| Summary |
Foraminiferal data coupled with lithostratigraphic, chronostratigraphic, and geophysical data are used to characterize the late Holocene geologic development of the Ocracoke Island back-barrier. Ocracoke Island is a 24 km-long island separating southern Pamlico Sound from the Atlantic Ocean. It is situated between Ocracoke Inlet to the southwest and Hatteras Inlet to the northeast. Howard's Reef and Green Island, two submarine sand shoals, are located directly behind Ocracoke Island and parallel the island. Seventeen vibracores were collected along three shore-normal transects from environments including tidal creeks, overwash fans, back-barrier salt marsh, estuarine sand flats and the sand shoals at Howard's Reef and Green Island. Seven paleoenvironments were distinguished using cluster analysis of foraminiferal subsamples. The barrier island sand/ overwash sand environment is a massive to laminated, fine to medium-grained, moderately to well sorted, sandy unit typically barren of foraminifera, ranging from 1 m to 0 m above mean sea-level (MSL). The marsh environment is an organic-rich, typically rooted, sandy mud unit ranging in depth from 0 m to 1.3 m below MSL. This environment contains a typical marsh foraminiferal assemblage characterized by finely agglutinated taxa Arenoparrella mexicana, Haplophragmoides wilberti, and Trochammina inflata, among others. The estuarine flats environment ranges in depth from 0 m to 0.8 m below MSL. This environment is characterized by varying lithologies including fine to medium-grained massive sand and muddy sand and a representative estuarine foraminiferal assemblage characterized by Ammonia parkinsoniana, Elphidium excavatum, and Haynesina germ̀nica. The shallow submarine sand flats environment is characterized by varying lithologies of gravel, sand and mud, with variable amounts of fine to gravel sized shell hash. It ranges in depth from 0.8 to 2.5 m below MSL. This unit is typically barren of foraminifera. Three normal salinity paleoenvironments were recognized (flood tide delta, normal salinity low energy/ low energy inner shelf, and normal salinity high energy/ inlet) ranging in depth from 1 m to 7 m below MSL. These three paleoenvironments were recognized through biofacies analysis and typically have coarser lithologies of medium to coarse sand with variable percentages of gravel-sized shell hash, but are also recognized in muddier lithologies. The normal salinity low energy/ low energy inner shelf paleoenvironment typically occurs as the basal sediments of cores obtained in Pamlico Sound, while the normal salinity high energy/ inlet paleoenvironment typically occurs as basal sediments of cores obtained on Ocracoke Island. The normal salinity low energy/low energy inner shelf biofacies is a high diversity assemblage characterized by Elphidium poeyanum, Haynesina germ̀nica, Elphidium excavatum, and Elphidium galvestonense along with rare species of Bolivina lowmani, Rectobolivina advena, Trifarina angulosa, and Gulf Stream planktonics. The normal salinity high energy/ inlet paleoenvironment is characterized by a high diversity assemblage containing robust taxa such as Quinqueloculinajugosa, Quinqueloculina seminula, and Quinqueloculina lamarckiana along with Elphidium species. The flood tide delta paleoenvironment is characterized by a moderate diversity normal salinity assemblage of Glabratellina lauriei, Elphidium mexicanum, and Hanzawaia strattoni coupled with in situ Gemma gemma clam species. Related studies conducted in the northeastern North Carolina coastal system have suggested that the eastern margins of Pamlico Sound experienced normal marine influence from approximately 1100 to 500 years BP, as indicated by litho- and biostratigraphic data coupled with ¹⁴C and optically stimulated luminescence (OSL) age estimates. Foraminiferal data along with ¹⁴C and OSL age estimates from this study further support the hypothesis for a collapse in the continuous barrier island complex in the Ocracoke Island study area, allowing normal mid-Atlantic Coast inner shelf foraminifera to live in southern Pamlico Sound for approximately 600 years. |
