long the shore of Chuar beach It is hard to imagine that had you been hiking in the Chuar Valley in the Late Precambrian, warm incoming tide waters would have lapped up against your ankles. Yet, this is the vision that we are left with after scratching our heads about the Chuar Group. It was mentioned in the last bqr article on the Grand Canyon Supergroup (Timmons and others, Volume 12, No. 1, Winter 1998­1999) that these beautiful Chuar Group rocks represent a marine environment‹yet other geologists have interpreted them otherwise. Why do we think the Chuar Group is marine and what features in the rock tell us there was an ocean here at this time? Furthermore, what does this 1600 meters (5248 feet) of strata tell us in terms of time, life, climate, and tectonics? We have learned from Mike Timmons and others that Chuar Group sediment was recording the incipient breakup of the supercontinent Rodinia. From looking at the sedimentary rocks and the fossils encased therein in great detail, what is now beginning to unfold is a picture of Chuar beach; linked to the greater ocean, and supporting a diverse and cosmopolitan assemblage of simple organisms. Looking even closer at these rocks (on an elemental scale and isotopic scale) we are finding that the geochemistry of the world ocean fluctuated during Chuar time and may be telling us of ancient glacial episodes.

It is always good to remember, not only what it is we as scientists are doing (especially in the delicate Grand Canyon), but also why we are doing it and how we go about it. The Chuar Group is an example of how the Grand Canyon is indeed one of the world¹s best geologic laboratories and it continues to yield new insights that can change the way we understand our planet. The Chuar Group is unique because it is exposed nowhere else in the world and it is well exposed indeed! These rocks record a time in Earth history rarely preserved in the rock record, particularly in the western U.S.. Furthermore, the preservation of the Chuar Group is remarkable for its age: it is unmetamorphosed and only very mildy deformed by movement on the Butte fault. Just as in the Phanerozoic strata above the Great Unconformity, the Chuar Group is like a book: it holds a record of what was going on in the world at this time (the Neoproterozoic). Within this record are sedimentological, stratigraphic, geochemical and fossil clues that allow us to hypothesize about processes of sedimentation, tectonism, life, and climate. Lastly, the Chuar Group may help us to explain the big changes from the Precambrian world to the Phanerozoic one (e.g.; evolution of life, atmosphere, ocean, and tectonics). All of these characteristics make the Chuar Group an extremely important succession of rocks that could become considered a world-class ³type section² for this time period. The following text is a summary of the more telltale details that we have come across in the Chuar rocks. By the way, the highlighted words are linked to a glossary, in the case that you do not understand some of this geo- or bio-lingo.

Charles D. Walcott was the first to study the stratigraphy and paleontology of the Chuar Group, in the 1880s and 90s. He described the Chuar ³terrane², discovered and named the microfossil Chuaria circularis, and noted the presence of stromatolites (Walcott, 1894, 1899). A more recent comprehensive study of the Chuar Group was conducted in the late 1960¹s and early 1970¹s (Ford and Breed, 1973a and references therein) and these studies yielded information on the general stratigraphic, sedimentologic, and paleontologic characteristics; however, environmental interpretations were ambiguous or lacking. Subsequent studies have resulted in a range of paleoenvironmental interpretations of all or part of the Chuar Group; including shallow marine, coastal plain, alluvial plain, fluvial (river), and lacustrine (lake) (e.g., Reynolds, et al., 1988; Cook, 1991; Ford and Dehler, in press). Our ongoing work is focussed on the details of the Chuar Group: to try and settle the debate about paleoenvironments and to glean new information about Neoproterozoic Earth history. This next step in understanding the Chuar Group has involved looking at this vast thickness of rocks up close and personal. Unfortunately, in most cases, these rocks are shy and don¹t want to let you get to know them. By spending significant amounts of time with them, though, it seems as if they are beginning to open up to us. Within these sedimentary rocks, we look for two main types of features: sedimentary structures and fossils. The sedimentary structures tell us something about sedimentary process (what was moving the sediments‹wind, water, gravity, or ice) and the fossils tell us something about paleobiology, and combined these things tell us about paleoecology and paleoenvironments.

