Saturday, July 26, 2008

Time Beyond Imagining - A Brief History of the Colorado Plateau, Part 7: Mountains Rise

Where did all the dirt come from??? During the previous post, we discovered that something had caused thousands of square miles of land across Arizona, Utah, and Colorado to be inundated with mud and sand more than 1,000 feet deep. These rocks formed above sea level, as deltas, floodplains, river channels, and sand dunes. In the Grand Canyon, these layers of red sediments are called the Supai Group and Hermit Shale. In southern Colorado and Utah, related rocks are referred to as the Hermosa Group, the Halgaito Formation, the Cedar Mesa Sandstone, and the Organ Rock Formation (the Organ Rock Formation makes up the base of the spires of Monument Valley).

On the other hand, today's photograph shows a canyon near the eastern edge of Colorado National Monument, one of those oft-overlooked gems that are scattered about the Colorado Plateau. The grayish scrub-covered rocks in the bottom of the canyon are composed of the same 1.7 billion year old metamorphic rocks that were seen in the Grand Canyon and Black Canyon of the Gunnison. Unlike the Grand Canyon, there is no Grand Canyon Supergroup, no Tonto Group, no Redwall Limestone, no Supai Group or Hermit Formation. None of the rocks of the Grand Canyon are found here at all. The rocks forming the canyon walls are Mesozoic in age (less than 245 million years old), with the lower slopes composed of Triassic Chinle Formation, and cliffs of Wingate Sandstone (more on these later). So we have a second mystery: why are the Paleozoic rocks missing in southwest Colorado when they are thousands of feet thick in adjacent regions?

Pioneering geologists in the region soon recognized that the source of the sediments found in Arizona were mountains that had risen in southwest Colorado during Pennsylvanian and Permian time. The mountains were pushed so high that all the overlying Paleozoic sediments and a considerable amount of the Proterozoic metamorphic rocks were washed onto the coastal plains, leaving a denuded surface of metamorphic rocks that were not covered by sediments until Mesozoic time. These mountains are referred to today as the Ancestral Rocky Mountains or the Uncompahgre Uplift. As we look at the unconformity between the Proterozoic and Mesozoic rocks, we are looking at the eroded roots of what must have been an extraordinary mountain range rising from the interior of the North American continent.

Which leads to the next vexing question: what were mountains doing within the continental interior? Mountains most often form along a continental margin (think of the Andes, Cascades, or Coast Ranges). For a long time, no particularly good explanation could be offered to explain the development of these mountains.
More in the next post! And a salty surprise!

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