Geologic Interpretation of 

Gravity and Aeromagnetic 

Maps of Tintic Valley and 

Adjacent Areas, Tooele and 

Juab Counties, Utah 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 516-D 

Geologic Interpretation of 

Gravity and Aeromagnetic 

Maps of Tintic Valley and 

Adjacent Areas, Tooele and 

Juab Counties, Utah 

By D. R. MABEY and H. T. MORRIS 

GEOPHYSICAL FIELD INVESTIGATIONS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 516-D 

Geophysical data indicate the thickness o; 

Cenozoic sedimentary and volcanic rocks 

and the distribution of igneous rocks 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1967 
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary 

GEOLOGICAL SURVEY 

William T. Pecora, Director 

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 
CONTENTS 

Page Geologic setting-Continued Page 

Abstract ________________________ ·- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ D 1 Structure of the Tintic Valley area______________ D4 

Introduction_______________________________________ 1 Ore deposits----------------------------------- 4 

Geologic setting____________________________________ 1 Gravity and magnetic surveys_______________________ 4 

Precambrian rocks______________________________ 2 Gravity and magnetic maps______________________ 5 

Paleozoic rocks__ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 East Tintic Mountains anomalies_________________ 5 

Tertiary volcanic rocks___ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 3 Tintic Valley anomalies_________________________ 6 

Tertiary intrusive rocks_________________________ 3 West Tintic and Sheeprock Mountains anomalies__ 8 

Tertiary and Quaternary alluvium and valley-fill Interpretation of regional structure___________________ 9 

deposits ______ ---------- ______ --------_______ 4 References cited _____________________ ------_________ 9 

ILLUSTRATIONS 

Page 

PLATE 1. Geologic maps and section showing gravity and magnetic data, Tintic Valley and adjacent areas, Tooele and Juab 

Counties, Utah-------------------------------------------------------------------------------- In pocket 

FIGURE 1. Index map of Utah showing location of Tintic Valley area and previous geophysical surveys____________________ Dl 

TABLE 

Page 

TABLE 1. Stratigraphic sections of rocks exposed in the Simpson, Sheeprock, West Tintic, and East Tintic Mountains______ D2 

III 

GEOPHYSICAL FIELD INVESTIGATIONS 

GEOLOGIC INTERPRETATION OF GRAVITY AND AEROMAGNETIC MAPS OF TINTIC VALLEY 

AND ADJACENT AREAS, TOOELE AND JUAB COUNTIES, UTAH 

By D. R. MABEY and H. T. MoRRIS 

ABsrrRA.C.l' 

A 30-milligal gravity low in Tintic Valley indicates that about 

7,000 feet of low-density sedimentary and volcanic rocks of Cenozoic age is present in the valley and that these rocks are thickest 

east of the center of the valley. Normal faults with large 

vertical throw are inferred along the east side of the valley, and 

normal faults with smaller throw are inferred on the west side. 

Small gravity lows, also reflecting Cenozoic rocks, occur northwest of Rockwell Ranch and along Vernon Creek. 

A west-trending zone of magnetic anomalies across Tintic 

Valley indicates a belt of abundant igneous rock, which also ex· 

tends west of the valley to the Wasatch Range. Over the East 

Tintic Mountains an 800-gamma magnetic high has two major 

closures. One is over intrusive igneous rocks exposed near 

Silver City ; the other is over volcanic rocks west of Tintic 

Mountain. Both magnetic features are believed to reflect a large 

intrusive body underlying the southern half of the East Tintic 

Mountains. Smaller magnetic highs are produced by intrusive 

rocks in the Sheeprock and West Tintic Ranges and at Desert 

Mountain. A zone of high magnetic intensity extending across 

Tintic Valley at Mcintyre Siding indicates that volcanic rocks 

underlie the southern part of the valley. Elsewhere volcanic 

rocks are reflected by both positive and negative magnetic 

anomalies. 

The geophysical data do not support the concept of a basement 

uplift in the Sheeprock Mountain area, but they are consistent 

with a structural interpretation that assigns the Precambrian 

sedimentary rock in this area to the upper plate of a major 

overthrust. 

INTRODUCTION 

Aeromagnetic and gravity maps of the area adjacent 

to Tintic Valley, Utah, were prepared as part of a continuing geophysical study of the Basin and Range province (fig. 1). These maps, which cover an area of 

about 600 square miles including parts of the East 

Tintic, West Tintic, Sheeprock, and Simpson Mountains, are overprinted on a generalized geologic map 

compiled from original and published (chiefly Stokes, 

1963) sources. The east boundary of the rna p overlaps the west edge of a gravity map of the central and 

southern Wasatch front by Cook and Berg (1961) and 

an aeromagnetic map of approximately the same area 

by Mabey, Crittenden, Morris, Roberts, and Tooker 

(1964). 

0 40 MILES L____j 

FIGURE 1.-Location of Tintic Valley area and areas covered by 

previous geophysical surveys. 

GEOLOGIC SETTING 

Tintic Valley is a fault-rimmed intermontane basin 

in the east-central part of the Basin and Range province (pl. 1). It is bordered on the east by the southern 

half of the East Tintic Mountains, a tilted fault-block 

range, and on the west by the West Tin tic and Sheeprock Mountains, which form part of a complex horst. 

The valley and ·adjacent ranges include the eastern part 

of a belt of intrusive rocks and mining districts that 

extends from the East Tintic Mountains to the Deep 

Creek Range; other concealed stocks and mineralized 

areas may underlie lavas and Quaternary deposits 

within this belt. 

Dl 
D2 GEOPHYSICAL FIELD INVESTIGATIONS 

The rocks exposed in the mountains adjacent to Tin tic 

Valley range in age from Precambrian to Recent and 

include rocks deposited during each period of the Paleozic Era. No rocks of Mesozoic age crop out, but regional geologic relations suggest that Triassic and Jurassic rocks may deeply underlie some of the Precambrian and Paleozoic rocks below thrust faults of large 

displacement, at least in the southeastern part of the 

East Tintic Mountains. The Tertiary and Quaternary 

rocks in the area are of volcanic and continental origin 

and vary widely in composition and thickness. 

