Tectonic inferences from the ca. 1255–1100 Ma Unkar Group ...€¦ · BACKGROUND OF GRAND CANYON...

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© 2005 Geological Society of America 1573

ABSTRACT

The Unkar Group of the Grand Canyon Supergroup is one of the best-preserved remnants of Mesoproterozoic sedimentary rocks in the southwestern United States. It provides an exceptional record of intracra-tonic basin formation and associated tecton-ics kinematically compatible with protracted “Grenville-age” NW-directed shortening. New U/Pb age determinations from an air-fall tephra at the base of the Unkar Group dates the onset of deposition at ca. 1255 Ma, and 40Ar/39Ar K-feldspar thermochronology in the Grand Canyon indicates that base-ment rocks cooled through 150 °C between ca. 1300 and 1250 Ma, refi ning exhumation rates of basement rocks just prior to Unkar deposition. Abrupt thickness and facies changes in conglomerate and dolomite of the Bass Formation (lower Unkar Group) associated with NE-striking monoclinal fl exures indicate NW-directed synsedimen-

tary contraction at ca. 1250 Ma. A large disconformity (~75 m.y. duration) is inferred between the lower and upper Unkar Group and is located below the upper Hakatai Shale, as documented by detrital zircons. A second style of Unkar Group deformation involved the development of half grabens and full grabens that record NE-SW extension on NW-striking, high-angle normal faults. Sev-eral observations indicate that NW-striking normal faulting was concurrent with upper Unkar deposition, mafi c magmatism, and early Nankoweap deposition: (1) intrafor-mational faulting in the Bass Formation, (2) intraformational faulting in the 1070 Ma (old Rb/Sr date) Cardenas Basalt and lower Nan-koweap Formation, (3) syntectonic relation-ships between Dox deposition and 1104 Ma (new Ar/Ar date) diabase intrusion, and (4) an angular unconformity between Unkar Group and Nankoweap strata. The two tectonic phases affecting the Unkar Group (ca. 1250 Ma and ca. 1100 Ma) provide new insight into tectonics of southern Laurentia: (1) Laramide-style (monoclines) deformation in the continental interior at ca. 1250 Ma records Grenville-age shortening; and (2) ca. 1100 Ma detrital muscovite (Ar/Ar) and zir-con (U/Pb) indicate an Unkar Group source

in the Grenville-age highlands of southwest-ern Laurentia during development of NW-striking extensional basins. We conclude that far-fi eld stresses related to Grenville-age orogenesis (NW shortening and orthogonal NE-SW extension) dominated the sedimen-tary and tectonic regime of southwestern Laurentia from 1250 to 1100 Ma.

Keywords: Grenville, Unkar Group, Nan-koweap Formation, Precambrian monocline, intracratonic rifting, Grand Canyon.

INTRODUCTION

The late Mesoproterozoic (1.3–1.0 Ga) was

characterized by the development of orogenic

belts worldwide that record the assembly of the

supercontinent of Rodinia (Dalziel, 1991; Hoff-

man, 1991; Moores, 1991). The Grenville oro-

gen of NE Laurentia (Rivers et al., 2002) and the

Texas Grenville (Mosher, 1998; Bickford et al.,

2000) record protracted convergence along the

“southern” (present coordinates) plate margin.

The term “Grenville” has been used in many

ways, but here, we consider a broad interval

of Grenville-age orogenesis recorded by island

arc (1360–1232 Ma), and continent-continent

(1150–1120 Ma) collisions (Mosher, 1998;

Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and

Nankoweap Formation, Grand Canyon: Intracratonic deformation and

basin formation during protracted Grenville orogenesis

J. Michael Timmons†

New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, New Mexico 87801, USA

Karl E. Karlstrom‡

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

Matthew. T. Heizler§

New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, New Mexico 87801, USA

Samuel A. Bowring#

Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA

George E. Gehrels††

Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

Laura J. Crossey‡‡

Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

GSA Bulletin; November/December 2005; v. 117; no. 11/12; p. 1573–1595; doi: 10.1130/B25538.1; 16 fi gures; 1 table; Data Repository item 2005196.

†E-mail: [email protected].‡E-mail: [email protected].§E-mail: [email protected].#E-mail: [email protected].††E-mail: [email protected].‡‡E-mail: [email protected].

TIMMONS et al.

1574 Geological Society of America Bulletin, November/December 2005

Rivers et al., 2002) as recorded in west Texas.

This protracted deformation was the culmina-

tion of a long-lived convergent/transpressional

margin that persisted between 1.8 and 1.0 Ga

(Karlstrom et al., 1999, 2001).

Modern examples of far-fi eld intracratonic

deformation (interior to orogenic belts) in

response to plate-margin interaction are com-

mon and include Lake Baikal, Tibetan Plateau,

and upper and lower Rhine grabens (discussed

in the following). In each of these cases, defor-

mation in the continental interior is driven by

and is kinematically linked with plate-margin

deformation. Ancient examples of far-fi eld

intracratonic deformation in southwestern Lau-

rentia associated with plate-margin interaction

have been proposed for several time periods,

such that this region seems to be an exception-

ally important laboratory for understanding

intracratonic deformation. Important examples

include: regional 1.4 Ga magmatism and defor-

mation (Nyman et al., 1994; Kirby et al., 1995);

1.1 Ga intracratonic rifting in the midcontinent

rift (Gordon and Hempton, 1986) and Central

Basin platform (Adams and Keller, 1996); ca.

750 rifting of the continental interior ~200 km

from the plate margin (Timmons et al., 2001);

Ancestral Rocky Mountains (Kluth and Coney,

1981; Ye et al., 1996; McBride and Nelson,

1999); and Laramide orogeny (Hamilton, 1981;

Bird, 1988; Livaccari, 1991; Erslev and Rog-

ers, 1993; Humphreys, 1995).

This paper examines the stratigraphic and

structural record of intracratonic contractional

and extensional tectonism and syntectonic

sedimentation in the 1.25–1.1 Ga Unkar Group

and ca. 900 Ma Nankoweap Formation of the

Grand Canyon. We recognize a complex inter-

play between synsedimentary contractional

and extensional faults in the Unkar Group,

which demonstrates a kinematic link to Gren-

ville orogenesis (Fig. 1). We present new geo-

chronology that improves our understanding of

regional tectonic events and sedimentary pat-

terns. Finally, this paper illustrates contrasting

responses to plate-scale contractional deforma-

tion from proximal deposits in the foreland to

distal deposits hundreds of kilometers into the

continental interior. Combined with analysis of

correlative units of the Apache Group of Ari-

zona (Shride, 1967; Wrucke, 1989), Grenville

deposits in west Texas, and the Lower Pah-

rump Group of California (Prave, 1998), this

study leads to a new paleogeographic under-

standing of southwestern Laurentia, which

may be useful for continued tests of confl icting

Rodinia reconstructions (Brookfi eld, 1993;

Dalziel, 1997; Karlstrom et al., 1999; Piper and

Jia sheng, 1999; Sears and Price, 2000; Win-

gate et al., 2002).

BACKGROUND OF GRAND CANYON GEOLOGY

The Grand Canyon Supergroup is exposed as

isolated fault-bounded remnants along the main

stem of the Colorado River and its tributaries in

the Grand Canyon (Fig. 2). It rests nonconform-

ably on basement metamorphic and igneous

rocks of the Granite Gorge Metamorphic Suite

(Ilg et al., 1996). The Grand Canyon Super-

group is formally divided into the Upper Meso-

proterozoic (1255–1100 Ma) Unkar Group

and Neoproterozoic (ca. 800–742 Ma) Chuar

Group (Van Gundy, 1951). The unconformity-

bounded Nankoweap Formation separates the

two groups. The Sixtymile Formation caps the

Chuar Group, and all Proterozoic rocks are

overlain in angular discordance (up to 15°) by

Middle Cambrian Tapeats Sandstone.

The 1255–1100 Ma Unkar Group is ~2100 m

thick and is divided into the Bass Formation,

Hakatai Shale, Shinumo Sandstone, Dox For-

mation, and Cardenas Basalt (Fig. 3; Elston,

1979; Hendricks and Stevenson, 1990). The

succession contains both fl uvial and shallow-

marine deposits, with one main disconformity

within or below the Hakatai. In general, Unkar

rocks dip northeast (10–30°) toward normal

faults that dip 60° southwest (Sears, 1990).

John Wesley Powell (1875) fi rst described

the gently tilted strata along the river corridor.

Walcott (1894) named the upper Chuar and

lower Unkar terranes. Noble (1914) divided the

Unkar Group into fi ve formations, excluding

the Cardenas Basalt, but the stratigraphy was

later revised by Beus et al. (1974) to include

the Cardenas Lavas within the Unkar Group.

The use of the term Cardenas Lava(s) is well

engrained in the literature (Hendricks and Ste-

venson, 1990, 2003); however, we use the term

Cardenas Basalt after Larson et al. (Larson et

al., 1994) as the preferred name for the volu-

minous basalt and basaltic andesite at the top of

the Unkar Group. Noble (1914), Sears (1973),

and Timmons et al. (2001, 2003) mapped the

structures in the Unkar Group. Numerous work-

ers conducted further stratigraphic studies of

the formations of the Unkar Group (Beus et al.,

1974), including unpublished MS theses that

are summarized and updated in the following

text (Fig. 3).

SYNOPSIS OF UNKAR GROUP STRATIGRAPHY

Dalton (1972) described the Hotauta Con-

glomerate and Bass Limestone. He recognized

the heterolithic composition of the Bass Lime-

stone, and suggested that it should have forma-

tion status, including the Hotauta Conglomerate

as a member. Dolomite is the dominate carbon-

ate facies in the Bass Formation, with subordi-

nate conglomerate, breccia, sandstone, and mud-

stone (60–100 m; Fig. 4). These intercalations

and primary structures, such as wave rippled

sandstone and mud-cracked surfaces, indicate

that Bass Formation deposition occurred during

repeated subaerial exposure and fl ooding, and

that it represents relatively low-energy intertidal

to supratidal depositional environments in a

general transgressive sequence (Dalton, 1972;

Beus et al., 1974; Hendricks and Stevenson,

1990). White, very fi ne-grained tephra are inter-

bedded with dolomite and mudstone toward the

base of the section. One of these ash fall beds

yielded zircons for U/Pb geochronology (see

geochronology section).

