Y 3. At7 A CR RME-2040 GEOLOGY AND MINERALOGY

AIRBORNE RECONNAISSANCE IN THE VALLEY OF FIRE-MUDDY MOUNTAINS REGION, NEVADA

By Donald C. Barrett Neil Mallory

September 1955

r'sl7 BfRAMY Salt Lake Branch Office, AEC b 5 Grand Junction Operations Office Salt Lake City, Utah

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UNITED STATES ATOMIC ENERGY COMMISSION Technical Information Service metadc783669 LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, "person acting on behalf of the Commission" includes any em- ployee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor.

This report has been reproduced directly from the best available copy.

Printed in USA. Price 50 cents. Available from the Office of Technical Services, Department of Commerce, Washing- ton 25, D. C.

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AEC Technical Information Service Extension Oak Ridge, Tennessee RME -2040

U. S. ATOMIC ENERGY COMMISSION GRAND JUNCTION OPERATIONS OFFICE DIVISION OF RAW MATERIALS SALT LAKE BRANCH OFFICE

AIRBORNE RECONNAISSANCE IN THE VALLEY OF FIRE- MUDDY MOUNTAINS REGION, NEVADA

by Donald C. Barrett and Neil Mallory

September 1955 Salt Lake City, Utah 1

AIRBORNE RECONNAISSANCE IN THE VALLEY OF FIRE- MUDDY MOUNTAINS REGION, NEVADA

CONTENTS

Page

Abstract ------5

Introduction ------5

Physiography------5 Climate------7

Geology------7 Precambrian------7 Paleozoic------7 Mesozoic ------8 Triassic and Jurassic ------8 Cretaceous------9 Cenozoic ------10

Regional structure ------10

Uranium occurrences in sedimentary rocks ------10

Airborne procedures------11

Results ------11

Ground investigations------13

Conclusions ------14

References------14

3 AIRBORNE RECONNAISSANCE IN THE VALLEY OF FIRE- MUDDY MOUNTAINS REGION, NEVADA

ILLUSTRATIONS

Page

Figure 1. Index map of Nevada ------6

2. Airborne reconnaissance, Valley of Fire- Muddy Mountains, Clark County,

Nevada ------12

4 AIRBORNE RECONNAISSANCE IN THE VALLEY OF FIRE- MUDDY MOUNTAINS REGION, NEVADA

ABSTRACT

The Valley of Fire-Muddy Mountains region, located 20 to 30 miles northeast of Las Vegas, Nevada, was given approximately 25- percent airborne radiometric coverage during the fall of 1954. Rocks in the area range from Cambrian to Miocene in age. Uranium was known to occur at two sedimentary horizons at the time the airborne program began. One new uranium-bearing sedimentary horizon was discovered by the airborne reconnaissance. Of 15 anomalies detected, only .11 were posted as the remainder were claimed at the time of the survey.

INTRODUCTION

The Valley of Fire is about 30 miles northeast of Las Vegas, Nevada (fig. 1). An airborne radiometric survey of this area was begun October 15, 1954, and completed December 26, 1954. The area was given approximately 25-percent coverage.

This area came to the attention of the Manhattan Project during World War II as a potential source of domestic uranium (Carper, 1946). At that time, the workings of the Perkins brothers' claim, the Carnotite No. 1, consisted of about 10 small open cuts and a shaft 20 feet deep. Carnotite was noted in five of the pits. In addition to the uranium indications on the Perkins brothers' claims, radioactive petrified logs had been found in the Valley of Fire State Park. These previously known uranium localities and a recent general reconnaissance of the area which verified the presence of extensive exposures of the Shinarump and Chinle formations in the area led Commission geologists to recommend the Valley of Fire region for an airborne radiometric survey.

PHYSIOGRA PHY

Maximum relief in the area is about 3, 800 feet. Muddy Peak rises to 5, 790 feet and is the highest point in the region. Several other peaks and ridges are over 4, 000 feet in elevation, whereas bottoms of intermontane valleys are locally less than 2, 000 feet above sea level.

