along the surface varies, a very irregular relief
develops which may be taken as a minimum measure
of the thickness of the carbonate layer dissolved.
The carbonate dissolved at grain contacts and along
stylolites is transported to the adjacent sediments by
diffusion. Here the carbonate will usually reprecipitate in the pores, with a possible reduction in
porosity of up to about 20%. Investigation of
interbedded layers of limestone and dolomite (the
rock is also called dolostone) show that the former is
more susceptible to pressure solution than the latter.
Dolostone usually has a strong framework of
interconnected dolomite rhombs. Stylolites are therefore especially well developed in carbonates that are
almost exclusively calcitic. Some late diagenetic dolomite rhombs may precipitate along developing
stylolite surfaces as a result of a dynamic system
with pressure gradients on a microscopic scale.
CO 2 and organic acids generated from
decomposing organic matter are rapidly neutralised.
The pH decreases with depth because of the increasing
amounts of CO 2 that can be dissolved in the porewater
as the pressure increases. The solubility of calcite
increases with increasing pressure but at normal
hydrostatic pressure gradients temperature is the
overriding factor determining the solubility gradients.
Calcite precipitates only very locally during upwards
flow, where there are abrupt pressure drops.
Compaction-driven flow is normally directed
upwards and since the solubility then is reduced, dissolution rather than precipitation will occur. The
capacity of compaction-driven porewater to transport
carbonate is in any case rather limited because of the
low solubility gradients and moderate fluid fluxes at
greater depth.
5.7.7 Classification of Carbonate Rocks
Early petrologists often subdivided limestones according
to the size of the dominant mechanically deposited
grains: Calcilutite (grains <63 μm), calcarenite (grains
between 63 μm and 2 mm) and calcirudite (grains
>2 mm). Later Folk’s (1959, 1962) classification was
widely accepted because of its applicability to a wide
range of carbonate rock types and the ease with which its
terms could be utilised and understood. Folk’s classification is based on the idea that, in principle, the
sedimentation of carbonate sediments is comparable to
that of terrigenous material. However, today most
workers prefer to use the classification by Dunham
(1962) because it is not based on the composition of
the matrix but on the nature of the framework, which is
more applicable in revealing the depositional processes.
5.7.8 Folk (1959, 1962) Classification
Disregarding admixture of terrigenous material, Folk
(1959, 1962) distinguished between three basic
components of limestones:
1. Sediment grains (allochems). The principal
allochems are: skeletal grains, ooids, peloids and
fragments of carbonate rocks (intraclasts or
extraclasts).
2. Microcrystalline lime mud (micrite) comprising
clay-size particles (grain size <4 μm). In modern
carbonate environments such as the Bahamas most
of the mud consists of micron-sized needles of
aragonite produced by green algae like Halimeda
and Penicillus. These are transported as clay fraction material.
3. Sparry calcite cement (sparite) which is carbonate
crystals that have been diagenetically precipitated
in the pore space after deposition. While the carbonate mud and some of the grains have a brown
stain due to organic material, the cement stands out
as clear and transparent in thin section.
Almost all carbonate deposits contain more than
one type of material, and Folk’s classification is
based on the relative proportions of the three
endmembers: allochems, microcrystalline lime mud
and sparry calcite cement (Fig. 5.58). Allochems represent the framework of the rock making up the bulk of
most limestones. The matrix between the allochems
may consist of lime mud if there is little bottom current. In limestones this mud may have recrystallised
into small calcite crystals (microcrystalline mud),
which is called micrite and is thus an indicator of a
low-energy environment.
Deposits consisting of well-sorted allochems have
primary porosity which may be filled with cement during
diagenesis. The cement is precipitated from aqueous
solutions and consists of clear, transparent crystals
(spar) which are easy to distinguish from micrite,
which tends to be brownish because of the organic
202
N.-M. Hanken et al.
develops which may be taken as a minimum measure
of the thickness of the carbonate layer dissolved.
The carbonate dissolved at grain contacts and along
stylolites is transported to the adjacent sediments by
diffusion. Here the carbonate will usually reprecipitate in the pores, with a possible reduction in
porosity of up to about 20%. Investigation of
interbedded layers of limestone and dolomite (the
rock is also called dolostone) show that the former is
more susceptible to pressure solution than the latter.
Dolostone usually has a strong framework of
interconnected dolomite rhombs. Stylolites are therefore especially well developed in carbonates that are
almost exclusively calcitic. Some late diagenetic dolomite rhombs may precipitate along developing
stylolite surfaces as a result of a dynamic system
with pressure gradients on a microscopic scale.
CO 2 and organic acids generated from
decomposing organic matter are rapidly neutralised.
The pH decreases with depth because of the increasing
amounts of CO 2 that can be dissolved in the porewater
as the pressure increases. The solubility of calcite
increases with increasing pressure but at normal
hydrostatic pressure gradients temperature is the
overriding factor determining the solubility gradients.
Calcite precipitates only very locally during upwards
flow, where there are abrupt pressure drops.
Compaction-driven flow is normally directed
upwards and since the solubility then is reduced, dissolution rather than precipitation will occur. The
capacity of compaction-driven porewater to transport
carbonate is in any case rather limited because of the
low solubility gradients and moderate fluid fluxes at
greater depth.
5.7.7 Classification of Carbonate Rocks
Early petrologists often subdivided limestones according
to the size of the dominant mechanically deposited
grains: Calcilutite (grains <63 μm), calcarenite (grains
between 63 μm and 2 mm) and calcirudite (grains
>2 mm). Later Folk’s (1959, 1962) classification was
widely accepted because of its applicability to a wide
range of carbonate rock types and the ease with which its
terms could be utilised and understood. Folk’s classification is based on the idea that, in principle, the
sedimentation of carbonate sediments is comparable to
that of terrigenous material. However, today most
workers prefer to use the classification by Dunham
(1962) because it is not based on the composition of
the matrix but on the nature of the framework, which is
more applicable in revealing the depositional processes.
5.7.8 Folk (1959, 1962) Classification
Disregarding admixture of terrigenous material, Folk
(1959, 1962) distinguished between three basic
components of limestones:
1. Sediment grains (allochems). The principal
allochems are: skeletal grains, ooids, peloids and
fragments of carbonate rocks (intraclasts or
extraclasts).
2. Microcrystalline lime mud (micrite) comprising
clay-size particles (grain size <4 μm). In modern
carbonate environments such as the Bahamas most
of the mud consists of micron-sized needles of
aragonite produced by green algae like Halimeda
and Penicillus. These are transported as clay fraction material.
3. Sparry calcite cement (sparite) which is carbonate
crystals that have been diagenetically precipitated
in the pore space after deposition. While the carbonate mud and some of the grains have a brown
stain due to organic material, the cement stands out
as clear and transparent in thin section.
Almost all carbonate deposits contain more than
one type of material, and Folk’s classification is
based on the relative proportions of the three
endmembers: allochems, microcrystalline lime mud
and sparry calcite cement (Fig. 5.58). Allochems represent the framework of the rock making up the bulk of
most limestones. The matrix between the allochems
may consist of lime mud if there is little bottom current. In limestones this mud may have recrystallised
into small calcite crystals (microcrystalline mud),
which is called micrite and is thus an indicator of a
low-energy environment.
Deposits consisting of well-sorted allochems have
primary porosity which may be filled with cement during
diagenesis. The cement is precipitated from aqueous
solutions and consists of clear, transparent crystals
(spar) which are easy to distinguish from micrite,
which tends to be brownish because of the organic
202
N.-M. Hanken et al.