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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,

Tile 3

Tidal Passes and Pleistocene Riverbeds: Modern tidal passes that were once Pleistocene riverbeds (e.g., Davies, 1980; USGS, unpublished data) separate islands in the Florida Keys and connect the waters of the Everglades and Florida Bay with those of the Gulf Stream and Atlantic Ocean. What was this area like during the Pleistocene when sea level was lower and the shelf was dry land? In plan view, the southernmost tip of a peninsular highland would have terminated in a broad, shallow, half-moon-shaped depression (Florida Bay) ringed on its south edge by a discontinuous band of rocky hills (the Florida Keys) about the same height as the highland. A second, linear depression would have paralleled the band of hills on their seaward side (Hawk Channel), dividing these features from the outer part of the shelf. More bands of rock ridges, parallel to the linear depression and discontinuous but lower and narrower (the offshore coral ridge-and-swale topography, one of two primary coral reef architectures), would have ornamented the outer shelf. A tall irregular rock ridge at least 30 m high would have marked the shelf edge before the landmass sloped steeply to the upper-slope terrace, then precipitously into the ocean.

Florida Bay topography consists of what has been termed a loose collection of shallow, semi-independent basins, giving it a highly irregular surface. Consisting of pelleted grainstone (classification of Dunham, 1962) or bryozoan limestone (e.g., Merriam, 1988), bay bedrock today is generally about 2 to 3 m below sea level (Fig. 57). Pelleted grainstone is composed of hardened fecal pellets and rounded skeletal grains. Bryozoa are mostly colonial calcareous marine invertebrate animals that, upon death, form cemented skeletal mounds (e.g., Multer et al., 2002). Both rock types developed during the sea-level highstand around 125 ka, at the same time the Key Largo Limestone (coral reef) and Miami Limestone (oolite) of the Florida Keys formed. Between that highstand and the subsequent (present) marine transgression, sea level remained below the elevation of all but the outermost part of the shelf (e.g., Lidz, 2004). Rivers crisscrossed the expanse of dry bay floor, draining the highland. Many were likely ephemeral, flowing only during the summer months of the rainy season.

Drawing shows configuration of riverbed channels in Florida Bay Pleistocene bedrock
Figure 57. Drawing shows configuration of riverbed channels in Florida Bay Pleistocene bedrock (wide blue areas), as proposed by Davies (figure modified from Davies, 1980). A likely third tine in the channel fork exiting the bay was bifurcated by Lignumvitae Key (thin blue lines, USGS, unpublished data). Note location of ancient riverbeds relative to modern rivers draining Taylor Slough. Bay area bedrock depths below sea level are approximate and were determined by probing with a rod (Davies, 1980). [larger version]

General direction of water flow during Pleistocene lowstands of sea level is believed to have been southward. Several meandering channels thought to be Pleistocene riverbeds (Davies, 1980) have been located in the floor of Florida Bay by probing to bedrock with a rod. Davies identified forked bay-floor channels south of the Taylor Slough that exit through Channel Two and Channel Five, two of the wider channels located in the tidal pass between Lower Matecumbe and Long Keys (Figs. 56B, 57, 58; see Bedrock Surface map). A third northern fork, visible on aerial photos and verified by probing, bifurcated around Lignumvitae Key in the bay, and egressed through Indian Key Channel and an unnamed channel north of Lower Matecumbe Key (Figs. 57, 58). A core taken in 1985 in the unnamed channel west of Lignumvitae Key recovered ~3 m of cross-bedded quartz sandstone overlying homogenous carbonates (USGS, unpublished data). Shallow depths of the quartz sandstone indicated sand import and emplacement had occurred during the Pleistocene. The unnamed channel was once a riverbed. Tidal channels elsewhere in the keys probably also represent bedrock depressions that once confined and carried rivers seaward. Sinuous remnants of old riverbeds are expected to exist throughout the bay floor.

Aerial photo (1991) shows sinuous intra-island tidal channels in areas around Lower Matecumbe Key
Figure 58. Aerial photo (1991) shows sinuous intra-island tidal channels in areas around Lower Matecumbe Key (middle Keys; from Lidz et al., 2003; compare with Fig. 57 and interpreted Benthic Ecosystems for Tile 3). White dotted line marks seaward edge of a nearshore rock ledge. Note width of the ledge relative to that of Lower Matecumbe Key. Also note tidal-delta sediments within channels and at channel edges. The four tidal channels in this photo are believed to represent bedrock depressions through which a forked Pleistocene river exited Florida Bay when the shelf was exposed (Davies, 1980). [larger version]

In the present transgressive setting, direction of surface and groundwater flow from the Everglades is generally southward (e.g., Douglas, 1947; Porter and Porter, 2002). Two primary river systems, the Shark River Slough and the Taylor Slough, empty from the Everglades into Florida Bay (Fig. 57). The northernmost major tidal passes in the keys occur southwest of Lower Matecumbe Key in the middle Keys (Fig. 56B). Sides of the tidal channels are lined with fine-grained sediments flushed seaward from Florida Bay (Fig. 58). Water level in Florida Bay is generally higher than the level of the Atlantic Ocean most of the time (see Hydrology), which produces a net bay-to-ocean slope or gradient and a tendency for strong currents through the passes (Fig. 25A; Smith, 1994). The currents excavate sand and mud from the bay side of the keys and drop their suspended loads on the ocean side, forming lobate tidal deltas readily visible in aerial photos (Figs. 46B, 58). Tidal deltas are a prominent type of sand body in the Florida Keys (Ball, 1967). Regardless of width of a tidal pass, depth to bedrock in the middle Keys passes is estimated from field observations not to exceed ~4.5 m (Lidz et al., 2003).

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

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