Sediment sorting and beach erosion along a coastal foreland: Magnetic measurements in Point Pelee National Park, Ontario, Canada

In-situ and laboratory-based magnetic measurements were made on sand samples from the beach on the eastern side of Point Pelee National Park (PPNP) in northwestern Lake Erie, in order to examine both the composition and spatial distribution of magnetic minerals. Magnetic remanence measurements on bulk and particle size fractions showed that the magnetic properties were dominated by multidomain ferrimagnetic grains, probably magnetite and/or maghemite. Particle size-specific measurements on different grain size fractions showed that the < 250 μm fraction of the beach sands was responsible for the majority of the bulk magnetic signal and that these magnetic grains, and other dense minerals, were concentrated into the finer fractions by sorting processes. Selective entrainment and erosion of lighter quartz minerals lead to concentration of heavy magnetic minerals as placer deposits in the finer fractions. Highest in-situ magnetic susceptibility values were observed in the mid-back beach sands where the beach was the narrowest and had experienced significant erosion in the past. Lowest magnetic susceptibility values were in beach areas which were wider and appeared to be less sensitive to erosion; therefore magnetic susceptibility provided a useful proxy for mapping erosion along the PPNP beaches. Mapping in-situ magnetic susceptibility along the 8.9 km length of eastern PPNP identified 2.4 km (31%) of the beach where erosion was high and where efforts to minimize erosion should be directed. This technique can be applied to a wide range of sedimentary settings and permits quantitative identification of areas of erosion and better targeting of mitigation measures aimed at reducing the impacts of erosion.