prater Creek and (or) 

Rattlesnake Ash-flow Tuff are present in Bear Valley 

and in Logan Valley, where they locally rest with 

discordance on the Miocene Strawberry Volcanies.

The units of ash-flow tuff are rhyolitic (Walker, 1979, 

table 2) and, in the classification of Nockolds (1954), are 

most like average alkali rhyolite.
110 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

The total volume of such upper Miocene ash-flow tuff 

layers, including only the more densely welded ledgeand rim-forming parts of the flows, is about 450 to 500 

km3 (Walker, 1970); if nonwelded parts of the ash flows 

are included, the volume could be as much as 1,000 to 

1,500 km3.

Stratigraphically above the clastic basin fill are thin, 

rim-forming, upper Miocene or Pliocene olivine basalt 

flows and flow breccia that were erupted from lava cones 

generally situated around the margins of the basin.

JOHN DAY VALLEY

To the north of Paulina Basin, in John Day Valley, 

middle Miocene flows of the Picture Gorge Basalt interfinger with and are overlain conformably by clastic rocks 

of the Mascall Formation, which is, in turn, unconformably overlain locally by gravelly sedimentary rocks and 

an interbedded layer of ash-flow tuff, collectively referred to by many investigators (Thayer and Brown, 

1966; Oles and Enlows, 1971; Enlows, 1976) as the 

Rattlesnake Formation of late Miocene and Pliocene(?) 

age (fig. 5.6). Merriam (1901) estimated that 244 to 305 m 

of Mascall Formation clastic rocks are present, and 

Enlows (1976) measured a total thickness of about 192 m 

for the Rattlesnake Formation in the area originally 

considered the type area.

Because of considerable confusion in stratigraphic 

nomenclature for upper Miocene rocks of both the John 

Day region and Harney Basin many kilometers to the

FIGURE 5.6. West end of John Day valley, Grant and Wheeler 

Counties, Oreg. Inclined layered flows at right are part of the Picture 

Gorge Basalt of the Columbia River Basalt Group. Flows are 

conformably overlain by bedded tuffaceous sedimentary rocks of the 

middle Miocene Mascall Formation (white outcrops on left) that, in 

turn, are unconformably overlain by fanglomerate and by the 

ledge-forming upper Miocene Rattlesnake Ash-flow Tuff'on skyline. 

Buttes on skyline are approximately 300 m high. View westward.

south, Walker (1979) redefined several of the stratigraphic units and applied formation names to several 

widespread ash-flow tuffs that had their source in buried 

vents, probably calderas, in Harney Basin (Walker, 

1970a; Blank and Getting, 1974). The ash-flow tuff 

interbedded in what had been called the Rattlesnake 

Formation was elevated to formation status and named 

the Rattlesnake Ash-flow Tuff. Walker's (1979) justification for redefining this unit includes the following: (1) 

This ash-flow tuff layer is one of several key marker 

horizons over thousands of square kilometers of southcentral and southeastern Oregon, whereas the locally 

accompanying fanglomerate deposits are very restricted 

in areal extent; (2) the exposures in and near John Day 

Valley are near the distal end of the ash-flow tuff sheet 

and are not representative of the major part of the unit. 

The original type area (or locality), on Rattlesnake Creek 

about 1.6 km west of Cottonwood, Oreg., was redesignated as a reference locality, and a new type locality was 

established on the west side of U.S. Interstate Highway 

395, about 10 km north of Burns, Oreg. (Walker, 1979).

In contrast to Merriam's estimate of 244 to 305 m for 

the thickness of the Mascall Formation, Thayer and 

Brown (1966) inferred that the formation at Picture 

Gorge, at the west end of John Day Valley, was originally 

1,830 to 2,130 m thick, although it is now preserved in 

thinner erosional remnants in isolated structural basins. 

However, most measured sections of the Mascall Formation are no more than about 300 m thick, and onlap 

relations and sedimentary facies throughout the region 

indicate that this is more nearly the correct maximum 

thickness for the formation. Only slight changes need be 

postulated in the relation of the Mascall Formation to 

flows of the Columbia River Basalt Group inferred by 

Thayer and Brown (1966, fig. 3), in combination with 

some fault repetition of the basalt section exposed on Flat 

Creek, as suggested by Swanson and others (1979, p. 

G13), to drastically reduce the thickness estimates by 

Thayer and Brown from approximately 2,000 m to only a 

small fraction of that figure.

Downs (1956, p. 205) briefly described a section of the 

Mascall Formation at its type locality near Picture 

Gorge, Grant County, Oreg. This section is 119 m thick 

and consists of several varieties of altered and unaltered, 

partly indurated, water-laid and ash-flow tuff composed 

of glass shards and mineral grains and some laminated 

and crossbedded tuffaceous sandstone; both the tuff and 

sandstone locally contain a Barstovian (middle Miocene) 

vertebrate fauna. Local swampy conditions are indicated 

in the Mascall Formation by the presence of lenticular 

lignite beds, as much as 1.5 m thick, a few kilometers to 

the east (Thayer, 1957). Farther to the east, gravelly 

sandstone and conglomerate, the pebbles of which are 

largely 2 to 3 cm in diameter, highly polished, and mostly
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 111

of pre-Cenozoic rocks, become more abundant in the 

formation; a layer of ash-flow tuff, 14 to 15 m thick, locally 

forms the basal part of the formation. A K-Ar age of 15.4 

Ma was determined by Evernden and others (1964, 

sample KA-1203) for the Mascall Formation of the John 

Day Valley area. This K-Ar age was confirmed by 

radiometric dating of a nearby (interbedded?) flow of the 

Columbia River Basalt Group. On these bases the Mascall Formation is here regarded as middle Miocene in age. 

Rocks referred to by previous workers as the Rattlesnake Formation include an upper deposit of coarsegrained fanglomerate of remarkably uniform lithology 

that disconformably overlies vitric ash-flow tuff, now 

referred to as the Rattlesnake Ash-flow Tuff (Walker, 

1979), and a disconformably underlying lower deposit of 

fanglomerate with thick lenticular interbeds of sandstone 

and mudstone (Enlows, 1976).

FOX BASIN

In Fox Basin, about 20 km north of John Day Valley, 

pebble conglomerate and tuff of the Mascall Formation 

are preserved in a shallow structural depression partly 

confined by northwest-trending faults and a gentle synclinal warp (Brown and Thayer^ 1966). According to 

Thayer (1957, p. 236) a series of "Columbia River Basalt 

flows wedges out southeastward over yellowish rhyolite 

tuff and Mascall-type pebble gravels in the northwest 

corner of Fox Valley, and thin basalt flows lie on and have 

been faulted together with tuffaceous beds in the southern part of the basin."

JUNTURA BASIN

In Juntura Basin, at and just south of the southern 

margin of the Blue Mountains, a maximum composite 

thickness of about 670 m of silicic tuff, palagonite tuff and 

breccia, and fine-grained tuffaceous sedimentary rocks, 

including ashy diatomite, rests disconformably, possibly 

with slight angular discordance (Bowen and others, 

1963), on Miocene basalt and andesite, in part representing flows of the Strawberry Volcanics. The section of 

clastic rocks and diatomite thins northward in the Blue 

Mountains province and, in places, appears to contain 

larger proportions of palagonitic tuff. The basin deposits, 

including the Juntura Formation of Barstovian age, the 

Devine Canyon Ash-flow Tuff, dated at about 9.2 Ma 

(Walker, 1979), and the Drewsey Formation, which 

contains a Hemphillian fauna (Shotwell, 1963), are in part 

equivalent in age to the Strawberry Volcanics and to 

parts of the Columbia River Basalt Group. Even though 

the Juntura Basin section impinges on the Strawberry 

Volcanics and is approximately coeval, clear interfingering or gradational relations have not been recognized.

HARPER BASIN

The northern edge of Harper Basin, which is the next 

major Miocene and younger(?) basin to the east of 

Juntura Basin, extends into the southernmost margin of 

the Blue Mountains south of the Juniper and Ironside 

Mountains. The basin is a partly faulted downwarp 

developed in middle Miocene silicic and mafic volcanic 

rocks that include the Strawberry Volcanics and units 

named the Littlefield Rhyolite and Hunter Creek Basalt 

by Kittleman and others (1965). The basin fill consists of 

fine- and coarse-grained epiclastic and volcaniclastic 

rocks, diatomite, and several thin basalt flows (fig. 5.7) 

that together aggregate to nearly 600-m thickness in the 

deepest part of the basin; the volcanic ash in many beds 

has been diagenetically altered to zeolites and clay 

minerals. Kittleman and others (1965) delineated several 

units, including the Drip Spring and Bully Creek Formations. The section thins northward and pinches out on 

older Miocene volcanic rocks; along the northern margin 

of the basin it is dominated by several layers of ash-flow 

tuff of different lithologic types, and it commonly occurs 

in disconnected patches (Brooks and others, 1976). The 

distinctive, crystal-rich Devine Canyon Ash-flow Tuff, 

dated at about 9.2 Ma, is present in the section, as well 

as isolated patches of an older welded tuff, possibly 

equivalent to the Dinner Creek Welded Ash-flow Tuff of 

Haddock (1967, 1968) (equivalent to the Dinner Creek 

Welded Tuff as used by Greene and others, 1972), and 

one or more younger pumiceous, vitric ash-flow tuff 

layers, possibly representing distal ends of the Prater 

Creek and (or) Rattlesnake Ash-flow Tuffs of Harney 

Basin, 100 km to the southwest. The Dinner Creek 

Welded Ash-flow Tuff, which Haddock (1966) considered 

to have a volume of more than 25 km3 and to have been

FIGURE 5.7. Bedded tuffaceous sedimentary rocks, partly diagenetically altered to clay minerals and zeolites, exposed in northern part 

of Harper Basin, northern Malheur County, Oreg. Part of the upper 

Miocene Bully Creek Formation. Bluff in background is about 100 m 

high. View northward.
112 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

erupted from a fissure now occupied by a rhyolite dike at 

Castle Rock, is well exposed in Malheur Gorge (fig. 5.8) 

and in discontinuous patches to the north and northeast 

through Willow Creek basin and into Unity Basin. It has 

been dated by K-Ar methods at 15.3±0.4, 14.9±0.4, 

14.7 ±0.4, and 13.1 ±0.4 Ma from widely separated areas 

on the south flanks of the Blue Mountains. Considering 

these discordant ages, more than one ash flow may have 

been identified by various workers in the region as the 

Dinner Creek Welded Ash-flow Tuff. If so, one or more 

of these tuff layers may be the same as some of those 

recognized in the Mascall Formation in Paulina Basin and 

the John Day Valley area (fig. 5.2).

BOISE BASIN

Along the southeastern margin of the Blue Mountains 

and in the adjacent Owyhee Upland and Columbia 

Intermontane provinces are extensive outcrop areas of 

Miocene and younger sedimentary rocks and minor 

volcanic rocks that represent the northward and northwestward extensions of sedimentary and volcanic sequences of Boise Basin (Bond, 1978). Many formation 

names have been applied to these rocks, depending on 

their geographic position within the basin and their 

lithology, and several regional correlations have been 

attempted (Malde and Powers, 1962; Kittleman and 

others, 1965; Armstrong, 1975). In Oregon, they generally have been referred to as the northward extensions of 

the Miocene Deer Butte and Pliocene Chalk Butte 

Formations of Corcoran and others (1962), or of the Bully 

Creek and Grassy Mountain Formations of Kittleman and 

others (1965); in Idaho, they have been referred to as the 

Payette (Lindgren, 1898; Russell, 1902), Poison Creek, 

Chalk Hills, Glenns Ferry, Bruneau, and Idaho Formations, the "Caldwell-Nampa sediments" of Mitchell and

FIGURE 5.8. Faulted middle Miocene Dinner Creek Welded Tuff 

(arrows) in Malheur Gorge, northern Malheur County, Oreg. Highest 

bluff in background is about 500 m high. View westward.

