neoproterozoic uinta mountain group of northeastern utah: pre...

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1

Dehler, C.M., Sprinkel, D.A., and Porter, S.M., 2005, Neoproterozoic Uinta Mountain Group of northeastern Utah: Pre-Sturtian geographic, tectonic, and biologic evolution, in Pederson, J., and Dehler, C.M., eds., Interior Western United States: Geological Society of America Field Guide 6, p. 1–25, doi: 10.1130/2005.fl d006(01). For permission to copy, contact [email protected]. © 2005 Geological Society of America

Geological Society of America

Field Guide 6

2005

Neoproterozoic Uinta Mountain Group of northeastern Utah:

Pre-Sturtian geographic, tectonic, and biologic evolution

Carol M. DehlerDepartment of Geology, Utah State University, 4505 Old Main Hill, Logan, Utah 84322, USA

Douglas A. SprinkelUtah Geological Survey, P.O. Box 14610, Salt Lake City, Utah 84114, USA

Susannah M. PorterDepartment of Earth Science University of California Santa Barbara, Santa Barbara, California 93106, USA

ABSTRACT

The Neoproterozoic Uinta Mountain Group is undergoing a new phase of strati-graphic and paleontologic research toward understanding the paleoenvironments, paleoecology, correlation across the range and the region, paleogeography, basin type, and tectonic setting. Mapping, measured sections, sedimentology, paleontology, U-Pb geochronology, and C-isotope geochemistry have resulted in the further characteriza-tion and genetic understanding of the western and eastern Uinta Mountain Group.

The Red Pine Shale in the western Uinta Mountain Group and the undivided clastic strata in the eastern Uinta Mountain Group have been a focus of this research, as they are relatively unstudied. Reevaluation of the other units is also underway. The Red Pine Shale is a thick, organic-rich, fossiliferous unit that represents a restricted environment in a marine deltaic setting. The units below the Red Pine Shale are dom-inantly sandstone and orthoquartzite, and represent a fl uviomarine setting. In the eastern Uinta Mountain Group, the undivided clastic strata are subdivided into three informal units due to a mappable 50–70-m-thick shale interval. These strata repre-sent a braided fl uvial system with fl ow to the southwest interrupted by a transgressing shoreline. Correlation between the eastern and western Uinta Mountain Group strata is not complete, yet distinctive shale units in the west and east may be correlative, and one of the latter has been dated (≤770 Ma). Regional correlation with the 770–742 Ma Chuar Group suggests the Red Pine Shale may also be ca. 740 Ma, and correlation with the undated Big Cottonwood Formation and the Pahrump Group are also likely based upon C-isotope, fossil, and provenance similarities. This fi eld trip will examine these strata and consider the hypothesis of a ca. 770–740 Ma regional seaway, fed by large braided rivers, fl ooding intracratonic rift basins and recording the fi rst of three phases of rifting prior to the development of the Cordilleran miogeocline.

Keywords: Neoproterozoic, Uinta Mountain Group, intracratonic rift, vase-shaped

microfossil, Bavlinella faveolata, Leiosphaeridia sp.

2 Dehler et al.

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INTRODUCTION

This fi eld trip guide is a review and an update of the existing

data sets regarding Uinta Mountain Group geology and reports

the latest ideas about depositional environments, correlation,

paleogeography, biologic evolution, and tectonic setting in north-

eastern Utah during Neoproterozoic time. The focus of the fi eld

trip will be on the previously understudied units (Red Pine Shale

and undivided eastern clastic strata), as well as a reevaluation of

previous interpretations about the other better-studied units, and

how all of these units relate to one another in time and space.

The Uinta Mountain Group is interesting for many reasons:

(1) it is one of few exposed strata in the region for understanding

the early tectonic evolution of the Late Neoproterozoic western

Laurentian margin; (2) it likely records the inception of climate

change leading into the low-latitude glaciations of the Sturtian

episode; and (3) it contains a wealth of microfossils throughout

the succession that can inform us of pre-Sturtian biologic evolu-

tion and how it may relate to (1) and (2) above. Lastly, (4) the

Uinta Mountain Group is a “sleeping giant” in terms of being

explored for its information on Precambrian geology. It has been

under the “curse of the Proterozoic sandstones” (Link et al.,

1993) for too long, and ongoing and future research will hope-

fully lift the curse.

A general overview of the Uinta Mountain Group is pro-

vided fi rst, followed by a two-day road log that takes the reader

clockwise around the Uinta Mountains. On Day 1, the western

and central strata of the north fl ank will be visited, and on Day

2 the easternmost strata and the strata of the south fl ank will be

viewed and discussed.

UINTA MOUNTAIN GROUP STRATIGRAPHY

The Neoproterozoic Uinta Mountain Group is a 4–7-km-

thick siliciclastic succession that is exposed only in the Uinta

Mountains and makes up the core of the Uinta Mountain anti-

cline (Fig. 1). The strata exposed in the western Uinta Mountains

are characteristically different than those in the eastern Uinta

Mountains, perhaps due to structural subbasins imparting control

on depositional style. Hansen (1965) indentifi ed two structural

domes within the overall Uinta anticline, one in the western and

one in the eastern part of the range, and these roughly correspond

to the changes in stratal character.

In the western Uinta Mountains, the Uinta Mountain Group

comprises >4 km of sandstone and sedimentary quartzite, with

lesser shale and rare conglomerate (Wallace, 1972) (Figs. 1

and 2). These strata show much lateral and vertical variability

and have undergone many subdivisions (see Sanderson, 1984).

In the eastern part of the range, the Uinta Mountain Group is

dominantly sandstone with lesser shale and a distinctive basal

conglomerate and breccia (Jesse Ewing Canyon Formation;

Sanderson and Wiley, 1986). The base of the Uinta Mountain

Group is exposed only in the eastern Uinta range, and calculated

thicknesses from air photos, seismic profi les, and thermal altera-

tion indices (TAI) indicate 4.5–7 km of strata (Hansen, 1965;

Stone, 1993; Sprinkel et al., 2002).

It is uncertain how the eastern and western Uinta Mountain

Group strata correlate; however, similar petrographic patterns are

evident across the range and throughout the group. Geochemical

and provenance studies show arkosic sandstone and shale in the

north part of the range were derived from the Wyoming craton to

the north, and quartz arenite in the southern part of the range was

derived, in part, from a Paleoproterozoic source to the east (e.g.,

Wallace, 1972; Sanderson, 1978; Sanderson, 1984; Ball and

Farmer, 1998; Condie et al., 2001).

The age of the Uinta Mountain Group is likely entirely

Neoproterozoic. It unconformably overlies the metamorphic

quartzitic and schistose units of the Red Creek Quartzite and

Owiyukuts Complex (ca. 1.7 to ca. 2.7 Ga; Hansen, 1965; Sears

et al., 1982) and is unconformably overlain by lower Paleozoic

strata. A 770 Ma detrital zircon population from the middle east-

ern Uinta Mountain Group (Fanning and Dehler, 2005) indicates

that the majority of the Uinta Mountain Group is younger than

770 Ma. The uppermost unit in the Uinta Mountain Group, the

Red Pine Shale, yielded a microfossil assemblage and C-isotope

variability similar to that of the 742 Ma upper Chuar Group in

Arizona, therefore putting a possible upper age limit on the

Uinta Mountain Group (Vidal and Ford, 1985; Karlstrom et al.,

2000; Porter and Knoll, 2000; Dehler, 2001; Dehler et al., 2006).

Paleomagnetic data from the Uinta Mountain Group indicate

deposition in equatorial latitudes, and the Uinta Mountain Group

paleopole sits right on the Chuar Group apparent polar wander

path, also suggesting a similar age (Weil et al., 2005).

Western Uinta Mountain Group Stratigraphy

The western Uinta Mountain Group has been subdivided

several different ways (Williams 1953; Wallace and Crittenden,

1969; Wallace, 1972; Sanderson, 1984). The nomenclature used

here is mainly after Wallace (1972), in combination with what

has been considered mappable on a 1:125,000 scale by Bry-

ant (1992). These units include the lowermost formation of

Moosehorn Lake (including the basal undivided Uinta Mountain

Group), the formation of Red Castle, the formation of Dead Horse

Pass, the Mount Watson Formation, the formation of Hades Pass,

and the Red Pine Shale (Figs. 2 and 3). Only the Mount Watson

Formation has been formally named following the Stratigraphic

Code (Sanderson, 1984). The Red Pine Shale was formalized

by Williams (1953) prior to adoption of the Stratigraphic Code

(North American Stratigraphic Commission on Nomenclature,

1983). Other informal units proposed by Wallace (1972) may be

mappable at larger scales, but will not be featured in this paper.

Basal Undivided Uinta Mountain Group and Formation of

Moosehorn Lake

The lowermost 60 m of exposed Uinta Mountain Group in

the western range is an undivided interval of white quartz arenite.

It is likely a lateral equivalent of the formation of Red Castle, and

Neoproterozoic Uinta Mountain Group of northeastern Utah 3

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fl d006-01 page 4 of 26

is only exposed in the southern and central parts of the western

Uinta range (Fig. 1) (Wallace 1972). The base of this unit is not

exposed, and it grades upward into the formation of Moosehorn

Lake. In the mapping of Bryant (1992) and in this paper, this unit

is included in the basal part of the formation of Moosehorn Lake

(Figs. 2 and 3).

The formation of Moosehorn Lake (~150–300 m thick) is

a dark olive-green to yellow green shale with thin lenticular to

tabular interbeds of pebbly arkosic arenite (Fig. 4). Common

sedimentary features include ripplemarks, mudcracks, and soft-

sediment deformation (Wallace and Crittenden, 1969; Wallace,

1972). It is exposed in the Bald Mountain area of the higher west-

ern Uinta Mountains (Fig. 2). It is overlain by the Mount Watson

Formation or formation of Dead Horse Pass, and northward it

grades into and is overlain by the lower part of the formation

of Red Castle (Fig. 4) (Wallace and Crittenden, 1969; Wallace,

1972). The formation of Moosehorn Lake was interpreted to

represent a spectrum of marine environments (see Day 1, Stop

2) (Wallace, 1972). The undivided underlying quartz arenite was

interpreted to represent a braided stream system that is related to

the stream system in the formation of Red Castle.

