The Cenozoic: Volcanism and Tectonism

The Cenozoic era (consisting of the Paleogene and Neogene 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.

Erosion of the mountains and highlands that had formed during the Mesozoic produced thick layers of conglomerates, sandstones, and mudstones across much of the Northwest Central. Volcanic ash 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 1.13) which today supplies water for farming and communities across much of the Great Plains. Due to crustal deformation during the Mesozoic, several basins formed inland lakes or depressions into which sediments were deposited. The best-known example is the Green River Basin of western Wyoming, which is famous for its well-preserved fossils found in lakebed shales and mudstones.

Figure 1.13: Extent and saturated thickness of the Ogalalla Aquifer.

Subduction at the West Coast ceased with the development of the San Andreas Fault System. Due to the complex interplay of plate motions, the portion of the subducting plate beneath the Southwest 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 emplacement of numerous granitic bodies and volcanic eruptions across the western US, including the Absaroka Range in Wyoming and Montana and the Challis Volcanic Field in Idaho. These large packages of volcanic rocks also host important mineral deposits. Ash from these eruptions fell long distances from its source, and is a major component of terrestrial sediment on the Great Plains, much of which is abundantly fossiliferous.

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 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.

At the end of the Neogene, around eight million years ago, epeirogenic uplift (resulting from upwelling mantle heat pushing the crust upwards) began, raising the Rocky Mountains and Colorado Plateau to its current “mile-high” elevation and initiating the downcutting of the Grand Canyon in Arizona. Another example of downcutting is the more recent development, 500,000 years ago, of the Badlands in South Dakota, where Cretaceous and Cenozoic sedimentary rocks are eroded into badland topography.

The development of the Yellowstone hot spot appears to have begun with the eruption of the voluminous Columbia Plateau flood basalts in present-day Washington and Oregon around 14 million years ago. As the North American plate traveled over this mantle plume, the crust melted and produced a trail of volcanic rock that crosses southern Idaho, forming the Snake River Plain and ending at Yellowstone National Park in northwestern Wyoming (Figure 1.14). The trail of volcanic eruptions from the hot spot works its way east along this path, with major explosive caldera eruptions occurring on a cycle of around 600,000 years. Multiple minor eruptions occur between the larger explosions; for example, Craters of the Moon National Mounment in southern Idaho is a recent (15,000 to 2000 years old) volcanic flow associated with rift zones formed by the Yellowstone hot spot. The latest caldera at Yellowstone National Park is 630,000 years old, and contains many younger minor volcanic flows and domes. The recent geological history of volcanism at Yellowstone has led the area to be classified as a supervolcano. While there is concern that the hot spot could generate another violent eruption, researchers using seismic tomography have not observed large volumes of melt below the area that could result in a large eruption. The hot spot has now reached a boundary of thicker overlying crust, which will significantly affect the amount and timing of the melt it produces, and the odds of an explosive eruption occurring during the next several thousand years are very low.