Canyonlands National Park Geology

Canyonlands National Park is a showcase of geology. In each of the districts, visitors can see the remarkable effects of millions of years of erosion on a landscape of sedimentary rock. For hundreds of millions of years, material was deposited from a variety of sources in what is now Canyonlands National Park. As movements in the earth’s crust altered surface features and the North American continent migrated north from the equator, the local environment changed dramatically.

Over time, southeast Utah was flooded by oceans, crisscrossed by rivers, covered by mudflats and buried by sand. The climate has resembled a tropical coast, an interior desert, and everything in between. Layer upon layer of sedimentary rock formed as buried materials were cemented by precipitates in ground water. Each layer contains clues, like patterns or fossils, that reveal its depositional environment. For example, the red and white layers of Cedar Mesa Sandstone occur where floods of iron-rich debris from nearby mountains periodically inundated coastal dunes of white sand. Only a trace of iron is needed to color a rock red.

It is difficult to imagine such major changes and the time scale they spanned. Equally surprising is the fact that all of these rock layers were flat when they were deposited. Only recently, speaking in geologic time, have these layers eroded to form the remarkable landscape seen today.

Until about 15 million years ago, most of the canyonlands area was near sea level. Local uplifts and volcanic activity had created features like Capitol Reef's Waterpocket Fold and the La Sal Mountains near Moab, but then movements in the earth's crust caused the whole area to rise. Today, the average elevation is over 5,000 feet above sea level.

The uplifting of this region, known as the Colorado Plateau, marked a shift from a depositional environment to one of erosion. The Colorado and Green rivers began to downcut and are now entrenched in canyons over 2,000 feet deep. Sediment-filled storm run-off drains into these rivers, scouring the surrounding landscape of into a network of tributary canyons, pour-offs and washes.

How sedimentary rock weathers depends largely on its exposure to water. An erosion- resistant caprock of White Rim Sandstone may protect a weaker layer of shale until only a thin spire remains. Examples of such "standing rocks" can be seen in both the Island in the Sky and the Maze districts. In addition to floods, the expansion of freezing water is a powerful erosive force. As ice loosens surface material and widens cracks, everything becomes more vulnerable to the next big storm.

Another significant factor in the shaping of Canyonlands is the Paradox Formation, a layer of sea water evaporates from the Pennsylvanian Period. Deeply buried, the salts in this layer can liquefy under the weight of the overlying rock, flowing, like toothpaste, away from the source of greatest overburden. In response, the upper layers may bow up, creating a salt dome, or erode and collapse, creating a salt valley.

This phenomenon is especially visible in the Needles, where parallel cracks or "joints" formed in the surface rock as buried layers slumped toward Cataract Canyon. These cracks are perpendicular to an older system of cracks created by the "Monument Uplift." The resulting crosshatched pattern of joints has eroded so that great blocks of sandstone have been reduced to thin spires of rock.

Desert varnish is the thin red to black coating found on exposed rock surfaces in arid regions. Varnish is composed of clay minerals, oxides and hydroxides of manganese and/or iron, as well as other particles such as sand grains and trace elements. The distinctive elements are Manganese (Mn) and Iron (Fe).

The color of rock varnish depends on the relative desert varnish

amounts of manganese and iron in it: manganese-rich varnishes are black; manganese-poor, iron-rich varnishes are red to orange; those intermediate in composition are usually a shade of brown. Varnish surfaces tend to be shiny when the varnish is smooth and rich in manganese.

Desert varnish consists of clays and other particles cemented to rock surfaces by manganese emplaced and oxidized by bacteria living there. It is produced by the physiological activities of microorganisms which are able to take manganese out of the environment, then oxidize and emplace it onto rock surfaces. These microorganisms live on most rock surfaces and may be able to use both organic and inorganic nutrition sources. These manganese-oxidizing microorganisms thrive in deserts and appear to fill an environmental niche unfit for faster growing organisms which feed only on organic materials.

The sources for desert varnish components come from outside the rock, most likely from atmospheric dust and surface runoff. Streaks of black varnish often occur where water cascades over cliffs. No major varnish characteristics are caused by wind.

Thousands of years are required to form a complete coat of manganese-rich desert varnish so it is rarely found on easily eroded surfaces. A change to more acidic conditions (such as acid rain) can erode rock varnish. In addition, lichens are involved in the chemical erosion of rock varnish.

The Grabens

The grabens in the Needles District of Canyonlands National Park are a system of linear collapsed valleys caused by the movement of underlying salt layers toward the Colorado River canyon. The grabens begin near the Confluence of the Green and Colorado rivers and run roughly parallel to Cataract Canyon for 25 km, veering slightly west before they end. Graben is a German word meaning ditch or grave. In the geologic sense it is a grabens collapsed or down-dropped block of rock that is bordered on its long sides by faults. Grabens are normally associated with "horsts," which are the up-thrown blocks of rock. In German, Horst means aerie, referring to the high nesting sites of predatory birds.

