THE STRATIGRAPHIC RECORD OF THE HOLOCENE TRANSGRESSION IN WESTERN LONG ISLAND, NEW YORK

The response of barrier islands to sea level rise during the Holocene, and the preservation of the transgressive sequence is controversial. Did barriers migrate by shore face retreat or were they drowned in-place? Our study of 32 vibrocores from the inner continental shelf of western Long Island provides evidence for what happened there in the Holocene transgression. Back bay-estuarine deposits are overlain by nearshore deposits. There is little evidence of reworking by wave action except in 4 shelly lag deposits These lag deposits contain inner shelf foraminifera and glauconite. The vibrocore stratigraphy in our study suggests that the bay that formed behind a paleobarrier was rapidly submerged below wave base allowing the record of transgressive deposition to be preserved. Preserved in this transgressive sequence are 4 coarse lag deposits. These layers contain glauconite and benthic foraminifera, and are laterally consistent across the area. We propose that they are tempestites formed during strong storm events. The identification of tempestites is a promising insight into hurricane frequency in the Northeast. The shell material in these layers will be dated in a future study.

There are two competing hypotheses to explain the response of barrier islands to sea level rise. The long-held hypothesis of "shoreface retreat” (Johnson 1919, Swift et al 1971) proposes that barrier islands migrate landwards as sea level rises. All backbarrier deposits would be eroded by wave action in that scenario. A second hypothesis, "in place drowning", was proposed by Sanders (1980, 1981). This model proposes that backbarrier sediments are preserved as sea level rises until it overtops the barrier island. In this model the backbarrier sequence is preserved. Rampino and Sanders (1980) utilized vibrocores in a study of nearshore central Long Island. Our study, in western Long Island, confirms the conclusions of Rampino and Sanders (1980). This suggests that there are two possible modes of Holocene transgression which may be determined by local conditions.