Two transects, consisting of seven 8- to 14-m deep core holes, were drilled across the major ecologic zones of Grecian Rocks (25°06′06″N, 80°18′03″W) in the Key Largo Coral Reef Marine Sanctuary to determine its internal anatomy and age. Approximately
750 by 200 m in size, the reef accumulation was found to be controlled by a local Pleistocene topographic feature. Facies in the underlying Pleistocene correspond closely with the various coral facies in the overlying Holocene reef. Grecian Rocks Reef is composed of five major ecologic
zones: (1) a deep seaward rubble zone ranging in depth from 6–8 m; (2) a poorly developed spur and groove zone composed of massive head corals and Millepora (4–6 m water depth); (3) a characteristic high-energy oriented Acropora palmata zone extending from the surface
down to 4 m; (4) a distinct broad reef flat composed of in situ A. palmata and coral rubble, followed by (5) a narrow low-energy back-reef zone of unoriented A. palmata, thickets of A. cervicornis, and various massive head corals in water 0–3 m deep. An extensive
grass-covered carbonate sand flat 3–4 m deep extends in a landward direction from zone 5. Cores revealed that all the zones except the massive coral head zone are superficial coatings over a carbonate sand and rubble accumulation. A thin 1-m thick layer of lime mud and peat was found
11.5 m below sea level on the Pleistocene bedrock beneath the sand and rubble in the reef flat core hole. Carbon-14 analyses of coral from 7 m below the reef surface indicate that the reef began growing approximately 6,000 years before present.
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