Geology Reference
In-Depth Information
Fig. 20.6
Field photos of illustrative bedforms. (
a
) Small linear
ripples from ~1.5 m water depth, Abacos. (
b
) Straight-crested
ripples, with a smaller secondary set superimposed, forming
ladder-back ripples, Abaco tidal deltas. Handle on
yellow
and
black
tape measure is 18 cm across. (
c
) Cuspate to lingoid C is
~15 cm across. (
d
) Symmetric ripples and scattered fi nger coral
fragments. Ripples have spacing of 30-40 cm. Fish Cays, ~1.5 m
water depth. (
e
) Simple dunes, with height of 20-30 cm, and
superimposed ripples. Schooner Cays, ~2 m water depth. (
f
) Crest
of an asymmetric medium to large subaqueous dune. Height of
dune is ~50 cm. (
g
) Surface photo of the crest of a subaqueous
dune from Lily Bank. From this perspective, these features appear
as “stripes” in the water. (
h
) Crest of shoal and subaqueous dunes
exposed at low tide. (
i
) Partly stabilized bottom, off the crest of an
active shoal, Abaco tidal deltas, ~3 m water depth. Handle on
yellow
and
black
tape measure is 18 cm across
current direction and magnitude, and the ease with
which current ripples can be modifi ed because of
their small size .
Beyond these current-generated ripples, wave-
generated
symmetric ripples,
frequently straight-
crested and around 2-10 cm in height, can be found
locally associated with tidal shoals (Fig.
20.6d
). These
occur either on the slightly deeper fl anks of shoal com-
plexes or, more ephemerally, on the shallowest inter-
tidal crests of some of the bar forms where small waves
break. The combination of waves breaking on shoal-
marginal shallow bar crests and the strong daily tidal
currents results in a paucity of symmetric wave ripples
across most parts of shoal complexes.
At a larger scale, several distinct types of subaqueous
dunes are found on carbonate tidal sand shoals
(Gonzalez and Eberli
1997
) , including: (1)
small 2-D
dunes
with crests that are straight to sinuous (with
amplitudes between 0.2 and 1.0 m; note that Ashley
(
1990
) would not refer to these as “small” dunes) and
spaced between 1 and 3 m apart, with superimposed
ripples (Fig.
20.6e
); (2)
small 3-D dunes
, with heights
of decimeters, and spacing on the order of 1 m, with
crest bifurcations every few m; and (3)
complex
composite dunes
with heights up to 0.1 m and which
migrate normal to the current. The largest scale of
these bedforms include
medium to large 2-D dunes
and
compound dunes
of Ashley (
1990
) . These features
