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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,


Summary: Massive fossil coral reefs majestically rim the shallow shelf off the Florida Keys. The keys themselves consist of an inland fossil reef with fossil tidal bars at either end. Offshore, the colorful coral reefs and organisms we regard as being among our national treasures cap the massive ancient reefs. Embedded in this scenic submarine tapestry are records of complex, dynamic natural processes that govern how the reefs and their associated biota and sediments formed and changed through geologic time.

This database-synthesis project was undertaken to examine these processes in depth and to capture and consolidate more than 60 years of geologic and biologic research conducted by the USGS and by others in a contiguous area of the Florida Keys National Marine Sanctuary. The goal was to produce a digitized regional database and comprehensive one-volume reference source for the area between central Key Largo and Halfmoon Shoal. This report is the result.

The emphasis in this report is on geologic and biologic changes along the Florida reef tract as sea level rose and fell over tens of thousands of years, but impacts of historical events on time scales of human interest are also included—from installation of lighthouses and development of the Overseas Railway to offshore exploration (for oil and gas) and marine archaeology (recovery of shipwrecks and their treasures). The difficulty of distinguishing deleterious effects of human activities from those of the many adverse natural stressors affecting the reef ecosystem is also now of interest, as well as concern. Information in this report may contribute to understanding the causes of reef decline observed today.

Scientific perspectives offered encompass global, regional, and local scales:

  • climatology, sea-level fluctuations, ancient environments;

  • the rock record in modern coral reefs, including geomorphology, stratigraphy, ground- and surface-water hydrology, and karst topography;

  • the biologic record, including natural changes in (a) coral species, (b) annual skeletal-coral bands, and (c) skeletal sand-grain percentages as responses to changing environment;

  • and the imprint of human activities.

Although specific to reefs of the Florida Keys, principles and perspectives set forth in the updated maps and shelf-edge-model datasets can be applied to understanding development of shallow-water coral reef systems anywhere, whether modern or ancient.

From a different viewpoint, knowledge gained from the study of modern coral reefs and their sediments is not limited to learning how older, local (underlying) reefs and sediments formed in the past. Large-scale geometric patterns and biogeologic processes observed in the modern carbonate setting are important tools to interpret what is seen in the very ancient limestone record (e.g., see papers in Scholle et al., 1983), even when the primary carbonate components are different (e.g., Shinn et al., 1983). Use of artificial components in modern sediments to replicate microscopic aspects of the rock record has also proven to be of benefit (e.g., Brown, 1969; Shinn et al., 1977b; Shinn and Robbin, 1983). These tools of modern patterns and processes are invaluable when applied, as intended in the field of geology, to fulfill the basic tenet of geology: the present is the key to the past.

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

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