So How (& when) was the grand Canyon Formed?
WAYNE RANNEY, GEOLOGIST AND AUTHOR OF "CARVING GRAND CANYON"
The quality of new research that was presented at the recent Colorado River workshop in Flagstaff was certainly impressive. Having been a long time observer and writer of popular literature on the subject, I cannot recall such an active and exciting period of investigation. Although each person attending the workshop might come away with an entirely different impression of it, I offer my thoughts on some of the exciting ideas that floated my way.
As a geologist, it strikes me as quite peculiar that in the second decade of the new millennium, the age of the Grand Canyon cannot be resolved any clearer than between 70 and 5.3 Ma (million years). To me this is simply astonishing. And while the majority of workers see a relatively youthful canyon, Brian Wernicke, Bill Dickinson and Carol Hill presented different lines of evidence suggesting a river and/or canyon as old as 70 Ma. Wernicke referred to the “Arizona River,” flowing through an early Grand Canyon, while Dickinson envisions a “California River” flowing from the Mojave region towards the Uintah basin (see Jon Spencer’s article for their methodology). Stay tuned to this evolving line of research as things are getting real interesting.
While the above studies highlight the evidence for an old river, there was also plenty of support for a young one. Kyle House and others presented what I thought was one of the most exceptional studies regarding the age of the lower Colorado River. Working in the Bullhead City/Laughlin area, they may have finally laid to rest the dilemma on the origin of the Bouse Formation. They show that a series of spillover episodes broke through bedrock divides from north to south and progressively filled the Cottonwood, Mojave and Chemuhuevi valleys with water, rocks, and ultimately Colorado River sediments. I found myself at the edge of my seat as they figuratively turned their gaze upstream on the river, and pondered what hydrologic and geologic incidents might have occurred beyond the Grand Canyon to set off these relatively rapid spillover events on the lower river.
Regarding the rivers enigmatic configuration and possible integration, two new and divergent ideas stand out. Charles Ferguson outlined a possible fluvial connection between the Bidahochi basin in Arizona and the Snake River in Idaho. Using fossil fish evidence from both basins and invoking a correlation of the Powder Rim gravel with Arizona’s rim gravel, he suggested a north-directed link between the Colorado/Green and Snake rivers might be as young as 5 or 6 Ma. Contrast this with Ivo Lucchitta and Dick Holm who revisited rounded river clasts exposed on Crooked Ridge and postulated an entirely different system flowing southwest from the San Juan Mountains towards the Grand Canyon perhaps 20 to 15 Ma. And to think that I once believed this field would mature and become less interesting as new research ‘lifted the veil of mystery’ on the origins of the Colorado River and its Grand Canyon.
I can’t help but notice that if a researchers specialty looks like a “hammer,” the lines of inquiry and the answers they propose look a lot like “nails.” Structural geologists invoke mantle uplift to explain the canyon, geomorphologists look for clues on earth’s surface, while karst experts peer beneath to the realm of groundwater. (It makes you wonder what we’d know about this story if a climatologist had been invited)? What an extraordinary mix assembled in Flagstaff: creative minds that utilize innovative thinking, geeky laboratory technology, and even some good old-fashioned fieldwork. And even though an answer still eludes us, the great gorge exerts a powerful draw to some very lucky geologists.