Bennett (1979), and the Cenozoic "undifferentiated 

rocks" of Malde and Powers (1962). Collectively, most of 

the middle Miocene and Pliocene sedimentary rocks and 

interbedded flows have been considered to be a part of 

what is referred to as the Idaho Group, and an overlying 

Pleistocene and Holocene sequence of interstratified 

basalt flows and gravels has been called the Snake River 

Group.

The basin fill ranges in thickness from about 900 m in 

areas to the south, in the deepest parts of Boise Basin, to 

near 0 along the northern margin of the basin. There it 

pinches out, principally by onlap on middle Miocene 

volcanic rocks, including basalt flows mapped as part of 

the Columbia River Basalt Group in Idaho, and on 

basaltic to rhyolitic volcanic rocks identified as the 

Strawberry Volcanics, Littlefield Rhyolite, Dinner 

Creek Welded Tuff, or associated unnamed rocks in 

Oregon.

Most of the formations of the Idaho Group consist of 

siliceous volcanic ash and fine-grained tuffaceous detrital 

material in both bedded and massive sequences, particularly toward the central part of the basin, and of 

somewhat coarser grained, commonly conglomeratic and 

partly locally derived detritus, intermixed with tuffaceous sandstone and siltstone, near the basin margins. 

Both lacustrine and fluviatile deposits are represented. 

Locally, thin basalt flows are interbedded in the Idaho 

Group; along the northwestern margin of the basin, near 

the boundary of the Blue Mountains and the Owyhee 

Upland province, several thin flows of diktytaxitic, 

subophitic to intergranular olivine basalt and basaltic 

andesite cap the Miocene and Pliocene(?) sedimentary 

rocks (Brooks and others, 1976). The olivine basalt flows 

were erupted from several small cinder and lava cones 

along the margin of the basin. The capping basalts are 

undated but are probably either late Miocene or early 

Pliocene in age. Other parts of the section have been 

dated as Miocene, Pliocene, and Pleistocene from contained vertebrate fossils and from K-Ar ages (Armstrong, 1975) that range from about 13 to less than 1 Ma.

Much of the fine-grained debris in the basin fill is 

siliceous volcanic ash or its alteration products, including 

clays, zeolites, and secondary silica minerals. Most of this 

detrital material is related to large-volume eruptions of 

tuff from calderas in Harney Basin during the late 

Miocene. Some of the ash may be related to middle 

Miocene eruptions of ash flows now recognized as part of 

either the Mascall Formation or the Dinner Creek 

Welded Tuff; some may be related to Miocene eruptions 

in the McDermitt-White Horse caldera complex, nearly 

250 km south-southwest of the central part of Boise Basin 

and to Miocene calderas south of Ontario, Oreg. The total 

volume of siliceous debris in these sedimentary sequences in the basin is not known; detailed information on the
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 113

configuration of the basin floor and the thickness of the 

basin fill apparently is almost nonexistent, and so volume 

estimates are approximate at best. Furthermore, the 

relative proportions of silicic ash and other detrital 

constituents are unknown. Certainly, thousands of cubic 

kilometers of sediment was deposited in Boise Basin, and 

much of it still remains after extensive erosion. As much 

as 75 percent of the fill is composed of fresh or altered 

vitric ash and pumice. The very large total volume of 

siliceous volcanic ash indicates voluminous silicic volcanic 

activity in adjoining areas to the west and north and 

possibly to the south.

BASINS NORTH OF HARPER AND BOISE BASINS

Several isolated and comparatively large depositional 

basins are present to the north of Harper and Boise 

Basins in and adjacent to the Blue Mountains, including 

Mormon, Unity, and Durkee Basins and Willow Creek 

basin (fig. 5.1), all containing middle Miocene and younger volcanic and volcaniclastic rocks, as well as some 

coarse-grained conglomeratic fluviatile rocks. Additional, structural and erosional depressions filled with epiclastic and volcaniclastic rocks and a few interstratified 

basalt and andesite flows have been recognized in the 

Burnt River valley and along several other drainages in 

the region.

In and adjacent to Mormon Basin, Wolff (1965) reported 250 to 300 m of Miocene volcanic rocks, including the 

Dinner Creek Welded Ash-flow Tuff, coarse-grained 

mudflow deposits, and tuffaceous sedimentary rocks, as 

well as several interbedded basalt flows and rhyolite flow 

breccia, all probably temporal equivalents of parts of the 

Strawberry Volcanics. Disconformably overlying this 

volcanic assemblage are coarse-grained fluvial conglomerate, sandstone, and mudstone deposits that Wolff 

(1965, p. 148) considered to be of Miocene age. Unconformably overlying these coarse-grained clastic rocks is a 

unit of partly lacustrine, light-colored tuffaceous siltstone 

and sandstone with interlayered lenses of diatomite; 

coarse-grained conglomeratic facies are present near the 

basin margins. The tuffaceous siltstone and sandstone 

unit has an aggregate maximum thickness of more than 

180 m, but it thins and pinches out on older rocks along 

the margins of the basin. Wolff (1965, p. 173) considered 

these rocks to be of Pliocene age, partly on the basis of 

correlation with tuffaceous sedimentary rocks in adjacent 

areas that contain a vertebrate fauna originally identified 

as early Pliocene in age. In current geochronologic 

terminology, that vertebrate fauna would most likely be 

considered of late Miocene age. Thin, gently dipping 

olivine-bearing basalt and basaltic andesite flows that 

were erupted from small central vents cap the sedimentary basin fill in places.

Unity Basin, 15 to 20 km west-northwest of Mormon 

Basin, and the Willow Creek basin (fig. 5.1), west of 

Mormon Basin, are largely fault-controlled depressions 

that contain fill a few tens to several hundreds of meters 

thick consisting of beds of conglomerate, claystone, and 

fine-grained water-laid tuff (fig. 5.9) rich in glass and 

pumice (Thayer and Brown, 1966). Miocene and Pliocene 

fossils considered of possible late Miocene age have been 

recovered from beds in Unity Basin (Thayer, 1957, p. 

238). East of Unity Basin, along the Burnt River valley, 

fluvial gravel, indurated ash beds, tuffaceous sedimentary rocks, and one or more thin layers of vitric ash-flow 

tuff have been incorporated in a unit designated as 

Miocene and Pliocene "tuffaceous sedimentary rocks" by 

Brooks and others (1976).

Durkee Basin, about 30 km northeast of Mormon Basin 

(fig. 5.1), is a grabenlike structure partly confined by 

northwest-trending normal faults. The basin contains 

sedimentary and volcanic deposits that total more than 

350 m in thickness; epiclastic deposits, which coarsen 

toward the basin margins and locally are conglomeratic, 

in places contain vertebrate fossils of Clarendonian (late 

Miocene) age. Prostka (1967) described the basin fill as 

mostly fine- to medium-grained, well-bedded, poorly 

consolidated sand, silt, and varying amounts of intermixed ash and pumice. Both fluvial and lacustrine deposits are present. Also recognized are ash beds, in part 

diagenetically altered to zeolites, potassium feldspar, 

clay minerals, and silica minerals (Gude and Sheppard, 

1978), as well as diatomite and one or more layers of 

ash-flow tuff. One welded tuff locally underlies and 

elsewhere is interbedded in the sedimentary sequence. 

According to Gude and Sheppard (1978), the diagenetic

FIGURE 5.9. Poorly to moderately well bedded upper Miocene tuffaceous sedimentary rocks in Unity Basin, Baker County, Oreg. Bluff 

in background is about 50 m high. View northeastward.
114 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

minerals indicate that a closed, saline, alkaline lake 

occupied part of Durkee Basin during the Pliocene (late 

Miocene time in current terminology). Local thin flows of 

platy olivine basalt, erupted from small central vents, are 

present in areas marginal to the basin and appear to 

predate the sedimentary basin fill.

BASINS NORTH AND NORTHWEST OF DURKEE BASIN

North and northwest of Durkee basin, silicic pyroclastic and tuffaceous sedimentary rocks fill several separate 

structural basins developed in flows of the Columbia 

River Basalt Group and pre-Cenozoic rocks. These structural basins, including Baker Valley, lower Powder River 

valley, and Richland Valley, contain sections about 150 m 

or more thick. As pointed out by Brooks and others 

(1976), the clastic basin fill apparently does not interfinger with the flows of the Columbia River Basalt Group; 

but if the Clarendonian (late Miocene) age postulated by 

Prostka (1967) is correct, some of these clastic rocks may 

be temporal equivalents of younger late Miocene flows of 

the central part of the Columbia Plateau.

In general, the basin fill consists of a lower lacustrine 

facies of thin-bedded, light-colored tuffaceous claystone, 

siltstone, and sandstone, tuff, and diatomite, and an 

upper fluvial facies of pebble conglomerate interbedded 

with fine-grained tuffaceous sandstone and siltstone. In 

places, much of the volcanic ash and pumice is diagenetically altered to zeolites, clay minerals, and secondary 

silica minerals. Several thin (maximum 8-m-thick) layers 

of vitric ash-flow tuff, mostly composed of abundant 

well-sorted, pinkish-brown to buff and nearly colorless 

glass shards, and some pumice lapilli and rare feldspar, 

biotite, and rock fragments, are interstratified with the 

sedimentary rocks (fig. 5.10). The ash-flow tuff layers 

show varying degrees of compaction and welding of 

shards and pumice, in a few places exhibiting welldefined eutaxitic texture and elsewhere showing essentially no flattening or deformation of shards and lapilli. 

The source of the ash-flow tuff deposit is not known; 

lithologically, they appear to be most like the upper 

Miocene Prater Creek and Rattlesnake Ash-flow Tuffs of 

Harney Basin (Walker, 1979), about 8.4 and 6.5 Ma, 

respectively, and they appear to be in a similar or near 

similar stratigraphic position. However, much additional 

fieldwork and geochemical and radiometric-age data are 

necessary to either firmly establish or reject these 

possible correlations.

Grande Ronde Valley, to the north of Baker Valley, is 

a complex graben developed in flows of the Columbia 

River Basalt Group; the graben contains as much as 600 

m of fine-grained, poorly consolidated lacustrine deposits, mostly claystone, siltstone, and sandstone (Hampton 

and Brown, 1964). Poorly sorted, bouldery alluvial-fan

deposits are present in many places along the precipitous 

margins of the graben. A Pleistocene(?) age was assigned 

to the lacustrine deposits by Hampton and Brown (1964), 

but in current age terminology some of the earliest of 

these deposits probably would be considered Pliocene 

and, possibly, even latest Miocene.

MIOCENE VOLCANIC ACCUMULATIONS

Interspersed among the sedimentary basins along the 

south and southeast flanks of the Blue Mountains are 

accumulations of Miocene eruptive rocks in the form of 

shield volcanos, cones transitional between shields and 

stratovolcanos, large complex domal masses, local flow 

sequences, and several kinds of related intrusions. In 

places, these volcanic rocks impinge on and intermingle 

with the Miocene sedimentary basin-fill deposits, but 

elsewhere they form separate and distinct eruptive piles. 

