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Figure 90. Interpreted seismic reflections representing Pleistocene and Holocene surfaces traced from profiles across shelf margin (vertical line) off the upper Keys (from Lidz et al., 1997a). Note obvious reef-and-trough system at the margin (versus that off the lower Keys, Fig. 89) and absence of such a system on the terrace. However, also note linear, buried low-relief features on the terrace believed to correspond to the lower Keys outlier-reef tracts. Profiles are in consecutive order from northeast to southwest (see Fig. 91A for locations). Island of Key Largo is at left. Straits of Florida are at right. Infilled shelf-edge trough and lack of seaward outlier reefs indicate that the parts of the margin illustrated are aggradational, that is, those parts can only expand seaward slowly by coral growth. Poor seismic resolution of internal structure of trough facies is consistent with reef-rubble infill. A double trough behind The Elbow indicates that part of The Elbow may have been an outlier reef. A shelf-edge reef higher than platform bedrock would have provided a protective rock-island barrier during shelf flooding. Distance along margin between end profiles is ~10 km. Diamonds indicate profiles used in block diagrams (Tile 7/8).

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