ABSTRACT

The Holocene history of south-central Long-Island and the adjacent inner continental shelf, as recorded in the stratigraphy of transgressive deposits, was studied, using information obtained from more than 400 boreholes, cores, and marsh probings in the barrier- and backbarrier-areas and vibracores and numerous seismic-reflection and bathymetric profiles on the inner shelf.

A sequence of deposits which records the Flandrian transgression is preserved in the backbarrier areas of south-central Long Island. The vertical sequence of these trangressive deposits was produced by the movement of successive environments of coastal deposition landward and upward with the rising sea. The vertical sequence is as follows reading from bottom to top: the buried Pleistocene surface, brackish- to salt-marsh peat, open-lagoonal silty clays, backbarrier sands, backbarrier-fringe salt-marsh peat, and barrier island sands. The entire transgressive sequence forms a lens-shaped deposit. In going seaward from the mainland shore to the barriers, the sequence thickens from 0 to 10 m; seaward of the barriers the sequence becomes thin again. In places beneath the barrier islands, t e transgressive sequence has been completely reworked by the lateral migration of tidal inlets.

Radiocarbon dating of basal peat and organic silty clays has provided a chronology of relative sea-level rise on the southern Long Island coast for the past 8000 years. Relative sea level on the Long Island coast is interpreted to have been rising at about 2.5 mm·yr^-1 between 7000 and 3000 yr BP and slowed markedly to about 1 mm·yr^-1 during the last 3000 years. Prior to 7000 yr BP, the rate of submergence may have been as high as 5 mm·yr^-1.

The extensive preservation of backbarrier sediments, dated between 7000 and 8000 yr BP, on the inner shelf of southern Long Island suggests that the barriers have not retreated by continuous shoreface erosion alone but have also undergone discontinuous retreat by in-place "drowning" of barriers and stepwise retreat of the surf zone landward. This would have locally prevented the backbarrier sediments from being reworked and thus would lead to their preservation on the inner shelf.