MAGNETIC MINERAL COMPOSITION AND DEPOSITIONAL PROCESSES ON A CAROLINA BAY RIM

Carolina bay wetlands are typically bordered on the eastern and southeastern margins by a raised sand rim representing shore face and eolian sediments. Due to the homogeneity of the source sands and short transport distance, on the order of meters to a few tens of meters, there are no obvious field indicators to distinguish subaqueous from eolian sediments in soil profiles on a sand rim. The aim of this study is to identify any discontinuities in sediment characteristics that might reflect differences in depositional environments on the bay rim at Flamingo Bay, SC. We examined samples at 5-cm intervals for the upper 90 cm, followed by additional samples at 115, 145, and 165 cm. For each of these we measured percent sand and weight of the magnetic fraction as extracted by a hand magnet. We then classified the mineralogy of the magnetic fraction using SEM-EDS spectra and examination under a light microscope. Finally, for those heavy minerals in the magnetic fraction, we used a light microscope to measure long-axis diameters on the 5 largest grains as a quick way to test for any marked changes in particle size. For comparison, we did the similar analyses on a set of samples collected from the modern shoreface and eolian dune of a barrier island.

The magnetic fraction of the bay rim is dominated by ilmenite, with lesser amounts of quartz containing ilmenite inclusions. At Flamingo Bay, concentrations of heavy magnetic minerals range from 0.23 to 0.96 mg per gram of total sand. The barrier island's lower shoreface sample has a magnetic heavy mineral concentration of 1.7 mg per g of sand, whereas the upper shoreface and dune samples have concentrations of 10.9 and 10.1 mg/g. At Flamingo bay, trends in both the abundance and size of heavy minerals suggest discontinuities in sedimentation at 20 cm depth (the base of the plow zone) and again at 70-80 cm depth. Above 70-80 cm, the bay rim sands have a slightly higher concentration of heavy minerals and a higher and more variable maximum diameter among the heavies. This suggests a change in depositional processes for the upper 75 cm compared to underlying sediment.