8.5 SANDSTONES
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which contain some feldspars and matrix, and the orthoquartzites, which are almost
pure silica. The quartzites are, therefore, by definition, sands that are mature in texture
and mineralogy. Typical quartzites are white, pale gray, or pink in color. They range from
unconsolidated to splintery in their degree of lithifaction. Grain size is variable, but sorting is generally good and individual grains are normally well rounded.
The main detrital grains are quartz, derived from igneous and metamorphic rocks, and
cherts (made of quartz, chalcedony, or chrystobalite) reworked from sediments. Rare
feldspar and mica grains may be present. Heavy minerals are generally the stable residue such as zircon, tourmaline, apatite, and garnet. Intraformational autochthonous detrital grains are often quite common in quartzites. Glauconite, phosphate pellets, and
skeletal debris are typical examples. Probably the majority of glauconitic sands are referrable to the quartzite group (were it not for the presence of that mineral).
Because of their uniform grain size, rounded grain shape, and low clay content, quartzites possess high porosities and permeabilities at the time of their deposition. Cementation is generally by calcite or secondary silica, but where it is absent, quartzites make
the best aquifers and hydrocarbon reservoirs of all the sandstone types. It is probable
that most, if not all, quartzites are polycyclic in origin. That is to say that they have been
through more than one cycle of weathering, erosion, transportation, and deposition to
achieve the necessary maturity to qualify. It is a matter of observation that the majority of quartzites were deposited in marine sand-shoal environments. Extensive quartz
sand sedimentation occurred in the Lower Paleozoic shelf seas around the border of the
Canadian Shield and the northern margin of the Saharan Shield (e.g., Bennacef et al.,
1971). Quartzose sands are also found in eolian deposits such as the Permian Rotliegende of the North Sea basin (Glennie, 1972) and the early Mesozoic dune sand formations of the Colorado Plateau (Baars, 1961). Eolian and marine shoal environments
provide the optimum conditions for quartzites to be deposited by selective winnowing.
Nevertheless, they do also occur in many other environments, including modern deepsea sands (Hubert, 1964) and ancient turbidites (Sturt, 1961). Plate 3C illustrates a thin
section of the Simpson Group (Silurian) orthoquartzite of Oklahoma.
8.5.2.2 Arkoses
The term arkose was first proposed by Brogniart (1826) for a coarse sand composed of
quartz and substantial quantities of feldspar from the Auvergne in France. This name
is still used for rocks of that general description. Essentially the arkoses are sands which
are relatively mature in texture (i.e., low in clay matrix) and are immature in mineralogy
as shown by the abundance of feldspar. The clay content must generally be less than 15 %
and there must be more than about 25 % of feldspar. Few arkoses have more than 60%
feldspar because this would presuppose a source in which feldspar was more abundant
than quartz. The converse is generally true and the ratio of quartz to feldspar begins to
rise as soon as the source rock is submitted to weathering.
Arkoses, therefore, are the product of the incomplete degradation of acid igneous
and metamorphic rocks such as granites and gneisses. It was once argued that arkoses
were indicators of cold and/or arid climates in which physical weathering processes
dominated chemical ones. Krynine (1935) showed, however, that modern alluvial arkoses occur in tropical rain forest climates. These form because rapid runoff and erosion
cut gullies through the weathered zone and eroded unweathered granite. Nevertheless,
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which contain some feldspars and matrix, and the orthoquartzites, which are almost
pure silica. The quartzites are, therefore, by definition, sands that are mature in texture
and mineralogy. Typical quartzites are white, pale gray, or pink in color. They range from
unconsolidated to splintery in their degree of lithifaction. Grain size is variable, but sorting is generally good and individual grains are normally well rounded.
The main detrital grains are quartz, derived from igneous and metamorphic rocks, and
cherts (made of quartz, chalcedony, or chrystobalite) reworked from sediments. Rare
feldspar and mica grains may be present. Heavy minerals are generally the stable residue such as zircon, tourmaline, apatite, and garnet. Intraformational autochthonous detrital grains are often quite common in quartzites. Glauconite, phosphate pellets, and
skeletal debris are typical examples. Probably the majority of glauconitic sands are referrable to the quartzite group (were it not for the presence of that mineral).
Because of their uniform grain size, rounded grain shape, and low clay content, quartzites possess high porosities and permeabilities at the time of their deposition. Cementation is generally by calcite or secondary silica, but where it is absent, quartzites make
the best aquifers and hydrocarbon reservoirs of all the sandstone types. It is probable
that most, if not all, quartzites are polycyclic in origin. That is to say that they have been
through more than one cycle of weathering, erosion, transportation, and deposition to
achieve the necessary maturity to qualify. It is a matter of observation that the majority of quartzites were deposited in marine sand-shoal environments. Extensive quartz
sand sedimentation occurred in the Lower Paleozoic shelf seas around the border of the
Canadian Shield and the northern margin of the Saharan Shield (e.g., Bennacef et al.,
1971). Quartzose sands are also found in eolian deposits such as the Permian Rotliegende of the North Sea basin (Glennie, 1972) and the early Mesozoic dune sand formations of the Colorado Plateau (Baars, 1961). Eolian and marine shoal environments
provide the optimum conditions for quartzites to be deposited by selective winnowing.
Nevertheless, they do also occur in many other environments, including modern deepsea sands (Hubert, 1964) and ancient turbidites (Sturt, 1961). Plate 3C illustrates a thin
section of the Simpson Group (Silurian) orthoquartzite of Oklahoma.
8.5.2.2 Arkoses
The term arkose was first proposed by Brogniart (1826) for a coarse sand composed of
quartz and substantial quantities of feldspar from the Auvergne in France. This name
is still used for rocks of that general description. Essentially the arkoses are sands which
are relatively mature in texture (i.e., low in clay matrix) and are immature in mineralogy
as shown by the abundance of feldspar. The clay content must generally be less than 15 %
and there must be more than about 25 % of feldspar. Few arkoses have more than 60%
feldspar because this would presuppose a source in which feldspar was more abundant
than quartz. The converse is generally true and the ratio of quartz to feldspar begins to
rise as soon as the source rock is submitted to weathering.
Arkoses, therefore, are the product of the incomplete degradation of acid igneous
and metamorphic rocks such as granites and gneisses. It was once argued that arkoses
were indicators of cold and/or arid climates in which physical weathering processes
dominated chemical ones. Krynine (1935) showed, however, that modern alluvial arkoses occur in tropical rain forest climates. These form because rapid runoff and erosion
cut gullies through the weathered zone and eroded unweathered granite. Nevertheless,