USGS Logo Geological Survey Professional Paper 132—A
Rock Formations in the Colorado Plateau of Southeastern Utah and Northern Arizona



A significant unconformity separates the Paleozoic and Mesozoic sections. There is no perceptible divergence between beds, but an unmistakable surface of erosion lies beneath the Moenkopi formation wherever it has been studied in Arizona, Utah, and Nevada. The unconformity is particularly well shown near Cataract Canyon and about the mouth of Fremont River, where the top of the Coconino sandstone forms wide structural platforms. Recent erosion has stripped away the soft Triassic sediments, and the more resistant Coconino preserves large areas of the pre-Triassic erosion surface almost unchanged, with patches of the basal conglomerate of the Moenkopi still adhering to it here and there in the shallow valleys and rounded divides. Farther back from the main streams recent canyons are cut through the contact, revealing it as a distinct wavy line, overlain by conglomerate in which all the pebbles are of sandstone.

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Last Updated: 08-Aug-2008