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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,

Organization of Report

Overall Setting and Environment: Bedrock on the south Florida shelf is porous Pleistocene limestone (Fig. 21A, 21B). The 125 ka Key Largo Limestone (coral reef) and Miami Limestone (oolite) form the Florida Keys and are capped by Holocene soilstone crust (Fig. 7A, 7B). The keys are located ~5 to 7 km landward of an 80-ka coral reef that forms the shelf margin. Direction of prevailing energy (winds and waves) is from the southeast. In the upper Keys, the direction is therefore onshore or landward. In the lower Keys, the direction is tangential to the curved coastline. The marine environment off the upper Keys is considered to be restricted, or one in which circulation and current energy are low compared to conditions in open-marine environments (e.g., Ginsburg, 1956; Enos, 1977). A restricted environment off Key Largo is due in part to a lack of natural tidal creeks through the elongate island and thus limited tidal flushing (Fig. 20B; see Summary Illustration map). Therefore, sediments on the upper Keys shelf are lime-mud-bearing carbonate sands owing to lack of winnowing or removal of fine-grained material.

Schematic drawing and schematic cross section (not to scale) illustrates the seven primary bedrock features along the south Florida shelfFigure 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. [larger version]

The environment off the middle and lower Keys exhibits a more open-marine setting. Tidal passes are numerous and wide (Fig. 20B, 20C). Water is deeper with a nearshore westward flow and diurnal tidal exchange from Florida Bay and the Gulf of Mexico. A Gulf-to-Atlantic slope in sea level exists, providing a net southward flow of surface water from the southwest Florida shelf toward the reef tract off the middle Keys (Smith, 1994, 1998). Offshore currents are strong. Fine-grained sediments are resuspended in the water column by winter and summer storms and are transported off the shelf, leaving coarser-grained material on the middle and lower Keys shelf (e.g., Lidz et al., 1985; Incze, 1998).

Reefs of the Florida Keys are generally considered to be in a marginal reef environment (e.g., Jaap, 1984), in that water quality and temperature are not always optimal for coral growth. Coral reefs require clear, warm, low-nutrient waters with normal oceanic salinity (salt content) to survive. Reefs off the middle Keys are those most highly impacted by tidal influx of waters from Florida Bay and the Gulf of Mexico. Florida Bay waters are high in nutrient content from their being shallow (generally 2-3 m deep, Davies, 1980) and close to the mainland. Sediments in the bay are primarily fine-grained lime muds that often become suspended by high winds spawned by winter cold fronts or summer storms. Resultant turbidity can reach the offshore reefs. Bay waters are variable in salinity owing to mainland freshwater runoff during heavy rainfalls. Because of limited circulation, bay waters also become abnormally hot during the summer (Glynn, 1984) and abnormally cold during winter months (Roberts et al., 1982). Corals become stressed at temperatures higher than 33°C (91°F; Shinn, 1966) and lower than 15°C (59°F; Mayer, 1914). If temperature extremes do not reverse quickly (within hours), coral mortality is the likely result. All of these factors adversely affect corals.

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

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