Geology Reference
In-Depth Information
ferent limits are used to define subgroups within the
calcite/dolomite/non-carbonate triangle. Subdividing
carbonate rocks according to their chemical composi-
tion (e.g. CaO, MgO, SiO
2
, and P
2
O
5
) is essential for
the economic use of limestones and dolostones (e.g.
Oates 1998). Mineralogical-chemical classifications are
discussed in Sect. 18.1.
Physical properties:
Reservoir rock studies need
classifications which consider petrophysical properties
such as porosity and permeability. These classifications
(Choquette and Pray 1970; Mazzullo et al. 1992;
Kopaska-Merkel and Mann 1993) differ significantly
from conventional limestone classifications because
they focus on genetic complex criteria (type, pore ge-
ometry and distribution) which are controlled by the
total of the factors reflecting the history of the rock
(see Sect. 7.3).
subjective. Most of the currently used limestone clas-
sifications are descriptive-genetic and emphasize the
descriptive data that provide evidence of environmen-
tal and depositional controls.
The term
eventstone
, introduced by Hüssner (1985),
is strictly genetic. It designates limestones that were
deposited relatively quickly as compared with the back-
ground sedimentation. Eventstones include turbidites
and tempestites and exhibit various depositional fabric
types. The name should not be used in a descriptive
way (see Sect. 12.1.2).
A distinctly genetic classification was proposed by
Plumley et al. (1962). The classification is based on
the inferred degree of water agitation. The authors dis-
tinguish five major limestone categories, with a grad-
ing spectrum ranging from quiet water to strongly agi-
tated water deposits (see Sect. 12.1.1.2).
(3) Depositional, biological and diagenetic aspects
Classifications are differently focused on deposi-
tional, biological-paleontological and diagenetic crite-
ria seen in carbonate rocks. Most classifications using
texture and fabric claim that the groupings reflect dif-
ferences in depositional processes. In contrast, classi-
fications of reef carbonates demand an understanding
of biological controls and of the interplay between or-
ganisms, sedimentation and cementation. Diagenesis
modifies and obliterates primary depositional textures,
and creates new fabrics that require specific nomen-
clatorial treatment (see Pl. 47).
Textural classifications
Nearly all limestone classification systems used to-
day were developed during the 1950s and 1960s in re-
sponse to the needs of oil companies who had discov-
ered major hydrocarbon reservoirs in carbonate rocks.
The long-standing bestselling topic
Classification of
Carbonate Rocks
(Ham 1962) published by the Ameri-
can Association of Petroleum Geologists, includes the
two standard classifications of Folk and Dunham as
well as less frequently used systems (Leighton and
Pendexter 1962; Powers 1962; Thomas 1962). Folk's
Practical classification of Limestones
and Dunham's
Classification of carbonate rocks according to deposi-
tional structure
have become largely accepted and
widely used systems. Both classifications were modi-
fied during the last few decades: New grain types were
added to Folk's system and quantitative boundaries
changed (Flügel 1982; Strohmenger and Wirsing 1991),
additional terms were introduced to Dunham's classi-
fication allowing a more detailed differentiation of reef
carbonates (Embry and Klovan 1971; Tsien 1981;
Cuffey 1985) and diagenetically altered limestones
(Wright 1992). Some of these proposals were criticized,
others, particularly those dealing with names coined
for reef carbonates, were apparently misunderstood be-
cause of the different geological and biological ap-
proach in reef studies.
(4) Autochthonous and allochthonous carbonates
Most limestone classifications differentiate between
two major groups: (1) limestones originating from the
activity of benthic sessile organisms and (2) limestones
composed of grains, lime mud and/or carbonate cement.
The first group contains
autochthonous carbonates.
The
second group has been called
allochthonous carbon-
ates
(or 'allochemical' carbonates sensu Folk 1959).
Characteristic autochthonous carbonates are reef lime-
stones and carbonates whose formation is influenced
by microbes (microbialites). Note that the term 'alloch-
thonous carbonates' is also used in the context of mass-
flow deposits (see Sect. 15.7.2).
8.2 Reef Limestones and Microbial
Carbonates (Autochthonous
Carbonates)
(2) Descriptive versus genetic views
Because a geologist must commonly classify and
name a sample before its origin can be determined, de-
scriptive classifications are preferable, at least during
field work. Genetic classifications require an under-
standing of the complicated processes responsible for
the formation of a given rock type and are, hence, more
Starting with Grabau (1904), all major classifications
have considered limestones which were organically
bound at the time of deposition as an independent group.
