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Geological Society of America Bulletin

doi: 10.1130/B30094.1 published online 20 May 2010;Geological Society of America Bulletin

Carol M. Dehler, C. Mark Fanning, Paul K. Link, Esther M. Kingsbury and Dan Rybczynski LaurentiaBig Cottonwood Formation, northern Utah: Paleogeography of rifting western Maximum depositional age and provenance of the Uinta Mountain Group and

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1686

ABSTRACT

U-Pb detrital zircon analyses provide a new maximum depositional age constraint on the Uinta Mountain Group (UMG) and correlative Big Cottonwood Formation (BCF) of Utah, and signifi cantly enhance our insights on the mid-Neoproterozoic paleo-geographic and tectonic setting of western Laurentia. A sandstone interval of the Out-law Trail formation with a youngest popu-lation (n = 4) of detrital zircons, from a sampling of 128 detrital zircon grains, yields a concordia age of 766 ± 5 Ma. This defi nes a maximum age for deposition of the lower-middle Uinta Mountain Group in the east-ern Uinta Mountains and indicates that the group is no older than middle Neo-proterozoic in age (i.e., Cryogenian). These data support a long-proposed correlation with the Chuar Group of Grand Canyon (youngest age 742 Ma ± 6 Ma), which, like the Uinta Mountain Group and Big Cotton-wood Formation, records nonmagmatic in-tracratonic extension. This suggests a ~742 to ≤766 Ma extensional phase in Utah and Arizona that preceded the regional rift epi-sode (~670–720 Ma), which led to develop-ment of the Cordilleran passive margin. This is likely an intracratonic response to an early rift phase of Rodinia. Further, because the Chuar Group and the Uinta Mountain Group–Big Cottonwood Formation strata record intracratonic marine deposition, this correlation suggests a regional ~740–770 Ma transgression onto western Laurentia.

The detrital grain-age distributions from 12 samples include the following grain-age populations and interpreted provenance: 2.5–2.7 Ga (late Archean southern Wyo-

ming province); 1.6–1.8 Ga (Paleoprotero-zoic Yavapai province); 1.5–1.6 Ga (Early Meso proterozoic North American mag-matic gap), 1.4–1.45 Ga (Colorado province A-type granite-rhyolite belt); 0.93–1.2 Ga (eastern Grenvillian orogen); and mid-Neoproterozoic volcanic grains (766 Ma). Sediment was transported by: (1) a major longitudinal west-fl owing river system tap-ping the Grenville orogen, (2) local south-fl owing drainages off the southern Wyoming craton, and (3) northerly and westerly fl owing marine currents. The Uinta Moun-tain Group river system was one of several major transcontinental drainages that deliv-ered Grenvillian zircon grains to the proto–Pacifi c Ocean. We propose that this river system ultimately supplied sediment to peri-Gondwanan margins along the proto-Pacifi c to Antarctica, Australia, and South America, providing an alternative source for explain-ing the problematic provenance of Grenvil-lian grains in these areas.

INTRODUCTION

Geochronologic constraints on Neoprotero-zoic successions are imperative for under-standing the geologic history of the western Laurentian margin in its Rodinian and wider context. The Neoproterozoic Uinta Mountain Group (UMG) and correlative Big Cotton-wood Formation (BCF) of northern Utah form a signifi cant part of this margin. These strata are included in Succession B of North America (>720 to <1000 Ma), which is hypothesized to record intracratonic basin development mark-ing the early stages of the breakup of Rodinia (Stewart, 1972; Young et al., 1979, 1981; Rain-bird et al., 1996, 1997). Until this study, these km-thick successions have lacked geochrono-logic control because they contain no known

tuffaceous beds or microfossils with calibrated short stratigraphic ranges.

The application of detrital zircon U-Pb geo-chronology has signifi cantly advanced Protero-zoic sedimentologic and stratigraphic research. Maximum depositional ages obtained from the youngest zircon age populations are now known for Proterozoic basins on many continents (e.g., Ross et al., 1992; Rainbird et al., 1997; Evans et al., 2000; Southgate et al., 2000; Smithies et al., 2001; Goodge et al., 2002; Ross and Ville neuve, 2003; Jackson et al., 2005; Maclean et al., 2006; Cawood et al., 2007; Cross and Crispe, 2007; Link et al., 2007; Amato et al., 2008), although they do not always yield an age that represents the timing of deposition. Detrital zircon U-Pb age populations also provide fundamental prov-enance information toward under standing the paleogeographic development of Laurentia’s Proterozoic rift margins, although data are lim-ited for middle Neoproterozoic Laurentia (Rain-bird et al., 1997; Stewart et al., 2001; Cawood et al., 2007).

Provenance interpretations of the Uinta Mountain Group and Big Cottonwood Forma-tion, based on sedimentologic, petrographic, geochemical, and Nd-isotope data, support a general model of quartz sand derived from eastern Proterozoic and distal Archean sources and feldspathic sand derived from proximal Archean sources to the north (Wallace, 1972; Sanderson, 1984; Ball and Farmer, 1998; Condie et al., 2001). Preliminary detrital zircon U-Pb and Hf data (n = 4 samples) generated from the Uinta Mountain Group show a domi-nant Meso protero zoic (1.1 Ga–Grenvillian ) age population with Hf isotope values sug-gesting derivation from enriched Grenvillian basement now in the subsurface of south-eastern North America (Mueller et al., 2007). Prior to this study, Paleo protero zoic and mid-Neoproterozoic detrital zircon ages had not

For permission to copy, contact [email protected]© 2010 Geological Society of America

GSA Bulletin; September/October 2010; v. 122; no. 9/10; p. 1686–1699; doi: 10.1130/B30094.1; 7 fi gures; 1 table; Data Repository item 2010129.

†E-mail: [email protected]

Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern

Utah: Paleogeography of rifting western Laurentia

Carol M. Dehler1,†, C. Mark Fanning2, Paul K. Link3, Esther M. Kingsbury3, and Dan Rybczynski1

1Department of Geology, Utah State University, Logan, Utah 84322, USA2Research School of Earth Sciences, Australian National University, Canberra 0200, Australia3Department of Geosciences, Idaho State University, Pocatello, Idaho 83209, USA

Published online May 20, 2010; doi:10.1130/B30094.1 on May 29, 2010gsabulletin.gsapubs.orgDownloaded from


Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah

Geological Society of America Bulletin, September/October 2010 1687

been reported from the Uinta Mountain Group, and no detrital zircons had been analyzed from the Big Cotton wood Formation.

In this paper, we present new SHRIMP (sensitive high-resolution microprobe) detri-tal zircon U-Pb data for 12 samples from the Uinta Mountain Group and the Big Cotton-wood Formation, along with stratigraphic and sedimentologic data collected in the past decade (Figs. 1 and 2; Table 1) (Ehlers and Chan, 1999; De Grey and Dehler, 2005; Brehm, 2007; Brehm, 2008; Kingsbury, 2008; Rybczynski, 2009). We hypothesize that the Uinta Mountain Group and Big Cottonwood Formation intracratonic basin can be placed into a mid-Neoproterozoic context (i.e., Cryo-genian [850–635 Ma], Plumb, 1991; Knoll et al., 2004; U.S. Geological Survey Geologic Names Committee, 2006). We also test corre-lations within Succession B strata in western North America, as well as extracontinental correlations, thus providing a refi ned view of the paleogeographic and tectonic setting of the Uinta Mountain Group–Big Cottonwood For-mation basin and surrounding region.