5 N E V A D A I

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INDEX MAP OF NEVADA

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AREA COVERED BY AIRBORNE RECONNAISANG

CLARK COUNTY, NEVADA 0 5 0 20 30 4 50ee

Scale miles

6

FLOWNo0200 U. S. Highway No. 91 forms the northern and western boundaries of the surveyed area. To the south and east, Lake Mead forms the boundary, thus the only practical access into the Muddy Peak region is from the north. Only two passable dirt roads lead southward from U. S. Highway No. 91 toward Lake Mead. The junction of one of these roads and Highway No. 91 is just northeast of Nellis Air Force Base. The other dirt road leaves U. S. Highway No. 91 at Crystal, about 30 miles northeast of Las Vegas, Nevada, and continues on to Overton, Nevada, which it enters from the south. This road passes north of Muddy Peak and through the southern end of the Valley of Fire. Several poor dirt roads lead directly to the Valley of Fire south of Glendale, one of which leaves the valley at Overton and is the access road to the Carnotite No. 1 claim.

Climate

Winters in the Muddy Mountains-Valley of Fire region are generally milder than in most of Nevada, although snow is common at higher ele- vations. In the town of Overton, the winters are very mild. During the summer, however, temperatures above 1100 F. are common. The average rainfall, which occurs as thunder showers mainly in July and August, is 5.01 inches.

Natural vegetation is sparse and trees suitable for mine timber are not available in the Muddy Mountains.

GEOLOGY

Precambrian

Although not exposed in the immediate area, Precambrian crystalline rocks occur to the east and south of the Valley of Fire and the Muddy Mountains. These basement rocks consist of various types of schist and gneiss which have been intruded by coarse-grained granitic rocks.

Paleozoic

In the Virgin Mountains east of Overton, about 400 feet of quartzite, sandstone, and shale overlie the Precambrian complex. Above these clastic sediments is 775 feet of dark-gray compact limestone which is

7 tentatively correlated with the Muav limestone of Middle Cambrian age of the Grand Canyon district (Longwell, 1928).

Longwell (1928) believes that the limestone and dolomite, exposed at the base of an overthrust, are probably Cambrian and if so, are the oldest strata exposed in the Muddy Mountains. The oldest formation having recognizable fossils is the Muddy Peak limestone of Middle Devonian age which consists of dense, hard limestone and dolomite, siliceous in appearance, with alternating light and dark beds.

The Mississippian section of the Muddy Mountains region is divided into two formations on the basis of fossils. The lower unit, the Rogers Spring limestone, consists mainly of 600 feet of gray crystalline lime- stone with some cherty zones. The Bluepoint limestone of Upper Mississippian age is lithologically similar to the lower unitexcept that it is finer grained and has a greater percentage of dark-gray carbonaceous beds.

Above the Bluepoint limestone is the Callville limestone of Pennsylvanian age, which consists of 2, 000 feet or more of thin-bedded gray limestone interbedded with sandstone at the top.

Longwell (1949) lists 2, 000 feet of unnamed red beds beneath the Kaibab limestone of Permian age, which had been correlated earlier with the underlying Supai formation of the Colorado Plateau.

The Kaibab limestone is roughly divided into two limestone members separated by a 100-foot gypseous bed. Its thickness ranges from 600 to 800 feet.

Mesozoic

Triassic and Jurassic-- The Lower Triassic Moenkopi formation of Nevada may be divided into two units. The lower unit, which seems to be limited to the western extent of the formation, consists mainly of marine limestone and thin-bedded calcareous shale interbedded with two crossbedded sandstones. This unit grades upward into nonmarine gray, pink, and chocolate-colored shales containing abundant gypsum. The thickness is about 1, 500 feet. Overlying the Moenkopi is the Shinarump member of the Chinle formation of Upper(?) Triassic age. The conglomerate ranges from 10 to approximately 200 feet in thick- ness. Silicified fragments of wood are distributed throughout the

8 formation, Lithologically it resembles the Shinarump exposures of the Colorado Plateau. It is strongly crossbedded with lenses of sand, pebbles, and cobbles erratically distributed throughout.

