Geyser Creek

Fanglomerate (Tertiary)

La Sal Mountains, Eastern Utah

By W. D. CARTER and J. L. GUALTIERI

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1224-E

Stratigraphic and structural history 

of the Geyser Creek Fanglomerate in 

the La Sal Mountains region
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

Thomas B. Nolan, Director

U.S. GOVERNMENT PRINTING OFFICE : 1965

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

Washington, D.G. 20402 - Price 15 cents (paper cover)
CONTENTS

Page

Abstract___________________________.__---_____.______ El

Introduction._____________________________________________________ 1

Geologic setting.__________________________________________________ 2

Type section._____________________________________________________ 4

Stratigraphic relations_____________________________________________ 8

Structural relations------_-_______--_--___-_------_____--_----_--_- 10

Age-__-__-----------------__---_-_------------__-----__-__------- 11

References. _-____________________-_---____-__-_-___________-_-_-_- 11

ILLUSTRATIONS

Pnge 

FIGURE 1. Generalized structure map showing location of Pine Flats area,

Utah_______________________________._________________ E3

2. Geologic map showing distribution and type section of the

Geyser Creek Fanglomerate.___________________________ 7

in

CONTRIBUTIONS TO STRATIGRAPHY

GEYSER CREEK FANGLOMERATE (TERTIARY), LA SAL 

MOUNTAINS, EASTERN UTAH

BY W. D. CARTER and J. L. GUALTIERI

ABSTRACT

The Geyser Creek Fanglomerate, a name here applied to an extensive conglomerate deposit of Tertiary age, crops out within the Taylor Creek syncline on the 

southeast flank of the North Mountain group of the La Sal Mountains. The 

formation, ranging in thickness from very thin to 1,000 feet, consists mainly of 

well-consolidated, yellowish-brown conglomerate and sandstone in lenticular 

beds. Pebbles, cobbles, and boulders derived from the igneous rocks and 

upturned sedimentary rocks of North Mountain compose the conglomerate.

The formation unconformably overlies the Mancos Shale of Late Cretaceous 

age and is unconformably overlain by the Harpole Mesa Formation of early 

Quaternary age. Tilting of the beds and angular unconformities, which mark 

the upper and lower contacts, indicate that the fanglomerate was folded prior 

to glaciation in the mountains. The fanglomerate is considered to be of probable 

Pliocene age and correlative with a similar conglomerate in Castle Valley.

INTRODUCTION

Fanglomerate deposits, of probable Tertiary age, have been reported 

on the north and west flanks of the La Sal Mountains of east-central 

Utah. Baker (1933, p. 56) described conglomerate near Pack Creek 

in San Juan County, Utah, as a consolidated mixture of angular to 

round pebbles, cobbles, and boulders of igneous and sedimentary 

origin, resting on folded Mancos Shale of Late Cretaceous age. 

Because it is lithologically different from more recent alluvial deposits 

in the Moab Valley and because it lies on Mancos Shale in the ridge 

between Cane and Pack Creeks, Baker tentatively assigned the conglomerate at Pack Creek to the Tertiary System.

In 1952, Hunt found a similar deposit of late Pliocene or early 

Pliocene age in Castle Valley, Grand County, Utah (Hunt, 1958, p. 

314). The north end of that deposit dips about 17° S. and rests unconformably on the salt-anhydrite core of the Castle Valley anticline. 

In contrast, the southernmost exposures of the deposit are nearly

El
E2 CONTRIBUTIONS TO STRATIGRAPHY

vertical or dip steeply to the north near the igneous rocks of North 

Mountain. Hunt attributed the folding of the conglomerate to collapse as a consequence of solution and removal of the underlying salt 

during the Pleistocene Epoch.

Kichmond (1962, p. 25-34), in studies of the Quaternary stratigraphy of the La Sal Mountains, found that the oldest deposit of 

undoubtedly Quaternary age, the Harpole Mesa Formation, consists 

of poorly consolidated gravel and boulder fans, till, and eolian sand 

and silt, all of which dip away from the three laccolithic centers of the 

mountains. He found that the Harpole Mesa Formation was younger 

than the Castle Valley f anglomerate deposits but lithologically similar 

to and correlative with the conglomerate at Pack Creek. He therefore 

included the latter in the Harpole Mesa Formation.

In 1956 during the course of a geologic study of the eastern La Sal 

Mountains area, made on behalf of the U.S. Atomic Energy Commission, the authors found fanglomerate deposits on the southeast flank 

of North Mountain near Geyser and Deep Creeks that are similar to the 

deposit described by Hunt. This report names and defines the Geyser 

Creek Fanglomerate.

