Redefining the Model of Barrier Island Formation along a Paraglacial Coast: Plum Island, Massachusetts

A geophysical and sedimentological study of Plum Island in northern Massachusetts has refined our understanding barrier formation in northern New England. Previous studies suggest various sources of sediment for building the Plum Island and adjacent barrier system, including erosion of drumlin sediments, sand discharged from the Merrimack River, and the Holocene reworking of lowstand deltaic sediments and braid-plain deposits. Although identifying different sediment sources, all models agree that the island evolved through the process of vertical accretion. Analysis of sediment from vibracores, deep (~36 m) drill auger cores and Geoprobe cores along the barrier show a generally coarsening upward sequence from the fine mud of the Presumpscot formation though fluvial deposits and into the barrier island sequence. Occasionally, impenetrable shallow till is encountered that may represent buried drumlins, onto which Plum Island became pinned during its formation. Sediment core and Ground Penetrating Radar (GPR) data suggest that the barrier lithosome ranges in thickness from 5 to 15 m and is composed of pervasive southerly dipping layers along the southern 2/3 of the island's length. These layers are interpreted as spit (thickness: 4-6 m) and inlet fill sands (thickness: 6 to > 8 m). These southerly dipping layers are frequently underlain by thinner sequences of northerly dipping GPR reflectors that represent the northern recurve of the southerly prograding spit. The predominant southerly dipping reflectors are consistent with dominantly southerly longshore transport system driven by northeast storms. Although previous studies have shown that existing barriers are commonly reworked by tidal inlet migration, this is the first study in New England to show that spit accretion and inlet migration were dominant processes in the formation of the barrier.