ou have all seen the Great Unconformity in the Grand Canyon, just as John Wesley Powell noticed it in 1869 on his pioneer voyage. This geologic feature is a dramatic break in the rock record. Below this feature are older (Paleoproterozoic) igneous and metamorphic ³basement rocks;² above are younger (Phanerozoic) layered sedimentary rocks. In most parts of the Grand Canyon, and in fact, throughout much of the southwestern u.s., the Cambrian Tapeats Sandstone forms the layer just above the unconformity. One such place is Blacktail Canyon, where the Great Unconformity represents about 1.2 billion years of missing time (1.75 Ga (billion year old) gneiss is overlain directly by 0.5 Ga sandstone). This amount of time adds up to 25% of Earth¹s history! What went on in the Southwest during this vast time gap? Luckily, the Grand Canyon Supergroup records parts of this history and offers us a look into a time when very significant changes were happening on our planet. During Grand Canyon Supergroup time, a hypothesized supercontinent called Rodinia assembled at about 1.1 Ga, and then broke apart around 0.75 Ga. The first life forms with a nucleus, single-celled creatures called Eukaryotes, were becoming more diverse, testing the waters for their future evolutionary extravaganza known as the ³Cambrian Explosion.² Changes in global seawater composition reflect complex changes in climate, tectonics, atmosphere, and biosphere.

Piecing together puzzles with missing pieces

In geology, perhaps more than some other natural sciences, researchers are given only pieces of the puzzle. We use the concept of OEmultiple working hypotheses¹ when attempting to decipher the fragmentary record of the ancient geologic past (Chamberlin, 1890). By using this method, the scientist proposes different hypotheses, which try to explain all the available observations. These hypotheses must be consistent with the physical processes that we observe in the present, and assume to have also operated in the past. As new data become available, some hypotheses will be discarded and some become stronger and may be advanced to ³theory² status. Hopefully, one hypothesis will ultimately represent the best and most logical explanation of those posed.

Meet the Grand Canyon Supergroup

We know from Brad Ilg¹s research that the Paleoproterozoic basement rocks in the Grand Canyon area were down at depths of 20 km during deformation and metamorphism at 1.7 Ga (Ilg and Karlstrom, 1997). These middle crustal rocks probably remained at great depths until 1.4 Ga, then they were uplifted and exposed at the Earth¹s surface after about 1.3 Ga. Sometime after erosion had exposed these basement rocks at the surface, low basins developed on the continent and began to fill with sediment

Unkar Group and Nankoweap Formation: Record of Mesoproterozoic (1.1 Ga) NE extension and NW contraction

The Unkar Group is divided into five formations: the Bass, Hakatai, Shinumo, Dox, and Cardenas Basalt (Hendricks and Stevenson, 1990, and references therein). This sequence of strata is ~6800 feet (2073 meters) thick, and is quite variable in rock type. The Bass Limestone contains a conglomerate at the base (Hotauta Conglomerate) and limestones, dolomites, and a few volcanic ash layers make up the rest of the unit; the Hakatai Shale is predominantly made up of shale, mudstone, and sandstone; the Shinumo Quartzite is a wall of sandstone and quartzite; the Dox formation is made up of shale, mudstone, and sandstone, and rare stromatolite beds; and the capping Cardenas Basalt is interbedded with the uppermost Dox Formation (Ochoa Point Member) (see figure). These strata collectively record a long (?) period of deposition along a tide- and wave-affected, fluctuating marine shoreline. This transgressing and regressing shoreline was fed by a river system, and was associated with an outpouring of basaltic lava during the (preserved) close of Unkar time.

The Nankoweap Formation is a 370-foot (113-meter) thick cryptic unit that comprises sandstone and lesser amounts of siltstone, shale (rare black shales), and mudstone (Elston and Scott, 1976). This formation has been informally subdivided into an upper and lower member. This is based both on variations in grain size (the lower member is finer than the upper member), but more importantly by an angular unconformity that separates the two members and signifies tectonic activity during Nankoweap time (Elston and Scott, 1976).