REGIONAL PRECAMBRIAN GEOLOGY

Uinta Mountain Group

The Uinta Mountain Group of north-central Utah is a thick siliciclastic succession that crops out in the east-west–trending Uinta Mountains, east of Salt Lake City (Fig. 1A). There, the UMG is a 4-km-thick succession of cross-bedded orthoquartzite and sandstone, siltstone, and shale; it has no exposed base and is unconformably overlain by Paleozoic strata (Fig. 2A). It is sub divided into six for-mations, four of them informal, and is inter-preted to record fl uvial and marine deposition (Fig. 2A and Table 1) (e.g., Wallace, 1972; Sanderson, 1984). The lower Red Castle for-mation and much of the Hades Pass quartzite indicate transverse fl uvial and deltaic systems fl owing southward (Table 1; Wallace , 1972; Kingsbury, 2008). All other units in the west-ern Uinta Mountain Group are largely marine siliciclastic shelf deposits that were infl uenced by west-fl owing longshore currents, west-ward- and southward-prograding deltas, and northerly directed tidal currents (Fig. 2A and Table 1; Wallace , 1972; Ehlers, 1997; Dehler et al., 2007; Brehm, 2008; Kingsbury, 2008).

In the eastern Uinta Mountains, near the Colorado-Utah border, a thicker section of the Uinta Mountain Group (~7 km) is exposed and comprises cross-bedded orthoquartzite and sandstone with lesser siltstone, shale, and conglomerate; it lies unconformably on the

Paleo protero zoic Red Creek Quartzite and is unconformably overlain by Paleozoic strata (Figs. 1 and 2A; Hansen, 1965). The basal Jessie Ewing Canyon Formation represents multiple south-fl owing alluvial fans and associated fan deltas that interacted with regional west- and north-fl owing braided fl uvial, deltaic, and shal-low marine environments (Table 1; e.g., Sander-son and Wiley, 1986; Brehm, 2007; Dehler

et al., 2007). The overlying ~6+ km of eastern Uinta Mountain Group strata represent part of a large west- and southwest-fl owing braided river system that was subjected to at least two marine transgressions represented by laterally extensive (10s–100s of km) intervals of shale and fi ne-grained sandstone of the Outlaw Trail formation and the Red Pine Shale (Fig. 2A, Dehler et al., 2007; Rybczynski, 2009).

Wyoming Craton

(>2.5 Ga)

Medicine Hat(2.6–3.3 Ga)

Trans-Hudson Orogen

(3.2–2.5 Ga, 1.92–1.77 Ga)

Superior Craton

(>2.5 Ga)

Central Plains

(1.8–1.7 Ga)

Yavapai(1.8–1.7 Ga)Mojave

(2.0–1.7 Ga)

Grouse Creek

(>2.5 Ga)

Selway(2.4–1.6 Ga)

Farm

ing

ton

Zo

ne

Great Falls

tectonic zone

(1.86–1.77 Ga)

200 km

50°

45°

40°

MT USACANADA

ND

SD

WY

CO

NE

ID

UT

95°100°105°110°

Belt

Cheyenne

Interiorseaway

N

B

Salt Lake City

Vernal

UT

CO

fault system

1 2

57

10

Uinta Mountain GroupBig Cottonwood Fm.

North Flank

43

11

ABrowns Park

8,9Uinta Anticline

126

To Grenville Orogen and foreland

Figure 1. (A) Inset showing the outcrop extent of the Neoproterozoic units sampled in this study. Green stars with numbers 1 through 12 indicate sample locations from this study and are keyed to Figures 2A and 3 and Table DR1 [see footnote 1]. Blue dots are sample loca-tions of Mueller et al. (2007). 1 and 2—Big Cottonwood Formation; 3—Jesse Ewing Canyon Formation; 4—Outlaw Trail formation; 5—Red Castle formation; 6—Moosehorn Lake formation; 7—Mount Watson Formation; 8—Deadhorse Pass formation; 9 and 10—Hades Pass quartzite; and 11 and 12—Red Pine Shale. (B) Paleogeographic reconstruction of the Uinta Mountain and Big Cottonwood basin at ~742 to <770 Ma showing possible source areas for arkosic and quartz-rich sediment and hypothesized extent of intracratonic seaway (transgression shown in blue). Larger arrows indicate paleofl ow of major fl uvial systems. Medium-sized arrows show paleofl ow of transverse drainages. Smallest arrows show paleo-fl ow of longshore and tidal currents. Red arrows indicate modifi cation to previous models (black arrows). See text for further comparison with previous models. (Base map modifi ed from Foster et al., 2006.)



Dehler et al.

1688 Geological Society of America Bulletin, September/October 2010

The western and eastern units of the Uinta Mountain Group are correlated using lithostratigraphy and low-resolution sequence stratigraphy (Fig. 2A). The Red Pine Shale, the uppermost formation in the Uinta Mountain Group (≤1800 m thick), is the only mappable unit exposed throughout the Uinta Mountains; the base of this shale unit can be mapped along strike for 165 km on the north side of the range (Bryant, 1997; Rybczynski, 2009). Fur-ther correlation in the Uinta Mountain Group is based on three upward-fi ning cycles (km thick; S1–S3, Fig. 2A), which represent fl uvial to marine deposition. It is possible that some of the sequence boundaries, in particular the basal one (S1), can be traced westward into the Big Cottonwood Formation (Crittenden, 1976; Christie-Blick, 1997; Fig. 2).

The Uinta Mountain Group represents depo-sition in an intracratonic extensional basin with a roughly east-west–trending northern basin edge and an open, low-relief southern margin (Fig. 1B, Table 1; Dehler et al., 2007), and is entirely developed on autochthonous Lauren-tian continental crust (Karlstrom and Houston, 1984). The previously used term “rift basin” to describe the Uinta Mountain Group basin is problematic because there are no volcanic rocks or signifi cant rift-type facies associated with the Uinta Mountain Group (cf. Link, 1987; Prave, 1999). Compiled data from previous work shows that the Uinta Mountain Group basin was, at least in part, extensional. Relatively coarser grained deposits and a greater percent-age of immature sandstones are found upsection throughout the Uinta Mountain Group strata on the northern side of the range, inferring an east-west–trending, basin-bounding fault (Hansen, 1965; Wallace, 1972; Brehm, 2008; Rybczyn-ski, 2009). Approximately 7 km of northward-thickening Uinta Mountain Group strata are exposed in the hanging-wall of what was likely a south-dipping growth fault (now reactivated as the Uinta-Sparks Laramide reverse fault), sug-gesting that the basin was a north-tilted half-graben (e.g., Sears et al., 1982; Bruhn et al., 1986; Stone, 1993; Dehler et al., 2007; Kings-bury, 2008; Rybczynski, 2009). This interpreted northern basin-bounding fault is roughly par-allel with the 1.7 Ga south-dipping Cheyenne belt suture zone (between the Wyoming Craton to the north and Paleoproterozoic basement to the south) and represents syn–Uinta Mountain Group reactivation on parts of that zone (Bruhn et al., 1983; Stone, 1993).

