Saguaro NP | GeologyVirtualTrips

SAGUARO NATIONAL PARK

FIELD TRIP STOP - LARGEST CACTUS IN NORTH AMERICA; COMPLEX GEOLOGY ABOVE AND BELOW A DETACHMENT FAULT

LOCATION: Saguaro (suh-wahr-oh) National Park is located in southeastern Arizona. This park is unusual in that it is divided into two separate areas – a western portion called the Tucson Mountain District located 10 miles west of Tucson, and an eastern area named the Rincon Mountain District about 10 miles east of the city.

GEOLOGIC FEATURES (and MORE):

Saguaro Cactus, Detachment Fault, Metamorphic Core Complex (MCC), Rhyolite, Catalina Gneiss, Basin and Range Province, Laramide Orogeny

DESCRIPTION:

The Saguaro Cactus

The Saguaro is the largest cactus found in North America reaching a maximum height of 50 ft and weights greater than 7 tons. It is only found in the Sonoran Desert of southern Arizona, southeast California, and Sonora, Mexico. Plant life in the Sonoran Desert is largely due to its subtropical temperatures and two seasons of rainfall. Following a rainfall, each cactus, using its shallow roots, can soak up to 200 gallons of water which can supply water needed for up to a year.

Nearly 2 million Saguaro cacti are found within the park boundaries along with about 25 other species of cacti, the most diverse assemblage of cacti in the nation. However, cold temperatures can affect these numbers since temperatures below freezing for more than 20 hours can kill Saguaros.

The Saguaro is a slow-growing cactus, reaching 3 to 6 inches in 10 years, 3 feet in 25 years, 6 ft in 35 years, 16 ft between 60-75 years, and more than 40 feet tall in 150 years or more. Interestingly, The Saguaro only begin sprouting branches (arms) at about 75 years. Inside the Saguaro are “ribs” which are like the bones of the cactus allowing the cactus to reach such tall heights.

The Geologic Setting

The main attraction of the Park is the Saguaro Cactus, abundant in both Districts. If you are expecting a large number of rock exposures close to the park roads, then you will be disappointed. Though the geological history between the two Districts is quite different, it is not immediately evident from road outcrops. Please note that the detailed geologic history of this area in incredibly complex. For the purposes of this report, only a brief, larger scale geological history is provided here.

Within the Upper Mesozoic, during the Laramide Orogeny, subduction of the Pacific Plate was responsible for volcanic activity (Rhyolitic lava) within the area of the park that would become the western Tucson District. Several granitic bodies were also emplaced into the crust miles below the surface. The 1.7 byo Pinto Gneiss, scattered within the park, was thought to be part of this preexisting crustal material.

After the Laramide Orogeny, about 20-30 mya, stretching of the crust produced a Detachment Fault (The Catalina Fault) dipping gently to the west, 6-8 miles below the surface. Rocks above the fault, including the previously formed volcanics, then slid to the southwest while rocks below the fault, mainly granites, moved relatively up dip towards the northeast by 16-19 miles (and thus significantly closer to the surface). This is why rocks found in the Tucson District (mainly volcanics) were originally formed geographically to the east while granites, once buried deep and far to the west rose to become the Rincon and Catalina Mountains to the east. (These mountains are part of what is called a Metamorphic Core Complex.)

The incredible friction, pressure, and heat produced by fault movement, metamorphosed the granite into the metamorphic rock, a mylonitic gneiss (including the Catalina Gneiss), so prominent in the Rincon District.

Ultimately, a second period of crustal extension began about 15 mya which broke the crust into a series of blocks that were separated by steeply dipping normal faults. This was to become known across the western U.S. as the Basin and Range Province. The Tucson and Rincon Mountains represent two of these uplifted crustal blocks separated by a valley.

STUDENT QUESTIONS:

(1) Why are Saguaro cacti found in a geographically limited area?

(2) What is the age of the Laramide Orogeny? What does it represent in terms of plate tectonics?

(3) How does Rhyolite differ from Granite?

(4) How does a Detachment Fault differ from a Thrust Fault?

(5) How does Mylonitic Gneiss formed compared to other varieties of gneiss?

(6) CHALLENGE: How are detachment faults formed?

(7) CHALLENGE: What is a Metamorphic Core Complex?

(8) CHALLENGE: At the end of the Laramide Orogeny, crustal extension occurred. Why did compression stop?

