UPPER PLEISTOCENE CLIMATIC DRYING EVENT ON THE COLORADO PLATEAU? PALEOCLIMATIC STUDY OF THE PLIO-PLEISTOCENE SEDIMENTS OF THE FISHER VALLEY BASIN, UTAH USING ROCK MAGNETIC METHODS

The ~150 m thick Pliocene-Pleistocene sediment sequence of the Fisher Valley Basin (FVB), near Moab, Utah is potentially one of the most continuous accumulations of upper Cenozoic sediments on the Colorado Plateau. These sediments were the result of weathered detritus from the surrounding Paleozoic and Mesozoic red beds, being deposited within the accommodation space created by the Plio-Pleistocene rise of the Onion Creek Salt Diapir. Consisting of repeated cycles of gravel, eolian sand, and calcic horizons, this section has the potential to provide much information on the regional paleoclimate of the Colorado Plateau, which generally lacks late Cenozoic sedimentary deposits. Rock-magnetic methods were used due to the ubiquity of magnetic minerals as well as their increasingly recognized sensitivity to climatic factors. Rock magnetic experiments were conducted such as: 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 experiments indicated the presence of hematite, magnetite, and goethite, the latter two of which are interpreted to have formed in-situ during pedogenesis

Notable cyclicity is evident within high-resolution magnetic susceptibility (Χ) and frequency-dependent magnetic susceptibility (% Χ_FD) measurements when compared to depth, suggesting the preservation of Milankovitch cycles that control the depositional record of this sequence. The Lava Creek ash layer (~0.631 Ma) is preserved in the section and above this horizon % Χ_FD values display a distinct decreasing trend with increasing height. This observation is interpreted to indicate a decreasing trend in the relative proportion of ultrafine (superparamagnetic) magnetite particles, which are known to develop in-situ during soil-development. These data, in conjunction with the notable increase in the frequency and thickness of calcified layers above the Lava Creek ash bed, leads to the tentative conclusion that the area experienced a period of drying after ~0.63 Ma. However, ambiguities still exist on how long this climatic period lasted and whether it is a regional feature of the Colorado Plateau or if it is simply a local event.