Mineral Resources of the

Sewemup Mesa Wilderness Study Area,

Mesa and Montrose Counties, Colorado

U.S. GEOLOGICAL SURVEY BULLETIN 1736-B

Chapter B

Mineral Resources of the

Sewemup Mesa Wilderness Study Area,

Mesa and Montrose Counties, Colorado

By SANDRA J. SOULLIERE, CHARLES G. PATTERSON, 

ANNE M. LEIBOLD, and DOLORES M. KULIK 

U.S. Geological Survey

CLAY M. MARTIN 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1736

MINERAL RESOURCES OF WILDERNESS STUDY AREAS 

GRAND JUNCTION REGION, COLORADO
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U. S. GEOLOGICAL SURVEY 

Dallas L Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging in Publication Data

Mineral resources of the Sewemup Mesa Wilderness Study Area, Mesa and 

Montrose Counties, Colorado.

(Mineral resources of wilderness study areas Grand Junction region, 

Colorado ; ch. B) (U.S. Geological Survey bulletin ; 1736) 

Bibliography: p. 

Supt. of Docs, no.: I 19.3:1736-8

I. Mines and mineral resources Colorado Sewemup Mesa 

Wilderness. 2. Sewemup Mesa Wilderness (Co!o.) I. Soulliere, Sandra J.

II. Series. III. Series: U.S. Geological Survey bulletin ; 1736-B. 

QE75.B9 no. 1736-B 557.3 S [553'.09788'17] 87-600431 

[TN24.C6]
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 on 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 the Sewemup Mesa (CO-070-176/030- 

310A) Wilderness Study Area, Mesa and Montrose Counties, Colorado.

CONTENTS

Abstract Bl 

Summary Bl 

Introduction B3

Investigation by the U.S. Bureau of Mines B4 

Investigation by the U.S. Geological Survey B4 

Appraisal of identified resources B4 

Mining and leasing activity B4 

Rajah mine B4 

Copper Rivet mine B6 

Prospects B6

Identified mineral resources B6 

Assessment of potential for undiscovered resources B6 

Geology B6

Geologic setting B6 

Description of rock units B7 

Geochemistry B8

Sample collection and analytical methods B8 

Results of survey B8 

Geophysics B9

Gravity and aeromagnetic data B9 

Remote sensing data B9 

Mineral and energy resources B9

Uranium, vanadium, and copper fill 

Copper and silver Bll 

Industrial minerals Bll 

Other metals Bll 

Energy resources B13 

References cited B13 

Appendix B14

FIGURES

1. Map showing location, identified resources, and mineral resource potential of the 

Sewemup Mesa Wilderness Study Area B2

2. Index map showing location of the Sewemup Mesa Wilderness Study Area B3

3. Map showing oil and gas leases for the Sewemup Mesa Wilderness Study Area B5

4. Complete Bouguer gravity anomaly map of the Sewemup Mesa Wilderness Study 

Area BIO

5. Residual intensity magnetic anomaly map of the Sewemup Mesa Wilderness Study 

Area B12

Contents V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS-GRAND JUNCTION REGION, COLORADO

Mineral Resources of the

Sewemup Mesa Wilderness Study Area,

Mesa and Montrose Counties, Colorado

By Sandra J. Soulliere, Charles G. Patterson, 

Anne M. Leibold, and Dolores M. Kulik 

U.S. Geological Survey

Clay M. Martin

U.S. Bureau of Mines

Abstract

In 1983 and 1986 the USGS (U.S. Geological Survey) and the 

USBM (U.S. Bureau of Mines) conducted studies to appraise the identified resources (known) and assess the mineral resource potential 

(undiscovered) of 18,835 acres of the Sewemup Mesa (CO-070- 

176/030-310A) Wilderness Study Area (hereafter referred to as "study 

area"). There are no mines, prospects, economic or marginally 

economic resources in the study area; however, there are inferred 

subeconomic resources of sand and gravel, and sandstone, and occurrences of evaporite minerals. Geological, geochemical, and 

geophysical studies indicate a moderate mineral resource potential 

for undiscovered uranium, vanadium, and copper in the Morrison Formation at the southeast edge of the study area. A moderate mineral 

resource potential exists for copper and silver in a zone of faulted 

sedimentary rocks along the northern, western, and southern sides of 

the study area. The entire wilderness study area has moderate energy resource potential for oil and gas and low resource potential for all 

metals, geothermal energy, anhydrite, gypsum, and potassium salts. 

There is no energy resource potential for coal resources.

SUMMARY

The Sewemup Mesa Wilderness Study Area is about 50 

mi (miles) southwest of Grand Junction in Mesa and 

Montrose Counties, Colorado (fig. 1). The study area includes 18,835 acres and is bounded by the Dolores River on 

the east, Salt Creek on the north, Sinbad Valley on the west, 

and Roc Creek on the south. Colorado State Highway 141 

and a gravel road along Salt Creek and into Sinbad Valley 

provide access along the base of Sewemup Mesa. Sewemup

Mesa, the main topographic feature of the study area, slopes 

west and rises about 1,500 ft (feet) above the floor of Sinbad Valley. Tributaries of Salt Creek and the Dolores River 

drain the mesa, forming numerous shallow canyons. The 

southwestern part of the study area includes terrain of low 

to moderate relief in part of Sinbad Valley.

The study area abuts the Uncompahgre Plateau, a 

major landform and structural feature of western Colorado 

(fig. 2), and is underlain by flat-lying to monoclinally folded 

sedimentary formations of late Paleozoic and Mesozoic age 

(see Appendix for geologic time chart). These are, in ascending order, the Hermosa Formation, Cutler Formation, 

Moenkopi Formation, Chinle Formation, Wingate 

Sandstone, Kayenta Formation, Navajo Sandstone, Dewey 

Bridge Member of the Entrada Formation, upper Entrada 

Formation, undivided Entrada, Wanakah Formation, and 

Morrison Formation. Precambrian igneous and metamorphic rocks probably underlie the area at depths of 10,000 to 

15,000 ft. Normal faults, shallow synclines, and salt anticlines are the dominant structural features in and near the 

southern and western parts of the study area. An isolated 

salt plug is indicated by a nearly circular pattern of collapse 

faults that cut through the Mesozoic rocks in the southern 

part of the study area.

