Geology Reference
In-Depth Information
The scheme most frequently used to describe the
morphology of carbonate sand accumulations in the
Bahamas utilizes several different classes: marine sand
belts, tidal bar belts, platform interior sand blankets,
and eolian ridges (Ball
1967
). This scheme has been
amplifi ed and refi ned in several reviews (e.g., Halley
et al.
1983
; Handford
1988
; Wanless and Tedesco
1993
; Tucker and Wright
1996
) , resulting in addition
of several new classes, including tidal deltas (expressed
by Halley et al.
1983
), shorefaces and “ooid sediment
production restricted through shoaling” (Wanless and
Tedesco
1993
). [Some of the classes (eolian ridges,
shorefaces) are not dominated by tidal processes, and
so are not discussed further in this chapter.]
In these summative schemes, however, there can be
considerable ambiguity. For example, the Joulter Cays
sand shoal complex has been variously described as a
marine sand belt (Halley et al.
1983
, although they also
suggested that parts formed a tidal bar belt - p. 472),
an area with “ooid sediment production restricted
through shoaling” (Wanless and Tedesco
1993
) , and a
“marine sand belt to sand fl at … with some spit-like
tidal bars” (Tucker and Wright
1996
) . Similarly , the
archetypal tidal bar belt, the Schooner Cays shoal
complex (Ball
1967
), only has well-developed tidal
bars oriented roughly normal to fl ow in its eastern
third; the western part of the system is dominated by
bars with a markedly different morphology.
These ambiguities suggest that lumping any partic-
ular shoal complex into one class may result in either
misleading oversimplifi cation or misrepresentation.
As such, this summary takes a slightly different
approach to characterizing shoals, building up in scale
from sediments, to bed forms, to bar forms found in
the shoals, focusing on the infl uences of tides on their
genesis. A few examples of the spatially variable
geometries present in several shoal complexes further
illustrate some of the geomorphic forms and deposi-
tional processes in these systems.
intraclasts and lithoclasts may occur locally. Unlike
siliciclastic tidal sands, which must be reworked from
earlier deposits or transported to the depositional
system from elsewhere, these sedimentary particles
can be (and many are) produced
in situ
, with relatively
little net transport .
Skeletal grains
are the remains of any of a diverse
group of fl ora and fauna (Fig.
20.2a, b
). On sand shoals
in the Bahamas and Caicos, the most abundant grain
types include whole skeletons or fragments of bivalves,
gastropods, miliolid and peneropolid foraminifera,
and, less abundantly, fragments of coral, red algae, or
Halimeda
(a green alga) or other minor skeletal com-
ponents. Many skeletal fragments on tidal sand shoals
are broken or abraided, and may have been transported
to the area by the strong currents or they may come
from organisms that resided in the area. Skeletal grains
may serve as nuclei for ooids.
Peloids
are non-discript aggregates of cryp-
tocrystalline carbonate less than 2 mm in diameter
(Fig.
20.2c
). Peloids can be of various origins; they
may represent fecal pellets, aggregates from erosion
and transport of semi-cohesive muds, or extensively
bored or micritized skeletal or oolitic grains in which
all primary textures have been obliterated (Bathurst
1975
). On the bank top and in many tidal shoals, pelo-
ids occur in every subenvironment and water depth.
Hardened peloids may form a substantial percentage
of grains in some tidal shoals, and peloids can serve as
nuclei for ooids.
Ooids
are sand-sized ovoid to circular carbonate
particles less than 2 mm in diameter that include one or
more concentric laminae (collectively forming a cortex)
around a nucleus (Fig.
20.2d-g
). These non-skeletal
grains can be found in strata of almost any age, from
Archean to recent (e.g., Opdyke and Wilkinson
1990
;
Sumner and Grotzinger, 1993), and are found today on
most platforms in the Bahamas and Caicos. Nuclei
from these areas are either peloids or skeletal frag-
ments, and oolitic laminations are made of tangentially
arranged aragonite needles that can make up more
than 80% of the volume of some grains. The number
and type of laminations varies considerably, from
classic
concentric ooids
coated with up to 90 or more
concentric laminations (Fig.
20.2e, f
; Newell et al.
1960
) ,
to
superfi cial ooids
(Fig.
20.2g
; Illing
1954
; Bathurst
1967
) with only a few, thin (1-5 mm) laminations, and
irregular ooids
(Fig.
20.2d
; Illing
1954
; Wanless and
Tedesco
1993
) that illustrate grain degradation such as
20.3.2 Sediment Composition and Early
Diagenesis
Tidal shoals of the Bahamian Archipelago are com-
posed exclusively of carbonate sands, with no silici-
clastics. Grain types include ooids, composite grains,
peloids, and skeletal fragments present in varying
amounts within and among the shoals (Fig.
20.2
);
