Geology Reference
In-Depth Information
rounded by an external cortex, the outer part of which
is concentrically smoothly laminated. Most ooids are
smaller than 2 mm in diameter; many ooids between
0.5 and about 1 mm in size. Modern calcareous ooids
consist of aragonite and/or calcite. The bestknown non-
carbonate ooids are iron ooids. The cortices of ooids
exhibit a variety of microfabrics caused by different
orientations of carbonate crystals in the laminae. Tan-
gentially or randomly arranged crystals form
tangen-
tial ooids
(usually aragonite; Pl. 3/3, Pl. 120/1, Pl. 136/
7), radially arranged crystals form
radial-fibrous ooids
(High-Mg calcite and/or aragonite; Pl. 13/2, Pl. 120/2,
Pl. 130/3).
Micrite ooids
are spherical grains composed
of a nucleus and a micritic cortex, in places exhibiting
a vague concentric structure (Pl. 13/5, 6; Pl. 134/7).
Spheroids
are small grains composed predominantly
of radially arranged fibrous crystals.
low supply of new nuclei and high cortex growth rate
both promote the growth of large ooids. High average
water velocity and velocity gradients also enhance ooid
growth (Sumner and Grotzinger 1993).
Conspicuously large ooids appear to be more abun-
dant in Precambrian open-marine carbonates than in
Phanerozoic carbonates (Pl. 16/8). Neoproterozoic gi-
ant ooids may have formed through a combination of
lower nuclei supply (because of the lack of skeletal frag-
ments during the Precambrian), higher growth rate, and
higher average agitation level (which might be caused
by a predominance of carbonate ramps over rimmed
carbonate platforms during the Neoproterozoic; Sumner
and Grotzinger 1993).
A useful method for determinating the true grain
size of ooids in thin sections was suggested by Fabri-
cius (1967).
Comparison with other grains
Oncoids exhibit a more irregular, often non-concen-
tric lamination; many oncoids are considerably larger
than ooids. Pisoids are characterized by concentric lami-
nae, but differ from ooids in their larger size (usually
several millimeters) and often very irregular shape.
Nucleus and cortex:
Nuclei include lithoclasts, pe-
loids, skeletal grains and minerals, e.g. quartz grains.
Fecal pellets are common nuclei, particularly of ooids
formed in low-energy environments (Jones and Good-
body 1984). Carozzi (1960) drew attention to the fre-
quent association of ooids and rounded elliptical or
spherical micritic grains representing rounded bio- or
lithoclasts (Fig. 4.25). These grains (called 'pseudo-
oolites') also occur as nuclei of ooids and indicate a
long abrasion phase prior to overgrowth by ooid lami-
nae. The cortex/nucleus ratio is an important descrip-
tive criterion of ooids.
Criteria
Descriptive criteria providing clues to the environ-
mental conditions and the depositional setting of ooid
formation are the microfabric of the cortex, nuclei types,
the shape and size of the ooid, associated fossils and
sedimentary textures.
Shape and size:
Ooids occur as
single
or
compound
(complex) ooids (Pl. 136/7). The latter consist of two
or more ooids bound together to form the nuclei for a
new larger ooid (polyooid). 'Multiple ooids' result from
the intergrowth of several ooids. Most ooids are spheri-
cal, but egg-shaped ooids are also common. The shape
often results from the shape of the nuclei. Size and sort-
ing provide clues to the hydrodynamic conditions. Ooid
size is controlled by the supply of nuclei (Bathurst
1975), growth rate (Swett and Knoll 1989), mobiliza-
tion and agitation (Carozzi 1989) and abrasion (Med-
wedeff and Wilkinson 1983). The maximum size of
most ooids (about 1 mm) may reflect a balance between
growth and abrasion. Experiments suggest that size dif-
ferences between radial ooids and radial-concentric
ooids may be explained by ooid transport processes
(Davies et al. 1978). Heller et al. (1980) observed a
critical size associated with a change in radial ooids to
tangential ooids at 0.6 mm; this probably reflects a
change in the mode of transport from suspension to
bed load. Computer-based models demonstrate that a
Superficial ooids
(Carozzi 1957) are ooids, in which
the thickness of the cortex is distinctly less than half of
the diameter of the entire ooids (Pl. 3/1, Pl. 29/6).
'Normal' ooids
are characterized by a cortex whose
thickness is equal or greater than half the diameter of
the ooid. Superficial ooids often only reveal one or two
laminae. Very thin oolitic films are a common feature
of Bahamian ooids (Bathurst 1967). The average thick-
ness of ooid laminae is 1-3
m. Cortex thickness is
related to environmental controls, particularly hydro-
dynamic factors. Small nuclei tend to have thick coat-
ings, that probably form quickly, because smaller grains
are more easily put into motion. Larger grains which
are seldom moved have only superficial coatings or no
coating at all (Bahamian ooids, Harris 1979).
Mineralogy and microfabric
(Fig. 4.20):
The cor-
tex of modern ooids consists of aragonite, Mg-calcite
or calcite. Aragonite and Mg-calcite can co-occur in
alternating laminae within one and the same ooid (Land
