Previous studies of the Uintan, Duchesnean, and Chadronian rocks and their mammalian faunas of Trans-Pecos Texas are supplemented by more recent magnetostratigraphic research to provide a more up-to-date chronostratigraphic framework. In the Vieja area, reinter-pretations of the work of Testarmata (1978; Testarmata and Gose, 1979), plus new magnetics, show that the Vieja Group (Colmena Chambers-Capote Mountain Tuffs) spans the interval from Chron C19r (42 Ma) to C13n (33 Ma). The early Duchesnean Candelaria l.f. occurs in Chron C19n (about 41.5 Ma), the late Duchesnean Porvenir l.f. and early Chadronian Little Egypt l.f. in Chron C17n (37-38 Ma), and the mid-Chadronian Airstrip l.f. in C16n (35.5 Ma). In the Agua Fria area, the Devil's Graveyard Formation spans the interval from C21n (46.5 Ma) to C13n (33 Ma), with a gap between C16n (35.5 Ma) and C18n (39 Ma) (wherein lie the Duchesnean faunas of the Skyline and Cotter channels). Revisions of Walton's (1992) magnetic interpretations place the late Bridgerian Junction l.f. in C21n (46.5 Ma), the early Uintan Whistler Squat l.f. in early C20r (45-46 Ma), the late Uintan Serendipity and Purple Bench l.f. in C19r (42 Ma) and C18r (41 Ma), respectively. The mid-Chadronian Coffee Cup l.f. occurs in Chron C15r (about 35 Ma).
The Trans-Pecos region of Texas is the only place in North America where the middle-late Eocene is recorded by superposed mammalian faunas datable by both radiometrics and magnetic stratigraphy.
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