Geology Reference
In-Depth Information
8 Classification - A Name for Your Sample
There are too many papers that simply conclude that
the carbonate rocks of a given study can be arranged
according to the classification of one or more authors.
All too often these workers have failed to recognize that
a classification is simply a tool for organizing infor-
mation, not the source of a conclusion (Blatt et al. 1972).
designated as 'genetic' or 'descriptive'. The concept
tries to consider differences in the kinetic energy
(caused by wave or current action) that might have ex-
isted within the water column either at the interface of
deposition or a meter or two above it. Geologists like
this rather simple dual distinction between high and
low energy sediments!
Classifying and naming carbonate rocks is a rather dry
pursuit, but nonetheless essential to microfacies analy-
sis when interpreting rock properties and paleoenvi-
ronments. Classifications should include objective,
measurable and reproducible features that allow genetic
inferences to be made from descriptive criteria. All the
limestone classifications commonly used in facies
analyses are based on textural and compositional crite-
ria. The classifications proposed by Dunham (1962) and
Folk (1962) have proved most practical. Modifications
suggested by Embry and Klovan (1971), Wright (1992)
and Strohmenger and Wirsing (1991) are of help. Nam-
ing and classifying reef limestones, nonmarine carbon-
ates as well as recrystallized carbonate rocks call for
specific concepts. Major problems involved in the clas-
sification of limestones are the 'low-energy micrite'
paradigm, the confusion between depositional and di-
agenetic textures, and the underestimation of the com-
plexity of biological controls on the formation of reef
carbonates.
Demands arising from the industrial use of carbon-
ate rocks as raw materials led relatively early to classi-
fications focused almost entirely on the chemical and
mineralogical composition. Most textbooks written be-
fore World War II discuss only these classifications.
Classifications based on environmental conditions and
depositional settings began with Pia (1933). During
their work on Arabian reservoir rocks, Bramkamp and
Powers (1958) were probably the first to stress the im-
portance of distinguishing between carbonate sediments
deposited in quiet-water environments and those formed
as current-washed deposits. The distinction between
'low energy' and 'high energy' carbonates has become
a 'leitmotif' of almost all later classifications, whether
8.1 Basic Concepts
There are different approaches to carbonate classifica-
tions, including (1) the choice between textural and non-
textural classification concepts, (2) the preference for
strongly descriptive or genetic classifications, (3) the
divergent consideration of various aspects of deposi-
tion, biotic contribution and diagenesis, and 4) the dif-
ferentiation between biologically controlled limestones
formed in place, and limestones produced by the accu-
mulation of grains.
(1) Non-Textural versus Textural Classifications
Non-Textural Classifications
Mineralogical and chemical composition:
The min-
eralogical composition of carbonate rocks (rocks con-
taining more than 50% by mass of carbonate minerals)
provides a relatively simple means for subdivisions (see
Sect. 3.1.1.1). The distinction between calcium carbon-
ate minerals (predominantly calcite) and dolomite forms
a basic element in almost all chemical carbonate rock
classifications. The term limestone is generally used
for rocks composed predominantly of calcite (>50%),
the term dolomite (or dolostone) characterizes rocks
consisting predominantly of the mineral dolomite
(>50%). The industrial use of carbonate rocks, in par-
ticular requires categories describing intergradations
caused by differences in the relative amounts of calcite
and dolomite and by various amounts of non-carbon-
ate minerals. Differences in chemical composition are
expressed as Ca/Mg weight ratios or as percentages of
calcite and dolomite or non-carbonate constituents. Dif-
