Apatite fission-track data show that Permian strata forming the Colorado Plateau surface at the Grand Canyon underwent burial temperatures of ∼90-100 °C in the Late Cretaceous, indicating that ∼2.7-4.5 km of Mesozoic strata covered the area at that time. This is similar to the ∼2.5-3.5 km thickness of Mesozoic section preserved to the north in southern Utah and confirms that those strata once extended south over a much broader area of the Colorado Plateau. Cooling from maximum burial temperatures began about 75 Ma, indicating that Laramide-age erosion removed ∼1.3-4.5 km of Mesozoic strata from the area. This erosion was probably caused by topographic relief created by Laramide reactivation of monoclines and reverse faults. Permian strata in the Waterpocket monocline in southern Utah underwent burial temperatures of ∼90 °C in the Late Cretaceous, consistent with the idea that the Permian to Upper Cretaceous section exposed in the monocline formerly extended south over the Grand Canyon region.

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