Geology Reference
In-Depth Information
Cay (Fig.
20.9b
) extends east-west just north of the
ooid shoal complexes.
Joulter Cays shoal complex
occurs on the northern
fl ank of the Pleistocene high of Andros Island, in an
embayment in the platform bordered by the deep-water
Tongue of the Ocean and an unrimmed, skeletal-sand
rich shelf (Harris 1979) (Fig.
20.14
). The shoal com-
plex includes a broad sand fl at (~400 km
2
), bordered
on its eastern margin by an active mobile sand belt
with small ebb tidal deltas and on its northern margin
by a number of longitudinal sand ridges. The shoal
includes a number of Holocene islands across its
extent, but the highest islands (up to 6 m above sea-
level) are on the eastern margin, due to active long-
shore transport, accretion and cementation on beaches,
as well as eolian processes. The sand fl at is penetrated
from the east by a number of tidal channels, and gradu-
ally slopes down to the platform interior to the west.
Much of the vast expanse of the sand fl at is exposed at
spring low tide and stabilized by seagrass and algae.
Surface sediments are dominantly non-skeletal. Clean,
well-sorted oolitic sand is most abundant on the mobile
sand belt on the eastern and northern fl anks of the shoal
(average abundance of ooids = 83%; Harris 1979).
Much of the sand fl at is burrowed, and includes fi ne
micritized ooids and peloids.
As it represents an aggraded shoal, Joulter Cays
shoal complex highlights the importance of prolifi c
carbonate sediment production on carbonate shoals.
Aggradation may have progressively fi lled accommo-
dation space on the shoal crests, decreasing the tidal
prism, which in turn decreased currents, leading to
further aggradation. Indeed, the sedimentology of
Joulter Cays shoals illustrates decreased energy in the
interior of the sand belt (Harris 1979). Similarly,
Joulter Cays shoal occurs on a windward margin. The
dispersion and refraction of impending waves sets up
a northward longshore transport, which is most likely
responsible for the formation of islands. Similar com-
binations of tidal and wave-driven hydrodynamics on
the outskirts of the shoal facilitate the prolifi c genera-
tion of oolitic sands in this belt (Carney and Boardman
1993
). Aggradation and island formation, and associ-
ated feedbacks, may have facilitated the widespread
aggradation. Given that many ancient ooid shoal com-
plexes include evidence for aggradation and stabiliza-
tion, the shoal may represent the “ultimate fate” of
many ooid shoals (but - note the contrast with Lily
Bank, above).
Fig. 20.12
Image of part of Lily Bank, westernmost Abaco
Island chain, Little Bahama Bank, from the area illustrated in
Fig.
20.11a
. In this area, Lily Bank includes a downdip (
top
of the
image) inactive shoal complex of generally fl ow-parallel tidal
sand ridges and an updip complex of active ooid sands with abun-
dant parabolic bars and subaqueous dunes. Image acquired
8/19/2002. Image copyright GeoEye.com. See text for discussion
(Rankey et al.
2006
; cf. Caston
1972
; Hine
1977
) .
Finally, the system illustrates barform geometries
analogous to those found in some siliciclastic middle
estuary systems (Dalrymple and Rhodes
1995
; Wood
2004
) ,
highlighting
some
geomorphic
similarities
between carbonates and siliciclastics.
Tongue of the Ocean shoal complex
is the largest
oolitic shoal system in the world, but is also the least
well studied system in the Bahamas due to its isolation
(Palmer
1979
). It lies on the shallow shelf of the Great
Bahama Bank bordering the broad deep-water embay-
ment of Tongue of the Ocean (TOTO). As it wraps
around the TOTO for more than 130 km (Fig.
20.13
),
the shoal complex includes various barform geome-
tries. To the east, parabolic bars are quite extensive,
and some appear to have sand waves in the intervening
channels (unlike those on Lily Bank, which have sea-
grass-stabilized bottoms) (e.g., Fig.
20.13b, c
). TOTO
is best known for its spectacular longitudinal tidal sand
ridges (Fig.
20.13d
), west of the parabolic bars. These
tidal sand ridges average over 10 km long, have widths
of several 100 m, and are separated by deeper channels
that average over 1.2 km wide and can be up to 8 m
deep. The isolated longitudinal sand barform of Green
