Fossil mammals and pedogenic development preserved in two units of the volcanogenic Wildcat Creek beds provide evidence of terrestrial environments in south central Washington. A partial rhinocerotid or hyracodontid tooth recovered from the Milk Creek tuff compares to taxa younger than the middle Chadronian. A published zircon fission track age of 33.7 Ma supports this estimate.
The upper Wildcat Creek beds produced 34 fossils belonging to at least 14 taxa of non-marine mammals. Fossil discoveries and prior collections data indicate the fossils occur in the upper 200 m, within unit B.
The correlation between the Wildcat Creek beds and the John Day Basin of eastern Oregon was first indicated by the discovery of a lower jaw of Eporeodon sp. (Grant 1941). Specimens recovered since that time also support this correlation. The fossil assemblage preserved in the upper Wildcat Creek beds, the Wildcat Creek local fauna, contains Cormocyon copei, Enhydrocyon sp., Parenhydrocyon josephi, a cricetid, cf. Palaeolagus sp., ?Miohippus equinanus, Miohippus equiceps, Diceratherium annectens, a tayassuid, Hypertragulus sp., Eporeodon sp., Merycoides sp., Mesoreodon sp., and Promerycochoerus superbus.
These taxa occur primarily between the Deep Creek Tuff and Tin Roof Tuff of the Turtle Cove Member of the John Day Formation, and indicate a northern extension of that late early Arikareean (Ar 2) fauna. This correlation is supported by bracketing Ar39/Ar40 radiometric ages of 26.97 and 27.16 Ma. Of the taxa recovered, only ?M. equinanus is not known from Oregon during this time, and the presence here indicates the taxon persists later in the Pacific Northwest.
The pedogenic succession from Argillisols, in the Milk Creek tuff, to Gleysols and gleyed Protosols, in the lower and upper Wildcat Creek beds, indicates a moist environment that changed from well-drained in the late Eocene, to poorly drained conditions of the upper Wildcat Creek beds, about 27 Ma.