GEOLOGIC SETTING

Gently folded strata surround the laccolithic igneous cores of the 

La Sal Mountains in the northeastern part of the Colorado Plateau 

(fig. 1). These strata, ranging in age from Pennsylvania!! to Late 

Cretaceous, are separated by unconformities that record a long history 

of continental deposition interrupted by periodic folding and marine 

invasions. The most recent transgression took place during Late 

Cretaceous time and was followed by uplift, folding of the sedimentary 

rocks, and intrusion of the igneous rocks of the La Sal Mountains. 

Mountain building was 'accompanied by erosion that established the 

major drainage pattern and formed alluvial fans on the mountain 

flanks. Glaciers steepened the topography of the upper valleys and 

deposited moraine and outwash in the lower valleys of the plateau 

surrounding the mountains. Postglacial erosion has cut deep valleys 

through the glacial cover and has exposed the underlying strata.

Surrounding the La Sal Mountains are a group of parallel northwestward-trending folds. The most conspicuous are breached 

anticlines marked by large elongate valleys surrounded by steep cliffs 

in which the stratigraphy is well displayed. Angular unconformities 

along the margins of the anticlines indicate repeated folding due to 

plastic flow of thick saline strata of Pennsylvanian age that make up 

the cores of the folds. In places, irregular bodies of contorted salt, 

anhydrite, and limestone crop out in the valley floors. Solution and
GEYSER CREEK FANGLOMERATE, EASTERN UTAH

109°

E3

108°

38'

FIGURE 1. Generalized structure map of the northeastern part of the Colorado 

Plateau showing location of the Pine Flats area, Utah.

removal of the upper parts of the saline cores of the anticlines during 

late Cenozoic time resulted in collapse of overlying beds. Between the 

anticlines are broad synclines that in some areas have become deeply 

dissected to form high mesas separated by narrow canyons.

The laccolithic La Sal Mountains are divided into three topographic 

groups: North Mountain, Middle Mountain, and South Mountain. 

The igneous cores of the North and South Mountain groups were 

intruded along two anticlines and uplifted upper Paleozoic and 

Mesozoic sedimentary rocks that were subsequently exposed by erosion. 

The Middle Mountain intrusives, however, are in what was once a 

broad intervening syncline and have uplifted and exposed upper
E4 CONTRIBUTIONS TO STRATIGRAPHY

Triassic, Jurassic, and Cretaceous strata. It is here that the laccolithic nature of the intrusives is best displayed. Narrow subsidiary 

synclines now separate Middle Mountain from the other groups.

TYPE SECTION

The Geyser Creek Fanglomerate is herein named for Geyser Creek, 

and eastward-flowing tributary to Roc Creek. The formation consists 

of fanglomerate and sandstone, which are best exposed in low escarpments along Deep Creek and Geyser Creek near their junctions with 

Roc Creek. The distribution of the formation and its probable south 

limit are shown in figure 2. Between the Deep Creek and Geyser 

Creek canyons and also in areas to the north and south, the formation 

is covered by terrace gravels of the Harpole Mesa Formation.

The formation is best exposed at the type section (fig. 2) , measured 

on the north side of Deep Creek, about 2,000 feet west of its junction 

with Taylor Creek. It crops out as a distinctive yellowish-brown rock 

in a cliff that marks the east limit of the deposits and stands above 

smooth slopes of dark-grayish-green Mancos Shale. The following 

measured section is typical of the formation.

Type section of the Geyser Creek Fanglomerate as measured in the north side of 

Deep Creek in NEty sec. 36, T. 26 8., R. 25 E., Grand County, Utah

Harpole Mesa Formation (dips gently to east).

Angular unconformity. Thickness

Geyser Creek Fanglomerate (dips 16°-25° W.) : (feet)

14. Conglomerate and quartz sandstone, yellowish-brown; mediumto coarse-grained matrix weakly cemented by calcium carbonate. 

