Capitol Reef National Park

Capitol Reef National Park

Geology

Geology

CAPITOL REEF

The most scenic portion of the Waterpocket Fold, found near the Fremont River, is known as Capitol Reef: capitol for the white domes of Navajo Sandstone that resemble capitol building domes, and reef for the rocky cliffs which are a barrier to travel, like a coral reef.

Nearly 10,000 feet of sedimentary strata are found in the Capitol Reef area. These rocks range in age from Permian (as old as 270 million years old) to Cretaceous (as young as 80 million years old.) The Waterpocket Fold has tilted this geologic layer cake down to the east. The older rocks are found in the western part of the park, and the younger rocks are found near the east boundary.

This layer upon layer sequence of sedimentary rock records nearly 200 million years of geologic history. Rock layers in Capitol Reef reveal ancient environments as varied as rivers and swamps (Chinle Formation), Sahara-like deserts (Navajo Sandstone), and shallow oceans (Mancos Shale).

CATHEDRAL VALLEY

The tilt of the Waterpocket Fold dies out at Thousand Lake Mountain near the northwestern boundary of the park. Rock layers in Cathedral Valley have a gentle inclination of three to five degrees to the east and appear nearly horizontal.

Deep erosion has carved Cathedral Valley's free-standing monoliths, or temples, out of the soft reddish-orange Entrada Sandstone, which was originally deposited as sandy mud on a tidal flat. Some of the cathedrals are capped by thin, hard beds of a greenish gray marine sandstone, the Curtis Formation.

The scenery of the Entrada Sandstone temples of Cathedral Valley is complemented by evidence of other geologic processes at work. The flowing and disolving of gypsum, a soluable mineral from the underlying Carmel Formation, created Glass Mountain and the Gypsum Sinkhole. Glass Mountain is an exposed plug of gypsum. The Gypsum Sinkhole formed when a gypsum deposit dissolved. Dikes and sills, which are thin bodies of igneous rock and small volcanic plugs, are found in Upper Cathedral Valley. These features formed during volcanic activity three to six million years ago.

EROSION

Most of the erosion that carved today's landscape occured after the uplift of the Colorado Plateau sometime within the last 20 million years. Most of the major canyon cutting probably occured between one and six million years ago.

Even in this desert climate, water is the erosional agent most responsible for the carving of the landscape. The pull of gravity, in the form of rock falls or rock creep, plays a major role in the shaping of the cliff lines. Wind is a minor agent of erosion here.

The landforms are a result of different responses of various rock layers to the forces of erosion. Hard sandstone layers, like the red Wingate and the white Navajo Sandstones, form cliffs. Softer shale layers, like the Chinle Formation, form slopes and low hills. The barren slopes found in many areas are due in part to the presence of bentonitic clays in the shale which make an inhospitible environment for plants.

Black boulders, found scattered throughout the Fremont River valley and along other drainages, are recent geologic arrivals to Capitol Reef. These volcanic rocks came from the 20 to 30 million year old lava flows which cap Boulder and Thousand Lake Mountains. The boulders made their way to Capitol Reef during the end of the Ice Age when the high plateaus supported small mountain glaciers. Landslides, debris flows, and possibly heavy stream outwash from these glaciers carried the boulders to lower elevations in the park.

Capitol Reef National Park was established primarily to preserve geologic features, such as the scenic rock domes and narrow canyons. Park boundaries encompass most of the Fold. Capitol Reef is a place to enjoy the scenic majesty formed by geologic processes, and also to appreciate the interrelationships between the Earth and all life found in the varied environments within the park - - from the forested slopes of Thousand Lake Mountain, to the green oasis of Fruita, to the barren Bentonite Hills.

Biological Soil Crusts

Cyanobacteria occur as single cells or as filaments. The most common form found in Colorado Plateau soils are the filamentous type, which are usually surrounded by sticky, mucilaginous sheaths.

When moistened, cyanobacteria become active, moving through the soil and leaving a trail of sticky material behind. The sheath material sticks to surfaces such as rock or soil particles, forming an intricate web of fibers throughout the soil. In this way, loose soil particles are joined together, and an otherwise unstable surface becomes very resistant to both wind and water erosion.

