BULL. OEOL. SOC. AM., VOL. 61 HABDY AND ZELLER, PL. 1

EXPLANATION

SCALE OF MILE

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GEOLOGY OF THE WEST-CENTRAL PART OF THE GUNNISON PLATEAU, UTAH

BY CLYDE T. HARDY AND HOWARD D. ZELLER

ABSTRACT A detailed study of the west-central part of the Gunnison Plateau, Utah, has disclosed stratigraphic and structural relations important in the geological history of central Utah. The area mapped includes the east- ern half of the Axtell No. 2 quadrangle, Manti area (U. S. Dept, of Agriculture, Soil Conservation Service). The bedrock ranges in age from the Arapien shale (Upper Jurassic) to the Green River formation (Eo- cene). The North Horn formation (Cretaceous-Tertiary), the Flagstaff limestone (Paleocene-Eocene), and the Green River formation (Eocene) successively overlap the Arapien shale and the Indianola group in the northern part of the area. In the southern part of the area, the North Horn formation and the Flagstaff limestone successively overlap the Price River conglomerate with angular discordance. The latter relation establishes a post-Price River and pre-North Horn orogenic movement in central Utah. A conspicuous west-dipping monocline, broken by many high-angle faults and a graben, extends along most of the western margin of the area. This structure is similar to the monocline and graben in the Wasatch Plateau to the east. At one point in the northern part of the area, the North Horn and the overlying Flag- staff oppose the Arapien shale across a high-angle fault. The Green River formation extends across the fault and overlaps the Arapien. This relation suggests faulting between Flagstaff and Green River time because the Colton formation, which normally occurs between the Flagstaff and Green River, cannot be differenti- ated in this immediate area. Numerous small intrusive masses of monzonite porphyry of post-Upper Jurassic age occur in the Arapien shale.

CONTENTS TEXT Page Page Geologic history 1277 Introduction 1262 References cited 1278 Location 1262 Major geologic features 1262 Topography and drainage 1264 Previous investigations 1264 Acknowledgments 1264 Flgure Paee Stratigraphic systems 1264 l- Index map of central Utah 1262 Jurassic system 1264 2- Chart showing lithologic changes in the Arapien shale 1264 Flagstaff limestone and the North Horn Cretaceous system 1266 formation 1269 Indianola group (undifferentiated) 1266 3- Schematic diagrams showing development Price River formation 1267 of pre-Green River fault 1276 Cretaceous-Tertiary system 1269 North Horn formation 1269 Tertiary system 1270 ate Facin« Pa«e Flagstaff limestone 1270 *• Geologic map of the west-central part of Colton formation 1272 tne Gunnison Plateau, Utah 1261 Green River formation 1272 2- Angular unconformities 1276 Igneous rocks 1274 3- Graben and monocline 1277 Structure 1275 Regional structural relations 1275 Major structural features 1275 TABLE Monocline and associated structures. . . . 1275 Graben 1275 Table Page Pre-North Horn structure 1277 1. Formations in west-central part of the Pre-Green River fault 1277 Gunnison Plateau, Utah 1263 1261

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FIGURE 1.—INDEX MAP OF CENTRAL UTAH

INTRODUCTION The area described in this report (Fig. 1) extends from near Levan, Utah, southward In 1947 and 1948 the writers independently to Fayette, Utah, which is about 5 miles north- investigated the stratigraphy and structure west of Gunnison. It extends eastward to the of adjacent areas in the west-central part of divide in the central part of the Gunnison the Gunnison Plateau, Utah. These studies Plateau. The area includes the eastern half are summarized in this paper. of the Axtell No. 2 quadrangle, Manti area (U. S. Dept, of Agriculture, Soil Conservation LOCATION Service), and is about 6 miles wide and 16 miles long. The Gunnison Plateau, about 100 miles south of Salt Lake City, extends southward MAJOR GEOLOGIC FEATURES from near Nephi, Utah, for 35 miles to Gun- nison, Utah, and is about 11 miles in width. The exposed bedrock (Table 1; PL 1) ranges The plateau is separated from Mt. Nebo, at in age from Upper Jurassic (Arapien shale) to the southern extremity of the Wasatch Moun- Eocene (Green River formation). The Arapien tains, by Salt Creek Canyon, east of Nephi, shale forms rugged foothills in the northwestern Utah. part of the area and contains numerous small

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conspicuous graben with displacement of about 1925, p. 445, 448-449; Spieker, 1946, p. 123- 1000-1500 feet. Along the western margin of 124, 127, 130-133, 139). Spieker recently sum- the central part of the Gunnison Plateau, the marized his views on the geologic history of Flagstaff limestone and related formations dip central Utah in a guidebook of the Utah westward in a pronounced monoclinal flexure. Geological Society (Spieker, 1949).

