Geology Reference
In-Depth Information
islands, which highly restrict the possible fl ow patterns.
However, tidal sand shoals can also form in less-
restricted settings, such as the
tidal deltas of the Abaco
Islands
. In this area of ~50 km
2
along the northwestern
Abaco island chain, ten inlets with associated fl ood
and ebb tidal deltas of various sizes occur (Fig.
20.11
)
(Reeder and Rankey
2009b
). Here, the sandy deltas are
almost exclusively parabolic bars forming around
inlets in the island chain. The parabolic bars terminate
at the edges of the islands and are convex outwards
from the main channels which extend perpendicularly
to the inlet opening. In some cases, a multi-inlet system
creates complex shoal geometries by combining multiple
parabolic bars into one.
The crests of the deltas, some of which may be
exposed at the lowest tides, are largely bare oolitic
sands (>75%) covered with ripples and subaqueous
dunes. The inner lobes of the deltas are located between
the islands and the sandy crests; they are stabilized by
dense seagrass and may contain patch reefs (ebb del-
tas). The sediments of the inner lobes are bioturbated
peloidal-skeletal sand and mud with few ooids (<1%).
The main channels passing through the inlets consist
of a hard bottom including some sponges and corals,
but no loose sediments.
The sedimentology of the tidal deltas is closely
linked to the hydrodynamics of the region (Reeder and
Rankey
2009b
). The main channels experience tidal
velocities >1.5 m/s, explaining the lack of loose sedi-
ments. They also form part of a mutually evasive fl ow
pattern, whereby the main channel of the fl ood lobe
experiences high velocities during fl ood tide, but the
ebb tides are focused down marginal channels fl anking
the lobe (broadly comparable to the model of Hayes
1975
). The presence of the shoal also increases veloci-
ties over the shoal, allowing adequate agitation on the
bar. This sets up a 'spin cycle' on the shoal, leading to
the agitation, cortex formation, and transport of ooids
on the shoal itself, but a lack of ooids off the shoal. As
the velocities pass through the inlets or over the shoal,
the fl ow expands outward, so the velocities in the
deeper, more peloidal, seagrass stabilized regions of
the inner lobes and outside of the tidal deltas are much
lower (~0.25 m/s).
The size of the deltas varies systematically with
the size of the inlet opening. Each inlet experiences a
different volume of water exchange between low and
high tides (tidal prism), and in each delta, the distance
to the delta apex is closely related to the tidal prism
Fig. 20.10
Remote sensing image of Double Breasted Cays,
Abacos, Bahamas, and schematic patterns of dominant path of
ebb (
blue
) and fl ood (
red
) tides (Modifi ed from Reeder and
Rankey (
2008
) )
In some carbonate tidal systems, pre-existing
topography shaped by Pleistocene bedrock highs
determines the location of shoals and markedly infl u-
ences their sedimentology and hydrodynamics. For
example, the
Double Breasted Cays sand shoal
, in the
Abaco island chain, occurs between two parallel bed-
rock highs. In this area, two channels fl anking the
Pleistocene island ridges set up a mutually evasive
fl ow pathway whereby the fl ood tidal currents are
stronger in the southern channel while the ebb tidal
currents are focused in the northern channel. This tidal
channel confi guration sets up a net circular fl ow pattern
around the central oolitic shoal, a dynamic termed the
'spin cycle' by Reeder and Rankey (
2008
) (Fig.
20.10
).
In this carbonate system, the fl ow pattern establishes a
situation in which the sediment can be agitated and
transported to permit the growth of oolitic cortices,
without the particles being removed from the geomor-
phic system. Reeder and Rankey (
2008
) suggested
that this general process was an important element for
the generation of ooids in the tidal-dominated settings
of the Bahamas.
The Double Breasted Cays oolitic shoal is one
extreme, where the shoal is situated between bedrock