tween about 1100 and 800 Ma, mc from 1000 to 540 Ma (Elston and I

The Late Precambrian is an irr the history of western North Arne1 searchers have proposed that duri nental masses were assembled intl

. . -, - . 54

assembly at 1.0 Ga, Rodinia begal . - GEOLOGIC STRUCTURE OF THE GRAND rifting that took place between 751

records part of this long history, a

. . CANYON SUPERGROUP ing and processes of intracratonic I ..-.r?' seawater, and the rapid diversifica '~xplosion' (Dalziel 1997).

j. Michael Timmons, Karl E. Karlstrom, andJames W. Sears

nocline. This monocline is a gr ibab Plateau that formed in Lac

(see below). Farther wesl hic Suite (GGMS) emerg

r Granite Gorge, but there pergroup beyond Deer Creek.

Early workers recognized that in fault-bounded, downdrc erosion. We can piece thf the Meso and Neoprotert

5.1. Precambrian rocks in a an ancestry are also shown.

INTRODUCTION / 1 @&bout 1100 and 800 Ma, mostly in the Neoproterozoic Era, which lasted &Q60 to 540 Ma (Elston and McKee 198.2; Bowring et al, 1996).

have proposed that during this time ~ o r t h America and other conti:' :masses were assembled into a supercontinent called Rodinia. Following

1.0 Ga, Rodinia began to break apart in one or more episodes of took place between 750 to 550 Ma. The Grand Canyon Supergroup

story, and it is a key locality for unraveling the tim- ses of intracratonic rifting, the evolution in the character of global the rapid diversification of life on earth leading to the Cambrian

distribution of Mesoproterozoic sedimentary rock ex p in the eastern Grand Canyon. The Unkar Group occurs in isolated wedge-

femnants (grabens and half grabens) in areas along the river, between md 137 (shown in Fig. 5.2). The Chuar Group is exposed in the Chuar 7

eau that formed in ~aramide time by the reactivation of a-~r~ecam- ee below). Farther west, Paleoproterozoic rocks of the Granite Gorge c Suite (GGMS) emerge again near Granite Park (mile 207) and in

Granite Gorge, but there are no more exposures of the Grand Canyon

erosion. We can piece these fragments togethe; to decjpher so& as- 6@f the Meso and Neoproterozoic tectonic history of the Grand Canyon

L I

.I, Precambrian rocks in eastern Grand Canyon. Major structures with Pre- L- '

OF THE MIDDLE CRUST IN THE . SOUTHWEST , ".

STRUCTURAL GEOLOGY OF THE UNKAR GROUP

to fluvial silicielastic rocks. The section is cross-cut Iq a illrd dikes 1.1 Gal and covered by the uppermost fomaticm af the U W 1.07-Ga Cardenas Lavas a s t o n and M a e a 1982). The Unlnar

that fadting and se$imemaaa mixz fwlt in PrsteroWc time is 1 ' t 'E48

' -- - .

STRUCTURAL GEOLOGY OF THE UNKAR GROUP 81

ting the timing of movement of extensional faults found within the is difficult. These faults are often truncated by the Cambrian Tapeats

we have no way to test if (indiatbg p r e - c h u ~

timing interpretations, there is inwea6ing ev- active throughout depositi~n of the Grwd

<

Nankoweap Formatiop

For example, in the Tanner graben we see a fault re- ar offset of the lower member of the Nankowea~ For-

5.7). These offsets are filled and covered by the upper member of ap Formation, clearly implying that faulting occurred during early

leinuation of the Butte fault, as expresses by the East Kaibab n far to the north across the Utah border. The Butte fault vends to the

orrhwest and dips moderately ~ ~ 7 0 8 ) to the west. It presently has ap-

@@ be seen in a traverse from rhe Colorado River to t h e ~ h u a r Group g-dong G a d Creek. Movement was predominately dip-slip, as shown ~mides oriented parallel to the dip of the fault. &&inate normal faults within the Chuar Croup are parallel, and from the

r m r d (see below) it a~wrently moved at the same time as the

khns (as opposed to half SbenG such as the Basalt Canyon fadt. Dis- ats on these faults are on h~ mIa of meters to tens of meters and thus &ange the averall aBymmmy of the Butte system. ~ G h w syncline is P broad, open, tmugh-shaped fold just west of and

&wed. i i the Paleszoic <over, indicating that the sBn'dlne is Pmtero- 6 d unrelated to the r@adivacion <reverse sense) of the ER~tte fault

k5chernatic.E-w c r ~ s s section near Nankdweap Butte. Syndlnal axial plane bult plane are parallel in s u f e (northl and are conjugate pairs (-500 an- m plane$. Note thickness variation of upper members across syncline,

