8.5 SANDSTONES
367
overgrowths form with well-defined crystal facies (Plate 4B). Sorby (1880) first drew
attention to this phenomenon in the Permian Penrith sandstone of England (see also
Waugh, 1970a,b).
As cementation continues, however, pore spaces are completely infilled with quartz
(Plate 4C). It is not always apparent whether the resultant fabric has been produced
only by continuous development of the initial quartz overgrowths. Alternatively, solution may have taken place where grains are in contact, and the dissolved silica reprecipitated in the adjacent pore space. This process is termed pressure solution.
The genesis of secondary silica cement has been extensively studied because this is
the most common type of porosity destroyer in sandstones (McBride, 1990; Worden,
2000). Particular attention has been paid to finding the depth below which effective porosity is absent in a particular sedimentary basin. This may be used to predict the "economic basement" below which it would be futile to search for aquifers or hydrocarbon
reservoirs.
Attention has been directed toward the source of silica, the physicochemical conditions that govern its precipitation, and the relationship between silica cementation and
pressure solution. There is no doubt that silica cements may have been precipitated
from solutions which derived their silica from organic debris such as radiolaria, diatom
tests, and siliceous sponge spicules. Likewise, some silica-rich solutions must have been
expelled from compacting clays. A number of successful attempts have been made to
grow silica overgrowths artificially. These have been achieved at high temperatures and
pressures (Heald and Renton, 1966; Paraguassu, 1972), and also at normal temperatures
and pressures too (Mackenzie and Gees, 1971). Study of the relationship between secondary silica, porosity, and depth of burial is inextricably linked with the phenomenon
of pressure solution, or pressure welding. Rittenhouse (1971) has given a quantitative
analysis of porosity loss that integrates pressure solution with grain shape and packing.
Taylor (1950) showed how, with increasing depth of burial, the number of grain contacts
per grain increased from about one or two near the surface up to five or more at great
depth. Simultaneously, Taylor showed how the nature of the grain contacts changed with
increasing depth of burial. At shallow depths tangential or point contacts are typical.
These grade down into long contacts where grain margins lie snugly side by side. At
greater depths still, concavo-convex and sutured grain boundaries prevail where there
has been extensive pressure solution. These changes in the number and nature of grain
contacts are accompanied by a gradual decrease in porosity (Fig. 8.26). Great care must
be taken in studying the relationship of secondary silica to pressure solution. Sippel
(1968) and Sibley and Blatt (1976) have shown that cathodoluminescence examination
of sands reveals far greater amounts of secondary quartz than revealed by examination
with a polarizing microscope. Many sands that appear to have lost porosity by extensive
pressure solution have, in fact, lost it by extensive secondary quartz cementation. This
calls into question the accuracy of all modal analysis of sandstone composition and most
studies of pressure solution and silica cementation.
8.5.3.3.3 Clay cements
Minor amounts of authigenic clay in a sand inhibit the precipitation of carbonate and
silica cements. But significant amounts of clay may not only occupy pores, but may also
367
overgrowths form with well-defined crystal facies (Plate 4B). Sorby (1880) first drew
attention to this phenomenon in the Permian Penrith sandstone of England (see also
Waugh, 1970a,b).
As cementation continues, however, pore spaces are completely infilled with quartz
(Plate 4C). It is not always apparent whether the resultant fabric has been produced
only by continuous development of the initial quartz overgrowths. Alternatively, solution may have taken place where grains are in contact, and the dissolved silica reprecipitated in the adjacent pore space. This process is termed pressure solution.
The genesis of secondary silica cement has been extensively studied because this is
the most common type of porosity destroyer in sandstones (McBride, 1990; Worden,
2000). Particular attention has been paid to finding the depth below which effective porosity is absent in a particular sedimentary basin. This may be used to predict the "economic basement" below which it would be futile to search for aquifers or hydrocarbon
reservoirs.
Attention has been directed toward the source of silica, the physicochemical conditions that govern its precipitation, and the relationship between silica cementation and
pressure solution. There is no doubt that silica cements may have been precipitated
from solutions which derived their silica from organic debris such as radiolaria, diatom
tests, and siliceous sponge spicules. Likewise, some silica-rich solutions must have been
expelled from compacting clays. A number of successful attempts have been made to
grow silica overgrowths artificially. These have been achieved at high temperatures and
pressures (Heald and Renton, 1966; Paraguassu, 1972), and also at normal temperatures
and pressures too (Mackenzie and Gees, 1971). Study of the relationship between secondary silica, porosity, and depth of burial is inextricably linked with the phenomenon
of pressure solution, or pressure welding. Rittenhouse (1971) has given a quantitative
analysis of porosity loss that integrates pressure solution with grain shape and packing.
Taylor (1950) showed how, with increasing depth of burial, the number of grain contacts
per grain increased from about one or two near the surface up to five or more at great
depth. Simultaneously, Taylor showed how the nature of the grain contacts changed with
increasing depth of burial. At shallow depths tangential or point contacts are typical.
These grade down into long contacts where grain margins lie snugly side by side. At
greater depths still, concavo-convex and sutured grain boundaries prevail where there
has been extensive pressure solution. These changes in the number and nature of grain
contacts are accompanied by a gradual decrease in porosity (Fig. 8.26). Great care must
be taken in studying the relationship of secondary silica to pressure solution. Sippel
(1968) and Sibley and Blatt (1976) have shown that cathodoluminescence examination
of sands reveals far greater amounts of secondary quartz than revealed by examination
with a polarizing microscope. Many sands that appear to have lost porosity by extensive
pressure solution have, in fact, lost it by extensive secondary quartz cementation. This
calls into question the accuracy of all modal analysis of sandstone composition and most
studies of pressure solution and silica cementation.
8.5.3.3.3 Clay cements
Minor amounts of authigenic clay in a sand inhibit the precipitation of carbonate and
silica cements. But significant amounts of clay may not only occupy pores, but may also