Geomorphic and stratigraphic analysis of Fire Island, New York

Abstract

Barrier islands along the East Coast of the United States are believed to be migrating landward in response to sea-level rise. Modes and rates of displacement of Fire Island along the south shore of Long Island, N.Y., were investigated through geomorphic and stratigraphic analyses. Inlet processes are principally responsible for bayshore sediment accretion and hence landward displacement, while overwash has contributed to increasing the island's elevation. Portions of marsh areas covered by washover deposits were often raised above the tidal level permitting colonization by barrier-flat vegetation. Photographic analysis showed that only the 1938 hurricane resulted in bayshore accretion by washover (a small amount at one locality). Most of the bayside marshes formed on relict flood-tidal delta shoals. Former inlet sites are characterized by typical geomorphic features, including relict inlet ridges, indentation of the bay shoreline, wide marsh plains, and relict flood-tidal delta islands in the bay. Historical records and relict inlet features indicate that as much as 85% of the area has been affected by inlet activity. The geomorphic and vegetative data supported by the stratigraphic interpretations showed that the western section of Fire Island, N.Y., is migrating landward more slowly than the eastern section of the barrier chain; this trend is probably due to hurricane tracks and to an increased sediment supply to the western part. Barrier migration is occurring continuously over geologic time, but considered on a short-term basis, displacement is sporadic and related to inlet processes. Over the past 1000 years, Fire Island has experienced shoreface erosion as well as bayshore erosion and submergence.