Post-Pleistocene history of the United States inner continental shelf: Significance to origin of barrier islands

Abstract

Segments of the U.S. Atlantic coast differ markedly in shoreline and shelf configuration because of both geologic setting and Quaternary geologic history. Regional trends in shelf structure, sediment sources, and history correlate with regional trends in adjacent shoreline configuration, and the Holocene transgressive history of the shelf has a direct bearing on the evolution of the present coastline. The shelf sedimentary record, particularly in the mid-Atlantic region, contains substantive evidence that precursors to existing barrier islands were common in Holocene time. Nearshore linear shoals are associated with barrier-island coastlines, and their continuity in position on the shelf, coupled with the presence of an underlying lagoonal substrate, implies persistent retreat of a barrier coastline during sea-level rise. Retreat paths of shoals off large capes and estuaries similarly indicate a consistency in shoreline type and orientation through time. Collectively, this information strongly suggests that barrier islands of the U.S. Atlantic coast originated far out on the shelf, and it invalidates certain criteria (coastal linearity, lithology of sediments beneath the modern lagoon) formerly cited as evidence for mode of formation. Understanding how and the processes by which barriers evolved relate directly to present and future behavior of barriers. Sediments eroded from the foreshore and shoreface are transported both along the coast and landward, aggrading as nearshore shoals, spits, tidal deltas, dunes, and overwash fans. Barrier islands lengthen both by sand accumulation (spit extension) and by submergence (deepening and headward extension of the lagoon). Development of barrier islands is influenced by rate of sea-level advance and sediment supply. Once initiated, barrier islands lengthen and retrograde through a combination of processes, including spit elongation, overwash, and flooding of back barrier lagoons.