Geology Reference
In-Depth Information
Fig. 20.9
Remote sensing images of barforms. (
a
) Joulter sand
fl at. Note the tidal channels extending onto the platform (to the
left
in this image), the Holocene islands, and the ebb tidal delta.
Image acquired 12/27/2003, and is copyright DigitalGlobe.com.
(
b
) Isolated longitudinal sand bars, on Green Cay shoal. See text
for detailed discussion
bottoms. These rocky (Pleistocene or coral-covered)
fl oors, along with the bedrock islands, help focus
the tidal currents in these inlets, allowing the tidal
velocities to exceed the equilibrium velocities (1 m/s)
observed in siliciclastic analogs, which can lead to
larger tidal deltas in carbonate settings.
Sand fl ats
are expansive, fl at-topped, sandy, inter-
tidal to shallowest subtidal accumulations penetrated
by tidal channels that are oriented roughly normal to
the platform margin (Fig.
20.9a
). Sand fl ats (Joulter
Cays, Fish Cays) have discontinuous islands near the
platform margin that are fl anked platformward by
broad intertidal to shallow subtidal fl ats that gradually
slope down to the subtidal platform interior without a
well-defi ned break in slope. Islands associated with
most sand fl ats are of Holocene age, are associated
with longshore or on-bank transport, and are exten-
sively cemented by meteoric processes (Harris
1979
;
Halley and Harris
1979
, see above). Sand fl ats can be
quite extensive; the Joulter Cays sand fl at reaches up to
20 km onto the platform, and the Fish Cays sand fl at is
up to 7.5 km wide. In both examples, however, the
broad shallow fl at is broken along strike by tidal chan-
nels several 100 m wide and 3-5 m deep that either
taper and branch or broaden into diffuse, shallow
depressions as they extend onto the platform. The large
areas of the crests of sand fl ats are intertidal, and can
be stabilized by seagrass and calcareous green algae
(Joulter Cays), and may also include pastures of red
algae, seagrass, and sponges (Fish Cays). Constituent
sediments refl ect this assemblage, and these intertidal
fl ats include fewer ooids than higher-energy channels
or ocean-facing margins of the shoal complexes
(Harris
1979
; Rankey and Reeder
2010
) .
