Miocene Epoch The Miocene Epoch extends from about 23 to 5.3 million years ago. Mountain building characterized the Miocene, with the Cascades in western North America, the Andes in western South America, and the Himalayas in Asia all taking root. Worldwide climate was drying, land bridges formed, and animal migration patterns went from continent to continent. Late in the Miocene, a large freshwater lake covered the Verde Valley of central Arizona. Clays, silts, sands and lime muds deposited in the lake form the rocks of the Verde Formation. Montezuma Castle National Monument near Camp Verde is an excellent place to view these ancient lake deposits.
Eocene Epoch The Eocene Epoch, of the Cenozoic Era, lasted from 56 to 34 million years ago. The Eocene is noted for the warmest climate in the entire Cenozoic. The first modern mammals appeared in the Eocene. The Rocky Mountains and the Colorado Plateau were uplifted during the Eocene. Mountainous terrain dominated southern Arizona. Rim Gravel deposits exposed in central Arizona with sediments derived from highlands to the south. (Courtesy of Ron Blakey, Professor Emeritus at Northern Arizona University.)
Cretaceous Period The Cretaceous Period began about 145 million years ago and ended with the demise of the dinosaurs 65.5 million years ago. The Cretaceous Seaway extended into southeastern-most Arizona. Triceratops and tyrannosaurs were just two of the many dinosaurs roaming the Cretaceous landscape. Birds began to appear on the scene, frogs and salamanders continued to diversify, and placentals and marsupials made their first appearance among mammals. Southern Arizona was prone to caldera formation and great pyroclastic outflow sheets were emplaced. This picture shows exposure of volcanic tuff near Gates Pass in the Tucson Mountains of southern Arizona.
Late Jurassic Extending from about 200 to 145 million years ago, the Jurassic Period is a time of massive sand deposition in northern Arizona. The Jurassic constitutes the middle period of the era: the age of reptiles. The climate of the late Jurassic was warmer than today s climate. Late Jurassic sandstones exposed in Northeastern Arizona. (Red Hills at Saguaro National Park, courtesy of Susan Hollis).
Middle Jurassic The Middle Jurassic lasted from about 176 to 161 million years ago; it was preceded by the Early Jurassic and followed by the Late Jurassic. Sea level was high and the continents were flooded in part by shallow seas. Northern Arizona was dominated by an erg - a large sand sea. Southern Arizona was dominated by mountainous terrain. The Entrada Sandstone formed as part of an erg - sand sea - during the Middle to Late Jurassic. (Photo above by Wayne Ranney.)
Early Jurassic The Early Jurassic occurred 200 to 175 million years ago. Dinosaurs dominated the land, while cyads and conifers were chief among the common plants. Sharks, looking much as they do today, shared the oceans with bony fishes, and phytoplankton and algae prospered in the warm seas. Pangaea, the supercontinent, was splitting apart with the continental fragments taking their own trajectories. The Navajo Sandstone of northern Arizona and southern Utah is characteristic of terrestrial Jurassic-age sediments of the western Colorado Plateau.
Late Triassic Period The Triassic Period ranged from 250 to 200 million years ago. The Great Dying - a period of global extinction at the end of the Permian about 252 million years ago - provided opportunities for new species to evolve and take root. The dinosaurs first appear in the Triassic. Plants proliferated and their fossil remains are preserved in the silty-sandy Chinle Formation exposed in northeastern Arizona. Northern Arizona was a swampy lowland fed by streams from the rugged hills of southern Arizona. The Petrified Forest Member of the Chinle Formation contains the fossilized remains - petrified wood - of trees and plants that lived over 200 million years ago.
Early Triassic At 250 million years ago, the Permian extinction wiped out many of worlds terrestrial and marine lifeforms and the Triassic Period (~250 to 200 million years ago) is just getting underway. New, opportunistic species, among them cyads and conifers, competed with other species to gain a foothole on land or in the sea. Lowlands dominated Arizona and stream systems slowly meanderd across the subdued topography. Sands of thetriassic Moenkopi Sandstone were deposited by shallow streams as evinced by ripple marks, cross beds, and mud cracks. Native Americans used flagstones from the Moenkopi Sandstone to build the pueblos at Wupatki National Monument.
