generations on top of older skeletal generations. The
models also show backstepping (landward coral growth)
and lateral stacking (linking) of reef tracts by infilling of
broad backreef troughs (Figure 3a and b). Offbank sediment accumulations extended non-reef areas seaward
over lesser distances.
Coral ages indicate that the proto-Florida platform was
subaerially exposed from the post-125-ka regression to
$7 ka. Though the 80-ka highstand was the last that
enabled Pleistocene marine deposition on the shelf, position of the sea then only reached a maximum of $9 m
below present (Lidz, 2006). Present depths to bedrock
are <9 m on most of the shelf seaward of the Keys and
average $6–8 m on the Marquesas–Quicksands ridge.
Thus, the platform interior and westward-extending ridge
remained dry land for $115 kyr. During that time, the
landmass promontory protected five shelf-edge reef tracts
from the cold Gulf of Mexico waters (Figure 4a and b).
Coastal Florida Bay waters did not exist. Geomorphic settings of the post-125-ka Pleistocene highstands provided
shelf-edge accommodation space, shelter, and favorable
conditions for reef calcification and accretion. Relative
to the long landmass duration, the collective time of offshore Pleistocene reef accrual was comparatively short
($20 ka; Lidz, 2006).
For unknown reasons, the Holocene transgression produced only $2 m of coral growth on the five 80-ka shelfedge reef tracts before the rising sea fully inundated the
shelf. As the sea rose, it created and gradually submerged
ephemeral islands of mangrove/mud, coral rock, and sand.
At $4 ka, seawater was far enough inland to infiltrate
inter-reef paleochannels of the Key Largo Limestone, creating tidal passes that would allow saltwater access to the
Florida Bay depression. During these events, outer-shelf
reefs (e.g., Grecian Rocks) behind the shelf-edge reef
responded by backstepping landward to keep pace with
the rise (Neumann and Macintyre, 1985). Though initiated
on hard substrate, some reefs also backstepped over carbonate sand and rubble (e.g., Looe Key Reef; Shinn,
1980; Lidz et al., 1985).
Then conditions changed. At $2 ka, coastal bay and
cold gulf waters began to flow seaward through the passes,
impacting health of reefs opposite the passes (Porter and
Porter, 2002). Terrestrial and atmospheric stressors
(Jameson et al., 2002), including African dust (Shinn
et al., 2000), joined the assault. Corals responded by
declining, then by succumbing rapidly within the last
30 years. As of this writing (October 2008), more than
95% of Florida Keys reefs are senescent (non-accretionary), coral mortality having resulted mainly from diseases.
Only small, isolated colonies of reef-building coral species are alive. Reef-framework structure is decaying
due to bioerosion, ocean acidification, and natural (hurricanes) and anthropogenic (boat groundings) mechanical
breakdown. Where live corals exist, underlying skeletal
framework is mostly overgrown by sea fans, sea whips,
algae, fleshy encrusting organisms, sponges, and stinging
corals, all associated with reefs in decline.
Two structural architectures
Florida Keys reefs consist of two repetitive architectures:
a narrow low-relief outer-shelf ridge-and-swale geometry
(Figure 5) and a broad high-relief shelf-edge reef-andtrough geometry (Figure 4a and b). Both are linear, discontinuous, and submerged and parallel the curved shelf
edge. Cores from transects across the outer-shelf rock
ridges have recovered skeletal Holocene corals overlying
Pleistocene corals (Shinn et al., 1977). Cored swales contain unconsolidated Holocene sands overlying a calcretecoated subaerial-exposure surface that caps cemented
Pleistocene sands. Eight such coral ridges are visible in
aerial photographs (Lidz et al., 2007). The ridge-andswale structure is tens of meters wide and thin relative to
the reefs and troughs.
