BIOEROSION TRACE FOSSILS AND A PLEISTOCENE SEA LEVEL EVENT IN THE KEY LARGO LIMESTONE, FLORIDA KEYS

The Pleistocene Key Largo Limestone in southern Florida contains abundant, largely unstudied bioerosion trace fossils (e.g, Gastrochaenolites, Meandropolydora, Trypanites). Widely exposed in the Florida Keys, this unit offers an excellent opportunity for study of Pleistocene bioerosion rates and sea-level change. Gastrochaenolites torpedo borings (attributed to the boring bivalve Lithophaga sp.) in fossil corals were counted and measured to obtain data of the volume and density of the borings. Using age/size relationships, an estimate of boring duration was determined. An overall Pleistocene bioerosion rate was calculated at 0.18 to 0.36 m per 1,000 years. A rate of 0.041 m per 1,000 years was calculated for time-averaged data, and this rate is likely a more accurate approximation of the Pleistocene bioerosion rate.

Bioerosion trace fossils also can provide insight into relative sea-level changes during the Pleistocene. A sea-level event at roughly 125 ka has been documented by other workers in the Bahamas, Mexico and Barbados, but this event has not yet been reported in the Pleistocene units of southern Florida. The Key Largo Limestone shows small amounts of caliche and abundant G. torpedo borings oriented normal to the apparent unconformity surface. As caliche indicates subaerial exposure, its presence within G. torpedo borings suggests that the borings occurred during the regression portion of the sea-level event. Previously published U/Th dates place this unconformity surface at approximately 125 to 138 ka.