New Mexico Geological Society Annual Spring Meeting — Abstracts

Environmental magnetic data are being obtained from over 80m of Pleistocene age lacustrine sediments (core GLAD-VC3-04) retrieved in 2004 from the Valles Caldera, New Mexico. The purpose of this study is to asses past climate variation within the Valles Caldera over the time period of sedimentation by quantifying variations in magnetic mineralogy as a proxy for changes in sediment influx resulting from climatically driven surficial processes. Typical intensities of natural remanent magnetization (NRM) of VC3-04 range from 3 - 6 mA/m. Intensities of anhysteretic remanent magnetization (ARM) that reflect total magnetite population, range from 15 - 30 mA/m. Results of alternating field demagnetization and other rock magnetic tests indicate that fine grained magnetite (single-domain and pseudo-single domain magnetite) carries the principal magnetization signal in much of the section. The VC3-04 core is not oriented and thus paleomagnetic evaluation of the NRM is limited to inclination values. The oldest part of the VC304 core is about 552 + 3 ka, based on 40Ar/39Ar isotopic age determination on sanidine in a tephra layer at 76.4m depth and thus the entire core is within the current Brunhes chron. Although this chron is exclusively of normal polarity, several geomagnetic “events” reflecting short-lived high amplitude directional deviations in the geomagnetic field (e.g., the Calabrian Ridge II at 515 +/- 3 ka) have been documented for the Brunhes chron. Samples obtained from the depth interval 17.22 m - 17.30 m yield moderate negative inclinations, which we interpret represent the Calabrian Ridge II event.