ABSTRACT

In October 2012, Hurricane Sandy caused severe erosion on beaches and dunes of Fire Island (New York, USA). Major shoreline changes occurred with erosional dominance in the upper shoreline and aggradation in the lowermost section of the beach due to the deposition of eroded upper-beach and dune sediment. Sand laminae with a high concentration of heavy minerals (“black sand laminae”) were observed in three excavated trenches located on a washover terrace and fan on the east side of Fire Island. The mineral composition of these laminae reveals the presence of high quantities of magnetite, ilmenite (as opaque minerals), and garnet (as the main translucent mineral). These heavy-mineral-enriched laminae were formed as waves eroded and transported sand from the primary dune and smaller relict dunes under specific hydrodynamic conditions that promoted grain sorting according to differences in size and specific gravity. Based on the concentrations of certain heavy minerals, the threshold for primarily density-driven sorting probably lies between the specific gravity of less dense opaque mineral (ilmenite, 4.7) and almandine (4.3), the most common transparent heavy mineral. The number of laminae and concentrations of heavy minerals vary between trenches on the overwash terrace and appear to be controlled by their distance from sediment sources. The trench with the greatest number of laminae and higher heavy-mineral concentrations is located farthest from the main dune but is just 10 m inland of a relict dune that acts as the primary source of sediment of the washover deposit in this trench. A conceptual model for deposition of heavy-mineral layers is presented based on geomorphological and sedimentological evidence allowing the definition of a density threshold.

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