Mineral Resources of the South Fork and 

Sand Hollow Wilderness Study Areas, 

Crook County, Oregon

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Chapter A

Mineral Resources of the South Fork

and Sand Hollow Wilderness Study Areas,

Crook County, Oregon

By JAY A. ACH, JAMES T. BATESON, RICHARD J. BLAKELY, 

and HARLEY D. KING 

U.S. Geological Survey

JERRY E. OLSON 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1744

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

CENTRAL AND FAR WESTERN OREGON
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade names and trademarks 

in this publication is for descriptive 

purposes only and does not constitute 

endorsement by the U.S. Geological Survey

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For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Mineral resources of the South Fork and Sand Hollow 

Wilderness Study Areas, Crook County, Oregon.

(U.S. Geological Survey bulletin ; 1744-A)

Bibliography: p.

Supt. of Docs. no. : I 19.3:1744-A

1. Mines and mineral resources Oregon South Fork 

Wilderness. 2. Mines and mineral resources Oregon 

Sand Hollow Wilderness. 3. South Fork Wilderness (Or.) 

4. Sand Hollow Wilderness (Or.) I. Ach, Jay A. 

II. Series.

QE75.B9 no. 1744-A 557.3 s 88-600122 

[TN24.07] [553'.09795'83]
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Areas

The Federal Land Policy and Management Act (Public Law 94-579, October 21, 

1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct 

mineral surveys of certain areas to determine the mineral values, if any, that may be 

present Results must be made available to the public and be submitted to the President 

and the Congress. This report presents the results of a mineral survey of parts of the 

South Fork (OR-005-033) and Sand Hollow (OR-005-034) Wilderness Study Areas, 

Crook County, Oregon.

CONTENTS

Summary Al

Abstract 1

Character and setting 1 

Identified resources 1

Mineral resource potential of the South Fork Wilderness Study Area 1 

Mineral resource potential of the Sand Hollow Wilderness Study Area 2 

Introduction 2

Area description 3 

Previous and present investigations 3 

Acknowledgments 3 

Appraisal of identified resources 3 

Assessment of mineral resource potential 5 

Geology 5 

Geophysical studies 6

Magnetic data 6 

Gravity studies 7 

Radiometric data 7 

Conclusions 8 

Geochemical studies 8

Mineral and energy resource potential of the South Fork Wilderness Study Area 9 

Mineral resource potential 9 

Energy resource potential 9

Mineral and energy resource potential of the Sand Hollow Wilderness Study Area 10 

Mineral resource potential 10 

Energy resource potential 10 

References cited 11 

Appendixes

Definition of levels of mineral resource potential and certainty of

assessment 15

Resource/reserve classification 16 

Geologic time chart 17

FIGURES

1. Index map showing location of the South Fork and Sand Hollow Wilderness Study Areas, 

Crook County, Oregon A2

2. Map showing mineral resource potential and geology of the South Fork and Sand Hollow 

Wilderness Study Areas, Crook County, Oregon 4

Contents

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

CENTRAL AND FAR WESTERN OREGON

Mineral Resources of the South Fork and 

Sand Hollow Wilderness Study Areas, 

Crook County, Oregon

By Jay A. Ach, James T. Bateson, Richard J. Blakely, and

Harley D. King

U.S. Geological Survey

Jerry E. Olson

U.S. Bureau of Mines

SUMMARY 

Abstract

At the request of the U.S. Bureau of Land Management, 

approximately 25,392 acres (17,001 and 8,391 acres, respectively) of the South Fork and Sand Hollow Wilderness Study 

Areas (OR-005-033 and OR-005-034) in central Oregon were 

evaluated for mineral resources (known) and mineral resource 

potential (undiscovered). Throughout this report, "wilderness 

study areas" and "study areas" refer to the 25,392 acres for 

which mineral surveys were requested. Fieldwork for this 

report was carried out in 1985 and 1986. No mines, significant prospects, or mining claims are located inside the study 

areas, and no identified resources were found; however, both 

areas are under lease for oil and gas. The west-central part 

of the South Fork Wilderness Study Area has a low mineral 

resource potential for mercury. The northwestern and southwestern parts of the Sand Hollow Wilderness Study Area also 

have a low mineral resource potential for mercury. Both 

study areas have a low potential for oil and gas resources and 

for geothermal energy resources.

Character and Setting

The South Fork and Sand Hollow Wildemesss Study 

Areas are located approximately 60 mi east of Bend and 

about 10 mi south of Paulina, in central Oregon (fig. 1). 

Rolling hills and gentle slopes are typical of most of the

study areas. The South Fork of the Crooked River cuts a 

deep canyon through the South Fork study area.

Volcanic rocks of the Clarno Formation are the oldest 

rocks exposed in the study areas. The Clarno Formation is 

overlain by the John Day Formation, and the Picture Gorge 

Basalt, part of the Columbia River Basalt Group, overlies 

the John Day Formation. The uneven, faulted surface of 

the Picture Gorge Basalt is unconformably overlain by 

tuffaceous sedimentary rocks that are in turn overlain by 

two thin ash-flow tuff units, the Devine Canyon Ash-flow 

Tuff and the Rattlesnake Ash-flow Tuff. On Twelvemile 

Table (fig. 1), thin flows of the basalt of Twelvemile Table 

overlie the Rattlesnake Ash-flow Tuff.

Identified Resources

No identified mineral resources exist within or adjacent to the wilderness study areas. Agate and common opal 

occur in areas along the north boundary of the South Fork 

study area. No other mineralized zones occur within the 

areas studied, and no mines or prospects are present. 

Bentonite is periodically mined from sedimentary rocks of 

the John Day Formation within 2 mi of the southwest 

boundary of the Sand Hollow study area. Similar sedimentary rocks are not exposed within the study areas.

Mineral Resource Potential of the South Fork 

Wilderness Study Area

The South Fork Wilderness Study Area has low minPublication authorized by the Director, U.S. Geological Survey, April 15,1988. eral resource potential for mercury in the west-central part

Mineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A1
of the area. This potential is indicated by anomalously high 

mercury concentrations in stream-sediment samples.

