Evidence of Shoreface Retreat and In-Place “Drowning” During Holocene Submergence of Barriers, Shelf off Fire Island, New York

At different times within the Holocene period, the barriers on the shelf off Fire Island, Long Island, New York, have responded to submergence through the contrasting processes of in-place drowning and landward retreat.

In-place drowning is indicated by evidence of a relict shoreline 7 km seaward of the present beach at a depth of −24 m. Sedimentologic criteria for locating this inferred relict shoreline consist of characteristic relict shoreface sediments on the seaward side and of characteristic relict backbarrier sediments on the landward side of a lenticular belt of sand from which no samples are available. Based on published submergence curves, this inferred relict shoreline is tentatively dated at 8,500 to 9,000 yr B.P.

At about 7,500 yr B.P., the breaker zone is inferred to have “jumped” 5 km landward from this relict barrier to form a new chain of barriers 2 km seaward of the modern shore at a depth of −16 m. Two cores collected seaward of the present beach in 14 to 16 m of water contain backbarrier salt-marsh peat which has been dated at 7,750 ± 125 and 7,585 ± 125 radiocarbon years. The peat underlies offshore sand which forms part of the shoreface of the modern barrier. These cores are evidence that the −16-m barrier migrated continuously landward and eventually became the modern barrier.

Inlet-filling sands can serve as indicators of former locations of barriers and as criteria for determining whether barriers have been drowned in place or have migrated landward. If a barrier migrates continuously landward, it should leave behind a blanket of inlet-filling sands. If a barrier drowns in place, inlet-filling sands should form only narrow, linear lenses parallel to the shore.