Five tuffs of the Green River Formation of Wyoming, Colorado and Utah were dated by ⁴⁰Ar/³⁹Ar laser fusion of biotites and hornblendes. The dated tuffs are: tuff #3 and tuff #6, both from the upper part of the Wilkins Peak Member near Green River, Wyoming; the Curly tuff and the Wavy tuff from Gate Canyon, Utah, below and above the Mahogany Oil Shale bed respectively; and an unnamed tuff at the base of the Mahogany Oil Shale bed in Cathedral Creek, Colorado. Single grains of biotite and hornblende were fused for each age determination, except for a few multi-grain analyses from tuff #3. Weighted mean ages* of biotites are: 49.08±0.32 Ma (n=22, Curly), 49.38±0.14 Ma (n=16, unnamed), 48.13±0.71 Ma and 48.22±0.30 Ma (n=38, n=26, Wavy), 50.71±0.33 Ma (n=19, tuff #3) and 49.33±0.18 Ma (n=31, tuff #6). An isochron age for tuff #3 is 50.78±0.34 Ma. The single grain age determinations were used to estimate the full scatter of the age population, and three scattered age populations were rejected on this basis: hornblende samples from the Wavy and the Curly tuffs, and an additional biotite sample of tuff #3 (from a layer different from the one reported above). High ⁴⁰Ar/³⁶Ar intercept values of the respective isochrons suggest that these samples may have suffered a later alteration.

    The reported ages bear on correlation both between and within the basins: 1. The similar ages of tuff #6, the Curly tuff and the unnamed tuff suggest that the base of the Laney Member in the Green River Basin correlates with the base of the Mahogany Oil Shale bed in the Piceance Creek and Uinta basins. Therefore the transition from a saline, evaporite-depositing lake to a fresh-water lake is synchronous within those basins. 2. The ages of tuffs #3 and #6 near the Green River Basin depocenter, combined with the magnetostratigraphy of Clyde et al. (2001, PPP, v. 167, p. 175) at the edges of the basin indicate that the Laney Member at the basin edges is coeval with the upper Wilkins Peak in the depocenter. 3. The base of paleomagnetic sections in the Uinta and the Washakie basins is constrained by the ages of the Wavy and #6 tuffs respectively, to be younger than Chron C21n.

    * Ages are relative to Fish Canyon sanidine age of 28.02 Ma. 1s analytical errors are calculated as in Karner and Renne (1998, GSA Bulletin, v. 110, p. 740).