TOPOGRAPHY AND DRAINAGE ACKNOWLEDGMENTS The Gunnison Plateau is a relatively narrow The authors are indebted to E. M. Spieker, upland which rises from the Sevier Valley near under whose direction the work was done, for Gunnison, Utah, to an elevation of almost advice and criticism both in the field and in 10,000 feet near the northern end. The sur- the preparation of preliminary reports. William face of the plateau is maturely dissected. The N. Gilliland and Siegfried Muessig offered in- margins are, in general, steep although lower valuable consultation in the field in 1947 and foothills parallel the conglomerate escarpments in 1948, respectively. Carl A. Lamey identified along the western side. Elevations range from the igneous rocks, and N. M. Denson read the about 5200 feet in Sevier and Juab valleys, manuscript. west of the plateau, to about 8900 feet in the central portion. STRATIGRAPHIC SYSTEMS The west-central part of the Gunnison Plateau is dissected by eight major canyons Jurassic System which trend nearly at right angles to the margin of the plateau. Small streams in the Arapien shale.—The Arapien shale was de- five northern canyons are fed by springs and fined by Spieker (1946, p. 123-124) as a forma- flow at diminishing rates throughout the sum- tion with two members: (1) the lower Twelve- mer. This area lies almost entirely west of the mile Canyon member, and (2) the upper Twist drainage divide in the central part of the Gun- Gulch member. In the west-central part of the nison Plateau. Gunnison Plateau, the Twelvemile Canyon member forms rugged foothills which extend PREVIOUS INVESTIGATIONS from the northern boundary of the area south- ward to about 1 mile north of Criss Creek. At E. E. Ho well prepared several excellent cross this point it disappears beneath the Green sections of the Gunnison Plateau, which were River formation (PI. 2, fig. 1). The Twist included by Button (1880, pi. facing p. 162) Gulch member crops out in the valley of the in the Report on the geology of the High Plateaus north fork of Little Salt Creek and is exposed of Utah. Meinzer (1911, p. 67-74) briefly de- continuously from there to the northern end scribed the western margin of the Gunnison of the Gunnison Plateau. This unit, which is Plateau in a report pertaining to water re- less resistant than the overlying Indianola sources of Juab, Millard, and Iron counties, conglomerate, forms a distinct break in slope Utah. Recent work in the Gunnison Plateau below the prominent cliff of conglomerate. was initiated by Gilliland in 1946 as part of a All the lithologic types recognized elsewhere in survey of the Gunnison quadrangle (Gilliland, the Twelvemile Canyon member are present in 1951). R. E. Hunt (unpublished dissertation) this area (Spieker, 1946, p. 124). The lowermost completed a study of the northern part of the exposed stratigraphic unit is a thin-bedded dark- Gunnison Plateau in 1950, and numerous gray limestone, with some inter bedded gypsum, theses of The Ohio State University, in addi- which weathers light gray. It is closely folded tion to Hunt's dissertation, describe the geology and highly fractured. Hardy and R. E. Hunt of the eastern part of the Gunnison Plateau have measured a minimum thickness of about (Hunt, Taylor, Babisak, unpublished theses). 800 feet for this unit in Chicken Creek Can- In the Wasatch Plateau and near-by areas yon, east of Levan, Utah. A succession of gray in central Utah Spieker and others have recog- shale and red shale with gypsum overlies the nized all formations found in the western part limestone in the west-central part of the plateau. of the Gunnison Plateau (Spieker and Reeside, An alternation of gray shale, sandstone, and

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some limestone overlies this unit. The latter Thickness succession is estimated to be at least 1500 feet ^ . , , , , . , ™ , , , , to coarse-grained, clay nodules; thick. The sandstone in a few places forms sandstone, buff to brown, in upper massive beds which commonly contain clay part 10.0 nodules, ripple marks, and some cross-bedding. (22) Shale and sandstone, like Unit 13. ... 7.5 The limestone is generally sandy or argilla- (21) Shale, green gray, calcareous, fissile... 3.0 ceous and rarely exhibits mud cracks. Within (20) shale; green graVj calcareous; sand- this unit many intrusive masses of monzonite stone, calcareous, thin-bedded 82.5 porphyry are found. A section, typical of the (19) Sandstone, green gray, calcareous, gray shale unit, was measured in Little Salt weathers yellow brown, some clay Canyon. It is nearly complete for this area, nodules' forms Ied8e 13 -° 1270 feet, and extends from the gray and red (18) Shale and sandstone, like Unit 13.... 8.5 shale with gypsum to an angular unconformity (17) Sandstone, green gray, calcareous, 4- wit...h th,, e overlyin. . g Gree„ n r>Rive. r ,formation . . to 6-inch beds, forms massive cliff.. 17.5 (16) thin bedded The , Twis,. t Gulc, ,. . h member, ,consist s i predomi-i, , - ^"^f^.^'PltUy> 11Ke unit 13 - ' .10., _U nan tly of reddish-gray sandstone, red siltstone, ,.,, „ , , „ , , , , . . , .. . ., (15) Sandstone, gray, calcareous, 2- to 5- and some red shale . TIt is relatively uniform inch beds, clay nodules, forms prom- throughout central Utah and no distinct units inent cliff 40.0 are recognized within the member. A section (14) Shale, blue gray, calcareous, clayey, of 1839 feet was measured and is believed to be weathers in rounded knobs 17.5 nearly complete, although the contact with (13) Shale, blue gray, calcareous, fissile; the underlying Twelvemile Canyon member sandstone, one-quarter inch beds... 13.0 is covered at this point. (12) Sandstone, gray, calcareous, weathers ^ brown 1.0 SECTION No. 1, LITTLE SALT CREEK C11) Limestone, gray, arenaceous; shale. .. 14.5 Section of Twelvemile Canyon member of Arapien (10) Shale> Uke Unit 9> sandstone; lime- shale in Little Salt Creek stone, arenaceous 6.0 Green River formation <9> Shale> blue S^V, calcareous 46.5 Twelvemile Canyon member of Arapien shale ® Shale> &*?> Poorly exposed 21.5 Thickness (7) Sandstone, gray, calcareous, fine- (Feet) grained, thin-bedded, weathers in (33) Covered interval, some fine-grained cl"Ps 26.0 sandstone 221.0 ((,) Sandstone, gray, calcareous, medium- (32) Limestone, gray, dense, weathers yel- grained, 1-foot beds, ripple marks low brown 5.0 and cross bedding 3.0 (31) Sandstone, gray, calcareous, lower 18 (5) Limestone, like Unit 2 39.0 feet forms cliff 62.0 (4) Limestone, gray green, argillaceous, (30) Sandstone, shale, and limestone; green mud cracks and possible trails 23.0 gray to gray 23.5 (3) Sandstone, gray, calcareous, cross- (29) Sandstone, light green gray, calcareous, bedded 2.0 medium- to fine-grained, weathers (2) Limestone, gray green, argillaceous, yellow, gray clay nodules 4.5 thin-bedded, platy, mud cracks.... 22.0 (28) Limestone, light gray, arenaceous, (1) Covered interval, shale, gray, cal- thm-bedded 17.5 careous; limestone, thin-bedded, (27) Sandstone, light gray, calcareous, platy; sandstone, thin-bedded 291.0 forms ledge 21.5 , , ... , Total Twelvemile Canyon member (26) Sandstone, shale, and limestone; poorly measured 1269 5 exposed, thin-bedded, more sand- s PP " ' The base of the Twelvemile Canyon member (2S) S^^K^W^Sm. h- not been found in """nd U^h south of erous (Ostrea sp.), some ripple the Wasatch Mountains. Near the mouth of marks 6.5 Ljttle Salt Creek, the Green River formation (24) Shale, some sandstone, like Unit 13... 74.0 overlies the Twelvemile Canyon member with (23) Sandstone, dark blue gray, medium- angular discordance; a red shaly bed at the

