particle morphology of sediments Particle morphology, or form, refers to the sum of the surface characteristics of sedimentary particles, and is an important source of information on the origin of sediments and sedimentary rocks. Processes of weathering, erosion, and transport may all leave distinctive imprints on particles, in the form of fractures, worn surfaces, and particular surface textures. Careful study of particle morphology can therefore yield valuable data on past processes and environments.
Casual examination of any stone shows that its surface, in all its minute detail, has a very complex form. A full description would clearly be impractical, and Earth scientists have identified a few measurable characteristics which provide the most useful summary. Three types of characteristics may be defined: shape, or the relative dimensions of the particle; roundness, or the overall smoothness of the particle outline; and texture, or small-scale surface features. (In some publications the definitions of ‘form’ and ‘shape’ have been used in the opposite senses to those used here.) Roundness is superimposed on form and is a characteristic of particle edges, and texture is superimposed on roundness. Studies of how two or more morphological characteristics vary to-gether can often yield much more information than one characteristic studied in isolation.
The shape of a particle can be visualized as the shape of the smallest rectangular box into which the particle can be fitted. The three dimensions of the box are referred to as the long (
L), intermediate (
I), and short (
S) axes (or alternatively, the
a,
b, and
c axes, respectively). The shape of the box (which is independent of its size) is then defined in terms of the relative dimensions of the axes. In the 1950s, Edmund Sneed and Robert Folk showed that all possible box shapes can be arranged on a triangular diagram, with three limiting cases:(1) a cube, with
L =
I =
S;(2) a rod, with
L > 0, and
I =
S = 0; and(3) a square plane (or disc), with
L =
I > 0, and
S = 0 (Fig. 1a).Several indices, descriptive of particle shape, can be plotted on this diagram without altering its basic geometry. Three of the most useful are:
S/
L, which separates cubes from rods and discs; (
L–
I)/(
L–
S), which differentiates rods and discs; and
I/
L, which separates rods from discs and cubes (Fig. 1a). Another useful family of indices is based on the behaviour of particles in a fluid. One such index is maximum projection sphericity ((
S2/
LI)
0.33), which reflects the balance of drag and gravitational forces acting on an immersed particle. Some researchers favour other shape diagrams, such as the two-axis plot devised by the German geologist T. Zingg in the 1930s, in which
I/
L is plotted against
S/
I. In fact, the triangular and two-axis plots are equivalent, and one can be transformed into the other by stretching it like a sheet of elastic. The triangular version is preferable because it more accurately reflects the three-way variation of particle shape.
Samples of particles from different environments commonly have distinct distributions when plotted on shape diagrams, reflecting their weathering and transport histories. For example, frost-shattered particles tend to be rod- or disc-shaped, whereas subglacially crushed and abraded particles tend to be more cubic (Fig. 1b). An additional influence is exerted by the joint distribution in the parent material. For example, fissile slates tend to yield much flatter particles than massive granites.
Roundness can be measured in several ways. One method is to measure the radius of curvature of the sharpest edge of the particle, and to compare it with the radius of the smallest circle that can be inscribed within the particle outline. Two disadvantages of this method are that radii of curvature can be very difficult to measure for sharp-edged particles, and the sharpest edge may not be representative of the particle as a whole. Because of this, many geologists favour comparison charts or descriptive tables which define summary roundness categories. Alternatively, computer programs have been produced that give mathematical descriptions of roundness from digitized particle outlines. While effective for sand grains and smaller particles, this method is costly and time-consuming to implement. Roundness is increased by abrasion and chemical weathering processes, which blunt particle edges, and decreased by fracturing, which creates new, unworn edges. Roundness studies can thus assess the relative importance of abrasion, weathering, and fracturing in the history of a particle.
Most studies of texture have focused on particles smaller than sand size. Quartz-grain microtextures have received particular attention, and many distinctive textural features, such as conchoidal fractures and stepped surfaces, have been classified. Early work focused on establishing textures diagnostic of particular environments or processes (e.g. ‘glacial’ or ‘aeolian’ textures), but it is now recognized that the origin of the grains (igneous, metamorphic, or diagenetic) is also an important controlling variable. For pebbles and larger particles, surface textures, such as weathering pits and percussion fractures, provide important clues to particle history. In glacial studies, striations and polished facets are often recorded as evidence for subglacial transport.
An additional aspect of particle morphology worthy of attention is the asymmetry of wear patterns. Particles from fluvial or coastal environments are generally worn evenly on all parts of their surfaces, whereas the abraded surfaces of particles that have undergone subglacial transport or deposition are usually concentrated at one end and fracture surfaces at the other. Such asymmetric ‘stoss-and-lee’ stones reflect the asymmetric distribution of stresses in the subglacial environment, particularly the shearing of debris-rich ice or till over or around the stone. Wear patterns have yielded detailed information on subglacial processes and are also potentially very useful in other environments.
Douglas I. Benn