There are no mines or economic or marginally 

economic resources in the study area, but two mines are just 

outside the study area: the Rajah mine near Roc Creek, 

south of the study area, and the Copper Rivet mine about

Sewemup Mesa Wilderness Study Area, Colorado B1
109*02'30' 108*57'30' 108*52'SO"

38*35'00' 

38*32'30'

38'30'00'

38«27'30" 

38*25*00'

APPROXIMATE LIMIT 

OF SINBAD VALLEY

APPROXIMATE 

OUTLINE OF 

SEWEMUP MESA

- \ \

Copper Rivet 

\ mine

MONTROSE COUNTY

APPROXIMATE 

BOUNDARY OF 

SEWEMUP MESA 

WILDERNESS STUDY AREA 

(CO-070-176/030-310A)

Figure 1 (above and facing page). Map showing location, identified resources, and mineral resource potential of the Sewemup Mesa Wilderness 

Study Area, Mesa and Montrose Counties, Colo.

0.25 mi northwest of the study area. The Rajah mine 

produced uranium, vanadium, and copper ore from the Salt 

Wash Member of the Morrison Formation. Copper and silver were mined from the Wingate Sandstone at the Copper 

Rivet mine. Numerous small prospect pits and trenches 

were found outside the study area, northwest of the Rajah 

mine. Nearly all of the study area is under lease for oil and 

gas (fig. 3); it contains one dry exploration drill hole.

Inferred subeconomic resources of sand and gravel, 

and sandstone, and occurrences of evaporite minerals exist

in the study area. However, development of these materials 

is unlikely because readily available sources exist elsewhere 

in the region at locations closer to potential markets.

Geological and geochemical features of the study area 

suggest the possible presence of three types of undiscovered 

mineral resources: (1) uranium, vanadium, and copper in 

the Salt Wash Member of the Morrison Formation (Rajah 

mine type); (2) copper and silver in faulted sedimentary 

rocks (Copper Rivet mine type); and (3) the industrial 

minerals, gypsum, anhydrite, and potassium salts.

B2 Mineral Resources of Wilderness Study Areas-Grand Junction Region, Colorado
L/B

N/D

B

D

EXPLANATION

Geologic terrane having moderate energy resource potential for oil and gas, with certainty level B Applies to entire study area

Geologic terrane having moderate mineral resource potential for uranium, vanadium, 

and copper in the Morrison Formation, with 

certainty level B

Geologic terrane having moderate mineral resource potential for copper and silver, with 

certainty B

Geologic terrane having low mineral resource 

potential for all metals (except as noted 

above), gypsum, anhydrite, potassium salts, 

and geothermal energy, with certainty level 

B Applies to entire study area

Geologic terrane having no energy resource 

potential for coal, with certainty level D 

Applies to entire study area

LEVELS OF CERTAINTY

Data indicate geologic environment, and suggest level of resource potential

Data indicate geologic environment, indicate 

resource potential, but do not establish 

activity of resource-forming processes

Available information clearly defines the level of 

resource potential

Fault U, upthrown side; D, downthrown side. 

Dashed where approximately located

109«02'30' 10B C 35'00'

The potential for uranium, vanadium, and copper resources in the Morrison Formation is considered to be 

moderate, on the basis of the presence of fractured host 

rocks and occurrence of nearby small deposits. The potential for undiscovered copper and silver resources is 

moderate along the northern, western, and southern sides 

of the study area because of the presence of favorable, frac 

tured host rocks, although the area lacks systematic 

geochemical anomalies. The mineral resource potential for 

all metals, gypsum, anhydrite, and potassium salts is low. 

The study area lacks geochemical anomalies indicative of 

metallic mineralization. Gypsum, anhydrite, and potassium 

salts were not observed in the study area, and where they 

occur nearby, they form thin, discontinuous beds.

Favorable host rocks for oil and gas may underlie the 

study area, and favorable structural traps are present. The 

study area therefore has moderate energy resource potential for oil and gas. There are no known geothermal sources 

and no warm springs were noted in the study area; the resource potential for geothermal energy is low. The study 

area lacks coal-bearing formations in the surface and sub0123 MILES

Figure 2. Index map showing location of the Sewemup Mesa 

Wilderness Study Area, Mesa and Montrose Counties, Colo.

surface; therefore there is no mineral resource potential for 

coal.

INTRODUCTION

At the request of the BLM (U.S. Bureau of Land 

Management), the USGS and the USBM studied 18,835 

acres of the 19,140-acre Sewemup Mesa Wilderness Study 

Area (CO-070-176/030-310A). Throughout this report 

"study area" and "wilderness study area" refer only to the 

smaller area. This report, the product of several separate 

studies by the USBM and the USGS, presents an evaluation 

of the mineral endowment (identified resources and 

mineral resource potential) of the study area. Identified resources were studied by the USBM and are classified according to the system of the USBM and USGS (1980), which 

is shown in the appendix. Mineral resource potential, 

studied by the. USGS, is the likelihood of occurrence of undiscovered concentrations of metals and nonmetals, of unappraised industrial rocks and minerals, and of 

undiscovered energy sources (coal, oil, gas, oil shale, and 

geothermal sources). Mineral resource potential and the 

level of certainty of each resource assessment were classified according to the system of Goudarzi (1984; see appendix).

The Sewemup Mesa Wilderness Study Area includes 

18,835 acres of high mesa terrain in Mesa and Montrose 

Counties, Colorado (fig. 1). Located about 10 mi south of 

the community of Gateway and about 50 mi southwest of

Sewemup Mesa Wilderness Study Area, Colorado B3
Grand Junction, Sewemup Mesa is bounded by the Dolores 

River on the east, Salt Creek on the northwest, Sinbad Valley on the west, and Roc Creek on the south. The mesa rises 

about 1,500 ft from the floor of Sinbad Valley and is dissected by numerous shallow canyon systems. The southwestern part of the study area includes terrain of low to 

moderate relief in part of the collapsed salt anticline region 

of Sinbad Valley. Total relief in the study area is approximately 2,800 ft; elevation ranges from about 4,600 ft in 

the canyon of the Dolores River to 7,400 ft on the west ridge 

of the mesa. Colorado State Highway 141 and a gravel road 

along Salt Creek into Sinbad Valley provide access along 

the base of the mesa. The top of the mesa can be reached 

only by aircraft or by foot.

Previous geologic studies providing background for 

this report include geologic maps of three quadrangles 

covering the Sewemup Mesa study area published at a scale 

of 1:24,000 by the USGS (Shoemaker, 1955,1956; McKay, 

1955). Earlier mineral resource potential studies include 

Soulliere and others (1983) and a report by Mountain States 

Mineral Enterprises and Wallaby Enterprises (unpub. data, 

1982) for the BLM.

