Geology Reference
In-Depth Information
between the coral stems, and fine-grained bioclasts and
peloids. Most bioclasts are of coral debris. The sedi-
ment in sample B consists of homogeneous micrite with
echinoderm and shell debris. These differences are of
greater significance for the definition of MFTs than are
the taxonomic differences because they reflect differ-
ent environmental and depositional conditions.
The corals of sample A grew on the inner part of a
carbonate ramp. The sediment between the coral stems
was formed by physical and biological erosion of cor-
als and by the microbial precipitation of fine-grained
carbonate. Baffling of sediment took place in protected
interspaces within the coral colony. The MFT should
be called coral
(Retiophyllia)
bafflestone with fine-
grained coral skeletal debris/microbial micrite matrix.
The corals of sample B developed on the lower and
deeper part of a slope between a platform and a distally
steepened ramp. Carbonate production took place by
accumulation of mollusk shells, echinoderms and fine-
grained carbonate on the slope. The MFT is a coral
(Stylophyllum)
bafflestone with fine-grained echino-
derm-micrite matrix. The message of this example is
that less conspicuous differences (such as matrix types)
may be more relevant in generating MFTs than more
conspicuous features (such as coral types).
Box 11.1.
Thin-section criteria used in establishing
microfacies types of allochthonous carbonates.
Depositional texture of the rock
• Mudstone, wackestone (floatstone), packstone, grain-
stone (rudstone); Sect. 8.3.2
• Grain size, sorting and packing; Sect. 6.1.2, e.g. Pl.
93
Qualitative composition
• Fine-grained matrix, grain types, carbonate cement
(sparite); Sect. 4.1
• Type of the fine-grained matrix (micrite, microspar,
calcilitite); Sect. 4.1.1, 4.1.3, 4.1.4
• Basic micrite types (auto- and allomicrite, seafloor
and internal micrite); Sect. 4.1.2
• Micrite texture (homogeneous, inhomogeneous, pel-
oidal, fine bioclastic, laminated, silty); Sect. 4.1.5
• Grain categories (skeletal grains, peloids, cortoids,
oncoids, ooids and pisoids, aggregate grains, resedim-
ents); Sect. 4.2
Quantitative composition
• Relative proportions of fine-grained matrix, grains and
interparticle carbonate cement (modal analysis); Sect.
6.2.1
• Abundance and percentage of major grain categories;
Sect. 6.2.1
• Quantitatively dominant grain categories
• Ranking of grain categories according to frequency;
Sect. 6.2.1.4
Depositional fabrics
• Biofabrics; Sect. 5.1.2
• Bedding and lamination; Sect. 5.1.3
• Burrowing and bioturbation; Sect. 5.1.4
• Fenestral fabrics; Sect. 5.1.5
Microbial carbonates:
The MFTs of carbonate tex-
tures believed to be due to microbially induced pro-
cesses (see Sect. 9.1) should be differentiated using the
criteria summarized in Box 9.1 and following the clas-
sification of microbialites and stromatolites presented
in Box 9.2. Microbial MFTs exhibit both layered and
non-layered fabrics (see Pl. 50). Most layered fabrics
correspond to bindstones. The names of MFTs should
include both textural and compositional criteria. The
latter can be indicated by the dominant constituents,
e.g. thrombolitic bindstone (Pl. 8/6), or peloidal lami-
nated bindstone (Pl. 50/2) corresponding to a lami-
nated fine-grained agglutinated stromatolite. A particu-
lar microbial MFT characterized by an intimate asso-
ciation of abundant carbonate cement with microbiota
is depicted in Fig. 8.3.
The significance of depositional texture
Depositional texture types, particularly those based
on the original or expanded Dunham classification (see
Sect. 8.3.2) permit a preliminary subdivision of lime-
stone samples. However, the name determined by the
application of the limestone classification rarely corre-
sponds to a microfacies type. Consider the examples
illustrated by Pl. 100/1, 2 and Pl. 101. All samples are
grainstones following the Dunham classification, and
all samples are skeletal grainstones. Using the Folk clas-
sification, the thin sections would be designated as
biosparites, but taking into account compositional cri-
teria, the three samples must be placed into two sepa-
rate microfacies types.
Grain size and sorting can be helpful in differentiat-
ing textural groups. Examples are shown in Pl. 15. The
depicted grainstones correspond in composition, but
size and sorting of the aggregate grains varies indicat-
ing slight hydrodynamic differences in the depositional
environment.
Which criteria are suitable for establishing the micro-
facies types of allochthonous limestones?
Both simple and more sophisticated methods can
be used to distinguish between separate MFTs. Simple
definitions are based on a combination of Dunhams
classification and the relative abundance of different
grain types. Most commonly used criteria are deposi-
tional texture and fabric, as well as qualitative and quan-
titative compositional data. Box 11.1 summarizes these
criteria.
