The northwestern margin of the Santa Catalina Mountains, located within Catalina State Park and the Coronado National Forest, contains a variety of spectacular geologic features. Because of the relatively sparse desert vegetation, all of them are easy to recognize and photograph. Some of these features occur throughout much of the western United States.
The Santa Catalina Mountains and the adjacent Oro Valley, Tucson, and San Pedro basins are typical of the topography that dominates much of western North America from central Mexico to Oregon. East-west stretching of this part of the continent between 25 and 5 million years ago caused the Earth’s crust to thin and break along faults (cracks) into long, north-and northwest-trending blocks. Some of these blocks were uplifted to form mountain ranges, such as the Santa Catalinas; other blocks were down-dropped to produce the adjacent basins. Geologists, therefore, refer to this vast region as the Basin and Range geologic province. The Santa Catalina Range is bordered by normal faults—deep, steeply sloping cracks in the Earth’s crust along which movement, primarily vertical, has occurred. This western side of the range is bounded by one such fault, which lies at the base of the mountain. Rocks west of this fault dropped an estimated 10,000 to 13,000 feet relative to those in the Santa Catalinas east of the fault.
Geologic evidence indicates there has been no movement along this fault in the last 5 million years. The bedrock of this part of the Santa Catalina Mountains is mainly the Oracle, Catalina, and Wilderness Suite granites. These granites were injected deep within the Earth’s crust as great molten masses. The molten rock, which cooled and solidified over millions of years and miles below the Earth’s surface, has since been exposed by erosion. These three granites have weathered and eroded into very different landscapes. Each landscape is a unique combination of landforms, which have developed in many cases because of differences in the hardness and fracture (joint) patterns of the bedrock.
Many of the geologic features you see are granitic landforms, some massive and others small-scale, that together make up these distinctive landscapes. Erosion was wearing back the western face of the Santa Catalina Mountains even as the range was being uplifted. About one million years ago today’s landscape began to take its present form. Throughout much of southern Arizona rivers were extending their headwaters into higher basins that previously had no outlets to the sea. These expanding streams removed much of the sedimentary fill and altered and integrated the drainage of the basins they invaded.
These processes produced particularly dramatic results in the Cañada del Oro Basin. Prior to this time the Cañada del Oro exited the heart of the Santa Catalina Mountains near the present town of Oracle and drained to the northwest. A more rapidly expanding stream, eroding along the shattered rock zone of fault at the base of the mountains, began to extend its headwaters northward from the Tucson Basin. This new stream cut its valley through the alluvial fan sediment that filled this part of the basin and reexposed the western margin of the Santa Catalina Mountains. In time, this stream intercepted the ancestral Cañada del Oro and caused it to flow south, a geologic process called “pirating”. The newly formed Cañada del Oro underwent at least four major episodes of down-cutting and left remnants of its former floodplains as part of today’s landscape. Called stream terraces, these stepped surfaces range in age from 500,000-4,000 years. The present landscape is a mosaic of exhumed pediments, stream terraces, remnants of alluvial fans, and the embayed front of the range—all produced over a span of at least five million years. These are typical features of the Tucson Basin and the much larger Basin and Range province of the western United States.
This cache is located in geocache-friendly Catalina State Park, along the Canyon Loop Trail. An entrance fee or annual pass is required to enter the park. Permission to place the cache was given by Catalina State Park, Steve Haas, Park Manager (520-628-5798). Be aware of normal desert hazards.
Questions: To receive credit for this earthcache, please send answers to the following questions to the CO. You may log your cache immediately.
Q1 – Using the information on the plaque in front of you, describe what geologic process caused the Catalina Mountains to rise to their current height?
Q2 – What geologic process caused the Tucson Valley to form, creating the steep mountain face to the east?
Q3 – What geologic processes filled in the Tucson Valley floor and created the Sutherland Wash in front of you?
Q4 – Looking at the stream in front of you, what evidence do you see of recent sedimentation and erosion in the area?
Optional – include a picture of the mountains on the day of your visit when you log the cache.
Source material:
Material for this Earthcache is extracted from “A Guide to the Geology of Catalina State Park and the Western Santa Catalina Mountain,” by John V. Bezy, National Park Service. Published by the Arizona Geological Survey. Paper copies are available at the park entrance station. The free EBook may be downloaded at:
http://repository.azgs.az.gov/sites/default/files/dlio/files/nid1453/dte-12_e-book_june2011.pdf
Another excellent reference is “A Natural History of the Santa Catalina Mountains, Arizona; with an Introduction to the Madrean Sky Islands,” by Richard C. Brusca , Wendy Moore and Bill Broyles, Arizona-Sonora Desert Museum Press. Available at Amazon.com
A good general reference to this geology is “Basin and Range,” by John McPhee, Farrar, Straus and Giroux, 1981.