8.5 SANDSTONES
345
Rock fragments are composite grains of volcanic minerals and glass. Volcaniclastic sands
are generally poorly sorted because if extensively transported and reworked they are
rapidly destroyed. Eolian volcaniclastic sands are an exception to this rule. Dunes of
basalt sand occur, for example, in Iceland, and around the volcanic crater of Waw en
Namus in the Libyan Sahara.
Fine-grained volcanic ash tends to undergo intensive postdepositional alteration. This
may give rise to the montmorillonitic bentonites and kaolinitic tonsteins discussed in the
previous section. Palagonite is a mineraloid produced by the alteration of basalt glass.
It is sometimes sufficiently abundant in the Pacific to make a sediment termed palagonite mud, and also occurs lithified in Pacific volcanic islands. Traces of volcanic ash are
a common constituent of modern pelagic sediments.
Volcaniclastic sands may have good primary porosity and permeability, especially if
deposited on beaches or dunes. They lose porosity on burial very quickly, however, because of their unstable mineralogy. During shallow burial they undergo hydration and
carbonation leading to the formation of authigenic carbonate, clay, laumontite, and
other zeolites. Further burial results in dehydration of the clays, albitization of feldspars, and the development of additional zeolites (Galloway, 1974). Thus volcaniclastic
rocks are generally poor-quality aquifers or petroleum reservoirs.
Volcanic and volcaniclastic rocks are of interest in mineral exploration, however, because they appear to be the primary source of some metalliferous deposits; the metals
having been originally disseminated in the volcanic rocks, subsequently mobilized during weathering, and concentrated and reprecipitated during shallow meteoric diagenesis. Uranium roll-fronts and some red-bed stratiform copper deposits are associated
with volcanic centers (see Section 6.3.2.2.4). More detailed accounts of the genesis and
petrology of volcaniclastic sediments are found in Fisher and Schmincke (1984), Suthren
(1985), and Orton (1996).
8.5 SANDSTONES
This section describes the important group of sedimentary rocks, the sandstones. Specifically, it is concerned with the quartzose or siliciclastic sands as distinct from the volcaniclastic and carbonate sands. About 30% of the land's sedimentary cover is made of
terrigenous sand and sandstone. This group of rocks is a fruitful topic for study both for
itself and to aid the exploitation of economic materials within sandstones. Because they
are often highly porous, sandstones are frequently major aquifers and petroleum reservoirs. They are more uniform in stratigraphy and petrophysical character than carbonates and it is, therefore, easier to predict their geometry and reservoir performance.
The following account discusses the problems of sandstone nomenclature and classification, summarizes the features of the more common sandstone types, and concludes
by discussing the effect of diagenesis on sand porosity and permeability.
8.5.1 Nomenclature and Classification of Sandstones
The nomenclature and classification of sandstones has always been a popular academic
pursuit. Indeed Klein (1963) and Pettijohn et al. (1972, pp. 149-174) have written reviews of this topic that include classifications of sandstone classifications. It is important
345
Rock fragments are composite grains of volcanic minerals and glass. Volcaniclastic sands
are generally poorly sorted because if extensively transported and reworked they are
rapidly destroyed. Eolian volcaniclastic sands are an exception to this rule. Dunes of
basalt sand occur, for example, in Iceland, and around the volcanic crater of Waw en
Namus in the Libyan Sahara.
Fine-grained volcanic ash tends to undergo intensive postdepositional alteration. This
may give rise to the montmorillonitic bentonites and kaolinitic tonsteins discussed in the
previous section. Palagonite is a mineraloid produced by the alteration of basalt glass.
It is sometimes sufficiently abundant in the Pacific to make a sediment termed palagonite mud, and also occurs lithified in Pacific volcanic islands. Traces of volcanic ash are
a common constituent of modern pelagic sediments.
Volcaniclastic sands may have good primary porosity and permeability, especially if
deposited on beaches or dunes. They lose porosity on burial very quickly, however, because of their unstable mineralogy. During shallow burial they undergo hydration and
carbonation leading to the formation of authigenic carbonate, clay, laumontite, and
other zeolites. Further burial results in dehydration of the clays, albitization of feldspars, and the development of additional zeolites (Galloway, 1974). Thus volcaniclastic
rocks are generally poor-quality aquifers or petroleum reservoirs.
Volcanic and volcaniclastic rocks are of interest in mineral exploration, however, because they appear to be the primary source of some metalliferous deposits; the metals
having been originally disseminated in the volcanic rocks, subsequently mobilized during weathering, and concentrated and reprecipitated during shallow meteoric diagenesis. Uranium roll-fronts and some red-bed stratiform copper deposits are associated
with volcanic centers (see Section 6.3.2.2.4). More detailed accounts of the genesis and
petrology of volcaniclastic sediments are found in Fisher and Schmincke (1984), Suthren
(1985), and Orton (1996).
8.5 SANDSTONES
This section describes the important group of sedimentary rocks, the sandstones. Specifically, it is concerned with the quartzose or siliciclastic sands as distinct from the volcaniclastic and carbonate sands. About 30% of the land's sedimentary cover is made of
terrigenous sand and sandstone. This group of rocks is a fruitful topic for study both for
itself and to aid the exploitation of economic materials within sandstones. Because they
are often highly porous, sandstones are frequently major aquifers and petroleum reservoirs. They are more uniform in stratigraphy and petrophysical character than carbonates and it is, therefore, easier to predict their geometry and reservoir performance.
The following account discusses the problems of sandstone nomenclature and classification, summarizes the features of the more common sandstone types, and concludes
by discussing the effect of diagenesis on sand porosity and permeability.
8.5.1 Nomenclature and Classification of Sandstones
The nomenclature and classification of sandstones has always been a popular academic
pursuit. Indeed Klein (1963) and Pettijohn et al. (1972, pp. 149-174) have written reviews of this topic that include classifications of sandstone classifications. It is important