Wednesday, July 23, 2008

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 6: Seeing Red (Really!)

Parts of the Grand Canyon are really red. Extremely red. So red, that a hike through the red layers leaves stains on socks that will never, never come out. So red that it spills over and turns the surface of the underlying rocks red too (see the last post about this). Red enough that the river that carries this sediment to the sea (or at least used to), the Colorado, means Red River.

There are several red formations in the Grand Canyon, but the subject of today's post is a group of layers that are collectively known as the Supai Group and Hermit Formation. A group is a collection of formations that share some characteristic, such as similar rocks, age or depositional environment. There are four formations that make up the Supai Group, and they are a bunch of tongue-twisters: the Watahomigi, the Manakacha, the Wescogame, and the Esplanade. They are red. Did I mention that?

All five formations are visible in the first photo. The lower prominent cliff is the Redwall Limestone, which takes the color by staining from the overlying Supai Group (it is actually gray). The Supai Group formations and the Hermit Formation are a series of alternating ledges and cliffs that are bright red, leading to another prominent cliff, the white Coconino Sandstone near the rim of the canyon. The individual formations are not easily distinguished. The easily eroded Hermit Formation is best seen in the second picture, where it is exposed just below the white cliff of Nankoweap Butte (as seen from Pt. Imperial on the North Rim). Collectively, these layers are the thickest of those seen from the rim of the canyon, totaling something like 1,000 feet. The first three formations date from the Pennsylvanian Period (just over 300 million years ago), and the Esplanade Sandstone and the Hermit Formation are early Permian (just less than 300 million years ago).

What does the red mean? It means iron oxides, which form in the presence...of oxygen. Very few of the layers discussed so far in this series formed above sea level. The red beds formed in floodplains, river channels, delta deposits, and sand dunes, all above sea level. They are terrestrial deposits. So the Supai Group and the Hermit Formation cover thousands of square miles in northern Arizona to a depth of more than a thousand feet, which leaves us with a huge question...where did all this sand and mud come from? And why are Pennsylvanian and Permian deposits on other parts of the Colorado Plateau very different from those in the Grand Canyon?

More in the next post!
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