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Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

Systematic Mapping of Bedrock and Habitats along the Florida Reef Tract—Central Key Largo to Halfmoon Shoal (Gulf of Mexico)

USGS Professional Paper 1751

by Barbara H. Lidz, Christopher D. Reich, and Eugene A. Shinn

Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
Depth to Pleistocene Bedrock Surface
Reef & Sediment Thickness
Benthic Ecosystems & Environments
Sedimentary Grains in 1989
Summary Illustration Index Map
Evolution Overview
Tile-by-Tile Analysis
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,

Primary Products

Seven Coral Reef Ecosystems: Regional mapping of geologic elements and radioisotope dating of corals and soilstone crusts have identified several different episodes of shelf exposure and coral reef growth in south Florida. As noted in the Regional Geologic Setting section, the most recent part of the bedrock consists of five marine units that date back to more than 324 ka (Fig. 7B). Coring has shown that the three most recent units (Q3-Q5) contain extensive remnants of coral reefs, but that the older Q1 and Q2 Units do not (Perkins, 1977). The highstand at around 125 ka (Q5-Unit time) produced the Key Largo Limestone reef and Miami Limestone oolite of the Florida Keys. Dates on corals from reefs at and seaward of the shelf margin indicate more recent periods of coral growth around 105, 90, and 80 ka (e.g., Ludwig et al., 1996; Toscano and Lundberg, 1999). Between each of these periods, sea level fell, exposing the corals and the shelf.

Corals between about 77.8 ka (Multer et al., 2002) and 9.6 ka (Mallinson et al., 2003) have not been found on the shelf, suggesting a lack of coral growth during that time. Correlation of this gap in the Florida coral record with global evidence of maximum sea-level elevations in the past shows that sea level remained below elevation of the Florida shelf during the time of the gap (e.g., Chappell and Shackleton, 1986; Toscano and Lundberg, 1998). Thus, six coral reef ecosystems ranging from about 366 to 77.8 ka cumulatively formed the most recent parts of the bedrock (Lidz, 2004). The seventh ecosystem lines the outer shelf today.

The oldest coral belonging to the present (Holocene) ecosystem is about 9.6 ka (Mallinson et al., 2003). This date comes from coral recovered from ~31 m below present sea level on the Tortugas Bank in the Gulf of Mexico and represents the earliest time Holocene corals began growing on the lower-elevation parts of the Florida shelf. Holocene corals did not begin to flourish along the outer shelf until about 7 ka (e.g., Shinn et al., 1977a). All named coral reefs, some of which are shown on the Summary Illustration index map, belong to this Holocene ecosystem. A comparatively thin layer of corals of this system caps the Pleistocene shelf-margin reef and the immense 30-m-high outlier reefs seaward of the shelf margin (e.g., Toscano and Lundberg, 1998).

Coastal & Marine Geology Program > Center for Coastal & Watershed Studies > Professional Paper 1751

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