Formation of Red Castle

The formation of Red Castle (>730 m thick) comprises

dominantly arkosic arenite with subordinate subarkosic and

quartz arenite. Common sedimentary features include trough and

-18-20-22-24-26-28-30-32 -16

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Figure 2. Stratigraphic columns for the western and eastern Uinta Mountain Group (UMG). Data for the western Uinta Mountain Group strati-graphic column is from Wallace (1972), Sanderson (1978, 1984), and Dehler et al. (2006). Data for the eastern Uinta Mountain Group is from Hansen (1965), Sanderson and Wiley (1986), Hansen and Rowley (1991), De Grey (2005), Nagy and Porter (2005), and Dehler et al. (2006). Note that there is a break in section in the eastern Uinta Mountain Group column. The Jesse Ewing Canyon Formation is overlain by ~7 km of undivided Uinta Mountain Group strata and this succession is exposed solely on the north side of the Browns Park graben. On the south side of the Browns Park graben, the divided eastern Uinta Mountain Group is exposed, although neither the basal contact nor the Jesse Ewing Canyon Formation are exposed there. Future work will determine how these strata correlate across the graben and how the eastern and western Uinta Mountain Group strata correlate across the range.


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formation of Hades Pass

Mount WatsonFormation formation of

Dead Horse Pass

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formation ofMoosehorn Lake

formation of Outlaw Trail

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Tintic Quartzite

Deseret Formation Madison Limestone

Lodore Sandstone

WESTERNUINTA MOUNTAINS

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Figure 3. Correlation chart for the Uinta Mountain Group. Stratigraphic nomenclature is from Wallace (1972), Sanderson (1978, 1984), De Grey (2005) and De Grey and Dehler (2005). The distribution of formations is based on mapping by Bryant (1992), Sprinkel (2002), and De Grey (2005). The 770 Ma detrital zircon age is from Fanning and Dehler (2005) and the ca. 740 age of the top of the Red Pine Shale is based on cor-relation with the 742 Ma Chuar Group (Dehler et al., 2006).

Figure 4. The east face of Bald Mountain along the Mirror Lake Scenic Byway. Bald Mountain is comprised of the Mountain Watson Forma-tion, which is thick- to medium-bedded quartz arenite and sedimentary quartzite; note the massive-weathering cliffs near the base and top of the mountain with an intervening slope-forming unit. The underlying formation of Moosehorn Lake forms the road cut above the vehicle (near center of photo). The bed top in the foreground is a medium-bed-ded sandstone interbed within the formation of Moosehorn Lake. The contact between the formation of Moosehorn Lake and the Mountain Watson Formation is at the base of the lower massive-weathering cliff near the bottom of Bald Mountain. Photo by Bart Kowallis.

6 Dehler et al.

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planar-tabular cross-bedding, mudcracks, ripplemarks, and mud

chips (Wallace, 1972). It is exposed in the higher Uinta Moun-

tains from the area of Red Castle Lake west to the area of Dead

Horse and Amethyst lakes (Fig. 1). This unit is conformably

overlain by the formation of Hades Pass. The formation of Red

Castle is interpreted to represent a braided stream system. Paleo-

current analysis indicates fl ow from the north, northeast, and east

(Wallace and Crittenden, 1969; Wallace, 1972).

Mount Watson Formation

The Mount Watson Formation (~550–1000 m thick)

comprises dominantly gray to white quartz arenite and lesser

subarkosic arenite, with subordinate lenticular gray-green shale

and reddish arkosic arenite interbeds (see Day 1, Stop 2). It is

exposed in the Mount Watson area of the high western Uinta

Mountains (Bryant, 1992; Fig. 1). It is laterally gradational with

the formation of Dead Horse Pass and is overlain by the forma-

tion of Hades Pass, both in gradational contact (Figs. 4 and 5)

(Wallace, 1972; Sanderson, 1978, 1984). The Mount Watson

Formation was interpreted to represent a mixture of fl uvial,

coastal, and marine environments by Wallace (1972) and to rep-

resent a braided sheetwash plain by Sanderson (1978, 1984).

Formation of Dead Horse Pass

This formation (900 m thick) is dominated by quartz arenite

and orthoquartzite, with subordinate shale and siltstone. Com-

mon sedimentary features include “giant planar crossbed sets,”

planar-tabular and trough crossbeds, thin beds of alternating

sandstone and shale, ripplemarks, mudchips, and soft-sediment

deformation. Shale intervals are locally interbedded with thin

beds of sandstone. Shale beds exhibit mudcracks, interference

ripples, terraced current ripplemarks, and mudcracks (Wallace,

1972; Sanderson, 1978; Sanderson, 1984). Shale intervals are tens

to hundreds of meters thick; the thickest shale interval (~200 m)

is informally named the Gilbert Peak Shale member by Wallace

(1972). This unit is exposed in the central higher Uinta Mountains

in the Dead Horse Pass area at the head of Rock Creek (Fig. 1)

(Wallace, 1972). Using the mapping divisions of Bryant (1992),

this latter formation includes the Gilbert Peak Shale member and

the Mount Agassiz formation of Wallace (1972). Where this unit

becomes shale- and siltstone-poor and rich in subarkose arenite to

the west, it is called the Mount Watson Formation (Fig. 3). To the

north, this unit grades into a poorly sorted arkosic arenite and is

called the formation of Red Castle (Wallace, 1972).

Paleoenvironmental interpretations are similar to those of the

Mount Watson Formation. Wallace (1972) interpreted this unit to

represent mostly coastal and marine environments, and Sanderson

(1978, 1984) interpreted this unit as a braided sheetwash plain.

Formation of Hades Pass

The formation of Hades Pass (1825–3600 m thick) com-

prises red to purple quartz arenite, subarkosic arenite, and

arkosic arenite (Fig. 5). It has subordinate shale intervals that are

grayish red, grayish olive green, or yellow and are ≤50 m thick.

Sedimentary structures in the sandstone units include medium

to thick beds of planar-tabular and trough crossbedding (some

indicating reverse current fl ow) and soft-sediment deformation

(Wallace and Crittenden, 1969; Wallace, 1972). It is recognized

in the central and western parts of the higher Uinta Mountains

(Fig. 1). The upper contact with the overlying Red Pine Shale is

gradational (Wallace and Crittenden, 1969; Wallace, 1972).

This unit is interpreted to represent a probable fl uvial origin.

Paleocurrent data indicate a dominantly east-west fl ow direction,

yet the dominant sandstone type is arkosic arenite. Interestingly,

in the westernmost exposure of this unit, paleocurrent directions

show dominantly northwest paleocurrent fl ow (Wallace and Crit-

tenden, 1969; Wallace, 1972).

Red Pine Shale

The Red Pine Shale (Williams, 1953) comprises organic-

rich gray shale, siltstone, and subordinate sandstone (quartz

arenite to arkosic arenite) (see Day 1, Stops 1 and 3; Day 2, Stops

5 and 6) (Fig. 6). It is overlain unconformably by the locally

laterally discontinuous Cambrian Tintic Quartzite or the overly-

ing Mississippian Madison Formation (Figs. 1 and 2). It ranges

in measured thickness from ~300 m to >1200 m on the south

fl ank and is between ~500 m and ~1825 m thick on the north

fl ank (Williams, 1953; Wallace, 1972; Bryant, 1992; Dehler et

al., 2006). The facies characteristics and associations indicate

offshore deposition near or below fair-weather wavebase, in a

deltaic system (Dehler et al., 2006).

Eastern Uinta Mountain Group Stratigraphy

Stratigraphic research on the eastern Uinta Mountain Group

has resulted in the subdivision of the basal ~225 m into the Jesse

Ewing Canyon Formation (Sanderson and Wiley, 1986) and the

new division of the majority of correlative overlying strata into

the formations of Diamond Breaks, Outlaw Trail, and Crouse

Canyon and undivided Uinta Mountain Group (Figs. 2 and 3)

(e.g., 1:24,000 scale mapping; De Grey, 2005; Dehler et al.,

2006). Constraints on the thickness of the eastern Uinta Moun-

tain Group are poorly known, yet seismic data interpretation,

thermal maturation data, and mapping suggest that the Uinta

Mountain Group is ~4.5–7 km thick in the northernmost area of

exposure (juxtaposed to the Uinta Fault zone) and between 4 and

5 km southward (Fig. 2) (Hansen, 1965; Stone, 1993; De Grey,

2005; Sprinkel and Waanders, 2005).

Jesse Ewing Canyon Formation

The Jesse Ewing Canyon Formation (~225 m thick) com-

prises lithic clast-supported conglomerate and breccia, lithic

and quartz arenite, and red to black shale (Fig. 7; see Day 2,

Stop 2) (Sanderson and Wiley, 1986). Abrupt north-to-south

facies changes show coarse conglomerate and breccia beds

thinning southward into thick intervals of gray to red to green

shale and subordinate interbeds of sandstone (Sanderson and

Wiley, 1986; Dehler et al., 2006). Paleocurrent data measured


fl d006-01 page 7 of 26

from different types of crossbedding in interbedded sandstone

units indicate a southwesterly fl ow direction (Sanderson and

Wiley, 1986).

The Jesse Ewing Canyon Formation unconformably over-

lies the Paleoproterozoic-Archean(?) Red Creek Quartzite and

is overlain in gradational contact by undivided Uinta Mountain

Group (Sanderson and Wiley, 1986) (Fig. 3). It is exposed in a

~56 km2 area in the Jesse Ewing Canyon area, south of Clay

Basin and north of Browns Park, very near and west of the

Utah-Colorado border (Fig. 1). This unit is truncated to the south

by the Tertiary Mountain Home fault, part of the Browns Park

graben, and is not found southward, nor is the base of the Uinta

Mountain Group exposed elsewhere except in this area north of

the graben (Fig. 1). The Jesse Ewing Canyon formation laterally

pinches out to the west and east, and, in these areas, undivided

Uinta Mountain Group sandstone rests directly on the Red Creek

Quartzite (Hansen, 1965; Sprinkel, 2002).

Sanderson and Wiley (1986) suggested that this unit repre-

sents alluvial fan and related deposits. Dehler et al. (2006) sug-

gest that parts of this unit indicate subaqueous-mass-fl ow and/or

fan delta deposition along a wave-affected shoreline.

