Geologic Field Guides to the Southeastern Picacho Mountains and Picacho Peak, Pinal County, Arizona

If you drive Interstate 10 between Tucson and Phoenix, about half way you pass between the Picacho Mountains (on the northeast side) and Picacho Peak (on the southwest side). Picacho Peak State Park is a frequent destination for picnics, rock climbing, and viewing spring wildflowers.

The Arizona Geological Survey has recently made available for free download Geologic Field Guides to the Southeastern Picacho Mountains and Picacho Peak. (Link)

From the guide:

The Picacho Mountains consist largely of a compositionally diverse suite of Laramide to middle Tertiary biotite granite, muscovite granite, and heterogeneous to gneissic granite. At the southern end of the range, most of the crystalline rocks have been affected by middle Tertiary mylonitic deformation. Mylonitization is inferred to have accompanied normal faulting and ascent of the bedrock from mid-crustal depths to near the Earth’s surface. [Mylonitization is modification due to dynamic recrystallization following plastic flow.]

Ascent occurred in the footwall of a moderate to low-angle normal fault commonly known as a “detachment fault”. The crystalline rocks of the Picacho Mountains are part of the footwall of a south- to southwest-dipping detachment fault that is exposed only at the base of a small klippe of volcanic rock on a hill top in the southeastern Picacho Mountains. [A klippe is an isolated block of rock separated from the underlying rocks by a fault.]

Picacho Peak, itself, looks like the remnant of a volcano. However, it is an erosional remnant of volcanic rocks that were displaced from over the Picacho Mountains by a detachment fault.

Picacho Peak is composed of multiple andesitic lava flows interbedded with thin sequences of medium- to thin-bedded, well-sorted, medium- to coarse-grained arkosic sandstone and granule sandstone. See the guide for detailed descriptions.

Earth fissures occur in the Picacho Peak area due to groundwater withdrawal and soil compaction. (See: Earth Fissure Map of the Picacho & Friendly Corners Study Area)

For some spectacular views, see a drone flight around Picacho Peak (3 minutes):


See also:

Picacho Peak weather station – how not to measure temperature


Rocks in the Chiricahua National Monument and Fort Bowie National Historic Site

The Arizona Geological Survey has made available for free download a 48-page booklet which explains geologic features of two areas of southeastern Arizona. This well-illustrated booklet is intended for the layman but is also interesting to experienced geologists.

The Chiricahua mountains are known for their thousands of rock pinnacles formed in volcanic rocks by weathering during the last glacial epoch. In a previous article, I explain “The Explosive Geology Of The Chiricahua Mountains.”



The new booklet provides photos and descriptions that allow you to find many volcanic features along major roads and trails in the monument. None of these features are designated by markers along the trail.

The geology of Fort Bowie is quite different and consists of sediments and granite intrusive rocks.

As described in the booklet:

“Fort Bowie was built to guard Apache Pass, a natural passage between the Dos Cabezas and Chiricahua Mountains that connects the San Simon and Sulphur Springs Valleys. The

dependable springs, including Apache Spring, that have attracted humans to this narrow passage for thousands of years are also the result of geology, specifically the Apache Pass fault.”

The booklet starts out by describing the past 30 million years of geologic history which includes plate tectonics and volcanism – the processes which gave rise to the features visible today.

The features of the Chiricahua Mountains are put in context as follows:

“The landscape of Chiricahua National Monument, like that of much of the Earth’s surface, is a complex mosaic of large and small geologic features. Some of these features were produced by processes that were more active in past geological time but have now slowed or ceased. Other features are the result of past and currently active processes; only a few owe their origin solely to recently active processes. Welded tuff, fiamme, surge beds, fossil fumaroles, and the dacite caprock, for example, were all produced during the eruption of the Turkey Creek caldera, about 27 million years ago. Some joints and spherulites formed as the ash sheet cooled. Other joints and the region’s mountain ranges and intervening basins are the result of Basin and Range faulting during the period 25 to 5 million years ago. Willcox Playa, talus cones, pinnacles, and slot canyons were produced by processes that were more active during the wetter, cooler climate of the glacial epochs from 1.6 million to 10,000 years ago. Tafoni, rock varnish, lichens, case-hardened surfaces, chicken heads, exfoliation shingles, horizontal ribs, solution ponds and the rounded form of the columns are all forming today. Unraveling the evolution of such complex landscapes makes geology a particularly challenging science.” If you are unfamiliar with some of the names of features, read the booklet.