Why not "Lake" Chuar?
Sedimentary structures are found throughout the Chuar sequence, yet are best preserved and most abundant in the middle part of the Chuar Group: the upper Carbon Canyon Member, the Carbon Butte Member, and the lower Awatubi Member (see Fig. 1). One of the most informative and common sedimentary features are ripplemarks, preserved on bedding-plane exposures, that are symmetrical by nature. These are common features to see on the faces of dipping beds along the Butte fault, especially between Carbon and Lava Chuar Canyons. The shape of these ripplemarks requires even flow of water in opposite directions to maintain this symmetry. Therefore, we know these sediments (silt and sand) were being pushed around in an oscillatory current within a wave zone. Waves can form in lakes, oceans, and even rivers, so, these features alone do not help us to pinpoint a paleoenvironment. Interestingly though, these ripplemarks are draped with mud that exhibit mudcracks. These mudcracks require a wet environment to deposit the mud and a dry one to dry out and crack the mud. We have observed many cycles of mud-draped and mud-cracked ripplemarks stacked on top of each other. The most likely sequence of events to create these cycles and suite of features is in a wave and tidal regime along an ocean shoreline (see Fig. 2). As the tide comes in, the sand or silt becomes rippled in the oscillatory flow. When the tide turns around at high tide (flood tide), the fine mud settles out and armors the newly rippled sand and silt. Then the tide goes out and when low tide is reached (ebb tide), these armored ripplemarks are exposed above the waterline, become dry and crack. Then the cycle repeats itself. Whether the tide fluctuated once or twice a day is not known, but was likely the latter (we are still searching for tidal features to tell us this).

In the Carbon Butte Member (a big red cliff of sandstone marks the base) are a suite of sedimentary features that also suggest a tidal- and wave-influenced environment. We have found crossbedding in the sandstone, which indicates that there were small underwater dunes being influenced by a fairly strong current moving in opposite directions (suggesting tidal currents). Associated with these small crossbeds are very large, very low-angle crossbeds that you can trace for at least a half of a mile. These are most likely the sides of shifting tidal channels or they were large underwater sand dunes. The Carbon Butte Member also hosts soft sediment deformation features, indicative of either very fast deposition and/or seismic shaking. The best place to see these features is up Kwagunt Canyon, either right along the Butte fault or up west of the Butte fault where Kwagunt Creek flows right over the lower cliff-forming red sandstone. Stromatolites are a small yet consistent part of the Chuar Group stratigraphy. There are 6 different types of stromatolites (see Ford and Breed, 1973a and Cook, 1991), some being more recognizable and accessible than others. Probably the best known of the stromatolites is Boxonia, a specimen which looks like a giant brain (see Fig. 3) and can most easily be seen near the Butte fault in Carbon Canyon (this specimen has tumbled down from Carbon Butte to the north). You can think of these spectacular ³brains² as microbial reefs or mounds. Had you been snorkeling in this Precambrian sea, you may have scraped your belly on one of these bad boys. What the "brains" and other stromatolites tell us about paleoenvironment is that the water level had to be at least as deep as the height of a mound. Therefore, this ³loose brain² in Carbon Canyon, being about 2 meters (3.6 ft) tall, tells us that the water depth had to be at least 2 meters deep. Another environmental condition required by stromatolites is that they remain in the light so that photosynthesizing microbes can do their thing. This also requires that the water not be too cloudy with sediment. The fact that this and some of the other stromatolite types have been found in marine deposits of Proterozoic age worldwide suggests, but does not confirm, a marine origin for Chuar Group stromatolites. Another good place to see the ³brains² is on the hike up to Nankoweap Butte (They are better exposed on the Kwagunt side.). While you are in Kwagunt Canyon, walk north up the Butte fault until the creek splits and then tool around on the slope facing the Butte fault. Here you will find Bacalia, a rather "Dr. Suessish" stromatolite typically less than .5 meters in any dimension, and below this, a dolomite bed that contains very strange features that we interpret as large mudcracks (see Fig. 4). These strange cracks, arranged in polygonal shapes in bedding-plane view, likely represent a time when sea level was lowered and this part of the land was exposed for a long duration.

