Geologic Formation and Evol.
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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
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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.
The Colorado Plateau Through the Ages
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
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Geologic Formation and Evolution
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
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Geologic Formation and Evolution
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.
Major Bedrock Units
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
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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.
Faults and Folds
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
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