Uinta Mountain Group Undivided above Jesse Ewing

Canyon Formation

Approximately 7 km of sandstone and interbedded shale

conformably overlie the Jesse Ewing Canyon Formation

(Hansen, 1965) and have yet to be subdivided or measured

and described in detail. These strata, and the underlying Jesse

Ewing Canyon Formation, are stratigraphically isolated on the

north side of the Browns Park graben from newly subdivided

Uinta Mountain Group strata on the south side of the graben

(Figs. 1, 2, and 3). This undivided Uinta Mountain Group unit

consists of pebbly sandstone interbedded with red shale inter-

vals. The sandstone intervals are typically tens of meters thick,

and the shale intervals are meters thick. Common sedimentary

features are trough crossbeds and soft-sediment deformation.

Hansen (1965) interpreted these strata to represent shallow- to

marginal-marine and subaerial environments in a rapidly sub-

siding trough.

Figure 5. View northeast of Hayden Peak (12,479 ft). The formation of Hades Pass (upper, darker unit) caps the peak with most of the mountain comprising the underlying lighter-colored Mount Watson Formation. Hayden Peak is faulted on the north and south. The rocks that form Hayden Peak dip northward. The axis of the Uinta arch is mapped by Bryant (1992) approximately through the Hayden Peak overlook stop.

Figure 6. View of a north-facing outcrop of the Red Pine Shale from the Castle Rocks overlook. Hades Creek is at the base of the outcrop. Note sandstone interbeds toward top of exposure. This unit is inter-preted to represent a marine-deltaic system.

Figure 7. Photo of Jesse Ewing Canyon Formation showing the lateral facies change from predominantly conglomerate and breccia to the north to dominantly shale to the south. Although previously interpreted as an al-luvial fan deposit, a signifi cant amount of the Jesse Ewing Canyon Forma-tion was deposited in a subaqueous environment, possibly marine.

8 Dehler et al.

fl d006-01 page 8 of 26

Formation of Diamond Breaks

The formation of Diamond Breaks (500–1000 m thick) is

the lowermost subdivided informal unit in the undivided eastern

Uinta Mountain Group, and it is not yet certain how this unit

correlates with the undivided Uinta Mountain Group and Jesse

Ewing Canyon Formation to the north (Figs. 1 and 2). This

formation comprises dominantly quartz arenite, with subordinate

arkosic and subarkosic arenite with subordinate thin intervals of

red shale (see Day 2, Stop 3) (De Grey, 2005). It is sharply, but

conformably, overlain by the formation of Outlaw Trail, and the

base is not exposed (Fig. 8). The formation of Diamond Breaks

contains facies associations that represent various depositional

environments within a braided river system.

Formation of Outlaw Trail

The formation of Outlaw Trail (50–70+ m thick) comprises

green to gray to red shale, interbedded with thin to thick arkosic

sandstone beds (Fig. 9) (see Day 2, Stop 3). It is exposed along

the north face of the Diamond Breaks and can be traced for tens

of kilometers laterally (Fig. 8) (Dehler et al., 2006). The forma-

tion of Outlaw Trail has been interpreted to be the low energy,

interdistributary area of a proximal to medial delta plain environ-

ment, such as a bay, lagoon, swamp, or lacustrine environment

(Dehler et al., 2006).

Formation of Crouse Canyon

The formation of Crouse Canyon sharply overlies the forma-

tion of Outlaw Trail (Fig. 8). The thickness of the formation of

Crouse Canyon in this area reaches up to 1170 m and is estimated

to be ~3200 m if extended to the top of the Uinta Mountain

Group (Fig. 2) (De Grey and Dehler, 2005). This formation is

similar to the formation of Diamond Breaks (see Day 2, Stops

3 and 4). Depositional environments are similar to those in the

formation of Diamond Breaks.

Undivided Uinta Mountain Group above formation of

Crouse Canyon. The upper 2030 m of the eastern Uinta Mountain

Group have not been described in detail, and are here included

in the formation of Crouse Canyon. Preliminary observation and

geologic mapping indicate that these strata are very similar to the

underlying formation of Crouse Canyon. This unit also very likely

represents a braided stream environment (De Grey, 2005).

Paleontology of the Uinta Mountain Group

Overview

The Uinta Mountain Group was deposited during a transi-

tional time in the early evolution of the biosphere. After a long

interval characterized by limited diversity and low abundance,

eukaryotes were diversifying and expanding into prokaryote-

dominated environments. Although animals had not yet origi-

nated, protistan clades including red algae, green algae, lobose

and fi lose testate amoebae, and, possibly, fungi, ciliates, and dino-

fl agellates, had appeared by Uinta Mountain Group time (Porter,

2004, and references therein). Biological and ecological complex-

ity was also increasing: complex multicellularity, biomineraliza-

tion, and sex had all been invented, and multi-tiered food webs

had begun to appear (Butterfi eld, 2000; Porter and Knoll, 2000;

Porter et al., 2003). At the end of the Neoproterozoic, this diversi-

fi cation culminated in a remarkable radiation of both animals and

protists, commonly known as the “Cambrian explosion.”

The Uinta Mountain Group records little of these events,

however. Although fossils are moderately to well preserved

throughout the succession, assemblages found thus far are lim-

ited in both diversity and morphological complexity. Most beds

record simple, smooth-walled microfossils collectively grouped

under the genus Leiosphaeridia and/or fi lamentous microfossils

probably representing the sheaths of bacteria. Other beds pre-

serve monospecifi c blooms of the bacterial aggregate, Bavlinella faveolata. More complex eukaryotic fossils, including vase-

shaped microfossils (VSMs) and ornamented acritarchs, occur

in the Uinta Mountain Group but are relatively rare (e.g., two out

of 31 fossiliferous samples examined by Nagy and Porter [2005]

Figure 8. View looking south at the Diamond Breaks on the south side of Browns Park. The white lines indicate contacts between the three informal units of the eastern Uinta Mountain Group proposed by De Grey (2005). Abbrevia-tions denote Neoproterozoic formations of Diamond Breaks (Zud), Outlaw Trail (Zuo), and Crouse Canyon (Zuc).


fl d006-01 page 9 of 26

had these “complex” fossils). The limited presence of eukaryotes

in the Uinta Mountain Group cannot be explained by either pres-

ervation or by age; the coeval Chuar Group, for example, records

comparably preserved, but much more diverse, fossils. Instead, it

is likely that the eukaryotes were excluded from the Uinta Moun-

tain Group due to unfavorable environmental conditions.

Western Uinta Mountain Group Paleontology

Much more paleontological data exist for the western part

of the Uinta Mountain Group. All early paleontological work on

the unit (Hofmann, 1977; Nyberg, et al., 1980; Nyberg, 1982a,

1982b; Vidal and Ford, 1985) is limited to these strata, and the

majority of the samples from recent work (Nagy and Porter,

2005; Dehler et al., 2006) come from here. The Red Pine Shale is

the most thoroughly studied; it has yielded a variety of fi laments,

Bavlinella faveolata, Leiosphaeridia sp., ornamented acritarchs,

vase-shaped microfossils, and the macroscopic carbonaceous

compression fossil, Chuaria circularis.

Though not as well studied, the Mount Watson Formation

and the formation of Moosehorn Lake are also fossiliferous.

The former has yielded Leiosphaeridia sp. and the ornamented

acritarch Trachysphaeridium laufeldi (Vidal and Ford, 1985), and

the latter has yielded fi laments and Leiosphaeridia sp. (Nyberg,

1982a; Nagy and Porter, 2005). VSMs may also be present in

the formation of Moosehorn Lake (Nyberg, 1982b, Dehler et al.,

2006), but this has yet to be confi rmed.

Eastern Uinta Mountain Group Paleontology

The eastern Uinta Mountain Group has been studied only

recently, by Nagy and Porter (2005) and Sprinkel and Waanders

(2005). Sampling focused on the undivided strata; like the

western Uinta Mountain Group, most samples yielded simple

fi laments, Leiosphaeridia sp., and Bavlinella faveolata. Only one

specimen, from the Leidy Peak locality (Day 1, Stop 4), yielded

more complex fossils, including ornamented acritarchs and

possible VSMs. A single sample from the Jesse Ewing Canyon

Formation yielded fi laments and Leiosphaeridia sp.

CORRELATION

It is unclear how the western and eastern Uinta Mountain

Group correlate because of the lack of detailed stratigraphic

information in the central and eastern parts of the range.

Research efforts on these key areas of the Uinta Mountain

Group are underway in the form of mapping, measuring section,

shale geochemistry, biostratigraphy, and C-isotope stratigraphy

(Dehler et al., 2006), and a preliminary correlation chart is shown

in Figure 3.

The most complete stratigraphy is available in the eastern

Uinta range where the base and the eroded top of the Uinta

Mountain Group are exposed. The <770 Ma formation of Out-

law Trail, in the lower-middle part of the eastern Uinta Mountain

Group, may be an eastern correlative of the shale-rich Dead

Horse and Mount Watson formations, since it is one of the only

signifi cant shale intervals in the eastern Uinta Mountain Group

and could correspond with the most shale-rich part of the western

Uinta Mountain Group. It is hypothesized that the eastern Uinta

Mountain Group section represents older and correlative strata to

the majority of the western Uinta Mountain Group, except there

is no lithostratigraphic equivalent of the Red Pine Shale, and

there are mapping relationships that suggest that the Red Pine

Shale is truncated eastward (Sprinkel, 2002).

Regional correlation has long been suggested between the

Uinta Mountain Group, the Big Cottonwood Formation, and

the Pahrump Group and Chuar groups (e.g., Link et al., 1993).

Until recently, however, there were no robust geochronological

constraints on any of these units and microfossil information

was limited (e.g., Crittenden and Peterman, 1975; Vidal and

Ford, 1985). It is now known that the Chuar Group is 742 to ca.

770 Ma, the majority of the Uinta Mountain Group is younger

than 770, and the upper Uinta Mountain Group is very similar to

the 742 Ma upper part of the Chuar Group (Fig. 10) (Karlstrom

et al., 2000; Williams et al., 2003; Fanning and Dehler, 2005;

Dehler et al., 2006). The geochemical and petrographic similari-

ties between the Uinta Mountain Group and the Big Cottonwood

Formation to the west in the Wasatch Range strongly suggest that

these units are related (Condie et al., 2001), as is the fact that

the Big Cottonwood Formation is part of the same reactivated

Laramide structure (the Uinta anticline or Uinta arch), suggest-

ing that these units may have formed in the same basin along the

same Neoproterozoic structure (Fig. 1). Similar fossils and C-

isotope variability are observed in the Beck Springs Dolomite of

the middle Pahrump Group (Prave, 1999; Corsetti and Kaufman,

2003), suggesting that it is also the same age (Fig. 10) (Dehler

Figure 9. Photo of the formation of Outlaw Trail in the newly sub-divided Uinta Mountain Group in the eastern Uinta range. This is a 50–70-m-thick unit (thickening to >100 m to the west) and is fi ne-grained, organic-rich, fossiliferous, and a contains a distinctive set of sedimentary structures that suggest deposition along a shoreline.

