Geologic History of Utah and the Colorado Plateau
I’ve put a lot of thought and study into the geologic history of Utah and the Colorado Plateau over the last decade. I hope in this article I can add some insight into the subject, and make the geologic history of Utah come alive for the geologist and novice alike.
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 figure 1, 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.
RON BLAKEY’S PALEOGEOGRAPHIC RECONSTRUCTIONS OF THE COLORADO PLATEAU
(Colorado Plateau paleogeographic reconstructions)