Previous age constraints on the Uinta Moun-tain Group are sparse, yet correlation with the Chuar Group data sets constrains it to ~740 Ma to 770 Ma (Fig. 2B). The Uinta Mountain Group has long been correlated with the Chuar Group of

Grand Canyon, Arizona (e.g., Young, 1981; Vidal and Ford, 1985; Link et al., 1993), the top of which is now known to be 742 ± 6 Ma (U-Pb age on reworked tuff at the top of upper-most member; Karlstrom et al., 2000). The distinct microfossil assemblage and C-isotope

variability in the Red Pine Shale of the upper Uinta Mountain Group is very similar to that of the upper part of the Chuar Group (Vidal and Ford, 1985; Porter and Knoll, 2000; Dehler et al., 2005; Nagy and Porter, 2005). The fi rst appearance of vase-shaped microfossils and the

BIG COTTONWOOD FMWASATCH RANGE

LW

Big

Co

tto

nw

oo

d F

orm

atio

nM

FF122PL02(2)

90PL05(1)

MF

C

S3?

S2?

UINTA MOUNTAIN GROUPEASTERN UINTA MTNS

JEC

Fm

RCQ

OT

Had

es P

ass

DB

SCUMG-9(4) (766 Ma)

91PL05(3)

Cro

use

Cyn

fm.

RPS

S1

S2

S3

Red Pine datum

fault

140PL02(10)

UINTA MOUNTAIN GROUPWESTERN UINTA MTNS

Had

es P

ass

qtz

ite.

Mt.W

atso

n F

m.

base not exposed

RP03B(12)

69PL05(11)

C

Red

Pin

e

Shal

e

76PL05(5)

36PL06(7)

31PLP06(9)

74PL05(8)

DH

MA

RedCastle

S2?

S3

MH

I.Lake

?

73PL05(6)

Crystalline basementConglomerate, sandstone, shaleSandstoneShale

Key:

ADetrital zircon sample (location number)

1 km

Base of sequenceS1

Detrital zircon sample (Mueller et al., 2007)

Figure 2 (on this and following page). (A) Stratigraphic columns of the Big Cottonwood For-mation (BCF) and Uinta Mountain Group (UMG) showing: intervals where detrital zircon samples were obtained; the existing means of correlation between the eastern UMG, the western UMG, and the BCF; and general stratigraphic trends of the units. S1, S2, and S3 indicate low-order fi ning upward sequences, which show fl uvial or proximal marine units at the base, becoming proximal to distal marine at the top. All units are Neoproterozoic unless otherwise noted. C—Cambrian; MF—Mutual Formation; MFF—Mineral Fork Forma-tion; LW—Paleoproterozoic(?) Little Willow Complex; DH—Deadhorse Pass formation; MA—Mount Agassiz formation; MH—Moosehorn Lake formation; RCQ—Paleo protero-zoic(?) Red Creek Quartzite; JEC—Jesse Ewing Canyon Formation; DB—Diamond Breaks formation; OT—Outlaw Trail formation; RPS—Red Pine Shale. (Modifi ed from Ehlers and Chan, 1999; Dehler et al., 2007.)





abundance of the colonial bacteria Bavlinella faveolata in both the Red Pine Shale and the upper Chuar Group suggest a pre-Sturtian age (>~726–660 Ma; Dehler et al., 2007; Hoff-man and Li, 2009; Nagy et al., 2009) (Fig. 2B). Paleo magnetic data from the Uinta Mountain Group also suggest a mid-Neoproterozoic age (and correlation with the Chuar Group) based on comparison with the apparent polar wander path (APWP) for Laurentia (~800–740 Ma from Chuar Group data; Weil et al., 2006). The micro-fossil Cerebro sphaera buickii was recently found in the lower Chuar Group (Nagy et al., 2009). This acritarch is thought to be an index fossil for pre-glacial strata younger than ~777 Ma (Hill et al., 2000). Maximum depositional ages for the Uinta Mountain Group and the Chuar Group are

900 and 942 Ma, respectively, and come from U-Pb analysis of detrital zircons (Timmons et al., 2005; Mueller et al., 2007).

Big Cottonwood Formation

The Big Cottonwood Formation, exposed in the central Wasatch Range east of Salt Lake City and areas to the west (Slate Canyon, East Tintic Mountains, Stansbury Island, and Carrington Island), is a 5-km-thick succession of sand-stone, orthoquartzite, and argillite with a basal conglomerate interval (Figs. 1 and 2A). This unit is positioned structurally below the thrust sheets of the Sevier orogenic belt and, although associated with thrust faults (several km of dis-placement), has not been signifi cantly displaced

with respect to its original location on the craton (Crittenden, 1976; Ehlers et al., 1997). Ehlers and Chan (1999) interpreted this formation to represent fl uvial and marine deposition based on facies architecture and analysis, and the presence of tidal rhythmites in some intervals. The Big Cottonwood Formation is interpreted to have been deposited together with the Uinta Mountain Group to the east in a tide-dominated estuary whereby a west-fl owing fl uvial system was intermittently drowned by the open ocean (Ehlers et al., 1997). Paleocurrent data show three modes of fl ow directions (NW, SW, and SE), similar to paleofl ow trends in the Uinta Mountain Group units (Table 1).

The Big Cottonwood Formation basin has been interpreted as an intracratonic rift basin

UINTA MOUNTAIN GROUPCHUAR GROUP

1.1 Ga

742 Ma

<942 Ma OLDERPROT

STURT?

Kwag

un

t Fm

.G

aler

os

Fm.

δδ13C (‰ PDB)

200

m

STURT*

>1.7 Ga

<766 Ma

Older

Red

Pin

e Sh

ale

low

er fo

rmat

ion

s

–35 –30 –25 –20 –15 –10

δ13C (‰ PDB)

C<777 Ma

C

Dolomite Organic-rich shale

Sandstone

Breccia/conglomerate

Vase-shaped microfossil

Bavlinella faveolata

Acritarch undifferentiated

Cerebrosphaera buickii

Unconformity

Basalt

Basement

Diamictite

–35 –30 –25 –20 –15 –10

Key:

BFigure 2 (continued). (B) Regional correlation of the Uinta Mountain Group, Utah, with the Chuar Group, Arizona, illustrating their shared features and similar ages of ~740–770 Ma. In the Chuar Group, the presence of Cerebrosphaera buickii (Nagy et al., 2009) suggests strata younger than ~777 Ma (Hill et al., 2000) and the ash at the top is ~742 Ma (Karlstrom et al., 2000). In the Uinta Mountain Group, the maxi-mum depositional age of ~766 Ma in the Outlaw Trail formation indicates the majority of the group, if not all, is younger than this age (this paper). The Red Pine Shale and the upper Chuar Group host similar fossil assemblages, including vase-shaped microfossils, considered to indicate a pre-Sturtian age (before the fi rst glacial episode of the Cryogenian; Vidal and Ford, 1985; Dehler et al., 2007). Corresponding organic carbon-isotope anomalies in the upper strata of both units show relatively positive isotope values declining to background values (Dehler et al., 2005; Dehler et al., 2007; Brehm, 2008). Total 7 km of Uinta Mountain Group not shown (see Fig. 2A). Sturtian deposits above Red Pine Shale extrapolated from relationships between Big Cottonwood Formation and Mineral Fork Formation to the west (Link et al., 1993). Older ages in sub-Chuar strata are from Timmons et al. (2005). Age of basement below Uinta Mountain Group is from Hansen (1965). Pink band shows proposed correlation between the Uinta Mountain Group and the Chuar Group. PDB—Pee Dee belemnite; Prot—Proterozoic; Sturt—Sturtian.