SEDIMENTARY R E C ~

ample, measured sections between h j her and the stromatolite at the base a

1

POST-SIXTYMILE 8 PHANEROZOIC ~4

. - *

D FOR PROLONGED INTERACTION 85

ured sections between the basal sandstone of the Carbon Butte Mem-

Suayrmle Formation

Formation marks a dramatic change in the style of deposition in ter marine shale to coarse sedimentary breccia

both higher energy deposition and probable tec- on 1979). The Sixtymile Formation is exposed only of the Chuar syncline. The lowermost member of cterized by large (10-m scale) dismembered blocks strome beds (slumped or gravity slide deposits)

deposition. Several hale and tuffaceous

rgence and tectonism. The upper breccia on p incised paleocanyons cut in shale that trend sting continued influence of the Butte fault on

POST-SIXTYMILE EROSION AND PHANEROZOIC REACTIVATION

no record preserved between the Sixtymile Formation and the uncom- at the base of the Cambrian. Neoproterozoic faulting and tilting proba-

ixtymile deposition to bring the Cardenas Lavas and Dox wall adjacent to upper Chuar Group shale. By Middle Cam- ad again reduced .the Grand Canyon Supergroup and un-

rocks to a nearly smooth, flat plain. Ridges of Shinumo e rise up to 600 feet (180 m) above the generally level plain along the

f the tilted blocks. The sea advanced over this plain from west to east

and drowned the islands of Shinumo Sandstone in sand and mud of the Gam- brian sea, about 510 million years ago.

After Paleozoic and Mesozoic strata buried the Grand Canyon region, many

REGIONAL IMPLICATIONS AND CONTINUED PROBLEMS FOR

SUPERGROUP TECTONISM

,

around 800 Ma. Much of North America was affected by the 1.3- to 1 .l-Ga Gr

In Arizona, sedimentary rocks of deposited between 1.4 Caand 1.

88 GEOLOGIC STRUCTURE OF THE GRAND CANYON SUPERGROUP

'the rift separation took place (Burchfiel et al. 1992). At the base of this thick prism of sedimentary rock are preserved patches of rock that are 1.45 Ga (Belt Supergroup), >1.1 Ga (lower Pahrump Group of Death Valley), and 800-700 Ma (Windemere Group, Uinta Mountain Group), suggesting that episodic rifting occurred over an extended period of time. Some workers sug- gest that the actual continental separation took place around 700 Ma (Ross et al. 1989), whereas others suggest that it took place about 550 Ma (Levy and Christie-Blick 1991). Evidence for long-lasting E-W extension in the Chuar Group may support the idea of intracratonic rifting in the 800- to 700-Ma time interval, but unfortunately does not clarify the timing of continental dis- memberment.

What were the continental masses to the south and west of North Amer- ica in the late Precambrian? Continental plate reconstructions based on pale- omagnetic data and lithostratigraphic correlations of Proterozoic rocks remain a heated topic of debate. One model proposed by Sears and Price (1978) places Siberia off of western North America in the Late Proterozoic prior to rifting and development of the Western Cordillera. This plate reconstruction remains the preferred model of one of the coauthors. Other workers have postulated that West Australia and East Antarctica were our neighbors to the west and that portions of South America were to the south (Fig. 5.10, inset) (Moores 1991; Hoffman 1991; Dalziel 1991, 1997). This latter model suggests that North America was part of a supercontinental mass, named Rodinia, which was assembled around 1.0 Ga and broke apart between 750 and 550 Ma. If

tectonism. Often, the subtle record preserved in sedimentary basins in continental interiors can provide clues to the nature of events at distant plate margins. It re- mains to be seen if the Grand Canyon Supergroup is recording the far-field ef- fects of (a) 1100-Ma collision and coeval failed rifting (supercontinent assembly) and (b) -800-Ma incipient rifting of a supercontinent.

SUMMARY

breakup. This rifting apparently failed in the Grand Canyon region, tually led to successful rifting and formation of the Cordilleran m between North America and the ancestral Pacific Ocean by 550

ACKNOWLED from a Masters thesis by Sears, 2 xative NSF-hnded project EAR- nan, Sam Bowring, and Andy Kn, We thank Carol Dehler who is ( in the Chuar Group.

ACKNOWLEDGMENTS 89

ACKNOWLEDGMENTS

from a Masters thesis by Sears, and new data came from early stages project EAR-9706541 (to Karlstrom, Maya Elrick, and kndy Knoll). Timmons' work is part of a Mas- ehler who is doing concurrent sedimentology and

t