Running head: Grand canyon 1

Grand Canyon Stratigraphy

Christina Tinsley

Southern Utah University

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Grand Canyon Stratigraphy

The Grand Staircase is a series of five monoclines that form the steps of the staircase.

Stretching from the North Rim of the Grand Canyon in Arizona all the way to central Utah in

Bryce Canyon. From the inner gorge of the Grand Canyon to the majestic spires of Bryce

Canyon, the staircase is truly one of the wonders of the world. Earth’s geologic history is

displayed like a stratigraphic clock, from the Precambrian all the way through the Cenozoic.

The Grand Staircase was not formed in a peaceful, static setting. Some layers of rock

were deposited, only to be eroded away their history never to seen. Others were twisted,

deformed, fractured, buried, metamorphosed, uplifted, in a dynamic series of events that would

form the stratigraphy we see today. http://www.earthscienceeducation.org/Da-

HamblinGurgelEtcAtlasPages/Hamblin2004-p085-GrStaircaseSchematic.jpg

The five steps of the staircase are named primarily for their color. Starting with the oldest

the Chocolate Cliffs, and working up

to the Vermillion cliffs, white Cliffs,

Gray Cliffs, and ending with the Pink

Cliffs. The base of the Chocolate

Cliffs is made up of the Permian age

Kaibab Limestone, the top most layer

of the Grand Canyon.

In order to understand the

formation of the Grand Staircase, you have to go back to the to the formation of the Precambrian

and Paleozoic that form the basement of the last step. The Grand Canyon is one of the greatest

geologic wonders of the world. It is one of those rare places where the geologic of the Colorado

Figure 1. Accretion Terranes in Western North America.http://www.earthscienceeducation.org/Da-HamblinGurgelEtcAtlasPages/Hamblin2004-p085-GrStaircaseSchematic.jpg

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Plateau history can be seen in one place. Both the Precambrian and entire Proterozoic can be

seen in the strata of the canyon. Each layer documents both the depositional environment and the

tectonic history in which it was formed. For this reason, it truly is a geologist’s playground.

Precambrian

The oldest rocks in the canyon date back to the late Precambrian Era, in the Proterozoic

Eon. They were formed approximately 1840 to 1710 million years ago. The basement rocks of

the canyon are the remnants of oceanic plates converging on the ancient North American cratons.

The west is made up of the accretion of volcanic arcs and

oceanic terranes, colliding with North America along

convergent plate boundaries. As pressure increased the rock

and sediment was compressed and folded, some to great

depths. A mountain range some five to six miles high is

thought to have formed during this time of orogenesis. The

basement rocks were heated and deformed at extremely high

temperatures, yielding metamorphic gneisses and schists.

Interweaving between the gneiss and schist, magma was

pushed up between the weaknesses in the rock and later

cooled intrusively to become Zoroaster Granite. These

metamorphic and intrusive volcanic rocks are referred to as the Grand Canyon Metamorphic

Suite. Exposed to the elements the mountain range was continuously eroded. As erosion lowered

the elevation and weight of the mountains, isostatic uplift buoyed up the once buried

metamorphic rocks to the surface once again. This period of erosion and lack of new deposition

marks the first unconformity in the stratigraphy of the canyon (Foos, 1999).

Figure 2. Accretionary Terranes https://en.wikipedia.org/wiki/TransHudson_orogeny#/media/File:Wyoming,_Mojave,_Yavapai,_Mazatzal,_Trans-Hudson.gif

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The Grenville Province collided with North America at approximately 1.4 to 1.0 Ga. The

Grenville Orogeny (see figure 1) created an intracratonic basin on the back side of the orogeny

and generated east-west extensional normal faults. The Grand Canyon Super Group, starting with

the Unkar Group were deposited in this low lying basin. Limestones; representing an offshore

setting, shale; representing a nearshore setting, and sandstone; representing a continental beach

setting were deposited as sea levels rose and fell. The entire formation is riddled with magmatic

intrusions, yet the Super Group has not been metamorphosed. The Shinumo Quartzite found in

the super group is not in fact a traditional quartzite. The cement that bonded it together has a very

high silica content, making it as hard as a true quartzite without ever being subjected to

metamorphosis.

The Grand Canyon Super Group is widely studied by geologists, because after deposition

there was a vast period of non-deposition and erosion. There are only small pockets where the

formations can still be found today. These portions are highly tilted and backed by fault zones.

There were at least two different periods of rifting and extension within the basin. The first as a

result of the Grenville collision and the second when the supercontinent of Rodinia broke apart.

These periods of extension created normal faults (Timmons, Karlstrom, Dehler, Heizler, &

Geissman, 2001). The remaining pockets of strata escaped erosion because they are located on

the down blocks of these faults, which dropped as much as 5000 feet in some places. In most of

the canyon, the Tapeats Sandstone sits directly on top of the Schist, leaving a gap of geologic

history spanning approximately 1200 million years. This gap in history is known as the Great

Unconformity (Foos, 1999).