The five major shelf-edge tracts form the reef-andtrough structure on a wide ($2.5 km) upper-slope terrace
$40 m below the present sea level. The landward tract is
the shelf margin. The others are outlier reefs, so called
because empty backreef troughs render them detached
from the shelf (Lidz et al., 1991). The exposed landmass
promontory protected these five tracts so well for so long
that they accumulated imposing relief ($30 m). Their
corals date to $106, 94–90, and 86–77.8 ka with an
inferred age of $127 ka at their bases. Thin Holocene
reefs with spurs and grooves cap these reefs. The reefand-trough geometry is hundreds of meters wide.
Four coral nuclei
Pleistocene outer-shelf beach dunes, antecedent reefs, and
a nearshore coral-and-oolite rock ledge are well-known
coral substrates along the Florida reef tract. Ledge patch
reefs, usually individual live head corals surrounded by
sand halos, are in the shallowest of Keys reef settings
($3 m deep).
Thousands of patch reefs, many with coalesced halos,
live in the middle of Hawk Channel, mainly off the lower
Keys (Figure 4b). No geologic studies have focused on
their origin, mid-channel alignment, distribution, and
location. Analysis of benthic habitats and bathymetry
(Lidz et al., 2006) has resolved their four-part characterization and has revealed a fourth type of coral nucleus
new to the Florida record. The patches align along landward edges of two long inner-shelf bedrock troughs within
the Hawk Channel depression. The troughs are presumed
to consist of grainstone/wackestone, as is found elsewhere
in channel bedrock (Shinn et al., 1994).
Coral spurs and zonations
Holocene corals also formed outer-shelf patch reefs along
and parallel to the shelf edge. Though linear, these reefs
had seaward spur-and-groove structures in which coral
spurs grew perpendicular to incoming waves (Figure 4b;
see also Fig. 1 in Chapter Spurs and Grooves). The highenergy shelf-edge setting dictated that the surf-loving
branching species Acropora palmata would build the
spurs. Nearly all margin spurs consist of densely interwoven skeletal A. palmata (Shinn, 1963).
FLORIDA KEYS
409
models also show backstepping (landward coral growth)
and lateral stacking (linking) of reef tracts by infilling of
broad backreef troughs (Figure 3a and b). Offbank sediment accumulations extended non-reef areas seaward
over lesser distances.
Coral ages indicate that the proto-Florida platform was
subaerially exposed from the post-125-ka regression to
$7 ka. Though the 80-ka highstand was the last that
enabled Pleistocene marine deposition on the shelf, position of the sea then only reached a maximum of $9 m
below present (Lidz, 2006). Present depths to bedrock
are <9 m on most of the shelf seaward of the Keys and
average $6–8 m on the Marquesas–Quicksands ridge.
Thus, the platform interior and westward-extending ridge
remained dry land for $115 kyr. During that time, the
landmass promontory protected five shelf-edge reef tracts
from the cold Gulf of Mexico waters (Figure 4a and b).
Coastal Florida Bay waters did not exist. Geomorphic settings of the post-125-ka Pleistocene highstands provided
shelf-edge accommodation space, shelter, and favorable
conditions for reef calcification and accretion. Relative
to the long landmass duration, the collective time of offshore Pleistocene reef accrual was comparatively short
($20 ka; Lidz, 2006).
For unknown reasons, the Holocene transgression produced only $2 m of coral growth on the five 80-ka shelfedge reef tracts before the rising sea fully inundated the
shelf. As the sea rose, it created and gradually submerged
ephemeral islands of mangrove/mud, coral rock, and sand.
At $4 ka, seawater was far enough inland to infiltrate
inter-reef paleochannels of the Key Largo Limestone, creating tidal passes that would allow saltwater access to the
Florida Bay depression. During these events, outer-shelf
reefs (e.g., Grecian Rocks) behind the shelf-edge reef
responded by backstepping landward to keep pace with
the rise (Neumann and Macintyre, 1985). Though initiated
on hard substrate, some reefs also backstepped over carbonate sand and rubble (e.g., Looe Key Reef; Shinn,
1980; Lidz et al., 1985).