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contact probably represents residual material Cretaceous System accumulated on the old erosion surface. The Indianola group (undifferentiated).—This term Twist Gulch member seems to grade into the has been designated by Spieker (1946, p. 127) overlying Indianola group (undifferentiated) to represent a thick series of conglomerate without observed erosional discontinuity. The and sandstone which crop out north of In- Morrison (?) formation, which is recognized by dianola, Utah, on Hjork and Dry creeks. Spieker and others along the southeastern The Indianola group (undifferentiated) crops margin of the Gunnison Plateau, has not been out in the northeastern part of the area de- differentiated with certainty in this area scribed in this report and is continuous into (Spieker, 1949, p. 18). the northern part of the Gunnison Plateau, Spieker (1946, p. 123) recognized that the which is only about 15 miles from the type Twelvemile Canyon member is probably locality. Near Little Salt Creek the Indianola equivalent to the San Rafael group. Hardy disappears beneath the North Horn formation (1952, p. 26) believes that the thick gray shale and the Flagstaff limestone. unit of the Twelvemile Canyon member, which The Indianola group in this area consists occurs elsewhere in central Utah, is a probable almost entirely of massive cliff-forming con- correlative of the Carmel formation, although glomerate composed primarily of cobbles and definite paleontological evidence is lacking; boulders of dark blue-gray limestone and red the age of the Carmel is indeterminate between and gray quartzite. Interbedded red sandstone Middle and Upper Jurassic. The Twelvemile and shale occur in the conglomerate and dis- Canyon member of the Gunnison Plateau tinguish it from the overlying Price River probably does not differ appreciably in age formation. Beds of light-gray and buff sand- from that exposed elsewhere in central Utah. stone, with layers of red shale, and gray or buff If the Twelvemile Canyon member is a cor- shale with interbedded light-gray calcareous relative of the Carmel formation of the San sandstone are also present. A section of In- Rafael group, the Twist Gulch member might dianola totaling 3600 feet was measured in be expected to be a correlative of the Entrada, Bear Canyon from the underlying Twist Curtis, and Summerville formations of the San Gulch member of the Arapien shale to the Rafael group (Hardy, 1952, p. 27-28). The age angular unconformity with the Flagstaff of the Twist Gulch member, therefore, is cer- limestone. tainly Upper Jurassic. SECTION No. 3, BEAR CANYON SECTION No. 2, LITTLE SALT CREEK Section of Indianola group (undifferentiated) Section of Twist Gulch member of the Arapien measured at the southernmost outcrop along the shale measured in the nerth fork of Little Salt Creek west margin of the Gunnison Plateau Indianola group Flagstaff limestone Twist Gulch member of Arapien shale Angular unconformity Thickness (Feet) Indianola group (undifferentiated) Thickness (7) Sandstone and siltstone, like Unit 1... 154.5 (Feet) (6) Conglomerate, red, pebbles and cobbles (21) Conglomerate, red to gray, cobbles of quartzite (average diameter 1 and boulders of red and buff quart- inch); matrix red calcareous sand- zite and dark-gray dense limestone stone, forms massive ledge 10.0 (maximum diameter 1 foot; average diameter 2-6 inches) 579.0 (5) Sandstone and siltstone, like Unit 1... 104.5 (20) Sandstone and conglomerate, light (4) Sandstone, like Unit 2 15.0 gray to white; shale, red; conglom- (3) Sandstone and siltstone, like Unit 1... 1122.0 erate, pebbles and cobbles of red and yellow quartzite, dark-gray (2) Sandstone, red, coarse-grained to grit, limestone, red-brown sandstone; forms massive ledge 10.0 sandstone, calcareous, coarse- to (1) Sandstone and siltstone, red to red gray, fine-grained; grit and sandstone, red calcareous, friable, thin-bedded; sand- and tawny, in upper part; shale, stone, coarse-grained in places 423.0 gray; unidentifiable plant fossils in upper 25 feet 401.5 Total Twist Gulch member ex- (19) Shale, gray to buff; sandstone, light posed 1839.0 gray, calcareous 165.5

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Thickness Indianola is not exposed inasmuch as the , . , . ... ,. . ,, ? « North Horn formation and the Flagstafs f (18) Shale, like Unit 16 ...... 4.0 limestone overlap the Indianola with angular (17) Sandstone, light gray to red, like ^ discordance. A reiatively high area of Indianola „ . . . „ conglomerate on the erosion surface beneath (16) Shale, yellow gray and red ...... 4.0 ° . ^ the unconformity is evidently related to the (15) Shale, red ...... 31.5 marked t^irm[ng of the Flagstaff limestone (14) Sandstone, gray to brown, calcareous . 4.0 and the North Hom formation in the northern (13) Conglomerate, red, boulders (average part of the area. S"S5i?S;liiiS£i fi£ Fra*mental p'ant r™ were found 300° grained ...... 189 .0 feet above the base of the Indianola conglom- (12) Conglomerate, gray to red, cobbles and «ate in a 25-foot bed of buff sandstone. Schoff boulders of quartzite and limestone (1951, p. 626) found a marine zone of Colorado

