Chiricahua Mountains
Chiricahua MountainsRoger Weller, geology instructor
wellerr@cochise.edu
last edited: 11/17/09
Chiricahua National Monument
JoAnn Deakin
Mississippi State University
and Cochise College
Introduction
The Chiricahua National Monument is best known for its interesting and sometimes spectacular volcanic pinnacles' called "fairy chimneys" by many locals. The monument is located in the southeast corner of Arizona and part of the Coronado National Forest, where it forms an intersection point of four biomes: the Sonoran Desert, the Sierra Madre Zone, the Chihuahuan Desert and a southern most point of the Rocky Mountain Zone. The Chiricahua Mountains were formed by high angled normal faulting due to crustal thinning, typical of the Basin and Range province, which extends into Northern Mexico.
These
mountains are surrounded by down dropped valleys (grabens) that form the Sulphur
Springs Valley to the west, the San Simon Valley to the northeast and the San
Bernardino Volcanic Field to the southeast. As with many of the mountain ranges
in this area, the Chiricahuas are termed "sky islands" (Parent, 1994). Sky
islands are areas of cooler temperatures and increased precipitation due to
orographic lifting as compared to the surrounding desert floors. These
mountains serve as oases that include natural springs which feed year-round
streams and deeply forested pine, cedar and aspen canyons, and are home to a
diverse animal population.
Paleo Proterozoic Tectonic
Setting
Proterozoic rocks of Arizona are classified into eight separate terranes. The five northwest terranes comprise the Yavapai supergroup. The metamorphic rocks in this supergroup are considered the basement rocks or crustal rocks and exhibit dates of 1.6 to 1.8 Ga (Baldridge, 2004). Prior to their accretion, the southern coast of North America extended as far south as the state of Wyoming. Geologists believe that during a three million year period from approximately 1.8 to 1.5 Ga, these eight terranes were accreted to the southwest portion of the continent (Keep 1996). The first orogenic event that affected this area is known as the Yavapai orogeny, which generally comprises the first five accretions. This large encompassing event was followed by the Mazatzal orogeny, dated 1.6Ga to 1.7Ga, which essentially deformed the last three terranes. Baldridge (2004) states that these rocks are younger towards the southeastern portion of Arizona, and were part of a foreland thrust, or magmatic arc. (See figure 1) Evidence for the Mazatzal Orogeny in southeast Arizona is displayed by the oldest known rock in the area; the Pinal Schist. Weller (2009) describes the Pinal Schist as a "strongly foliated sericite schist interbedded with quartzose grits of sedimentary origin and a maximum thickness of 6000m.” The Pinal Schist was first described by Ransome (1904) as a "very fine grained metasedimentary rock from the greenschist facies with bimodal suites of volcanic rocks. To the east of the region, the outcrops are bimodal volcanic, and to the west, the rocks are more of a quartz wacke turbidite."(page 26) This schist outcrops between the Chiricahua National Monument and the city of Wilcox, Arizona in the Dos Cabeza Mountains (Pallister, du Bray and Hall, 1997).
There are several competing models for the formation of the Pinal Schist.
Bowring and Karlstrom (as cited in Keep, 1996) proposed that it was deformed due
to the accretion of a terrane. This work was published after the work of Conway
and Silver (as cited in Keep, 1996) who proposed that deposition occurred along
a series of continental margin basins. Copeland and Condie (as cited in Keep,
1996) posit that the origin of the Pinal Schist was in a continental margin
arc/intra arc system. Anderson (as cited in Keep, 1996) proposed a back arc
basin model for its original deposition. Keep (1996) believes the evidence
supports deposition in a rifted basin, which then contracted due to some type of
margin inversion. She uses the evidence of rare outcrops of Pinal equivalents
in Sonora, Mexico, as well as gentle folding with no overturned folds, foliation
plots and the presence of felsic volcanic magmatics, to support her theory.
Although she does speculate on what could cause this type of basin contraction
(sea floor spreading pulses), she presents no evidence for such a mechanism in
the region. In any case, the origin of the paleogeographic setting of these
terranes is still a controversial issue (Baldridge, 2004).
(figure 1)
http://jan.ucc.nau.edu/~rcb7/pcpaleo.html
Whatever the original depositional setting of the Pinal Schist, by 1.4 Ga the majority of Arizona had been added to the continent, and the area underwent intrusion by felsic magmas. These Precambrian rocks, which out-crop throughout the southeastern portion of Arizona, can be found in the Huachuca, Whetstone, Rincon, Dos Cabeza Mountains, and east of the Chiricahua National Monument near Hilltop. Pallister et al.(1997) suggests that these rocks effectively "stitched the terranes to the continent." (page 1)
Paleozoic Sequence
An 800 million year unconformity separates the Proterozoic rocks from the
Paleozoic sequence overlying them. The stratigraphy (figure 2) shows deposition
from about 570 until 250 million years ago. Ransome (1904) first described the
stratigraphic sequence of the region; the unfossiliferous Bolsa quartzite lies
unconformably on the Pinal schist and is overlain by the Arbrigo formation which
contains abundant fossil trilobites, brachiopods and pteropods. Ransome (1904)
submitted samples of these fossils to Dr. Charles Walcott who identified them as
Cambrian in age. The remainder of the Paleozoic sediments are marine in origin,
and represent deposition patterns consistent with marine deposition.