The Hotauta Member conglomerate at the

base of the Bass Formation in the eastern

Grand Canyon contains more than 80% clasts

of granite and quartzite that range from pebble

to cobble size (Table 1). Quartzite clasts have

no local equivalents in the Grand Canyon, indi-

cating a distant source. Granite clasts may be

locally derived and mixed with quartzite clasts,

or transported with quartzite clasts from a dis-

tant source. Conglomerate beds and intraclastic

breccias indicative of higher-energy depositional

environments are interbedded with low-energy

carbonate and mudstone deposition within the

Bass Formation. Clast composition and size in

the upper conglomeratic units are similar to the

basal Hotauta Member, but with the addition of

intraclasts of carbonate and siliciclastic compo-

sition derived from the lower Bass Formation

(~17.3% of the conglomerate clasts, Table 1).

Interbedded breccias are composed exclusively

of intraclastic material and thus are interpreted

to be locally derived (Fig. 4). The recycling

of carbonate, and perhaps clasts from basal

conglomerates in the eastern Grand Canyon,

refl ects localized uplift (by faulting) and erosion

of lower units.

The Hakatai Shale (137–300 m) is a mud-

stone- to coarse sandstone–dominated package

that appears to be in gradational contact with

the Bass Formation (Reed, 1976). The Haka-

tai Shale has been subdivided informally into

three members. The Hance Rapids member is

dominated by thin-bedded subarkose to quartz

arenite. The middle member (Cheops Pyramid)

is mostly mudstone. The uppermost Stone

Creek member is dominated by coarse arkose.

The Hakatai Shale is more heterolithic than

the name implies, because it contains numer-

ous sandstone beds. Marker beds generally are

absent in the section; however, two prominent

sandstone markers (~0.5 m thick) are pres-

ent ~50 m above the base. Mud cracks, ripple

marks, tabular-planar cross-bedding, salt casts,

TECTONIC INFERENCES FROM THE CA. 1255–1100 MA UNKAR GROUP

Geological Society of America Bulletin, November/December 2005 1575

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TIMMONS et al.


and tool marks, indicate shallow-water deposi-

tion, probably in a marginal-marine/tidal-fl at

environment (Reed, 1976).

The contact of the Hakatai Shale with the

overlying Shinumo Sandstone is sharp and

locally truncates cross-beds and channel forms,

which suggests a hiatus (Fig. 3; Daneker,

1975). The Shinumo Sandstone (syn. Shinumo

Quartzite; 355–410 m) forms resistant cliffs

composed of lower subarkose and upper quartz

arenite with subordinate interbedded mudstone

(Daneker, 1975). A basal lag of conglomerate

contains basement clasts up to 5 cm across.

Like the Hotauta Conglomerate, the basal con-

glomerate includes quartzite clasts that have no

known equivalents in the Grand Canyon region.

The Shinumo Sandstone is divided infor-

mally into the basal Surprise Valley, Ribbon

Falls, Papago Creek, Cottonwood Camp, and

75-Mile Rapid members from bottom to top.

The predominant rock type in the Shinumo

Sandstone is quartz arenite; however, subarkose

is more common in the Surprise Valley member.

Sedimentary structures in sandstone beds are

dominated by centimeter- to meter-scale planar-

tabular cross-stratifi cation and trough cross-beds

(Fig. 5) that record north-directed paleocurrent

directions. Subordinate bidirectional paleocur-

rent indicators, such as bidirectional cross-bed

sets, are also observed. Trough cross-beds sug-

gest a more northerly transport direction and are

more common near the top of the section and its

contact with the Dox Formation. Upper massive

sandstone beds of the Shinumo contain dramatic

convolute bedding. The presence, abundance,

and repetition of very thick (meters to tens

of meters) contorted beds in upper beds have

been cited as evidence for earthquake activity

and fl uid migration during Shinumo deposi-

tion (Sears, 1973; Daneker, 1975; Middleton

and Blakey, 1998; Timmons et al., 2001).

The depositional environment proposed for the

Shinumo is nearshore, marginal marine, and/or

fl uvial/deltaic (Daneker, 1975). Sears (1973)

reports that faulting during Shinumo deposition

resulted in thickness changes across discrete

fault zones. This conclusion was based on map-

ping in Bright Angel Canyon and a reported

thickness change of ~61 m over 460 m distance

across the Bright Angel monocline.

The contact between the Shinumo Sandstone

and the Dox Formation is gradational and is

marked by a change in topographic expression

and color. The transition also marks a strik-

ing upward gradation from quartz arenite to

mudstone and fi ne-grained arkose. The basal

Dox Formation includes channel facies arkosic

sandstone that represents fl uvial deposition by

a large river system, and hence a regression to

fl uvial/deltaic facies.

The Dox Formation is divided into the

Escalante Creek, Solomon Temple, Coman-

che Point, and Ochoa Point Members, based

10 km

East Kaibab m

onocline

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Butte fault

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Bass Canyon

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Chuar Group

Unkar Group

Laramide monocline

Proterozoic monocline

Granite Gorge Metamorphic Suite

Proterozoic normal fault, ball on the downthrown side

Cardenas Basalt and DiabaseCardenas Basalt and Diabase

1120 31'W

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Figure 2. Generalized geologic map of eastern Grand Canyon showing outcrops of Precambrian rocks. Neoproterozoic rocks of the Chuar Group record synsedimentary movement of the Butte fault system. Unkar-age rocks are preserved in grabens and half grabens bounded by northwest-striking normal faults. Also shown are the locations of NE-striking Precambrian monoclines in the Unkar Group. Laramide monoclines reactivated both NW and N-S structural trends. Also shown are locations for measured sections of the Bass Formation and sample locations for thermochronologic and geochronologic specimens presented in this paper.



on color changes and topographic expression

(Stevenson, 1973). Marker beds generally are

absent with the exception of a zone of contorted

bedding near the base that may suggest a con-

tinuation of Shinumo-age seismicity, and two

beds in the Comanche Point Member, a convo-

lute sandstone bed and a thin (30 cm) carbonate

bed (Stevenson, 1973).

The Dox Formation is interpreted here as a

marginal marine deltaic to tidal fl at sequence

that records a regression from marine Shinumo

followed by transgression within the Dox For-

mation. Fluvial-dominated distributary chan-

nel deposits with unidirectional paleocurrent

indicators dominate the base of the Dox section

(Escalante Creek Member; Fig. 5). Upper mem-

bers of the Dox Formation record the transition

from channel sandstone beds in intrafl uvial

mudstones to thin-bedded sheet sand deposits

with wave-ripples that are mud-draped and

mud-cracked. Uppermost beds are interpreted

as tidal-dominated mud-fl at facies.

The contact between the Dox Formation and

Cardenas Basalt shows interfi ngering of basalt

fl ows with Ochoa Point sandstone and mud-

stone. Small folds and load structures beneath

the basalt, and intrafl ow red-bed sandstone

beds through the Cardenas section (Hendricks,

1972; Stevenson, 1973) suggest that basaltic

volcanism was contemporaneous with red-bed

deposition.

The Cardenas Basalt is ~300 m thick and

consists of >10-m-thick fl ow units (Hendricks,

1972). Three marker layers are described as

informal members: the bottle-green member,

fan-jointed member, and lapillite member (Luc-

chitta and Hendricks, 1983). Hyaloclastite of

the bottle-green member is ~90 m thick, highly

altered, and contains secondary chlorite, epi-

dote, talc, and zeolites. Basaltic andesite (~50 m

thick) comprises the fan-jointed member with

porphyritic to aphanitic and vesicular textures

(Hendricks and Lucchitta, 1974). The lapillite

member ranges in thickness from a few meters

to several tens of meters thick and is composed

of scoriaceous lapilli and blocks (~10 cm) and

volcanic bombs (<1 m) in matrix, suggesting

proximity to a vent location (Lucchitta and

Hendricks, 1983). The lapillite member is

interbedded with massive fl ows of basalt that

comprise the remaining thickness of the Carde-

nas Basalt.

Intrusive rocks of the Unkar Group are similar

in mineralogy and chemistry to the basalts, sug-

gesting that intrusive and extrusive rocks were

coeval and shared a common source. Intrusive

rocks occur as dikes and sills within the Unkar

Group, with sills ranging in thickness from a

few tens of meters to 300 m; dikes typically are

much thinner and locally follow fault planes.

aN

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ar G

roup

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ar G

roupGra

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500

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1000

1500

2000

2500

3000

Cardenas Basalt

Dox Formation

Hakatai ShaleBass Formation

GalerosFormation

KwaguntFormation

Nankoweap Formation

Hotauta Member

Shinumo Sandstone

Cambrian

Granite Gorge Metamorphic Suite and Granitoids

Tonto Group

Carbon Butte MemberAwatubi Member

Walcott Member

Tanner Member

Duppa MemberCarbon Canyon Member

Jupiter Member

Ochoa Point MemberComanche Point MemberSolomon Temple Member

Escalante Creek Member

Sixtymile Formation.

approx.ages (Ma)

510-

ca. 900

1104

sandstone

basalt

conglomerate

shale andsiltstone

limestoneand dolomite

ca. 1100 Ma diabase sills

unconformity

ash beds

convolute bedding

cross-beds

Explanationbreccia

a

aa

1842ca. 1650-

3500

4000

s stromatolite

742-(U/Pb)

(inferred pmag)

(Ar/Ar)

approx.thickness(m)

1255(U/Pb)

ca. 750

ca. 770

(Ar/Ar marc.)

(U/Pb monz)

s

s

s

s

s

whitered

<1187(U/Pb DZ)

<1167(U/Pb DZ)

<1170(U/Pb DZ)

Figure 3. Stratigraphic column of the Grand Canyon Supergroup showing geochronologic constraints and approximate ages (modifi ed from Elston, 1989). Mafi c intrusive rocks cross-cutting rocks of the Unkar Group are commonly associated with Cardenas magmatism. The 1600-m-thick Chuar Group unconformably overlies the Nankoweap Formation and includes two main formations, the Galeros and Kwagunt Forma-tions with seven members and several key marker beds (Dehler et al., 2001). Overlying the Chuar Group is the Sixtymile Formation.

TIMMONS et al.


12

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Extrusive rocks tend to be more silicic than the

extrusive rocks, however, the lower bottle-green

member is compositionally very similar to

intrusive rocks, suggesting some differentiation

of the parent magma after emplacement of the

sills and eruption of lowermost fl ows (Lucchitta

and Hendricks, 1983). Unkar Group igneous

rocks are correlated to mafi c intrusions of simi-

lar age in the southwest (Howard, 1991).

Van Gundy (1937, 1951) fi rst recognized

the unconformity-bounded section of quartzitic

sandstones and mudstones above the Cardenas

and named the strata the Nankoweap Group,

removing it from the Unkar Group of Noble

(1914). Mapping in the eastern Grand Can-

yon by Maxson (1961) introduced the name

Nankoweap Formation. Later workers divided

the Nankoweap into two informal members,

including the lower ferruginous member and

upper member separated by a disconformity

with tens of meters of relief (Elston and Scott,

1976) separating rocks with greatly differing

paleomagnetic directions (Elston and Scott,

1973). The Nankoweap type section in Basalt

Canyon measures ~100 m thick (Gebel, 1978).