It¹s funny, but the domineering and colorful shales in the Chuar Group don¹t tell us much of anything when looking at them ³in the field². We have learned that the beautiful Martian colors you see on Chuar hillsides are mostly created by post-depositional processes (i.e.; color typically is not diagnostic of paleoenvironment). By looking closely at these shales back in the lab, however, we may find geochemical trends indicating things such as unconformities, burial depth, and changes in Chuar water chemistry through time.

Well, why not ³Lake² Chuar?
From sedimentary structures we deduce that the Chuar Group sediments were under the influence of tides and waves. Tides, especially ones large enough to leave their mark in the rock record, are a process inherent to the ocean and not to lakes. Therefore, the tidal features found throughout the Chuar Group allow us to interpret deposition in a marine setting (see Fig. 5). In addition, stromatolites (and microfossils) in the Chuar Group can be matched to stromatolites (and microfossils) found in Neoproterozoic marine deposits elsewhere in the world.

Life in Chuar waters
Although there were no animals or land plants during Chuar time, the earth was not barren. Oceans teemed with life‹many eukaryotic lineages had diversified by Chuar time (about 750-800 Ma‹see below), and bacteria had been around for at least 2.5 billion years, indicating that life was here on Earth nearly since the beginning (4.6 Ga). The Chuar Group provides us with an exceptional window on the diversity of life and the dynamics of ecosystems during this time just prior to the poorly understood ³Cambrian Explosion². Both bacteria and eukaryotes are well-represented in its sedimentary record.

In the shallow, high energy, warm water environments of the Chuar Group (represented by dolomites), bacteria probably related to cyanobacteria grew on the surfaces of dolomite grains. They are preserved at a number of horizons in the Walcott Member (see Fig. 1; Schopf et al., 1973). As mentioned earlier, also growing in shallow, lower energy waters were stromatolites. These are formed by mats of microorganisms, mostly cyanobacteria, which, as they grow, trap and bind grains or promote localized carbonate (limestone and/or dolomite) precipitation, thus causing the stromatolite to be laminated in cross section. If you look closely at the dolomite layers throughout the Chuar Group, you should find this wide variety of stromatolites in all but the Carbon Butte and Duppa Members (Fig. 1). Modern examples can be found in Shark Bay, Western Australia, in the Gulf of California, off Baja Peninsula, and off Lee Stocking Island in the Bahamas.

A diversity of algae also lived in Chuar waters. They are represented in Chuar Group shales by acritarchs, i.e., organic-walled microfossils of unknown taxonomic affinity, the majority of which are probably algal cysts. Many algae form cysts ‹ a dormant stage in their life cycle ‹ when they are exposed to stressful conditions, such as a lack of nutrients, or a change in salinity or temperature. The cyst wall is made of resistant material, and thus is commonly the only stage of the life cycle preserved in the fossil record. Chuar Group acritarchs include the megascopic Chuaria circularis (Fig. 6a), a flattened, carbonaceous disc, 0.4 to 5 mm in diameter found on the bedding planes of shales throughout the Chuar Group. Although earlier interpreted as a brachiopod, a snail, a foraminifer, and a trilobite egg, C. circularis is now thought to be a smooth, spherical algal cyst (Ford and Breed, 1973b and references therein). It has been found in Neoproterozoic marine rocks all over the world. The ecosystem that inhabited Chuar Group waters consisted not only of algae and bacteria, but included single-celled predators, or protozoa, as well. Evidence for this comes from Œvase-shaped microfossils¹ (Melanocyrillium Bloeser, 1985), which represent the tests of testate or 'shelled' amoebae

