Geology Reference
In-Depth Information
So, based upon this exercise, it is possible for
a 1 km wide shelf to produce suffi cient oolitic
sediment to generate a realistically sized dune
deposit on the adjacent island, assuming effi cient
transport mechanisms are in place. However,
if the oolitic generation phase was signifi cantly
condensed temporally and spatially to only
1000 years and 100 m wide, then a thickness of
6 cm yr
1
needs to be generated. This is about
60 times faster than the fastest rate reported by
Schlager (1981). Perhaps, this scenario is unre-
alistic? Perhaps, Schlager's (1981) numbers are
incorrect? Perhaps, there is another model that
needs to be considered? Nevertheless, it does
appear possible to create the sediment from the
immediately adjacent shelf.
Fig. 6.
View from the top (45 m at this particular loca-
tion) of the Gregory Town ridge towards the west. This
ridge consists of oolitic limestone of Late Pleistocene age.
Note preserved dune morphology, with the lee side facing
the bank interior. Large westward-dipping foresets (arrow)
further indicate that the constituent sedimentary particles
originated from the east. Road cut is 4 m high.
Ooid production on a wider platform that
has since collapsed
In the second model, based on the present-day
example along the Berry Islands where the outer
platform is over 15 km wide (Fig. 1), ooids were
formed on a wide outer shelf, which no longer
exists, and carried onto the islands by the easterly
trade winds. Such a carbonate factory could have
been more effi cient than that of the fi rst model
because of a more extensive production area. It
would thus better explain the occurrence of the
large oolitic ridges forming the backbone of the
windward islands.
One good example is the Gregory Town ridge in
the northern and central part of Eleuthera (Fig. 6).
This massive, NW-SE-trending crest is 8 km long,
1.5 km wide and culminates at about 55 m. It is
parallel to the Eleuthera shoreline and its top
stands less than 2 km away from the present-day
platform margin, which shows a scalloped (i.e.
convex bankward) morphology in this area (Fig. 7
Mullins & Hine, 1989; Mullins
et al.
, 1991). On
the ocean side of the island, the ridge comprises
well-exposed subtidal and beach deposits up to
4 m above mean sea level. In contrast, only aeolian
sediments, which locally display rainfall-induced
fenestral porosity (Bain & Kindler, 1994; Kindler
& Strasser, 2000), can be seen at higher elevations
and on the bank side.
The Gregory Town ridge consists of fi ne-grained,
well-lithifi ed oolitic grainstone characterized by
the predominance of 'normal' (i.e. thickly coated)
ooids (Fig. 8). Amino-acid ratios obtained from
ridges of identical lithology and stratigraphic
position on Eleuthera (Hearty, 1998) and San
Salvador (Hearty & Kindler, 1993), the occurrence
former
margin ?
N
Glass
window
Gregory town
ridge
10 km
Fig. 7.
Satellite view of northern Eleuthera showing
scalloped (i.e. convex bankward) margin. The extreme
narrowness of the outer platform in this area is probably
related to bank-edge erosion or collapse. The missing part
of the platform was probably the site of ooid production
during MIS 5e. Image from: https://zulu.ssc.nasa.gov.
of an overlying
terra rossa
palaeosol, and the pres-
ence of associated marine deposits at about 4 m
all indicate that this ridge was deposited during
the last interglacial period, i.e. Marine Oxygen
Isotope Stage (MIS) 5e. The primary morphology
of the dune is well preserved, with a lee side fac-
ing towards the bank interior and large aeolian
foresets dipping towards the southwest (Fig. 6).
The orientation of the lee face of the dune and
the dip of the foresets clearly show that the con-
stituent particles originated from the outer plat-
form to the northeast. Today, this outer platform