The soil-binding action is not dependent on the presence of living filaments. Layers of abandoned sheaths, built up over long periods of time, can still be found clinging tenaciously to soil particles, providing cohesion and stability in sandy soils at depths up to 10cm.

Nitrogen fixation is another significant capability of cyanobacteria. Vascular plants are unable to utilize nitrogen as it occurs in the atmosphere. Cyanobacteria are able to convert atmospheric nitrogen to a form plants can use. This is especially important in desert ecosystems, where nitrogen levels are low and often limiting to plant productivity.

The sheaths have other functions as well. When moistened, they swell up to ten times their dry size. This ability to intercept and store water benefits both the crustal organisms as well as vascular plants, especially in arid regions with sporadic rainfall.

Sheaths, and the organisms they surround, also contribute organic matter and help make essential nutrients available to vascular plants. Negatively charged clay particles, often found clinging to the sheaths, bind positively charged nutrients, preventing them from being leached out of the upper soil horizons or becoming bound in a form unavailable to plants. Like soil stability, this function is not dependent on the presence of living filaments, but only the presence of sheath material.

Air pollutants, both from urban areas and coal-fired power plants, also harm these crusts.

Tracks in continuous strips, such as those produced by vehicles or bicycles, are especially damaging, creating areas that are highly vulnerable to wind and water erosion. Rainfall carries away loose material, often creating channels along these tracks, especially when they occur on slopes.

Wind not only blows pieces of the pulverized crust away, thereby preventing reattachment to disturbed areas, but also disturbs the underlying loose soil, often covering nearby crusts. Since crustal organisms need light to photosynthesize, burial can mean death. When large sandy areas are impacted during dry periods, previously stable areas can become a series of shifting sand dunes in just a few years.

Impacted areas may never fully recover. Under the best circumstances, a thin veneer of biological soil crust may return in five to seven years. Damage done to the sheath material, and the accompanying loss of soil nutrients, is repaired slowly during up to 50 years of cyanobacterial growth. Lichens and mosses may take even longer to recover.

Crust Tips

More information is available at www.soilcrust.org.

Triassic Tracks

Recent discoveries in the Moenkopi Formation (Early Triassic) of Capitol Reef National Park (CRNP), and Glen Canyon National Recreation Area (GCNRA), Utah have revealed important new terrestrial and subaqueous vertebrate track localities. These well-preserved tracks occur on multiple stratigraphic horizons and are the oldest and most laterally extensive track-bearing horizons documented in the Western U.S. Ichnogenera (Chirotherium), (Rhynchosauroides), and (Rotodactylus), are the dominant forms. Rare fish fin drag marks (Undichna) and fish skeletal remains have been identified in the Torrey Member and equivalent strata of the Moenkopi Formation.

Tracks are preserved either as positive relief "casts" filling impressions in the underlying mudstones or on plane bed surfaces as negative relief "impressions". Exposed traces occur on the undersides of resistant sandstone ledges where the mudstone has eroded away and in finer grained sediments such as mudstones and siltstones. The Torrey Member represents deposition on a broad, flatlying coastal delta plain. Both nonmarine (fluvial) and marine (principally tidal) processes influenced deposition. Even-bedded mudstones, siltstones, claystones, and fine grained sandstones, containing abundant ripple marks and parallel laminations dominate lithologic types. Ichnites indicating swimming/floating behavior are associated with the walking trackways. The water depth was sufficiently shallow to permit the vertebrates to touch the substrate with manus and pedes when moving through the water.

Tracks form locally dense concentrations of toe scrape marks which sometimes occur with complete plantigrade manus and pes impressions. Well preserved, skin, claw, and pad, impressions are common. Occasional, well developed, tail-drag marks frequently occur in many of the trackway sequences. Fish fin drag marks and fish skeletal material are preserved with tetrapod swim tracks. In addition to vertebrate ichnites, fossil invertebrate traces Arenicolites, Paleophycus, Fuersichnus, Kouphichnium (horseshoe crab), centipede, and fossil plants of Equisetum are abundant. Lateral correlations of the ichnostratigraphic units identified in the Moenkopi Formation throughout Utah's National Parks will aid interpretations about the paleoecology, and diversity of the Western Interior during the Middle Triassic - "the dawn of the dinosaurs".

