New Clues Date Grand Canyon to Dino Days

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Grand Canyon viewed from Hopi Point, on the south rim. New evidence suggests the western Grand Canyon was cut to within 70 percent of its current depth long before the Colorado River existed. CREDIT: National Park Service.

Content provided by Becky Oskin, OurAmazingPlanet

The age of the Grand Canyon is a puzzle, because the Colorado River has washed away many of the clues.

So for 150 years, geologists have pondered the processes shaping the

canyon, one of the world's great wonders and a living laboratory for

understanding Earth history.

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The gorge's rugged beauty, with its sheer cliff and steep slopes, looks

young. And the general scientific consensus, updated at a 2010

conference, holds that the copper-colored Colorado River

carved the Grand Canyon beginning 5 million to 6 million years ago.

Many strong lines of evidence support this theory, including a pile of

gravel and limestone pancaked with lava at a place called Muddy Creek.

This geologic layer cake, at the western mouth of the canyon, locks down

the Colorado River from exiting the canyon before 6 million years ago.

However, recent advances in dating techniques have upended the notion of a uniformly young Grand Canyon.

The new approach determines when erosion uncovered rocks in the canyon.

The big picture: there were two ancestral canyons, one in the west and

one in the east. And the western canyon may be as old as 70 million

years.

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Grand Canyon's ancestors

The latest sally is a study reporting samples from the western Grand

Canyon were close to the Earth's surface 70 million years ago. The

evidence suggests the western Grand Canyon was cut to within 70 percent

of its current depth of 3,200 feet (1,000 meters) long before the

Colorado River existed. The results appear today (Nov. 29) in the

journal Science.

"Our data suggests that there was in fact a large canyon present for

most of the Grand Canyon by about 70 million years ago in its western

segment, and that canyon was carved to nearly modern depths," said

Rebecca Flowers, lead study author and a geology professor at the

University of Colorado, Boulder. "In the eastern canyon, the canyon was

higher, and lowered into its modern configuration sometime after 20

million years ago."

This much older western "paleocanyon" was incised by an ancient river flowing west to east.

This Cretaceous river carved the western Grand Canyon to within a few

hundred meters of its modern depth, and the eastern Grand Canyon to a

higher level.

When combined with rock sample ages Flowers collected in the eastern Grand Canyon during this study and in 2008,

the overview gives the Grand Canyon a complicated history. However, the

research can fit into the constraints presented by the Muddy Creek

barrier and other evidence supporting a young canyon, Flowers told

OurAmazingPlanet.

"The presence of the [Muddy Creek] detritus represents the integration

of the river system," Flowers said. That is, the Muddy Creek simply

represents the Colorado River appropriating the paleocanyons and created

a single drainage 6 million years ago.

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Two canyons?

Geologist Richard Young, who has studied the Grand Canyon for nearly 50

years, said scientists have considered the idea of two Grand Canyon

precursors — one west, one east — since the research community's first

symposium in 1964.

North America during the Late Cretaceous, 68 million years ago. The flat Colorado Plateau can be seen in the southwest, bounded by the Sevier Mountains on the west and the Western Interior Sea to the east. CREDIT: U.S. Geological Survey

"We agreed that there were two canyons, one in the west and in the

east, we don't disagree on that," he said. The problem is that Dr.

Flowers wants to make the western canyon very old, Young told

OurAmazingPlanet. (Related: Grand Canyon in Pictures)

"It really looks like they're onto something, but it's hard to make

sense out of it," said Young, a professor at the State University of New

York in Geneseo. "It's really good work and it's really interesting, so

obviously there's something we're missing in the story. I'm sure we're

going to be talking about it forever," he said.

Recent work by geologist Karl Karlstrom supports the idea for a

paleocanyon in the east. "We showed very conclusively that there was a

paleocanyon in the eastern Grand Canyon that was carved between 25 and

15 million years ago," said Karlstrom, a professor at the University of

New Mexico in Albuquerque.

But Karlstrom is a strong advocate for a Grand Canyon quickly carved

by the Colorado River starting 6 million years ago, not older rivers

coming from the west. The western Grand Canyon region was cut across

nearly at right angles by one or more paleocanyons with rivers that

flowed north around 70 million years ago, but these paleorivers did not

follow the modern course of Grand Canyon, Karlstrom said.

"The best answer is that Grand Canyon was carved by the west-flowing

Colorado River in the last 5 to 6 million years and that earlier

paleocanyons were likely re-used and deepened once the river found its

present path," he said.

Southwest during the end of the dinosaurs

The American Southwest

had a radically different appearance 70 million years ago. Most of the

region's famed dinosaur fossils come from the Jurassic, and the

canyon-cutting identified by Flowers and colleague Ken Farley of Caltech

began in the Late Cretaceous.

Seen from the air, the flat Colorado plateau might be recognizable, but

the rainbow-hued pillars and monuments of national parks such as

Arches, Zion and Bryce had yet to take shape. Close to the west rose a

volcanic arc similar to today's Andes — the precursor to California's

Sierra Nevada Mountains. A wrinkled belt called the Sevier mountains was

northwest of the plateau. To the east was the Western Interior Seaway.

Rivers flowed out of mountains generally heading northeast into the

ocean.

The infant Rocky Mountains didn't start their rise in the east until

about 10 million years later, though this timing is debated. The Basin and Range

province, which built the classic Southwest monuments and valleys

immortalized in film and art, began tearing apart 20 million years ago.

Rivers crossing the Colorado Plateau reversed their course, flowing east

to west, around this time.

"We know the river systems must have evolved dramatically during this

time. The controversial part of it is how they evolved," Flowers said.

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