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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

Introduction:
Table of Contents
Project Overview
Project Objective
Geologic Setting
Primary Datasets
Primary Products - Overview Maps & Evolution Overview:
Bedrock Surface map.
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
Satellite image of the Florida Keys showing location of tiles.
Organization of Report
Tiles: 1, 2, 3, 4,
5, 6, 7/8, 9/10,
11
Summary
Acknowledg-
ments
References
Disclaimer
Related
Publications

Tile 6

American, Maryland, and Pelican Shoals: A group of five nearly equidistant Holocene shelf-edge reefs occupies the margin off Sugarloaf, Saddlebunch, and Boca Chica Keys (Figs. 86C, 91B). American, Maryland, and Pelican Shoals are within ~20 km of Looe Key Reef (Fig. 87A, 87B). Like Looe Key Reef, the three shoals have spurs and grooves, but they are less developed than at Looe Key Reef (Fig. 88). Eastern and Western Sambo are the two western reefs in the group. Eastern Sambo falls at the left edge of the Tile 6 sector (Fig. 86C).

Data from seismic profiles across the shelf margin in the upper and lower Keys (Figs. 89, 90, 91A, 91B) show both consistent and contrasting overall morphologies (Lidz et al., 1997a). Common to both areas is the presence of a discontinuous shelf-margin reef fronted by a seaward upper-slope terrace ranging in depth from 30 to 40 m below present sea level. Terrace sediments margin-wide average 3 to 4 m thick (see Sediment Thickness map). Contrasts are more numerous. For example:

  • A deep backreef trough flanks the landward side of The Elbow (upper Keys) and becomes shallower to the southwest (Fig. 90).

  • Seismic facies on the terrace indicate presence of linear features now buried by sediments near The Elbow, whereas immense outlier reefs developed in linear tracts to the southwest (Figs. 18, 89).

  • Elevation of the shelf-edge reef varies throughout the keys but is generally a few meters higher in the northeast than in the southwest (Figs. 89, 90). During the rising Holocene sea level, the higher shelf-edge segments (where not broken by reentrants) would have provided landward shelf reefs protection from wave exposure for a longer period than would have the lower shelf-edge segments.

  • The trough behind The Elbow is V-shaped and relatively narrow, but troughs behind the multiple outlier reefs are U-shaped and wider (Figs. 89, 90). Lidz et al. (1997a) noted The Elbow as being offset seaward from other parts of the shelf-edge reef. A double trough behind part of the reef (Fig. 90, line 9) is suggestive that the offset part may at one time have been an outlier reef that developed over a more seaward beach dune or southward-projecting spit.

The Tile 6 terrace surface near American, Maryland, and Pelican Shoals shows variations ranging from a generally non-descript low-gradient slope, to a horizontal surface, to a 'corrugated' or karstified surface that supports outlier reefs at the seaward edge (Fig. 89, left column). Structures on the terrace vary in relief, ranging from a slightly elevated reef-flat-like feature (Fig. 89, line 12) to a buried dune-ridge feature (line 3) to prominent outlier reefs that are both buried and exposed (lines 4 and 2). Number of outlier-reef tracts ranges from zero at Pelican Shoal (Fig. 94), to one at Eastern Sambo (Fig. 89, line 2), to two at Maryland Shoal (Fig. 95), to three and four tracts off Rock Key and Sand Key Reefs (Figs. 18, 89, lines 16a-d, 17, 18, and Tile 7/8). Like the shelf-edge reef, the outlier tracts are discontinuous.

(A) Seismic-reflection profile (1989) and (B) interpretation
Figure 94. (A) Seismic-reflection profile (1989) and (B) interpretation show upper-slope terrace off Pelican Shoal (lower Keys; from Lidz et al., 2003; Figs. 86C, 87B, 91B). Profile crosses the shelf margin nearly normal to a single outlier-reef tract at Eastern Sambo and Maryland Shoal (Fig. 89, lines 2 and 4, and Fig. 95A) and ~20 km east of the multiple outliers off Sand Key Reef (Fig. 18 and Tile 7/8). Note facies change in sediment wedge on terrace. Horizontal layers are interpreted to result from a stillstand of sea level that eroded tops of the inclined strata and redeposited the sands in an intertidal zone (Lidz et al., 2003). Note smoothness of the reflection marking a mounded bedrock surface buried at the seaward terrace edge. The mound is interpreted to represent a cemented dune ridge that was not colonized by corals. For comparison, note jagged nature of reflection across known Holocene shelf-edge corals (e.g., Fig. 95A, 95B). Latitude and longitude in degrees and decimal minutes based on GPS coordinates. Hours (military time) below coordinates serve as navigational correlation points along seismic line. [larger version]

