Wednesday, July 16, 2008

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 4: Something is Missing!

The Grand Canyon is assumed by many to be an encyclopedia of earth history, that the rocks exposed there reveal a complete sequence of sedimentary layers showing the march of world events. It is not quite like that...

Imagine that you are interviewing two candidates for a job opening. One of them has a perfect resume, with a complete job history going back for years, with no time periods for which a job was not being performed. The other candidate has a rather sloppy resume, and there are years at a time where no job is mentioned. It might be easy to choose whom you would hire, but on the other hand, who do you find the most interesting, the most mysterious? Who do you want to know more about, even if you would never hire him or her?

Geology on the Colorado Plateau is much the same as a choppy resume. Each layer is a mere glimpse at a short period of earth history, but huge gaps exist between many of the layers. These gaps, mentioned in previous posts, are called uncomformities. They result when exposure produces erosion that removes previously existing rocks. There are at least 14 of these gaps in the Grand Canyon section.

In the previous post, we talked about the Tonto Group, the three Cambrian-aged layers that were laid down sequentially as the shallow epicontinental sea covered much of the North American continent. These layers formed around 545-515 million years ago. If there were a complete unbroken record, the next layers would be Ordovician (490-445 million years ago, roughly) and the layers above those would be Silurian (around 445 to 415 million years ago). But they aren't there. Something like 100 million years has gone missing!

The next layer in the sequence at Grand Canyon is the Devonian-aged Temple Butte Limestone. It is a fresh-water limestone that was deposited across a rough eroded surface. In many parts of the Grand Canyon, it doesn't even exist as a continuous layer, because it was eroded itself before the next formation was laid down on top of it. I have not actually seen it myself in my travels inside the canyon (and certainly no digital images, thus a mysterious looking canyon sunset for a picture today!).

The layer is of great interest because although vertebrates are known from middle to late Cambrian time, and they evolved widely in Ordovician and Silurian time, the Temple Butte Limestone is the first Grand Canyon formation that has fish fossils. They are the bony plates of fish who scuttled back and forth in the channels near the Devonian shoreline.

Geology is interesting to me, not because the layers of the earth provide all the answers to all the geological mysteries, but because so much of the record is missing. A geologist has to be a very intuitive detective, working with limited information and tantalizing clues. But what a great journey!
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