Chapter 17
Chapter 17: Dry Regions: The Geology of Deserts
Feature Articles
The Rest of the Story: The Basin and Range Province
by Stephen Marshak
Tectonic stretching of the North American continental crust during the last 25 million years has yielded a broad rift called the Basin and Range Province, so named because topographically, it consists of long linear mountain ranges separated from one another by flat plains (basins) of alluvium. The high Sierra Nevada Mountains form the western edge of the province, while the face of the Colorado Plateau defines its eastern edge.
When the Basin and Range Province first formed, normal faulting yielded a series of tilted, fault-bounded blocks. The wedge-shaped basins, or half-grabens, between these blocks quickly began to fill with alluvium that had eroded off the blocks. Bajadas covering the floor of the basins began lapping up the sides of the mountain ridges. Eventually, only the crests of the mountains remained, and these began to grow smaller because of scarp retreat and the formation of pediment. The pediments themselves have slowly been covered by alluvial fans. Thus, the ranges of the Basin and Range Province are rows of inselbergs, and the basins are alluvium-filled half-grabens. In some places, the centers of some of these basins have become playas or salt lakes. Overall, the floor of the Basin and Range Province lies low relative to its surroundings, so the province acts as an interior basin that geographers refer to as the Great Basin.
The Basin and Range Province experiences desert-like conditions because it not only lies in the rain shadow of the Sierra Nevada Mountains, but much of it lies far from the sea. Though dry throughout, variations in elevation and therefore temperature lead to major differences in the appearance of the landscape and the life it hosts. Thus, different portions of the desert are carpeted with scruffy, aromatic sagebrush. Others consist of playa (salt lakes), bordered by alluvial fans. Others are rocky landscapes dotted with a great variety of cacti and other desert conditions and unreliable water sources.
The province presented a major obstacle to settlers heading to California. Only with the construction of the transcontinental railroad did crossing the Basin and Range Province become routine.
The Rest of the Story: Navajo Sandstone
by Elizabeth Lane Mason
The famous Slickrock mountain bike trail winds its way over miles of Navajo Sandstone near Moab, Utah. Here the buff-colored Navajo eroded to mimic the great dunes that created it in the Jurassic Period, creating a petrified dune field of sorts. People come from all over the world to test their skills on this unique landscape. But bikers aren’t the only ones drawn to the excellent exposure of the Navajo. Geologists flock to the desert playground as well.
During the 160 million years that Pangaea straddled the equator and Utah was located near the western edge of the supercontinent, the wind-deposited Navajo sands built into a very thick layer. Of these deposits, between 500 and 700 meters are now preserved in the cliffs of Zion National Park. Recent scrutiny of the sandstone by geologists is yielding hints about the climate of the early Jurassic. It seems the Navajo may contain a prehistoric record of the weather.
The cross beds of the Navajo are traces of the lee sides of dunes that migrated in the direction of the prevailing winds. Because the prevailing wind direction shifted with the seasons, geologists can identify annual depositional cycles. During the winter, the dominant winds were northwesterly and the dunes migrated southeastwards. In the summer, northeasterly winds pushed the dunes to the southwest.
Scientists found evidence that mighty summer rainstorms traversed the desert landscape recorded in sediments deposited during the summer. The summer rainstorms soaked the dunes causing slumps in the lee faces. The rainwater seeped into the sand creating cohesive blocks that were unstable and slid a short distance down the lee face. This movement caused characteristic faults and folds that record the storm events.
One section of the Navajo represents 36 years of dune migration and has 24 slumps. Twenty of these occurred during the summer, and four were triggered by winter storms. The thickest slab measures 84 cm. Calculations reveal that a minimum of 17 cm of rain would be needed to saturate and destabilize a slab of loose sand of that thickness, giving scientists a clear picture of a cloudy day 200 million years ago.
REFERENCES
Loope, D.B., C.M. Rowe, R.M.Joeckel. 2001. “Annual monsoon rains recorded by Jurassic dunes.” Nature 412: 64-66.