Thursday, August 6, 2009

Time Beyond Imagining - A Really Scrambled Landscape on the Colorado Plateau

The final chapters of the geological history of the Colorado Plateau involve huge changes, and a true scrambling of the landscape by tectonic movements, and by erosional activity. Volcanism has been an ever present entity during the Cenozoic era. Each of these elements can be seen in the photo above, the plateau's most famous gorge, the Grand Canyon. On the horizon is the San Francisco Peaks volcanic field, which includes what may have recently been the highest mountain in the contiguous United States. The canyon (seen here from the less visited North Rim) is an obvious monument to erosional forces, and a more subtle monument to the work of tectonics.

Most discussions of the Grand Canyon emphasize the ancient nature of the rocks, but the very appearance of the canyon bespeaks geological youth. Incredibly steep canyon walls lining the river for more than 200 miles, compared to an exceedingly flat canyon rim. The canyon is more than a mile deep in places which means the Colorado River has quarried and carried away a tremendous amount of rocks, gravel, and silt. It becomes even more incredible when one realizes that something like two miles of additional sedimentary layers once mantled the flat surface making up the canyon rim. But the weird part of the whole story is: the Grand Canyon shouldn't be here.

Like the previous blog entry on the Black Canyon of the Gunnison, the Colorado Plateau is replete with canyons that make no particular sense. And the Grand Canyon is the grand apotheosis of a canyon that should have formed somewhere else, in a far less dramatic manner.

The Colorado River changes character when it flows into the Grand Canyon region. For several hundred miles upstream, the river gorge winds back and forth in a series of entrenched meanders, which suggests that the original path of the river was over a relatively flat plain. Then, in the Marble Canyon just east of Grand Canyon, the smaller tributary canyons do something strange: they flow into the canyon oriented upstream, as if the river once flowed in the opposite direction (Think of two tributaries coming together in the shape of a Y. Normally the two limbs of the Y point downstream. In Marble Canyon, the main canyon is flowing the opposite direction). Then, the river suddenly plunges westward, crossing the Kaibab Plateau, which is more than 8,000 feet high, making a number of ninety degree turns, and showing none of the meandering nature of the upper canyon. At the Grand Wash Cliffs, just east of Las Vegas, the river emerges into the desert environment of the Basin and Range Province, eventually flowing into the ocean at the Gulf of California (at least it used to; all its water is taken out and utilized long before any if it reaches the sea these days).

How did the river cross the Kaibab Plateau? It would be logical to assume that the river had already established a flow across the uplift before the region was lifted. Then, trapped in a channel, the river carved the canyon as the land rose. A great idea, but the timing is wrong: the Kaibab Plateau is a Laramide structure, which formed around 50 million years ago. But sediments west of the uplift show that the river did not exist there until less than 17 million years ago. And...the Gulf of California is less than 5 million years old. So what the heck was going on?

In my next post, I will try to unscramble this conundrum, but don't look forward to getting a satisfactory answer....
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