GEOLOGIC HISTORY AND PALEOECOLOGY OF SOUTHWEST FLORIDA'S HOLOCENE VERMETID GASTROPOD REEFS AND IMPLICATIONS RELATED TO PAST AND PRESENT SEA-LEVEL RISE

The carbonate-producing coastline of southwest Florida attributes its distinctive geomorphology to oyster and vermetid reef construction, which eventually becomes substrate for mangrove forest progradation. Understanding the unique development process of the mangrove-forested islands is crucial to the management of this ecosystem. This study aims to understand the environmental and ecological constraints on Holocene vermetid reef development and the deeper examination of this relict reef community exposed at the sea surface. Also, the stratigraphy and development of the vermetid reefs will help constrain the regional middle to late Holocene sea-level curve. Radiocarbon dating has concluded that the age of surface vermetid rock is 1,300 Cal yr BP. Four sediment cores taken thus far show a marine transgression, followed by the progradation of reef facies, and ending with the development of mangrove-forested islands. Vermetus nigricans is the vermetid gastropod responsible for the construction of the wave-resistant framework and its structure often includes incorporation of the oyster Crassostrea virginica and barnacles. The geomorphology of this region differs when the prominent framebuilder changes from vermetid to oyster. Vermetid-based mangrove islands have a more lobate shape, while the oyster-based mangrove islands exhibit a more linear shape. This transition occurs mid-estuary with the vermetid-dominated lobate geomorphology occurring more seaward and the oyster-dominated linear geomorphology occurring more landward. Also, there are unique vermetid-containing landforms further northwest that reside on the outer coast that resemble linear tidal cheniers. Here, vermetids played a minor role in landform construction but occasionally established boundstones. Our data suggest that the distribution of landward oyster reefs and the relict vermetid reefs along the seaward edge may be the result of a well-defined salinity gradient within the Ten Thousand Islands.