A Grand Canyon as Old as the Dinosaurs?

The origin of the Grand Canyon has been a topic of scientific controversy for nearly 140 years. Now, with geochronologic data from the canyon and surrounding plateaus, geologists from the California Institute of Technology present significant evidence that lends new insight into its history of formation.

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 faculty at the University of Colorado; Chandler Family Professor of Geology Brian Wernicke; and Keck Foundation Professor of Geochemistry Kenneth Farley. They studied the sedimentary rock layers, or strata, of both the canyon and a large area of the surrounding plateaus. These strata, much like the rings of a tree, hold keys to Earth's history.

The researchers found that a large canyon, directly above and just as deep as the deepest part of the modern-day Grand Canyon, had already formed by 55 million years ago in overlying strata.

While traditional interpretations stress the Colorado River's power in cutting today's canyon like a giant scalpel, 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, on average, every bit as fast.

"Anyone who stands at the rim of Grand Canyon and looks into its deepest part, the Upper Granite Gorge, is confronted with one of the most humbling spectacles on Earth and a quiet and powerful case for the immensity of geologic time," remarks Wernicke. In order to flow in its present-day, mile-deep gorge, the Colorado River had to cut through the flat-lying strata exposed in the canyon walls and on the surrounding high, bare plateau. These strata have been virtually undisturbed for at least half a billion years, says Wernicke, pointing to erosion as the only possible explanation for the canyon's formation. "The Colorado River, with some help from the wind, ultimately carries the detritus away pebble by pebble, sand grain by sand grain. From this point of analysis, the unanswered questions about how and why the canyon formed start to pile up."

The widely-held notion among geologists is that the Colorado River connected drainages on the western slopes of the Rocky Mountains with the then newly-formed Gulf of California, incising the modern plateau surface to create the canyon when the plateau was uplifted from near sea level beginning about six million years ago.

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 above the present position of the Upper Granite Gorge, within strata much younger than those currently exposed in the canyon walls. "When this canyon was formed, it looked like a much deeper version of the present-day Zion Canyon, which cuts through much younger strata than the Grand Canyon does," Wernicke says.

Then from 28-15 million years ago, a rapid pulse of erosion lowered the already-formed canyon and its surrounding plateaus from an upper position within younger strata of the Mesozoic era down through older rocks of the Paleozoic era and close to what 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[ring] 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 give the same (U-Th)/He apatite dates as samples collected on the plateau surface nearby. The results also show that from 55-28 million years ago, both the canyon and the plateau resided at temperatures near 55° C, then cooled to near-surface temperatures by about 15 million years ago.

"Because both canyon and plateau samples 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. After a long period of comparatively slow erosion from 55-28 million years ago, both the plateau and the "proto"-Grand Canyon were rapidly eroded down about 6,000 feet, to a point relatively close to the current erosion surface, by about 15 million years ago. This was followed by a period of relatively slow erosion of perhaps an additional 1,000 feet.

According to Wernicke, 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. 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 month 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 50 and 63 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 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 Grand Canyon in particular, and geology in general, is being challenged by these new, high-tech avenues of research."

Source: CalTech

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