and bank configuration focus tidal currents, ooid shoals
are formed.
Tectonic and stratigraphic studies have established that
up to 10 km of largely shallow-water carbonates, Jurassic
to Holocene in age, overlie rifted Jurassic transitional crust
in the NW Bahamas and oceanic crust SE of Tongue of the
Ocean (TOTO, Sheridan et al., 1988), although others
have argued for African continental crust (Mullins and
Lynts, 1977). A carbonate “megabank” developed in the
Early Cretaceous that may have included the Bahamas,
Florida, and Yucatan (Austin and Schlager, 1988). This
bank was fragmented into smaller platforms separated by
subsiding blocks in the mid-Cretaceous. The southern part
was further fragmented during collision with Cuba in Late
Cretaceous–Middle Eocene (Masaferro and Eberli, 1999).
Seismic stratigraphy shows that leeward lateral accretion
of sediment derived from the bank tops has filled such
depressions, merging platform fragments during the
Cenozoic to form the GBB (Eberli and Ginsburg, 1987,
1989; Masaferro and Eberli, 1999). Deep channels (Straits
of Florida, Santaren Channel, Old Bahamas Channel,
Providence Channels) and reentrants (TOTO, Exuma
Sound) remain. These dissect the banks and isolate them
from terrigenous input except for wind-blown dust that
reddens the soils.
Coral reefs were common around the margins of the
Bahama Banks at least as far back as the Pliocene (Beach
and Ginsburg, 1980). Pleistocene reefs are exposed on
most of the major islands (Cant, 1977), some extending
to 5 m above present sea level (Hearty, 1998). Lithified
Pleistocene eolianite dune ridges form the bulk of the
Bahaman islands, however, reaching 63-m elevation on
Cat Island (Government of the Bahamas). Subtidal Pleistocene deposits extend up to 6 m above sea level on the
large islands. Multiple Holocene beach-accretion ridges,
anchored by Pleistocene buttresses, form many low-lying
areas. Origins from dunes, beaches, and reefs dictate that
the largest islands (Andros, Eleuthera, Great Abaco) and
highest (Cat Island) lie at the eastern margins of the banks,
where tides, trade winds, and wave fetch maximize skeletal sediment production (Ball, 1967).
Most islands have multiple nested rows of Pleistocene
dunes, generally younging toward the windward margin.
This is well illustrated on New Providence and adjacent
islands where six prominent ridges are visible on topographic maps (Hearty and Kindler, 1997). Strings of small
cays formed by eroded dunes near the shelf break are common around the Bahamas, even on the shallower leeward
margins, e.g., from Bimini south beyond Cat Cays. Similarly, the eroded remnants of dunes, beaches, and reefs
dominate the submarine topography at the margins of
the Bahama Banks. This combination of linear, arcuate,
and spur-and-grove ridges are the substrate for modern
reef growth, as well as vast areas of coral-encrusted hard
grounds.
Environmental parameters that impact the reefs most
directly include climate and hydrology. Bahaman climate
ranges from humid subtropical with dry winters in the
NW (18–28
C monthly average temperature, 73–79%
humidity, 135.5 cm annual rainfall on Grand Bahama
Island) to subarid tropical in the SE (25–34
C, 18–25%,
60.4 cm/year, Caicos). About half of the area lies within
the tropics; the Tropic of Cancer bisects Little Exuma
Island. Sea-surface temperatures range from 21.7 to
28.3
C annually on the Andros lobe of GBB (Cloud,
1962) compared to 26–29
C on Caicos Bank.
The Bahamas lie in the northeasterly trade winds belt,
but continental low pressures over North America produce
predominately southeasterly winds during the warmer
months (Smith, 1940). Continental cold fronts occasionally impinge on the northwestern Bahamas in winter, producing strong NW winds and cooling the bank water with
air temperatures down to 3
C. Hurricanes struck the Bahamas–Turks area 121 times from 1901 to 1963 (compiled
from Cry, 1965), an average of two per year. Twenty-one
of these storms passed directly over Andros Island. The
damage to reefs from wave pressure and sediment abrasion can be severe, although it is quite variable, depending
on storm intensity, direction, duration, and frequency.
