Geologic Formation and Evolution

Plate Tectonics

The Colorado Plateau began to rise from North America about 50 mya. The Pacific Ocean plate and the North American Continental plate collided just west of what is now California. This convergent plate boundary, complete with oceanic-contintal convergence and subduction caused stresses and reverberations all the way to present day Colorado and New Mexico. The entire Southwest portion of the United States was folded, uplifted and fractured. The Sierra Nevadas arose, along with the Rocky Mountains. Amongst all this sideways pressure, the great monolith of the Colorado Plateau began to rise. The Colorado Plateau has been rising so relentlessly that its elevation is more than one and a half km high.

Precambrian Era.
The Colorado Plateau has the widest geologic time scale of any exposed spot on the earth. The oldest Precambrian in the Inner Gorge to the present day layers are all on display to us in the Colorado Plateau.

With the formation of the earth around 4.6 bya began the Precambrian era, during which the oldest of the visible layers were formed. The first oldest layer is metamorphosed vishnu schist, about 2 byo. The next oldest is granite gneiss that intruded the schist and is only really visible in the more eroded areas of the plateau like the Grand Canyon and the Black canyon of the Gunnison. Most of these rocks were probably formed by the heating, crumpling and recrystalization of sandstone, shale, volcanic ash and lava.

This is immensely thick, tens of kilometers; making it difficult to tell if there is a different layer underneath it. After the granite intrusion there was a great beveling, caused by erosion. These layers were covered with younger rock layers of conglomerate, sandstone and shale. But before the end of the Precambrian there was another great beveling.

Paleozoic Era.
Thus began the Paleozoic Era, one of relative calmness and sameness. The flat land rose and fell, several times. During the lower levels of fluctuation, the Colorado Plateau dropped enough that the western sea was able to advance and cover the land, only to retreat with the upthrusting of the Colorado Plateau. This produced sand dunes blowing across what is now the Colorado Plateau. The deposits in this area and time were relatively thin - a few hundred meters during the Cambrian period. None for the Ordovician and Silurian periods, next to nothing during the Devonian period, and a little more during the Mississippian, Pennsylvanian, and Permian periods maybe a couple thousand meters, vs the many thousand meters found in a deep marine trough to the west.

The seas that covered the Colorado Plateau during the Paleozoic era were alive with critters. Some of them include brachiopods, trilobites, corals, clams and snails, feathery bryozoans, and fishes, the first vertebrates. There are many fossils of these and we can track their evolution, whether it was developing fish or the evolution of exoskeletons and shells used for protection by them all. All the plant and animal shells greatly added to the sediments, producing layers of limestone. As they continued to evolve (into the Devonian period) there are signs of early plants and animals crawling up onto the land. Later in the Permian period we find footprints in the wet dune sands.

Overall the the Paleozoic era was geologically "low key". There was not any volcanism, or mountain building and slow accumulation of sediments across a nearly horizontal surface.

Mesozoic Era.
One of the staples at the end of the Paleozoic era was the rise of the Colorado Plateau and the drainage of the western sea. Mountains that had been formed in Colorado during the Pennsylvanian period shed their sediments onto the Colorado Plateau. Volcanism increased a little as smaller volcanoes burped ash onto the Colorado Plateau. Well to the west, molten magma pushed up forming the granites of today's Sierra Nevada range. The culprits in cutting off the moisture swept eastward from the Pacific. This is a crucial key in the development of the ecosystem, for the lack of moisture created a sahara across the Plateau region. The sand dunes created during this period comprise most of the scenery-making rocks of the Colorado Plateau. The dinosaurs dominated this area tromping across desert and delta alike. They dominated for about 200 my. And as shallow seas covered the area one more time marine reptiles, and nicely spiraled relatives of the squid and octopus called ammonites, swam above the light silt of the sea floor.

Toward the end of the Mesozoic era convection currents below the crust forced the break of North America from Europe. As the Atlantic Basin widened, the collision on the other side of North American with the East Pacific Plate added to the stresses in the crust. In what has come to be known as the Laramide Orogeny, the Rockies were birthed to the east and north of the Colorado Plateau. The development of the Rockies provided headwaters for the great river that with its tributaries, carved out the Colorado Plateau. Not only was the river cutting down into the rock layers but the Plateau was also pushed up by the Laramide Orogeny causing the rivers to act more youthful longer, causing in turn, greater erosion.

There are many views on how exactly the demise of about half of the living plant and animal species (including the dinosaurs) was brought about 65 mya, but with this change came the spread and diversification of the survivors, mostly mammalian.

