Evolution of a Barrier System in Response to Slow Sea Level Rise and Backbarrier Infilling: Plum Island, Massachusetts

Hein, Christopher J.¹; Carruthers, Emily ¹; FitzGerald, Duncan ¹; Barnhardt, Walter A.²; Stone, Byron D.^3
1 Dept. of Earth Sciences, Boston University, Boston, MA.
² Woods Hole Science Center, United States Geological Survey, Woods Hole, MA.
³ United States Geological Survey, Reston, VA.

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. New geophysical and sedimentological data combined with a chronology based on radiocarbon dates reveal that sediment supply, geological framework, and backbarrier processes controlled the early evolution of the Plum Island barrier system. Subsequently, the barrier built through aggradation and progradation during a regime of slow sea-level rise. Bedrock, glaciomarine clay, and till deposits form the underpinnings of the barriers in this region and the drainage of several rivers govern the extent of the backbarrier and dimensions of the tidal inlets. Glacio-isostatic rebound forced a regional lowstand of about -45 m at 10.5 kya. During the subsequent Holocene transgression, proto-Plum Island formed about 4 to 5,000 yrs BP from sediment sourced from the reworking of nearshore fluvial deposits combined with sand discharged by the Merrimack River. Evolution of the barrier was strongly influenced by backbarrier infilling, spit accretion and tidal inlet processes.

Sediment cores and Ground Penetrating Radar (GPR) data demonstrate that the barrier lithosome is 5 to 15 m thick and can be divided into three sectors:

1)A northern section dominated by channel cut and fills (depth: 8 to 10 m) associated with the Merrimack River Inlet.

2) A central section characterized by flat-lying to southerly dipping reflectors (thickness: 3-4 m) along the backside of the barrier fronted by seaward dipping clinoforms indicating southerly spit accretion and barrier progradation.

3) A southern third of the barrier is dominated channel migration and filling of the Parker River Inlet and southerly spit accretion influenced by nearby glacial till deposits.

Along the southern section of the island’s length a complex inlet fill sequence is observed. The dominant southerly dipping sequences often contain several distinct sets of overlapping reflectors and 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 Parker Inlet. Using existing marsh stratigraphy and high resolution imaging of the paleo-Parker Inlet, this study models the effects of a diminishing tidal prism due backbarrier infilling, causing inlet shoaling and spit accretion.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009