classification should be based on descriptions that use these
tools. When these instruments are used, it is usually possible
to identify the individual grains forming the rock. Thus,
most classifications require that the most significant sedimentary particle in the rock be described. For instance, if
a rock is composed of ooids, it is termed an oolitic limestone. If the limestone also contains a minor element such
as skeletal fragments, then it is called a skeletal-oolitic
limestone.
Two of the most widely used classifications are those of
Folk (1959, 1962) and Dunham (1962). Both classifications subdivide limestones primarily on the basis of their
matrix content and their major component grains.
Most limestones are classified by Folk (1959, 1962) as
allochemical rocks if they contain over 10% allochems
(transported carbonate grains). Based on the percentage of
interstitial material, the rocks may be further subdivided
into two groups: sparry allochemical limestones (containing
a sparry calcite cement of clear coarsely crystalline mosaic
calcite crystals) and microcrystalline allochemical limestone (containing microcrystalline calcite mud, micrite,
which is subtranslucent grayish or brownish particles less
than about 5 mm in size) (Figure 1). Further subdivision is
based on the allochem ratios of Folk (1962) as is illustrated
in Scholle and Ulmer-Scholle (2003) (Figure 2).
Thus, Folk’s classification (Figures 1 and 2) is most
suited for thin section study. Note that he terms rocks with
appreciable matrix as micrites while matrix-free rocks that
contain sparry calcite cement are termed sparites. Sparites
and micrites are further subdivided on the basis of their
most common grains.
In contrast, Dunham’s classification (Figures 3 and 4)
and its modification by Embry and Klovan (1971) and
James (1984) update and illustrations deal with depositional texture. For this reason, Dunham’s scheme is better
suited for rock descriptions that employ a hand lens or binocular microscope. For example, if the grains of
a limestone are touching one another and the sediment
contains no mud, then the sediment is called
a grainstone. If the carbonate is grain supported but contains a small percentage of mud, then it is known as
a packstone. If the sediment is mud supported but contains
more than 10% grains, then it is known as a wackestone,
and if it contains less than 10% grains and is mud
supported, it is known as a mudstone.
If one compares the two classifications, a rock rich in
carbonate mud is termed a micrite by Folk and
a mudstone or wackestone by Dunham. Moreover,
a rock containing little matrix is termed a sparite by Folk
and a grainstone or packstone by Dunham. The wide range
of percentage of mud matrix that a carbonate may have
and still be termed a packstone by Dunham sometimes
reduces the utility of this classification. Embry has modified the classification of Dunham and Klovan (1971) to
include coarse grained carbonates (above figure). In their
revised scheme, a wackestone in which the grains are
greater than 2 mm in size is termed a floatstone and
a coarse grainstone is called a rudstone.
Both terms are extremely useful in the description of
limestones. Embry and Klovan modified the boundstone
classification of Dunham in order to graphically express
the role that organisms performed during accumulation.
They introduced terms such as bafflestone, bindstone,
and framestone, which are useful in concep, but these
can be extremely difficult to apply to ancient limestones
where diagenesis and sample size can limit one’s ability
to determine an organism’s function.
Other modifications followed Folk’s (1965) recognition
that though micrite is more commonly a product of sedimentary accumulation, it can also be cement, and/or
a product of diagenesis. To this end, Reid et al. (1990)
and Wright (1992) and others are not alone in
reemphasizing the need to recognize the role of micrite
and other allochems as a product of sedimentary matrix,
internal cavity sedimentary fill, diagenesis, and cementation, and that some carbonate grains are the products of
later diagenetic modifications. The reader is urged to recognize the multiplicity of origins of carbonates, though
their sedimentary origin is commoner.
A short history of limestone classifications
The classification of limestones has been constantly updated
as new analytical techniques have been developed and new
information has become available. This has resulted in
new understandings about the origin and depositional
setting of carbonate particles and carbonate sediments.
Limestone classifications underwent a rapid evolution
shortly after Wolf (1961) published an excellent summary
of the early classification schemes for carbonates. In this,
he recognized that many of these classifications unfortunately were inefficient and commonly misleading; in particular, the descriptive terms used to describe carbonates
included detrital, clastic, fragmental, granular, fossiliferous,
calcarenite, coquina, etc. Each one of these terms conveys
one particular aspect of information, but ignored, among
other things, the presence or absence of cement or matrix.
Earlier classifications include Twenhoffel’s of 1932
which recognized three major limestone groups:
1. Organic origin
2. Chemical origin
3. Mechanical origin
In the 1930s, workers differentiated between the following
types of limestones:
1. Hard
2. Mud or soft
In the 1950s, workers recognized:
1. Uncemented
(a) Calcilutites (mud sized)
(b) Calcarenites (sand sized)
(c) Calcirudite (gravel sized)
(d) Mixture of (a), (b), and (c).