URL to the booklet: http://repository.azgs.az.gov/sites/default/files/dlio/files/nid1731/dte-11chirichua_mtns.pdf


Tucson Mountains geology – an update


The Tucson Mountains form the rampart on the west side of the city. If you drive over Gates Pass, take a look at the road cuts, especially near the top, you will see a chaotic jumble of different rocks, mainly volcanics. According to the Arizona Geological Survey: “Tucson Mountain Chaos is a formal geologic name, describing one of the more confusing, complex, and controversial areas in southern Arizona.”

Like many mountain ranges in Southern Arizona, the Tucson Mountains have experienced several episodes of volcanic eruption. Major eruptions occurred during Triassic-Jurassic time (~190-200 Ma), early Laramide (74 Ma), later Laramide (62 Ma), and one late Tertiary (~20 Ma). (Ma means million years ago.) There were also several interspersed minor eruptions.

The early Laramide (74 Ma) eruption was very explosive and produced great volumes of rhyolite tuff (Cat Mountain tuff). The rapid eruption caused the volcano to collapse in on itself to form a caldera. That collapse produced megabreccia called the Tucson Mountain Chaos. (Breccia is simply a bunch of angular fragments cemented together.) Within that breccia are small to very large fragments of other rocks including house-sized blocks of limestone. The breccia could have formed in three ways (and there are proponents of each way): moat in-filling of the caldera, landslides, or fluidized material brought up from below.

TM geo map3The Tucson Mountain caldera is not a typical caldera with equal subsidence all around. Rather, it is a “trap door” caldera with the “hinge” area on the southeast and major subsidence on the west. The western ring fault (called the Museum Fault) parallels Kinney Road from about Old Tucson to just past the Arizona-Sonora Desert Museum, then swings east around a granite pluton. This structure was first proposed by Peter Lipman of the United States Geological Survey in 1994 and later supported by geophysical investigations.

Beginning about 25 to 30 million years ago, Arizona and the West experienced crustal stretching which began to tear things apart. It was proposed, about 10 years ago, that the Tucson Mountain volcano and caldera formed over where the Santa Catalina Mountains now stand on the east side of Tucson. It was posited that crustal stretching slid the caldera to its present location. (You can see an explanation and cross-sections of that story in a 2009 article from my Wryheat blog.) That was such a neat story that the Arizona-Sonora Desert Museum constructed a mechanical model which Docents (including me) used to interpret the story. That particular “kit” has been retired because subsequent evidence shows that the story is probably in error. The main evidence against the sliding story is that the chemistry of the volcanics in the Tucson Mountains is incompatible with the proposed generating pluton in the Santa Catalina Mountains. There are also some structural inconsistencies.

TM section

One other thing: there was a Tucson Mountain dinosaur. Dinosaur bones were found within one of the blocks of megabreccia about 1800 feet NNW of Gates Pass. This dinosaur is classified as a large Hadrosaur (duck-billed dinosaur). This dinosaur lived in Tucson some time between 72 and 83 million years ago.


Kring, D.A., 2002, Desert Heat – Volcanic Fire, The Geologic History of the Tucson Mountains and Southern Arizona, Arizona Geological Society Digest 21

Lipman, Peter, 1993, Geologic map of the Tucson Mountains Caldera, southern Arizona, U.S.G.S. IMAP 2205. (link)

Lipman Peter W., 1994: Tucson Mountains caldera; a Cretaceous ash-flow caldera in southern Arizona. U S (link)

Marshall, L. and Stokes, P., 2012, the Tucson Mountains Caldera: Using Gravity and Magnetic Anomalies to Test Trapdoor Subsidence and Locate Subsurface Plutonic Bodies. (link)

Spencer, G.L. et al., 2005, The late Cretaceous Tucson Mountains dinosaur, New Mexico Museum of Natural History and Science Bulletin, 29 (link)

See also:

Arizona Geologic History: Chapter 1, Precambrian Time When Arizona was at the South Pole

Arizona Geological History: Chapter 2, Cambrian and Ordovician Time

Arizona Geological History: Chapter 3: Devonian to Permian Time

Arizona Geological History Chapter 4: Triassic Period

Arizona Geological History Chapter 5: Jurassic Time

Arizona Geological History Chapter 6, The Cretaceous Period

Arizona Geological History Chapter 7: The Cenozoic Era

Old mines of the Tucson Mountains

New Release – AZGS Guides to Northern Arizona Geology

The Arizona Geological Survey has just released an electronic version of a classical work for free download: “Geology of Northern Arizona – with notes on archaeology and paleoclimatehttp://repository.azgs.az.gov/uri_gin/azgs/dlio/1647

This is a two-volume set originally published in 1974, contains about 800 pages. Here are the contents:

