commonly encountered in low-energy, muddy depositional environments in the platform interior.
5.6.4 Peloids
The term peloid is used for a polygenetic group of
spherical, ellipsoidal or angular grains with diffuse
margins but no internal structure. The grains consist
of fine-grained carbonate irrespective of size or origin.
Because of the diverse origin of the particles, the term
peloid is purely descriptive and does not denote origin.
Various origins are possible: faecal pellets, micritised
grains, intraclasts, a type of abiogenic precipitate, and
a microbially mediated precipitate. Most peloids, however, are formed by extensive micritisation of preexisting carbonate grains which is related to the
activities of microboring algae and fungi within the
photic zone. Thus, peloids generally indicate shallow,
non-agitated waters.
5.6.5 Ooids
Ooids are round grains of carbonate formed through
chemical or biological precipitation by the rolling
action of waves and/or tidal currents in shallow
water. Sediments consisting of ooids are called oolites.
Ooids are by definition less than 2 mm, but most are
0.3–1.0 mm in diameter (Fig. 5.36a,b). They have a
concentric structure with layers of carbonate around a
a
b
0.3 mm
0.3 mm
Pe
Pe
Fig. 5.35 (a) Rounded rod-shaped pellets (Pe). Upper Cretaceous, France. (b) Dark pellets (Pe) in a sandy matrix. Silurian, Oslo
Region, Norway. Thin sections as seen in plane polarised light
Fig. 5.36 (a) Recent ooids from Bahamas. They are well
sorted with grain size mostly between 0.5 and 1 mm.
(b) Ooids are composed of a nucleus overlain by successive
concentric coatings of aragonite. Thin section, plane
polarised light. Recent, Bahamas. (Photograph courtesy of
Arve Lønøy)
5 Carbonate Sediments
181
5.6.4 Peloids
The term peloid is used for a polygenetic group of
spherical, ellipsoidal or angular grains with diffuse
margins but no internal structure. The grains consist
of fine-grained carbonate irrespective of size or origin.
Because of the diverse origin of the particles, the term
peloid is purely descriptive and does not denote origin.
Various origins are possible: faecal pellets, micritised
grains, intraclasts, a type of abiogenic precipitate, and
a microbially mediated precipitate. Most peloids, however, are formed by extensive micritisation of preexisting carbonate grains which is related to the
activities of microboring algae and fungi within the
photic zone. Thus, peloids generally indicate shallow,
non-agitated waters.
5.6.5 Ooids
Ooids are round grains of carbonate formed through
chemical or biological precipitation by the rolling
action of waves and/or tidal currents in shallow
water. Sediments consisting of ooids are called oolites.
Ooids are by definition less than 2 mm, but most are
0.3–1.0 mm in diameter (Fig. 5.36a,b). They have a
concentric structure with layers of carbonate around a
a
b
0.3 mm
0.3 mm
Pe
Pe
Fig. 5.35 (a) Rounded rod-shaped pellets (Pe). Upper Cretaceous, France. (b) Dark pellets (Pe) in a sandy matrix. Silurian, Oslo
Region, Norway. Thin sections as seen in plane polarised light
Fig. 5.36 (a) Recent ooids from Bahamas. They are well
sorted with grain size mostly between 0.5 and 1 mm.
(b) Ooids are composed of a nucleus overlain by successive
concentric coatings of aragonite. Thin section, plane
polarised light. Recent, Bahamas. (Photograph courtesy of
Arve Lønøy)
5 Carbonate Sediments
181