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.

Wednesday, May 26, 2010

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.

Tuesday, May 25, 2010

1st Day of the CR_Evolution Workshop

The first day of the Colorado River Workshop has now ended and things are as I suspected they might be for me - exhaustive! I got home at 7:30 last night and couldn't decide if I was wide awake ready for more, or just merely ready for bed. It didn't matter - I woke up at some unruly hour in the middle of the night and couldn't get the river or the canyon out of my head. I missed my morning walk but am using the extra time to write a few words about some of my impressions. I'll be back at the 2nd day of the proceedings at 8 AM today and am listed as a co-author on two of the talks.

Overall, everyone was pretty good natured about presenting their ideas, although the two sets of black boxing gloves visible on one of the entry tables wasn't entirely a joke. At least no one screamed but there was a few hearty discussions concerning the lower river (downstream from Hoover Dam). The format of the Workshop is like this: An agenda has been made and you can read it here. Each speaker gets five minutes to present their findings or ideas and then five minutes of discussion after that. That's not a typo. It's five minutes. And fortunately it's policed pretty stringently. It's enough time to show three or four slides although others push that to ten (and then end up being surprised that the five minutes went by so quickly). A cell phone type ringer goes off when each five minutes are up. It's a pretty good system and keeps things moving without having to endure long-winded talks.

There was at least one earth-shattering idea presented in the morning. Brian Wernicke of Cal Tech proposes that a river carved the entire Grand Canyon to within a few hundred meters of its present depth between 80 and 70 million years ago! Not only that, but this river was going in the the opposite direction towards Colorado and Wyoming. For those of you who have read "Carving Grand Canyon" this may not seem like such a new idea, since Don Elston and Andre Potochnik have also proposed such a vision. But Wurnicke uses a technique called thermochronology, which essentially measures when the now removed, overlying rocks were stripped off the ones we can see in the canyon today. (The two previous ideas spoke to surface evidence only). Wernicke has been a giant in the study of the Basin and Range for the last 20 years and this is his first entry into the Grand Canyon debate.

He calls his river the California River (get it - the modern Colorado River flows into California but the ancient California River flowed into Colorado). You can read his abstract here (although it's unfortunate he did not include a map shown in his talk that depicts the California River going from the Sierra's through the Grand Canyon exactly in the alignment familiar to us today and on into northern Colorado. (Remember the Sierra's were much closer to Grand Canyon before Nevada was stretched into the Basin and Range only after 20 Ma. The Sierra's in this depiction were in the approximate position of Las Vegas). I hope to obtain a copy of this map when I see him today.

On the other end of the time spectrum, Kyle House and colleagues provide outstanding evidence from the lower Colorado River between Hoover Dam and Blythe for the catastrophic filling of basins that overflowed sequentially downstream. These basins were separated by bedrock sills and essentially mirror the dammed basins today (Lake Mojave, Lake Havasu, etc.) All of this occurred in the 5 Ma time frame. You can read their abstract here. This seems to argue for a very young river, at least for its lower reach. Amazing the two frames between Wernicke and House.

Wish I had time to write more this morning but I gotta go hear more. Stay tuned.

Thursday, May 20, 2010

Workshop on the Evolution of the Colorado River next week in Flagstaff

Next week, on May 24, 25 and 26 about 60 geologists will convene in Flagstaff to discuss, debate and pontificate about how the Colorado River may have come into existence. Although there likely will not be a single, defining answer at the end of all this discussion, the field does seem to move incrementally towards greater truths about the mysteries and puzzles of the Southwest's master stream.

Many of the old names will be familiar to those who have poked around in this everlasting debate; Ivo Lucchitta and Dick Young being the main two and the only ones who also attended the symposiums in 1964 and 2000. But many more newer faces will certainly liven up the atmosphere. Look here for a list of the attendees and the abstracts which have been submitted. I will be in attendance for all of the proceedings and hope to post blogs each and every day as the events happen. However, a gathering of minds such as this is bound to be an exhaustive affair and I make no promises! Do look here as often as you can and I will post many interesting readings from the workshop.

