MAGNETIC SUSCEPTIBILITY AND TEXTURAL VARIATIONS OF OVERTOP DEPOSITS RECORD THE WANING SURGE OF HURRICANE SANDY, NEW JERSEY

Among prominent geological signatures of Hurricane Sandy (2012) are sediment horizons enriched in dense (>2.9 g/cm³) minerals. These heavy-mineral concentrations (HMCs) blanketed extensive areas of Atlantic beaches immediately following the storm and comprise the basal portions of event horizons within the remaining overtop and overwash deposits. Therefore, they serve as sedimentological indicators of storm surge conditions. As part of a larger paleotempestological study, four plastic casts were collected adjacent to trenched overtopping sequences in Ocean County, New Jersey, 60 and 80 km north of the hurricane landfall. Due to time consuming nature of traditional heavy-mineral analyses, textural and magnetic properties of cast sections were used to examine variations within and between individual event horizons, thereby complementing the field measurements (trenches, cores, and georadar images). Grayscale profiles demonstrate distinct peaks of dark HMCs overlain by light-colored quartz-rich laminations. Along triplicate cast profiles sampled at 4-cm intervals, the bulk low-field magnetic susceptibility (MS) yielded average values of >90 μSI in proximity to basal HMCs. In contrast, thicker quartzose sublayers, as well as non-storm deposits, have substantially lower ranges of 4-28 μSI. Microscopic and granulometric analysis of selected trench and cast samples indicate that magnetite, ilmenite, and garnet (mostly almandine) comprise the finer, denser fraction, which is hydrodynamically equivalent to coarse quartz grains. Up-section, the bulk mean grain size of individual layers increases by 15-30%. Along with a substantial decrease in HMC content, which is supported by lower MS values, these trends can be attributed to a reduction in near-bottom surge velocities during the waning stages of the storm. This study demonstrates that textural and MS peaks in trenches and casts offer a means of utilizing lithological anomalies for spatial correlation and flow-regime hindcasting of event horizons.