The South Fork study area also has low potential for 

petroleum and natural gas. A dry hole is located about 2 

mi north of this study area. A log of the hole indicates that 

potential source rocks are thin or absent and potential reservoir rocks, although probably present, may be of poor 

quality. Additionally, no favorable reservoir structures are 

known in the study area.

A low potential also exists for geothermal energy in the 

study area. This potential, which is unlikely to be explored 

or exploited, is indicated by two springs near the study area 

that have slightly elevated temperatures.

Mineral Resource Potential of the Sand Hollow 

Wilderness Study Area

A low mineral resource potential for mercury exists in 

the northwestern and southwestern parts of the Sand Hollow Wilderness Study Area. This potential is indicated by

anomalously high mercury concentrations in stream-sediment samples.

The Sand Hollow study area has low potential for petroleum and natural gas. The dry hole mentioned above is 

located about 5 mi northeast of this study area. Potential 

source rocks are thin or absent and potential reservoir rocks 

are likely to be of poor quality. No favorable reservoir 

structures are known in the study area.

A low potential also exists for geothermal energy in the 

study area. This potential, which is unlikely to be explored 

or exploited, is indicated by the two springs mentioned 

above that have slightly elevated temperatures.

INTRODUCTION

This mineral resource study is a joint effort by the U.S. 

Geological Survey and the U.S. Bureau of Mines. Mineral 

assessment methods and terminology are discussed in 

Goudarzi (1984). Identified resources are classified according to a system that is a modification of that described

APPROXIMATE BOUNDARY

SAND HOLLOW 

WILDERNESS STUDY AREA 

(OR-005-034)

APPROXIMATE BOUNDARY 

OF

SOUTH FORK 

WILDERNESS STUDY AREA

(OR-005-033) OCHOCO

NATIONAL

FOREST

Figure 1. Index map showing location of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon. 

A2 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
by McKelvey (1972) and described by the U.S. Bureau of 

Mines and the U.S. Geological Survey (1980). See the appendixes for the definition of levels of mineral resource potential, certainty of assessment, and classification of identified resources. Studies by the U.S. Geological Survey are 

designed to provide a reasonable scientific basis for assessing the potential for undiscovered mineral resources by determining geological units and structures, possible environments of mineral deposition, presence of geophysical and 

geochemical anomalies, and applicable ore-deposit models. 

The U.S. Bureau of Mines evaluates identified resources at 

individual mines and known mineralized areas by collecting data on current and past mining activities and through 

field examination of mines, prospects, claims, and mineralized areas.

Area Description

The U.S. Bureau of Land Management requested that a 

total of 25,392 acres of the South Fork and Sand Hollow 

Wilderness Study Areas (OR-005-033 and OR-005-034), 

17,001 and 8,391 acres, respectively, be evaluated for their 

mineral resource potential. The study areas are about 60 mi 

east of Bend and about 10 mi southwest of Paulina, in 

central Oregon (fig. 1). The topography of the areas is 

dominated by rolling hills that extend to the flat tablelands 

of Twelvemile Table to the east. The South Fork of the 

Crooked River has cut a steep, 600- to 700- ft-deep canyon 

through the South Fork study area. Elevations range from 

approximately 3,860 ft at the canyon bottom to 4,986 ft at 

the top of Steen's Ridge. The region has a semiarid climate 

(Strahler, 1969), and an average of about 15 in. of precipitation a year (Meteorology Committee, Pacific Northwest 

River Basins Commission, 1969). The vegetation consists 

mainly of range grasses, sagebrush, and juniper trees. Deer 

and coyotes are the larger mammalian residents of the 

study areas.

Access to the study areas from county roads to the west 

is provided by graded dirt and gravel roads and jeep trails. 

The two study areas are separated by a graded dirt road.

Previous and Present Investigations

Parts of the study areas are included in geologic maps by 

Walker and Repenning (1966), Walker and others, (1967), 

Brown and Thayer (1966), Swanson (1969), and Greene 

and others (1972). Davenport (1971) investigated the 

Rattlesnake Ash-flow Tuff and other tuffs in the vicinity. 

Dickinson and Vigrass (1965) studied the pre-Tertiary 

rocks in the area around Suplee, approximately 15 mi east 

of the study areas.

Previous reconnaissance studies of the geology and 

mineral resources of wilderness study areas in the region 

were carried out by Terradata (1983) and Barringer Resources, Inc. (1984). The uranium resource potential for 

the region was evaluated as part of the U.S. Department of 

Energy National Uranium Resource Evaluation (NURE) 

program (High Life Helicopters, Inc./ QEB, Inc., 198la, b, 

c, d).

The U.S. Geological Survey carried out field investigations in the study areas during the summers of 1985 and 

1986. This work included geologic mapping and geochemical sampling. An analysis was made of available 

geophysical data.

The U.S. Bureau of Mines conducted a library search for 

information on mines and prospects within and near the 

study areas. Additional data were obtained from records of 

Crook County, the U.S. Bureau of Land Management, and 

the U.S. Bureau of Mines. Field studies by U.S. Bureau of 

Mines personnel were carried out in 1986 (Olson, 1987). 

Fieldwork consisted of a general reconnaissance of the 

areas for mining-related activity that may not have been 

recorded. Additionally, bentonite deposits within 2 mi of 

the southwest boundary of Sand Hollow study area, agate 

and common opal occurrences near the north boundary of 

the South Fork study area, and occurrences of possible 

zeolites were investigated. From these areas, four samples 

were analyzed for bentonite, and seven samples were analyzed for zeolites. Complete analytical data are on file at 

the U.S. Bureau of Mines, Western Field Operations Center, E. 360 Third Ave., Spokane, Wash. 99202.

Acknowledgments

The U.S. Geological Survey wishes to thank the staff of 

the Prineville, Oregon, District Office of the U.S. Bureau of 

Land Management for providing logistical support, background information, and the use of the Bureau's facilities at 

the Paulina Guard Station. Calvin Westmoreland and 

family graciously provided access to their ranch as well as 

good conversation and a fine cup of coffee.

