MOUNT OGDEN GRANITE

by

Dennis Charles Temple

A thesis submitted to the faculty of the University of Utah in partial fulfillment of the requirements for the degree of

Master of Science

Department of Mineralogy

University of Utah June 1969 This Thesis for the

Master of Science Degree

by

Dennis Chafrles Temple

has been approved

June 1969 ACKNOWLEDGEMENTS

Especial thanks are given to Dr. Bronson Stringham, under whose supervision this thesis was undertaken and carried to the completion of the field work and petrographic studies.

Dr. James A. Whelan was instrumental in the writing and final preparation of this thesis, through his encouragement and kind supervision•

The assistance of Dr. A. J, Eardley was of great help in under­ standing structural relationships.

The Anaconda Company materially assisted in the final prepa­ ration of this thesis.

iii TABLE OF CONTENTS

ABSTRACT , vii

INTRODUCTION 1

Purpose •••• 1

Location •••• 1

Accessability • • 1

Field and Laboratory Studies 2

Previous Work 3

Geologic Setting • 3

PRECAMBRIAN ROCKS 5

Gneissic Granite • 5

Meta-Diorite • • 9

Hornblende-Labradorite Gneiss 11

Feldspar Gneiss •*.. •.• 13

Pegmatites 14

Boundary Between Gneissic Granite and Migmatite •. 15

Age Determination 16

CAMBRIAN STRATIGRAPHY 17

Tintic Quartzite 17

Ophir Shale 18

Cambrian Limestone and Dolomite •. • • 18

QUATERNARY SEDIMENTS 19

Lake Bonneville Terraces 19

Canyon Gravels 19

iv STRUCTURE • 20

Thrust Sheets 20

Taylor Thrust . 20

Ogden Thrust • ..• 21

Wasatch Fault 22

ECONOMIC GEOLOGY 2*

GEOLOGIC HISTORY 26

REFERENCES 29

VITA • 30

v LIST OF ILLUSTRATIONS

PLATE 1, LOCATION MAP Folder

PLATE 2. GEOLOGIC MAP Folder

PLATE 3. GEOLOGIC SECTIONS Folder

vi ABSTRACT

The area of gneissic granite on the west side of the Wasatch

Mountains, immediately east of Ogden, Utah, was mapped on U. S.

Geol. Survey 7-1/2 minute quadrangle topographic sheets. The gneissic granite has foliation of variable intensity and is finer grained than the migmatite to the south. There is a gradational contact between gneissic granite and migmatite.

Within the gneissic granite there are numerous inclusions that have been grouped into three general types: Meta-diorite,

Hornblende-labradorite gneiss, and Feldspar gneiss.

Unconformably overlying the Precambrian rocks are the follow­ ing Cambrian sediments: Tintic quartzite, Ophir shale, and Cambrian limestone and dolomite. Quaternary deposits occur as Lake Bonneville terraces and canyon gravels.

Structures consist of the Ogden and Taylor thrusts, the normal

Wasatch fault, and east-west trending folds. Within the map area the Ogden thrust is younger and cuts the Taylor thrust. Both of these structures have been transversely folded. The steep western edge of the Wasatch Mountains is determined by the Wasatch fault.

vii INTRODUCTION

This study was undertaken to determine the rock types present and their relationships in that part of the Precambrian Farmington

Canyon complex exposed on the west side of the Wasatch Mountains, immediately east of Ogden, Utah, Gneissic granites were known to exist on the west side of Mt, Ogden, but the relationship of these rocks with the metamorphic rocks to both the north and south was not known. Since past mapping in this part of the north-central

Wasatch range was done on a regional scale, a detailed study of gneissic granite facies was made to better understand the geology of the Wasatch Mountains,

Location

The area studied (Plate 1) is within the Wasatch Mountains, immediately east of Ogden, Utah, It occupies a triangle, in parts of T5 and 6N, R1W and T5N, R1E, Salt Lake Base Line and Meridian, with its west side along the valley front, Gneissic granite is restricted to the west side of the range except at the southeast corner, where it extends one mile down the east side of the range

In Strawberry Canyon.

