Running head: Cenozoic 1

The Cenozoic Section of the Grand Staircase

Christina Tinsley

Southern Utah University

Cenozoic 2

The Cenozoic Section of the Grand Staircase

The Cenozoic Era spans from about 65 Ma to the present day. The Cenozoic is well

documented in the geologic and fossil record due to recent deposition and lack of

metamorphism. Since much more detail can be attained geologists generally refer to the epochs,

rather than periods. The Cenozoic is broken into two

periods, the Tertiary and the Quaternary, The Tertiary

is broken into the Paleogene and Neogene, which are

further broken into epochs; the Paleocene, Eocene,

Oligocene, Miocene, Pliocene. The Quaternary is

broken in two epochs; the Pleistocene and

Holocene. Continued orogenic events on the

western Cordilleran subduction zone created mountains and plateaus, highland uplifts altered

weather patterns, and shifting oceanic plates disrupted oceanic currents. All interplaying to create

the last step in the Grand Staircase.

Global Tectonic and Climatic Setting

The Continents of the Cenozoic would have looked much like they do today. The last

remnants of Pangaea continued to break apart, into the construction that is familiar to inhabitants

today. Tectonics in the Cenozoic had a particularly strong influence on climate change in the

Cenozoic. During the Eocene tropical conditions extended 10 to 15 degrees further north and

south than tropical environments seen today. Temperatures were considerably warmer across the

globe, as high carbon dioxide levels created a greenhouse effect. Tectonic events altered the

climate from warm, equitable global conditions of the early Cenozoic to the colder, seasonal

Figure 1. Cenozoic Era Timelinehttp://serc.carleton.edu/eslabs/climatedetectives/5b.html

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glaciations seen in the Late Cenozoic by changing the route of oceanic currents and altering

weather patterns.

Three major tectonic events occurred during the Cenozoic that would effectively alter the

climate across the globe. The first of these events, was the collision of India with Asia, which

would start the uplift of the Tibetan Plateau and the highest mountain range seen in modern

times, the Himalayas. The Himalayas, and other uplifts of the Cenozoic, changed the weather

patterns by blocking the moist air currents and winds. The second, occurred as the African and

Arabian plate collided with Eurasia, closing off the remains of the Tethys seaway and raised the

Alps, Pyrenees, and the Apennines mountains. Equatorial currents had been able to flow

unimpeded around the globe through the Tethys seaway. These warm waters were able to

circumnavigated the globe, keeping the global climate mild even in polar regions. The third,

occurred as Antarctica detached from South America and Australia and moved south towards the

South Pole. Oceanic currents began to circle Antarctica, inhibiting the mixture of warm

equatorial currents. Without warm water from the equator, Antarctica’s polar ice caps soon

formed.

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Along with a disruption of weather and oceanic patterns, orogenic uplift caused a rapid

increase in the erosion of silicate rocks. The weathering of silicates generates a chemical reaction

that removes carbon dioxide from the atmosphere. Therefore, the massive mountains created

during the Cenozoic are also, in part, responsible for the global cooling of climatic conditions as

they were eroded (Buchdahl, 2016).

Regional Tectonics

The Cordilleran Orogeny continued throughout the Cenozoic Period. As the Farallon

plate subducted beneath the North American Plate. The steep subduction angle of the Sevier

segment ceased and changed instead to the shallow, sub-horizontal subduction zone of the

Laramide Orogeny. The exact reason for this change is unknown, though there have been many

hypotheses. The most commonly accepted hypothesis today, is that as the Farallon plate

subducted beneath the North American plate, the mid ocean ridge in the Pacific Ocean grew

closer to the subduction zone. As oceanic crust moves away from a mid ocean ridge, it cools and

subsides. Younger, hotter crust is more buoyant. As the progressively hotter oceanic crust

subducted beneath the North American plate, the angle of subduction decreased until the

Farallon plate sheared just below the continental crust.

Evidence for this change can be seen in the volcanic record of western North America.

The Nevadan and Sevier orogenies steep subduction angles forced the oceanic plate to depths

where partial melt occurred, creating continental arc systems, like that of the Sierra Nevada

volcanic arc (Fillmore, 2011, pp. 259-266). During the Laramide, continental volcanism in the

Sierra Nevada’s and on the western coast ceased.

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Unlike the Sevier Orogeny, which created thin skinned deformation of the upper strata,

the Laramide Orogeny created basement uplifts as it scraped beneath the continental plate. The

low angle of subduction caused large arch like folds and single step monoclines throughout the

Colorado Plateau. Generated the uplifts that make up the basement of the modern Rocky

Mountains and stretched farther inland than any previous orogenic event.

The Farallon plate has been all subducted today. The Juan de Fuca and the Cocos plate

are the only remaining segments. However, the Pacific Plate, the western twin to the Farallon,

was formed at the same mid ocean ridge, and therefore can be studied in place of the Farallon.

Studies from the Pacific plate indicate that in the late Cretaceous the rate of subduction increased

dramatically, from 7 to 8 cm per year, to about 15 cm per year. The combination of a hot,

buoyant plate and increased subduction rate are dually responsible for the Farallon Plate reaching

so far inland.

