Roger Weller, geology instructor
firstname.lastname@example.org last edited: 1//17/16
Parent Rock of Brown Canyon Sand Sample
Mississippi State University
and Cochise College
Huachuca Mountains. Carr Peak with foreground low hills
composed of Precambrian granite.
Photo is copyright free for non-commercial educational uses. R.Weller/Cochise College
Brown Canyon is located in the Huachuca Mountains of the Coronado National Forest (Figure 1). The Huachuca Mountains are a southeast to northwest trending range located in Southern Arizona. The northern range of the Huachuca Mountains is part of the Fort Huachuca Military Reservation, which extends to the Garden Canyon Recreational area. The remainder of the range is open to the public except for the Ramsey Canyon Preserve, which is owned and operated by the Nature Conservancy. The range extends south into the international border with Mexico. Brown Canyon is a scenic little canyon with good views and high desert vegetation such as manzanita, live oak and pinion pine, as well as high desert grasses. It is a relatively shallow, open canyon carved among rolling hills. The upper reaches provide excellent views of the sheer cliffs of the Dragoon Mountains’ western face, as well as of the Chiricahuas to the east, and the Whetstones to the north. The Brown Canyon trail connects with the Hamburg and Pomona mine trails, and can be used to access Ramsey Canyon.
Figure 1. Raised Topographic Map of the Huachuca Mountains. Taken from R.Weller
Mineralogy of Sand Micrograph
The sand sample obtained from Brown
Canyon, latitude 31.47 N and 110.31 W in southeastern Arizona clearly shows the
remains of a weathered granite (Figure 2). The sample is probably derived from
a porphyritic granite of Precambrian age exposed in the canyon. The sample
seems to be quartz rich, with angular clasts of colorless and milky quartz, as
well as mica and some feldspar of either sodic or potash (or both) origin. The
quartz sand grains are angular portraying the type of weathering they have
undergone. There also seem to be some grains that appear to be a rose color of
quartz. The mica looks as if it could be biotite, and is slightly greenish in
color. The feldspars are difficult to discern in the sample, but the yellowish
powdery material coating the entire sample gives away the presence of clay
minerals, which could be from the weathering of the feldspars in the original
rock. In some parts of the photo, what looks to be white to gray opaque
minerals with flat cleavage surfaces are visible; these may be feldspars, but it
is difficult to determine.
A second set of photos of this sand
showed much more than the first photo simply because the author could take time
to look through the sample. Muscovite and feldspars in these new photos were
identified very easily. The feldspars showed cleavage surfaces, and are
probably albite in composition. Some fragments of biotite were also seen, as
well as crystals that looked likely to be magnetite or some other metallic
constituent. This second set of photos confirmed the presence of rose quartz.
(See figures 3a - 3c)
Figure 2. Brown Canyon sand.
Photograph courtesy of Dr. R.M.Clary, Mississippi State University
Figure 3a. Muscovite crystal from Brown Canyon sand.
Figure 3b. Feldspar crystal (albite?)from Brown Canyon
Figure 3c. Possible magnetite
crystal in Brown Canyon sand.
Parent Rock Composition
In the Huachuca Mountains, the most likely origin of the
granite sand pictured below is probably a widespread intrusive igneous rock: a
porphyritic granite. This rock contains orthoclase crystals as much as three
inches in length, embedded in a medium to coarse grained groundmass. (Anderson,
1961) Hayes and Raup (1968, MAP I-509) describe this granite as a "yellowish to
pinkish-gray coarse grained porphyritc granite with some areas of unmapped
alaskite." In places where the large feldspar phenocrysts are exposed, the
rock exhibits a type of flow pattern, as the phenocrysts seem to be oriented in
the same direction. The granite also contains threads and dikes of aplite in
many places. Aplite’s essential components are quartz and alkali feldspar
combined with a texture classified as allotriomorphic- granular or xenomorphic
-granular. The threads are fine-grained and sugary; being entirely composed of
anhedral crystals (Jackson, 1970). The anhedral crystals are indicative of the
late stages of crystallization, when little space is available for crystal
growth. Aplites are thought to form in a process called pressure quenching.
In this process, granitic magmas become fractionated and enriched in silica,
alkalis and water vapor (Raymond, 1995).
Hayes and Raup (1968) mapped this area and
stated that there are "unmapped areas of alaskite", which this author believes
are the aplite dikes mention above. Outcrops of this granite are highly
weathered, and retrieval of fresh surfaces is difficult without extensive
equipment. The rock is highly fractured and breaks deep into surface exposures
easily along weathered planes. For several inches into the surface, oxidation
and fractures continue. One surface, exposed through sheer diligence, shows
muscovite, quartz and highly altered feldspars. (figure 4) This finding is very
indicative of hydrothermal alteration, and not surprisingly, the area is host to
many abandoned mines along veins of hydrothermal deposition. In other places,
feldspar crystals are so large they weather out onto the surface, and can be
identified by a glint of sunlight off weathered crystal faces. A visual
analysis of the granite shows a quartz rich rock with high amounts of sodic rich
feldspar and muscovite as a distinct accessory mineral. Hayes and Raup (1968)
labeled the granite a quartz monzonite porphyry and this author has no
disagreement with their assessment.
