The Rb-Sr radiometric age determination method has been applied to problems of dating diagenesis-epigenesis in the southern San Juan basin, New Mexico. Nonmarine and marine units of the Morrison Formation (Late Jurassic) and Dakota and Mancos formations (Cretaceous) have been examined. Clay minerals from the -2 µ fraction, when carefully characterized by X-ray and SEM techniques, are especially useful if maintained at low oxidation potential conditions since the time of the last diagenetic or epigenetic effect on the rocks. Montmorillonites, illites, and mixed layer montmorillonite-illite from the Westwater Canyon and Brushy Basin Members of the Morrison Formation yield a composite date of 130-140±15 m.y.B.P., although the errors for individual units are larger. Chlorite-rich fractions penecontemporaneous with uranium mineralization in the Grants Mineral Belt yield dates of 139±10 m.y.B.P. for early epigenesis and, where remobilization has occurred, dates commonly cluster about 110-120 m.y.B.P. Dates on the Dakota Formation (93±8 m.y.B.P.) and Mancos Formation (83±7 m.y.B.P.) are consistent with K-Ar dates reported for these units. In general, low CEC clay minerals formed during or slightly later than sedimentation yield the most useful information; an exception is kaolinite-rich material which may prove to be more useful in determining the age of source area material.

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