Geology Reference
In-Depth Information
Isolated longitudinal sand bars
are individual
elongate, narrow bars oriented normal to the shelf
margin (Fig.
20.9b
) that occur apparently “in the middle
of nowhere.” Several of these occur in the Bahamian
Archipelago, and include Ambergris, Mackie, and
Green Cay shoals. These features are the least well-
understood barforms, in large part due to the fact that
they occur away from easily-accessible islands. These
elongate barforms can reach up to 30 km long, have
current- or wave-agitated crests <1 km wide (narrower
in many places), and can have several meters total
relief. Unlike longitudinal tidal sand ridges, these iso-
lated forms have no fl anking, fl ow focusing channels,
and for much of their extent have straight crests with
no superimposed parabolic forms or switchbacks. The
crests are ornamented with low relief subaqueous
dunes of various orientations, and slope gradually to
waters 4-5 m deep. Of these bars, the sedimentology
has been studied in detail only on Ambergris shoal on
the Caicos platform (Rankey et al.
2008
) . There, the
bar crest includes well- to very-well-sorted, coarse
ooid sands; the fl anks include less well-sorted and less
ooid-rich sands, with discontinuous hardgrounds.
sandy substrate in the form of subaqueous dunes and
little to no seagrass. In a few channels in the Exuma
Island chain, subtidal columnar stromatolites are found
in tidal inlets (Dill et al.
1986
) .
Holocene islands,
another associated type of deposit,
form on the crests of several sand shoals. Many of these
islands have well-developed sandy, oolitic beaches,
include a succession of beach ridges and eolian dunes,
and are lithifi ed by early meteoric cementation (Halley
and Harris
1979
; Budd
1984
). In some cases, shoreline
spit-like geometries suggest their genesis may initiate
longshore transport, producing a feedback mechanism
allowing the islands to grow.
Platform interior
sediments occur bankward of
many shoals, in lower energy environments. These
regions generally have a fl at bottom, with no active
physical sedimentary structures (subaqueous dunes or
bars). In some areas, such as on Little Bahama Bank
platformward of Lily Bank and on Great Bahama Bank
platformward of Joulter Cays, sediments are mud
dominated. In contrast, grain-dominated fi ne to coarse
sands occur in the interior of the Berry Islands area, on
Crooked-Acklins Platform, platformward of Fish
Cays, and on the platform west of the Exuma Islands.
Nonetheless, although the character of these sediments
is highly variable, these areas are almost always bur-
rowed, and often (although not always) are partly sta-
bilized by seagrass or calcareous algae; cementation
can occur locally (Taft et al.
1968
) . These attributes,
along with infrequent ripples, illustrate that these
regions are less frequently agitated and that the rate of
biological reworking is greater than the rate of physi-
cal reworking.
20.3.5 Associated Geomorphic Elements
The occurrence and distribution of
channels
between
bars is implicit in the preceding discussion, but these
merit some discussion of their attributes as well.
Channels can be of variable width, depth and orienta-
tion, and some reach as deep as the Pleistocene bed-
rock, which forms a hard substrate resistant to erosion.
The sedimentology of the channel fl oors is dictated by
the speeds of the currents passing through them. Many
channels passing through inlets between rocky islands
(either of Pleistocene bedrock or lithifi ed Holocene
sands) have rocky (Pleistocene) fl oors with only
patchy, rubbly sediments and local patch reefs that
pass into bare sands with ripples or small dunes away
from the restricted opening. Where channel fl oors are
mostly seagrass-covered, bioturbated muddy sands,
the current velocities rarely exceed 0.5 m/s, and the
margins are commonly composed of erodible sedi-
ments instead of the rocky inlet margins. In channels
where currents are suffi cient for active sediment trans-
port (e.g., in the oceanward parts of Joulter Cays tidal
channels; amongst parabolic bars of eastern Tongue
of the Ocean), however, the channel fl oors consist of
20.3.6 Examples of Shoal Complexes
Although they all occur in shallow, agitated environ-
ments, the tidal sand shoals of the Bahamas and Caicos
include a wide variety of sizes and shapes, collectively
formed by complex associations of individual bedforms
and bars. The variability is related to historical or spatial
contingencies, such as bedrock topography and the pres-
ence of ridges or islands, age of the shoal, orientation
relative to predominant winds and waves, and shelf-
break curvature. This section explores how different
bedforms and bars combine to create vastly different shoal
systems by discussing the details of a few examples of
tidal sand shoals from the Bahamian Archipelago.