The rocks are of several types of eruptive materials, 

including basaltic to rhyolitic calc-alkaline flows and flow 

breccia of the Strawberry Volcanics, rhyolitic flows, 

breccia, tuff, and intrusions of the igneous complex at 

Dooley Mountain, basaltic to andesitic peperite, palagonitic tuff, minor flows of high-alumina (plagioclase-rich) 

ophitic basalt, and several unnamed domal and intrusive 

complexes of dacite and rhyolite.

STRAWBERRY VOLCANICS

The largest and most diverse group of these eruptive 

rocks, named the Strawberry Volcanics by Thayer

FIGURE 5.10. Poorly exposed, fine-grained, tuffaceous sedimentary 

rocks containing interlayers of pumiceous ash-flow tuff at Virtue 

Flat, Baker County, Oreg. Vertebrate fossils indicate middle to late 

Miocene age for the sedimentary rocks. Source of ash flow is 

unknown; lithologically, it resembles distal end of either the Prater 

Creek or Rattlesnake Ash-flow Tuff of Harney Basin. Outcrops range 

in height from about 3 to 12 m. View westward.
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 115

(1957), is mostly coeval with the Columbia River Basalt 

Group; radiometric ages mostly indicate an age range of 

about 20 to 10 Ma (fig. 5.2) for the formation. There is 

some question as to whether the K-Ar ages older than 

about 16 Ma were obtained on rocks representing lower 

parts of the Strawberry Volcanics or on some older unit. 

The Strawberry Volcanics is here considered to be 

middle and late Miocene in age. The Strawberry Volcanics is exposed widely in southeastern Grant County and 

parts of adjoining counties on the south flanks of the Blue 

Mountains (fig. 5.1; Brown and Thayer, 1966; Robyn, 

1977,1979). Most of the volcanic rocks are flows and flow 

breccia of olivine basalt, basaltic andesite, and pyroxene 

andesite, although they also include local large masses of 

dacite or rhyodacite. Chemically, they show great diversity (fig. 5.11); Si02 contents range from about 51 to 72 

weight percent. Comparisons with average compositions 

of Rock Creek, Frenchman Springs, and Pomona chemical types (Swanson and others, 1979), all parts of the 

Columbia River Basalt Group and all about the same age 

as the Strawberry Volcanics, show (fig. 5.11) that the 

Strawberry Volcanics is almost invariably lower in CaO, 

Ti02 , and MgO contents and that its rocks tend to be 

higher in Na20 and K20 contents for comparable Si02 

contents. Comparisons with the average composition of 

the Prineville chemical type, also apparently coeval with 

part of the Strawberry Volcanics, show similar alkali, 

higher A1203 , and lower Ti02 and Fe203 contents.

Numerous vents for these rocks are present in the 

vicinity of Strawberry and Lookout Mountains, 25 km or 

more southeast of John Day, and several large masses of 

silicic rocks were erupted from local large domal complexes east and southeast of Bear Valley.

The total volume of the calc-alkaline Strawberry Volcanics is difficult to estimate, largely because the amount 

of relief on the surface on which these eruptive rocks 

were deposited is poorly known. As shown by Brown and 

Thayer (1966), they cover an area north of lat 44° N. of 

more than 3,400 km2 and locally are much more than 

2,000 m thick. Thayer (1957) recognized a section more 

than 2,200 m thick dominated by basaltic to andesitic 

hypersthene-bearing lava on the north slopes of Strawberry Mountain; Thayer and Brown (1973) indicated a 

total maximum thickness of 2,700 to 3,000 m for a section 

composed mainly of rhyolite and andesite in Ironside 

Mountain, a few kilometers southwest of Willow Creek 

basin. Sections hundreds of meters thick also are exposed 

on canyon walls in areas south, southwest, and southeast 

of Logan Valley.

If the average thickness of these volcanic rocks over 

3,400 km2 of the outcrop area north of lat 44° N. is 

assumed to be 2 km, then their total volume is 6,800 km3 . 

An additional volume of these rocks is present south of lat 

44° N. on the south flank of the Blue Mountains.

OTHER VOLCANIC ACCUMULATIONS

In Paulina Basin and westward around the margins of 

the Maury Mountains, flows, breccia, and local peperite 

of basalt and basaltic andesite are interstratified with 

tuffaceous sedimentary rocks that in places contain a 

Barstovian (Mascallian) fauna. Apparently, none of these 

rocks has been dated by radiometric methods, but their 

Barstovian faunal age and stratigraphic relations indicate 

that they are coeval with parts of the Columbia River 

Basalt Group. Although some of these flows show chemical similarities with flows of the group, others are 

chemically quite different and, as shown by Osawa and 

Goles (1970), contain trace-element abundances that 

differ significantly from any known abundances in the 

Columbia River Basalt Group.

Southwest of Paulina, on Camp Creek, basalt flows 

with good columnar jointing, presumably correlative 

with the flows at Picture Gorge, are sandwiched between 

silicic tuff and tuffaceous sedimentary rocks; the basalts 

pinch out southward on underlying sedimentary rocks of 

the John Day Formation and are, in turn, overlain by 

bedded tuff and tuffaceous sedimentary rocks that contain a Barstovian vertebrate fauna. An analysis of one of 

the flows shows 48.8 weight percent Si02 , 15.3 weight 

percent A1203 , 10.6 weight percent CaO, and low K20 

and Ti02 contents. This composition is most like the 

average composition of flows of the Picture Gorge Basalt 

given by Swanson and others (1979, table 2, column 1), 

although it is somewhat less silicic.

Flows of ophitic, high-alumina (plagioclase-rich) basalt 

are interstratified in the middle Miocene part of the 

sedimentary section both in the west part of Paulina 

Basin and on Bear Creek at the west end of the Maury 

Mountains. In major-oxide chemistry (fig. 5.11) these 

rocks are unlike any of the chemical averages of flows of 

the Columbia River Basalt Group (Swanson and others, 

1979, table 2). A few flows at the west end of the Maury 

Mountains are like Picture Gorge flows in their traceelement abundances, but interstratified ophitic flows are 

chemically unlike any flows of the Columbia River Basalt 

Group (Osawa and Goles, 1979, table 4, samples 

27A-27E). The major-oxide chemistry of the ophitic 

flows is more nearly comparable to that of flows in the 

Steens Basalt (Gunn and Watkins, 1970; Walker, 1970b) 

of southeastern Oregon, particularly in their high A1203 

and CaO and relatively low K20 contents.

Middle Miocene silicic volcanic rocks are widely exposed in the mountain ranges south of Baker, particularly 

in the Dooley Mountain area and in ranges both north and 

south of Mormon Basin. The silicic rocks consist of a wide 

variety of flows, breccia, intrusive rocks, and related 

air-fall and ash-flow tuff, presumably all of rhyolitic 

composition. Gilluly (1937) named the silicic rocks on
116 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Dooley Mountain and in adjacent areas the Dooley 

Rhyolite Breccia and considered those of Dooley Mountain to mark the site of a volcanic eruptive center.

These rocks have been.radiometrically dated in only 

one place on Dooley Mountain, so that their age range 

and precise correlation with other Miocene units are not

EXPLANATION

+ Analyses of Strawberry Volcanics from Thayer (1957)

 Analyses of Strawberry Volcanics from Robyn (1977) 

x Analyses of Strawberry Volcanics at Ironside 

Mountain (from Thayer and Brown, 1973)

 Unpublished (USGS) analyses of Miocene flows 

adjacent to the Maury Mountains, along south 

margin of Blue Mountains province

® Average Prineville chemical type of basalt from 

Uppuluri (1974)

A Average (68 analyses) of Rock Creek chemical type. 

Columbia River Basalt Group (Swanson and 

others, 1979)

-\- Average (8 analyses) of Frenchman Springs chemical

type, Columbia River Basalt Group (Swanson and

others, 1979) 

V Average (30 analyses) of Pomona chemical type.

Columbia River Basalt Group (Swanson and

others, 1979) 

X Average (18 analyses, including several of Steens

Basalt) of Miocene basalt flows of southeastern

Oregon (Walker, 1979)

19

17

15

UJ

0 

CC 

UJ

o_ 11

1-

X

o

UJ

^ 12

 ^CONTEr 

o

UJ

d 

X 8

0

6

4

2

n

\ \"" i 

AIA

- x .x.V + +

- \ *X ' * ' .

X 

X

1 1 1

1 1 1

 CaO

 v

" *.-

2» ~

x * _

+ .+

X

* -

1 1 l x

50 60 70 80

16

14

12

10

Fe2 03

50 60 70 80 

SiO2 CONTENT, IN WEIGHT PERCENT

- X

50 60

Na0

K.O

MgO

70 80

FIGURE 5.11. Harker diagrams of the Strawberry Volcanics and temporally related Miocene flows along south margins of Blue Mountains, 

showing comparisons with selected average compositions of chemical types within the Columbia River Basalt Group.
MIOCENE AND YOUNGER ROCKS OF THE BLUE MOUNTAINS REGION 117

known. A K-Ar age of 14.7±4 Ma (Fiebelkorn and others, 

1983) on intrusive rhyolite exposed in a quarry near the 

summit of Dooley Mountain indicates approximate temporal equivalence with the Dinner Creek Welded Tuff, 

the Mascall Formation, and both the Picture Gorge and 

Grande Ronde Basalts of the Columbia River Basalt 

Group.

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Stradling, Dale, eds., Columbia River Basalt Symposium 2, Wash. 

[Procs.]: Cheney, Eastern Washington State College Press, p. 

223-237. 

-1979, Revisions to the Cenozoic stratigraphy of Harney Basin,

Southeastern Oregon: U.S. Geological Survey Bulletin 1475, 35 p.

Waters, A.C., 1968, Reconnaissance geologic map of the Madras 

quadrangle, Jefferson and Wasco Counties, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-555, 

scale 1:125,000.

Wheeler, Greg, 1982, Problems in the regional stratigraphy of the 

Strawberry Volcanics: Oregon Geology, v. 44, no. 1, p. 3-7.

Williams, Howel, 1957, A geologic map of the Bend quadrangle and a 

reconnaissance geologic map of the central portion of the High 

Cascade Mountains: Oregon Department of Geology and Mineral 

Industries, in cooperation with the U.S. Geological Survey, scales 

1:125,000 and 1:250,000.

Wolff, E.N., 1965, The geology of the northern half of the Caviness 

quadrangle, Oregon: Eugene, University of Oregon, Ph.D. dissertation, 200 p.
6. CENOZOIC TECTONISM AND VOLCANISM OF 

THE BLUE MOUNTAINS REGION

By GEORGE W. WALKER and PAUL T. ROBINSON 1

CONTENTS

Page

Abstract 119 

Introduction 119 

Basement beneath the Cenozoic rocks 120 

Cenozoic era 121 

Paleocene(?) and Eocene 121 

Oligocene and early Miocene 127 

Miocene to Holocene 128 

Summary and conclusions 131 

References cited 133

ABSTRACT

The extensive record of Cenozoic volcanism preserved in the Blue 

Mountains region of northeastern Oregon and adjacent Washington and 

Idaho indicates a more or less continuous eruptive history interspersed 

with a few apparent brief hiatuses and several periods of deformation. 