The stratigraphic sections are summarized in table 1. 

PREC~BR~ ROCKS 

The oldest rocks exposed near Tintic Valley consist 

of a thick sequence of clastic sedimentary rocks interlayered with a few thin diabase sills or basalt flows. 

Cohenour (1959, p. 17-18) described two major seTABLE !.-Stratigraphic sootioos of rocks emposed in the Simpson., Sheeprock, West Tintic, and East Tintic Mountains 

[Figures in parentheses indicate thickness, in feet] 

ERA SYSTEM 

QUATERNARY 

CENOZOIC 

TERTIARY 

PERMIAN 

MISSISSIPPIAN 

DEVONIAN 

SIMPSON MOUNTAINS 

(Thomas, 1958) 

Alluvium and colluvium 

Monzonite and quartz monzonite 

stocks 

Raul Canyon Basalt 

Judd Creek Latite 

SHEEPROCK AND 

WEST TINTIC MOUNTAINS 

(Cohenour, 1959; Groff, 1959) 

Younger alluvium 

Lake Bonneville Group 

Moraine 

Older alluvium 

Salt Lake Formation 

Rhyolite 

Granite stocks, plugs, and dikes 

Aplitic granite stocks and plugs 

Monzonite and quartz monzonite 

intrusive· bodies 

Latite and andesite flows and 

pyroclastic rocks 

EAST TINTIC MOUNTAINS 

(Morris, 1957; Morris and 

Lovering, 1961) 

Younger alluvium 

Lake Bonneville Group 

Older alluvium 

Salt Lake(?) Formation 

Basalt dikes, sills, and flows 

Monzonite of Silver City stock 

and other intrusive bodies 

Laguna Springs Latite 

Quartz monzonite of Swansea stock 

and other intrusive bodies 

Packard Quartz Latite 

Apex Conglomerate 

Park City and Phosphoria Formations undivided (+1620) Diamond Creek(?) Sandstone (685) 

Oquirrh Formation (+13,250) 

(Fault contact) --------t- Manning Canyon Shale (1000-1100) - Great Blue Formation (4130) Great Blue Formation (2400-2600) 

Humbug and Deseret Formations Humbug Formation (600-675) undivided (1150) Deseret Limestone (700-1100) Gardison Limestone (450-550) 

Madison Limestone (400) Fitchville Formation (250-350) 

Pinyon Peak Limestone and ----J--.PinY<m Peak Limestone (70-300) Victoria Formation undivided (775) Victoria Formation (25.0-300) 

Bluebell Dolomite (330-600) 

PALEOZOIC 1-------------+Laketown and Fish Haven Dollontitet&--<:f---------------!7 

undivided (1450) 

ORDOVICIAN 

CAMBRIAN 

PRECAMBRIAN 

Swan Peak Quartzite (700) Kanosh Shale (55-175) 

Pogonip Group (+760) 

Undifferentiated carbonate rocks (2300) 

Pioche Shale (22) Tintic Quartzite (2800) 

Sheeprock Series, including Dutch 

Peak Tillite (+9400 feet) 

Swan Peak Quartzite (35-465) Kanosh Shale (100-250) Juab Formation (160-205) Wahwah Formation (455-475) Fillmore and House Formations 

undivided (1085-1110) 

Ajax Formation (1016) 

Opex Formation (938) Cole Canyon Dolomite (900) 

Marjum Formation (1171) Wheeler Formation (400-430) 

Swasey Limestone (130-170) Condor Formation (105-135) Dome Limestone (35-80) Burnt Canyon Formation (175-225) Millard Formation (245-355) 

Busby Quartzite (85-170) Pioche Shale (300-350) Tintic Quartzite (2575) 

Mutual(?) Formation (+3000) 

Sheeprock Series, including Dutch Peak Tillite (+10,000) 

Ajax Dolomite (500-730) 

Opex Formation (140-350) Cole Canyon Dolomite (830-900) Bluebird Dolomite (150-220) 

Herkimer Limestone (350-430) 

Dagmar Dolomite (60-100) Teutonic Limestone (400-420) 

Ophir Formation (275-430) 

Tintic Quartzite (2300-3200) 

Big Cottonwood Formation ( + 1675) 
INTERPRETATION OF GEOPHYSICAL DATA, TINTIC VALLEY AREA, UTAH D3 

quences of these rocks in the Sheep rock Mountains. The 

older sequence, termed the Sheeprock Series, is 9,000-

10,000 feet thick and consists of a lower sequence of slaty 

phyllite and quartzite, a middle sequence of schistose 

tillite, and an Uipper sequence of quartzite and phyllite. 

The younger sequence, provisionally correlated with the 

Mutual Formation of the Wasatch Mountains, is about 

925 feet thick and consists of a lower argillite member 

and an upper member of conglomeratic quartzite. In 

the East Tintic Mountains the Precambrian rocks resemble the lower part of the Sheeprock Series and have 

been correlated with the Big Cottonwood Formation 

(Morris and Lovering, 1961, p. 10-13) of late Precambrian age. Quartzite, slate, and argillite, identified 

as the Sheeprock Series by Thomas (1958, p. 4-7), make 

up large parts of the Simpson Mountains. No tillite 

was recognized by Thomas in the exposures of Precambrian rock in the Simpson Mountains; therefore, the 

stratigraphic position of the tillite in this area is unknown. 

The base of the upper Precambrian rocks is not exposed in the area adjacent to Tintic Valley. Presumably these stratified rocks overlie gneiss and schist comparable to the Farmington Canyon Complex (Eardley 

and Hatch, 1940, p. 61) or the Little Willow Formation 

(Crittenden and others, 1952, p. 3), which are both 

exposed in the Wasatch Range. 