SSaSSSiSSSiJ"": «B.5 ^ about 9000 feet above the base of the (11) Conglomerate, red, like Unit 9 ...... 173.0 **" * *C ^ .^ ^f " J194^-P ' v ' , „ 130) tentatively regards the beds below this (10) Sandstone, red, calcareous, fine- . ,-, „,, grained, forms ledge ...... 8.0 marin e zone as UTPPT er Cretaceous. The (9) Conglomerate, red, cobbles (maximum Indianola group as identified in this area is diameter 6 inches) of dark blue-gray therefore considered tentatively as Upper limestone and quartzite; shale, red; Cretaceous. sandstone, red, calcareous, coarse- „. „. , ,. T,, T, • „. tofine-gndned' ...... 148.5 Pnce River formation.— The Price River (8) Conglomerate, like Unit 2 ...... 4.0 f°rmation was defined by Spieker and Reeside ... TT . . ,, - (1925, p. 445) with reference to strata which (7v ;) Conglomerate6 , like Unit 3 ...... 16.5 ' \ . . „ <.!_,.<•,-,, crop out in Price Canyon, northwest of Castle- (6) Conglomerate, like Unit 2 ...... 152.5g ^ Utah The prke Riyer formation is (5) Sandstone, red, calcareous; shale ..... 24.5 found only in ^ southern half of the area (4) Conglomerate, like Unit 2, massive where it forms a prominent escarpment which cliff in upper 20 feet ...... 115.5 trends northward (pl 2> fig 2) ^^^

(3) C ex sures occur in each of the ^S^v&ffilS£^ P° ^ major shale, red, calcareous, in upper canyons which dissect the southern plateau part ...... 29.0 surface. The formation also appears in the low (2) Conglomerate, gray to red, pebbles and hills near the southwest margin of the area. S^^SS£3[SES£ The Trice Ri ver formation in this area rated by shale units ...... 613.5 consists largely off conglomerate which contains (1) Covered interval ...... 30.0 pebbles and cobbles of quartzite and limestone. Interbedded with the conglomerate are me- dium- to coarse-grained sandstone lenses , , which weather to dark brown. Beds of red Twist Gulch member of Araplen shale Com lon 6 In About 5000 feet of Indianola strata is exposed " j^ f^ola conglomerate. in the northeastern part of this area. In Chicken ™e Pebbles and cobbles of the Price River Creek Canyon, about 3 miles to the north, ^glomerate consist dominantly of white, lnk and red ( uartzlte SmaUer amounts of the uppermost conglomerates lack the abun- Phmeston' e occurl - dance of limestone fragments which seem to be chiefly as pebbles, although rarel the llm characteristic of the Indianola in the west- y estone is found as cobbles 6 or 8 mches across The central part of the Gunnison Plateau. Hunt ' limestone is commonly (unpublished dissertation) has measured at either dark blue §ray and dense °r light gray and least 7000 feet of Indianola in the northern fine-grained. In some beds limestone con- part of the Gunnison Plateau. stitutes about 10 per cent of the pebbles and The Twist Gulch member of the Arapien cobbles. Pebbles of light-gray medium-grained shale appears to be conformable with the sandstone and buff arenaceous limestone also Indianola conglomerate. The upper part of the occur in lesser amounts.

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of fossils in the upper part of the Price River Price River formation and conformably below formation and in the lower few hundred feet the Flagstaff limestone in the central part of of the overlying North Horn formation leaves the area. Excellent exposures are seen in Hell's

MtLLOR HELCCANYOS KITCHEN N CANYON

FIGURE 2.—CHART SHOWING LITHOLOGIC CHANGES IN THE FLAGSTAFF LIMESTONE AND THE NORTH HORN FORMATION

the exact age of the upper part of the Price Kitchen (PL 2, fig. 2) and Timber canyons. River in doubt. The North Horn is absent in the south where The massive conglomerate which appears the Flagstaff limestone rests directly on the in the southern part of the area is not Con- Price River formation. It is present, however, tinous along the outcrop with conglomerates in the north where it is in angular discordance which are considered Price River in other with the Indianola conglomerate. The best areas; however, it is believed to represent the exposures of the North Horn are along the Price River inasmuch as it occurs beneath south side of Little Salt Creek Canyon. strata which must certainly represent the North The North Horn formation in general repre- Horn formation. The angular unconformity sents an alternation between fluviatile and between the Price River and the North Horn, lacustrine conditions (Spieker, 1946, p. 133). in the west-central part of the Gunnison It includes an assortment of lithologic types Plateau, is probably local and of no great time which consist of variegated shales with sand- significance. stones, conglomerates, and fresh-water lime- stones (Fig. 2). Cretaceous-Tertiary System In the vicinity of Hell's Kitchen and Timber canyons, the basal unit of the North Horn is a North Horn formation.—-This formation massive conglomerate about 100 feet thick. was defined by Spieker (1946, p. 132) from Pebbles of white quartzite and dark blue-gray exposures in the east-central part of the and yellow limestone are abundant. Overlying Wasatch Plateau. The unit was originally this unit is about 85 feet of light-gray arenace- considered as the lower member of the Wasatch ous limestone with at least one bed of conglom- formation. The North Horn formation occurs erate lithologically similar to the basal clastic in angular discordance with the underlying unit. In these canyons the upper part of the