Tuesday, May 04, 2010

Rafting the Rapids and Geology of the Salt River in Central Arizona's Transition Zone

With all of the winter snowfall we've had in Arizona this year, the rivers are running really big this spring. I was fortunate to receive an invitation to run the Salt River with friends during the period from April 25 to May 1. It had been 18 years since I was on the "River of Rocks" and it was a thrill to see it again after long time. The river is rocky, wild, and unpredictable but our group was well experienced and we had a marvelous time. I was the only geologist along on the trip but Bryan Brown covered the ornithology and river history, while Gary Reznick and Lee Midgley paddled their whitewater canoes. The pictures tell the story of this river through time.

A first view of the Salt River and its canyon near the US Highway 60 bridge, north of Globe, Arizona. The canyon is sometimes referred to as the "Little Grand Canyon". Shown are two of the first rapids, Bump and Grind (top) and Maytag (below). The rock cliff in the foreground is part of the Dripping Springs Quartzite (Proterozoic).

Looking downstream from above Overboard Rapid. The rocks belong to the Proterozoic Apache Group, a sequence of strata time correlative with the Unkar Group in Grand Canyon. Black rock near the river is a diabase intrusive dated at 1,100 Ma, gold colors above this are Dripping Springs Quartzite, capped by whitish Mescal Limestone (with another diabase sill sandwiched in here), and Troy Quartzite capping the skyline.

A wider view to the northwest. This canyon and river are exquisite landscapes that are rarely seen or visited in Arizona. The land north of the river belongs to the White Mountain Apache Indian Tribe.

Bryan and I found this rather cryptic lava flow (at least from river level) perched well above the modern river channel but perhaps occupying an old alignment of the river bed. Dating of this flow might reveal the rate of downcutting for the Salt River here.

A view of the lava flow from our camp at Grumman Rapid.

After running Overboard Rapid on our second day, we entered a narrow gorge cut into the intrusive diabase. This magma forced its way between layers of quartzite and created a space for itself while in the subsurface. Grand Canyon exposes a similar diabase intrusion of the same age, lending credence to the idea that the Apache Group and Unkar Group rocks are related.

Downstream view of the rocks above Cibeque Creek, a major side drainage off of the Mogollon Rim to the north.
Gary paddles in front of the famous Salt Banks, a spring deposit of travertine and various salts that "erupt" right next to the river. Some believe that the Salt River was named for this spring deposit but a more likely story is that the river made agricultural fields salty throigh time.

Overhang of colorful travertine above an alcove in the spring.

Unusual spring deposits at the Salt Banks.

An upstream view back towards the Salt Banks from Walnut Canyon. The scenery is fantastic.

Spring was in full bloom for us on this trip. I photographed this blooming cactus on a side hike up n Walnut Canyon.

On day three we first encountered the Ruin Granite, a body of rock that is worn smooth by the runoff on the Salt River. The granite was emplaced about 1,400 Ma at a depth of five to ten miles. It was then uplifted and eroded before the Apache Group rocks buried it, beginning about 1,200 Ma.

View upstream of the Ruin Granite gorge at the mouth of Canyon Creek, another marvelous Mogollon Rim drainage from the north. We traveled with another group from Ridgeway, Colorado for a few days.

View downstream to Canyon Creek Rapid in the granite gorge. Note how scoured the rocks appear above the river. Between January 21 and 24 of this year, the Salt River experienced an enormous flood that reached 90,000 cfs above the Roosevelt reservoir. We saw evidence for this tremendous flood everywhere on our trip.

The juxtaposition of classic Sonoran Desert vegetation and scenery with a perennial river like the Salt is like no other river trip I know of (except the Salt's cousin, the Verde River). I must have taken over 200 photos of these enormous cactus near the river.

See what I mean!

The Ruin Granite is a very coarse-grained rock with well exposed crystals of orthoclase feldspar (pink), plagioclase feldspar (white), and quartz (milky). Here, a few stringers of epidote (pistachio green) were found.

Sculpted exposures of Ruin Granite up in Canyon Creek make for an other-worldly view.

Exiting the gorge above Gleason Flats, we got a view of majestic Canyon Creek Butte, capped by the Apache Leap Tuff, which was erupted about 18.5 Ma in a paleocanyon that trended northeast. More will be said about this Oligocene/early Miocene canyon later.

