The Colorado Plateau

The Colorado Plateau provides a record of animal and plant evolution during the Paleozoic and Mesozoic eras. It stretches across four states (Figure 10‑31). During the Paleozoic and Mesozoic, the Colorado Plateau (Figure 10‑31) region was near or below sea level and included coastal plains, swamps, large river valleys, tidal flats, deserts, and shallow seas. Sediment deposition formed over ten thousand feet of geologic strata over 500 million years and a record of invertebrate, fish, plant, archosaur, and early mammal evolution. 

With shifting coastlines and rivers and climate changes, deposition patterns and geologic materials changed. The layers in the Grand Canyon and the upper geologic formations of the plateau reflect these changes as well as the evolving animal kingdom. 

Erosion processes in the uplifted plateau formed deep river canyons such as the Grand Canyon (right side of Figure 10‑30), Uneven uplift and subsequent erosion formed a series of mesas and cliffs called the Grand Staircase (Left side of Figure 10‑30) in the region north of the Grand Canyon. These processes exposed geologic strata and the fossils within them.

As with the Chengjiang Lagerstatten and Burgess Shale in the sea, rich beds of fossils are found in select fossiliferous locations with extensive preservation of fossils. Section 10-5 focuses on Petrified Forest National Monument in the southern part of the plateau (Figure 10‑31), which has Triassic strata and early relatives of crocodiles. Section 10-6 focuses on Dinosaur National Monument in the Morrison Formation at the northern end of the Plateau (Figure 10‑31), with extensive preservation of dinosaurs from the Jurassic Period.

The lowest and oldest geologic layers of the Colorado Plateau are in the Grand Canyon (Figure 10‑32). These strata document animal evolution in the Paleozoic Era. The Precambrian Vishnu Schist (1.75 Ga) at the bottom of the Grand Canyon has no animals. The Cambrian Tonto Group (Tapeats Sandstone, Bright Angel Shale, and Mauv Limestone) alternates between land and sea formations. The Tapeats Sandstone deposited in a land environment. Because there were no land animals or plants at the time in Earth’s history, there are no fossils. The Bright Angel Shale and Mauv Limestone formed in the sea during and have many marine fossils such as trilobites. The Ordovician and Silurian layers eroded away prior to the deposition of the Devonian Temple Butte Formation, which formed in eroded sections of the Mauv Limestone and has marine fossils, as does the Mississippian Redwall Limestone above it. Limestone forms in shallow seas from the successive deposition of marine organisms with calcium carbonate shells. Other formations such as the Redwall Limestone in the Grand Canyon also formed in shallow seas.

The Pennsylvanian Supai Group formed in marginal marine and shallow marine environments as the seas were retreating from the area. There are ferns and animal tracks in these layers. The seas had retreated during the period of the Permian Hermit Shale Formation and Coconino Sandstone, which have no marine fossils but have tracks of small reptiles and amphibians. The Hermit Shale formed from alluvium from erosion of an ancient mountain range. The sandstone formed in a desert environment with loose sand. The seas then returned, and the Upper Permian Toroweap and Kaibab Limestone have many brachiopods, corals, mollusks, sea lilies, worms, and fish teeth.

Geologists have been trying to figure out how the Grand Canyon formed since the first geologist, Robert Newberry visited the canyon in 1860.  The primary confusion concerns the Kaibab uplift near the middle of the canyon, which is higher than either end of the canyon. How did the river pass over a highpoint? Because the layers on opposite sides of the canyon lined up, Newberry concluded that the Colorado River carved the canyon. This might sound like a simple conclusion, but prior to Newberry’s observation, people had not considered that a river could carve a canyon. They thought that faults formed and then a river followed the faults. 

John Wesley Powell was the second geologist to explore the canyon. He travelled down the Colorado River in a boat. He developed the antecedence theory, which was that the river formed before the Kaibab uplift took place, which is not correct. Geologist William Morris Davis determined that all of the side streams carved their own canyons, which were just as deep as the Grand Canyon. He realized that giant boulders carved the canyon during floods, and that the erosion wasn’t just due the slow abrasion of sand and water in the river. Because the Muddy Creek Formation at the outlet of the Grand Canyon is only 6 million years old, and the Colorado River cuts through it, Eliot Blackwelder determined that at least part of the Grand Canyon was carved less than 6 million years ago. Other parts of the canyon might have formed 70 million years ago.

Eddie McKee has proposed the most popular theory of canyon formation to date, which is that the Grand Canyon formed from two separate rivers. One of the rivers was from Utah and Colorado, and it turned south and east at the Kaibab uplift toward the Rio Grande. The other river was originally in the Grand Canyon and flowed to the west. According to McKee’s theory, it kept cutting away at the western end of the Kaibab uplift. Because of the extensive limestone layers in the Canyon, it is possible that a series of limestone caverns, channels, and faults provided a path that connected the two rivers and that the part of the river from the east started to flow toward the west. Eventually, the flow eroded a channel and diverted the entire flow of the Colorado River to the west. This is still a hypothesis, and geologists still cannot say with certainty that this is how the Grand Canyon formed.

Once the Colorado River entered the Grand Canyon, it eventually worked its way toward the Gulf of California. Radiometric dating reveals that the Colorado River arrived in the Gulf of California 5.3 Ma. Since then, massive floods deposited millions of tons of sediment in the Imperial Valley and the Gulf of California. Deposited sediments move slowly to the north because the San Andreas fault slides up the Imperial Valley each year. Scientists track the dates of deposition over the last five million years with radiometric dating, which shows that sediments in the valley are older to the north.

The Permian Kaibab Limestone is at the top of the Grand Canyon. The series of plateaus that form the Grand Staircase formed in layers above the Grand Canyon. The Moenkopi Formation lies above the Permian Kaibab Limestone. Above the Moenkopi Formation, the Vermillion Cliffs (Figure 10‑33) include the Chinle Formation at the base and the red Moenavi and Kayenta Formation above it. These formed during the Triassic Period when the reptiles and ancestors of crocodiles and dinosaurs inhabited the region. Petrified Forest National Park contains the strata of the Chinle Formation in another part of the Colorado Plateau. At the time, it was a tropical river valley and has many amazing fossils of early crocodile like animals. The Moenavi Formation is just above the Chinle Formation (Figure 10‑33) and formed in a river system that passed through the Arizona area during the early Jurassic. At the same time, the northern part of the Colorado Plateau was a desert. The Kayenta Formation (Figure 10‑33 and Figure 10‑36) is also an early Jurassic formation. It is a main attraction in Zion, Capitol Reef, and Canyonlands parks. It has many early Jurassic amphibians, Crocodylomorpha (crocodile), Ornithodira (bird and dinosaurs), and Tritylodonta (early mammal) fossils.

The Grand Staircase consists of a series of plateaus between the White Cliffs, Gray Cliffs, Straight Cliffs, and Pink Cliffs. The Claron Formation is the upper step of the Grand Staircase and forms the Pink Cliffs (Figure 10‑34) at Bryce Canyon National Park. This formation is composed of limestone, which formed after the region was uplifted by the Laramide Orogeny. It formed during the first half of the Cenozoic Era in inland lakes in North America.

Figure 10‑35 shows three cliffs in the Grand Staircase: Vermillion Cliffs in middle, White Cliffs, and Gray Cliffs in back. National Parks were established in many of the deep river canyons on the Colorado Plateau. Formations extend across the different canyons and are exposed at different elevations in different canyons (Figure 10‑36).

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Painted Desert near Holbrook, Arizona on Colorado Plateau in Petrified Forest National Park.  Credit: Adbar. Used here per CC BY-SA 3.0.