78
3 PARTICLES, PORES, AND PERMEABILITY
in Schlee, 1957) show that long axes tend to parallel current flow. Some data from crossbedded sands, however, show this orientation on the foreset, but on the topset and bottomset, the pebbles are elongated perpendicular to the current direction (Johansson,
1965; Sengupta, 1966; Bandyopadhyay, 1971; Gnaccolini and Orombelli, 1971). Studies of sand-grain orientation were held back when it was only possible to measure each
grain individually. Indirect methods of orientation measurement have now evolved
(Shelton and Mack, 1970; Sippel, 1971; Schafer and Teyssen, 1987) that enable aggregate samples to be measured with speed.
Potter and Pettijohn (1977, pp. 47-55) gave a detailed review of the relationship between grain orientation and current direction. Most studies show that in fiat-bedded
sands grains are elongated parallel to current direction, and that this axis coincides with
maximum permeability. Few sediments are made of spherical particles however, perhaps with oolitic sands being the only example. Most quartz grains are slightly elongated, while platy grains of mica, clay, bioclasts, and plant debris also occur. The shapes
of skeletal sand grains are still more complex. Thus when a sediment is deposited the
flaky grains settle parallel to the horizon and contribute to the laminated appearance.
These flakes will inhibit vertical permeability. In the horizontal plane permeability
will be higher, the highest permeability paralleling the long axes of the grains. Thus
permeability correlates with paleocurrent direction (Fig. 3.28). This relationship has
been seen in flat-bedded sands in fluvial and turbidite deposits (Shelton and Mack,
1970; Martini, 1971; von Rad, 1971; Hiscott and Middleton, 1980). On beaches grains
are aligned perpendicular to the shoreface (Curray, 1956; Pryor, 1973).
3.2.3.1.5 Effect of sedimentary structures on porosity and permeability
Finally it is appropriate to move up from consideration of the control of the fabric on the
porosity and permeability of a sediment, to consideration of sedimentary structures. Of
these the most studied has been cross-bedding. For cross-bedded sands the relationships
between permeability and fabric are more complex (Fig. 3.29). Grain-size variations
appear to exert a greater influence than grain orientation. Thus in avalanche cross-beds,
such as occur on braid bars, grain size and thus permeability will increase downward. In
eolian sands, however, the reverse is often seen, with a sandy foreset grading down into
a silty toeset. In such units permeability will decrease downward (Fig. 3.30).
Fig. 3.28. Block of fiat-bedded sediment showing the relationship between fabric and current orientation. Permeability is usually lowest in the vertical (C) axis, and highest in the horizontal (A) axis parallel to the current.
3 PARTICLES, PORES, AND PERMEABILITY
in Schlee, 1957) show that long axes tend to parallel current flow. Some data from crossbedded sands, however, show this orientation on the foreset, but on the topset and bottomset, the pebbles are elongated perpendicular to the current direction (Johansson,
1965; Sengupta, 1966; Bandyopadhyay, 1971; Gnaccolini and Orombelli, 1971). Studies of sand-grain orientation were held back when it was only possible to measure each
grain individually. Indirect methods of orientation measurement have now evolved
(Shelton and Mack, 1970; Sippel, 1971; Schafer and Teyssen, 1987) that enable aggregate samples to be measured with speed.
Potter and Pettijohn (1977, pp. 47-55) gave a detailed review of the relationship between grain orientation and current direction. Most studies show that in fiat-bedded
sands grains are elongated parallel to current direction, and that this axis coincides with
maximum permeability. Few sediments are made of spherical particles however, perhaps with oolitic sands being the only example. Most quartz grains are slightly elongated, while platy grains of mica, clay, bioclasts, and plant debris also occur. The shapes
of skeletal sand grains are still more complex. Thus when a sediment is deposited the
flaky grains settle parallel to the horizon and contribute to the laminated appearance.
These flakes will inhibit vertical permeability. In the horizontal plane permeability
will be higher, the highest permeability paralleling the long axes of the grains. Thus
permeability correlates with paleocurrent direction (Fig. 3.28). This relationship has
been seen in flat-bedded sands in fluvial and turbidite deposits (Shelton and Mack,
1970; Martini, 1971; von Rad, 1971; Hiscott and Middleton, 1980). On beaches grains
are aligned perpendicular to the shoreface (Curray, 1956; Pryor, 1973).
3.2.3.1.5 Effect of sedimentary structures on porosity and permeability
Finally it is appropriate to move up from consideration of the control of the fabric on the
porosity and permeability of a sediment, to consideration of sedimentary structures. Of
these the most studied has been cross-bedding. For cross-bedded sands the relationships
between permeability and fabric are more complex (Fig. 3.29). Grain-size variations
appear to exert a greater influence than grain orientation. Thus in avalanche cross-beds,
such as occur on braid bars, grain size and thus permeability will increase downward. In
eolian sands, however, the reverse is often seen, with a sandy foreset grading down into
a silty toeset. In such units permeability will decrease downward (Fig. 3.30).
Fig. 3.28. Block of fiat-bedded sediment showing the relationship between fabric and current orientation. Permeability is usually lowest in the vertical (C) axis, and highest in the horizontal (A) axis parallel to the current.