The U.S. Bureau of Mines thanks Dennis Davis, U.S. 

Bureau of Land Management, for providing information 

pertaining to the geology and general mineral character of 

the areas studied. U.S. Bureau of Mines geologist Fredrick 

Johnson ably assisted the U.S. Bureau of Mines author in 

prefield and field work.

APPRAISAL OF IDENTIFIED RESOURCES

By Jerry E. Olson 

U.S. Bureau of Mines

No identified mineral resources exist in the South Fork 

and Sand Hollow Wilderness Study Areas. The study areas 

do not lie in an established mining district and no records 

of historic or active claims exist.

Mineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A3
No mineralized zones were found within the study areas. extend into the areas studied. Common opal and agate

Low-grade bentonite deposits are periodically mined in occur in several locations along the north boundary of the

Camp Creek Valley within 2 mi of the southwest boundary South Fork study area and may be of recreational interest,

of the Sand Hollow study area but are confined to probable Several sites were sampled to determine whether zeolite

lacustrine rocks of the John Day Formation that do not minerals are present, but analytical results were negative.

120°07'30" 120°001 00"

APPROXIMATE BOUNDARY

OF

SAND HOLLOW

WILDERNESS

STUDY AREA

(OR-005-034)

3 Geo,

-L/COG

(entire study area)

APPROXIMATE BOUNDARY

OF

SOUTH FORK 

WILDERNESS 

STUDY AREA 

(OR-005-033)

Figure 2. Map showing mineral resource potential and geology of the South Fork and Sand Hollow Wilderness Study Areas, 

Crook County, Oregon.

A4 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
EXPLANATION

Area having low mineral resource potential (L)

Levels of certainty of assessment 

B Data suggest level of potential 

C Data give good indication of level of potential

Commodities

Hg Mercury

OG Oil and natural gas

Geo Geothermal

[ ] Deposit type

1 Epithermal mercury deposits in volcanic and 

volcanoclastic rocks

Correlation of map units

QUATERNARY

U~ pii Pliocene

 Miocene

Miocene to > Eocene(?)

Mi 

EC

>

Oligocene(?) 

ancd Eocene

- TERTIARY

Geologic map units

Qal Alluvium (Quaternary)

Tb Basalt of Twelvemile Table (Tertiary)

Tt Ash-flow tuffs (Tertiary)

Ts Tuffaceous sedimentary rocks (Tertiary)

Tpg Picture Gorge Basalt (Tertiary)

Tjd John Day Formation (Tertiary)

Tc Clarno Formation (Tertiary)

 Contact Dashed where approximate 

~l Fault Dashed where approximate; bar and 

ball on downthrown side

Figure 2. Continued.

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL

By Jay A. Ach, James T. Bateson, Richard J. Blakely, and

Harley D. King,

U.S. Geological Survey

Geology

Pre-Tertiary basement rocks are not exposed within or 

near the South Fork and Sand Hollow Wilderness Study 

Areas (see appendixes for geologic time chart). However, 

older rocks are exposed in the area around Suplee, approximately 15 mi east of the study areas. There, two small 

outcrops of highly folded, fossiliferous limestone constitute 

the only known exposures of Devonian rocks in Oregon 

(Kleweno and Jeffords, 1962; Buddenhagen, 1967). South 

of Suplee, highly folded and faulted Mississippian, Pennsylvanian, and Permian marine and non-marine clastic 

rocks, chert, and limestone are exposed (Merriam and 

Berthiaume, 1943; Mamay and Read, 1956; Buddenhagen, 

1967). East of Suplee, Dickinson and Vigrass (1965) 

mapped and described a thick section of Permian through 

Cretaceous rocks. The Permian rocks east of Suplee consist primarily of an undetermined thickness of felsites 

(light-colored, fine-grained igneous rocks) that contain 

minor interbeds of limestone, chert, and volcaniclastic 

sedimentary rocks. Some Permian metamorphosed mafic 

rocks (greenstones) that include possible pillow basalts are 

also exposed there; such rocks could represent the upper 

part of an ophiolite sequence. The Mesozoic section east 

of Suplee consists of 33,000 to 34,000 ft of marine sedimentary rocks, including mudstone, siltstone, sandstone, 

and less abundant limestone and conglomerate. These 

rocks are at least partially volcaniclastic in origin.

Well logs from the Texaco Federal No. 1 drillhole, 

located about 2 mi north of the South Fork study area, 

indicate that the Mesozoic sedimentary sequence beneath 

the study areas is not as thick as it is in the area around 

Suplee. This hole was drilled to a total depth of 7,998 ft 

in 1971 near the crest of a Cenozoic anticline. From the 

surface, the hole first penetrated 1,500 to 1,560 ft of the 

lower Tertiary Clarno Formation. The well log describes 

the interval from 1,560 to 1,890 ft as Cretaceous siltstone 

and sandstone and the interval from 1,890 to 3,500 ft as 

Jurassic sandstone, claystone, and bentonite. Thompson 

and others (1984) studied microfossils from this hole and 

revised the well stratigraphy. They found that the rocks in 

the interval from 1,500 to 2,040 ft are nonmarine and 

uppermost Cretaceous and Paleocene, from 2,040 to 2,220 

ft the rocks are marine and mid-Cretaceous, and from 2,220 

to 3,500 ft the rocks are nonmarine and mid-Cretaceous. 

Both studies indicate that the interval from 3,500 ft to the 

bottom of the hole consists of pre-Cretaceous, predomiMineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A5
nantly volcanic rocks. Newton (1974) notes that some 

traces of gas were detected in the Cretaceous interval in this 

hole. The log for this hole indicates that these gas traces 

were barely above backround levels. Such minor traces of 

gas are common in dry holes and have little significance 

(Dan E. Wermiel, oral commun., 1987).

The oldest unit exposed in the study areas belongs to the 

Eocene and lowermost Oligocene (?) Clarno Formation 

(George W. Walker, oral commun., 1987). In the areas 

studied, the Clarno Formation consists predominantly of 

rusty brown andesite lava flows and breccia and interbedded tuffaceous sedimentary rocks.