Depositional Environment

The Cenozoic saw a change from a depositional to an erosional environment due to

uplifts by the Cordilleran compression. Deposition occurred when sediments were brought down

from the highlands by erosional forces such as wind and water in the form of stream and rivers.

The Cenozoic Era is displayed in the Pink Cliffs of the Grand Staircase. The red, pink, and white

cliffs that have become infamous in the Bryce Canyon and Cedar Breaks area come rocks

deposited in the Cenozoic, particularly the Claron formation. The Claron formation was

deposited in a fluvial setting. Erosion had become the dominate force in the Cenozoic. Streams

and rivers of fluctuating sizes and depths crisscrossed the area, working their way to sedimentary

basins.

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The Claron formation is predominately made up of limestones, segmented by bands of

poorly sorted conglomerates, calcareous sandstones, and shales. The many different variations of

color come from varying degrees of iron and magnesium

within the rocks. The beds with the highest calcareous iron

content contain the brightest reds, while the porous white

sands contain the least. These white bands may have

contained iron at some point, but it may have been leached

out by ground water (Weaver, 2016). These bands of

varying density and porosity make the landscape for Bryce

Canyon possible. Water drips down into cracks and

crevices in the rocks, freeze-thaw helps to widen these

cracks which steadily erode away material between the cracks. The resistant ceiling rocks and

bands keep the structure from being eroded entirely, resulting in spires, pinnacles, and hoodoos

that seem to defy logic.

Another common feature seen among the sedimentary strata of the Cenozoic are igneous

rocks of various compositions. The earlier Farallon plate magmatism is of an intermediate origin,

whereas later Basin and Range magmatism is of a more basaltic composition.

Farallon Plate and Volcanism. By about 50 Ma., the rate of subduction slowed to the

point that the plate began to flounder into the mantle and break apart. As the mafic oceanic crust

began to sink into the mantle partial melting occurred. As the less dense magma rose to the

surface it assimilated with the silicic crustal material, resulting in a magma of intermediate

composition.

Figure 4. Fairyland in Bryce Canyon.Photography by Christina Tinsley

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Large scale magmatism tends to occur on the borders of the Colorado Plateau during the

Cenozoic. The Plateau has remained relatively

undeformed, primarily due to the thickness of

the deposits that make up its composition. The

crust of the plateau ranges from about 40 to 50

km. The Basin and Range Province to the west

is highly fractured and has a crustal thickness

of about 30 km. Crust to the East, has the same

relative thickness, but is riddled with faults,

making it weak. As magma continued to rise, it bubbled in into these points of weakness around

the plateau. Large intrusive magmatic bodies are evidenced in the laccoliths and batholiths, as

well as large scale, explosive volcanic events that occurred primarily around the borders of the

plateau (Fillmore, 2011, pp. 296-297).

One such area is in the Maryvale volcanic field near Bryce Canyon. Volcanism in the area

produced one of the most impressive gravity slides that has ever been documented. The

Markagunt Megabreccia is a large mass of volcanic and volcanoclastic rock that overlies younger

rocks due to movement in a landslide event. The exact trigger of the gravity slide is unknown.

Three possibilities have been suggested as possible triggers. First, as the magma body pushed its

way up and out between the strata, the overlying layers mushroomed up in response. As the Iron

Peak laccolith grew, the relative slope became too steep to support its own weight causing the

collapse. Second, inflation of the crust caused by magmatic intrusions inside of the caldera. And

third, the southern end of the Marysvale volcanic field could have collapsed in on itself, thereby

removing the material needed to keep the mountain stable, triggering the slide.

Figure 5. Location and timing of magmatism.http://geology.isu.edu/Alamo/devonian/basin_range.php

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What is known is that the slide encompassed an area a large as 300 to 500 sq. miles. It

was large enough to pick up and move entire mountains in its wake and fast enough to generate

enough heat to melt the ground on which it

traversed. The compression generated by the

slide was great enough to cause folding and

faulting in the toe of the slide and extensional

faulting in the zones of detachment. Low

angel thrust faults like that of the Rubys Inn

thrust fault, which forced older cretaceous

strata over the younger Tertiary strata throughout Bryce Canyon and along the Paunsagaunt

Plateau. Thrust faulting from this event may be seen in the rock record as far a Cedar City, yet

conclusive studies have not yet been completed (Biek, 2013).

Uplift of the Colorado Plateau. How the Colorado Plateau was uplifted remains a

constant and continuing debate among geologists. The one aspect of the debate that is agreed

upon is the lithosphere was in some way involved in the process. The lithosphere is comprised of

the crust and solid upper mantle, just beneath the lithosphere is the asthenosphere. The

asthenosphere is hotter and exhibits ductility. Between the lithosphere and the asthenosphere lies

a zone of rock that is partially melted. This is the same area that allows the continental plates to

move. Both sides agree that the Laramide orogeny caused the Colorado Plateau to be uplifted.

