Comment and Reply on "Late Triassic paleogeography of the southern Cordillera: The problem of a source for voluminous volcanic detritus in the Chinle Formation of the Colorado Plateau region"

Abstract

On page 569 of our article (Stewart et al., 1986) we emphasized the critical dependence of our ideas upon the validity of the isotopic and paleontologic geochronology, and on the calibration between the two methods. Wyman also emphasizes this same point but introduces other information—namely, isotopic dates on uranium ores in the Chinle Formation. He indicates, citing the work of Miller and Kulp (1963), that the average date for uranium ores in the Chinle Formation in the Cameron area, Arizona, is 175 Ma and in Lisbon Valley, Utah, is 150 Ma. These averages, however, seem meaningless because they incorporate individual dates that range from 101 to 218 Ma at Cameron, and from 95 to 207 Ma at Lisbon Valley. In any case, recent data (not cited by Wyman) gives a concordant U-Pb age of 206 ±1 Ma for uranium ores from the Chinle Formation in Lisbon Valley (Ludwig et al., 1986).