Geology Reference
In-Depth Information
a
b
Thickness
of oolite
Raleigh Co.
0-4 ft
4-8 ft
8-12 ft
>12 ft
Cross
bedded
oolite
Burrowed
packstone
Cross
bedded
oolite
Burrowed
packstone
Cross
bedded
oolite
Wyoming Co.
Mercer Co.
0
2
2
0
km
km
Fig. 20.19
Patterns of thickness and facies in ancient ana-
logs. See text for discussion. (
a
) Mississippian example from
West Virginia, modifi ed from Cavallo and Smosna (
1997
) .
(
b
) Mississippian example from the Illinois Basin, modifi ed
from Carr (
1973
) .
Dots
indicate well locations
Although many tidal sands of the Bahamas include
abundant ooids, tidal sands are not always oolitic.
One well-studied example that includes classic tidal
sedimentary structures is the Upper Carboniferous
(Mississippian) Salem Limestone of the United States
Midcontinent (Sedimentology Seminar
1966
; Brown
et al.
1990
). Although it is oolitic in some areas, in the
St. Louis, Missouri, area, this unit consists largely of
skeletal (foraminifera, crinoid, bryozoan) grains, pelo-
ids, and other skeletal debris. Although ooids are not
abundant, evidence for tidal currents is found in the
presence of ubiquitous current ripples with variable
directions (Fig.
20.22a
), muddy toes (Fig.
20.22b
) or
topsets in cross-bed sets, and cross-bedded units with
tidal bundles (Fig.
20.22
c e.g., Brown et al.
1990
) .
that form the most spectacular seascapes in the tropics,
and they are made exclusively of carbonate grains,
with many shoals consisting largely of ooids. These
tidal sands are locally derived (and in many cases,
formed on the shoals themselves) and, refl ecting the
dominant physical infl uences, include a hierarchy of
bedforms and barforms. Barforms in these systems
include a wide array of geometries and sizes, both
within and among shoals, but most shoal complexes
include close relations between sediments, hydrody-
namics, and morphology due to feedbacks. Ancient
tidal sand systems can preserve overall morphology
comparable to Holocene analogs, although clearly not
all do. Preservation of geomorphic forms is probably
enhanced by the early cementation of these systems,
but diagenesis can also lead to dissolution of grains
and complicated patterns of the distribution of porosity
and permeability.
As illustrated above, the sedimentology and geo-
morphology of many carbonate tidal sand systems in
the Bahamas have been extensively documented. In
spite of the wealth of study, important gaps remain in
20.5
Summary and Perspectives
In the clear, shallow, blue-green waters of the Bahamas,
tidal sands represent more than a snorkeler's or sailor's
paradise. These sands are strongly infl uenced by tides
