Zircon fission-track ages of Pearlette family ash beds in Meade County, Kansas

Pearlette family volcanic ash beds at two faunally important late Cenozoic localities near Meade, Meade County, Kansas, are very similar in chemical and mineralogic composition, yet their zircon microphenocrysts have markedly different fission-track ages. Zircon microphenocrysts from type B Pearlette volcanic ash underlying sediments that contain the Borchers local fauna of Hibbard are various shades of pink and have a fission-track age of 1.9 ± 0.1 m.y. In contrast, zircon microphenocrysts from type O Pearlette volcanic ash overlying sediments that contain the Cudahy local fauna of Hibbard are colorless and have a fission-track age of 0.6 ± 0.1 m.y. These fission-track ages are in good agreement with K-Ar ages on the probable source material in Yellowstone National Park.