How the Grand Canyon Took Shape 5.6 Million Years Ago Is Still Debated, but Ancient Lakes May Help Explain

The Colorado River has flowed through the Grand Canyon for more than 5 million years, but the two were not initially united. For millions of years, before returning to its current course, the river followed a path that did not cross the Grand Canyon. Scientists now believe they have found an explanation for how the river ended up in the canyon.

A new study published in Science suggests that the Colorado River became part of the Grand Canyon when several ancient lakes connected to the river gradually filled and drained into the canyon. The age of zircon crystals in the Bidahochi Basin in northern Arizona lends weight to this overflow hypothesis, shedding light on the origins of the river’s altered course.

“After integration, the transported sediments would have given the newly formed Colorado River the tools to begin carving out the canyon. Some reaches were likely newly excavated and others where paleocanyons were located would have been significantly deepened by the integrated Colorado River over millions of years,” said study author Ryan Crow, a geologist with the U.S. Geological Survey. Discover.

How the Colorado River and the Grand Canyon met

A precursor to the modern Colorado River existed in western Colorado at least 11 million years ago. Researchers have proposed numerous routes for the first river, estimating that it may have flowed almost parallel to the current river, as well as northwest and southeast of the Grand Canyon.

The river only emerged from the Grand Canyon as it does today 5.6 million years ago, leaving a significant gap in the river’s history. Researchers have proposed several theories to make sense of the river’s altered path through the canyon, from headwater erosion to karst piracy, which involves transporting large volumes of water through cave systems.

The new study, however, uncovered evidence that the overflow of a paleolake system was the primary process by which the Colorado River was channeled toward the Grand Canyon.

The study authors reached this conclusion after using uranium-lead dating on zircon crystals in volcanic ash beds and sandstones of the Bidahochi Formation; This area is home to a large basin that once contained a system of ephemeral lakes – temporary bodies of water that can either drain or recharge water, and are commonly found in arid regions.

These lakes, which once covered much of northeastern Arizona according to the National Park Service (NPS), deposited sediments like silt, clay and sand, which represent the lower part of the Bidahochi Formation. Ash and lava ejected by nearby volcanoes also reached the basin; the ash formed fine-grained layers in the lake sediments.

Learn more: The sharp-toothed worm and other creatures lived in the Grand Canyon 500 million years ago

Fingerprint search

Uranium-lead dating of zircon is often used to determine the age of rocks, and this method is particularly useful because zircon grains act like fingerprints in the geologic record, allowing researchers to trace sediment sources and river connections throughout ancient history.

The study authors found that the composition and age distribution of zircon grains in the upper Bidahochi Formation (representing a time period from 7 to 6 million years ago, in the late Miocene) match evidence found in early Colorado River deposits. This suggests that the Bidahochi Formation shares a sediment source with the river and was likely connected to the river 6.6 million years ago.

Additional evidence, including increased sediment accumulation and similarities between fossil fish found in the Bidahochi Formation and surrounding rivers, also indicates that the Colorado River flowed and filled the basin for more than a million years before expanding downstream into the Grand Canyon.

Sculpting the Grand Canyon

The study authors believe that the overflow of lakes in the Grand Canyon is likely what determined the current course of the Colorado River, although other factors such as groundwater flow or erosion may have played a lesser role.

In the nearly 5 million years since this change, the Colorado River has continued to carve the Grand Canyon wider and deeper; One way to do this is through deep cuts, which happens when the river carries large rocks and boulders that act like scissors, eating away at the riverbed, according to the NPS. It is this lasting influence of the river that has made the Grand Canyon the geological wonder we see today.

“Previous work shows that over the last million years, the Colorado River has cut through rock at an average rate of about 100 to 160 meters. [328-525 feet] per million years, so the process of carving the canyon continues, and the canyon we see today is the result of about 5 million years of river incision and erosion,” Crow told Discover.

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