356
8 ALLOCHTHONOUS SEDIMENTS
in the early phase of sea floor spreading (see Section 10.1.3). Crustal thinning is held
responsible for submarine volcanics and the generation of ultrabasic ophiolites and pillow lavas. It has been argued that the cherts form from blooms of radiolaria caused by
an abundance of volcanically generated silica, whereas the greywackes are formed from
sediments shed off rising volcanic island arcs.
Quartz-waekes, on the other hand, are more usually derived from preexisting sediments. Sands contribute the quartz, shales produce the clay, and the lithic fraction comes
from the indurated equivalents of both. Thus quartz-wackes are typically found, not
so much in fiysch settings, but in proximal continental deposits. Quartz-wackes occur in
fanglomerate and alluvial environments. One example is provided by the continental
Mesozoic sandstones of the Sirte basin, Libya, which were derived from Paleozoic sediments. Another example is the Cretaceous sands of offshore South Africa, which were
drived from the Paleozoic sediments of the Cape Fold Belt.
Continental quartz-wackes are often red-brown in color due to impregnation of the
clay matrix by red ferric oxide. With increasing transportation, the quartz-wackes lose
some of their clay content and assume a rock type often termed subgreywacke. This is
found in both fluvial and deltaic deposits. Examples of subgreywackes occur in the fluvial Devonian and deltaic Pennsylvanian (Upper Carboniferous) sandstones on both
sides of the North Atlantic.
This brief review of sandstone petrography shows that the composition of a newly
deposited sand is a product of provenance and process. The chemical maturity of a recently eroded sediment will depend on the source rock and the extent of weathering.
Chemically mature sediments are generally polycyclic in origin and owe their maturity
to derivation from preexisting sedimentary formations. Chemically immature sands are
generally first-cycle material derived from igneous and high-grade metamorphic rocks.
Recently eroded sediment is commonly poorly sorted and rich in argillaceous matrix.
Eolian and aqueous processes increase the textural maturity of a sand, glacial processes
may reverse it.
The next section of this chapter shows how postdepositional changes affect sandstone
composition, and attempts to relate these to the evolution of porosity and permeability in the terrigenous sands. After deposition a sand may be buried and turned into
sandstone by lithifaction. This consists of physical compaction and chemical diagenesis.
These are now discussed in turn.
8.5.3 Diagenesis and Porosity Evolution of Sandstones
8.5.3.1 Introduction
The term diagenesis has been applied in varying ways to the postdepositional, yet premetamorphic processes that affect a sediment. Dunoyer de Segonzac (1968) has reviewed the history and semantics of this term. For the purposes of the following account
the definition of Pettijohn (1957, p. 648) is used:
Diagenesis refers primarily to the reactions which take place within a sediment between one
mineral and another, or between one or several minerals and the interstitial or supernatant
fluids.
This definition limits diagenesis to essentially chemical processes distinct from physical
processes, such as compaction. Note that this definition is broader than that used by or-
8 ALLOCHTHONOUS SEDIMENTS
in the early phase of sea floor spreading (see Section 10.1.3). Crustal thinning is held
responsible for submarine volcanics and the generation of ultrabasic ophiolites and pillow lavas. It has been argued that the cherts form from blooms of radiolaria caused by
an abundance of volcanically generated silica, whereas the greywackes are formed from
sediments shed off rising volcanic island arcs.
Quartz-waekes, on the other hand, are more usually derived from preexisting sediments. Sands contribute the quartz, shales produce the clay, and the lithic fraction comes
from the indurated equivalents of both. Thus quartz-wackes are typically found, not
so much in fiysch settings, but in proximal continental deposits. Quartz-wackes occur in
fanglomerate and alluvial environments. One example is provided by the continental
Mesozoic sandstones of the Sirte basin, Libya, which were derived from Paleozoic sediments. Another example is the Cretaceous sands of offshore South Africa, which were
drived from the Paleozoic sediments of the Cape Fold Belt.
Continental quartz-wackes are often red-brown in color due to impregnation of the
clay matrix by red ferric oxide. With increasing transportation, the quartz-wackes lose
some of their clay content and assume a rock type often termed subgreywacke. This is
found in both fluvial and deltaic deposits. Examples of subgreywackes occur in the fluvial Devonian and deltaic Pennsylvanian (Upper Carboniferous) sandstones on both
sides of the North Atlantic.
This brief review of sandstone petrography shows that the composition of a newly
deposited sand is a product of provenance and process. The chemical maturity of a recently eroded sediment will depend on the source rock and the extent of weathering.
Chemically mature sediments are generally polycyclic in origin and owe their maturity
to derivation from preexisting sedimentary formations. Chemically immature sands are
generally first-cycle material derived from igneous and high-grade metamorphic rocks.
Recently eroded sediment is commonly poorly sorted and rich in argillaceous matrix.
Eolian and aqueous processes increase the textural maturity of a sand, glacial processes
may reverse it.
The next section of this chapter shows how postdepositional changes affect sandstone
composition, and attempts to relate these to the evolution of porosity and permeability in the terrigenous sands. After deposition a sand may be buried and turned into
sandstone by lithifaction. This consists of physical compaction and chemical diagenesis.
These are now discussed in turn.
8.5.3 Diagenesis and Porosity Evolution of Sandstones
8.5.3.1 Introduction
The term diagenesis has been applied in varying ways to the postdepositional, yet premetamorphic processes that affect a sediment. Dunoyer de Segonzac (1968) has reviewed the history and semantics of this term. For the purposes of the following account
the definition of Pettijohn (1957, p. 648) is used:
Diagenesis refers primarily to the reactions which take place within a sediment between one
mineral and another, or between one or several minerals and the interstitial or supernatant
fluids.
This definition limits diagenesis to essentially chemical processes distinct from physical
processes, such as compaction. Note that this definition is broader than that used by or-