GEO_PLATE_C-16.HTML
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Plate C-16
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Map
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The Great Bahama Bank is a pile of coral reef limestone
at least 4500 m thick that has been accumulating since
Cretaceous time. The entire pile was deposited in warm
shallow water, so the foundation crust must have been slowly
subsiding at an average rate of at least 3.6 cm per 1000 years
(Newell, 1955). The crustal block basement underneath the
Bank is continental in structure and may have rifted from North
America when that continent began to break off from Africa
and the Atlantic Ocean originated.
The Bahama Bank is now separated from the mainland by
the Straits of Florida, which reach depths of 760 m. The eastern
edge of the Gulf Stream sweeps northward past the western
margins of the Bank at a speed of 3 km per hour, about half as
fast as the main current near Miami Beach. On the Atlantic Ocean
side, the platform drops steeply to depths exceeding 4000
m just 20 km to the east. Depths within the bank proper are seldom
greater than 6 meters.
Most bedrock under Great Bahama Bank is coral-reef
limestone, as indicated by a deep well on Andros Island. The nearly
vertical submarine cliff that forms the west bank edge must be built
of Tertiary-age corals. It has the same form as the ribbon reefs
of the Australian Great Barrier Reef (Plate C-18) or mid-oceanic
atolls such as Jaluit atoll (Plate C-19). The west cliff descends abruptly
to depths of 30 to 60 m, then less rapidly to the floor of the Straits of
Florida. The loose shifting sands of the present Bahama Bank could
not maintain such steep gradients.
Various species of coral live on Great Bahama Bank, as
they do in the nearshore zone of Florida, but their growth is
so slow that they do not build significant reefs. Since the
Pleistocene, this part of the Atlantic has been in the
"marginal" zone of coral growth, where
winter temperatures inhibit healthy coral growth. A more
important limiting factor on the Bank is turbidity. No suspended
detrital sediment from the continent reaches the Bahamas,
accounting for the clear water in the Straits of Florida. However,
the warm Gulf Stream surface water is supersaturated (by 40
percent) with dissolved calcium carbonate, and when the water
washes up and over the bank rim, it is warmed. Agitation and
evaporation cause massive chemical precipitation of a cloud of
aragonite crystals. These accrete concentrically on nuclei of shell
or coral fragments, growing into oolites, which are sand-sized
pellets with a layered structure similar to hailstones. The clouds of
limy precipitate and shoals of oolitic sand effectively inhibit coral
growth today.
Most of this oolitic sand originates within a few hundred
meters of the bank margins, but currents distribute the sand into
the interior of the bank. The small islands and cays are made up
mainly of biogenic limy sand and mud derived from foraminifera,
corals, mollusks, and calcareous plants (Illing, 1954; Newell,
1955).
During glacial ages, when sea level was about 120 m lower
than at present, the Bahama Banks would have been great emerged
flat-topped limestone plateau islands. Their surfaces were
riddled with karst sinkholes and the oolitic sand was blown into
giant dunes of
eolianite
sand that has now recemented. As sea level
rose in Late Holocene time, the karst sinkholes have flooded (known
as "blue holes"), and the islands such as North and
South Bimini are the tops of Pleistocene eolianite dunes.
The Plate is an enlarged scene of the Bimini Islands at the
northwest corner of the Great Bahama Bank (South Bimini is
83 km due east of Miami; see Figure C-15.1) taken
from a Landsat-4 image made by the Thematic Mapper.
Although this false-color image (Bands 1, 2, 4) was
generated to emphasize shallow water and submerged shoals
in realistic blue shades, the vegetation on the islands is in
characteristic reds. Sandy Cay, an emerged barrier sand patch
in lower center of Plate image, takes on the bright white color
of oolitic sand.
Figure
C-16.1
, a Landsat MSS image, depicts the largest Bahama
Island, Andros, with its mangrove swamps. Sand sedimentation
extends up to 100 km westward there, but the waters just to the
east deepen to 2400 m in the Tongue of the Ocean, a channel that
divides Andros from Nassau (upper right).
Figure C-16.2
is an Apollo camera view
of Eleuthera Island, with its shoals to the west, the Atlantic on the ,
and another deep-water channel (Exuma Sound) to the south.
Figure C-16.3
is an
view of a typical coral island (Stocking Island) in the Bahamas.
Landsat TM 40182-15125, January 18, 1983.
Continue to Plate C-17
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