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Geology of Grand Canyon National Park: Sedimentation and Erosion on Planet Earth’s Grandest Landform - Article 1

RANNEY (Wayne)

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Geology of Grand Canyon National Park: Sedimentation and Erosion on Planet Earth’s Grandest Landform - Article 1

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  • ISBN : 978-2-86781-817-2
  • Nombre de pages : 20
  • Format : 21 x 29,7
  • Sortie Nationale : 2014/10
The Grand Canyon, located in the state of Arizona at the southwest edge of the Colorado Plateau geographic province, exposes rocks representing nearly half of all Earth history. Basement rocks are composed of a metamorphic assemblage (the Granite Gorge Metamorphic Suite) and an igneous group (the Zoroaster Plutonic Complex) of nearly equal age, dating between 1840 to 1660 Ma (million years ago). The Grand Canyon Supergroup, a package of now tilted Meso- to Neoproterozoic sedimentary and volcanic rocks, overlies this basement. Flat-lying sedimentary rocks ranging in age from Cambrian to Permian cover these older rocks, composing the classic stratified appearance of the canyon. A thick section of Mesozoic-age rocks once covered these but were mostly eroded. Details of the timing and exact processes involved in the excavation of Grand Canyon are still debated, but a broad outline is known. The canyon formed as a result of erosion by the Colorado River or some ancestor(s) to it. Tectonic uplift began in the Laramide Orogeny (70 to 40 Ma) and an initial river system developed with flow toward the northeast (opposite to that of the modern river). A few hypotheses suggest that this early drainage system carved the canyon to near its present depth; remnant sediments in a few tributary canyons attest to landscape incision at this time. Most hypotheses, however, center on fluvial integration in two separate and distinct drainage systems near the end of the Miocene, between 5 and 6 Ma. This was facilitated by the opening of the Gulf of California along the San Andreas Fault system, which created an extensional corridor where the lower Colorado River developed by sequential closed-basin spillover. River water upstream from the Grand Canyon ultimately found a connection to this corridor through some process: 1) headward erosion and stream piracy; 2) closed-basin spillover; 3) karst collapse; or 4) a combination of processes. After this integration, the through-going Colorado River deepened the Grand Canyon. Spectacular lava flows (<1 Ma) spill into the western part of the canyon and record the growth and destruction of lava dams, and the resulting outburst floods.