Grand Canyon born on East coast

[Brian G Turner]

I just liked this article - the idea of this immense movement of rock across what is now the continent of North America.

http://www.nature.com/nsu/030915/030915-3.html

Grand Canyon born on East coast

Like many of their tourist visitors, some of the rocks that make up the Grand Canyon came across North America from the East Coast, a new study reveals1.
Until now, the origin of the sands that covered approximately 350,000 square kilometers of the western United States and solidified into sandstone between 150 million and 300 million years ago has been a mystery.

"There's been no way to test hypotheses," explains geologist Bill Dickinson of the University of Arizona in Tucson. Most researchers assumed that the sands came from the now-flattened Ancestral Rocky Mountains, which stretched from southern New Mexico to northern Colorado 300 million years ago.

Now Dickinson and his Arizona colleague George Gehrels have discovered that around half of the sand was once part of the Appalachian Mountains, thousands of kilometres away. They propose that huge rivers carried the sand westwards, depositing it on an ancient shoreline in Wyoming, from whence winds blew it south into the dune fields.

Geologists are enthusiastic about the results. "I'm very pleased," says Bob Dott, emeritus professor at the University of Wisconsin-Madison who has studied the geological history of the western US for decades. "The big question was where did all the sand come from, and this paper has been the first to document it with hard data."

Traditionally, geologists have looked at a sandstone's grain types to discern its rocky parentage. Other clues, such as which way the wind or water that deposited the grains was flowing, pointed them in the right direction. But for some rocks, such as the ancient dunes of the western United States, these methods cannot narrow the possibilities much.

So Dickinson and Gehrels instead scrutinized grains of zircon, a uranium-bearing mineral, in the sandstones. As soon as zircon crystallizes from molten magma, its radioactive uranium begins to decay into lead. The amount of lead in a zircon grain therefore reveals when it formed. These ages can then be matched to zircon ages from different mountain ranges.

Half of the Grand Canyon samples were formed either around 1.2 billion years ago or around 500 million years ago. These ages match granite in the Appalachian Mountains. Only a quarter of the grains came from the Ancestral Rockies; the rest hark from the interior of Canada. "I was very surprised by what we found - I thought the study would basically confirm the going hypothesis," says Gehrels.

Clock watching

Recent technical advances in uranium-lead dating have drastically cut the number of grains and the amount of time required to get results. This is the first study to apply the new methods comprehensively to sandstone origins. "It answers questions you couldn't even ask before," says Paul Link of Idaho State University in Pocatello, who is using the techniques on rocks in Idaho's Snake River area.

The technique could also help geologists to probe the past positions of the tectonic plates that move across Earth's surface. Matching zircon ages from the sandstone of one continent to the mountains of another provides evidence that the two must have once been joined.

"It gives you something like a barcode to make the correlation between a potential source rock and a sedimentary basin," says Ray Price, professor emeritus at Queens University in Kingston, Ontario.