The Bahamas are bathed by the north equatorial current
that bifurcates to produce currents of 30–42 cm/s along
the eastern islands and 46 cm/s in Old Bahama Channel
to the south (Carew and Mylroie, 1997). The Gulf Stream
skirts the western Bahamas with velocities up to 200 cm/s
(Bergman et al., 2010). The range of semidiurnal tides is
about 1 m at the platform margins everywhere in Bahamas. Resonance in the deep embayments, such as TOTO,
can amplify the tides and produce strong currents at the
margins, generating ooid shoals instead of reefs (Ball,
1967). Water at the bank margins has normal marine salinities, about 36 ppt, but sluggish circulation on the larger
banks, where residence time can reach 240 days, produces
salinities reaching 43 ppt in the hotter months (Broecker
and Takahashi, 1966).
Reef distribution
Bank/barrier reefs in the Bahamas occur almost exclusively on the windward (eastern) sides of banks. They
are best developed on margins facing the open Atlantic
swell (Rankey et al., 2009) and where large islands provide protection from the flux of bank-top water. The fluctuations of bank water in temperature, salinity, nutrients,
and turbidity are detrimental to most corals, although
patch reefs thrive on some areas of the banks. Lee margins,
where wave energy is minimal and wind-driven flux of
bank water is maximum, have few reefs that are small
and deeper.
Reefs occur intermittently for 160 km along the northeast-facing margins of LBB, from beyond Walker’s Cay
at the north to Elbow Cay at the middle of Great Abaco
Island. The narrow, steeply sloping shelf further south,
which faces ESE, has few reefs (Feingold et al., 2003;
Rankey et al., 2009). Reefs on the LBB extend 220 km further north than Florida reefs despite the partiality of the
Gulf Stream to Florida. Coral cover averaged only 14%
86
BAHAMAS
are formed.
Tectonic and stratigraphic studies have established that
up to 10 km of largely shallow-water carbonates, Jurassic
to Holocene in age, overlie rifted Jurassic transitional crust
in the NW Bahamas and oceanic crust SE of Tongue of the
Ocean (TOTO, Sheridan et al., 1988), although others
have argued for African continental crust (Mullins and
Lynts, 1977). A carbonate “megabank” developed in the
Early Cretaceous that may have included the Bahamas,
Florida, and Yucatan (Austin and Schlager, 1988). This
bank was fragmented into smaller platforms separated by
subsiding blocks in the mid-Cretaceous. The southern part
was further fragmented during collision with Cuba in Late
Cretaceous–Middle Eocene (Masaferro and Eberli, 1999).
Seismic stratigraphy shows that leeward lateral accretion
of sediment derived from the bank tops has filled such
depressions, merging platform fragments during the
Cenozoic to form the GBB (Eberli and Ginsburg, 1987,
1989; Masaferro and Eberli, 1999). Deep channels (Straits
of Florida, Santaren Channel, Old Bahamas Channel,
Providence Channels) and reentrants (TOTO, Exuma
Sound) remain. These dissect the banks and isolate them
from terrigenous input except for wind-blown dust that
reddens the soils.
Coral reefs were common around the margins of the
Bahama Banks at least as far back as the Pliocene (Beach
and Ginsburg, 1980). Pleistocene reefs are exposed on
most of the major islands (Cant, 1977), some extending
to 5 m above present sea level (Hearty, 1998). Lithified
Pleistocene eolianite dune ridges form the bulk of the
Bahaman islands, however, reaching 63-m elevation on
Cat Island (Government of the Bahamas). Subtidal Pleistocene deposits extend up to 6 m above sea level on the
large islands. Multiple Holocene beach-accretion ridges,
anchored by Pleistocene buttresses, form many low-lying
areas. Origins from dunes, beaches, and reefs dictate that
the largest islands (Andros, Eleuthera, Great Abaco) and
highest (Cat Island) lie at the eastern margins of the banks,
where tides, trade winds, and wave fetch maximize skeletal sediment production (Ball, 1967).