The John Day Formation overlies the Clarno Formation. 

Fine-grained sandstone, siltstone, and claystone form the 

bulk of the John Day Formation. These rocks occur in light 

pastel shades of green, gray, tan, and white, and are commonly bentonitic. Locally, thin flows of dark gray to black, 

aphanitic to microcrystalline, flow-foliated basalt or andesite are present. The age of the John Day Formation 

ranges from about 37 to about 19 m.y. (million years), 

latest Eocene to early Miocene (George W. Walker, oral 

commun., 1987).

In most of the area studied, the lower Miocene Picture 

Gorge Basalt of the Columbia River Basalt Group is the 

oldest unit exposed at the surface. This unit is approximately 15 to 16 m.y. in age (Watkins and Baksi, 1974; 

Fiebelkorn and others, 1982) and overlies the John Day 

Formation. The Picture Gorge Basalt is composed of thick, 

highly fractured flows of aphanitic to finely crystalline 

olivine basalt. Columnar joints are commonly well developed in individual outcrops. Within the study areas, rocks 

mapped as the Picture Gorge Basalt may include some 

basalt of the John Day Formation.

Tan to light-gray to white, tuffaceous, fine-grained sandstone and siltstone are interbedded with and overlie the 

Picture Gorge Basalt. Locally these sedimentary rocks 

contain minor amounts of small, rounded, white pumice 

fragments and may include some air-fall tuff. Greene and 

others (1972) consider this unit to be correlative, in part, 

with the middle to upper Miocene Mascall Formation of 

Merriam (1901).

Two thin ash-flow tuff units overlie the tuffaceous sedimentary rocks. Because they are too thin to be shown 

individually, these two tuffs are grouped together in one 

unit (Tt) on the map (fig. 2); however, they are easily 

distinguished in the field. The lowermost tuff unit is the 

Devine Canyon Ash-flow Tuff. This rhyolite welded tuff 

is light- to medium -gray to greenish gray and contains 2 to 

17 percent phenocrysts of sanidine and quartz (Davenport, 

1971). The distinguishing feature of this unit is 1 to 5 

percent light yellow-brown to orange-brown accretionary 

lapilli that are approximately 0.1 to 0.3 in. in diameter. 

This tuff has a late Miocene potassium-argon age of 9.2 to 

9.3 m.y. (Walker, 1979).

The upper tuff unit is the Rattlesnake Ash-flow Tuff, a 

medium gray peraluminous rhyolite welded tuff. The well 

developed eutaxitic texture, produced by dark gray, offwhite, and medium brown fiamme (flattened, stretched 

pumice fragments), is distinctive. The unit is phenocryst 

poor and has one percent or less sanidine and sparsely 

scattered quartz and clinopyroxene grains. Potassium-argon ages for the unit average 6.4 m.y. (Walker, 1979), late 

Miocene. The Rattlesnake Ash-flow Tuff is separated from 

the Devine Canyon Ash-flow Tuff by 0 to 25 ft of tuffaceous sedimentary rocks and nonwelded ash.

In the eastern part of the Sand Hollow study area, the 

Rattlesnake Ash-flow Tuff is overlain by several thin flows 

of basalt, the basalt of Twelvemile Table. This basalt is 

composed of small olivine phenocrysts in a microcrystalline matrix of augite and calcic plagioclase and minor 

magnetite.

Quaternary deposits consist of alluvium in the canyon 

bottoms, talus on canyon slopes, and local landslide deposits along the canyon walls.

Normal faults having offsets of tens to hundreds of feet 

developed in the areas in the middle to late Miocene, after 

the eruption of the Picture Gorge Basalt and before deposition of the younger units. The Picture Gorge Basalt and 

older units also are folded in a northeast-trending anticline 

located immediately north of the study areas. This faulting 

and folding resulted in a paleotopography of high hills 

composed of the Picture Gorge Basalt surrounded by 

younger basins. The tuffaceous sedimentary rocks fill 

paleobasins developed in the Picture Gorge Basalt, and the 

ash-flow tuffs overlie the sedimentary rocks but also lap 

onto or directly overlie the basalt.

In the western part of the Sand Hollow study area, a 

second episode of faulting produced numerous small offsets in the the upper Miocene ash-flow tuff unit.

Geophysical Studies

Magnetic, gravity, and radiometric data from central 

Oregon were compiled and examined in order to aid assessment of the mineral resource potential of the Sand Hollow 

and South Fork Wilderness Study Areas. The sparsely distributed data in all three data sets are adequate for addressing the regional structural and tectonic setting of the study 

areas but do not permit assessment of mineral resource 

potential at deposit scales except in limited areas directly 

beneath detailed profiles.

Magnetic Data

The Sand Hollow and South Fork Wilderness Study 

Areas lie within a terrane of young, unmetamorphosed 

volcanic rocks that commonly have large magnetic susceptibilities and large remanent magnetizations. In low-level

A6 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
aeromagnetic profiles, these rocks typically produce highamplitude, short-wavelength magnetic anomalies that tend 

to obscure anomalies caused by deeper magnetic sources.