10 Dehler et al.

fl d006-01 page 10 of 26

et al., 2001a, 2001b). All of these units are, in part, marine, and

all but the Beck Springs Formation record intracratonic rifting

(Link et al., 1993; Timmons et al., 2001; Condie et al., 2001;

Prave, 2004, personal commun.). These correlations indicate

a rift-related, intracratonic seaway fl ooding a good part of the

western Laurentia margin at ca. 750 Ma (Chuar-Uinta Mountain

Pahrump groups [ChUMP] hypothesis of Dehler et al., 2001b).

These correlations would make the Mineral Fork Formation,

stratigraphically above the Big Cottonwood Formation, younger

than ca. 740 Ma (Fig. 10). This is consistent with new U-Pb ages

from zircons in the Pocatello Formation, which indicate a 709–

667 Ma age (Fanning and Link, 2004). The Pocatello Formation

has long been correlated with the Mineral Fork Formation (e.g.,

Link et al., 1993).

PALEOGEOGRAPHY

The Uinta Mountain Group is most commonly interpreted as

a large braided fl uvial system with a trunk stream fl owing west-

ward and tributary streams fl owing southward (e.g., Sanderson,

1984; Condie et al., 2001). The streams are fl owing within an

east-west–trending structural trough and ultimately meeting the

sea, somewhere around the Wasatch Range to the west (Fig. 11)

(represented by the Big Cottonwood Formation) (Sanderson,

1984; Sanderson and Wiley, 1986; Ehlers and Chan, 1999;

Condie et al., 2001). In contrast, Wallace (1972) interpreted the

western Uinta Mountain Group as part of a fl uvio-marine basin,

with the shoreline coincident with the modern east-west divide,

and the basin margin extending beyond the south fl ank of the

modern range (Fig. 11). Results from the Red Pine Shale, and

similar mudstones eastward in the Uinta Mountain Group, also

suggest marine-deltaic deposition in a seaway that extended as

far east as Flaming Gorge, and maybe, at times, to the Colorado

border (formation of Outlaw Trail?). This seaway could have

been of regional scale and extended as far southward as the

Grand Canyon and Death Valley areas (“ChUMP seaway”) (Link

et al., 1993; Dehler et al., 2001a, 2006).

BASIN TYPE AND TECTONIC SETTING

The most current views on the tectonic setting of the Uinta

Mountain Group are that it was an east-west–trending intracra-

Figure 10. Regional correlation between the Uinta Mountain Group and the Pahrump Group of Death Valley, the Chuar Group of Grand Canyon, Arizona, and the Big Cottonwood Formation of the Wasatch Range, northern Utah. The recent dates from the Chuar and Uinta Mountain groups, in combination with robust correlation of C-isotope and fossil data sets or provenance data, strongly suggest that these units were both deposited at about the same time, and may even be part of the same regional seaway (Dehler et al., 2001b). Correlation with the Big Cottonwood Forma-tion is based upon sandstone compositional similarities (e.g., Condie et al., 2001). Correlation with the middle Pahrump Group is based upon C-isotope and fossil data sets (Dehler et al., 2001b). Abbreviations: Pahrump Group: BSD + “tb”—Beck Springs Dolomite and the transitional beds; K Pk—Kingston Peak Formation; Chuar Group: UG—Umkar Group; Kw. Fm—Kwagunt Formation; S.F.—Sixtymile Formation; Big Cottonwood Formation: LWF—Little Willow Formation. See key for explanation of symbols and fi ll patterns.


fl d006-01 page 11 of 26

tonic rift occupying roughly the same area as the modern Uinta

Mountain range (Fig. 11) (e.g., Ball and Farmer, 1998; Condie et

al., 2001). Facies, petrographic, and geochemical analyses sug-

gest that the basin was bounded by an active fault on the northern

edge, but the structural setting of the other basin margins remains

unknown (Wallace, 1972; Sanderson, 1984; Condie et al., 2001).

Interestingly, there are no paleocurrent data indicating northward

fl ow, just southerly and westerly fl ow directions (Wallace, 1972;

Sanderson, 1978; Condie et al., 2001). This suggests that the

southern basin margin was extremely low relief and/or farther

away than the southern modern range boundary. Seismic profi les

and mapping in the eastern Uinta Mountains indicate that the

Uinta Mountain Group may thin from the north (~7 km thick) to

the south (4–5 km thick) (Hansen, 1965; Stone, 1993; De Grey,

2005). The thickest part of the Uinta Mountain Group is coinci-

dent with the Uinta-Sparks fault zone on the north fl ank and sug-

gests that this and/or related structures were active normal faults

during Uinta Mountain Group deposition.

Figure 11. (A) Paleogeographic inter-pretation of the Uinta Mountain Group by Wallace and Crittenden (1969). Note: Interpreted strandline roughly coincides with east-west divide of Uinta Moun-tains. (B) Paleogeographic and tectonic interpretation by Condie et al. (2001). Arrows denote generalized paleocurrent directions. Work by Ball and Farmer (1998) suggests that the quartz arenites and shales found close to the axis of the range (“strandline” of Wallace and Crit-tenden [1969]) are derived from east-ward Proterozoic cratonic sources as opposed to being reworked sands from the Archean source to the north.

12 Dehler et al.

fl d006-01 page 12 of 26

Many workers have suggested that the Uinta Mountain

Group was deposited in an aulacogen (e.g., Sears et al., 1982;

Stone, 1993), but this is unlikely as it overlies cratonic rocks,

is not associated with oceanic crust, and is ~500 km from the

edge of the Proterozoic craton edge (Condie et al., 2001). Fur-

thermore, this interpretation requires the presence of the Cordil-

leran (or some earlier) passive margin at Uinta Mountain Group

time, which did not develop until infra-Cambrian time (Bond

and Komintz, 1984; Colpron et al., 2002).

Considering regional correlations with the Big Cottonwood

Formation, and the Pahrump and Chuar groups (Fig. 10), there

appears to have been a phase of intracratonic rifting and marine

fl ooding at ca. 750 Ma. This is signifi cantly earlier than the two

phases of rifting suggested by Prave (1999) of 700 Ma and

600 Ma. Therefore, it is hypothesized that there were at least

three phases of rifting associated with the protracted breakup of

the western Laurentian margin.

FIELD TRIP DAY 1—SALT LAKE CITY TO RED CANYON LODGE NEAR FLAMING GORGE

Travel on Day 1 will be from Salt Lake City over the Mir-

ror Lake Highway Scenic Byway to Red Canyon Lodge (Figs. 1

and 12). This segment of the trip offers opportunities to view

and discuss the stratigraphy, sedimentology, paleontology, and

geochemistry of the Uinta Mountain Group in the western and

central Uinta Mountains.

Road Log to Stop 1, Day 1

CumulativeMileage Directions

0.0 Start from the Salt Lake City convention center

at 7:30 a.m., and take I-80 east.

4.7 Intersection of I-80 and I-15; take I-80 east

toward Cheyenne, Wyoming.

10.5 Cross Wasatch fault (Bryant, 1990).

29.4 Take U.S. Hwy 40 south to Heber City and Ver-

nal.

33.1 Take Exit 2 to Park City and Kamas.

33.4 Turn left at the bottom of the exit and travel east

on State Road (SR) 248.

45.2 Intersection of SR 248 and SR 32. Turn left on

SR 32.

45.4 Intersection of SR 32 and the Mirror Lake Sce-

nic Byway (SR 150). Turn right on SR 150 to

Mirror Lake.

50.1 The Mississippian Madison Limestone is

exposed on both sides of the road in this area.

This unit rests unconformably on the Uinta

Mountain Group in this area.

50.9 Pull over on a large dirt turnout on the right side

of the road.

Stop 1—Introduction to the Western Uinta Mountain Group Stratigraphy

The Neoproterozoic Red Pine Shale of the Uinta Mountain

Group is exposed along the creek on the right. This is the fi rst

glimpse coming from the west of an outcrop of the Uinta Moun-

tain Group. The Red Pine Shale is the uppermost unit in the

western Uinta Mountain Group (Figs. 2 and 3). As the fi eld trip

continues over SR 150 to the east, the older strata are exposed as

the road cuts toward the core of the Uinta anticline. The next low-

ermost unit exposed along the road is the red to pink formation of

Hades Pass, and below that the white Mount Watson Formation

and green to white basal strata can be seen along the road in the

highest part of the range.

The western Uinta Mountain Group is subdivided by

sandstone composition and lithology (sandstone versus shale)

(Fig. 2). There is a very distinct pattern across the range; the

units are more arkosic on the north fl ank and more quartz rich

on the south fl ank (Sanderson, 1978; Wallace, 1972). These

compositional differences, plus the presence of shales and key

sedimentary structures, show facies changes in a north-south and

an east-west pattern. All of these units appear to be gradational

between one another, and all facies appear to be genetically

related (Fig. 3).

Interpretations are variable for these western units. Sand-

erson (1984) interpreted the whole Uinta Mountain Group to

represent a braided system, and Wallace (1972) viewed the strata

to indicate a dynamic fl uvio-deltaic marine system. In the next

two days, many of these units will be visited to critically evaluate

these paleoenvironmental interpretations.

This is a very typical outcrop of the Red Pine Shale. It is pre-

dominantly shale with subordinate thin sandstone and siltstone

beds. The shale is organic-rich and fossiliferous. The sandstone

and siltstone beds are lenticular to tabular and exhibit a range of

sedimentary structures suggesting deposition in a deltaic system.

The facies exposed here represents a pro-delta environment.



59.4 The formation of Hades Pass crops out on a

slope on either side of SR 150.

67.0 Murdock Basin Road intersection is on the

right. There is a good view of Bald Mountain

(11,943 ft) to the north. The Mount Watson

Formation is exposed on the left and is on the

footwall block of the Hoyt Canyon normal

fault (Laramide age). To the south, the forma-

tion of Hades Pass forms the cliffs, and the

Hoyt Canyon fault is between here and the

Hades Pass cliffs. The formation of Hades Pass

(1825–3600 m thick) varies from quartz arenite


fl d006-01 page 13 of 26

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14 Dehler et al.

