Sedimentary architecture of a spit-end (Salisbury Beach, Massachusetts): The imprints of sea-level rise and inlet dynamics

In the vicinity of tidal inlets, the facies architecture of barrier islands is largely controlled by inlet channel shifting and the transfer of sand between the ebb-tidal delta and the adjacent shore, including processes of inlet sediment bypas sing. Merrimack River Inlet is situated in a mixed-energy setting and is characterized by episodic shifting of the ebb channel prior to being stabilized in 1881. Here, we delineate the internal structure and evolution of the updrift barrier-spit terminus by integrating data obtained from ground-penetrating radar transects and sediment cores to document the effects of inlet-associated sedimentation processes. Our results document complex radar and sedimentological facies, which define two distinctive associations of facies, the inlet-distal and -proximal, within the core of the present barrier. These facies are associated with three phases of barrier-terminus evolution: (i) barrier retrogradation, characterized by the preservation of washover and backbarrier marsh facies deposited during sea-level rise, (ii) downdrift barrier progradation, characterized by channel fill and swash platform deposits overlain by foreshore deposits generated by long shore currents, and swash bars migrating to the foreshore from the swash platform, and (iii) foreshore seaward progradation, characterized by cross-beds. This study illustrates the highly complex nature of inlet associated spit-ends of barriers. Likewise, our results shed light on the mechanisms of spit elongation and recurving in a setting where the estuary opening is narrowing.

Research highlights
► We image the architecture of a barrier-spit terminus integrating data from ground-penetrating radar (GPR) and sediment cores.
► Spit-ends are very complex coastal depositional systems modeled by diverse agents.
► Facies associations, inlet-distal and inlet-proximal, interact differently with the adjacentinlet.
► Facies associations were regulated sea-level or inlet dynamics.
► This paper captures for the first time an entire spit-tidal inlet fill sequence in GPR.