Magnetic data in the vicinity of the study areas are from 

two sources. An aeromagnetic survey was flown in 1983 

at an altitude of 7,500 ft above sea level along east-west 

flightlines spaced 3 mi apart (U. S. Geological Survey, 

1984). These profiles are too high in altitude and too 

widely spaced to provide detailed information about mineral resource potential, but contour maps of these data show 

various magnetic anomalies of regional interest. The 

westernmost part of the study areas is located on or near the 

east edge of a large magnetic high. The shape and amplitude of the anomaly indicate that its source lies at shallow 

depth (less than 1 mi) beneath the topographic surface and 

has horizontal dimensions on the order of 10 mi. This 

anomaly is located at the west tip of a conspicuous arcuateshaped belt of anomalies that extends over 100 mi to the 

east (Committee for the Magnetic Anomaly Map of North 

America, 1987). In some locations, anomalies of this belt 

correspond with outcrops of Triassic mafic and ultramafic 

rocks (Walker, 1977). In this belt, near Suplee, Permian 

greenstones containing possible pillow basalts are exposed 

(Dickinson and Vigrass, 1965), and approximately 25 mi to 

the west of the study areas, possible serpentinite was reported between depths of 4,000 ft and 4,200 ft in the 

Sunray-Midcontinent Bear Creek No. 1 well. Together, 

this information suggests that the belt of magnetic anomalies is related to a belt of mafic to ultramafic rocks of 

Permian and (or) Triassic age; such a belt may represent a 

Permian and (or) Triassic suture zone. Although similar 

rocks do not crop out in or near the Sand Hollow and South 

Fork Wilderness Study Areas, the magnetic data suggest 

that the westernmost part of the study areas may be underlain at shallow depth by similar mafic or ultramafic rocks.

The second source of magnetic data was compiled under 

contract to the U.S. Department of Energy as part of the 

National Uranium Resource Evaluation (NURE) program 

(High Life Helicopters/QEB, 1981a, 1981b, 1981c, 1981d). 

These data were measured 400 ft above average terrain 

along east-west profiles spaced 6 mi apart and north-south 

profiles spaced 24 mi apart. (North of lat 44° N. and west 

of long 120° W., east-west profiles are spaced 3 mi apart.) 

Only two NURE profiles actually cross the study areas and 

are too widely spaced to describe the spatial distribution of 

magnetic anomalies between them. Because of the low 

altitude of the aircraft, these profiles are greatly influenced 

by highly magnetic volcanic rocks at the surface and nearsurface. Some of the individual highs and lows along the 

magnetic profile may indicate mineralized areas, but are 

more likely the result of lateral variations in magnetite 

content in shallow basaltic rocks.

The NURE magnetic profiles show clear evidence of the 

large, shallowly buried source that was interpreted earlier

from high-level magnetic data and presumed to be the west 

end of a belt of mafic and ultramafic rocks of Permian and 

(or) Triassic age extending approximately 100 mi to the 

east. The NURE data indicate that the east edge of the 

buried source is located very near the west boundary of the 

study areas and at a depth of less than 1 mi below the 

topographic surface.

Gravity Studies

Gravity data from the vicinity of the study areas were 

obtained from the National Geophysical Data Center, National Oceanic and Atmospheric Administration, Boulder, 

Colorado. Measurements are scattered at 3-to 5-mi spacing 

in this region, and only two measurements are actually 

within the study areas. The observed gravity data, based on 

the International Gravity Standardization Net datum 

(Morelli, 1974), were reduced to free-air gravity anomalies 

by using standard formulas (Telford and others, 1976). 

Bouguer, curvature, and terrain corrections (out to a distance of 103.6 mi from each station) at a standard reduction 

density of 2.67 g/cm3 were added to the free-air anomaly at 

each station to determine complete Bouguer gravity anomalies.

Bouguer anomalies typically reflect shallow density 

contrasts of interest to resource appraisals, but also include 

contributions from deep-crustal masses that correlate with 

topography in a manner consistent with the concept of 

isostasy. To reduce the effect of deep sources related to 

isostasy, an isostatic residual-gravity map was constructed 

from the Bouguer gravity data by removing a regional 

gravity field computed from a model of the crust-mantle 

interface, assuming Airy-type isostatic compensation (Jachens and Griscom, 1985). A regional gravity perspective of 

the setting of the study areas can be seen on Jachens and 

others (1985).

The Sand Hollow and South Fork Wilderness Study 

Areas lie in a north-trending trough-shaped gravity low 

approximately 10 mi wide and 50 mi long. The linear 

nature of the trough suggests that it may be related to 

faulting of the pre-Tertiary basement rocks that lie beneath 

the volcanic cover. The trough is located between two 

positive anomalies of regional scale. These anomalies 

appear spatially related to the arcuate-shaped belt of magnetic anomalies discussed earlier and support the interpretation that mafic and ultramafic rocks may exist at shallow 

depth within or near the study areas.

Radiometric Data

Radiometric data were collected as part of the NURE 

survey (High Life Helicopters, Inc./QEB, Inc., 1981a, 

1981b, 198 Ic, 1981d). Recordings were made of gammaray flux from radioactive isotopes of potassium, thorium,

Mineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A7
and uranium along east-west flightlines spaced approximately 6 mi apart and at an altitude of 400 ft above average 

terrain. Only two profiles actually cross the study areas, at 

lat 43° 58.7' N. and at lat 44° 01.5' N. Count rates were 

low along these lines except in two locations approximately 

at lat 43° 58.67' N., long 120° 02.21' W. and at lat 43° 

58.59' N., long 120° 08.09' W. Slight anomalies of both 

equivalent thorium and equivalent uranium occur in these 

two locales, but they were not considered significant by the 

NURE study. Both anomalies occur near outcrops of fluvial and lacustrine sediments of Tertiary age. Because only 

two flightlines cross the study areas and because gamma 

rays are strongly attenuated by passage through earth materials, these data do not preclude the presence of anomalous amounts of radioactive elements away from the flightlines or buried a few feet or more beneath the ground 

surface.

Conclusions

Regional magnetic and gravity data suggest that the 

westernmost part of the Sand Hollow and South Fork 

Wilderness Study Areas may be underlain at depth by 

mafic and ultramafic rocks of Permian and (or) Triassic age 

related to an arcuate belt of similar rocks extending 100 mi 

to the east. Chromite deposits of economic importance are 

known to exist in the eastern part of this belt of anomalies. 

The Canyon Mountain (ultramafic) Complex in Grant 

County, for example, is one of the largest producers of 

chromite ore in the state of Oregon (Ferns and Huber, 

1984). Although these chromite deposits are located over 

30 mi from the study areas, the magnetic and gravity data 

suggest that similar host rocks may exist at depths of 3000 

feet or more within or near the westernmost part of the 

study areas. The 8,000-ft-deep Texaco Federal No. 1 hole, 

located 2 mi north (and 8 mi east of the west boundary) of 

the study areas, did not intersect ultramafic rocks, although 

given the sketchy nature of drill logs, the existence of such 

rocks here cannot be ruled out absolutely. This hole is 

located east of the east edge of the magnetic and gravity 

anomalies, however, and mafic and ultramafic rocks are not 

predicted in this area by geophysical models.