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to arkosic arenite and has some signifi cant shale

intervals (tens of meters thick). This unit was

interpreted by Wallace (1972) to represent a

probable fl uvial origin; however, considering

reported bidirectional paleofl ow indicators (Wal-

lace, 1972) and the presence of a signifi cant

amount of mud during deposition, alternate

depositional environments, such as a tidal area

and/or delta plain would be more likely.

68.0 Slate Gorge overlook. Interbedded shale and tab-

ular siltstone and sandstone beds in the forma-

tion of Moosehorn Lake can be observed here.

Bryant (1992) mapped the Hoyt Canyon fault

south of here (near Stop 1), where the formation

of Hades Pass to the south is juxtaposed against

the formation of Moosehorn Lake and Mount

Watson Formation to the north.

74.6 Turn left (N) into the Bald Mountain photo

viewpoint turnout and parking lot. It is recom-

mended to view the Mount Watson Formation

here and then carefully walk along the left side

of the road 0.2 mi to the Bald Mountain picnic

area, where the formation of Moosehorn Lake

can be visited. Take the trail toward Bald Moun-

tain to view the outcrop character of the Mount

Watson Formation.

Stop 2—Basal and Middle Western Uinta Mountain Group

The Mount Watson Formation (≤1000 m thick) comprises

light-colored orthoquartzite, quartz arenite, and subarkosic

arenite (Figs. 4 and 5). Sedimentary structures exposed at this

locality are thin- to medium-bedded trough crossbeds, soft-sedi-

ment deformation, and symmetric and interference ripplemarks.

Paleofl ow direction at this stop is dominantly to the northwest

and symmetric ripplecrest orientations indicate a locally north-

south–trending shoreline. Other sedimentary features observed

by Wallace (1972) are distinctive 15–20-m-thick sets of thickly

bedded planar tabular foresets, trough crossbeds tens of meters

wide and long in plan view, and thin shale beds.

There are subordinate intervals of green and red shale in this

unit, which thicken to the east, where the unit is called the forma-

tion of Dead Horse Pass. This shaley correlative may correlate with

a green shale unit, the formation of Outlaw Trail, in the eastern part

of the range (Figs. 3 and 9) (Dehler and Sprinkel, 2005). North-

ward and northwestward, the Mount Watson Formation grades

into the formation of Red Castle, which is dominantly arkosic

arenite with fewer interbedded shale intervals (Figs. 2 and 3).

There are two different environmental interpretations for the

Mount Watson Formation and equivalent units. Sanderson (1978,

1984) interpreted these strata to represent a braided sheetwash

plain and extended this interpretation to the entire Uinta Mountain

Group. Wallace (1972) interpreted this formation to be a dynamic

system with a fl uvial component, as well as facies representing

shoreline, deltaic, and offshore deposition. Wallace’s interpreta-

tion describes tributary streams carrying arkosic sediment from

the north, with reworking of local stream deposits into quartz

arenite and subarkosic sediment along an east-west–trending

shoreline coincident with the modern Uinta Mountain divide. He

specifi cally recognized deltas, tidal fl ats, mudfl ats, and offshore

bars. Sanderson’s interpretation describes tributary streams car-

rying arkosic sediment from the north, feeding a larger braided

system from the east carrying quartzose sediment. Provenance

studies by Ball and Farmer (1998) indicate that the quartz arenite

source was derived from a chemically different source than the

arkosic source, indicating that the quartz arenite was not derived

through mechanical reduction of arkosic sediment.

The formation of Moosehorn Lake (<360 m) comprises

green to gray to yellow shale, with subordinate thin to medium

beds of arkosic siltstone and sandstone (Fig. 4). Sedimentary

structures that can be viewed here are thin to medium bedding

and soft sediment deformation. Wallace (1972) interpreted this

unit to represent tidal fl ats, submerged to emergent delta plains,

and/or lagoonal environments, with the interbedded sandstone

beds indicating delta front or offshore bar deposition.

Only simple fossils have been found in this formation:

fi laments and smooth-walled microfossils of the genus Leios-phaeridia (Fig. 13A–13C and 13F; Nyberg, 1982a; Nagy and

Porter, 2005). The former likely represent the empty sheaths

of fi lamentous bacteria; the affi nities of the latter, often referred

to as “leiosphaerids,” are more problematic because the genus

likely contains a range of unrelated taxa. Some specimens may

represent prokaryotes; others have been interpreted as the resting

stages of simple eukaryotic algae. Nyberg (1982a) also reported

VSMs from this unit, but the two specimens he illustrated have

irregular morphologies more reminiscent of mineral grains than

of the rounded tests of VSMs. Specimens representing “possible

VSMs” are reported by Dehler et al. (2006); their preservation is

not good enough to allow confi dent attribution.

The axis of the Uinta anticline was mapped by Bryant

(1992) near here. Bryant (1992) also mapped faults on Murdock

Mountain to the southeast that place Mount Watson Formation

next to the formation of Moosehorn Lake.

Figure 13. Examples of fossil species from the Uinta Mountain Group. (A–C) Leiosphaeridia sp.: (A) a solitary specimen of Leiosphaeridia; (B) a Leiosphaeridia specimen exhibiting a medial split, a likely ex-cystment structure; (C) aggregates of Leiosphaeridia cells. (D–E) Bav-linella faveolata. (F–H) Filaments, both unbranched (F) and branched (G–H). (I)—Satka colonialica. (J–M) Examples of “complex” acri-tarchs found in the Uinta Mountain Group: (J) Trachysphaeridium laminaritum; (K) a species with an outer envelope around an inner spheroid; (L) a species with ornamented walls; (M) a species covered in short spines. (N) Two vase-shaped microfossils, attached at their ap-ertures. (O) The modern testate amoeba, Diffl ugia lucida, undergoing asexual reproduction. (1) and (J) are from the Chuar Group; they are representative of the same species found in the Uinta Mountain Group. (O) is courtesy of Ralf Meisterfeld.


fl d006-01 page 15 of 26

16 Dehler et al.

fl d006-01 page 16 of 26



74.8 Bald Mountain Picnic area is on the left. From

here, a short hike accesses the basal formation of

Moosehorn Lake and the overlying Mount Wat-

son Formation. After Stop 2, leave Bald Moun-

tain parking, turn left on SR 150, and continue

north to Mirror Lake and Evanston, Wyoming.

74.9 Duchesne County line. Outcrop of quartz arenite

of the Mount Watson Formation is on the right

and faulted formation of Moosehorn Lake on

Murdock Mountain is ahead and to the right.

Just before turnoff to Hayden Peak overlook, the

contact between the formation of Moosehorn

Lake and the overlying Mount Watson Forma-

tion can be viewed on the left.

75.6 Hayden Peak and Moosehorn Lake overlook

(alternative Stop 2). Hayden Peak (12,479 ft)

is in full view to the northeast. Hayden Peak is

capped by the formation of Hades Pass (red-

dish rocks) and is underlain by the light-colored

Mount Watson Formation. Faults cut the rocks

north and south of Hayden Peak. To the south-

east, the green shale and sandstone beds are the

formation of Moosehorn Lake.

77.5 Pass Lake. View of formation of Hades Pass in

cliffs the on peaks to the north.

79.2 Fine-grained interbeds of Mount Watson

Formation.

82.8 Formation of Hades Pass is exposed in road cut

on the left.

96.9 The road crosses the approximate position of the

North Flank fault.

121.1 Evanston city limits. Continue through outer

Evanston to I-80 eastbound onramp.

122.9 Turn right onto I-80 east toward Rocks Springs

and Cheyenne. Travel east to the Fort Bridger

exit.

152.2 Take the Fort Bridger exit and travel east in the

I-80 business loop through Fort Bridger.

157.4 Urie, Wyoming. Turn right at intersection and

travel south to Mountain View, Wyoming.

160.4 Mountain View, Wyoming. Stay on Wyoming

SR 414. The road near the south end of Moun-

tain View will split; keep left to stay on SR 414

to Lone Tree, Wyoming.

182.1 Lonetree, Wyoming.

190.5 Burnt Fork, Wyoming

192.9 Spirit Lake Road is on the right; turn right on

Spirit Lake Road, and travel south. To the east

(left) is Phil Pico Mountain.

199.6 Pennsylvanian-Permian Weber Sandstone is

exposed in road cuts. We continue to go down

section as we travel up the road in Birch Canyon.

201.3 Madison Limestone forms the cliff to the east

and west of the road. Just south of here is the

angular unconformity between the underly-

ing Red Pine Shale and the overlying Madi-

son Limestone. There are no Cambrian strata

exposed in this area.

201.8 Red Pine Shale exposed in road cuts. Samples

collected here have yielded Leiosphaeridia

spp., fi laments, Satka colonialica, and Tra-chysphaeridium laminaritum (Sprinkel and

Waanders, 2005; Nagy and Porter, 2005). Satka colonialica is characterized by spherical to elon-

gate envelopes with impressions of cells that

were once inside (Fig. 13I). Trachysphaeridium laminaritum is an organic-walled microfossil

with small, tightly packed, round depressions

covering its surface (Fig. 13J). Many specimens

have a regular circular opening, likely represent-

ing an excystment structure; i.e., an “escape

hatch” through which a cell can escape the resis-

tant outer wall (the “cyst”) that has protected it

during its resting period. Both species are known

from early to mid-Neoproterozoic successions,

including the Chuar Group, Arizona, and the

Visingsö Beds, Sweden (Vidal and Ford, 1985).

206.1 Spirit Lake Lodge Road on left; stay on the U.S.

Forest Service road (USFS 221).

207.2 Turn left at the intersection of USFS 221 and

the access road to the borrow pit in the Red

Pine Shale. After Stop 3, continue east on the

USFS 221.

Stop 3—Last Gasp of the Red Pine Shale?

Just west of here by ~10 km, in the Hoop Lake area, there

are thick exposures (>500 m) of Red Pine Shale comprising

interbedded arkosic sandstone, siltstone, and organic-rich shale.