(A) Seismic-reflection profile obtained in 1989 and (B) interpretation
Figure 95. (A) Seismic-reflection profile obtained in 1989 and (B) interpretation show shelf margin at Maryland Shoal (lower Keys, Figs. 86C, 87B, 91B) and a single outlier reef at the outer edge of the upper-slope terrace. Note the jaggedness of Holocene corals at the crest of the shelf-edge reef. Holocene sediments have buried a small outlier reef near the base of the shelf-edge reef. The combination of location at the seaward terrace edge, presence of a single outlier at Maryland Shoal and at Eastern Sambo (Fig. 89, lines 4 and 2), and the smooth-mound reflection at Pelican Shoal is good seismic evidence that the mound may be a cemented dune ridge. However, coring the mound is the only way to verify this interpretation. Cemented dune ridges are believed to underlie the outlier reefs margin-wide (Lidz et al., 1997a, 2003). Latitude and longitude in degrees and decimal minutes based on GPS coordinates. Hours (military time) below coordinates serve as navigational correlation points along seismic line. [larger version]

The relatively uncomplicated seismic record at Pelican Shoal may be representative of general shelf-edge morphology before development of outlier reefs on the terrace. Lidz et al. (2003) used the Pelican Shoal profile (Fig. 94A) as the generic basis for models to reconstruct shelf-margin evolution at Carysfort Reef (Fig. 36A), The Elbow (Fig. 40A), Pelican Shoal (Fig. 96A), and Sand Key Reef (Tile 7/8). These areas were selected for modeling based on presence of the discontinuous shelf-edge reef, the amount of coral-age data available, and the variability in seismic data. The seismic data as illustrated in the models indicate that these areas consist primarily of two coral reef architectures: a ridge-and-swale structure on the shelf, and a reef-and-trough structure at the margin.

Model of evolution at Pelican Shoal
Figure 96. (A) Model of evolution at Pelican Shoal shows a Pleistocene backstepped reef complex at the margin and Holocene backfilled progradation of the shelf surface (ages >125 ka modified from Lidz, 2004). Note presence of coral-ridge bands and sediment-filled swales on the outer shelf. Based on seismic and other data, models developed for Carysfort Reef (Fig. 36A), The Elbow (Fig. 40A), Pelican Shoal, and Sand Key Reef (Tile 7/8) all show that these areas of the margin have prograded during the Holocene (the most recent 10 ka). Ls = limestone. ka = thousands of years. Q1-Q5 Units = names assigned by Perkins (1977) to the five marine sections that compose the most recent part of the south Florida Pleistocene rock record. Marine-isotope substages refer to periods of time that correspond to major changes in the paleotemperature record (Fig. 37A, 37B). Long curved arrows indicate offshelf sediment transport. Short curved arrows indicate landward sediment transport and infilling of backreef troughs. (B) With continued sediment accumulation, a future seismic record at Pelican Shoal would show a classic prograded margin with landward-dipping beds behind a buried shelf-edge reef. If sediments were to fill space on the terrace to sea level (e.g., Tucker, 1985), a new shelf margin would be formed above the point at which the present upper-slope gradient slopes below terrace level. [larger version]

Based on coring and commercial-excavation data at local sites (e.g., Shinn et al., 1977a), Lidz et al. (2003) theorized that cemented sand dunes most likely provided elevated sediment-free nuclei for initial coral growth on the shelf and terrace. Pleistocene sand dunes are widespread in the Bahamas and Caribbean, where present environmental setting is similar to that in Florida (Ball, 1967; McKee and Ward, 1983), indicating that dunes should be present region-wide in the Florida Pleistocene as well. Locker et al. (1996) have documented ~14-ka beach-dune and paleoshoreline complexes that parallel the shelf margin between 124 and 50 m below present sea level. At that time, the present marine transgression was underway.