Dehler et al.


that was bounded on its northern edge by a westward continuation of the basin-bounding fault of the northern Uinta Mountain Group basin. Evidence for this includes: the northern pinchout of the Big Cottonwood Formation across a Laramide-age syncline, clasts of crys-talline basement in the overlying Mineral Fork Formation suggesting that there was no BCF present to the north of the modern BCF outcrop

by Mineral Fork time (i.e., Sturtian), and that the basal BCF conglomerate could refl ect rift onset and be correlative with the basal conglomer-ate of the Uinta Mountain Group (Crittenden, 1976; Christie-Blick, 1997; Ehlers et al., 1997). The Big Cottonwood Formation, much like the Uinta Mountain Group, does not contain vol-canic rocks or a signifi cant amount of immature facies that would be expected in a classic rift

basin. Big Cottonwood Formation and Uinta Mountain Group sediments were likely depos-ited in different parts of the same intracratonic extensional basin (Link et al., 1993).

The timing of deposition of the Big Cotton-wood Formation is unknown, but the unit is cor-related with the Uinta Mountain Group based on lithostratigraphic and paleomagnetic data (e.g., Link et al., 1993; Ehlers and Chan, 1999;

TABLE 1. PALEOFLOW, PALEOENVIRONMENTAL, AND PROVENANCE INFORMATION

*atadtnerrucoelaPnoitamroF †§ Paleoenvironment# Petrography

Detritalzircon

provenance References

Red Pine Shale, middle Southerly** Prodelta Quartzose shale Mixed Brehm (2008);

Rybczynski (2009)

Red Pine Shale, lower Southerly** Delta front Feldspathic arenite Archean Brehm (2008)

Hades Pass, lower and upper

n = 18* Fluvial deltaic Subfeldspathic arenite Mixed

Wallace (1972);Lee (2000);

Kingsbury (2008)

n = 143†

Mount Watson, basal

n = 20*

Marine, fl uvial? Quartz arenite MixedWallace (1972);

Sanderson (1984);Kingsbury (2008)

Dead Horse Pass

n = 170*

Barrier bar system Subarkosic siltstone Mixed Wallace (1972);Kingsbury (2008)

Moosehorn Lake

n = 3*

Marine Quartz arenite Mixed + Neoprot. Wallace (1972);Rybczynski (2009)

Red Castle

n = 291*

Fluvial, tidal Arkose Archean Wallace (1972);Kingsbury (2008)

Outlaw Trail

n = 120*

Estuary, fl uvial Subfeldspathic arenite and shale Mixed + Neoprot. Dehler et al. (2007);

Rybczynski (2009)

Jesse Ewing Canyon

n = 37*

Marine, fan delta Subfeldspathic arenite Mixed + Neoprot. Brehm (2007)

n = 182†

Alluvial fan NA NA Sanderson andWiley (1986)

Big Cottonwood

n = 114†

Fluvial tidal

Quartz arenite (basal),

subfeldspathic siltstone(middle)

Archean (basal), mixed

(middle)

Chan et al. (1994);Ehlers and Chan (1999)

*Paleofl ow data from studies associated with this paper.†Paleofl ow data from other studies with palefl ow data expressed as rose diagrams.§Flow direction (black arrow) shows specifi c fl ow direction of interval sampled (where available).#Paleoenvironments shown as underlined are linked to detrital zircon samples.**Inferred from architecture, provenance





Dehler et al., 2001a). Further evidence for a Big Cottonwood Formation–Uinta Mountain Group correlation includes similarity of paleo-current data, depofacies, and detrital zircon age populations (Figs. 2A and 3; Table 1; Ehlers, 1997; Ehlers and Chan, 1999; this paper). The Big Cottonwood Formation is unconformably bracketed by the underlying Paleoprotero-zoic(?) Little Willow Complex and the overly-ing Neoproterozoic (Sturtian glacial) Mineral Fork Formation (Fig. 2A; Crittenden, 1976; Christie-Blick and Link, 1988).

METHODS

Stratigraphic Units Sampled for Detrital Zircons

Twelve samples were collected from the Uinta Mountain Group (n = 10) and Big Cot-tonwood Formation (n = 2) (De Grey, 2005; Brehm, 2007; Brehm, 2008; Kingsbury, 2008). The samples were collected to ascertain the detrital zircon provenance from a spectrum of sandstone compositions throughout the stratigraphic successions and from sandstone units representing different depositional envi-ronments and/or different parts of the basin (Figs. 1 and 2A; Table 1; GSA Data Repository Table DR11). This sampling strategy comple-ments the detrital zircon study by Mueller et al. (2007), whose data were presented from four samples of a limited geographic, paleo envi-ronmental, and stratigraphic range (Figs. 1A and 2). Eight of our samples are from the west-ern Uinta Mountain Group, and two are from the eastern UMG. Collectively they span from the base to the top of the entire group and were collected along and across strike of the east-west–trending northern margin of the Uinta Mountain Group basin (Figs. 1A and 2A). The samples from the Big Cottonwood Formation are from the basal (fl uvial-tidal) interval and from the middle interval where tidal rhythmites are present (Fig. 2A and Table 1).

Geochronology

U-Pb zircon analyses were performed on the 12 samples described above (Tables 1, DR1, and DR2 [see footnote 1]). Our samples were sepa-rated from several kg of sandstone, siltstone,

and/or shale. A heavy mineral concentrate was prepared from the total rock using standard crushing, washing, heavy liquid (specifi c grav-ity 2.96 and 3.3), and paramagnetic procedures (Williams, 1998). Representative fractions of the total zircon-rich heavy mineral concentrate were poured onto double-sided tape, mounted in epoxy together with chips of the reference zircons (Duluth Gabbro-FC1 and Sri Lankan-SL13, see below), sectioned approximately in half, and polished. Refl ected and transmitted light photomicrographs were prepared for all zircons. Cathodoluminescence (CL) scanning electron microscope (SEM) images were pre-pared for all zircon grains and used to examine the internal structures of the sectioned grains. The U-Th-Pb analyses were made using a combination of SHRIMP I, SHRIMP II, and SHRIMP reverse geometry (RG) at the Re-search School of Earth Sciences, the Austra-lian National University, Canberra, Australia. For the provenance studies, the zircons were analyzed sequentially and randomly with total number of grains analyzed ranging from 30 to 72 grains, except for one special case where 128 grains were analyzed (see discussion of sample SCUMG-9 below). In some cases, as determined from CL imaging and resultant U-Pb ages, the zircon populations were deemed

to be remarkably homogeneous, in which case the total number of grains run was less than in other samples. The analyses consisted of four to six scans through the mass range, with a refer-ence zircon analyzed for every fi ve unknown zircon analyses (Williams, 1998, and references therein). The data have been reduced using the SQUID Excel Macro of Ludwig (2003).