Volume 1
Geology of northern Arizona with notes on archaeology and paleoclimate, pt. 1, Regional studies p. 1-407


A preliminary report on the older Precambrian rocks in the upper Granite Gorge of the Grand Canyon

Preliminary report on the Unkar Group (Precambrian) in Grand Canyon, Arizona

The late Precambrian Chuar Group of the eastern Grand Canyon

Upper Precambrian igneous rocks of the Grand Canyon, Arizona

Rb-Sr age of the Cardenas Lavas, Grand Canyon, Arizona

Precambrian polar wandering from Unkar Group and Nankoweap Formation, eastern Grand Canyon, Arizona


Paleozoic rocks of Grand Canyon

The Toroweap Formation: A new look


Mesozoic stratigraphy of northeastern Arizona

Mesozoic vertebrates of northern Arizona


K-Ar chronology for the San Francisco volcanic field and rate of erosion of the Little Colorado River

Cenozoic volcanism and tectonism of the southern Colorado Plateau
Southwest paleoclimate and continental correlations

A resume of the archaeology of northern Arizona


Synopsis of the Laramide and post-Laramide structural geology of the eastern Grand Canyon, Arizona

Structural evolution of northwest Arizona and its relation to adjacent Basin and Range province structures

The Bright Angel and Mesa Butte fault systems of northern Arizona


Review of the development of oil and gas resources of northern Arizona

Volume 2:
Geology of northern Arizona with notes on archaeology and paleoclimate, pt. 2 Area studies and field guides


River guide of Colorado River–Lee’s Ferry to Phantom Ranch

Kaibab trail guide to the southern part of Grand Canyon, northern Arizona


Geologic resume and field guide, north-central Arizona

Interference and gravity tectonics in the Gray Mountain area, Arizona

Geology of Shadow Mountain, Arizona


Geo1ogy of the eastern and northern parts of the San Francisco volcanic field, Arizona

Field guide to the geology of the San Francisco volcanic field, Arizona

Geology of the Elden Mountain area, Arizona

Xenoliths of the San Francisco volcanic field, northern Arizona


The volcanic history of the San Francisco Mountain, northern Arizona

Glacial and pre-glacial deposits in the San Francisco Mountain area, northern Arizona

Field guide to the geology of the San Francisco Mountain, northern Arizona

Phreatomagmatic deposits of the Sugarloaf tephra, San Francisco Mountain, northern Arizona

Preliminary geochemical study of the lava flows of Humphrey’s Peak, San Francisco Mountain, northern Arizona


Miocene-Pliocene volcanism in the Hackberry Mountain area and evolution of the Verde Valley, north-central Arizona

Paleontology, biostratigraphy, and paleoecology of the Verde Formation of Late Cenozoic age, north-central Arizona

Field guide for southeast Verde Valley–northern Hackberry Mountain area, north-central Arizona,


The geology of Hopi Buttes, Arizona

The Buell Park kimberlite pipe, northeastern Arizona

Field guide for Hopi Buttes and Navajo Buttes area, Arizona

Field guide for the Black Mesa–Little Colorado River area, northeastern Arizona

Ground water in the Navajo Sandstone in the Black Mesa area, Arizona

Paleoenvironmental and cultural changes in the Black Mesa region, northeastern Arizona


Economic geology and field guide for the Jerome district

Other new releases:


Field Guide – Oak Creek-Mormon Lake Graben Northern Arizona

The Oak Creek-Mormon Lake Graben lies between Flagstaff and Sedona, Arizona. The Arizona Geological Society and geologist Paul A. Lindberg have produced a 13-page field guide to the geology of the area (shown on the map below).

This geologic field trip guide circumnavigates a loop of ~120 miles from Flagstaff to Sedona along Highway 89A and returns to Flagstaff along the Lake Mary Road. The guide contains many illustrations and photographs and may be downloaded from:

Lindberg introduces us to the local geological setting:

“The Oak Creek-Mormon Lake graben (a rift valley formed by extension of the earth’s crust) has been faulted into the southwestern margin of the Colorado Plateau as basin and range crustal extension has migrated eastward across Western U.S. over time. The graben may be as young as 2-3 million years old, based upon the youthful appearance of numerous V-shaped canyons (Oak Creek, West Fork, Munds, Woods and Rattlesnake Canyons) that cut the minimally eroded original surface of the largely basalt covered core of the graben. That morphology is in sharp contrast to more maturely eroded landforms along the northeast margin of 10 Ma Verde graben near Sedona. Timing of the genesis of the Oak Creek-Mormon Lake graben may be contemporaneous with the main eruptive cycle of San Francisco Peaks north of Flagstaff, Arizona.”

