Geology Reference
In-Depth Information
destruction and their redeposition. Redeposition must
be one of the criteria used in the definition of the MFT.
The qualitative biotic composition together with the pre-
dominance of echinoderms and mollusk shells is an-
other criterion. Further criteria valuable in comparing
the sample with texturally and compositionally similar
samples are the presence or absence of brachiopod
shells, and the presence not only of crinoids but also of
echinoids. Borings in the echinoderm fragments may
have taken place prior to or subsequent to deposition
of the sediment, and are therefore less useful in defin-
ing the MFT. The differences in preservation of echi-
noderm, mollusk and brachiopod grains tell a lot about
burial diagenesis but do not assist in MFT discrimina-
tion. In contrast, the infilling of interstices between
grains by inhomogeneous micritic matrix is an impor-
tant MFT criterion. The matrix type together with the
packstone texture, reflects the deposition of allochtho-
nous bioclastic sediment by a mass flow event. This
explanation is in agreement with field data: the sample
comes from carbonate beds dipping with a paleoslope
that separates a non-rimmed platform from a distally
steepened carbonate ramp (Stanton and Flügel 1989).
Box 11.2.
Skeletal grain criteria relevant in defining
microfacies types of limestones.
1
Which fossil groups are represented by the skeletal
grains (describes the thin-section diversity)?
2
Which fossil groups are abundant, common and
rare?
3
If calcareous algae or foraminifera occur, which ma-
jor systematic units (e.g. dasyclad green algae or
corallinacean red algae; orbitolinid or nummulitinid
foraminifera) are present?
4
Are there differences in the preservation (e.g. frag-
mentation, solution, recrystallization) of the skel-
etal grains?
5
Is there an apparent loss or predominance of grains
of a specific skeletal mineralogy (e.g. no aragonitic
grains)?
6
Are the sizes of the skeletal grains within compa-
rable ranges or are some grains conspicuously larger
or smaller than the average grains?
7
Do the skeletal grains represent benthic (mobile,
sessile) or planktonic organisms?
8
Do the grains exhibit biogenic encrustations?
9
Which fossil groups appear commonly associated
(e.g. radiolarians and sponge spicules, or echino-
derms and bivalves)? Are there striking grain asso-
ciations (that might point to specific paleoclimatic
conditions)?
Quantitative composition and multivariate discrimina-
tion of microfacies types
Many authors differentiate MFT according to the
frequency of compositional constituents using point
counter data or abundance estimations (see Chap. 6).
The abundance of micrite, sparite and different grain
categories determined by modal analysis characterizes
lithofacies types that are commonly expressed by lime-
stone classification groups.
Establishing MFT requires a different approach fo-
cused on
the abundance of grains and the grain types.
Abundance can be expressed by the frequency (or rela-
tive frequency) of grain categories, by considering the
most abundant grain category, or by ranking grain cat-
egories according to their frequency in the sample.
Abundance and frequency form basic data for
mul-
tivariate discriminations of MFT
and facies types (Sect.
6.3.2). Hierarchical methods (resulting in a dendro-
gram) as well non-hierarchical numerical classifications
(producing a number of unaffiliated clusters) are used
to discriminate between samples and establish facies
groups. Hierarchical methods are either agglomerative
(forming clusters by combining the most similar indi-
viduals first, and continuing to combine similar clus-
ters into partitions of progressively higher rank until
all individuals are in a single set) or divisive (subdivid-
ing an initial set into classes of progressively lower hi-
erarchical rank). The use of hierarchical methods for
the discrimination of ancient carbonate rocks can be
10
Show the fossil concentrations in your sample (e.g.
shell coquinas, Sect. 3.1.1.3; accumulations of
sponge spicules, Pl. 78.2, or of crinoids, Pl. 95.4)!
Are these concentrations allochthonous or autoch-
thonous (Fig. 10.48)?
than shells which are strongly fragmented; some echi-
noderms exhibit borings (question 4). Predominantly
High-Mg calcite echinoderms and calcitic brachiopod
shells occur but are differently preserved; no indica-
tion of aragonitic grains (question 5). Grain size varies
within a wide range; it reflects growth size for echino-
derm elements but the shell size is a result of mechani-
cal destruction (question 6). The skeletal grains indi-
cate the existence of sessile benthos (crinoids, brachio-
pods) and mobile benthos (echinoids, recorded by rare
spines), question 7. Biogenic encrustations are absent
(question 8). The sample represents an example of a
fossil concentration (question 10). No answer is pos-
sible for question 9.
Which of the criteria will define a specific microfa-
cies reflecting environmental and depositional condi-
tions? The biofabric characterized by a concentration
of echinoderms and shells and the admixture of vari-
ously fragmented bioclasts indicate a deposition of skel-
etons of organisms from a single environment, their