The lower member is dominated by hematite-

cemented sandstone and siltstone with lenses

of lithic sandstone derived from the underlying

Cardenas. The upper member is composed of

siltstone and thin-bedded fi ne-grained red-bed

sandstones toward the base and more massive

meter-thick sandstone beds toward the top of

the section. The capping white sandstone is

composed of a fi ne-grained quartz-cemented

quartz arenite. Abundant sedimentary features

are identifi ed in the section and include planar-

tabular and trough cross-bedding, ripple marks,

mud cracks, soft sediment deformation, and

rare salt casts (Gebel, 1978). Combined, these

features suggest that the Nankoweap Formation

was deposited in a moderate- to low-energy,

shallow-water environment, perhaps a struc-

turally controlled marine or lake environment

(Elston and Scott, 1976; Gebel, 1978).

UNKAR GROUP THERMOCHRONOLOGY AND GEOCHRONOLOGY

U-Pb Geochronology of the Bass Formation

The combination of a new U/Pb zircon date

on an ash bed (direct date) and Ar/Ar dates

on intrusive rocks (direct dates), with thermo-

chronologic results from basement rocks and

detrital grains (indirect dates) provides new

constraints on the age of the Unkar Group.

Samples for U/Pb zircon dating were collected

from ash-fall deposits interbedded in the lower

Bass Formation. Sample K7-77-7 is from a very

fi ne-grained (clay-sized) white horizon with

no obvious sedimentary features to suggest

transport of material. The suspected tephra are

a few to several tens of centimeters thick, and

are located in the lower half of most measured

sections of the Bass Formation. Direct correla-

tion of individual tephra beds between measured

sections is not possible and suggests that these

beds are discontinuous over the study area.

Seven small (<100 microns) clear-euhedral

TABLE 1. CLAST COUNTS

Quartzite Granite Vein qtz Meta lithic Chert Carbonate Sandstone Shale Other lithic Total clasts

No. (%) No. (%) No. (%) No. (%) No. (%) No. (%) No. (%) No. (%) No. (%)

Upper Bass conglomerates

Vishnu Canyon (section #4) 9 11.0 35 43 0.0 3 3.7 8 9.8 0.0 15 18.0 2 2.4 10 12 82Vishnu Canyon (section #3) 22 26.0 32 38 4 4.7 0.0 9 11.0 8 9.4 3 3.5 7 8.2 0 85South Kaibab (section #6) 24 22.0 55 51 3 2.8 3 2.8 3 2.8 16 15.0 0.0 0.0 3 3 107Bright Angel (section #7) 17 15.0 56 49 4 3.5 13 11.0 5 4.3 19 17.0 0.0 0.0 1 1 115Bright Angel (section #7) 22 19.0 68 58 1 0.9 8 6.8 7 6.0 11 9.4 0.0 0.0 0 117Clear Creek (section #5) 10 8.5 34 29 9 7.6 4 3.4 17 14.0 26 22.0 0.0 0.0 18 15 118Representative % composition 17.0 45 3.4 5.0 7.9 13.0 2.9 1.4 5 624

Lower Bass conglomerates

Vishnu Canyon (section #4) 37 35 49 46 17 16 2 1.9 1 0.9 106

Imbrication ripple foresets planar tabular foresets

Trough axes Parting lineations

A

n = 70

24%12%

B

12%6%

n = 186

Figure 5. Rose diagrams of paleocurrent indicators from (A) planar-tabular and trough cross-stratifi cation in the Shinumo Sandstone that preserves dominant transport direction toward ~350°. Planar-tabular beds suggest bipolar current directions (to NE and SW) as suggested by cross-stratifi cation in the fi eld. (B) The Dox Formation yields a variety of paleocurrent indicators, including current ripples, planar-tabular foresets, trough foresets, and parting lineations. Both paleocurrent directions for parting lineations are plotted. Paleo-currents are strongly oriented to the NNW as suggested by trough cross-beds and supported by N-directed parting lineations.

TIMMONS et al.


zircons were separated from sample K7-77-7,

fi ve of which were analyzed. Analytical details

are presented in Table DR11 and Schmitz et al.

(2003). Four of the fi ve zircons cluster near con-

cordia and one is discordant but yields a precise

upper intercept of 1254.8 ± 1.6 Ma (Fig. 6). We

recognize the possibility that these grains could

represent inherited or detrital zircon grains

based on the dominance of ca. 1250 Ma detrital

zircons in the upper Unkar Group (discussed

in the following). However, based on regional

lithologic correlations (see discussion section),

and the simple zircon population recovered, we

interpret 1254.8 Ma as the depositional age of

the lower Bass Formation.

Basement Thermochronology

The 40Ar/39Ar data from basement rocks gen-

erally are compatible with the tephra age and

suggest exhumation prior to Bass deposition.

Timmons et al. (2001) reported that the onset of

Unkar Group deposition postdated ca. 1250 Ma

and fi nal closure to Ar loss in K-feldspar-bear-

ing basement rocks. The 40Ar/39Ar studies of

K-feldspars show that individual feldspars con-

tain multiple diffusion domain sizes, and thus

have variable closure temperatures or retentivity

(150–325 °C) dependent on domain size within

a single grain (Lovera et al., 1989; McDougall

and Harrison, 1999). Because different diffu-

sion domains in a K-feldspar are closed to Ar

loss at different temperatures, single feldspar

crystals can record a segment of the thermal his-

tory rather than a single point. Smaller diffusion

domains release Ar at temperatures between

150 and 200 °C, whereas the largest diffu-

sion domains remain retentive at temperatures

approaching 325 °C.

Metamorphic and igneous feldspars were

collected along the river corridor to evaluate

basement thermal history in the Grand Canyon

(Fig. 2). Twelve K-feldspars were analyzed

and their age spectra are given in Figure 7

(Table DR2, see footnote one). The age spectra

typically are complex and do not in all cases

allow unambiguous age or thermal-history

interpretations. Electron microprobe analyses

of selected samples reveal variable degrees

of alteration to sericite and distinct albite/

K-feldspar intergrowths at scales ranging from

submicron to tens of microns. We attribute gen-

eral age spectra complexity to this alteration.

Many spectra reveal steep age gradients

from ca. 600–1300 Ma during the initial 10%

of 39Ar released, and steep gradients commonly

are followed by an age decrease or undulatory

pattern for the middle part of the spectra (e.g.,

Figs. 7A, 7B, and 7E). Following the age

decrease, or saddle part of the spectra, the ages

rise during the fi nal ~50% of gas release. Due

to these complexities, several samples cannot

be used for multiple diffusion domain modeling

(MDD method; Lovera et al., 1989). The causes

for the initially old apparent ages followed by

decreasing apparent ages are not well under-

stood, and we suspect that it is caused primarily

by 39Ar recoil and secondary alteration of the

K-feldspars. This spectrum pattern also is gen-

erated by models that mimic recrystallization

of large diffusion domains below their closure

temperatures (Lovera et al., 2002). Aside from

the overall age spectrum complexity, some sam-

ples (Figs. 7A, 7C, and 7I) have ages that are

too young based on the 1255 Ma age from the

Bass Formation. If the ages were meaningful,

these samples would suggest that the basement

was at elevated temperatures (>200 °C) during

deposition of the Bass Formation. Given that

these samples were collected below exposures

of the Bass Formation, they could not have been

at these temperatures during or after Bass depo-

sition (Fig. 4). For these samples, the sericite

alteration of the K-feldspars presumably causes

apparent ages that are geologically too young

with respect to the inferred post–Bass Forma-

tion thermal history.

In contrast to samples that appear too young

and/or too complex, samples K7-115-3, T02-

98-14, and T02-98-16 (Figs. 7F, 7K, and 7L)

are considered to be well enough behaved to

be used for MDD modeling (Fig. 8). Arrhe-

nius plots (Figs. 8D–F) are constructed using

the fraction of 39Ar released and the labora-

tory-heating schedule with the assumption of

a plane-sheet diffusion geometry. Of the three

samples chosen for MDD modeling, sample

T02-98-14 has initial diffusion coeffi cients that

yield a well-defi ned linear array, which indi-

cates an activation energy (E) of 38.1 kcal/mol

(Fig. 8E). The other spectra have poorly defi ned

initial linear segments that probably are related

to simultaneous degassing of several diffusion

domains and/or slight alteration. We chose to

model all the samples with E = 38.1 kcal/mol in

order to facilitate sample comparison, but rec-

ognize that this is a poorly constrained assump-

tion, since it has been shown that K-feldspars

can have a range of activation energies

(McDougall and Harrison, 1999). The poor

resolution of E will primarily affect the absolute

temperature of the thermal history and has very

little effect on the cooling rates inferred from the

models (Lovera et al., 1997). Log r/ro plots for

each sample (Fig. 8G–I) are constructed from

the diffusion coeffi cients and reference Arrhe-

nius Law (Lovera et al., 1991). Like nearly all

1GSA Data Repository item 2005196, details of the methods used, data tables, and age calcula-tions for Ar/Ar geochronology and thermochro-nology, and tables for U/Pb zircon analyses, is available on the Web at http://www.geosociety.org/pubs/ft2005.htm. Requests may also be sent to [email protected].

Figure 6. U-Pb data from a thin ash (K-77-7) in lower Bass Limestone near Hance Mine, Grand Canyon. Zircons are small (<100 µm) euhedral crystals. Seven analyses, including three that cluster near concordia, give a precise upper-intercept date at 1254.8 ± 1.6 Ma, interpreted as the age of the tuff. MSWD—mean square of weighted deviates.



A H02-81-3 ksp

600

800

1000

1200

1400

Int. Age = 1214.0 ± 1.9 Ma

B T02-86-2 ksp

Int. Age = 1266.8 ± 2.0 Ma

100

1000 K/Ca

C H02-87.5-3 ksp

Int. Age = 1109.9 ± 1.8 Ma

D K6-91.1-1 ksp

600

800

1000

1200

1400

Int. Age = 1286.0 ± 2.0 Ma

E K7-95.5-1 ksp

Int. Age = 1296.8 ± 1.7 Ma

100

1000K/Ca

F T02-98-14 ksp

Integrated Age = 1329 ± 2 Ma

G T02-98-16 ksp

600

800

1000

1200

1400


H K7-99-4 ksp

Int. Age = 1287.9 ± 1.9 Ma

100

1000K/Ca

I T02-99.5-1 ksp


J H02-107-1 ksp

0 20 40 60 80 100

600

800

1000

1200

1400


K K7-115-3 ksp

0 20 40 60 80 100

Int. Age = 1243.9 ± 1.4 Ma

100

1000 K/Ca

L T02-136.2-2 ksp

0 20 40 60 80 100


Cumulative 39Ar Released

App

aren

t Age

(M

a)A

ppar

ent A

ge (

Ma)

App

aren

t Age

(M

a)A

ppar

ent A

ge (

Ma)

Figure 7. Age spectrum and K/Ca diagrams for basement K-feldspars along the Colorado River corridor. Variably complex spectra are attributed to different thermal histories, complex K-feldspar/albite intergrowths, and degree of alteration to white mica. Horizontal line at 1255 Ma represents the age of the ash layer within the Bass Formation that unconformably overlies the basement. Intermediate parts of the age spectra that record ages younger than this are considered unreliable based on maximum post–Bass deposition basement temperatures (<200 °C) estimated from Supergroup stratigraphic thickness. Initial steep age gradients probably record heating to ~150 °C during the Neoproterozoic burial history of the basement. Samples are keyed to the map in Figure 2.