Conglomerate fragments are angular to subrounded pebbles and 

boulders of sandstone and diorite ; they are as much as 18 in. 

long. Near base several cobbles averaging 3 in. in length are 

weathered to powdery limonite ; thin calcite crust marks contact 

of cobble with surrounding conglomerate 7.5

13. Sandstone, grayish-orange-pink, very fine to coarse-grained, poorly 

sorted. Composed mainly of quartz (80-90 percent), sparse 

black and brown chert; angular to subangular fragments of 

calcite and aragonite ___ ______ ___ ___ _ _ _________ _ 1. 5

12. Conglomerate similar to unit 14. Boulders of diorite and sandstone as much as 12 in. long, 6 in. wide, 5 in. thick. Most 

sandstone boulders are gray, very fine to fine grained, massive, 

and closely resemble Wingate Sandstone. Upper 5 ft partially 

covered ____________ ____________________ 8

11. Covered interval 16. 5
GEYSER CREEK FANGLOMERATE, EASTERN UTAH E5

Thickness Geyser Creek Fanglomerate (dips 16°-25° W.) Continued (feet)

10. Conglomerate and quartz sandstone, light-brown, fine- to coarsegrained, poorly sorted; matrix weakly cemented by calcium 

carbonate. Boulders, subangular to well-rounded, are as much 

as 24 in. in diameter. About 30 percent of the boulders is 

diorite. Another 30 percent includes both lavender to reddishbrown sandstone (probably from Kayenta Formation), 

light-gray to light-brown fine-grained sandstone (Wingate Sandstone), and reddish-brown ito reddish-orange-brown silty sandstone (Chinle Formation). About 20 percent includes pebbles 

and boulders from younger formations, angular fragments of red 

chert (Summerville Formation), light-gray to light-brown 

fine- to medium-grain sandstone (Salt Wash Sandstone Member of Morrison Formation), white to light-gray fine-grained 

orthoquartzite (Burro Canyon Formation), light-green platy 

siltstone (Brushy Basin Shale Member of Morrison Formation, or Burro Canyon Formation), and dark-gray-green limestone (Mancos Shale). Remaining 20 percent not identifiable, 

mainly of sandstone and siltstone. Thin sandstone lenses also 

present____________________________ _ 24

9. Conglomerate composed mainly of pebbles and cobbles ranging 

from 6 to 12 in. in diameter. About 40 percent consists of 

unweathered igneous rock; remainder is sandstone and siltstone. 

Some silty sandstone pebbles have weathered to powdery 

limonite- _ __ ____________________ 2.5

8. Sandstone, light-brown, fine-grained to medium-fine-grained, poorly 

sorted, with calcium carbonate cement. Lenticular, containing 

sparsely scattered pebbles and coarse-grained sandstone along 

base of lenses. Coarser-grained sandstone is more firmly 

cemented and forms riblike projections on outcrops. Pebbles 

include partially weathered diorite and sandstone___ 9

7. Conglomerate, sandstone, and siltstone, yellowish-brown, lenticular. Conglomerate consists of weathered moderately rounded 

fragments, as much as 4 in. and averaging 2 in. in diameter, 

composed of diorite (about 60 percent) and reddish-brown and 

yellowish-brown (limonitic) sandstone and siltstone (about 40 

percent). Sandstone, medium-fine-grained to very fine grained; 

in places grades to siltstone, so poorly sorted that siltstone 

locally contains coarse sand grains. Calcium carbonate cement 

appears to be more abundant in sandstone lenses that project 

several inches from face of outcrop _________________ 9.5

6. Conglomerate consists of fragments, as much as 3 in. in diameter, 

composed chiefly of red sandstone and siltstone, in part weathered to powdery limonite and partly weathered diorite; matrix 

is very fine to very coarse grained sandstone, well indurated 

with calcium carbonate; a lens___________________ 1
E6 CONTRIBUTIONS TO STRATIGRAPHY
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E8 CONTRIBUTIONS TO STRATIGRAPHY

Thickness 

Geyser Creek Fanglomerate (dips 16°-25° W.) Continued (feet)

5. Conglomerate, yellowish-brown, crossbedded in lower part, flat 

bedded at top. Matrix poorly sorted yellowish-brown (limonitic) 

sandstone; weathered pebbles, cobbles, and boulders as much 

as 18 in. in length primarily of sandstone and siltstone. 

Diorite pebbles (about 10 percent) are less weathered. Basal 

contact sharp and marks an old channel scour____ 11. 5

4. Sandstone, conglomeratic, light-green to yellowish-brown; fine- to 

coarse-grained sandstone containing conglomerate as thin lenses. 

