The geology of Arizona began to form in the Precambrian. Igneous and metamorphic crystalline basement rock may have been much older, but was overwritten during the Yavapai and Mazatzal orogenies in the Proterozoic. The Grenville orogeny to the east caused Arizona to fill with sediments, shedding into a shallow sea. Limestone formed in the sea was metamorphosed by mafic intrusions. The Great Unconformity is a famous gap in the stratigraphic record, as Arizona experienced 900 million years of terrestrial conditions, except in isolated basins. The region oscillated between terrestrial and shallow ocean conditions during the Paleozoic as multi-cellular life became common and three major orogenies to the east shed sediments before North America became part of the supercontinent Pangaea. The breakup of Pangaea was accompanied by the subduction of the Farallon Plate, which drove volcanism during the Nevadan orogeny and the Sevier orogeny in the Mesozoic, which covered much of Arizona in volcanic debris and sediments. The Mid-Tertiary ignimbrite flare-up created smaller mountain ranges with extensive ash and lava in the Cenozoic, followed by the sinking of the Farallon slab in the mantle throughout the past 14 million years, which has created the Basin and Range Province. Arizona has extensive mineralization in veins, due to hydrothermal fluids and is notable for copper-gold porphyry, lead, zinc, rare minerals formed from copper enrichment and evaporites among other resources.
Stratigraphy, Tectonics & Geologic History
Precambrian: Before 539 million years ago
The oldest rocks in Arizona likely date to the late Archean or early Proterozoic, although evidence of earlier geology was overwritten during the Yavapai orogeny and the Mazatzal orogeny—major mountain building events 1.8 to 1.6 billion years ago. During this period in the Proterozoic, the Yavapai, Pinal and Vishnu schist rocks formed due to intense metamorphism and were intruded with granites. These ancient deformed rocks are found at the base of the Grand Canyon, in Salt River Canyon, and in mountain ranges throughout the state.
Arizona's oldest rocks overall are metamorphosed volcanic rocks, including basalt and rhyolite and related sedimentary rocks, that now constitute the bottom of the Grand Canyon and formed beginning 1.8 billion years ago. The Verde district at Jerome, in Yavapai County also preserves rocks from this period. There, ancient submarine hydrothermal vents precipitated sulfides directly onto an ancient seabed, creating massive copper and zinc ore sulfide deposits.
The Mazatzal orogeny occurred from 1.7 to 1.61 billion years ago. During this event, igneous rocks that were more enriched in quartz than previous rocks formed the Mazatzal Mountains and the New River area. The Pinal Schist also formed during this period and was intruded by granite and granodiorite. The oldest Proterozoic metamorphic and igneous rocks were intruded with granites and pegmatite between 1.45 and 1.4 billion years ago. The Oracle Granite, near Tucson, the Ruin Granite, close to the Ray-Superior area, and the Zoroaster Granite at the base of the Grand Canyon all contain one to two inch long, pink orthoclase crystals. The orogeny generated the Texas Zone, an area of fractures, fissures and faults in rock, trending northwest from Texas to California. In Arizona, these veins commonly host exotic minerals including beryllium and tantalum, and elements like lithium, bismuth, uranium and tungsten. These minerals were emplaced by hot hydrothermal fluids moving through the weak zones in the rock.
The major Grenville orogeny in the east of the Proto-North American continent impacted areas as far west as Arizona, producing large rift basins between 1.2 and 1 billion years ago. Rift basins were structurally related to the formation of copper deposits and the Keweenawan basalt flaws in Michigan, and regionally, the basins filled with thick layers of sediment in Arizona. The Apache Group near the bottom of the Grand Canyon, includes fluvial shale and sandstone, as well as limestone from a shallow sea, exposed in the Salt River Canyon. The limestone contains stromatolite remains, mounds of blue-green algae. The Apache Group was intruded with basalt and diabase between 1.05 and 1.14 billion years ago. The Mescal Limestone metamorphosed, forming asbestos and superheated diabase magma "baked" the limestone to marble. The 1.07 billion year old limestone, shale, quartzite, sandstone and basalt of the Unkar Group is also found in the Grand Canyon.