APPRAISAL OF IDENTIFIED RESOURCES

By Clay M. Martin 

U.S. Bureau of Mines

Investigation by the U.S. Bureau of Mines Mining and Leasing Activity

In May and August 1986, the USBM conducted a 

mineral investigation of the Sewemup Mesa Wilderness 

Study Area. The field investigation of the study area was 

preceded by a survey of relevant literature about the geology and mineral resources of the region. Information about 

surface and mineral ownership, oil and gas leases, claims, 

and prospecting activity was researched from records of the 

BLM state office in Lakewood, Colo.

Field investigation included the examination and sampling of mineralized areas in and near the study area, scintillometer readings to detect anomalous radioactivity at 

sample sites and along foot and vehicle traverses, and the 

collection of minus-80-mesh stream-sediment samples to 

determine the extent of mineralized areas. One hundred 

and three rock samples and 17 stream-sediment samples 

were collected in and near the study area (Martin, 1987). 

All rock and stream-sediment samples underwent semiquantitative inductively coupled plasma analysis for 30 elements at the USBM labs in Reno, Nev. Samples from the 

Rajah and Copper Rivet mines were analyzed for gold by 

neutron activation analysis. Results of the analyses are in 

Martin (1987).

Investigation by the U.S. Geological 

Survey

Prior to field work, the USGS researched published 

and unpublished information about the study area and 

reviewed existing geophysical data. Field work in 1983 consisted of a check of previous geologic mapping, the collection of stream-sediment samples for geochemical analyses, 

and a search for mineralized and altered areas in the study 

area.

There are no mines in or recorded mineral production 

from the Sewemup Mesa Wilderness Study Area. The study 

area is along the western margin of the Uravan mineral belt 

(fig. 2), an elongated, arcuate area in Mesa, Montrose, and 

San Miguel Counties that contains more abundant large, 

closely spaced, high-grade carnotite deposits than the surrounding region (Fischer and Hilpert, 1952). Carnotite ore 

has been mined for uranium, vanadium, and radium in the 

Uravan district since the end of the nineteenth century. 

Nearly all carnotite production from the region was from 

the Salt Wash Member of the Morrison Formation.

During the late 19th and early 20th centuries, the Sinbad Valley area was prospected and mined for copper and 

silver. No mines or prospects are in the study area. 

However, two mines are just outside the study area (fig. 1). 

At the Copper Rivet mine, about 0.25 mi north of the study 

area, copper-silver sulfide and carbonate ore was mined 

from a fault zone in the Wingate Sandstone. Extensive 

prospecting and some mining occurred at the Rajah mine 

near Roc Creek, south of the study area. Blocks of patented 

lode claims cover the Rajah mine area. Numerous prospect 

trenches and pits and two adits are west and southwest of 

the study area. Hundreds of unpatented lode claims for 

uranium were in and near the study area. Most of the study 

area is under lease for oil and gas and one dry exploration 

hole has been drilled inside the study area (fig. 3).

Rajah Mine

The deposits at the Rajah mine, just south of the study 

area, were mined and extensively prospected during several 

different periods. At the mine, uranium, vanadium, and 

copper are associated with fault zones in the Morrison Formation. Workings at the Rajah mine include 26 adits, many 

of which are collapsed, and many surface pits and trenches

B4 Mineral Resources of Wilderness Study Areas Grand Junction Region, Colorado
109°00' R.19W. H.18W.

38°30'

Oil and gas lease information from the Bureau of Land Management: 

current as of February 1987.

COLORADO

MAP LOCATION

5 MILES 

J

EXPLANATION

APPROXIMATE BOUNDARY OF THE SEWEMUP 

MESA WILDERNESS STUDY AREA

OIL AND GAS LEASES

-9- DRILL HOLE. DRY

Figure 3. Map showing oil and gas leases for the Sewemup Mesa Wilderness Study Area, Mesa and 

Montrose Counties, Colo.

Sewemup Mesa Wilderness Study Area, Colorado B5
(Martin, 1987). Surface investigation of the mine area indicated no anomalous radiation or mineralized rock in any 

formations more than a few feet outside fault zones. 

Uranium mineralization seems to have been confined to 

faults and adjacent beds where sandstone has been replaced 

and impregnated by uranium, vanadium, and copper 

minerals. Copper concentrations from samples in the Rajah 

mine area range from less than 0.003 percent to over 1 percent; silver concentrations ranged from the lower detection 

limit of 0.2 ppm to 90 ppm (parts per million). Gold content 

of the samples ranged from nil to 174 ppb (parts per billion). 

Only 9 of 57 rock samples taken hi this area showed more 

than 0.1 percent UaOg (Martin, 1987). Several faults that 

bear uranium hi the vicinity of the Rajah mine extend into 

the southeastern part of the study area but are unrnineralized at the surface. Rock samples and surface scintillometer 

surveys along the fault zones hi the study area showed no 

anomalous radiation.

Copper Rivet Mine

The Copper Rivet mine is the only mine in the Salt 

Creek graben area having recorded production. Mine workings follow a vein in the northern boundary fault of the Salt 

Creek graben, about 0.25 mi north of the study area. Between 1908 and about 1942, the mine sporadically produced 

copper-silver sulfide and carbonate ore from a fault zone hi 

the Wingate Sandstone. The workings consist of three adits 

that are described hi detail by Martin (1987). Nineteen 

samples were taken from the Copper Rivet mine area. Copper concentrations ranged from less than 0.008 percent to 

over 1 percent; silver concentrations ranged from the lower 

detection limit of 0.2 parts per million (ppm) to more than 

200 ppm (the upper detection limit). Gold content of the 

samples was generally insignificant, with most sample concentrations ranging from 0 to 2 ppb. One select dump 

sample from the middle adit assayed 45 ppb gold. Assay 

results from the lower parts of the adits may not accurately 

indicate concentrations in the upper inaccessible parts of 

the fault zone or in the unmined fault beyond the adit face.

Prospects

Fault zones hosting copper minerals occur near the 

head of Salt Creek, to the northwest of the Rajah mine area, 

and southwest of the study area. The area has been 

prospected intermittently since the end of the 19th century, 

but no known production has occurred. Workings consist 

of two adits and numerous prospect trenches and pits. 

