Saturday, July 11, 2009

Time Beyond Imagining: A Scrambled Landscape

It's kind of strange, what's happened to the Colorado Plateau. For more than a billion years, the region was one of the most stable areas on the planet, lying submerged under shallow seas, or lying exposed as desert dunes or river floodplains. Something like two or three miles of sediments accumulated, providing us today with one of the more complete records of the earth's history for the entire existence of complex life forms. Then, all of the sudden (in geological time), it got scrambled up. The Laramide Orogeny, starting a mere 70 million years ago, upended the sediments into a series of domes, basins and monoclines. The entire region was lifted around two or three miles, and the inevitable, relentless forces of erosion began to strip away the sedimentary cover. Though we have mentioned the early volcanism and the deposition of lake sediments at Bryce Canyon and Cedar Breaks in recent posts, the real story of the Cenozoic Era in the plateau region is one of subtraction of sedimentary rocks on a massive scale.

The reasons for the disruption of the crust can be traced to my home state of California, along with Washington and Oregon. A subduction zone had been present here for tens of millions of years, but aside from developing a chain of volcanoes that dropped ash all over the region, the effect inland had been muted. During the Laramide Orogeny, it is thought that a huge segment of subducted oceanic crust did not sink into the underlying mantle as would be normal, but instead got caught scraping along the base of the continent, leading to the deformation and uplift that characterized the orogeny. As the oceanic crust finally did sink, it allowed the burst of volcanism to sweep across the region, as mentioned in the last few posts. It left behind a strange erosional landscape in the American West that includes mountains completely buried in their own debris, whole swaths of deep crust forming the summits of mountain ranges, older rocks pushed over younger rocks, rivers that run nowhere, other rivers that cross mountain ranges in illogical places, and canyons whose locations make no sense at all. And there is that ultimate expression of river erosion for the world, the Grand Canyon of the Colorado River.

This is the kind of thing that brings home the concept that we are living in one of the best times to actually be able to comprehend the history of our world: the story of a billion years of geological history pushed upward and exposed for our study and understanding.

This all brings us to a sort of puzzle for the day. The photo above is a beautiful canyon in western Colorado. It has been carved through a massive uplift, and the walls along the bottom of the canyon expose Early Proterozoic gneiss and granite; it is one of the most accessible places for observing these rocks, in comparison say to Grand Canyon where one has to hike 4,000 feet downhill to see the rocks. We obviously drove to the site. The canyon is well over 2,000 feet deep, including more than 1,000 feet cut through the very resistant metamorphic rocks. The name of the canyon in the Ute language is "Canyon with Two Mouths". And that's the problem of the day. The canyon does have two mouths, but it's missing something in-between: a river. There are two pathetically small creeks flowing out of the canyon in two different directions, and in no universe could those little rivulets have carved this gorge. So, where did it come from? What carved it?
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