CR Evolution II: Flagstaff 2010 Workshop
Origin and Evolution of the Colorado River System II
JON SPENCER, SENIOR GEOLOGIST, ARIZONA GEOLOGICAL SURVEY
About 60 geologists assembled for a three day workshop at the U.S. Geological Survey office in Flagstaff, Arizona, to present new data and discuss new developments regarding the origin and evolution of the Colorado River and Grand Canyon (Fig. 1; https://sites.google.com/site/crevolution2/home). It was clear from the start that geologists have been very active in the 10 years since the previous, similar symposium had been held at Grand Canyon Village (Young and Spamer, 2001). New ideas and remarkable new data sets were used to challenge conventional wisdom and, in other cases, to defend long-held ideas. As a result of many discussion and comments in smaller groups, it is likely that each conference participant views the conference, in retrospect, in a somewhat different light. Below is my view (and only my view), of particularly interesting new information and insights.
Genesis of the modern Colorado River
In every gift shop with “dinosaur” toys, observant shoppers are likely to find toy models of glyptodonts. Two shown here are Glyptotherium and Doedicurus. Among the many lineages of glyptodonts in South America, perhaps the most bizarre is Doedicurus, with the massive mace at the end of the bony tube at the rear half of the tail.
Isotope geochronology is a field of study based on a set of laboratory techniques that can be used to determine the age and cooling history of rocks. Gradual technological development has allowed geologists to determine the geologic age of increasingly large numbers of samples with increasing accuracy, and occasionally provides entirely new approaches to old geologic problems. A technique developed largely in the past 15 years, with major contributions from Professor George Gehrels and co-workers at the Arizona Laserchron Center at the University of Arizona, has allowed geologists to determine the age of hundreds of thousands of sand grains from hundreds of sediment and sedimentary rock samples. This is done by separating the rare sand grains of the mineral zircon from all the other sand grains, and then using a laser to vaporize a small pit in each grain so that uranium and lead isotopes are liberated and then measured in a laboratory instrument called a mass spectrometer. Analyses of dozens of grains from a sediment or sedimentary rock sample determines the ages of the various major rock units or rock assemblages in the ultimate source areas of the sands.
In a previous interpretation (Lucchitta, 1979) it had been proposed that the Colorado River developed after the Gulf of California began opening ~5-8 million years ago, and that the river formed because streams responded to the topographic lowering of the Gulf region by eroding and incising into the underlying sediment and rock. This erosion and incision resulted in erosional degradation of headwater regions, which migrated into the Colorado Plateau and captured drainages that had previously drained elsewhere. This is the “headward erosion” hypothesis for the origin of the Colorado River in the past 5-8 million years, which envisions “bottom-up” river integration.
With gradual headward erosion and sequential capture of ever-more distant drainages, the sand carried by a river would gradually incorporate sands derived from increasingly diverse source areas. At the 2000 Colorado River symposium an alternative hypothesis had been debated that envisioned a primary role for “lake spillover” and “top-down” river integration (Meek and Douglass, 2001). In this alternative, river integration would result in derivation of the first arriving Colorado River sands from throughout the Colorado River drainage basin.
A study of modern Colorado River sands and the oldest known Colorado River sands (in the 5 Ma Imperial Formation in the Salton Trough), done by Professor David Kimbrough (California State University, San Diego) and co-workers using the Arizona Laserchron Center laboratory and presented at the workshop, determined that the detrital-zircon fingerprint of the oldest Colorado River sands are virtually identical to that of modern Colorado River sands. This provided strong support for the top-down, lake-spillover hypothesis for integration of the modern Colorado River.
An ancestral Grand Canyon?
Grand Canyon is generally considered to be a textbook example of a young landscape, especially when compared to the eastern USA. Its spectacular cliffs and flat plateaus appear to reflect geologically recent incision by the Colorado River. And, in fact, several lines of geologic evidence indicate that the Grand Canyon was carved by the Colorado River over the past five million years (this is considered geologically “recent”). However, a few geologists have suggested that there might have been an ancestral Grand Canyon (e.g., Polyak et al., 2008). Such a canyon would have been significantly older than the modern Grand Canyon, and likely much smaller, but would have occupied the same approximate position as the modern Grand Canyon.
At the Colorado River workshop Cal Tech Professor Brian Wernicke presented new evidence for an ancestral Grand Canyon. This evidence consisted of laboratory data produced by studying a rare mineral called “apatite” that is present in small quantities in granitic rocks and sandstones. Apatite grains contain small but significant amounts of uranium, which produces helium during its gradual radioactive decay into lead. A few laboratories in the world can now measure helium in single apatite crystals. By measuring the amount of uranium and helium in apatite grains, geologists can determine the time since the grain cooled below a certain temperature (which varies somewhat depending on radiation damage to the apatite crystal but is in the range of 35°-70° C). For rock samples from the bottom of Grand Canyon, geologists can use this technique to determine the time when significant canyon incision in overlying rocks resulted in cooling of underlying rocks that are now at the bottom of the canyon.
