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In-Depth Information
Values for HMC are highest along the northern
edge of the platform (Fig. 4b). Slightly increased
values occur in the mud-dominated facies belt
in the lee of Andros Island. The cause of this
latter increase remains uncertain, but could
have a biological (e.g. pellet formation) or a
diagenetic origin.
Two small zones with slightly elevated LMC
values occur on the bank (Fig. 4c). One near
the western tip of Andros, which is probably
related to the input of Pleistocene eroded mater-
ial from the island. The second zone relates to an
inlet on the eastern side of the platform in which
the ooid shoals occur also, evidencing strong
currents that enter the platform from the Tongue
of the Ocean.
Island shows up on all maps, however (Figs 2
and 6a-c). Another feature that shows up on all
maps is the sharp transition from mud-dominated
sediments (facies type 1.5) to grainstone facies
(facies type 4) on the western boundary of the
platform (Figs 2 and 6a-c). A detailed comparison,
however, is diffi cult, because the majority of the
maps use a classifi cation scheme partly based on
the occurrence of single grain types and carbonate
muds. A signifi cant difference is that the domin-
ant grapestone occurrence in the northern part
of the platform as shown by Newell
et al
. (1959)
and Purdy (1963b) could not be confi rmed by
this study (cf. Fig. 2 with 6b). On the Enos (1974)
map this northern grapestone-dominated facies
belt was classifi ed as a peloid-dominated grain-
stone facies, which agrees more with our results
(cf. Fig. 2 with Fig. 6c). Despite the fact that the
Enos (1974) map also used the Dunham classifi ca-
tion scheme (1962), large differences in facies dis-
tribution exist between that map and this study.
Further detailed studies on the composition of
the different grain-size fractions in combination
with a detailed petrographic analysis are needed
to quantify the skeletal and non-skeletal compon-
ent distribution on the platform.
Facies patterns and isotope signals
Mud-dominated, aragonite-rich sediments
(facies types 1.5-4.5) form the majority of the
sediments in the interior of GBB (Figs. 2, 3a, 4a).
These facies belts are surrounded by skeletal and
non-skeletal aragonite sediments on the north-
ern and western outer edges of the platform and
in the southern parts of the study area (Fig. 2).
The predominantly aragonite sediments on GBB
have similar
18
O compositions (Swart
et al
., 2009). A comparison between the pellet
distribution and the distribution of the mud-rich
sediments (cf. Figs 2, 3c and 5a) shows that muds
and related sediment particles are important sedi-
ment components on the platform. The muds
and their derivates seem to dominate the iso-
tope distribution observed on GBB (Swart
et al
.,
2009).
13
C and
CONCLUSIONS
The facies, grain-size and mineralogy distribu-
tion patterns on GBB show a current-dominated
sedimentation pattern with a clear distinction
between the outer edge and the inner platform.
Inner-platform facies distribution relates to the
present-day bathymetry, which might represent in
part an inherited Pleistocene topography. Seismic
studies are needed to verify this.
Grain-size differences refl ect the bathymetry
and show a more coarse-grained outer platform
and a mud-dominated interior. The grainstone
distribution north and south of Andros Island
registers the presence of strong currents that
enter and cross the shallow-water environ-
ment. The correlation between the distribution
of the 63-125 μm grain-size fraction and the
distribution of pellets suggests a predominantly
biological origin of this grain size.
The mineralogy of the sediments shows a clear
dominance of aragonite mixed with HMC and
minor LMC. Concentration of both calcite types in
the fi ne fraction suggests the preferential transport
of aragonite-rich muds from the platform through
various processes.
Comparison between new facies map and
literature data
The facies maps that are widely used in the
literature are based on the data sets presented
by Illing (1954), Ginsburg
et al
. (1958; Fig. 6a),
Newell & Rigby (1957), Newell
et al
. (1959), Cloud
(1962), Purdy (1963a) and Traverse & Ginsburg
(1966; Fig. 6a). The most widely used facies
distribution maps of Purdy (1963b; Fig. 6b) and
Enos (1974; Fig. 6c) make use of these data sets.
The facies, grain-size and mineralogy maps pre-
sented in the present study show a more diverse
distribution of all sediment parameters along the
platform. The gross facies distribution with a
skeletal-dominated, coarse-grained platform edge
and a mud-dominated interior in the lee of Andros
