THE INFLUENCE OF FRAMEWORK GEOLOGY ON THE EVOLUTION OF ONSLOW BEACH, NC AT MULTIPLE TIME SCALES

The geologic framework of a barrier island has a significant influence on barrier evolution at time scales from years to millennia. Onslow Beach, a 12 km long barrier island located between the New River Inlet (south) and Browns Inlet (north), is characterized by significant along-beach variations in underlying geology. Specifically, an Oligocene hard-bottom scarp intersects the shoreline in the middle of the barrier where it forms a headland. The shoreline and barrier lithosome at the headland has a different evolutionary history than the embayments. Long-term rates of shoreline erosion from the NC Division of Coastal Management (NCDCM) show a decrease at the headland, but do not consistently show an increase at the embayments. Mapping the mean high water line and quantifying rates of volume change across the barrier from terrestrial laser scanning data collected semiannually for three years indicates the headland is eroding more rapidly than both embayments. These short-term data conflict with the decadal record of shoreline retreat from the NCDCM; however, the R² value from the decadal record is high (>0.85) at the embayments and low (<0.1) around the headland indicating significantly more variability in shoreline response near the headland than the embayment. The beach slope at the headland is also consistently steeper than the embayments and the maximum depth of erosion during a year, measured using 72 cores, is largest around the headlandand. Cross-shore transects of vibracores (33 cores) collected along the barrier show the highest preservation of estuarine and washover deposits at the embayments. Radiocarbon dates at the base of the barrier island lithosome indicates emplacement since 1000 cal yr. BP. Our results support previous work at Onslow Beach that highlighted the importance of placing short-term trends of shoreline evolution into a wider geologic context.