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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 7/8

Margin Evolution: The south Florida margin is windward, facing the direction from which prevailing winds blow. The shallow, gently sloping shelf does not fit the classic model of steeply inclined, reef-rimmed windward margins (Hine and Mullins, 1983). These facts are important when examining how the Florida margin builds or progrades seaward.

The Holocene surfaces seen in seismic data from the four model sites highlight, for the first time, the most striking evidence for windward-margin evolution in Florida. Holocene infilling of Pleistocene backreef troughs, where present, has extended those areas of the shelf seaward by hundreds of meters (top panels in Figs. 36A, 40A, 96A, 108A; Lidz, 2004). This type of stratigraphy can be said to characterize a backfilled prograded margin. Such processes have not occurred where reentrants are present, resulting in discontinuous or laterally alternating sites of progradation and relative stasis. Given that the outer shelf has been submerged since, roughly, only the most recent 7 ka (Table 4, Table 5), the infilling process has "stepped" the margin seaward instantaneously, relative to geologic time. Progradation over distances of several kilometers within a few thousand years is typical of highstand-shedding leeward carbonate margins (e.g., Ginsburg, 2001). Progradation by any kind of sedimentary action, especially with the rapidity evidenced in the Florida data, is not possible at classic steeply inclined windward margins (e.g., Hine and Mullins, 1983).

Freestanding outlier reefs off Carysfort (Fig. 36A) and Sand Key Reefs (Fig. 108A) display keep-up profiles with the shelf-edge reef, but reef-like features seaward of the Carysfort outlier and The Elbow (Fig. 40A) show give-up (buried) profiles (Neumann and Macintyre, 1985). The outlier reefs forecast a new episode of rapid seaward-stepping progradation when their landward troughs become filled. The geologic result would be a coalesced reef-complexes margin that would manifest seismically as parallel coral facies (Figs. 36B, 108B). The coral facies would span distances of several kilometers in cross-sectional profile. Landward- and seaward-dipping bedding would again be present.

Under thermal-maturation conditions, multiple-reef zones, if correctly identified over such distances, would be of high interest as potential hydrocarbon-bearing reservoirs if there were organic-rich sediments in the vicinity. Thermal-maturation conditions develop when organic deposits are buried deep beneath thick overlying strata for millions of years. Heat and pressure compress the organics, causing them to "mature" into oil and gas.

In contrast, rapid progradation will likely not occur where outlier reefs are absent (Fig. 96B), or where kilometers-wide reentrants break the shelf-edge reef, such as at Alligator and Tennessee Reefs in the middle Keys (Figs. 49A, 67). The Florida windward margin thus has divergent modes of seaward expansion, at alternating sites. Under highstand conditions of sea level, the shelf edge as viewed from the air should become increasingly laterally irregular with time—at least until reentrants and the 2-km-wide accommodation space on the upper-slope terrace are filled (Figs. 109A, 109B, 110). Given stable highstand conditions, carbonate platforms will build to and just above the sea surface (e.g., Tucker, 1985). Whether there was margin progradation of any kind at any time during the Pleistocene is not known. Seismic reflections at the sites modeled indicate that highstand progradation potential did exist to the extent that backstepping Pleistocene coral reefs were slowly filling landward troughs, but time for coral growth (i.e., transgressive intervals) or rate of coral growth was insufficient, and the infill process at those sites was incomplete (Lidz, 2004).

Summary diagram based on seismic data shows present margin surface and two scenarios for possible future development at the four sites Figure 109. Summary diagram based on seismic data shows present margin surface and two scenarios for possible future development at the four sites modeled: (A) coral growth and sedimentation, and (B) sedimentation with no coral growth. Long curved arrows indicate offshelf sediment transport. Short curved arrows indicate landward sediment transport and infilling of backreef troughs. Carysfort Reef: Filling of the existing trough behind the outlier reef would extend the margin seaward. However, if corals once again grow on the outlier reef, elevation of the new margin could be as high as the present shelf edge, unless corals also resume growth on Carysfort Reef. If corals do not colonize either reef (B), sediment would eventually bury both, and this part of the margin would develop a pronounced slope. The Elbow: If corals renew growth on the linear terrace features, new outlier reefs and backreef troughs could form. If there is no coral growth on these features, sediments should build up on the terrace, raising elevation of its horizontal surface. Pelican Shoal: On the terrace, no hard, sediment-free surface for coral colonization presently exists. Sediments will continue to accumulate, extending the dip angle of the sediment wedge upward and seaward across the terrace. The terrace at this site lies between two outlier-reef segments in a single discontinuous reef tract (Fig. 89, lines 4, 3, and 2). The Pelican Shoal site could become a reentrant in a new shelf margin with infilling of the trough behind the outlier reefs at Maryland Shoal and Eastern Sambo. Sand Key Reef: When the troughs behind the outlier reefs become filled with sediment and/or coalescing fringing reefs, the shelf margin would instantaneously "step" seaward, relative to geologic time. Considering present status of The Elbow and its infilled backreef trough (Fig. 39A, 39B), some parts of the south Florida shelf may have built seaward in this fashion in the past. These scenarios are dependent on the position of sea level and its duration at that position. [larger version]

Plan views of the Sand Key shelf-margin and outlier reefs traced from the aerial photo Figure 110. (A-C) Plan views of the Sand Key shelf-margin and outlier reefs traced from the aerial photo (Fig. 106B) show how irregularities along the margin edge could develop as backreef troughs become filled with sediment. (D, E) Generalized plan view of hypothetical composite-margin section is drawn from present morphologies at Alligator and Tennessee Reefs (Figs. 49A, 67), and Maryland and Pelican Shoals (Figs. 87B, 89, lines 4 and 3). Shelf-edge promontories could develop as infilling of troughs behind single outlier reefs extends parts of the margin seaward. Promontories on either side of an existing reentrant would enlarge the reentrant. A new reentrant could form where outlier reefs are lacking, as off Pelican Shoal. These models relate only to changing submarine morphology as sediment fills present accommodation space. Corals may or may not grow depending on many factors, including fluctuation of sea level. [larger version]

Models of what the future geologic record might look like with continued accretion of corals and sediments at the four sites (Figs. 36B, 40B, 96B, and 108B) are summarized for easy comparison (Figs. 109A, 109B, 110). Slight differences among these drawings, such as renewed vs. non-renewed coral growth at the shelf edge or on terrace ridges, underscore the fact that we do not know precisely what type of accretions will actually accumulate at any given site in the future.

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

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