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1 Machlus 1 Data Repository Item Analytical methods: Mineral separates of biotite were obtained by wet sieving and concentrating with a magnetic separator. Final hand picking was done under a binocular microscope. Samples were loaded into aluminum disks, together with Fish Canyon sanidines (FC) as monitor standards, and irradiated in the Cd-lined in-core facility (CLICIT) at the Oregon State University reactor. Samples 15B and 245B were irradiated for 10 hours; sample 12B was irradiated for 3.5 hours. Monitors and unknown samples were loaded in different pits within each irradiation disk. Following Renne et al. (1996), a weighted mean F, F 40 Ar*/ 39 Ar as defined in Dalrymple et al. (1981), was calculated for the FC and unknowns in each location of the irradiation disk (2-6 FC sanidines per pit). The F value for the FC at each unknown sample location was calculated as the arithmetic mean of the surrounding F-values of FC pits. Neutronflux ("J") was calculated from this F-value, relative to FC sanidine age of Ma (Renne et al., 1998a). Single-step laser fusion data were collected in the Ar geochronology lab at Lamont- Doherty Earth Observatory. Grains were fused with 7 Watts from a CO 2 laser for approximately 30 seconds. Ar isotope ratios were corrected for mass discrimination, interfering nuclear reactions, procedural blanks, and atmospheric Ar contamination. The weighted mean age was calculated from the ratio of the weighted mean F-value of the unknowns to the F-value of the standards, calculated for the unknown sample location (Renne et al., 1996; Karner and Renne, 1998; Renne et al., 1998a, b). Error propagation was undertaken using the Karner and Renne (1998) method, which includes the range of measured standard monitor ages, the errors in standard calibration, and the errors of the decay constants. Full isotopic results are given in Table DR1. Individual age determinations for tuff 3 are shown in figure DR1. Calculating the preferred age: The ages reported for sample 245B (Table 1) are the weighted mean ages because the single crystals are highly radiogenic, and therefore isochron ages are inaccurate. However, isochron ages for both samples are used for selecting age populations, following Chen et al. (1996). Inverse isochron ages of the selected age populations are such that their respective MSWD (Mean Square Weighted Deviations) are closest to one, and the number of individual ages is maximized. Inverse isochron ages for age populations of sample 245B are given in Table DR2. Run ID of individual ages for each age population (Table 1) are given below: 245B-a: (all). 245B-b: , 07 17, 23, 27, B-c: , 07 11, 14, 15, B-d: , 16, 23, 27, B-a: (all). 15B-b: , 02, B-c: , 15, 18.

2 Machlus 2 Details of the stratigraphy The dated tuffs are from levels ~50 m apart in lacustrine deposits near the top of the Wilkins Peak Member of the Green River Formation in the center of the greater Green River basin (tuffs 3 and 6 of Culbertson, 1961; Fig. 2). They can be traced toward the basin margin (Roehler, 1992), but are not found in the sections for which pertinent magnetostratigraphy has been established (P1 and P2 in Fig. 2; Clyde et al., 1997; 2001), and in which the Green River Formation interfingers with fluvial deposits of the Wasatch and Bridger formations (Bradley, 1964; Roehler, 1992; Fig. 3). The stratigraphic level of the tuffs in sections P1 and P2 can, however, be inferred on the basis of a basinwide change in facies immediately above tuff #6. In the basin center, the facies change is between evaporite-bearing deposits of the Wilkins Peak Member and micro-laminated organic-rich carbonates ( oil-shales ) of the Laney Member. In the vicinity of both marginal sections P1 and P2, lithological transitions mark a change from fluvial deposits of the Wasatch Formation into lacustrine deposits of the Green River Formation (Sullivan, 1980; Roehler, 1989a, 1989b, 1992; Zonneveld et al., 2000; Fig. 3). The marginal transitions were correlated with the basinal one (Sullivan, 1980; Roehler, 1989a, 1989b, 1992; Zonneveld et al., 2000) and together were interpreted as maximum lake expansion (Bradley, 1964; Roehler, 1992). This correlation is supported by isopach maps for the upper Wilkins Peak, lower and upper Laney members (Roehler, 1992) that indicate only one depocenter for the relevant area that includes both Clyde et al. (1997; 2001) sections and the dated tuffs. The duration of lake expansion is unknown and the lithological transition is likely to be diachronous. Tuff 6 can thus serve only as a maximum age for the marginal transitions. References: Bradley, W.H., 1964, Geology of Green River Formation and associated Eocene rocks in southwestern Wyoming and adjacent parts of Colorado and Utah: U.S. Geological Survey Professional Paper, v. 496-A, p. A1-A86. Chen, Y., Smith, P.E., Evensen, N.M., York, D., and Lajoie, K.R., 1996, The Edge of Time: Dating Young Volcanic Ash Layers with the 40 Ar- 39 Ar Laser Probe: Science, v. 274, p Clyde, W.C., Sheldon, N.D., Koch, P.L., Gunnell, G.F., and Bartels, W.S., 2001, Linking the Wasatchian/ Bridgerian boundary to the Cenozoic global climate optimum; new magnetostratigraphic and isotopic results from South Pass, Wyoming: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 167, p Clyde, W.C., Zonneveld, J.P., Stamatakos, J., Gunnell, G.F., and Bartels, W.S., 1997, Magnetostratigraphy across the Wasatchian/ Bridgerian NALMA boundary (early to middle Eocene) in the western Green River basin, Wyoming: Journal of Geology, v. 105, p Culbertson, W.C., 1961, Stratigraphy of the Wilkins Peak Member of the Green River Formation, Firehole Basin Quadrangle, Wyoming, Article 348: U.S. Geological Survey Professional Paper 424-D, p. D170-D173. Dalrymple, G.B., Alexander, E.C., Jr., Lanphere, M.A., and Kraker, G.P., 1981, Irradiation of samples for 40 Ar/ 39 Ar dating using the Geological Survey TRIGA reactor: U.S. Geological Survey Professional Paper 1176, 55p.