Pb/U ratios have been normalized relative to a value of 0.01859 for the FC1 reference zircon, equivalent to an age of 1099 Ma (see Paces and Miller, 1993). Uncertainties given for individual analyses (ratios and ages) are at the one sigma level; however, the uncertainties in concor-dia age are reported as 95% confi dence limits. Wetherill concordia plots (Fig. DR1 [see foot-note 1]), probability density plots with stacked histograms, and concordia age calculations were carried out using ISOPLOT/EX (Ludwig, 2003). For grains that are less than 1000 Ma, the 206Pb/238U age was used for the relative prob-ability plots, and for those over 1000 Ma, the 207Pb/206Pb age was employed. Analyses that are more than 20% discordant were not included in the relative probability plots. However, samples 76PL05 and 90PL05 are dominated by Archean age grains, many of which are metamict, have lost radiogenic Pb, and so are greater than 20% discordant. From Wetherill concordia plots, it

700 270023001900

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6

8

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6

Age (Ma)

Num

ber

Relative probability

Location 2: 122PL02, middle Big Cottonwood Formation

Big Cottonwood Canyonsubfeldspathic siltstone; tidal

(56 analyses; 62 grains)

Location 1: 90PL05, lower Big Cottonwood Formation

mouth of Big Cottonwood Canyonquartz arenite; fluvial-tidal

(25 analyses; 52 grains)

Figure 3 (on this and following page). (A) Relative probability plots, with stacked histograms for the two samples from the Big Cotton-wood Formation.

1GSA Data Repository item 2010129, Table DR1: Sample locations; Tables DR2.1 through 2.12: Data for all 12 detrital zircon sample SHRIMP analyses; Table DR3: Petrographic data for 12 detrital zircon samples; Figure DR1: Wetherhill plots for all 12 de-trital zircon sample SHRIMP analyses, is available at http://www.geosociety.org/pubs/ft2010.htm or by request to [email protected].



Dehler et al.


4

2

Relative probability

5

1

3

6

Location 12: RP03-B, middle Red Pine ShaleHades Creek. south flank, Uintas

quartz siltstone; distal deltaic(25 analyses, 30 grains)

Location 7: 36PL06, Deadhorse Pass fm.Gilbert Peak, High Uintas

subarkosic siltstone; marine (28 analyses; 42 grains)

Location 3: 91PL05, Jesse Ewing Canyon Fm.type section, Jesse Ewing Canyon

coarse-grained subfeldspathic arenite;marine

(40 analyses, 72 grains)

Location 8: 74PL05, Mount Watson Fm.Bald Mountain, High Uintas

quartz arenite; marine(53 analyses; 57 grains)

Location 6: 73PL05, Moosehorn Lake fm.Provo River headwaters

fine-grained quartz arenite; marine(52 analyses; 61 grains)

Location 11: 69PL05, lower Red Pine Shaletype section, Red Pine Creek

feldspathic arenite; proximal deltaic(35 analyses, 40 grains)

Location 5: 76PL05, Red Castle fm.Christmas Meadows, Stillwater River; fluvial

coarse-grained arkosic arenite; fluvial(18 analyses; 30 grains)

Location 10: 140PL02, upper Hades Pass fm.Henrys Fork, High Uintas

subfeldspathic arenite; fluvial-deltaic(46 analyses, 60 grains)

Location 4: SCUMG-9, Outlaw Trail fm.Browns Park, eastern Uintas

subfeldspathic arenite and shale; estuary(106 analyses, 128 grains)

Location 9: 31PL06, lower Hades Pass fm.Gilbert Peak, High Uintas

subfeldspathic arenite; fluvial-deltaic(67 analyses; 72 grains)

B

Figure 3 (continued). (B) Relative probability plots, with stacked histograms for the ten samples from the Uinta Mountain Group. See Figure 2A for corresponding stratigraphic data and Tables DR2 and DR3 [see footnote 1] for additional detrital zircon analytical data.





is clear that this discordance arises from radio-genic Pb loss at or near the present day, because the data defi ne a simple lead loss discordia line. In these specifi c cases, the 207Pb/206Pb age is considered to refl ect the crystallization age of the detrital zircons (Fig. DR1 [see footnote 1]), and thus were included in our spectra. If one were to exclude all such discordant analyses, the data set would be biased against these sig-nifi cantly older grains. It should be noted that the analyses for sample 36PL06 also show sig-nifi cant discordance. However, in this case, the Wetherill concordia plot (Fig. DR1 [footnote 1]) shows that the radiogenic Pb loss did not oc-cur at or near the present day. In this case, the 207Pb/206Pb ages for such discordant analyses are not considered to refl ect the crystallization age of the zircon, and so they were excluded from the relative probability plots.

SCUMG-9 Sample from Outlaw Trail formation

As with the other samples, zircon grains (n = 50) from sample SCUMG-9 (Swallow Canyon-UMG-9) were fi rst analyzed in a ran-dom manner in order to determine the detrital zircon age distribution (Fig. 3). Within these 50 randomly selected grains, one had a concordant age of ~770 Ma, and we analyzed a second area on this sectioned grain to confi rm the notable young age. For the other 49 grains analyzed in this session, fi ve failed to yield meaningful data, and the remaining 44 grains had U-Pb ages ≥880 Ma, with most ≥1000 Ma. It must be noted, and in fact stressed very strongly, that a single grain cannot truly defi ne the maximum age of deposition. There are many variables, not the least of which includes the potential for the youngest grain being a contaminant of the laboratory processing. We hand selected ad-ditional grains from that fi rst mineral separa-tion and collected a second sample suite from the same stratigraphic interval; we then car-ried out another complete mineral separation. Zircon grains from both samplings were hand selected on the basis of similar physical charac-teristics—least rounded, subhedral to euhedral grains, with zoned igneous internal structure on the basis of the CL imaging (Fig. 4). From a population of many hundreds of grains (com-bined from two fi eld samplings), and an addi-tional 78 SHRIMP U-Pb analyses, three more young grains with the same ~770 Ma age were identifi ed (Fig. 4; Table DR2 [see footnote 1]). The four grains have widely different morphol-ogies, ranging from subrounded, more equant grains (grains 1 and 38) to elongate, to sub-hedral grains (grains 12 and 41) (Fig. 4). It is notable that grain 12 has internal cavities con-

sistent with rapid crystallization in a vol canic to subvolcanic setting. The style of internal CL zoning is consistent with a felsic igneous paragenesis; the zircons are not from a mafi c igneous source where broader or subdued CL structure would be present (Corfu et al., 2003).

RESULTS

Maximum Depositional Age of the Uinta Mountain Group

After analyzing a total of ~128 zircon grains (Table DR2 [see footnote 1]) in sample SCUMG-9, we identifi ed the four youngest analyses that are within analytical uncertainty of each other, and these give a concordia age at 766.4 ± 4.8 Ma (Figs. 4 and 5). Similar single-grain U-Pb dates are recorded in the Jesse Ewing Canyon Formation sample (91PL05) and the Moosehorn Lake formation sample (73PL05) (Fig. 3B). Unfortunately, these are isolated single-grain analyses and so individu-ally are not yet considered to be depositional age constraints. Nevertheless, the presence of grains less than 800 Ma in three samples from the lower-middle Uinta Mountain Group in both the eastern and western parts of the range, in different stratigraphic levels, including the very basal formation, adds further support to a general age assignment for the Uinta Mountain Group at <770 Ma.

The fact that there are only four out of 128 grains analyzed (3.1%) in SCUMG-9 that pro-vide the ~766 Ma date in the Uinta Mountain Group demonstrates the diffi culty of recogniz-ing small but important detrital zircon popula-tions in detrital zircon studies. This is in general agreement with Vermeesch (2004), who cal-culated that in the worst case scenario, at least 117 grains are required in order to have 95% confi dence that one has identifi ed a group of ages that comprises 5% of the total population (cf. Stewart et al., 2001; Mueller et al., 2007).

