Barrier island evolution, middle Atlantic shelf, U.S.A. Part II: Evidence from the shelf floor

Abstract

Where the Atlantic coastal gradient is low and the substrate is unconsolidated, shoreface processes lead to an equilibrium shoreface surface, characterized by a straightened plan view and a concave-up profile. Maintenance of this surface during the post-glacial sea-level rise may lead to barrier formation by mainland beach detachment, where the outer oceanic shoreface is maintained by wave and current processes. Downwelling storm currents sweep sand from the shoreface of the middle Atlantic bight, transport it downcoast and seaward and deposit it on the adjacent inner shelf. Some of this sand is returned by the action of asymmetrical wave orbital currents during fair weather. However, there is a net loss in most areas. The loss is not made good by river sand input, since on the Atlantic Coast all river sand is trapped by estuaries. As a result the shoreface in these areas is undergoing erosional retreat. Mean horizontal retreat rates are on the order of 1–3 m yr −1 but most movement occurs in brief episodes with recurrence rates on the order of several events per century. Erosional shoreface retreat, coupled with aeolian overshoot, storm washover and the movement of sand through inlets into the lagoons, has lead to landward migration of the Atlantic barriers through the Holocene period of sea-level rise. The landward retreat of the barrier as a whole is as dependent on inlet formation as it is on erosional shoreface retreat. Repeated inlet breaching and the downdrift migration of inlets yields coalescing flood tidal deltas within the lagoon. The resulting surface forms a platform on which the subaerial barrier deposits can advance under the impetus of storm washover and aeolian action. The backbarrier deposits eventually re-emerge at the shoreface, where their upper beds are eroded and recycled. The barrier thus migrates over a pavement of its own flood tidal deltas and washover fans; sand is added to the system partly through the erosion of updrift headlands but mainly by means of scour in inlets, which reaches down to the underlying Pleistocene. The modern shelf sand sheet is the debris blanket resulting from the retreat process. The leading edge occurs as a thin veneer of rip-current fallout on the shoreface; this is periodically stripped off by winter storms to expose the underlying backbarrier strata. Further seaward, shelf sands eroded from the shoreface rest disconformably on backbarrier deposits; shoreface, beach and dune strata are always missing. Every grain of sand in this shelf sand sheet has occupied a position on the beach or upper shoreface within the recent past. However, the sheet as a whole has been reconstituted; its primary structures are those of the inner shelf floor. Backbarrier sand and mud deposits, characterized by channeling and landward-dipping reflectors, can be traced for over 100 km seaward across the middle Atlantic shelf, beneath the modern sand sheet. The backbarrier stratum together with the overlying shelf sand layer constitutes the record of barrier retreat across the Atlantic shelf surface during Holocene time. The sheet-like nature of this record indicates that, viewed at the appropriate time scale, barrier migration is a continuous process.