Instantaneous record of the geomagnetic field direction of various facies from pyroclastic flow deposits: Tests for consistency in paleomagnetic directions

A comparative study of the paleomagnetism of various lithofacies of pyroclastic flow deposits has been undertaken to examine the accuracy of their remanent magnetization direction at the time of deposition. The Aso-2 pyroclastic flow deposit, which erupted at 141 ka from Aso caldera, Japan, was chosen as the site of our investigation because the deposit formed over a short time period and the paleomagnetic directions of the deposit are expected to record substantial contemporaneous volcanism. Paleomagnetic samples of densely welded, lava-like rheomorphic, and non-welded pyroclastic deposits were collected at nine sites from the Aso-2 pyroclastic flow deposit. Remanence directions from densely welded pyroclastic deposits display good within- and between-site consistency and are considered to accurately record the ambient geomagnetic field direction at the time of emplacement. Rheomorphic pyroclastic deposits had directions that show good within-site consistency, and it is considered that this material obtains its remanence direction parallel to the ambient field corresponding to the time of its emplacement. Remanence directions from non-welded pyroclastic deposits show large confidence limits or deviations from their expected directions. Such deposits would likely be prone to modification of remanence direction introduced from random rotations of remanence-carrying material during syn- or post-depositional stages. In conclusion, we suggest that among various lithofacies of pyroclastic flow deposits, remanence directions observed from non-welded pyroclastic deposits may need to be interpreted cautiously.