3.1 PHYSICAL PROPERTIES OF PARTICLES
45
Table 3.1
Wentworth Grade Scale, Showing Correlation with ~b Scale and Nomenclature
for Unconsolidated Aggregates and for Lithified Sediments
Particle diameter
~b values
(mm diam.)
Wentworth grades
Rock name
-6
Cobbles~
64
Conglomerate
Pebbles J
-2
4
-1
2
Granules
Granulestone
Very coarse
0
1
Coarse
1
0.5
Medium
2
0.25
Fine
3
0.125
Very fine
4
0.0625
8
0.0039
sand
Sandstone
Silt
Siltstone
Clay
Claystone
scheme, at least down to silt grade (Assalay et aL, 1998). Silt and clay can be differentiated by whether they are crunchy or plastic between one's teeth, although more sophisticated methods of analysis are available. Well-cemented sediments are measured
from thin-section microscope study (Mazzullo and Kennedy, 1985). There is the quickand-dirty method and the slow enthusiast's method. The first involves no more than
measuring the diameter of the field of view of the microscope at a known magnification,
counting the number of grains transected by the cross-wires, and dividing by twice the
diameter. This process is repeated until tired- or until statistical whims are satisfied.
Then calculate the average grain size of the thin section with this formula:
Average grain size =
Z(2d/n)
N
where n is the number of grains cut by the cross-wires, d is the diameter of the field of
view, and N is the total number of fields of view counted. This method is quick and foolproof. It is adequate for most sedimentological studies aimed at broader aspects of facies analysis. It gives only the average grain size and does not describe the sorting of the
sample. This method is quite inadequate for detailed granulometric studies. Then it is
necessary to measure the length and/or some other size parameter of individual grains.
This is not only very time consuming, but care must be taken over the orientation of the
thin section and of the selection of sample. It is improbable that the thin section cuts
the longest axis of every particle, so there is an inherent bias to record axes that are less
45
Table 3.1
Wentworth Grade Scale, Showing Correlation with ~b Scale and Nomenclature
for Unconsolidated Aggregates and for Lithified Sediments
Particle diameter
~b values
(mm diam.)
Wentworth grades
Rock name
-6
Cobbles~
64
Conglomerate
Pebbles J
-2
4
-1
2
Granules
Granulestone
Very coarse
0
1
Coarse
1
0.5
Medium
2
0.25
Fine
3
0.125
Very fine
4
0.0625
8
0.0039
sand
Sandstone
Silt
Siltstone
Clay
Claystone
scheme, at least down to silt grade (Assalay et aL, 1998). Silt and clay can be differentiated by whether they are crunchy or plastic between one's teeth, although more sophisticated methods of analysis are available. Well-cemented sediments are measured
from thin-section microscope study (Mazzullo and Kennedy, 1985). There is the quickand-dirty method and the slow enthusiast's method. The first involves no more than
measuring the diameter of the field of view of the microscope at a known magnification,
counting the number of grains transected by the cross-wires, and dividing by twice the
diameter. This process is repeated until tired- or until statistical whims are satisfied.
Then calculate the average grain size of the thin section with this formula:
Average grain size =
Z(2d/n)
N
where n is the number of grains cut by the cross-wires, d is the diameter of the field of
view, and N is the total number of fields of view counted. This method is quick and foolproof. It is adequate for most sedimentological studies aimed at broader aspects of facies analysis. It gives only the average grain size and does not describe the sorting of the
sample. This method is quite inadequate for detailed granulometric studies. Then it is
necessary to measure the length and/or some other size parameter of individual grains.
This is not only very time consuming, but care must be taken over the orientation of the
thin section and of the selection of sample. It is improbable that the thin section cuts
the longest axis of every particle, so there is an inherent bias to record axes that are less