Geochemical Studies

A reconnaissance geochemical investigation of the study 

areas was made using samples of stream sediment, the 

nonmagnetic fraction of heavy-mineral concentrates from 

stream sediments, and rock. The stream-sediment and 

concentrate samples contain material derived from rock 

units of the drainage basins within the study areas. 

Sampled drainage basins range in area from less than one 

to several square miles.

From the two study areas, a total of 49 stream-sediment 

samples, 39 heavy-mineral-concentrate samples, and 32 

rock samples were analyzed for 31 elements by six-step 

semiquantitative emission-spectrographic methods (Myers 

and others, 1961; Grimes and Marranzino, 1968), and 

additional analyses were made using atomic-absorption 

spectroscopy and inductively coupled argon plasma-atomic 

emission spectroscopy (Crock and others, 1987). The 

analyses of stream-sediment and heavy-mineral-concentrate 

samples identify drainages that have anomalously high 

concentrations of metallic and metal-related elements. 

Rock samples were collected from areas of altered bedrock 

and from each lithologic unit in order to obtain information 

on trace-element signatures associated with potentially 

mineralized areas and to provide data on background traceelement concentrations. For this study, anomalous geochemical values were determined by inspection of histograms of the data from both the study areas and the surrounding region and by comparing the data to average 

crustal abundances in silicic and mafic volcanic rocks. For 

most elements, a value was considered anomalous if it 

exceeded the mean value for the element by two standard 

deviations.

Anomalous concentrations of mercury (greater than 0.18 

ppm (parts per million)) were found in seven stream-sediment samples from northwestern and southwestern parts of 

the Sand Hollow study area. These samples have mercury 

concentrations of 0.27 to 4.6 ppm. Two stream-sediment 

samples from the west-central part of the South Fork study 

area also contain anomalous mercury concentrations (0.23 

and 0.55 ppm). One stream-sediment sample from a site 

approximately 1/2 mi north of the South Fork study area 

contains 0.28 ppm mercury; the drainage basin from which 

this sample was collected does not extend into the study 

area. Cinnabar, the primary ore mineral of mercury deposits, was not observed during microscopic examination of 

the heavy-mineral concentrates from the sites.

Data from Barringer Resources, Inc. (1984) indicates 

that the magnetic fraction of a heavy-mineral concentrate of 

one stream-sediment sample contains 0.03 ppm silver. This 

sample was collected just over a mile southwest of the Sand 

Hollow study area. Igneous rocks commonly contain 0.04 

to 0.1 ppm silver (Rose and others, 1979); the 0.03 ppm 

silver present in this sample is not considered anomalous.

Previous regional uranium surveys, mentioned earlier, 

which included aerial gamma-ray emission studies (High 

Life Helicopters, Inc./ QEB, Inc., 198la, b, c, d), found two 

small areas with equivalent thorium and equivalent uranium anomalies in the study areas. No anomalous concentrations of thorium were found in stream-sediment or rock 

samples. No stream-sediment geochemical data is available to confirm or deny the existence of the uranium 

anomalies. No other geological evidence exists that would 

indicate uranium resources in these anomalous areas.

A8 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
Mineral and Energy Resource.Potential of the 

South Fork Wilderness Study Area

Mineral Resource Potential

Investigations by the U.S. Geological Survey indicate 

that an area in the west central part of the South Fork 

Wilderness Study Area has low mineral resource potential 

for mercury in epithermal deposits, certainty level C. (See 

appendixes for definitions of levels of mineral resource 

potential and certainty of assessment.)

A low potential for mercury is based solely upon the two 

mercury anomalies in the stream sediment data from the 

study area. Permeability of the rocks is an important prerequisite for the emplacement of mercury deposits (Brooks, 

1963, p. 21). The Picture Gorge Basalt and the densely 

welded tuffs lack permeability, and therefore are unlikely 

host rocks for mercury deposits. With respect to mercury, 

Brooks (1963) states that the Columbia River Basalt Group 

is "remarkably unmineralized." Additionally, the rocks in 

the anomalous areas have not been greatly affected by local 

faulting or fracturing that would increase permeability and 

provide pathways for mineralizing fluids or vapors. The 

tuffaceous sedimentary rocks lying between the Picture 

Gorge Basalt and welded tuff may have higher permeability 

and may be more likely host rocks. However, no visible 

alteration was observed in the tuffaceous sedimentary 

rocks. Since nearly all mercury deposits worldwide are 

epithermal deposits (Brooks, 1963), it can be safely assumed that any mercury deposit, if present in the study 

area, would likely be epithermal.

Small sand and gravel deposits suitable for construction 

use are present in the study area but are too small to constitute a resource. No undiscovered sand and gravel resources are expected beyond known deposits.

Energy Resource Potential

Investigations by the U.S. Geological Survey indicate 

that the South Fork Wilderness Study Area has low resource potential for petroleum and natural gas, certainty 

level C. The geothermal resource potential is low, certainty 

level B; however, this level of potential may be too high.

The South Fork Wilderness Study Area is considered to 

have a low potential, certainty level C, for oil and gas. This 

is consistent with Fouch (1983) who also rated the area as 

having low potential for oil and gas. Besides being near a 

dry hole, the area is considered to have low potential because it lies outside of, or on the fringe of, known Cretaceous basins, because few likely source rocks are present, 

and because potential reservoir rocks probably have low 

porosity and permeability.

Central Oregon has been explored for oil since the 

1930's, but no discoveries of oil or gas have been made

(Thompson and others, 1984). Exploration efforts and 

petroleum resource appraisals are hampered by the meager 

data available on subsurface geology in central Oregon 

(Thompson and others, 1984; Fritts and Fisk, 1985a, b). 

