Lecture 21 -The Basin and Range Province

Outline

I. Basin and Range Province

II. Deserts

III. Death Valley National Park

IV. Joshua Tree National Park  

I. Basin and Range Province

Extends from southeastern Oregon southward to include all of Nevada, western Utah, southwestern Arizona and even southwest Texas. The Basin and Range Province contains 7 national parks: Death Valley and Joshua Tree in California, Big Basin in Nevada, Saguaro in Arizona, Guadalupe Mountains and Carlsbad Caverns in New Mexico, and Big Bend in Texas. All these parks share the following common features:

1. normal faulted mountain ranges alternating with sediment filled basins
2. arid climate resulting in desert processes and landscapes

The normal faulted mountain ranges result from tensional forces that developed in western North America in late Cenozoic time and continue today. These tensional forces are responsible for raising the Teton Mountains in Wyoming and the Sierra Nevada in California. The crust between California and the Rocky Mountains is being stretched and thinned, in addition to generating a parallel series of normal faulted mountain ranges and basins, hot mantle material has ascended to shallow depth producing recent volcanism. The rocks that make up the mountains and valleys of the Basin and Range vary considerably from place to place since they experienced different geologic histories prior to late Cenozoic extension.

II. Deserts

Deserts make up about 25% of the total world land mass. They are characterized by low rainfall (less than 25 cm per year) and high temperatures, evaporation, and wind. They generally lie between about 20-30° north or south latitudes and are produced by atmospheric effects (sites of cold, dry downwellings of air). Western North America has 4 different deserts characterized by different vegetation: Great Basin, Mojave, Sonoran, and Chihauhaun.

Even though deserts have very low rainfall, water is still the most important agent of erosion acting on the landscapes. The water table tends to be very deep in deserts and streams are not maintained by springs. They are mostly intermittent and run during desert storms. Large volumes of water are carried down desert mountains during flash floods and carry a great amount of sediment with them. This sediment accumulates in large fan structures at the base of the mountains called alluvial fans. When alluvial fans from neighboring canyons coalesce into a single apron of sediment we call this a bajada. Beautiful examples of both of these structures are abundant in Death Valley National Park. As bajadas grow they can almost bury the mountain range that they were derived from producing a pediment. Many desert areas develop in regions of no external drainages and playas, dried up lake beds that are periodically flooded and evaporated (depositing salt deposits) develop.

Wind is also an important agent of erosion in deserts and can produce sand dunes with large cross bedding if sand is available or desert pavement where wind removes all the finer grained material and leaves behind a ground surface covered by gravel. Desert pavement is very common in US deserts.

III. Death Valley National Park

Located mostly in the Mojave Desert, Death Valley became a national park in 1994 (proclaimed a national monument in 1933). Elevations range from over 11,000 ft (Telescope peak in the Panamint range) to almost 300 ft below sea level. The mountain and valley areas result from normal faulting. Playas are common in the numerous valleys of Death Valley. The rapid evaporation that follows lake formation results in deposition of evaporites such as salt. During the last glacial period, when the climate was much wetter, a large lake, Lake Manly occupied much of Death Valley. Evidence for a lake over 145 km long and 180 m deep exists as wave cut shorelines high on the mountain slopes. This lake evaporated about 11,000 years ago leaving behind large salt deposits. Other interesting features in Death Valley include young volcanics (basaltic cinder cone at south end near Mormon Point), sand dunes (Mesquite Flat north of Stovepipe Wells), and springs (near Furnace Creek Wash).

IV. Joshua Tree National Park

Also located in the Mojave Desert of southeastern California, Joshua Tree became a national park in 1994 at the same time as Death Valley. It too had been a national monument since the 1930s. The park contains normal faulted mountain ranges and intervening valleys like all parks in the Basin and Range Province, but it is best known forts Mesozoic aged granites that have been weathered into rounded shapes by exfoliation. Fracture patterns in the rock have played a large role in shaping the landscape. Desert features such as playas, alluvial fans, bajadas, and pediments are all common.

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