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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 Datasets Introduction Seismic Profiles—Data Acquisition and Processing Seismic Profiles—Data Integration Aerial Photomosaics Thin Sections of Surface Sediments Seismic Profiles—Data Integration: The 1997 seismic-reflection profiles filled a major gap in high-resolution geophysical data along the south Florida shelf (Fig. 6C). To complete the comprehensive bedrock-surface and reef/sediment-thickness maps, the 1997 data were merged with four analogous high-resolution datasets from specific reef tract areas surveyed by the USGS in prior years (Lidz et al., 1985; Shinn et al., 1990; Lidz et al., 1991; Lidz et al., 1997a, b). The combined datasets, ~1,655 line kilometers of data, represent the most extensive contiguous seismic coverage available in the Florida Keys National Marine Sanctuary. Bedrock-surface and sediment-thickness data were read from the seismic records at 5-min intervals. These data provided distances (depths) from the water surface to the images or reflections marking the bedrock and sediment surfaces in milliseconds. Where significant changes in depth to bedrock or sediment thickness occurred between the 5-min intervals, readings were taken at shorter intervals. Interpretation of millisecond depth is based on the speed of sound in seawater (two-way travel time, 750 m/s). Sediment/reef thickness in milliseconds was measured as the difference between depth from sea level to the seafloor surface and depth to the reflection identified as Pleistocene bedrock. Seafloor-surface depth, bedrock depth, and sediment/reef thickness in milliseconds were then converted to depths in meters in an Excel spreadsheet. These calculations included compensation for a sound source/receiver offset of 7 m. Any variations in travel time in the Florida shelf sediments are inconsequential due to shallow water depth and thin stratigraphic sections (relative to depths and thicknesses encountered by the oil industry). Water depths to bedrock and sediment/reef thicknesses were plotted at the appropriate Global Positioning System (GPS) points on trackline navigation maps at the same scale as a published aerial photomosaic (Lidz et al., 1997a, scale 1:24,000). Isolated data on bedrock depth below sea level, interpolated from non-geolocated maps of Enos (1977, scale 1:80,645), supplemented the merged bedrock dataset. (See Methods in Lidz et al., 2003, for criteria applied to select usable data of Enos, 1977.) Data points on the final integrated datasets were then contoured manually with the aid of prior field knowledge and aerial photomosaics that provided visual clues regarding geomorphic shapes and trends (see Bedrock Surface and Sediment Thickness maps). continue to: Aerial Photomosaics

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