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Chapter 21: Dry Regions: The Geology of Deserts
Feature Articles
The Rest of the Story: Navajo Sandstone
by Elizabeth Lane MasonThe 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.
Loope, D.B., Rowe, C.M., Joeckel, R.M., 2001, "Annual monsoon rains recorded by Jurassic dunes": Nature, v. 412, p. 64-66.
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