Accessability

The only paved road within the area is State Highway 39 in Ogden Canyon, Dirt roads are present along the front of the range for the length of the area studied. Foot trails are present in the bottom of Waterfall Canyon and in Taylor Canyon, The trail in Taylor

Canyon forks approximately one half mile up the canyon with the south fork going first to Malans Peak and then to the upper portion of

Waterfall Canyon, Another trail is present along the south side of

Ogden Canyon for approximately one mile from its start at the foot of the range north of Taylor Canyon, On the east side a work road in Snow Basin Ski Area extends to the crest of the range south of

Ogden Peak,

Field and Laboratory Studies

Field mapping was done during the fall of 1961, spring and fall of 1962, and the summer of 1963. The work was first recorded on aerial photographs and then transferred to U.S. Geol. Survey, 7-1/2 minute quadrangle topographic sheets (Plate 2). Due to the rugged nature of the terrain and the lack of roads, many days were spent on foot in order to reach all the outcrops within the area. Outside the Precambrian area, the work was done by field sightings with more reliance on the photographs, which show the contacts clearly.

While the field work was in progress, forty-one thin section slides were prepared and studied at the University of Utah, from specimens typical of the rocks present. During the winter of 1965 additional thin section study was done at Arizona University. All rock descriptions are based on these studies. Previous Work

The first published geological study of this area was made by members of the U. S. Geological Exploration of the 40th Parallel, who described the sedimentary rocks present but did no detailed work on the Precambrian rocks which they correlated with the Precambrian rocks to the south. Eliot Blackwelder (1910) reported on this area, but did not attempt to separate the various Precambrian rocks.

A. J. Eardley (1944) defined the general area of granite rocks, while doing his study of the North Central Wasatch Mountains.

Eardley and Hatch (1940) described the major Precambrian rock types in this area.

Geologic Setting

The area under study is located in the north central portion of the Wasatch Mountains, which in turn is the westernmost range of the Rocky Mountain Provence. The eastern edge of the Basin and Range provence is marked by the Wasatch fault along the west side of this area. All of the sediments that overlie the Precambrian rocks of interest are tilted to the east due to elevation of the west side of the Wasatch range. Also, this area is within the northern Utah highland of Eardley (1944). Due to these two factors, Precambrian rocks now have a vertical exposure of nearly 5000 feet in this area

(Plate 3).

The following types of Precambrian rocks are exposed: Gneissic granite, Meta-diorite, Hornblende-labradorite gneiss, Feldspar gneiss, 4

Migmatite, and Pegmatite dikes. Along the northeastern side of this area the following Cambrian rocks are exposed: Tintic quartzite,

Ophir shale, and limestone and dolomite of the Hartmann, Lynch and

Bluebird formations which were not differentiated in this study.

Quaternary sediments occur as Lake Bonneville beach terraces and canyon gravels.

All of the rocks in this area, except the Quaternary deposits, have been affected by moderate folding due to north-south com­ pression. The Taylor thrust and Ogden thrust cut through the area and finally the Wasatch fault has elevated these rocks to their present position. PRECAMBRIAN ROCKS

Gneissic Granite

The area studied is defined by exposures of gneissic granite.

The west side is limited by Lake Bonneville beach terraces, except between Taylor Canyon and Ogden Canyon, where Cambrian sediments extend to the valley* From Ogden Canyon northward to Jumpoff

Canyon the gneissic granite is present along the lower slopes of the range. South of Ogden Canyon* the gneissic granite is present in Taylor Canyon and below the crest of the range until just south of Ogden Peak, where it extends a few hundred feet onto the east side. Southward in Strawberry Canyon, the gneissic granite extends about one mile down the canyon. The southern boundary is defined by the presence of migmatite, extending from the south side of

Strawberry Canyon in a nearly straight line to between Strongs and

Hues canyons at the west edge of the range. A wedge of Cambrian sediments is present along the bottom one half mile of the south side of Taylor Canyon that taper southward until it feathers out at the south edge of the area studied* Tintic quartzite in this wedge is responsible for the cliffs present south of Taylor Canyon*

The Heta-diorite, Komblende-labradorite gneiss, and Feldspar gneiss fades are inclusions in the gneissic granite*

Due to variations in foliation and abundance of mafics, no one description of hand specimen*appearance is representative of the whole mass* The majority of outcrops are red to green-gray, weakly foliated granite. The red tones are due to weathering, oxidation of mafic constituents and pale red microcline. On fresh faces, a random mixture of light and dark minerals refects a green-gray color. Variations of up to 20% in the abundance of mafic con­ stituents is responsible for similar variation in color lightness.

Fine fracturing parallel to foliation and faults create irregularly tabular weathering fragments.

Changes in general appearance can be expected within 1000 feet of any point. The most prominent variation is in foliation.

Throughout most of the area foliation is weak and reflected by orientation of mafic constituents. Randomly scattered are abundant locations of moderate foliation, where all of the minerals appear to be oriented. Some of these areas can be related to faults, in­ clusions, or the migmatite south of the study area. No attempt was made to map foliation intensity due to the abundance of variation, irregular distribution, and very gradational contacts. The variation in abundance of mafic constituents, mentioned above, does not appear to be related to foliation.