Most islands have multiple nested rows of Pleistocene
dunes, generally younging toward the windward margin.
This is well illustrated on New Providence and adjacent
islands where six prominent ridges are visible on topographic maps (Hearty and Kindler, 1997). Strings of small
cays formed by eroded dunes near the shelf break are common around the Bahamas, even on the shallower leeward
margins, e.g., from Bimini south beyond Cat Cays. Similarly, the eroded remnants of dunes, beaches, and reefs
dominate the submarine topography at the margins of
the Bahama Banks. This combination of linear, arcuate,
and spur-and-grove ridges are the substrate for modern
reef growth, as well as vast areas of coral-encrusted hard
grounds.
Environmental parameters that impact the reefs most
directly include climate and hydrology. Bahaman climate
ranges from humid subtropical with dry winters in the
NW (18–28
C monthly average temperature, 73–79%
humidity, 135.5 cm annual rainfall on Grand Bahama
Island) to subarid tropical in the SE (25–34
C, 18–25%,
60.4 cm/year, Caicos). About half of the area lies within
the tropics; the Tropic of Cancer bisects Little Exuma
Island. Sea-surface temperatures range from 21.7 to
28.3
C annually on the Andros lobe of GBB (Cloud,
1962) compared to 26–29
C on Caicos Bank.
The Bahamas lie in the northeasterly trade winds belt,
but continental low pressures over North America produce
predominately southeasterly winds during the warmer
months (Smith, 1940). Continental cold fronts occasionally impinge on the northwestern Bahamas in winter, producing strong NW winds and cooling the bank water with
air temperatures down to 3
C. Hurricanes struck the Bahamas–Turks area 121 times from 1901 to 1963 (compiled
from Cry, 1965), an average of two per year. Twenty-one
of these storms passed directly over Andros Island. The
damage to reefs from wave pressure and sediment abrasion can be severe, although it is quite variable, depending
on storm intensity, direction, duration, and frequency.
The Bahamas are bathed by the north equatorial current
that bifurcates to produce currents of 30–42 cm/s along
the eastern islands and 46 cm/s in Old Bahama Channel
to the south (Carew and Mylroie, 1997). The Gulf Stream
skirts the western Bahamas with velocities up to 200 cm/s
(Bergman et al., 2010). The range of semidiurnal tides is
about 1 m at the platform margins everywhere in Bahamas. Resonance in the deep embayments, such as TOTO,
can amplify the tides and produce strong currents at the
margins, generating ooid shoals instead of reefs (Ball,
1967). Water at the bank margins has normal marine salinities, about 36 ppt, but sluggish circulation on the larger
banks, where residence time can reach 240 days, produces
salinities reaching 43 ppt in the hotter months (Broecker
and Takahashi, 1966).
Reef distribution
Bank/barrier reefs in the Bahamas occur almost exclusively on the windward (eastern) sides of banks. They
are best developed on margins facing the open Atlantic
swell (Rankey et al., 2009) and where large islands provide protection from the flux of bank-top water. The fluctuations of bank water in temperature, salinity, nutrients,
and turbidity are detrimental to most corals, although
patch reefs thrive on some areas of the banks. Lee margins,
where wave energy is minimal and wind-driven flux of
bank water is maximum, have few reefs that are small
and deeper.
Reefs occur intermittently for 160 km along the northeast-facing margins of LBB, from beyond Walker’s Cay
at the north to Elbow Cay at the middle of Great Abaco
Island. The narrow, steeply sloping shelf further south,
which faces ESE, has few reefs (Feingold et al., 2003;
Rankey et al., 2009). Reefs on the LBB extend 220 km further north than Florida reefs despite the partiality of the
Gulf Stream to Florida. Coral cover averaged only 14%
86
BAHAMAS