Geology Reference
In-Depth Information
nant members of the community (Fig. 8.1). In general,
the distribution of growth forms of benthic shallow-
water organisms reflect patterns of functional signifi-
cance. Many authors underline the adaption of growth
habits of reef organisms to water energy, e.g. strength,
type, and duration of currents and waves. Specific
growth forms are used as indicators of the existence of
particular or dominating energy stages (see Sect.
12.1.1). There is, however, growing evidence that nu-
trient levels and resulting predator patterns as well as
substrate conditions are major and probably dominat-
ing controls on the biogenic fabrics that are used in
more sophisticated subdivisions of reef limestones
(Wood 1995).
vestigations of cores, and in laboratory studies. The use
of Folk's classification is more restricted to laboratory
studies based on thin sections or peels.
8.3.1 Prerequisites
Before classifying anything, it is necessary to deter-
mine what constituents occur (grain categories, matrix
and cement types).
The second fundamental principle concerns the ques-
tion of whether the constituent grains are grain- or mud-
supported (see Pl. 43).
8.3.2 Original and Expanded Dunham
Classification
8.3 Classifications Based on
Depositional Texture
Figure 8.4 shows the original version of the Dunham
classification, Fig. 8.5 the expanded and revised ver-
sions (Embry and Klovan 1971; Wright 1992). The cri-
teria of the rock categories are depicted and discussed
in Pl. 43 and Pl. 44.
The most widely used classifications are those of Dun-
ham (1962) and Folk (1959, 1962). Dunham's classifi-
cation can equally well be applied in the field, in in-
Carbonates
Dunham (1962)
Groundmass:
F i n e c a r b o n a t e m a t r i x
sparry cement
+ s p a r
Bioconstruction
Matrix-supported
Grain-supported
Grains: < 10% > 10%
MUDSTONE WACKESTONE
PACKSTONE
GRAINSTONE
BOUNDSTONE
Folk (1959, 1962)
Allochems:
< 1%
110%
1050%
> 50%
fossiliferous
sparse
packed
poorly washed
MICRITE
BIOMICRITE
BIOSPARITE
BIOLITHITE
Terrigenous
Matrix-supported
Grain-supported
Sand: < 10% 1025%
> 25%
sandy
WACKE
SUBWACKE ARENITE
MUDSTONE
S A N D S T O N E
Fig. 8.4. Fossiliferous limestones classification after Dunham (1962) and Folk (1959, 1962). Both classifications distinguish
allochthonous limestones (mudstone, wackestone, packstone, grainstone) and autochthonous limestones (here called bound-
stone or biolithite). Limestones whose components were deposited as discrete grains are grouped according to mud-support
or grain-support and the abundance of grains. The Dunham classification stresses the depositional fabric, the Folk classifica-
tion tries to evaluate hydrodynamic conditions. Both classifications consider the dominating groundmass types. Note the
divergent categorization of limestones with abundant, densely packed grains and a fine-grained matrix or mixed fine-grained/
sparry groundmass (packstone).
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