From the initial analyses of sample 31PL06, grain 8 gave a slightly discordant age of just less than 700 Ma (207Pb/206Pb date of 697 ± 9 Ma, ~5% discordant). This is anomalously young, because the other 56 grains analyzed are Grenvillian or older (Fig. 3; Table DR2 [see footnote 1]). A replicate analysis of this grain confi rms the original date. In the course of this replication, a further 15 grains were targeted in an attempt to fi nd another similarly young grain, but only Grenvillian or older were re-corded. Grain 8 is not a simple euhedral grain that might be expected for a volcaniclastic zir-con dating the youngest deposition. The grain is dark colored and high in U (~1780 and 2645 ppm, respectively, for the two analyses) and

200 μm

200 μm

200 μm

200 μm

Grain 1

Grain 12

Grain 38

Grain 41

Figure 4. Combined transmitted light photo-micrographs and cathodoluminescence (CL) scanning electron microscope images of the young grains (766 Ma) analyzed from the SCUMG sample, Outlaw Trail formation. Grains labeled as per Table DR2.4 [see foot-note 1]. The areas analyzed are shown on the CL image.



Dehler et al.


has likely lost radiogenic Pb. This single grain is the only one of this age that was found out of the overall total of 704 grains analyzed and, while interesting, no geological signifi cance is placed on this single grain. Indeed, many geological data sets indicate that the Uinta Mountain Group is ~740–770 Ma, including correlations with the Big Cottonwood Forma-tion (which is overlain by presumed Sturtian Mineral Fork Formation) and the Chuar Group (~740–~770 Ma) (Figs. 2A and 2B).

Detrital Zircon Age Populations

Overall, from a total of 704 grains analyzed, 565 are considered signifi cant in terms of con-cordance and/or have meaningful 207Pb/206Pb ages, and these have been used to construct relative probability plots (Fig. 3; Table DR2 [see footnote 1]). Peaks are identifi ed at ~2700, 1650–1850 Ma, 1500–1600 Ma, 1400–1450 Ma, and 930–1200 Ma, in addition to the small popu-lation of ~766 Ma grains. Archean and Gren-villian grains occur in about equal proportions, with ~32% of the total grains analyzed in the age range 2500–2755 Ma, whereas ~36.5% are between 930 and 1300 Ma.

Archean Detrital Zircon Age Populations (2.5–2.8 Ga)

All 12 samples contain some Archean grains with a dominance of grains at 2600–2700 Ma (see Fig. 3). Three samples (lower Big Cottonwood, lower Red Pine Shale, and lower Red Castle formations) contain solely Archean grains.

Two of these are feldspathic samples from the Uinta Mountain Group (76PL05 and 69PL05), and one is a quartz arenite from the basal Big Cottonwood Formation (90PL05) (Figs. 3 and 6; Tables DR2 and DR3 [see footnote 1]). Those from the BCF have suffered signifi cant

radiogenic Pb loss, but still defi ne a prominent 207Pb/206Pb age peak at ~2685 Ma. Concordant grains in the lower Red Castle Formation de-fi ne a single peak at 2600 Ma, whereas zircons from the lower Red Pine Shale have a more dis-persed age spectrum with prominent peaks at ~2645 Ma and 2710 Ma (Fig. 3). All three sam-ples have few to no grains older than 2800 Ma.

We infer that the source for the Uinta Moun-tain Group feldspathic deposits was on the southern margin of the Wyoming province (SAT—Southern Accreted Terranes of Mueller and Frost, 2006). Petrographic, facies, paleocur-rent, and detrital zircon data together indicate that fi rst-cycle sediment was derived from igne-ous and/or metaigneous sources exposed along the southern part of the Wyoming Craton and deposited in deltaic systems on the northern side of the Uinta Mountain Group basin (Table 1; Table DR3 [see footnote 1]). The strong Archean peak in the Red Pine Shale, the youngest unit of the Uinta Mountain Group, and in the lower Red Castle formation, one of the oldest units in the western Uinta Mountain Group, and the presence of similar Archean peaks in all other samples, indicates that local northern source area(s) continuously contributed immature sedi-ment throughout deposition of the Uinta Moun-tain Group (Figs. 2A and 3; Table 1).

Unlike the Uinta Mountain Group feld-spathic samples mentioned above, the basal Big

0.116

0.120

0.124

0.128

0.132

0.136

0.98 1.02 1.06 1.10 1.14 1.18 1.22 1.26

760

206 Pb

/238 U

207Pb/235U

Data-point error ellipses are 68.3% confidence

800

720Concordia age = 766 ± 5 Ma

(1σ, decay-constant errors included)

MSWD (of concordance) = 0.66,

Probability (of concordance) = 0.42

Figure 5. Concordia plot from SCUMG-9 (Outlaw Trail formation) showing concordia age of 766 ± 4.8 Ma (supplemental data in Table DR2.4 [see footnote 1]). MSWD—mean square of weighted deviates.

Qt

F L100

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

Mixed

Mixed + NeoproterozoicArchean only

10

00

Figure 6. Ternary diagram showing relationship between sandstone composition and detrital zircon age populations. Note that there is not a direct correlation between composition and detrital zircon provenance. See Table DR3 [footnote 1] for petrographic data.





Cotton wood Formation sample with the uni-modal Archean-age population is a texturally mature, fi ne-grained quartz arenite of fl uvial-tidal origin (Ehlers and Chan, 1999) (Fig. 6, Table DR3 [see footnote 1]). The depofacies, petrography, and detrital zircon provenance of this sample suggest that quartz-rich sands were produced from fl uvial and tidal rework-ing of feldspathic sands similar to those repre-sented in the Uinta Mountain Group samples with unimodal Archean grain-age populations. This does not appear to be the case in the Uinta Mountain Group, however, where quartz are-nite samples show a mixture of Archean and Protero zoic grain-age populations (Table 1). The westerly and northerly paleofl ow directions in the Big Cottonwood Formation are supportive of reworking by longitudinal rivers or tides (by comparison to facies and paleofl ow in the Uinta Mountain Group, Table 1). Alternatively, these grains could have been derived from a quartz-rich source on the Wyoming craton, now eroded or covered, that supplied sediment to the incipi-ent basin in the Big Cottonwood area (Fig. 6; Table DR3 [see footnote 1]). The wide range in sandstone composition of these three Uinta Mountain Group–Big Cottonwood Formation samples with unimodal Archean grain popula-tions shows that composition is not necessarily a refl ection of provenance, as has been the general model for UMG provenance (Fig. 6).

Whole rock Nd-isotope data from Big Cot-tonwood Formation, Red Pine Shale, and Red Castle shale samples (Condie et al., 2001; sam-ple locations not reported) suggest a dominantly Paleoproterozoic or mixed Paleoproterozoic–Archean source rather than the Archean prove-nance shown in our detrital zircon data. Because unimodal Archean detrital zircon populations were found in coarse- and fi ne-grained sand-stone samples, grain size alone does not explain the differences in zircon provenance, at least in the sand fraction (Fig. 6; Table DR3 [see foot-note 1]). As our understanding of the complex sourcing of the Uinta Mountain Group and Big Cottonwood Formation basin unfolds, it may be that the shale deposits sampled by Condie et al. (2001) were derived from Paleoproterozoic or mixed sources. In any case, the whole rock Nd analyses, because they are a composite average analysis of the total rock samples, record differ-ent chemical systematics than the more resilient single zircon data presented herein.