3 Machlus 3 Deino, A., and Potts, R., 1992, Age-probability spectra for examination of single-crystal 40 Ar/ 39 Ar dating results; examples from Olorgesailie, southern Kenya Rift: Quaternary International, v , p Karner, D.B., and Renne, P.R., 1998, 40 Ar/ 39 Ar geochronology of Roman volcanic province tephra in the Tiber River valley; age calibration of middle Pleistocene sea-level changes: Geological Society of America Bulletin, v. 110, p Renne, P.R., Deckart, K., Ernesto, M., Feraud, G., et al., 1996, Age of the Ponta Grossa dike swarm (Brazil), and implications to Parana flood volcanism: Earth and Planetary Science Letters, v. 144, p Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and DePaolo, D.J., 1998a, Intercalibration of standards, absolute ages and uncertainties in 40 Ar/ 39 Ar dating: Chemical Geology, v. 145, p , 1998b, Corrigendum; Intercalibration of standards, absolute ages and uncertainties in 40 Ar/ 39 Ar dating: Chemical Geology, v. 149, p Roehler, H.W., 1989a, Correlation of surface sections of the intertongued Eocene Wasatch and Green River formations along the west flank of the Rock Springs Uplift in southwest Wyoming: U.S. Geological Survey Miscellaneous Field Studies Map MF-2104, 1 sheet., 1989b, Correlation of surface sections of the intertongued Eocene Wasatch and Green River formations, Great Divide Basin, southwest Wyoming: U.S. Geological Survey Miscellaneous Field Studies Map MF-2102, 1 sheet., 1992, Correlation, composition, areal distribution, and thickness of Eocene stratigraphic units, greater Green River basin, Wyoming, Utah, and Colorado: U.S. Geological Survey Professional Paper 1506-E, p. E1-E49. Sullivan, R., 1980, A stratigraphic evaluation of the Eocene rocks of southwestern Wyoming: Geological Survey of Wyoming Report of Investigations 20, 50 p. Zonneveld, J.P., Gunnell, G.F., and Bartels, W.S., 2000, Early Eocene fossil vertebrates from the southwestern Green River basin, Lincoln and Uinta counties, Wyoming: Journal of Vertebrate Paleontology, v. 20, p FIGURE CAPTION Figure DR1. Cumulative age-probability curves (ideograms [Deino and Potts, 1992]; in thick lines), individual age determinations and their respective percent radiogenic Ar for samples 15B (A) and 12B (B). Open, shaded and black circles mark groups of age determinations interpreted as different age populations (see Table 1 for age populations 15Ba-c designated in A). Diamonds mark the age estimate for each sample. Errors are 1σ.

4 Machlus 4 Run ID Sample 15B (tuff 3): 40 Ar/ 39 Ar 37 Ar/ 39 Ar x Ar/ 39 Ar x 100 TABLE DR1: Ar ISOTOPIC DATA 38 Ar/ 39 Ar 40 Ar*/ 39 Ar ± 1σ 40 Ar* x 10 (F) (%) Age ± 1σ 40 Ar Mol 39 Ar Mol Ca/K N* Crystal size > 354µm. Sample 12B (tuff 3): Crystal size > 250µm.

5 Machlus 5 Sample 245B (tuff 6): Crystal size > 354m. Note: * N: Number of crystals per single age determination. Included in the preferred weighted mean age of tuff 6.

6 Figure DR1. Machlus 6

7 Machlus 7 TABLE DR2: INVERSE ISOCHRON AGES Sample* N Age (Ma) ±1σ 40 Ar/ 36 Ar ±1σ MSWD Tuff 6: 245B-a B-b B-c B-d Note: * Minerals: B biotites; a d see text and Table 1 for explanation. N = number of individual age determinations. Analytical error.

8 Machlus 8 Chron TABLE DR3: GPTS CALIBRATION DATA B* Normal Polarity Intervals (Ma) C* Normal Polarity Intervals (Ma) D* Normal Polarity Intervals (Ma) n n n n n n n n n n n n Note * B, C, D: Calibration schemes as in Fig. 1.

Header. Volcanic matrix from three basalts and one dacite sample from the Black Mountains and Black Hills, respectively, were separated to perform

Header. Volcanic matrix from three basalts and one dacite sample from the Black Mountains and Black Hills, respectively, were separated to perform DR000 Data Repository Item Geochronology Sample Preparation and Analysis Volcanic matrix from three basalts and one dacite sample from the Black Mountains and Black Hills, respectively, were separated