Geology Reference
In-Depth Information
Fig. 20.16
Character and interpreted geologic history of Joulter
Cays ooid shoal complex, Great Bahama Bank, modifi ed from
Harris (
1979
) . (
a
) Thickness (
colors
) and depth to bedrock (
red
lines
) in the area. Note that the shoal lies above a bedrock high
that plunges to the north. (
b
-
d
) Thicknesses of various facies
(
b
- ooid sand,
c
- muddy ooid sand,
d
- muddy fi ne peloidal
sand) in the shoal complex. See text for discussion
kilometers long, oriented roughly normal to the trend
of the complex. Cores from this sand fl at and channel
system suggest it averages between 5 and 8 m thick and
is dominated by burrowed oolitic sand (Fig.
20.18d
),
which makes up an average of 60% of the succession.
Cross-bedded ooid sand makes up the remainder, but
are laterally discontinuous, occurring preferentially on
the margins of the channels (Evans
1987
) .
In contrast, fl anking part of the eastern margin of
the sand fl at and channel belt, a 32 km × 0.8 km bar-
rier bar system developed, apparently post-dating
much of the system to the west. This system is up to
11 m thick and attains higher elevations than the sand
fl at-channel system to the west. In the barrier bar sys-
tem, cross-bedded facies are common, making up
60% of the succession, and the less abundant bur-
rowed facies are discontinuous. The cross-bedded
deposits have complex internal geometries, related to
migration of inlets (along strike) and subaqueous
dunes (across strike, with the ebb and fl ood tides);
individual outcrops reveal decimeter- to meter-scale
tabular- or sigmoidal-cross bedded oolitic grainstone
(Fig.
20.18e
) with dips that can exceed 20°. The east-
ern margin of this complex slopes towards the paleo-
open ocean (topographic gradients of 0.3-3.0 m/km
to the east), and this ooid-rich succession passes
abruptly (across a few 100 m; Halley et al.
1977
) to
subtidal skeletal sands to the east. The barrier bar
system probably refl ects a combination of amalgama-
tion of ebb tidal deltas and longshore transport
(Halley et al.
1977
; Evans
1987
; Grasmueck and
Weger
2002
; Neal et al.
2008
) .
Joulter Cays is not the only Holocene example to
have ancient analogs; trends similar to those recog-
nized in tidal sand ridges from Schooner Cays or
Tongue of the Ocean have been recognized in several
