New uranium-lead (U-Pb) analyses of carbonate deposits in the Navajo Sandstone in southeastern Utah (USA) yielded dates of 200.5 ± 1.5 Ma (earliest Jurassic, Hettangian Age) and 195.0 ± 7.7 Ma (Early Jurassic, Sinemurian Age). These radioisotopic ages—the first reported from the Navajo erg and the oldest ages reported for this formation—are critical for understanding Colorado Plateau stratigraphy because they demonstrate that initial Navajo Sandstone deposition began just after the Triassic and that the base of the unit is strongly time-transgressive by at least 5.5 m.y.

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