Geologic Processes

The processes that led to the development of the grabens began approximately 300 million years ago in the Pennsylvanian period with the deposition of evaporates (salts) in a shallow inland sea. These deposits, known as the Paradox member of the Hermosa Formation, were later covered by the limestone layers of the upper Hermosa and Rico formations. In the Needles District, the Paradox layer can be 3,000 to 5,000 feet thick.

Sea levels eventually dropped, and white sands blew in from the west, forming large sand dunes. At the same time, red mud and silt was deposited by rain and snow melt from the Uncompahgre Mountain to the east. The resulting red and white beds alternated, forming the lower beds of the Cutler Formation, or the Cedar Mesa Sandstone that is dominant in the Needles District today.

Sediment from a variety of environments continued to accumulate on top of these layers for millions of years. Approximately 60 million years ago, a tectonic plate collision called the Laramide Orogeny created the Rocky Mountains. Shortly after, a regional upwarp called the Monument Uplift caused the sedimentary layers in the Needles to tilt gradually westward. This event also formed joints, or long parallel fractures in the rock, throughout the Needles. In the vicinity of the grabens there are two joint sets: one trending roughly northeast to southwest, and one trending northwest to southeast. Some of these joints became the faults that border the grabens.

Around 10 million years ago, the uplift of the Colorado Plateau gave rise to the Colorado River and its tributaries. As the Colorado river cut its way downward through the rock layers, it carried away millions of tons of sediment towards the Pacific Ocean.

Necessary Ingredients

Finally, about 55,000 years ago, the ingredients were in place and the grabens began to form. Four factors have been identified as critical to the formation of grabens:

The ability of evaporates to flow plastically. The evaporates in the Needles District are slowly flowing westward towards Cataract Canyon due to the pressure exerted by the rock layers above.
The erosion of the Colorado River down to the Paradox Formation, creating a low pressure zone the evaporates are drawn to.
The gradual tilt created by the Monument Uplift, which allows gravity to act on the evaporates.
The interaction of water with the evaporates, dissolving the salts and facilitating their ability to flow.

The grabens are a very young geologic feature. Graben growth is thought to be a slow process where small, seismically undetectable movement occurs: as little as one inch per year. The grabens continue to drop and slide toward the river today, and are a fascinating feature of the Needles District.

Canyonlands is a place of relative geologic order. Layers of sedimentary deposits systematically record chapters in the park's past. With some exceptions, these layers have not been altered, tilted or folded significantly in the millions of years since they were laid down by ancient seas rivers or winds.

Upheaval Dome is quite a different story. In an area approximately three miles (5km) across, rock layers are dramatically deformed. In the center,

the rocks are pushed up into a circular structure called a dome, or an anticline. Surrounding this dome is a downwarp in the rock layers called a syncline. What caused these folds at Upheaval Dome? Geologists do not know for sure, but there are two main theories which are hotly debated.

A thick layer of salt, formed by the evaporation of ancient landlocked seas, underlies much of southeastern Utah and Canyonlands National Park. When under pressure from thousands of feet of overlying rock, the salt can flow plastically, like ice moving at the bottom of a glacier. In addition, salt is less dense than sandstone. As a result, over millions of years salt can flow up through rock layers as a "salt bubble", rising to the surface and creating salt domes that deform the surrounding rock.

When geologists first suggested that Upheaval Dome was the result of a salt dome, they believed the land form resulted from erosion of the rock layers above the dome itself. Recent research suggests that a salt bubble as well as the overlying rock have been entirely removed by erosion and the present surface of Upheaval Dome is the pinched off stem below the missing bubble. If true, Upheaval Dome would earn the distinction of being the most deeply eroded salt structure on earth.

When meteorites collide with the earth, they leave impact craters like the well-known one in Arizona. Some geologists estimate that roughly 60 million years ago, a meteorite with a diameter of approximately one-third of a mile hit at what is now the Upheaval Dome. The impact created a large explosion, sending dust and debris high into the atmosphere. The impact initially created an unstable crater that partially collapsed. As the area around Upheaval Dome reached an equilibrium, the rocks underground heaved upward to fill the void left by the impact. Erosion since the impact has washed away any meteorite debris, and now provides a glimpse into the interior of the impact crater, exposing rock layers once buried thousands of feet underground.

Whatever the origin of Upheaval Dome, it is the result of erosion of a structural dome. Rock layers now at the surface within the dome were once buried at least a mile underground and are not visible anywhere else in the nearby area. While some call this feature a crater, it is not a crater in the traditional sense of the word, but simply another example of the erosion which created Canyonlands National Park.