No Red Pine Shale has been formally mapped in this area, but it

is clearly still present (Fig. 1). We are not far from the last expo-

sure of the Red Pine Shale on the north fl ank. It is unclear to us

at this time if the Red Pine Shale undergoes a facies change from

predominantly shale to sandstone with shale interbeds, or if the

Red Pine Shale has been removed by erosion or faulted out and

the sandstone and shale interbeds are the formation of Hades Pass

stratigraphically downsection. On the south fl ank of the Uinta

Mountains, the Red Pine Shale is exposed from the western end

of the range to the east side of Ashley Creek (Fig. 1). There, the

Red Pine Shale is truncated and in angular discordance with the

Mississippian Madison Limestone.

Preliminary δ13Corg

data from the Henry’s Fork section,

~25 km west of here, shows values from −19.6 to −26.5‰, and


fl d006-01 page 17 of 26

the Ashley Creek section, ~40 km south of here, values range

from −17.1 to −27.7‰ (Fig. 2). Values from these localities are

similar to the values in the composite δ13Corg

curve generated

from western shale localities (to be discussed at Stop 6, Day 2)

and are encouraging for potential use in correlation of the Red

Pine sections across the range (Fig. 2).

Hoop Lake paleontology samples have yielded the same

simple fossil assemblages seen elsewhere: fi laments, including

some branching specimens (Fig. 13G and 13H), Leiosphaeridia sp., and Bavlinella faveolata. This last fossil is usually found by

itself, and is thought to have been an opportunistic taxon, creat-

ing blooms in environments adverse to other taxa (Knoll et al.,

1981). Here B. faveolata is found in association with fi laments

and Leiosphaeridia sp.



208.8 Intersection of USFS 221 and access road USFS

014 to Long Park Reservoir. The Uinta Moun-

tain Group along the Long Park Reservoir access

road comprise mostly medium to thick beds of

red quartz sandstone interbedded with green-

gray shale beds. The general dip of the Uinta

Mountain Group is ~12°N; however, near here

the Uinta Mountain Group is folded and steeply

tilted. These sandstone and interbedded shale

beds are similar to the succession of rocks seen

at Sheep Creek Canyon (our next stop). Con-

tinue on USFS 221.

216.7 Intersection of USFS 221 and the Sheep Creek

Canyon Geological Area loop road. Turn left

onto the loop road (USFS 218) to the Sheep

Creek Canyon Geological Area.

217.5 Intersection with USFS 93 (Death Valley Road).

Continue west on USFS 218.

217.8 Beginning of switchback. View to the west-

northwest of Sheep Creek and Mahogany Draw

in the Sheep Creek Canyon Geological Area

(Schell, 1969; Sprinkel et al., 2003). The deep

red strata are the sandstone beds of the Uinta

Mountain Group. Interbedded with the sand-

stone beds are green-gray shale beds that contain

microfossils. The steep hill north of the road is

Windy Ridge. Windy Ridge consists of steeply

dipping and overturned beds of the Mississip-

pian Madison Limestone. The road is built on

the Uinta Mountain Group. The Uinta fault zone

is near the base of the Madison Limestone.

220.2 Pull over in a large turnout at the boundary of

Sheep Creek Canyon Geological Area and Pali-

sades Memorial Park overlook. After Stop 4,

turn around and retrace route (USFS 218) to get

to SR 44.

Stop 4—The North Flank Fault and the Uinta Mountain Group

The view is to the west and north: the Uinta Mountain Group

consists of red sandstone, siltstone, and mudstone that form the

bowl of an amphitheater. The rim of the amphitheater is formed

by the gray cliff of the Mississippian Madison Limestone, which

is locally referred to as “The Palisades.” The southwestern branch

of the Uinta fault zone placed the Uinta Mountain Group (on the

south) next to the Madison Limestone (Fig. 14). West of the

Sheep Creek area, the fault likely continues along the base of

the Madison cliff to west of Long Park Reservoir, where it may

cut down into the Uinta Mountain Group and place the prob-

Figure 14. Photo looking north at the Uinta Fault zone. The Uinta Moun-tain Group is on the hanging wall, and the Madison Limestone (cliff at top) is on the footwall of a Laramide-age reverse fault dipping to the south. This fault is one of several that bound the northern Uinta range. The Uinta Mountain Group stratigraphy undergoes character changes in this part of the range and is therefore a key area for understanding how the eastern and western Uinta Mountain Group stratigraphy correlate across the range. It is still unclear if and how structure played a role in the stratigraphic changes. Photo by Andy Brehm.

18 Dehler et al.

fl d006-01 page 18 of 26

able upper-middle part of the Uinta Mountain Group over the

Red Pine Shale. An alternative interpretation is that interbedded

sandstone and shale exposed here is equivalent to the Red Pine

Shale to the west, representing a facies change, which would not

require the thrust fault to continue to the west.

Green-gray shale beds in this area mostly contain the simple

fossils seen elsewhere: Leiosphaeridia sp. and fi laments, some of

them branching (Fig. 13G and 13H). A single sample from this

locality, however, preserves a more diverse assemblage of relatively

complex acritarchs, typical of Neoproterozoic shallow water envi-

ronments (Butterfi eld and Chandler, 1992). These include species

that possess an outer envelope around an inner spheroid (Fig. 13),

species with ornamented walls (Fig. 13J and 13L), and species with

a diversity of spines or “processes” (Fig. 13M). “Probable VSMs”

have also been reported from these samples, but their poor preserva-

tion prevents confi dent assignment (Dehler et al., 2006).

Bedding in the Sheep Creek Canyon Geological area, and

beyond, forms the north fl ank of the Uinta arch, a broad asym-

metrical anticline ~30 mi wide and 150 mi long (Fig. 1) (Han-

sen, 1965; Sprinkel, 2003). The Uinta arch consists of two large

domes that are aligned east-west and are separated by a shallow

structural saddle, which is crossed by U.S. Hwy 191–Utah

Hwy 44 from Vernal to Manila (Hansen, 1965; Sprinkel, 2003).

Thus, the Sheep Creek Canyon Geological Area is on the eastern

part of the western dome.



223.7 Intersection of Spirit Lake Road (USFS 221) and

Deep Creek Road (USFS 539). Continue east

on USFS 218, which is on the Uinta Mountain

Group.

226.7 Intersection of Sheep Creek Canyon Geological

Area loop road (USFS 218) and SR 44. Turn

right on SR 44 and travel east to Greens Lake

Recreation area and the Red Canyon overlook.

227.5 Dowd Mountain turn off; stay on SR 44.

237.9 Intersection of Green Lakes Recreation area and

Red Canyon overlook: turn left on USFS 095

and travel north.

240.4 Either drive to the Green Lakes Recreation area,

which includes the Red Canyon overlook, or

park at the Red Canyon Lodge and walk the

short rim trail to the overlook. After this stop,

stay overnight at Red Canyon Lodge.

Stop 5—East Meets West: Character Changes in the Uinta Mountain Group

This is a profound area for trying to understand the Uinta

Mountain Group stratigraphy (Fig. 3). This overlook sits about

where Hansen (1965) puts a structural trough between two

domes that make up the Uinta anticline. The strata take on a

different look in this area, and it could be that the domes were

once subbasins that, in part, controlled deposition of the Uinta

Mountain Group. As will be shown on Day 2, the eastern Uinta

Mountain Group appears to represent a fl uvial-dominated sys-

tem, whereas in the western Uintas, the strata refl ect marine-

deltaic, as well as fl uvial, deposition.

The Uinta Mountain Group seen from the overlook consists

of interbedded sandstone and red shale beds. Sedimentary struc-

tures in the local vicinity are trough crossbeds and low-angle

crossbedding, similar to much of what we will see on Day 2.

Sedimentological and stratigraphic analyses have yet to be con-

ducted in this area to determine the depositional environment and

local correlation. The strata in this area are particularly indistinct

and will be a challenge to correlate even locally. These strata are

likely equivalent to the middle-upper part of the western Uinta

Mountain Group (formation of Hades Pass or Red Pine Shale)

(Fig. 3).

FIELD TRIP DAY 2—RED CANYON LODGE, FLAMING GORGE AREA, OVER DIAMOND PLATEAU, AND RETURN TO SALT LAKE CITY

Travels on Day 2 are from Red Canyon Lodge over Flam-

ing Gorge, down Jesse Ewing Canyon, across Browns Park,

over the Diamond Mountain Plateau, through Vernal, up the

north Fork of the Duchesne River, and back to Salt Lake City

(Fig. 12). Stops will exemplify the different formations and

proposed subdivisions of the eastern Uinta Mountain Group,

and we will discuss correlation between the Uinta Mountain

Group and other strata regionally (Figs. 3 and 10). We will also

examine different facies and facies associations and discuss

interpretation of paleogeography and paleoclimate based on

sedimentology, paleontology, stratigraphy, geochemistry, and

geochronology.



0.0 Leave Red Canyon Lodge and travel south to the

intersection with SR 44. Turn left on SR 44 to

Flaming Gorge Dam and Vernal.

3.7 Intersection of SR 44 and U.S. Hwy 191. Turn

left on Hwy 191, and travel north to Flaming

Gorge Dam.

12.1 Dutch John, Utah. This town was built by the

Bureau of Reclamation to house workers con-

structing Flaming Gorge Dam.

14.3 Pull over along the side of the road on the right,

adjacent to the outcrop of sandstone and green

shale on the right (east) side of the road.


fl d006-01 page 19 of 26

Stop 1—Organic-Rich Shale Beds in the Uinta Mountain Group

This outcrop exhibits a signifi cant amount of variability in

grain size and color. At the southern end of the outcrop, there are

many shale intervals (meters thick) that are green to gray, indicat-

ing the presence of organic matter. Interbedded with these shale

beds are thin to medium beds of green to red sandstone, pebbly

sandstone, and local conglomerate. The pebble-sized clasts are

angular to rounded and are composed of quartzite derived from

the underlying Red Creek Quartzite.

These shale beds contain the same simple assemblages seen

elsewhere: fi laments and Leiosphaeridia sp., some of the latter as

solitary cells (Fig. 13A and 13B), some in aggregates (Fig. 15C).

Also present is the ornamented acritarch Trachysphaeridium laminaritum (Fig. 13J; Sprinkel and Waanders, 2005; Nagy and

Porter, 2005).



15.3 The Uinta-Sparks fault zone, dipping to the

south, is a reverse fault that places the Neopro-

terozoic Uinta Mountain Group over the Triassic

Chinle Formation and Jurassic Nugget Sand-

stone (Sprinkel, 2002).

