OOIDS
Maurice Tucker
Durham University, Durham, UK
Synonyms
Ooliths
Definition
Ooids are spherical-subspherical grains, consisting of one
or more regular concentric lamellae around a nucleus, usually a carbonate particle or quartz grain.
Sediment composed of ooids is referred to as an oolite.
The term ooid has been restricted to grains less than 2 mm
in diameter and the term pisoid (formerly pisolith) is used
for similar grains of a larger diameter. Coated grain is
a general term frequently used for ooids and pisoids, and
includes oncoids, grains with a microbial coating.
Ooids typically form in agitated shallow waters of subtropical seas where they are frequently moved as sandwaves, dunes and ripples by tidal and storm currents,
and wave action. They are commonly found in association
with reefs and reefal limestones; thus they often form
along shelf-margins (e.g., the Bahamas), but they also
occur in tidal-deltas and beach-barriers of shallow ramps
(e.g., Trucial Coast, Arabian Gulf ).
The majority of modern ooids range from 0.2 to 0.5 mm
in diameter, are composed of aragonite and have a high
surface polish. The characteristic microstructure is a tangential arrangement of acicular crystals, 2 mm in length.
Rarely ooids (and pisoids) form in quieter-water marine
locations, such as lagoons and tidal flats, where a radial
fabric is more common. Ooids can form in areas of strong
wave action in lakes, where they usually have a dull, commonly, cerebroid (bumpy) surface. They may be composed of aragonite or calcite, or be bi-mineralic (low-Mg
calcite–aragonite). These may be associated with stromatolite bioherms.
Ancient marine ooids
Ooids in the rock record are generally composed of calcite
(low Mg), but of these some were originally calcite (possibly high Mg), whereas others were originally aragonite.
Primary calcite ooids, whether in high-energy or lowenergy facies, typically have a radial texture of wedgeshaped, fibrous crystals, with an extinction cross under
crossed polars. Ancient ooids originally of aragonite will
have been altered during diagenesis to a greater or lesser
extent. Commonly, the aragonite has been dissolved out
completely, to leave oomoulds, or these holes may be
filled with calcite cement. Some ooids have a fine-grained
micritic texture, mostly the result of micritisation by endolithic microbial organisms.
Origin of ooids
Current ideas for the origin of ooids invoke inorganic or
biochemical processes. For the former, the sea water in
shallow tropical areas is supersaturated with respect to
CaCO 3 , so that this, together with water agitation, CO 2
degassing and elevated temperature, is considered sufficient to bring about carbonate precipitation on nuclei. In
a biochemical origin, the microbial processes in the
organic mucilage (EPS) that coats and permeates the ooids
create a microenvironment conducive to carbonate precipitation. The factors determining the primary mineralogy of
ooids are water chemistry, especially PCO 2 , Mg/Ca ratio,
carbonate saturation, and possibly the degree of water agitation. It is believed that aragonite and high-Mg calcite
ooids are precipitated when PCO 2 is low and Mg/Ca ratio
high, with opposite conditions for low-Mg calcite ooids.
High carbonate supply, as would occur in high-energy
locations, is thought to favour aragonite precipitation over
high-Mg calcite.
There is a secular variation in the original mineralogy
of ooids through the Phanerozoic, with aragonitic ooids,
which may be associated with calcitic ooids (likely highMg calcite originally) in the late Precambrian/early
Cambrian, mid-Carboniferous through Triassic and
Tertiary to the Recent, and calcitic ooids (presumed to
have low-to-moderate Mg content) dominant in the midPalaeozoic and the Jurassic–Cretaceous. This pattern,
which correlates with the first-order, global sea-level
curve, suggests a geotectonic mechanism(s) causing subtle variations in PCO 2 and/or Mg/Ca ratio in seawater. High
sea-level stands, correlating with calcite seas, are times of
high rates of sea-floor spreading, and high PCO 2 from
increased metamorphism at subduction zones, and low
Mg/Ca ratio from increased extraction of Mg
2þ at midocean ridges through seawater pumping.
Bibliography
Tucker, M. E., and Wright, V. P., 1990. Carbonate Sedimentology.
Oxford: Blackwell Science.
