Geology Reference
In-Depth Information
Fig. 20.13
Remote sensing images of Tongue of the Ocean
shoal complex, Great Bahama Bank. (
a
) Overview of the arcuate
trend ringing the deeper water Tongue of the Ocean. Image
acquired 1/29/1985. (
b
) Image from the eastern part of the com-
plex, with well-developed parabolic bars. (
c
) Close up of part of
the eastern part, illustrating the complex patterns of parabolic
bars and subaqueous dunes in this area. Image acquired 10/21/02.
Image copyright GeoEye.com. (
d
) Close up of the southern mar-
gin, with numerous longitudinal tidal sand ridges
The possible role of winds on the generation of ooid
shoals has been highlighted in studies of the
Ambergris
shoal
on Caicos Platform south of the Bahamas (Wanless
et al.
1989
; Wanless and Tedesco
1993
; Rankey et al.
2008
). This 20 km long isolated longitudinal sand bar-
form is oriented roughly parallel to the predominant
trade winds. The shoal includes coarse sand-sized ooids
and low amplitude (generally < 0.5 m) subaqueous dunes
on its crest, which can be exposed at low tide. The crest
is immediately fl anked by a rocky hardground up to
300 m wide with a slightly deeper (3 m) wave-rippled
bottom to the north and a 2-4 m deep fl at, intraclast-rich
bottom to the south, before passing into burrowed deeper
(>4 m) platform interior sands. Wanless et al. (
1989
)
and Wanless and Tedesco (
1993
) emphasized the role
of “agitation by wind-generated waves and the cross-
platform current” on the morphology and sedimentol-
ogy of the Ambergris shoal complex, but Rankey et al.
(
2008
) pointed out that many of the geometries of super-
imposed barforms suggested a “more complicated situ-
ation,” infl uenced in part by tides. Similarly, in a more
general sense, the geometry of other isolated longitudi-
nal sand bars, including Mackie Shoal (long axis normal
to predominant winds) and Green Cay shoal (shoal in
the stoss of an island), suggest that wind-driven waves
may not be a fundamental control on the geometry of
these systems. Yet, to date, there have been no measure-
ments of waves or tides to test these alternative concepts,
and these represent the least well-understood of all shoal
complexes.