PALEOZOIC. ROCKS 

Carbonate and clastic rocks of Paleozoic age crop out 

extensively throughout the area studied. In the East 

Tintic Mountains the aggregate thickness of Paleozoic 

rocks is close to 30,000 feet. Groff ( 1959) reported at 

least 11,500 feet of carbonate and clastic rocks of Cambrian through Pennsylvanian age in the West Tintic 

Mountains, and Cohenour (1959, p. 12) described more 

than 19,500 feet of Paleozoic rocks in the Sheeprock 

Mountains. Although the sections of Paleozoic rocks 

exposed in these mountain ranges are generally alike, 

correlation of specific formational units across the 

major valleys is complicated by the juxtaposition of 

dissimilar sedimentary rock facies by exposed and concealed thrust faults. 

TERTIARY VOLCANIC ROCKS 

Lavas and pyroclastic rocks, ranging in composition 

from rhyolite and quartz latite to basalt, blanket the 

Precambrian and Paleozoic rocks chiefly on the east 

sides of the westward-tilted ranges. The extensive lavas 

in the East Tintic Mountains have been dated as middle 

to late Eocene on the basis of regional stratigraphic 

relations, physiographic considerations, and by isotope 

methods (Morris, 1957, p. 29-34). Parts of the less extensive lava and pyroclastic deposits in the West Tin tic, 

Sheeprock, and Simpson Mountains are probably comparable in age, although no specific correlations are 

possible from the data at hand. 

TERTIARY INTRUSIVE ROCKS 

Stocks, plugs, dikes, and other intrusive bodies ranging in composition from granite to diabase cut all the 

pre-Miocene rocks exposed near Tin tic Valley. The oldest intrusions that have been dated are middle to late 

Eocene (Morris, 1957, p. 29-39), and the youngest may 

be Miocene (Cohenour, 1959, p. 100). These intrusive bodies form part of a belt of intrusions 20-30 miles 

wide extending from the central Deep Creek Range at 

the Nevada border eastward to the East Tin tic Mountains and possibly beyond to the southern Wasatch 

Range (Mabey and others, 1964). 

The intrusions exposed in the East Tintic Mountains 

are chiefly porphyritic monzonite and quartz monzonite, but they also include some diabase dikes and sills. 

The largest intrusion in this range is the Silver City 

monzonite stock, which crops out over an irregular area 

of about 2 by 3 miles. 

Some of the intrusions in the southern part of the 

West Tintic and Sheeprock Mountains are also porphyritic monzonites and quartz monzonites, but others include a wide dike of rhyolitic vitrophyre 1 and plugs of 

aplitic granite in the southern Sheeprock Mountains, 

and several pluglike intrusive bodies of uncertain composition in the southern vV est Tintic Mountains. 

The large intrusive body cut by Cottonwood and Rock 

Canyons in the western part of the Sheeprock Mountains is a leucocratic granite containing scattered pe.gmatitic segregations of bery 1 and other uncommon 

minerals (Williams, 1954; Cohenour, 1959, p.100-101). 

It is similar to a coarse-grained leucocratic granite exposed in Desert Mountain. 

The small intrusive bodies in the Simpson Mountains 

consist chiefly of plugs and dikes of latite and monzonite. The la.rgest plug crops out over an area of only 

800 by 1,400 feet (Thomas, 1958, p. 15). 

1 After this paper had been prepared for publication, several large 

diatremes composed of rhyolite and rhyolitic vitrophyre and containing 

giant blocks of Precambrian and Paleozoic sedimentary rock were dis~ 

covered in the upper part of Cherry Creek near the center of the mapped 

area. The presence of these intrusive bodies, then, makes inaccurate 

the geology as shown on plate 1, particularly the distribution of Precambrian sedimentary rocks and the position of the southeastern edge 

of the Sheeprock overthrust. Although the igneous matrix material 

of the diatremes is highly leucocratic and its contained magnetite is 

commonly pyritized, these volcanic rocks may account for the magnetic 

anomalies on either side of the central and upper parts of Cherry Creek. 

(Seep. DS.) 
D4 GEOPHYSICAL FIELD INVESTIGATIONS 

TERTIARY AND QUATERNARY ALLUVIUM AND 

VALLEY-FILL DEPOSITS 

The intermontane basins, particularly Tintic Valley, 

are filled with an irregular thickness of fanglomerate, 

volcanic debris, lacustrine sediments, and alluvium. 

The oldest of these deposits are part of the Salt Lake ( ~) 

Formation of Pliocene age. They consist most typically 

of conspicuously bedded bentonitic clays and tuffaceous 

marly limestone interlayered with lenses of sand and 

gravel and wedge-shaped masses of coarse fanglomerate. Pleistocene and Recent deposits overlie faulted 

and tilted beds of the Salt Lake(~) Formation, and consist of fanglomerate, loess, and alluvium. Sediments 

deposited in ancient Lake Bonneville during Wisconsin 

time mantle all but the youngest alluvium below an 

elevation of a;bout 5,150 feet. 

STRUCTURE OF THE TINTIC VALLEY AREA 

The major structura;l features of the Tintic Valley 

area are thrust faults, folds, and transcurrent faults 

that are older than the Eocene ·lavas. and Basin-andRange normal faults that cut even th~ most recent deposits and form the boundaries of the major topographic features. The structures that formed before 

the lava was extruded originated in response to tectonic 

uplift during late Mesozoic time, which resulted in the 

detachment of several great sheets of stratified rocks 

that glided east toward a Cretaceous sedimentary basin 

that Spieker (1949, p. 20) and Schoff (1951, p. 643-

644) recognized about 35 miles east of the Tin tic Valley 

area. The folds, thrusts, and transcurrent faults of the 

East Tintic Mountains formed during compressional · 

deformation of one of these thrust sheets. Other sheets 

also show minor folding, thrusting, and tear faulting. 