752
OOIDS
Maurice Tucker
Durham University, Durham, UK
Synonyms
Ooliths
Definition
Ooids are spherical-subspherical grains, consisting of one
or more regular concentric lamellae around a nucleus, usually a carbonate particle or quartz grain.
Sediment composed of ooids is referred to as an oolite.
The term ooid has been restricted to grains less than 2 mm
in diameter and the term pisoid (formerly pisolith) is used
for similar grains of a larger diameter. Coated grain is
a general term frequently used for ooids and pisoids, and
includes oncoids, grains with a microbial coating.
Ooids typically form in agitated shallow waters of subtropical seas where they are frequently moved as sandwaves, dunes and ripples by tidal and storm currents,
and wave action. They are commonly found in association
with reefs and reefal limestones; thus they often form
along shelf-margins (e.g., the Bahamas), but they also
occur in tidal-deltas and beach-barriers of shallow ramps
(e.g., Trucial Coast, Arabian Gulf ).
The majority of modern ooids range from 0.2 to 0.5 mm
in diameter, are composed of aragonite and have a high
surface polish. The characteristic microstructure is a tangential arrangement of acicular crystals, 2 mm in length.
Rarely ooids (and pisoids) form in quieter-water marine
locations, such as lagoons and tidal flats, where a radial
fabric is more common. Ooids can form in areas of strong
wave action in lakes, where they usually have a dull, commonly, cerebroid (bumpy) surface. They may be composed of aragonite or calcite, or be bi-mineralic (low-Mg
calcite–aragonite). These may be associated with stromatolite bioherms.
Ancient marine ooids
Ooids in the rock record are generally composed of calcite
(low Mg), but of these some were originally calcite (possibly high Mg), whereas others were originally aragonite.
Primary calcite ooids, whether in high-energy or lowenergy facies, typically have a radial texture of wedgeshaped, fibrous crystals, with an extinction cross under
crossed polars. Ancient ooids originally of aragonite will
have been altered during diagenesis to a greater or lesser
extent. Commonly, the aragonite has been dissolved out
completely, to leave oomoulds, or these holes may be
filled with calcite cement. Some ooids have a fine-grained
micritic texture, mostly the result of micritisation by endolithic microbial organisms.
Origin of ooids
Current ideas for the origin of ooids invoke inorganic or
biochemical processes. For the former, the sea water in
shallow tropical areas is supersaturated with respect to
CaCO 3 , so that this, together with water agitation, CO 2
degassing and elevated temperature, is considered sufficient to bring about carbonate precipitation on nuclei. In
a biochemical origin, the microbial processes in the
organic mucilage (EPS) that coats and permeates the ooids
create a microenvironment conducive to carbonate precipitation. The factors determining the primary mineralogy of
ooids are water chemistry, especially PCO 2 , Mg/Ca ratio,
carbonate saturation, and possibly the degree of water agitation. It is believed that aragonite and high-Mg calcite
ooids are precipitated when PCO 2 is low and Mg/Ca ratio
high, with opposite conditions for low-Mg calcite ooids.
High carbonate supply, as would occur in high-energy
locations, is thought to favour aragonite precipitation over
high-Mg calcite.
There is a secular variation in the original mineralogy
of ooids through the Phanerozoic, with aragonitic ooids,
which may be associated with calcitic ooids (likely highMg calcite originally) in the late Precambrian/early
Cambrian, mid-Carboniferous through Triassic and
Tertiary to the Recent, and calcitic ooids (presumed to
have low-to-moderate Mg content) dominant in the midPalaeozoic and the Jurassic–Cretaceous. This pattern,
which correlates with the first-order, global sea-level
curve, suggests a geotectonic mechanism(s) causing subtle variations in PCO 2 and/or Mg/Ca ratio in seawater. High
sea-level stands, correlating with calcite seas, are times of
high rates of sea-floor spreading, and high PCO 2 from
increased metamorphism at subduction zones, and low
Mg/Ca ratio from increased extraction of Mg
2þ at midocean ridges through seawater pumping.
Bibliography
Tucker, M. E., and Wright, V. P., 1990. Carbonate Sedimentology.
Oxford: Blackwell Science.
752
OOIDS