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GEOLOGY HAPPENINGS February 2020

Mostly Mesozoic: Geologic Time and Moab
by Allyson Mathis
Westwater Canyon. BLM photo by Bob Brennan
The ages of the rock layers (formations to geologists) found around Moab are measured on a different time scale than the one we humans typically use. With the calendar having turned over to 2020, there has been a lot of commentary about the new decade. Most of us now view Y2K, the start of the new millennium 20 years ago, as almost ancient history. Historical events, such as the settling of Moab by Anglo-Americans in 1878 seem impossibly long ago, and the time when ancestral Puebloan people created rock art along the Colorado River more than a thousand years ago is even more so.

Geologic time is measured in millions of years, stretching to billions of years. The oldest rocks near Moab, the igneous (“fire-born”) and metamorphic (altered through heat and pressure) rocks in Westwater Canyon, are about 1,700 million (1.7 billion) years old. The Wingate Sandstone that makes up the red cliffs soaring above the Moab Valley and holds up prominent mesas like Dead Horse Point is approximately 200 million years old. Just this span of geologic history as documented in some of the rocks found near Moab makes even the entire extent of human occupation on the Colorado Plateau of approximately 12,000 years a mere blimp.

The earth is
Mesozoic Rocks along the Green River in Canyonlands National Park. The cliffs consist of the Jurassic Wingate Sandstone capped by the Kayenta Formation. The slopes are made of the Triassic Moenkopi and Chinle Formations.
4,540 million (4.54 billion) years old. Long before geologists were able to determine the exact age of rocks, they began putting together the geologic time scale based on the relative age of sedimentary rocks and using distinctive fossils in them. The older rocks that contained fossils of trilobites and crinoids were assigned to the Paleozoic (“ancient life”) Era. Those with dinosaur fossils were termed Mesozoic (“middle life”), and the ones containing fossils of mammals and other organisms more similar those alive today were put into the Cenozoic (“modern life”). The rocks older than Paleozoic that did not have abundant and/or obvious fossils were lumped into the Precambrian because they were formed before the Cambrian Period in the earliest Paleozoic.

Determination Towers are made of the Carmel Formation and Entrada Sandstone.
Later, geologists developed ways to precisely measure the age of rocks and determined that the Paleozoic took place between 541 and 252 million years ago, the Mesozoic 252 to 66 million years ago, and the Cenozoic during the last 66 million years. They also discovered that the Precambrian includes most of the earth’s history.

No single locale contains rocks that represent the entire expanse of geologic time because the earth is a dynamic planet. The theory of plate tectonics explains how older parts of the crust is recycled by being subducted down into the mantle, and new crust is formed where plates spread apart. Weathering and erosion also destroys older rocks and leads to the deposition of new ones.

The area around Moab is notable not for having the oldest or youngest rocks on the Colorado Plateau, but it does contains rocks that were formed during each of the four major subdivisions of geologic time, from the Precambrian to the Cenozoic. Despite this great range in the age of rocks near Moab, the Mesozoic section is the area’s main geologic calling card. Most rock layers that are exposed in the Moab area are Mesozoic in age from the red siltstones of the Moenkopi Formation to the gray Mancos Shale exposed in the Book Cliffs to the north.

The La Sals are made of igneous rocks that crystallized 28–29 million years ago.
Precambrian
Precambrian rocks make up the core (aka, the basement) of continents, such as that underneath the Colorado Plateau, but these rocks are only exposed in a few areas in the region: in Westwater Canyon north of Moab, in Colorado National Monument near Grand Junction, and in Grand Canyon. These basement rocks that were formed and altered by intense heat and pressure record the story of how North America became larger as moving tectonic plates collided.

Paleozoic
With the exception of the deeper canyons in along the Colorado River south of Moab where late Paleozoic sedimentary rocks such as the Cedar Mesa and White Rim sandstones may been seen, Paleozoic Rocks are generally not exposed on the earth’s surface near Moab. The best place on the Colorado Plateau to see Paleozoic rocks is Grand Canyon where the river has cut into these older rocks.

Mesozoic
Paleozioc rocks exposed in Grand Canyon. Most of these rocks are older than the ones near Moab.
Most of the significant rock layers exposed in southeastern Utah are Mesozoic in age. A dozen different formations provide a detailed geologic history of the Mesozoic Era, including the rich fossil record of the time, especially that of dinosaurs. The Entrada Sandstone in Arches National Park is Mesozoic, as the Navajo Sandstone in Sand Flats, and the Kayenta Formation on the Moab Rim. The total thickness of all the Mesozoic rock layers in southeastern Utah is between 4,000 and 8,000 feet.

Cenozoic
None of the sedimentary rock layers near Moab are Cenozoic in age, but Cenozoic geologic history is recorded in the landscape itself as it was sculpted in the recent geologic past. The most prominent example of Cenozoic rocks near Moab are the igneous rocks in the La Sal Mountains. The youngest geologic deposits near Moab are gravel deposits of the Colorado River formed in the last few hundred thousand years. Farther afield, the Claron Formation in Bryce Canyon was deposited in the early Cenozoic.
Simplified Geologic Time showing the age of some of the rock layers found near Moab. The height of eras on the left-hand column is arbitrary, but is proportional to the age of the earth in the right-hand column. 88% of earth’s history is within the Precambrian. Fm. = Formation. Ss. = Sandstone

You can read more geology articles HERE

A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab. Allyson enjoys contemplating deep time in Moab and beyond..

 
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