Paleoproterozoic Detrital Zircon Age Populations (1.65–1.85 Ga)

Paleoproterozoic detrital zircons are present in nine of the samples. They range in age from 1650 to 1850 Ma and were derived from source areas in the juvenile Paleoproterozoic crust of

the Mojave, Yavapai, and Mazatzal–Central Plains provinces. Most of these grains, like other Proterozoic grains in this study, were transported from the east, northeast, and southeast via fl u-vial, tidal, and/or long shore currents (Table 1; Fig. 1B). The presence of the mixture of Archean and Paleoproterozoic detrital zircon grains typi-cally coincides with a change in depofacies and demonstrates a trend seen on many stratigraphic scales. For example, the basal Red Pine Shale (sample 69PL05) is a delta-front deposit with a unimodal Archean zircon age population (Figs. 2A and 3; Table 1). Above this interval, the Red Pine Shale (sample RP03B) shows transgression to a distal prodelta setting with mixed Archean and dominantly Paleoproterozoic detrital zircon grains (Dehler et al., 2007; Brehm, 2008). The Paleoproterozoic provenance signature is con-sistent with Nd-isotope values from many of the Uinta Mountain Group and Big Cottonwood Formation shale samples analyzed by Condie et al. (2001). Petrographic results from these nine samples again show that sandstone compo-sition does not consistently correlate with a spe-cifi c source area (Fig. 6).

Early Mesoproterozoic (1.5–1.6 Ga) Detrital Zircon Age Population

There is a minor but significant 1500–1600 Ma age peak in four of the samples from the Uinta Mountain Group and Big Cotton-wood Formation (Fig. 3). The ultimate source for these grains may be from Australia (e.g., Gawler Range Volcanics), but a more proxi-mal and likely source could be reworked lower Belt Supergroup equivalents (e.g., Ross and Villeneuve, 2003; Link et al., 2007) and/or the Pinware terrane in northeastern Canada (Rain-bird et al., 1997; Heaman et al., 2004). This grain-age population has thus far only been found in the middle part of the two successions (Uinta Mountain Group and Big Cottonwood Formation) and is associated with very promi-nent Grenvillian peaks; further analyses could reveal that this is a signifi cant detrital zircon stratigraphic-marker interval, perhaps indicat-ing a regional paleogeographic change. This idea is consistent with the proposed correlation between the Uinta Mountain Group and Big Cottonwood Formation in Figure 2A.

Early-Middle Mesoproterozoic (1.4–1.45 Ga) Detrital Zircon Age Populations

Seven of the samples yielded an age popula-tion of 1.4 to 1.45 Ga. These zircons probably were ultimately sourced from A-type igneous rocks that were emplaced across Laurentia be-tween 1330 and 1480 Ma (Van Schmus et al., 1993) or reworked from the Belt basin to the north (Link et al., 2007).

Middle-Late Mesoproterozoic (9.3–1.2 Ga) Detrital Zircon Age Populations

Nine samples contain grains ultimately de-rived from the 930–1200 Ma Grenville-age ter-rane. The large proportion of Grenvillian grains reaffi rms the “Grenville fl ood” as reported by Mueller et al. (2007), and extensively noted pre-viously in similar age rocks elsewhere in Lau-rentia, Baltica, and Siberia (Rainbird et al., 1992, 1997; Maclean et al. 2006; Cawood et al., 2007). The Grenvillian grains are associated with the Paleoproterozoic and other Mesoproterozoic grain-age populations and a wide range of paleo-fl ow directions indicating a mixing of popula-tions from multiple source areas. On the other hand, some samples show a correlation between large populations of Grenvillian grains and over-all westerly paleofl ow (Table 1), suggesting deri-vation from the Grenville orogen and its foreland to the east (e.g., Link et al., 1993; Mueller et al., 2007; Baranoski et al., 2009). The river system that fl owed through and/or into the Uinta Moun-tain Group–Big Cottonwood Formation basin supplied major numbers of Grenvillian zircons to the western margin of Laurentia and the proto-Pacifi c (Fig. 7).

The Big Cottonwood Formation tidal rhyth-mite sample (122PL02) is dominated by a Gren-villian detrital zircon population (Fig. 3A). This is in stark contrast to the other sample from the basal BCF, which has only Archean zircon grains. This is another good example (like the Red Pine Shale samples) of what is likely caus-ing the provenance change in the Big Cotton-wood Formation–Uinta Mountain Group strata: the marine or relatively distal depofacies have mixed populations, and the fl uvial-infl uenced or more proximal (northern) depofacies are domi-nated by Archean grains.

Neoproterozoic (~766 Ma) Detrital Zircon Age Population

Three samples contain detrital zircons derived from a 760–770 Ma source. The morphology and internal CL structure of the grains indicate a felsic volcanic source for these zircons (Fig. 4). These younger grains are from samples that have mixed provenance, and come from sub-arkosic and/or quartz-rich sandstone and shale from similar marine depofacies (Table 1; Figs. 3B and 6). Sampled intervals of the Outlaw Trail (SCUMG-9), Jesse Ewing Canyon (91PL05), and Moosehorn Lake (73PL05) formations show north-northwesterly paleofl ow indicating fi nal transport was by marine currents coming from the south-southeast (Table 1).

There are several possible sources for the ~760–770 Ma grains. In eastern Laurentia, rhyolite in the Mount Rogers Formation is ~760 Ma (Aleinikoff et al., 1995), and some



Dehler et al.


plutonic rocks of the Crossnore Complex in the North Carolina Blue Ridge are between 750 and 760 Ma (Su et al., 1994). There are no known felsic sources of this age in western Laurentia. Considering the paucity of ~766 Ma grains in the Uinta Mountain Group, it is possible that the zircons were the product of an ashfall event from eastern Laurentia or the proximal con-jugate Rodinia rift margin, such as from the 777 ± 7 Ma Boucaut Volcanics of the Nackara Arc in South Australia (Preiss, 2000). Regard-less of the primary source, the 766 Ma grains appear to be unique to marine depofacies and were ultimately carried via marine currents dur-ing transgressions.

DISCUSSION

Paleogeography and Tectonics

The maximum depositional age of the Out-law Trail formation, which, along with the Jesse Ewing Canyon and Moosehorn Lake for-mations, hosts the 760–770 Ma zircon grains, refi nes previously proposed correlations within Succession B of western North America. De-trital zircon age populations from the Big Cotton wood Formation are nearly identical to the Uinta Mountain Group age populations, further supporting their correlation and paleo-geographic relationships. The maximum age of the UMG and correlative BCF is similar to the hypothesized age of the lower Chuar Group of Grand Canyon (~770 Ma) and strengthens this correlation. These correlations place the Uinta Mountain Group, Big Cottonwood Formation, and Chuar Group at the top of Succession B in southwestern Laurentia. If correlation with the

middle Pahrump Group of Death Valley holds (e.g., Link et al., 1993; Rainbird et al., 1996; Dehler et al., 2001b), it is possible that all of these units were deposited after ~770 Ma, in a series of coeval, and possibly connected marine basins in southwestern Laurentia (the ChUMP Interior Seaway; Dehler, 2008). This age range is consistent with constraints on Succession B strata in Canada, which also record intra-cratonic basin development at this general time (>723 Ma to <1070 Ma; Rainbird et al., 1996, and references therein).

