Saturday, July 5, 2008

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 2

Look at a picture of one of the most famous geologic localities in the world, the Grand Canyon in Arizona: in this case, from the South Rim, in the vicinity of Desert View on the eastern edge of the park (we were on the North Rim during our recent trip, but this view works better for today's discussion). Sedimentary layers stacked thousands of feet deep, recording hundreds of millions of years of deposition in shallow seas, river floodplains, and desert dune fields. But take a closer look: there are two utterly separate stories revealed here. One story is told by the horizontal layers making up the upper walls of the canyon. Their story will be told on a different day. Concentrate instead on the tilted layers visible in the bottom of the canyon.

The upper layers of the Grand Canyon, the Paleozoic rocks, are about 4,000 feet thick. The tilted layers below, the Grand Canyon Supergroup, are closer to 12,000 feet thick, some three times thicker. How in the world did more than two miles of sedimentary layers get tucked into the bottom of the Grand Canyon, between the very ancient metamorphic rocks and the flat-lying Paleozoic rocks?

The basic story is as follows. Following the formation of the Mazatzal/Yavapai mountains in early Proterozoic time, erosion reduced the mountains to a nearly flat, featureless plain (see my previous post on the Oldest Rocks). At various times the land subsided or sea level rose, leading to the deposition of thousands of feet of sedimentary rocks: sandstone, shale, limestone, and some occasional volcanic flows. As in the older rocks, few fossils are found; life on the planet consisted mostly of single-celled organisms. Exposed land was barren of life.

The southwest part of the continent was stretched about 700-800 million years ago by forces related to the break-up of the supercontinent Rodinia. The crust broke into a series of fault blocks that tilted the sedimentary rocks, and raised a series of mountain ridges that would have resembled the faulted mountains of central Nevada and eastern California (the Basin and Range Province). These mountains may have been thousands of feet high, but as in previous events, erosion brought the mountains down, and the layers of the Grand Canyon Supergroup disappeared across the region, except in a few places where downdropped blocks protected the sediments from erosion. Only a few ridges, a few hundred feet high, projected above the plains. This surface was later inundated by rising Paleozoic seas.

So, much of the story of the Grand Canyon involves what is missing more than what is present. Some 500 million years elapsed between the Mazatzal mountains and the formation of the layers of the Grand Canyon Supergroup. Another 250 million years is missing before the deposition of the Paleozoic sequence. These gaps in the rock record, indicated by buried erosional surfaces and abrupt changes in the rock types (metamorphic topped by horizontal sediments; tilted sedimentary layers topped by horizontal sediments) are called unconformities. The two unconformities in the depths of the Grand Canyon are among the most famous in the world.

I love these particular rocks...they are difficult to access, only by long steep trails, and I have only stood on them once. But it was my first extended geology field trip; a five day backpack down the New Hance Trail and up the Grandview Trail in 1976. I came out of the canyon convinced that I wanted to be a geologist. I was able to place my hand on the unconformities, I understood for the first time the idea of the earth's crust as a history book, I touched for the first time some of the oldest fossils on the continent. For icing on the cake, I even saw the Northern Lights for the first time in my life while we were camped on Horseshoe Mesa!

What is next? Life finds a way....the Paleozoic story of Grand Canyon and the Colorado Plateau!
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