Malachite and azurite are commonly found as impregnations and replacements in sandstone and in gouge in faults 

associated with the Sinbad Valley anticline. No significant 

concentrations of uranium or vanadium are found in this 

vicinity. This is probably due to the general absence of the

Morrison Formation in the area. Eighteen samples were 

taken from prospect pits in the area. The samples contained 

0.002 percent to over 1 percent copper, 0.2 ppm to 54 ppm 

silver, and 0 to 4 ppb gold (Martin, 1987).

Identified Mineral Resources

There are no economic or marginally economic resources in the study area. However, there are large inferred 

subeconomic resources of sandstone present in the study 

area hi a wide range of textures, colors, cementations, and 

bedding thicknesses. Many of the sandstone units would be 

suitable for use as dimension stone, flagstone, or concrete 

aggregate. However, vast quantities of sandstone of equal 

or higher quality are available throughout the region at locations closer to markets, which minimizes the economic significance of sandstone from the study area.

The Hermosa Formation, which contains large 

amounts of evaporitic gypsum, anhydrite, and potash elsewhere, crops out over about 1.5 sq mi (square miles) of the 

western part of the study area, and the study area is underlain by a thick sequence of Hermosa Formation rock. 

Resources of these evaporitic minerals may underlie the 

study area, but none were identified during this study.

Small inferred subeconomic resources of sand and 

gravel are in the canyons forming the boundaries of the 

study area. These resources would be suitable for construction aggregate; however, none of these deposits are easily 

accessible, and they are far from potential markets. Ample 

resources of sand and gravel exist closer to current markets 

elsewhere in the region.

ASSESSMENT OF POTENTIAL FOR 

UNDISCOVERED RESOURCES

By Sandra J. Soulliere, Charles G. Patter son, 

Anne M. Leibold, and Dolores M. Kulik 

U.S. Geological Survey

Geology

Geologic Setting

The Sewemup Mesa Wilderness Study Area is on the 

northeastern margin of the Paradox Basin in the Canyon 

Lands section of the Colorado Plateaus physiographic 

province (Fenneman, 1931). The regional structure of the 

Colorado Plateaus province consists of uplifted blocks 

bounded by locally faulted monoclines, domical uplifts, and 

large salt anticlines. Sewemup Mesa is between the uplifted

B6 Mineral Resources of Wilderness Study Areas-Grand Junction Region, Colorado
block of the Uncompahgre Plateau and a breached salt anticline of Sinbad Valley. The Uncompahgre Plateau is a 

large uplift consisting of partially exposed Precambrian igneous and metamorphic rocks overlain by flat-lying to 

monoclinally folded sedimentary rocks of late Paleozoic 

and Mesozoic age. The salt anticline at Sinbad Valley has 

an intrusive core of salt and gypsum derived from the 

Paradox Member of the Hermosa Formation. Dissolution 

and erosion of the salt core resulted in the collapse of the 

crest of the anticline and the formation of the valley.

Precambrian igneous and metamorphic rocks probably underlie the study area at depths of 10,000 to 15,000 ft 

(Baars and Stevenson, 1981). Flat-lying to monoclinally 

folded Paleozoic and Mesozoic sedimentary rocks are exposed in the study area. The oldest rocks, of Pennsylvanian 

age, crop out hi Sinbad Valley and near Roc Creek. Rocks 

of Permian, Triassic, and Jurassic age are exposed on the 

flanks, walls, and plateau of Sewemup Mesa and in the Roc 

Creek drainage. Normal faults, shallow synclines, and salt 

anticlines are the dominant structural features along the 

southern and western part of the boundary of the study area. 

Salt Creek meanders across a fault graben in the northern 

part of the study area. This 3.5-mi-long downdropped block 

trends perpendicular to the axis of the Sinbad Valley anticline (Shoemaker, 1955). The faults that bound the graben 

have downward displacement near Sinbad Valley and die 

out to the northeast near the Dolores River. In the southern 

part of the study area, just north of Roc Creek, the presence 

of an isolated salt plug is indicated only by the nearly circular pattern of collapse faults that cut through the overlying Mesozoic rocks (fig. 1).

Description of Rock Units

Geologic quadrangle maps at a scale of 1:24,000 by 

McKay (1955) and Shoemaker (1955, 1956) include the 

Sewemup Mesa Wilderness Study Area. The following 

description of rock units is modified from Soulliere and 

others (1983), which includes a complete geologic map of 

the Sewemup Mesa Wilderness Study Area.

The Hermosa Formation of Middle and Late Pennsylvanian age is divided into lower, or Paradox, and upper 

members. The Paradox Member consists of gypsum, darkgray shale, micaceous sandstone, arkose, and gray marine 

limestone in surface exposures, but is mainly salt at depth. 

Highly contorted beds of the Paradox Member crop out on 

the floor of Sinbad Valley and hi the Roc Creek drainage. 

The upper, unnamed member is gray arkose and marine 

limestone. A steeply dipping and contorted belt of the upper 

member is exposed near the center of Sinbad Valley.

The Lower Permian Cutler Formation conformably 

overlies the Hermosa Formation and consists of maroon 

and purple arkosic conglomerate and sandstone with some 

red-brown sandy mudstone. About 1,000 ft of the formation

is exposed along Roc Creek. Ripple marks, cut and fill structures, and irregular bedding indicate a fluvial environment 

of deposition. The Moenkopi Formation of Early and Middle(?) Triassic age unconformably overlies the Cutler Formation. The formation is divided into three members, and 

has a total thickness of 300 ft along the Dolores River and 

1,000 ft in Sinbad Valley. The lower member consists of 

evenly bedded, brick-red, micaceous mudstone, sandy 

siltstone, and a massive white gypsum bed, as much as 6 ft 

thick, near the base of the member. The middle member 

consists of purplish and red-brown conglomerate and 

arkosic sandstone that is interbedded with silty shale. The 

thin- to medium-bedded sandstone is cross laminated and 

has channeling, current lineation, and ripple marks. The 

upper member consists of uniformly bedded red-brown and 

chocolate-brown shale and scattered thin beds of gypsiferous sandstone.

The Chinle Formation of Late Triassic age unconformably overlies the Moenkopi. It forms steep, stepped slopes 

and consists of discontinuous lenses of sandstone and limestone-pebble and mudstone-pebble conglomerate interbedded with bright-red and red-brown mudstone and 

siltstone. As much as 700 ft of the Chinle Formation is 

present in the study area. The Upper Triassic Wingate 

Sandstone conformably overlies the Chinle Formation. 