(Fig. 6b). Most modern testate amoebae live in freshwater or moist terrestrial environments, although some inhabit marine environments, living at the sediment-water interface. They eat bacteria, algae, other protozoa, fungi, and even (very small!) animals, by engulfing them with their pseudopodia (finger-like extensions of the cell). By analogy with their modern counterparts, Neoproterozoic testate amoebae probably lived on the sea floor, and ate any or all of the organisms in Chuar waters ‹ algae, bacteria, and even other amoebae. These testate amoebae represent the earliest evidence for protozoa in the fossil record, and therefore also provide evidence for a relatively complex ecosystem ‹ one that contains predators as well as primary producers.

Climate during Chuar time

Geochemical signatures in sedimentary rocks are the record of chemical changes that occurred during deposition of the sediments. These geochemical imprints can be indicators of fundamental changes on Earth such as climate fluctuations and life cycles (e.g.; glaciations and extinction events). Although many geochemical impressions in Precambrian rocks have been obscured by metamorphism, the Chuar Group is unmetamorphosed and hence, holds a record of what the water chemistry was like during deposition of Chuar sediments. For example, the amount of the element carbon, found in the black shales and dolomites of the Chuar Group, has not changed since it was buried so long ago. By looking at the ratio between the two stable isotopes of this carbon (12C, the lighter one, and 13C, the heavier one), we have found (with the help of Dave Des Marais at NASA and Zachary Sharp at UNM) that the 13C-to-12C ratio has fluctuated significantly throughout Chuar time (Fig. 1). Based on the hypothesis that that the Chuar Group is a marine succession, these fluctuations in carbon-isotope ratios through time (carbon curve) can be used as an approximation for the carbon cycle (linked to life and atmosphere) of the Neoproterozoic ocean!

Many geologists have recognized Precambrian glacial deposits (tillite) (e.g.: Harland 1964). Oddly enough, it has been demonstrated that many of these tillites were originally deposited in low latitudes. This combination of tillites and low latitudes has instigated an array of hypotheses to explain how glaciers could form in equatorial regions (see Hoffman 1998a for a review of these hypotheses). Currently, the most published hypothesis is the "Snowball Earth" hypothesis (original Harland, 1964; extended Kirschvink, 1992; revived Hoffman, 1998a) which suggests that the Earth¹s oceans were completely frozen over for possibly up to 10 million years at a time. During these oceanic deep freezes, the ocean was cut off from the atmosphere (i.e.; oxygen supply) causing many of the organisms on the planet to die off. Coupled with these Precambrian tillites in the rock record are anomolously large variabilities in carbon-isotope ratios ("excursions"). Some Precambrian sedimentary sequences lack tillites yet show these carbon excursions and are interpreted to also be recording glaciations. We are finding the Chuar Group has several of these (large) excursions (Fig. 1)! We also know from paleomagnetic data that the Chuar Group was deposited at near-equatorial paleolatitudes. This information has several implications: 1) the Chuar Group was deposited near the equator during a time of fluctuating climate associated with glaciations; and 2) we can match the Chuar Group to other Neoproterozoic sequences with these same carbon excursions to piece together a global history of climate and life (and whatever else is controlling this curve‹tectonics?); and 3) we can use the Chuar Group to learn more about the ³Snowball Earth² hypothesis. Does the Chuar Group record a ³Snowball Earth² scenario? This is hard to say. What we can say is that the Chuar Group likely records global climate changes. What will be exciting is to combine detailed paleontology with these excursions to see if we can find extinction events. This would be consistent with a "Snowball Earth" hypothesis.

