8.5 SANDSTONES
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from the German "grauewacke," which was applied to the Paleozoic sandstones of the
Harz Mountain. Petrographic descriptions of examples from the type area have been
given by Helmbold (1952) and Mattiatt (1960).
The greywackes are characteristically hard, dark gray-green rocks that break with a
hackly fracture. Under the hand lens, greywackes are very poorly sorted with particles
ranging from very coarse sand grains down into clay-grade matrix. They have been aptly
described as microconglomerates. Grains are commonly angular and of poor sphericity.
Quartz is overshadowed by an abundance of other detrital minerals. Feldspar is present,
but so also are mafic grains such as horneblende and, occasionally, pyroxenes. Some
of the larger grains are lithic rock fragments, and, depending on the source, these may
have been derived from volcanics or older metasediments such as quartzite or slate. Micas are abundant and include both muscovite and biotite, as well as microcrystalline
diagenetic chlorite and sericite. A diverse suite of unstable heavy minerals is also typical. Plate 3E illustrates a Jurassic greywacke from the northern North Sea.
All of these detrital grains are set in the abundant matrix. This is a microcrystalline
paste of clay minerals, chlorite, sericite, quartz, carbonate (often siderite), pyrite, and
occasionally carbonaceous matter. Corrosion of detrital grain boundaries is sometimes
seen in the form of a characteristic chevaux de frise of micas. This rims not only the unstable grains but sometimes even quartz.
The quartz-wackes differ from the typical greywacke just described in that they lack
the diverse suite of unstable detrital minerals. Their absence is coupled with an increase
in the number of quartz and sedimentary lithic grains, though the clay paste is still present, and in hand specimen quartz-wackes and greywackes are hard to distinguish. The
quartz-wackes correspond in part to the subgreywackes and lithic wackes of some texts
(Plate 3F).
Particular attention has been given to the origin of the matrix in wackes (e.g., Dott,
1964). The poor sorting of these sandstones presupposes that the matrix may be largely
syndepositional. On the other hand, the presence of authigenic minerals, and the corrosion of detrital grains, shows that some of the matrix is diagenetic in origin (Cummins,
1962; Brenchley, 1969). In this context it is important to note that the typical wackes are
largely Pre-Cambrian and Paleozoic in age. This suggests that time, deep burial, and/or
high geothermal gradients are needed to generate greywackes. These are thus perhaps
metamorphic rocks. This argument is of particular importance when considering deepwater sands. Most ancient greywackes occur in flysch facies, which are commonly interpreted as turbidites. Many modern deep-sea sands are clean and well sorted. These may
have been transported into deep water by turbidity flows, but then undergone reworking by geostrophic contour currents (Hubert, 1964). This reworking may have removed
any clay matrix that may have originally been present. The problem is threefold: Is clay
necessary for turbidity flows or not? If it is necessary, then modern deep-sea sands may
be due to normal bottom traction currents.
The greywackes, as stated in the previous paragraphs, are commonly found in preMesozoic flysch facies. These typically occur in subductive troughs and it is apparent,
both from their petrology and regional setting, that greywackes are often derived from
the rising island arcs of volcanic origin. Hence the unstable suite of mafic minerals and
the relatively high percentage of iron and magnesia in greywackes.
The Steinmann Trinity is a picturesque name given to a commonly observed association of greywackes, cherts, and volcanics (Steinmann, 1926). This is often present
355
from the German "grauewacke," which was applied to the Paleozoic sandstones of the
Harz Mountain. Petrographic descriptions of examples from the type area have been
given by Helmbold (1952) and Mattiatt (1960).
The greywackes are characteristically hard, dark gray-green rocks that break with a
hackly fracture. Under the hand lens, greywackes are very poorly sorted with particles
ranging from very coarse sand grains down into clay-grade matrix. They have been aptly
described as microconglomerates. Grains are commonly angular and of poor sphericity.
Quartz is overshadowed by an abundance of other detrital minerals. Feldspar is present,
but so also are mafic grains such as horneblende and, occasionally, pyroxenes. Some
of the larger grains are lithic rock fragments, and, depending on the source, these may
have been derived from volcanics or older metasediments such as quartzite or slate. Micas are abundant and include both muscovite and biotite, as well as microcrystalline
diagenetic chlorite and sericite. A diverse suite of unstable heavy minerals is also typical. Plate 3E illustrates a Jurassic greywacke from the northern North Sea.
All of these detrital grains are set in the abundant matrix. This is a microcrystalline
paste of clay minerals, chlorite, sericite, quartz, carbonate (often siderite), pyrite, and
occasionally carbonaceous matter. Corrosion of detrital grain boundaries is sometimes
seen in the form of a characteristic chevaux de frise of micas. This rims not only the unstable grains but sometimes even quartz.
The quartz-wackes differ from the typical greywacke just described in that they lack
the diverse suite of unstable detrital minerals. Their absence is coupled with an increase
in the number of quartz and sedimentary lithic grains, though the clay paste is still present, and in hand specimen quartz-wackes and greywackes are hard to distinguish. The
quartz-wackes correspond in part to the subgreywackes and lithic wackes of some texts
(Plate 3F).
Particular attention has been given to the origin of the matrix in wackes (e.g., Dott,
1964). The poor sorting of these sandstones presupposes that the matrix may be largely
syndepositional. On the other hand, the presence of authigenic minerals, and the corrosion of detrital grains, shows that some of the matrix is diagenetic in origin (Cummins,
1962; Brenchley, 1969). In this context it is important to note that the typical wackes are
largely Pre-Cambrian and Paleozoic in age. This suggests that time, deep burial, and/or
high geothermal gradients are needed to generate greywackes. These are thus perhaps
metamorphic rocks. This argument is of particular importance when considering deepwater sands. Most ancient greywackes occur in flysch facies, which are commonly interpreted as turbidites. Many modern deep-sea sands are clean and well sorted. These may
have been transported into deep water by turbidity flows, but then undergone reworking by geostrophic contour currents (Hubert, 1964). This reworking may have removed
any clay matrix that may have originally been present. The problem is threefold: Is clay
necessary for turbidity flows or not? If it is necessary, then modern deep-sea sands may
be due to normal bottom traction currents.
The greywackes, as stated in the previous paragraphs, are commonly found in preMesozoic flysch facies. These typically occur in subductive troughs and it is apparent,
both from their petrology and regional setting, that greywackes are often derived from
the rising island arcs of volcanic origin. Hence the unstable suite of mafic minerals and
the relatively high percentage of iron and magnesia in greywackes.
The Steinmann Trinity is a picturesque name given to a commonly observed association of greywackes, cherts, and volcanics (Steinmann, 1926). This is often present