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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,

Tile 2

Conch Reef: Conch Reef, composed of Holocene corals on the Pleistocene shelf-edge reef, is located southeast of Tavernier Key (Fig. 42B). Nearly emergent at low tide, Conch Reef is ornamented with spurs and grooves (Fig. 48B). Spurs dip seaward and disappear in about 9 m of water, giving way to a channel 12 to 18 m deep that is filled with carbonate sand. The channel parallels the reef.

Seaward of the channel is an outlier reef in ~16.5 m of water with scattered massive live head corals. The reef crest of the outlier rises to a water depth of about 12 m. This feature, like other outlier reefs along the margin, parallels the shallower, more landward, shelf-edge reef. On its seaward side, the outlier reef slopes to a depth of about 36.5 m, where it merges with sandy bottom that extends seaward for a kilometer or more before dropping off into the depths of the Florida Straits. Having the appearance of a typical coral reef, the outlier is ornamented with numerous gorgonians, large sponges, and scattered massive head corals up to 2 m high. Most of the surface is coated with turf algae.

Radiometric dates on a 16.5-m-long USGS core drilled in 13.7 m of water on the seaward edge of the outlier revealed that the feature is Pleistocene (~87 ka, Table 5). The core penetrated to a depth of ~35 m below sea level and has ~15 cm of Holocene overgrowth on a calcrete or soilstone crust that caps the Pleistocene corals. The subaerially formed calcrete developed when the Pleistocene reef was exposed during the period of lowered sea level prior to about 7 ka. Designated as core AQ and described in Multer et al. (2002), the core is housed at the USGS facility in St. Petersburg, Florida.

Ages of radiometrically dated Pleistocene corals from the Florida reef tract.Table 5. Ages of radiometrically dated Pleistocene corals from the Florida reef tract. Site locations shown on Summary Illustration index map. Ages at the bottom of the table have been modified from the isoleucine amino-acid dates of Mitterer (1974) and Perkins (1977) as shown in Lidz et al. (2003). Because the amino-acid age-dating method is sensitive to temperature, pH, and other environmental factors, the method is considered unreliable (Schroeder and Bada, 1976). Although discrepancies exist between dates on corals from the older Q3 and Q4 Units obtained by the more accurate uranium-series and δ18O age-dating methods (Fig. 37A), both of these dating methods also have limitations and neither is precise (see Muhs, 2002; Muhs et al., 2004). Nonetheless, both methods indicate the stratigraphic units are likely mid-Pleistocene (~340-300 ka and ~230-220 ka, isotope Stages 9 and 7, respectively). Comparison with Table 4 data shows that dates between ~77.8 (Multer et al., 2002) and 9.6 ka (Mallison et al., 2003) have not been obtained on corals landward of the Holocene shelf edge. These dates therefore bracket a period of time between Pleistocene isotope substage 5a and the Holocene (Fig. 37B), when a sea level lowered below the Florida shelf precluded coral growth on the shelf. Authors cited listed in References. For more information, contact Barbara Lidz. [larger version]

An enclosed underwater habitat named the Aquarius rests in about 19 m of water in a sand patch within a rock depression along the crest of the outlier reef seaward of Conch Reef. The 16.5-m-long core was drilled adjacent to the Aquarius habitat. Located at 24°57'00'' N, 80°27'13'' W, the Aquarius is a self-contained facility in which as many as six divers can live under saturated conditions of ambient water pressure. The habitat was first deployed in Salt River Canyon on St. Croix, U.S. Virgin Islands, in 1988 and has been in operation off Conch Reef in the Florida Keys National Marine Sanctuary since 1992. Diving scientists, known as aquanauts, generally conduct 5- to 10-day-long projects from the habitat. The habitat supports as many as a dozen missions each summer. Numerous publications have resulted from observational biologic and geologic surveys based at the habitat.

The University of North Carolina (Wilmington) operates the habitat for NOAA from an onshore facility on Key Largo. Extensive information on the habitat, its mission, research projects, and other comprehensive facts can be found on the Aquarius website. Particulars on the geology of the Conch Reef area can be found in Shinn et al. (1989a). Aronson et al. (1994) provide descriptions of benthic communities at Conch and Carysfort Reefs in the upper Keys.

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

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