Geology Reference
In-Depth Information
Fig. 20.14
Morphology of Joulter Cays shoal complex, Great
Bahama Bank. (
a
) Entire shoal, bounded by deep water to the
east, and platform to the west. Remote sensing image from
4/29/1986. (
b
) Interpretive diagram illustrating general facies
patterns (Modifi ed from Harris (
1979
) )
sets lie directly on bedrock (Cruz
2008
; Neal et al.
2008
) and illustrate that the cross-bed sets can dip
either bankward (on the east side) or basinward (on the
west side).
Joulter Cays shoal complex illustrates spatial and
temporal variability in depositional facies that devel-
oped in response to the Holocene relative rise in sea
level (Figs.
20.16
and
20.17
). The shoal complex was
deposited just north of present-day Andros Island, a
high of Pleistocene bedrock, and probe data suggests
that a nose of Pleistocene high plunges northward
(Fig.
20.16a
) beneath the present shoal. Cores from
this area (Harris
1979
) illustrate a typical vertical suc-
cession of a basal interval of lithoclast packstone and
peloid wackestone, a medial fi ne-grained peloid pack-
stone, and an upper ooid packstone. This succession
includes an upwards-coarsening succession with an
increase in the abundance of ooids. Within the sand
fl at, the thickness of ooid packstone thins bankward, as
the fi ne peloid packstone thickens. In this shoal, ooid
grainstone is abundant only on the eastern, windward
margin, where it locally overlies bedrock and interfi n-
gers with the packstones onto the platform.
The shoal complex is interpreted to refl ect three
general stages of growth (Harris
1979
) : (1) an early
stage during initial fl ooding, in which the lithoclast
packstone and wackestone accumulated in subtle lows
on the Pleistocene bedrock surface; (2) a shoaling
stage of generation and accumulation of ooids, initi-
ated by bedrock highs that focused tidal currents; and
20.4
Possible Stratigraphic Record
and Geologic Examples
20.4.1 Holocene Shoals
Although the sedimentology and morphology of
Holocene ooid shoals in the Bahamas and Caicos have
been studied for over a 100 years, there are strikingly
few extensive published analyses of their potential
application for interpreting the stratigraphic record.
The two areas that have been most extensively studied
are the Cat Cay shoal complex (Ball
1967
; Cruz
2008
)
and the Joulter Cays shoal complex (Harris
1979
) .
The Cat Cay shoal complex is on the western,
leeward margin of Great Bahama Bank, fl anking the
Straits of Florida. This ooid-rich shoal complex extends
~14 km along strike, and is ~2 km wide. In this area, a
bedrock high runs roughly parallel to the margin, west
of the shoal complex, passing eastward to a flat
top-Pleistocene surface ~6 m underneath the shoal
complex (Fig.
20.15
; Cruz
2008
, compare with Purdy
1961
). Within the shoal complex, a core transect
revealed a basal burrowed skeletal-peloidal fi ne sand
that is sharply to gradationally overlain by a ~4 m thick
succession of cross-bed sets that thin upwards (Ball
1967
). The gradational lower contact and the admixed
ooids in the basal peloidal sands suggest a burrowed
contact (Ball
1967
) . More recent high-resolution seis-
mic data suggest that, at least locally, some cross-bed