2. Primary hard (chemical or biochemically cemented)
3. Secondary hard (diagenetically modified)
194
CLASSIFICATION OF CARBONATES
tools. When these instruments are used, it is usually possible
to identify the individual grains forming the rock. Thus,
most classifications require that the most significant sedimentary particle in the rock be described. For instance, if
a rock is composed of ooids, it is termed an oolitic limestone. If the limestone also contains a minor element such
as skeletal fragments, then it is called a skeletal-oolitic
limestone.
Two of the most widely used classifications are those of
Folk (1959, 1962) and Dunham (1962). Both classifications subdivide limestones primarily on the basis of their
matrix content and their major component grains.
Most limestones are classified by Folk (1959, 1962) as
allochemical rocks if they contain over 10% allochems
(transported carbonate grains). Based on the percentage of
interstitial material, the rocks may be further subdivided
into two groups: sparry allochemical limestones (containing
a sparry calcite cement of clear coarsely crystalline mosaic
calcite crystals) and microcrystalline allochemical limestone (containing microcrystalline calcite mud, micrite,
which is subtranslucent grayish or brownish particles less
than about 5 mm in size) (Figure 1). Further subdivision is
based on the allochem ratios of Folk (1962) as is illustrated
in Scholle and Ulmer-Scholle (2003) (Figure 2).
Thus, Folk’s classification (Figures 1 and 2) is most
suited for thin section study. Note that he terms rocks with
appreciable matrix as micrites while matrix-free rocks that
contain sparry calcite cement are termed sparites. Sparites
and micrites are further subdivided on the basis of their
most common grains.
In contrast, Dunham’s classification (Figures 3 and 4)
and its modification by Embry and Klovan (1971) and
James (1984) update and illustrations deal with depositional texture. For this reason, Dunham’s scheme is better
suited for rock descriptions that employ a hand lens or binocular microscope. For example, if the grains of
a limestone are touching one another and the sediment
contains no mud, then the sediment is called
a grainstone. If the carbonate is grain supported but contains a small percentage of mud, then it is known as
a packstone. If the sediment is mud supported but contains
more than 10% grains, then it is known as a wackestone,
and if it contains less than 10% grains and is mud
supported, it is known as a mudstone.
If one compares the two classifications, a rock rich in
carbonate mud is termed a micrite by Folk and
a mudstone or wackestone by Dunham. Moreover,
a rock containing little matrix is termed a sparite by Folk
and a grainstone or packstone by Dunham. The wide range
of percentage of mud matrix that a carbonate may have
and still be termed a packstone by Dunham sometimes
reduces the utility of this classification. Embry has modified the classification of Dunham and Klovan (1971) to
include coarse grained carbonates (above figure). In their
revised scheme, a wackestone in which the grains are
greater than 2 mm in size is termed a floatstone and
a coarse grainstone is called a rudstone.
Both terms are extremely useful in the description of
limestones. Embry and Klovan modified the boundstone
classification of Dunham in order to graphically express
the role that organisms performed during accumulation.
They introduced terms such as bafflestone, bindstone,
and framestone, which are useful in concep, but these
can be extremely difficult to apply to ancient limestones
where diagenesis and sample size can limit one’s ability
to determine an organism’s function.
Other modifications followed Folk’s (1965) recognition
that though micrite is more commonly a product of sedimentary accumulation, it can also be cement, and/or
a product of diagenesis. To this end, Reid et al. (1990)
and Wright (1992) and others are not alone in
reemphasizing the need to recognize the role of micrite
and other allochems as a product of sedimentary matrix,
internal cavity sedimentary fill, diagenesis, and cementation, and that some carbonate grains are the products of
later diagenetic modifications. The reader is urged to recognize the multiplicity of origins of carbonates, though
their sedimentary origin is commoner.
A short history of limestone classifications
The classification of limestones has been constantly updated
as new analytical techniques have been developed and new
information has become available. This has resulted in
new understandings about the origin and depositional
setting of carbonate particles and carbonate sediments.
Limestone classifications underwent a rapid evolution
shortly after Wolf (1961) published an excellent summary
of the early classification schemes for carbonates. In this,
he recognized that many of these classifications unfortunately were inefficient and commonly misleading; in particular, the descriptive terms used to describe carbonates
included detrital, clastic, fragmental, granular, fossiliferous,
calcarenite, coquina, etc. Each one of these terms conveys
one particular aspect of information, but ignored, among
other things, the presence or absence of cement or matrix.
Earlier classifications include Twenhoffel’s of 1932
which recognized three major limestone groups:
1. Organic origin
2. Chemical origin
3. Mechanical origin
In the 1930s, workers differentiated between the following
types of limestones:
1. Hard
2. Mud or soft
In the 1950s, workers recognized:
1. Uncemented
(a) Calcilutites (mud sized)
(b) Calcarenites (sand sized)
(c) Calcirudite (gravel sized)
(d) Mixture of (a), (b), and (c).
2. Primary hard (chemical or biochemically cemented)
3. Secondary hard (diagenetically modified)
194
CLASSIFICATION OF CARBONATES