The types and volumes of volcanic products varied both geographically 

and over time. The earliest volcanism, represented mostly by the 

dominantly Eocene Clarno Formation in Oregon and the partly coeval 

Challis Volcanics in Idaho, has been attributed by some workers to a 

west:northwest-trending volcanic arc, commonly identified as the 

Challis arc. The distribution of the Clarno and coeval rocks in Oregon 

seem to indicate, however, that they are more likely related to a 

north-south-trending volcanic arc, the axis of which lay to the east of the 

present Cascade Range.

Subsequent volcanic activity both in and marginal to the Blue 

Mountains appears to be related to (1) younger arc volcanism, with the 

axis of eruptive activity in or near the Cascade axis; (2) extensional 

tectonics that generated bimodal volcanism along the southern margin 

of the Blue Mountains province and large volumes of Miocene flood 

basalt erupted principally from two major dike swarms located within 

the province; and (3) numerous isolated eruptive centers within the 

province that probably also are related to extensional tectonism.

INTRODUCTION

Since 1980, several summaries have synthesized the 

vast amount of data on the distribution, composition, and 

age of igneous activity in the western Cordillera, as well 

as on its structural evolution (Hamilton and Myers, 1966;

'Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada.

Christiansen and Lipman, 1972; Lipman and others, 

1972; Snyder and others, 1976; Armstrong, 1978; Christiansen and McKee, 1978; Smith, 1978). The purpose of 

this chapter is to consider in more detail the tectonic and 

volcanic history of the Blue Mountains region of Oregon 

and contiguous areas of Washington and Idaho. We 

examine the relations of Cenozoic igneous activity and 

coeval structural deformation of the region, review 

evidence for the persistence and continuity of volcanism, 

discuss the types, ages, and distribution of volcanic 

products, including volume relations where data permit, 

and consider how the underlying pre-Cenozoic basement 

affected the Tertiary volcanism. The preceding chapters 

in this volume clearly demonstrate that some parts of the 

Cenozoic column are far better known than others and 

that our knowledge of the age and distribution of 

different rock types is uneven.

The Blue Mountains region is a complex uplifted 

crustal block composed of Cenozoic and older rocks 

fringed on the north, west, and south by Miocene and 

younger volcanic rocks and terrestrial sedimentary 

rocks. The Blue Mountains crustal block has no welldefined eastern topographic or structural boundary and 

appears to merge into the uplifted Columbia Intermontane province of Idaho.

On the south, the Blue Mountains block is bounded 

partly by the northwest-trending Brothers fault zone, 

which separates the Blue Mountains from the High Lava 

Plains part of the Basin and Range province. The fault 

zone is expressed at the surface by closely spaced 

(commonly 0.5-3 km apart) en echelon normal faults of 

small to moderate displacement, mostly 10 to 100 m. 

Basaltic and rhyolitic vents of late Miocene through 

Pleistocene age are abundant within and adjacent to the 

fault zone. The ages of these vents indicate that recurrent 

crustal breaking has taken place along the zone since at 

least Miocene time. The normal faults of the zone and the 

many volcanic vents are interpreted as the surface 

manifestations of a large, deeply buried structure. Some 

vertical offset apparently has occurred along this zone, 

inasmuch as pre-Cenozoic and early Cenozoic rocks occur 

at the surface only in the block to the north; the pattern

119
120 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

of normal faults within and near the fault zone and the 

relation of many small monoclinal folds to the faults also 

indicate some lateral displacement. The normal faults 

denote only adjustment of near surface brittle volcanic 

and tuffaceous sedimentary rocks to movement on the 

deeply buried structure. Analysis of surface features 

permits either right- or left-lateral displacement of the 

deep-crustal fault, although right-lateral (dextral) movement seems more likely (Walker and others, 1967; 

Walker, 1969; Lawrence, 1976). The zone is interpreted as 

a transcurrent-fault zone separating a major block (or 

blocks) on the south, in which significant east-west 

extension has occurred, from the Blue Mountains block to 

the north, which has comparatively less extension and 

more restricted late Cenozoic volcanism.

The less well defined northern margin of the Blue 

Mountains block consists of a series of northeast-trending, faulted monoclines and associated anticlines and 

synclines that extend from near the Deschutes River on 

the west to the vicinity of Pendleton and Walla Walla on 

the east (fig. 6.1). In many places, the north boundary is 

represented by abrupt changes in dip on flows of the 

Columbia River Basalt Group; west of Lewiston, Idaho, 

the inclination of flows is generally to the west, although 

some local flexures disrupt the regional dip. The northeast-trending monoclines and faults are locally intersected by northwest-trending folds and faults, including 

such enigmatic structures as the Olympic-Wallowa lineament. Most of the northwest-trending faults within the 

Blue Mountains block exhibit dip-slip movement, but 

some show strike-slip, mostly dextral, movement.

Bounding structures on the southern and northern 

margins of the Blue Mountains block disrupt Miocene and 

older rocks; toward the west end of the Brothers fault 

zone, on the southern margin of the block, upper Miocene, Pliocene, and, locally, lower Pleistocene rocks also 

are disrupted.

Along its western margin the block is onlapped by 

upper Miocene and Pliocene(?) sedimentary rocks, volcaniclastic rocks, and basalt flows of the Madras Formation (the Deschutes Formation of Farooqui and others, 

1981) and by overlying Quaternary flows, poorly consolidated sedimentary rocks, and ash. The time of inception 

of the bounding structures is poorly delimited, but these 

structures are thought to reflect interactions at the 

boundary between the North American plate and several 

crustal plates lying to the west during Miocene and more 

recent time.

BASEMENT BENEATH THE CENOZOIC ROCKS

The pre-Tertiary basement rocks in the Blue Mountains region vary lithologically and represent both allochthonous and autochthonous terranes (Jones and others,

1977; Brooks and Vallier, 1978; Dickinson, 1979b). The 

entire region appears to represent a zone, at least 500 km 

wide, that is transitional between regions underlain by 

old continental crust on the east and by young oceanic 

crust on the west. Within this transition zone, several 

petrologic-tectonic terranes have been recognized, all of 

which have been partly metamorphosed and locally 

intruded by Mesozoic granitic plutons. The different 

terranes are arranged in belts subparallel to a northeasttrending regional gravity gradient (Berg and Thiruvathukal, 1967; Thiruvathukal and others, 1970, fig. 1) 

and to the trend of the Blue Mountains uplift itself. Data 

from a single refraction profile extending approximately 

north-south across this gravity gradient were interpreted by Hill (1972; 1978, p. 160) to indicate a change in 

crustal thickness from approximately 25 km beneath 

Pasco Basin on the north to about 45 km beneath the Blue 

Mountains uplift to the south (Thiruvathukal and others, 

1970). However, recent detailed seismic-reflection and 

deep-refraction studies in Pasco Basin indicate a crustal 

thickness of about 42 km, rather than 25 km (W.D. 

Mooney, oral commun., 1985).

Initial 87Sr/86Sr ratios of Cenozoic rocks increase from 

about 0.703-0.704 in the Cenozoic volcanic region of 

east-central Oregon and in the Cascade Range on the 

west to about 0.706-0.709 in the Challis Volcanics and 

Absaroka Volcanic Supergroup on the east (Leeman, 

1982). Both the Absaroka Volcanic Supergroup and 

Challis Volcanics are east of the line representing an 

initial 87Sr/86Sr ratio of 0.706, which has been inferred to 

be related to the edge of Precambrian continental crust in 

the Western United States (fig. 6.2). The Cenozoic 

volcanic rocks of the Blue Mountains and coeval volcanic 

rocks of the Cascade Range in Washington are west of 

this line and are underlain in part or entirely by accreted 

terranes and dismembered oceanic crust. Armstrong and 

others (1977) determined an initial 87Sr/86Sr ratio in 

Mesozoic plutons of the region, and Fleck (1983) recognized an abrupt increase in the 87Sr/86Sr ratio from less 

than 0.704 to greater than 0.710 within a distance of 7 km 

across the boundary between the Seven Devils terrane 

(Seven Devils Group) on the west and the Belt/Yellowjacket terrane (Belt Supergroup and Yellowjacket Formation) on the east.

Within this 500-km-wide zone of transition, four major 

pre-Tertiary petrologic-tectonic terranes have been delineated, all of which may represent allochthonous sequences that have collided with and been accreted to the 

North American cratonic margin during Mesozoic time 

(Brooks and Vallier, 1978; Dickinson and Thayer, 1978; 

Dickinson, 1979b; Silberling and others, 1984). The 

Cenozoic volcanic and sedimentary pile rests discordantly 

on these different terranes, which include sedimentary 

and volcanic rocks of Devonian to Cretaceous (mostly
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 121

Permian to Jurassic) age, and on the late Mesozoic 

granitic plutons that invade them. The terranes were 

named by Brooks and Vallier (1978) the Wallowa Mountains-Seven Devils Mountains volcanic-arc terrane, dismembered oceanic-crust terrane, Jurassic flysch, and 

Juniper Mountain-Cuddy Mountain volcanic-arc terrane, 

and by Dickinson (1979b) the Seven Devils, central 

melange, Mesozoic clastic, and Huntington terranes. 

More recently, these sequences were named by Silberling and others (1984) the Wallowa, Baker, Izee, and Olds 

Ferry terranes.

The relations of these terranes to each other are poorly 

understood. As summarized by Brooks and Vallier (1978, 

p. 133), the Wallowa Mountains-Seven Devils Mountains 

and Juniper Mountain-Cuddy Mountain volcanic-arc terranes "may merge beneath Cenozoic lavas south of the 

Seven Devils Mountains, or they may be joined along a 

major suture. The dismembered ocean-crust terrane and 

the Juniper Mountain-Cuddy Mountain volcanic-arc terrane were joined technically in Late Triassic and Early 

Jurassic time, and flysch derived mainly from erosion of 

the Juniper Mountain-Cuddy Mountain volcanic-arc terrane was deposited across the junction."

According to Brooks and Vallier (1978), compressional 

deformation of the flysch and closure of the Jurassic basin 

probably occurred during the late phases of the accretionary process. Final emergence of the Mesozoic rocks 

occurred in Late Jurassic or Early Cretaceous time, some 

time after flysch deposition ceased. Emergence of these 

terranes provided a source for the upper Albian to 

Cenomanian epiclastic, commonly conglomeratic sedimentary rocks in the Mitchell and Bernard Ranch areas 

(fig. 6.1). Recent studies by Thompson and others (1984) 

of pollen and foraminifers from samples obtained from 

several wells south of the Mitchell area indicate the 

presence of Upper Cretaceous (Cenomanian through 

Maestrichtian) strata in the subsurface. Deposition of 

these Cretaceous near-shore fluvial and deltaic marine 

assemblages was followed by further uplift and erosion.

CENOZOIC ERA

Discussion of the Cenozoic tectonic and volcanic history 

of the Blue Mountains and adjacent areas has generally 

considered the rocks in major groups defined largely by 

age. These broad categories generally correspond to the 

coverage in the preceding chapters: (1) Paleocene(?) and 

Eocene rocks, (2) Oligocene and lower Miocene rocks, 

and (3) middle Miocene and younger rocks. Both volcanic 

and terrestrial sedimentary processes were active 

throughout the region for most of the Cenozoic, and both 

were affected by recurrent tectonism. Sites of volcanism 

and sedimentation have shifted geographically throughout the Cenozoic, in places merging in common depositional basins, elsewhere or at other times being 

separated by considerable distances.

Volcanism appears to have been more or less continuous within the province, although the types of volcanic 

products varied both geographically and over time. 

