FISC - St. Petersburg
Overview MapDepth to Pleistocene Bedrock Surface: Coral reefs and sediments (now cemented limestone bedrock) accumulated over many thousands of years. Their resultant buildups contain records of past environmental quality and physical/biological processes. In Florida, ancient reefs form much of the bedrock (e.g., Sanford, 1909; Hoffmeister and Multer, 1964; Perkins, 1977). The bedrock is Pleistocene (Fig. 7B).
Detailed regional bedrock topography in the Florida Keys was derived from geophysical data that provided the depth of the bedrock surface below present sea level. High-resolution seismic-reflection profiles were generated using acoustic (sound-wave) instruments. Water, sediments, and rock have different acoustical properties and produce different lines on a seismic profile. The sound waves penetrate through the water column and sediments, reflecting the sediment (seafloor) surface and the contact between the sediments and underlying bedrock. The sediment thickness can be calculated and the topography of the bedrock surface can be determined and contoured from the profiles (see Bedrock Surface map and Open-File Report 00-046. The lines representing these surfaces on the profiles are known as seismic reflections.
Although discontinuous, bedrock coral reefs are hundreds of meters wide, tens of meters high, and more than 200 km in extent. Such dimensions indicate that conditions during the various periods of their accumulation were highly favorable for coral growth region-wide (see Seven Coral Reef Ecosystems section).
Mapping surface contours of the pre-existing rock surface is the first step toward reconstruction of the geologic history and processes at work along the Florida reef tract. The bedrock surface shows:
Supplemental information is obtained by coring the bedrock. Cores recover datable material (soilstone crusts that formed when the shelf was exposed, mangrove peat that formed at offshore shorelines as the shelf flooded, and corals that changed species in a reef or that grew landward with rising sea level). Most importantly, radioisotope dates on bedrock corals indicate the different times when the shelf was flooded in the past (i.e., times of high sea levels). Dates on soilstone crusts now submerged similarly indicate times when the shelf was exposed by lowered sea levels.
The Florida peninsula is regarded as having been tectonically stable during the Pleistocene (Fig. 7B; e.g., Davis et al., 1992; Ludwig et al., 1996; Toscano and Lundberg, 1999). When viewing the bedrock map, envision the sea rising over an uneven rock floor. The map shows that elevations are several meters lower to the southwest than northeast; thus, bedrock flooded earlier in the southwest. Dates on overlying Holocene corals confirm earlier flooding to the southwest (see Summary Illustration index map).