Microscopic study of the slides showed the variations described above, but were consistent in the mineral species present. The normal composition is: quartz 20%, microperthite 25%, microcline 25% albite 10%, biotite 15%, and hornblende 5%, with the following accessory minerals in total amount less than 1%; magnetite, zircon, apatite and shpene. 7

The quartz has sutured outlines. It is moderately fractured, with aphanitic specks and bubbles along the fractures. Wavy ex­ tinction is usual. Grain size is 0.3 mm to 1,5 mm. There are microcrystalline inclusions of apatite and zircon in the quartz.

The microperthite is composed of bands 0,01 mm wide, with irregular feathery outlines and ends. There is undulatory ex­ tinction at right angles to the bands. Grain size is 0,3 mm to 1,2 mm. Inclusions of zircon are present. Cleavages contain black dust and a few have weak sericite alteration.

The microcline is well cleaved and has a general speckled appearance. Grain size is 0.3 mm to 1.5 mm. It embays both micro­ perthite and plagioclase. Inclusions of apatite and zircon are present. Trace amounts of aphanitic dusty specks are present.

The albite has good 0.03 mm albite twinning and is 0.1 to 0.8 mm in grain size. It is embayed by quartz. There is weak to moderate sericitic alteration along cleavages and twinning planes.

Biotite is in moderately oriented crystals, with wavy, irregular outlines, up to 1,5 mm long. Weak to moderate chloritic alteration containing microcrystalline magnetite is present along edges, cleavages and fractures.

The hornblende is in cross fractured, cleaved and irregular crystals up to 1.0 mm. Weak to moderate chloritic alteration is present along cleavages and fractures. Microcrystalline quartz and magnetite inclusions are present in areas of stronger chloritic 8 alteration. It generally forms intergrowths with the feldspars.

The minor constituents are present in interstitial locations and are approximately 0.01 mm in size, except where noted above.

To the southwest of Ogden Peak, brown almandite near 0.2 mm in diameter is present in variable amounts up to 5%, Limonite halos are present on weathered surfaces. The restricted area of notable almandite is outlined on the map.

The foliation noted above in both hand specimens and microscopic descriptions is near parallel to the south boundary, N. 70° to 85° W., in the southern part and N. 50° to 60° E. in the northern part of this area. The dips are generally steeper than 70° to the south, with a few noted dipping northward. Superimposed upon this foliation are local directions that parallel major structures. Near the Taylor and

Ogden thrusts, parallel foliation is accompanied by moderate to strong diaphthoresis or retrograde metamorphism. These zones are generally forty feet wide and decrease in intensity away from the fault plane.

The normal constituents are altered to chlorite, clay and white mica, with the quartz being the only original rock mineral not alter­ ed. Near the western edge of the range a few outcrops have foliation that is parallel to the valley and dipping approximately 65° W, This is parallel to the Wasatch fault, mapped to the west, under Lake

Bonneville benches, by Gilbert (1890) and Eardley (1944).

Within the gneissic granite are numerous inclusions of variable composition that have been grouped into three general types: 9

Meta-diorite, Hornblende-labradorite gneiss and Feldspar gneiss.

Heta-Diorite

In the area south of the head of Taylor Canyon there are numerous irregular to elongated meta-diorite inclusions. Near the head of the south fork of Taylor Canyon are two inclusions, 700 feet by 300 feet and 500 feet by 200 feet. On the ridge one mile above Malans Peak are two inclusions, 1700 feet by 400 feet and 400 feet by 100 feet.

On the ridge between the heads of Malans Basin and Strongs Canyon are two larger inclusions separated by 3000 feet. They are 1500 feet by 500 feet and 1200 feet by 1000 feet. On the ridge 3000 feet above Strongs Peak there are two thin inclusions, 6000 feet by 200 feet and 3200 feet by 200 feet. On the ridges below these two are three inclusions approximately 200 feet by 100 feet.

Approximately 5000 feet east of Bues Reservoir is an inclusion 300 feet by 100 feet. Cue inclusion, 200 feet by 100 feet is located

1000 feet south of DeMoisy Peak, Near the head of Strawberry Canyon are four inclusions, approximately 200 feet by 100 feet, and one 400 feet by 200 feet. Approximately one mile down Strawberry Canyon, from its head, is an inclusion that is exposed for 2500 feet by

1200 feet.

