2nd Day of the CR_Evolution Workshop

The 2nd day of this workshop proved to be even more exciting than the first since the topics covered the area of the Grand Canyon specifically and the upper Colorado River in Colorado. Much of the discussion again focused on the evidence gleaned from studying the thermochronology of the rocks (basically studying when they were uncovered by erosion). At this workshop, everyone gets their five minutes but then there are hour-long or even 90 minute discussions where everyone in the room can return to ideas that were presented in a talk. In this way, important concepts are revisited and discussed by all in the room.

I have been impressed at how focused these discussions have been. I think everyone who is attending probably felt that this might be an impossible outcome from this workshop since there are so many divergent views. But the discussions have been as enlightening as the individual talks. Sue Beard discussed the evidence for the Kingman Arch, a pre-Basin and Range high that sat southwest of the Grand Canyon. She showed an image that depicted some Laramide (70 to 40 Ma) paleovalleys that were cut into Tonto Group rocks on the southern Grand Wash Cliffs. (I had just flown over Music Mountain a week before and had seen these remnant valleys from the air). These paleovalleys came off of the Kingman Arch and flowed northeast towards the Grand Canyon (but perhaps, before the canyon was here). See the image below from their talk.

Image from Sue Beard and Jim Faulds workshop abstract showing the area of the Kingman Arch (heavy dashed line, left); three paleovalleys with their names trending onto the Colorado Plateau (small dashed lines); outcrops of the 18.5 Ma Peach Springs Tuff (dark pink) and its suspected source caldera (light pink); and the western end of the Grand Canyon (upper right). The parallel colored lines are the southwestern limit of various Grand Canyon strata.

Ivo Lucchitta showed similar space images from near the Echo Cliffs where, northeast of The Gap, there is a sinuous high-standing ridge of gravel that trends northeast for about 35 miles. Ivo thinks that this may be an ancient main-stem river channel (the San Juan?) that flowed toward the lower end of the Little Colorado River. Some of the clasts in the deposit are identified as coming from the San Juan Mts. in Colorado. Rich Hereford who studied this same deposit 30 year ago thinks it may just be coming from Black Mesa with the clasts being reworked into this.


Composite Landsat and DEM image of Crooked Ridge from Lucchitta and Holm workshop abstract. Note the Echo Cliffs and The Gap (lower left) and Black Mesa (upper right). Crooked Ridge is a sinuous line of gravel between the two. Ive believes this river ran down the monocline of the Echo Cliffs and then turned west into the Little Colorado River.

My name was on two abstracts at this workshop but both of my co-authors gave the talks. (I've been busy transcribing notes). Ron Blakey showed evidence for the age of the Mogollon Rim (> 27 Ma) and we suggested that this feature could have trended all the way to the western end of the Grand Canyon. Carol Hill summarized our work concerning the Laramide to present paleogeography of the Grand Canyon, which includes northeast-directed rivers in some portions of the canyon, and a karst connection between the Marble Canyon section and the rest of Grand Canyon. This karst connection is a tough sell to a group specializes in surface processes but Carol is on to something that may be overlooked in this story.

Having said that, one of the big surprises to me at this workshop is the amount of evidence being presented for the Colorado River going across the Kaibab Arch well before it was integrated with the lower river. This idea means that the ancestral river would have had to leave its present track in the vicinity of Kanab Creek (destination unknown). The time-frame for this is between 20 and 6 Ma. This gets around the problem of how did the river cross the upwarp but still leaves its exit as a problem.

What else? Gosh there was so much. It looks like the Colorado River "instantly appeared" into its delta area around 5.3 Ma. Lots of great evidence for this. And Charles Ferguson really came up with an outrageous idea (not wrong, just outrageous). He thinks that a main-stem river was headed from the basin of Lake Bidahochi (Winslow area) to the Pacific Northwest through the Green and Snake rivers! Not only that, but he believes that this system was intact until integration of the river through Grand Canyon at about 5.3 Ma! That might be tough sell but Charles thinks he has the evidence based on fossil fish assemblages and the gravels. I'll be reporting once more for the 3rd and final day of this workshop.