Tuesday, October 29, 2013

GSA 125 Denver - Monday - The Bouse Formation (Morning) and West of Grand Canyon (Afternoon)

The Special Session regardng the Grand Canyon and Colorado River is called, "Cenozoic Landscape Evolution of the Colorado Plateau-Rocky Mountain Region and the Carving of the Grand Canyon", (Parts I, II, III, and IV).  The entire two-day special session could be considered as the 4th conference ever held to address the issue for how the Colorado River and the Grand Canyon formed. The previous three were held in Flagstaff, 1964; Grand Canyon, 2000; and Flagstaff, 2010. Of course, you can read the results from the three previous symposiums in my book "Carving Grand Canyon", reviews for which here.

Talks the first morning were focused on the lower Colorado River, downstream from the Grand Canyon. The entire Session is set up to work from the lower river regions to upper river reaches.

Along the lower Colorado River corridor, from the Hoover Dam area to north of Yuma, AZ, are deposits known as the Bouse Formation. They have been controversially interpreted as either marine deposits from the early Gulf of California, or deposits from lakes that sequentially filled and spilled from water upstream. The Gulf of California idea is one from the 1960 to 1990's, with the fill and spill being more recent (except Eliot Blackwelder in the 1930's who was the first to propose a young river formed from spillover). Major contributors to the modern fill and spill hypothesis are Kyle House, Phil Pearthree, and Jon Spencer. They have done a bang-up job of looking at sediments in the lower river corridor and showing a sequence that grades upwards from local closed basin alluvium, course debris composed of material from the basin divides, fine-grained limestone grading upwards to delta sands and mud, and finally Colorado River cobbles and gravel. The interpretation is that formerly closed basins were sequentially filled with spillover water and sediment to establish a course for the lower river.

All seemed well and good with this approach until Kris McDougall (USGS - Flagstaff) and Rebecca Dorsey (Univ. of Oregon) related that paleontological evidence from the Bouse Formation has shown marine affinities in the lower basins. This means that a marine origin for the Bouse cannot be totally ruled out at this time, at least for the Blythe basin. It seems that every time a step is taken forward in understanding this river system that an "avalanche" downslope comes along. Dorsey also reminded listeners that modern faults lie astride the Bouse's various basins and that very recent uplift of the deposits cannot be discounted. More work will be needed in this area.

The afternoon session moved upstream to the Hualapai and Muddy Creek basins and the area around the Grand Wash trough, immediately west of Grand Canyon. Jim Faulds talked about the 1.5 miles of salt that lies beneath Red Lake and other basins and showed that these massive salt deposits record closed basin evaporation and deposition before the Colorado River was integrated (approximately 15 to 6 Ma). Others workers discussed the idea that an ancestral Colorado River might have followed the course of today's lower Virgin River but nothing definitive has come of that.

Then came the "fireworks". Tim Raub came to the defense of river systems that have persisted and existed on Earth's surface for 10's of millions, even hundreds of millions of years. Two examples came from Australia, the Murray/Darling system, the Fitzroy system (280 Ma), an Old Red Sandstone system in Scotland (310 Ma), and the paleo-Susquehanna and Potomac systems (56 Ma). Of course, he was defending the idea that the Colorado River could have been in place for up to 70 Ma as proposed by Brian Wernicke and Rebecca Flowers.

I will discuss more controversies in later posting but now it is time for me to attend the 2nd day of this fascinating conference.

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