20 Coastal Geomorphology (Posters)

Friday, 13 March 2009
1:30 PM - 5:30 PM

RECONSTRUCTING FORMER SOUTHWEST FLORIDA'S BARRIER ISLAND DOWN-CORE HISTORY FROM THE DISTRIBUTION OF ESTUARINE AND OPEN SHELF FACIES

Paper number: 20-2
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

Southwest Florida's coastal geomorphology is dominated by barrier islands. The history of older late Holocene barrier islands, however, is typically lost during transgression due to reworking of island sediments. This study recreates this history using the distribution of sedimentary facies that represent former protected estuarine environments and open shelves as indicators of older ephemeral islands and their disappearance. Work was conducted within the Rookery Bay National Estuarine Research Reserve, south of Naples, where 4 imbricated or currently eclipsed barrier islands exist (from out- to inboard: Keewaydin, Little Marco, Cannon, and Johnson Islands) whose core stratigraphy reflects the evolution of island activity. Six 4+ meter-long cores were taken (2 each from Keewaydin and Little Marco; 1 each from Cannon and Johnson) from current protected back-barrier settings either within mangrove forests or from apron beaches. Standard vibracoring techniques were employed, and facies were described from stratigraphic, sedimentologic, paleontologic, and taphonomic data. Geochronology was obtained from AMS radiocarbon dates of both autochthonous organics and from carbonate shells of high taphonomic grade. Aerial imagery for these islands also exists dating back to 1940.

Keewaydin cores show open shelf environments in their oldest marine histories, but have thick sequences of barrier island facies that predate the existence of the current island. Cores from Little Marco, whose southern terminus was active through the early 1960s, exhibit estuarine facies predating the island's most recent activity. Additionally, one core shows estuarine conditions as old as ~ 5200 Cal ybp at over 4 m below current MSL, suggesting even older barrier islands were seaward of this location. Cores from Cannon and Johnson, being the most proximally positioned cores, are dominated by estuarine facies and exhibit no open shelf conditions. Sediments indicating the timing of these islands' activity are found within the core, but we currently have no dates. Refining this historic interpretation will require better geochronology; many of our radiocarbon dates are suspect and may represent reworked specimens.