The youngest features that predate extrusion of the 

lava are east-trending normal faults of large throw. 

· .. The Basin-and-Range faults are arcuate in plan, but, 

m general they trend northward or northeastward. 

The most prominent of these faults defines the west edge 

of the East Tintic Mountains, where local scarps in 

Pleistocene and younger fanglomerates attest to recent 

. vertical displacement. Many of the late normal faults 

elsewhere in the Tintic Valley area are not easily recognized where they cut poorly consolidated alluvium 

and fanglomerate, and they are known chiefly from 

geophysical surveys. 

ORE DEPOSITS 

Several mining districts that have produced large 

quantities of base-metal and silver ores are located in 

the East Tintic, Sheeprock, and Simpson Mountains. 

The most productive districts are close to monzonite and 

quartz monzonite stocks that cut carbonate rocks of 

Paleozoic age. North of the stocks in the central East 

Tintic Mountains are the rich Tin tic and East Tintic districts, which have produced ores of silver, lead, zinc, 

copper, and gold valued at about $450 million, and 

whi~h still contain large reserves of silver, lead, and 

zinc. The smaller metalliferous districts in the Sheeprock and Simpson Mountains have been ·far less produc· 

tive, but they contain much unexplored ground. Of 

particular interest to the geophysicist and exploration 

geologist are the large areas covered by postmineralization valley-fill deposits and alluvium near the major 

ore-producing centers. Presumably these areas may 

contain extensions of the major districts or undiscovered 

districts of comparable richness. 

GRAVITY AND MAGNETIC SURVEYS 

A regional gravity survey along the central and 

southern Wasatch front by Cook and Berg (1961) included 40 stations in Tintic Valley and the East Tintic 

Mountains. To better define the gravity anomalies in 

Tintic Valley and the East Tintic Mountains, and to 

extend the coverage over the West Tin tic Mountains 

and the southern part of the Sheeprock Range, 232 new 

gravity stations were established in 1963. The new data 

from these stations were tied to those of the base network of the original survey, and 12 of the original 

stations were reoccupied to test the accuracy of the tie. 

The gravity datum for the survey by Cook and Berg 

was referenced to the U.S. Coast and Geodetic Survey 

pendulum station at Salt Lake City (Duerksen, 1949). 

A subsequent tie of one of the base stations used by Cook 

and Berg to the gravity control network in North 

America (Behrendt and Woollard, 1961) indicated that 

the datum they used was 2.2 mgals ( milligruls) below the 

North American network. The datum used in this report is based on a value of 979800.1 mgals determined 

by Behrendt and Woollard for the Liberty Park (Salt 

Lake City) base station. 

The gravity data were reduced to the complete Bouguer anomaly using an elevation correction factor corresponding to a density of 2.67 g per cm3 (grams per cubic 

centimeter) for the rocks ·above sea level. Terrain corrections were computed on an electronic digital computer (Kane, 1962). The corrections were made for 

terrain in a 40-km square with the station at the center. 

Latitude and curvature corrections were computed 

using the methods described by Swick (1942). 

Total magnetic intensity was m~asured with an AN I 

ASQ-3A magnetometer. East-west flight lines generally 1 mile apart were flown ·at a barometric elevation 

of 9,000 feet 'rubove sea level. The data were oompiled 

on U.S. Geological Survey topographic quadrangle 

maps and were reduced to a scale of 1: 125,000 (pl. 1). 
INTERPRETATION OF GEOPHYSICAL DATA, TINTIC VALLEY AREA, UTAH D5 

GRAVITY AND :MAGNETIC MAPS 

The dominant feature on the gravity anomaly map is 

a 30-mgallow in Tin tic Valley. This low, two smaller 

lows near Cherry Creek, and the low east of the East 

Tintic Mountains probably are produced primarily by 

the density contrast between sedimentary ·and volcanic 

rocks of Cenozoic age and ~the more dense older rocks. 

In the mountains the anomaly values have a range of 

about 15 mgals. 

The aeromagnetic map of part of north-central U·tah 

(Mabey and others, 1964) reveals a zone of magnetic 

anomalies south of the 40th parallel of latitude that is 

approxnnrutely coextensive with an area of Tertiary 

volcanic rock. The area of magnetic map presented 

here lies along the north edge of this zone. A high over 

the East Tintic Mountains is the largest magnetic 

anomaly in the area. At the 9,000-foot flight level, the 

anomaly consists of two closed magnetic highs. The 

smaller of the two closures is over an exposed intrusive 

stock, and the larger closure is over volcanic rocks on the 

west side of the East Tintic Mountains. In other areas 

smaller local anomalies are associated with both volcanic and intrusive rock. Relatively gentle magnetic 

gradients and low-amplitude anomalies occur over exposed sedimentary rocks of Precambrian and Paleozoic 

age. 

EAST TINTIC MOUNTAINS ANOMALIES 

Moderate gravity anomalies and large magnetic 

anomalies occur in the East Tintic Mountains. The 

Bouguer gravity values range from ·a high of -178 

mgals on Paleozoic rock near the north edge of the map 

to a low of -190 mgals on Tertiary igneous rocks between Silver Pass and Buckhorn Mountain. Over the 

Tertiary igneous rocks in the central and southern parts 

of the range, the magnetic map is dominated by an extensive area of high intensity with the maximum values 

about 800 gammas above the general level. North of 

this extensive high there are magnetic highs and lows 

over the Tertiary volcanic rock. 

