Geological Evidence of Former Tidal Inlets along a Retrograding Barrier: Duxbury Beach, Massachusetts, USA

Duxbury Beach is a retrograding barrier system located along the South Shore of Massachusetts Bay. The barrier fronts a large embayment consisting of a broad supratidal marsh in the northern sector grading to intertidal flats and open water to the south. Radiocarbon dates indicate that the barrier began forming sometime prior to 3,700 ¹⁴C years BP. The paucity of sediment combined with rising sea level have produced a relatively low barrier ranging in width from 95 to 285 m. The foredune is discontinuous and storm washovers are common. Historical maps and coastal charts indicate the presence of several former tidal inlets along the length of the barrier, although none exist today. Ground-Penetrating Radar (GPR) surveys suggest a minimum of 18 paleo-inlets along the barrier as indicated by large-scale channel cut-and-fill structures. Former inlet channels range from 60 to 285 m in width and are typically 3 to 6 m deep. The ephemeral character of the inlets and their diminutive size are a product of their inability to capture a significant portion of the bay tidal prism. Seven channel fill patterns were documented in the GPR records; the three major types include prograded, conformable, and complex. The patterns of subsurface reflectors reveal that former inlet channels infilled by a variety of processes including simple spit extension from one or both sides and channel aggradation through landward sediment transport. Statistical analyses indicate that generally the position and geometry of former tidal inlets are unrelated to present barrier dimensions or offshore bathymetry, however, there are correlations with bay morphology and hydraulics as well as barrier stratigraphy. In one instance, a relatively wide paleo-inlet coincides with a moderately large bay tidal channel. Deepening of another inlet appears to be related to migration of its channel toward a resistant till headland. This study demonstrates that tidal inlet fill deposits can comprise a significant portion of the coastal lithosome along retrograding barriers (>25% at Duxbury Beach).