SEDIMENT SIZE ANALYSIS OF OUTER BANKS SAND SAMPLES

The Outer Banks barrier island chain is a dynamic environment worked by continual aeolian and oscillatory fluid processes. The Atlantic side of the barrier islands is dominated by winds and longshore drift currents that, when oblique to the shoreline, form spits. The Cape Hatteras spit is cuspate, indicating that longshore drift currents approach it from both angles. Winds also transport sediment over great distances and are responsible for forming many dunes. For example, the Jockey’s Ridge sand dune, at 60 ft tall, is the largest sand dune on the eastern seaboard. In this study, we use grain size analysis of sediment from different facies of the barrier island deposits to study the different fluid flow processes that shape the barrier island system.

Sand samples were collected from two main locations: 3 samples were taken from the swash zone on Hatteras Island Spit and 4 samples were collected in a cross-section across the Outer Banks at Nags Head, traversing the island through Jockey’s Ridge. Coordinates and environmental conditions of each site were recorded. Laboratory examinations consisted of petrographic analysis with a binocular microscope, followed by grain size analysis using standard sieve procedures and investigation of the fine size fractions with the SediGraph.

All the samples are dominated by quartz, but differ in secondary constituents (e.g., Fe-oxides, biogenic fragments) as well as sorting, rounding, and skewedness. Grain size analysis shows that the aeolian sediments are characteristically homogeneous and well sorted, whereas the sediment transported by waves is not as well sorted, is comprised of more angular grains, and has greater variation in secondary constituents.