Then conditions changed. At $2 ka, coastal bay and
cold gulf waters began to flow seaward through the passes,
impacting health of reefs opposite the passes (Porter and
Porter, 2002). Terrestrial and atmospheric stressors
(Jameson et al., 2002), including African dust (Shinn
et al., 2000), joined the assault. Corals responded by
declining, then by succumbing rapidly within the last
30 years. As of this writing (October 2008), more than
95% of Florida Keys reefs are senescent (non-accretionary), coral mortality having resulted mainly from diseases.
Only small, isolated colonies of reef-building coral species are alive. Reef-framework structure is decaying
due to bioerosion, ocean acidification, and natural (hurricanes) and anthropogenic (boat groundings) mechanical
breakdown. Where live corals exist, underlying skeletal
framework is mostly overgrown by sea fans, sea whips,
algae, fleshy encrusting organisms, sponges, and stinging
corals, all associated with reefs in decline.
Two structural architectures
Florida Keys reefs consist of two repetitive architectures:
a narrow low-relief outer-shelf ridge-and-swale geometry
(Figure 5) and a broad high-relief shelf-edge reef-andtrough geometry (Figure 4a and b). Both are linear, discontinuous, and submerged and parallel the curved shelf
edge. Cores from transects across the outer-shelf rock
ridges have recovered skeletal Holocene corals overlying
Pleistocene corals (Shinn et al., 1977). Cored swales contain unconsolidated Holocene sands overlying a calcretecoated subaerial-exposure surface that caps cemented
Pleistocene sands. Eight such coral ridges are visible in
aerial photographs (Lidz et al., 2007). The ridge-andswale structure is tens of meters wide and thin relative to
the reefs and troughs.
The five major shelf-edge tracts form the reef-andtrough structure on a wide ($2.5 km) upper-slope terrace
$40 m below the present sea level. The landward tract is
the shelf margin. The others are outlier reefs, so called
because empty backreef troughs render them detached
from the shelf (Lidz et al., 1991). The exposed landmass
promontory protected these five tracts so well for so long
that they accumulated imposing relief ($30 m). Their
corals date to $106, 94–90, and 86–77.8 ka with an
inferred age of $127 ka at their bases. Thin Holocene
reefs with spurs and grooves cap these reefs. The reefand-trough geometry is hundreds of meters wide.
Four coral nuclei
Pleistocene outer-shelf beach dunes, antecedent reefs, and
a nearshore coral-and-oolite rock ledge are well-known
coral substrates along the Florida reef tract. Ledge patch
reefs, usually individual live head corals surrounded by
sand halos, are in the shallowest of Keys reef settings
($3 m deep).
Thousands of patch reefs, many with coalesced halos,
live in the middle of Hawk Channel, mainly off the lower
Keys (Figure 4b). No geologic studies have focused on
their origin, mid-channel alignment, distribution, and
location. Analysis of benthic habitats and bathymetry
(Lidz et al., 2006) has resolved their four-part characterization and has revealed a fourth type of coral nucleus
new to the Florida record. The patches align along landward edges of two long inner-shelf bedrock troughs within
the Hawk Channel depression. The troughs are presumed
to consist of grainstone/wackestone, as is found elsewhere
in channel bedrock (Shinn et al., 1994).
Coral spurs and zonations
Holocene corals also formed outer-shelf patch reefs along
and parallel to the shelf edge. Though linear, these reefs
had seaward spur-and-groove structures in which coral
spurs grew perpendicular to incoming waves (Figure 4b;
see also Fig. 1 in Chapter Spurs and Grooves). The highenergy shelf-edge setting dictated that the surf-loving
branching species Acropora palmata would build the
spurs. Nearly all margin spurs consist of densely interwoven skeletal A. palmata (Shinn, 1963).
FLORIDA KEYS
409