STRATIGRAPHIC DEVELOPMENT OF GEORGIA ESTUARINE SEQUENCES ON HOLOCENE AND MODERN TIMESCALES

The stratigraphy and sedimentology of salt marshes record the history of sea-level rise and marsh development along passive margin coasts. New mapping tools (i.e., RTK GPS, multibeam sonars), when combined with traditional geologic techniques, provide the means to collect high-resolution data for both retrospective and predictive studies of back-barrier marsh behavior, although outstanding questions remain about the use of some new methods in marshes (i.e., LiDAR).

Groves Creek marsh is typical of Spartina alterniflora salt marshes of the southeastern US, with a main subtidal channel, numerous intertidal channels and broad, vegetated, intertidal marsh platforms. The marsh was mapped at cm-scale resolution (2.7 M points) and sampled with 0.5-6 m long cores. Detailed examination of general trends in morphology, sampling site elevations relative to sea level metrics and marsh subenvironment interconnectivity can provide critical insight into salt marsh sedimentological processes and products. Stratigraphic, sedimentological and microfossil evidence illustrate that environmental conditions have changed significantly over the past few thousand years as sea-level has risen and estuarine marshes have developed, creating a distinct fining-upward sequence. Shell-rich lag layers define the transition from sandy subtidal to muddy intertidal environmental conditions, probably reflecting distinct changes in sediment character and supply associated with climatic changes. The upper meter of the sediment column reflects conditions similar to those at present: homogeneous, fine-grained (>85% mud) sediments, high organic carbon, annual (⁷Be) and centennial (²¹⁰Pb/¹³⁷Cs) accumulation rates pacing local recent sea-level rise (~0.3 cm/y) and δ13C values (-18.5‰) reflecting S. alterniflora input. In contrast, below the lag layer, deeper sediments are >80% sand, well-bedded and low in organic carbon.