The Role of Backbarrier Filling in the Evolution of a Barrier Island System

Barrier islands develop through a variety of processes, including spit accretion, barrier elongation, breaching and inlet filling. New geophysical and sedimentological data collected along a barrier system in the western Gulf of Maine provide a means of documenting a unique process of barrier evolution involving backbarrier infilling and ensuing closure of an ancient tidal inlet. Plum Island is located along a mixed-energy, tide-dominated coastline bounded by estuaries and backed by an extensive system of salt marsh and tidal creeks. Following the regional glacioisostatic lowstand of approximately -45 m at 12 ka, the Holocene transgression reworked a late Pleistocene regressive braid plain and lowstand delta. Sediments driven onshore during the transgression and derived from the Merrimack River fed the developing barrier system. Radiocarbon dates suggests that backbarrier sands began accumulating at the modern site of Plum Island at approximately 9 ka. At this time, the barrier was composed of several discrete islands separated by inlets and situated offshore of modern Plum Island. Shallow seismic and ground penetrating radar (GPR) data confirm the existence of lower stand riverine/tidal channels extending onto the shallow shelf. Coincident with the slowing of relative sea level rise, Plum Island began to form in its current location approximately 5.5 ka, establishing its modern form by about 2.5 ka. Sediment cores and GPR data demonstrate that the barrier lithosome is 5 to 15 m thick and evolved through initial aggradation followed by southerly spit accretion and progradation. The discovery of a multiple inlet channel system reoccupying the area carved by the lowstand Parker River indicates that central Plum Island underwent a complex developmental history. Cores through the inlet sequence consist of fine to medium sand with repetitive interbedded coarse sand units, marking high-energy depositional events associated with spit accretion and displacement of the inlet southward. GPR profiles reveal that the inlet sequence contains conformable sets of southerly dipping reflectors punctuated by sharp truncation surfaces, cut and fill structures and smaller packets of northerly dipping reflectors. These large-scale structures are evidence of inlet migration, ebb-delta breaching, onshore bar migration, channel shoaling, and closure of a tidal inlet. Discovery of the paleo-inlet within the Plum Island lithosome illuminates the larger process of backbarrier infilling and its effect barrier and tidal inlet morphodynamics. Sediment influx to the backbarrier from nearby river systems and the offshore led to bay sedimentation, formation of tidal flats and marshes, and a vast reduction in the bay tidal prism. Ultimately the inlet closed by a southerly building spit.