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North Horn consists largely of light-gray The North Horn formation lies below the medium- to coarse-grained sandstones, and Flagstaff limestone as recognized throughout light-gray irregularly bedded limestones. Some the Gunnison Plateau and above conglom- beds of limestone in the upper part are mottled erates assignable to the Price River formation red, yellow, and lavender. This lithologic type in areas where it is not in angular discordance is typical of the basal part of the Flagstaff with older formations. limestone into which the underlying North In the eastern part of the Wasatch Plateau, Horn grades. the North Horn formation ranges from Upper In the northern part of the area, the North Cretaceous to Paleocene in age. Spieker Horn formation consists dominantly of red (1949, p. 27) believes that west of this area it calcareous siltstone, some gray to red sandstone, is younger, but probably not younger than conglomerate, and red shale. Mottled red and Paleocene. gray limestone identical with that found in the Flagstaff also occurs near the base. The Tertiary System North Horn formation locally pinches out over the Indianola conglomerate in this area. Flagstaff limestone.—The Flagstaff limestone Great variations in thickness characterize was originally defined as the middle member the North Horn formation. In the northeastern of the Wasatch formation (Spieker and Ree- corner of the area, the formation is relatively side, 1925, p. 448^149) but was later raised thin and consists of yellow-brown friable to formational rank (Spieker, 1946, p. 135). sandstone, some shale, and conglomerate. It The type locality is Flagstaff Peak in the thickens rapidly southward to 422 feet in southern part of the Wasatch Plateau. The Little Salt Creek Canyon where it is dominantly Flagstaff limestone occurs over most of the red siltstone, gray sandstone, conglomerate, north half of the area, although at most places and red shale. In Timber Canyon there is at it is covered by the Colton and Green River least 500 feet of strata which consist chiefly formations. In the southeast the Flagstaff of gray limestone and sandstone with algal forms prominent dip slopes on the plateau nodules, in addition to beds of conglomerate upland. in the lower part. The North Horn thins In the northern part of the area the Flagstaff southward from Timber Canyon and is not limestone is characterized by mottled gray, found in the south. yellow, and red argillaceous and sandy lime- In the northern part of the area the North stone (Fig. 2). The uppermost limestone bed, Horn formation overlies with angular dis- about 10 feet thick, is unusually persistent and cordance the Indianola conglomerate and the traceable throughout the northern half of the Twist Gulch member of the Arapien shale. area and beyond. It is a thin-bedded, light- The Price River formation, which normally gray, dense limestone which contains gastro- underlies the North Horn in central Utah, pods, ostracods, and veins of calcite. In Criss is not recognized in this locality. The North Canyon the upper half of the formation is Horn formation is in angular discordance with dominantly yellow calcareous sandstone with the underlying Price River formation in the some yellow limestone. The persistent lime- southern part of the area. The contact between the North Horn formation and the overlying stone unit nevertheless occurs at the top of the Flagstaff limestone is both conformable and section. A section, measured in Bear Canyon, gradational (Fig. 2). In the north the contact is typical of the Flagstaff in the northern half is arbitrarily placed above the dominantly of the area. red clastic sediments and below the strata which are dominantly limestone. In the south a SECTION No. 5, BEAR CANYON considerable thickness of red arenaceous lime- Section of North Horn formation and Flagstaff stone is included in the basal part of the Flag- limestone on the spur along the south side of Bear staff. The North Horn strata below this basal Canyon unit are characterized by light-colored massive Colton formation beds of sandstone and limestone. Flagstaff limestone

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In Timber Canyon the basal part of the overlies the Indianola conglomerate. Here the Flagstaff is represented by about 190 feet of basal conglomerate of the Flagstaff consists red irregularly bedded limestone. There are dominantly of cobbles of red and green quartzite also several persistent sandstone layers, some and only rarely contains limestone cobbles of which contain conglomeratic lenses. South- similar to those found in the underlying ward the basal part consists chiefly of irregu- Indianola. Possibly this conglomerate repre- larly bedded arenaceous limestone which com- sents the North Horn formation. monly weathers to a mottled yellow, or red The Flagstaff limestone of the Wasatch and yellow. The upper part of the Flagstaff Plateau was thought by Spieker (1946, p. 136; between Timber Canyon and the southern 1949, p. 32-33) to be probably Paleocene. margin of the area consists largely of light- Recent studies of the molluscan fauna by La gray limestone with some gray sandstone. Rocque (1951, p. 1457) indicate it is Paleocene The greatest thickness of the Flagstaff and Eocene (?). The similarity of the strati- limestone measured in the northern part of the graphic succession of the Gunnison Plateau area occurs in Criss Canyon where about 730 to that of the Wasatch Plateau suggests that feet is found. Northward the formation thins the age is probably not greatly different in the to about 305 feet in the vicinity of Bear Canyon Gunnison Plateau. and thence to about 25 feet in the northeastern Colton formation.—The Colton formation corner of the area. Southward from Criss was defined by Spieker (1946, p. 139) as equiva- Canyon it thins to about 650 feet in Timber lent to the original upper member of the Wa- Canyon and finally to about 500 feet in Mellor satch formation. At the type locality near Canyon near the southern margin of the area Colton, Utah, it comprises 1500 feet of sand- (PL 2, fig. 3). Between Mellor and Timber stone and red to variegated shale which inter- canyons, the thickness of limestone above the tongues westward with lacustrine beds of the basal red arenaceous limestone apparently is Green River formation. The Colton formation essentially uniform, although the basal unit is is widely distributed in the eastern part of the about 360 feet thick in Timber Canyon and area and also occurs near the southwestern only about 60 feet thick in Mellor Canyon. A margin. In the northwest, however, the Colton section of the Flagstaff limestone from Mellor could not be differentiated from the Green Can}'on appears with the section of the Price River formation. River formation. The Colton formation consists typically of The red and yellow fades of the Flagstaff gray, green, and red variegated shales; coarse- is developed notably in the northern part of grained green-buff sandstone; light-gray me- the area and is not found immediately to the dium-grained or brown sandstone; and some east or north in either the Gunnison or the dark- to light-gray dense limestone. Wasatch plateaus. The Flagstaff in the Wasatch About 600 feet of Colton occurs near the Plateau typically consists of light-gray fresh- head of Criss Creek and about 350 feet near water limestone and gray shale. It ranges in the southeastern corner of the area. thickness from 300 to 1500 feet. The Colton formation is conformable with The Flagstaff limestone in this area is con- the Flagstaff limestone below and the Green formable with the overlying and underlying River formation above. The lower contact is formations except in the northern part where quite distinct; the upper contact is probably it is locally in angular discordance with the not mapped everywhere at the same horizon underlying Indianola conglomerate (Fig. 2). as considerable intertonguing with the Green Throughout most of the area, the contact with River formation is evident. the overlying Colton formation is quite dis- The Colton formation in the Wasatch Plateau tinct. In the northwestern part, however, is probably lower Eocene (Spieker, 1946, p. where the Colton formation cannot be separated 139; 1949, p. 34), and it is unlikely that the from the Green River formation, the upper age differs appreciably in the west-central contact of the Flagstaff can nevertheless be Gunnison Plateau. readily placed. In the northeastern part of the Green River formation.—The Green River area only a relatively thin section of Flagstaff formation was named by Hayden (1869, p.

