Southeastern Section - 64th Annual Meeting (19–20 March 2015)

Paper No. 11
Presentation Time: 1:00 PM-5:00 PM


JONES, Kelsey R.1, SAVARESE, Michael2, BUYNEVICH, Ilya V.3, CONNORS, Olga C.1, ECHOLS, Ronald J.1, PICKETT, Wesley4 and SEGAL, David1, (1)Marine & Ecological Sciences, Florida Gulf Coast University, Fort Myers, FL 33965, (2)Marine & Ecological Sciences, Florida Gulf Coast University, 10501 FGCU Blvd South, Fort Myers, FL 33965-6565, (3)Department of Earth & Environmental Science, Temple University, Philadelphia, PA 19122, (4)Earth and Environmental Science, Temple University, Philadelphia, PA 19122,

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.