Geology Reference
In-Depth Information
Fig. 4.22.
Microfossils within and on ooids
.
A
- Ooids offering a hard substrate for encrusting organisms. In areas dominated
by mobile calcareous sands, ooids and bioclasts often are the only substrates available for encrusting organisms, particularly
foraminifera. Note the multi-stage structure of the ooid. Ooid growth started on a nucleus formed by an ooid. The develop-
ment of radial-fibrous laminae was disturbed by the encrustation of foraminifera, followed by a new set of laminae with
radial-fibrous microfabric. The outer part of the ooid exhibits a tangential-concentric microfabric reflecting the change to
more agitated water energy. The strong overgrowth by nubeculariid foraminifera indicates the absence of grain transport.
Late Triassic (Carnian): Central Carinthia, Austria.
B
- Fossils as ooid nuclei. Due to the high mobility of ooid grains, oolitic limestones are commonly poor in autochthonous
fossils. Exceptions are microfossils occurring as ooid nuclei, e.g. benthic foraminifera which provide clues to the age of
oolites and their depositional environment. The nucleus of the radial ooid is an endothyranid foraminifera. Early Carbonifer-
ous (Kirchbach limestone, Viséan): Carnic Alps, Austria. Scale is 1 mm.
subtidal marine environments. Non-marine ooids are
known from lacustrine and terrestrial sediments. Some
of these 'ooids' are pisoids (Sect. 4.2.6). Ooids form in
wave-agitated regions of the seas (high-energy ooids),
and in marginal-marine or non-marine low-energy set-
tings ('quiet-water ooids').
Excellent examples of modern marine ooids (Box
4.14) include those from the Bahamas, the Persian Gulf,
the Red Sea and the Great Barrier Reef. Very impor-
tant studies on ooids formed in a hypersaline setting
have been carried out in the Great Salt Lake in Utah, in
coastal pools of the Gulf of Suez, and the southern Red
Sea.
The Quaternary
ooid sand shoals of the Bahama
Bank
originate and accumulate in different subenviron-
ments, including mobile fringes, sand flats, tidal chan-
nels, platform shelfs, open margins, and platform inte-
riors. The shoals consist of long and narrow or spill-
over bars and tidal channels that trend nearly perpen-
dicular to the length of the shoal. Tidal flow and wave-
generated current cause the ooid sands to be in motion
over the entire width of the shoal. On average ooids
range in diameter from about 0.25 to 1.00 mm and very
rarely exceed 2 mm. The variations in the depositional
settings are produced by a decrease in grain agitation
from the ocean-facing margin of the interior of the ooid
shoals. Local variations relate to topography and the
stabilization of the sea bottom by vegetation. The main
environments differ in the composition, abundance and
association of carbonate grains, the distribution patterns
of organisms, as well as the type and frequency of physi-
cal and biogenic sedimentary structures. Ooids occur
together with peloids, aggregate grains and skeletal
grains. The distribution of ooid sand shoals is restricted
to
high-energy
zones along open bank margins. High-
energy deposits are tidal-current or wind-wave-gener-
ated (Wanless and Tedesco 1993).
Tidal-current generated sands are characterized by
ooid growth in water depths ranging from less than 1 m
to as much as 10 m. Well-developed tangential ooids
originate in marine sand belts, tidal bar belts, flood-
tidal belts and in areas stabilized by algae and seagrass.
Tide-dominated ooid sand bodies have been studied in
the Bahamas and in the Persian Gulf (summarized in
Halley et al. 1983). Examples of ancient oolites formed
in tidally influenced zones are known from the Missis-
sippian (Smosna and Koehler 1993) and Jurassic oo-
lites of southern England (Klein 1965, Wilson 1968).
Wind-wave-generated oolitic sands (southeastern
Bahamas, Caicos platform) contrast strongly with tidal-
generated oolitic sands, restricted to shoals near the plat-
form margins (northern Bahamas). Wind-wave gener-
ated sands are characterized by thick and widespread
sheet-like ooid grainstones. Regular spherical, tangen-
tial ooids form stratified deposits on wind-wave agi-
tated shallow subtidal shoreface settings on the plat-
form interior, and on ocean-wave-agitated shorefaces
facing the platform margin. Irregularly shaped ooids
