Geology of Arches National Park
Arches National Park Geology
Geologic History
Over 2,000 natural rock arches can be found within Arches National Park. Located in eastern Utah, just south of I-70 and north of Moab, this high desert park covers over 75,000 acres with elevations ranging from 4,085 feet at the Visitor’s Center to 5,653 feet at Elephant Butte. Pennsylvanian (286 to 320 million years ago) through Cretaceous (66 million years ago) sedimentary rocks are exposed in Arches National Park, representing marine, nearshore, and continental depositional environments and the arches are mostly found within Jurassic-aged sandstone. Many of the geologic units found within Arches National Park can also be found throughout the southwestern United States.
The oldest rocks exposed in Arches National Park are within the Pennsylvanian Paradox Formation. During this time, two super continents (Gondwana and Laurasia) collided, causing compression of the land that created a shallow, trough adjacent to the uplift called the Paradox Basin, which was periodically flooded with marine water. During the Early Pennsylvanian, the climate changed from a warm, humid environment to a more arid climate resulting in increased evaporation, which concentrated salt in the seawater. Periodically new seawater began to flow into the basin, depositing sequences of shale, dolomite, anhydrite, sodium and potassium salts, and black shale.
Later in the Pennsylvanian Period, the Paradox Basin filled with sediment and the area was connected to an open sea. During this time, limestone and dolomite were deposited instead of salt. The Uncompahgre Mountains began to rise during the Upper Pennsylvanian Period and elevated the region to the northeast, filling the basin with erosional debris. The eroded sediments accumulated, particularly within troughs formed by faults. This accumulation of erosional debris within fault-derived troughs and the special properties of salt are important to the formation of the arches within Arches National Park. The salt in the troughs was more buoyant than the heavy, thick sediments lying on top of it, which caused the salt to be squeezed into adjoining ridges and ultimately to rise. The salt flowing from beneath the thick sediments allowed for a greater accumulation of erosional debris in the troughs, which began to form downfolds, and the ridges began to form salt rolls that later became the cores of salt anticlines.
During the lower Triassic Period the area of Arches National Park was covered by a shallow, marine sea and the reddish-brown mudstone, sandstone, and siltstone layers of the Moenkopi Formation were deposited. Tectonic activity decreased on the western margin of the supercontinent formed by the earlier collision of Gondwana and Laurasia. Sea level rose relative to the land and then dropped again, changing the shoreline and causing red beds to be deposited. These red beds are rust colored because when these sediment beds were exposed to air the iron in the sediment oxidized, a common feature of sedimentary rocks of this age throughout the southwest region.
During much of the Jurassic, the climate of the region that is now Arches National Park was desert-like, much like the modern Saharan Desert, with vast sand seas called ergs. The Wingate Formation was deposited during the lower Jurassic Period due to the erosion of sandstones, which were transported by wind and contains crossbedding and well-sorted quartz grains. Above the Wingate is the Kayenta Formation, also a Jurassic deposit, with abundant current ripple marks suggest a fluvial (river) environment. Above the Kayenta Formation is the Navajo Sandstone, a thick unit containing abundant groups of inclined layers, known as cross beds, which preserve ancient wind patterns. The Navajo Sandstone and the Entrada Sandstone are two of the major arch-forming formations within Arches National Park.
Tectonic collision increased off the western coast during the Middle Jurassic and the rock layers were warped upward. The sea came onto the continent from the north and sandstone formations were deposited in an extensive dune field. During the Upper Jurassic Period, the sea level rose and fell multiple times, destroying the vast aolian sand seas that once covered the area. An epicontinental sea, one that is large, shallow, and extends inland upon the continental shelf, once again covered Utah during the Cretaceous Period and rising mountains in Utah stopped the inundation of marine water to the west. This shallow sea redistributed the sediments that had been deposited from river systems, and it advanced, retreated and advanced many times during the Cretaceous depositing additional sediments. This process created many different environments including incised river valley systems, estuaries, coal swamps, lagoons, delta systems, beach and offshore marine deposits.
