Long Ride West: Many western sediments came from Appalachians

North America may once have hosted a continent-crossing river system as grand as today’s Amazon, two new studies suggest. That notion is bolstered by the discovery that material in several thick layers of sandstone in the western United States originated in the Appalachians.

About 190 million years ago, what is now southwestern Utah was covered with sand dunes. The so-called Navajo Sandstone of today is the preserved remnant of dunes that covered as many as 660,000 square kilometers, an area almost the size of Texas. In some Utah locations, that rock formation is up to 750 meters thick.

Scientists have long debated where all that sand came from, says Peter W. Reiners, a geochemist at Yale University. In an effort to end the debate, he and his colleagues used two radioactive dating techniques on small mineral grains called zircons that they harvested from the sandstone. First, the researchers zapped a zircon with a laser and chemically parsed the vapor to estimate when the mineral crystallized. Then, they analyzed what was left of the mineral grain to determine when in the past the zircon had cooled below 180C, the temperature at which a zircon starts trapping the helium being produced by the radioactive decay of other atoms. Rocks cool below that temperature as they approach Earth’s surface during episodes of mountain growth.

The researchers found that about two-thirds of the zircons they analyzed had cooled between 400 million and 250 million years ago. Of that fraction, most had originally crystallized between 1.2 billion and 950 million years ago. Those periods correspond with the uplift of the Appalachians and the formation of the rocks from which they grew. Because no other large-scale mountain growth took place on the continent during that time, it’s a safe bet that most of those zircons came from the Appalachians, Reiners asserts. He and his colleagues report their findings in the September Geology.

If this conclusion is correct, the Appalachian zircons took a long, circuitous route to reach Utah. After eroding from exposed rocks, the zircons–and presumably many of the sandstone’s other mineral grains–were carried to a region north and northwest of Utah. The pattern of ripple marks and other features locked in southwestern Utah’s sandstone indicates that the mineral grains had been blown in from those directions.

The most likely transportation system across the continent, says Reiners, would have been a river system capable of moving massive amounts of sediment.

Similar analyses of sandstone at several western sites back this scenario. In a study to be published in an upcoming Sedimentary Geology, William R. Dickinson and George E. Gehrels of the University of Arizona in Tucson report that about half the zircons they analyzed came from the Appalachians, and about one-fourth had eroded from ancient rocks in central Canada. “We didn’t expect that many of [these zircons] would have come from so far away,” Dickinson says. The team’s research suggests that a westward-flowing transcontinental river system was in place several times between 275 million and 150 million years ago.

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