Geology Reference
In-Depth Information
Storms and hurricanes frequently affect the area
and exhibit highly variable source direction,
with winter storms generally coming from the
northwest whereas the hurricane directions are
highly variable. Only major storms affect sedi-
mentation on the platform because of its great
lateral extent and its protection by marginal shoals
(rocky and sand shoals, and reefs) and islands
(Tucker & Wright, 1990). The annual precipitation
over the Bahama Platform amounts to 1355 mm
(Carew & Mylroie, 1997), concentrated between
May and November, with a maximum in September
and October (Gebelein, 1974). The winter months
(December-April) are relatively dry. The average
monthly rainfall varies between 100 and 250 mm
(Miller
et al
., 1983).
Seasonal atmospheric circulation patterns
strongly infl uence temperature and precipitation
patterns on GBB. Circulation responds to changes
in the position and strength of high- and low-
pressure systems, the Intertropical Convergence
Zone (ITCZ), the Subtropical Divergence Zone
(STDZ), respectively, and the associated trade-
wind belt. During May to November, the atmo-
spheric highs and lows are less pronounced over
the North Atlantic Ocean, and the ITCZ and the
STDZ move northward. The trade winds weaken
and high precipitation prevails over the northern
Bahamas. The atmospheric regime migrates
southward during the boreal winter (December-
April) and the ITCZ shifts toward the Equator or
slightly south of it (0-5°S). During this period
the atmospheric high- and low-pressure sys-
tems strengthen and the easterly trade winds are
enhanced. The result is a stronger infl uence of
colder and dryer air masses. During this winter
period, the evaporation minus precipitation
(E
exchange with oceanic water is very small
(Bathurst, 1975).
Carbonate mineralogy and grain size
Carbonate mineralogy on GBB is well documented
and shows a dominance of aragonite on the plat-
form top with minor high-magnesium calcite
(HMC) and low-magnesium calcite (LMC) (Purdy,
1963a; Husseini & Matthews, 1972; Milliman,
1974; Mullins, 1986; Milliman
et al
., 1993).
Ginsburg (1956) noted, 'Variations in the
physical environment of carbonate depositional
environments (bathymetry, areal geography, and
hydrography) are refl ected in the grain size and
constituent particle composition of the sedi-
ments'. On the platform top grain size has been
used as a relative indicator of current intensity
(Purdy, 1963a). The
in situ
production of coarse-
grained skeletal material and changes in the origi-
nal particle size on the platform, e.g. grapestone
formation and particle breakdown, interfere with
this assumption (Purdy, 1963a).
METHODS
Grab samples
Approximately 300 samples were obtained using
a Shipek sampler during four cruises aboard the
RV Bellows between 2001 and 2004 (Fig. 1b).
The design of this sampler prevents muds from
escaping from the sample during retrieval
from the sea bottom. It thus differs from the
Van-Veen sampler used during earlier studies
(Purdy, 1963a,b) in that it better retains the mud
fraction.
P) averages around 225-250 cm yr
1
while
it only reaches about 50 cm yr
1
in the summer
(Smith, 1995).
Facies classifi cation
Immediately after collection, samples were
assigned to a modifi ed Dunham scheme (Dunham,
1962). In order to complement the classifi cation
we added divisions between wackestone and mud-
stone (mud-rich wackestone; facies number 1.5),
between packstone and wackestone (mud-rich
packstone, facies number 2.5), and fi nally between
packstone and grainstone (mud-lean packstone
or poorly washed grainstone; facies number 3.5)
(Fig. 2). Samples from all cruises were preserved
in cold storage and after the 2004 cruise all sam-
ples were re-examined to ensure consistency
between cruises.
Salinity
In summer, salinity varies from 36‰ near the edge
of the platform to 46‰ in the platform interior,
protected environment west of Andros Island
(Smith, 1940; Ginsburg
et al
., 1958; Gebelein,
1974). Maximum winter salinity reaches 38‰
(Gebelein, 1974). Reasons for the persistence
of increased salinity in the bank interior are
the great lateral extension of the platform and
the existence of marginal shoals and islands
reducing the exchange of platform and open
ocean waters (Tucker & Wright, 1990); tidal
