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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

Primary Products

Overview Map—Reef and Sediment Thickness: Dates on coral reefs and sediments of the present ecosystem indicate they have been accumulating since the sea began to cover the Florida shelf approximately 7 to 6 ka. As in the case of the bedrock components and fossil reefs, thickness and distribution of the modern reefs and sediments contain records of environmental quality and physical/biological processes. Coral reefs flourished between ~7 and 2 ka (e.g., Shinn et al., 1989a) and began to decline as Florida and Biscayne Bays filled with sea water between 4 and 2 ka (Lidz et al., 1991). Flooded tidal passes between the keys allowed bay water, unsuitable for coral growth, to access the offshore reefs. The modern reef system is Holocene (Fig. 7B).

Updated regional isopach map that shows contoured thickness of Holocene carbonates from the northern Florida Keys to west of The Quicksands.
Sediment Thickness map: USGS geophysical data were combined with selected analogous data of Enos (1977; see Data Integration section) to produce an updated regional isopach map that shows contoured thickness of Holocene carbonates from the northern Florida Keys to west of The Quicksands. (From Lidz et al., 2003). Reef data are based on drill cores. Contours are in meters. Dogleg and diagonal lines divide upper, middle, and lower Keys.

Colors represent different thicknesses ranging from bare bedrock (white) to thinnest (pale pinks) to thicker (dark reds) to thickest (rust). White area at shelf margin denotes seaward sediment-free side of a massive discontinuous Pleistocene coral reef. This fossil reef marks the shelf margin at the 30-m depth contour. An upper-slope terrace lies at the toe of this reef in 30 to 40 m of water. Note areas of thinnest coverage (pale pinks) generally correspond to areas of highest bedrock elevations, such as on the Marquesas-Quicksands ridge (in the large black rectangle).

Sediments are generally ~3 to 4 m thick shelf-wide, indicating offshelf transport in the deeper southwestern area. The open-marine environment there is subject to stronger currents than the more restricted northeastern marine setting. The thickest shelf accumulations (~27-30 m) occur in a backreef trough behind the shelf margin, where both reef and trough are present. Thicker sediments (>55 m) are known to fill an outer-shelf sinkhole located off north Key Largo (Fig. 6C; Shinn et al., 1996). The thickest (>40 m) sediment buildup contoured on the map occurs on the upper slope southwest of the Marquesas Keys in 80 to 190 m of water.

The isopach map shows sites and types of localized sedimentary processes: accumulation in enclosed bedrock lows (north of New Ground Shoal and in sinkhole off Key Largo) and behind rock barriers (east of Halfmoon Shoal), and offshelf transport (sediment wedge southwest of the Marquesas Keys). Deep water around the Marquesas-Quicksands ridge and strong currents in Boca Grande Channel indicate that ridge sands form in place. Both maps show the location of NOAA's newly named Captain Roy's Reef off north Key Largo and the generally margin-parallel nature of Pleistocene and Holocene geomorphic trends seaward of the keys. [larger version]

The Sediment Thickness map (also see Open-File Report 00-164) was derived from geophysical data and probing with a rod. Reef thicknesses were based on drill cores. Areas of thinnest coverage occur:

  • along the landward side of Hawk Channel (the major bedrock depression between the keys and shelf margin),
  • at the seaward edge of the shelf-margin fossil reef,
  • on all but the widest of the reefs seaward of the margin, and
  • on the Marquesas-Quicksands ridge in the Gulf of Mexico; the Marquesas Keys developed on a forked bedrock crest and around a shallow mud-filled depression.

The thickest (>40 m) sediment buildup visible on the map occurs on the upper slope southwest of the Marquesas Keys in 80 to 190 m of water. Thicker sediments (> 55 m) are known to fill an outer-shelf sinkhole located off north Key Largo (Fig. 6C; Shinn et al., 1996).

Although less extensive in distribution and dimensions than the older bedrock reefs, reefs of the modern ecosystem are comparable. Thickness of their skeletons indicates they grew vigorously, but their ages indicate growth occurred only over ~5 ka (e.g., Shinn et al., 1977a, 1989b). The thickest Holocene reef drilled (~17 m) is at Southeast Reef in the Dry Tortugas (Fig. 6A, 7B; Shinn et al., 1977a) and indicates that environmental quality upon initial flooding of the shelf (at ~7 to 6 ka) was very favorable for coral growth. The least healthy modern reefs are off the middle Keys, where wide tidal passes allow diurnal exchange of turbid water from Florida Bay (e.g., Hallock and Schlager, 1986) and cold winter water from the Gulf of Mexico onto the reef tract (e.g., Roberts et al., 1982).

When viewing the sediment thickness map, envision the physical and sedimentary processes responsible for the distribution of reefs and sediments. Reefs grow on bedrock highs where sediments do not collect. Sediment accumulates in enclosed bedrock lows, such as those north of New Ground Shoal and the sinkhole off Key Largo, and piles up behind rock barriers, such as east of Halfmoon Shoal. Sediment is being transported off the shelf, forming a thick sediment wedge southwest of the Marquesas Keys, and strong currents are preventing accretion in Boca Grande Channel (Shinn et al., 1990). Deep water on three sides of the Marquesas-Quicksands ridge and current-swept, sediment-free Boca Grande Channel indicate that ridge sediments are being formed in place. In the case of The Quicksands where the sediments consist primarily of Halimeda grains (Hudson, 1985), sediments can be considered biologic (algae) and geologic (in-situ source) indicators of physical processes.

Considered together, the bedrock and sediment-thickness datasets show that bedrock elevation is several meters lower in the southwest than in the northeast (e.g., Perkins and Enos, 1977), yet sediment cover is about 3 to 4 m thick shelf-wide (Lidz et al., 2003). In other words, except for localized enclosed bedrock depressions, sediments are not thicker in extensive open areas where bedrock is deeper, which reveals differences in physical processes in the southwest and northeast. Prevailing winds and waves impinging on the shelf margin generally arrive from the southeast, i.e., onshore in the northeast areas, but tangential to shore in the southwest. Sediments in the southwest region are being removed from the shelf due to the generally westward winds and waves, a more open exposure to strong oceanic currents, and other conditions associated with the deeper-water setting and location at the end of the island chain.

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

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