Table 6. Data and assumptions used to infer youngest possible age of upper-slope terrace off the Florida Keys.Table 6. Data and assumptions used to infer youngest possible age of upper-slope terrace off the Florida Keys. Older part of table modified from Lidz et al., 2003. By inference from dates and depths of corals in the Sand Key outlier reef and from proxy sea-level curve data (Fig. 37A, 37B), the latest possible time the terrace could have been formed (at ~190 ka) was during the post-195-ka regression. This time is 20 ka earlier than the time previously inferred from the same data without the marine-isotope δ18O curve (Lidz et al., 2003). If the terrace is ~190 ka and given a mean outlier-reef relief of 29 m and the 21.7-m-thick dated interval in the Sand Key outlier reef, a reasonable deduction is that the initial 7.3 m of outlier-reef growth accrued during the next post-190-ka rise in sea level that reached and exceeded elevation of the upper-slope terrace. That transgression occurred during the Stage-6/5 transition that culminated in deposition of the 125-ka Key Largo and Miami Limestone of the Florida Keys. Without reference to any particular coral species or growth rate, it is clear from the thickness (~8 m, Perkins, 1977, his plate 3) of the post-Q3-Unit section in the Big Pine Key core that there was ample time for a comparable 7.3 m of Stage-6/5 (substage-5e) coral framework to accumulate at the base of the outlier reefs. That said, the upper-slope terrace could be older than 190 ka. The only way to verify its age is to core and date the material at and below its surface, an unlikely procedure given the water depths and strong currents of the offshore setting. Authors cited listed in References. [larger version]

Prior to 14-ka, however, the Florida shelf had already been subaerially exposed for ~54 ka (68 ka minus the 14-ka age of the paleoshoreline complexes), providing ample time for beach dunes to form (see Bands of Outer-Shelf Coral-Rock Ridges section; Lidz, 2004). Beach dunes develop under conditions of subaerial exposure, abundant sand availability, low rainfall, and high capability of sand mobilization. Such conditions likely existed on the vast reaches of a pre-Florida platform exposed by low stands of Pleistocene sea level. Lidz et al. (2003) inferred that the most recent opportunity for formation of sand dunes beneath the outer-shelf reefs and upper-slope terrace outlier reefs was around 175 ka. That time estimate was derived from seismic, core, aerial photomosaic, and high-precision radiometric-age data correlated with maximum elevations of global sea levels over the past 325 ka (Fig. 37). Later correlation, however, with a well-accepted marine oxygen-isotope paleotemperature curve (Fig. 80A) showed that sea level at 180 ka was below the Florida shelf. The latest possible time of terrace formation was therefore more likely around 190 ka, after a highstand at ~195 ka (Lidz, 2006). That high sea-level stand was the last Pleistocene sea level to exceed present terrace depths (30-40 m) and to precede the well-dated outlier reefs (Table 6).

Models of shelf evolution were constructed primarily on the basis of: (a) building incremental, dated (where available) components of geologic strata as they appear in seismic profiles of the shelf margin, correlated with periods of sea level high enough to have flooded the shelf (Table 6 and Table 7); and (b) knowledge of the shelf-wide coral-rock ridges on the outer shelf. Ridge data were gleaned from field observation, aerial-photographic evidence (for example, as seen in Figs. 33A, 33B, 87A, 87B), and the cores recovered at Marker G (Fig. 34A).

Table 7. Summary of late Quaternary geochronology and sea-level status along the Florida reef tract.Table 7. Summary of late Quaternary geochronology and sea-level status along the Florida reef tract. Corrected 2-sigma ranges for conventional radiocarbon ages are given in Table 3 and Table 4. Statements of precision, where available for abbreviated radiometric dates, are given in Table 5 or in references cited in this table if not listed in Table 5. The earliest periods of outlier-reef growth are inferred at ~127 and 125 ka in Table 6. Authors cited listed in References. Table and footnotes modified from Lidz (2004). [larger version]