21.0 Intersection of U.S. Hwy 191 and the Clay

Basin road. Turn right (east) on the Clay

Basin road. The road parallels the Cretaceous

Mesaverde Group on right and the Cretaceous

Fort Union Formation on left. The road trav-

els mostly on the lower part of the Fort Union

Formation but drops onto the Mesaverde Group

just west of Clay Basin. There, the Uinta-

Sparks fault zone is exposed and places the

Archean-Paleoproterozoic Red Creek Quartzite

over the Mesaverde Group.

31.0 Clay Basin and the Clay Basin gas fi eld. Clay

Basin is fl oored by the Baxter Shale and sur-

rounded on the north by cliffs of the Cretaceous

Mesaverde Group and Tertiary Wasatch Forma-

tion. The hills to the south are composed of Red

Creek Quartzite and the Uinta Mountain Group.

The Uinta-Sparks fault zone is near the base of

the hills to the south.

33.6 Intersection; turn right and continue through

Clay Basin to Browns Park.

39.0 The Uinta-Sparks fault zone here places the

Uinta Mountain Group over the Cretaceous

Baxter Shale; however, the zone is several

hundred feet wide here. Continue on Browns

Park Scenic Byway road and descend down

Jesse Ewing Canyon to Browns Park. The Jesse

Ewing Canyon Formation and undifferentiated

Figure 15. Rose diagrams from crossbedding in the formation of Diamond Breaks (A) and the formation of Crouse Canyon (B). The paleofl ow direction is similar for both of these units and indicates predominantly southwesterly fl ow.

20 Dehler et al.

fl d006-01 page 20 of 26

Uinta Mountain Group are exposed in the hills

on both sides of the road.

40.7 Pull over along the right side of the road where

an unnamed fault places the Neoproterozoic

Uinta Mountain Group down to the south next

to the Archean-Paleoproterozoic Red Creek

Quartzite. It is best to park up the road from this

fault just a few hundred meters and walk down

along the road for the next ~0.5 mi to look at the

spectacular exposures of this basal unit. Watch

for vehicles!

Stop 2—The Base of the Uinta Mountain Group

On the footwall of this unnamed fault is the Red Creek

Quartzite, a likely correlative with the Farmington Canyon

Complex in the Wasatch Range. This Paleoproterozoic-Archean

(?) unit has several metamorphic facies that were mapped by

Hansen (1965).

On the hanging wall of this fault is some of the best expo-

sure of the Jesse Ewing Canyon Formation, the basal unit of the

eastern Uinta Mountain Group (Fig. 2) (Sanderson and Wiley;

1986). Although it is a clast-supported cobble breccia at this spot,

laterally and vertically, this unit varies radically and includes beds

of cobble conglomerate, pebble conglomerate and breccia, sand-

stone, siltstone, and signifi cant red to green to gray shale intervals

(Fig. 7) (Sanderson and Wiley, 1986). These relationships are

nicely exposed down and along the road for the next 0.5 mi. The

coarser-grained facies become subordinate within this 0.5 mi

distance, and shale becomes dominant. Although red in appear-

ance, much of this shale is gray to black on the inside and contains

microfossils including leosphaerids and carbonaceous fi laments.

Sedimentary structures in the Jesse Ewing Canyon Forma-

tion are abundant. In the shale intervals, parallel laminations,

ripplemarks (symmetric and asymmetric, ladder), soft-sediment

deformation, mudcracks, evaporite pseudomorphs, potential

bipolar crossbeds, tool marks, mud chips, load casts, wavy bed-

ding, and hummocky cross-stratifi cation have been observed. In

the coarse-grained intervals, trough and tabular crossbedding,

fi ning- and coarsening-upward sequences, and weak imbrica-

tion are common.

Sanderson and Wiley interpreted this unit to represent an

alluvial fan complex. We suggest, based upon the lateral facies

changes, the high organic content, and the presence of microfos-

sils, that the depositional environment was, in part, subaqueous.

It could be that many of the breccia and conglomerate beds also

represent subaqueous deposition in the form of coarse-grained

turbidites and mass-fl ow deposits.

Recent mapping and stratigraphic studies imply that the

Jesse Ewing Canyon Formation exhibits differential subsidence

along an active (normal?) fault. Measured sections on the north

side of this fault are ~200 m thick (Sanderson and Wiley, 1986),

whereas sections on the south side of this fault indicate a mini-

mum thickness of 410 m.



41.0 Sign 831; fi laments and leiosphaerids were

recovered from these shales (Sprinkel and

Waanders, 2005; Nagy and Porter, 2005).

41.2 Mountain Home fault. The Brown Park Forma-

tion is faulted down next to red shale beds of the

Jesse Ewing Canyon Formation. This represents

the valley bounding fault that forms the north

side of the Browns Park graben. There is no

Jesse Ewing Canyon Formation or crystalline

basement exposed south of this structure. Cor-

relation of the northern Uinta Mountain Group

strata with the signifi cant thickness of Uinta

Mountain Group strata on the south side of the

graben has not yet been successful.

42.7 Intersection of Browns Park Scenic Byway and

the road to the John Jarvie House. Turn right to

the John Jarvie House.

44.1 At the bridge, turn left on the Taylor Flat Road

and cross the Green River.

44.3 At the intersection, turn west (right) off Taylor

Flat Road to the trailhead of Outlaw Trail.

45.8 Park the vehicles at the trailhead and hike

~0.5 mi upstream along the south side of Green

River to view the formation of Outlaw Trail. At

end of Stop 3, hike back to vehicles and retrace

route back to the Taylor Flat Road.

Stop 3—Formation of Outlaw Trail and Subdivision of the Eastern Uinta Mountain Group along the Diamond Breaks

The bedrock unit the trail follows to get to the formation

of Outlaw Trail is the underlying formation of Diamond Breaks.

The formation of Crouse Canyon, which overlies the formation

of Outlaw Trail, is very similar in facies characteristics, compo-

sition, paleofl ow direction, and depositional environment to the

underlying formation of Diamond Breaks (Figs. 2, 3, 15, and

16; De Grey and Dehler, 2005). Dominant sedimentary struc-

tures in the underlying and overlying formations include trough

crossbeds, low-angle crossbeds, and planar-tabular crossbeds

(Fig. 16). Soft-sediment deformation, mudcracks, mudchips, and

asymmetric ripplemarks are also present. Paleocurrent data from

these units indicate fl ow to the south-southwest (Fig. 15) (De

Grey, 2005). The facies associations in these sandstone forma-

tions indicate a low- to moderate-energy braid plain with higher

energy main channels of a braided river (De Grey, 2005).

The formation of Outlaw Trail is a mappable green to red

shale unit with subordinate sandstone beds that is ~50–70+ m

thick and allows the Uinta Mountain Group to be subdivided

into three units in the eastern Uinta Mountains (Figs. 8 and 9)


fl d006-01 page 21 of 26

(Connor et al., 1988; De Grey, 2005; Dehler et al., 2006). It is an

oxidized to organic-rich shale unit with interbeds of subarkosic

to arkosic siltstone and arenite (Fig. 9). Sandstone and siltstone

beds are typically thin to medium bedded, yet the sandstone beds

become thicker to the west, as does the whole unit. Sedimen-

tary features include planar-tabular crossbeds, planar-horizontal

laminae, symmetric, asymmetric, and interference ripplemarks,

mudcracks, gypsum pseudomorphs, and soft-sediment deforma-

tion. Sandstone beds thicken westward, along with the westward

thickening of the whole formation. The shale becomes more var-

iegated and locally organic westward (Dehler et al., 2006). Fila-

ments and leiosphaerids were recovered from these shales (Nagy

and Porter, 2005). This unit represents a delta plain and is likely

a marine fl ooding event from the west.

A possible local correlation is between the formation of

Outlaw Trail and the Mount Watson Formation and equivalents

(Fig. 3). These western units (especially the formation of Dead-

horse Pass) are the thickest and most abundant shale intervals

besides the uppermost Red Pine Shale. The stratigraphic position

in about the middle of the succession supports this correlation.

A 770 Ma detrital zircon population from sandstone in

the formation of Outlaw Trail indicates that this unit can be

no older than 770 Ma (Fanning and Dehler, 2005). If the local

correlation with the middle western Uinta Mountain Group is

A

B

Figure 16. Two of the most common facies within the sandstone formations of the eastern Uinta Mountain Group. (A) Thick, massive tabular to lenticular sandstone beds. (B) Medium-bedded trough crossbeds. These sedimentary structures indicate deposition in a braided-river system.

22 Dehler et al.

fl d006-01 page 22 of 26

correct, then this age association can be used in the western part

of the range as well.



47.3 Taylor Flat Road. Turn right to continue to

Crouse Canyon Road.

50.9 At the intersection with the road to Sears

Canyon, stay on Taylor Flat Road to Browns

Park Road. The strata exposed along the

road between here and Crouse Canyon is the

lowermost of the subdivided units along the

Diamond Breaks, the formation of Diamond

Breaks. Watch for trough crossbedding, low-

angle crossbedding, and large channelforms

representing a braided stream system.

56.1 Swallow Canyon. The Green River cuts through

a spur of Uinta Mountain Group. Swallow Can-

yon is not far south of the approximate projec-

tion of the eastern part of the Uinta axis (Hansen,

1965; Sprinkel, 2002).

57.7 Junction of Taylor Flat and Browns Park roads.

Turn right and travel south and up Crouse Can-

yon.

58.7 Mouth of Crouse Canyon. Near the mouth of

Crouse Canyon is the poorly exposed formation

of Outlaw Trail. As we travel up Crouse Canyon,

we will travel stratigraphically up through the

Uinta Mountain Group. The average dip of the

Uinta Mountain Group from here southward is

~12°S. There are great views of the lithofacies

and sedimentary structures in the Uinta Moun-

tain Group in the canyon.

68.9 Intersection of Browns Park Road and the road

to Crouse Reservoir. Turn right onto the connect-

ing road to Crouse Reservoir. The road travels

west through Uinta Mountain Group and surfi -

cial deposits.

70.8 At the intersection, turn right to Crouse Reservoir.

71.2 Pull over along the road. After Stop 4, turn

around and travel due south to Vernal.

Stop 4—Upper Eastern Uinta Mountain Group

This stop is nearing the top of the upper eastern Uinta

Mountain Group. Currently, we are extending the formation of

Crouse Canyon all the way to the top of the group (Figs. 2 and 3).