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Thickness Thickness (Feet) (Feet) (28) Sandstone, gray yellow green, fine- (1) Covered interval ...... 60.0 grained, iron stain on weathered sur- face; shale ...... 27 .5 Total Green River formation .... „, , . . ,„ ~ (tawny fades) exposed ...... 994.0 (27) Shale, green gray, clayey, calcareous ---- 10.0 (26.,,). TLimestone . , ..ligh. t gray ...... c5. „0 Twelvemile Canyo' n member of Arapien shale (25) Shale, like Unit 6 ...... 10.0 The lower contact of the Green River (24) Conglomerate, cobbles of red and gray formation with the Colton is conformable, quartzite (average diameter 3 inches), although considerable intertonguing occurs. forms cliff ...... 15.0 ^^ ^ ^^ formation jg not differentj_ (23) Sandstone, light green gray, calcareous, t d h G RJ fo be conform. fine-gramed ...... 1.0 ' ^f , ., ... able with the Flagstaff limestone although (22) Shale and siltstone, light green gray 11,11. * i. u and mottled with light brown, cal- locally there must have been some erosion. careous; limestone; sandstone; clay- Near the lower part of Salt Creek Canyon, stone, calcareous ...... 35.0 the tawny fades overlies the Twelvemile (21) Limestone and sandstone, tawny; shale. 50.0 Canyon member of the Arapien shale with (20) Sandstone, tawny, calcareous; lime- angular discordance. South and east, the stone; shale calcareous, plant fossils; formation (Tertiary) overlies conglomerate, lenticular bed 5 feet c * H u ^ }' in thickness ...... 84.0 the Green River (Spieker, 1949, p. 36). (19) Shale, gray, calcareous; limestone, In the northern part of the area a few gastro- light gray, dense ...... 28.0 pods were found in that part of the Green (18) Sandstone, tawny, friable, coarse- River formation below the tawny facies. grained to grit ...... 6.0 Fossil rushes, ferns, and a few deciduous (17) Sandstone and shale, plant fossils ..... 13.0 leaves were found in the tawny facies along (16) Shale, green gray; sandstone, gray, Little Salt Creek Canyon. Some ostracods abundant plant fossils ...... 11.0 were also found in the lower part of the tawny (15) Limestone and sandstone, plant fossils facies, and a small gastropod, Helicina ? (leaves, branches, twigs) ...... 3.0 sp. (A. La Rocque, personal communication), (14) Shale, like Unit 13 and Unit 19 ...... 6.0 was found m tne upper part. (13) Sandstone, like Unit 15 ...... 3.0 Bradley (1931, p. 9) considers the Green (12) Shale, light olive gray, calcareous; shale, River formation in Wyoming and northeastern like Unit 13 ...... 4.0 Utah as middle Eocene. It is probably the (11) Shale, dark gray green, calcareous ..... 1.0 same age in central Utah because the fossils (10) Sandstone, tawny, calcareous; shale, found there do not differ from those described green gray ...... 4.5 by Bradley (A. La Rocque, personal com- (9) Shale, gray blue green to gray; silt- munication) . Therefore, the Green River stone, arenaceous, calcareous ...... 8.0 formation in this area fe considered middle (8) Sandstone, gray and brown; limestone; Eocene shale ...... 10.0 (7) Covered interval ...... 25.0 IGNEOUS ROCKS (6) Shale, gray green and mottled yellow brown; siltstone; abundant plant In the northwestern part of the area small 08 ...... intrusive bodies of monzonite porphyry are (5) Limestone, gray; shale, green gray, Arapien shale. The structural clayey, calcareous ...... 4.0 foun. ^d. in, th. e , , ^ . relationships of these masses to the surround- 3.5 ing sediments are generally obscured, although along Deep Creek and Little Salt Creek canyons (3) Sandstone, tawny, calcareous; sand- ^ 11- , • , -n A , • , stone, green gray,fine-grained ,con - two bodles are obviously sills. A third mass, glomeratic in part ...... 4.0 which occurs in Maple Canyon in the north- (2) Conglomerate, gray, cobbles of green central part of the area, appears to be dis- and red quartzite and limestone ..... 1.0 cordant. The age of the intrusions is not known ;

