Geodiversity Atlas—Sonoran Desert Network Index

photo of a distant rocky desert peak with cactus and grassland in the foreground.
Coronado National Memorial, Arizona. View looking north towards Montezuma Peak, predominantly composed of the exposures of the Jurassic Huachuca Granite

Geology and Stratigraphy of the Sonoran Desert Network Parks

The Sonoran Desert Network Inventory & Monitoring Network consists of 10 park units in central and southern Arizona and one park unit in southwestern New Mexico. These units are characteristic of the upper Sonoran subdivision of the Sonoran Desert Ecoregion and the Apache Highlands Ecoregion, and include Casa Grande Ruins National Monument, Chiricahua National Monument, Coronado National Memorial, Fort Bowie National Historic Site, Gila Cliff Dwellings National Monument, Montezuma Castle National Monument, Organ Pipe Cactus National Monument, Saguaro National Park, Tonto National Monument, Tumacácori National Monument, and Tuzigoot National Monument. The parks that comprise the Sonoran Desert Network protect a combined 179,801 hectares (444,299 acres) of wilderness and vary in size from 145 hectares (360 acres) in Tumacácori to 133,825 hectares (330,689 acres) in Organ Pipe Cactus.

The park units of the Sonoran Desert Network are composed of a complex assemblage of sedimentary, igneous, and metamorphic rocks with widely varying ages, from approximately 2-billion-year-old Precambrian surface exposures to relatively recent (ca. 700 CE) volcanic rocks in the Pinacate region near the international border. During the Cenozoic Era, significant changes in plate motion occurred from the Eocene through the Miocene ~35–15 Ma (mega-annum, million years ago) that caused the underlying crust of western North America to stretch, thin, and break along low-angle fractures called detachment faults (Bezy 2005). During this time, numerous volcanoes were active in the Sonoran Desert, resulting in calderas (collapsed magma chambers) such as the Turkey Creek caldera of Chiricahua, Montezuma caldera of Coronado National Memorial, Gila Cliffs Dwelling caldera of Gila Cliff Dwellings National Monument, and Tucson Mountains caldera of Saguaro; lava vents; and cinder cones. Large-scale regional extension, coupled with intense heat from below, produced uplifts of highly sheared and metamorphosed rock called metamorphic core complexes and placed tremendous stress on the underlying crust of the Sonoran Desert region (Bezy 2005).

The modern landscape of the Basin and Range province began developing during the Miocene Epoch (~12–5 Ma) as extensive tectonic forces continued to stretch and thin the continental crust, producing large-scale normal faults (Bezy 2005; Graham 2011c). The result was roughly parallel, fault-bounded mountain ranges (horsts), many approaching elevations of 3,048 m (10,000 ft), separated by broad valleys (grabens) flanked by broad sloping “bajadas” of coalesced alluvial fans. The precipitous topographic relief of the Basin and Range landscape provides for drastic differences in climate along slopes, with the relatively cool and moist summits containing biological communities more characteristic of Canada than those of the valley bottoms below. These montane habitats are termed “sky islands”, because they rise above the desert floor to resemble islands surrounded by a sea (Graham 2011a). These “sky islands” are analogous to actual marine islands from the perspective of biogeography, speciation, and landscape connectivity (Scarborough and Brusca 2015). In combination with the bimodal precipitation regime and mid-continental position, the tremendous topographic variation over short distances helps create the amazing natural and geological diversity of the Sonoran Desert.

A Brief Geologic History—Sonoran Desert Network

A few examples of events and Network resources in each geologic time period are highlighted below, from youngest to oldest.

Cenozoic deposits are mapped in every park unit of the Sonoran Desert Network except Fort Bowie. The oldest Cenozoic rocks include Oligocene igneous and volcaniclastic sedimentary rocks such as the Faraway Formation, Jesse James Canyon Tuff, and Rhyolite Canyon Tuff in Chiricahua; the Bloodgood Canyon Tuff, Bearwallow Mountain Andesite, and Gila Conglomerate in Gila Cliff Dwellings; and the Safford Dacite and Pantano Formation in Saguaro. A diverse sequence of Miocene rocks underlies Organ Pipe Cactus and includes the Childs Latite, Batamote Andesite complex, Daniels Conglomerate, Growler Mountain Rhyolite, rhyolite of Montezuma’s Head, and rhyolite of Pinkley Peak. Rocks of the Miocene–Pliocene Verde Formation are a major bedrock constituent in both Montezuma Castle and Tuzigoot.

