Late Oligocene and early Miocene, nearly flat-lying and dominantly volcaniclastic, continental strata in the lower part of the Chalk Canyon Formation are exposed in central Arizona along the southwestern margin of the Transition Zone and the adjacent northeastern margin of the Basin and Range Province. The strata have yielded both of the two known oreodont records reported from Arizona. These occurrences represent the Cave Creek and Black Canyon Local Faunas. Both records are assigned tentatively to the early late Arikareean (Ar3), small-bodied oreodontid species Merychyus calaminthus, based primarily on their occlusal and basicranial anatomies and on the comparatively small sizes of the individuals they represent. Both fossil sites are in discontinuous exposures separated by about 28 km (17 mi). Overlying olivine-rich basaltic lava flows still in the lower part of the formation in the Cave Creek area are as old as 23.3 ± 2.7 Ma in age, based on whole-rock Potassium-Argon radiometric age determinations. On the other hand, a pumice-rich air-fall tuff bed at the base of the formation is only 20.98 ± 0.06 and 21.03 ± 0.06 Ma old, based on ⁴⁰Argon/³⁹ Argon (Ar/Ar) analysis. A blue ash layer in the Black Canyon City area and at the approximate level of the oreodont site is 23.7 ± 0.9 Ma old, based on the ²³⁸Uranium²⁰⁶Lead analysis of zircon crystals. The oreodont records and nearly all available radiometric data suggest that the fossilbearing strata in the lower part of the Chalk Canyon Formation are correlatives of the Harrison Formation in the northwestern Nebraska Panhandle. The Agate Ash in the Harrison Formation is 23.23 ± 0.13 Ma in age on the basis of Ar/Ar analysis.