Few deep wells have been drilled, outcrops of pre-Cenozoic rocks are rare, and the thick Cenozoic volcanic cover 

causes difficulty in the interpretation of geophysical data 

(Newton, 1974). Additionally, the region is structurally 

complex and has Cenozoic structures that differ from 

underlying Mesozoic structures. Little is known about preMesozoic structures (George W. Walker, oral commun., 

1987).

The primary exploration targets in central Oregon have 

been within Cretaceous basins (Newton, 1968; Fritts and 

Fisk, 1985a, b). Two such basins exist. The larger and 

older of the basins is the Columbia River Basin of mid- to 

Late Cretaceous age (Newton, 1966, 1977; Fritts and Fisk, 

1985a, b). Most of this basin lies to the north of the study 

area. Only the south margin passes through the vicinity of 

the study area (Fritts and Fisk, 1985a, b). The smaller and 

slightly younger (latest Cretaceous) Ochoco Basin lies to 

the west and probably never extended as far east as the 

study area (Thompson and others, 1984).

Oil and gas leases cover the study area. In 1971, Texaco 

drilled the Federal No. 1 test hole approximately 2 mi north 

of the South Fork study area. Newton (1974) reported 

slight gas shows between 1,700 and 3,600 ft in this hole, 

but the well log indicates only a negligible increase over 

background gas values in this section of the hole. From 

approximately 11 to 25 mi to the southwest and west of the 

study areas, five other holes have been drilled by various 

companies. No oil or gas shows were reported from four 

of these holes. The Sunray-Midcontinent Bear Creek No. 

1, drilled in 1958, showed slight gas shows at 7,919 ft 

(Newton, 1969). Well logs for all holes are on file at the 

Oregon Department of Geology and Mineral Industries, 

Portland, Oregon.

For purposes of an oil and gas resource potential 

assessment, it is assumed that the subsurface stratigraphy of 

the wilderness study area is similar to that in the Texaco 

Federal No. 1 hole, previously described. The relatively 

thin, 180-ft-thick sequence of mid-Cretaceous marine 

sedimentary rocks indicates that the study area lies outside 

the Ochoco Basin and on the very margin of the Columbia 

River Basin. Further, the relative paucity of Cretaceous 

and pre-Cretaceous marine sedimentary rocks indicates that 

few potential source rocks exist in the area, since preCretaceous rocks are primarily volcanic in origin 

(Thompson and others, 1984). The 2,000-ft-thick 

Cretaceous sedimentary section could include potential 

reservoir rocks, but since the rocks in this section are of 

mostly non-marine character and may be poorly sorted 

(George W. Walker, oral commun., 1987), their porosity 

and permeability may be low, thus reducing their reservoir

Mineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A9
rock potential. Because it lies outside of or on the fringe 

of known Cretaceous basins, because few likely source 

rocks are present, and because potential reservoir rocks 

may have low porosity and permeability, the area is 

considered to have low resource potential for oil and gas. 

The resource potential for geothermal energy in the 

study area is low, certainty level B. A certainty level of B 

is assigned because the existence of a resource is unlikely 

and the assessment of potential may be too high. Two 

warm springs, having water temperatures of 73° and 88° F, 

occur on or within 1/2 mile of the north border of the study 

area (Bliss, 1983). Mariner and others (1983) classify these 

springs as representing an isolated system that has a reservoir area of less than 1 sq km. These springs are probably 

the result of deep circulation along normal faults in an area 

of normal heat flow (Mariner and others, 1983), indicating 

low reservoir temperatures. Therefore a geothermal resource, if present, would be a low-temperature resource. 

The study area does not lie in a region of high heat flow 

(Riccio, 1978). Previous state and regional geothermal 

surveys have not indicated any geothermal potential for the 

study area (Bowen and others, 1978; Riccio, 1978; Muffler, 

1979; National Geophysical Data Center, 1982; Reed, 

1983; Bliss, 1983). Under the definitions used in this study 

(see appendixes), the geothermal resource potential is 

considered low.

Mineral and Energy Resource Potential of the 

Sand Hollow Wilderness Study Area

Mineral Resource Potential

The investigations by the U.S. Geological Survey indicate that areas in the northwest and southwest parts of the 

Sand Hollow Wilderness Study Area have low mineral 

resource potential for mercury in epithermal deposits, certainty level C. (See appendixes for definitions of levels of 

mineral resource potential and certainty of assessment.)

Mercury anomalies in the stream sediment data indicate 

a low potential for mercury in two areas (fig. 2). Some of 

the highest mercury concentrations occur in samples from 

sites along faults or from sites in drainage basins that have 

faults upstream. This indicates that faults may have played 

an important role in acting as conduits for mercury vapors 

or mercury-bearing solutions and for localizing mercury. 

Other samples from sites along faults do not have anomalous mercury concentrations, suggesting leakage of fluids 

or vapors may have only occurred along certain faults or 

certain portions of faults.

As discussed previously, permeability of the rocks is an 

important prerequisite for the emplacement of mercury 

deposits (Brooks, 1963). The Picture Gorge Basalt and the 

densely welded tuffs lack permeability and therefore are

unlikely host rocks for mercury deposits. The tuffaceous 

sedimentary rocks lying between these other two units may 

have higher permeability and be more likely host rocks. 

However, no visible alteration was observed in these sedimentary rocks.

Low-grade bentonite deposits have been mined periodically within 2 mi of the southwest boundary of the Sand 

Hollow study area, but are confined to probable lacustrine 

rocks of the John Day Formation. Such rocks do not extend into the Sand Hollow study area, and the area has no 

mineral resource potential for bentonite.

Small sand and gravel deposits suitable for construction 

use are present in the study area but are too small to constitute a resource. No undiscovered sand and gravel resources are expected beyond known deposits.

Energy Resource Potential

The investigations by the U.S. Geological Survey indicate that the Sand Hollow Wilderness Study Area has low 

resource potential for petroleum and natural gas, certainty 

level C. The geothermal resource potential is low, certainty 

level B; however, this level of potential may be too high.

