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04/10/2008 07:00:00

A Grand Canyon as Old as the Dinosaurs?

PASADENA, Calif.--How the Grand Canyon was carved has been a topic of scientific controversy for nearly 140 years. Now, with new geochronologic data from the canyon and surrounding plateaus, geologists from the California Institute of Technology present significant evidence that the canyon formed nearly 50 million years earlier than previously thought.

The results will be published in the May issue of the Geological Society of America Bulletin in a paper by Rebecca Flowers, a former Caltech postdoctoral scholar now on the faculty of the University of Colorado; Chandler Family Professor of Geology Brian Wernicke; and Keck Foundation Professor of Geochemistry Kenneth Farley.

The team studied the sedimentary rock layers, or strata, of both the canyon and a large area of the surrounding plateaus. These strata were deposited near sea level sometime in the Paleozoic era (540-250 million years ago) and were subsequently uplifted and eroded to form the canyon. But questions like when and why the canyon itself formed have remained open.

The long-held interpretation sets canyon incision at about six million years ago, when the plateau that hosts it began to rise from near sea level to a current elevation of almost 7,000 feet. This view highlights the erosive power of the Colorado River, which cut into the plateau surface like a giant buzzsaw and progressively deepened the canyon at the same time the entire region was rising.

Now, using a radiometric dating method called uranium-thorium-helium [(U-Th)/He] dating, developed in Farley's lab, the researchers paint a different scenario. Uplift and carving of a deep canyon took place more than 55 million years ago, above the present position of the Grand Canyon's Upper Granite Gorge, within strata much younger than the Paleozoic rocks currently exposed in the canyon walls.

"When this canyon was formed, it looked like a much deeper version of present-day Zion Canyon, which cuts through strata of the Mesozoic era," Wernicke says. Then from 28 to 15 million years ago, a pulse of erosion deepened the already-formed canyon and also scoured the surrounding plateaus, stripping off the Mesozoic strata to reveal the Paleozoic rocks that we see today.

The key to the discovery lay in the ancient sandstones of the canyon walls, which contain scant grains of the phosphate mineral apatite that in turn host trace amounts of the radioactive elements uranium and thorium. These elements decay, spitting out helium atoms at well-constrained rates via alpha-particle emission. Although some of those atoms are lost through diffusion early in the grain's history, by measuring the abundances of all three elements, (U-Th)/He dating ultimately yields the time that an apatite crystal cooled below 70 degrees Celsius. Paired with information from boreholes about how Earth's temperature increases with depth, dates from apatite grains in rocks that are now at the surface communicate the last time those rocks were buried a mile deep.

A key finding of the Caltech team is that samples collected from the bottom of the Upper Granite Gorge region yield the same (U-Th)/He apatite dates as samples collected on the plateau surface nearby. "Because both canyon and plateau samples have resided near the same depth since 55 million years ago, a canyon of about the same dimensions as today must have existed at least that far back, and possibly as far back as the time of the last dinosaurs at the end of the Cretaceous period 65 million years ago," Wernicke states.

Wernicke says that the most surprising aspect of their new findings is that, since the Grand Canyon was originally cut, the adjacent plateaus have also eroded downward by about a mile, on average, every bit as fast as the bottom of the canyon. "And so the small, ephemeral streams that cover the arid plateau seem to be just as effective as the mighty Colorado at eroding away rock," he notes.

The erosional history proposed by the Caltech team jibes with other recent studies that also involve innovative radiometric dating techniques and speak to the early history of the canyon, Wernicke says. The first, undertaken by researchers led by Karl Karlstrom at the University of New Mexico and published last November in the same journal as the new Caltech study, demonstrated that the amount of downcutting of the Colorado in the Upper Granite Gorge was about 350 feet over the last 700,000 years. Extrapolated back in time, this rate is too slow to have carved the entire canyon in only six million years. Another University of New Mexico study, led by Carol Hill and Yemane Asmerom and published this March in the journal Science, demonstrated by dating cave deposits throughout the canyon that a water table, and therefore an erosion surface, lay somewhere near the canyon rim 17 million years ago, very close to the end of the pulse of erosion suggested by Caltech's (U-Th)/He dating.

The new work also echoes even earlier ideas of Richard Young of the State University of New York at Geneseo, Wernicke notes. In the 1980s, Young led a team that discovered that a group of ancient tributary canyons just south of the western Grand Canyon (Lower Granite Gorge region) were in fact originally formed between 63 and 50 million years ago, about the time the (U-Th)/He data suggest for initial cutting above the Upper Granite Gorge area. "The current wave of research thus strengthens the link between the formation of the tributary canyons and the evolution of the Grand Canyon proper, including the Upper Granite Gorge region," Wernicke says.

Wernicke credits much of the recent discoveries to cutting-edge dating techniques. "Although vigorous debate is sure to continue," he notes, "conventional wisdom about the history of the Grand Canyon in particular, and geology in general, is being challenged by these new, high-tech avenues of research."

Written by elisabeth nadin