In the East Tintic Mountains nea.r the north edge of 

the mapped area, relatively low gravity values and a 

magnetic high occur over volcanic rocks west of the 

Eureka Lilly-Selma fault zone. Part of the magnetic 

high doubtless reflects the effect of terrain, as the highest 

magnetic intensity was measured where the terrain 

clearance was smallest; however, the main part of the 

anomaly is not related to terrain. The gravity and magnetic anomaly west of the Eureka Lilly-Selma fault 

zone oould be produced by the thick lobe of volcanic 

rocks, but the absence of a comparable magnetic high 

over similar thick volcanic rocks a few miles to the east 

and the generally gentle gradients associated with the 

anomaly suggest that the magnetic anomaly is not produced by lavas and .pyroclastic deposits, but that it reflects a concealed intrusive mass. Indirect evidence for 

an intrusive mass may poss~bly be found in the interpretation that the deep, north-plunging base-metal and 

silver ore bodies of the northern part of the Tintic 

mining district were deposited in part from solutions 

rising from a concealed source, possibly an unexposed 

igneous intrusion, in this area. 

The magnetic intensity over ·the volcanic rocks northeast and east of Silver Pass is generally low, but local 

variations of over 200 gammas were defined at the 9,000-

foot elevation. An earlier survey of part of this area 

( unpub. data) with flight lines 1,000 feet above the 

surface revealed local negative anomalies of over 300 

gammas associated with inversely magnetized latite 

flows. The net magnetic effect of these volcanic rocks 

appears to be negative, indicating that much of this 

rock has an inverse remanent magnetization. 

The gravity values northeast of Silver Pass do not 

appear to be strongly influenced by local variation in 

thickness of the volcanic rock, although the volcanic 

rocks, which cover an irregular terrain, are absent in 

some places and and are more than 2,800 feet thick in 

others. 

A magnetic high centered over the monzonite stock 

at Silver City extends over several of the larger exposures of intrusive rock in this area, including the older 

Swansea Quartz Monzonite. Alteration has destroyed much of the magnetite in the Swansea Quartz 

Monzonite, and the susceptibility is much lower than 

that of the Silver City monzonite stock. However, the 

Swansea and Silver City stocks and other related intrusive bodies are reflected on the map as one anomaly. 

The position of the magnetic maximum suggests that 

the intrusive rock is more aJbundant or has greater depth 

extent along the west side of the range. The magnetic 

data suggest that the intrusive rocks in the Silver Pass 

area are sills and dikes, which converge with the Swansea-Silver City stocks at depth. 

Monzonite porphyry crops out at Diamond Gulch in 

a magnetic saddle. The relatively low magnetic intensity over this outcrop suggests that it. has limited depth 

extent or low susceptibility. Geologic field relations 

suggest that the outcrops are part of a sill-like mass. 

An e:rlensive magnetic high is centered over the west 

side of the range west of Tintic Mountain. Along the 

north edge of this high, a stock of monzonite porphyry 

is exposed, and smaller bodies of similar rocks are exposed to the south and east. Tertiary volcanic rocks 

cover most of the area of the anomaly. 

Susceptibilities were determined for 17 samples of 

intrusive and extrusive igneous rocks in the East Tintic 
D6 GEOPHYSICAL FIELD INVESTIGATIONS 

Mountains. Susceptibilities of 11 samples of volcanic 

rocks range from 0.27 to 5.41 X 10-3 cgs (centimetergram-second) units and average 2.90 X 10-3 cgs units. 

Three samples of the Silver City monzonite have susceptibilities of 2. 7 4, 3.48, and 5.40 X 10-3 egs units; one 

of monzonite porphyry from Sunrise Peak stoek is 

2.07 X 10-3 cgs units; two of ·the hydrothermally altered 

Swansea Quartz Monzonite are both 0.02 X 10-3 cgs 

units. These susceptibilities indicate that either the 

unaltered monzonite or the volcanic rocks could produce 

the magnetic high over the East Tintic Mountains if a 

sufficient thickness-in excess of 10,000 feet-of either 

were present. Several lines of evidence suggest that 

the greater part of the anomaly is produced by 'an underlying magnetic mass, presumably an igneous intrusive 

body. Paleozoic sedimentary rocks are exposed in a 

window near the area of the highest magnetic intensity, 

and it seems reasonable to infer that the volcanic rocks 

over the area adjoining this window are relatively thin. 

The highest intensity does not occur where the terrain 

clearance is lowest, which suggests ·that the surface rocks 

are not the primary eause of the anomaly. Over Tintic 

Mountain, which is the highest point in the range, the 

magnetic intensity is decreased, which suggests that 

volcanic rock there is inversely magnetized and thus 

is not contributing to the large positive anomaly. A 

strong local high of about 500 gammas occurs over exposed monzonite in the Silver City area, showing that 

the monzonite does produce anomalies of this amplitude. No cl~ar evidence indicates that similar anomalies are produced by the volcanic rock. 

We conclude that a large mass of intrusive rock with 

magnetic properties similar to monzonite underlies the 

East Tintic Mountains from Mammoth south to the 

head of Furner Creek. Local features within the composite magnetic high may reflect either variations in the 

thickness or magnetization of the volcanic rock, the 

geometry of the underlying magnetic mass, or compositional variations within the mass. The magnetic high 

is partly bounded by linear zones of steep magnetic 

gradient. The most prominent of these zones trend 

northeast and northwest. Numerous major faults within 

the Paleozoic rocks in the region trend northeast, and 

some of them have substantial components of both rightlateral and left-lateral displacement. Northwest-trending faults are not so prominent in the Paleozoic rocks; 

however, a zone of northwest-trending normal and 

strike-slip faults north of Boulter is on a projection of 

the trend of the steepened magnetic gradient bounding 

the magnetic high on its northeast margin. The linear 

zones bounding the magnetic anomaly and· their relation to trends of known faults suggest fault control for 

the margins of the intrusive mass believed to be producing the anomaly. 

The gravity values in the area of high magnetic intensity in the East Tintic Mountains are the lowest 

observed in the ranges in the area of this survey. Although thick accumulations of pyroclastic rocks may 

be part of the cause of the low gravity values, the lowest values are over the Tertiary intrusive rock and probably reflect a density contrast between intrusive rock 

and the generally more dense pre-Tertiary rock. 

