Utah Desert Sandstone History Reveals Jurassic Monsoons

Contact: Clinton Rowe, geosciences -- (402) 472-1946; David Loope, geosciences -- (402) 472-2647

UNL GEOSCIENTISTS LOOPE, ROWE AND JOECKEL UNCOVER MONSOON EVIDENCE IN NAVAJO SANDSTONE

(News release Web site: http://www.unl.edu/pr/releases.html)

Lincoln (Neb.) -- Researchers at the University of Nebraska-Lincoln discovered that monsoon rains fell in a vast desert dunefield, and helped create the fabulous Navajo sandstone deposits in Utah, Arizona, Nevada and Wyoming.

Evidence of heavy summer rains, during the Jurassic Period (208 to 146 million years ago), is present in the geologic history of the Navajo sandstone cliffs in Utah, according to David Loope, a UNL geoscientist and a world-known expert on windblown sediment.

Loope's geologic field work with Matt Joeckel of the Conservation and Survey Division, as well as accompanying climatological research by Clint Rowe, is highlighted on the cover of the July 5 edition of "Nature," the international weekly journal of science. "Annual monsoon rains recorded by Jurassic dunes," reports that these Jurassic summer rains created slumps in the ancient sand dunes, and were later covered by dry sand deposits.

That information is new, and could offer hints about climate worldwide during the same period, Loope said.

"The paper reports new details of the nature of the weather and climate during that time," Loope said from Kanab, Utah, where he continues his research this summer. "Geologists are always trying to extract more information about Earth's history. In this case, an amazing amount of detail is recorded in these cliffs and we can tell how far the dunes moved each year, and tell that the dominant wind was in the winter."

Loope said evidence of the desert monsoons is clear.

"The reason we know the rain came at all is that when enough rain falls, the dune will slump, producing a distinctive contorted layer that is buried by smooth, undisturbed layers when the dune starts to move again," he said. Field observations at one site showed that during 37 years of dune migration, 24 "slumps" were generated. The Navajo sandstone forms canyons and cliffs throughout the southwest, and is obvious to visitors at Utah's Zion, Capitol Reef and Arches National Parks, where some of the towering sandstone formations reach upward of 2,000 feet.

Loope said evidence of the monsoon rains appears to be restricted to the lower part of the Navajo sandstone. At this time, Utah was only 15 degrees north of the equator and lay toward the western margin of the supercontinent Pangaea, the largest landmass known from Earth's history. Studies on ancient deserts help reveal how climate patterns have changed during the Earth's history.

"We hope to be able to go into more detail, and Clint Rowe will be looking into computer-modeling the early Jurassic climate. Combining field observations with computer models, we will try to figure out how the wind systems changed during this time period," Loope said. Rowe said previous modeling has looked at average seasonal shifts in the atmospheric circulation.

"Now, since we know that both seasonal -- monsoonal -- and shorter term wind variations are recorded in the dune layers, we should be able to gain greater insight into regional weather patterns during the Jurassic," Rowe said. Those climate models and simulations may have use in research in other areas of the globe and continents, which were aligned much differently then.

The Nature article marks the third major science publication in one year for Loope. In July 2000, Nature published his research on volcanic ash at Scottsbluff, and also that month Natural History recorded his research in the Gobi Desert.

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