The Unkar Group is only preserved between river miles 53 (2 miles up Nankoweap Canyon) and 137. It exists in fault-bounded, down-dropped blocks, where it was protected from erosion prior to deposition of the Tapeats Sandstone (see figure). We can piece these fragments together to decipher some aspects of the Mesoproterozoic tectonic and sedimentary history of the Grand Canyon region, but the original extent and distribution of the sediments and the shape of the basin(s) remain unknown.

Intimately linked with Unkar sedimentation and basaltic magmatic activity are faults that appear to have been active during Unkar and early Nankoweap time. These faults apparently record a period of ne-crustal stretching and basin formation that overlapped in time with nw-crustal shortening that was caused by the Grenville orogeny (mountain building event) to the southeast in the Texas region (see regional discussion). Contractional faults horizontally shorten and vertically thicken a section of rocks. These Proterozoic reverse faults and monoclines, along which nw­se crustal shortening occurred, angle northeast and are exposed in Bright Angel, Bass, Vishnu, and Red Canyons (Sears, 1973). Structures of this type and age are known only in the Unkar Group and older rocks, suggesting that this type of contractional deformation is restricted to Unkar age. Extensional, or ³normal² faults, form during horizontal stretching of the Earth¹s crust. A family of faults that cut across Unkar strata angle northwest (see figure). Jim Sears (1973) suggested that Unkar sedimentation and magmatism were synchronous with both ne extension and nw contraction. We agree, and our new work emphasizes that Unkar age extension was significant, regional in scale, and different in style from Chuar age extension, described below. The Palisades Fault (exposed in Palisades Canyon and at the Morning Star mine

Chuar Group and Sixtymile Formation: Record of E-W extension on N-S trending normal faults

The Chuar Group is quite possibly the most beautiful and striking geologic unit in the Grand Canyon. Tucked away in the headwaters of several right-bank tributaries from Nankoweap Canyon down to Basalt Canyon, it is separated from the Colorado River by the north-south trending Butte fault and a stack of Paleozoic rocks (see figure).

This unique package of rocks has no correlative (rocks of the same age) in Arizona and may be one of only a handful of rocks of this age in the western United States. This 5248-foot (1600-meters) unit is subdivided into the lower Galeros Formation and the upper Kwagunt Formation (Ford and Breed, 1973, Dehler, unpublished data, 1998). The Galeros Formation is best viewed in Nankoweap, Carbon, Lava Chuar, and Basalt Canyons where they appear as multi-colored shales and mudstones interbedded with lesser amounts of sandstones and stromatolitic beds. The Kwagunt Formation can be viewed in Nankoweap, Kwagunt, and north Carbon canyons where a prominent red sandstone cliff (Carbon Butte Member) demarcates the base of the formation. This formation is not unlike the Galeros Formation in rock type; but has fewer dolomite beds, an unusually thick sandstone unit (11­18 meters), and a significant thickness of organic-rich black shales containing a diverse assemblage of microfossils including the alga, Chuaria circularis. Collectively, the Chuar Group, not unlike the Unkar Group, represents deposition associated with a fluctuating marine shoreline that was affected by waves and tides. Interestingly, water depth remained relatively constant throughout Unkar and Chuar deposition. The Chuar Group differs from the Unkar Group in that it contains a significant amount of organic-rich sediments, a rich assemblage of microfossils, and at least four different forms of stromatolites. Stay tuned for a more detailed article on the evolution of the Chuar basin and life along a Proterozoic marine shoreline.

The Sixtymile Formation has some similarities with the underlying Chuar Group. This formation is composed of siltstones and sandstones similar to those in the Chuar Group, but also contains a significant amount of breccias and lesser conglomerates. Slump folds and large boulders in this formation signify local relief along the Butte fault marking the ³Grand Canyon Disturbance² (Elston and McKee, 1982), which was the culmination of a long period of fault movement. Perhaps the preeminent structure in the Grand Canyon is the Butte fault system of the eastern Grand Canyon. It is exposed for a length of 18 km east of and parallel to the Colorado River between mile 53 and 68.5. The present Chuar Group exposures are just west of the Butte fault. The Butte fault angles north-northwest, dips 60­70° to the west, and now forms the main fault of the Laramide-age (~60 Ma) East Kaibab monocline. During the Precambrian, the Butte fault¹s movement history was longer and involved larger displacements (see figure). Removing the displacement of Paleozoic strata across the East Kaibab monocline restores the Butte fault to its Proterozoic configuration, suggesting that displacement across the fault was as much as 3 km at the end of the Proterozoic.