However, when volumes of erupted material are considered, the volcanism is seen to have been significantly 

episodic. Major outpourings of calc-alkaline andesitic and 

basaltic rocks occurred during the period from about 54 to 

38 Ma. Chiefly rhyolitic and dacitic rocks were erupted 

during the period from about 37 to 23 Ma, with principal 

vents near or to the west of the western margin of the 

Blue Mountains province. Large volumes of tholeiitic 

basalt and moderate volumes of calc-alkaline andesitic 

and rhyolitic rocks were erupted during the period from 

about 20 to 7 Ma. Within this youngest group, different 

coeval chemical groups of rocks reflect different tectonic 

settings. Many of the lower(?) and middle Tertiary 

volcanic rocks, particularly those of the John Day Formation and upper Miocene ash-flow tuff units, originated 

from vents outside the Blue Mountains province.

Pliocene and younger volcanism, though widespread in 

south-central Oregon and in the Cascade Range, has been 

confined to comparatively small eruptions of basalt along 

the western and southern margins of the Blue Mountains 

province (see Swanson, 1969; Brooks and others, 1976). 

Small to moderate volumes of silicic volcanic rocks were 

erupted in the Blue Mountains about 12 Ma, and a small 

dacite intrusion northwest of John Day has been radiometrically dated at 3.1 Ma by Robyn (1977). Young silicic 

volcanism occurred extensively in the Brothers fault zone 

about 50 km to the south, as well as in areas still farther 

south. Among its products are upper Miocene ash-flow 

tuff deposits of moderately large volume that were 

erupted from calderas in Harney Basin and extend long 

distances northward into the Blue Mountains province.

PALEOCENE(P) AND EOCENE

Our knowledge of the earliest Cenozoic history of the 

Blue Mountains region relies heavily on fragmentary 

information derived from poorly exposed arkosic, carbonaceous, and conglomeratic sedimentary rocks along the 

axis of the Blue Mountains uplift. Deeply weathered, 

poorly exposed, and poorly dated coarse-grained conglomerate in areas adjacent to the Wallowa and Elkhorn 

Mountains may also be part of the lowermost Cenozoic 

sequence.

The arkosic sedimentary rocks, which rest discordantly on pre-Cenozoic basement rocks and have been paleobotanically dated as Paleocene(?) age (see chap. 2), 

consist of well-bedded, moderately well sorted, locally 

carbonaceous terrestrial sandstone, siltstone, and shale 

that represent nearshore, partly deltaic deposits. They
122 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

O ~~~ 

Heppner

Approximate boundary __ _ 

of Blue Mountains province ^^/T^

 ~ t ' . ;3F 

45

0 10 20 30 40 50 KILOMETERS

FIGURE 6.1. Cenozoic tectonic map of the
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 123

118°

EXPLANATION

 Fault Dashed where approximately located. Bar and 

ball on downthrown side 

Folds Dashed where approximately located. Arrow on 

axis indicates direction of 

local plunge 

Anticline

Syncline

Monocline axis Arrows show

abrupt increase in dip 

Monocline axis Arrows show

abrupt decrease in dip 

Volcanic vents

Intermediate to mafic cone or

plug

Large silicic dome 

Dike or sill of basalt and

andesite

Compiled principally from Newcomb (1970), 

Bond (1978), Swanson and others (1981),and 

Walker (1977)

Blue Mountains province and adjacent areas.
124 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

contain detritus apparently derived from a granitic or 

metamorphic terrane, presumably lying to the east, and 

little, if any, Cenozoic volcanic debris. Coarse-grained 

conglomerate is discontinuously exposed in areas east of 

the arkosic sedimentary rocks and consists largely of 

rounded cobbles and boulders of quartzite, some metavolcanic rocks (greenstone), and rare granitoid clasts. It 

does not contain clasts of lower Cenozoic volcanic rocks 

and is interpreted to be essentially coeval with the 

arkosic sedimentary rocks and possibly to represent an 

onshore, coarser grained fluvial facies. All of these 

terrestrial sedimentary rocks were deposited on an 

eroded and locally weathered surface that had tens or 

hundreds of meters of relief. The mineralogy and lithology of the detritus, and its apparent eastward coarsening, indicate probable derivation from outcrops in the 

vicinity of the Idaho batholith and thus a possible relation 

to its uplift and subsequent deroofing. Granitic clasts in 

the Cretaceous (Cenomanian) Bernard Formation (Dickinson and Vigrass, 1965, p. 68) imply that the batholithic 

masses of northeastern Oregon were initially deroofed 

during Late Cretaceous rather than Paleocene or early 

Eocene time. Along the axis of the Blue Mountains uplift, 

south of Heppner, the arkosic sedimentary rocks are

124'

42°

100 200 300 KILOMETERS

FIGURE 6.2. Regional distribution of Eocene and lower Oligocene 

calc-alkaline volcanic rocks in and east of the Cascade Range. 

Separate volcanic fields include: AV, Absaroka Volcanic Supergroup 

(Eocene); CF, Clarno Formation (Eocene and lowermost Oligocene?); 

CV, Challis Volcanics (Eocene); LA, volcanic rocks of Lakeview area, 

which locally are intruded by 34- to 33-Ma adamellite stocks 

(Muntzert and Field, 1969; Fiebelkorn and others, 1983) and are 

unconformably overlain by tuffs and tuffaceous sedimentary rocks 

correlative with part of the uppermost Eocene(?) to lower Miocene 

John Day Formation; RG, volcanic rocks of Republic graben; WC, 

volcanic rocks of Washington Cascades. Lines of 87Sr/86Sr=0.704 and 

0.706 from Leeman (1982).

overlain, apparently conformably (Pigg, 1961, p. 16), by 

porphyritic andesite flows of the Clarno Formation.

Cenozoic volcanism in the Blue Mountains commenced 

earlier than 50 Ma (fig. 6.3) near what is now the axis of 

Blue Mountains uplift. This earliest known volcanism, 

represented by altered plagioclase-phyric andesite of the 

Clarno Formation, is of early Eocene age: 53.7±1.0 Ma 

(P.T. Robinson and E.H. McKee, quoted in Fiebelkorn 

and others, 1983). Calc-alkaline andesitic and dacitic 

volcanism continued mostly near the axis of the uplift and 

in several areas to the southeast (see fig. 2.1) until latest 

Eocene time. Radiometric ages on these rocks range 

from about 54 to 38 Ma, but the age range depends on 

what rocks are included in the Clarno Formation (see 

chap. 2; fig. 2.3).

The Challis Volcanics of central Idaho, which are at 

least partly coeval with the Clarno Formation, were 

erupted between about 50 and 39 Ma (Armstrong, 1975). 

They consist mostly of rhyodacitic and rhyolitic ash-flow 

tuff related to cauldron complexes, but they include flows 

of dacite, latite, andesite, and basalt. The isolated 

outcrops of trachytic andesite flows and related breccia at 

Kamiah Butte and the well-indurated breccia at Potato 

Hill, both in western Idaho (fig. 6.1), may represent 

outlying parts of either the Challis Volcanics or the 

Clarno Formation, or they may be unrelated. Partly 

coeval volcanism, also mostly andesitic but including 

basaltic, dacitic, and rhyodacitic material, was occurring 

in other adjacent regions (fig. 6.2), for example, to the 

north in and adjacent to the Republic graben near the 

United States-Canada border (Muessig, 1962; Armstrong, 1978; Ewing, 1980), to the south near Lakeview, 

Oreg. (Walker, 1980), and in the Cedarville area, Calif., 

as well as in the Cascade Range in Washington (fig. 6.2; 

Vance, 1982; Ort and others, 1983; Tabor and others, 

1984). Basaltic and andesitic volcanism that is partly 

coeval also was occurring in areas west of the Cascade 

Range, as represented by mafic flows and breccia of the 

upper Eocene and lower Oligocene Goble Volcanics 

(Livingston, 1966; Beck and Burr, 1979), upper Eocene 

basalt and basaltic andesite of the Tillamook Volcanics 

(Wells and others, 1983), and, possibly, parts of the 

Eocene to Miocene(?) Skamania Volcanic Series of Trimble (1963), which strata Wise (1970) included in his 

Ohanapecosh Formation, and the volcanic parts of the 

upper Eocene and Oligocene Fisher Formation.

Several workers (Snyder and others, 1976; Armstrong, 

1978; Vance, 1982) have suggested that the Clarno, 

Challis, and Absaroka volcanic rocks are related to an 

extensive magmatic arc, generally referred to as the 

Challis arc, that extended westward from Wyoming and 

Montana, through east-central Oregon, and northwestward to west-central Washington. Magma for the Challis 

Volcanics and Absaroka Volcanic Supergroup appears to
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 125

Rattlesnake, Prater 

Creek, and Oevine 

Canyon Ash-flow Tuffs

Tygh Valley 

Formation '

Q.

2 Chenoweth Formation1

_» 

"CD 1

o Oeschutes Formation

Juntura Formation

Dinner Creek Welded 

Tuff 2

Dooley Rhyolite Breccia

Mascall Formation

Payette Formation

Strawberry Volcanics

^ Saddle Mountains 

8 Basalt

ID

Wanapum Basalt « " 

.> g 

^cS Grande Ronde Basalt

CD

.2 Picture Gorge Basalt

0 Imnaha Basalt

John Day Formation

Clarno Formation

Challis Volcanics

I i 1

H---H

h*1

h»H 

(- H 

(- -1

(-W

t-»H

CD

KM 5

HM 2 

l-«-f OO

t*< 1

^ (3

1~7^-H illll

h»4

>*1 'sKiP^'C

I ?

? ?

- liiii

7 O :^^:V::' :-^"::j::::! '

h«H »

- K 

3 

S

 i; i

~ & en

 CD 

. A 

 '») 

 O

oKH 

h-»

till

CD

2 

§

3 

8

~- 1

O.

O) , 

CD

J3

O

Q.

H !r?^: " :

'..'7" ; : ;KH

1 »H

I-»H 

I-M 

W 

KH

t «H

1 1

10 20

'Of Farooqui, Beaulieu, and others (1981). 

2OfHaddock(1966,1967).

30

MILLIONS OF YEARS

40 50 60

FIGURE 6.3. K-Ar ages of volcanic and volcaniclastic rocks, showing apparent gaps in volcanic activity (stippling). 

Within formations, dated samples are not necessarily arranged according to stratigraphic position. Dots, calculated 

age; bars, analytical error; dashed lines, approximate K-Ar age range; ?, questionable age limits.
126 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

have originated in old continental lithosphere, whereas 

magma for the Clarno Formation and, probably, for the 

Cascade Range originated in oceanic lithosphere that had 

previously been somewhat dismembered during amalgamation and accretion at the edge of the continent. 

Noblett (1981) considered rocks of the Clarno Formation 

to be a product of subduction-related volcanism along a 

continental margin and to be a late phase of Challis arc 

volcanism. His model postulates that the Challis arc 

migrated westward during Eocene time from Idaho and 

Montana, across central Oregon, to and through the locus 

of Clarno volcanism. However, the similar ages of 

volcanism in these widely separated areas, as established 

by radiometric-age determinations, seems to refute the 

concept of time-transgressive Eocene arc volcanism. 

Also, the partly coeval calc-alkaline andesitic to rhyolitic 

volcanic rocks in the Paisley Hills and the LakeviewCedarville area, 150 km south of the Blue Mountains near 

the Oregon-California State line, and those in western 

Oregon and Washington do not fit the geometry of the 

proposed Challis arc, although they may constitute 

completely separate volcanic piles of different origin. 