However, the debate hinges on whether or not Laramide compressional thrust faults

caused the deformation that created the steps of the Grand Staircase or if the later Basin and

Range Extension normal faults created down dropping steps. Both events would have the same

effect; with the highlands to the north and the drainage to the south.

Figure 6. Hanging wall of the Ruby’s Inn thrust faultPhotography by Christina Tinsley

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The exact timing for the Colorado Plateau uplift is not known. What is known

definitively is that approximately 65 Ma in the late cretaceous sediments were deposited in an

oceanic environment. Those same deposits now sit at an elevation of approximately 7000 feet

above sea level. It is also known that the Colorado River drainage system did not begin prior to 6

MA ago when Basin and Range extension began and by 4.4 Ma the Colorado River was well

developed. Ash deposits have been found in the Hualapai Limestone, dating to about 6.Ma. What

is interesting about these deposits is that they were formed in a lake that is now intersected by the

Colorado River, giving further proof to the Basin and Range hypothesis (Fillmore, 2011, pp. 300-

308).

Basin and Range Extension. During the Miocene the tectonic style changed from that of

a subduction zone, a transform fault boundary. Only small segments of the Farallon Plate

remained, the majority destroyed as it was subducted beneath the North American plate. As

compressional forces were removed and

the San Andreas, transform fault began its

Northwestern ascent up the Pacific

coastline, extensional forces of the Basin

and Range began creating Northwest

bounding normal faults. Extension was

caused by a release of compression,

allowing the crust to relax and settle. And,

by drag created by the San Andreas as it

sheared along the coast.

Figure 7. Basin and Range Provincehttp://geology.isu.edu/Alamo/devonian/basin_range.php

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Normal faults are fracture zones exhibiting a hanging wall that drops down relative to the

base of the footwall. Normal faults create a topography of fault block mountain and down

dropped basins, referred to as horst and graben topography. The words horst and graben are of

German descent and literally translate to, high place and ditch or trench. Much of the landscape

of the basin and range is made up of these exact structures. However, many exhibit a listric low

angle, concave up-curvature of the plane, which causes the blocks to gradually become tilted.

These create half grabens, which are bounded by steep mountains on one side and a gently

sloping hillside on the other.

All normal faults allow for extension and thinning of the crust. Listric faults allow for

even greater rate extension to occur. These

low angle normal fault in many cases are the

reactivation of low angle Sevier thrust

faults, but acting in reverse, so that the

hanging wall drops instead of raises. As a

result of their relatively flat fault plane, the

hanging wall and the foot wall move almost

horizontally away from each other. The exact rate of extension varies, depending on the structure

and composition of the geology of the area, but on average about 400 km of extension has

occurred in the last 30 million years. Extension is still happening today, at a rate of about 10 mm

per year (DeCourten, 2003, pp. 180-189).

Between the Basin and Range and the Colorado Plateau lies a zone referred to as the

transition zone or hinge line. This zone contains the many active normal faults; including the

Wasatch, Sevier, and Hurricane faults. The formation of the hinge line dates back to at least the

Figure 8. Tilted strata bordering the Hurricane Normal Fault. Photography by Christina Tinsley

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Cambrian Period, but may have cratonic roots as well dating into the Precambrian. During the

Cambrian the ancient shore line resided at approximately the same area. To the west of the hinge

line massive deposits of limestone were deposited, some exceeding depths of 10,000 feet. Since

that time this same approximate zone can be seen in the deposition patterns throughout Utah’s

geologic history (Hintze & Kowallis, 2009).

Conclusion. The geologic history of the Grand Staircase is complex and intricate. People

travel thousands of miles to se some of the greatest geological wonders in the world, found right

here in Utah and Arizona on the Grand Staircase. The Grand Canyon, Zion National Park, Cedar

Breaks, Bryce Canyon, Canyonlands, Arches, the list could go on and on. The formation of the

geology that makes up these natural wonders took millions of years and required a collaboration

of natural forces to deposit, erode, mold, buckle, and fracture the landscape into the majestic

region seen today.

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References

Biek, B. (2013). The early Miocene Markagunt Megabreccia- Utah's largest catastrophic

landslide. 1-5.

Buchdahl, J. (2016, April 15). 5.2.2.3. Cenozoic Climates. Retrieved from Global Climate

Change: http://www.global-climate-change.org.uk/5-2-2-1.php

DeCourten, F. L. (2003). Adventures in Great Basin geology: The broken land. Salt Lake City:

The University of Utah Press.

Fillmore, R. (2011). Geological evolution of the Colorado Plateau of Eastern Utah and western

Colorado. Salt Lake City: The University of Utah Press.

Hintze, L. F., & Kowallis, B. J. (2009). Geologic history of Utah. Provo: Brigham Young

University Geology Studies.

The geologic timeline for the Cenozoic Era. (2016, April 18). Retrieved from EarthLabs:

http://serc.carleton.edu/eslabs/climatedetectives/5b.html

Weaver, L. (2016, April 18). The Claron / Wasatch Formation. Retrieved from

UtahGeology.com: http://utahgeology.com/utah-geologic-formations/

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