The Wild Wild West

The western half of the United States is home to some of the most spectacular geology In the world. The arid climate combined with an intricate tectonic history result in fantastic, textbook examples of structural features like fractures, faults, and folds. Along with this, the dominance of “layer-cake” bedding of sedimentary rocks in the region makes analyzing faults and folds extremely easy as layers are easily followed with the eye. 

This project aims to give you a brief tour around this fascinating region and to highlight some of these geologic features. 

Fractures form in rocks as a brittle reaction to compressional or extensional forces. Often times, when talking about tectonic extension, fractures will form perpendicular to the direction of movement (Wilkerson 2024).

One of the most famous places to see large scale extensional fractures is Arches National Park in Utah where water and wind have eroded sandstones into magnificent, free-standing arches.

The formation of Arches National Park's fractures comes from the dissolution of large salt pillows that formed from the immense pressure of the overlaying sedimentary rocks (Wilkerson 2024). As these large salt pillows dissolved, and the rocks above began to settle, the overlying rock was fractured parallel to length of the salt pillow valley. The dominant direction of fractures in Arches NP are NW-SE as pictured below.

Once fractures are formed, the increase in surface area allows wind and water to begin eroding the rocks faster. Once water is able to flow from fracture to fracture laterally, and given enough time, an arch will form.

Faults are zones/planes within the surface of the earth where rocks have been moved relative to each other as a result of tectonic compression, extension, or combination of the two. There are three main types of faults.

There are very few places on earth with so many examples of every type of fault, One of such places is the Basin and Range Province in the western United States. While this project is not about that region specifically, its vastness dominates a majority of the western United States, and many geologic features lie within its boundaries.

Besides the large scale listric faulting of the basin and range province, one great example of a normal fault is an unnamed fault on the western front of the Resting Spring Range (Below).

The fault planes' intersection with the ground surface is shown in red, and the truncated bedding contact of the Bonanza King limestone (dark grey rock) and the Carrara Formation (tan formation) is shown in brown. It is clear to see the large section of Bonanza King and Carrara that has been displaced and is now lower than its surrounding rocks.

One of the most conceptually spectacular structures in the west is the Keystone Thrust. The Keystone thrust runs predominantly north, and places ancient dark colored cambrian limestone above significantly younger, lighter colored Jurassic sandstone. This large age gap also results in a spectacular clash of colors when viewed from above (as seen below)

Below is a cross section of the Keystone Thrust. As one can see, the eastward vergence of the hanging wall has caused the Bonanza King formation (Grey above) to be thrusted up and over the top of the Aztec Sandstone (green above). Spectacular to say the least.

Last but not least, an easy to see transform fault can also be found in the Resting Spring Range. This transform fault bisects the range in the central region and offsets formations such as the Bonanza King and Carrera

The transform fault is "Right-Lateral" meaning that when facing the fault, objects on the other side will move to your right and vice versa if you were to cross the fault and turn around. In this instance, the entire range itself has been sifted, creating a stagger when trying to follow beds across the fault.

It is impossible to mention the west without mentioning Zion National Park. Large regional fractures, similar to those in Arches National Park, combined with base level drop of the Colorado river, resulted in the deep carving canyons of Zion. Below is a 3d model of Zion National Park with a vertical exaggeration of z=1.5. Notice the dominant NNW-SSE treading fractures in the lower right quadrant.