Rogers and Novitsky-Evans (1977) considered the calcalkaline basaltic andesite and andesite of the Clarno 

Formation to be a continental-margin assemblage formed 

above a subduction zone.

We might speculate that some of the outcrops in the 

Eocene volcanic belt have been brought fortuitously into 

position by major clockwise rotation during post-Eocene 

time. As much as 70° of rotation has been documented for 

rocks of the Coast Range of Oregon and Washington 

(Simpson and Cox, 1977; Beck and Plumley, 1980; Magill 

and Cox, 1980). These rocks could also have migrated a 

significant distance to the northeast from their original 

site of deposition. The rotation of the coeval rocks of the 

Clarno Formation apparently is considerably less, possibly no more than 17°±10° (Beck and others, 1978; C.S. 

Gromme, oral commun., 1984). This relatively small 

amount of rotation should have had little affect on the 

geographic position of the Clarno Formation. Even 

where rotation of crustal blocks has been significant, 

there may be little or no change in geographic position of 

the rocks if the rotation is confined largely to microblocks 

within a zone of transition between major crustal plates.

Any model for the origin of the various Eocene 

calc-alkaline rocks of the northwestern United States 

must take into account the generally synchronous occurrence of the volcanism, its chemical similarities and 

differences, and, most importantly, the geometry of 

possible volcanic belts and their relation to the position 

and orientation of the continental margin. The affects of 

rotation and changes in the geographic position of crustal 

blocks along the western margin of the region must also 

be considered. We believe that the available data argue

against the proposed west- to northwest-trending Challis 

arc. If such an arc existed, the distribution of the Eocene 

rocks virtually mandates a fore-arc trench and related 

subduction zone parallel to the arc on the south or 

southwest side. Such a model would require essentially 

north-south convergence of the North American plate 

and an oceanic plate in the Pacific over a front more than 

1,000 km long in Eocene time. The orientation of this 

proposed arc is at a large angle to the continental margin, 

and so the trench and subduction zone would also have 

been nearly at right angles to the present margin. This 

requirement alone seems to refute the concept of a 

trench, subduction zone, and related island or magmatic 

arc all trending west-northwestward to westward across 

the region. Dickinson (1979a, fig. 1), Engebretson (1982), 

and Wells and others (1984) inferred that during Eocene 

time the direction of convergence was more northerly 

than in either Cretaceous or Oligocene time, but their 

convergence directions are still at a small angle to the 

postulated west-northwestward to westward trend of arc 

and trench.

Taylor (1977) considered that the Clarno volcanic rocks 

of east-central Oregon reflect a northeast-southwest 

andesite belt that at the time of formation was undergoing northwest-southeast compression. Such a belt trending northeast-southwest is necessary if the Eocene 

andesites of the Lakeview-Cedarville area are related to 

those of the Clarno Formation. This model requires a 

fore-arc trench approximately 100 km to the northwest of 

the arc, with both arc and trench trending generally 

parallel to the present continental margin. Plate convergence could be essentially east-west, in accord with 

convergence directions as postulated by Dickinson 

(1979a). This model would almost certainly exclude a 

genetic connection along a single subduction zone between the Clarno Formation and coeval volcanic rocks to 

the east in Idaho, Montana, and Wyoming; it instead 

implies different origins for these similar and coeval 

volcanic assemblages.

From our evaluation of the data, we conclude that the 

Eocene volcanism of the Blue Mountains region and 

adjacent areas can best be explained by eruptions along 

a north- to northeast-trending arc that was situated above 

a subduction zone oriented nearly parallel to the present 

continental margin. The Eocene andesites of the 

Lakeview-Cedarville area near the California-Oregon 

State line are inferred to represent the southward 

extension of this belt, although some left-lateral offset 

along such features as the Brothers fault zone appears 

necessary to bring them into line with coeval rocks in the 

Blue Mountains. Alternatively, the geometry of a trencharc assemblage may have been modified by later tectonic 

dismembering, particularly in the region south of the 

volcanic belt in the zone of significant crustal extension.
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 127

OLIGOCENE AND EARLY MIOCENE

Oligocene and lower Miocene sequences in the Blue 

Mountains region are chiefly intermediate to silicic airfall tuff, ash-flow tuff, and tuffaceous sedimentary rocks 

mostly representing part of the John Day Formation. 

Locally, flows of alkali olivine basalt and trachyandesite 

are interlayered with the pyroclastic deposits, and rhyolite flows and domes are common along the western 

margin of the Blue Mountains province. Included in this 

assemblage are several local piles of plagioclase-, pyroxene-, and olivine-phyric andesitic rocks, some of which 

have been previously assigned to the Clarno Formation 

(see chaps. 2, 3). However, many of these rocks are less 

altered than typical Clarno Formation rocks and have 

radiometric ages equivalent to those of the John Day 

Formation.

Isotopic ages of the Oligocene and lower Miocene rocks 

range from about 37 to 20 Ma (fig. 6.3). Vertebrate fossils 

from the uppermost part of the John Day Formation in 

the Mutton Mountains area, about 10 km north of Warm 

Springs, are Hemingfordian in age (Woodburne and 

Robinson, 1977), probably about 20 to 19 Ma. Andesitic 

rocks that predate the Columbia River Basalt Group and 

are radiometrically dated at 20 to 19 Ma are included in 

the section of Miocene and younger rocks.

Volcanism in the Blue Mountains region during Oligocene and early Miocene time appears to have been much 

more restricted geographically than Eocene volcanism. 

The great bulk of the pyroclastic material making up the 

John Day Formation apparently was derived from vents 

west of the Blue Mountains province, in or near the 

present Cascade Range (Robinson and others, 1984). 

Within the Blue Mountains province itself, small flows of 

alkali olivine basalt and trachyandesite (minimum 30 Ma) 

were erupted from several vents, and more voluminous 

flows and domes of rhyolite were emplaced, particularly 

along the western margin of the region. The local 

andesite flows and breccia of John Day age, which are 

lithologically similar to Clarno sequences, may simply 

represent a waning phase of the Eocene volcanic activity. 

However, some of these rocks are as young as 28 to 27 Ma 

and thus are 9 to 10 m.y. younger than the 37-Ma base of 

the John Day Formation, as established by Swanson and 

Robinson (1968). Very few vents have been identified for 

this andesitic volcanism, but they apparently lie chiefly in 

areas to the east of the silicic vents.

Distribution of the Oligocene and lower Miocene rocks 

demonstrates that early Cenozoic structural trends, 

particularly the Blue Mountains uplift, developed before 

their eruption and deposition; subsequent deformation 

followed these earlier Cenozoic structural trends. An 

episode of regional uplift and consequent erosion in latest 

Eocene time produced an erosion surface on the Clarno

Formation with several hundred meters of relief. In 

many areas, a red saprolite formed on this surface, and 

this paleosol has been used where present to mark the 

boundary between the Clarno and John Day Formation 

(Waters, 1954). The uplift was concentrated along preexisting structural trends and apparently did not affect 

the bounding structures on the northern and southern 

margins of the Blue Mountains uplift.

Fisher (1967, p. 120) recognized both northeast- and 

northwest-trending folds within the Blue Mountains 

uplift that are either pre-Oligocene or earliest Oligocene 

in age. The northeast-trending folds are dominant and 

are attributed to northwest-southeast compression (Taylor, 1981, p. 112). Much of this deformation apparently 

occurred during Eocene time; in the area between 

Madras and Mitchell, Oreg., however, it partly postdates 

eruption of the basal ash-flow sheet of the John Day 

Formation, which has been dated at about 37 Ma (Swanson and Robinson, 1968). The distribution and thickness 

variation of the John Day Formation and its stratigraphic 

relations to the underlying Clarno Formation indicate 

that a major topographic barrier existed along the Blue 

Mountains axis throughout most of John Day time.

Folds that developed during the Oligocene and early 

Miocene mostly trend northeast, parallel to the older 

structures. These northeast-trending folds also are inferred to indicate northwest-southeast compression, although the stress regime appears to have weakened after 

Eocene deformation.

The initiation of John Day volcanism about 37 Ma 

signified a major change in the composition and locus of 

volcanic activity in the region. Shortly after cessation of 

Clarno volcanism in the Blue Mountains, volcanos in the 

vicinity of the present-day Cascade Range began erupting andesitic, dacitic, and rhyolitic pyroclastic material, 

which was deposited in the John Day area as ash falls. 

Simultaneously, vents between the Cascade Range and 

the Blue Mountains uplift were erupting rhyolitic ash 

flows, lava flows, and minor air-fall pyroclastic material. 

Most of the ash-flow tuff was derived from vents west of 

the present-day outcrops in the Blue Mountains, whereas 

silicic lava flows and domes represent local eruptions 

within the western part of the Blue Mountains region. 

Other vents, mostly east of the major rhyolitic volcanos, 

erupted alkaline basalt and trachyandesite. Partly coeval 

dacitic and rhyolitic rocks indicate that volcanism was 

occurring in several parts of the Basin and Range many 

tens of kilometers to the south near Lakeview, in the 

Coyote and Rabbit Hills and at Hart Mountain, about 50 

km northeast of Lakeview, and in the Steens Mountains, 

80 km northeast of Lakeview.

This pattern of volcanism continued until approximately 25 to 20 Ma, when rhyolitic, basaltic, and trachyandesitic eruptions within the Blue Mountains ceased.
128 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

Andesitic, dacitic, and rhyolitic eruptions continued in 

the Cascade Range; air-fall tuff from this volcanism forms 

the upper part of the John Day Formation. Deposition of 

the John Day Formation ceased at about 20 to 18 Ma, 

coincident with the start of a probable hiatus in Cascade 

Range volcanism (McBirney and others, 1974); after a 

short period of folding and erosion, the Columbia River 

Basalt Group was erupted, beginning about 17 Ma (see 

chap. 4; Watkins and Baksi, 1974; McKee and others, 

1977).

The westward shift in the locus of volcanic activity 

from Clarno to John Day time is interpreted as the result 

of a westward migration and possible reorientation of the 

early Tertiary subduction zone along the continental 

margin. The postulated north- to northeast-trending 

volcanic arc of Eocene time apparently passed through 

the western part of the Blue Mountains. In latest Eocene 

time activity along this trend diminished significantly, 

and a new arc was developed in the vicinity of the 

present-day Cascade Range, possibly as early as 41 Ma 

(Lux, 1982).

MIOCENE TO HOLOCENE

The compressional tectonism of Mesozoic and early 

Cenozoic time was replaced in middle Miocene time by 

mostly extensional tectonism, with only local compression. This change in tectonic style, presumably related to 

the development of transform-fault systems along parts 

of western North America, significantly altered the 

character of volcanism both within the Blue Mountains 

region and in areas to the south. The Brothers fault zone 

(fig. 6.4), lying between the Blue Mountains province and 

the Basin and Range province to the south, has been 

interpreted as a transcurrent- (transform?-) fault zone 

that separates crustal blocks characterized by appreciable differences in the amount of extension, in structural 

pattern, and in type of volcanism. Some investigators 

also have speculated that the Olympic-Wallowa lineament north of the Blue Mountains (fig. 6.4) is the surface 

manifestation of another, older transcurrent-fault zone, 

separating continental crust on the north from dismembered oceanic crust on the south.

