DEVELOPMENT OF THE PLUM ISLAND, MA BARRIER SYSTEM IN RESPONSE TO SLOW SEA LEVEL RISE

High resolution seafloor mapping, seismic stratigraphy and sedimentological studies of the nearshore zone are combined with detailed onshore ground penetrating radar (GPR) and stratigraphic studies to investigate the impacts of sediment supply, geological framework, and backbarrier processes on the formation and evolution of the Plum Island barrier system. Following a -45 m lowstand at 12 kya, the Holocene transgression has reworked the remnants of a late Pleistocene regressive braid plain and lowstand delta. As the shoreline migrated landward, a transgressive unconformity was eroded into the top of the delta and sediments were driven onshore. The distribution and thickness of mobile sediment (i.e., above the unconformity) indicates dominant southward transport of sediment by northeast storms. New radiocarbon dates suggest that barrier sediment began accumulating approximately 5.5 ka when rates of RSL rise decreased. The sediment was sourced from the reworking of offshore shelf sediments and sand discharged by the Merrimack River. Southerly and seaward dipping GPR reflectors (sequence thickness = 3-4 m) indicate that, following initial barrier aggradation, the barrier evolved through southerly spit accretion and barrier progradation. The central section of the barrier is dominated by a complex inlet fill package associated with the paleo-Parker River Inlet. The fill sequence consists of fine to medium sand with repetitive interbedded coarse sand units, marking the high energy depositional events associated with spit accretion and the displacement of the inlet southward. Additionally, this inlet sequence contains conformable sets of southerly dipping reflectors, punctuated by cut and fill structures and smaller packets of northerly dipping reflectors. These sequences capture events of inlet migration, ebb-delta breaching, channel shoaling and inlet infilling. Closure of this inlet occurred during a period of relatively slow sea-level rise that began about 3 ka, coinciding with backbarrier infilling and attendant tidal prism reduction. A fully developed dune system presently occupies the location of the paleo-inlet. Accelerating sea level rise may reverse the long-term trend of tidal prism reduction leading to an enlarging inlet system at the expense of the adjacent barriers.