The Chuar Group is folded into a broad trough-shaped syncline just west of, and parallel to, the north-south trace of the Butte fault (see figure). The Chuar syncline is asymmetrical with steeper dips on the east limb, adjacent to the Butte fault (see figure). Several aspects of this feature suggest that Chuar sedimentation, fault movement, and syncline development were synchronous. New measured sections across the Chuar syncline show stratigraphic trends indicative of syncline growth during progressive deepening of the basin on the downthrown side of the Butte fault. Chuar Group extension was east-west directed and post-dated, by nearly 200 million years, the northeast-southwest directed extension recorded in the Unkar Group.

The Big Picture

Assembly of Rodinia: The Unkar Group

It has been hypothesized that approximately 1.1 billion years ago North America was part of an assembling supercontinent called Rodinia (see figure) (Moves, 1991; Hoffman, 1991; Borg and DePaulo, 1994; Dalziel, 1995). This supercontinent predated the widely accepted and better understood supercontinent, Pangea, which assembled from the dispersed pieces of Rodinia in the late Paleozoic (~300 Ma). In North America, the event associated with the assembly of Rodinia is often called the Grenville orogeny, which resulted from the collision and suturing of North America to other continental mass(es). Remnants of this old continental suture can be found from Texas to the Grenville Province of New York and ne Canada. Many scientists postulate that East Antarctica and West Australia were docked to the western margin of North America, placing North America within the center of the Rodinian Supercontinent (see figure) (Moores, 1991; Brookfield, 1993; Karlstrom and others, 1999). Perhaps due to the Grenville collisions, North America was also being cracked and split, as suggested by widespread extension and basin development. The Midcontinent rift (a subsurface rift basin in the mid-west), Central Basin Platform (a subsurface rift basin in Texas), Apache Group (in central Arizona), and the Unkar Group all show extension at high angles to the Grenville Collision (see figure). They all have 1.1 Ga mafic intrusions and volcanic rocks, and show evidence for fault movement, allowing the crust to extend or pull apart in an northeast-southwest sense, perhaps in response to the northwest-directed continental collision on the southeast margin of North America.

The Breakup of Rodinia: The Chuar Group

For about 250­300 million years following the Grenville orogeny, the southwest geologic record is virtually nonexistent. Perhaps about 800 million years ago, East Antarctica and West Australia began to pull away from North America. Precisely when this rifting occurred remains in debate (Bond and Kominz, 1984; Ross and others, 1989). In the Grand Canyon Chuar Group and other exposures along the western margin of North America, we see evidence for continental scale extension that may be signaling the initial phases of this supercontinent rifting. If we can get better dates on the Chuar Group, we may be able to help resolve the timing of supercontinent breakup.

The Grand Canyon Supergroup offers a unique record that fills part of the gap in Powell¹s ³Great Unconformity,² a time period we are just beginning to decipher. Emerging models point toward a prolonged history of tectonism in western North America that is cryptically recorded by an interaction of sedimentation and faulting in the Grand Canyon Supergroup, with numerous unconformities marking important erosional and tectonic events. The Unkar Group and associated mafic magmatism appears to record the presence of a basin within the continent that formed in response to nw contraction and ne extension related to the Grenville collisions to the south. This was followed by a hiatus of ~200 my before Chuar Group deposition recorded renewed continental rifting in an e-w sense, probably related to the early stages of supercontinent break up.

