Geology Reference
In-Depth Information
The reproducibility of the method was determined
through the replicate analysis of samples and is
approximately
edge of the platform towards fi ner and muddier
sediments in the vicinity of Andros Island
(Fig. 3a-d). The tidal fl ats with scrub mangroves
on the western side of Andros Island further con-
fi rm the muddy character of the sediments in this
inner platform environment. The coarsest mean
grain-sizes are on the northern, western and south-
ern margins of the bank. The mean grain-size map
(Fig. 3a) precisely displays open and protected
environments on the platform. Note especially the
zones with relatively coarse-grained sediments
associated with the ooid belts at the eastern edge
of the platform, N and SSE of Andros Island, near
the Tongue of the Ocean.
2%.
Sediment composition
In order to identify individual facies groups, the
coarse fraction (63 to >1000 μm) of each sample
was analysed for its variation in skeletal (coral frag-
ments, calcareous green algae, benthic foraminifera,
gastropods, echinoderm spines and fragments,
serpulids, ostracods, bryozoans, sponge spines
and others) and non-skeletal components (peloids,
ooids, grapestones, clasts) using a stereo binocular
microscope (sieve samples) as well as a polarizing
microscope (thin-sections). Only the distributions
of pellets and ooids are described here.
For the identifi cation of the skeletal and non-
skeletal components, the following literature was
used: Bolli
et al
. (1994), Haq & Boersma (1978),
Illing (1954), Flügel (2004), Loeblich & Tappan
(1978), and Scholle & Ulmer-Scholle (2003).
Mineralogy
Aragonite is by far the dominant mineral across
the platform and varies between 77.7 and
100%, with a mean of 93.3% (Fig. 4a; Table 1).
High- and low-magnesium calcite (HMC and
LMC) content varies between 0 and 22.3%, and
0 and 3.9%, respectively, with a mean of 6.5%
for HMC and 0.2% for LMC (Fig. 4b and c;
Table 1). The analysis of the fi ne-fraction
(<63 μm) of the 2004 dataset showed an appre-
ciable increase in the mean values of HMC and
LMC, with values ranging from 4.8% (bulk
samples) to 10.6% (fi ne fraction) for HMC and
from 0.5% (bulk) to 6.7% (fi ne fraction) for LMC
(Table 1).
RESULTS
Facies
The facies distribution (Fig. 2) shows a domi-
nance of mud-free to mud-lean sediments in the
north, west and south of the northern part of GBB.
A gradual transition exists between these sedi-
ments and mud-rich wackestones present near
the west side of Andros Island. The facies pattern
is more complex than that presented in earlier
studies (Ginsburg
et al
., 1958; Newell
et al
., 1959;
Purdy, 1963b; Ball, 1967; Enos, 1974). A some-
what concentric distribution of various facies
types is striking. The muddier sediment types
(mud-rich wackestone (facies 1.5) to packstone
(facies 3)) clearly dominate the platform interior.
These sediments form a lobe adjacent to Andros
Island, but also form an elongate zone close to
the western edge of the platform. The cleaner
sediments (packstones to grainstones) form a con-
centric belt on the western edge of the platform
extending to the northern and southern regions.
The grainier tongues (facies 3 and 3.5) project-
ing from NW Andros Island in a WSW and WNW
direction are also notable features.
Facies variations
The distribution of pellets largely coincides
with the mud-rich facies distribution; as they are
more abundant in restricted areas of the platform
(Fig. 5a). As might be expected, ooids prefer-
entially occur on the edge of the platform and
in the more open areas on the platform (south
and north of Andros Island; Fig. 5b) and thus
show a large overlap with the more grainy facies
types on the platform. The largest quantities of
Halimeda
were present in deeper waters, on the
northern edge of the platform. On the platform
itself
Halimeda
sp. occurs in minor quantities
only. Grapestones were found on the more open
northern part of the platform and along the south-
ern side of the investigated area (not shown).
Part of the grapestones from the northern sector
of the platform consist of non-indurated grains
bound by soft algal mucus that thus differ from
the indurated ones found in the southern part of
the platform.
Grain-size
The grain-size distribution on GBB shows a dis-
tinct trend from coarse sediments on the western
