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Geology HAPPENINGS June 2018

The Underground Secret of the Grabens
By Allyson Mathis
NPS photo Neal Herbert

In the canyon country around Moab, some areas differ distinctly from the surrounding landscape. The La Sal Mountains towering over the surrounding redrock country is one area. Arches National Park, with its extremely high concentrations of natural arches and characteristic rock fins, is another. And the Grabens in the Needles District of Canyonlands National Park are a third such area. In fact, the Grabens are one of the most unusual topographic areas on our planet and are actively being studied by geologists from all around the world.

Like the other exceptional landscapes around Moab, the landscape of the Grabens is tied directly to the geology deep underground. To a geologist at least, the beauty of canyon country is more than skin deep.

The Grabens look like a series of roughly parallel canyons with straight vertical walls. But they were not formed by stream erosion. In fact, some of the Grabens even have drainage patterns that cut across them but are no longer interconnected, and others are internally-drained basins that do not connect with other streams in the area.

The geological term graben comes from the German word for “ditch.” A graben is a trough that forms when a section of land drops down between two faults (faults are surfaces along which blocks of the earth’s crust have moved relative to one another).

Both horsts and grabens occur in the Needles District, although the area is known by the name of just one half of this geologic combination. Horsts are fault-bounded blocks that have moved up relative to the down-dropped grabens. Horst and grabens usually form in areas where the earth’s crust is pulled apart, such as in the Basin and Range province in Nevada and western Utah. There, the crust is being stretched over a wide area due to complicated plate tectonics movements in the American west.

The Needles’ grabens were formed by a much smaller scale of movements than those that formed the high ranges and broad basins in Nevada. In Canyonlands, the horst-and-graben terrain covers an area of approximately 75 square miles mostly on the east side of Cataract Canyon, extending five miles or so from the gorge. Individual grabens are usually less than a third of mile wide, range from about 80 to more than 300 feet deep, and are usually no more than a few miles long. The faults that created them do not penetrate deep into the earth’s crust. This area with its high concentration of faults is completely unlike anything else in canyon country. How did it form?

The primary secret to the Grabens’ origin and evolution is found approximately 1500 feet beneath the surface in a rock layer called the Paradox Formation, which consists primarily of salt. Salt as a rock type has several unique properties, including the ability to flow ductilely (like Silly Putty) rather than break or fracture, and it will do so under geologically low pressures.

The Grabens are also related to the formation of Cataract Canyon, which came to be the way most landscapes in southern Utah were formed—eroded by flowing water. The Colorado River has cut into the earth carving Cataract Canyon, with the incision reaching nearly to the depth of the salt in the Paradox Formation. With the removal of billions of tons of sandstone and other rocks above the Paradox Formation where the canyon is now, the rock on either side lost its lateral support. And like books on a shelf that are no longer held in place after a book has been removed, the rocks started to slide. The soft salt layer underneath them acted like a lubricated surface for this sliding. While the salt layer underneath was able glide plastically, ordinary rock types like the sandstone above it could not. The land was stretched as it slid towards the gorge, and the brittle sandstones and limestones fractured and faulted. Alternating blocks dropped downward, leaving the land looking like an opened accordion. The area within the Needles Fault Zone (as geologists call the Grabens area) has been stretched (or extended) by as much as 25% compared to its original width due to the gliding in the salt layer due to the instability caused by the presence of Cataract Canyon.

The reason that most of the grabens are on the east (Needles) side of the river (versus being on both sides) is because there is a very slight tilt of the rocks layers downward to the west-northwest on the Needles side that aids the gravitational sliding towards the canyon.Allyson Mathis

The older (and larger) grabens are located closest to the river. The Grabens get younger towards the east, with Devils Lane and Devils Pocket being among the youngest ones found in the park. The Needles Fault Zone is still geologically active, although the movement appears to occur mostly by gradual creep rather than by earthquake events. Extension and subsidence in the area is occurring at rates up to approximately 0.1 inches per year.

All geologists agree that the Needles Fault Zone and the Grabens are very young and that they began forming after rapid river incision carved Cataract Canyon as recently as one million years ago. It is thought that the Grabens themselves began forming within the last half million years.

Sometimes people think that the geology of canyon country is concerned mostly with events that happened in the very distant past, but landscapes like the Grabens show that geology is still happening today. And we will continue to explore these events in Geology Happenings.

 
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