TIMMONS et al.


50

100

150

200

250

300

350

400

K7-115-3

0

50

100

150

200

250

300

350

400

T02-98-16

0600 800 1000 1200 1400

0

50

100

150

200

250

300

350

400

T02-98-14

Age (Ma)

600 800 1000 1200 1400

Age (Ma)

600 800 1000 1200 1400

Age (Ma)

J K L

-0.4-0.20.00.20.40.60.81.01.21.41.61.82.0

log(

r/r o)

500600700800900

1000110012001300140015001600

App

aren

tAge

(Ma)

-10

-9

-8

-7

-6

-5

-4

-3

log(

D/ r

2 )s-1

-0.4-0.20.00.20.40.60.81.01.21.41.61.82.0

log(

r/r o)

500600700800900

1000110012001300140015001600

App

aren

tAge

(Ma)

-9

-8

-7

-6

-5

-4

-3

log(

D/r

2 )s-1

0 20 40 60 80 100

Cumulative39

Ar released

-0.4-0.20.00.20.40.60.81.01.21.41.61.82.0

log(

r/r o)

0 20 40 60 80 100

Cumulative39

Ar released

500600700800900

1000110012001300140015001600

App

aren

tAge

(Ma)

6 7 8 9 10 11 12 13 14 15

10000/T(K)

-10

-9

-8

-7

-6

-5

-4

-3

log(

D/r

2 )s-1

A D

E

F

G

H

I

B

C

ModelMeasured

ReferenceE=38.1 kcal/molDo/ro = 4.3 /sec2



K7-115-3

T02-98-14

T02-98-16

(%) (%)

Tem

per

atu

re (o

C)

Tem

per

atu

re (o

C)

Tem

per

atu

re (o

C)

Figure 8. Multiple diffusion domain (MDD) results from the least complex basement K-feldspars. (A–C) Measured and model spectra. (D–F) Measured and model Arrhenius plots. (G–I) Measured and model log(r/r

o) plots. (J–L) Contours of time-temperature history paths

output by MDD analysis. The activation energy (E) of the K-feldspars (D–F) is estimated from the initial linear segment recorded by T02-98-14 and the reference D

o/r2

o is determined by forcing the reference line through the diffusion coeffi cient recorded by the fi rst heating step.

Model thermal histories (J–L) indicate signifi cant cooling between 1300 and 1225 Ma and also suggest variable Neoproterozoic temperature maxima between ~100 and 150 °C. Based on the overlying Bass Formation age of 1255 Ma (vertical line in J–L), the models predict cooling that is too late for samples K7-115-3 and T02-98-16. This perhaps represents poor intercalibration between Ar/Ar and U/Pb methods and/or inaccuracy of apparent Ar ages due to alteration of the K-feldspars. Sample T02-98-14 records a thermal history that is compatible with the Bass Formation age and indicates signifi cant basement cooling, and presumably exhumation, between 1300 and 1270 Ma.



K-feldspars, the log r/ro plots reveal signifi cant

differences in diffusion domain sizes and vol-

ume fractions and also provide a means to better

visualize the kinetic parameters relative to the

Arrhenius plots, because information about

the amount of 39Ar represented by each diffu-

sion coeffi cient can be represented on the log

r/ro diagram. Thermal histories (Fig. 8J–L) are

determined following the methods of Quidel-

leur et al. (1997) and are constructed by forward

modeling of the measured age spectrum within

the temperature range defi ned by the kinetic

data. Model age spectra are shown along with

the measured spectra in Figure 8A–C.

The derived thermal histories share several

characteristics, but most fundamentally record

cooling from ~250–300 °C to below 150 °C

between 1300 and 1225 Ma. For samples K7-

115-3 and T02-98-16, the timing of cooling may

be problematic with respect to the 1255 Ma zir-

con age on the ash layer within the Bass Forma-

tion. Provided the zircon data faithfully record

Bass Formation deposition, it is expected that

the basement at the unconformity would have

cooled prior to 1255 Ma. Two explanations for

the timing problem could be poor intercalibra-

tion between the U/Pb and 40Ar/39Ar methods

(e.g., Renne et al., 1998; Min et al., 2000) or

using K-feldspars that are unsuitable for MDD

analysis. We have tried to deal with intercalibra-

tion issues by adopting the most recent values

for the total 40K decay constant (5.476e – 10/a

[a is standard for annum or year]) and age of

Fish Canyon sanidine (28.27 Ma) by Kwon et

al. (2002). These values still require signifi cant

inspection, but are designed to close the observed

gap between U/Pb ages and 40Ar/39Ar ages. The

fi ne-scale age spectrum complexity of the chosen

K-feldspars may suggest caution with respect to

highly rigorous treatment of the thermal histo-

ries. We suggest that the K-feldspar data record

signifi cant cooling within about a 10–50 m.y.

period prior to Bass deposition, but realize that

this assertion requires further testing.

Another aspect of the K-feldspar data that

may be geologically useful is the steep age

gradients observed in the initial part of the age

spectra (Fig. 7). The MDD analysis predicts

post–1100 Ma temperatures for the base-

ment K-feldspars between ~100 and 150 °C

(Fig. 8I–L). Based on the burial history recorded

by the supergroup sediments (~3.6 km), this

temperature range would be expected. How-

ever, the timing of the maximum post–1100 Ma

temperatures is variable and may refl ect not

only sediment accumulation, but also transient,

perhaps fl uid-driven, temperature excursions.

More work is required to understand in detail

the basement thermal history from 1255 Ma to

deposition of the Cambrian Tapeats Sandstone.

Cardenas Basalt Geochronology

Final deposition of Unkar strata is marked

by voluminous basaltic and andesitic fl ows

of the Cardenas Basalt, which are chemically

similar to and likely synchronous with diabase

intrusions within the lower Unkar Group (Hen-

dricks and Lucchitta, 1974) and Apache Group

(Howard, 1991). Elston and McKee (1982)

reported an Rb/Sr age for whole-rock samples

of the Cardenas Basalt of 1070 ± 70 Ma. This

age was refi ned by Larson et al. (1994) to 1103

± 66 Ma based on additional Rb/Sr data. New

Ar-Ar data from two diabase sills yield precise

ages that may also better constrain the timing of

Cardenas activity (Fig. 9; Table DR3 [see foot-

note one]). Both a hornblende and a biotite were

dated from a diabase at Stone Creek (Fig. 9A–

B). H98-131.6-2 biotite, which grew within the

contact metamorphic assemblage below the

sill, yields an overall fl at age spectrum with

initial ages slightly older than the plateau age of

1104 ± 2 Ma (Fig. 9A). The hornblende (H98-

131.6-4), which is a phenocryst phase within the

sill, has a similar age spectrum, but the apparent

plateau age is signifi cantly older at 1124 ± 2 Ma

(Fig. 9B). Considering that the sill should cool

quickly, we would expect these minerals to yield

concordant apparent ages. Isochron analysis of

the hornblende data indicates the possibility of

excess 40Ar within the hornblende (Fig. 9C).

Steps D–L defi ne a linear array that suggests

an age of 1115 ± 4 Ma and an initial trapped

argon component with a 40Ar/36Ar value of

470 ± 60, which is signifi cantly higher than the

atmospheric value of 295.5. This hornblende

isochron age just overlaps with the biotite pla-

teau age at 2σ uncertainty and appears to sup-

port an excess Ar problem for the hornblende.

Our preferred interpretation for the Stone Creek

sill age is given by the biotite result at 1104

± 2 Ma. Biotite sample 98GC25 is from a sill

located in Bass Canyon and yields a very com-

plex age spectrum (Fig. 9D). The spectrum may

be related to recoil redistribution of 39Ar during

irradiation, and we suggest that the total gas age

of 1114 ± 1 Ma may record the intrusion age.

This interpretation by itself would be of limited

value, however, the apparent age of ca. 1100 Ma

is consistent with other sill ages and regional

mafi c magmatism at this time (Howard, 1991).

Interestingly, K/Ar and Ar/Ar dates for the

Cardenas Basalt span a wide interval, from

700 to 1090 Ma, and were postulated to refl ect

cooling during movement on the Butte fault,

coincident with deposition of the Sixtymile For-

mation and “Grand Canyon orogeny” (Elston,

1979; Elston and McKee, 1982). However,

Larson et al. (1994) suggested that the range

in K/Ar dates represents apparent ages and is

an artifact of an alteration/heating event at low

temperatures, perhaps related to Neoproterozoic

rifting (Timmons et al., 2001). Hence the desig-

nation of a Grand Canyon “orogeny” should be

discontinued and replaced by “Neoproterozoic

(ca. 750 Ma) extensional episode” that includes

all penecontemporaneous syntectonic deposits

along western Laurentia (Young et al., 1979;

Elston and McKee, 1982; Ross et al., 1989;

Ross, 1991; Prave, 1999).

Detrital Mineral Geochronology

Geochronology of detrital zircon and musco-

vite grains provides another useful tool for eval-

uating the age of sedimentary units and yields

some information about possible source terrains.

As part of a large effort to conduct detrital min-

eral age analyses from several samples within

the Grand Canyon Supergroup, we present a

relevant subset of both 40Ar/39Ar muscovite ages

(Table DR4a, DR4b, see footnote one) and U/Pb

zircon ages (Table DR5, see footnote one) from

selected units. Total fusion and age spectrum

plateau analyses from muscovite single crystals

from the Escalante Creek Member of the Dox

Formation (Fig. 10A) yield ages ranging from

ca. 1120–1260 Ma, with a well-defi ned node at

1140 Ma. These young apparent ages are some-

what of a surprise given that the local metamor-

phic basement yields micas with apparent ages

between 1400 and 1650 Ma (Karlstrom et al.,

1997). However, numerous samples from sev-

eral units corroborate this fi nding and support

the interpretation that the Dox Formation is

younger than 1140 Ma. Based on the entire data

set, and petrographic and microprobe exami-

nation, it is not likely these young muscovite

grains represent alteration ages or ages that have

been thermally affected by, for instance, 1.1 Ga

magmatism or volcanism.

