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Figure 21. (A) Schematic drawing shows the two primary types of limestone and three of the unconformities (red lines) that form the south Florida shelf. The Key Largo Limestone (Sanford, 1909) is composed mostly of fossil coral reefs that successively accumulated during six periods of high Pleistocene sea levels (Perkins, 1977; Toscano and Lundberg, 1999; Lidz et al., 2003). The youngest part of the reef is ~125 ka. The oolitic Miami Limestone (Hoffmeister et al., 1967) is also ~125 ka (Fig. 7A, 7B). The sea level that produced these two formations was approximately 6.7 m higher than present sea level and may have been as much as 10.7 m higher (e.g., Halley and Evans, 1983). Today, the Pleistocene reef and oolite comprise the Florida Keys. A seventh coral reef ecosystem developed during the most recent 10 ka, known as the Holocene, and is the system we see today (Seven Coral Reef Ecosystems). (B) Schematic cross section (not to scale) illustrates the seven primary bedrock features along the south Florida shelf. The most recent possible time of formation of the upper-slope terrace is inferred to be around 190 ka (Lidz, 2006). The time was revised from ~175 ka (Lidz et al., 2003) on the basis of the widely accepted marine oxygen-isotope curve of Imbrie et al. (1984), shown in a later figure. The nearshore rock ledge began to form at ~4 ka (Lidz et al., 2006). Holocene corals would grow on elevated Pleistocene bedrock, including on the outer part of the rock ledge off the lower Keys.

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