viernes, 8 de octubre de 2010
Colorado River Once Flowed in Opposite Direction
* Zircon crystals tell of a dramatically reversed landscape 55 million years ago.
* The so-called California River ran more than 600 miles from southwest Arizona to northeast Utah.
* The zircons bolster other geological signs of an northeastern flowing river in the Paleogene Age.
Aerial view of the Colorado River, Imperial Valley. Minerals suggest a river once flowed in the same path but in the opposite direction some 55 million years ago. Where the Colorado River today is carving the Grand Canyon, there once flowed another river in the opposite direction, according to a new study comparing tiny grains of the mineral, zircon, in Utah and Arizona.
Telltale lead and uranium isotopes in the zircons from ancient sediments suggest a vast drainage system that fed what researchers call the California River. It started in today's Mojave Desert of southeast California and southwest Arizona and headed northeast all the way to northern Utah, some 55 million years ago.
Zircons with the same lead and uranium signatures were found in both old river sediments in northern Utah as well as rocks southwest Arizona. This means there had to be a mighty river to carry the minerals about 700 kilometers (435 miles) northeast.
"Zircons are very resistant to weathering and so can travel a long, long way," commented geologist Christopher Henry of the University of Nevada in Reno and the Nevada Bureau of Mines and Geology. That's why they are handy in reconstructing the ancient lay of the land. "Basically I'd say these people are right."
The source of the zircons appears to be granite-like rocks in what were called the McCoy Mountains, the remains of which are in southwest Arizona. This would have been part of the headwaters of the California River (so named because that's where it originated).
Rain and snow would have broken up a lot of McCoy Mountain's rocks into sand, and that sand would have been carried into the river, and eventually all the way to present-day Uinta Basin in northeastern Utah.
But it wasn't enough to just find the zircons, said Steve Davis, the lead author of a paper on the research in the October issue of the journal Geology. He and his co-authors also had to play devil's advocate and try to statistically disprove the connection between the distantly placed zircons.
"We had to prove how sure we were that they didn't come from the same place," said Davis. They failed to do that and so bolstered the opposite conclusion: that the rocks are originally from the same place and those in Utah were carried by a long river.
The zircons are not the only evidence of an opposite-running river, said Davis. There are also other studies which found what are called paleocurrent indicators that basically tell geologists which direction the water was flowing at a given location in the ancient river.
Paleocurrent indicators include such things as the angles at which river rocks are stacked, or the ways river bottom sand ripples are piled. Paleocurrents in Utah and Arizona all suggest something very opposite the Colorado was going on 55 million years ago.
The zircons, on the other hand, give a broader picture of where the river started and how far it flowed, which, Davis reports, could have been up to 1,000 kilometers (621 miles).