Middle Permian Period The Permian Period extends from ~ 299 to 251 million years ago). It was a time when the Earth s continents were annealed to form a single supercontinent, Pangaea, that stretched from pole to pole. Arizona was innudated then by a shallow sea. The world s ocean, Panthalassa, was home to brachiopods, ammonoids, and crinoids, whose fossil remains are commonly found in marine limestone. A worldwide extinction, with an estimated 90% of species becoming extinct, signaled the end of the Permian about 252 million years ago. The 270 million year old Kaibab Limestone caps the Colorado Plateau of northern Arizona. (Kaibab Limestone pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)
sd Cenozoic Early Permian The Early Permian occurred from about 299 to 270 mllion years ago. Earth s continents were still colliding to form the supercontinent, Pangaea. Shallow marine waters inudated basins in Arizona and New Mexico. Fine-grained shales and thin limestone rock formed in these quiet basins. Seed-producing gymnosperms, a family of plants that include the conifers and the cycads, become the dominant plant group. Exposed in Grand Canyon, the clay and silt particles that comprise the Hermit Shale were deposited in quiet waters in deep basins. (Hermit Shale pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)
Middle Pennsylvanian The Pennslyvania period lasted from about 318 to 299 mllion years ago. Earth s continents were colliding to form the supercontinent, Pangaea. The Mississippian and Pennslyvanian periods are lumped together in the Carboniferous Period, socalled because of the large volume of coal that formed during the period. North America was situated near Earth s equator so the climate was generally hot and humid. Arizona was surrounded by shallow seas and inundated in the southeast corner. Amphibians were diverse and common, continuing an ascendancy that began in the Devonian. As closed basins of central and northern Arizona were inundated by brackish waters, thick salt deposits - rock salt and sylvite (potash) formed. These deposits were subsequently buried by clastic sedimentary rocks such as shale, siltstone or sandstone.
Early Pennsylvanian The Pennsylvanian Period lasted from about 318 to 299 million years ago. The central portion of Arizona was covered by sand deposits that transitioned into fine grained sediments westward towards the encroaching sea. The shallow seas of western Arizona would give rise to calcareous mudstones and limestones. The Earth s continents were slowly colliding and hot, humid conditions would have characterized the northern landmass. The red-stained Manakacha Sandstone of the lower Supai Formation contains cross beds and ripples. The red color stems from hematite - an iron oxide - cement. (Manakacha Sandstone pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)
Early Mississippian The Mississippian Period lasted from from roughly 359 to 318 million years ago. North America and Eurasia formed the supercontinent Laurasia. It was a period of shallow seas covering Arizona. The central part of the continent was home to lush, terrestrial swamps and rich organic deposits that resulted in large coal deposits. The Redwall Limestone is exposed in the wall of Grand Canyon. Near Sedona, Arizona, the Redwall lies 400+ feet below ground level. Dissolution of the Redwall has resulted in the formation of large caverns. (Redwall Limestone pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)
Late Devonian The Devonian of the Era ranged from 416 to 359 million years ago. The first seed-bearing plants spread across dry land and terrestrial lifeforms blossomed. Shallow seas were home to ammonoids, jawless fishes, bony fishes and sharks. Arthorpods, including wingless insects, diversify. The Earth s terrestrial surface comprises two supercontinents, Gondwana in the south and Laurasia in the north. Temple Butte Limestone outcrops as a cliff-former in western Grand Canyon. In the east, fossils of freshwater fish occur, in the west fossil remains are of marine fish. (Temple Butte Limestone pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)
Middle Cambrian The Cambrian Period, the earliest in the Era, lasted from 542 to 488 million years ago. The land surface was largely devoid of life. The climate was generally warm and oxygen levels in marine waters reached new heights. There was an explosion of new life forms in the world s seas and arthropods, which includes insects and crustaceans, first appear. Bright Angel Shale comprises green to purple-red, slope-forming siltstones and shales. The fine sediments that make up the Bright Angel Shale were deposited offshore in a shallow muddy sea. (Bright Angel Shale pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)
Late Proterozoic The Proterozoic began 2.5 billion years ago and ended 543 million years ago. Stable continents first appeared and the supercontinent, Rodinia, formed and was subsequently torn apart by tectonic forces. Primitive blue-green algae began loading Earth s atmosphere with oxygen. Other single-celled and multi-celled organisms took hold about 1.8 billion years ago, presaging the Cambrian Explosion. Arizona highlands contributed sediments into nearby shallow seas. The Vishnu Schist formed about 1.75 million years ago as thousands of feet of volcanic ash, mud, sand, and silt were laid down in a shallow sea. Deep burial and heat subsequently transformed the sediments into the metamorphic rocks we see today. (Vishnu Schist pedestal from Grand Canyon s Trail of Time, courtesy Grand Canyon National Park.)