Tuesday, July 09, 2013

Colorado River Geologists Win Award For Basin Spillover Research

ADDENDUM: on August 26, 2013 I added an interview published in Arizona Highways with head researcher Kyle House. You can access that interview here. It is quite good and gives a nice rendition of their work.

A trio of geologists who worked out the evolutionary history of the Lower Colorado River (from Hoover Dam to the Gulf of California) have won the prestigious Kirk Bryan Award from the Geological Society of America. An article appearing in a St. George journal can be read here.

Kyle House, Phillip Pearthree, and Michael Perkins published an article in 2007 that detailed their findings while working along the lower river corridor. They described a sequence of deposits in four basins that each contained (from bottom to top) A) Closed basin material only; B) coarse cobbles from bedrock sills located on the upstream margin of each basin; C) freshwater lake deposits; and D) capped with unequivocal Colorado River gravels and sand. Their conclusion was that once formerly disconnected basins were sequentially filled and spilled to integrate the lower river. A marvelous piece of geologic detective work!

Photo courtesy of Kyle House
Deposits exposed near Laughlin, Nevada along the lower Colorado River (below). The lower half of the outcrop contains rocks deposited exclusively in a closed basin before the lower Colorado River was formed; the gray cobble deposit midway in the outcrop represents debris from a bedrock sill that was overtopped and destroyed when an upstream lake basin "filled and spilled"; and the thin whitish cap on top is a remnant of the Bouse Formation, freshwater lake deposits laid down when this basin filled with water. Once this basin filled, it spilled to the next one farther south (left).

You can read this story in the new edition of my book, "Carving Grand Canyon". Here is an excerpt from pages 134 to 137:

Evidence for a Young Colorado River

Knowing that some geologists favor ideas for an old ancestor to the Colorado River, it might seem exceedingly incongruous for others to present evidence for a quite young river. Yet startling results in this area were forwarded at the 2010 workshop. Working in the area along the lower river near Laughlin, Nevada, and Bullhead City, Arizona (and not coincidentally where Blackwelder first suggested evidence of a young river), Kyle House, Philip Pearthree, and others showed convincingly that closed, disconnected basins became sequentially filled with water, overtopped their bedrock divides, and created a course for the lower Colorado River. Their results show that this fill-and-spill episode spans a 1.5-million-year period that began after 5.6 million years ago and finished by 4.1 million years ago. These studies also helped to clarify the problematic origin of the Bouse Formation.

Photo courtesy of Jon Spencer

Picture of the Bouse Formation near Cibola, Arizona. The Bouse contains evidence for freshwater lakes along the course of the lower Colorado River. These lakes were rapidly filled (geologically) and then spilled to integrate the river here.
Their work concluded that four distinct basins contain a similar sequence of deposits that grade from the bottom with (a) material derived only from the enclosing mountains, (b) coarse debris derived from bedrock exposures upstream of the basin edge, (c) fine-grained lake deposits, and (d) unmistakable deposits from the Colorado River. The interpretation is that water rapidly arrived (geologically speaking) in the Las Vegas basin and eventually overtopped a bedrock divide in Black Canyon, thus rapidly filling the basin in modern Cottonwood Valley. The spillover from the Las Vegas basin formed the river through Black Canyon (Hoover Dam area) as it rapidly filled the Cottonwood Valley. Eventually the Cottonwood Basin overtopped a bedrock divide in the Pyramid Hills (Davis Dam area) and spilled water into the Mojave Valley (Laughlin, Bullhead City, Fort Mojave, and Needles areas). This spillover rapidly filled the Mojave Valley, which was large enough to merge with the former lake in Cottonwood Valley, creating a larger and deeper lake. This lake ultimately overtopped another bedrock divide at Topock Gorge, subsequently draining the Mojave Valley and filling the Chemehuevi Valley downstream (Lake Havasu City and Blythe area). This basin was breached by spillover at the Chocolate paleodam, making a final connection with the Gulf of California.

In outlining the sequence of deposits (and the events that created them) the authors provided support for a lacustrine (lake) origin of the Bouse Formation, noting only that it was substantially eroded after the lakes drained. Overlying the Bouse remnants are diagnostic and unmistakable Colorado River sand and gravel deposits that culminated in the southernmost basin about 4.1 million years ago, according to them. They wondered what might have brought the rapid arrival of river water to the Las Vegas basin and turned their gaze figuratively and literally upstream toward the Grand Canyon and beyond. Could some other upstream basin also have filled and spilled? Or was it perhaps stream capture related to headward erosion or karst collapse? The results from the lower Colorado River area only intensify questions for how the upper Colorado River became integrated, but the origin of the lower Colorado River now seems rather certain.

Congratulations to these three for winning this award. The "fill and spill" theory is an exciting development in the understanding of the origin and evolution of the Colorado River. By extension, this also helps to better understand the history of the Grand Canyon. When these researchers wondered what might have caused the relatively rapid arrival of water into these disconnected basins, they figuratively and literally looked upstream on the river toward Grand Canyon. They wondered if basin spillover might have happened there as well. However, the evidence for that is not all that good.

1 comment:

Dr. Jerque said...

If you are interested in visiting some of the key sites in the story of the lower Colorado River check out this link to a 2005 GSA field guide to the area: