3.1 PHYSICAL PROPERTIES OF PARTICLES
53
this work and its interpretation and application. First of all there are many cases where
it is quite inadequate to describe a sediment as "a medium-grained well-sorted sand."
Within the sand and gravel industry rigid trade descriptions of marketable sediments
are required. This includes hard core, road aggregates, building sands, sewage filter-bed
sands, blasting sand, and so on.
Sand and gravel for these and many other uses require specific grain size distributions.
These must be described accurately using statistical coefficients such as those described
in the preceding section. Within the field of geology, accurate granulometric analyses
are required for petrophysical studies that relate sand texture to porosity and permeability (see Section 3.2.3). The selection of gravel pack completions for water wells also
requires a detailed knowledge of the granulometry of the aquifer.
Beyond these purely descriptive aspects of granulometry there is one interpretive aspect that has always led geologists on. This is the use of grain size analysis to detect the
depositional environment of ancient sediments. Figure 3.9 shows the diversity of modern sands from various recent environments. Glacial outwash is coarse, poorly sorted,
and the particles angular (Fig. 3.9A). Fluvial sands are moderately sorted and rounded
(Fig. 3.9B). Beach sands are well sorted and rounded, with scattered skeletal debris
of low sphericity (Fig. 3.9C). Eolian sands are extremely well sorted and well rounded
(Fig. 3.9D).
This much is obvious. The problem is to develop statistical parameters that can differentiate the depositional environment of any given sand sample. This avenue of research has not been notably successful, despite intensive efforts. Modern environments
whose sediment granulometry have been extensively studied include rivers, beaches,
and dunes. Reviews of this work have been given by Folk and Ward (1957), Friedman
(1961, 1967), Folk (1966), Moiola and Weiser (1968), Erlich (1983), and McLaren and
Bowles (1985). These reviews show that statistical coefficients can very often differentiate sediments from various modern environments. For example, a number of studies
show that beach and dune sands are negatively and positively skewed respectively (e.g.
Mason and Folk, 1958; Friedman, 1961; Chappell, 1967). Several complicating factors
have emerged, however. Many sediments are actually combinations of two or more different grain size populations of different origins (Doeglas, 1946; Spencer, 1963). These
admixtures may reflect mixing of sediments of different environments. It is more likely,
however, that the presence of several populations in one sample reflects the action of
different physical processes. For example, within Barataria Bay, Louisiana, multivariate analysis defined sediments into populations influenced by wind, wave, current, and
gravitational processes (Klovan, 1966). Yet these were all deposited in the same lagoonal
or bay environment. Similarly, Visher (1965) demonstrated the variability of sediment
type within fluvial channels, and showed how these differences are related to sedimentary structure, that is, to depositional process. The C-M diagrams of Passega (1957,
1964) are another approach to this same problem. Plots of C, the first percentile, which
measures the coarsest fraction, against M, the median, are most illuminating. They reveal different fields for pelagic suspensions, turbidites, bed load suspensions, and so on
(Fig. 3.10).
A further problem of defining the granulometric characteristics of modern environments is that of inheritance. It has often been pointed out that if a fine-grained sand of
uniform grain size is transported into a basin then that is the only granulometric type
53
this work and its interpretation and application. First of all there are many cases where
it is quite inadequate to describe a sediment as "a medium-grained well-sorted sand."
Within the sand and gravel industry rigid trade descriptions of marketable sediments
are required. This includes hard core, road aggregates, building sands, sewage filter-bed
sands, blasting sand, and so on.
Sand and gravel for these and many other uses require specific grain size distributions.
These must be described accurately using statistical coefficients such as those described
in the preceding section. Within the field of geology, accurate granulometric analyses
are required for petrophysical studies that relate sand texture to porosity and permeability (see Section 3.2.3). The selection of gravel pack completions for water wells also
requires a detailed knowledge of the granulometry of the aquifer.
Beyond these purely descriptive aspects of granulometry there is one interpretive aspect that has always led geologists on. This is the use of grain size analysis to detect the
depositional environment of ancient sediments. Figure 3.9 shows the diversity of modern sands from various recent environments. Glacial outwash is coarse, poorly sorted,
and the particles angular (Fig. 3.9A). Fluvial sands are moderately sorted and rounded
(Fig. 3.9B). Beach sands are well sorted and rounded, with scattered skeletal debris
of low sphericity (Fig. 3.9C). Eolian sands are extremely well sorted and well rounded
(Fig. 3.9D).
This much is obvious. The problem is to develop statistical parameters that can differentiate the depositional environment of any given sand sample. This avenue of research has not been notably successful, despite intensive efforts. Modern environments
whose sediment granulometry have been extensively studied include rivers, beaches,
and dunes. Reviews of this work have been given by Folk and Ward (1957), Friedman
(1961, 1967), Folk (1966), Moiola and Weiser (1968), Erlich (1983), and McLaren and
Bowles (1985). These reviews show that statistical coefficients can very often differentiate sediments from various modern environments. For example, a number of studies
show that beach and dune sands are negatively and positively skewed respectively (e.g.
Mason and Folk, 1958; Friedman, 1961; Chappell, 1967). Several complicating factors
have emerged, however. Many sediments are actually combinations of two or more different grain size populations of different origins (Doeglas, 1946; Spencer, 1963). These
admixtures may reflect mixing of sediments of different environments. It is more likely,
however, that the presence of several populations in one sample reflects the action of
different physical processes. For example, within Barataria Bay, Louisiana, multivariate analysis defined sediments into populations influenced by wind, wave, current, and
gravitational processes (Klovan, 1966). Yet these were all deposited in the same lagoonal
or bay environment. Similarly, Visher (1965) demonstrated the variability of sediment
type within fluvial channels, and showed how these differences are related to sedimentary structure, that is, to depositional process. The C-M diagrams of Passega (1957,
1964) are another approach to this same problem. Plots of C, the first percentile, which
measures the coarsest fraction, against M, the median, are most illuminating. They reveal different fields for pelagic suspensions, turbidites, bed load suspensions, and so on
(Fig. 3.10).
A further problem of defining the granulometric characteristics of modern environments is that of inheritance. It has often been pointed out that if a fine-grained sand of
uniform grain size is transported into a basin then that is the only granulometric type