PALEOMAGNETISM AND CYCLOSTRATIGRAPHY OF LATE MIOCENE - EARLY PLIOCENE BASIN FILL DEPOSIT IN SONOITA, ARIZONA

The post-tectonic landscape evolution of the Basin and Range Physiographic province characterizing southeastern Arizona is preserved in the sedimentation records of ancient endorheic basin fill deposits. Incision into the upper basin fill in Sonoita, AZ exposes 51m of unconsolidated, Late Miocene – Early Pliocene sediment composed of calcareous paleosol horizons interbedded with reddish-brown, clay-to-sand sized sediments. Menges & McFadden (1981) provide a detailed stratigraphic column of basin fill capped by Martinez alluvium with magnetostratigraphic age constraints of 5.3 – 2.0 Ma, however key parameters to verify the study quality are not reported. Our goal in this exploratory study is to (1) verify the magnetostratigraphy to constrain the ages of a new carbonate paleosol derived paleoclimate record from this deposit and (2) determine if Milankovitch climate cycles are captured in the sedimentary record. Based on the previous study, we assume an average sediment accumulation rate of 2 cm/kyr (20 cm every ~10 kyr), yet we increased the sampling resolution of the unoriented samples for cyclostratigraphy to 14 cm to account for slower rates. For the oriented samples we maintained the previous sampling interval (50 cm) to compare magnetostratigraphy results with Menges & McFadden (1981). Preliminary magnetic susceptibility (MS) and MS vs temperature analyses of the sediment show a strong magnetic signal primarily carried by magnetite. These analyses indicate that this deposit is suitable for cyclostratigraphy using MS and magnetostratigraphy using alternating field demagnetization. Results from the magnetostratigraphy survey will provide high resolution, independent age constraints on a paleoclimate record reconstructed from 34 carbonate paleosols, which enable paleoclimate and magnetic reversal comparisons to global records. The presence/absence of climate-based cycles from the magnetic cyclostratigraphy survey will inform future studies to disentangle processes shaping the post-tectonic landscape in southeast Arizona. Finally, the high-fidelity sedimentary records across numerous basins in Basin and Range topography show commonalities in basin fill morphology, enabling future studies to elucidate the timing and processes driving erosion and deposition across southeast Arizona.