Geology Reference
In-Depth Information
6.1.2.2 Application of GrainSize Analyses to
Carbonate Rocks
are bioclasts, fossils, detrital minerals (e.g. terrigenous
quartz) and intrabasinal particles (intraclasts, peloids,
ooids, oncoids). Because specific densities of bioclasts
vary as a function of intraparticle porosity and because
the transport potential of skeletal grains depends on
shape and size, not all bioclast types can be used equally
in establishing the clasticity index. Carozzi recommends
echinoderm fragments, in particular crinoid columnals.
In the case of bimodal sediments, both modes are mea-
sured. The index is regarded as a relative measure of
the energy level in the depositional environment. It is
Box 6.2 summarizes important points which should
be considered when performing grain-size analyses of
microfacies data. Not mentioned in the Box is the
in-
dex of clasticity
introduced by Carozzi (1958). This in-
dex is defined as the mean apparent diameter (expressed
in millimeters) of the 10 largest grains present in the
thin section. The index can be only used for grains
which have obviously undergone transport. Grains used
Box 6.2.
Guide to grain-size studies of carbonate rocks.
Field work
Sampling:
Samples should be large enough to show vertical and lateral textural variability (Davies and Conley
1977). Take several samples if variability appears great. Choose beds showing grain-supported fabrics and grains within
sand to pebble-size range. The evaluation of grain size using Passega diagrams requires at least 20-30 samples taken
from a depositional sequence and representing all textures present.
Laboratory work
Thin sections and peels:
Investigate large thin sections, with a size of at least 5 x 5 cm. Peels are useful if the outlines
of the grains are clearly visible.
What to measure?
In general, all grains should be measured, but skeletal grains, cortoids and synsedimentary re-
worked particles (intraclasts) are more useful than peloids or large oncoids.
How to measure?
Follow a strict sampling plan and use the ribbon counting method or the point counting method
(see Figs. 6.7 and 6.8). Point-counting data are biased toward coarser grain sizes as compared with data resulting from
ribbon-counting measurements. Only the latter method also considers small and inconspicuous grain sizes. Measure the
largest apparent diameter of the grains. To reach at least some statistical basis measure more than 100 but, better, up to
300 or more grains.
Conversion of mm to
φ
values:
Conversion is necessary to produce grain-size diagrams. Use conversion tables or
nomograms.
Grain-size distributions:
Describe grain-size distributions by cumulative curves (plotted on probability paper) and
try to separate these curves into linear segments (following the method of Visher 1969). The Visher method aims to
separate subpopulations, which should reflect the proportions of grains transported by saltation, traction and suspension
processes.
Grain-size parameters:
Calculate average size and sorting using the formulas presented in Table 6.1. Inclusive stan-
dard deviation and inclusive skewness are of particular importance. There are several computer programs available
which calculate the relevant parameters.
Diagrams:
Draw Friedman-diagrams showing possible relations between sorting and average grain size. Draw Passega
diagrams showing the relations between the coarsest grains and median diameter. An advantage of the Passega diagram
is that both necessary parameters can be determined from thin sections of limestones.
Cluster analysis:
In order to obtain more objective results than by comparing only combinations of grain-size param-
eters, all data should be treated by statistical analyses showing similarities and allowing groups to be recognized which
may reflect controlling depositional processes.
Interpretation
Be cautious:
Consider the possibility that grain-size ranges of skeletal grains are predominantly dependent on indi-
vidual growth rates and microstructures of organisms, or are caused by bioeroders which produce specific grain sizes. If
you use grain-size parameters, take care, because the concepts for discriminating depositional environments or pro-
cesses by grain sizes have been developed for siliciclastic sediments, and are based on sieve and settling methods. Some
authors emphasize the necessity of transforming thin-section data (number percent) into sieve data (weight percent)
prior to drawing and evaluating grain-size diagrams. Comparisons of thin-section data and transformed data of calcarenites
show a shift of data (higher percentage of coarser grains in thin sections), but the general pattern shown in the diagram
is not essentially changed (Fig. 6.6).
Additional indications:
If your mind is set on a very specific answer (look, that is a beach sediment, or seems to
point to high-energy conditions although other criteria do not show this), be critical and look for further indications, e.g.
the hydrodynamic conditions revealed by the composition of the sediment, and by paleontological and paleoecological
criteria (Sect. 12.1).