Detrital zircon age analyses corroborate the

mica results and show that much of the Unkar

Group (excluding the Bass Formation) was

deposited between 1100 and 1170 Ma. U-Pb

geochronology of detrital zircons was con-

ducted by laser ablation multicollector–induc-

tively coupled plasma–mass spectrometry

(LA-MC-ICPMS), utilizing a Micromass Iso-

probe and a New Wave DUV 193 laser ablation

system. The common Pb correction is made by

using the measured 204Pb and assuming an ini-

tial Pb composition from Stacey and Kramers

(1975). Errors that propagate from the measure-

ment of 206Pb/238U, 206Pb/207Pb, and 206Pb/204Pb

are reported at the 1σ level. Additional errors

that affect all ages include uncertainties from

(1) U decay constants, (2) the composition of

common Pb (assumed to be ±1.0 for 206Pb/204Pb

and ±0.3 for 207Pb/204Pb), and (3) calibration

TIMMONS et al.


110100

K / C a

H98

-131

.6-2

bio

tite

1020

3040

5060

7080

9010

0

1060

1080

1100

1120

1140

1160

1180

1200

ABC

D

EF

G

HI

J

110

4.0

± 1

.5 M

a* (

MS

WD

= 2

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± 1

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Apparent Age (Ma) Apparent Age (Ma)

Apparent Age (Ma) 36Ar/40Ar

Fig

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9. (

A–C

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nd K

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ram

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0

40

80

00GC59 muscovite fusionand plateau ages

1060 1100 1140 1180 1220 1260 1300Age (Ma)

Rel

ativ

e P

roba

bilit

yAnalyses

Dox Formationdetrital micas

n = 82

0.12

0.16

0.20

0.24

0.28

0.32

0.36

0.5 1.5 2.5 3.5 4.5 5.5

206 P

b/23

8 U

LC-02-81-2 (n = 85) Hakatai

0

10

20

30

40

800 1200 1600 2000 2400 2800 3200

1900

1700

1500

1300

1100

900

0.05

0.15

0.25

0.35

0.45

0 2 4 6 8 10 12

T01-75-4 (n = 89) Shinumo

0

5

10

15

20

800 1200 1600 2000 2400 2800 3200

2600

2200

1800

1400

1000

0.1

0.2

0.3

0.4

0.5

0.6

0 4 8 12 16

207Pb/235U

206 P

b/23

8 U

T02-75-1z (n = 93)

0

2

4

6

8

10

12

14

800 1200 1600 2000 2400 2800 3200

2600

2200

1800

1400

1000

0.05

0.15

0.25

0.35

0.45

0.55

0.65

0 4 8 12 16 20

207Pb/235U

K00-53-3 (n = 85) Nankoweap

0

4

8

12

16

800 1200 1600 2000 2400 2800 3200

3000

2600

2200

1800

1400

1000

Age (Ma)

B C

Age (Ma)

D

Age (Ma)

EEscalante Creek Member, Dox Formation

Age (Ma)

A

Figure 10. (A) Relative probability diagrams of 40Ar/39Ar mus-covite age determinations from the Dox Formation. 40Ar/39Ar ages are determined from single crystal detrital muscovites from the Escalante Creek Member of the Dox Formation. Diagram combines 56 total fusion ages with 26 plateau ages from low-resolution step heating of single crystals. The step-heating data reveal that total gas ages are typically 0.8% younger than plateau ages, and therefore all total fusion results have been increased by this percentage. The well-devel-oped node at 1140 Ma indicates that Dox deposition occurred post–1140 Ma, but prior to eruption of Cardenas lavas and intrusion of diabase (vertical line) at ca. 1115–1104 Ma. (B–E) Combined concordia and relative probability plots of detrital zircons from Grand Canyon Unkar Group and Nankoweap Formation. Zircons compliment detrital mica age determina-tions and also suggest a strong Grenville-age source for sedi-ment, as well as sources from expected 1450–1350, 1750–1650, and 1840 Ma local basement, and minor Archean grains.

TIMMONS et al.


correction. These systematic errors add an

additional 2% (1σ) uncertainty to 206Pb/238U and

>1.4 Ga 206Pb/207Pb ages.

One-hundred zircon grains were analyzed

from four samples from the Hakatai, Shinumo,

Dox, and Nankoweap Formations (Fig. 10).

Forty-eight analyses with >20% discordance

or >10% reverse discordance have been elimi-

nated from consideration, leaving 352 reliable

ages. This subset of detrital zircon analyses

yielded minimum ages for samples that were

determined from the youngest cluster of age

determinations. Sample LC-02-81-2 was col-

lected from the upper Hakatai Formation and

yielded eighty-fi ve zircon grains for analysis.

The youngest cluster of ages suggests that the

maximum age of deposition was ca. 1187 Ma

(Fig. 10B). Interestingly, this sample was

dominated by zircons of Grenville age (1200–

1350 Ma) and appeared to have little input

from more proximal Paleoproterozoic and early

Mesoproterozoic crustal rocks.

Sample T01-75-4 from the Shinumo Sand-

stone yielded eighty-nine zircons representative

of a more cosmopolitan source terrain. The

maximum age of deposition for the Shinumo is

ca. 1176 Ma based on these analyses (Fig. 10C).

This sample contained zircons similar to the

upper Hakatai sample and is interpreted to

refl ect continued input from Grenville-age crust.

In addition, the Shinumo sample had abundant

zircons that refl ected proximal crustal ages, sug-

gesting a mixed source.

Sample T02-75-1z was collected from the

Escalante Creek Member of the Dox Forma-

tion. This sample contained ninety-three zircons

for analysis and yielded a maximum age of ca.

1165 Ma, similar to the inferred maximum age

of the Shinumo (Fig. 10D). This sample also

contained numerous zircons of Paleoprotero-

zoic and early Mesoproterozoic (ca. 1.4 Ga and

1.6 –1.8 Ga) source rocks, but also contained

signifi cant numbers of zircons that are older

than proximal crustal sources. This implies that

there has been recycling of old zircons from

proximal Paleoproterozoic metasedimentary

rocks, or zircons have been transported from

some distal older crustal block, presumably

from the south, based on paleocurrent analysis.

Sample K00-53-3 was collected from the

upper member of the Nankoweap Formation

and yielded 85 zircons for analysis. The zircons

were considerably younger than the underlying

Unkar Group and suggested a maximum age

of ca. 949 Ma (Fig. 10E). This maximum age

is probably conservatively old, and the actual

age of the upper Nankoweap Formation may

be closer to inferred ages of ca. 850–900 Ma

determined from paleomagnetic studies (Luc-

chitta and Hendricks, 1983; Weil et al., 2003).

The lower Nankoweap Formation (ferruginous

member) remains undated, and could poten-

tially be older than 942 Ma, depending on the

duration of the hiatus separating lower and

upper Nankoweap rocks.

TIMING OF DEFORMATION IN THE GRAND CANYON SUPERGROUP

Sears (1973) recognized three main types of

faulting in the Unkar Group: (1) faults related to

intrusion of ca. 1.1 Ga diabase, (2) faults related

to regional shortening, and (3) faults related to

extension and domino-style tilting of Unkar

strata. Like Elston (1979), Sears interpreted

the tilting of Unkar strata on NW-striking faults

to coincide with the Late Precambrian “Grand

Canyon Revolution” of Maxson (1961). Sears

(1973) also described NE-striking, steeply dip-

ping contractional faults that folded Unkar strata

into monoclines. These monoclines are clearly

truncated by the Middle Cambrian Tapeats

Sandstone, indicating that these structures are

also Precambrian in age, but the relative impor-

tance of the extensional and contractional faults

was not well understood.

More recent work in the Chuar and Unkar

Groups (Timmons et al., 2001) has further

refi ned our understanding of multiple, but

punctuated episodes of deformation and sedi-

mentation and has led us to abandon the con-

cept of a single Neoproterozoic Grand Canyon

orogeny. Instead, rocks of the Grand Canyon

Supergroup record multiple extensional events

separated by nearly 300 m.y. of geologic time

(Timmons et al., 2001). The details of the older,

1300–1100 Ma, deformational events recorded

in the Unkar Group, as presented in this paper,

suggest a regional tectonic response to progres-

sive plate-margin deformation in the late Meso-

proterozoic.

Lower Unkar Group NW-SE Contraction (>1140–1250 Ma)

Contractional faults that offset Unkar Group

deposits are located in side canyons along the

Colorado River, including Red, Vishnu, Bright

Angel, and Bass Canyons (Fig. 2). Unkar

Group rocks in these locations are folded

into monoclines that trend NE and face NW

(nearly orthogonal to known N-NW–trending

Laramide monoclines). Monoclines in Vishnu

and Bright Angel Canyons follow the structural

grain of the Granite Gorge Metamorphic Suite,

whereas in Bass Canyon the monocline cross-

cuts the metamorphic grain at high angles. All

monoclines have a stratigraphic separation less

than 200 m and are beveled by upper Unkar or

Tapeats strata.

Field observations suggest that contractional

faulting of the Unkar Group predates the 1.1 Ga

magmatic activity in the area. In Bright Angel

Canyon, 1.1 Ga dikes intrude along NE-striking

reverse faults and feed sills within the Hakatai

Shale, however, neither the sills nor the dikes

appear to be offset by Precambrian contractional

movement. In Bass Canyon, mapping shows the

monocline truncated by a large dike presumed to

be part of the 1.1 Ga magmatism. Precambrian

reverse faults and monoclines are not observed

to affect any deposits younger than the Shinumo

Sandstone. In Red Canyon, the monocline dies

out in the Hakatai Shale; in Vishnu Canyon,

the monocline diminishes within the Hakatai

Shale; in Bright Angel Canyon the monocline

penetrates the Shinumo Sandstone and is trun-

cated by the Tapeats Sandstone; and in Bass

Canyon the well-developed monocline in the

Bass Formation and Hakatai Shale is covered by

undeformed Shinumo Sandstone (Fig. 2).

Inspection of depositional patterns of the

Bass Formation highlights unusual trends in

facies and thickness distribution that suggest

a complex interplay between deposition and

tectonism. The formation thickens to the west

with important and abrupt thickness changes in

Vishnu and Bright Angel Canyons that suggest

syndepositional response to preexisting relief

on the basement surface due to faulting (Fig. 4).