Significance

There are Three Lines of Evidence of Tidal Influence

Geology

The Torrey Member of the Moenkopi Formation has been the subject of investigation for almost 50 years. However, these studies were more broad based regional studies, and only recently has the Torrey Member been studied in stratigraphic detail with emphasis on the extensive tetrapod track-bearing surfaces of predinosaurian communities present within it. At present, the multiple track-bearing horizons are known to extend throughout much of Utah's National Parks. Currently, the Torrey Member vertebrate tracks are the oldest and most laterally extensive megatracksite horizons ever recorded.

Following the deposition of the Sinbad Member in a clear shallow sea, a change in tectonic and/or climatic conditions caused the progradation of a major delta succession into Utah. This delta complex is preserved as the Torrey Member.

Basal deposits of the Torrey Member include interbedded siltstones, dolomites, and very fine-grained sandstones that were laid down in advance of the prograding delta. This sequence grades upwards into ledgeforming coarser grained sandstones and interbedded siltstones. Several trackbearing horizons are present within this delta-plain facies. The facies includes channel deposits of large-scale trough cross bedded fine to medium grained sandstone that was deposited within the fluvial-dominated reaches of the upperdelta-plain. Multiple tetrapod track horizons have been identified within these deposits.

Channel bodies dominated by ripple to large-scale trough cross bedded sandstones and interbedded mudstones are organized into inclined heterolithic packages. Also present within these sandstone and mudstone-dominated channels are large-scale soft sediment deformational features and clay-draped ripple- and dune-scale bedforms. Tetrapod tracks and fish-fin drag marks are typically associated with these deposits. These inclined barforms are likely pointbar deposits that experienced tidal influence and may represent the more seaward lower delta-plain expression of the sandstone-dominated fluvial channels.

Chirotherium Chirotherium Chirotherium

A threefold lithofacies classification model produced by Smith (1987) was adapted to describe depositional environments of the Torrey Member delta-plain channels. Outcrop measured sections (a west to east trend) are similar to Smith's, (1987) lithofacies classification for meandering river estuarine systems.

Vertebrate Ichnology

Chirotherium Tracks: Relatively narrow, quadrupedal trackways indicating the normal tetrapod walking gait; in the walking gait a small pentadactyl manus impression regularly occur immediately in front of, but never overlapped by a much larger, pentadactyl pes which generally resembles a reversed human hand. Manus and pes are digitigrade, and in large forms the pes tends to be plantigrade; digits I-IV point more or less forward, manus digits IV is always shorter than III being largest; the footprints may or may not show specialized metatarsal pads. Clear impressions often show a granular or beaded skin surface (skin impressions). Associated swim tracks are common and often indicate current flow directions and water depths.

Rotodactylus

Rotodactylus Tracks: Long-striding, trackways of a medium pentadactyl reptile are well preserved with rare skin and claw impressions. These tracks commonly occur with smaller Rhynchosauroides footprints. The manus is always closer to the midline and in some cases overstepped even in the walking gait by the much larger pes in a moderately narrow trackway pattern; pace angulation (pes) as high as 146 degrees in a running trackway and as low as 93 degrees in a walking trackway. The pes impression indicates a foot with an advanced digitigrade posture, and with a strongly developed but slender digit V rotated to the rear where it functioned as a rotated backward but it has a propping function. Digit IV on both manus and pes is longer than III; digit I may fail to impress; claws are evident and distinct on digits I-IV. Scaly plantar surface (well defined skin impressions) are often preserved in exquisite detail and is characterized by transversely elongate scales on the digit axis bordered by granular scales.

