SLOW ENERGY, FAST TRANSGRESSIONS: STATUS OF KNOWLEDGE IN FLORIDA'S BIG BEND

Surface geology, climate, and geomorphic processes are presented with evidence for a modern marine transgression along the Gulf Coast of Florida. The Big Bend region, between Apalachee and Tampa Bay, is a shallow estuarine environment along a 470-km coastline. It is characterized by low elevation, low gradient, and low energy with limited sediment supply. Spring-fed rivers and seepage from the Floridan aquifer moderate nearshore salinities. The shallow nearshore and marsh-dominated intertidal zone consist of thin sediments over porous and locally exposed limestone. Apparent shoreline stability has masked the impact of sea-level rise on this coast. Records of intertidal sedimentation processes indicate wide fluctuations from site to site and year to year. Remotely sensed imagery and vegetation studies reveal a relatively stable intertidal zone with a marine transgression of the adjoining lowland forests. Analysis of historic navigation charts confirmed these results and showed local variations in the extent of marine transgression. Local differences were related to underlying karst topography, freshwater supply, human activities, and variation in tidal energy. Interactions among biota in response to fluctuations in sea level and climate may have served to stabilize the intertidal zone. A potential analog for marine transgressions is suggested in field observations of upland soil loss, lowered surface elevation, and expansion of tidal flow across the newly altered terrain. Dissolution and fracturing of the underlying karst may serve as a conduit for the rapid advancement of tidal flow across the coastal lowlands. A preliminary diagram illustrating the state of knowledge and information gaps for the Big Bend region will be presented for open discussion.