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Paleozoic

Beginning in the Cambrian,

approximately 570 Ma. the strata began to take

on their more familiar horizontal bedding

structure. The Paleozoic Era on the Colorado

Plateau is defined by transgressional marine

deposition on a passive continental margin. The

different facies of rock formations formed by a

series of transgressions and regressions of sea

level. The Tapeats Sandstone, Bright Angel

Shale, Muav Limestone, and Undivided Dolomite are all examples of marine depositional

environments at different depths. There is a definitive pattern of transgression and deposition and

subsequent regression and erosion displayed throughout the Paleozoic layers of the Colorado

Plateau. Disconformities throughout the rock record display these erosional periods of sea level

regression (Foos, 1999).

The rise in sea levels in the Cambrian may have attributed to the explosion of new life on

Earth. The shallow marine environments created more temperate climates along with the

increased nutrient content of the continental shelf is thought to have been a major contributor to

the increase in species diversity and trophic stability (Brasier, 1982).

Deposition continued uninterrupted in the Cambrian until a period of glaciation began,

approximately 507 Ma. Polar ice caused sea level to drop and a period of erosion began. The

Ordovician, Silurian, and a great deal of the Devonian period is completely missing from the

Figure 3. Early Cambrian 525 Ma. http://cpgeosystems.com/images/SWNA_525Ma-sm.jpg

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geologic during this period either caused by erosional forces or due to lack of deposition

(Waggoner, 1994).

Sea levels began to rise again at the beginning of Mississippian, resulting in the Redwall

Limestone formation and the Surprise Canyon formations. At the end of the Mississippian, sea

levels dropped again. Marked by another disconformity.

During the Pennsylvanian sea levels transgressed and the Supai Group was deposited,

before regressing again at the beginning of the Permian. Sea levels rose again during the

Permian, where the last of the Paleozoic strata were deposited.

Causes for the fluctuation in sea level can be caused by many different things or a

combination of events. Buoying of the sea floor at the site of a divergent plate boundary can

cause sea levels to rise. Tectonic activity within the plates as they shift and move creates heat

that will raise the continent, thereby lowering sea levels. At the continents age and cool they will

subside into the mantle allowing sea levels will rise onto the continent. Glaciation will also cause

global sea level regression.

From the Cambrian, there have been 14 periods where

erosion has dominated, creating unconformities in the rock

Figure 3 Mississippian 340 Ma Figure 4. Pennsylvanian 310 Ma. Figure 5. Permian 280 Mahttp://cpgeosystems.com/swnamtectonic.html

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record. Either by lack of deposition during the period or because erosion has scoured away any

trace of existence. Transgressive sequences are marked in light gray of Figure 6. These are

periods where sea levels have risen onto the continents. As sea levels rise there will be a series of

facies that can be found. Starting with a beach setting, sandstones will form. As water levels

continue and sea levels deepen, mudstones and shales will form. And finally, limestones will

form in the offshore reef environment. Regressions will have the opposite effect; limestones,

shales, mudstones, sandstones. The facies of the Grand Canyon Strata document these changes in

sea level.

The Laramide Orogeny is responsible for the uplift of the Colorado Plateau as it is today.

The Farallon Plate subducted beneath the North American continent at a very shallow angle, for

reasons that are still being debated. Compressional forces caused the ancient normal faults to

reactivate and reverse. Bringing the former down blocks of the fault as much as three thousand

feet upwards in some places (Timmons, Karlstrom, Dehler, Heizler, & Geissman, 2001).

The same forces that created the strata of the Colorado Plateau are still shaping it today.

Though not as fast moving as they once were, tectonics is still at work on the canyon, just as it

has been for millions of years.

References

Blakey, R. (2012 , February ). Paleogeographic and Tectonic History of Southwestern North

America:. Retrieved from Colorado Plateau Geosystems:

http://cpgeosystems.com/swnamtectonic.html

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Brasier, M. (1982). Sea level changes, facies changes and the late Precambrian-early Cambrian

evolutionary explosion. Retrieved from Academia:

http://www.academia.edu/437826/Sea_level_changes_facies_changes_and_the_late_Prec

ambrian-early_Cambrian_evolutionary_explosion

Foos, A. (1999). Geology of Grand Canyon National Park, North Rim. Retrieved from

http://www2.nature.nps.gov/geology/education/foos/grand.pdf

Ranney, W. (2013). Academia. Retrieved from Geology of Grand Canyon National Park:

Sedimentation and erosion on planet Earth's greatest landform :

https://www.academia.edu/8937766/Geology_of_Grand_Canyon_National_Park_Sedime

ntation_and_Erosion_on_Planet_Earths_Greatest_Landform

Rice, R. (2015). Tectonic Origin of The Colorado Mineral Belt. Retrieved from

http://academic.emporia.edu/aberjame/student/rice1/CMB.htm

Timmons, J., Karlstrom, K., Dehler, C., Heizler, M., & Geissman, J. (2001, February).

Proterozoic multistage (ca. 1.1 and 0.8 Ga) extension recorded in the Grand Canyon

Supergroup and establishment of northwest- and north-trending tectonic grains in the

southwestern United States. Retrieved from Research Gate: gsabulletin.gsapubs.org

Waggoner, B. (1994, 11 22). Cambrian: Tectonics and Paleoclimate. Retrieved from

http://www.ucmp.berkeley.edu/cambrian/aucambrian.html