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however, they have not been found to intrude shale (PI. 2, fig. 1; PI. 1, structure section beds younger than the Arapien shale. A-B). The Flagstaff is nearly horizontal on the The monzonite porphyry is composed of east side of Beckers Hollow, south of Criss phenocrysts of hornblende in a groundmass Canyon (PI. 3, fig. 2). Immediately to the consisting largely of orthoclase and plagioclase. west across Beckers Hollow, however, is a The orthoclase is sericitized, and the plagio- down-faulted block of the Green River clase is extensively altered. Chlorite occurs as formation with a westward dip. Between Criss an alteration product of biotite and to a lesser Creek and Timber Canyon, the Flagstaff extent of hornblende. Apatite is a principal limestone and the Green River formation accessory of the rock in addition to biotite, dip 13-17° W. although, north of Maple Creek, muscovite, and sphene. Specular hematite and the Green River dips about 26° W. on the magnetite are also abundant. flank of the monocline. In a horizontal distance of 1500 feet down dip, at this point, the Green STRUCTURE River changes abruptly to vertical and over- turned attitudes. In the lower part of Mellor Regional Structural Relations Canyon, near the western margin of the area, The Gunnison Plateau is in the area of the Flagstaff limestone dips about 9° W. over structural transition between the Colorado the Price River formation (PI. 1, structure Plateaus and the Great Basin (Spieker, 1949, section C-D). p. 5; Fig. 1). The major structural feature of The structure of the plateau upland is the plateau is a broad asymmetric syncline relatively simple. In general, the beds dip only with the steeper limb on the east. This structure about 2-6 E. and SE. Near the western margin plunges southward from near the northern end of the area, a few normal faults of no great of the plateau and extends beneath the alluvium displacement seem to parallel the faults as- of the Sevier Valley in the vicinity of Gunni- sociated with the monocline. son, Utah. Both the eastern and western The monocline is broken by a major fault margins of the plateau seem to be denned by in the vicinity of Timber Canyon and north- major faults, although a conspicuous west- ward, near the western margin of the plateau dipping monocline appears along part of the (PL 1). This fault is well exposed on the north western margin. This monocline and related side of the lower part of Hell's Kitchen Canyon. features are similar to the structure of the At this point, a limestone characteristic of the western margin of the Wasatch Plateau. upper part of the Green River formation directly opposes a reddish-gray conglomerate Major Structural Features characteristic of the basal exposed part of the Price River formation. Stratigraphic displace- Monocline and associated structures.—A west- ment is about 1800-2000 feet. This fault paral- dipping monocline which involves the Flagstaff lels the outcrop of the Price River conglom- limestone and related formations extends along erate south of Hell's Kitchen Canyon and the western margin of the Gunnison Plateau apparently extends beneath the alluvium just from Timber Canyon northward to the vicinity north of Mellor Canyon. of Buck Canyon, north of Little Salt Creek Graben.—-A major graben occurs on the (PL 1). This structure is not found near the shoulder of the west-dipping monocline in the northern boundary of the area because the western part of the area (PL 1). It extends younger rocks involved in the folding have been removed by erosion, and the older folded from Criss Creek south to the boundary of the sequence is exposed. In the vicinity of Buck area. It is generally 1 mile, or slightly more, Canyon, the Flagstaff limestone on the mono- in width and about 11 miles long. At the north- cline changes in attitude from nearly horizontal ern extremity this graben trends nearly north. to a dip of 31° W. South of Little Salt Creek, South near Maple Creek, the trend changes to the Flagstaff and the North Horn dip 15° W. about N. 24° E. and, near Hell's Kitchen over the Indianola group and the Arapien Canyon, it again changes to N. 7° W.

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In the vicinity of Criss Creek (PI. 3, fig. 2) istic of the upper part of the Green River the fault along the eastern margin of the opposes the Price River conglomerate near graben has a displacement of about 800-1000 the base of the escarpment. The displacement feet. Green River beds west of the fault are of this fault appears to decrease southward

WE w Tgr /

Tf Ja

KTnh I KTnh / ' Ja

2 — High - ongle faulting and erosion 4 — Flexing of monocline and formation of reverse drag E W E W

Tf Tgr

KTnh Tf Ja Ja KTnh

— Pre-Green River relations 3 — Deposition of Green River over fault (Colton not differentiated)

FIGURE 3.—SCHEMATIC DIAGRAMS SHOWING DEVELOPMENT OF PRE-GREEN RIVER FAULT

found 760 feet below the top of the Flagstaff and at Mellor Canyon, near the southern on the east. In the region of Timber Canyon boundary of the area, it is about 1000 feet as (PI. 3, fig. 1), the displacement may be as much indicated by a down-faulted block of the as 1700-1800 feet where a limestone character- Flagstaff limestone. Field relationships indicate

PLATE 2.—ANGULAR UNCONFORMITIES FIGURE 1.—ANGULAR UNCONFORMITY, LOOKING SOUTHWEST, FROM LITTLE SALT CREEK The tawny fades of the Green River formation, nearly horizontal, overlies the east-dipping Twelvemile Canyon member of the Arapien shale in the northwestern part of the area. FIGURE 2.—ANGULAR UNCONFORMITY, LOOKING NORTH, ON THE NORTH SIDE OF HELL'S KITCHEN CANYON Angular discordance is evident between the massive conglomerates of the Price River formation, in the lower part of the cliff, and the North Horn beds above, in the central part of the area. Both units dip toward the east. FIGURE 3.—ANGULAR UNCONFORMITY, LOOKING NORTH, ON THE NORTH SIDE OF MELLOR CANYON The Flagstaff limestone overlaps the east-dipping Price River conglomerate in the southern part of the area.

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------ANGULAR UNCONFORMITIES

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------GRABEN AND MONOCLINE

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that the fault along the western margin of the 1500 feet. Probably these units were truncated graben is of less displacement. and eroded on the west of the fault prior to The graben floor is 800-1000 feet lower Green River time (Fig. 3). The Green River topographically than the plateau upland and formation, east of the fault, directly overlies is fractured in a complex pattern. A remarkable the Flagstaff limestone since the Colton forma- series of west-dipping fault blocks of the tion is not present in the area. West of the Colton and Green River formations occur fault, the Green River formation is found in in the graben near Hell's Kitchen and Timber angular discordance with the Arapien shale canyons (PI. 3, fig. 1). In this area the dis- (PL 2, fig. 1). The faulting is therefore dated placements are about 150-200 feet, and the as post-Flagstaff and pre-Green River. blocks dip about 34° W. Northward, similar Reverse drag, apparent on the fault, might small tilted fault blocks of the Green River have resulted either from compression or occur on the graben floor. subsequent movement on the fault. Pre-North Horn structure.—In the northern part of the area, more than 7000 feet of pre- GEOLOGIC HISTORY North Horn strata is present. These include the Indianola conglomerate and the Arapien The major geologic events, which are evident shale. Except for the lower part of the Arapien in the west-central part of the Gunnison shale, which is highly contorted, these units Plateau, are similar to those outlined by strike generally NE. and dip about 40° SE. Spieker for the geologic history of central In Little Salt Creek, however, the Indianola Utah (Spieker, 1949), except for the pre- conglomerate and the Twist Gulch member, North Horn post-Price River angular un- of the Arapien strike N. 60° E. and dip 40° SE. conformity, which is not found elsewhere. In the central part of the area, on the north Deposition of nearly 4500 feet of thin-bedded side of Hell's Kitchen Canyon, the Price River argillaceous marine limestone, gray shale with conglomerate dips eastward at a slightly greater interbedded sandstone and gypsum, and red angle than the overlying North Horn (PI. 2, sandstone and siltstone occurred in Late fig. 2). The Price River, at this point, dips Jurassic time. These strata constitute the about 5° E. In Mellor Canyon, near the south- Arapien shale. Coarse boulder conglomerate, ern boundary of the area, the Price River 7000 feet or more thick, assigned to the Upper strikes nearly N.-S. and dips about 20° E. Cretaceous Indianola group, follows this suc- (PI. 2, fig. 3). cession without evidence of widespread dis- Pre-Green River fault.—Near the mouth of conformity. The lower portions of the conglom- the north fork of Little Salt Creek, the North erate may be Lower Cretaceous, or perhaps Horn formation and the Flagstaff limestone Upper Jurassic. At least 7000 feet of conglom- overlie the Arapien shale in angular discordance erate is found in the mapped area, making a on the east side of a normal fault; these units total thickness of about 11,500 feet of strata are not found west of the fault (PI. 1, structure below the angular discordance with the Price section A-B). The Flagstaff dips steeply toward River formation. the fault, but the actual truncation of these The early Laramide orogeny (Spieker, beds by the fault is obscured. It is unlikely 1947, p. 149, 152-155), widely recognized in that 455 feet of Flagstaff and North Horn central Utah, is dated by Spieker between could pinch out in a horizontal distance of middle and late Montana times. It is marked

