Thursday, May 27, 2010

Final Day of the CR_Evolution Workshop

The third and final day of the workshop was reserved for participants to discuss some of the progress made since the Grand Canyon Symposium in 2000 and to identify those areas that need more work and clarification. It is difficult at this time to say where progress has been made. Certainly the techniques used to glean information out of the sparse deposits has improved. Remember, in 2000 everyone gave their presentations with slides on Kodak carousels (and at the 1964 symposium plate tectonics did not yet exist). But advances have been made.

Andres Aslan and Rex Cole presented information on the evolution of the upper Colorado River. It was in place and flowing towards the Colorado Plateau by about 11 Ma and the evidence comes from Colorado River gravels preserved beneath the lava flows capping Grand Mesa. This is a big move forward to understand the upper river. The question becomes, where did that river go? Was there a lake somewhere on the central Plateau? Or did the river continue south towards Grand Canyon and Hopi Lake? Or as others have suggested, did that river turn north and go into an ancestral Green River. Seems that the answer to one question yields additional unanswerable questions. Still, we now have solid evidence for the existence of an ancestral upper Colorado River flowing onto the Colorado Plateau before 11 Ma.

Regarding the lower river, Kyle House and colleagues presented perhaps the most solid story presented here, concerning the river's history in the reach between Las Vegas and Blythe. They have deposits that show the presence of isolated basins that were sequentially filled with lake water, then breached at bedrock divides producing distinctive outburst flood deposits. They are preserved around the Laughlin/Bullhead City area and cluster in age around 5.5 to 5.0 Ma. Irrefutable evidence (we don't get to use that term too often in this story) for the creation and existence of the higher parts of the lower river. Other work from the area around Anza Borrego shows that reworked fossils from the Colorado Plateau arrived in a delta to the Colorado River by 5.3 Ma. So the picture of the lower river is becoming better known as well.

We are still left however, with the mystery of the Grand Canyon. As reported earlier in this blog, evidence was presented for a Grand Canyon cut to near its present depth by 70 Ma, a truly mind-boggling idea. This flies in the face of other lines of evidence for a much younger canyon but it seems that the old canyon idea will not go away, even with the passing 3 years ago of Don Elston. Some at this workshop questioned how a landform could just sit there for all of that time and not appreciably get bigger (or at least develop a soil or something). Other thermochronologic evidence from John Lee suggested that only a paleovalley (and not a canyon) was developed about 18 Ma over eastern Grand Canyon. These discrepancies will have to be worked out.

Concerning the persistent "Muddy Creek Problem" (no Colorado River sediment is found in the deposit, so there could be no Grand Canyon as recently as 6 Ma), yours truly asked the whole group, "If you were looked towards the Grand Wash Cliffs 6 Ma, what would it look like?" Ivo Lucchitta replied, "A wall of rock across the cliff face". Meaning that there was nothing at all of the canyon at that time. But we now know that the river arrived at its delta only 700,000 years later (5.3 Ma). Is it reasonable to assume that we could go from no river to the one we have today in such a short time? Only if spillover is right I guess. If we could answer this one question, (and if you attended the workshop and want to give me your best guess) we might be on our way to understanding if headward erosion, spillover, or sapping is what caused the outburst floods downstream from here.

Another new focus this year was the presentation that rather recent mantle-driven uplift may have affected the southwestern edge of the Plateau in just the last 5 Ma. Presented by Karl Karlstrom, this work suggests that movement on the Toroweap and Hurricane faults in western Grand Canyon may be the result of uplift to the east where removal of the Farallon slab has allowed the hot mantle to heat the overlying crust. The uplift could be what is driving the incision of the Grand Canyon according to this view.

Many came here to bury the idea of spillover from a presumed Hopi Lake (Lake Bidahochi if you prefer). But John Douglass gave a rousing defense of its possibility, even showing the group that in every geology textbook surveyed, not one mention of spillover was included, while stream piracy, antecedence, and superposition are all mentioned. He argued that we as a science are not trained to appreciate the role that spillover can have on the landscape. And given the evidence from the lower Colorado River where spillover seems to be verified, one does have to wonder.

Speaking of unknowns, karst processes were also presented as possible players in the formation of the Grand Canyon. Carol Hill and Laura Crossey did not collaborate on such an idea, yet their results seemed to dovetail toward the roll that groundwater might have played in creating the canyon. In spite of their endorsements for this process, the community remains largely in favor of the surface evidence only, seeming to make John Douglass's point about inherent biases in our training.

At the end of the third day, it was decided that this group should meet more ofter to share data and ideas. Sue Beard motioned that since 2014 will be the 50th anniversary of the first symposium in 1964, that we meet then. It was seconded and passed by those in attendance! So the topic will not soon go away.

I was impressed with the respectful manner that most everyone brought to this workshop. Although very observant ones may have seen a few brief moments of impatience towards those who hold certain disfavored ideas, these were very minor and did not disrupt the general tenor of the community. Everyone here obviously loves their work and would not be involved with it were it not so satisfying. The future looks promising for more cooperation and coordination in this effort to understand the Grand Canyon of the Colorado River.

2 comments:

Gaelyn said...

Wow, what a great workshop. I would have liked to have been a fly on the wall. Glad you'll gather with more frequency. Some interesting ideas can come from a gathering of the minds.

Russell Ownby said...

Great info Wayne...thanks for posting it up!