OPTICAL DATING AND HOLOCENE EVOLUTION OF BEACH RIDGES ON ST. JOSEPH PENINSULA, CAPE SAN BLAS, ST. VINCENT ISLAND AND LITTLE ST. GEORGE ISLAND, NORTH GULF OF MEXICO, FLORIDA, USA

Thirty vibracores were obtained in the study area to investigate the supra-tidal evolution of the central-west barrier islands using optically-stimulated luminescence (OSL) dating of quartz. These landforms belong to the Apalachicola Complex and are located along the coast west of the city of Apalachicola. We found that the oldest ages occur on the north side of St. Vincent Island and at Richardson's Hammock, a truncated older barrier island attached to St. Joseph Peninsula at its southeastern end, bordering St. Joseph Bay to its east. These ages allow a determination of the minimum time of emergence of these land features at around 3,000 years ago.

St. Joseph Peninsula prograded northwards from Richardson's Hammock but also from another island nucleus situated just north of Eagle Harbor on the present-day peninsula. Minimum formation OSL ages of the beach ridges decrease progressively northward from each of these nuclei. Cape San Blas, a tombolo connecting St. Joseph Peninsula to the mainland, evolved westwards from the mainland, starting some time before about 1,000 years ago. The youngest ridges we dated on the western (younger end) of the tombolo are about 300 years old. St. Vincent Island, located well to the east of the St. Joseph/Cape San Blas landform, is a well developed strandplain. It shows a consecutive, ordered, sequence of ages starting at 3,000 on the north end to about 300 years ago on its southern Gulf of Mexico shoreline. Holocene aeolian sedimentation on this island was shown be still active at ridges that are stranded quite far inland, at distances more than 1 km away from the southern shoreline. Little St. George Island, the westernmost landform in the study area evolved from an initial nucleus sometime after about 2,000 years ago, and shows a progressive decrease in beach ridge age from north to south. All systems show sequential age progressions in the directions expected based on geomporphology of the land surfaces and ridge orientation. The timing of vertical accretion of aeolian sand was obtained in nearly all cores through multiple OSL age determinations from near core base upward to near the present day land surface.