Arizona Geological History Chapter 7: The Cenozoic Era

Cenozoic Era, Arizona was squeezed, then stretched; steamed and frozen.

The Cenozoic era represents the most recent 65 million years. (See the geologic time chart for the subdivisions.)

Paleomap 50


Construction of the Rocky Mountains, volcanism, and emplacement of our major copper deposits, all of which began in Cretaceous time,  continued in the Cenozoic Era until about 40 million years ago.   During this time, the oceanic crust of the Pacific Ocean was being subducted beneath the westward-moving North American continental plate.  The resulting compression caused southern and western Arizona to be topographically higher than the Colorado Plateau, the opposite of current topography.

The compression produced large thrust faults which led to mountain building. The Front Range of the Rocky Mountains in Colorado has a structure similar to the diagram below.




By about 20 million years ago, Arizona was covered with thousands of feet of volcanic rocks, locally punctured by calderas.

The photo below (from the Arizona Geological Survey) shows erosional remnants of a volcanic ash-flow in the Chiricahua Mountains. These rocks were expelled from the Turkey Creek caldera 27 million years ago. The spire forms, called “hoodoos,” result from mass wasting by ice and water.



Sometime between 30- and 20 million years ago the north American tectonic plate overrode a spreading center called the East Pacific Rise. This area is similar to the spreading center of the Mid-Atlantic ridge that gradually separated Africa from South American, and Europe from North America. Today, this western spreading center runs up the Gulf of California and separates Baja from mainland Mexico. It is also the driver of the San Andreas fault in California. By over-riding the spreading center, the tectonic regime changed from compression to extension. Arizona began to be pulled apart to form the Basin and Range physiography of today.

Initially, crustal extension was characterized by widespread normal faulting and fault-block rotation. Movement occurred along high-angle normal faults some of which may flatten at depth into low-angle detachment faults. Later extension resulted in high-angle faults which bound our valleys and make some of the valleys as much as 15,000 feet deep to bedrock.


Detachment fault from Arizona Geology

All of this faulting sometimes makes the life of exploration geologists very interesting when hunting for porphyry copper deposits, because some of those deposits were cut and fanned out like a deck of cards. Finding all the pieces takes some geologic detective work.

Perhaps the most famous local case of geological detective work is that of John Guilbert and David Lowell who studied the San Manuel mine north of Tucson. They noticed that the arrangement of mineralization and alteration formed shells around the generating intrusive. But the model they constructed implied that the deposit was lying on its side, and half of it was missing. It was removed by faulting. By applying their model, Lowell and Guilbert found the other half.

South of Tucson, the Mission-Pima mine and the San Xavier mine seem to be slices removed from top of the Twin Buttes deposit by low-angle faulting.. The Sierrita mine, located on the opposite side of a major high-angle fault from Twin Buttes is still intact (we think).

Middle Cenozoic veins host gold, silver, and base-metal deposits. Copper-gold mineralization is associated with the detachment faults. Manganese and uranium deposits occur in the basins resulting from the extension.

Volcanic activity resumed 2- to 3 million years ago with eruption of basalt which produced flows and cinder cones (see map below). The rocks of the San Francisco volcanic field near Flagstaff, the Springerville-Show Low field, the San Bernardino field east of Douglas, and the Pinacate field in Mexico are examples of this episode. The most recent volcanism was at Sunset Crater near Flagstaff. It erupted about 1,000 years ago. The San Francisco field is considered active and the most likely place in Arizona to have another eruption. The map below, from the Arizona Geological Survey shows the extensive Cenozoic volcanism.


The Grand Canyon was formed during the late Cenozoic. The Colorado Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction into Utah and Colorado. Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest. Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago.

Climate in the early Cenozoic continued to be hot and steamy, about 18̊F warmer than today, even though atmospheric carbon dioxide had been decreasing for 80 million years due to coal formation in the Cretaceous. Around 55 mya, there was a sudden temperature spike that lasted for about 100,000 years. (That’s geologically sudden = 10,000 years.) The spike is known as the Paleocene-Eocene Thermal Maximum (PETM). Data, derived from drill cores brought up from the deep seabed in the Atlantic and Pacific Oceans, show that the surface temperature of the planet rose by as much as 15̊F over the already warm temperatures. The cause is controversial.

Carbon dioxide levels rose from 1000 ppm to 1700 ppm–more than four times higher than today’s level of 400 ppm, but that rise began after the start of the temperature spike.

Isotopic analysis of carbon suggests that the culprit was methane, which is 65 times more powerful as a greenhouse gas than carbon dioxide. There are two hypotheses as to the source of methane: microbially generated methane buried in sediments along the slopes of the continental shelves; and methane clathrates. Methane clathrates are crystalline structures of methane bound to water. They form at near freezing temperatures under high pressure. They are stable up to 64̊F under high enough pressure. This form of methane exists along our coasts today, frozen in the sediment at low temperatures and high pressures. They are being investigated as a source of energy.

It is speculated that volcanism and tectonic disturbance released pressure that was holding the methane in clathrates or in sediments themselves. This “sudden” release of methane caused the temperature spike. (There is nothing to prevent this from happening again.)

After that temperature spike subsided, temperatures remained warm until about 34 mya when global temperatures began to drop. Antarctica had separated itself from Africa, Australia, and South America which caused the southern circumpolar ocean current to be established which isolated Antarctica from warm tropical waters. Global temperatures continued to drop. About 2.6 mya, continental ice formed at lower latitudes and initiated the glacial epochs and interglacial periods of our current ice age.


Shellito, Cindy, 2006, Catastrophe and Opportunity in an Ancient Hot-House Climate, Geotimes, October 2006.

