Geochemical Investigations into a Miocene/Pliocene Tephra Which May Constrain the Timing of Cervidae in North America

Craigs Hill is an outcrop of Pliocene to present deposits located in Ellensburg, Washington, that sits on a resistant portion of the Miocene Ellensburg formation. Within Craigs Hill, there has been a recent discovery of a paleosol containing cervid fossils of the genus Bretzia. An aliquot of hornblende crystals from a tephra unit directly above the paleosol has been dated to 4.9 ± 0.1 Ma by 40Ar/39Ar analyses, indicating the fossils could represent some of the oldest Cervidae occurrences in North America. Using LA-ICP-MS, 47 zircon grains from the same tephra unit overlying the paleosol were analyzed for their U-Th-Pb isotopic compositions. The youngest cluster of zircons define a Tera-Wasserburg lower intercept age of 4.15 ± 0.10 Ma (2σ; n=18). The U-Pb age data of zircon also show that the tephra unit is not a simple air-fall tephra but was reworked and contains detrital zircons ranging from Pliocene to Proterozoic. The approximately 800,000-year discrepancy in the zircon and hornblende dates could be due to excess argon in the hornblende and/or older detrital hornblende grains included in the analyzed aliquot. The new U-Pb age for the tephra suggests this outcrop may not contain a new early occurrence of Bretzia. The Nd isotopic composition of the tephra magma source was determined from ~150 phosphate minerals that were digested, chemically processed, and analyzed by MC-ICP-MS. The Nd isotopic composition, referenced against present-day CHUR, yielded an εNd value of +6.83 ± 0.20 The εNd values from rocks derived from the Yellowstone and Mt. Adams volcanic centers are between -24.8 to +4.2 and between +5.35 to +7.24, respectively. The εNd value of +6.83 from this study is consistent with values found in the Mount Adams volcanic field. We conclude that the tephra layer of Craigs Hill is most likely related to a Cascade Arc magmatic event at 4.15 ± 0.10 Ma.