FIGURE 1. 1.1 Map of the Tornillo Basin (modified from Atchley et al., 2004 after Lehman, 1986). Star indicates location of the Devil’s Graveyard Formation exposures examined in this study. 1.2. Stratigraphy of the Tornillo Basin and the related North American Land Mammal Ages. A.C.B. = Alamo Creek Basalt (Modified from Lehman, 1991).
FIGURE 2. Generalized stratigraphic column of the lower and middle members of the Devil’s Graveyard Formation. Symbols denote detrital zircon sample locations and known fossil localities. X-axis represents erosional profile. Colors represent visual expression of outcrop correlating to paleosol type (Emery-Wetherell et al., 2016). Note that the detrital zircon U-Pb sample identifications listed at the far right do not stratigraphically coincide with the associated fossil localities, which are identified by the bone symbols.
FIGURE 3. Zircon U-Pb results from tuffaceous sandstones of the Devil's Graveyard Fm. analyzed via LA-ICP-MS. Results were filtered to exclude single grain ages with >15% discordance between 235U/ 207Pb age and 238U/ 206Pb age, or >10% propagated analytical error in 23 U/ 206Pb age. Weighted means were calculated for each sample using Tuff extractor plugin from the ISOPLOT package (Ludwig and Mundil, 2002; Ludwig, 2003). Number of grains used in TuffZirc algorithm displayed as n= # of ages used by TuffZirc / # of ages total for sample. Data are displayed as kernel density estimators (KDEs) with a bandwidth of 1 Myr (Vermeesch, 2018). Individual zircon ages and 2s standard errors are displayed as vertically stacked boxes. Individual ages in red were used in the calculation of the weighted mean. Weighted means and errors are displayed as a grey box with a bar representing the mean.
FIGURE 4. Comparison of new and published age estimates for the Devil’s Graveyard Fm. and the Purple Bench tuff. Walton (1992) placed the Purple Bench tuff at the C20R-C20N chron transition (43.43 Ma; recalibrated to GST2012; Gradstein et al., 2012), while Prothero (1996) placed the Purple Bench tuff within the C18R chron (41.15- 40.14 Ma; recalibrated to GST2012; Gradstein et al., 2012).