COMPLEX PROGRADATION HISTORY OF CAYO COSTA BARRIER ISLAND, SOUTHWEST FLORIDA: INSIGHTS FROM GEORADAR IMAGING

Cayo Costa (Lacosta) barrier island along the Florida Gulf Coast has been the subject of geomorphological and chronological research aimed at reconstructing regional sea-level history. However, questions remain as to the indicative meaning of late Holocene beach ridges due to the effects of differential aeolian aggradation. High-resolution geophysical surveys were conducted across several key shore-normal transects using an 800 MHz ground-penetrating radar (GPR) system. Subsurface records clearly delineate sections of sand-dominated barrier lithosome alternating with organic-rich muddy swale fill (>1 m thick). Several modes of sigmoidal-oblique offlap (progradation phases) are punctuated by onlapping (retrograding) lithosomes. This results in complex depositional patterns in a regime of net progradation, with beachface accretion sequences (>3 m thick) alternating with transgressive tracts (overwash and ridge accretion) that range from 0.5-2.5 m in thickness and exceed 20 m in width. Truncated reflections are interpreted as erosional disconformities, likely related to moderate storms and may have caused shore-wide initiation of beach ridges. Shallow cores confirm key GPR reflections, including heavy-mineral anomalies and shell-hash accumulations. The hummocky radar facies indicate aeolian aggradation, making beach-dune contact a more reliable indicator (higher high tide) with respect to past sea-level positions than beach ridge height. Our findings demonstrate the need for continuous subsurface imaging in reconstructing paleo-shoreline features, especially in protected areas, such as Cayo Costa State Park, where opportunities for groundtruth may be limited.