Geology Reference
In-Depth Information
Fig. 20.3
Characteristics of several types of sand present in
Bahamian tidal sand shoals. (
a
) Underwater photo of an intrac-
last, formed as part of a cemented crust. Clast is ~40 cm across.
Schooner Cays. (
b
) Thin section photomicrograph of rim of
fi brous aragonite that cements ooid sand, forming clasts such as
that in (
a
), Fish Cays. (
c
) SEM image of an ooid (
right
, note
laminations) and fi brous aragonite (
left
) from the same sample
as in (
b
). (
d
) SEM close-up of the ooid-cement boundary.
(
e
) Low-angle aerial photo of a parabolic barform with an oolitic
sand island, Schooner Cays. Island is ~160 m across at its widest.
(
f
) Cemented Holocene beach-dune succession; this is common
on the fl anks of islands such as that illustrated in (
e
) (Budd
1984
) , Schooner Cays. (
g
) Outcrop photo of fi ning-up lamina-
tions in oolitic beachrock sand, Fish Cays. Approximately 3 cm
of the width of a hand-held GPS unit is in the lower right. (
h
and
i
) Thin section photomicrograph of clear equant calcite cement
with meniscus morphology, indicative of vadose-zone freshwa-
ter diagenesis, from outcrop illustrated in (
g
)
20.3.3 Bedforms
Meteoric cementation
occurs in areas where sand
shoals have aggraded above sea level and are exposed
to the effects of freshwater (Fig.
20.3e-g
). These
slightly acidic waters dissolve unstable aragonite (e.g.,
in ooids) in near-surface layers, and re-precipitate
much of the calcium carbonate as low-magnesium cal-
cite lower in the column, near grain contacts or as pen-
dant cements (Fig.
20.3h, i
; Halley and Harris
1979
;
Budd
1984
). As with marine cements, meteoric cemen-
tation can be quite rapid, as indicated by observations
of bottles, cans, and other trash cemented within beach
rock, and entire islands can be cemented within a
1,000 years (Halley and Harris
1979
) .
Although carbonate sediments are “born, not made”
(James
1983
), they can be suspended by waves and
moved by currents (Braithwaite
1973
; Wanless et al.
1981
; Kench and McLean
1996
; Prager et al.
1996
) .
Tidal sand shoals of the Bahamian Archipelago show
ample evidence of transport by reversing tidal currents,
or less frequently and importantly, by tidal surges or
waves related to the passage of fronts or storms.
As sands are transported, the processes create
bedforms of various sizes, from cm-scale ripples to
cm- to m-scale subaqueous dunes to even larger barforms.
