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 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

August 2000

News Notes
 Geologic History

Grand Canyon quandary

About 75 experts assembled in Grand Canyon Village, Ariz., June 7-9 for a meeting of the minds on the origin of the Grand Canyon. They debated the evolution of the landscape, formation of the canyon, age of Quaternary deposits and timing of fault movements. Not since 1964, when the last organized gathering of scientists debated the issue, has the geologic marvel received such concentrated scrutiny.

Interest in a Grand Canyon meeting intensified at a Geological Society of America conference two years ago.  At an informal gathering, several geologists decided that a reconsideration of new data was long overdue. As a result, this year’s meeting focused on examining the diverse data sets collected in recent years, including extensive potassium-argon age dating.

The majority of professional researchers at the meeting agreed that the modern Colorado River probably did not flow to the Gulf of California until 4.5 million to 6 million years ago. Most also acknowledged strong evidence that a significant amount of the deep-canyon cutting in the upper Colorado River could have occurred within the last million years.

“I was impressed by the diverse lines of evidence presented to support that the last 1,000 to 1,500 feet of the eastern Grand Canyon were cut in the last million years,” says geologist Stephen Reynolds of Arizona State University. Most of the canyon was eroded between 4 million and 6 million years ago. But over the past three decades, scientists have increasingly suspected that significant changes in the eastern canyon occurred over a short time frame. Only now are multiple lines of support emerging, says conference organizer Richard Young of the State University of New York at Geneseo.

     Grand Canyon conference field trip to eastern Arizona
      (Navajo Reservation) to examine sediments deposited
      in Lake Hopi (Bidahochi Formation), considered a 
      possible temporary basin to store water of an early 
      (ancestral) upper Colorado River between 5 million and
      16 million years ago. Richard Young, SUNY Geneseo.

According to co-organizer George Billingsley of the U.S. Geological Survey (USGS), the large amount of Quaternary activity along Hurricane fault supports the relatively young age of the eastern canyon. It is now thought the normal fault did not exist before 2.5 million years ago. As the major north-south trending fault in the western Grand Canyon significantly lowered the terrain to the west, the Colorado River could have responded by rapidly downcutting in the east to regain equilibrium. “I think it’s the Hurricane fault that caused the downcutting,” Reynolds says,
adding that all known evidence for downcutting is upstream, east of the fault.

Estimated Quaternary downcutting rates also support a geologically recent origin. The rates are rapid enough to account for the latest stage of eastern canyon formation during the last million years, Young says. Based on the research by Ivo Lucchitta and Tom Hanks of the USGS and consultant Sid Davis, Quaternary deposits upstream from the Grand Canyon, near Glen Canyon, produced the youngest age constraint, limiting parts of the eastern canyon erosion to within the last 700,000 years.

One theory on overall Grand Canyon formation combines evidence for older and younger ages of origin. In the past, canyon formation was attributed to cutting or to a large-scale erosional stripping of sedimentary rocks off the Colorado Plateau, a process called the “Great Denudation.” Graduate student Andre Potochnik of Arizona State University suggests that rivers flowing to the northeast more than 30 million years ago established parts of the course of the Colorado River. Mesozoic rocks may have been stripped around this time, Reynolds says, but the final notching of the deeper canyon could have occurred much later.

Bridget Mulvey

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