Orange-red, massive, fine-grained, well-sorted sandstone, 

as much as 350 ft thick, forms the steep cliffs of Sewemup 

Mesa. Massive crossbeds, vertical joints, and desert varnish 

are common features of the Wingate. The Upper Triassic(?) Kayenta Formation conformably overlies the Wingate Sandstone. As much as 300 ft thick in places, the 

formation consists of discontinuous lenses of purplish-red 

sandstone interbedded with shale and conglomerate, forming a series of benches and ledges. Irregular to flaggy bedding and trough crossbedding are characteristics of the 

Kayenta Formation. The Navajo Sandstone of Triassic(?) 

and Jurassic age lies conformably on the Kayenta and is a 

massive, fine-grained, clean sandstone of eolian origin that 

locally contains large-scale tangential crossbeds. The 

Navajo Sandstone weathers to rounded cliffs and is as much 

as 100 ft thick; it thins to the east. A 6-ft-thick bed of limestone is locally present in the Navajo on Sewemup Mesa 

(Shoemaker, 1956).

The Middle Jurassic Entrada Sandstone is less than 100 

ft thick in most places. The Dewey Bridge Member at the 

base of the Entrada is unconformable with the Navajo 

Sandstone. It consists of red to buff, nonresistant, horizontally bedded siltstone, mudstone, and sandstone. In places 

it contains small (as much as 1 in. (inch) across) pebbles of 

white and gray chert, pebbles and cobbles of gray, red, and 

yellow quartzite, and scattered barite nodules. The Dewey 

Bridge grades upward into the Slick Rock and Moab Members of the Entrada. The salmon-colored, fine-grained 

sandstone of the upper members of the Entrada forms

Sewemup Mesa Wilderness Study Area, Colorado B7
smooth, rounded cliffs in the study area. Large-scale sweeping crossbeds indicate the eolian origin of this sandstone. 

The Wanakah Formation of Middle Jurassic age lies conformably on the Entrada Sandstone. About 130 ft thick at 

the top of Sewemup Mesa, the Wanakah Formation consists 

of sandy and silty shales interbedded with fine-grained 

sandstone and siltstone. Beds are predominantly red but 

may be green, brown, light yellow, or white. Outcrops of the 

Summerville Formation exhibit even, thin, horizontal bedding.

Only the lower part of the Upper Jurassic Morrison 

Formation is exposed in the study area; the lower part consists of the Tidwell and the Salt Wash Members. Peterson 

(1980) recognized the Tidwell Member as consisting of 

light-gray, laminated to thin-bedded, fine-grained 

sandstone with interbedded greenish-gray and reddishbrown shale and mudstone. The Tidwell was not differentiated from the Salt Wash Member in the study area. The 

Salt Wash Member consists of lenses of gray to buff, finegrained, fluvial sandstone and conglomerate as much as 40 

ft thick interbedded with reddish shale and mudstone and 

gray limestone. Approximately 80 ft of the Salt Wash and 

Tidwell Members are exposed at the highest elevation on 

Sewemup Mesa.

Geochemistry

Sample Collection and Analytical Methods

Stream-sediment and heavy-mineral-concentrate 

samples were collected from 20 sites in the study area (SoulHere and others, 1983). Most of the sampled sites were at 

the mouths of ephemeral streams that flow from the top of 

Sewemup Mesa to either the Dolores River on the east or 

Salt Creek on the northwest. The streams have drainage 

basins from 1 to 3 sq mi in area and cover approximately 90 

percent of the wilderness study area.

All stream-sediment and panned-concentrate samples 

were collected from drainage basins underlain by sedimentary rocks. Four stream-sediment and panned-concentrate 

samples were taken from terrain underlain by the Hermosa 

Formation (Pennsylvanian) and from streams whose basins 

also include outcrops of Permian and Triassic sedimentary 

rocks. All other samples were collected in terrain of 

Mesozoic sedimentary rocks that include either the Chinle 

Formation or the Moenkopi Formation.

Stream-sediment samples were collected and analyzed 

to determine the presence of easily soluble metals. At each 

sample site, about 25 oz (ounces) of stream sediment were 

collected from four or more points hi the active stream 

channel, usually along point bars or sections of lower 

gradient. In the laboratory, the sediment was coarsely 

ground with a mortar and pestle to release clay-cemented

aggregate and then sieved to minus-100-mesh (0.0059 in.). 

Five grams of sample were analyzed by Scintrex pulsed 

laser fluorimetry for uranium following the techniques of 

Rose and Keith (1976). Two hundred milligrams of sample 

were then analyzed by inductively coupled plasma atomic 

emission spectroscopy (ICP) for 45 elements using the 

methods of Taggart and others (1981).

The heavy-mineral-concentrate samples were obtained 

by collecting sediment from four or more points within the 

active stream bed. Approximately 15 Ib (pounds) of sediment was screened through a 10-mesh (0.039 in.) stainless 

steel screen at the sample site. The sediment was then 

panned until approximately half of the remaining sediment 

in the pan was quartz and feldspar; this constituted a 

panned-concentrate sample for analysis. Ten milligrams of 

the panned-concentrate sample were analyzed semiquantitatively for 66 elements by optical emission spectrographic 

methods (Myers and others, 1961).

Results of Survey

Interpretations of the geochemical analyses included 

the determination and evaluation of anomalous values from 

histograms and single element plots. Histograms revealed 

significant anomalous values for 14 of the 22 elements 

detected in the panned-concentrate samples. Single-element plots from data for the panned-concentrate samples 

revealed three single elements (zirconium, barium, molybdenum) and a suite of elements (iron, manganese, 

chromium, cobalt, nickel, scandium, vanadium, yttrium, 

lanthanum, copper, lead) that displayed coherent patterns 

pertinent to the mineral resource assessment. High zirconium concentrations (1,000-1,500 ppm) found in samples 

from the southeast side of Sewemup Mesa are assumed to 

be the result of detrital zircon in the Triassic and Jurassic 

sedimentary rocks. Four samples having anomalous barium 

concentrations (10,000-15,000 ppm) are from the northern 

half of the study area but are not clustered around any apparent geologic or geographic feature. Authigenic barite 

has been noted in the Chinle and Moenkopi Formations 

(Cadigan, 1971; Stewart and others, 1972) and is a possible 

source for the high barium values. Further studies would be 

necessary to confidently explain the source of the barium 

and apparent randomness in anomaly locations. Only one 

sample contained molybdenum (5 ppm); the source of the 

molybdenum is unknown. Uranium was not detected in any 

of the panned-concentrate samples. The association of 

anomalous concentrations of iron, manganese, chromium, 

cobalt, nickel, scandium, vanadium, yttrium, lanthanum, 

copper, and lead is probably from detrital magnetite hi four

Any use of trade names is for descriptive purposes only and does not constitute endorsement by the U.S. Geological Survey.