The presence of conglomerate and breccia inti-

mately interbedded with carbonate rocks of the

Bass Formation suggests local relief, recycling

of older carbonate beds, and possibly erosion

of basement rocks during deposition. Clast size

and composition of conglomerate beds in the

Bass Formation suggest high-energy transport

of local and distal basement clasts prior to

deposition of carbonate beds and recycling of

lower carbonate and conglomerate beds during

deposition of upper conglomerate lenses. Field

observations suggest that faulting plays some

role in the development of relief between differ-

ent crustal blocks that infl uenced depositional

patterns and facies associations.

Faulting during Bass Formation deposi-

tion is documented by detailed mapping and

measured sections. In Red Canyon, a gentle

monocline (30° dip in ramp) folds rocks of

the Bass Formation and Hakatai Shale. At this

location, the fault does not penetrate exposed

beds of the Bass Formation, however, the trace

of the monocline trends toward the northeast.

Siliciclastic beds of the upper Bass Formation

preserve very gentle (10°) angular pinch-outs

in the ramp of the monocline (Fig. 11). Trun-

cation surfaces are discrete and no lag deposits

were observed in the section. The pinch-outs

are consistent with erosion of the hanging-

wall block during progressive monocline



development and burial of the disconformity

by subsequent sedimentation.

In Vishnu Canyon, deposits of the Bass For-

mation are observed to thin dramatically onto

the hanging wall of the monocline (Fig. 12).

The reverse fault clearly offsets the lowermost

deposits of the Bass Formation (including

Hotauta Member) with no obvious changes in

thickness. This is, however, not the case in the

upper part of the section. On the west, the sec-

tion is thick, with multiple beds of conglomerate

and sandstone. Correlative beds on the east side

of the fault lack similar thickness of conglomer-

ate and sandstone; in fact, upper conglomerate

beds thin to zero thickness, and sandstone

beds thin to a few centimeters thick (Fig. 4).

Furthermore, we observed soft sediment slump

features and olistostrome deposits in associ-

ated carbonate and interbedded siliciclastic

rocks over this same horizon (Fig. 12B–C).

Sedimentary horizons in the Bass and Hakatai

above the attenuated section do not show similar

thickness changes, suggesting a hiatus in fault

activity. Beds above the disturbed zone are,

however, also folded by the monocline, imply-

ing renewed Proterozoic movement that post-

dated these upper deposits. The amplitude of the

monocline diminishes into the Hakatai Shale,

and the Tapeats Sandstone ultimately truncates

the structure. Therefore movement across this

structure took place during lower Bass deposi-

tion and again sometime between deposition of

the Hakatai and Tapeats.

In the Bass Canyon monocline (Fig. 2),

folding of bedding took place before Shinumo

Sandstone deposition. The Bass Formation is

tightly folded into a NW-facing monocline,

with beds that are vertical to overturned. Strati-

graphic units within the Bass Formation are

correlated across the structure without changes

in facies distribution or thickness (Fig. 4), sug-

gesting movement after deposition of the Bass

Formation. Farther upsection in the Hakatai

Shale to Shinumo contact, we observe a well-

developed monocline with beds that dip as steep

as 45° to the NW. Overlying Shinumo beds are

in fault contact with this monocline but show no

evidence of folding, suggesting this monocline

dies out in the upper Hakatai. The monocline

also is truncated by a NW-trending graben that

juxtaposes undeformed diabase with overturned

beds of the Bass Formation, further showing

that the monocline predates 1100 Ma magma-

tism. Collectively these observations suggest

that this monocline developed after deposition

of the Bass Formation and before fi nal deposi-

tion of the Shinumo Sandstone.

The duration of contractional deformation

remains diffi cult to determine. The apparent

conformity of Dox with Shinumo deposition

suggests that contraction was long-lived and

lasted from at least 1250 Ma until ca. 1165 Ma

(assuming little difference in age between

Shinumo and Dox deposits). Huntoon and Sears

(1975), report 183 m of east-side-up reverse-

sense movement along the Bright Angel fault

zone after emplacement of ca. 1100 Ma diabase

sills and dikes. If accurate, this would imply that

contractional deformation was concurrent with

diabase intrusion and orthogonal extensional

deformation.

Unkar Group NE-SW Extension ca. 1250–1150 Ma

The main extensional deformation that tilted

Grand Canyon Supergroup strata before depo-

sition of the Cambrian-age Tapeats Formation

is well known (Powell, 1875; Walcott, 1889).

Unkar Group rocks are cut by Proterozoic nor-

mal faults of variable displacement that strike

NW and commonly form full or half grabens

(Fig. 2). Unkar Group strata generally dip

northeast toward steeply southwest-dipping

normal faults and form coherent ≤20°-dipping

tilt blocks with half-graben geometries.

The timing of this deformation has been

variably interpreted as late Neoproterozoic

(Elston, 1979) and late Mesoproterozoic

(Sears, 1973). Our data are interpreted to sug-

gest that early extensional deformation over-

lapped with, but was generally younger than,

contractional deformation, and that the main

tilting and normal faulting of the Unkar Group

took place before deposition of the ca. 900 Ma

upper Nankoweap Formation and Chuar

Group (Fig. 3). Mutually crosscutting relation-

ships between normal faults and diabase sills

and dikes (Sears, 1973) suggest that mafi c

magmatism at ca. 1.1 Ga overlapped in time

with extensional faulting in the Unkar Group

(Fig. 13A). Intraformational faults near the

Palisades fault die out in the Cardenas Basalt

(Fig. 13B). There is a 3°–5° angular uncon-

formity between the Unkar Group and Nan-

koweap Formation (Gebel, 1978; Fig. 13C),

and in the Tanner graben several Unkar-age

normal faults die out upsection in the Nan-

koweap Formation (Gebel, 1978; Fig. 13D).

Some of these observations are discussed in

more detail in the following.

The Palisades fault is a key fault for deci-

phering pre–Chuar Group extension (Fig. 2).

The fault strikes 310° and dips steeply to the

southwest. Proterozoic stratigraphic separation

across this structure is ~1100 m down-to-the-

southwest after ~300 m of Laramide reverse

slip is restored. Timmons et al. (2001) argued

that the continuity of Butte fault hanging-wall

strata and discontinuity of Butte fault footwall

strata across the Palisades fault indicate that the

Palisades fault is truncated by the Butte fault

and that faulting and tilting of Unkar Group

rocks predated Butte fault movement and Chuar

Group deposition. Corollary observations

of intraformational faults subordinate to the

Truncated bedding

~1.2 m

Figure 11. Field photo of the ramp of a Precambrian monocline in Red Canyon. View is toward the NE, and beds of the uppermost Bass Formation dip gently to the NW. Sedimen-tary pinch-outs of beds are observed indicating monoclinal development, and erosion of mildly deformed depositional surfaces was contemporaneous with deposition of the Bass Formation. The observed thinning of beds and sedimentary pinch-outs are interpreted to refl ect little or no structural thinning. All sedimentary facies show a thinning onto the monocline, and the gentle dips suggest little structural modifi cation. This suggests that NW-directed shortening and deposition were synchronous.

TIMMONS et al.


Palisades structure (Fig. 13B) and Basalt Can-

yon fault support the interpretation that faulting,

igneous activity, and sedimentation were inti-

mately linked. Post–Unkar Group movement of

the Palisades and other large extensional faults

occurred to further tilt and ultimately structur-

ally isolate Unkar outcrops. Interestingly, the

Neoproterozoic-age Chuar Group does not

share this same history of tilting and graben for-

mation. Rather, Chuar rocks record the develop-

ment of a N-S–trending Neoproterozoic growth

syncline (Timmons et al., 2001).

The angular discordance between the Nan-

koweap Formation and the Unkar Group indi-

cates a period of faulting and rock tilting after

eruptions of the Cardenas Basalt and prior to

deposition of Nankoweap strata (Fig. 13C).

Here, the 3°–5° angular discordance between

the two rock packages accounts for part of the

total 10°–15° tilt preserved by Unkar deposits.

Extension recorded within the undated lower

Nankoweap Formation seems be a continuation

of Unkar-related extension, as supported by the

similarity between red-beds in the Dox Forma-

tion, interfl ow red-beds in the Cardenas Basalt,

and red-beds of the lower Nankoweap Forma-

tion (Elston, 1979).

Evidence for extensional deformation is

recorded by unconformities and intraforma-

tional faults in the lower Nankoweap Forma-

tion. Elston and Scott (1976) and Link et al.

(1993) reported a major unconformity within

the Nankoweap Formation and suggested that

faulting and erosion preceded deposition of

the upper member of the Nankoweap Forma-

tion. Intraformational normal faults within the

Tanner graben are truncated by strata of the

upper Nankoweap Formation and Chuar Group

(Fig. 13D), suggesting extension during early

phases of Nankoweap deposition. Adjacent to

major faults, such as the one in Basalt Canyon,

sedimentary beds pinch out against the fault.

Further, a coarse-grained (5–10 mm clasts)

lag deposit (<1 cm thick) in the Nankoweap

Formation in Basalt Canyon contains clasts of

disturbedbeds

A

B C

conglomerate

dolomite

sandstone olistostrome~4m

Figure 12. Field photos from Vishnu Canyon highlighting some of the features represented in Figure 4. (A) Sedimentary units in the Upper Bass Formation. View is oblique to the north and slightly oblique to the axis of the monocline. The upper conglomerate bed and associated sandstone thin from ~10 m thick to the left of the photo to 0 m and 0.1 m thickness of conglomerate and sandstone (respectively) to the right of the photo. (B) Also shown are disturbed carbonate beds that abruptly end and are associated with a <5-m-thick bed of disturbed siliciclastic rocks. The disturbed interval is bounded by intact beds of carbonate and siliciclastic rocks and contains numerous (meter scale) blocks (C) that both disrupt bedding and exhibit soft-sediment deformational features that suggest slumping of material from the SE.



the underlying Cardenas Basalt and indicates

exposure of the basalt (by faulting) during early

Nankoweap deposition. The duration and tec-

tonic importance of the disconformities below,

within, and at the top of the Nankoweap For-

mation remain poorly understood. We suggest,

however, that the lower Nankoweap Formation

records a continuation of Dox-age deposition

and faulting that predates ca. 950 Ma deposition

of the upper Nankoweap Formation.

The onset of extensional deformation in the

Unkar Group has been diffi cult to determine,

but may have begun early during Unkar depo-

sition. The Phantom graben (Fig. 2) is a NW-

striking symmetrical graben that juxtaposes

rocks of the lower Unkar Group with basement

metamorphic rocks. The Bass Formation at this

location exhibits unusual thickness and facies

changes suggestive of synsedimentary faulting.

