Washington: Geologists have found evidence of a giant river which flowed in the opposite direction some 55 million years ago in the same region where the present-day Colorado River is carving the famous Grand Canyon in the US.
Researchers at the Carnegie Institution of Washington discovered the ancient river system by comparing sedimentary deposits in Utah and southwest Arizona.
By analysing different types of uranium and lead atoms in sand grains made of the mineral zircon, they were able to determine that the sand at both locations came from igneous bedrock in the Mojave region of southern California.
According to the researchers, 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 km northeast, the Discovery News reported.
It appears that the ancient river, named the California River, could have started in Mojave Desert of southeast California and southwest Arizona and headed northeast all the way to northern Utah, some 55 million years ago, said the researchers.
But it wasn't enough to just find the zircons, said Steve Davis, the lead author of the study which is published in the journal Geology.
"We had to prove how sure we were that they didn't come from the same place," said Davis.
They failed to statistically disprove the connection between the distantly placed zircons and reached the opposite conclusion that the rocks are originally from the same place
and those in Utah were carried by a long river.
"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, the experts believe.
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, they said.
According to Davis, a post-doctoral researcher at the Carnegie Institution, the zircons are not the only evidence of an opposite-running river.
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, according to Davis, all suggested 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 said, could have been up to 1,000 km.
First Published: Tuesday, October 12, 2010, 16:33