Geology Reference
In-Depth Information
remain for a substantial amount of time; (3) an
environment affected by strong bottom currents
where grains can be repeatedly rotated; (4) con-
ditions suitable to inorganic and/or microbial
precipitation, for example areas where cool
oceanic water mixes with warm bank water
supersaturated with respect to aragonite. Modern
tangential ooids are preferentially generated in
intertidal and shallow subtidal environments
along platform edges, such as tidal deltas, lagoons,
beaches or tidal fl ats (Siewers, 2003).
On GBB, ooid sands are forming in plat-
form margin shoals that are parallel to the slope
break (Ball, 1967; Harris, 1979; Ginsburg, 2005).
Most signifi cant modern examples include the
region south of Bimini (70
bedforms on GBB (Purdy, 1963; Swart
et al.
, 2009;
Reijmer
et al.
, 2009). Predominant water move-
ment and sediment transport is thus towards the
west, but residual southward and eastward motion
takes place during northwesterly gales in the
winter (Bathurst, 1975). The infl uence of tropical
storms and hurricanes is less well constrained.
Harris (1979) noted that most storms affecting
the northern Bahamas produce a dominant bank-
ward wind on northeast-facing shorelines, and
that associated water movement and sediment
transport is also towards the platform interior.
In contrast, Boss & Neumann (1993) observed
that Hurricane Andrew had a minimal impact on
both subaqueous sand bodies and islands. With
the exception of channels between islands, shoals
or reefs, tidal currents on and off GBB are sluggish
and their infl uence on the net lateral transport
of sand-sized sediments appears to be complex
(Gonzalez & Eberli, 1997). The usual greater velo-
city of the fl ood current (Bathurst, 1975) suggests
that net sediment transport must be bankward.
In summary, due to the predominance of easterly
winds, the main sediment transport vector on
GBB, at present, is from east to west.
2 km), the southwest
end (or Cul-de-Sac) of the Tongue of the Ocean
(100
20-25 km), the Schooners Cays (45
15 km),
the Berry Islands (12
6 km) and the Joulters Cays
(25
1-2 km) (Fig. 1). In most of these areas,
the ooid factory covers a signifi cant portion of
the platform, but in the Joulters Cays, ooids are
produced in a 1-2 km-wide mobile fringe jux-
taposed to the island and situated only 5 km
bankwards from the slope break (Harris, 1979).
Modern ooid accumulations are located in or near
the environment where ooids are produced, but
the common occurrence of these grains in aeoli-
anites (Kindler & Hearty, 1996) and in deep-water
turbidites (Haak & Schlager, 1989) indicates that
signifi cant transportation may take place.
The windward islands
According to Ball (1967), the prevailing easterly
winds are responsible for the existence of islands
along the eastern edge of GBB (Fig. 1), because
they generate sustained wave action that provides
'building material' for the constituent dunes.
These islands (Eleuthera, Cat, Long, Exumas;
Fig. 1) are narrow (<1-12 km wide), elongated (up
to 140 km) rock bodies, lying near (<1-7 km) the
bank margin and oriented parallel to it. They con-
sist of vertically stacked or laterally juxtaposed
carbonate units and palaeosols (Fig. 2). The former
accumulated during interglacial highstands of sea
level, whereas the latter developed mostly during
glacial lowstands (Carew & Mylroie, 2001). Details
on the stratigraphy of the Bahamian islands can
be found in Kindler & Hearty (1997) and Hearty &
Kaufman (2000). Carbonate units are essentially
composed of aeolian sediments, but marine
deposits, recording ancient sea-stands, occur up to
18 m above modern sea level (Hearty
et al.
, 1999;
Kindler & Hearty, 2000). Aeolianite foresets dip
always towards the interior of islands, emphasiz-
ing further that the source material for these dunes
originates from the shore and that only onshore
winds are effective in dune build up (MacKenzie,
1964; Ball, 1967).
Sediment transport vectors on Great
Bahama Bank
The main physical parameters controlling
sediment distribution on the GBB are the wind,
wind-induced waves and currents, and tidal
currents. In the Bahamas, prevailing winds are
from northeast to southeast during most of the
year (Sealey, 1994), with strong northwesterly
winds, related to cold fronts, further affecting the
area in the winter. In addition, gusts associated
with thunderstorms can blow in any direction.
Winds transport marine sand concentrated in bars
and beaches onto or off the islands (Ball, 1967;
McKee & Ward, 1983). In the fi rst case, grains build
up coastal dunes that are preserved as aeolianites,
and increase the island volume. In the second
case, sand is washed back into the sea and may
possibly be exported to a deeper depositional
setting. Winds further generate waves and cur-
rents that determine the subaqueous transport
of sediment and the distribution of sedimentary