Sparse pebbles 2 in. in diameter. Limonite abundant in lower 

part of unit. Flat bedded and crossbedded-____________ 4. 5

3. Conglomerate, yellowish-brown, intensely stained 'by limonite. 

Angular to subangular fragments as much as 24 in. in diameter 

but mostly 2 to 3 in. in diameter, coarser in top 3 ft, composed 

mainly of poorly cemented to well-cemented siltstone and sandstone, flat bedded and crossbedded_ _______ _ 6

2. Siltstone, sandy, light-yellowish-green; with fragments of fossils 

and aragonite from Mancos Shale. Lower 9 in. are reddish 

brown (limonite), very friable___ ________-___ 6

1. Conglomerate, yellowish-brown, with rounded pebbles, cobbles, 

and blocks up .to 18 in. in length. Larger fragments mainly of 

sandstone and siltstone; sparse pebbles of diorite. Matrix sandy 

with scattered fragments of shell, aragonite, and calcite. Limonitic, friable sandstone at base of exposure. Basal contact with 

Mancos Shale covered by debris__________________ 10

Total thickness of Geyser Creek Fanglomerate_______ 117. 5 

Angular unconformity. 

Mancos Shale (dips45°-55° W.)

STRATIGRAPHIC RELATIONS

Mancos Shale of Late Cretaceous age is well exposed both east and 

west of the known limits of the Geyser Creek Fanglomerate. On the 

east., Mancos Shale dips 30°-55° SW., and the overlying Geyser Creek 

dips l7°-22° SW. Although the contact between the Mancos Shale 

and overlying Geyser Creek Fanglomerate is not clearly exposed, it 

can be located within a few feet. The greater dip of the Mancos beds 

and the presence of Mancos fragments in the reworked zone at the 

base of the Geyser Creek indicate that the east contact is an angular 

unconformity.

Near the flank of North Mountain, Mancos Shale and older formations are exposed west of the Geyser Creek Fanglomerate in the headwaters of Deep Creek Canyon. The contact of the Mancos Shale with 

the overlying Geyser Creek is covered by soil and vegetation and was 

located only approximately. Scattered outcrops, however, indicate 

that the Geyser Creek wedges out 2,000 feet east of ridges formed by 

the Dakota and Burro Canyon Formations near North Mountain and 

probably dips gently to the northeast.
GEYSER CREEK FANGLOMERATE, EASTERN UTAH E9

The measured section shows that the vertical distribution of rock 

fragments is more or less inverse to the local stratigraphy, so that 

progressively older sedimentary rocks of the hogbacks and finally the 

diorite core of North Mountain are represented in successively younger 

conglomerate layers. For example, in the lower units (1 and 2) the 

dominant rock types are shale, limestone, and fossil fragments from 

the Mancos which underlies and crops out along the margins of the 

fanglomerate. No fossil fragments of sufficient size or preservation 

were obtained to determine how many different horizons of the Mancos 

might be represented in the Geyser Creek Fanglomerate. Kemnants 

of Mancos decrease upward in the overlying units. Poorly cemented 

sandstones of the Dakota and Burro Canyon Formations probably 

contributed most of the quartz sand and chert pebbles that compose 

the sandy material of the fanglomerate units. More durable rocks 

from older formations were the principal source of blocks, cobbles, 

and pebbles of the conglomerate lenses. Diorite fragments are generally absent or sparse in the lower conglomerate lenses (units 1 

through 6). They show a marked increase and are most abundant 

(about 60 percent) in unit 7. In the upper units they generally 

compose about half the fragmental material. Unit 10 is the thickest 

(24 ft) and most complex of the sequence. Fragments of all formations exposed in the source area that contain rock types capable of 

surviving weathering and erosion appear to be represented. Coarse 

fragments in units 12 through 14 are mainly diorite and probably 

Wingate Sandstone. Such a vertical distribution of fragments was 

apparently caused by headward stream erosion in a restricted source 

area.

The Geyser Creek Fanglomerate is overlain by the Harpole Mesa 

Formation of early Quaternary age, which in this area consists of 

a thin mantle of poorly consolidated terrace gravels covering a slope 

that dips gently eastward from the mountain area. These gravel beds 

increase in thickness toward the mountains and cover the north and 

south limits of the Geyser Creek. The east contact of the two is 

an angular unconformity, for along its eastern exposure the Geyser 

Creek dips !7°-22° SW., in contrast to the gentle eastward dip of the 

overlying gravel beds. The upper contact of the Geyser Creek is 

not exposed along the west margin of the basin; probably Geyser 

Creek strata dip more steeply to the east than those of the overlying 

Harpole Mesa Formation.

The full thickness of the Geyser Creek Fanglomerate is not known, 

but it may range from 0 to as much as 1,000 feet thick. The deposit 

wedges out on the west along both Deep and Geyser Creeks. It is 

exposed in the side of Geyser Creek Valley (SE*4 sec. 1, T. 27 S., R.
E10 CONTRIBUTIONS TO STRATIGRAPHY

25 E., San Juan County, Utah) near the axis of a structural trough, 

where it is estimated to be at least 280 feet thick. There, however, the 

base of the formation is not exposed, and interpretation of the covered 

part suggests that the Geyser Creek Fanglomerate may be 600 to 1,000 

feet thick in the deepest part of the trough. On the east margin the 

formation is 120 feet thick at the type section.