What time is it anyway...
How old is the Chuar Group?
There are two ways to determine geologic time; relatively and absolutely. Relative age determination involves recognizing the ordering of geologic events. For example, the dark green dike (igneous intrusion) at Hance Rapids crosscuts the Hakatai Shale, therefore the shale had to be there first (the shale is older than the intruding dike). We know from absolute age determination, based on radioactive elements decaying at constant rates, that the Hance dike is about 1.1 Ga. There are no intrusions (e.g.; dikes) in the Chuar Group, so we cannot use these techniques, yet these intrusions below in the Unkar Group tell us that the Chuar Group is at least younger than 1.1 Ga (a maximum age). But even better, there are a few bright white beds found in the Chuar Group that appear to be volcanic ashes (tuffs). One of these at the very top of the Chuar Group (Walcott Member at Nankoweap Butte), has been recently dated by Sam Bowring at MIT and yielded a U-Pb date of ~742 Ma. This is the first direct age on the Chuar Group! Fossil data and the new carbon curve are consistent with this date. Now we can combine the age (of the top of the Chuar Group) with this fossil information and the carbon curve to visualize what the Chuar world was like, and when. With all of this data, we can really do some high-resolution correlation with other Neoproterozoic successions that have absolute ages, carbon curves, and fossil data. This will give us a keener view of the greater Neoproterozoic (~742 Ma) world.

Reflections
Next time you¹re hiking in the Chuar Valley, think of it as taking a stroll along an ancient equatorial shoreline. You¹ll be sharing the environment with small photosynthesizers and Earth¹s first predators, and enjoying the lapping tides and mellow surf. Imagine glaciers at the poles and, at times, glaciers closer by. You might even feel a tremor as the Butte fault makes room for more Chuar sediment. If you look at your watch, the time is ~750 Ma.
Enjoy your beach walk!

References Cited

Bloeser, B. 1985, Melanocyrillium, a new genus of structurally complex Late Proterozoic microfossils from the Kwagunt Formation (Chuar Group), Grand Canyon, Arizona: Journal of Paleontology, v. 39, p. 741-765.

Cook, D., 1991, Sedimentology and shale petrology of the upper Proterozoic Walcott Member, Kwagunt Formation, Chuar Group, Grand Canyon, Arizona: Unpublished Masters Thesis, NAU, 158 p.

Ford, T. D., and W. J. Breed, 1969, Preliminary geologic report of the Chuar Group, Grand Canyon, Arizona, in Four Corners Geological Society Guidebook to Grand Canyon, Appendix by C. Downie, p. 114-122.

Ford, T. D., and W. J. Breed, 1973a, Late Precambrian Chuar Group, Grand Canyon, Arizona: Geological Society of America Bulletin v. 84, p.1243-1260.

Ford, T. D., and W. J. Breed, 1973b, The problematical Precambrian fossil Chuaria: Paleontology, v. 16, no. 3, p. 535-550.

Ford and Dehler, in press, Grand Canyon Supergroup: Nankoweap Formation, Chuar Group, and Sixtymile Formation, in Beus, S., and Morales, M., (eds), Grand Canyon geology, Oxford University Press, Edition 3. Harland, W.B., 1964, Critical evidence for a great Infracambrian glaciation: Geologische Rundschau, v. 54, p. 45-61.

Hoffman, P.F., Kaufman, A.J., and Halverson, G.P., 1998a, Comings and goings of global glaciation on a Neoproterozoic tropical platform in Namibia: GSA Today, v. 8, no. 5, p. 1-9.

Krischvink, J.L., 1992, Late Proterozoic low latitude glaciation: The snowball earth, in Schopf, J.W., and Klein, C., (eds), The Proterozoic biosphere, A multidisciplinary study: Cambridge, Cambridge University Press, p. 51-52.

Reynolds, M.W., Palacas, J.G., and Elston, D.P., 1988, Potential petroleum source rocks in the Late Proterozoic Chuar Group, Grand Canyon, Arizona: U.S.G.S. Circular 1025, p. 49-50.