Stewart (1978) summarized current hypotheses on the 

origin of this Neogene extensional stress regime and its 

relation to regional tectonics, particularly as applied to 

the Basin and Range. One hypothesis relates to "oblique 

tensional fragmentation within a broad belt of rightlateral movement and distributed extension along the 

west side of the North American plate," another to 

"spreading caused by upwelling from the mantle behind 

an active subduction zone (back-arc spreading)," and a 

third to presumed subduction of the East Pacific Rise 

beneath part of North America. Eaton and others (1978,

p. 86) explained the extensional tectonics in the western 

Cordillera by "the rise, and probably the lateral divergent flow, of hot asthenospheric material accompanying 

major mantle upwelling." This model, although it accounts for many of the phenomena that characterize the 

Basin and Range province, such as normal faults, high 

heat flow, relatively thin crust, and low Pn seismic 

velocity, is inconsistent with apparent north-south compressional features north of the Brothers fault zone and 

with the apparent thickening of crust from the Columbia 

Plateau and Pasco Basin southward beneath the Blue 

Mountains (Hill, 1978).

By middle Miocene time, calc-alkaline volcanism in the 

Cascade Range was well established. The presence of 

this persistent, mostly andesitic and basaltic volcanic 

activity, along a belt several tens of kilometers wide and 

more than 1,000 km in length from northern California to 

the United States-Canada border, is persuasive evidence 

for the existence of a subduction zone along western 

North America. The Blue Mountains region is geographically behind (east of) this magmatic arc, and the pattern 

of Miocene and younger deformation in and adjacent to

124° 122° 120° 118°

48C

46C

44°

42°

^ Little, if any, north - south 

compression; little east 

O west extension

* ?\ *hU.

\ ^Ox Some east-west extension; 

\ (fat) north -south compression

Major east-west 

extension

 - _ __ _ OREGON 

CALIFORNIA ~ ""I" ~ "

0 100 200 300 400 KILOMETERS

FIGURE 6.4. Geographic relations of the Blue Mountains province to 

the Brothers fault zone, Olympic-Wallowa lineament, and Cascade 

Range. Queried arrows on lineament and fault zone, uncertain 

direction of movement. D, downthrown side; U, upthrown side.
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 129

this region appears to be most consistent with a combination of back-arc spreading and right-lateral movement 

of crustal blocks in the transition zone between the 

Pacific and North American plates. Transcurrent movement along transform zones was probably responsible for 

creating local compressional stresses.

Major crustal extension commenced at about 17 to 16 

Ma and has continued throughout the late Cenozoic. 

Some deformational evidence, as well as fault-plane 

solutions of recent seismic events, indicates that the 

regional stress regime responsible for this warping and 

block faulting still persists (Couch and Lowell, 1971). The 

Blue Mountains province was less affected by this extension than were areas in the Basin and Range province to 

the south; the consequent differential extension was 

apparently accommodated along the Brothers fault zone. 

Faults and folds within the Blue Mountains generally 

parallel preexisting northeast-trending structures, but 

northwest-, northeast-, and east-trending structures are 

also present. Thayer (1957) concluded that structures in 

the John Day region resulted principally from northsouth compression and east-west extension.

Robyn and Hoover (1982) identified three separate 

types and ages of Cenozoic deformational structures: (1) 

northeast- and east-trending fold axes and reverse faults 

generated during the period 36 to 17 Ma; (2) northnorthwest-trending fissures and north-trending strikeslip faults generated from about 17 to 10 Ma; and (3) 

northwest- and northeast-trending conjugate faults that 

formed from 10 Ma to the present. Robyn and Hoover 

(1982) related all of these structures to interaction 

between the northwestward-moving lithosphere of the 

Basin and Range and a relatively immobile block of 

lithosphere beneath the Blue Mountains. According to 

this interpretation, many of the Miocene and younger 

faults and folds can be best explained by adjustment of 

brittle surface and near-surface layers of volcanic and 

tuffaceous sedimentary rocks to transcurrent movement 

along bounding faults.

In the northernmost Basin and Range, south of the 

Brothers fault zone, major east-west extension was 

accompanied by voluminous basaltic and rhyolitic (bimodal) volcanism from the middle to late Pliocene and, 

locally, into the Pleistocene. The basalts are dominantly 

olivine tholeiite or plagioclase-rich high-alumina types. 

Vents for the mafic rocks include numerous small lava 

cones or shields, cinder cones, and localized dike swarms; 

numerous domes and intrusions and several calderas 

mark the sites of eruption for the silicic rocks.

In the Blue Mountains province north of the Brothers 

fault zone, where east-west extension was much less, 

Miocene volcanism produced voluminous tholeiitic flood 

basalts, moderate volumes of calc-alkaline andesitic to 

rhyolitic lava, and indeterminate volumes of rhyolitic

lava, tuff, and tuffaceous sedimentary rocks. Younger, 

upper Cenozoic volcanic deposits similar to those in the 

northern Basin and Range are absent. According to 

Robyn (1979) the high-alumina basalt, tholeiite, and 

olivine tholeiite of eastern Oregon show a southward 

decrease of normative quartz similar to compositional 

variations observed across linear volcanic belts associated with subduction. Much of the Miocene silicic ejecta 

was incorporated into bedded pumiceous and ashy sedimentary rocks and interlayered ash-flow tuff; lesser 

amounts occur in domal masses, such as those at Dooley 

Mountain and the rhyolitic complexes within the Strawberry Volcanics. The total original volume of this rhyolitic to dacitic material was very large, possibly nearly 

comparable to that of the coeval flood basalts, if the 

thickness estimates by Thayer and Brown (1966) are 

accurate. Vents for some of the silicic rocks within the 

Blue Mountains province, including those for middle 

Miocene ash-flow tuff, apparently are buried beneath 

rhyolitic complexes like that at Dooley Mountain, 20 km 

south of Baker; others are represented by such intrusions 

as those at Castle Rock and at Black Butte, a few 

kilometers northwest of Castle Rock. Most of the Miocene andesitic rocks are platy and contain phenocrysts of 

augite and hypersthene; some of the more mafic flows 

associated with the andesites are olivine bearing. Hornblende-porphyritic andesite flows and breccia of early to 

middle Miocene age (approx. 20-19 Ma) occur in several 

ranges north and east of Unity Basin; commonly, these 

rocks have been misidentified as representing part of the 

Eocene and Oligocene(?) Clarno Formation.

Along the northern margins of the Blue Mountains and 

in areas to the north, the amount of east-west extension 

appears to be even less than in areas to the south, and 

Miocene volcanism was largely basaltic. Tholeiitic flood 

basalt of the Columbia River Basalt Group is localized 

mostly north of the axis of the Blue Mountains uplift, 

although flow sequences extend in places far south of the 

axis. All major vent systems for the voluminous flows of 

the Columbia River Basalt Group apparently lie within 

the area underlain by dismembered oceanic crust and 

accreted terranes rather than continental crust, as indicated by Thompson (1977). The feeder dikes for these 

flows are principally in two major dike swarms (fig. 6.1). 

The northwest-trending Monument swarm in John Day 

Basin was the main source for flows of the Picture Gorge 

Basalt, and the north- to northwest-trending Chief Joseph swarm, exposed mainly in drainages tributary to 

the Grande Ronde River, was the main source of flows of 

the Grande Ronde, Wanapum, and Saddle Mountains 

Basalts. Isolated small groups of basalt feeder dikes also 

have been identified in many other parts of the Blue 

Mountains, in several widely separated areas in western 

Idaho (fig. 6.1), and along the Snake River in Washing-
130 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

ton. Vents for the Prinevilie type of flood basalt and for 

plagioclase-rich, ophitic basalt flows along the southern 

margin of the Blue Mountains uplift have not yet been 

recognized and presumably are buried beneath younger 

rocks in Deschutes Basin or, possibly, in or near the 

Brothers fault zone.

Robyn (1979) recognized three centers of Miocene 

calc-alkaline hypersthene andesite volcanism in the Blue 

Mountains (fig. 6.5; Sawtooth Crater, Strawberry Volcano, and Dry Mountain), all in a straight line or belt with 

a northeastward trend and with 100-m spacing between 

centers. A fourth center, also composed largely of middle 

to upper Miocene hypersthene andesite, has been recognized at Gearhart Mountain, lying precisely on the 

southwestward extension of this linear belt at approximately the same spacing as between the other centers 

(fig. 6.5). This uncommonly straight, narrow linear belt 

or line of volcanos, all of middle or late Miocene age, does 

not appear to coincide with any single throughgoing 

structure or group of structures recognizable in surface 

exposures. Robyn (1979) suggested that this line of 

volcanos is situated at the intersection of two geologic 

provinces and generally parallels and lies to the south of 

the northeast-trending gravity gradient (Thiruvathukal 

and others, 1970; Hildenbrand and others, 1982). Therefore, the belt appears to be underlain by crust that is 

somewhat thicker than that in areas to the west and 

northwest. Whether this Miocene hypersthene andesite 

belt extends farther southwestward into northern California remains to be ascertained. The possible effect of 

the Brothers fault zone, which appears to transect the 

belt, must also be considered. Robyn (1979) noted that 

the oblique relation of this linear trend to Miocene 

andesitic centers in the western Cascades is similar to 

that of the volcanos adjacent to the Ryukyu and Izu 

Trenches in the northwestern Pacific. Differences in the 

structures and volcanic features of these widely separated areas indicated to Robyn (1979, p. 159), however, 

that the Miocene andesite volcanos of eastern Oregon, 

unlike those of the modern northwestern Pacific, are not 

subduction related. He proposed, instead, that the Oregon lava was generated by small degrees of melting to 

form high-alumina basalt magma which subsequently 

differentiated to form the calc-alkaline andesite and 

associated dacite. Their relation to tectonic events is 

unclear, although Robyn (1979, p. 159) inferred that the 

andesite and related rocks were generated at a major 

crustal boundary.

Basaltic flows and flow breccia of Pliocene age are 

present in a few restricted areas in northern Malheur 

County and adjacent parts of Baker County, Oreg., on 

the northwestward projection of the Snake River graben, 

and in and adjacent to Deschutes Basin. The precise age 

of these rocks is not well documented, but they generally

rest on or are interbedded with sedimentary beds containing Pliocene vertebrate faunas. Most of the flows 

show little evidence of deformation. They consist mostly 

of thin, open-textured (diktytaxitic), commonly plagioclase-rich olivine basalt and some basaltic andesite, and 

many of them can be related to small, well-defined shield 

volcanos or cones of mafic cinders and agglutinate. In the 

western part of the region, near the boundary between 

the Blue Mountains province and the Deschutes-Umatilla 

Plateau province (see fig. 1.1), upper Pliocene or Pleistocene flows of diktytaxitic olivine basalt overlie the 

Deschutes Formation (Swanson, 1969; Robinson, 1975; 

Peterson and others, 1976; Robinson and Stensland, 

1979). Many of these flows form intracanyon fill in the 

deeply incised canyons of the Deschutes and Crooked 

Rivers. Some of these flows and flow breccias of small to 

moderate volume appear to be the terminal phases of 

volcanism in the Blue Mountains province, although 

calc-alkaline basaltic, andesitic, and, locally, dacitic and 

rhyolitic volcanism persisted into the Holocene in areas of 

the northwestern Basin and Range to the south of the 

Blue Mountains and in the Cascade Range to the west.