Rhynchosauroides Tracks: Dense concentrations of Rhynchosauroides tracks are commonly associated with the trackways of Chirotherium and Rotodactylus. These small lacertoid footprints are generally characterized by deeply impressed manus and a faintly impressed pes. Trackways exhibit a relatively wide pattern with pentadactyl footprint relatively distant from the midline. The pace angulation is low, below 90 degrees - 100 -120 degrees if figured from the manus pattern. Most often only 3 to 4 digits are preserved with occasional tail drag marks. The digits are slender and relatively longer in the pes than in the manus and both sometimes exhibit distinct claw impressions. Swim tracks are common.

Undichna

Undichna Fish Trails: The Moenkopi Formation is known for its exceptional vertebrate fossil record. Fish are rare and have been little studied in detail, and fish trails (fish fin drag marks) have never been recorded. The purpose of this study is to describe the first known occurrence of fish trails (fish fin drag marks), Undichna from the Early Triassic Torrey Member of the Moenkopi Formation. This ichnogenus has been reported in abundance from the Late Paleozoic, Permian, Cretaceous, and more recently from the Eocene. Undichna from the Torrey Member of the Moenkopi Formation represents the first and only known occurrence of fish trace fossils in the Triassic in the Western U.S.

The fish fin trace fossils are preserved as convex hyporelief sandstone casts with filled imprints preserved in underlying mudstone. Exposed traces occur on the undersides of resistant sandstone ledges where the mudstone eroded away. Undichna commonly occur with locally dense concentrations of swim traces of Chirotherium.

Occurring in clusters, one isolated fish fin trace consists of a single, slightly asymmetrical, sinusoidal trail. The trace is 56 cm. Long and includes 6.5 cycles with wavelengths varying from 9 to 10 cm and amplitudes of 3.5 to 4.5 cm.

The trails were most likely produced by a fish with a large caudal or anal fin able to reach the sediment without any other fin doing so. The low wavelength to amplitude ratio is most consistent with a caudal fin. This occurrence of Undichna is similar to other previous descriptions and it confirms that the preservation of these trails are favored in fine-grained sediments. Importantly, these traces coupled with Chirotherium, and Rhynchosauroides, swim tracks, all indicate fluctuating water depths.

Invertebrate Ichnology

Fuersichnus, Palaeophycus, and Arenicolites: The Torrey Member of the Moenkopi Formation assemblage studied is considered herein as an example of the Glossifungites ichnofacies and commonly occur with vertebrate swim tracks. This ichnofacies has been restricted to firm but unlithified nonmarine and marine surfaces. The Glossifungites ichnofacies is characterized by low diversity and high density assemblages which include Fuerichnus, Palaeophycus, Arenicolites, and Skolithos.

Fuersichnus, Palaeophycus, Arenicolites

The ichnogenus Fuersichnus is a relatively rare trace fossil that has been documented from Triassic and Jurassic nonmarine deposits and only recently documented in marine deposits from the Upper Cretaceous . The ichnogenus consists of horizontal to subhorizontal, isolated of loosely clustered, U-shaped, curved to banana-like burrows, characterized by distinctive striations parallel to the trace axis. It is interpreted as a dwelling structure probably produced by crustaceans or polychaetes.

The ichnogenus Palaeophycus a common trace fossil that has been documented from Pre-Cambrian to Holocene nonmarine and marine deposits. Branched, and irregularly winding, cylindrical or subcylindrical tubes, that sometimes cross-cut one another. These horizontal galleries most often have vertically striated lined burrows or rarely nearly smooth surface textures. Palaeophycus represents passive sedimentation within an open dwelling burrow constructed by a predaceous or suspension-feeding animal.

The ichnogenus Arenicolites are simple U-tubes (paired tubes) without spereite, perpendicular to bedding plane; usually varying in size, tube diameter, distance of limbs, and depth of burrows; limbs rarely somewhat branched, some with funnel-shaped opening; walls commonly smooth. A common trace fossil documented from Triassic to Cretaceous from marine and nonmarine deposits. The Torrey Arenicolites are very consistent in size, shape, and distance apart from each other and are interpreted as made by annelid worms.

What is Ichnology?

