Geology Reference
In-Depth Information
clasticity index (Sect. 6.1.2.2) and the frequency in-
dex. These indices are determined subsequent to a rough
classification of the samples into microfacies types by
means of qualitative criteria. The frequency index is
counted as the sum of the number of specific particles
occurring in six adjacent fields in the thin section ar-
ranged in two rows of three fields. In grain-supported
rocks, the area required to be counted should be at least
10 times larger than the average size of the grains. In
practice, a field of 300-400 square millimeters is com-
monly used. Each grain category is separately counted.
Grains considered include bioclasts and fossils, intra-
clasts, ooids, and oncoids as well as detrital minerals
(e.g. quartz, glauconite). The frequency index of bio-
clasts, transported fossils and detrital minerals is re-
garded as an indication of the traction or suspension
load of transporting agents in the depositional environ-
ment. For non-transported benthic or planktonic fos-
sils the index expresses the frequency of a given popu-
lation. The frequency and clasticity index and rock com-
position are used to distinguish microfacies types. For
shallow-marine limestone these types are arranged ac-
cording to their position in an ideal shallowing-upward
sequence corresponding to the assumption that all or
most shallow-marine carbonate successions consist of
superposed shallowing-upward sequences (Carozzi
1958, 1986).
The method has been intensively used and was ap-
plied to carbonate ramps and platforms, reefs, mass flow
deposits as well as to lacustrine limestones (see case
studies in Carozzi 1988). The technique is time con-
suming and the presentation of the results is somewhat
arduous; but if the results are statistically tested and
put in the context of other facies criteria, the frequency
index and the other indices used by Carozzi provide
valuable information about short-term changes in wa-
ter energy and the inferred paleobathymetry.
Visual comparison charts for sedimentary rocks have
been published by various authors (Folk 1951; Terry
and Chilingar 1958; Baccelle and Bosellini 1965;
Schäfer 1969). Matthew et al. (1991), confronted with
the problem of quantifying archaeological ceramics, de-
veloped charts whose use in microfacies studies is es-
pecially recommended. The comparison charts were
prepared in different ways: (a) by cutting black paper
and scattering the pieces over a defined area (Terry and
Chilingar), (b) by image presentation of point-count-
ing results (Baccelle and Bosellini), or (c) by a com-
puter program considering covering of grains and dif-
ferent ranges of grain size (Matthew et al. 1991).
The comparison charts of Baccelle and Bosellini,
Matthews et al. and Schäfer assist positively in reliable
estimations of frequencies. The conceptual background
of the charts is shown in Figs. 6.10 and 6.11. The
Bacelle and Bosellini charts are included in the CD-
ROM accompanying this topic.
Advantages and disadvantages of the charts:
The
most commonly used
Baccelle and Bosellini chart
has
the advantage that it shows images which can be at-
tributed to common carbonate grain types. A pitfall of
the charts is, that variations in grain size are only in-
sufficiently taken in consideration. Checking the im-
ages by digital image analysis result in somewhat dif-
ferent percentage values (± 5%), particularly for well-
rounded grains.
The
Matthew et al. charts
have the advantage that
they show percentages in the context of grain size
ranges. The latter include ranges which are common in
calcarenites and calcirudites. The ranges comprise dif-
ferent ranges between 0.5 mm and 10 mm. If you want
to use these charts, (1) select the appropriate charts ac-
cording to the shapes and association of the particles
(the charts display various round and mixed round and
polygonal figures), (2) determine the grain-size ranges
in your sample, and (3) decide whether it is better to
use black/white or white/black charts.
The charts proposed by
Schäfer
are particularly de-
signed for limestones with shells or echinoderms, e.g.
mollusk coquinas and crinoid limestones. Particles are
shown in black on a white background.
6.2.1.2 Estimating
Counting methods described in Sect. 6.2.1.1 need a con-
siderable amount of time when applied to a large num-
ber of samples. The time factor is often a large draw-
back to collecting and handling quantitative microfa-
cies data. What should be done instead? Just use esti-
mates as many people do?
Point counting, visual comparison charts or free-lance
estimating?
Series of experiments carried out with students and
postgraduates in Erlangen produced good results with
regard to the comparability of point counting measure-
ments and estimates. Rough estimates are somewhat
lower than those using the Baccelle and Bosellini charts.
Spots in front of your eyes: Using comparison charts
Estimations of the frequency of grains and matrix
in thin-sections are common methods in facies analy-
sis, although not as accurate as point counting or im-
age analysis systems.