Quaternary surficial deposits are widely mapped in the park units of the Sonoran Desert Network and include alluvial fan deposits (Chiricahua, Saguaro), alluvium and colluvium (Casa Grande Ruins, Chiricahua, Coronado National Memorial, Gila Cliff Dwellings, Montezuma Castle, Organ Pipe Cactus, Saguaro, Tonto National Monument, Tumacácori, Tuzigoot), debris flows (Coronado National Memorial, Tonto National Monument, Tumacácori), river terraces (Casa Grande Ruins, Gila Cliff Dwellings, Tumacácori, Tuzigoot), and terrace gravels (Coronado National Memorial, Montezuma Castle).
Mesozoic strata occur in six park units of the Sonoran Desert Network, with some of the oldest rocks consisting of the Triassic–Jurassic Recreation Red Beds in Saguaro. Jurassic units include the Huachuca Granite in Coronado National Memorial, rocks of La Abra and conglomerate of Scarface Mountain in Organ Pipe Cactus, and quartz monzonite of Mount Benedict in Tumacácori. Strata associated with the Cretaceous Bisbee Group (Morita Formation, Glance Conglomerate, Cintura Formation, Mural Limestone) underlie Chiricahua, Coronado National Memorial, Fort Bowie, and Saguaro. Other Cretaceous formations include the Gunsight Hills Granodiorite, Aguajita Spring Granite, and Bandeja Well Granodiorite in Organ Pipe Cactus; Cat Mountain Tuff, Wrong Mountain Quartz Monzonite, Amole Arkose, Amole plutonic rocks, and volcanics of Yuma Mine in Saguaro; and the Salero Formation in Tumacácori.
Paleozoic strata are found at four of the park units of the Sonoran Desert Network, with some of the oldest rocks represented by the Cambrian Coronado Sandstone in Fort Bowie and Cambrian Bolsa Quartzite in Saguaro. Ordovician and Devonian units in Fort Bowie include the El Paso and Portal Formations. The Mississippian Escabrosa Limestone is mapped in both Fort Bowie and Saguaro. Sedimentary strata of the Pennsylvanian–Permian Naco Group (Horquilla Limestone, Earp Formation, Scherrer Formation, Colina Limestone, Epitaph Dolomite) are widely distributed, occurring in Chiricahua, Coronado National Memorial, Fort Bowie, and Saguaro.
Rocks of Precambrian age are limited to three park units of the Sonoran Desert Network, with some of the oldest units represented by Paleoproterozoic rocks of the Pinal Schist in Saguaro and granitic rocks of Cottonwood Creek in Tonto National Monument. Rocks of Mesoproterozoic age include the Dripping Spring Quartzite of Saguaro and Tonto National Monument, the Continental Granodiorite and the granodiorite of Rincon Valley in Saguaro, and the Mescal Formation in Tonto National Monument. Neoproterozoic units include granodiorite stocks and aplite intrusions that underlie Fort Bowie.

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    Type Sections—Sonoran Desert Network

    small image of a report cover with a tiny color photo

    The geologic history above is excerpted from a report titled, "National Park Service geologic type section inventory: Sonoran Desert Inventory & Monitoring Network". Type sections are essential reference locations for the geoscientists who study geologic history and paleontology. A summary of the type sections in each park can be found at the links below.

    • Casa Grande Ruins National Monument, Arizona (no designated stratotypes identified)

    • Chiricahua National Monument, Arizona [Site Under Development]

    • Coronado National Memorial, Arizona [Site Under Development]

    • Fort Bowie National Historic Site, Arizona (no designated stratotypes identified)

    • Gila Cliff Dwellings National Monument, New Mexico [Site Under Development]

    • Montezuma Castle National Monument, Arizona (no designated stratotypes identified)

    • Organ Pipe Cactus National Monument, Arizona (no designated stratotypes identified)

    • Saguaro National Park, Arizona [Site Under Development]

    • Tonto National Monument, Arizona (no designated stratotypes identified)

    • Tumacácori National Historical Park, Arizona (no designated stratotypes identified)

    • Tuzigoot National Monument, Arizona (no designated stratotypes identified)

    The full Network report is available in digital format from:

    Please cite this publication as:

    • Henderson TC, Santucci VL, Connors T, Tweet JS. National Park Service geologic type section inventory: Sonoran Desert Inventory & Monitoring Network. Natural Resource Report. NPS/SODN/NRR—2022/2453. National Park Service. Fort Collins, Colorado.

    NPS Stratotype Inventory

    Last updated: August 25, 2023

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