Correlations with strata farther afi eld suggest that the transgressions documented in the Uinta Mountain Group–Big Cottonwood Formation basin were eustatically driven. In addition to the development of marine intracratonic basins in Laurentia at ~740–770 Ma, major marine in-undation also commenced at ~760–770 Ma in the Adelaide rift complex of Australia (Preiss, 2000) and is coincident with the development of other marine intracratonic basins of this gen-eral age (e.g., Akademikerbreen Group, north-east Svalbard and Eleonore Bay Group, east Greenland [Knoll et al., 1986; Halverson et al., 2007]). These correlations suggest a pre-Sturtian and/or early Cryogenian interval of high global sea level that was likely caused by increased seafl oor spreading rates as Rodinia rifted apart (e.g., Asmerom et al., 1991; Preiss, 2000; Li et al., 2003; Li et al., 2008).

The Uinta Mountain Group, Big Cottonwood Formation, and Chuar Group together record intracratonic extension and sedimentation in southwestern Laurentia at ~740–770 Ma that was likely related to the breakup of Rodinia (Dehler et al., 2001a; Timmons et al., 2001; this paper). This would have taken place prior to bi-

modal volcanism and normal faulting recorded in the ~680–720 Ma formations along the Cor-dilleran margin (e.g., Pocatello, Kingston Peak, Mineral Fork, Perry Canyon, and Edwards-burg formations) and subsequent formation of a west-facing passive margin after 650 Ma (Christie-Blick, 1997; Lund et al., 2003; Fan-ning and Link, 2004, 2008). The Adelaide rift complex (~777 Ma) and rift basins in south China (745–780 Ma) and Namibia (758.5 ± 3.5 Ma) are roughly coeval with the ~740–770 Ma Laurentian intracratonic basins, sup-porting the idea of a time-transgressive breakup of Rodinia (Hoffman and Halverson, 1996; Hoffman et al., 1998; Preiss, 2000; Eyles and Januszczak, 2003; Li et al., 2003).

Provenance

The detrital zircon age populations presented in this paper generally reaffi rm the interpreta-tions of Condie et al. (2001), among others, that sediment was sourced both from the north, from the Archean Wyoming province, and from the east, from a major river parallel with the strike of the Cheyenne Belt and the fabric of the Paleoproterozoic accreted crust of Colorado (Fig. 1B). This large west-fl owing fl uvial system contained detrital zircons from 1650 to 1850 Ma juvenile Paleoproterozoic crust of the Mojave, Yavapai, and Central Plains provinces, 1450 Ma A-type granites of Colorado, and 930–1200 Ma Grenvillian terranes (Fig. 1B). In addition to derivation from the Wyoming Province to the north, Archean detrital zircons were also likely delivered from the Superior craton to the north-east (Fig. 1B), which is consistent with a com-mon southwesterly paleofl ow associated with the alluvial and fl uvial depofacies of the east-ern Uinta Mountain Group (Table 1; Sanderson and Wiley, 1986; De Grey and Dehler, 2005; Rybczynski, 2009).

A minor, but signifi cant, number of previ-ously unrecognized mid-Neoproterozoic zir-con grains (~760–770 Ma) were derived from an unknown source and fi nally transported in a north-northwesterly direction within the Uinta Mountain Group–Big Cottonwood Formation system during marine transgressions. The prov-enance change associated with these marine units, defi ned by detrital zircon populations, paleocurrent data, and depofacies, suggest that during transgressive episodes, young grains were reworked, and/or different sources were tapped to the south-southeast and integrated with sediment continuously derived from the north and east via deltaic and/or fl uvial systems (Fig. 1B; Table 1). During regressions there was less mixing of grain populations, especially proximal to the basin edges.

Figure 7. Paleotectonic recon-struction at ~750 Ma modi-fi ed from Goodge et al. (2008) showing transcontinental Uinta Mountain Group river system tapping Grenvillian sources, crossing the Mid-Continent rift (gray areas), entering the Uinta Mountain Group–Big Cotton-wood Formation fl uvial-marine system (outcrop extent shown in larger black area), and de-livering Grenvillian grains to adjacent peri-Gondwanan conti-nental margins (black arrows). Also shown are the potentially coeval Shaler and related river systems in northern Lau-rentia (Rainbird et al., 1992, 1997). See text for discussion. Abbreviations: UMG—Uinta Mountain Group; BCF—Big Cottonwood Formation; MM—Mackenzie Mountain basin; Am—Amundsen basin; CG—Chuar Group. Short dashed line shows hypothesized shore-line during maximum transgression onto western Laurentia at ~766 Ma.

Grenville

highlands

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Ocean ?

East Antarctica

Australia

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Divide

MM Am. Baltica

CGGre

nville hig

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East Antarctica

Australia

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The Fate of Grenvillian Sediment Downstream of the Uinta Mountain Group–Big Cottonwood Formation River System

The Uinta Mountain Group–Big Cottonwood Formation river system was a continental-scale river system that transported Laurentian zircons, in particular the abundant 930–1300 Ma grains derived from the Grenville province, westward to western Laurentia and ultimately to the incipi-ent marine basin formed by the early rifting of Rodinia (Fig. 7) (cf. Mueller et al., 2007). This river system may have been coeval with other pan-continental river systems in northern Lau-rentia that were also tapping the Grenville oro-gen (Young et al., 1979; Rainbird et al., 1992, 1997). Limited geochronologic control on the Shaler and Mackenzie Mountain supergroups (>723 Ma and <1070 Ma), which record these northern fl uvial systems, makes this paleogeo-graphic interpretation uncertain.

Though Rodinia reconstructions are under debate, the confi guration shown by Goodge et al. (2008) places the Uinta Mountain Group–Big Cottonwood Formation basin such that it may have been an avenue for Grenvillian zir-cons to reach not only the western Laurentian margin but also peri-Gondwanan Neoprotero-zoic continental margins. This may have pro-vided a way for large quantities of Grenvillian zircons to be reworked into latest Neoprotero-zoic sands on several continental margins of eastern Gondwana (Ireland et al., 1998) that otherwise have no known viable Grenvillian source. This idea requires that the proto–Pacifi c Ocean separating Laurentia from Antarctica and Australia maintained subdued topography and, hence, rifting had not yet commenced in this part of the ocean basin.

ACKNOWLEDGMENTS

Support was provided by the U.S. Geological Sur-vey, National Cooperative Geologic Mapping Pro-gram (Dehler and Link); the Utah Geological Survey Mapping Division; and the National Science Founda-tion grant EAR-0819759 (Link, Dehler, and Yonkee). We thank graduate students Laura De Grey Ellis, Andy Brehm, and Caroline Myer Brehm for contri-butions. The manuscript benefi ted from reviews by Paul Mueller, Lang Farmer, Ron Blakey, Tony Prave, Robert Rainbird (associate editor), Scott Sampson, Paul Myrow, Gerry Ross, and an anonymous reviewer. Discussions with John Goodge and Paul Hoffman also improved this manuscript. Mineral separation assistance came from Joe Larsen, Debbie Pierce, and Mark Schmitz.

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