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Sandy Surprise for Double-Dating Geologists
29 August 2003 (All day)
The spectacular landscapes of Utah and Arizona owe their grandeur to vast dunes that solidified into rock millions of years ago. Most geologists assumed that such huge piles of sand must have come from the erosion of the nearby and long-vanished range called the Ancestral Rocky Mountains. But now a handful of grains from Utah, and a new technique for dating them, shows that the sands actually hail from clear across the continent.
Knowing the source of grains that make up sedimentary rocks can help reveal aspects of ancient landscapes. Traditionally, geologists have tried to do this by matching the compositions of the remaining source rock with the various kinds of grains that ended up in the distant sandstone. The strategy falls short, though, when several potential parent rocks fit the profile. That's the case for the Navajo Sandstone, a 190-million-year-old petrified desert that covered much of Utah and Arizona. So geologists Jeffrey Rahl and Peter Reiners of Yale University in New Haven, Connecticut, took a higher-tech approach.
Rahl and Reiners applied two dating techniques that can shed light on the early days of the source rocks, revealing a history that might eliminate some contenders. Using grains of the mineral zircon from the Navajo, Rahl and Reiners measured how much uranium had decayed into lead. This revealed that around half of the grains had crystallized from deep magma between 950 million and 1200 million years ago. They then measured the amount of helium, which is produced by the decay of uranium and thorium. Helium is trapped only after the grain cools to near surface temperatures, so the amount present indicated that the grain was pushed to the surface between 250 million and 500 million years ago.
To the researchers' surprise, these two dates exactly matched dates previously found for zircon in the Appalachian Mountains in the eastern United States, they report in the September issue of Geology. That means mighty rivers must have once crossed North America, carrying grains westward until they were picked up by winds and deposited in the desert dunes, they conclude.
"Most people would have suggested these grains came from the Ancestral Rocky Mountains," says geologist George Gehrels of the University of Arizona in Tucson. But the idea of rivers carrying sediment across a continent isn't unprecedented: That's what is happening today with the Amazon carrying grains from the Andes across South America, he says.