Parashant: A Boundary Between Geologic Provinces

Shaded relief map with outline of geologic provinces. (USGS Webpage at https://geomaps.wr.usgs.gov/parks/province/index.html)

Geologists have long recognized that portions of the earth’s landscape can be distinguished by their unified, and often unique, geological characteristics. During the earth’s more than 4.5-billion-year history many distinct events have changed its form and appearance, most prominent being the collisions of huge lithospheric plates that folded and fractured the surface, and in the process raised immense mountain ranges. All along though, the ongoing forces of erosion and weathering slowly worked to lower the mountains and carve new and greater channels for rivers to redistribute the erosive debris to other places. After all this time, the geologic story remains imprinted on our landscape as landscapes and drainage patterns that define distinctive geologic provinces. As seen on the shaded relief image, differences in topographic relief provide obvious distinctions between some areas of the United States, particularly the rough-appearing texture of the western U.S. vs. the smooth, fairly uniform appearance of the central states and much of the east. The boundaries of distinct areas highlighted on the shaded relief image are known as geologic provinces and portions of two of them, the Basin and Range and the Colorado Plateau, are found adjacent to one another in the Grand Canyon-Parashant National Monument.

The boundary between these two large geologic provinces creates some of the most spectacular scenery in the Parashant Monument. As visitors travel the area, the enormous vertical escarpment of the Grand Wash Cliffs clearly marks the transition, with the pinyon-juniper uplands of the Colorado Plateau province visible on top to the east and the Mojave Desert in the Basin and Range province visible to the west.

 

Normal Fault Diagram (USGS)

Basin and Range

The Basin and Range province is widespread across the southwestern United States from as far east as western Texas and southern New Mexico, into northern Mexico and southern and western Arizona and the west half of Utah, virtually all of Nevada, portions of eastern California and as far north as the Snake River Plain of Idaho (Spencer and Reynolds 1989). A basin and range landscape is fairly distinctive and easily seen from an aerial perspective: isolated ranges of mountains, tending to trend in the same direction, are separated by broad, flat valleys or basins. The landscape looks this way because the earth’s crust has been stretched and extended, resulting in large, regional faults. These roughly north-south trending faults have broken and detached portions of the earth’s crust and created steep mountain ranges and flat, distinctive intermontane valleys. Most of the faulting caused by crustal extension here is classified by geologists as normal, with the mountains represented by the so-called upthrown fault block and the valleys within the downthrown block. The fault planes along which the two sides of a fault move may extend well below the earth’s surface, usually at a fairly steep angle of 60 degrees (although the dip can lessen with depth). In places, the relief or vertical difference between the two sides can be hundreds of meters.

With time, sediments eroded from the exposed bedrock in the mountains and higher areas of the upthrown block are moved by rivers, landslides and even wind onto the downthrown block in the valley, a continual process to level out the terrain as nature works to establish dynamic equilibrium. Thicknesses of hundreds and even thousands of meters of eroded materials may collect in the basins, attesting to the initially great vertical differences between valley lows and highest peaks of the mountains. Sediments in the Grand Wash Trough, on the downthrown side of the Grand Wash Fault, are believed to be more than 3 thousand meters thick in some areas.

 

Colorado Plateau

The Colorado Plateau, on the other hand, appears as a relatively flat surface that has undergone comparatively little deformation. Northern Arizona, southern and eastern Utah, western Colorado and northwest New Mexico comprise the Colorado Plateau, which is bounded on the east and to the north by the Rocky Mountains, and to the west and south by the Basin and Range province. The rocks and topography of the Plateau describe well over a billion years of geologic history represented by igneous and metamorphic rocks exposed in the deepest canyons up through millions of years of uplift, exposure, sediment deposition and erosion.

In contrast with the Basin and Range province, or with the Rocky Mountain province to the north and east, the Colorado Plateau has been remarkably stable. Relatively little folding or faulting has affected this high, thick crustal block within the last 600 million years or so, even though the area was uplifted with the Basin and Range as much as 3 kilometers in the last 20 million years. But while the crust in the Basin and Range stretched and cracked, breaking into a multitude of down-dropped valleys and elongate mountains, the neighboring Plateau was able to preserve its structural integrity and remained a single tectonic block (United States Geological Survey (USGS) Webpage at https://geomaps.wr.usgs.gov/parks/province/coloplat.html). In fact, the dominant expression of geologic forces working within the Colorado Plateau has been and continues to be erosion.

Erosion has created many of the most photogenic and splendid landforms of the Colorado Plateau, attested to by the number of spectacular protected places in the region including Monument Valley, Canyonlands and Arches National Parks, Natural Bridges, Parashant and Vermillion Cliffs National Monuments, and many others. Nearly flat, sedimentary rock formations are cut by steep-walled canyons that expose colorful and patterned rock layers representing tens and even hundreds of millions of years of geologic time. These canyons allow water to quickly drain off the rocky surfaces and out of the rock formations. As the land within the Colorado Plateau rose during the past 10 million years, the streams responded by cutting ever deeper. The most well-known of these streams, the Colorado River, began to carve the Grand Canyon less than 6 million years ago.

Grand Wash Fault

The Grand Wash Fault marks the boundary between the Basin and Range and the Colorado Plateau. It is a high-angle normal fault that is the dominant structural feature of the area and trends from southwest Utah into west-central Arizona, through the Monument. Vertical displacement of the Grand Wash Fault blocks is estimated to be as much as 3,300 meters near the Colorado River and even more further south into west-central Arizona (Lucchitta 1987; Billingsley and Workman 2000).

Along the eastern edge of this generally north-south trending fault is the Grand Wash Cliffs, a scarp ranging in height from more than 600 meters up to about 1,700 meters. The cliffs are the result of upward movement along the fault, while the downthrown block to the west is represented by a basin known as the Grand Wash Trough (Lucchitta 1987). The landscape east of the Grand Wash Cliffs is within the Colorado Plateau, and geologic formations comprising the cliffs are, for the most part, the same as those in the Grand Canyon exposed further to the east and south. The landscape immediately west of the Grand Wash Cliffs begins the familiar Basin and Range topography. The generally north-south trending Beaver Dam range and the Virgin Mountains, the easternmost mountains in this part of the Basin and Range, are separated from the Colorado Plateau province by the Grand Wash Fault (Billingsley 1995). This pattern is repeated by other fault block sequences, basins, and mountain ranges through Nevada and into California.

So, the Grand Wash Cliffs represent a rather abrupt transition between two major geologic provinces. With such an abrupt boundary comes an obvious question: How did this landscape come to be the way it is? To really answer that question, it has been necessary for geologists to study the many different types of rocks and geologic formations found in the area, and to determine how they got here in the first place. And for us to understand the geologists’ explanations, we need to know something about plate tectonics and what the earth – and what eventually became the Grand Canyon-Parashant National Monument - looked like at different times in its long, long history.