In the hand specimen, the meta-diorite is a dark green-gray, with a medium to fine grained, weak to moderately foliated, granite texture, The variation in foliation is apparently related to the shape of inclusions, with the strongest foliation parallel to the long axis of the elongated bodies. Variations in color from near black to green-gray are due to chloritic alteration and a 20% range in the amounts of mineral species present.

Microscopic studies show the same variations discussed above in the form of oriented mafics, weak to strong chloritic and argillic alteration, and differences in the amounts of mineral species present.

The types of minerals are constant and separate these inclusions from the others present. Unaltered specimens normally contain: labradorite,

40%; hornblende, 25%; augite, 20%; quartz, 10%; and biotite 5%; with apatite and shpene as the prominent accessories.

Labradorite is present in 0.3 mm to 1.0 mm crystals with weak sericite alteration along twinning planes.

Hornblende is in ragged crystals from 0,4 mm to 0.8 mm long and has weak chloritic alteration of edges and along numerous cross fractures and cleavages.

Augite occurs as the most euhedral crystals in the specimens and is 0.2 mm to 1.0 mm in size. It is normally intergrown with labrdorite and hornblende.

Quartz is fractured and has wavy extinction. Crystals occur up to 1.2 mm, but many are much smaller and occur in interstitial positions.

Biotite occurs as wavy, ragged crystals up to 0.7 mm long and appears to bend around the other constituents. Ueak chloritic alteration of biotite is common. 11

Apatite and sphene occur as inclusions in quartz and in inter­

stitial positions.

The term meta-diorite is used for these inclusions to define both the occurrence as inclusions in gneissic granite and the composition that is found. The contacts are sharp where found end

do not indicate one rock type intruded the other. Due to the

scattered distribution and random orientation of these bodies it is

felt that they are inclusions of an older terrain in the gneissic granite. This rock type is mentioned by Eardley and Hatch (1940),

Hornblende-labradorite Gneiss

In several locations throughout the gneissic granite are several hornblende-labradorite gneiss inclusions. Following are the locations and sizes of the inclusions mapped. Near the mouth of Jumpoff Canyon are three inclusions. The western one is 1500 feet from the other two and is 200 feet by 75 feet. The two are 400 feet by 100 feet and

100 feet by 100 feet. On the ridge above Hidden Valley is an in­ clusion that is exposed for 300 feet by 75 feet. On the north side of

Taylor Canyon are three inclusions, 300 feet by 100 feet, 200 feet by

100 feet and 100 feet by 100 feet. Approximately 2000 feet northwest of Ogden Peak is an inclusion that is exposed for 1400 feet by 200

feet. Approximately 1500 feet south of Ogden Peak are three in­ clusions; 150 feet by 100 feet, 200 feet by 75 feet, and 300 feet by

100 feet. The largest of these inclusions is in the mouth Of Water­

fall Canyon and is 3500 feet by 1500 feet. 12

Hand specimens of hornblende-labradorite gneiss are identified by their green-black to dark gray color, medium to coarse grain, and moderate to strong foliation. This rock type is differentiated from meta-diorite by more consistent foliation and the abundance of crystalline hornblende.

Microscopic studies show hornblende-labradorite gneiss to be consistent in mineral species, although there are variations of as much as 20% in amounts. The normal type contains; hornblende, 50%; labradorite, 40%; and quartz, 10%.

Hornblende is present in euhedral, well oriented crystals, generally 1,0 mm long and a few 1.5 mm long. Weak chloritic alteration can be found at edges and along a few fractures and cleavages,

Labradorite is present as euhedral crystals from 0.1 mm to 1.0 mm in size and with albite twins 0.03 mm wide. It is often oriented parallel with hornblende. Moderate to strong sericite alteration is common.

Quartz is crushed, with crystals ranging from cryptocrystalline to 0.8 mm. The crystals are rounded, with wavy extinction and large crystals contain serveral optical orientations. It has an interstitial relationship with the other constituents.

The mineralogy of hornblende-labradorite gneiss differs from meta-diorite in the absence of pyroxene and the abundance of horn­ blende. The foliation mentioned above is oriented parallel to the long axis of inclusions and disregards the foliation of gneissic 13 granite. It is felt they are inclusions of an older terrain in the gneissic granite. The same rock type was described by Eardley and

Hatch (1940), from Bountiful Peak and Antelope Island.

Feldspar Gneiss

At two locations within the gneissic granite, in the north portion and near the south border are feldspar gneiss inclusions. The locations and sizes of these inclusions are described below. Low on the ridge south of Jumpoff Canyon are five inclusions. They are 300 feet by 100 feet, 550 feet by 100 feet, 700 feet by 100 feet, 1100 feet by 200 feet, and 200 feet by 75 feet. On the ridge east of Ben Lomond High

School is an inclusion 1700 feet by 700 feet. Far to the south, approximately 2200 feet south of DeMoisy Peak is a single inclusion

300 feet by 100 feet.

