Geology Reference
In-Depth Information
Fig. 19.1
General overview of the Bahamian Archipelago. The
locations of some of the largest tidal fl ats are schematically
highlighted by the
red
areas, and areas mentioned in the text are
labeled.
Dashed white lines
represent lines of equal annual
rainfall (labels in mm) (Data from Pierson 1982). Bathymetric
data from General Bathymetric Chart of the Oceans (GEBCO)
of the Intergovernmental Oceanographic Commission (IOC)
and the International Hydrographic Commission (IHC)
Contours are illustrated as
solid lines
for 0, 500, 1,000, and
1,500 m; other depths are shaded
Subtidal regions are those which are more-or-less
permanently submerged, and in the Bahamas include
nearshore marine regions, ponds, and tidal channels
(Fig.
19.3
). The nearshore marine regions may be
sparsely covered with seagrasses (
Thalassia
and
Syringodium
) and calcareous algae (Fig.
19.3a, b
), and
include grey peloidal lime mud with scattered skeletal
fragments, including miliolid and soritid foraminifera
and mollusks. These sediments are commonly exten-
sively bioturbated by the ghost shrimp
Callianassa
(Fig.
19.3a
). On the Bahamian platforms and Caicos
platform, these shallow marine settings bounding the
tidal fl at complexes extend kilometers offshore. Within
the Andros tidal-fl at complexes, ponds are ubiquitous
(Fig.
19.3c
), bounded by creek levees, beach ridges,
palm hammocks, or intertidal fl ats. They are much less
common on Caicos and Crooked-Acklins tidal fl ats
further south. On Andros, the ponds are completely
drained only during periods of prolonged offshore
winds, thus they are essentially subtidal. Pond sedi-
ments are yellow-brown peloidal mud with scattered
foraminifera and gastropods (Fig.
19.3c
) and, being
extensively burrowed, form deposits similar to those
offshore from the tidal fl at, but with lower biodiversity.
In contrast, deposits of subtidal tidal channels
(Fig.
19.3d, e
) can include a coarse basal lag of
cemented crust and fragments of Pleistocene bedrock
(generally only the lowest few cm), gastropods and
foraminifera, and intraclasts of indurated laminated
levee deposits. In more outboard areas with wider tidal
creeks, the creeks can exceed 2 m depth, and expose
the top-Pleistocene bedrock. Creeks generally shallow
and narrow landward, however, and inland creeks can
simply be subtle, muddy depressions extending into
ponds (Fig.
19.3f
).
Intertidal
areas lie between mean low tide and mean
high tide, and are fl ooded and exposed twice daily, in
the absence of persistent onshore or offshore winds. In
the Bahamas, intertidal regions support mangrove and
microbial marshes (“algal marshes” of older literature)
between the subtidal open ponds and the supratidal
levees fl anking the creeks. In general, low intertidal
