Region 1: The Basin and Range

While the formation of the Basin and Range province is a recent event that began only 30 million years ago, the bedrock that makes up the up-thrust ranges and down-dropped basins is very old. Since its formation, the bedrock of the basins has been covered by young deposits such as loose sediment washed down from the mountains and evaporite deposits from dried-out lakes. The ranges, however, expose rocks whose ages span from Precambrian to Cenozoic. Many of the sedimentary rock layers exposed in the mountains of the Basin and Range are the same as the rocks exposed by the Colorado River in the Grand Canyon, such as the Coconino Sandstone, Hermit Shale, and Kaibab Limestone, all of which are Permian in age.

The oldest rocks in the Basin and Range can be found in southern Nevada and the eastern Mojave Desert of California. They include granite as well as 1.7-billion-year-old metamorphic rocks such as gneiss, schist, and marble. The Pahrump group of rocks in Death Valley and nearby Nevada contains limestone in which stromatolites may be found.

An excellent place to see both Paleozoic and Mesozoic rock is near Las Vegas, Nevada. The dark gray limestone of the Bonanza King Formation formed in the Cambrian sea during a time period when the land that is now the Western States was still completely submerged—or not even yet part of North America. Today, the Paleozoic limestone has been thrust over the younger Jurassic Aztec Sandstone along a reverse fault that was created in the Mesozoic when the western edge of North America became a convergent plate boundary. The red Aztec Sandstone gives Red Rock Canyon its name (Figure 2.2). The sand that makes up this stone was deposited as sand dunes, which is evident from the cross-bedded layers that result from sand sliding down the side of a dune.

Stromatolites

Stromatolites are regularly banded accumulations of sediment created by the trapping and cementation of sediment grains in bacterial mats (especially photosynthetic cyanobacteria). Cyanobacteria emit a sticky substance that binds settling clay grains and creates a chemical environment that leads to the precipitation of calcium carbonate. The calcium carbonate then hardens the underlying layers of bacterial mats, while the living bacteria move upward so that they are not buried. Over time, this cycle of growth combined with sediment capture creates a rounded structure filled with banded layers.

Stromatolites peaked in abundance around 1.25 billion years ago, and likely declined due to predation by grazing organisms. Today, stromatolites exist in only a few locations worldwide, such as Shark Bay, Australia. Modern stromatolites form thick layers only in stressful environments, such as very salty water, that exclude animal grazers. Even though there are still modern stromatolites, the term is often used to refer specifically to fossils. For more information, see the Fossils chapter in this Guide.

During the Mesozoic, subduction occurred along the western coast. The resulting volcanoes are long since gone, but the granite that formed beneath those volcanoes remains, especially in the western parts of the Basin and Range. The stacked towers of granitic boulders found in Joshua Tree National Park (Figure 2.3), the Granite Mountains of the eastern Mojave, and Granite Peak in Nevada are all examples of granitic intrusions that formed during the Mesozoic.

Figure 2.2: The Aztec Sandstone in Nevada.

Cross-bedded Sand Dunes

Cross-bedded sand dunes form as air movement pushes sediment downwind, creating thin beds that slope gently in the direction of the flow as migrating ripples. The downstream slope of the ripple may be preserved as a thin layer dipping in the direction of the current, across the natural flat-lying repose of the beds. Another migrating ripple will form an additional layer on top of the previous one.

The formation of the San Andreas Fault around 30 million years ago ended the compression of southern California and Nevada, and a period of extension began, leading to the wide-spread formation of basins and ranges. The extensional forces thinned the crust, allowing magma to reach the surface, which resulted in a period of intense volcanism This volcanic activity continues to the present day. In addition to creating the faults that make up the fault-block mountains of the Basin and Range, the tension resulted in a period of intense volcanism, which still continues today.

Figure 2.3: “Giant Marbles” in Joshua Tree National Park.

This volcanic activity gave rise to a wide variety of igneous materials. Hole in the Wall and Wild Horse Mesa of the eastern Mojave National Park provide evidence for an explosive eruption of rhyolitic ash, which created an ash-flow tuff (Figure 2.4). Ash-flow tuffs are the result of pyroclastic flows—explosions that contain pulverized rock and superheated gases, which can reach temperatures of up to 1000 °C (1830 °F). The violent expansion of hot gas shreds the erupting magma into tiny particles that cool in the air to form dense clouds of volcanic ash. The tremendous explosions that are necessary to create ash-flow tuffs are caused by rhyolitic magma, which is felsic in nature. High silica content makes the magma quite viscous, preventing gas bubbles from easily escaping, thus leading to pressure build-ups that are released by explosive eruptions. The ash flows from these violent explosions tend to hug the ground, eventually solidifying into tuffs. Tuffs and other pyroclastic materials are vesicular (porous) due to gases expanding within the material as it cools.

The largest deposit of ash-flow tuff in the West is the Bishop Tuff from California’s Long Valley Caldera eruption 760,000 years ago. The Bishop Tuff is pink in color, vesicular, and contains pieces of pumice (Figure 2.5). Along the Owens Gorge, just north of Bishop, California, the Bishop Tuff can be found in columnar joints.

Figure 2.4: Hole in the Wall ash-flow tuff.

Figure 2.5: Bishop Tuff.

Not all volcanic rocks of the Basin and Range were created under such explosive conditions. Mafic magmas, which create rocks such as basalt, are less viscous and therefore can flow more easily. These gentler, effusive eruptions create features such as cinder cones and basalt flows. Lunar Crater in Nevada contains cinder cones and basalt lava flows that date from the Miocene to the Pleistocene, 20,000 years ago. Amboy Crater, Pisgah Crater, and the Cima Volcanic fields of the eastern Mojave also produced basalt and scoria.

In addition to the ancient stromatolites preserved in limestone, the Basin and Range also contains younger sedimentary rocks. Because the region is part of the Great Basin, water does not flow to the ocean, but instead evaporates from many playa lakes in the basins. At Mono Lake, a large playa lake located in Mono County, California, bubbling hot springs create a spongy limestone called tufa, which composes the strange towers that can now be seen since the water level of the lake has fallen (Figure 2.6).

Figure 2.6: Tufa towers at Mono Lake, California.