In Arizona Geological Society Digest 17:

Lucchitta, Ivo, 1989 History of the Grand Canyon and of the Colorado River in Arizona.

Lynch, D.J., 1989, Neogene volcanism in Arizona.

Menges, C. M., 1989, Late Cenozoic Tectonism in Arizona and its impact on regional landscape evolution.

Pearthree, P., House, K., (now with USGS), and Perkins, M., Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona, Geological Society of America Special Paper 439

Scarborough, R., 1989, Cenozoic erosion and sedimentation in Arizona.



Arizona Geological History Chapter 6, The Cretaceous Period

The Cretaceous Period (145- to 65 million years ago) was hot and steamy. There was no ice at the poles. Global temperature is estimated to have been about 18 F warmer than today. Atmospheric carbon dioxide began a 145-million-year decline from about 2,000 ppm to the 380 ppm of today, in part, due to carbon sequestration by formation of coal deposits. Flowering plants appeared.

Paleomap 94

The North American continent was split by a sea connecting the Gulf of Mexico with the Arctic Ocean. Transgressions and regressions of this sea formed conditions ripe for coal formation similar to those in the Paleozoic Era In Southern Arizona, the lower Cretaceous Bisbee Group, consisting of the basal Glance conglomerate, the Morita formation sandstones and mudstones, the distinctive Mural Limestone (which forms the cliffs just east of Bisbee), and the sandstones and mudstones of the Cintura Formation record the changes in sea level. Upper Cretaceous rocks, the Fort Crittenden Formation lie unconformably (representing erosion or structural change) upon the Bisbee Group. The lower Fort Crittenden is dominated by marginal wetland to deep-water lake deposits, whereas the upper Fort Crittenden is characterized by wetland to deltaic deposits. These rocks contain organic geochemical evidence of wildfires which suggest that seasonal aridity and wildfires were common occurrences.

There are no early Cretaceous rocks recognized in northern Arizona. Thick sequences of upper Cretaceous rocks were deposited on what is now the Colorado Plateau. These represent near-shore marine, coastal, and river-deposited sands, mudstone, and coal. Coal is mined from the Dakota sandstone at Black Mesa in Navajo County, AZ. This is overlain by the Mancos Shale, and several other sedimentary formations.

The Laramide orogeny of late Cretaceous to early Tertiary time (80- to 40 million years ago) built the Rocky Mountains and closed the inland Cretaceous sea. Subduction of oceanic crust under continental rocks along the west coast caused compression and uplift of the continent.

This was the time of emplacement of most of the porphyry copper deposits in the western U.S. Volcanism was extensive, and included the volcano that produced the rocks of the Tucson Mountains.

sonorasaurusDinosaurs roamed the land, including Arizona’s Sonorasaurus thompsoni, a new species of brachiosaurid dinosaur whose remains were first discovered in the Whetstone mountains by UofA graduate geology student Richard Thompson in 1994. Sonorasaurus is estimated to have been about 50 feet long and 27 feet tall, about one third of the size of other brachiosaurus. It may have been a juvenile or just a small dinosaur species. Sonorasaurus was an herbivore. Tooth gouges on its bones suggest it was killed and eaten by a larger dinosaur. A single blade-like tooth of a huge meat eater called Acrocanthosaurus was found near the bones and suggests that this was the predator that killed Sonorasaurus. You can see an exhibit dedicated to Sonorasaurus at the Arizona-Sonora Desert Museum.

The end of the Cretaceous Period saw another major extinction of life. Dinosaurs, pterosaurs, many marine reptiles, some marine invertebrates, some groups of mammals, and a few plant groups became extinct. The reasons are still controversial. We know that an asteroid impacted near Yucatan, Mexico and formed the Chicxulub crater about 65 million years ago. The impact is said to have vaporized rock into clouds of dust, that cooled temperatures, and created clouds of sulfurous gas, which may have killed plants with acid rain. The impact is also said to have deposited a thin clay layer containing iridium and strained quartz. However, the extinction occurred during an 800,000-year eruption of basalts that form the Deccan Traps in India. Volcanic eruptions can also product dust and sulfur dioxide emissions (and layers of iridium which characterize the K/T boundary). More precise dating shows that the Chicxulub impact occurred 300,000 years before the mass extinction. Evidence suggests that the extinctions occurred over a period of several million years.

Cretaceous Trivia:

The white cliffs of Dover, England are Cretaceous age chalk deposits.

Paul Spur, a rail stop between Bisbee and Douglas exists because Mural limestone was mined for smelter flux.

Mural Hill Bisbee 1902

Hills carved from Cretaceous beds east of Bisbee. View is northward across Mule Gulch. The prominent white band is the upper member of the Mural limestone, forming the top of Mural Hill on the left and showing the dislocation due to the Mexican Canyon fault. Cochise County, Arizona. December 1, 1902. Plate 9-B in U.S. Geological Survey. Professional paper 21. 1904, figure 7 in U.S. Geological Survey Folio 112. 1904.


Dickinson, W.R., et al., 1989, Cretaceous Strata of Southern Arizona, in Geologic Evolution of Arizona, Arizona Geological Society Digest 17.

Finkelstein, D.B, et al., 2005, Wildfires and seasonal aridity recorded in Late Cretaceous strata from south-eastern Arizona, USA, Sedimentology, Volume 52, Issue 3 , Pages587 – 599, International Association of Sedimentologists

Krantz, R.W., 1989, Laramide Structures of Arizona, in Geologic Evolution of Arizona, Arizona Geological Society Digest 17.

Nations, J.D., 1989, Cretaceous History of Northeastern and East-Central Arizona, in Geologic Evolution of Arizona, Arizona Geological Society Digest 17.