Geology Reference
In-Depth Information
Wind related to tropical depressions, cold fronts (in
the Bahamian archipelago), or shamals (in the Arabian
Gulf) can also impact tidal-fl at sedimentation by infl u-
encing both suspended-sediment concentrations, and
patterns of water movement across the fl ats. For
example, tropical depressions commonly stir up and
suspend muddy offshore sediments, and storm surges
can fl ood the tidal fl at with sediment-laden water,
which then settles as the storm passes (Shinn et al.
1969
; Hardie
1977
; Wanless et al.
1988a,
b
) . Numerous
workers have considered these to be the most impor-
tant physical process impacting tidal-fl at sedimenta-
tion (e.g. Hardie
1977
; Shinn
1986
; Gebelein
1975
) .
These sedimentologic interpretations notwithstand-
ing, relatively few direct observations of the impacts
of storms on carbonate tidal fl ats have been published
(Ball et al.
1967
; Perkins and Enos
1968
; Wanless
et al.
1988a
; Rankey et al.
2004
) , but the limited
impact observed from some tropical depressions sug-
gest that the tide stage and forward velocity of storms
at landfall strongly infl uence the impacts that these
events have on the tidal fl ats.
Persistent onshore cold-front wind, although less
destructive, can result in the fl ooding of much of the
Bahamian tidal fl ats for days. Conversely, strong off-
shore wind can cause continuous tidal-fl at drainage
(e.g. Hardie and Garrett
1977
) . Sustained wind so
strongly modulate the tide in shallow, more restricted
parts of the Bahamas, that these fl ats might be consid-
ered “wind tidal fl ats” rather than tidal fl ats.
Wind also infl uences the Arabian Gulf systems.
Here, strong, northerly, shamal wind drives southward
longshore transport on the Qatari shoreline (Shinn
1973a,
b
). This wind also pushes water onshore in
southern Gulf (UAE), fl ooding the sabkha with a meter
or more of water (Schneider
1975
) .
Although it does not directly infl uence where they
occur, the climatic setting of tidal fl ats infl uences biota,
salinity, early fl uid fl ow, and diagenetic features.
Climate is therefore an important factor infl uencing the
types of sediments, their susceptibility to erosion, and
early diagenesis on tidal fl ats. For example, on the tidal
fl ats of Andros Island with mean annual rainfall
>120 cm, tidal ponds and algal marshes are abundant
and evaporites are ephemeral to absent. In the Arabian
Gulf, with annual rainfall < 10 cm, tidal ponds and
creeks are essentially absent, algal marshes are very
limited in extent, gypsum and anhydrite are common
depositional and diagenetic features, bioturbation of
intertidal sediment is less intense than in the Bahamas,
and eolian transport provides ample sediment. Although
less marked than these contrasts, subtle facies changes
from the northern Bahamas to the southern Bahamas
and Caicos islands (discussed below) have been inter-
preted to refl ect the southward increase in aridity.
19.3
Humid Tidal Flats of the Bahamian
Archipelago
Most understanding of Holocene humid carbonate
tidal fl ats has come from the study of those in the
Bahamian Archipelago, reaching from Little Bahama
Bank in the north to Caicos platform in the south
(Fig.
19.1
). The most expansive tidal fl ats here fall on
the platformward side of larger islands (Great Abaco,
Andros, Grand Bahama, Crooked, Acklins, North,
Middle and East Caicos), although narrower or less
areally expansive tidal fl ats occur on parts of other
islands as well (e.g. Long Island, San Salvador).
Precipitation in this region decreases from ~150 cm/
year in the northern Bahamas to ~75 cm/year on
Caicos, and a majority falls in the wet season between
May and October (northern Bahamas) and September
and December (Caicos).
19.3.1 Zonation and Subenvironments
Carbonate tidal fl ats of the Bahamas and Caicos
include spatially complex patchworks of subtidal,
intertidal, and supratidal zones (Shinn et al.
1969
;
Hardie
1977
; Shinn
1986
; Wanless et al.
1989
; Rankey
2002
), their characteristics being generally related to
elevation relative to mean tidal level. In a classic study
of the Three Creeks area of Andros Island, Ginsburg
et al. (1977) developed the “exposure index,” a quanti-
tative measure of the percentage of time that given
elevations are exposed, which is closely related to the
ecologic and sedimentologic characteristics across the
tidal fl at (Fig.
19.2
). Although the details of the expo-
sure index - subenvironment relationship varies some-
what with climate, sediment supply, and shoreline
orientation along the archipelago (Wanless et al.
1989
) ,
broadly similar subenvironments, each with generally
distinct associations of biota and sedimentary struc-
tures (Fig.
19.2
), occur in tidal fl ats across the region;
as such, they are discussed together here.
