44
3 PARTICLES, PORES, AND PERMEABILITY
Wadell's definition is perhaps statistically more sound, but for practical purposes Wentworth's definition is much simpler and faster to use. Additional definitions have been
proposed by Russell and Taylor (1937) and Powers (1953).
Considerable attention has been paid to the factors that control the roundness and
sphericity of sediment particles. Many studies show the sphericity and roundness of sediments to increase away from their source area (e.g., Wentworth, 1919; Laming, 1966).
Kuenen, in a series of papers (1956a,b, 1959, 1960), described the results of the experimental abrasion of both pebbles and sand by various eolian and aqueous processes.
These studies indicated that the degree of abrasion and shape change along rivers and
beaches was due as much to shape sorting as to abrasion. The experiments themselves
showed that eolian action was infinitely more efficient as a rounding mechanism than
aqueous transportation over an equivalent distance. There is little evidence for chemical solution to be a significant rounding agent. This is shown by the angularity of very
fine sand and silt. Margolis and Krinsley (1971) demonstrated that the high rounding
commonly seen in eolian sands is due to a combination of abrasion and of synchronous
precipitation of silica on the grain boundary.
3.1.3 Particle Size
Size is perhaps the most striking property of a sediment particle. This fact is recognized
by the broad classification of sediments into gravels, sands, and muds. It is easy to understand the concept of particle size. It is less easy to find accurate methods of measuring
particles (Whalley, 1972; Lewis and McConchie, 1994).
3.1.3.1 Grade Scales
Sedimentary particles come in all sizes. For communication it is convenient to be able
to describe sediments as gravels, sands (of several grades), silt, and clay. Various grade
scales have been proposed that arbitrarily divide sediments into a spectrum of size
classes. The Wentworth grade scale is the one most commonly used by geologists (Wentworth, 1922). This is shown in Table 3.1, together with the grade names and their lithifled equivalents. A common variation of the Wentworth system is the phi (~b) scale proposed by Krumbein (1934). This retains the Wentworth grade names, but converts the
grade boundaries into phi values by a logarithmic transform:
th = -log2d,
where d is the diameter. The relationship between the Wentworth and phi grade scales
is shown in Fig. 3.3.
3.1.3.2 Methods of Particle Analysis
The size of sediment particles can be measured from both unconsolidated and indurated sediment. The most common way is by visual estimation backed up, if necessary,
by reference to a set of sieved samples of known sizes. With experience, most geologists can visually measure grain size within the accuracy of the Wentworth grade scale
3 PARTICLES, PORES, AND PERMEABILITY
Wadell's definition is perhaps statistically more sound, but for practical purposes Wentworth's definition is much simpler and faster to use. Additional definitions have been
proposed by Russell and Taylor (1937) and Powers (1953).
Considerable attention has been paid to the factors that control the roundness and
sphericity of sediment particles. Many studies show the sphericity and roundness of sediments to increase away from their source area (e.g., Wentworth, 1919; Laming, 1966).
Kuenen, in a series of papers (1956a,b, 1959, 1960), described the results of the experimental abrasion of both pebbles and sand by various eolian and aqueous processes.
These studies indicated that the degree of abrasion and shape change along rivers and
beaches was due as much to shape sorting as to abrasion. The experiments themselves
showed that eolian action was infinitely more efficient as a rounding mechanism than
aqueous transportation over an equivalent distance. There is little evidence for chemical solution to be a significant rounding agent. This is shown by the angularity of very
fine sand and silt. Margolis and Krinsley (1971) demonstrated that the high rounding
commonly seen in eolian sands is due to a combination of abrasion and of synchronous
precipitation of silica on the grain boundary.
3.1.3 Particle Size
Size is perhaps the most striking property of a sediment particle. This fact is recognized
by the broad classification of sediments into gravels, sands, and muds. It is easy to understand the concept of particle size. It is less easy to find accurate methods of measuring
particles (Whalley, 1972; Lewis and McConchie, 1994).
3.1.3.1 Grade Scales
Sedimentary particles come in all sizes. For communication it is convenient to be able
to describe sediments as gravels, sands (of several grades), silt, and clay. Various grade
scales have been proposed that arbitrarily divide sediments into a spectrum of size
classes. The Wentworth grade scale is the one most commonly used by geologists (Wentworth, 1922). This is shown in Table 3.1, together with the grade names and their lithifled equivalents. A common variation of the Wentworth system is the phi (~b) scale proposed by Krumbein (1934). This retains the Wentworth grade names, but converts the
grade boundaries into phi values by a logarithmic transform:
th = -log2d,
where d is the diameter. The relationship between the Wentworth and phi grade scales
is shown in Fig. 3.3.
3.1.3.2 Methods of Particle Analysis
The size of sediment particles can be measured from both unconsolidated and indurated sediment. The most common way is by visual estimation backed up, if necessary,
by reference to a set of sieved samples of known sizes. With experience, most geologists can visually measure grain size within the accuracy of the Wentworth grade scale