Geologic History of the Southwestern U.S.

Cenozoic (66 million years ago to present)

The Cenozoic era was an age of diversification and evolution of mammals, birds, insects, flowering plants, and coral reefs. 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.

Paleogene (66 to 23 million years ago)

The Paleogene was a time of active volcanism around the world, and North America was no exception. 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 inter-layered 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 which today supplies water for farming and communities on the Great Plains.

Paleogene basins

Due to crustal deformation during the late Mesozoic and early Cenozoic, numerous basins formed inland lakes or depressions into which sediments were deposited. Some 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.

Perhaps the best-known basin, however, 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.

Paleogene volcanic fields

Between 40 and 30 million years ago, a transition from subduction and crustal compression to crustal extension began to occur. Due to the complex interplay of plate motions, the portion of the subducting plate beneath the southwestern U.S. overrode hot, upwelling mantle. Melting of the lower crust resulted in the intrusion of numerous granitic bodies and the formation of large volcanic fields during the late Eocene to early Miocene. The 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. These are found in the San Juan and Thirty-nine Mile volcanic fields in Colorado and the Marysvale in Utah. 

Neogene (23 to 2.6 million years ago)

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.

Volcanic activity continued in the Southwest throughout the Neogene and extending in the Pleistocene. The Raton Volcanic Field in the Raton Basin formed about 9 million years ago (during the late Miocene) and erupted in several phases, ending about 30,000 years ago. Some well-known landmarks in the region were formed by the activity of this volcanic field, including the Capulin Volcano in northern New Mexico and Fishers Peak near Trinidad, Colorado. 

Quaternary (2.6 million to 12,000 years ago)

At the start of the Quaternary period, approximately 2.5 million years ago, continental ice sheets began to form in northernmost Canada. Throughout this period, the northern half of North America has been periodically covered by continental glaciers that originated in northern Canada. The Quaternary period is divided into two epochs: the Pleistocene and Holocene. During the Pleistocene, ice sheets advanced south and retreated north several dozen times, reaching their last maximum extent 25,000–18,000 years ago. The Holocene epoch is the most recent (and current) period of retreat, and is referred to as an interglacial interval. The beginning of the Holocene is considered to be 11,700 years ago, or about 9700 BCE.

The Pleistocene ice sheets did not extend into the Southwest. At their greatest extent in the western U.S., continental ice sheets reached into Washington, Idaho, Montana, and the Dakotas. However, local alpine glaciers covered the highest parts of the Rocky Mountains as far south as northern New Mexico. These glaciers carved U-shaped valleys with steep headwalls (cirques) and sharp drainage divides (arêtes). They also left unsorted deposits of glacial till in lateral moraines along the sides of valley walls, and in terminal and recessional moraines marking both the farthest extent of the ice and the places where the ice front paused during retreat. A small number of glaciers still occur in the Colorado Rockies today. These glaciers exist only because they have formed in sheltered areas where they are fed by drifting snow and avalanches and are protected from heat and sun that could fully melt them. As the climate warms, they are likely to eventually disappear.

Glacial lakes formed in low areas between or in front of glaciers, and also during times between glacial advances. One such lake, Lake Bonneville in Utah, was the precursor of today’s Great Salt Lake.

The Quaternary period has been dominated by the advance and retreat of continental ice sheets and alpine glaciers; glacial periods alternate with warm interglacial periods. While the “ice age” continues today, the Earth is in an interglacial stage, since the ice sheets have retreated for now. The glacial-interglacial cycling of ice ages indicates that the world will return to a glacial stage in the future, unless the impacts of human-induced climate change radically shift these natural cycles.