LATE HOLOCENE BACK-BARRIER DEVELOPMENT OF PORTSMOUTH ISLAND, OUTER BANKS, NORTH CAROLINA

Lithologic, foraminiferal, and radionuclide data are being used to reconstruct the late Holocene geologic history of the back-barrier environment of Portsmouth Island, northern Core Banks, North Carolina. Fourteen vibracores were collected along four shore-normal transects from tidal salt marshes, along tidal creeks and into Pamlico Sound. Four litho/biofacies were distinguished. Facies A is a highly organic, sometimes heavily rooted, sandy mud unit varying in depth from 0 m to 0.5 m below sea level containing a typical marsh foraminiferal assemblage characterized by Trochammina inflata and Haphophragmoides wilberti, with other subsidiary species. Facies B, varying in depth from 0.5 m to 3.0 m below sealevel, is fine grained, well sorted, quartz sand, sometimes rooted and sometimes containing organics. Foraminifera were generally absent from this facies. Facies C is a fine grained, well sorted quartz sand, containing fine shell hash that extends from 1.0 m to over 7 m below sea level. The moderate diversity foraminifera assemblage from this unit is typical of the inner shelf environment off the modern North Carolina coast and is dominated by Elphidium excavatum with several subsidiary species (e.g., Quinqueloculina seminula, Elphidium mexicanum, Elphidium subarticum, Hanzawaia strattoni, and Nonionella atlantica). Facies D is interbedded within Facies C at 4 m below sea level and is a gravelly coarse shell lag containing the inner shelf foraminiferal assemblage. Other regional studies suggest that eastern portions of Pamlico Sound may have been open to fully marine influence around 1200 to 500 years B.P. as indicated by C-14 age estimates, sedimentologic data and foraminiferal assemblages. Part of the barrier island collapse that allowed Gulf Stream foraminifera into Pamlico Sound at this time may have occurred along Northern Core Banks.