Schopf, J. W., T. D. Ford, and W. J. Breed, 1973, Microorganisms from the Late Precambrian of the Grand Canyon, Arizona, Science, v. 179, p. 1319-1321.

Timmons, J.M., Karlstrom, K.E., and Dehler, C.M., 1999, Grand Canyon Supergroup: Six unconformities make one ³Great Unconformity: A record of supercontinent assembly and disassembly: Boatman¹s Quarterly Review, v. 12, no. 1, p. 28-32.

Walcott, C. D., 1894, Precambrian igneous rocks of the Unkar terrane, Grand Canyon of the Colorado: U.S.G.S. 14th Annual Report for 1892/3, part 2, p. 492-519.

Walcott, C. D., 1899, Precambrian fossiliferous formations: Bulletin of the Geological Society of America, v. 10, p. 199-244.

Glossary of geological and biological terms

algae: photosynthetic eukaryotes.
amoebae: a microscopic, single-celled eukaryote consisting of a naked blob of protoplasm that can change in shape as it moves and eats
bacteria: the group of organisms which do not possess a membrane-bound nucleus (a.k.a. prokaryotes). They are single-celled, live virtually everywhere, and some kinds give us nasty colds and diseases. One of the big players in stromatolites, especially cyanobacteria.
brachiopod: a kind of bivalved animal which possesses a lophophore‹a folded, often horseshoe-shaped feeding structure that encircles the mouth and bears ciliated tentacles. Despite the common possession of a bivalved shell, brachiopods are unrelated to clams and their relatives.
crossbedding: a type of sedimentary structure where angled, planar to concave-up features in the rock represent the ancient face of a migrating dune (subaqueous or subaerial) or the side of a migrating channel
dolomite: magnesium-rich limestone (Ca, Mg)2 (CO3) eukaryote: a group of organisms which possess a membrane-bound nucleus. The group encompasses all life (amoebae, algae, fungi, plants, and animals) with the exception of the bacteria.
foraminifer: a group of single-celled eukaryotes that secrete distinctive, chambered calcium carbonate shells or 'tests'.
geochemistry/geochemical: the chemical nature of natural processes and results occurring on Earth and beyond
Neoproterozoic: geologic time period between 1000 and 545 million years ago, the latest subdivision of the Precambrian
paleoenvironmental: ancient environment such as a shoreline, lake, or river
paleomagnetic: pertaining to a past intensity and direction of the Earth¹s magnetic field
Phanerozoic: geologic time period between 544 million years ago and present
Precambrian: the geologic time period between 4.6 billion years ago and 544 million years ago
primary producers: organisms capable of making organic carbon compounds by fixing CO2
protozoa: (typically) one-celled, non-photosynthetic eukaryotes that generally lack a cell wall sedimentary structures: features in sediment/sedimentary rock made by processes such as wind, water, gravity, ice, or biota.
sedimentological: the natureof sediments/sedimentary rock stable isotopes: certain elements, such as carbon, have varying numbers of neutrons in their nuclei. 12C (6 neutrons) and 13C (7 neutrons) are two different stable isotopes of carbon. They do not radioactively decay like 14C (8 neutrons), another isotope of carbon‹yet an unstable one.
strata/stratigraphic: layered rock/the nature of layered rock
stromatolites: distinctively laminated rocks formed in association with microbial mats (Walcott, 1899)
taxonomic: pertaining to the classification of organisms on the basis of the evolutionary relationships. tectonics: referring to the large-scale structural elements of a region and the processes by which they formed (e.g; deformation from differential stresses due to plate motion)
trilobite: a group of animals, related to crustaceans, insects, and spiders, that was diverse and abundant during the Early Paleozoic (544 Ma+) and became extinct at the Permo-Triassic boundary (~225 Ma)
type section: the most continuous and well-preserved succession of rocks representative of a particular time period in the area where those rocks are found.