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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 7/8 Geography Sector-Specific Studies Geologic Highlights Margin Evolution Flooding the Florida Shelf Addendum Introduction New Evidence for Interpretation of the Nearshore Rock Ledge New Coral Reef Nuclei When Did the Upper-Slope Terrace Form? Why Did the Pleistocene Reefs Become So Imposing? Age of the Marquesas - Quicksand Ridge Ephemeral Islands and Lighthouses Why Did the Pleistocene Reefs Become So Imposing?: The five offshore tracts of shelf-edge and outlier reefs have been dated to late Pleistocene marine-isotope substages 5c, 5b, and 5a (Ludwig et al., 1996; Toscano, 1966; Toscano and Lundberg, 1998, 1999; Fig. 80A, 80B; Tables 5, 6, 7). The reefs are 30 m high in seismic relief, cumulative, and accreted over many thousands of years during each of six consecutive periods of highstands (Table 7). Time has been the reason most often invoked for their size, but what, other than time, might have been the primary margin-wide cause of so many thousands of years of optimal conditions? One need only think in terms of the present setting and status of reefs today (Figs. 3, 4). The shelf is submerged. Shallow-water Florida Bay exists. Tidal passes are present between the keys. The Gulf of Mexico and many other sources of waters deleterious to the reefs are now natural stressors to the modern reef system. Each of the natural stressors now also bears the imprint of man. What made the late Pleistocene highstand conditions different from those of today was a lack of the stressors present today (Lidz, 2006). The highest apex of late Pleistocene sea level after deposition of the Key Largo and Miami Limestone never submerged the shelf, reaching only to ~9 m below present sea level (Toscano and Lundberg, 1999). Contours on the Bedrock Topography map show that elevations of 9 m and above would have formed a continuous emergent barrier promontory along the shelf that stretched from the Everglades across Florida Bay, the Florida Keys and Boca Grande Channel, to the west end of the Marquesas-Quicksands ridge in the Gulf of Mexico. Unlike today, the Pleistocene corals were unencumbered by effects of turbid, shallow, nutrient-rich, excessively hot or cold coastal, bay, or gulf waters. Corals of the late Pleistocene thrived in warm, clear, low-nutrient, oceanic waters of the Gulf Stream. Though they died when sea level fell, they again flourished during the next highstand in front of a large, long, protective promontory that extended westward from the southern tip of a peninsular landmass. continue to: Age of the Marquesas - Quicksand Ridge

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