B8 Mineral Resources of Wilderness Study Areas Grand Junction Region, Colorado
samples taken from the western side of the study area. Although the source of this magnetite is unknown, all four sites 

are underlain by the Hermosa Formation.

Histograms and single element plots showed significant 

geochemical anomalies hi the minus- 100-mesh fraction of 

four stream-sediment samples. Locations of anomalous 

values for molybdenum and for iron-suite elements correspond well with the locations of molybdenum and ironsuite element anomalies identified in the 

panned-concentrate samples. Barium was present in 

anomalous concentrations only in these four samples; zirconium was not detected. Uranium was not detected in any 

of the samples either by emission spectroscopy, TCP, or 

fluorimetry.

Geophysics

Gravity and Aero magnetic Data

Gravity and magnetic studies were done as part of the 

mineral resource evaluation of the Sewemup Mesa Wilderness Study Area. These studies provide information on the 

subsurface distribution of rock masses and the structural 

framework of the area.

The gravity data were obtained from files of the U.S. 

Defense Mapping Agency (1974) of the U.S. Department 

of Defense. Bouguer anomaly values were computed using 

the gravity formula from the International Association of 

Geodesy (1967) (Cordell and others, 1982) and a reduction 

density of 2.67 g/cm (grams per cubic centimeter). Terrain 

corrections were made by computer for a distance of 167 

km (kilometers) from the station using the method of Plouff 

(1977). The data are shown on figure 4 as a complete 

Bouguer anomaly map with a contour interval of 5 mGal 

(milligals).

Magnetic data shown in figure 5 are reproduced from 

part of the Moab quadrangle residual Intensity magnetic 

contour map with the 1975 reference field removed (U.S. 

Department of Energy, 1983). Flight elevation was 400 ft, 

flight-line spacing was approximately 3 mi, and flight lines 

were flown hi an east-west direction.

The gravity data in the region surrounding the study 

area mostly reflect the distribution of salt in the sedimentary section. A major gravity low (A) occurs over the 

Paradox Valley anticline and low gravity anomalies of 

smaller amplitude occur over the Pine Ridge anticline (B 

and B') and the Sinbad Valley anticline (C) adjacent to the 

western part of the study area. A gravity low (D) at the 

southeastern part of the study area suggests that a salt dome 

is present hi the shallow subsurface there. A more detailed 

Bouguer anomaly map (Joestling and Byerly, 1958) shows 

the anomaly extending over the exposed Roc Creek plug. 

Other gravity lows (E, F) north and east of the study area

suggest that salt is also present beneath these areas. No data 

are available within the study area to determine if similar 

buried salt structures might be present there. The gradient 

in the northeasternmost corner of the map is associated with 

Precambrian rocks in the Uncompahgre uplift.

The magnetic data mostly reflect the distribution of the 

Precambrian rocks beneath the essentially nonmagnetic 

sedimentary cover rocks. The magnetic highs at the northeastern corner of the map are part of a more extensive 

northwesterly trend (U.S. Department of Energy, 1983) associated with the Precambrian rocks of the Uncompahgre 

uplift. The high magnetic anomaly (A) which trends 

northwest through the southern and central part of the map 

may indicate that the basement rocks are offset along faults 

parallel to those of the Uncompahgre uplift. However, 

Joesting and Byerly (1958) suggest that this and other 

similar anomalies of small amplitude and low gradient between the Uncompahgre front and Big Gypsum Valley are 

related to small contrasts in the magnetization of basement 

rocks, to basement relief, or to very deep seated sources.

Remote Sensing Data

A remote sensing study of the Sewemup Mesa Wilderness Study Area was done by Keenan Lee (unpub. data, 

1987) and was based on Landsat 2 multispectral-scanner 

imagery. These data were processed and analyzed for lineaments and for areas of limonitic anomalies that might relate 

to hydrothermal alteration, migrating hydrocarbons, or 

uranium. From the lineament analysis, no major basement 

faults were detected. No limonite anomalies of interest are 

apparent in the imagery, but the narrow outcrop widths 

compared with the image resolution suggest that any such 

anomalies would go undetected.

Mineral and Energy Resources

The potential for mineral resources in the Sewemup 

Mesa Wilderness Study Area was assessed by comparing 

geochemical, geophysical, and geological characteristics of 

the study area with those of nearby mineralized areas and 

with those of mineral deposit models. These characteristics 

suggest three kinds of mineral resources may be present in 

the study area: (1) uranium, vanadium, and copper in the 

Salt Wash Member of the Morrison Formation (Rajah mine 

type); (2) copper and silver in Copper Rivet mine type 

deposits; and (3) industrial minerals (gypsum, anhydrite, 

and potassium salts).

Sewemup Mesa Wilderness Study Area, Colorado 89
108°45'

38°30' -s

38°15'

Figure 4 (above and facing page). Complete Bouguer gravity anomaly map of the Sewemup Mesa Wilderness Study Area, Mesa and Montrose 

Counties, Colo. Gravity lows A-F are referred to in the text.

B10 Mineral Resources of Wilderness Study Areas Grand Junction Region, Colorado
EXPLANATION

Approximate boundary of the Sewemup Mesa

Wilderness Study Area 

Contour Contour interval 5 mGal (milligals).

Hachures point towards area of lower gravity 

Gravity station

Uranium, Vanadium, and Copper

The mineral resource potential is moderate for 

uranium, vanadium, and copper in the Salt Wash Member 

of the Morrison Formation, with a certainty level of B. More 

than 12 percent of the uranium mined in the United States 

as of 1978 was supplied from the Colorado Plateau 

(Chenoweth, 1978). The deposits are principally found in 

the Salt Wash Member of the Morrison Formation. At the 

Rajah mine, just south of the study area, uranium, 

vanadium, and copper minerals are disseminated in 

sandstone of the Salt Wash Member along an east-westtrending portion of the fault zone bounding the salt collapse 

structure in the southern part of the wilderness study area. 

