The Geology of Saguaro National Park: 1.6 Billion Years Around Tucson

The park

Saguaro National Park has two separate districts — the Rincon Mountain District and the Tucson Mountain District — on either side of Tucson, in Pima County, Arizona. Proclaimed a national monument on March 1, 1933 and redesignated on October 4, 1994, it covers about 37,005 hectares (91,442 acres). It protects the giant saguaro — the tallest cactus in the United States and a worldwide symbol of the Southwest — along with the Sonoran Desert's varied life, mountain ecosystems, Rincon Creek, and cultural sites spanning prehistoric to modern times.

The park has one of the desert Southwest's largest concentrations of rare aquatic microhabitats — tinajas, seeps and springs — that support rare species such as the lowland leopard frog, yellow-billed cuckoo, gray hawk and southwestern willow flycatcher, and trees unusual in the desert, such as sycamore and ash.

1.6 billion years in four chapters

The park's rocks record about 1.6 billion years:

1. Precambrian — the continent takes shape;
2. Paleozoic — the ocean covers southeast Arizona;
3. Mesozoic — volcanic upheaval and mountain building;
4. Cenozoic — continental rifting.

Two districts, two stories. The Rincon Mountain District holds some of the oldest rocks — Precambrian igneous and metamorphic units such as the Pinal Schist, Wrong Mountain Quartz Monzonite, Continental Granodiorite and the Dripping Spring Formation. The Tucson Mountain District is mostly Cretaceous: the Cat Mountain Tuff, Amole Arkose, Amole pluton and the volcanics of Yuma Mine. Both districts are mantled with young alluvial fans, alluvium, colluvium and terrace gravels.

Features to look for

• Tucson Mountains Caldera — the eroded remains of a colossal eruption about 70 to 75 million years ago.
• A metamorphic core complex — the Rincon Mountain District is one of western North America's best exposures of this feature, unique to the North American Cordillera: metamorphosed basement rock beneath unmetamorphosed layers. Outcrops along Cactus Forest Drive are its "showpiece," and the district's geology helped geologists define what these complexes are.
• Desert landforms in both districts — pediments, alluvial fans, bajadas and rock varnish.
• Tinajas — short-lived pools in loose sediment or bedrock, vital water for wildlife, especially the leopard frog.

Fossils and caves

• Cambrian marine fossils — brachiopods and trilobites — have been found, though they are scarce. Packrat middens help scientists reconstruct the region's past environments, and new finds are possible. Park fossils are protected by the Paleontological Resources Preservation Act of 2009.
• Tilted beds of Horquilla limestone (Pennsylvanian–Permian, 320 to 245 million years old) and Escabrosa limestone (Mississippian, 359 to 323 million years old) crop out in the Tucson Mountains, where one cave has been reported. Caves are protected by the Federal Cave Resources Protection Act of 1988.

Hazards

Floods and debris flows have been major problems and will likely remain so. In the summers of 2005 and 2006, events expected only once in hundreds to more than a thousand years struck. Debris flows are more typical after fire strips vegetation, but 2006 showed they can happen without fire. They can wash out roads, forcing closures, speed erosion, endanger visitors, and fill tinajas with sediment.

Old mines

Saguaro has a rich mining history. The Park Service lists 177 abandoned mine sites with 541 features, mostly underground workings for gold, silver, copper and lead. They can be important cultural resources and habitat, but many are hazardous — unstable, and with metal contaminants that degrade water and landscapes. Stay out and stay alive.

Overview of the digital geologic data for Saguaro, from the Geologic Resources Inventory. NPS figure.

Learn more

Saguaro belongs to the Basin and Range physiographic province. The Park Service's Geologic Resources Inventory has produced maps, a report and scoping summaries for the park (NPS DataStore), and a Soil Resources Inventory is complete. See also the park's nature and science pages.

Saguaro National Park entrance. Image from the NPS's page.

Sources

Based on "NPS Geodiversity Atlas—Saguaro National Park, Arizona," National Park Service, drawing on Graham (2010), Tweet and others (2008) and other works it cites; a work of the United States government in the public domain. The source calls the Rincon Mountain District western and the Tucson Mountain District eastern; this page leaves the directions out.