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

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

Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
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
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,

Tile 7/8

When Did the Upper-Slope Terrace Form?: The upper-slope terrace, which is shelf-wide and presumed to be an erosional feature (Lidz et al., 2003), supports four tracts of outlier reefs off Rock Key and Sand Key Reefs. The deepest and oldest coral cored from the largest outlier was recovered from a depth of 21.7 m below present sea level and dated at 106.5 ka (Toscano, 1996). That outlier reef is ~29 m high (Fig. 106A), which leaves corals in the bottom ~7.3 m of the reef of an unknown age. The bottom corals were initially inferred to be 140 and 125 ka (Lidz et al., 2003).

Based on ages of the Q Units derived from amino-acid dates on the bivalve Mercenaria (Mitterer, 1974; Perkins, 1977), Lidz et al. (2003) inferred a youngest possible time of pre-outlier-reef terrace formation at ~175 ka, during the regression that followed a highstand of sea level at ~180 ka when the Q4 Unit is believed to have accumulated (Table 7). Studies of coral reefs now uplifted on the Huon Peninsula of New Guinea also indicate a highstand occurred around 180 ka (Bloom et al., 1974; Chappell, 1974a). Comparison (Lidz, 2006) of that time period with a widely accepted marine oxygen-isotope curve (Fig. 80A) showed that the 180-ka sea level, albeit at a highstand, was below elevation of the Florida shelf, including the upper-slope terrace. Thus, the terrace had to have pre-dated that time. The last pre-106.5-ka highstand that flooded the terrace and shelf occurred at ~195 ka (Fig. 80A). Lidz (2006) revised the youngest possible time of terrace formation to that highstand regression at approximately 190 ka.

The marine-isotope curve also showed that sea level at 140 ka was well below elevation of the Florida shelf, thus negating 140-ka corals at the bottom of the outlier reefs. The curve supports the initial inference of 125-ka corals at the base of the outliers and indicates that corals representing the isotope Stage 6/5 transition at about that time are now the most likely vintages to be present.

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

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