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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

Introduction: Table of Contents Project Overview Project Objective Geologic Setting Primary Datasets Primary Products - Overview Maps & Evolution Overview: Introduction 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 Organization of Report Tiles: 1, 2, 3, 4, 5, 6, 7/8, 9/10, 11 Summary Acknowledg- ments References Disclaimer Related Publications

Tile 5 Geography Sector-Specific Studies Geologic Highlights Moser Channel Bahia Honda Channel Ramrod, Torch, and Big Pine Keys Ramrod, Torch, and Big Pine Keys: Ramrod, Torch, and Big Pine Keys (lower Keys) are located near and north of Newfound Harbor Keys. The Newfound Harbor Keys are the westernmost of the middle Keys and the westernmost emergent part of the Key Largo Limestone coral reef (Figs. 77C, 82). The east-west trend of the Newfound Harbor Keys clearly reflects the margin-parallel nature of the curved 125-ka reef. Figure 82. Aerial photo (1991) shows area where the Key Largo Limestone coral reef (Newfound Harbor Keys, middle Keys) is in contact with the Miami Limestone tidal-bar oolite (south parts of Ramrod and Torch Keys, lower Keys) (Hoffmeister and Multer, 1968). The Newfound Harbor Keys represent the westernmost exposure of the Key Largo reef. Black arrows show submerged parts of the Pleistocene reef. Shallow areas around Ramrod and Torch Keys are Pleistocene oolite (Shinn et al., 1989b). Submerged reef and oolite compose the nearshore rock ledge in this area. Note east-west orientation of Newfound Harbor Keys vs. north-south trend of Ramrod and Torch Key tidal bars (see Benthic Ecosystems map and Benthic Ecosystems for Tile 5). Also note pockmarked nature of the shallow oolite vs. smooth surface of the shallow reef. Pockmark holes are filled with seagrass-covered sand. Mangroves fringe the islands. [larger version] At some time during the higher stand of sea level that created the Key Largo reef, the reef focused strong currents around its ends, causing ooids to precipitate from the water. Ooids are tiny spherules of calcium carbonate that precipitate in warm, high-energy, generally very shallow marine waters. The ooids collected as tidal bars more or less perpendicular to the reef. Known as the Miami Limestone, the cemented tidal bars form the lower Keys and underlie the city of Miami to the northeast. Ramrod, Torch, and Big Pine Keys are three of the largest inhabitable lower Keys islands. Black limestone pebbles are found throughout south Florida (Perkins, 1977) and the Bahamas (Beach and Ginsburg, 1980). Their typically angular shapes indicate they have not been transported by wind or water. Their common association with calcrete shows they were exposed to air. They often occur in multicolored breccias that consist of rock fragments embedded in a fine-grained matrix (Fig. 83A, 83B). The breccias are generally concentrated in karst potholes or solution pits (Fig. 84). Some calcretes contain layers that are distinctly blackened. Some pebbles may show gradual darkening from white to gray to black. Blackened calcrete layers and pebbles are particularly abundant on Ramrod Key (Figs. 82, 85A, 85B). Figure 83. (A) Large, angular, blackened pebbles embedded in naturally brown, layered, fine-grained calcrete or soilstone crust form a breccia. Breccias are consolidated coarse-grained clastic rocks whose fragments are angular and consist of any type of pre-existing rock. This sample is from Ramrod Key (lower Keys, Fig. 77C). (B) A well-cemented, multicolored, artificially blackened breccia was collected at a depth of ~6 m below sea level from quarry tailings in a solution pit on Big Pine Key. Note layered crust on left side of specimen. Blackened fragments include pre-existing calcrete and fossiliferous Key Largo Limestone. Left part of cut specimen was heated in the laboratory to reproduce darkening similar to colors of blackened pebbles widely found in the Pleistocene record (Shinn and Lidz, 1988). Original rock colors are visible in unheated section at right. [larger version] Figure 84. Sketch shows geologic components usually associated with sites of blackened limestone pebbles in Florida. [larger version] Figure 85. Photos show beds of blackened limestone on Ramrod Key. Note soilstone crust or calcrete on surface at edge of bank in (A). Underlying rock is Pleistocene oolite, the 125-ka Miami Limestone. [larger version] The darkened color is believed to result from thermal effects of natural wildfires (Shinn and Lidz, 1988). Chance observations of blackened limestone around campgrounds in Florida and the Bahamas sparked the fire hypothesis. Laboratory heating experiments gave it credence. Radiocarbon dates of about 5,000 yr B.P. to the present were obtained on unblackened sections of Ramrod Key calcrete containing twigs that had been turned to charcoal (Robbin and Stipp, 1977). The ages and charred twigs indicate the presence of wildfires. In Florida, known as the lightning capital of the country, there would have been no lack of natural torches for wildfire ignition. continue to: Tile 6

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