Geology Reference
In-Depth Information
(mainly faecal pellets) dominate the interior of the
platform. Very fi ne-grained sediments predomin-
ate in platform interior zones that are protected
by topographic barriers such as marginal islands
and submarine topography ('pelletoidal sands
with lime mud matrix and lime muds', Ginsburg
et al
., 1958; 'muddy sand and mud', Newell
et al
.,
1959; 'pellet-mud facies' and 'mud facies', Purdy,
1963a; 'pelletoidal packstones' and 'pelletoidal
wackestones', Enos, 1974). Some differences, how-
ever, exist in the interpretation and, as seen on the
maps, in the distribution of the facies types. For
example, Ginsburg
et al
. (1958) and Enos (1974)
show that the sediments in the platform interior
mainly contain pellets, peloids or pelletoids.
Newell
et al
. (1959) and Purdy (1963a,b), how-
ever, described sediments containing a larger per-
centage of grapestones. Enos (1974) also described
the occurrence of grapestones without showing
them on the map as a particular facies type. Enos
(1974) was the only study in which the Dunham
classifi cation (1962) was used and thus showed
the components to matrix ratio along the plat-
form. All maps known from the literature were
based on sampling along a series of profi les cross-
ing the bank and additional datasets. All samples
were taken without modern precision positioning
systems and thus might contain signifi cant uncer-
tainties in the exact location of facies types.
The major question, however, concerns the
processes that control the present facies distribu-
tion on western GBB, the Andros lobe. This paper
attempts to determine the links between the sedi-
ment distribution and environmental parameters
such as currents, tides, topography, evaporation
(salinity), precipitation, water exchange with the
surrounding ocean water masses and sediment
export.
et al
. (1995) showed that the average water
transport through the Santaren Channel
amounts to 1.9 Sv or 1.8 Sv, respectively. The trans-
port through the Northwest Providence Channel
reaches 1.2 Sv (Leaman
et al
., 1995). On the west-
ern, leeward sides the current system on GBB is
connected with the currents forming the sources
of the Gulf Stream. On the windward sides a link
exists with the North Atlantic Subtropical Gyre
situated eastward in the Sargasso Sea. The GBB
has a great lateral extent and shows a well-defi ned
marginal escarpment with islands. Shallow waters
cover most of GBB and consequently, the inter-
action with water masses from the open ocean is
limited (Smith, 1940). Islands such as Andros
Island defl ect westward-moving currents gener-
ated by the trade winds to the north and south
(Bathurst, 1975). As a result protected areas with
limited water exchange are present on the western
side of these islands (Bathurst, 1975). Water move-
ment on GBB is mostly infl uenced by the wind and
tides, but also by waves and storms (Smith, 1940;
Cloud, 1962; Purdy, 1963a; Traverse & Ginsburg,
1966; Winland & Matthews, 1974; Gonzalez &
Eberli, 1997).
Long-term net fl ow measurements over GBB
show a very slow current velocity of 2 cm s
1
toward the north (002°N; Smith, 1995). This is
thought to be the outcome of two almost oppos-
ing forces, a tide-induced fl ow toward the
east-southeast and a wind- and density-driven
northward fl ow (Smith, 1995). Tidal currents
vary from near 0 to 4 km h
1
in the tidal channels
(Newell & Rigby, 1957; Bathurst, 1975) and infl u-
ence the entire platform to some degree.
Water temperature, wind and precipitation
The surface seawater temperature (SST) on GBB
ranges from 18.5°C during winter to 28.5°C in
summer (Cloud, 1962; Bathurst, 1975). During the
summer, higher SSTs are reached locally. Mixing
of bank and ocean waters is very low (Purdy,
1963a). Smith (1940) noted that the difference in
SST between bank and surface ocean waters may
be as large as 3.6°C. The lateral variation in the
SST on the bank itself is very low, but on bank
margins the slow mixing with ocean water is dem-
onstrated by changes in SSTs (Purdy, 1963a).
The Bahama Platform is infl uenced by the
trade winds blowing from east or southeast in
the summer (March to August). In the winter
months, the winds have a northeasterly direction.
Currents and circulation
Most of the surface water masses of the Caribbean
Sea, which eventually form the Florida Current
and the Gulf Stream, enter in the Lesser Antilles
through various passages (Wajsowicz, 2002). A
smaller portion (approximately 20%) enters the
Caribbean Sea through the Windward Passage
between Cuba and Hispaniola (Schmitz &
Richardson, 1991). The Florida Current is fed as
well by water masses through the Old Bahama
Channel and Santaren Channel and through the
Northwest Providence Channel (Fig. 1a; Leaman
et al
., 1995). Atkinson
et al
. (1995) and Leaman
