BARRIER EVOLUTION IN RESPONSE TO INLET CLOSURE: AN EXAMPLE FROM A PARAGLACIAL BARRIER SYSTEM

Accelerated sea level rise is expected to force the conversion of backbarrier marshes and tidal flats to open water, resulting in an increase in tidal prism and an enlarging inlet system at the expense of the adjacent barriers. A geophysical and stratigraphic study of a barrier island in the western Gulf of Maine provides a means of documenting paleo-inlet processes and their effects on barrier morphodynamics and evolution.

Plum Island is part of the longest barrier chain in New England and subject of considerable sedimentologic and stratigraphic investigation during the past 40 years. Ground-penetrating radar (GPR) data, ground-truthed with sediment cores and combined with a chronology based on radiocarbon dates reveal that the barrier lithosome extents 3 to 10 m below modern mean sea level and has formed from the reworking of shallow shelf deposits during the Holocene transgression, supplemented with additional sediment delivered from the adjacent Merrimack River. Holocene sediment was first delivered to the site of modern Plum Island by 7 – 8 ka and proto-barriers had migrated to this site by 3.5 ka. These barriers evolved initially through aggradation, followed by southerly spit accretion and progradation. In central Plum Island, a nearly 450 m wide and 4 m thick complex paleo-inlet sequence was discovered. Detailed investigation of this complex channel system has revealed evidence of inlet migration, ebb-delta breaching, and bar welding associated with the migration of the inlet. A series of published and unpublished core logs were used to develop chronostratigraphic reconstructions of the Plum Island backbarrier. Time-transgressive backstripping of individual facies show that sediment influx to the open water backbarrier lagoon led to bay sedimentation, formation of tidal flats and marshes, and a vast reduction in the bay tidal prism. The backbarrier tidal prism was more than halved, reducing tidal fluxes through the paleo-inlet and resulting in the inlet channel shoaling and infilling. The eventual closure of the paleo-inlet allowed for the latest stages of development of the island by progradation and spit elongation.