| Summary |
The Broad Creek study area on the southeastern end of Roanoke Island is located within the large estuarine system dominating the northern half of coastal North Carolina. Estuaries in northeastern North Carolina are characterized by low salinities (0 to 25 ppt) and low astronomical tides ([less-than] 15cm). However, because of their large size and shallowness, they are strongly influenced by wind tides. The study area consists of three environments; a shallow elongated embayment (Broad Creek) protected from most wave energy, a tidal creek system, and an irregularly flooded brackish marsh dominated by Juneus roemerianus. Within the study area, sediment distribution patterns are similar to the sediment distribution patterns which occur within the larger scale estuarine system of northeastern North Carolina. Sedimentation processes operating within the study area include the following: 1) wave erosion during low wind tides and biocorrosion of the marsh shoreline recycling fine sediment and organic material; 2) permanent deposition of suspended sediments within the central, deeper portions of the bay during normal energy conditions; 3) deposition of suspended sediments on the marsh surface during high wind tides; 4) nondeposition, winnowing, and reworking of lag sands into thin shelf bars on outcrops of a muddy sand unit lateral to the central bay mud depositional environment within Broad Creek; 5) channel bottom scour within meanders of the tidal creek system; 6) deposition of highly organic rich mud on shoals lateral to the tidal creek channel; and 7) erosion and southerly transport of fine to medium grained sands along the high energy Roanoke Sound shoreline and deposition on a bay mouth bar forming across the southern end of Broad Creek Stratigraphic relationships and dates indicate that the present Broad Creek system is a product of a series of evolutionary changes which occurred over the past 6,000 years. The study area represents a relict drainage system formed during the late Wisconsin lowstand of sea level which has been progressively drowned by rising sea level through time within an increasing influence of marine conditions. The drowning influence was confined to the floodplains from 6,000 B.P. until approximately 2,500 B.P. Between 2,500 B.P. and 1,500 B.P. fresh marshes expanded rapidly across the low interstream divides within the study area as the floodplains were overtopped by the transgression. Between 1,500 B.P. and 1,000 B.P., the fresh marshes were replaced by brackish marshes which moved into the study area from the southeast. Brackish marshes continue to dominate the study area today and respond to rising sea level by growing vertically and migrating landward over the inorganic uplands. The Broad Creek embayment has expanded to its present size through erosion of its marsh shorelines. The eroded shorelines are undercut peat scarps with relief of 0.75 to 1.0 meter. This steplike shoreline geometry and the sedimentation processes operating within the study area represent a major departure from tidal flat shoreline geometries and sedimentation processes reported in similar scale systems characterized by astronomical tidal ranges [greater-than]1/2 meter. Since this natural astronomical tidal range break falls within the microtidal classification generally in use, a new tidal range subdivision to be called "nanotidal" is proposed for estuarine systems with a tidal range of [less-than]1/2 meter. Erosional peat scarp shorelines like those found within Broad Creek are typical throughout the northeastern North Carolina estuarine system. Considering the magnitude of erosion of marsh shorelines, it is feasible that this mechanism provides a significant means of introducing organic detritus into the estuarine system where regular tidal flushing is not significant. Preliminary calculations show similar amounts of organic detritus entering the northeastern North Carolina estuaries through erosion to amounts reported entering Georgia estuaries through tidal flushing. |