The 12 geologic stops focus on recent faulting and the encroachment of Basin and Range extensional structures on the Colorado Plateau. Each stop is detailed in the text, which is amply illustrated with photographs and colored geologic sketches.

Oak Creek Graben map

What Lies Beneath the Tucson Valley

The deposits within the Tucson Valley record at least 145 million years of geologic history. The Tucson Valley was formed by crustal extension beginning about 25 million years ago. That stretching transported a volcano across what is now the valley and those volcanics form the Tucson Mountains. Several times, the valley contained lakes, and at least twice it was buried in volcanic ash. For the story of how the valley formed, see my article: Tucson Mountain Chaos.

Southern Arizona contains many deep alluvial valleys, with bedrock many thousands of feet below the valley floor.. The Arizona Geological Survey has published a map, “Estimated Depth to Bedrock in Arizona” (DGM-52) which shows the valley patterns and depths statewide. In the case of the Tucson Valley, however, we don’t need to estimate the depth because in 1972, Exxon drilled an exploration hole which penetrated 12,556 feet and reached granite bedrock at 12,001 feet. (USGS Scientific Investigations Report 2004-5076).The location is shown on the Landsat photo below. Notice the linear, northeast-trending structures on the right side of the picture. These are large folds called synforms or synclines in the Catalina-Rincon Mountains metamorphic complex (see second graphic below). These synforms coincide with the deepest parts or sub-basins of the valley.


TVsynforms-98x150The upper 1,200 feet of the valley contain unconsolidated gravels derived from alluvial fans that contain the aquifers from which we pump part of our water supply. There are deeper aquifers as yet unexploited, but the water in deeper aquifers becomes laden with dissolved salts and metals. There are several volcanic ash beds between 1,150 and 1,350 feet. Below 2,000 feet are remnants of playa lakes with deposits of gypsum.

At 2,980 feet, there is a sharp boundary between the upper unconsolidated and undeformed alluvial sediments and denser, highly faulted basin fill indicating a change in tectonic style.

The sandstones and siltstones from 2,980 to 3,840 feet are interpreted to represent deposits from a braided stream. Below that, to 6,170 feet are more alluvial fan deposits.

The interval between 6,170 and 8,256 is called the Pantano Formation. It consists of alluvial deposits, lake beds, lava flows, and rock avalanche deposits. An andesite flow near the middle has been dated at 26 million years old. The avalanche deposits are similar to modern debris flows that occur on the slopes of the Catalina Mountains. However, the rock avalanche deposits in the drill hole are composed mainly of volcanic rocks that had a source east of the Catalinas. This evidence is consistent with the theory that the volcanics of the Tucson Mountains were transported from somewhere over or east of the Catalina Mountains.

The interval 8,256 to 10,026 consists of Mid-Tertiary aged volcanic and sedimentary rocks. The volcanics include both lava flows and ash deposits.

The interval 10,026 to 12,001 contain the Lower Cretaceous to Upper Jurassic marine sediments (sandstone, limestone, conglomerate) of the Bisbee Group (so named because it was first described from outcrops near Bisbee, AZ). In Bisbee, these rocks form the mountains at an elevation of 5,000 feet, but in Tucson they are two miles beneath the surface. Below the Bisbee Group is granite at least 138 million years old and more likely Precambrian-age, 1.4 billion years old.

Even though the Exxon well went to 12,556 feet it did not reach the underlying detachment fault which transported the Tucson Mountain volcanics to their present position, probably because the detachment fault was itself offset by younger, steep faults bounding the valley. The detachment fault crops out along the Catalina foothills.


I have so far described the rocks encountered in the Exxon hole from top to bottom, from youngest to oldest. So let’s flip things around and tell the story in chronological order.

Paleozoic marine sediments were deposited upon Precambrian granites. Some time prior to latest Jurassic time they were eroded away (since they don’t appear in the Exxon hole but do appear in the surrounding region.) Beginning in latest Jurassic time and continuing through the Cretaceous, northeast-southwest extension created the Bisbee Basin into which the marine sediments of the Bisbee Group were deposited.

There may have been some erosion along a sea shore as evidenced by coastal plain deposits. In mid-Tertiary time alluvial fan deposits indicate that surrounding mountains were eroding. These deposits are interspersed with lava flows. A violent volcanic eruption 26 million years ago deposited an ash in the basin (8,500 to 9,000 feet in the hole). By this time crustal extension was deepening the basin and accelerating denuding of the surrounding mountains and filling the basins with alluvial fan material, i.e., rocks and soil.

 Now, when we look out at the valley and see the city and the mountains, we see just a short slice of time in its history. And now you know what lies beneath the valley.