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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

Primary Products Introduction Overview Map—Depth to Pleistocene Bedrock Surface Overview Map—Reef and Sediment Thickness Overview Map—Benthic Ecosystems and Environments Overview Map—Sedimentary Grains in 1989 Overview Map—Summary Illustration Index Map Evolution Overview—Coral Ages and Shelf-Edge Models Introduction Seven Coral Reef Ecosystems Two Coral Reef Architectures Two Coral Reef Architectures: Data-based cross-sectional models of four areas along the shelf margin off the Florida Keys show the two primary geomorphic architectures built by the coral reefs: a broad reef-and-trough structure at the shelf edge and a narrow ridge-and-swale structure along the outer shelf. Where the Pleistocene reef is present at the margin, a bedrock trough backs the reef. Depending on location, the trough is partly to completely filled (Fig. 10). The reef-and-trough architecture is regional but discontinuous. Where present, this architecture built laterally alternating parts of the shelf edge landward and upward through vertical and lateral growth over time (Lidz, 2004). As the backreef trough behind the shelf-margin reef became filled at various locations, that part of the shelf edge was also built seaward. The reef-and-trough architecture is many hundreds of meters in dimension. A different type of architecture built the outer shelf upward over time. The outer shelf is lined with a series of thin narrow rock ridges. The ridges are too small (<1 m) to be seen in the seismic data but are clearly visible in aerial photographs (Fig. 19). A 5-core transect across two ridges in the lower Keys has shown they are composed of older, massive coral species overlain by similar species that belong to the present reef system (Shinn et al., 1977a). The cored ridges are separated by a swale filled with sediment. Sediment in the bedrock part of the swale is cemented and is overlain by loose sands of the modern ecosystem. The ridge-and-swale architecture is also regional and discontinuous and is broken by reentrants through the shelf-edge reef. This architecture built the outer shelf upward through cumulative vertical stacking of narrow reefs and sediments in intervening swales (Lidz, 2004). Architectural dimensions are in a few tens of meters. Figure 19. Sample mosaic of aerial photos (1975) shows seabed features seaward of the Newfound Harbor Keys (first islands north of Marker G on Fig. 6C, Tile 5, middle Keys). Arrows at right edge point to and show trend directions of the linear rock ridges or old reef lines visible on the outer shelf. These are believed to be narrow coral reefs separated by sediment-filled swales, such as have been cored near Marker G, located ~5 km northeast of Looe Key Reef. [larger version] Pre-existing cemented beach-dune ridges are suspected to underlie the deepest layer of corals in both types of architecture (e.g., Shinn et al., 1991; Lidz et al., 1997b). Coring and commercial excavations have shown that dune-ridge nuclei focused the development of linear coral ridges off Bal Harbor north of Miami (Shinn et al., 1977a). continue to: Organization of Report

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