Approximately 70 to 35 million years ago the Laramide Orogeny, an event in which oceanic crust slipped under the North American Plate, deformed rocks throughout Utah. The tectonic forces warped the entire geologic column in the area of Arches National Park creating anticlines and synclines. An anticline is a folded structure with an arch-like shape. In an anticline the oldest beds are at the core, while a syncline is the opposite. One way to visualize this process is to think of a rug where the ends are pushed toward one another. Folds form across the middle of the rug creating a series of U-shapes (synclines) and inverted U- shapes (anticline). When these folds were forming, cracks also formed in the sandstone deep beneath the surface. These anticlines create a favorable environment for arch formation because many of the rock layers are not lying flat.
The entire region began to rise 15 million years ago, and increasing erosion caused removal of the sedimentary rocks above the Entrada Sandstone. Once at the surface, erosional forces began to act upon the sandstone layers creating the famous arches with the national park.
Today many of the rocks in Arches National Park and throughout the southwestern United States are coated by a thin red-to-black coating called desert varnish. It is found mainly in arid environments and is thought to be formed from atmospheric dust and surface runoff. The color of desert varnish depends on its chemistry. More manganese-rich desert varnish is black, while more iron-rich desert varnish is red or orange. Black streaks of desert varnish can be seen in Arches National Park after rain events as pictured below.
Arch Formation
In order for sandstone arches to form several conditions need to be met. These conditions are brittle sandstone that has been jointed due to faulting activity, a dry climate and adjacent to salt anticlines that are undergoing dissolution. The vast majority of arches in Arches National Park are within the Entrada Sandstone, including at Devils Garden and Klondike bluffs.
Grains in the Entrada Sandstone are nearly spherical, creating a very porous (full of tiny spaces) rock. The Carmel layer, which underlies the Entrada Sandstone, is made of sand and clay. Clay grains are much smaller than sandstone grains and pack together more tightly, creating a less porous rock. Since the Carmel layer is less porous, it does not efficiently absorb water, which then pools at the at the base of the Entrada Sandstone, further eroding it.
Cracks in the Entrada Sandstone were formed by a series of uplifts and collapses. The rock layers above the Entrada Sandstone were eroded away. Water and wind erosion then widened the gaps between cracks, which creates fin-like structures. Arches National Park receives only 8-10 inches of precipitation per year, which is enough for erosion to occur all year long. Rainwater soaks into the fins in the Entrada sandstone and puddles above the less permeable Carmel layers, dissolving the natural cement joining the sandstone together. During the winter, water that seeps into cracks within the rocks expands as it freezes, causing the cracks to widen. Eventually, the roof of the rock collapses destroying these arches. These erosional forces are still occurring today, which means that these arches are not permanent. An example of a collapse of an arch is the fall of Wall Arch in 2008.
Ongoing Geologic Research Within Arches
Like other national parks, Arches provides many opportunities for scientific research. Scientists within the park monitor the arches, but there are other types of geologic research happening within the park as well. National parks allow scientists to compare less-developed landscapes that can act as a “control” area to nearby areas of developed land. Inside national parks mining and grazing is not allowed. Below are two examples of recent research within Arches National Park.
Water Research Within Arches National Park
From 2012 to 2016 scientists from the US Geological Survey, National Park Service and Environmental Protection Agency sampled water across the National Park System to examine whether pesticides, pharmaceuticals, and personal care products are contaminating the natural landscape. In addition to sampling water at Arches National Park, the scientists examined water in Bryce Canyon National Park, Canyonlands National Park, Capitol Reef National Park, Dinosaur National Monument, Hovenweep National Monument, Timpanogos Cave National Monument, and Zion National Park. They found that these contaminants are found even in isolated areas, but in lower concentrations than in urban or agricultural areas. The areas where contamination within the parks were highest were in areas nearby wastewater treatment plants or in areas within stagnant or low flowing waters. At current level the contaminants are not thought to affect wildlife or pose a public health risk.
Air Quality Within Arches National Park
Arches National Park is located in an area with an arid environment and can experience haze due to dust and pollutants. Severe wind and other soil disturbances can increase the dust concentrations in the air. Currently the US Geological Survey is undertaking air quality monitoring. This monitoring can allow scientists to understand the background emission levels, seasonal dynamics, and other influences on the level of dust in the air.
