GEOLOGIC HIGHLIGHTS OF SOUTHEASTERN ARIZONA AND VICINITY
YEARS AGO/
GEOLOGIC TIME SCALE GEOLOGIC AND LIFE HISTORY
QUATERNARY PERIOD
0–100 years — Holocene Industrialized society develops mineral and water resources.
10,000 years — Holocene Today's Sonoran Desert appears. Many mammals go extinct.
10,000–1.6 m.y. — Pleistocene Glacial-interglacial climate fluctuations occur with associated changes in lakes, rivers, flora and fauna. Large mammals, like
TERTIARY PERIOD
1.6–5.3 m.y. — Pliocene
5.3–15.0 m.y. — Miocene mammoths, mastodons and giant ground sloths, roam across southern Arizona. Hunters and gatherers arrive in Southern
Arizona about 11,000 years ago, e.g., Curry Springs mammoth kill site. Episodes of basaltic volcanism occur in the Pinacates, San
Francisco Peaks, White Mountains and San Bernardino Valley.
The present-day Basin and Range landscape of the Sonoran Desert begins to take shape 15 m.y. ago. Deep pervasive fracturing of the crust and earthquake activity break up a high, mountainous southern Arizona into blocks. Some of these crustal blocks drop down to form the Basins, while others tilt but remain high to be mountains. Present stream systems evolve. The initially closed basins fill up with sediments, washed off the surrounding mountains, and slowly integrate into regional drainage systems, e.g., Gila, Salt and Colorado Rivers. The climate becomes semiarid. The Gulf of California and the San Andreas fault develop as southwestern North America begins to get torn apart by plate tectonic processes. With the opening of the Gulf of
California, the Colorado River drains south and excavates the
Grand Canyon.

15.0–23.7 m.y. — Miocene
23.7–36.6 m.y. — Oligocene
36.6–57.8 m.y.— Eocene
57.8–66.4 m.y. — Paleocene
CRETACEOUS PERIOD
66.4–80.0 m.y.
CRETACEOUS PERIOD
(continued)
Subduction of ocean crust along the western margin of North
America continues until the East Pacific Rise interacts with the continental crust, resulting in the activation of the San Andreas fault and Basin and Range faults. Rivers carry sediments off the high mountains in southern Arizona and deposit gravels to the north onto what is now the edge of the Colorado Plateau. The horse evolves in North America, and other modern mammals diversify. Between ~20–30 m.y. tremendously explosive volcanic eruptions rip across the Southwest and northern Mexico. The landscape is built up higher by layers of volcanic rocks, e.g., tuffs, ash and rhyolite. These volcanic rocks now make up the
Chiricahua, Tucson, Galiuro, Superstition, Ajo, and Atascosa
Mountains in southern Arizona and the Sierra Madre Occidental in
Mexico. During this volcanism, plutons are emplaced into the upper crust., e.g., Kitt Peak and Stronghold (Dragoon Mountains) granites.
Volcanic mountains exist in southern Arizona. A tremendous geologic upheaval occurs in western North America called the
Laramide Orogeny. Intrusions of molten rock into the crust and associated volcanism form new continental crust, enlarging the
North American continent. These plutonic rocks are now exposed in Texas Canyon. The Catalina-Rincon, Graham, Tucson, Sierra
Nevada, and many other mountains are intruded, metamorphosing large portions of the upper crust. As the Pacific Ocean crust rapidly subducts under colliding North America, the continental crust is deformed and folded (buckling like an accordion). Beneath some volcanoes most copper and molybdenum deposits now being mined in Arizona are forming by hydrothermal interactions.
Dinosaurs go extinct around 66 m.y. ago.

80–144 m.y.
JURASSIC PERIOD
144–208 m.y.
TRIASSIC PERIOD
208–245 m.y.
PERMIAN PERIOD
245–286 m.y.
PENNSYLVANIAN PERIOD
286–320 m.y.
MISSISSIPPIAN PERIOD
320–360 m.y.
DEVONIAN PERIOD
360-408 m.y.
SILURIAN PERIOD
408–438 m.y.
ORDOVICIAN PERIOD
438–505 m.y.
CAMBRIAN PERIOD
505–570 m.y.
P RECAMBRIAN
PROTEROZOIC
570–2,500 m.y.
ARCHEAN
2,500–4,500 m.y.
Mountains, called the Mogollon Highlands, begin to build up in southern Arizona, as great quantities of granitic magma intrude the crust of western North America. Some molten rock erupts onto the
Earth’s surface, e.g., 20,000 feet of volcanic rock in the Santa Rita
Mountains. These high volcanic peaks form a mountain chain stretching from Alaska to Mexico. The Bisbee copper deposit forms beneath one of these volcanoes. Other parts of Arizona are covered by an extensive swampy mud flat across which meander rivers jammed with logs (now the Petrified Forest). Volcanic ash, blown in from the south and west, occasionally falls on these swamps. Later, a great sand desert extends across Northern
Arizona and into Utah and Colorado. Fossilized sand dunes from this desert are now exposed in the cliffs of Zion and Arches
National Parks. A shallow sea covers southeastern and northeastern Arizona about 100 m.y. ago. Abundant plant growth along the swampy margins of that seaway accumulates to form coal now being mined on Black Mesa. Flowering plants and birds evolve in Jurassic time. Dinosaurs and insects become abundant in
Triassic and Jurassic time.
North America is part of a supercontinent called Pangea. Reptiles become abundant.
Tropical shallow sea and coastal environments exist in Arizona for a few 100 million years. There are at least three cycles of shallow sea inundation (transgressions), followed by uplift and emergence of land (regression). Coral reefs, fishes and marine invertebrates are abundant in the warm, shallow waters that cover the continental crust. Thick deposits of limestone, mudstone (shale) and sandstone form in these marine and tidal/beach environments. Paleozoic marine sedimentary rocks from this time are now exposed throughout Arizona, notably in the Grand Canyon and Whetstone
Mountains. The first abundant fossils of marine life with hard parts appear in Cambrian rocks.
Arizona crust forms around 2 billion years ago, during an orogenic event, as the continental crust continues to grow in size by accretion. The first four billion years of geologic history is obscured by more recent events and changes. First, life probably evolved 3.5 to 3 billion years ago but remained much the same
(algae) for billions of years. Cycles of mountain building–erosion– deposition reshaped the land as continents must have drifted across the Earth.