The rifting of Rodinia produced a passive margin on the western edge of Laurentia. At the start of the Paleozoic, during the Cambrian, the area that now comprises the states of California, Oregon, Washington, and western Nevada did not exist as part of the North American continent (Figure 1.7). The continent’s western coastline was located at approximately the Utah-Nevada and Arizona-California state lines, where a broad continental shelf extended westward from the coast. The shore moved eastward over time as sea level rose, covering much of the western US under a shallow (epicontinental) sea, and sheets of sand and carbonate sediment were deposited on the shelf. During the late Cambrian and early Ordovician, the entire shelf was covered by a huge carbonate platform, much like the region around today’s Bahama Islands. At other times, such as the late Ordovician, transgression paused or reversed (regression), and the sea deposited thin sheets of nearly pure quartz sand. At the end of the Ordovician, global temperatures and sea level fell abruptly, probably due to glaciation caused by the movement of the large southern supercontinent Gondwana over the South Pole. As a result of these environmental changes, a major mass extinction of marine life took place at this time. Sea level continued to rise and fall throughout the first half of the Paleozoic, depositing marine sediments during transgressions and allowing the exposed carbonate rocks to weather during regressions.

Figure 1.7: The Southwestern US during the late Cambrian, approximately 500 million years ago. The entire region is located in the southern hemisphere—note the position of the equator.

In the late Devonian period (approximately 370 million years ago) a major geological change took place in the Southwest. A portion of the continental shelf adjacent to present-day Idaho and Nevada changed from a quiet passive margin to an active subduction zone, where oceanic crust plunged beneath the continent. Here, as oceanic crust descended deep into the upper mantle, the rock above the descending crust melted to form a line of volcanoes on the surface. Subduction also led to accretion—sediment, sedimentary rock, and even bits of the oceanic crust itself were scraped off the descending crustal plate and pushed onto the overlying plate (Figure 1.8). Just as a rug develops folds when pushed from the side, these rocks were wrinkled up into mountains. Volcanic islands carried along by the subducting plate also accreted to the edge of the continent. The landmass began to rotate, moving the North American plate into a more modern orientation (Figure 1.9).

Continental and Oceanic Crust

The lithosphere includes two types of crust: continental and oceanic. Continental crust is less dense but significantly thicker than oceanic crust. The higher density of the oceanic crust means that when continental crust collides with oceanic crust, the denser oceanic crust (made mostly of dense rocks such as basalt) will be dragged (or subducted) under the buoyant continental crust (made mostly of less dense rocks such as granite). Although mountains are created at these oceanic/continental crust collisions due to the compression of the two plates, much taller ranges are produced by continental/continental collisions. When two buoyant continental crusts collide, there is nowhere for the crust to go but up! The modern Himalayas, at the collision site of the Asian and Indian plates, are a good example of very tall mountains formed by a collision between two continental crusts.

Figure 1.8: Subduction along the western edge of the North American plate.

Figure 1.9: The Southwestern US during the late Devonian, approximately 375 million years ago.

By the Carboniferous, most of the West Coast had transformed into a subduction zone. During the Mississippian period (around 340 million years ago), an island arc collided with and accreted to that coast, generating a major mountain-building event: the Antler Orogeny (Figure 1.10). This orogeny was the first in a long series of mountain-building events that affected the margin of western North America. The cycle of sedimentation and collision initiated during the Antler event would be repeated many times and with many variations into the Cenozoic era, when most of the subduction zone along the western margin of North America was altered into a transform boundary (the most important component of which is the San Andreas fault system). Much of North America, including most of the Southwest, was gently uplifted in the late Mississippian, causing sea level to fall across the continent.

As North America began to collide with Gondwana (composed of present-day South America, Africa, India, Australia, and Antarctica), forces from the collision began to affect the continent’s topography. During the Pennsylvanian (300 million years ago), compressional forces from the collision and tension from coastal subduction combined to deform the continent’s interior, buckling the crust and creating deep basins between uplifted blocks. Shallow inland seas spread across the interior of the continent, covering parts of North America’s Precambrian shield (Figure 1.11). Associated uplift led to the expansion of terrestrial environments over areas that had once been marine. Geologists call the resulting landscape the Ancestral Rocky Mountains. Sediments that eroded from this range and other uplifted areas were transported to the inland sea and the continental margins, forming deposits of conglomerates, sandstones, shales, limestones, and evaporite minerals. Although these ranges are long eroded away and the inland basins filled with sediment, evidence for their existence is preserved in the patterns of sedimentary rocks remaining throughout the Southwest today.

Figure 1.10: Collision of a volcanic island arc with the West Coast during the Antler Orogeny.

Sea level fell again in the late Paleozoic, during the Pennsylvanian and Permian, as continental collisions progressed to form the supercontinent Pangaea. As accretion continued over time, the coastline moved farther seaward, running through western Montana, eastern Idaho, central Utah, and western Arizona (Figure 1.12). The climate at low latitudes of this supercontinent was hot and dry, and iron-rich limestones, sandstones, and mudstones were oxidized. This process generated rocks with a distinctive and characteristic red color, appropriately called “red beds.” Permian red beds are very characteristic of the Southwest, particularly in the Colorado Plateau region, where they are the most widespread geologic feature exposed. Many of these red beds represent ancient dunes. Meanwhile, the marine Permian Basin (Figure 1.13) of southern New Mexico and west Texas was ringed by reefs—the Guadalupe Mountains of southern New Mexico and western Texas contain the largest and best-preserved Paleozoic reef in the world. Regression of the sea near the end of the Permian period spelled the end of the Southwest’s magnificent reefs.

Figure 1.11: The Southwestern US during the Pennsylvanian, approximately 308 million years ago.

Figure 1.12: The early Permian, approximately 280 million years ago.

Figure 1.13: The Permian Basin is a large sedimentary basin in western Texas and southeastern New Mexico. It is made up of three main component parts: the eastern Midland Basin, the Central Basin Platform, and the western Delaware Basin. These structures existed from the Carboniferous to the Triassic periods.