The previous summary represents a synthesis of work by many geologists. However, we have added key new findings based on work in the last three years. First, in contrast to Elston and McKee (1982) we postulate multiple, prolonged extensional events, rather than a single ³Grand Canyon Disturbance.² Second we divide faults in the Grand Canyon Supergroup into two sets: 1) nw trending faults (Unkar age ~1.2-1.1 Ga) predate the Chuar Group as shown by angular unconformity beneath the Nankoweap Formation, and 2) The n-s traces of the Butte Fault system are Chuar-age faults (700­800 Ma). Third, we document that Chuar Group deposition was synchronous with Butte fault movement and Chuar Syncline development in the Neoproterozoic. Finally, the Chuar Group was deposited in marine, not lake, environments and thus the things we learn about fossils and sea water composition can be extrapolated to the world¹s oceans. For example, Chuar shales are far richer in organic material and fossil life than the underlying units, indicating diversification of life through time and possibly more livable environmental conditions locally, and perhaps globally.

More details will be available in the upcoming edition (1999) of ³Grand Canyon Geology: Chapter 5

Mike Timmons
Karl Karlstrom
Carol Dehler

References Cited

Bond, G.C. and Kominz, M.A., 1984, Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains: Implications for subsidence mechanisms, age of break-up, and crustal thinning. Gsa Bulletin, v. 95, p. 155- 173.

Borg, S.G. and DePaulo, D.J., 1994, Laurentia, Australia, and Antarctica as a Late Proterozoic supercontinent: constraints from isotopic mapping: Geology, v. 22, p. 307-310.

Brookfield, M.E., 1993, Neoproterozoic Laurentia-Australia fit: Geology, v.21, no. 8, p. 683

Chamberlin, T.C., 1965, The method of multiple working hypotheses: Science, v. 148, p. 754759.(reprinted from Science, Feb. 7, 1890; also published in the Journal of Geology, v. 5., p. 837-848.

Dalziel, I.W.D., 1995, Earth before Pangea: Scientific American, Jan., p. 58-63.

Elston D.P. and Scott, G.R., 1973, Paleomagnetism of some Precambrian basalt flows and red beds, eastern Grand Canyon, Arizona. Earth and Planetary Science Letters 18, p. 253-265.

Elston, D.P., 1979, Late Precambrian Sixty Mile Formation and the orogeny at the top of the Grand Canyon Supergroup, northern Arizona: Geological Survey Professional Paper 1092.

Elston, D.P. and McKee, E.H., 1982, Age and correlation of the Late Proterozoic Grand Canyon disturbance, northern Arizona: gsa Bulletin, v. 93, p. 681-699. Ford, T.D., and Breed, W.J., 1973b, Late Precambrian Chuar Group, Grand Canyon, Arizona: gsa Bulletin, v. 84, p. 1243-1260.

Hendricks, J.D. and Stevenson, G.M., 1990, Grand Canyon Supergroup: Unkar Group. in Beus, S.S. and Morales, M., Grand Canyon Geology, p. 29-47.

Hoffman, P.F., 1991, Did the breakout of Laurentia turn Gondwanaland inside-out?: Science, v. 252, p.1409-1412.

Ilg, B.R., Karlstrom, K.E., 1997, Geology for the geologically impaired, Boatman¹s Quarterly Review, v. 10, n. 2. Karlstrom, K.E., Harlan, S., Williams, M. and McClelland, J., in press, Refining Rodinia: Geologic evidence for the Australia-western us (auswus) connection for Proterozoic supercontinent reconstructions, gsa Today 1999.

Larson, A.A., Patterson, P.E., and Mutschler, F.E., Lithology, chemistry, age, and origin of the Proterozoic Cardenas Basalt, Grand Canyon, Arizona: Precambrian Research, v. 65, p. 255-276.

Moores, E.M., 1991, Southwest us-East /Antarctic (sweat) connection: a hypothesis: Geology, v. 19, p.-425-428.

Ross, G.M., McMechan, M.E., and Hein, F.J., 1989, Proterozoic history: The birth of the miogeocline, in Ricketts, B.D., ea., Western Canada sedimentary basin: A case history, Calgary, Alberta, Canadian Society of Petroleum Geologists, p. 79-104.

Sears, J.W., 1973, Structural Geology of the Precambrian Grand Canyon Series, Arizona: University of Wyoming (ms thesis), University of Wyoming, Laramie, Wyoming.

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