Hand specimens are white, medium to coarse grained, weakly foliated, crystalline feldspar gneiss. This inclusion type is easily separated from the other two by the accessory amount of mafics present.

Microscopic studies show feldspar gneiss to be a weakly oriented intergrowth of feldspar, with minor mafics. The northern inclusions contain: microcline, 40%; oligoclase, 35%; quartz, 20%; and biotite,

5%; with magnetite, apatite and sphene in trace amounts.

Microcline occurs as rounded intergrown crystals 0.2 mm to 1.5 mm in size. Weak to moderate sericitic alteration is common and rarely is more intense in one carlsbad twin. 14

Oligoclase is present in sutured crystals 0.2 mm to 1.2 mm in size. Locally the albite twins are bent. Weak to moderate sericitic alteration is common, often along twinning planes and cleavages.

Quartz occurs in intergrown crystals 0,1 mm to 0,8 mm in size.

Fine fractures are common and weak, wavy extinction is present.

Biotite appears fragmental and bent around the other constituents in ragged, wavy crystals up to 0.5 mm long. The appearance in part is caused by strong chloritic alteration.

Zircon is found in euhedral crystals up to 0.3 mm in oligoclase.

The single feldspar gneiss inclusion found near the south edge of the gneissic granite differs in that all of the feldspar is labradorite. In other respects it is the same and was mapped in this rock type because it is the same white color, whereas the other nearby inclusions are all dark green-gray colored. This feldspar gneiss, with a relatively basic plagioclase, may be related to the intruded terrain represented by the nearby meta-diorite inclusions.

Foliation is parallel to the long axis of inclusions. Where the contact with surrounding gneissic granite is exposed it is a thin, gently curved line, with no evidence of invasion of one rock type into the other. It is felt that these bodies are inclusions of an older terrain in the gneissic granite.

Pegmatites

Throughout the area studied are numerous pegmatite dikes. They 15 are generally less than one foot wide, but a few were seen on the north side of Ogden Canyon and on the north side of the first ridge north of Ogden Canyon that are up to four feet wide. Color varies from pink to white due to wide variations in the ratio of K-feldspar to quartz, with the end points being near monomineralic. In all but a few dikes composition is the simple type, with only quartz and

K-feldspar present, A few dikes near the southern boundary of gneissic granite are composite type, containing moderate amounts of mica and apatite. The trend of pegmatite dikes is generally parallel to foliation in gneissic granite, but occasionally they cross cut fol­ iation for short distances. Local crowding of the country rock and crosscutting relationships indicate that the pegmatites are younger than country rock. This is substantiated by radiometric age determin­ ations discussed below.

Boundary Between Gneissic Granite and Migmatite

A very interesting area is the boundary along the southern side of the gneissic granite. This boundary extends to the eastward from

Lake Bonneville beach terraces between Waterfall and Bues canyons in nearly a straight line to approximately one mile east of the range crest in Strawberry Canyon. No significant changes were noted along this four mile length. This contact is gradational and therefore differs from the contacts gneissic granite has with Paleozoic rocks, since no sharp lithologic or structural difference is present. Moving ; to the south from the main body of gneissic granite an increase in 16 the strength of foliation is first noted, followed by gneiss that is mineralogically the same as gneissic granite, and finally segregation

of light and dark constituents, forming migmatite. The band of gradual change is usually 1000 feet wide. A general increase in grain

size accompanies the transition from gneissic granite to migmatite.

The migmatite to the south of the area studied is similar to that described by Bell (1951) and appears to be continuous with the area

in Weber Canyon he studied. For mapping, the boundary chosen is the

first place where definite segregation of the constituents is found.

Age Determination

Two lead-alpha age determinations have been made on rocks in this area by J. R. Odekirk (1963). A sample from the north side of Ogden

Canyon, typical of gneissic granite found throughout the area studied,

had an age of 1582 million years (Middle Precambrain). It probably represents the time when the gneissic granite was crystallized.

