Geology Reference
In-Depth Information
et al. 1979; Major et al. 1988). These mineralogies
formed ooids in the Precambrian (Tucker 1984) and
during much of the Phanerozoic.
Cortical microfabrics: Most modern aragonitic oo-
ids formed in high-energy marine environments exhibit
a concentric or tangential microfabric, which is rarely
well preserved in ancient
concentric tangential ooids
.
Marine low-energy ooids are characterized by a ra-
dial-fibrous fabric which is also known from hypersa-
line environments and Low-Mg calcite ooids formed
in freshwater environments. The original mineralogy
of ancient ooids is inferred from preserved fabrics, ow-
ing to the fact that diagenetic calcification of aragonite
typically results in obliteration of textural details,
whereas ooids exhibiting radial arrangement of acicu-
lar crystals point to original calcite mineralogies (Kahle
1974).
about 0.3 mm. The radial-fibrous structure of ancient
calcitic ooids is considered to be primary because of
the similarities to the microfabric of modern High-Mg
calcite radial ooids from Baffin Bay, Texas (Land et al.
1979), the Amazon Shelf (Milliman and Barretto 1975),
and from deep waters of the Great Barrier Reef
(Marshall and Davies 1975). The significantly higher
Mg content of radial calcite ooids as compared with
tangential ooids may be suggestive of a High-Mg cal-
cite precursor and a transformation of High-Mg calcite
to Low-Mg calcite without destruction of micro-
structures. This primary radial microfabric must be dis-
tinguished from a
secondary radial structure
resulting
from a diagenetic overprint on primary radial and con-
centric ooids (Marshall and Davies 1975). This struc-
ture transects the concentric laminae. The radially ori-
ented large crystals may extend to the outer surface of
the ooid or they may be limited to parts of the cortex.
Traces of concentric layering can be preserved within
the radially oriented crystals.
Radial ooids
(Pl. 13/2, 4) are characterized by a cor-
tical fabric consisting of radial-fibrous crystals. In most
cortices this microfabric also exhibits concentric band-
ing (radial-concentric, banded radial), but banding may
be absent in small ooids with an average diameter of
Micrite ooids
often occur together with other ooid
types. The micritic fabric may be restricted to certain
Microfabric of the cortex
Mineralogy, modern examples
Environment
Concentric
Concentric laminae consisting
Aragonite:
Bahamas, Yucatan,
Very shallow, warm low
(tangential)
of tangentially arranged
Abu Dhabi,
latitudinal seas; common in
ooids
crystals whose long axes are
Persian Gulf
highenergy settings
aligned to the surface of
the laminae.
(Great Salt Lake/Utah)
Lacustrinehypersaline
High microporosity
LowMg calcite: Caliche ooids*
Terrestrial
Radial
Laminae consisting of
Aragonite:
Persian Gulf, Great
Shallow marine, common in
(radialfibrous)
radially arranged crystals;
Barrier Reef, (Yucatan,
lowenergy settings
ooids
long crystal axes perpendicular
Shark Bay,
to the laminae surface
Mediterranean)
Gulf of Aqaba
Seamarginal hypersaline pool
Great Salt Lake/Utah
Lacustrinehypersaline
Mgcalcite: (Baffin Bay/Texas)
Marinehypersaline
Calcite and LowMg calcite:
Nonmarine
e.g. Cave pearls*
Micritic
(random) Laminae composed of randomly
Aragonite:
Bahamas
Shallowmarine
ooids
arranged microcrystalline crystals
or
Laminae obliterated or absent,
due to a pervasive micritization
of the cortex
Fig. 4.20.
Major structural ooid types.
Concentric or tangential ooids originate commonly in high-energy settings; radial or
radial-fibrous ooids are formed in moderate to low-energy environments. Well-preserved radial-fibrous fabrics of ancient
ooids are generally regarded as primary features that have not been structurally changed over the course of transformation of
High-Mg calcite to Low-Mg calcite. Micritic ooids can be generated by the random growth of crystals, or the micritic
appearance of the ooids is caused by obliteration of original tangential or radial microfabrics by micritization and recrystal-
lization of the cortex. Locality names put in brackets refer to places where ooids are rare; asterisks refer to terrestrial grains,
which some authors call ooids, but which are treated as a separate grain category (pisoids, see Sect. 4.2.6) in this topic.