Figure 14 shows the Bass Formation through

the lower Hakatai Shale bounded by faults that

strike to the northwest. At the base of the sec-

tion, competent beds of dolomite are offset by

intraformational normal faults. Fault offsets are

estimated between 10 and 20 m of stratigraphic

separation within the Bass Formation. Silici-

clastic beds of the upper Bass Formation are

observed to be laterally continuous across the

entire width of the graben, suggesting a trunca-

tion of subordinate fault sets. This suggests that

contractional and extensional deformation were

contemporaneous and record a single strain fi eld

during Bass deposition.

Fault Kinematics

Orientations of normal faults in the Grand

Canyon Supergroup are shown in Figure 15.

Normal faults in the Unkar Group and Chuar

Group occur in two main populations that

have northwest to north strikes and are steeply

dipping. These faults are Precambrian in age,

because large offset normal faults can be traced

to the Tapeats or older sedimentary unit contact

where they are truncated, and in Phanerozoic

3003000, 65S, 65S Chuar Group

Nankoweap Formation

Ochoa Point,Dox

ComanchePoint, Dox

Dox Dox

CardenasBasalt

Cardenas

Tapeats

Chuar Group

Nankoweap Formation

Cardenas Basalt

Tapeats

Cardenas

Ochoa Point, Dox

Ochoa Point, Dox

Ochoa

Escalante Creek,Dox

A B

C D

0.4 m

Figure 13. Field photo mosaic showing important fi eld relationships documenting syn-Unkar and Nankoweap faulting. (A) Diabase sill that is both offset by and intrudes minor faults that parallel major Unkar-age faults in the area, indicating that intrusion of the diabase sill and normal faulting were concurrent. (B) Intraformational faults in the Tanner graben. Subordinate faults offset Unkar rocks by tens of meters; however, faults can be traced upsection into the Nankoweap Formation, where fault slip diminishes to zero. These faults record Unkar-style deformation that postdates eruption of Unkar basalt but predates deposition of the upper Nankoweap Formation and Chuar Group; view is toward the west. (C) Intraformational faults in Cardenas Basalt proximal to the Palisades fault. Subordinate faults offset the Dox-Cardenas contact by ~10–20 m, and can be traced through the lower Cardenas, but do not offset a sandstone marker bed in the Cardenas, suggesting normal faulting and mafi c magmatism were concurrent; view is toward the southeast. (D) Angular unconformity between the Cardenas Basalt and overlying Nankoweap Formation.

TIMMONS et al.


rocks, faults of similar orientation preserve

reverse sense of movement. Both Unkar and

Chuar Groups show conjugate faults (Fig. 15).

Paleostress axes for Unkar fault planes record

subvertical σ1, subhorizontal σ

2 that trends

toward 306°, and subhorizontal σ3 that has a

bearing of 037° (Fig. 15). Chuar Group faults

also record subvertical σ1; however, the orien-

tation of σ2 is 334° and σ

3 is 065° (Fig. 15).

The difference in fault plane populations is

interpreted as overprinting of Meso- and Neo-

proterozoic fault populations. Extension during

Chuar deposition had shifted to E-W extension.

REGIONAL SEDIMENTARY AND TECTONIC CORRELATIONS

Our updated geochronology allows us to

confi rm and refi ne proposed regional correla-

tions of Upper Mesoproterozoic sedimentary

successions and extensional events. Earlier

attempts to correlate regionally dispersed sedi-

mentary successions relied heavily on lithologic

correlations and paleomagnetic evaluations of

red-bed deposits and igneous rocks. Link et al.

(1993) reported that onset of Unkar deposition

occurred at ca. 1250 Ma, based on an interpola-

tion of Bass Formation paleomagnetic pole in

the 1300–1200 Ma North American apparent

polar wander (APW) path, which is in agree-

ment with our new geochronology. They also

noted that paleomagnetic pole positions from

the upper Unkar Group defi ne a counterclock-

wise, NNE-elongated loop that resembles the

loop defi ned by Keweenawan rocks of the mid-

continent rift system (Halls, 1974; Pesonen and

Halls, 1979).

Previous Late Mesoproterozoic Regional Correlations

Lithologic correlations between the Apache

Group and Unkar Group have long been postu-

lated. Shride (1967) suggested that the Mescal

Limestone (including carbonate and overlying

argillite member) and the Bass Formation and

Hakatai Shale were regional correlatives. He

also recognized similarities between units of

the Shinumo Sandstone and Troy Quartzite,

including “curiously twisted and gnarled” beds

observed by Powell (1875) and Noble (1914,

p. 51) in the Grand Canyon similar to the Che-

diski Member of the Troy. No lithologic correla-

tions have been postulated between the Pioneer

Shale and Dripping Spring Formations in the

Apache Group and Grand Canyon rocks. Like-

wise, it has been suggested that the Dox has no

depositional equivalents in the Apache Group

(Shride, 1967; Wrucke, 1989).

Further regional correlations between other

late Mesoproterozoic successions have been

attempted. Wrucke (1966) suggested that

carbonate rocks of the Middle Crystal Spring

Formation of the Pahrump Group correlate to

the Bass and Mescal Formations in Arizona,

based on lithologic similarities and similar

diabase intrusive relationships. The correlations

based on the combined data set (paleomagnetic

and lithologic correlation) are supported by our

geochronology from the Unkar Group.

Proposed Regional Correlation and Tectonic Model

Our new geochronology indicates that

sedimentary rocks over a large region of the

southwestern United States were deposited

synchronously and share a common sedimen-

tologic and tectonic history. We postulate that

the depositional record of the Pioneer Shale and

Dripping Spring Quartzite mostly predates the

sedimentary record in the Grand Canyon Unkar

Group, and may record part of the exhumation

history of the Grand Canyon region (Karlstrom

et al., 1997). Rocks of the Pioneer and Dripping

Spring Formations record derivation of coarse

clastics from some unknown northern basement

source (Cullom, 1996). Best estimates for the

age of the Pioneer Formation come from detrital

zircon studies and suggest that this section is

ca. 1328 ± 5 Ma, based on euhedral and clear

zircons from a tuff or possibly a reworked tuff

(Stewart et al., 2001).

Rocks of the Allamoore Formation, dated

at 1250 +20/–27 Ma, 1253 ± 15 Ma, and

1256 ± 5 Ma, Castner Marble, dated at 1260

± 20 Ma (Bickford et al., 2000), and Bass

Formation, dated at 1254.8 ± 1.6 Ma, appear

to be age equivalents, contain numerous

dolomitic/marble stromatolite-bearing beds,

and have been interpreted to record shallow

marine deposition (Figs. 1 and 16; Dalton,

1972; McConnell, 1975; Nyberg and Schopf,

1981; Pittenger et al., 1994). We support the

original correlation of Wrucke (1966, 1989) of

the middle Crystal Spring Formation carbonate

with other ca. 1250 Ma carbonate sequences

based on crosscutting 1.08 Ga diabase (Heaman

and Grotzinger, 1992), and the recognition of

an unconformity between the middle and upper

Crystal Spring Formation (Prave, 1998, per-

sonal commun.). We also support, after Shride

(1967), the correlation of late Mesoproterozoic

sedimentary rocks in central Arizona (Mescal

Formation), and carbonate facies in subcrops in

New Mexico (Debaca Group; Amarante, 2001)

BASS

Hakatai

Basement

Hakatai

Shinumo

Cambrian

Figure 14. In Bright Angel Canyon near Phantom Creek, a NW-striking graben preserves sedimentary rocks of the Bass Formation through lower Shinumo Sandstone. Minor faults within the graben offset lower beds of the Bass Formation and are truncated by siliciclastic beds of the uppermost Bass Formation and lower Hakatai Shale. Fault offsets at the base of the section are estimated at ~10–20 m. This suggests that early faulting of the Bass included movement of NW-striking faults in response to NE-SW extension.



with the ca. 1250 Ma carbonate sequences in the

southwestern United States.

Current paleogeographic/depositional mod-

els suggest that the pre-orogenic marine rocks

in west Texas were deposited during island-arc

convergence in a back-arc, orogen-parallel,

extensional setting (Rudnick, 1983; Pittenger

et al., 1994; Mosher, 1998). Assuming that the

1255 Ma age of the Bass Formation is accurate,

we argue that the regional distribution of ca.

1250 Ma carbonates, with mixed siliciclastic

beds (with shallow water indicators) and asso-

ciated stromatolite beds, suggests deposition in

shallow marine intertidal to subtidal conditions

in a back-arc–epicontinental seaway (Figs. 1

and 16). The record of regional contraction at

1250 Ma in the Grand Canyon, however, may

differ from the orogen-parallel extensional

back-arc setting for west Texas carbonate suc-

cessions. Monoclines of the lower Unkar Group

were developing during regional deposition of

carbonate sequences, and there is an apparent

paucity of convergent deformation in west

Texas deposits at ca. 1250 Ma. As monocli-

nal development waned in the Grand Canyon

region, contractional deformation, as a large-

scale thrust belt during continent-continent

phase convergence, is recorded in younger

deposits of the Hazel Formation (Bickford et

al., 2000).

New detrital mica and zircon ages provide a

much better constraint on the timing of deposi-

tion of the Dox Formation than previous studies

and allow correlation to other sequences in the

southwest United States. The 1140–1104 Ma

Dox Formation records the transition from

shallow marine deposits of quartz arenite

of the Shinumo Sandstone to fl uvial-deltaic

arkosic sandstones of the basal Dox Formation.

The transition is very abrupt (over 10 m), but

illustrates a regression of the Shinumo sea and

burial by fl uvial/deltaic facies of the lower Dox.

Sedimentary structures in the upper Shinumo

Sandstone exhibit general bimodal paleocur-

rent directions. The basal member of the Dox

Formation is characterized by large-scale (tens

of meters wide) channel facies sandstone inter-

bedded with intrafl uvial mudstone. The channel

facies record a consistent paleocurrent direction

(determined from trough bedding axes) toward

the north that persisted into upper beds in the

Dox (Fig. 4). We interpret the change in sand-

stone composition and change in depositional

systems to refl ect a fundamental reorganization

of the paleogeography related to progressive

Grenville contraction.

Late Mesoproterozoic (1.2–1.1 Ga)–age

micas in basement rocks of the Southwest

United States are rare, as much of the Southwest

records cooling histories that predate deposition

of the Dox Formation. For instance, basement

rocks in the Southwest record cooling through

the mica closure temperature window by or

soon after 1.4 Ga (Shaw et al., 1999). The few

exceptions include the Uncompahgre Uplift (ca.

1100 Ma; Timmons et al., 2002), small portions

of central Arizona, and the core of the Grenville

orogeny (Bickford et al., 2000). Of the known

1100 Ma mica thermochronologic terranes, the

Grenville Front provides the most likely south-

ern source for fl uvial detritus of a single prov-

enance during Dox deposition. We conclude

that the proximal facies of the Hazel Formation

(Soegaard and Callahan, 1994; Bickford et al.,

2000) in west Texas and the distal fl uvial/deltaic

facies of the Dox Formation represent age-

equivalent deposits (Fig. 16). This implies also,

that during Dox deposition, there was no high-

land separating Unkar and Apache Group basins.

