Geology Reference
In-Depth Information
Fig. 20.21
Thin section photomicrographs of ancient oolitic
sand. (
a
and
b
) Pleistocene oolite, from Crooked Island, Bahamas.
Note how ooids may be completely or partly dissolved, but their
form is preserved as molds by the early cement. (
c
and
d
) Oolite
from a Pennsylvanian oolite in the Bethany Falls Limestone,
Kansas. As in the Pleistocene example, some ooids are completely
dissolved, others are only partly dissolved. Unlike the Pleistocene
sample, this entire rock has been recrystallized. In this rock, most
porosity is in molds, and much of the original interparticle poros-
ity has been occluded by cement. In some examples such as this,
although porosity may be high, if the molds are not connected or
are connected by only small pore throats, permeabilities may be
low (note the white voids in C - these are voids that remained
unfi lled during vacuum impregnation of the sample)
changed through the Holocene, in response to both
local and global change. When did shoals originate?
Are all oolitic shoals of the same age? Why have
some complexes backstepped onto the platform
(Lily Bank), whereas others have aggraded and
fi lled much of the available accommodation (Joulter
Cays, Fish Cays), and still others may not have
changed during this rise?
3. How does the granulometry of Holocene shoals
compare with ancient shoals? Because the
detailed stratigraphic records of most Holocene
ooid shoals are not well documented, Holocene
analogs are most commonly applied in general,
qualitative terms. Likewise, the detailed granu-
lometry of ancient analogs are only rarely quanti-
tatively documented. Are relationships between
morphology and facies comparable to those doc-
umented herein preserved and discernable in the
rock record?
4. At a larger scale, all of the examples documented in
this chapter are from the tidally-dominated, fl at-
topped isolated platforms of the Bahamas, with
distinct (and geologically unique?) wave conditions,
tides, and geochemical settings. How analogous are
platform-top Bahamian Holocene examples to many
ancient systems, including ramp systems? Is it pos-
sible that Bahamian Holocene oolitic systems pro-
vide a limited or non-unique suite of actualistic
models or incomplete sampling of the possible range
of variability?
5. The Bahamas include some of the most expansive
accumulations of oolitic sand today. Recent studies
(Rankey and Reeder
2009,
2010
) have suggested
that this is related to the elevated supersaturation
