A High-Resolution Study of a Complex Mixed-Energy Barrier Lithosome: The Evolution of Plum Island in Response to Slow Sea Level Rise and Backbarrier Infillfing

Christopher J. Hein
Boston University Boston, Massachusetts
[email protected]

A stratigraphic study of a modern barrier lithosome, including the barrier, backbarrier, and abutting mainland allows an assessment of the relationship among bay-infilling, tidal prism reduction, and inlet closure. Plum Island, 13-km long barrier island in western Gulf of Maine, is the focus of geophysical and sedimentological studies combined with a chronology based on radiocarbon dates. These data reveal that sediment supply, geological framework, and backbarrier processes controlled the early evolution of the Plum Island barrier system. During the recent Holocene transgression, proto-Plum Island formed about 6 to 7,000 yrs BP from sediment sourced from the reworking of nearshore fluvial and deltaic deposits combined with sand discharged directly by the Merrimack River. Following stabilization at its present site, the barrier built through aggradation and progradation during a regime of slow sea-level rise. Evolution of the barrier was strongly influenced by backbarrier infilling, spit accretion and tidal inlet processes. Existing marsh stratigraphic data, high resolution GPR transects and sediment cores demonstrate that the island contains at least two paleo-channel deposits, representing inlet closures during the barrier’s early evolution (ca. 3 to 4 kya.). The dominant southerly dipping sequences often contain several distinct sets of overlapping reflectors, which are occasionally punctuated by cut and fill structures and smaller packets of northerly dipping reflectors. These features are interpreted to represent the processes associated with the migration and eventual closing of the paleo-Parker Inlet. This study models the effects of a diminishing tidal prism due to backbarrier infilling, causing inlet shoaling and spit accretion.

AAPG Search and Discovery Article #90094 © 2009 AAPG Foundation Grants in Aid