Paleogeography of the Southwest

Paleozoic The Age of Fishes

 Most of Arizona is underwater.   Thick layers of limestone, sandstone and mudstone are developing as the ancient seas invade and recede.

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 Pennsylvanian Period. 280 million years ago.

Pangea, a consolidated landmass of all the continents as we know them today, is intact.

Sedona Red Rocks are a result of Paleozoic geology.  The iron oxides have leached from the volcanic layers into the sandstone, mudstone and limestone layers creating soft reds, pinks and peach colored layers below.


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Permian:  This is the period of the Great Extinction. 90% of all life on Earth died during the final transition.  The Atlantic Ocean is being "born" as the tectonic plates rotate, dramatically changing the climate.  Arizona is about 14 degrees south of the equator at this point.

Stand on the floor of Monument Valley and you'll be in touch with the Permian layer.  Huge sand dunes form at the seas edge.  Today, the remnants are the mesas, buttes and spires. 

Kaibab limestone deposits are forming in the northwest quadrant of Arizona, Utah, and southeast Wyoming.

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Now the fun begins! The Mesozoic Era The Age of Dinosaurs

Early Triassic Paleogeography, Southwestern US (240 Ma). A broad fluvial and shallow marine depositional system (the Moenkopi Formation) covers much of the southern Western Interior. To the west, the Stikine and Quesnell elements of the McCloud arc are colliding and in turn collapse the Havallah back arc basin as the Sonoman orogeny continues. The Cache Creek forearc complex is trapped between the two arc elements

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Late Triassic Paleogeography, Southwestern US (215 Ma). The Chinle fluvial system covers much of the Western Interior. Chinle rivers are sourced in remnant highlands of the Appalachian Mountain system. As the Cordilleran arc develops, a new back arc basin forms behind the McCloud arc.

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Early Jurassic Paleogeography, Southwestern US (200 Ma). The great Navajo sand sea is spreading across much of the Western Interior. A continental arc is developing across the Southwest and some Navajo dunes spill westward into the topographically low arc. Farther north the arc is off shore, separated from the continent by a back arc basin. The Stikine and Quesnell elements are separated by the Cache Creek interarc basin.

Middle Jurassic Paleogeography, Southwestern US (170 Ma). A long, narrow, epicontinental seaway extends into the Western Interior from the north. Dunes rim the southeast margin. The elements of the McCloud arc collapses against the continent and id thrusting in Nevada creates uplands. The large Wrangellia oceanic plateau approaches from the south. Most interpretations consider Wrangellia to be an exotic terrain. However, the details of the history of this terrain and its collision with North America are greatly debated

Late Jurassic Paleogeography, Southwestern US (150 Ma). The uplift in Nevada reverses stream direction across the region and the Morrison fluvial system expands eastward across the Western Interior. Stresses related to the opening of the Gulf of Mexico oceanic basin cause oblique, transtensional basins to form across the southwest portion of North America. Thick continental and later marine deposits fill these basins. The collapse of the Stikine arc against Quesnell and perhaps the initial collision of Wrangellia trigger the first phase of the Nevadan orogeny.

Latest Jurassic Paleogeography, Southwestern US (145 Ma). A large saline lake spreads across the eastern Colorado Plateau. To the west, uplands sourced streams of the Morrison Formation. At the conclusion of the Nevadan orogeny, a continental arc extends along most of the western coast of North America

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Early Cretaceous Paleogeography, Southwestern US (130 Ma). The Western Interior is the site of an encroaching epicontinental seaway from the north. To the west lay uplands and a thrust belt in Nevada and western Utah. Transtensional basins in southern Arizona and California are the sites of thick marine and continental deposition. The Cordilleran arc is now a classic continental (Andean-style) arc along all or most of its extent. The arc consists of a fore arc-trench system, fore arc basin, and Andean arc. The fore arc-trench is the site of the famous Franciscan melange'. The Great Valley sequence is deposited in the fore arc basin and the Sierra Nevada batholith complex forms in the bowels of the arc

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 Note the major rivers in southern Wyoming, Utah, Colorado and Arizona.  They form  natural corridors for dinosaurs.   Dinosaur National Park is in that corridor where Utah, Colorado and Wyoming meet.   This is the time of their extinction and their remains are found in abundance.

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The Age of Mammals

Early Tertiary (Eocene) Paleogeography, Southwestern US (50 Ma). As the Rocky Mountains are uplifting, basins form and subside between major uplifts. Huge lakes fill the basins during the Eocene. Although the lakes are in basins, paleobotanical data suggests that their absolute elevation is near the current elevation of the deposits. This fact suggests that much of the Rocky Mountain region and the Colorado Plateau are uplifting in the Eocene. Mountainous terrain exists in Nevada, western Utah, and central and southern Arizona. Streams drain these uplands onto the Colorado Plateau and probably into the lakes. The coastal region continue to have right-lateral translation of terrains northward. Some of these terrains lie outboard of marginal seas along the west coast and form continental borderlands at the edge of the continent. The most famous of these, Salinia, probably start out in northern Mexico and   progressively translate northward; you can follow its progression on the four Tertiary maps

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Early Tertiary (Oligocene) Paleogeography, Southwestern US (35 Ma). Much of the drainage of the Rocky Mountain and Colorado Plateau regions is poorly developed and probably internal to the region. However, some streams found outlets to lower elevations and carved canyons into the margins of the Colorado Plateau. Volcanism is widespread and extreme across much of the Western Interior. The first stages of the Rio Grande Rift develop as parts of western North America evolved from compressional tectonics to extensional tectonics. California borderlands continue their northward translation and a series of complex basins form between and east of them. The East Pacific Rise, a Pacific spreading center, nears the coast of southwestern North America. Its impending collision will have profound effects on the evolution of the region


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Middle Tertiary (Early Miocene) Paleogeography, Southwestern US (20 Ma). Drainage systems of the Western Interior are still not well integrated. Note the absence of a through going stream on the Colorado Plateau. Extensive volcanism continus. On the west coast, normal subduction resumes, but only north of where the East Pacific Rise is colliding with North America. Since the Permian or Triassic, the Farallon and related plates are subducted below North America. Once the East Pacific Rise collides with the continent, the westerly plate, the Pacific Plate, comes into contact with the continent. Because both the Pacific Plate and North American Plate have westerly components of motion, a new stress regime was established. Extension dominates the regions east of where North America contacts the Pacific Plate. A large transform fault system, the San Andreas, is created. Blocks moving northwest along the Pacific side of the fault are caught in a bend of the fault system in southern California. These blocks are rotating in clockwise fashion to form the Transverse Ranges

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Latest Middle Tertiary (Late Miocene) Paleogeography, Southwestern US (10 Ma). The tectonic patterns of the earlier Miocene continue. Volcanism continues widespread and streams continue to gnaw at the Colorado Plateau. Extension spreads northward as the Mendocino triple junction, the point of contact between the Farallon, North American, and Pacific plates move northward. Note that Salinia is now in central California. The Cordilleran arc shuts down south of the triple junction as subduction ceases and transform motion begins.

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Map of Arizona


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Map of the reservations and Four Corners 


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