The gravity values on Paleozoic rock in the Pinyon 

Peak area at the north end of the mapped area are 

higher than the values on Paleozoic rock near the south 

end of the range, and a part of the southward decrease 

in gravity values between Pinyon Peak and Silver Pass 

is probably a broad regional gravity variation. 

TINTIC VALLEY ANOMALIES 

On the west side of the East Tintic Mountains the 

gravity values decrease abruptly into the large gravity 

low in Tintic Valley. The gravity anomaly is asymmetrie, with the lowest values and the steepest gradients 

on the east side. A continuous zone of steep gravity 

gradients forms the east margin of the low from the 

north edge of the map to Riley Canyon. South of Riley 

Canyon the gradient is gentler. Along the west side of 

the valley several zones of relatively steep gravity gradi· 

ents are apparent, but more detailed observations are 

needed to adequately define the gravity variations in 

this area. The amplitude of the gravity anomaly deereases where the valley is eonstricted at the north and 

south ends. 

The gravity low undoubtedly is produced primarily 

by low-density sedimentary and volcanic rocks of Cenozoic age. No deep holes have been drilled in the valley, 

and we have no direct eontrol on the thickness or density of the material underlying the valley. Material 

about 7,200 feet thick with a density 0.45 g per cm3 

lower than the enclosing rock would produce the measured gravity anomaly. The 0.45 g per cm3 density contrast assumes densities of 2.7 and 2.25 g per em3 for the 

Paleozoic and Cenozoic rocks respectively. This density contrast has proved applieable in other parts of the 

Basin and Range province. 

Over Tintic Valley the magnetic anomalies are 

smaller than over the East Tin tic Mountains, but sign ificant variations in the magnetic field do occur. A zone 

of high magnetic intensity extends across the south 

end of the valley. This zone is continuous with the large 

high over the southern part of the East Tin tic· Mountains and another area of high intensity over the southern part of the West Tintic Mountains. The fact that 

the large magnetic high over the East Tintic Moun-
INTERPRETATION OF GEOPHYSICAL DATA, TINTIC VALLEY AREA, UTAH D7 

tains extends over the east edge of the valley suggests 

that the magnetic mass producing the anomaly is not 

terminated at the range front. From this high zone in 

the valley the magnetic intensity generally decreases 

northward with minor irregularities. A 100-gamma 

high, largely within the valley, is present west of 

Boulter. 

The magnetic data provide indication of the nature 

of the material underlying the valley. The zone of 

high intensity across the southern part of the valley 

reveals that igneous rocks are present in the subsurface. 

The low gravity values over part of the zone suggest 

that the rocks involved, at least in part, have low density; presumably they are similar to the extrusive rocks 

that are exposed in rthis magnetic zone 4 miles west of 

Mcintyre Siding. The northward decrease in magnetic intensity in the valley probably reflects a decrease 

in the amount of m'agnetic material in the valley fill or 

an increasing depth to the top of a magnetic basement 

underlying the sedimentary rocks. The local high west 

of Boulter indicates that the volcanic rock exposed under the south end of the anomaly extends into the valley 

under the Quaternary cover. The amplitude of this 

anomaly compared with that of areas of exposed volcanic rock suggests an anomalous concentration of 

magnetic material; perhaps it is an eruptive center. 

An analysis of the gravity gradient along the east 

side of the valley indicates that a single high-angle 

fault separating two density units could not produce 

the observed anomaly. Three possible configurations 

could produce the anomaly : ( 1) two or more highangle step faults, (2) a low-angle interface, and (3) 

a major high-angle fault combined with decreased density of the Quaternary and Tertiary sedimentary rocks 

toward the center of the valley. 

In making the two-dimensional analysis of the gravity data along profile A-A' (pl. 1), we attributed the 

gravity low in Tintic Valley to a combination of Quaternary and Tertiary sedimentary rocks ·and Tertiary 

volcanic rock with density contrasts of 0.45 and 0.22 g 

per cm3 respectively, relative to the pre-Tertiary rocks. 

If the density of the sedimentary rocks decreases toward the center of the valley, some of the inferred faults 

may be steeper and the displacement of the range-front 

fault greater. We have assumed that the depression of 

the valley has occurred along normal faults that dip 

45°. The gravity data require that the fill thicken immediately west of the front of the East Tintic Mountains where we infer the presence of a range-front fault. 

The existence of a fault within the valley is indicated 

by the steepening of the gravity gradient near Tintic 

Junction. South of the profile the magnetic anomaly 

associated with the Swansea and Silver City stocks extends into the valley without any indication that the 

top of the stock is displaced by a large amount at the 

range front. However, the west boundary of the area 

of highest intensi·ty is approximately coincident with 

the western inferred fault. Along the profile, Tertiary 

volcanic rocks are exposed on the west side of the valley 

and low-amplitude magnetic highs suggest that volcanic 

rock underlies the sedimentary rocks in the valley. An 

irregular layer of volcanic rock, as much as 1,000 feet 

thick, has been assumed to underlie most of the valley ; 

however, the thickness of the volcanic rock may differ 

substantially from the assumed thickness. 

The pattern of the faulting indicated by the gravity 

da:ta is continuous ·along the east side of ·the valley north 

of Riley Canyon. The bend in the major zone of gravity relief west of Mammoth could be produced by an 

arc in the fault zone or by the intersection of two linear 

zones. The gravity data indicate that the displacement 

along the inferred faul,t zone continues to the north edge 

of the mapped area, but that ~toward the south the displacement diminishes along both faults. 

The structure of the west side of Tin tic Valley is complex and the gravity data are sparse. Along profile 

A-A' two faults •are inferred on the basis of two zones 

of steepened gra.vity gradients. Elsewhere the gravity 

data are rrot sufficiently detailed to define the structure; 

however, it is clear tl1:rut faults along the west side of the 

valley produce strong vertical displacement. The dominant trend of the individual faults is probably slightly 

west of north. 

