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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 - Quicksands Ridge Ephemeral Islands and Lighthouses Age of the Marquesas-Quicksands Ridge: The area known as The Quicksands, located in the Gulf of Mexico between Boca Grande Channel and an unnamed channel between Halfmoon Shoal and Rebecca Shoal, lies atop a rectangular bedrock ridge informally called the Marquesas-Quicksands ridge (Tile 11, Shinn et al., 1990). Ridge elevation is well known from seismic profiling. Elevation ranges from 1 to ~12 m but is generally less than 6 m (av. 6-8) below present sea level (Shinn et al., 1990; Lidz et al., 2003). Cores taken through a discontinuous line of skeletal Holocene reefs along the northern edge of the ridge showed that the corals are similar to those of the Pleistocene in that they lacked branching species. Corals beneath the Holocene accretions have the same appearance as the Key Largo Limestone of the middle and upper Florida Keys but were not dated. The bedrock surface of the ridge itself is calcrete-coated oolite similar to the calcrete-coated Miami Limestone oolite of the lower Keys, but those materials were also not dated. Shinn et al. (1990) inferred that the ridge oolite likely represents the westward extension of the Miami Limestone and that its age is thus the same (~125 ka, having accrued during marine-isotope substage-5e time). Younger Pleistocene corals, equivalent to substages-5c, -5b, and -5a time, have been radiometrically dated along the outer shelf off the Florida Keys. Two types of datasets from the South Florida shelf margin were used to demonstrate why Pleistocene reefs became so imposing: (a) pre-Holocene bedrock topography and depth below present sea level, and (b) the highest positions of sea level during late Pleistocene transgressive intervals (Lidz, 2006). The same datasets verified the inferred age of the ridge. Late Pleistocene sea levels reached their highest stands during isotope Stage 5—at substages 5e when the Key Largo and Miami Limestone formed, 5c, 5b, and 5a when corals, now dated, accrued in outlier and outer-shelf reefs (Table 6). Sea levels of substages 5c and 5b were well below general elevation of the ridge, at 10 to 15 m lower relative to present sea level. Contours of the Bedrock Topography map indicate that a deeper area (8-10 m) in the northeastern quadrant of the ridge may extend into a small central area but does not cross the ridge. Those depths are within reach of the minus 9-m substage-5a sea-level apex (Table 6), which could mean 80-ka limestone might be present at that spot. However, higher-elevation bedrock (av. 6-8 m deep) throughout most of the ridge would have remained emergent at a sea-level position 9 m lower than present. The emergent bedrock would have maintained a landmass extending as an intact promontory from the mainland well into the Gulf of Mexico during the last Pleistocene highstand at ~80 ka. Because most of the oolite was never submerged after its formation until the present (Holocene) marine transgression, much of the ridge bedrock beneath The Quicksands must date to substage-5e time (Lidz, 2006). continue to: Ephemeral Islands and Lighthouses

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