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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

New Evidence for Interpretation of the Nearshore Rock Ledge: The hues depicting depths on the NGDC colored bathymetric map shown in Lidz et al. (2006) provide another piece of evidence that reinforces the concept of ledge composition. Extent of the broad submerged bank of ooid tidal bars to the north and west of the emergent crests (Miami Limestone of the lower Keys) is very well defined on the map. That of a narrow submerged back reef behind emergent reef crests (Key Largo Limestone of the middle and upper Keys) is also well defined, as is the seaward contact at a water depth of 4 m between the elevated ledge edge and the bedrock depression under Hawk Channel. Coupled with other physical characteristics of the ledge, such as its generally being wider than islands of the middle and upper Keys (Fig. 48A) and the jagged nature of its seaward edge, all aspects of the ledge are consistent with a natural angle of repose of ooids and with a broad, irregular fore reef that faced incoming waves. As is evident at Grecian Rocks off the upper Keys (Shinn, 1980a), the forereef part of a reef that kept pace with rising sea level generally has a seaward slope and is broader than its crest due to the reef having backstepped or grown landward.

In other words, the bathymetric-map colors provide additional evidence that supports the present theory that the ledge represents the seaward extent of the Key Largo Limestone reef and the Miami Limestone oolite. All ledge characteristics, especially its landward-sloping gradient and coincident landward/shoreline edge, point to the only timing of erosion that is possible—post-Key Largo/Miami Limestone formation and thus the most recent part of the Holocene (time at which sea level was 4 m lower than present; Fig. 102B). Converting the maximum ledge width (2.5 km) and onset of erosion (4 ka) to human-interest scales shows that islands in the Florida Keys have been eroding landward at a rate of ~6.25 m/100 years for the last 4 ka—and under conditions of a rising sea level (Fig. 102A, 102C).

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

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