354
8 ALLOCHTHONOUS SEDIMENTS
some arkoses, such as the Torridonian (Pre-Cambrian) example of Scotland, contain
well-rounded feldspar grains (Plate 3D). It is unlikely that these grains are polycyclic.
The implication is that rounding occurred within the first and only cycle of sedimentation, and it is known that eolian action is far more effective at this than running water
(see Section 3.1.2). There is a strong presumption that the Torridonian arkoses were
wind-rounded eolian sands that were deposited fluvially in an arid climate.
The typical arkose is a pink or red colored rock; less commonly it is gray. Pink coloration is due to the feldspars, but the red colored examples owe this feature to absorption
of red ferric oxide into the clay matrix. Arkoses show a wide range of grain sizes and
are often poorly sorted. Arkose often forms in place on granites forming a transitional
weathering zone, termed "granite wash," in which it is hard to distinguish sediment from
igneous rock. This situation is a well-site geologist's nightmare. It is very hard to distinguish arkose from granite on the basis of drill cuttings, yet the contact can easily be
picked subsequently from geophysical logs. The grains of arkoses are typically angular
to subrounded and there is generally a significant amount of clay matrix. For this reason arkoses seldom stay unconsolidated for long, like the quartzites. Most ancient arkoses show some degree of lithifaction due to clay bonding. In extreme cases porosity
can be completely obliterated by a silica or carbonate cement. The feldspars in arkose
are of various types, depending on the nature of the source rock, but microcline and albite tend to be more abundant than the less stable calcic feldspars. Alteration of the
feldspars to kaolinite and sericite is a common feature, but it is hard to tell whether this
occurred by hydrothermal alteration of the source rock, during weathering, or through
subsequent burial diagenesis.
Micas are a common accessory mineral and arkoses often contain a diverse suite of
heavy minerals. These may give some indication of the source terrain, specifically
whether it was igneous or metamorphic. In addition to the ubiquitous stable suite of zircon, apatite, garnet, and tourmaline, it is not uncommon to find opaque iron ores as well
as many other heavy minerals.
Huckenholz (1963) described the petrography of the type arkoses of the Auvergne.
Most arkoses seem to occur in alluvium adjacent to granitoid basement. They are, therefore, the characteristic sediment type of fault-bounded intracratonic basins. Because
such sediments are deposited in an oxidizing alluvial fan environment, many arkoses
are of what is termed the "red bed" facies assemblage.
Typical examples of arkoses formed in this setting occur on the Pre-Cambrian shields
all over the world. Specific examples include the Torridonian (Pre-Cambrian) of Scotland (Selley, 1966) and the adjacent Jotnian, Dala, and Sparagmite Pre-Cambrian series of the Scandinavian Shield. The Triassic Newark group of the Connecticut trough,
USA, is another classic example of an arkose. Though probably the bulk of arkoses occur in fluvial environments, they are also found elsewhere. Arkoses mixed with coralgal
debris occur on modern beaches of the fault-bounded Red Sea. Ancient analogs occur
on the oil-soaked, reef-crowned granite highs of the Sirte basin, Libya.
8.5.2.3 Wackes
The wackes are, by definition, texturally immature sands with more than 15 % matrix.
Chemically mature wackes, with less than 25 % feldspar, are termed "quartz-wackes."
Chemically immature wackes are the classic greywackes. The name "greywacke" comes
8 ALLOCHTHONOUS SEDIMENTS
some arkoses, such as the Torridonian (Pre-Cambrian) example of Scotland, contain
well-rounded feldspar grains (Plate 3D). It is unlikely that these grains are polycyclic.
The implication is that rounding occurred within the first and only cycle of sedimentation, and it is known that eolian action is far more effective at this than running water
(see Section 3.1.2). There is a strong presumption that the Torridonian arkoses were
wind-rounded eolian sands that were deposited fluvially in an arid climate.
The typical arkose is a pink or red colored rock; less commonly it is gray. Pink coloration is due to the feldspars, but the red colored examples owe this feature to absorption
of red ferric oxide into the clay matrix. Arkoses show a wide range of grain sizes and
are often poorly sorted. Arkose often forms in place on granites forming a transitional
weathering zone, termed "granite wash," in which it is hard to distinguish sediment from
igneous rock. This situation is a well-site geologist's nightmare. It is very hard to distinguish arkose from granite on the basis of drill cuttings, yet the contact can easily be
picked subsequently from geophysical logs. The grains of arkoses are typically angular
to subrounded and there is generally a significant amount of clay matrix. For this reason arkoses seldom stay unconsolidated for long, like the quartzites. Most ancient arkoses show some degree of lithifaction due to clay bonding. In extreme cases porosity
can be completely obliterated by a silica or carbonate cement. The feldspars in arkose
are of various types, depending on the nature of the source rock, but microcline and albite tend to be more abundant than the less stable calcic feldspars. Alteration of the
feldspars to kaolinite and sericite is a common feature, but it is hard to tell whether this
occurred by hydrothermal alteration of the source rock, during weathering, or through
subsequent burial diagenesis.
Micas are a common accessory mineral and arkoses often contain a diverse suite of
heavy minerals. These may give some indication of the source terrain, specifically
whether it was igneous or metamorphic. In addition to the ubiquitous stable suite of zircon, apatite, garnet, and tourmaline, it is not uncommon to find opaque iron ores as well
as many other heavy minerals.
Huckenholz (1963) described the petrography of the type arkoses of the Auvergne.
Most arkoses seem to occur in alluvium adjacent to granitoid basement. They are, therefore, the characteristic sediment type of fault-bounded intracratonic basins. Because
such sediments are deposited in an oxidizing alluvial fan environment, many arkoses
are of what is termed the "red bed" facies assemblage.
Typical examples of arkoses formed in this setting occur on the Pre-Cambrian shields
all over the world. Specific examples include the Torridonian (Pre-Cambrian) of Scotland (Selley, 1966) and the adjacent Jotnian, Dala, and Sparagmite Pre-Cambrian series of the Scandinavian Shield. The Triassic Newark group of the Connecticut trough,
USA, is another classic example of an arkose. Though probably the bulk of arkoses occur in fluvial environments, they are also found elsewhere. Arkoses mixed with coralgal
debris occur on modern beaches of the fault-bounded Red Sea. Ancient analogs occur
on the oil-soaked, reef-crowned granite highs of the Sirte basin, Libya.
8.5.2.3 Wackes
The wackes are, by definition, texturally immature sands with more than 15 % matrix.
Chemically mature wackes, with less than 25 % feldspar, are termed "quartz-wackes."
Chemically immature wackes are the classic greywackes. The name "greywacke" comes