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Updated 10:00 AM December 3, 2007
 

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Ancient supercontinent studies raise conundrum

For a quarter-century or more, the prevailing view among geoscientists — supported by paleomagnetic records in rock — has been that the portion of the ancient supercontinent of Pangea that is now the Colorado Plateau in southern Utah shifted more than 1,300 miles north during a 100-million-year span that ended about 200 million years ago in the early Jurassic Period, when Pangea began to break up.
Jurassic Navajo Sandstone in Capitol Reef National Park, southern Utah. Photos courtesy Science.
Above, a sandstone cliff in Vermillion Cliffs National Monument, Ariz., composed of the deposits of three large, Jurassic sand dunes (person for scale in lower center). Each dune migrated toward the present southeast (toward the right in this photo). The dune sand accumulated about 200 million years ago, just above sea level in a slowly subsiding sedimentary basin. Circulating groundwater cemented the sand into sandstone. Below, Jurassic Navajo Sandstone in Zion National Park, southern Utah. The sloping lines within the sandstone (crossbeds) indicate the sediment accumulated in a large dune field.

But new research by a team of geoscientists from U-M and the University of Nebraska-Lincoln (UNL) challenges that theory, based on extensive climate modeling studies and sedimentary records found from Wyoming into Utah and Arizona.

In a paper published in the Nov. 23 issue of the journal Science, U-M geophysicist Rob Van der Voo and co-authors report findings that indicate the area must have remained at the equator during the time in question.

"It's a puzzle, a 'conundrum' is the word we like to use," says Robert Oglesby of UNL. "And in the Science paper, we're not solving the conundrum, we're raising the conundrum."

The puzzle revolves around ongoing research by UNL researcher David Loope in the Colorado Plateau. A sedimentologist and an expert on dune formation, Loope found that from central Wyoming into central Utah, ancient dunes preserved in the region's sandstone formations from 300 million to 200 million years ago all faced southwest, meaning that the winds over that extensive area were almost constantly from the northeast.

As his study progressed, he discovered that the direction of the dunes shifted to the southeast in what is now southern Utah, meaning the wind direction shifted to the northwest. What's more, those prevailing winds were consistent over the entire 100 million years in question and the shift in wind direction could only have occurred at the equator.

"I thought that was very curious," Loope says. "It didn't seem to fit with what we think we know about where the continents were."

Puzzled by the discrepancy between their research and the paleomagnetic records, they turned to Van der Voo, an expert on paleomagnetism.

Paleomagnetic records are found in igneous rocks that permanently record the direction of the Earth's magnetic field at the time they solidify from the molten state. They're an important tool for geoscientists in tracking the movement of Earth's tectonic plates over time, and records in North America indicate that the Colorado Plateau moved from the equator to about 20 degrees north latitude from 300 million years ago to 200 million years ago.

"The nicest thing would have been if we had a solution, but we don't," says Van der Voo, the Frank H. T. Rhodes Professor of Geological Sciences at U-M. "All we can say is that we have this enigma, so perhaps our model of Pangea for the period in question is wrong or the wind direction didn't follow the common patterns that we recognize in the modern world. Neither seems likely, but we're bringing this inconsistency to the attention of the scientific community in hopes of stimulating further research."

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