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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 11 Geography Geologic Highlights The Marquesas-Quicksands Ridge Ellis Rock and New Ground Shoal Halfmoon Shoal Rebecca Shoal The Quicksands Ellis Rock and New Ground Shoal: Along the north side of the ridge, Ellis Rock and New Ground Shoal represent parts of a narrow, discontinuous line of Holocene reefs with intervening carbonate muds and silts as thick as 7 m (Fig. 134). Ellis Rock is located at the tip of a hook-shaped bedrock spit at the edge of the ridge northwest of the Marquesas Keys (Fig. 130). The rock spit mimics the shape and orientation of the sand spit that forms the northwestern end of the Marquesas Keys, indicating that the Pleistocene sediments that formed the rock spit were likely also accreting to the west. Deep water separates the north edge of the ridge from New Ground Shoal, a linear topographic high paralleling the northwest part of the ridge (Fig. 117). Rotary rock cores drilled at both sites show the intermittent shoals are massive Holocene head corals that overlie corals typical of the Key Largo Limestone (Shinn et al., 1990). The Holocene accretions are ~7.6 m thick. This northern line of Holocene reefs would have been subject to strong currents (e.g., Shinn et al., 1990) and damaging effects of cold-water temperatures in the Gulf of Mexico (e.g., Mayer, 1914; Roberts et al., 1982). Bottom waters in coastal areas of the Gulf are also typically murky, unsuitable for coral growth. Visibility in the area of exploratory well site 826Y, located east of and at about the same latitude as Ellis Rock and New Ground Shoal, was less than 3 m (e.g., Dustan et al., 1991). North of this reef line, water depth drops from about 6 m at New Ground Shoal to an average of 23 m (Figs. 130, 134), and the sediments change from coral framework to lime mud and silt. continue to: Halfmoon Shoal

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