Geology Reference
In-Depth Information
8.3.2.2 Discussion
tors but also by the diagenetic history, he added five
categories predominantly describing diagenetic rock
types (Fig. 8.5): Three of these rock types are non-oblit-
erative with regard to the primary depositional fabric
(
cementstone, condensed grainstone
and
fitted grain-
stone;
Pl. 47/3
).
Two categories, differentiated by crys-
tal size (using Folk's 1962 scale), are obliterative be-
cause they have lost their depositional fabric as a result
of complete obliterative recrystallization or replace-
ment.
Sparstones
are composed of inequant sparry cal-
cite crystals forming a blocky mosaic, with a crystal
size larger than 10
m. The term
microsparstone
ap-
plies to limestones consisting of calcite mosaics with
crystals between 4
m and 10
m (Pl. 47/2).
Folk's classification has many
advantages but also some
pitfalls
in that not all the information included in mi-
crofacies features is revealed:
• The combined rock names (prefix for grains, suffix
for matrix) are simple to learn and the terms can be
easily used in a descriptive way.
• The rock names can be freely combined with a num-
ber of modifying and qualifying adjectives.
• The recognition of well-defined rock types facili-
tates facies-oriented studies because the basic binary
principle is oriented to major differences in the envi-
ronmental controls of deposition. However, paleoen-
vironmental interpretations which cling too strongly to
the idea of low versus high energy controls on the sedi-
mentation, may go wrong. The possibility, that low-
energy conditions are only simulated by lime mud trick-
ling down into empty pores of high-energy deposits or
that the fine-grained 'micritic matrix' does not corre-
spond to a low-energy lime mud but instead represents
a cryptocrystalline cement, should always be taken into
account.
• A major shortcoming of the original classification is
the loss of information about grain diversity because
less frequent grain types are not indicated in the rock
name. This difficulty can be overcome by using the
modified classification of Strohmenger and Wirsing.
• Limestones containing both groundmass types rep-
resent problems in all limestone classifications. In
Dunham's classification these types are incorporated
within the packstone group. Folk (1962), stressing the
wide textural spectrum of carbonates, proposed a tran-
sitional class ('poorly washed limestones'), defined as
having 1/3 to 2/3 spar and 2/3 to 1/3 micrite (Fig. 8.6).
Both Folk's and Dunham's classifications can be
sharpened by using additional criteria reflecting pack-
ing intensity (e.g. densely packed, loosely packed), the
existence of large grains (e.g. rudite wackestone), the
degree of winnowing (poorly or strongly winnowed),
the most frequent grain type, and the amount of terrig-
enous particles.
8.4.2 Some Nonmarine Carbonates Need
very Specific Names
Many nonmarine carbonates are characterized by
particular components, fabrics, and microfacies types,
which are difficult to describe using the standard tex-
tural classifications developed for marine carbonate
rocks. Some of these difficulties are caused by the strong
impact of abiogenic factors on the formation of non-
marine carbonates such as speleothems, travertine, tufa
and caliche. Particular rock names have been proposed
for these limestones (Gebhardt 1988; Pedley 1990;
Riding 1991; Wright 1992; Koban and Schweigert
1993). Lacustrine limestones are better suited for ap-
plying standard classification schemes, especially that
of Dunham, because carbonates formed within lakes
are more strongly controlled by biotic factors. Folk's
classification is less appropriate because of the strong
emphasis on hydrodynamic aspects, which may have
had less influence on the textures of lacustrine carbon-
ates.
Nonmarine carbonates formed at the margins of
aquatic environments (e.g. lakes) may be strongly af-
fected by pedogenic and meteoric processes overprint-
ing primary textures. The Miocene carbonates of the
Ries crater lake in Southern Germany are excellent ob-
jects for testing the efficiency of a classification, by
combining descriptive criteria (following the Dunham
scheme) and indications of overprinting (Arp 1995).
Examples are shown in Pl. 48.
8.4 Specific Classifications
8.4.1 Diagenetic Changes in Depositional
Textures
8.5 Classification of
Mixed Siliciclastic-Carbonate Rocks
Wright (1992) revised both the Dunham (1962) clas-
sification and its modification by Embry and Klovan
(1971). Because the textures of most limestones are
controlled not only by depositional and biological fac-
Sediments composed of mixtures of carbonate and si-
liciclastic material are common in near-coast environ-
