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GEOLOGY OF ARIZONA
Tectonic Setting
The state of Arizona lies in the Southwest Region of the United States in
and are called the Cordilleran. It lies on the southwestern side of the Rocky
Mountain region. The crustal material has been dated to an age of 1.8 to 1.6
billion years. We can look at the tectonic map and see that there are major thrust
faults along the western edge of the Rocky Mountains and to the far
southeastern and eastern edge of this 1.8-1.6 billion year old crustal material.
We can also note that the approximate thickness of the early to middle
Paleozoic rocks in Arizona and along the transcontinental arch is greater than
one kilometer thick. When compared to the material that lies to the northwest
and east of the transcontinental arch, we see that this arch provided a
topographic high that did not allow the extremely thick (greater than 6 km)
sedimentary rocks to build at this location. To the southeast we see slightly
thicker Paleozoic sediments. To the southwest the Paleozoic rocks exhibit a
rather complex history of faults, volcanism, down throw and uplift.
Moving on to the Mesozoic, we see that the Mogollon Highlands formed
through the center of Arizona along a broad east-west trending line. This
topographic high was created by uplift and allowed these materials to be eroded
and deposited to the northern edge of the current border with Utah. South of the
Mogollon Highlands, we see a band of sediments washed to the south,
paralleling the southern highlands border.
During the Cenozoic, we see that the Colorado Plateau is well formed
along the southwestern edge of the Rocky Mountains. The northern half of the
state is plateau, while the southern section is Basin and Range. We see that the
Basin and Range extends far to the north and to the south into Mexico. If we
now turn to the current tectonic map of the state, we see that Arizona is divided
into three tectonic or physiographic regions. We have the Colorado Plateau, the
Transition Zone (which is a line along the Mogollon Rim) which trends in a
northwest-southeast line. The lower half to third of Arizona is the Basin and
Range area. It consists of extensive faulting and volcanism that was extremely
active during the Tertiary.
If we move on to the Shaded Relief Map, we will see the differences in
elevation that help us distinguish these tectonic zones that closely parallel the
elevation of the state.
Minerals and Geologic Resources of Arizona
We know that Arizona is named the “Copper State”. The main reason for
this is that extensive volcanism to the south and southeastern parts of the state
have allowed deposits of copper and subsequent fluids and oxidation to create a
vast assemblage of copper minerals. There of course is native copper,
turquoise, azurite, chrysocolla, chalcopyrite, malachite and more.
There are large mines that have closed, like the Lavender Pit in the far
southeastern part of Arizona. But locations like Morenci, are still very active and
being operated by Phelps-Dodge. Along with this volcanism came fluids bearing
other metals, such as gold, silver, lead, iron, mercury, manganese, and tungsten.
Some gold is mined from veins, but small placer deposits exist. Placers
are old or current streambeds where material was eroded, freeing the gold, and
allowing it to be deposited in these stream channels.
One of the more interesting deposits found is helium, near I-40 and the
New Mexico border. This gas was probably created from the decay of light
radioactive elements like lithium and calcium. It could have come from deepseated granites and pegmatites which typically contain lithium, calcium and
lighter elements which are typically the last to come out of solution to form
minerals.
Looking at the cupric minerals that formed, Arizona has a rich history of
turquoise mining. There are the Kingman and Sleeping Beauty mines that are
well known for their turquoise deposits. But because these are used for jewelry
and are not industrial minerals, they are not as profitable as other types of
deposits.
The main location for coal in the state is on Navajo land in the
northeastern portion of the state, at Black Mesa. These Mesozoic coals are
remote and are not easily accessible. Also on native lands are Navajo oil fields
in the far northeastern corner of the state.
Some gypsum sites lie in the far northwestern edge of the state and
southeast of Phoenix. These evaporites of calcium sulphate are major sources
for sulfur and for the development of gypsum board, or drywall. Looking closely
we see scattered locations for peridot, clays, zeolites, sand, flagstone, dolomite,
marble and carbon dioxide.
Lastly, we see areas that contain uranium deposits (uranium oxide) which
typically lie on government lands. Back in the 1950’s to the 1970’s there was a
big push to get people back east to buy leases of government lands so they
could make money off of uranium deposits.
General Questions Concerning Arizona’s Geology
1. Where does the thickest sequence of sediments lie in the state? Are they
Cenozoic, Mesozoic, or Paleozoic in age?
2. What is the significance of the Mogollon Rim?
3. What are the three tectonic or physiographic regions of Arizona?
4. What happened in the southern most region of Arizona and what is the
area called?
5. Why is the geology in the Grand Canyon area called “layer-cake”
geology?
6. Why is the geology in the southern portion of the state considered to be
more complex than on the Colorado Plateau?
7. Where was the most recent volcanic eruption in Arizona? When did it
happen?
8. What is the age of the trees found in the Petrified Forest and what
formation is it found in?
9. What is the major geologic unit exposed in the Page area? What is its
age, the rock type and how did it form?
10. What geologic era do the Black Mesa Coal beds belong?
11. Name at least 5 cupric minerals that are found in Arizona.
12. Name at least 2 minerals that mined in Arizona and used for jewelry.
13. Are their any oil resources in Arizona? If so, where are they located and
who owns them?
14. What is the main mineral mined in Arizona?
15. Give 5 minerals that are non-cupric and are found in Arizona.
16. Provide 2 gases that are Arizona resources.
17. What era was the Basin and Range most geologically active in Arizona?
18. Where are two locations that are very likely to receive earthquakes?
What would be the sources for these earthquakes?
19. The formation of the landforms and the Colorado River channel has
occurred over the last how many million years?
20. Why do the Kaibab Formation and the Redwall Limestone in the Grand
Canyon form good cliffs?
21. Which lithology forms the fans and slopes on the cliffs of the Grand
Canyon? What does this mean about this rock type?
22. Does the Grand Canyon have more rocks present representing geologic
time, or more rocks that are “missing” showing a great gap in geologic
time?
23. In the Central Grand Canyon, the Shinmuno Quartzite and the Hakatai
Shale form the lower boundary of what?
24. In the Central Grand Canyon the Tapeats Sandstone and the Bright
Angel Shale form the upper contact of what boundary? What is the age of
this boundary?
25. Most of the volcanic fields in Arizona are what age?
26. What is the problem with having dams along the Colorado River?
27. Name two dams that are found in Arizona along the Colorado River.
28. What caused the Colorado River to cut down into the Colorado Plateau?
29. What kind of lava flow is the Bonito Flow at Sunset Crater? What is its
composition?
30. What is the major resource for water for the Phoenix area aqua duct
system?