ENVIRONMENTAL MAGNETISM AND PALEOMAGNETIC INVESTIGATION OF THE QUATERNARY HARPOLE MESA FORMATION IN THE FISHER VALLEY BASIN, GRAND COUNTY, UTAH

The Harpole Mesa Formation is a well-exposed, ~150 m thick cyclic sequence of Pliocene-Pleistocene sediments located in the Fisher Valley Basin (FVB) near Moab, Utah. Due to the general lack of upper Cenozoic sedimentary deposits on the Colorado Plateau, these sediments have the potential to provide much insight regarding the Quaternary regional paleoclimate. This study obtained high-resolution rock-magnetic properties as a function of depth, including: bulk magnetic susceptibility (ꭓ), magnetic susceptibility of anhysteretic remanent magnetization (ꭓ_ARM), and also environmental-magnetic ratios: percent frequency dependent magnetic susceptibility (%ꭓ_FD) and ꭓ_ARM/ꭓ. All rock-magnetic properties display notable cyclicity, which is tentatively interpreted to correlate to Milankovitch cycles. Moreover, both %ꭓ_FD and ꭓ_ARM/ꭓ show a noticeable decreasing trend above the Lava Creek Ash Layer (~0.63 Ma).

In order to better understand the magnetic mineralogy, representative specimens were subjected to rock magnetic tests including: bulk magnetic susceptibility vs temperature, magnetic hysteresis experiments, First Order Reversal Curves (FORC(s)), acquisition of isothermal remanent magnetization (IRM), and unmixing analysis on IRM acquisition curves. These data suggest that the decreasing trend in the upper sediment sequence reflects a greater proportion of higher coercivity material, interpreted to be hematite, which generally correspond to drier climates. By contrast, the lower sediment sequence has a greater contribution from lower coercivity fine-grained magnetite, which formed during soil development as a result of wetter climates. An increased contribution of hematite implies that these sediments experienced more prolonged drying after ~0.63 Ma, although more data are needed to confirm this hypothesis.

Progressive step-wise thermal demagnetization of oriented specimens up ~700 °C reveals the presence of normal and reverse polarity magnetozones above and below an exposure of the Bishop Ash (~0.73 Ma), which are interpreted to represent the Brunhes and Matuyama magnetozones respectively. A thermally stable normal magnetozone is also present in the basal 20 m of the sampled section, although more data are needed in order to identify to which subchron it correlates.