The bedrock image or reflection visible in the seismic record off Pelican Shoal shows a broad, flat, upper-slope terrace fronting the Pleistocene shelf-margin reef (Fig. 94A, 94B). To construct the evolutionary model for the Carysfort, The Elbow, and Sand Key Reef areas, a 'generic' Pelican Shoal surface was modified to approximate seismic bedrock reflections in those areas (Figs. 36A, 40A, bottom panels, and Tile 7/8). Then, using the reflection representing the bedrock surface as traced from the seismic profiles, 'layers' simulating bedrock coral growth were added between the terrace and reef surfaces. Each layer was correlated with coral dates from the shelf-margin reef and the outlier reef (Figs. 36A, 40A, top panels, and Tile 7/8) and with stands of interglacial sea levels that were sufficiently high to have flooded the shelf (Fig. 37). Lastly, except at the crest of Carysfort Reef where the water is too shallow to obtain seismic data, the Holocene component was drawn as traced from the seismic profiles. The model for Pelican Shoal was constructed in the same fashion (Fig. 96A). The Holocene component at Carysfort Reef was drawn from field knowledge.

Lidz (2004) proposed terminology for application to platform margins in general to characterize the various stratigraphies observed in the Florida Keys seismic data.

  • A backstepped reef complex is present at all four sites modeled (Figs. 36A, 40A, 96A, top panels, and Tile 7/8). Backstepping occurs when coral reefs grow upward and landward in response to rising sea level, as revealed at Looe Key Reef and Grecian Rocks (Fig. 92A, 92B).

  • Backfilled progradation of the outer shelf has occurred at all four modeled sites by Holocene infilling of Pleistocene troughs behind the discontinuous shelf-margin reef. Backfilling occurs when storms wash sand and reef rubble generated at the margin into landward troughs.

  • Future development of outlier reefs and infilling of their backreef troughs, or filling of troughs behind existing outlier reefs, would create coalesced reef complexes at Carysfort Reef, The Elbow, and Sand Key Reef (Figs. 36B, 40B, and Tile 7/8). Reef coalescence occurs when reef crests are no longer identifiable as discrete entities elevated above surrounding terrain.

With continued sediment accretion and no development of outlier reefs at Pelican Shoal, future seismic images of this and similar areas of the margin would represent a classic prograded margin (Fig. 96B), or one that had advanced seaward by spilling of sands over the shelf edge. Progradation is the seaward advance or building of the shelf and margin by deposition and accumulation of sediments derived from the shelf. A geologic process, progradation can be rapid in terms of geologic time. The western margins of the Great Bahama Bank (e.g., Ginsburg, 2001) and Little Bahama Bank (Hine and Neumann, 1977) are classic examples of prograded margins. In Florida, progradation occurred as Holocene reefs and sediments filled bedrock troughs behind many areas of the Pleistocene shelf-edge reef, such as at Maryland and Pelican Shoals (Figs. 95B, 96A). Other examples can be seen elsewhere in the profiles of margin morphology (Figs. 89, 90). The progradational process is presently occurring in front of the shelf-edge reef through sand chutes, which are low-elevation areas or breaks (sometimes called divides, Fig. 92A) in the shelf-edge reef through which sands sift slowly off the shelf due to tidal and current action. Sand chutes are present everywhere along the shelf edge (good examples can be seen in Figs. 33A, 33B, 48B, 75, 87A). In the case of the Florida windward margin, chute sediments would accumulate first on the upper-slope terrace before cascading over the seaward edge to greater depths.

The seismic and coral-age data document that where progradation occurred on the Florida margin, it was very rapid relative to geologic time, having taken place only after the shelf became flooded about 7 ka. Whether progradation occurred in the Pleistocene is not known. However, the models indicate that the potential had existed. Backstepping Pleistocene coral growth partially filled landward troughs (e.g., Figs. 36A, 40A, 96A). If the duration of highstand intervals and coral-growth phases had been longer, the troughs might eventually have become filled (Lidz, 2004).

One of the most significant outcomes of the Lidz (2004) models was the formal recognition that alternating sections of the Florida windward margin had prograded during the Holocene. Most reef-rimmed windward margins in the geologic record are steeply inclined and are regarded as aggraded. Aggradation is the building upward and seaward of the shelf and margin by the deposition and accretion of skeletal calcium-carbonate organisms such as reefs of coral. As opposed to the rapid (relative to geologic time) geologic process of progradation, aggradation is a slow biologic process.

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