The low potential for oil and gas in the Sand Hollow 

Wilderness Study Area is consistent with the conclusion of 

Fouch (1983) who also rated the area as having low potential for these commodities. Besides being near a dry hole, 

the area is considered to have low potential because it lies 

outside of or on the fringe of known Cretaceous basins, 

because few likely source rocks are present, and because 

potential reservoir rocks probably have low porosity and 

permeability. Oil and gas leases cover the study area. See 

South Fork "Energy Resource Potential" section above for 

further discussion.

The resource potential for geothermal energy in the 

study area is low, certainty level B. A certainty level of B 

is assigned because the existence of a resource is unlikely 

and the assessment of potential may be too high. As discussed for the South Fork study area, a warm spring occurs 

within 1/2 mile of the northeast corner of the Sand Hollow 

Wilderness Study Area, and another occurs about 4 mi to 

the northeast (Bliss, 1983). Riccio (1978) and National 

Geophysical Data Center (1982) indicate that a warm water 

well and (or) spring occur along Camp Creek, 1 to 4 mi 

west of the study area. The conclusions reached earlier 

regarding the potential reservoir system for the South Fork 

study area apply as well to the Sand Hollow study area. 

Any geothermal system in the area is likely to be small and 

of low temperature. Previous state and regional geothermal 

surveys have not indicated any geothermal potential for the 

study area (Bowen and others, 1978; Riccio, 1978; Muffler, 

1979; National Geophysical Data Center, 1982; Reed, 

1983; Bliss, 1983).

A10 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
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U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-493, scale 1:250,000.

Walker, G.W., and Repenning, C.A., 1966, Reconnaissance geologic map of the west half of the Jordan Valley quadrangle, 

Malheur County, Oregon: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-457, scale 1:250,000.

Watkins, N.D., and Baksi, A.K., 1974, Magnetostratigraphy and 

oroclinal folding of the Columbia River, Steens, and Owyhee 

basalts in Oregon, Washington, and Idaho: American Journal 

of Science, v. 275, no. 2, p. 148-189.

A12 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
APPENDIXES

DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

LEVELS OF RESOURCE POTENTIAL

H HIGH mineral resource potential is assigned to areas where geologic, geochemical, and geophysical characteristics indicate a geologic environment favorable for resource occurrence, where interpretations of data 

indicate a high degree of likelihood for resource accumulation, where data support mineral-deposit models 

indicating presence of resources, and where evidence indicates that mineral concentration has taken place. 

Assignment of high resource potential to an area requires some positive knowledge that mineral-forming processes 

have been active in at least part of the area.

M MODERATE mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable for resource occurrence, where interpretations of data 

indicate reasonable likelihood for resource accumulation, and (or) where an application of mineral-deposit models 

indicates favorable ground for the specified type(s) of deposits.

L LOW mineral resource potential is assigned to areas where geologic, geochemical, and geophysical characteristics 

define a geologic environment in which the existence of resources is permissive. This broad category embraces 

areas with dispersed but insignificantly mineralized rock, as well as areas with little or no indication of having 

been mineralized.

N NO mineral resource potential is a category reserved for a specific type of resource in a well-defined area.

U UNKNOWN mineral resource potential is assigned to areas where information is inadequate to assign a low, 

moderate, or high level of resource potential.

LEVELS OF CERTAINTY

A Available information is not adequate for determination of the level of mineral resource potential.

B Available information only suggests the level of mineral resource potential.

C Available information gives a good indication of the level of mineral resource potential.

D Available information clearly defines the level of mineral resource potential.

O

o_

LLJ

U

o

_J

LU 

>

A

U/A

UNKNOWN POTENTIAL

B

H/B

HIGH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW POTENTIAL

C

H/C

HIGH POTENTIAL

M/C

MODERATE POTENTIAL

L/C

LOW POTENTIAL

D

H/D

HIGH POTENTIAL

M/D

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

LEVEL OF CERTAINTY

Abstracted with minor modifications from:

Taylor, R.B., and Steven, T.A., 1983, Definition of mineral resource potential: Economic Geology, v. 78, no. 6, p. 1268-1270.

Taylor, R.B., Stoneman, R.J., and Marsh, S.P., 1984, An assessment of the mineral resource potential of the San Isabel National Forest, south-central Colorado: U.S.

Geological Survey Bulletin 1638, p. 40-42. 

Goudarzi, G.H., compiler, 1984, Guide to preparation of mineral survey reports on public lands: U.S. Geological Survey Open-File Report 84-0787, p. 7, 8.

Mineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A15
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured

Rese

Indicated

;rves

Marginal 

Reserves

Demonstrated 

Subeconomic 

Resources

Inferred

Inferred 

Reserves

Inferred 

Marginal 

Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

Hypothetical Speculative

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from McKelvey, V.E., 1972, Mineral 

resource estimates and public policy: American Scientist, v. 60, p. 32-40; and U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles 

of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831, p. 5.

A16 Mineral Resources of Wilderness Study Areas Central and Far Western Oregon
GEOLOGIC TIME CHART

Terms and boundary ages used by the U.S. Geological Survey in this report

EON

Phanerozoic

Proterozoic

pre-Archean2

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean

PERIOD

Neogene

Subperiod

Tertiary

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Mississippian

Devonian

Silurian

Ordovician

Cambrian

.......... (3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

AGE ESTIMATES OF 

BOUNDARIES (in Ma)

 0.010

 1.7

 5

24

1Q

55

 66

1 1Q

205

r-")A{\

 290 

-330

tf^r\

 410

 J\JU

i q7n

 1600 

 2500

 3400

 ^ccn 

'Rocks older than 570 Ma also called Precambrian, a time term without specific rank. 

Informal time term without specific rank.

Mineral Resources of the South Fork and Sand Hollow Wilderness Study Areas, Crook County, Oregon A17
GPO 585-045/78059
SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

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Miscellaneous Investigations Series Maps are on planimetric or 

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Oil and Gas Investigations Charts show stratigraphic information 

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Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

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Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

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