SELECTED REFERENCES:

-Bezy, John V. 2016. A Guide to the Geology of the Santa Catalina Mountains, Arizona: The Geology and Life Zones of a Madrean Sky Island. Arizona Geological Survey, Down to Earth-22. Accessed on December 26, 2024 from https://docslib.org/doc/455463/a-guide-to-the-geology-of-the-santa-catalina-mountains-arizona-the-geology-and-life-zones-of-a-madrean-sky-island

-Friends of Saguaro National Park. Saguaros at the Park. Accessed on December 26, 2024 from https://www.friendsofsaguaro.org/saguaros

-NPS. Saguaro National Park, Geology of the Rincon Mountains. Updated June 22, 2023. Accessed on Dec. 26, 2024 from https://www.nps.gov/sagu/geology-of-the-tucson-mountains.htm

-NPS. Saguaro National Park, Geology of the Tucson Mountains. Updated January 19, 2024. Accessed on Dec. 26, 2024 from https://www.nps.gov/sagu/geology-of-the-tucson-mountains.htm

-NPS. Saguaro National Park, Geology of Saguaro National Park. Updated March 20, 2024. Accessed on Dec. 26, 2024 from https://www.nps.gov/sagu/geology-of-the-tucson-mountains.htm

-NPS. Saguaro National Park, Geology of Saguaro National Park. Updated March 20, 2024. Accessed on December 26, 2024 from https://home.nps.gov/sagu/learn/nature/geology.htm

-NPS. 1972. Saguaro National Park, Rocks: Foundation and Soilmakers. Natural History Series. Accessed on December 26, 2024 from https://www.nps.gov/parkhistory/online_books/natural/4b/nh4be.htm

Figure A - Saguaro National Park maps of West and East Districts

Figure B: The effects of Detachment Faulting and Block Faulting due to extension in the area of Saguaro NP. Note the movement of crustal blocks as denoted by the relative movement of letters "A" and "B" written above and below the detachment fault within the block diagrams.(Diagrams modified, in part, from NPS publications.)

PHOTOS:

Figure 1 - Saguaro Cactus found outside of the Rincon Mountain (East) Visitor Center. Saguaro cacti, the largest in North America, can reach up to 50 ft. in height and live more than 150 years.

Figure 2 - View from Future Generations Overlook (Rincon District). Tanque Verde Ridge of the Rincon Mountains is in the background. Prickly Pear Cactus is seen in the foreground. Over 25 species of cactus are found in the park.

Figure 3 - View from Cactus Forest Overlook (Rincon District). Agua Caliente Hill, found west of the Rincon Mountains, is in the background. In order to deter poaching, more than 1000 microchips have been inserted into Saguaros across the park.

Figure 4 - Tanque Verde Ridge looking east from Cactus Forest Overlook (Rincon District). This ridge is part of the Rincon Mountains, a granitic gneiss uplifted most recently during the formation of the Basin and Range Province.

Figure 5 - Branches (arms) of Saguaro cacti usually begin to form after 75 years of life. (Rincon District)

Figure 6 - Three common types of cactus found in the park - Prickly Pear (lower right), Chain Fruit Cholla (lower left), and Saguaro (branched and the yet-to-branch) (Rincon District)

Figure 7 - .Outcrop of gneiss exposed at the northern part of the Cactus Forest Drive (Rincon District). It is one of the few rock exposures found immediately adjacent to the park road.

Figure 8 - Gneiss formed from the metamorphism of Granite. Specimen from the outcrop pictured directly above.

Figure 9 - Javelina Rocks Overlook (Rincon District), an exposure of the Catalina Gneiss. Note the foliation characteristic of metamorphic rock.

Figure 10 - The Catalina Gneiss exposed at Javelina Outlook is a result of the metamorphism of granites found in the Rincon Mountains. The rock is called Mylonite, a type of metamorphic rock formed through intense shearing along the Catalina Detachment Fault.

Figure 11 - Mylonite sample displayed in the East Visitor Center. Mylonite is a fine-grained, ductilly deformed, metamorphic rock associated with the shearing and pressures of fault zones.

Figure 12 - A garden of Saguaro Cacti found outside the Tuscon Mountain (West) Visitor Center

Figure 13 - The Sus Picnic Area (Tucson Mt. District). Prickly Pear and Saguaro Cacti dominate the biology while the Tuscon Mountains fill the background. These mountains, along with the Rincon Mountains in the Eastern District, were uplifted during the block faulting episode forming the Basin and Range Province.

Figure 14 - Strip away the outer softer tissue of a Saguaro Cactus and you will find these internal "ribs" which act as bones, providing support needed to accommodate tall growth.

Figure 15 - Photo taken at the Ez-Kim-In-Zin Picnic Area.

Figure 16 - Signal Hill is known for its petroglyphs carved into Rhyolite, a volcanic rock associated with intense uppermost Cretaceous volcanic activity. As for the East District of Saguaro National Park, rock outcrops are not readily accessible immediately adjacent to the main park loop road. Signal Hill is located about a half mile off the main park road.