CLAY AND SEDIMENT MINERALOGY FROM A SALT MARSH, SKIDAWAY ISLAND, GEORGIA

Salt marsh sediments from Skidaway Island, Georgia were studied as a part of a three-year undergraduate research program funded by the National Science Foundation (Atlanta Consortium for Research in the Earth Sciences, ACRES), to determine sediment texture, composition, and geochemistry. The clay mineralogy of the fine fraction (<63mm) and the bulk mineralogy of the coarse fraction (>63mm) of the sediments were investigated. One high marsh vibracore (2.2 m) and one low marsh vibracore (3.77 m) were retrieved and sampled every 5 cm. The high marsh core consists of brown sandy clays fining downward (0 to 1.3 m) with some brownish gray mottling from bioturbation. A dense root mass exists from 0-20 cm. Blue clays with areas of brownish gray mottling occur from 1.3-2.2 m in the high marsh core. Areas of oxidized mottles occur and increase in abundance downcore. The low marsh core consists of gray clays, which become increasingly compacted and cohesive with depth. Some areas exhibit brownish-gray mottling. Sand lenses occur at 1.7 m and 3.0 m indicating storm wash-over, tidal channel migration, or relative sea level changes. X-ray diffraction (XRD) analyses were performed on a Phillips Model 12045 X-ray diffractometer equipped with a MDI Databox. Air-dried oriented mounts were prepared according to Moore and Reynolds (1989). The results of the XRD analysis indicate that the low marsh clays are kaolinite. The clay mineralogy is the same in each unit and appears not to be facies dependant. Results of the high marsh XRD will be presented. A point count of the coarse fraction was performed to determine the mineralogical composition. The point count revealed very angular quartz with about 5% heavy minerals (rutile, garnet, and zircon). There were no micas or feldspars, which indicates that the clays are being transported and are likely not being formed in situ because kaolinites are a chemical weathering product of alumino-silicate minerals such as micas and feldspars (Dietrich and Skinner, 1979). Clay mineralogy, radioisotopic studies of the sediment (¹³⁷Cs and ²¹⁰Pb), and radiocarbon (¹⁴C) dating of shells and wood will help constrain the age of this unit.