After three miles across the open country of Gleason Flats, the river enters another canyon cut into the Redmond Formation, a suite of metamorphosed ash flows that are the oldest rocks in the Salt River Canyon. They are dated at 1,750 Ma. These rocks, along with the overlying Hess Canyon Group, are the less-altered cousins of the Vishnu Schist in the Grand Canyon. Again, note the scouring of the gorge immediately above the river. We estimated that the water was over 30 feet higher during the flood than seen here. The noise must have been deafening. The power must have been unreal.

Lee paddling her canoe downstream from Eye of the Needle Rapid.

Redmond Formation sculpted by the Salt River above Black Rock Rapid.

A view of Black Rock Rapid from our scout vantage point on river right. We scouted more than 12 rapids in the course of our trip.

Gary paddles for a closer view of the Whitetail Conglomerate, an Oligocene/early Miocene (∼30 to 20 Ma) valley fill deposit that preserves an older canyon before the modern Salt River Canyon. Rivers in this area used to travel northeast and these gravels help to document that earlier drainage alignment.

Scouting Pinball Rapid with the Redmond Formation along the river.

Brian pulls on the oars of our raft as we negotiate the swirling currents of the Salt River.

Lee paddling beneath the upturned beds of the Hess Canyon Group. These are Proterozoic sedimentary rocks whose metamorphism was slight enough that we can discern their original bedding and depositional characteristics. The same rocks in the Grand Canyon were cooked and pressurized into schist and gneiss and it is more difficult to see what they were originally.

Downstream view to the narrow gap where Quartzite Falls is located. Note the upturned beds of the White Ledges Quartzite (Hess Canyon Group). The Salt River is having a difficult time excavating the river channel here through this resistant bed of rock. Thus, the falls are created where the river pours over the ledge. However, in the early 1990's someone dynamited the falls which are now runnable but still difficult to run.

Gary and Lee paddling in the open country near Horseshoe Bend.

The wind was blowing very hard this day nd I found an excellent example of adhesion ripples on the river bank. These form when dry sand blows across wet sand and they adhere the the surface creating this unusual pattern. Adhesion ripples are found in sandstones that are hundreds of millions of years old.

Rockinstraw Mountain (elev. 5385 ft.) as seen from Horseshoe Bend. It is capped by Apache Leap Tuff but is visibly downfaulted back towards the photographer. This faulting documents the progressive lowering of the Basin and Range to the southwest of the Salt River Canyon. This lowering is what caused the drainage reversal from the old northeast-directed flow in the paleocanyon to southwest flow in the modern Salt River.

Pinnacles of rock related to the eruption of the Apache Leap Tuff 18.5 Ma. Erosion by the Salt River has exposed this former vent for these Cascade-style volcanoes.

Close-up view of the pinnacles, which are now closed to all stopping because of nearby nesting bald eagles.

And just around the corner, we saw two of the eagles by the side of the river.

Can you stand it? Another view of the Whitetail Conglomerate just upstream from Coon Creek. The unsorted nature of the deposit (where boulders are mixed in with pebbles and sand) is striking and probably represents deposition in nearby alluvial fans within the old paleocanyon.

A casualty of the January 21 flood on the Salt River. This old giant was undercut when its substrate was removed in the flood. But small shoots of green leaves were seen growing out of the toppled tree.

Shh! A secret waterfall and pool located in Chalk Creek.

Another Sonoran Desert mastepiece along the lower Salt River run.

A baby saguaro cactus growing next to a crumbling boulder of Ruin Granite. In some areas, the plagioclase feldspars weather out of the granite, causing the orthclase and quartz to become liberated as grus on the desert floor.

View downstream to the Klondike Cliffs and the edge of the Salt River Canyon Wilderness. The ride of the cliffs is capped by the Apache Leap Tuff.

Bryan stands next to an old guardian that we estimated to be about 38 feet high. Wow!

The last narrows before the take-out are carved into the Apache Leap Tuff near the confluence with Pinal Creek.

The open country near the Highway 288 bridge, where the Roosevelt reservoir was constructed on the Salt River in 1911. It was a fabulous trip with good weather, food and company. I wish everyone could see it.