Download Brief History of the White Tank Mountains and the Western Phoenix

Brief History of the White Tank Mountains and the Western Phoenix Valley
Since the Proterozoic (Precambrian) the White Tank Mountain gneiss and granites have
experienced different stages of uplift and deformation. The Laramide Orogeny occurred 75 to 50
MY ago, beginning in the Cretaceous and ending in the Paleocene (lower Tertiary).
In the southern part of Arizona the Mid-Tertiary Orogeny occurred about 24 MY ago and
formed many of the metamorphic core complexes (mountains with a central composition of
gneiss, granite, or schist). The latest orogeny (mountain building event), The Basin and Range
Orogeny, occurred about 8 – 15 MY ago and creates the most recent uplift in southern Arizona.
The greatest known amount of uplift and deformation in the southern part of Arizona happened
between 11 and 15 MY ago. The Basin and Range Orogeny continues today with measurable
uplift, but is minus the great volcanic eruptions and extensive faulting which created basins
(grabens) and mountains (horsts). The occasional earthquakes recorded in the southwestern
states marks movement related to the continuation of the Basin and Range uplift.
The mountains exhibiting the metamorphic core complexes may have remained as
topographic highs in the region over time, with the Mid-Tertiary orogeny creating the greatest
amount of metamorphic core mountain building. The White Tank Mountains are some of these
Mid-Tertiary Metamorphic Core complexes that were uplifted and metamorphosed by the MidTertiary Orogeny. They are accompanied by other similar mountain ranges that border the
Phoenix Valley.
The various glacial melts of the Pleistocene (2 - .01 MY ago) and erosional waters during the
Pliocene, resulted in extensive erosion and movement of Tertiary and Quaternary sediments
moving from the Colorado Plateau down through the Central Basin into the southern basin and
range area of Arizona. Extensive erosion from the north fed the Phoenix area with thick
sediments both during the upper Tertiary and the Quaternary.
More and more sediments poured into this lake, but with the wane of the flooding waters, the
lake was no longer replenished and soon began to dry up. As the water level lowered, minerals
that were dissolved in the lake began to precipitate out of solution, and the waters became more
and more saturated with these mineral salts. Eventually, the lake became like the Great Salt
Lake, but continued to evaporate. This resulted in thick deposits of salts such as, gypsum
(CaSO4.2H2O), halite (NaCl), anhydrite (CaSO4), borax (Na2B4O7.OH2O) and other salts.
If we dig deep into the western Phoenix Valley near Luke Air Force Base, we will find these
thick salt deposits at a depth of 900 feet or greater. These salts reach a thickness of 2000 feet,
and are overlain with Quaternary and Tertiary alluvium, which was washed into the valley after
these salt beds formed.
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Subsequent ephemeral waters pushed coarse eroded sediments into the valley, much coming
from this up-thrown mountain blocks, now showing dendritic drainage patterns, covering these
salt deposits and the entire valley floor. If we look to the south central part of the Phoenix valley
we see thick Tertiary alluvium pushed up against Precambrian granite and schist, at Camelback
Mountain. The erosion in the Central Highlands to the north, during the Eocene (55 – 38 MY) and
Oligocene (38 –24- MY), washed the coarse, thick sediments (the head) up against the granite
and schist, which form the camel’s hump. These kind of thick sedimentary deposits are found
throughout the Phoenix valley and generally cover the valley floor. Thick sequences of Tertiary
and Quaternary alluvium are a common occurrence throughout the basins in the southwest’s
Basin and Range physiographic region.
The geology of the White Tank Mountains is similar to other metamorphic core complexes in
the Phoenix Valley, like the Estrella Mountains and South Mountain, that contain gneiss, granite
and/or schists that are typically Precambrian in age but demonstrate Tertiary metamorphism.
Miocene volcanism and uplift (30MY) helped to reset the radiometric age of the eastern side of
the White Tank Mountains to a Tertiary age. The Potassium-Argon (K-Ar) absolute dating shows
that an event reset the radioactive decay in part of the mountain range to this relatively young age
(Tertiary), despite the Proterozoic gneiss and schist found there. The west side of the White
Tank Mountains is far less metamorphosed and uplifted which resulted in little or no effect on
their radiometric dates.