Fossils can be divided into body fossils, preserved parts of the plant or animal, and trace fossils, indirect evidence of their presence. Ichnology is the study of plant and animal traces. The implication of this definition is that the traces made by plants and animals reflect some sort of behavior or in the case of animals their mode of locomotion. The best known trace fossil are tracks but burrows, nests, and gnaw marks on bones or plants are also types of trace fossils. Ichnology can be divided into two major subdivisions: paleoichnology (the study of ancient traces) and neoichnology (the study of modern traces). Most ichnologists are involved in paleoichnology but a considerable number also study neoichnology for the comparison of modern equivalents (and their trace makers) to ancient traces. Technically speaking, wildlife biologists or ecologists who study tracking (identification of animals and their behavior on the basis of their tracks and feces) are neoichnologists, although they probably would not recognize such an designation if you told them.

Summary

Discussion

Several important discoveries have been made during the course of this research in GLCA and CARE. The Torrey Member of the Moenkopi Formation trackways are first described in detail from this stratigraphic unit and suggest a great potential for finding other footprint sites in this Formation. This unit is widely exposed not only in Capitol Reef, and Glen Canyon Recreation Area, but also, Zion, Canyonlands, and Arches, National Parks. Lateral correlations in Utah's National Parks of the Moenkopi's extensive track bearing horizons provide a good basis for correlation with the entire region.

As a non-renewable resource on public lands, fossil footprints provide an opportunity for public education, scientific research, monitoring programs, and an administrative opportunity and challenge for both scientists and land management authorities.

Additional Reading:

Blakey, R.C., 1973. Stratigraphic and origin of the Moenkopi Formation (Triassic) of Southeastern Utah. The Mountain Geologist 10(1):1-17.

Blakey, R.C., 1977. Petroliferous lithosomes in the Moenkopi Formation, Southern Utah: Utah Geology 49(2):67-84.

Hintze, L.F., 1988. Geologic History of Utah: Brigham Young University Geology Studies Special Publication 7: 202 pp.

McAllister, J.A., 1989. Dakota Formation tracks from Kansas: Implications for the recognition of tetrapod subaqueous traces. pp. 343- 348. in GIillette, D.D., and Lockley, M.G., eds., Dinosaur Tracks and Traces: Cambridge University Press, New York

McAllister, J.A., and Kirby, J. 1998. An occurrence of reptile subaqueous traces in the Moenkopi Formation (Triassic) of Capitol Reef National Park, South Central Utah, USA. Journal of Pennsylvania Academy of Science, 71, Suppl. And Index:174-181.

McKee, E.D., 1954. Stratigraphy and History of the Moenkopi Formation of Triassic Age: The Geologic Society of America Memoir 61:1-133.

Mickelson, D.L., Huntoon, J.E., Worthington, D., Santucci, V.L., Clark, T, 2000. Pre-dinosaurian community from the Triassic Moenkopi Formation Capitol Reef National Park: Geological Society of America (Abstracts) 32(7).

Peabody, F.E., 1948. Reptile and amphibian trackways from the Lower Triassic Moenkopi Formation of Arizona and Utah. University of California, Bulletin of the Department of Geological Sciences 27: 467 p.

Smith, D.G., 1987. Meandering river point bar lithofacies models: Modern and ancient examples compared: pp. 83-91 in Ethridge, F.G., Flores, R.M., and Harvey, M.D., (eds.). Recent Developments in Fluvial Sedimentology Contributions from the Third International Fluvial Sedimentology Conference, The Society of Economic Paleontologists and Mineralogists, Special Publications 39.

Smith, J.F. Jr., Lyman, L.C., Hinrichs, E.N., and Luedke, R.G. 1963. Geology of the Capitol Reef Area, Wayne and Garfield Counties, Utah: Geological Survey Professional Paper no. 363:99 pp.

Contributing Author:

Debra L. Mickelson
Department of Geological Sciences University of Colorado at Boulder, UCB 399
Boulder, Colorado 80399
[email protected]


Black Boulders

Large black boulders are strewn along several valleys that cross Capitol Reef National Park. In the Fremont River Valley they cover Johnson Mesa above the campground and scatter the hillsides of Fruita. The boulders are strikingly out of place among the tilted red and white bands of sandstone and shale that form the Waterpocket Fold. They originated in the high basalt and andesite cliffs that top Boulder Mountain and the Thousand Lakes Mountain plateaus west of the park.