STRUCTURAL RELATIONS

The Geyser Creek Fanglomerate crops out within the southern part 

of the Taylor Creek syncline, an open northwestward-trending fold 

approximately 5 miles long and 2 miles wide that lies between the 

North Mountain anticline on the west and the Sinbad Valley collapsed 

anticline on the east. The Valley View fault system, which marks 

the northeast side of the Paradox Valley collapse zone, extends northwestward through the junction of Geyser, Deep, and Taylor Creeks 

with Roc Creek. This fault zone dies out to the north in a monoclinal fold on the flank of the Sinbad Valley anticline. The fault and 

fold mark the east limit of the Taylor Creek syncline.

The outcrop pattern of the Geyser Creek-Fanglomerate suggests 

that the unit underlies an area that is approximately elliptical and 

fills a trough which conforms to the shape of the Taylor Creek syncline. Prior to folding and erosion, however, the unit probably was 

a fan-shaped deposit with an apex at the southeast end of North 

Mountain where prominent hogback ridges, mainly of sandstone, surround a diorite core. The hogbacks are cut by a deep water gap 

through which Deep Creek flows to the east. The water gap and the 

mountainous terrain within the hogbacks are the nearest available 

source and also the most probable source of the f anglomerate. Faulting, folding, and subsequent erosion have modified the shape of the 

fanglomerate deposit by removing both its apex on the west and its 

east margin, where it wedged out, presumably east of the Valley View 

fault.

West of the fault zone the Taylor Creek syncline is marked by 

outcrops of strata of Cretaceous age (Burro Canyon, Dakota, and 

Mancos Formations) which strike northwest and clip 30°-55° SW. 

The overlying Geyser Creek Fanglomerate conforms to the strike 

of the older strata but dips about l7°-23° SW.; that is, the dips are 

10°-15° gentler than those of the Mancos, and they are toward the 

source area rather than horizontal or dipping away from it. Angular 

unconformity between the Geyser Creek Fanglomerate and Mancos 

Shale therefore indicates that Geyser Creek deposition probably began 

after the Taylor Creek syncline had been partly developed. Mancos 

Shale and older Cretaceous strata probably dipped 20°-35° SW. along
GEYSE.R CREEK FANGLOMERATE, EASTERN UTAH Ell

the northeast margin of the syncline before the f anglomerate was deposited. Subsequently, increased folding of the syncline tilted both 

Cretaceous and Tertiary strata to their present attitudes. Folding 

probably was caused by solution and lateral migration of salt into 

the adjacent Sinbad and Paradox Valley anticlines.

AGE

The Geyser Creek Fanglomerate probably formed sometime between late Miocene and early Pleistocene time and is tentatively 

assigned to the Pliocene Epoch. It clearly is younger than Late 

Cretaceous, for the lowermost part of the f anglomerate contains shale 

and fossil fragments derived from the underlying Mancos Shale. No 

other fossils have been found. The presence of diorite fragments 

in the f anglomerate shows that the deposit is also younger than the 

La Sal Mountains intrusion to which Hunt (1956, p. 83) assigned a 

middle Miocene age. The Geyser Creek Fanglomerate is older than 

the Harpole Mesa Formation of early Quaternary age (Richmond, 

1962), by which it is overlain.

The Geyser Creek Fanglomerate correlates with the conglomerate 

in Castle Valley of Pliocene(?) age (Hunt, 1958). Although the two 

formations differ in lithology and in stratigraphic relation to underlying rocks, they are similar in that both are folded and are unconformably overlain by the Harpole Mesa Formation of early Quaternary age. Their differences are considered minor and dependent on 

local conditions in source areas and the basins in which they were 

deposited.

REFERENCES

Baker, A. A., 1933, Geology and oil possibilities of the Moab district, Grand and

San Juan Counties, Utah: U.S. Geol. Survey Bull. 841, 95 p. 

Hunt, C. B., 1956, Cenozoic geology of the Colorado Plateau: U.S. Geol. Survey

Prof. Paper 279, 99 p. 

 1958, Structural and igneous geology of the La Sal Mountains, Utah:

U.S. Geol. Survey Prof. Paper 294-1, p. 305-364. 

Richmond, G. M., 1962, Quaternary stratigraphy of the La Sal Mountains, Utah:

U.S. Geol. Survey Prof. Paper 324, 135 p.

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