Mesozoic Sedimentation and Volcanic Activity
The geologic column presented by Ransome (figure 2) shows an unconformity
between the Paleozoic and Mesozoic Eras that covers approximately 150 million
years. Few rocks from this period are preserved in the record of the Chiricahua
Mountains or similar mountain ranges of the region, which likely represents an
erosional period due to uplift. The Mesozoic sediments that are present, are
comprised of the rocks of the Bisbee group, and probably represent deposition in
a basin from a marine incursion that transgressed from the Gulf of Mexico. The
sediments represent a transgressive cycle as the oldest layer the Glance
Conglomerate is overlain by the Morita Formation (mostly shale) which, in turn,
is overlain by limestone. Dickerson and Lawton (2001) describe the Bisbee Basin
as the "result of a mid-Jurassic intracontinental rift system." (page 475)
They observe the Bisbee Basin to be the central segment of the rift
between the Cordilleran and the Caribbean plates. They
also observe the progressive transgression of the sea from the Gulf of Mexico
toward the west in the sedimentary record. Around 160 million years ago,
subduction to the west brought about the formation of volcanoes in the Huachuca
and Dragoon Mountains, creating calderas as well as faulting (Pallister et al.,
1997). A second round of volcanic activity generally associated with the
Larimide orogeny, continued mountain building in the region and the formation of
the large copper deposits found in the area. Morenci to the north and Bisbee to
the south
west, are two very large open pit
copper mines representative of these Mesozoic intrusions.
(figure 2) http://skywalker.cochise.edu/wellerr/ransome/pg054a.htm
Cenozoic Volcanic Activity
At the beginning of the Cenozoic Era, subduction continued
to the west and volcanic activity resumed in southeastern Arizona. During this
time small intrusive plutons of granite were emplaced in the mountains of the
Cochise Stronghold to the west as well as the Chiricahuas themselves. During
the Oligocene Era, smaller volcanic eruptions of andesite, basalt and rhyolite
occurred, are evident in the National Monument, and underlie the Faraway Ranch
Formation. The Faraway Ranch Formation is composed mainly of rhyolite, dacite
and andesite lavas. Some smaller explosive pyroclastic flows are also found in
this formation. On top of the Faraway Ranch Formation is the Jesse James Canyon
Tuff; a welded tuff with thickness in some places of 790 feet. At this time,
the Chiricahuas were uplifted sections of Paleozoic and Mesozoic sediments, with
small volcanoes and vents dotting the area. In his 1969 PhD Dissertation,
Marjaniemi described this area as being in the intersection of the Sierra Madre
Occidental and the Basin and Range tectonic province. His objective was to
identify the source area of the major ash-flow sheets in the area.
The Chiricahua Monument itself, at that time, was a valley
with slightly higher uplifted areas towards the south. Around 27 million years
ago, a volume of magma intruded under the mountains to the south of the
monument. The magma was silica rich and highly volatile.
Around
26.9 million years, the magma chamber erupted and the Turkey Creek Caldera was
born.
The eruption of the magma chamber is estimated to have
spewed more than 100 cubic miles of magma into the surroundings. In comparison,
Mount St. Helens, which erupted in 1982, expelled only one-tenth of a cubic mile
(Pallister et al., 1997). The immense amount of material expelled lead to the
collapse of the volcano forming the Turkey Creek Caldera. Much of the expelled
mass became clouds of ash and pyroclastic flows that filled the Chiricahua
Valley to a depth of 1600 m (Pallister et al., 1997).
A more in-depth look shows that the volcanic flows of this particular eruption
are divided into three main layers, all which date to the same time and must
have occurred in rapid succession. The bottom member is composed of a pumaceous
ash flow tuff, the middle member is classified as a densely welded pumaceous ash
flow tuff, and the upper member is a white ash rich surge bed overlain by a grey
medium welded tuff (Pallister et al., 1997). Overlying the Rhyolite Canyon Tuff
is Dacite lava, but the occurrence is rare, being found only on the top of Sugar
Loaf Mountain. Inside the collapsed caldera, lava, flows of ash, pumice and
pyroclastic tuffs surround the uplifted central section and the walls of the
caldera. Marjaniemi (1969) described the caldera as exhibiting differential
erosion. The highly resistant silicic rocks of the moat remain, but the dome
itself has been removed giving the caldera a cirque like structure. (Geologic
Map, figure 3)
(figure 3) (courtesy of Roger Weller, wellerr@cochise.edu)
Interesting Geologic Formations
The Chiricahua Mountains display the high angle normal faulting associated with
crustal thinning over the last 20 million years, which is characteristic of
Basin and Range topography. These mountains exhibit the signature keystone type
of block faulting common to many mountain ranges of the region. Basalt flows
are also characteristic of crustal thinning and block faulting. The San
Bernardino Volcanic Field, to the southeast, is a basalt flow of Pliocene to
Pleistocene age.