A second sample was taken from the north side of Weber Canyon

at the western edge of Devils Gate. The sample is similar to Bell's

(1951) "pink pegmatite". It is equivalent to pegmatite dikes found

within the gneissic granite. The age given is 674 million years,

which is very late Precambrian. This could represent the time when

foliation appeared in the gneissic granite and possibly development

of the gradational contact between gneissic granite and migmatite. CAMBRIAN STRATIGRAPHY

Tintic Quartzite

The eastern boundary of Precambrian rocks in this area is gener­ ally marked by abrupt cliffs of Tintic quartzite. On the southwest side of Ogden Peak there is a coarse basal conglomerate, approximately

70 feet thick, similar to that Eardley noted from Cottonwood Canyon to the east. The pebbles are well rounded and range from one inch to three inches in diameter, consisting of white, pink, and gray quartzite.

The matrix is a dark red silty sand. This conglomerate is absent along the rest of the contact. The Tintic quartzite rests in unconformable sedimentary contact upon the Precambrian rocks. Since individual thin beds within the quartzite can be traced parallel to this contact for considerable distances in the northern and southern parts of the area, the surface upon which it was deposited was, no doubt, fairly smooth, except for the depression containing conglomerate.

The Tintic quartzite is medium to fine grained, locally thin bedded, with well rounded quartz grains, cemented by silica. It is light tan to gray, with several dark red beds near the bottom that contain numerous pebbles. Iron staining is present in numerous fractures, and on exposed surfaces. In this area it is from 500 feet to 700 feet thick and is the most prominent unit present, forming

steep cliffs and the crest of the range in the vicinity of Ogden

Peak. 18

Ophir Shale

Overlying the Tintic quartzite is the Ophir shale. In this area it is approximately 100 feet thick and due to relative softness is generally covered. Where exposed, it is brown to olive green, with prominent, large wormlike markings on bedding surfaces and a dis­ tinctive micaceous sheen. For mapping purposes, the covered band between the Tintic quartzite and overlying carbonate outcrops was considered to be Ophir shale.

Cambrian Limestone and Dolomite

Overlying the Ophir shale is a thick sequence of limestone and dolomite. Eardley (1944) considered these rocks most similar to the Hartmann, Lynch and Bluebird formations of Gilluly (1932).

Since they are on the edge of the area considered and their contact with Precambrian rocks is restricted to Ogden and Taylor canyons, no attempt was made to separate the formations. QUATERNARY SEDIMENTS

Lake Bonneville Terraces

Along the entire front of the range, the Provo terrace of Lake

Bonneville marks the edge of valley fill. At the mouth of Ogden

Canyon terraces have been removed by the Ogden River, leaving a few small remnants perched on the canyon sides. The Provo terrace is at approximately 5000 feet, with some recent alluvium extending into the canyons and gullies above.

Canyon Gravels

In the central and upper portions of Taylor and Waterfall canyons are recent coarse, angular, poorly sorted and unconsolidated gravels.

They probably fill the canyon bottoms to less than 50 feet. During the spring runoff water is entrapped in the gravels and keeps the streams in these canyons flowing all year. During Pleistocene time there were small glaciers in Upper Ogden Bowl and Strawberry Bowl.

The morainal deposits from these glaciers were mapped as canyon gravels. STRUCTURE

Thrust Sheets

The most striking geologic features of this area are two thrust sheets. They are best exposed in Ogden Canyon, where the Tintic quartzite is repeated three times. The lower structure was named the Taylor thrust by Eardley for its trace parallel to the bottom of lower Taylor Canyon. The upper structure was named the Ogden thrust by Blackwelder. Both thrusts dip gently to the east-northeast and are sharply folded transverse to their general strike. They both cut across Paleozoic sediments and Precambrian rocks. The transverse folding of the thrusts is not parallel. On the south side of Taylor

Canyon, near where it forks, the Ogden thrust cuts across the Taylor thrust and is the only one present to the south. This evidence in­ dicates that the transverse folding was either comtemporaneous or took place after each of the thrusts. The Ogden thrust appears to be later than the Taylor thrust, although they both could have formed in response to the same compressional forces., From the evidence gathered by Eardley (1944) in his regional study, he dates the thrusting and transverse folding as Laramide.

Taylor Thrust

The trace of the Taylor thrust emerges from the Lake Bonneville beach approximately one mile north of Ogden Canyon, where it is in gneissic granite. At this point, the trace is obscured by cover, 21 except for a few small outcrops of brecciated and retrograde meta­ morphosed gneissic granite. To the east it truncates the lower exposure of Tintic quartzite and has a 50° dip north. It then follows the base of the middle exposure of Tintic quartzite, south­ easterly until it reaches Ogden Canyon, where it turns west, parallel to the canyon on the south side and again truncates Paleo­ zoic sediments. It disappears under valley alluvium just south of the mouth of Ogden Canyon.

