Geology Reference
In-Depth Information
of the diameter of the largest grain. Narrow grid spac-
ing tends to overrepresent the large grains. Limestones
with cm-sized grains should be measured in polished
slabs using wide grid spacing..
What about a spar-filled gastropod shell?
Particles
can be registered in point counting analyses by (a) all
grid points falling on the particle as well as on voids
within the particle, or (b) only those grid points which
fall on the particle but not on the voids included in the
particle (Fig. 6.8). Voids are separately counted, e.g. as
micrite or cement. The first case corresponds to
grain-
bulk
measurements, the second to
grain-solid
measure-
ments. If intraparticle porosity should be emphasized
and the amount of cement within grains, grain-solid
measurements are recommended. The same may be
necessary for coated grains exhibiting siliciclastic nu-
clei (Wilson 1965). The grain-solid definition is related
to the weight; the grain-bulk definition to the volume
of the grain.
The magic number or how many grid points must
be considered?
The number of points necessary for a
statistically reliable result depends on the distribution
and size of the particles. Accuracy is related to selected
grid density and the number of points counted. The
mean deviation decreases with an increasing number
of points and increases with an increasing percentage
of particles, that is up to 50%, whereupon the mean
deviation again drops. In order to keep sampling errors
small, several hundred (at least 300 to 500) points must
be counted; for limestones with variously sized and
poorly sorted grains as many as 1000 points and more.
Fig. 6.7.
Comparison of
A
- point counting,
B
- line method,
C
- area method, and
D
- ribbon method
used in evaluating
frequency values. Solid black grains are those not counted.
Note that ribbon counting measures only those grains which
are totally or almost totally included within the ribbon
(hatched area). After Middleton et al. (1985).
particles of that type (Chayes 1956). The number of
grid points intersected by specific grains is proportional
to the relative area of the grain. For detailed measure-
ments under the microscope special oculars are used
together with a specially adapted stage micrometer.
Computerized point-counting (Prior point counter)
facilitates the time-consuming work (Kobluk and Kim
1991, McKinney 1993). Several computer programs for
point counting are available, e.g. Point Count 99, de-
veloped for reef monitoring programs.
In practice, pointcounting of microfacies samples
is done on photographs of thin sections or on the screen
of reading fiches covered with a point grid. Grid spac-
ings used are (a) larger than the diameter of the largest
grain, (b) approximately equal to the diameter of the
largest grain, or (c) approximately half or one quarter
Fig. 6.8.
A
:
Dual point counting.
Point counting measure-
ments of grains deal with the entire grain (including not only
the shell but also voids filled with sediment or with cements
(grain-bulk measurements) or
B
only the solid parts of the
skeletal grain (grain-solid measurements). In the first case,
all voids are counted regardless of their infilling (indicated
by large dots). In the second case intraparticle voids (open
circles) are measured separately as internal infillings or in-
traparticle cement. It is necessary to state clearly which of
the definitions has been used in the study. After Jaanusson
(1972).