Cenozoic Era.
In Tertiary time, ancestors of today's horses, elephants, pigs, and camels roamed this area, feeding on the growth of the savannah environment. These animals in turn became the prey of the dog and cat families. Upthrusting of the Colorado Plateau continued into the early Cenozoic era, cracking and breaking along old faults created during the Precambrian era. Segments of the Colorado Plateau reached their current elevation as they creaked upwards. Alas, none of this happened with out our favorite friend erosion. Layer upon layer of sedimentary rocks were stripped off the exposed surface of the Colorado Plateau. Stripping occurred because the layers tend to alternate between more and less resistant rock and the erosion of the less resistant undermines the more resistant cap rock. This all causes the development of features like the "Grand Staircase," on the north side of the Grand Canyon, a series of successive layers eroding generally to the north. Most of the Colorado Plateau follows a gentle slope to the north causing the cliffs to retreat in that direction.

As this magnificent region continued to develop molten magma rose from the depths, squeezing out through faults and fissures, creating several stratovolcanoes: San Francisco Mt, Mt Trumbull, Mt Taylor, and the White Mountain Volcanoes. Later as the Colorado and its cohorts were fashioning the Plateau to the likes of today's patterns of drainage, the cinder cones popped up presenting some color contrast to the Plateau region. There were several "recorded" incidences where these cones would temporarily dam the mighty Colorado, creating lakes and revolutionizing the tiny, delicate ecosystems that developed along the river.

The last incident of volcanism, that of Sunset Crater, occurred a little over 900 ya. It is almost certain that the Colorado Plateau has not heard the last of the underworld.

Glaciers
Glaciers also helped shaped the Colorado Plateau as well. During the Pleistocene epoch none of the glaciers were in direct with the Colorado Plateau, but distance tends not be an issue for nature. Rainy cycles in conglomeration with the continental glaciation farther north sent a lot of water (which carries debris) down and on to the Colorado Plateau, adding to the massive amount and rate at which erosion takes place.

The Colorado Plateau is just about all sedimentary rocks, mainly sandstone. Many of these sediments were deposited during the times when it was covered by seas, and so contain a lot of salt. These salt deposits create a lot of anticlines. This is how: The salt was deposited over a fault, which is still active. The salty layers are then covered by heavy sediment, which crack from the movement of the hidden fault. These cracks allow water into the salty layers, which devolves the salt, causing the layers on top to fall and break. In the end, you are just left with a valley. This feature can be seen a lot in Arches National Park.

The Colorado Plateau has thousands of fault lines. Particularly in its center where there is a lot of crustal distortion, caused by the massive uplift. The two largest faults are the Moab fault and the Hurricane fault, which formed Arches Canyon and Zion Canyon, respectively. The faults are nearly vertical normal and reverse faults, which is how the Colorado Plateau was able to make its relatively smooth, vertical transition.

A fold is when there is not enough stress on the rock to form a fault but enough to bend it. The Grand Canyon shows a monocline, which is the simplest kind of fold. Because of all the stresses the Colorado Plateau has many folds and is not completely flat as some people assume plateaus are.

Fossils are not a significant part of the Colorado Plateau, but some are found, such as the ones in Dinosaur National Monument. These include, the Stegosaurus (with bony plates that could have released body heat), Brontosaurus (looks like an elephant mixed with a giraffe and travels in herds), Camptosaurus (about the size of a human) and the Allosaurus (with sharp claws and knife-like teeth). There is also an abundance of petrified wood, and Anasazi bones.

As salt accumulated over the many cycles of sea-evaporation (29 in over 10-15 my), it developed layer thousands of meters thick. Then mountains to the east started be weathered and eroded. Millions of tons of sediment were deposited on top of the salt. The weight compressed the salt and in doing so, squeezed it like a glacier. The salt migrated south, but was cut off by block faulting. As water cut across and into the Entrada sandstone surface, at some areas the water made it down to the salt and started to dissolve it. This made areas where the sandstone sank, jointing it laterally. More erosion caused the formation of indvidual walls of sandstone running parallel. It is these wall that would be carved out into the arches.

Here are the rock layers represented in the Colorado Plateau. As you can see there are many unconformities here, some with hundreds of millions worth of missing rock. The great unconformity seen only in the Grand Canyon, is an extreme angular unconformity. If all of these unconformities had been drawn to scale, their thickness would be greater than this entire diagram.