The trace of the Taylor thrust appears again in the bottom of

Taylor Canyon, where the north side is on Precambrian rocks and the south side on Cambrian limestone and dolomite. Low on the south side of the central portion of the canyon, near where it forks, these relationships end. Since the exact location of the fault is covered by debris in the canyon bottom, this portion was not mapped as a thrust, although it is likely that it is a portion of the Taylor thrust. At the place where the Taylor thrust meets the Ogden thrust there is an area of diaphthoresis approximately 200 feet across.

Ogden Thrust

To the east of the Taylor thrust in Ogden Canyon, at Coldwater

Canyon, the Ogden thrust enters the area mapped from the east, in an area of jumbled Cambrian limestone and dolomite blocks. The trace then turns south on the east side of Warm Water Canyon, where it separates gneissic granite from Cambrian limestone and dolomite. 22

At this point gneissic granite is above. From the head of Hidden

Valley southward in Taylor Canyon for 1800 feet, a slice of Tintic quartzite is present between two limbs of the thrust. This slice is above Cambrian limestone and dolomite and below gneissic granite.

Where the Ogden thrust crosses the bottom of Taylor Canyon, it cuts the Taylor thrust and then continues up the south side of the canyon in a southwesterly direction to the crest of Malans Peak, then the trace continues in a southerly direction, dipping gently to the south until it reaches the Lake Bonneville beach east of Bues

Reservoir. South of Malans Peak the Ogden thrust cuts down across the Tintic quartizite until the quartzite disappears east of Weber

State University. South of the quartzite the fault trace contains several ledges of chlorite, white mica and mylonized quartz.

Wasatch Fault

The location of the western edge of the Wasatch Mountains is determined by the Wasatch fault. This structure cuts all of the structures discussed above. The major zone of displacement is obscured by the Lake Bonneville beach terraces. Locally evidence was found of this structure near the west edge of the range.

North of Jumpoff Canyon the Tintic quartzite appears to be dragged down over gneissic granite and is strongly fractured. In this area the gneissic granite has moderate retrograde metamorphism, particularly near the quartzite. On the north side of Taylor 23

Canyon, the Tintic quartzite again appears to have been dragged down over gneissic granite and contains numerous near 65°, east and west dipping, cross fractures. Tracing of individual beds along the cliff shows many normal fault displacements of up to five feet on west dipping fractures. Further to the south, on the ridge west of Strongs Peak, gneissic granite is well fractured, with many planes striking parallel to the edge of the range and dipping near

65° west. The west edge of the range is not straight, but gener­ ally strikes N 10° W. ECONOMIC GEOLOGY

Two areas of mining interest are located in this region. The first is a group of six lode claims held by Norman Minerals Inc. of

Ogden, Utah, located at the quarter section corner between sections

14 and 15, T6N, R1W. On the south side of a large feldspar gneiss body mentioned above are a few quartz veins with variable fluorite.

The veins strike N 50° E and dip 90°. At the crest of the hill a bench has been cut on the north side of the veins, approximately

100 feet by 40 feet. Three samples were taken from the vein exposed in this bench, near the east end, at the middle and near the west end. Another sample was taken from the largest of three veins exposed in a recently cleared area approximately 300 feet west of the bench. Below is a table of assay results, listed from east to west.

Sample No. Vein width Oz.Au/ton Oz.Ag/ton %CaF2

69655 1" 0.005 None 52.73

69656 1" Trace None 52.37

69657 1" Trace None 32.36

69658 6" Trace None 0,23

The second area is located approximately one half mile up

Taylor Canyon, near the center of section 35, T6N, R1W. At this point there is an old, caved adit and dump on the south side of the canyon near the bottom. From the size of the dump it is estimated 25 that the adit is 50 feet long. Section 35 is owned by the Union Pacific

Railroad. The dump consists of moderate to strong limoriite stained, strongly silicified limestone, with moderate, fine grained pyrite in fractures and less disseminated. One sample of the dump was taken, which assayed as follows; Oz.Au/ton, trace, and Oz.Ag/ton, none.

All of the rocks seen are so well fractured it is unlikely a competent piece can be found that will exceed three feet on a side.

Therefore, it is felt that no building or ornamental stone can be developed in this area.

The body of canyon gravels found in the north fork of Taylor

Canyon is used as a reservoir. The railroad depot in Ogden, Utah uses the water that is piped from this body. GEOLOGIC HISTORY

For overall regional reiationships the reader is referred to

Eardley and Hatch (1940) and Eardley (1944). However, the study of the local area developed the following which bears on the regional studies, specifically the mode of crystallization of the gneissic granite and the age of thrusting.