Correlation of Parent Rock
This granite comprises about one-third of the
entire range along the eastern side of the Huachuca Mountains. The oldest rock
known in Southeastern Arizona, the Pinal Schist, the ancient crystalline
foundation upon which all this area is believed to rest, (Ransome, 1925) does
not outcrop in the Huachuca Mountains. In places where it can be found and
correlated, however, it is intruded by Precambrian granites and other igneous
rocks. (Darton, 1925) The Precambrian granite of Brown Canyon is believed to be
of this age and type of intrusion.
Ransome (1925) mapped many of the Precambrian
intrusive granites found in the Pinal Mountain Range in Gila, AZ, which were the
cause of the contact metamorphism of the Pinal Schist in that area. Correlating
the age of this Precambrian granite with the Pinal Schist and the Grenville
orogeny, places it between 1.5 and 1.2-1.0 Ga. Baldridge (2004) identifies
several pulses of magmatism that involved deep seated melting and intrusion of
granitic magmas into the crust from 1.46 to 1.40 Ga in this area. This
magmatism was extensive, and spanned a great portion of the continent from
California to Illinois (Baldridge, 2004). Anderson (as cited in Baldridge 2004)
stated that such magmatism was so profound that between 15 and 40 % of the
Proterozoic crust of North America formed during this time.
Weathering of Granite
The Huachuca Precambrian granite can be found in a number of places along the range and is highly weathered where it outcrops. It can mainly be seen in the canyon washes and intermittent stream beds, as well as in the year-round streams. In places along the Brown Canyon trail, which serves as a bike, horse and hiking trail, the large feldspar phenocrysts are weathered out, and can be picked up by the handful. In other canyons along the front range, the granite is so weathered it has formed grus; small loose slopes and sands in the lower parts of the arroyos. The granite is especially weathered on east-facing slopes and the immature angular clasts show that very little transportation of this rock has occurred. (See Figures 5 - 8)
Although the location is at 31 degrees
N latitude, due to its elevation, the area experiences hot daytime and below
freezing nighttime temperatures. It is not unusual for the area to experience
daily temperature differences of 50 degrees or more in a given day. Grus is a
product of physical weathering in granites due to this daily cycling of
temperature. Where rock has been previously weakened by chemical weathering and
ground water, the process is accelerated, as is the case in the Huachuca
Mountains. Chemical changes in granite due to weathering include the loss of
hornblende and mica initially, followed by the disappearance of oligoclase and
the simultaneous appearance of iron oxides and clay minerals (Blatt, Middleton
and Murray, 1980). This process seems to support the composition of the Brown
Canyon sand sample as hornblende is not apparent and it is difficult to isolate
feldspars in the sand sample.
Figure 5. Photo is copyright free for non-commercial educational uses. Granite grus.
Just credit photo to R.Weller/Cochise College
Granite grus and aplite dike. Photo is copyright free for non-commercial
Just credit photo to R.Weller/Cochise College
Figure 7. Large Feldspar Crystals. Weathered grus in background.
Figure 8. Exposed Precambrian quartz monzonite porphyry.
Parent Rock Exposure
The geologic map completed in 1968 (Hayes and Raup) shows that this granite is faulted several times toward the core of the range. The Nicksville normal fault lies along the extreme eastern front, and is mapped from Nicksville in the southeast to the end of the range in the northwest. This fault has a dip of about 60 degrees and is mapped as a normal fault. A series of small thrust faults trend the same way and for about the same length, but are parallel to each other and located closer to the mountain core. Each of these faults exposes younger sedimentary rocks toward the west. This series of thrust faults vary in dip from 45 to 60 degrees, thrusting rock from the east toward the west. This faulting caused the uplift of the granite, exposing it to the elements and building the eastern third of the range. Hayes and Raup (1968) have added an underlying main detachment fault, which extends from the most western exposure of the Cambrian sedimentary rocks which dips at 60 degrees, but turns to an almost horizontal shallow shear zone which underlies both of the large faults shown on the map below (Figure 8). However, it is clear from their map that they were not sure about this fact at the time, as they had drawn it into the cross section with question marks.
The Brown Canyon sand sample seems to be the product of weathered granite of Precambrian age. This mostly undeformed granite was exposed by Basin and Range faulting, which uplifted the Huachuca Mountains and comprises nearly one-third of the mountain range on the eastern side. This granite was mostly likely emplaced around 1.4 Ga as the result of crust stabilization. The physical and chemical weathering of the granite produced the sand product.
Map is a portion of the Geologic Map of Arizona, created by the Arizona
Bureau of Mines
and the United States Geologic Survey, 1969- original scale was 1:500,000
Photo is copyright free for non-commercial educational uses.
Just credit photo to R.Weller/Cochise College.
Anderson, C.A. (1961). Older Precambrian Structure in Arizona. Bulletin of the Geological
Society of America. V62, p 1331 - 1346.
Anderson, J.L. (1989). Proterozoic anorogenic granites of the south western United States. In Baldridge, S. (2004).
Baldridge, S. (2004). Geology of the American Southwest. Cambridge, Cambridge University Press.
Blatt, H., Middleton, G. and Murray, R. (1980). Origin of Sedimentary Rocks. New Jersey, Prentice-Hall, Inc.
Darton, N.H. (1925). A Resume of Arizona Geology. University of Arizona Bulletin, College of Mines and Engineering.
Hayes, P.T. and Raup, R.B. (1968) Geologic Map of the Huachuca and Mustang Mountains Southeastern Arizona. USGS Misc. Inv. Ser. I-509 (1- 48,000)
Jackson, K. C. (1970). Textbook of Lithology, New York, McGraw-Hill Book Company.
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