Three local gravity highs, which show as noses on the 

contour map, are superimposed on the general westward gravity rise on ·the west side of the valley. The 

northernmost correlates with the northern part of the 

magnetic high west of Boulter ·and indicates that the 

magnetic body is also a positive mass anomaly relative 

to enclosing valley fill. The ·anomaly is approximately 

alined with a ridge of volcanic rock to the south, and it 

may refledt a subsurface extension of this ridge. The 

gravity rrose immediately west of Boulter does not correlate with any structure apparent ·at the surface, hut it 

probably also reflects :a buried ridge. The third nose is 

coincident with an outcrop of volcanic rock in the valley 

south of Devil Creek and indicates that this outcrop is 

part of a north-trending partly buried ridge. 

The south end of the valley is constricted hy ·a largely 

buried hill of pre·-Tertiary rock :extending into the 

valley from the west. The gravity data suggest that 

this buried hill is hounded on the north and east by 

faults. 

The gravity and magnetic data suggest two major 

Cenozoic structures in Tin tic Valley. The first is related to the magnetic zone across the southern part of 
DS GEOPHYSICAL FIELD INVESTIGATIONS 

the valley. This zone is coincident with an alinement 

of minor intrusive bodies extending westward from 

Tintic Mountain through the southern part of the West 

Tintic and Sheeprock Mountains, and it is also coincident with Eocene volcanic rock present in the ranges 

and exposed in one area in the valley. The relatively 

high magnetic intensity in this zone probably reflects 

both volcanic rock underlying the valley and a deepseated source for the minor intrusive bodies. Within 

the valley, Bouguer gravity values over the magnetic 

zone are 20 mgals lower than over Paleozoic rock ·to the 

south and west, and steep gravity gradients suggesting 

faults form the south and west boundary of the low. 

Eocene hypabyssal and volcanic rocks, perhaps interlayered with and locally overlain by sedimentary rocks, 

are probably the primary source of the gravity ·anomaly. 

The volcanic rock may have accumulated in an easttrending rtrough, or it may have been preserved in a depression that formed after the Eocene volcanism. The 

second structure is the north-trending ·asymmetric 

graben formed during the episode of Basin-.and-Range 

faulting, which began in latest Oligocene or earliest 

Miocene time. This graben, which crosses the inferred 

trough of Eocene volcanic rock, was filled with both 

sediment·ary and pyroclastic rocks. 

WEST TINTIC AND SHEEPROCK MOUNTAINS 

ANOMALIES 

Variations of more than 10 mgals occur in the 

Bouguer anomaly values over the Sheep rock and West 

Tintic Mountains. Some of the variations have an apparent correlation wirth surface geology, but others do 

not. The highest values occur over or near windows in 

the Sheeprock thrust fault. The upper plate of the 

thrust is composed largely of clastic rocks which are 

probably less dense than the underlying predominantly 

carbonate rocks; therefore, higher gravity values would 

be expected over windows in the thrust. Low values in 

the west-central Sheeprock Mountains may reflect the 

presence of the granite in this area. The cause of the 

low west of Vernon Creek is not apparent. The lowest 

values may be effected by a local accumulation of lowdensity valley fill, but the anomaly also extends over 

Precambrian and Paleozoic rocks. An east-trending 

fault in the range east of Clyde Ranch may be reflected 

in the gravity darta, but more data are needed to define 

the gravity variations in this area. 

Considerable local gravity relief occurs over the alluvium-covered areas south of the Sheeprock and West 

Tintic Mountains. The gravity variations in this ·area 

probably reflect, in large part, relief on the surface of 

the pre-Tertiary rocks. Gravity highs occur over 

Paleozoic outcrops in the hills west of Jericho and east 

of Rockwell Ranch. A low indicating thicker cover of 

low density materi·al extends northwest from Rockwell 

Ranch. The gravity gradient along the northeast side 

of this low is continuous with a larger gradient east of 

the ranch and suggests a fault displa;cing the bedrock 

surface. Northeast of this postulated fa.ult the bedrock 

probably rises gradually toward the Sheep rock Mountains. A smaller low indicating a local structural depression occurs about 4 miles northeast of the ranch. A 

depression of the Paleozoic rock is indic3!ted by the 

gravity data between the West Tintic Mountains and 

the hills west of Jericho. 

In the West Tin tic Mountains magnetic highs occur 

over the major areas of extrusive and intrusive igneous 

rock. The positive magnetic zone that extends across 

the southern parts of the East and West Tintic Mountains and Tintic Valley terminates at the west edge of 

the exposure of igneous rock near Cherry Creek. The 

westernmost closed high in this zone is centered near the 

southeast corner of an exposed dike of rhyolite vitrophyre. This intrusive, which cuts both Precambrian 

and Tertiary rocks, is 1,500-2,000 'feet wide and more 

than 11,500 feet long, and it dips about 80° E. It may 

be a partial cause of the local magnetic high. The 

magnetic high near the south end of the West Tintic 

Mountains may reflect the presence of a granite or 

quartz monzonite stock that is mostly concealed beneath 

younger lava flows. 

An extensive magnetic high occurs over the granite 

stocks in the west-central Sheeprock Mountains and 

over outcrops of the monzonite and aplitic granite in the 

southwest Sheeprock Mountains. This magnetic 

anomaly indicates a major northwest-trending zone of 

intrusive rock lying ·along the southwest flank of the 

Sheeprock Mountains and extending southeast into the 

Sevier Desert. The crest of the closed magnetic high 

southeast of Clyde Ranch is about 2 miles west of the 

intrusive igneous rocks exposed along the southwest side 

. of the Sheeprock Mountains. Alluvium covers most 

of the surface near the crest of the magnetic high, but 

rhyolite vitrophyre crops out 4 miles north of Rockwell Ranch, and Tertiary volcanic rocks crop out in several places. Nine holes have been drilled in the area 

of the magnetic high by theN ew Jersey Zinc Co. (R. E. 