Two poorly exposed green shale beds (tens of meters thick) are

mapped in this area and are similar to the formation of Outlaw

Trail (Hansen and Rowley, 1991). It is not known whether these

shale units are laterally continuous and therefore whether this

upper part of the eastern Uinta Mountain Group can be subdi-

vided further. Otherwise, these sandstone strata exhibit the same

facies associations as the lower sandstone formations.

This east-west–trending valley is following a normal fault

that was active in the Quaternary and likely the Tertiary (Hansen

and Rowley, 1991). Seismic log interpretation shows a structure

possibly offsetting the Uinta Mountain Group by ~<100 m

(Stone, 1993). This would affect calculations of total thickness

of the Uinta Mountain Group. Other structures in the area show

remarkably small amounts of offset (≤10 m). These structures

are oriented between ~50° and near vertical, typically trend east-

west, dip to the north or south, and are associated with multi-ori-

entation joint sets (De Grey, 2005).



71.6 Continue heading south.

73.3 The Madison Limestone hogback with Lodore

Sandstone exposed below. The Uinta Mountain

Group is exposed below the Lodore Sandstone.

74.4 At this intersection, veer right and continue

south on Browns Park Road.

76.6 At the intersection of the Browns Park Road and

Jones Hole Road, turn right and travel across the

Diamond Mountain Plateau.

79.9 At this intersection, take the left fork to Vernal.

Follow the road off the Diamond Mountain Pla-

teau to Brush Creek.

92.4 Cross Brush Creek and stay on the road to Ver-

nal. This road will skirt around the east side of

the Buckskin Hills and past the Uintah County

landfi ll. Cross Ashley Creek and continue west

to 500 East.

101.9 Turn left on 500 East and travel south to U.S.

Hwy 40.

102.4 Turn right on U.S. Hwy 40 and travel west

through Vernal. Stay on Hwy 40 through Roos-

evelt and to Duchesne.

160.9 Duchesne. Turn right on SR 87 and travel north

to the intersection with SR 35.

166.8 At the intersection, turn left onto SR 35 to Tabi-

ona, Hanna, and beyond. The road follows the

Duchesne River.

186.9 Tabiona.

191.3 Hanna.

197.0 Intersection of SR 35 and USFS 144 to upper

Duchesne River drainage. Turn right on USFS

144 and travel north into the upper Duchesne

River drainage toward the Hades Canyon turn off.

200.4 Tintic Quartzite in road cut on right.

200.6 Red Pine Shale is exposed.


fl d006-01 page 23 of 26

201.2 Pull over along the side of the road. After Stop 5,

continue straight ahead.

Stop 5—Paleotopography Developed on the Red Pine Shale

The uppermost unit of the western Uinta Mountain Group,

the Red Pine Shale, is apparently onlapped by the Cambrian Tin-

tic Quartzite. On the east-facing side of the canyon, the cliff of

Tintic Quartzite dramatically thins to the north and pinches out

over this paleohigh in the Red Pine Shale. Up the canyon, the

Mississippian Madison Limestone sits directly on the Red Pine

Shale. Paleotopographic highs in the Uinta Mountain Group are

also exposed east of here along the Green River, just downstream

from the confl uence with the Yampa River. The combination

of these highlands and pervasive angular unconformities in the

Uinta Mountains suggest signifi cant regional uplift after ca.

750 Ma and prior to the Middle Cambrian, likely associated with

the rifting of the Laurentian margin.

The Red Pine Shale in this canyon is ~1200 m thick and

consists of a shale facies, an interbedded shale and siltstone and

sandstone facies, and a sandstone facies (Figs. 2 and 6). The shale

and siltstone and some of the sandstone are organic rich. Note the

Red Pine sandstone and siltstone beds forming the low cliff along

west side of Duchesne River up-canyon. Beyond that cliff, on the

west side, you may be able to see the thick succession (hundreds

of meters) of sandstone making up the middle and upper slope in

the upper Red Pine Shale.



203.5 Road junction of Hades Canyon to Grand View

trailhead to the Granddaddy Lakes area. Turn

right and travel up Hades Canyon to Castle

Rocks overlook near Splash dam.

207.6 Pull over at the Castle Rocks overview pullout.

When Stop 6 is over, turn around and retrace

route down the canyon to SR 35.

Stop 6—Paleoenvironments and Paleontology of the Red Pine Shale

The Castle Rocks viewpoint sits very close to an east-

west–trending normal fault, with the formation of Hades Pass on

the footwall (north side) and the Red Pine Shale on the hanging

wall (south side). The formation of Hades Pass will only be vis-

ible from a distance, yet is the cliffy white to reddish sandstone to

quartzite with signifi cant red shale interbeds. Where exposed, the

contact between the formation of Hades Pass and the Red Pine

Shale in gradational.

The Red Pine Shale that is exposed to the south from the

Castle Rocks overlook, on the south side of Hades Creek, is the

site of a large ~600 m thick measured section, and two other partial

sections to the west on the same ridge (Fig. 6) (Dehler et al., 2006).

Sections were also measured across the Duschesne River along the

lower cliff to the south and across and northward on the steep sand-

stone cliffs to the west (both of these sections are on the east facing

slope of the main canyon). From mapping and correlating these

sections, C. Dehler has calculated a thickness of 1200 m. These

sections were sampled for shale geochemistry, detrital zircon

analysis, paleontology, petrography, and C-isotope stratigraphy.

Common sedimentary structures include parallel- to ripple-

laminations, asymmetric and symmetric ripplemarks, mud-

cracks, normal and reverse grading, slump folds, load structures,

hummocky-cross stratifi cation, silica concretions, mud chips,

rare quartzite pebbles, cut-and-fi ll structures, planar-tabular

crossbeds, and rare associated topsets. Sandstone beds are thin to

thickly bedded, and shale and siltstone intervals are typically tens

to hundreds of meters thick (Fig. 6). This suite of sedimentary

structures indicates prodelta, delta front, and delta plain deposi-

tion in a marine basin (Dehler et al., 2006).

δ13Corg

analysis of organic-rich shales reveals variability of

13.3‰ (Peedee belemnite [PDB]), with values ranging from

−16.9 to −30.2 ‰ PDB (Fig. 2; Dehler et al., 2006). Preliminary

total organic carbon values range from 0.07 to 5.91%. A com-

posite δ13Corg

curve in Figure 2 includes data from the two best-

exposed and more completely sampled sections—the type sec-

tion on the northwest fl ank and the Hades Creek section (Fig. 1).

The variability in δ13Corg

values is similar to other δ13Corg

curves

from middle to late Neoproterozoic marine successions (e.g.,

Coats Lake Group, Kaufman et al., 1997; Chuar Group, Dehler

et al., 2005) suggesting that the Red Pine Shale is within this age

range and is likely marine in origin.

Detrital zircon data indicate that at least two sources of

sand were deposited into the Red Pine basin: an arkosic Archean

source from the Wyoming craton to the north and a Paleoprotero-

zoic source from the Colorado province to the east. These data

are similar to geochemical data from the Big Cottonwood Forma-

tion, suggesting another linkage between the Uinta Mountain

Group and the Wasatch strata to the west (Condie et al., 2001).

Like the rest of the Uinta Mountain Group, most of the

samples collected from the Red Pine Shale yield simple fossils.

Assemblages consisting only of fi laments and leiosphaerids are

common. Also common are monospecifi c assemblages of the bac-

terial aggregate Bavlinella faveolata (Fig. 13D and 13E). These

are preserved in black, sulfur-rich shales, consistent with the

hypothesis that B. faveolata was an anoxygenic photosynthetic

bacterium that lived in euxinic environments (this would also

explain the absence of eukaryotic fossils from B. faveolata assem-

blages; Vidal and Nystuen, 1990). The ~mm-scale carbonaceous

compression fossil Chuaria circularis has also been reported from

the Red Pine Shale (Hofmann, 1977; Nagy and Porter, 2005).

In addition to these simple fossils, a single, silicifi ed mud-

stone from the middle-upper part of the type section has yielded

abundant vase-shaped microfossils (VSMs) preserved as siliceous

internal molds. VSMs are tear-drop–shaped or hemispherically

24 Dehler et al.

fl d006-01 page 24 of 26

shaped tests with a circular or polygonal aperture at one end.

They have been shown to be the remains of fi lose and lobose

testate amoebae, two groups that are common today in freshwa-

ter and terrestrial environments (Porter and Knoll, 2000). VSMs

are locally abundant and widespread in rocks that just pre-date

the Sturtian glaciations, but, with one possible exception, do not

occur in post-Sturtian rocks. They thus provide a useful biostrati-

graphic marker for immediately pre-Sturtian times. Of particular

interest in the Red Pine Shale population are two VSM speci-

mens that are attached at their apertures (Fig. 13N). The same

pose is found in modern testate amoebae undergoing cell division

(Fig. 13O), suggesting these specimens were undergoing asexual

reproduction when they died.

Road Log to Salt Lake City


218.2 Intersection with SR 35; turn right on SR 35 to

Kamas. The road travels up and over Wolf Creek

Pass, down to Woodland, and then to Kamas.

252.2 Kamas. Turn left on SR 248 to U.S. Hwy 40.

264.0 SR 248 and Hwy 40. Take Hwy 40 north to I-80.

268.0 Take I-80 west to Salt Lake City.

End of Day 2 and end of fi eld trip.

ACKNOWLEDGMENTS

Sprinkel’s ongoing work is supported by National Cooperative

Geologic Mapping Program state geologic survey mapping

component (STATEMAP) funding. Dehler’s work and that of

her students is supported by the National Cooperative Geologic

Mapping Program university geologic mapping component

(EDMAP), the Utah Geological Survey, and a New Faculty Grant

from Utah State University. Porter’s work was supported by the

National Science Foundation and an Academic Senate grant

from the University of California–Santa Barbara. Robin Nagy is

thanked for useful discussions and for providing fossil images.

We thank Darlene Koerner and colleagues at Ashley National

Forest and the Bureau of Land Management crew at John Jarvie

Historic Ranch for their scientifi c and logistical support. The

University of New Mexico provided C-isotope lab analyses. Karl

Karlstrom, Laura Crossey, John Bloch, Viorel Atudorei, and Arlo

Weil provided energetic fi eld assistance. Bart Kowallis, Laura De

Grey, and Andy Brehm provided photographs and mileage.

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