The Dox Formation records intracratonic defor-

mation and transition of regional strain patterns

from compression near the orogen to orthogonal

extension in the continental interior. The relative

timing of contraction and extension in the Unkar

Group suggests early NW-directed shortening

and coeval perpendicular extension followed

by dominant NE-SW–directed extension dur-

ing Unkar deposition. Similar relationships

have been postulated closer to the orogen in

the Central Basin Platform (Adams and Keller,

1996), where a preserved Phanerozoic-inverted

Precambrian graben is present in subcrop.

Several modern examples of collisional-per-

pendicular extensional basins exist that share

many similar features to this ancient orogenic

belt. Mechanisms driving intracratonic defor-

mation remain a topic of debate; however,

many workers have concluded that intracratonic

rifting was a response to plate-margin conver-

gence. The NE-trending Baikal rift has been

interpreted as a passive rift formed in response

to the Indo-Eurasian collision (Tapponnier

and Molnar, 1979; Hutchinson et al., 1992).

Another example includes the upper and lower

Rhine grabens in Western Europe. Following

Sengör et al. (1978), many workers have refi ned

the timing and kinematics of the Rhine graben,

and they have concluded that graben formation

σ1

σ2

σ3

n=201

Chuar faults: n = 45Unkar faults: n = 156

Chuar

Unkar

Figure 15. Lower-hemisphere equal-area projection of minor faults in Unkar and Chuar Group strata, interpreted as Precambrian in age; shaded dots represent poles to fault planes of the Chuar Group, and conjugate pair great circles (shaded) were picked by eye after 1% contouring of fault-plane data (contouring not shown). Stars show orientation of poles to fault planes in Unkar Group rocks. The averaged conjugate pair determined for the Unkar Group allowed calculation of principal axes for Unkar rocks (large stars). Interestingly, fault planes in the Unkar Group overlap with Chuar Group data, however, after contouring, two distinct conjugate populations can be determined, suggesting faults were responding to different regional stresses.

TIMMONS et al.


and Alpine tectonics were penecontemporane-

ous and kinematically linked (Larroque and

Laurent, 1988; Laubscher, 2001).

Temporally associated with Grenville oro-

genesis and regional deformation are widely

dispersed mafi c intrusive and extrusive igneous

rocks (Fig. 1, inset). Mafi c igneous rocks span

a broad interval in time, from ca. 1235 Ma in

the Grenville of Ontario (Bethune, 1997),

1109–1087 Ma in the Midcontinent region (Van

Schmus, 1992), to 1165–1120 Ma in west Texas

(Keller et al., 1989; Barnes et al., 1999), and

new data indicate 1115–1104 Ma in the Grand

Canyon. Mafi c intrusions commonly occur as

sills in both Mesoproterozoic sedimentary rocks

and the crystalline basement in the southwestern

United States, which led Howard (1991) to pos-

tulate that sills record subvertical σ3 during NW-

directed shortening. NW-striking mafi c dikes of

Grenville age are also observed over much of

Laurentia and are commonly associated with

NW-striking rift zones and record NE-SW–

directed extension during orogenesis (Keller et

al., 1989; Bethune, 1997). In the Grand Can-

yon, both voluminous subhorizontal mafi c sills

and NW-striking mafi c dikes are observed, sug-

gesting a σ3 that is either subhorizontal and NE-

trending or subvertical. We suggest that both σ2

and σ3 were subequal, with perhaps more local

stresses controlling the orientation of σ2 and σ

3.

Both structural relationships are compatible

with NW-directed shortening.

The extent of Grenville deformation in the

continental interior becomes clearer when

considering seemingly disparate data sets.

Laramide contraction in the southwestern

United States reactivated a linked network that

involved segments of late Mesoproterozoic and

Neoproterozoic normal faults in the Grand Can-

yon (Huntoon, 1971; Sears, 1973; Huntoon et

al., 1996; Timmons et al., 2001). Marshak et al.

(2000) and Timmons et al. (2001) argued that

the strong northwest trend of faults observed

over much of Laurentia (Grand Canyon, Central

Basin Platform, and Sudbury dikes; cf. Fahrig

and West, 1986) records regional northeast

extension, which overlaps in time with Gren-

ville-age NW-directed contraction.

Further tests of ancestry of regional fault

networks in the Colorado Plateau and Rocky

Mountain region are under way via 40Ar/39Ar

studies. K-feldspar data document important

cooling events that may represent periods of

exhumation during (1) Grenville orogenesis

(1200–1100 Ma), (2) Late Precambrian rift-

ing at 800–700 Ma (western Cordillera) and

perhaps 600–550 Ma (Oklahoma Aulocogen

trend), and (3) Ancestral Rockies deformation

(ca. 350 Ma). At more local scales, feldspars

seem to resolve disparate cooling paths across

(lower ferriginous)

1400

1300

1200

1100

1000

900

800

700

AG

E (M

a)

upper unitCrystal Spr.

N. Utah / S.E. Idaho Death Valley Grand Canyon Central Arizona West Texas

1254 Ma (zircon ash)

ca. 1250 Ma (Ar/Ar K-spar)

Hakatai Shale

Bass Formation

Pikes Peak Granite

Scanlon Conglomerate

1.7-1.4 Ga basement 1.7-1.4 Ga basement ca. 1.4 Ga granite 1.6-1.4 Ga basement 1.7-1.4 Ga basement1.7-1.4 Ga basement

Red Pine Shale

Mutual Formation

Beck Spring Dolomite Chuar Group

Nankoweap Fm.

middle unit

lower unit

Crystal SpringFormation

Nankoweap Fm.

Cardenas BasaltDox Formation

Shinumo Sandstone Troy Quartzite

Pioneer Shale

Dripping SpringsMescal Limestone

Hazel Formation

Allamore Fm./Castner Marble

1383 Ma (zircon)

Carrizo Mtn. Group

Debaca Group(Las Animas Fm.)

Debaca Gp.

Sandstone

Mudstone

Carbonate

Diabase

Basement metamorphic gneiss and granite

Radiometric age determination

Granite

Explanation

Tumbledown Fm.

New Mexico andColorado

ca. 900 Ma (pmag)

? ???

? ?

? ?

(upper)

?

????

???

??

??

? ? Lanoria Formation? ? ??

??

(upper)

(lower)

?

??

??

Uinta Mountain Group

<942 Ma

Figure 16. Correlation of lower Mesoproterozoic and Neoproterozoic successions in the southwestern United States, showing three main tectono-stratigraphic packages and unconformities: (1) 1350–1250 Ma intracratonic successions; (2) 1250–1100 Ma syntectonic intracra-tonic deposition; (3) ca. 900 Ma unconformity bounded sedimentary rocks; and (4) ca. 800–700 Ma synrift deposits (Dehler et al., 2001). Radiometric ages are cited in the text (fi gure modifi ed after Link et al., 1993).



discrete faults, suggesting that faults controlled

exhumation of different crustal blocks at dif-

ferent times (Timmons et al., 2002; Timmons,

2004). The occurrence of a 1250–1100 Ma

cooling trend over much of the Rocky Moun-

tain region and Grand Canyon suggests a

dynamic response in the continental interior to

Grenville orogenesis.

SUMMARY

The lower Unkar Group records contempo-

raneous contractional and extensional deforma-

tion and deposition. NE-trending monoclines

deformed lower units of the Unkar Group and

developed during sedimentation. Mapping

suggests that convergent structures deformed

mostly rocks of the lower Unkar Group and not

the Neoproterozoic-age Chuar Group. Hence,

combined with sedimentologic evidence, con-

tractional structures are interpreted to be late

Mesoproterozoic in age. The Unkar Group is

also faulted and tilted by NW-striking normal

faults that record NE-SW–directed extension

prior to Chuar Group deposition. NE-directed

extension was the dominant kinematic regime

during late Unkar and early Nankoweap depo-

sition. Unkar Group deformation is temporally

and kinematically linked to Grenville oro-

genesis. In contrast, extensional deformation

recorded in the ca. 800–742 Ma Chuar Group

records E-W–directed extension related to the

incipient rifting of the Western Cordillera of

Laurentia, nearly 200 m.y. after Unkar defor-

mation and tilting (Timmons et al., 2001). The

terms Grand Canyon “Revolution” of Maxson

(1961) and “Orogeny” of Elston (1979) refer to

a single extensional event, but there were at least

two separate deformational episodes linked to

supercontinent assembly (ca. 1250–>950 Ma)

and rifting (ca. 800–750 Ma).

New geochronology confi rms and refi nes

published correlations for late Mesoproterozoic

rocks in the Southwest. Carbonate rocks were

deposited in an epicontinental seaway undergo-

ing Laramide-style convergence in the continen-

tal interior at ca. 1250 Ma. Subcrop rocks of the

Las Animas Formation, Central Basin Platform,

and DeBaca terrane correlate to exposed sec-

tions that include the Allamoore, Tumbledown,

and Hazel Formations of west Texas, the

Apache Group of central Arizona, the lower

and middle Crystal Springs Formation of Death

Valley, California, and the Unkar Group of the

Grand Canyon. Data from the Grand Canyon

indicate pulses of sedimentation with discrete

episodes of tectonism associated with the evolv-

ing Grenville orogen from ca. 1250–1100 Ma to

perhaps as young as 950 Ma, which probably

also affected correlative rocks of the region.

ACKNOWLEDGMENTS

We would like to acknowledge the following col-laborators who have been part of the overall scien-tifi c development of the Grand Canyon Supergroup project: Carol Dehler, John Bloch, Tony Prave, Steve Cather, Adam Read, Stacy Wagner, Paul Bauer, Mary Simmons, Colin Shaw, Brad Ilg, Mike Doe, Jake Armour, Casey Cook, Sarah Tindall, Arlo Weil, John Geissman, Annie McCoy, Micah Jessup, Mark Quigley, Andy Stone, Lisa Peters, Tim Lite, Andy Knoll, Suzanna Porter, and Jason Raucci. This work was made possible by the National Science Founda-tion grant EAR-9706541 to Karl Karlstrom, John Geissman, and Maya Elrick, and EAR-9902955 to Karlstrom and Heizler for thermochronology efforts. Further support comes from Ben Donegan (consult-ing geologist), Conoco, Inc., and Schlumberger, Inc. We would also like to acknowledge the helpful reviews of Gerry Ross and Jim Sears.

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Tectonic inferences from the ca. 1255–1100 Ma Unkar Group ...€¦ · BACKGROUND OF GRAND CANYON...

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