Geology Reference
In-Depth Information
6 Quantitative Microfacies Analysis
The previous chapters of this topic introduced various
aspects of recognizing qualitative microfacies criteria
and their signifcance, whereas this chapter will treat
quantitative criteria. The first part describes selected
principles of grain-size analysis and demonstrates the
potential of grain size in the context of microfacies stud-
ies. The second part deals with methods of frequency
analysis and their use in microfacies-based interpreta-
tions. The last part discusses the evaluation of micro-
facies by multivariate statistical analyses.
tending from <1/256 mm to >256 mm. A modification
of this scale is the φ
-scale
which allows grain-size data
to be expressed in units of equal value. The φ size is
the negative logarithm to the base 2 of the grain size
measured in millimeters (Krumbein 1936). A grain size
of 1 mm corresponds to a φ value of zero. Smaller grain
sizes are expressed by positive phi values, larger grain
sizes by negative φ values. Pierce and Graus (1981)
and McManus (1982) have discussed the use and mis-
use of the φ-scale. Phi values can be obtained from con-
version tables (Page 1955; Griffith 1967) or nomograms
(Friedman and Sanders 1978).
6.1 Grain-Size Analysis
Grain-size groupings in carbonate rocks:
The terms
calcilutite, calcarenite and calcirudite, proposed by
Grabau (1904), refer to the
sizes of carbonate particles
.
Calcilutite
consists of clay- and silt-sized particles
(<2
m to 62
m); it corresponds to a consolidated car-
bonate mud.
Calcisiltite
(Kay 1951) embraces the
coarser part of calcilutite (Sect. 4.1.4).
Calcarenite
is a
limestone consisting of sand-sized grains (0.062 to
2 mm); it corresponds to a consolidated calcareous sand.
Calcirudite
is a limestone consisting of grains which
are larger than 2 mm. The term embraces consolidated
calcareous gravels and rubbles as well as limestone con-
glomerates and breccias. Some authors use a lower limit
(1 mm).
The terms calcarenite and calcirudite are of com-
mon use in microfacies studies. Thin sections exhibit-
ing characteristic medium- and coarse-grained cal-
carenites are shown in Pl. 43/1, Pl. 44/2, and Pl. 45/1.
Typical calciruditic limestone types are depicted in Pl.
45/2 and Pl. 45/3. A comparison of these microphoto-
graphs shows considerable differences with regard to
the number of the sand- or pebble-sized grains, the char-
acter of the matrix and the mud- or grain-supported
fabric. These differences have been taken into account
by a classification used by Carozzi (1988) and his stu-
dents.
In the original definition the upper grain size of cal-
cirudite is not limited and embraces rocks consisting
of millimeter- to meter-sized clasts. It seems, however,
Grain size is a basic descriptive measure of sediments.
Grain-size distribution patterns may be characteristic
of sediments deposited in certain environments and can
yield information about depositional processes. In turn
grain sizes are a major control on porosity and perme-
ability. Various methods are in use ranging from sim-
plistic diagrams of grain-size parameters up to sophis-
ticated analyses of statistically treated data. Measures
for characterizing grain-size populations have been re-
viewed at length in many standard texts (e.g. Boggs
1995) and include a wide spectrum of analytical meth-
ods (McManus 1988; Syvitski 1991; Pye 1994). Most
descriptive and interpretative concepts are based on
studies of modern quartz sands deposited in specific
environmental settings. Grain-size criteria of modern
carbonate sediments have been examined predomi-
nantly for tropical and subtropical inter- and subtidal
sands accumulating near beaches, lagoons, on shallow-
marine banks and within reef complexes (Box 6.1).
Contrary to the abundant grain-size analyses of sand-
stones, only a few authors have studied grain-size dis-
tributions of ancient carbonate rocks.
Grain-size scales
: The commonly adopted size
scales are shown in Fig. 6.1. Sedimentary grains range
in size from a few microns to a few meters. Because of
this wide range, logarithmic or geometric scales rather
than linear scales are more useful for expressing size.
The
Udden-Wentworth scale
is a geometric scale ex-
