The Cenozoic era (consisting of the Paleogene, Neogene, and Quaternary periods, 66 million years ago to present) was an age of diversification and evolution of mammals, birds, insects, flowering plants, and coral reefs. The continents continued to spread apart to reach their present-day positions. Sea levels rose and fell, affecting the coastline, but the interior of North America remained relatively high. Sediment deposition, for the most part, occurred as fluvial and lake deposits. This was also a time of active volcanism in western North America. The Cenozoic geology of western North America is dominated by three large-scale processes: erosion, subduction and extension, and volcanic activity.

The Paleogene was a time of active volcanism around the world, and North America was no exception. The Paleogene and Neogene periods were warmer than today, with average global temperatures ranging from 17 to 25°C. Erosion of the mountains and highlands that had formed during the Mesozoic produced thick layers of conglomerates, sandstones, and mudstones across stream valleys, deltas, and lakes. Volcanic ash from nearby eruptions is commonly interlayered with these sediments. Many of these sedimentary layers were deposited by rivers, or in alluvial fans coming from the mountain systems. Several such layers are now important aquifers, including the enormous Ogallala Aquifer (Figure 1.18) which today supplies water for farming and communities on the Great Plains.

Due to crustal deformation during the late Mesozoic and early Cenozoic, numerous basins formed inland lakes or depressions into which sediments were deposited (Figure 1.19). Perhaps the best-known example is the Green River Basin of western Wyoming and its equivalents, the Uinta Basin in northeastern Utah and the Piceance Basin in northwestern Colorado. Lakebed shales and mudstones found in all of these basins are famous for their abundant and well-preserved fossils. Other important basins, including the Denver and Raton basins in Colorado and the San Juan and Baca basins in New Mexico and Arizona, produce coal, oil, and gas, and other industrial minerals.

Figure 1.18: Extent and saturated thickness of the Ogalalla Aquifer.

Figure 1.19: Sedimentary basins of the Southwest.

In the late Cenozoic, subduction at the West Coast was progressively replaced to the north and South by the developing San Andreas fault system. Subduction continues today beneath Oregon and Washington (and beneath Mexico south of the Gulf of California). Due to the complex interplay of plate motions, the portion of the subducting plate beneath the Southwestern US overrode hot, upwelling mantle. This, in turn, caused a number of major changes. In the early Paleogene, melting of the lower crust resulted in the intrusion of numerous granitic bodies and the formation of large volcanic fields, including the San Juan and Thirtynine Mile in Colorado and the Marysvale in Utah. These granite intrusions remain mostly buried beneath the surface, detectable only by magnetic surveys. Volcanic lava and ash flows, now deeply eroded, cover the countryside above the buried granite intrusions. Remains of ancient calderas, recording the collapse of now-eroded volcanoes, can be detected in the subsurface.

Between 40 and 30 million years ago, a transition from subduction and crustal compression to crustal extension began to occur. By the Neogene, the Farallon plate lay shallowly under the North American plate for hundreds of kilometers eastward of the West Coast. Now situated more fully beneath what are now the South Central, Southwestern, and Northwest Central states, this extra layer of crust caused uplift and extension of the region, as the added thickness of buoyant rock (relative to the mantle) caused the entire area to rise isostatically. The Farallon plate was subjected to increasing temperatures as it subducted, causing it to expand. As heat dissipated to the overlying North American plate, that rock expanded as well. Finally, the high temperatures in the upper mantle caused the Farallon plate to melt, and the resulting magma was injected into the North American plate, destabilizing it. These processes, along with the complex crustal movements taking place along the San Andreas fault, caused the surface of the North American plate to pull apart and fault into the mountainous blocks of the huge Basin and Range province that stretches from Idaho, Nevada, and Utah into California, Arizona, New Mexico, and Texas. In the late Miocene, around eight million years ago, epeirogenic uplift (resulting from upwelling mantle heat pushing the crust upwards) began. The uplift raised not just the mountains but also the base elevation of the entire region to its current level. This process raised the Rocky Mountains and Colorado Plateau to their current “mile-high” elevation, initiating the downcutting of the Grand Canyon in Arizona. The modern direction of the Colorado River’s flow toward the Gulf of California was established about six million years ago, at which time the river began to widen and deepen the canyon we know today.