Geologic Evolution of Utah, the Colorado Plateau & Western US.
By Lance Weaver. Geologist: Utah Geological Survey
As a geologist for the Utah Geological Survey, author of the website utahgeology.com, and owner of my own geologic tour business, I’ve spent over 20 years studying the geology of Utah. Growing up in St George, Utah placed me in a special situation of being able to spend my childhood roaming around the scenic denuded rocks of Southwest Utah in one of the very few places on earth where the entire rock record from the Precambrian to the Late Cenozoic is exposed within a 20-40 mile radius. In grade school I built forts in the rocks of the Triassic Chinle Formation behind my house and rode my bike over the pastry hills of the Moenkopi. As a High schooler I started rappelling the cliffs of the Kayenta Formation and Canyoneering the world famous slot canyons of the Jurassic Navajo Sandstone of Zion National Park. In College I hiked throughout the Grand Staircase of the Cretaceous, the Paleozoic rocks of the Grand Canyon and Virgin River Gorge.
I’ve put an enormous amount of thought and study into the geologic history of Utah and the Colorado Plateau over the last decade. In this article I showcase my unique paleogeography maps of the Colorado Plateau and offer geologic history reconstructions based on data from all available geologic maps, strat columns literally thousands of data points. Additionally I attempt to combat what I believe are many false ideas embedded in conventional models based on faulty logic and old data. These maps are vector based and created in Adobe Illustrator, loosely based Ron Blakey’s previous work but correcting many of his poor interpretations. If you have questions or comments, feel free to contact me by looking me up at the Utah Geological Survey.
Geologic Overview & Understanding Geologic Facies
The Geologic formations in Utah tend to be divided primarily by lithology facies. In Geology, a facies is a body of rock characterized by its formation, composition, and fossil content–which usually says something about the environment it was formed in. One particular geologic formation, might have multiple facies. For instance, a famous formation like the Moenkopi, has both near shore tidal facies, and offshore limestone and siltstone facies as well as eolian and coarse grained beach facies. Walther’s Law of facies, states that the strata formed by these separate depositional environments tend to become stacked on top of each other as sea level rises and falls. You’ll see in my interpretations, that I’m always always trying to account for every possible sequence in a depositional system’s environment. The offshore, the nearshore, the backshore, and the river systems. In most major deltaic basins, each of these environments is receiving & preserving sediment simultaneously, stacking these differing sequences on top of each other as sea level fluctuates. Too often I feel like we geologists fail to portray in our historic reconstructions the full dynamics of facies stacking. In the following geologic history, I will attempt to use lot of pictures and imagery to fully represent the possible depositional environments and facies stacking involved in the formation of Utah’s geologic units.
As you can see in the above illustration, a single depositional basin or shoreline often has four to seven separate facies. As sea level rises and falls, or as deltas prograde out to sea, those facies become stacked vertically in a variety of unique ways. Anytime a geologists stands before the coarse grained sediments of a fluvial (stream or river) deposit, they must ask where did the fine grained sediments in this system end up? Where was the shoreline? And as sea-level rose or fell through time are those deposits stacked above or below the one’s I’m looking at?
The Grand Staircase of Utah and Arizona
The Grand Staircase refers to an immense sequence of sedimentary rock layers that stretch south from Bryce Canyon National Park and Grand Staircase-Escalante National Monument, through Zion National Park, and into the Grand Canyon National Park. It is perhaps one of the best locations in the world to study Walther’s Law of facies stacking.
In the 1870s, geologist Clarence Dutton first conceptualized this region as a huge stairway ascending out of the bottom of the Grand Canyon northward with the cliff edge of each layer forming giant steps. By travelling from the bottom of the Grand Canyon, through Zion and the Vermillion Cliffs of Grand Staircase Escalante and onto Bryce, Cedar Brakes or the Cenozoic layers of central Utah one can walk essentially the entire rock record of the last 500 million years since complex life began to evolve on the planet in the Cambrian. By studying these layers geologist like myself can reconstruct what the landscape looked like through time.
Paleogeographic Reconstructions of the Colorado Plateau
Starting in the Triassic.
-add a linked table of contents
-add a section on radical theories
-expanding earth (from mantel density flux) as a driving mechanism for plate tectonics, Earth as a rocky gas giant, gradual and catastrophic TPW as driver of orogenesis.
-add a planetary geology section
-add sections for every geologic age. Cover each major formation/group within those sections
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