The gneissic granite differs from the migmatite to the south in grain size, texture and gross composition. According to Bell

(1951) the migmatite has a variable gross composition similar to quartz monzonite or granodiorite. The Middle Precambrian age represents the time during which the gneissic granite was emplaced in the rocks of the Farmington Canyon complex or their predecessors (Bell, 1951).

The foliation gives some clues as to the nature of emplacement of the gneissic granite. If it is a metamorphic rock, the foliation may represent original sedimentary structure. This possibility is not likely, since the inclusions have random discordant foliation and orientation. It is possible that the foliation was imposed upon the gneissic granite, which formed by metamorphic processes, at an earlier time. This possibility is not favored, because the inclusion boundaries should have become gradational during repeated metamorphism.

If the gneissic granite is intrusive and contemporaneous with the migmatite, the foliation may be an igneous feature. This is practical since the true limit of the gneissic granite is exposed only at its southern boundary. Also, if the gneissic granite is intrusive, the 27 foliation could have been imposed at the time of formation of the migmatite during the metamorphism which ended with the intrusion of pegmatite dikes in Late Precambrian time. The inclusions would have been protected by the gneissic granite. The low grade metamorphism

imposed on the gneissic granite would not have caused the boundaries to become gradational. Of the above possibilities, the last is thought to be the most likely. It best accounts for the features seen and includes the favorable arguments of the other possibilities.

It is felt that the numerous inclusions noted above are completely recrystallized remnants of the pregneissic granite terrain. This terrain consisted of several rock types as indicated by the several types of inclusions. Older (2400 million years) Precambrian rocks occur in the Raft River Tange of northwestern Utah (Sayyah, 1965).

These inclusions could possibly be remnants of this early Precambrian terrain.

Additional isotopic age-determinations of the gneissic granite, migmatite, and inclusions by the potassium-argon and rubidium-

strontium methods, on whole rocks and on various minerals, might

clarify the complete thermal history of this area.

Compressional forces were again active in this area during the

Laramide orogeny of late Cretaceous time. East-west striking folds were formed in the overlying Paleozoic sediments and the Taylor and

Ogden thrusts. Eardley (1944) concluded that the thrusts moved from

west to east. The right angle intersection of the Ogden and Taylor 28 thrusts in Taylor Canyon creates conclusions regarding the series of events that differ from the older regional studies. It is possible that the Taylor thrust pre-dates the Ogden thrust and was folded prior to the Ogden thrust, or both thrusts are contemporaneous and formed waves that strike parallel to the direction of movement. Due to the large angle of intersection between the two thrusts in Taylor Canyon, it is concluded that the Taylor thrust formed and was folded before the Ogden thrust was formed. Both thrusts were probably folded after the Ogden thrust formed. It is also possible that the two sets of folds did not form in exactly the same locations, which would help to explain irregularities seen in the trace of the two thrust sheets.

The present topography is developed in the upthrown, footwall block of the Wasatch fault. This structure is probably still active, although no displacement has been reported in this area during recent times. The lower course of Ogden Canyon is determined by an area of

sharp bending and faulting of the Tintic quartzite. This deform­ ation has caused abundant fractures, which facilitate erosional forces. REFERENCES

Bell, G. L. 1951, Geology of the Precambrian metamorphic terrane, Farmington Mountains, Utah: Unpublished Ph.D. thesis, Univ. of Utah, 101 p.

Blackwelder, Eliot, 1910, New light on the geology of the Wasatch Mountains, Utah: Geol. Soc America Bull., v. 21, p. 517-542,

Eardley, A, J,, 1944, Geology of the north-central Wasatch Mountains, Utah: Geol, Soc America Bull,, v, 55, p, 819-894,

Eardley, A, J, and Hatch, R, A,, 1940, Pre-Cambrian crystalline rocks of north-central Utah: Jour, Geology, v. 48, p. 58-72,

Gilbert, G, K., 1890, Lake Bonneville: U, S. Geol. Survey, Mon. 1, 438 p.

Gilluly, James, 1932, Geology and ore deposits of the Stockton and Fairfield quadrangle, Utah: U. S. Geol, Survey, Prof. Paper, 173, 167 p.

King, Clarence, Emmons, S. F,, and Hague, Arnold, 1877: Geol. Explor. 40th Parallel, v. 1, 2, 803 p.

Odekirk, J. R., 1963, Lead alpha age determinations of five Utah rocks: Unpublished M. S. thesis, Univ. of Utah, 31 p.

Sayyah, T. A., 1965, Geochronological studies of the Kinsley Stock, Nevada and the Raft River Range, Utah: Unpublished Ph.D. thesis, Univ. of Utah, 99 p.