Geology Reference
In-Depth Information
Facies patterns
Cay (Newell & Rigby, 1957; Ball, 1967; Harris,
1979, 1983), also support the presence of strong
currents that enter the shallow-water environ-
ments. The mud-free facies is positioned close
to these facies belts, and thus confi rms the infl u-
ence of currents on the grain-size distribution in
these parts of the platform. The grainstone belt
(facies 4) crossing the platform south of Andros
Island suggests that this zone acts as a conduit
for currents of Tongue of the Ocean water.
Another interesting feature is the agreement
between the pellet distribution and the abundance
of the 63-125 μm grain-size fraction showing
the biological control on grain-size distribution
(cf. Figs 3c and 5a).
Another process that infl uences the facies
distribution is the tidal currents that freely
enter the shallow-water platform. This is shown
by the trend towards mud-free sediments in the
southern part of the platform where no major
barriers exist that would prevent the removal of the
fi ne-grained material (Figs 2, 3a, 3b and 3d). The
observed export of shallow-water muds by phys-
ical processes occurring during passage of win-
ter cold fronts confi rms this inference (Wilson &
Roberts, 1992, 1995). The precise correlation
of variations in sediment accumulation on the
western slope of GBB with the scaling of atmo-
spheric and oceanographic processes underlines
the subtle character of the processes infl uencing
sediment export from the inner platform areas
(Roth & Reijmer, 2004, 2005).
Purdy (1963b) hypothesized that the facies on
GBB theoretically would be distributed in a series
of bilaterally symmetrical bands parallel to the
margin of the bank. However, the depositional
topography of the Pleistocene substrate creates
local current conditions that have resulted in
the somewhat patchy facies distribution pattern
observed (Purdy, 1963a,b). The early maps of
Newell & Rigby (1957) and Newell
et al
. (1959),
however, showed some concentric facies belts in
the lee of Andros Island. All samples taken during
these early studies were taken by hand or using
a Van Veen grab-sampler. The Van Veen sampler
has the disadvantage that it loses a proportion of
the very fi ne fraction (<63 μm) during the retrieval
process. This probably is the reason that differ-
ences in the mud content exist between the early
facies maps and our data. The distribution pattern
observed in the present study of GBB, however,
shows a roughly concentric series of facies belts
with an outer edge dominated by coarse-grained,
mud-free to poorly washed grainstones, and an
inner platform area with mud-rich wackestones
forming a U-shaped band, situated close to Andros
Island (Fig. 2).
Rankey & Morgan (2002) and Rankey (2002)
suggested that the tidal fl ats west of Andros Island
do not respond solely to the rate of sea-level rise,
but also to the variation in circulation or storm
intensity and direction. The tongue of less muddy
sediment that extends from northwest Andros
Island supports this scenario because it is clearly
positioned at a location where the infl ux of waters
from the east was observed on this side of the
platform (Newell & Rigby, 1957; Harris, 1979,
1983). Another factor that seems to infl uence
the distribution of mud-rich sediments is water
depth and the related parameter, water energy.
Mud-rich aragonitic sediments characterize the
deeper areas while the shallower areas comprise
coarser-grained sediments. As shown on the
facies map (Fig. 2), Andros Island also plays an
important role in the present-day sediment distri-
bution as it protects the inner platform environ-
ments from the currents induced by the easterly
trade winds and by tides. The facies change to
grainstone dominance (facies 3.5-4) observed
north and south of Andros Island precisely
registers the infl uence of currents on the overall
sediment distribution. The ooid-dominated tidal
bars south of Tongue of the Ocean (Ginsburg,
2005) and north of Andros Island, on Joulters
Topography and sediment distribution
Another important question is whether the plat-
form topography inherited from the last glacial
cycle infl uenced the facies distribution pattern
found today. The present-day distribution shows
a link between water depth on the platform and
the type of sediment present. Mud-rich facies
(facies 1.5) shows a clear relation to deeper areas
(Fig. 2). Coarse-grained, mud-free facies occur
either close to the edge of the platform, or on
elevated areas within the interior of the plat-
form (e.g. southeast of Bimini; Figs 1 and 3a-d),
or within the more open parts of the platform,
in the southern part of the study area. Newell
et al
. (1959) described two broad shoals running
across the Andros lobe, (i) the Bimini axis, which
extends from Bimini in the west to the northern
tip of Andros Island, and (ii) the Billy Island axis
running westward from the middle promontory
of Andros Island. The map of mean grain-size
