Geology Reference
In-Depth Information
by distributary systems (small birdfoot deltas) within
ponds at the end of some creeks (Fig.
19.7c
) illustrat-
ing appreciable landward sediment delivery. Similarly,
in the southern Three Creeks area where there are no
active creeks, ponds are much more extensive. Since
no sediment is being delivered into these areas and sea
level continues to rise (Rankey and Morgan
2002
) , the
ponds here areas are expanding (Fig.
19.7d, e
).
The channeled belt is fl anked on the oceanward side
by grainy beach ridges at the shoreline (Fig.
19.7b
).
The beach ridges and spillovers are generally less than
100 m wide, but reach up to ~250 m inboard in a few
exceptional cases, and include either fi ne, laminated
peloid-skeletal sand or coarse skeletal debris. Locally,
intraclasts of crusts are abundant.
The Three Creeks area is changing on historical
time scales. For example, Rankey and Morgan (
2002
)
documented shoreline erosion at rates of just under
1 m/year with varied geomorphic signatures: (1) pond
and levee backslope sediments are exposed in the
intertidal zone seaward of the beach ridge in many
areas, (2) stumps of
Casurina,
a freshwater conifer
living only on beach ridges, are also present in the
intertidal zone; and (3) beach-ridge spillover lobes
cover or block creeks (Fig.
19.7b
,
circle), illustrating
landward encroachment. The cause of the change from
aggradational or progradational (representing the
accumulation of the tidal-fl at record) to the present
erosional state is not well understood. Wright (
1984
)
suggested changes in sediment supply could have
infl uenced this change, but this hypothesis has remained
untested, as has the suggestion that it may be related
to a relative rise in sea level (Rankey and Morgan
2002
). At a longer time scale, the tidal fl ats have been
stepping landward; nearshore marsh deposits are found
beneath channeled belt sediments and, locally, several
100 meters offshore (see below; Shinn et al.
1969
; Shinn
1986
). Obviously, at some point the tidal fl at built sea-
ward or upward, only to undergo erosion again.
Island to the Bights (Fig.
19.8
), and beyond. This area
includes both erosional (northwest-facing) and progra-
dational (southwest-facing) shorelines, with the transi-
tion between each corresponding with a marked change
in shoreline orientation. Because these tidal fl ats have
such distinct geomorphic characteristics, they are
described separately.
Southwest-facing margin
. Offshore of the southwest-
facing Andros tidal fl ats is a broad expanse of shallow
subtidal, muddy sediments that are <1 m deep at low
tide and exhibit an extremely low bathymetric gradient
(Queen
1976
). The shoreline is straight along much of
its length along this margin, and is penetrated by only
a few broad, shallow tidal creeks. The present shore-
line has no grainy beach, but at least in the western
area, instead consists of a low scarp (Fig.
19.6a
), sug-
gesting that it presently is erosional.
Tidal channels extending into the tidal fl at are rel-
atively rare, with only fi ve channels along the 45 km
coast (Fig.
19.8a
). These channels, although much
less common than in the Three Creeks area, are much
larger, reaching nearly 3 km wide, but are less than a
few metres deep. Most do not branch for much of
their extent, but instead are wide, straight, and shal-
low. Creeks in the southeasternmost part of the tidal-
fl at complex (Fig.
19.8b
) pass from a classic
estuarine-like V-shape to narrower creeks with
branching distributary systems. and include stabilized
and vegetated levees (Fig.
19.8b
), unlike those in the
Three Creeks region.
Along much of the coast, landward of the present
shoreline, a broad supratidal plain gently passes north-
easterly into mangrove ponds and open freshwater
ponds (Fig.
19.8a
). The supratidal plain here is wide
and fl at, and covered by a thin blackened crust, scat-
tered microbial mats, and mangroves. It is thus broadly
similar to the inland algal marsh in the Three Creeks
area, but with fewer
Scytonema
pincushions and more
expansive crusts. Further inboard, however, ponds
become more common, and are the dominant landscape
element. These ponds are circular to highly irregular, can
form pond networks ranging in size up to 6 km across,
and are mostly shallow, but can be up to several meters
deep (Fig.
19.8a, c
). Networks of ponds may collec-
tively form larger bodies of water several kilometers
across, broken only where they abut hammocks. The
arcuate margins of some ponds appear to be infl uenced
by spit accretion, possibly controlled by winds.
19.3.2.2 West Andros Island, Bahamas
Relative to the tidal fl ats of the Three Creeks area,
those of west Andros Island are less well studied. This
broad area forms the westernmost tip of Andros Island,
extending up to 25 km outboard from patchily-exposed
Pleistocene bedrock. This area rims western Andros,
from ~25 km northeast of Williams Island (an ero-
sional vestage of the tidal fl at; Gebelein
1974
) , around
the tip, and >45 km along shore from near Williams