The 1, 500 to 3, 500 feet of variegated shales and sandstones which lie conformably on the Shinarump constitute the remainder of the Chinle formation. Near the base of this sequence is a con- glomeratic sandstone containing silicified logs.

The most conspicuous formation of the region is the Aztec sandstone of Jurassic(?) age, which is about 2, 500 feet thick; it is a strongly cross- bedded eolian sandstone. Although normally brick red, it has been bleached to light pink or white in many places.

Cretaceous--Longwell (1949) reports an angular unconformity between the Jurassic and Upper Cretaceous formations. The con- glomerate bed at the bottom of the Willow Tank formation of Late Cretaceous age rests with slight angular unconformity on the Aztec sandstone at most exposures in the Valley of Fire.

An unnamed sequence is seemingly interbedded with the upper part of the Aztec sandstone in the vicinity of the Carnotite No. 1 claim. The sequence includes three cherty limestone beds, some clay beds and a sandstone containing carbon trash. Between these beds is mottled red and green sandstone. The total thickness of this sequence is 58 feet in a section measured on the Carnotite No. 1 claim.

No depositional break can be seen between the lowermost clay bed and the typical Aztec sandstone below. The sandstone immediately below the clay bed has a mottled red and green appearance, but changes to a pink and white phase which grades into the normal brick-red Aztec sandstone at depth.

The contact between the unnamed beds and the basal conglomerate of the Willow Tank formation is not exposed in the area examined. Due to the extremely sharp folding and faulting in the area, the exact re- lationship of these unnamed beds with the Willow Tank formation is uncertain. However, the Willow Tank formation probably rests un- conformably on these beds as it does on the Aztec sandstone.

9 Above the Willow Tank conglomerate is 300 feet of grayish-green to buff clays interbedded with bentonitic siltstone and sandstone. The Baseline sandstone of Late Cretaceous age, which consists of friable variegated sandstone and conglomerate, lies conformably on these upper members of the Willow Tank formation.

Cenozoic

An angular unconformity separates the Baseline sandstone from the Overton fanglomerate of Tertiary(?) age which consists of extremely coarse fan debris and some interbedded lenses of sandstone and silt. Conformably overlying the fanglomerate are 1, 000 to 2, 700 feet of limestone, dolomite, magnesite, clay silt, volcanic ash, sandstone, and conglomerate, which comprise the Horse Spring formation of Tertiary(?) age.

Resting with angular unconformity on the Horse Spring formation is the Muddy Creek formation of Miocene(?) age. It ranges in thick- ness from 0 to 200 feet of clay, silt and sandstone and some interbedded gypsum and salt.

Lying unconformably above the Muddy Creek formation are the clays, sands, gravels, and unconsolidated alluvium of Pleistocene and Recent age.

REGIONAL STRUCTURE

The dominant structural features of the Muddy Mountain region are two major thrust faults (Longwell, 1949). Normal faults, minor thrust faults and numerous folds, make the structural geology of the area ex- tremely complex. A complete discussion of structure of the region is not pertinent to this report.

URANIUM OCCURRENCES IN SEDIMENTARY ROCK

Uranium has been found at three sedimentary horizons in the Valley of Fire-Muddy Mountains region. The lowest horizon is the Shinarump zone, which contains moderately radioactive carbon trash beds. Uranium minerals are not visible at any of the exposures in this zone except possibly in the radioactive petrified logs in the Valley of Fire State Park.

10 At the middle are the unnamed beds between the Aztec sand- stone and the Willow Tank formation. Here the uranium is associated with cherty limestone and a carbon trash bed. The most promising uranium showing in the area is in this sequence.

The uppermost formation known to carry uranium is the thin siliceous limestone bed near the base of the Horse Spring formation.

AIRBORNE PROCEDURES

All exposed rocks in the area, ranging in age from Cambrian to Miocene, were given approximately 25-percent airborne coverage. The rock types differ considerably. They include massive limestone; massive c.rossbedded eolian sandstone; fluviatile shale; sandstone and conglomerate, which contain minor amounts of lacustrine limestones and mudstone; and some Cenozoic lake beds.