When rocks undergo heat and pressure, they can become metamorphosed and create
newly formed minerals. These minerals can have Potassium minerals (such as muscovite) that
will start its radioactive decay to Argon after they are formed. This is how we know that the east
side of the White Tank Mountains, which experienced more metamorphism than the west side,
shows a younger Tertiary age, while the west side shows a more accurate age of the
Precambrian granites and gneiss. New minerals can yield a date that is much younger than the
original date of the precursor rocks. The metamorphic radiometric date can be far younger than
the age of the original sediments.
Some of the White Tank rocks and features are tonalites, distinct folds, quartzite veins and
faults. Some quartzite folds found in the Precambrian schists indicate well-pronounced
compressional stress. The tonalites are calcium and sodium rich rocks with 20 to 60 percent
quartz, but with abnormally high anorthosite and plagioclase (sodium and calcium rich feldspars).
Summary
The White Tank Mountains are continuing to experience the gradual uplift of the Basin and
Range Orogeny. The Precambrian gneiss and granites (600 MY), and quartzite veins show folds
and faults that may have been initiated 75 MY ago, but were mainly created by the Mid-Tertiary
Orogeny.
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The White Tank Mountains were the western boundary for the Phoenix Valley inter-mountain
lake that formed from northern outwash, which also carried great amounts of Tertiary and
Quaternary sediments into southern Arizona.
Over the past 30MY, the valley experienced flooding, the formation of a lake and its
subsequent evaporation that formed thick, 2000 feet, evaporites or salts including gypsum,
anhydrite, halite and borax. Thick, coarse Tertiary and Quaternary sediments filled the valley
floor covering these evaporites that can be found at a depth of 900 feet due east of the White
Tank Mountains and west of Phoenix proper.
Source
Chronic, Halka, 1981, Roadside Geology of Arizona, Mountain Press Publishing Company,
Missoula, Montana.
Chronic, Halka, 1986, Pages of Stone, Geology of Western National Parks and Monuments, 3:
The Desert Southwest, The Mountaineers, Seattle, Washington.
Please answer the following.
1. What is the age of some of the granite and gneiss in the Phoenix valley found in the
White Tank Mountains and at Camelback Mountain? The age some of the rocks were
reset to?
A)
B)
2. An Orogeny is a mountain building or uplifting event. When did the Phoenix area last
experience the effects of an Orogeny? What is its name?
When did it end?
A)
B)
C)
3. How was the Phoenix valley similar to the Great Salt Lake? What is the proof? Give the
name of some of the minerals found in the west valley at depth that support this.
A)
B)
C)
4. When metamorphism occurs, new minerals can form. How can this affect
radiometric dates? What does this tell us about determining the age of metamorphic
rocks?
A)
B)
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White Tank Mountains Regional Park, Waddell, Arizona
Part of the Basin and Range
There are three basins in the Phoenix area that are greater than 9000 feet
thick with sediments. These are: Luke Basin, Paradise Valley Basin and Higley
Basin. Earlier we discussed the thick sediments in the Luke Basin, where at 900
feet down we have a sequence of evaporites that have a thickness of 2000 feet.
These basins sit on the flanks of mountain ranges in the Greater Phoenix
area. There are the McDowell Mountains to the east of Scottsdale, the Phoenix
Mountains in the central part of the Phoenix Valley, and the White Tank
Mountains. Others exist, but we will focus on the White Tank Mountains, referred
to here as WTM. The diagram below shows the gross features of the basin and
range in the west part of the Phoenix Valley.
A
D
B
F
F
F
E
F
C
F
A Represents the White Tank Mountains
B Represents the Phoenix Mountains
C Represents the McDowell Mountains
F Represents the Basement rock consisting of gneiss, schists and some volcanic
material.
Note: All are mountain ranges that have been eroded. All were originally blocks
uplifted by tensional or pull-apart resulting in the black lines on the flanks of the
mountains that represent normal faults. All of these mountains are Proterozoic
Gneiss and Schist with some volcanic materials. The uplift occurred about 15 to
30 million years ago, where as the basement rock, gneiss and schist is about
1700MA. The arrows show the tensional or pull-apart.
D Represents Luke Basin
E Represents Paradise Valley Basin
Note: These consist of Tertiary and Quaternary sediments washed from these
mountains and washed into the area from the north.
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