SUBSURFACE CONTROLS ON SHORT- AND LONG-TERM FRINGING MARSH EVOLUTION: COMPARISON OF COASTAL SETTINGS ALONG THE CENTRAL ATLANTIC COAST, CARTERET COUNTY, NC

The degree to which short-term variation in morphology and underlying geology control short- and long-term fringing marsh evolution was examined along the central North Carolina coast. Cores were collected along transects at the fringing marshes and high-resolution surface elevation data was obtained to constrain marsh accretion and erosion within the area. Fringing marsh environments examined are separated into 4 types based on paleodepositional, geographic, and hydrologic setting. Paleodepositional environments examined include flood-tidal delta, barrier island, wash-over fan, and bay-head delta.

Subsurface geology is variable between sites. The fringing marsh at the flood-tidal delta locale, for example, overlies dark gray interbedded sands and clays, interpreted as tidal couplets, whereas the fringing marsh at the back-barrier site overlies a yellowish brown, parallel to cross-laminated fine-grained sand indicative of foreshore sedimentation. No marsh deposits predating the modern are present in any of the cores.

Short-term changes in fringing marsh surface elevations over a one-year period range from – 0.7 to + 1.5 mm and currently yield no clear-cut relationship to the underlying geology; however, these data continue to be collected to better understand short-term marsh response to both natural and anthropogenic forcing mechanisms. Bay-ravinement processes operating at timescales of seasonal (storms) to 1000's of years are attributed with the removal of organic and inorganic sediments from the study sites. Data that show truncation of depositional environments indicate varying depths of bay-ravinement for different marsh types. This suggests control of the underlying geology on the extent of marsh erosion. Furthermore, the absence of marsh deposits in the underlying geologic record and the variable extent of modern marsh erosion at the sites suggest bay-ravinement processes were largely unaffected by marsh accretion rates. Although the fringing marshes studied are presently able to keep up with relative sea-level rise (3 mm/yr), accretion rates alone cannot predict future marsh extent or the preservation potential of marsh deposits. The underlying geology and ravinement processes need to be taken into account.