Geology Reference
In-Depth Information
or on the beaches of the islands that occur on the eastern,
windward margins of the platforms. As a result of these
factors, waves in the platform interiors are small, with
most signifi cant wave heights less than 1 m (Reeder
and Rankey
2009a
; Rankey and Reeder
2010
) .
Tidal currents, however, can locally infl uence the
sediment distribution on these fl at-top platforms.
Although the platforms are microtidal (spring tidal
amplitude of approximately 1 m near shelf margins
throughout the area), the tidal exchange fl oods and
drains large platform areas, and in many locations cur-
rents are focused through localized channels. Whereas
current speeds in sandy siliciclastic channels reach a
dynamic equilibrium point at 1 m/s, within these chan-
nels and surrounding shoals, the current speeds can
exceed 1 m/s or even 2 m/s, especially where bedrock
islands occur (Gonzalez and Eberli
1997
; Reeder and
Rankey
2009b
). On the platform interior, however, the
tidal currents are much lower, rarely exceeding
0.50 m/s (Smith
1995
; Rankey and Reeder
2010
).
Many tidal sand shoals of the Bahamas are domi-
nated by ooids, so much of this chapter focuses on
these grains and their oolitic shoals. Because oolitic
laminations are mainly physiochemical precipitates,
the occurrence of oolitic tidal sand shoals in the
Bahamas is infl uenced by the geochemical characteris-
tics of Bahamian seawater. Recent global compilations
(Royal Society
2005
; Lee et al.
2006
) illustrate an ele-
vated carbonate saturation state and pH of waters in
this part of the Atlantic, factors that would favor abi-
otic precipitation of aragonitic ooids, given the right
hydrodynamic conditions (Rankey and Reeder
2009,
2010
; discussed below).
eastern and northern fl anks of the platforms form the
largest islands (Abaco, Andros, Eleuthera, Long,
Crooked, Acklins, Caicos chain) (Beach and Ginsburg
1980
; Hearty and Kindler
1993
; Aurell et al.
1995
) .
Many windward islands are bordered oceanward by
reefs and reef-derived sands. Tidal fl ats fl ank the lee-
ward sides of many islands, towards the platform inte-
rior (Rankey and Berkeley, this volume), and pass
laterally into shallow subtidal settings.
Across much of the archipelago, the best developed
and thickest tidal sands occur at or near bank margins,
where there is an open exchange between the platform
interior and the surrounding deep basins (Hine et al.
1981
). Areas with broad expanses of tidal sands
(Fig.
20.1
, red patches) can be found on windward
margins (Joulter Cays, Exuma tidal deltas), at the end
of deep-water embayments into the platform (Tongue
of the Ocean, Schooner Cays), leeward margins
(Cat Cay, Fish Cays, Berry Islands), and on margins
oblique to the dominant easterlies (Lily Bank, Abaco
tidal deltas) (e.g., Hine et al.
1981
) . In general terms,
shoals are most probable in concavities extending into
the platform, and the distribution of tidal sands sug-
gests that margin orientation relative to the dominant
winds is not the sole factor controlling where tidal
sands occur. Instead, although the region is microtidal,
the strong reversing tidal currents near the margins
facilitate the development of expansive tidal sand
bodies (Ball
1967
; Reeder and Rankey
2008
) . Shoal
areas generally lack well-developed reefs outboard,
due to inimical waters of elevated salinity or tempera-
ture fl owing off the banks during ebb tide (Newell
et al.
1959
; Neumann and Macintyre
1985
; Ginsburg
and Shinn
1993
), and many sediments are oolitic
(Illing
1954
; Newell et al.
1960
) , although there can
be a pronounced skeletal or peloidal component (Hine
et al.
1981
) (see next section).
Although why they occur where they do is generally
well known, the controls on the considerable variability
in morphology of ooid shoals are less understood. For
example, the complicated geometries present in shoals
have led to summative statements such as “the form
of the oolite shoals is less subject to generalization
than the distribution of these deposits” (Purdy
1961
,
p. 54-55), and “each marginal trend is unique” (Hine
et al.
1981
, p. 286). Some have even gone so far as to
suggest that the variability refl ects a “marginal-facies
mosaic” characterized by “facies complexities” (Hine
and Neumann
1977
p. 376-377).
20.3
Tidal Sands of the Bahamas
and Caicos
20.3.1 Setting and Distribution
The modern sedimentologic patterns on the tops of the
shallow platforms are the net result of complex inter-
actions among physical, biological, and chemical pro-
cesses, all acting during the late Holocene sea-level
rise within the geographic framework established by
Pleistocene bedrock (Newell and Rigby
1957
; Purdy
1963
; Enos
1974
; Hine
1977
; Hine and Neumann
1977
; Wanless et al.
1989
). In general terms, highs of
Pleistocene reefs, marine sands, and eolianites on the
