|Steve Obrochta, Department of Marine Science, Eckerd College, St. Petersburg, FL
Boudewijn Remick, Department of Marine Science, Eckerd College, St. Petersburg, FL
Gregg Brooks, Department of Marine Science, Eckerd College, St. Petersburg, FL
Dave Duncan, Department of Marine Science, Eckerd College, St. Petersburg, FL
Shore-parallel ledges represent the greatest natural topographic relief on the inner west-central Florida continental shelf. For many years, these ledges have been popular fishing sites. They also support a varied invertebrate fauna, including both hard and soft corals, sponges, hydroids (milleporina), and many classes of vagrant echinoderms. In order to gain a more complete understanding of their geologic history and relationships to the underlying limestone platform, ledges have been described and photographed, and a series of samples have been collected, thin sectioned, and petrographically analyzed.
The ledges and hardgrounds observed range from around 19 to 40 km off shore, while their vertical relief ranges from less than 1 meter to greater than 3 meters. The ledges have been bioeroded in excess of 1 meter, creating an overhang, which periodically calves off to produce an accumulation of rubble.
Results show that all samples except for one are comprised of 100% magnesium calcite, and all contain a significant siliciclastic framework. The framework consists dominantly of quartz, but feldspar is also present. Although the grains are generally of varied degrees of roundness, they are for the most part texturally and compositionally immature. Grain size and sorting of framework grains is difficult to determine in thin section, but both appear to vary considerably. Some samples collected from different parts of the same ledge system show striking differences in grain size, carbonate and siliciclastic mineralogy, and presence of fossils. Unfortunately, the exact location of each sample on the ledge was not noted during collection.
Two types of rocks predominate; relatively coarse quartz grains in a micritic matrix, and finer-grained quartz also in a micritic matrix, some of which contains fossils. None of the fossils have yet been identified. Quartz-rich rocks containing coarser quartz grains are from ledges that may represent ancient beach ridges, formed during lower sea-levels. Quartz-poor, finer-grained samples may represent the underlying limestone surface upon which the ridges are perched. Following sea-level rise, the ridges have been bioeroded, creating a "ledge." The steep face which generally points towards shore, may represent selective bioerosion on the lee, or protected, side of the ridge.
If the ledges are indeed ancient beach ridges, their selective preservation may have ramifications relating to the rates and timing of sea-level rise. Ledges and hardgrounds will continue to be studied in order to ascertain the controls and processes governing their geologic development and their relationship to sea-level fluctuations. Results will aid in our understanding of the modern coastal development of west-central Florida.