Wednesday, October 30, 2013

GSA 125 Denver - Tuesday - The River that Formed Grand Canyon Flowed Where? Stunning Proposal by University of Montana Geologist

Tuesdays sessions were devoted to ideas related to the evolution of the Grand Canyon, the Colorado Plateau, and the Rocky Mountains.

Lon Abbott (Univ. of Colorado) presented a fascinating talk on nine travertine deposits that are found on or near the Tonto Platform between Hermit and Boucher creeks. The most interesting of these are the ones perched on top of the Tonto. Curiously, they do not drape over the ledge into the Inner Gorge, suggesting to him that they predate the cutting of the Vishnu Schist here. Abbott determined (through a convoluted but successful series of determinations) that the age of these perched travertines are between 591,000 and 497,000 years old. This could be the age of the Inner Gorge in Grand Canyon, which is quite young. He was also able to suggest a scarp retreat rate for the Muav/Redwall cliff behind the Tonto of about 850 m/Ma (meters per million years). These are fantastic results that show fast rates in the possible recent sculpting of Grand Canyon.

Ryan Crow (Univ. of New Mexico) was next and reported on mantle-driven uplift of the Grand Canyon region. Chemical signatures in basalt rocks for the last 10 million years show a trend through time to sources derived from the lithosphere to aesthenosphere. He painted a picture of a propagating wave of uplift, that progressively raised the margin of the Colorado Plateau from SW to NE. This wave of uplift he suggested, could be the driver of canyon incision.

Lots of talks spoke to the possible causes of the variable stream gradients observed along the Colorado River system. Possible controls on gradient are rock strength (resistance to abrasion), knickpoint migration, uplift, and/or subsidence. Kelin Whipple (Ariz. State Univ.) gave evidence for the importance of rock strength in determining these gradients (the Colorado/Green rivers have four locations of steep gradients - Grand Canyon, Cataract Canyon, Desolation Canyon, and the eastern Uintah's).

Things began to heat up when Brian Wernicke gave his talk, followed immediately by Karl Karlstrom. These two currently are the point men for ideas of an old canyon (Wernicke) or a young canyon (Karlstrom). Wernicke invoked that Grand Canyon is generally a simple landform that did not require a complicated set of processes in its formation. He channeled Ken Hamblin who was the first person to study the lava dams in western Grand Canyon, who noted when a dam was formed, it was removed rapidly but with no additional incision into the canyon walls. Karlstrom responded that if any segment of the Grand Canyon could be shown to be young, then the entire idea for an old canyon falls apart. Karl's current idea is that the western canyon paleochannels went north along the Hurricane fault zone and then followed a paleocanyon beneath modern Toroweap Valley.

Overall, I was impressed that Brian Wernicke and Rebecca Flowers (and a few others in that camp) continue to beat the drum for an old Grand Canyon; this in spite of the fact that most other workers are showing evidence for a younger canyon. They are not backing down. They generally agree on the Helium dates for cooling of the rocks (unroofing) but diagree on the interpretation of that data. For his part, Karl has now backed off from insisting that the entire canyon is less than 6 Ma - he believes there was a paleocanyon in eastern Grand Canyon between 25 and 15 Ma.

Finally, a former NAU Geology grad proposed what likely is the biggest "bombshell" of the entire session. Just when you think that some things might be resolved or that nothing more exotic could be proposed, something comes along that just blows the roof off of everything. Dr. Jim Sears is a person I had heard of before but had never met. While attending the NAU Alumni Reception on Monday night he hinted to me his idea - that the Colorado River in Grand Canyon might have flowed 23 Ma into Nevada, then north through to Idaho, Wyoming, and Montana, then north still into Saskatchewan, toward modern day Hudson Bay and into the Hudson Strait to the Labrador Sea. That's right - river water from the Grand Canyon flowed into the Labrador Sea 27 Ma.

Slide from Dr. Jim Sears talk on Tuesday, October 29 showing the Bell River system as it existed in North America prior to reorganization by glacial cover. Original Beel River system on the left, reorganized (modern) drainage on the right.

Close up of the previous Bell River system with added branch of the river in the Grand Canyon region. I will report more on this research in a later posting.

The original proposal of a Bell River was published in the Journal of Geology, vol. 3, #5 in 1895. Here is the work:


Dan McShane said...

Ok, that is a bomb! Thanks fro the updates.

Dr. Jack Share said...

Looks like we'll be studying the Grand Canyon (or at least its geo-genesis) from my side of the craton as well as from your side.

citizenschallenge said...

Fascinating post, thanks.

In it you write:
"He (Abbott) was also able to suggest a scarp retreat rate for the Muav/Redwall cliff behind the Tonto of about 850 m/Ma (meters per million years)."
~ ~ ~

Is there an easy way to explain how he arrived at that retreat rate?

Wayne Ranney said...
This comment has been removed by the author.
Wayne Ranney said...

Dear Citizenschallenge:

Abbott arrived at that rate of retreat by first determining the age of travertine deposition (about 500 Ka). The spring water that created the travertine had to issue from the Redwall cliff, before it had retreated to its present position. It was a matter of measuring that distance by the 500 Ka years. His actually rate was between 724 and 946 m/Ma. I rounded that off to the 850 m/Ma figure in my posting.

citizenschallenge said...

Well that's straight forward enough.

Thanks for the info.