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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 1

Grecian Rocks: Grecian Rocks is one of many discontinuous linear coral reefs that developed on the outer shelf landward of the current shelf margin (Fig. 22B). Coring at Grecian Rocks (Fig. 31A, 31B) has shown that coral growth began on a bedrock topographic high and that different coral species built the reef upward and landward from different depths. The result is a coral zonation (Fig. 32). Surface corals at Grecian Rocks in the 1960s showed five distinct zones (Shinn, 1963, 1980a). Field observations in 2002 revealed most of the corals including stinging corals or hydrocorals were dead. Hydrocorals (Fig. 16A, 16B, 16C, 16D) often dominate a reef when no other corals are alive (Marszalek, 1977).

Photos of divers drilling core 4 and core 7 at Grecian Rocks.
Figure 31. (A) Drilling core 4 at Grecian Rocks (Fig. 22B) in about 3 m of water. The hydraulic drill can be used both under water and on land (e.g., drilling ancient reefs at Scorpion and Muleshoe Mounds in New Mexico, Ancient Reefs; Shinn et al., 1983). An aluminum tripod supports the drill and drill string of core barrels and assures a vertical core. Each core barrel is 1.5 m (5 ft) long and recovers a core 5.08 cm (2 in.) in diameter. Equipment is lifted to the surface by attaching and inflating an airbag (not shown) with air from a diver's tank. (B) Drilling core 7 at Grecian Rocks in about 8 m of water. Note the difference in bottom habitats at the two drill sites (hardbottom and sand, respectively). Compare water and core depths and bottom habitats in the cross section (Fig. 32). [larger version]

The coral zones at Grecian Rocks are prime examples of the influence on corals and coral reefs of pre-existing topography and changing conditions caused by a rising sea level. During the Holocene, the 80-ka Pleistocene reef at the shelf edge formed a series of discontinuous offshore rock ridges (or linear islands) behind which topographic lows were progressively being submerged (e.g., Lidz and Shinn, 1991; see Bedrock Surface map). The ridges served as protective barriers to large waves generated by storms in the Florida Straits and provided calm waters on parts of the outer shelf. At Grecian Rocks, head corals were the early colonizers. Mangrove peat, recovered from different depths and sites at Grecian Rocks (Fig. 32), indicates that a very low-wave-energy shoreline setting existed during peat accumulation. As sea level continued to rise, the offshore Pleistocene ridges drowned, eliminating the wave barriers, moving the surf zone landward, and creating high-energy conditions suitable for branching corals. Acropora palmata was the dominant species in Holocene reefs shelf-wide.

Cross section of Grecian Rocks (modified from Shinn, 1980a) shows reef components and uncorrected radiocarbon ages in years before present of corals recovered in cores.
Figure 32. Cross section of Grecian Rocks (modified from Shinn, 1980a) shows reef components and uncorrected radiocarbon ages in years before present of corals recovered in cores. Compare underwater settings and bottom cover at drill sites for cores 4 and 7 in photos (Fig. 31A, 31B). Coral zonation at Grecian Rocks in the 1960s showed five distinct zones. Field observations in 2002 indicated that most of the corals including hydrocorals were dead. [larger version]

Acropora palmata was also the dominant builder of coral spurs in spur-and-groove systems. At Grecian Rocks and many other reefs, early stands of A. palmata grew in oriented fashion, forming parallel spurs normal to incoming waves. Unoriented individuals later replaced them in a landward direction (Shinn, 1980a). Changes in coral reef structure from head to branching and in coral orientation from unilateral to random indicate changing water depth and wave energy. A spur-and-groove system forms in the surf zone and acts as a buffer, deflecting wave energy. Wave energy controls coral species, orientation, and spur dimension (Shinn, 1963). Where wave energy is low, head corals become established. Where surf is constant and rough, branching corals take over and become preferentially oriented in a landward direction. In oriented growth, coral blades lean away from incoming waves; blades crosswise to waves are physically destroyed. In high-energy settings, spur-and-groove components are narrow. In lower-energy settings, such as in deeper or calmer water, branching corals form randomly oriented stands, and spur-and-groove components are wide. Spurs and grooves formed at Grecian Rocks (Shinn, 1980a) but are less obvious in deep water than similar systems on shallow shelf-margin reefs (Fig. 15).

Coring has also shown that the thickest Holocene reefs developed landward of the shelf margin. Grecian Rocks, situated more than 1 km from the platform edge, is 14 m thick (Fig. 32; Shinn, 1980a). The Holocene reef at the shelf edge seaward of Grecian Rocks is less than 1 m thick (Lidz et al., 1997a). High-resolution seismic profiling and mapping confirm that, for the most part, reefs along the entire platform margin off the Florida Keys are less than 1 m thick (Lidz et al., 2003; see Sediment Thickness map). For reasons not understood, thick Holocene reefs failed to accumulate on the 80-ka Pleistocene shelf-edge reef (e.g., Toscano and Lundberg, 1998).

Coral ages from various sites confirm what we know from seismic data about Pleistocene topography and Holocene flooding of the south Florida shelf. Conventional 14C ages of 6580±90 yr B.P. at the base of core LK5 at Looe Key Reef in the lower Keys (Lidz et al., 1985) and 5950±100 yr B.P. in Grecian Rocks core 4 (Shinn, 1980a) indicate Holocene coral growth began first in the lower Keys (see Summary Illustration index map). Calibrated to correlate with high-precision dates, these ages yielded growth ranges of 7,270 to 6,860 cal. yr B.P. at Looe Key and 6,610 to 6,180 cal. yr B.P. at Grecian Rocks (Table 4). The older Looe Key range is consistent with lower bedrock elevation and thus earlier flooding in the lower than upper Keys (see Bedrock Surface map).

Table 4. Ages of radiometrically dated Holocene corals from the Florida reef tractTable 4. Ages of radiometrically dated Holocene corals from the Florida reef tract (footnotes modified from Lidz, 2004). Site locations shown on Summary Illustration index map. Comparison with Table 5 data (presented in Tile 2) shows that coral ages indicate sea level remained below the Florida shelf between Pleistocene isotope substage 5a (Fig. 37A, 37B) and the Holocene, i.e., from ~77.8 ka (Multer et al., 2002) to ~9.6 ka (Mallinson et al., 2003). Authors cited listed in References. [larger version]

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

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