Earliest Jurassic U-Pb ages from carbonate deposits in the Navajo Sandstone, southeastern Utah, USA

摘要

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.