The interesting volcanic columns in the monument were formed from weathering
along joints as the result of cooling of the volcanic flows. Weathering is
accelerated along these joints resulting in the pinnacles in the area. Volcanic
features such as fossil fumaroles, surge beds, solution pans and exfoliation
shingles can be seen throughout the park. Tafoni cavities (Bezy, 2001) are
weathered into the rock walls, and create spooned-out features along canyon
walls. Slot Canyons are also common, and attest to the weathering processes
that are taking place.
Some Views of the Chiricahua Mountains
R.Weller/Cochise College (Pinnacles in the Park)
R.Weller/Cochise College (Balancing Rocks)
Cultural Aspects
Paleo Indians were most likely the first inhabitants of
this area. Ten thousand years ago the climate of the area was cooler and
wetter, making the area more hospitable for habitation. These first inhabitants
are believed to have migrated from the north, over the Bearing Strait land
bridge. As the climate changed to dryer conditions over time, these inhabitants
evolved into the hunter-gatherers of the desert southwest. Parent (2006)
labeled these people the Cochise culture. Around 200 B.C., a more agrarian
culture labeled the Mogollon evolved, spurred by cultures from southern Mexico.
The Mogollon culture had a more stationary lifestyle evidenced by archeological
discoveries which found pottery making, farming and permanent homes (pit houses)
as normal day to day behaviors. These small settlements were scattered around
the Southwest and more are being discovered. As early as March 2009, a new
archeological site for these Mogollon people was unearthed during the
construction of a new housing development near the Ft. Huachuca military post in
the city of Sierra Vista. Some authors argue that this culture abandoned their
homes around A.D. 1300 (Parent 2006), and may have become part of the Anasazi at
a later date. The
Chiricahua Apaches migrated into the
area in the 1500's and moved seasonally from one area to another. Interestingly
the word "Chiricahua" actually translates as "mountains of the turkeys", which
makes sense as there are many of them inhabiting the mountain ranges.
As the Spanish began to
explore and settle in the region, conflicts became more frequent between the
Apaches and various settlers. Apaches were a warrior culture who raided not
only Spanish settlements, but other Indian tribes as well. In the middle
1800's, the 10th Cavalry of
Buffalo
Soldiers were stationed in Bonita Canyon which lies in the monument area. The
U.S. Army was racially segregated at the time with the black Buffalo Soldiers
commanded by white officers. Stories abound about the fear the Indians held for
these black men, as Indians were unaccustomed to seeing African people.
Eventually the Chiricahua Monument became a protected area under President
Calvin Coolidge in 1924, due to the rich history and beauty of the area.
References
Baldridge, S. (2004). Geology of the American Southwest. Cambridge, Cambridge University Press
Bezy, J.V. (2001). Rocks in the Chiricahua National Monument and the Fort Bowie National Historic Site. Tucson, Arizona Geological Survey
Dickerson, W.R. & Lawton, T.F. (2001) Tectonic setting and sandstone petrofacies of the Bisbee Basin (USA-Mexico). Journal of South American Earth Science, 14, 475 - 504
Keep, M. (1996) Pinal Schist, southeast Arizona, USA: The contraction of a Paleoproterozoic rift basin. Journal of Geological Society. 153(6), 979-993
Marjaniemi, D.K. (1969) Geologic History of an Ash Flow Sequence and Its Source Area in the Basin and Range Province of Southeastern Arizona. (Doctoral Dissertation, University of Arizona, 1969)
Pallister, J.S., du Bray, E.A., and Hall, D.B., (1997). Interpretive map and guide to the volcanic geology of Chiricahua Monument and vicinity, Cochise County, Arizona, Arizona: USGS Misc. Inv. Ser. I-2541 (1:24,000)
Parent, L. (1994). Chiricahua National Monument. Tucson, Western National Parks Association
Ransome, F.L. (1904). Description of the Bisbee Quadrangle, Arizona. US Geological Survey, Folio 112 17p.
Weller, R. (2009). Virtual Geology Field Trips., Chiricahua Mountains. Retrieved
Oct 1, 2009.
http://skywalker.cochise.edu/wellerr/geology_SEAZ/chiricahuas/Chiricahuas-list.htm