The type of topography controlled the method of flying; rim- type coverage was employed almost exclusively. The horizontal distance from the rim varied with the nature of the area being surveyed, but it remained within the limit of 25 feet minimum and about 75 feet maximum.

The airborne survey was conducted with a Piper PA-18 aircraft, equipped with an airborne scintillation counter. Generally good map coverage was available.

Hangar space and facilities for maintenance of aircraft are available at McCarran Field.

RESULTS

Fifteen airborne anomalies were detected. Only 11 of these were posted as the remainder were either claimed or on withdrawn lands at the time of their detection (fig. 2). Only two of the anomalies were in the Valley of Fire proper. The others were detected en route to the Valley of Fire from Las Vegas.

During the survey, a total of 71:20 hours flying time was logged. Of this, 49:00 hours were employed for rim flying; 14:45 for recon- naissance; and 7:35 for cross-country flying.

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+ 10/ AREA SURVEYED AND COVERAGE Fig.2 AIRBORNE RECONNAISSANCE VALLEY OF FIRE - MUDDY MOUNTAIN REGION I ANOMALY - Anomalous radioactivity observed from the air,does not CLARK COUNTY, NEVADA necessarily indicate uranium on the ground. 1954 1955

0 6 9 I ~COi* miles ea m .. 12 The airborne survey stimulated interest in uranium prospecting in the southern Nevada area. Numerous persons operating private airborne units conferred with the AEC flight crew from time to time. They were primarily interested in information on airborne methods and techniques of operation and the location of potential uranium areas. Ground prospectors were referred to the Commission representatives in Las Vegas, Nevada.

GROUND INVESTIGATIONS

Anomalous radioactivity was detected at several widely separated sedimentary horizons and in a late Cenozoic granite.

The uranium occurrences from the Chinle formation in the Valley of Fire are insignificant compared to the deposits found in this forma- tion on the Colorado Plateau. A few silicified logs in sandstone from the Valley of Fire State Park carry minor amounts of uranium. Along the west side of Weiser anticline, carbon trash beds in the Shinarump member of the Chinle have an unusually high background of radioactivity which is probably due to disseminated uranium although no uranium minerals have been identified from them.

Anomalous radioactivity has been detected in the Willow Tank formation, but no uranium deposits have been found in this formation. The unnamed strata, which are interbedded with the upper part of the Aztec sandstone and lie unconformably below the Willow Tank formation, constitute the principal ore-mineralized beds of the Valley of Fire area. This sequence crops out at only one locality. In tracing the contact between the Willow Tank formation and the Aztec sandstone, no other outcrops of the unnamed strata were found.

The largest group of airborne anomalies, seven in number, was detected along a thin limestone bed of the Horse Spring formation in an area northwest of Lake Mead and southwest of Muddy Peak (fig. 2). The radioactive zone is only about 1 foot thick but is very persistent and is traceable for several miles. However, none of the samples taken from this zone assayed as much as 0. 05 percent cU3O 8 .

The other group of four anomalies was located in the Henderson, Nevada, quadrangle a few miles east of Sunrise Mountain. The anomalous zones were underlain by a coarsely crystalline granite which gave evidence of cutting Tertiary (Miocepe ?) lake beds. The radioactivity may have been due to uranium and/or other elements, but the material was not of ore grade.

13 CONCLUSIONS

Due to intensified prospecting of the area during the airborne survey, some anomalous areas were claimed before they were detected by the Atomic Energy Commission airborne operations. Development is being carried on by private interests at some of the areas as well as at the Carnotite No. 1 claim in the Valley of Fire. None of the anomalies posted by the Commission have yet proved indicative of uranium of commercial grade.

REFERENCES

Carper, A. F., 1946, Examination of alleged uranium occurrences in southeast Nevada: U. S. Atomic Energy Comm. RMOO-87.

Longwell, C.R., 1928, Geology of the Muddy Mountains, Nevada, with a section through the Virgin Range to the Grand Wash Cliffs, Arizona: U. S. Geol. Survey Bull. 798.

Longwell, C. R., 1949, Structure of the northern Muddy Mountains 'area, Nevada: Geol. Soc. America, V. 60, No. 5.

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