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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 1 Geography Sector-Specific Studies Geologic Highlights Rodriguez Key Sinkhole Captain Roy's Reef Grecian Rocks Key Largo Dry Rocks Mosquito Bank, White Bank, Coral-Rock Ridges, and French Reef Carysfort Reef The Elbow Molasses Reef and Little Molasses Island Pickles Reef Mosquito Bank, White Bank, Coral-Rock Ridges, and French Reef: Aerial photographs taken in 1975 were cropped and joined to form photomosaics of the shelf-wide seabed surface from the keys to the shelf edge. Mosaics of the area in Tile 1 reveal various sand-body forms and geologic structures typical of the outer shelf and margin (Fig. 33A, 33B). Features at the margin in the Tile 1 sector consist primarily of a generally continuous section of the Pleistocene shelf-margin reef capped by skeletal coral reefs and numerous sand chutes of the present reef ecosystem (see Benthic Ecosystems for Tile 1). Figure 33A-B. Contiguous photomosaics (1975) show features and benthic environments on the outer shelf and margin seaward of Key Largo (compare with interpretive Benthic Ecosystems for Tile 1). Photomosaic covers nearly all of the seaward part of Tile 1 (Fig. 22B). Proceeding southwestward (right to left), (B) shows the backreef marine sand belt of White Bank, located seaward of the Mosquito Bank reentrant in the Hawk Channel bedrock depression (modified from Lidz et al., 2003). Note rock ridges (dotted lines) and sinuosity of ridges around possible karst or swale feature. (A) Continuing rock ridges or old reef lines capped by hundreds of patch reefs are clearly visible and thus are not marked by dotted lines. Because the shelf is generally less than 12 m deep (Enos, 1977), high-quality aerial photographs are extremely useful for interpreting seabed characteristics. Photos used in this project revealed important geologic features that were either (a) below seismic resolution (<1 m high, e.g., the series of outer-shelf rock ridges, Fig. 87, Tile 6) or (b) missed by seismic tracklines (e.g., a fourth tract of outlier reefs off Sand Key Reef in the lower Keys, Fig. 106, Tile 7/8; Lidz et al., 2003). [larger version] Mosquito Bank overlies a concave low-elevation indentation or reentrant in the bedrock depression beneath Hawk Channel (Figs. 22B, 33A, 33B). Sediments in Mosquito Bank are mostly muddy sands with patchy areas of lime mud that surround scattered patch reefs. White Bank is a backreef marine sand belt that has accumulated on the higher bedrock seaward of Mosquito Bank (Ball, 1967; Halley et al., 1983). Composed of coral, algal, and molluscan sand flats and sand waves, the sand belt is 1 to 2 km wide and 40 km long (Enos, 1977). Active (rippled) and stable bottoms (covered by sea grasses) both occur at water depths of 1 to 3 m, indicating that local hydrologic conditions control formation of the large scattered sand waves. The sand belt contains dipping layers called cross-bedding. A slope steeper at the landward than seaward edge (Enos, 1977) indicates landward sand transport. Ball et al. (1967) documented landward accretion after the 1960 passage of Hurricane Donna. Perkins (1977) referred to White Bank as a sand-shoal/patch-reef complex and considered it a modern analogue to similar underlying bedrock constituents. Shelf-wide, the outer shelf seaward of Hawk Channel is lined with numerous rock ridges (Fig. 33B). Where cored behind Marker G in the lower Keys (Fig. 34A), two ridges have been shown to consist of narrow lines of massive Pleistocene and Holocene coral species separated by a sediment-filled swale (Shinn et al., 1977a). Coring of coral-rock ridges seaward of Bal Harbor, located north of Miami (Fig. 22B), has shown they developed on cemented sand dunes (Shinn et al., 1977a). It is believed that all coral ridges shelf-wide likely overlie cemented Pleistocene beach dunes (Shinn et al., 1991; Lidz et al., 2003; Lidz, 2004). The depositional environment in Florida during the Pleistocene is considered to have been similar to that in the Caribbean and Bahamas, where emergent Pleistocene sand dunes occur widely (e.g., Ball, 1967; McKee and Ward, 1983). Beach dunes forming along the arcuate Florida shelf edge would have created longitudinal or transverse ridges (Fig. 34B). The discontinuous coral ridges are mostly linear off the lower and middle Keys but can be quite sinuous seaward of Mosquito Bank. Hundreds of patch reefs line the ridges (Fig. 33A, 33B). Figure 34. (A) Cross section of core transect across two rock ridges inshore from Marker G in the lower Keys (Tile 5, modified from Shinn et al., 1977a). The cores indicate the ridges are coralline, are growing vertically, and are separated by a sediment-filled swale. Dimensions of the ridge-and-swale architecture are in the tens of meters. Note uncorrected radiocarbon ages of recovered corals. The 37,480±1,300 14C date was obtained on a recrystallized coral and was believed to be much too young. Inferred age of the bedrock coral sections is between ~86.2 and 77.8 ka, which is the radiometric-age range for the youngest Pleistocene corals on the shelf (Toscano, 1996; Multer et al., 2002; Lidz et al., 2003). (B) Cemented beach-dune ridges are thought to underlie the coral ridges shelf-wide. Wind-blown sand dunes, now rock ridges, are well known throughout the Caribbean and Bahamas (e.g., Ball, 1967; McKee and Ward, 1983). [larger version] Other types of accumulations visible in the aerial mosaics include elongate or ovate zones of coral rubble behind reefs and numerous sand chutes at the shelf edge (Fig. 33A, 33B; see Benthic Ecosystems for Tile 1). Coral-rubble zones result from landward transport of coarse debris by storms and are found close to abundant quantities of loose rubble, usually behind major Holocene shelf-margin reefs. Low-elevation areas in the underlying Pleistocene reef form chutes through which sands near the shelf edge spill seaward. Also evident in the mosaics, reefs of the present ecosystem have numerous structural spurs of skeletal coral separated by sand-filled grooves (Figs. 15, 33A). Spurs and grooves form perpendicular to incoming waves and act as baffles to wave energy (Shinn, 1963). Present along most areas of the shelf-edge Holocene reefs, spurs and grooves are most prominent on the largest reefs. continue to: Carysfort Reef

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