Our discussion on carbonates included a general classification in
inorganic (travertine, tufa, oolite) and bioclastic (fossiliferous
limestone, reefs, coquina, chalk, micrite) carbonates. In reality, the
more precise main classification schemes, Folk’s and Dunham’, are
used and have also been discussed.
Folk’s scheme relies on the presence (or absence) of the three
components that make up a carbonate, individual (allochemical)
grains, cement, and matrix.
Allochemical grains can be separated in four major types:
- Bioclasts (broken and/or whole skeletal parts)
- Ooids (spherical grains formed by calcite precipitation around a nucleus)
- Peloids (including fecal pellets, whole or fragmented calcareous algae,
micritised grains, and mud clasts)
- Intraclasts (sand- or gravel-size pieces of limestone or dolostone coming
from the same basin [“intra”] of deposition after disruption [“clasts”]
The allochemical grains of a carbonate rock are held together by either
cement or matrix.
The most common carbonate cement is called sparry carbonate, or sparite.
This cement is a clear crystalline carbonate that has been precipitated
between the grains or has developed by recrystallization of carbonate clasts.
Matrix in most carbonate rocks is a murky, fine-grained calcium carbonate
mud called micrite. Unlike sparite, the fine-grained carbonate mud is
deposited with the clasts and, during lithification, is recrystallized.
Geology 252, Historical Geology, California State University, Los Angeles
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The Dunham’s classification is the most widely used scheme for the
description of limestone in the field, in hand-specimen, and in thin
section.
The primary criterion used in this scheme is the texture, which is
described in terms of the proportion of carbonate mud present and the
framework of the rock (see figure next page).
The first stage in using Dunham’s classification is to determine
whether the fabric is matrix- or clast-supported.
Matrix-supported limestone is divided into carbonate mudstone and
wackestone. If the limestone is clast-supported it is termed a
packstone if there is mud present or a grainstone if there is little or no
matrix. A boundstone has an organic framework such as a coral
colony. The original scheme did not include the subdivision of
boundstone into bafflestone, bindstone, and framestone, which
describes the type of organisms that build up the framework. these
categories, along with the addition of rudstone (which are clastsupported limestone conglomerate) and floatstone (matrix-supported
limestone conglomerate) were added a few years later
The nature of the grains or framework material makes up the
secondary part of the classification. A rock consisting entirely of ooids
with no matrix would be an oolitic grainstone, one composed of about
75% broken shelly fragments in a matrix of carbonate mud is a
bioclastic packstone, and rock composed mainly of large oyster shells
termed a bioclastic rudstone.
Naming a limestone using the combination of textural and
compositional criteria in the Dunham scheme provides information
about the likely conditions under which the sediment formed: for
example, a coral boundstone forms under quite different conditions to
a foraminiferal wackestone.
Geology 252, Historical Geology, California State University, Los Angeles
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