To everyone’s surprise, the Cal Tech research group determined that a large ancestral Grand Canyon existed by about 70 million years ago! This caused considerable consternation at the meeting and resulted in a lively and interesting discussion of alternative interpretations of various types of evidence. While most participants appeared to be skeptical, it was clear that a new technique was being used to challenge old ideas, and that the issue of an ancestral canyon was far from resolved.
Where did all the water go?
By far most of the water in the Colorado River is derived from the Rocky Mountains. Spring snowmelt especially brings much of the water that fills the river’s reservoirs. But where did all that water go before five million years ago, or generally during the Miocene (23-5 Ma)? There is much evidence that it did not enter the Lake Mead area and points now downstream. At the first Grand Canyon conference, in 1964, it was suggested that the water flowed into the Rio Grande. That suggestion is now considered unlikely because fish in Colorado Plateau rivers and fish fossils in the Bidahochi Formation on the Plateau are unrelated to fish in the Rio Grande (or in the Mississippi and its tributaries), but are closely related to fish in the Snake River in Idaho and the Sacramento River in California, and to fish fossils in Miocene sediments in southwestern Idaho (Spencer et al., 2008). This has raised the possibility that a river system
carried water and sediment off of the Colorado Plateau, through southwestern Wyoming, and across southern Idaho and northwesten Nevada to reach the ocean after traveling through northern California.
The Colorado Plateau region was largely covered by sand dunes that accumulated during the early part of the dry and cold Oligocene Epoch (35 to 23 Ma) (Cather et al., 2008). These sands were largely removed from the southern Colorado Plateau region after about 27 Ma and before siltstones of the Bidahochi Formation were deposited beginning at about 16 Ma. Were these sediments carried away to southern Idaho or to basins farther west?
AZGS research geologist Charles Ferguson proposed that poorly consolidated sandstone and river gravels preserved at scattered localities in southwestern Wyoming were deposited by a river system that was carrying these sediments to the northwest and away from the Colorado Plateau. This proposal was received with interest because it is testable. Detrital-zircon geochronologic analysis of Oligo-Miocene sandstone units in possible downstream areas can potentially identify sands derived from the Colorado Plateau region, although two previous studies of middle to upper Cenozoic sandstones from southern Idaho and southwestern Montana region did not identify Plateau-derived sands (Beranek et al., 2006; Stroup et al., 2008).
Many other issues were discussed at the workshop, and diverse data from numerous other study areas were reviewed. The workshop was a great success in promoting cross-disciplinary data presentation, hypothesis review, and discussion. The meeting ended with a suggestion for a fourth Colorado River workshop to be held in four years. This is a short time interval compared to the time between previous meetings, but the idea was favorably received because of many new and rapidly developing ideas about river evolution and the large number of researchers involved, and because new laboratory techniques are providing new types of data. It seems likely that future studies will continue to provide geologists with new insights into one of the world’s great rivers and its associated iconic landscape.
Beranek, L.P., Link, P.K., and Fanning, C.M., 2006, Miocene to Holocene landscape evolution of the western Snake River Plain region, Idaho: Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone hotspot: Geological Society of America Bulletin, v. 118, p. 1027-1050.
Cather, S.M., Connell, S.D., Chamberlin, R.M., McIntosh, W.C., Jones, G.E., Potochnik, A.R., Lucas, S.G., and Johnson, P.S., 2008, The Chuska erg: Paleogeomorphic and paleoclimatic implications of an Oligocene sand sea on the Colorado Plateau: Geological Society of America Bulletin, v. 120, n. 1/2, p. 13-33; doi: 10.1130/B26081.1
Lucchitta, I., 1979, Late Cenozoic uplift of the southwestern Colorado Plateau and adjacent lower Colorado River region: Tectonophysics, v. 61, p. 63-95.
Meek, N., and Douglass, J., 2001, Lake overflow: An alternative hypothesis for Grand Canyon incision and development of the Colorado River, in Young, R.A., and Spamer, E.E., eds., The Colorado River: Origin and evolution: Grand Canyon, Arizona, Grand Canyon Association Monograph 12, p. 199-204.
Polyak, V., Hill, C., and Asmerom, Y., 2008, Age and evolution of the Grand Canyon revealed by U-Pb dating of water table – type speleothems: Science, v. 319, p. 1377-1380.