Radabaugh, written commun., 1965). Quartz monzonite was penetrated at depths ranging from 115 to 845 

feet in five of the holes between the exposed intrusive 

rock and the crest of the anomaly. West and south of 

the holes penetrating quartz monzonite, but also near 

the crest of the magnetic high, four holes ranging in 

depth from 910 to 1,060 feet were bottomed in Tertiary 

sedimentary and volcanic rocks. Although the volcanic 

rock may contribute to the magnetic high, the magnetic 
INTERPRETATION OF GEOPHYSICAL DATA, TINTIC VALLEY AREA, UTAH D9 

data suggest the presence of an intrusive mass underlying an area extending as much as 3 miles west of the 

outcrops of intrusive rock. The intrusive mass is apparently overlain by several thousand feet of older 

and younger rocks along its southwest side. 

Several small monzonite bodies within the Precambrian and Paleozoic rocks west of Cherry Creek are 

completely off the magnetic anomaly. This suggests 

that these monzonite bodies have very low susceptibility, or that they have limited depth extent and may be 

sill-like apophyses from a parent body to the west. 

The magnetic anomalies that are attributed to intrusive rocks in the Sheeprock Mountains are mu~h smaller 

in amplitude than the major anomaly produced by intrusive rock in the East Tintic Mountains. This 

amplitude difference prob31bly indicates a lower content 

of magnetite in the intrusives in the West Tintic area. 

Six samples of the unaltered intrusive from the Sheeprock Mountains ranged in susceptibility from 0.37 X 10-3 

to 2.60X 10-3 cgs units and averaged 1.24X 1()-3 cgs units. 

Four samples from the East Tintic Mountains ranged 

from 2.07 X 1()-3 to 5.40 X 10-s cgs units and averaged 

3.44 X lQ-3 cgs units. Although the number of samples 

is very small, the measured susceptibilities are consistent with the postulated lower magnetite content of the 

West Tintic intrustive rocks. 

A magnetic high in the area of Desert Mountain appears to be related to the granite bodies that crop out 

there. The magnetic data indicate that the granite extends under the Precambrian sedimentary rock on the 

west side of Desert Mountain and also under the 

alluvium to the north. The magnetic low over the 

Tertiary volcanic rocks east of Desert Mountain suggests that these rock may have inverse remanent 

magnetization. 

INTERPRETATION OF REGIONAL STRUCTURE 

All the major magnetic anomalies appear to be related directly to Tertiary igneous rocks, and no magnetic effect of the Precambrian basement complex is 

recognized. The igneous rocks and related magnetic 

anomalies occur in a broad west-trending belt that is 

similar to belts through the Bingham and Park City 

areas on the north (Mabey and others, 1964) and 

through the Milford area on the south ( unpub. data). 

These belts coincide with mineral belts which account 

for 95 percent of the copper, lead, silver, gold, and zinc 

produced in Utah (Hilpert and Roberts, 1964). Thus, 

regional magnetic surveys are a valuable tool in studying the mineral belts, particularly in areas with an extensive cover of Cenozoic sedimentary rocks. 

Fuller ( 1964) considered the east-west interruptions 

of magnetic anomalies in the Western United States 

as an expression of deep fracture patterns that are partly 

decoupled from the upper crust. He cited one example of an east-west offset of a magnetic anomaly in 

eastern Nevada as indic:ating the possibility that 30 

miles of left-lateral movement had occurred on one of 

the inferred fractures. 

In western Utah the present gross distribution of the 

west-trending belts of igneous rocks does not appear 

to be controlled by faulting younger than the igneous 

rocks. Abundant evidence suggests that the present 

distribution of the Tertiary igneous rocks is virtually 

the same as the original distribution. The belts of igneous rocks were probably controlled by deep-seated 

structures that influenced either the formation or upward movement of the magma. The absence of major 

surface structures coincident with or parallel to these 

belts of abundant igneous rocks suggests that the controlling structures ·are largely decoupled from the upper 

crust. 

The difference in magnetic expression between the 

intrusive rocks in the Sheeprock and West Tin tic Mountains and those in the East Tintic Mountains may be 

related to a fundamental difference in rock type and 

method of emplacement. This difference may help explain why economic concentrations of minerals are 

much more abundant in the East Tintic Mountains. 

Although the utility of gravity observations in determining the depth to Precambrian crystalline basement rock in the Basin and Range province has not been 

proved, the highest Bouguer anomaly values in the 

vicinity of the Wasatch front occur over the exposed 

Precambrian complex on Antelope Island (Cook and 

Berg, 1961). If other factors remain constant, other 

basement highs should also be reflected as gravity highs. 

The Sheeprock uplift, which was defined by Eardley 

(1940, pl. 1, fig. 4} and illustrated as a basement high, 

occupies the northwestern part of the mapped area. 

The geophysical data do not support the concept of 

basement uplift in the Sheeprock Mountain area, but 

they are consistent with a structural interpretation that 

assigns the Precambrian sedimentary rocks in this area 

to the upper plate of a major overthrust that extends 

over Paleozoic rocks that lie on other thrust sheets. 

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gravity control network in North America : Geophysics, v. 

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Cohenour, R. E., 1959, Sheeprock Mountains, Tooele and Juab 

Counties [Utah] : Utah GeoL and Mineralog. Survey BulL 

63,201 p. 

Cook, K. L., and Berg, J. W., Jr., 1961, Regional gravity survey 

along the central and southern Wasatch front, Utah: U.S. 

Geol. Survey Prof. Paper 316-E, p. 75-89. 
DlO GEOPHYSICAL FIELD INVESTIGATIONS 

Crittenden, M. D. Jr., Sharp, B. J., and Calkins, F. C., 1952, 

Geology of the Wasatch Mountains east of Salt Lake CityParleys Canyon to Traverse Range, in Utah Geol. Soc. 

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Fuller, M. D., 1964, Expression of E-W fractures in magnetic 

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