Similar conditions exist in the southern part of the study 

area, where minor exposures of the Morrison Formation are 

cut by faults. However, scintillometer surveys and 

geochemical analyses of stream-sediment samples did not 

reveal near-surface uranium, vanadium, or copper in this 

area. The Sewemup Mesa Wilderness Study Area is adjacent to the extensive Uravan mineral belt (fig. 2). The 

Uravan mineral belt is composed of a number of uraniumvanadium mining districts that form a nearly arcuate pattern 

that is perpendicular to the paleocurrent directions of the 

Salt Wash. However, the mineral belt exhibits a rather 

abrupt cut-off in uranium and vanadium occurrence 

towards the study area.

Copper and Silver

The mineral resource potential is moderate for copper 

and silver in the study area, with a certainty level of B. The 

model locality for copper-silver occurrence is the Copper 

Rivet mine, which is about 0.25 mi northwest of the study 

area. Much of the mine description is derived from information in reports by Fischer (1936) and Schwochow (1978) 

and from a visit to the mine by C.G. Patterson. At the Copper Rivet mine, copper and silver minerals are localized 

along a northeast-trending fault associated with the Salt 

Creek graben. The deposit consists of copper sulfides and 

carbonates with calcite and barite gangue in fault-shattered 

Wingate Sandstone. The mineralization was apparently 

produced by hydrothermal fluids migrating up a fault related to the collapse of the Sinbad Valley salt anticline.

A combination of structure and host rock similar to that 

at the Copper Rivet mine exists along the southeastern wall 

of Salt Creek Canyon and along the western and southern 

parts of the boundary of the study area. In small prospect 

pits just south of the study area, uranium, vanadium, and 

copper minerals are found in faulted Chinle Formation, 

Wingate Sandstone, and Kayenta and Summerville Formations (Shoemaker, 1956). However, geochemical data 

showed low values for copper, silver, uranium, and 

vanadium in samples from areas where Copper Rivet mine 

type mineralization would be expected to occur. 

Anomalous concentrations of barium were found in many 

stream-sediment samples. Because barite is a common gangue mineral in deposits of this type, the presence of barium 

would be considered favorable. However, the source of the 

barium anomalies in the stream-sediment samples was not 

found during our investigations, and no traces of mineralized rock were noted when the areas were examined.

Industrial Minerals

The mineral resource potential is low for gypsum, anhydrite, and potassium salts in the study area, with a certainty level of B. Gypsum, anhydrite, and potassium salts are 

present in Sinbad Valley just outside the study area in the 

Paradox Member of the Hermosa Formation. This formation is exposed in the eastern part of the study area; 

however, no gypsum, anhydrite, or potash beds were observed there. Most of the study area is in a facies transition 

zone where gypsum, anhydrite, and potassium salt are interbedded with coarse clastic material. If gypsum, anhydrite, and potassium salt beds extend into the study area, 

they would probably be discontinuous and interbedded with 

conglomerate and arkose. An isolated salt plug is in the 

southern part of the study area, just north of Roc Creek. It 

is indicated by a nearly circular pattern of collapse faults 

that cut through the overlying Mesozoic rocks. There is no 

surface evidence of potassium salts in the study area, but it 

is possible that potassium salts could be found at depth. 

Wells drilled in the Sinbad Valley anticline average 42 percent mixed sodium and potassium salt (Shoemaker and 

others, 1958). More information is needed regarding the 

subsurface geology of this area in order to raise the level of 

certainty for the presence of these minerals within the study 

area.

Other Metals

A favorable geologic environment for the presence of 

metallic mineral resources is lacking in the study area. In 

addition, geochemical data do not indicate abnormally high 

concentrations of metals for this environment. Therefore, 

the study area is assigned a low mineral resource potential

Sewemup Mesa Wilderness Study Area, Colorado B11
108°45'

38°30' -,

38°15'

Figure 5 (above and facing page). Residual intensity magnetic anomaly map of the Sewemup Mesa Wilderness Study Area, Mesa and Montrose 

Counties, Colo. Anomaly A may indicate offset of basement rocks along faults parallel to Uncompahgre uplift.

B12 Mineral Resources of Wilderness Study Areas Grand Junction Region, Colorado
EXPLANATION

 Approximate boundary of the Sewemup Mesa 

Wilderness Study Area

 ^ ~"~ Contour Contour interval 20 nT (nanoTeslas).

Hachures point towards area of low magnetism

for all metals other than uranium, vanadium, silver, and copper; certainty level is B.

Energy Resources

The energy resource potential for oil and gas is 

moderate in the study area, with a certainty level of B on the 

basis of favorable structural and stratigraphic conditions in 

the subsurface. The study area is on the east flank of the Sinbad Valley anticline. Favorable host rocks of Pennsylvanian 

age may underlie the study area. Spencer (1983) assigned a 

low resource potential for oil and gas in the study area on 

the basis of sparse well control. The one well drilled in the 

study area was dry.

The study area has low energy resource potential for 

geothermal resources, with a certainty level of B. There are 

no known geothermal sources in the study area. No warm 

springs or other geothermal sources were noted during this 

investigation.

The study area lacks coal-bearing formations in the surface and subsurface; therefore there is no potential for coal, 

with certainty level D.

REFERENCES CITED

Baars, D.L., and Stevenson, G.M., 1981, Tectonic evolution of western 

Colorado and eastern Utah, in Epis, R.C., and Callender, J.F., eds., 

Western Slope Colorado: New Mexico Geological Society 

Guidebook no. 32, p. 75-80.

Cadigan, R.A., 1971, Petrology of the Triassic Moenkopi Formation and 

related strata in the Colorado Plateau Region: U.S. Geological Survey Professional Paper 692,70 p.

Chenoweth, F.W., 1978, Uranium in Western Colorado: Mountain 

Geologist, v. 15, no. 3, p. 89-96.

Cordell, Lindrith, Keller, G.R., and Hildenbrand, T.G., 1982, Bouguer 

gravity map of the Rio Grande Rift, Colorado, New Mexico, and 

Texas: U.S. Geological Survey Geophysical Investigations Series 

Map GP-949, scale 1:1,000,000.

Fenneman, N.M., 1931, Physiography of the United States: New York, McGraw-Hill Book Co., Inc., 534 p.

Fischer, R.P., 1936, Peculiar hydrothermal copper-bearing veins of the 

northwestern Colorado Plateau: Economic Geology, v. 31, no. 6, p. 

571-599.

