Lapen, TomZentner, NickCopeland, PeterFields, Shawn2023-07-132023-07-132023-05-08https://hdl.handle.net/10657/14999Craigs 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.enThe author of this work is the copyright owner. UH Libraries and the Texas Digital Library have their permission to store and provide access to this work. Further transmission, reproduction, or presentation of this work is prohibited except with permission of the author(s).Craigs HillTephraGeochemistryZirconU-PbGeochronologyEllensburgGeologyHonors Thesis