Amazon River Once Flowed Other Way, Study Says

Sean Markey
for National Geographic News
October 25, 2006
The entire Amazon Basin, the world's largest river drainage system, once
flowed in the opposite direction—a find that could pave the way to
new oil and gas exploration, scientists say.

Geologists in the United States and Brazil say the discovery that the river previously flowed east-to-west was accidental. The team was studying how swiftly sediment travels in the Amazon Basin from its headwaters in the Andes Mountains of Peru to the Atlantic Ocean.

Using airplanes and riverboats, the team traversed nearly 80 percent of the Amazon Basin to collect geologic samples (map of Brazil).

"All the current indicators in the ancient sediments"—including ripple marks and telltale mineral traces—"showed that the current, the river flow, was from the east to the west," said study author Drew Coleman, a geologist at the University of North Carolina at Chapel Hill.

Previous studies have shown that some segments of the Amazon Basin once flowed opposite their present course.

But the new study is the first to suggest that the entire drainage system flowed from the Atlantic to the Pacific for a period of time. (Related: "Amazon Logging Twice as Heavy as Thought, Images Show" [October 20, 2005].)

Researchers presented the study today at the annual meeting of the American Geological Society in Philadelphia, Pennsylvania.

Continental Breakup

The Amazon was flowing west-to-east, as it does now, when the shift occurred. That reversal is "almost certainly tied" to the breakup of the South American and African continents, which began about 130 million years ago, Coleman said.

"When that happened, the east coast [of South America] was uplifted—and the whole river flowed the other way," he said.

When the Andes Mountains started growing at about the end of the Cretaceous period (around 65 million years ago), the geologic tide begin to shift again in favor of the Amazon's current west-to-east course.

Precisely when the basin made these dramatic shifts remains unknown, Coleman says, but the changes were abrupt.

"It flip-flopped pretty quickly," he said, adding that the study reveals these shifts can occur on a "continental scale."

New Oil Reserves?

Experts say the Amazon drainage assumed its present west-to-east flow at least 16 million years ago.

"The Amazon fan right now is an active oil and gas exploration area," Coleman said. "And if things were flowing in the other direction, then it makes sense to look where the ancient Amazon fan was being deposited for those sorts of natural resources."

Any potential reserves would lie in the central Amazon Basin of Brazil and off the west coast of South America.

The study's findings highlight that "the surface of the Earth is very transient," study co-author Russell Mapes, a UNC graduate student, said in a press statement.

"Although the Amazon seems permanent and unchanging, it has actually gone through three different stages of drainage since the mid-Cretaceous,"—about a hundred million years ago—"a short period of time geologically speaking."

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