PLATE 3.—GRABEN AND MONOCLINE FIGURE 1.—GRABEN, LOOKING NORTHWEST, FROM JUST SOUTH OF HELL'S KITCHEN CANYON Note the fault blocks of the Colton and Green River formations on the graben floor between scarps of the Price River formation. FIGURE 2.—MONOCLINE, LOOKING SOUTH, FROM CRISS CANYON Small antithetic faults cut the west-dipping monocline on the right. The north-south marginal fault of the graben is at the left between a barren cliff and the brush-covered hills.

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by an angular unconformity in the northern believes that the flexing and faulting of the part of this area. The Price River formation, Wasatch monocline, east of the Gunnison which appears to be the oldest unit above the Plateau, was initiated between late Eocene unconformity, is not present here, and where and Miocene (Spieker, 1949, p. 80). The Crazy it does appear in the southern part of the area Hollow formation of post-Green River age, the Indianola group does not outcrop. Never- however, is involved in the folding of the theless, the Arapien shale, the Twist Gulch Wasatch monocline but is not present in this member of the Arapien shale, and the Indianola area. The similarity of the Wasatch monocline group were extensively folded prior to the dep- and the monocline on the western side of the osition of the Price River, and subsequent erosion produced considerable relief. The Price Gunnison Plateau suggests the two monoclines River formation was probably not deposited are probably of the same age. in the northern part of the area, although considerable thicknesses were deposited in RE TERENCES CITED adjacent areas to the northeast and south. A Bradley, W. H., 1931, Origin and microfossils of portion of the northern area remained positive the oil shale of the Green River formation of through North Horn (Upper Cretaceous- Colorado and Utah: U. S. Geol. Survey Prof. Paleocene) time and through most of Flag- Paper 168, 58 pages. Dutton, C. E., 1880, Report on the geology of the staff time (Paleocene-Eocene). Thus, in the high plateaus of Utah: U. S. Geog. Geol. Sur- northeastern part of the area, only the upper- vey Rocky Mountain Region, 307 pages. Gilliland, W. N., 1951, Geology of the Gunnison most beds of the Flagstaff limestone were quadrangle, Utah: Univ. Nebr. Studies, new deposited over the Indianola. ser., no. 8, 101 pages. In the southern part of the area, a thick Hardy, C. T., 1952, Eastern Sevier Valley, Sevier and Sanpete counties, Utah, with reference section of the Price River conglomerate is to formations of Jurassic age: Utah Geol. overlain by the North Horn formation in Mineralog. Survey Bull. 43, 98 pages. angular discordance. The North Horn forma- Hayden, F. V., 1869, Preliminary field report in the United States Geological Survey of Colo- tion, however, wedges out toward the south rado and New Mexico: U. S. Geol. Geog. Survey so that only the upper Flagstaff beds are above Terr., 3rd Ann. Rept., 155 pages. the folded Price River conglomerate. This La Rocque, A., 1951, Molluscan fauna of the Flag- staff formation, central Utah (Abstract): Geol. movement is later than the early Laramide Soc. America Bull., v. 62, p. 1457-1458. orogeny established by Spieker (1946, p. 152- Meinzer, 0. E., 1911, Ground water in Juab, Millard, and Iron counties, Utah: U. S. Geol. 155) and appears to have preceded the pre- Survey Water-Supply Paper 277, 162 pages. Flagstaff movement described by Spieker Schoff, S. L., 1951, Geology of the Cedar Hills, (1946, p. 155). Where the North Horn forma- Utah: Geol. Soc. America Bull, v. 62, p. 619- 645. tion is present it is conformably overlain by the Spieker, E. M., 1946, Late Mesozoic and early Flagstaff. Erosion, which followed the pre- Cenozoic history of central Utah: U. S. Geol. Green River faulting in the vicinity of Little Survey Prof. Paper 205-D, p. 117-161. 1949, The transition between the Colorado Salt Creek, locally removed the Flagstaff and plateaus and the Great Basin in central Utah: North Horn. Thus the Green River formation Utah Geol. Soc. Guidebook 4, 106 pages. Spieker, E. M., and Reeside, J. B., Jr., 1925, overlies the older folded beds in places. Deposi- Cretaceous and Tertiary formations of the tion of the Colton and Green River formations Wasatch Plateau, Utah: Geol. Soc. America followed in a fresh-water lake. Bull., v. 36, p. 435-454. The folding and faulting that affects the UTAH STATE AGRICULTURAL COLLEGE, LOGAN, Green River formation cannot be dated from UTAH; U. S. GEOLOGICAL SURVEY, WASHINGTON, D. C. evidence in this area because beds younger MANUSCRIPT RECEIVED BY THE SECRETARY OF THE than Green River are not present. Spieker SOCIETY, OCTOBER 17, 1952

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