Geologists long thought the boulders had moved from Boulder Mountain in Ice-Age glaciers and streams that carried the rocks down valley. Recent studies show that the glaciers were small and the streams lacked the power to move boulders nine feet or more in diameter such as those found around Fruita.

Many of the boulders are angular in shape, whereas rocks rolled by streams become rounded. Large landslides occurred and the remains of these slides (huge chunks of basalt and andesite) mantle the slopes of Boulder and Thousand Lakes Mountains. Some of the slides flowed into the heads of the Fremont and Escalante Rivers and were liquid enough to move as debris flows for tens of miles down canyons, like wet cement in a chute. Such dense flow can raft boulders without rounding them. Some debris flows incorporated enough river water to become floods that spread boulders through Capitol Reef and farther east of the park.

These enormous debris flows and floods dumped their freight of boulders across broad valley floors beyond the mountains. The Fremont and Escalante Rivers have since cut deeply into those valley floors, carving canyons. The old valley floors are now mesas 200 to 600 feet above the present river valleys. Since a river cuts down through sandstone and shale at a rate of only inches per century, it took tens of thousands of years to carve out the valleys. The high boulder-strewn mesas were valley floors 100,000 or even a million years ago.

Boulder-laden debris flows descended along the Fremont River and its southern tributaries, Pleasant and Oak Creeks. Numerous terraces and mesas, perched 100 to 400 feet above the modern valley floors, are capped by coarse black boulders. Similar boulders that originated at Thousand Lake Mountain cap benches high above the northern valleys of the park. Such benches are visible along the Hartnet Road from the Cathedral Valley Campground all the way to the Fremont River.

East of the park, black boulders form flat benches where ancient floods emerged at the mouths of Pleasant and Oak Creek Canyons, i.e.: Notom Bench. These benches are the ancient floors of these streams. Since then, the creeks and the Fremont River have excavated canyons 200 feet deeper into the tilted sandstone of the Waterpocket Fold. Deposits of black boulders extend into Upper Cathedral Valley. Just east of the park boundary along the Fremont River, a prominent flat terrace is mantled by several feet of river-worn black boulders, the ancient floor of Hartnet Draw.

Wet landslides have also slumped off the south side of Boulder Mountain and shed huge debris flows into tributaries of the Escalante River. Large ancient flows cap ridges like the hogsback, seven miles southwest of the town of Boulder, where Utah Highway 12 balances on a bouldery spine 500-600 feet above the canyons. Boulder is built on the youngest large debris flow. A prehistoric Ancestral Puebloan village also lies on this deposit, just north of Boulder. Established about 1050 AD, it shows that the debris flow is older than 900 years.

This bulletin was written by Richard Waitt of the United States Geological Survey, in cooperation with the National Park Service.

Stromatolite Fossils

An Unlikely Giant in Capitol Reef National Park

The discovery of giant stromatolite fossils in the Navajo Sandstone is part of a growing body of research challenging some long-held assumptions about the Paleo-environment of the Navajo erg.

The Navajo Sandstone, a prominent and well-exposed rock unit in the Colorado Plateau, was once an enormous, arid sea of blowing sands (called an erg) often compared to the present Sahara Desert. This early Jurassic dune field covered close to half a million square kilometers and reached a thickness in excess of 700 meters making it one of the largest dune fields in the history of Earth.

Although the Navajo erg is generally thought to have been an expansive and lonely desert, new fossils found in Capitol Reef National Park suggest otherwise. During an extended backpacking trip, the senior author stumbled across what he described looked "almost like a giant haystack" or "a giant limestone onion slowly being peeled." This turned out to be the serendipitous discovery of the first known stromatolite in the Navajo Sandstone and possibly the first stromatolite within an erg setting. Eisenberg, an independent geology consultant, spent the next several years researching the occurrence and the discovery was reported in the February, 2003 issue of Geology.