Fischer, R.P., and Hilpert, L.S., 1952, Geology of the Uravan mineral belt: 

U.S. Geological Survey Bulletin 988-A, 13 p.

Goudarzi, G.H., compiler, 1984, Guide to preparation of mineral survey 

reports on public lands: U.S. Geological Survey Open-File Report 

84-787,42 p.

International Association of Geodesy, 1967, Geodetic Reference System, 

1967: International Association of Geodesy Special Publication No. 

3,116 p.

Joesting, H.R., and Byerly, P.E., 1958, Regional geophysical investigations 

of the Uravan area, Colorado: U.S. Geological Survey Professional 

Paper 316-A, 17 p.

Martin, C.M., 1987, Mineral investigation of the Dolores River Canyon 

and Sewemup Mesa Wilderness Study Areas, Mesa and Montrose 

Counties, Colorado: U.S. Bureau of Mines Open File Report MLA47-87,47 p.

McKay, E.J., 1955, Geology of the Red Canyon quadrangle, Colorado: 

U.S. Geological Survey Geologic Quadrangle Map GQ-58, scale 

1:24,000.

Meyers, A.T., Havens, R.A., and Dunton, P.J., 1961, A spectrochemical 

method for the semiquantitative analysis of rocks, minerals, and 

ores: U.S. Geological Survey Bulletin 1084-1, p. 207-229.

Peterson, Fred, 1980, Sedimentology as a strategy for uranium exploration; Concepts gained from analysis of a uranium-bearing depositional sequence in the Morrison Formation of south-central Utah, 

in Turner-Peterson, C.E., ed., Uranium in sedimentary rocks; Application of the facies concept to exploration: Rocky Mountain Section, Society of Economic Paleontologists and Mineralogists, 

Denver, Colorado, short course notes, p. 65-126.

Plouff, Donald, 1977, Preliminary documentation for a FORTRAN 

program to compute gravity terrain corrections based on topography 

digitized on a geographic grid: U.S. Geological Survey Open-File 

Report 77-535.

Rose, A.W., and Keith, M.L., 1976, Reconnaissance geochemical techniques for detecting uranium deposits in sandstones of northeastern 

Pennsylvania: Journal of Geochemical Exploration, v. 6, p. 119-137.

Schwochow, S.D., 1978, Mineral resources survey of Mesa County~a 

model study: Colorado Geological Survey Resources Series 2, 

Colorado Geological Survey, 110 p.

Shoemaker, E.M., 1955, Geology of the Juanita Arch quadrangle, 

Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ81, scale 1:24,000.

____1956, Geology of the Roc Creek quadrangle, Colorado: U.S. 

Geological Survey Geologic Quadrangle Map GQ-83, scale 1:24,000.

Shoemaker, E.M., Case, I.E., and Elston, D.P., 1958, Salt anticlines of the 

Paradox Basin: Intermountain Association of Petroleum Geologists 

Guidebook, 9th Annual Field Conference, p. 39-59.

Soulliere, S.J., Leibold, A.M., and Patterson, C.G., 1983, Geology and 

mineral resource potential of the Sewemup Mesa Wilderness Study 

Area, Mesa and Montrose Counties, Colorado: U.S. Geological Survey Open-File Report 83-796, 26 p.

Spencer, C.W., 1983, Petroleum potential of wilderness lands, Colorado, 

in Miller, B.M., ed., Petroleum potential of wilderness lands in the 

Western United States: U.S. Geological Survey Circular 902-E, 33 p.

Stewart, J.H., Poole, F.G., and Wilson, R.F., 1972, Stratigraphy and origin 

of the Chinle Formation and related Upper Triassic strata in the 

Colorado Plateau region, with a section on Sedimentary petrology, 

by R.A. Cadigan: U.S. Geological Survey Professional Paper 690, p. 

56-61.

Taggart, I.E., Lichte, F.E., and Wahlberg, J.S., 1981, Methods of analysis 

of samples using X-ray fluorescence and induction coupled plasma 

spectroscopy: U.S. Geological Survey Professional Paper 1250, p. 

683-687.

U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles of a 

resource/reserve classification for minerals: U.S. Geological Survey 

Circular 831,5 p.

U.S. Defense Mapping Agency, Aerospace Center, 1974, World Relative 

Gravity Reference Network, North America, Part 2: Defense Mapping Agency Aerospace Center Reference Publication 25, with supplement updating gravity values to the international Gravity 

Standardization Net 1971, 635 p.

U.S. Department of Energy, 1983, Moab Quadrangle; Residual Intensity 

Magnetic Anomaly Contour Map, GJM406, plate 4.

Sewemup Mesa Wilderness Study Area, Colorado B13

APPENDIX
DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

Definitions of Mineral Resource Potential

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 unlikely. This broad 

category embraces areas with dispersed but insignificantly mineralized rock as well as areas with few 

or no indications of having been mineralized.

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 a reasonable likelihood of resource accumulation, and (or) where an application of 

mineral-deposit models indicates favorable ground for the specified type(s) of deposits.

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.

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

moderate, or high levels of resource potential.

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

area.

Levels of Certainty

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B 

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C 

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HIGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

2

LU 

O 

QC

O 

CO

LU

cc

LU 

LU

B C 

LEVEL OF CERTAINTY

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

B. Available information 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.

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.
RESOURCE/RESERVE CLASSIFICATION

IDENTIFIED RESOURCES

Demonstrated

Measured Indicated

Inferred

UNDISCOVERED RESOURCES

Probability Range

 (or) 

Hypothetical i Speculative

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

1

Reserves Inferred Reserves

Marginal Reserves

Demonstrated 

Subeconomic Resources

Inferred 

Marginal Reserves

Inferred

Subeconomic

Resources

+

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from 

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.
GEOLOGIC TIME CHART 

Terms and boundary ages used in this report

EON

Phanerozoic

Proterozoic

Archean 

pre-Ar

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

;hean2

PERIOD

Quaternary

Neogene 

Subperiod

Tertiary 

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Periods ' ' 

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

 -

BOUNDARY AGE 

IN 

MILLION YEARS

-0.010

- 1.7

- 5

1H

- 55

CO

- 96

OAC

-A, 240 

- 290

-~ 330

 -icn

- 410

u» Kf\f\

- ~ 570 1

- 900 

- 1600

- 2500

- 3000

1 Rocks older than 570 m.y. also called Precambrian, a time term without specific rank. 

* Informal time term without specific rank.

* U.S. GPO: 1988 573-047/56020