Geology Reference
In-Depth Information
4.2 Carbonate Grains
Terminology of carbonate grains
: Fig. 4.7 summa-
rizes the definitions between the major grain type cat-
egories commonly used to describe carbonate rocks.
Excellent colored thin-section photographs of grain
types can be found in Adams and MacKenzie (1998).
Subtypes and the controls on the formation of grains
are discussed in the following paragraphs of this chapter.
The nomenclatoral differentiation of carbonate
grains developed from the study of modern carbonate
environments in the 1960s and was rapidly augmented
by facies studies of ancient carbonate rocks. Today,
more than thirty useful and less useful terms are known,
having been proposed for specific grain types that dif-
fer in real or inferred origin, morphology, internal struc-
tures and size. Microfacies analysis needs well-defined
grain categories, but it should be kept in mind that a
careful description is more important than a desperate
search for an instant label for a grain.
Various names are in use for organic and non-organic
particles of limestones that are larger than the ground-
mass: Grains, particles, constituents, and 'allochems'.
The last term is a collective term for mechanically de-
posited grains that have undergone transportation in
most cases (Folk 1962).
Grosso modo
skeletal grains
and non-skeletal grains can be separated. Skeletal grains
comprise complete or fragmented fossils.
Common 'non-skeletal' grains are peloids, various
coated grains (Peryt 1983), grain aggregates and clasts.
The strict differentiation of 'skeletal' and 'non-skeletal'
grains, however, has been discarded, because the for-
mation of most coated grains and grain aggregates as
well as some ooids and peloids is substantially con-
trolled by biotic processes.
Small micritic grains, commonly without internal structure. Subrounded, spherical,
ovoid or irregular in shape. Size between <0.02 and about 1 mm, commonly
0.10 to 0.50 mm.
PELOIDS
Rounded skleletal grains and other grains covered by a thin micrite envelope.
Boundary between the central grain and the envelope indistinct. Size between
<1 mm to a few centimeters.
CORTOIDS
Large and small grains consisting of a more or less distinct nucleus (e.g. a fossil)
and a thick cortex formed by irregular, nonconcentric, partially overlapping
micritic laminae. Laminae may exhibit biogenic structures. No tendency to
increase sphericity during growth. Size from <1 mm to a few decimeters.
ONCOIDS
Spherical or ovoid grains, consisting of smooth and regular laminae formed as
successive concentric coatings around a nucleus. Laminae may exhibit tangential
and radial microfabrics. Size between 0.20 and about 2 mm, commonly
between 0.5 and 1 mm.
OOIDS
Large subspherical and irregularly shaped grains, consisting of a mostly non
biogenic nucleus and a thick cortex formed by conspicuously, often densely
spaced laminae exhibiting tangential and radial microfabrics. Pisoids occur as
isolated grains or are incorporated in crusts. Size generally >2 mm, up to >1 cm.
PISOIDS
Compound grains consisting of two or more originally separated particles
(e.g. ooids, skeletal grains) that have been bound and cemented together, forming
grapelike or rounded lumps. Intergrain spaces filled with micrite or spar. Outline
irregular lobular or rounded. Size 0.5 to more than 2 mm.
GRAIN AGGREGATES
Synsedimentary or postsedimentary lime clasts, reworked partly consolidated
carbonate sediment or already lithified material. Shape and size are highly
variable: angular to rounded. Size ranges between <0.2 mm and several
decameters. Very small clasts are hardly distinguishable from peloids.
CLASTS
Fragmented or complete skeletons of organisms. Size from 0.05 mm to many
centimeters.
SKELETAL GRAINS
Fig. 4.7.
Descriptive
terminology of th
e
major categories of carbonate grains.
Some of these grain types represent gradational
stages: Cortoids may develop into oncoids; spherical isolated pisoids may resemble ooids structurally, and very small micritic
clasts are often included within the peloid group.