Stromatolites are bizarre fossils whose biological origins were debated until only a few decades ago. Today, scientists generally agree that stromatolites are layered colonial structures predominately formed by cyanobacteria. Stromatolites are the oldest fossils on earth, dating back to more than three billion years ago. They were the dominant life form on earth for over 2 billion years and are thought to be primarily responsible for the oxygenation of the atmosphere. Living and fossil stromatolites are usually no more than half a meter tall and are found in marine environments. In contrast, the Capitol Reef Stromatolites are up to five meters in height and appear in thin carbonate beds associated with interdune deposits.

Please Note: The fossil stromatolites at Capitol Reef are not easily accessible and somewhat hard to find. If you're up for a long day-hike or an overnighter, one of the best locations to see the stromatolites is located along Cottonwood wash in the central-eastern section of the Park. Always check in with the park staff before going on any overnight or extended trips into the back country of the park. As always, removal or vandalism of any fossil is strictly prohibited.

The most important implication of the fossils is the suggestion of large bodies of standing water necessary to sustain the towering stromatolites. "We need to reevaluate the whole Paleo-environment," David Loope of the University of Nebraska-Lincoln says. "Until we had the stromatolites the general picture was hyper-arid," he says. Dr. Marjorie Chan of the University of Utah agrees, saying that despite the dry and dusty impression of the Navajo erg, "it in fact had water and lakes."

This is a dramatically different picture of the Navajo than previously thought. The Navajo erg "may not be analogous to the present Sahara" in that it had the "potential for heavy rain and long lived episodes of water," Loope says. Long lived episodes of water would also translate into extended periods of erg stabilization.

Researchers have long "suspected that the Navajo must have stabilized at some point," although this is the first direct evidence of such stabilization. Modern ergs are known to periodically stabilize, a recent example being the "greening period" of the Sahara between four to ten thousand years ago. Using growth rates for modern stromatolites, it can be determined that the fossil stromatolites grew over a period of a few thousand to over ten thousand years, putting them "right in the ballpark, …in the thousands of years range," with the duration of the Sahara stabilization.

If the Navajo erg stabilized for thousands of years, it would mark a major stratigraphic boundary within the Navajo Sandstone. Right now, "the Navajo is a big pile of sand and it's hard to know where you are stratigraphically,"

"The next step is the "correlation of these scattered outcrops" to help "unravel the internal geometry and history of the Navajo erg."

Further correlation of interdune carbonate deposits could also suggest a regional climatic event, helping to improve climatic models of the Early Jurassic.

"There is variability that we never realized that was there," Chan says. "Just when you think you know it all, we discover there's a lot we didn't know." The stromatolites in Capitol Reef National Park have renewed interest in the Navajo Sandstone and provide insight into the biology and environmental history of the Navajo erg, all from a walk in the Park.

Additional Reading:

Caine, J.S. and S.R.A. Tomusiak. 2003. Brittle structures and their role in controlling porosity and permeability in a complex Precambrian crystalline-rock aquifer system in the Colorado Rocky Mountain Front Range. Geological Society of America Bulletin 115(11):1410-1424.

Eisenberg, L. 2003. Giant stromatolites and a supersurface in the Navajo Sandstone, Capitol Reef National Park, Utah. Geology 31(2):111-114.

Riggs, N.R., S.R. Ash, A.P. Barth, G.E. Gehrels and J.L. Wooden. 2003. Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone. Geological Society of America Bulletin 115(11(:1315-1323.

Walsh, P. and D.D. Schultz-Ela. 2003. Mechanics of graben evolution in Canyonlands National Park, Utah. Geological Society of America Bulletin 115 (3):259-270.

Web sites http://www.sharkbay.org/terrestial_enviroment/page_15.htm

National Natural Landmark site http://www.petrifiedseagardens.org/stromat.htm

Contributors of this article:

Len Eisenberg 223 Granite Street Ashland, Oregon 97520

Jay Chapman 370 South 36th Street Boulder, Colorado 80305 [email protected]