124° 122° 120° 118°

48C

46°

44°

42° -

r"f

§' 

H

,DM

CALIFORNIA"

GM

OREGON

NEVADA

0 100 200 300 400 KILOMETERS

i i i i i

FIGURE 6.5. Middle and upper Miocene volcanic centers of eastern 

Oregon that are partly characterized by hypersthene andesite. DM, 

Dry Mountain; GM, Gearhart Mountain; SC, Sawtooth Crater; SV, 

Strawberry Volcanics. Modified from Robyn (1979, fig. 1).
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 131

SUMMARY AND CONCLUSIONS

Proposed models for the Cenozoic volcanism and tectonism in the northwestern United States are made 

uncertain by the paucity of information concerning the 

gross structure, thickness, and lithology of the crust 

beneath the Cenozoic cover, as well as data on the age, 

geographic distribution, volume, and geochemistry of the 

Cenozoic units that make up that cover. The Blue 

Mountains province and adjacent parts of Washington 

and Idaho appear to lie largely, if not entirely, to the west 

of the line marking an initial ^Sr/^Sr ratio of 0.706, 

implying that old (Precambrian) crust is absent in the 

subsurface. Isolated surface exposures indicate that 

basement to the Cenozoic rocks consists mostly of accreted terranes of Devonian through Jurassic age comprising marine sedimentary and volcanic island-arc 

sequences, ophiolite assemblages, granitic plutons, and 

upper Mesozoic marine sedimentary rocks, all interpreted to rest on dismembered oceanic crust. Within the Blue 

Mountains region, deformation and uplift of these older 

rocks was followed in earliest Tertiary time by volcanism 

and terrestrial sedimentation that have persisted 

throughout the Cenozoic.

Few reliable data exist on the ages of initial structural 

uplift of the Blue Mountains province or development of 

the Cenozoic fold and fault structures. The stress regimes that initiated uplift of the Blue Mountains and 

created fold and fault structures within the region 

presumably result from interactions between crustal 

blocks in a zone of deformation 1,000 to 1,500 km wide 

that is related to oblique convergence of the Pacific and 

North American plates; near-surface, local stresses also 

developed within and between these crustal blocks. 

Paleomagnetic data indicate that crustal blocks in this 

wide zone were rotated clockwise during the Cenozoic 

and that the greatest rotation occurred on blocks west of 

the Blue Mountains in the Cascade and Coast Ranges of 

Oregon and parts of Washington. The Blue Mountains 

province is characterized by regional uplift and is in part 

bounded by linear structures that at least locally exhibit 

transcurrent movement and may represent transform 

zones. The region has been subjected to several compressional regimes of different orientations and has undergone much less extension than the northern Basin and 

Range province. Apparent gaps in the Tertiary volcanic 

record appear to coincide with changes in tectonic style, 

such as that from mostly compressional to mostly extensional stress regimes during early to middle Miocene 

time; however, dating of many rock units of the region is 

not adequate to support definite models for episodic 

volcanism. Furthermore, the absence of reliable volume 

estimates for most of these intermediate to silicic volcanic 

rocks prevents valid comparisons of the magnitudes of

different kinds of volcanism by age and their significance 

in petrogenetic modeling.

The chemical composition of several Cenozoic sequences of the Blue Mountains province, including rocks of the 

Clarno Formation, flows of the Columbia River Basalt 

Group, and a geographically restricted part of the Strawberry Volcanics, is reasonably well established, but little 

is known of the Oligocene andesites that are coeval with 

part of the John Day Formation or of the silicic volcanic 

rocks of middle and late Miocene age. Estimates of the 

relative volumes of the different chemical types vary in 

their precision among the different age groups. Virtually 

nothing is known of the volume of the Paleocene(?) 

terrestrial sedimentary rocks. In addition, although the 

volume of Eocene calc-alkaline volcanic rocks can be 

approximated from their areal distribution and approximate thickness, the extent of these rocks beneath younger cover and the proportions of intermediate to silicic 

rock types on a regional basis are largely unknown. From 

the distribution of surface exposures and the thicknesses 

of measured sections, we estimate the total volume of 

these early Cenozoic calc-alkaline volcanic and volcaniclastic rocks to be in the range 10,000 to 20,000 km3. 

Oligocene and early Miocene volcanism was largely 

rhyolitic and was confined to the western part of the 

province, and most of the vents for these rocks are 

represented by large domal complexes. Most of the 

andesitic and dacitic pyroclastic material of the John Day 

Formation was erupted from vents west of the Blue 

Mountains province, probably from the site of the 

present Cascade Range. Some coeval andesitic volcanism 

occurred in the central and eastern parts of the province, 

but there are few data to establish its precise regional 

distribution or the volume of products. The distribution 

of Oligocene and lower Miocene rocks and random 

measurements of their sections indicate a total volume of 

intermediate and silicic volcanic and volcaniclastic rocks 

well in excess of 5,000 km3; much of this volume originated in vents along the western margin of the Blue 

Mountains or in the Cascade Range.

Miocene volcanism was apparently more diverse. Tens 

of thousands of cubic kilometers of tholeiitic basalt was 

erupted mostly from fissure-dike swarms near the axis of 

the Blue Mountains uplift, possibly equal volumes of 

bimodal basalt and rhyolite were erupted along the 

southern margin of the Blue Mountains and to the south 

in the northern Basin and Range, and somewhat lesser 

volumes of calc-alkaline andesite, dacite, and rhyolite 

were erupted in the southeastern and eastern parts of the 

province. The original volume of Miocene rhyolitic flows, 

breccia, domes, and ash-flow and air-fall tuff erupted in 

the region is virtually impossible to calculate: (1) Great 

volumes of volcaniclastic material have been removed by 

erosion, and some areas were completely stripped of
132 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

these rocks; (2) some, possibly much, of the silicic clastic 

material may have been derived from erosion of rocks of 

the Clarno and John Day Formations or introduced as 

airborne material derived from volcanos in the Cascades 

to the west; and (3) thickness estimates for this part of 

the section differ significantly among different investigators. The volume of silicic volcanic material was large, 

certainly totaling many thousands of cubic kilometers.

In spite of these major gaps in data, many geologists 

have speculated on the relation between tectonism and 

volcanism in this region, on the sequential structural 

development of the region within the framework of plate 

tectonics, and on the genesis of the different volcanic 

products. We conclude that conceptual models relating 

the Eocene Blue Mountains volcanism to widespread 

coeval activity to the east along a northwest-trending 

Challis arc need to be modified because they appear to 

conflict with currently available data. The overall distribution of these Eocene calc-alkaline rocks and the geometric relations of such a postulated arc to the 

continental margin and to presently accepted plateconvergence directions are all inconsistent with such a 

model. We propose, instead, that the Clarno volcanism is 

related to a north- or northeast-trending arc passing 

through the Blue Mountains and that the Absaroka 

Volcanic Supergroup and Challis Volcanics, lying tens to 

hundreds of kilometers to the east, resulted from entirely 

different processes. We also suggest that the Clarno arc 

persisted until the earliest Oligocene(?), when the locus 

of volcanic activity shifted westward to the site of the 

present Cascade Range. Many problems concerning the 

relations of Cenozoic volcanism to tectonism in the Blue 

Mountains region are beyond analysis using available 

data. Some of the more critical questions that require 

additional information for comprehensive study and review include the following:

(1) What are the thickness and composition of the crust 

beneath the Blue Mountains and adjacent areas? 

Seismic and gravity data indicate that it thickens 

southward from Pasco Basin toward the axial region 

of the Blue Mountains uplift. Does the thickened 

crust beneath the uplifted block result from stacking 

of accreted terranes, from incorporation of material 

other than dismembered oceanic crust, or from other 

causes? Does it thin appreciably south of the Brothers 

fault zone in the northern Basin and Range as a result 

of extension and stretching?

(2) What role has been played by tectonic rotation of 

blocks in the region? Paleomagnetic data imply various amounts of rotation of crustal blocks, especially 

before middle Miocene time, but the amounts of 

rotation and their geographic extent are still poorly 

documented. The impact of this apparent rotation on 

regional tectonic analysis is uncertain, and we do not

even know if any of the earliest Cenozoic terranes 

originated in other than their present position. How 

far have these terranes moved, if at all, or have they 

rotated approximately in place? What were their 

original geometric relations to early Cascade volcanism?

(3) What are the age and origin of the enigmatic linear 

structures, such as the Brothers fault zone and the 

Olympic-Wallowa lineament, and how do they relate 

to the development of the uplifted Blue Mountains 

block? The Olympic-Wallowa lineament is traceable 

in surface physiographic features for many tens of 

kilometers, and it locally disrupts middle Miocene 

rocks. These surface features are generally considered to represent only minor adjustments of surficial 

rocks that overlie a deeply buried structure. Some 

post-Miocene movement is implied by deformation of 

Miocene basalt flows (Tabor and others, 1984, p. 43). 

Surface structures along the Brothers fault zone 

disrupt rocks of middle Miocene (approx. 15-Ma) to 

Pleistocene (approx. 1-Ma) age. There are virtually 

no data, however, to indicate when this zone originated or whether it overlies an older, deeply buried 

crustal structure. Are these linear structures transform zones separating the Blue Mountains uplift from 

adjacent terranes, or are they tangential shears 

related to oblique convergence of the Pacific and 

North American plates? Are the Brothers fault zone 

and the Olympic-Wallowa lineament the same type of 

structure with somewhat different surface expression? Surface structures related to the Brothers fault 

zone can be explained by stresses related to either 

right- or left-lateral movement on a much deeper 

buried linear structure, but no convincing field evidence has as yet been recognized to support either 

dextral or sinistral displacement.

(4) When was the uplift of the Blue Mountains initiated, 

and when did uplift terminate? Is deformation still in 

progress?

(5) Could the Blue Mountains uplift be the result solely of 

isostatic adjustments where accretionary wedges of 

comparatively light-weight rocks have been crammed 

against the North American plate?

(6) Do apparent gaps in the volcanic record indicate 

episodic volcanism? If so, do they correspond with 

changes in tectonic regime? The significance of episodic volcanism can only be evaluated when more 

comprehensive data are available on the volumes of 

different chemical kinds of volcanic products.

(7) Where are the vents (calderas?) for the moderate 

volumes of ash-flow tuff and air-fall pumice lapilli tuff 

in the Clarno and John Day Formations? Presumably, most of these vents are west of the axis of the 

Blue Mountains uplift, in and adjacent to Deschutes
CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 133

Basin; all the ones known are now represented by 

plugs, domes, and related flows, and none is known to 

be related to ring fracture zones or to show evidence 

of cauldron subsidence. How do these silicic volcaniclastic rocks relate genetically to contemporaneous 

silicic ash flows and tuff in the western Cascades?

(8) Where are the vents for ash-flow tuff in the middle 

Miocene part of the section? Haddock (1966, 1967) 

postulated that the Dinner Creek Welded Ash-flow 

Tuff was erupted from fissure vents at and near 

Castle Rock, but where are the vents for other 

ash-flow tuff in the Mascall Formation? Are there 

ash-flow tuff vents related to the Dooley Rhyolite 

Breccia or to other rhyolitic bodies associated with 

the Strawberry Volcanics?

(9) How do the Miocene hypersthene andesite volcanos 

aligned along a northeastward trend relate to the 

apparent thickened crust underlying the Blue Mountains? Is their position controlled by some throughgoing structure, or do they simply signify a common 

type of crust in the Blue Mountains and areas to the 

south near Gearhart Mountain, which is within but 

near the margin of the Basin and Range province?

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134 CENOZOIC GEOLOGY OF THE BLUE MOUNTAINS REGION

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CENOZOIC TECTONISM AND VOLCANISM OF THE BLUE MOUNTAINS REGION 135

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750-757.

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