346
8 ALLOCHTHONOUS SEDIMENTS
that sandstones be named with a terminology that is tolerably familiar to, and agreed
on by, practicing geologists. Any nomenclatural system has to have arbitrary bounding
parameters that separate one rock type from another. These bounding parameters are
most useful when based on some underlying concepts of sand genesis. The basic problem of classifying sands is that they can be grouped according to their physical composition (i.e., grain size and matrix content) or according to their chemical composition
(i.e., mineralogy). There are more textural and mineralogical components deemed to be
significant in sandstone nomenclature than can be conveniently represented in an endmember triangle or tetrahedron.
Higher, statistically based classificatory schemes, such as factor analysis, may be more
logical. On the other hand, they lack the simplicity and visual appeal of end-member
classifications. Thus the majority of sand classifications are based on end-member triangles, the three components generally being chosen from quartz, clay, feldspar, or lithic
content.
One of the most fruitful concepts on which sandstone nomenclature is based is the
idea of maturity. The maturation of a sand takes place in two ways. It matures chemically and it matures physically. Sediments form from the weathering of mineralogically
complex source rocks. Throughout weathering and transportation relatively unstable
minerals are destroyed and chemically stable minerals thus increase proportionally.
Quartz is the most abundant stable mineral and feldspar is a common example of an
unstable mineral. An index of the chemical maturity of a rock might, therefore, be the
ratio of quartz to feldspar. As sediments are reworked, perhaps through two or more
cycles of sediment, they thus tend to mature to pure quartz sands.
Physical maturity, on the other hand, describes the textural changes that a sediment
undergoes from the time it is weathered until it is deposited. These changes involve both
an increase in the degree of sorting and a decrease in matrix content. Thus an index of
the degree of physical maturation might be the ratio of grains to matrix. Total clay content is a useful index of textural maturation, given certain reservations to be discussed
shortly.
Both physical and chemical maturation occur during the history of a sand population,
but they are not closely related (Johnsson and Basu, 1993). Thus a chemically mature
sand may be physically immature and vice versa. This is because chemical composition
is essentially a result of provenance, while textural composition is a result of process.
From the preceding analysis, it would appear that the most appropriate triangular classification to adopt would have stable grains at one apex, matrix at a second and unstable
grains at the third (Fig. 8.10). As a sand population increases in textural maturity it
would move away from the matrix apex. As it improves in mineralogical maturity it
would move away from the unstable grain apex. Since both types of maturation occur
simultaneously, albeit at different rates, the net tendency is for a sediment to move to
the stable grain apex. This may not be achieved in a single sedimentary cycle, but it is
the ultimate destination of any sand.
These concepts of maturity can be used as a basis for sandstone nomenclature
(Fig. 8.11). The total amount of clay material in a sand is obviously the best indicator
of matrix content. Feldspar is a common and often volumetrically abundant unstable
mineral which may be used as an index of chemical immaturity. Quartz is the obvious
choice as the index mineral for the apex of chemical stability.
8 ALLOCHTHONOUS SEDIMENTS
that sandstones be named with a terminology that is tolerably familiar to, and agreed
on by, practicing geologists. Any nomenclatural system has to have arbitrary bounding
parameters that separate one rock type from another. These bounding parameters are
most useful when based on some underlying concepts of sand genesis. The basic problem of classifying sands is that they can be grouped according to their physical composition (i.e., grain size and matrix content) or according to their chemical composition
(i.e., mineralogy). There are more textural and mineralogical components deemed to be
significant in sandstone nomenclature than can be conveniently represented in an endmember triangle or tetrahedron.
Higher, statistically based classificatory schemes, such as factor analysis, may be more
logical. On the other hand, they lack the simplicity and visual appeal of end-member
classifications. Thus the majority of sand classifications are based on end-member triangles, the three components generally being chosen from quartz, clay, feldspar, or lithic
content.
One of the most fruitful concepts on which sandstone nomenclature is based is the
idea of maturity. The maturation of a sand takes place in two ways. It matures chemically and it matures physically. Sediments form from the weathering of mineralogically
complex source rocks. Throughout weathering and transportation relatively unstable
minerals are destroyed and chemically stable minerals thus increase proportionally.
Quartz is the most abundant stable mineral and feldspar is a common example of an
unstable mineral. An index of the chemical maturity of a rock might, therefore, be the
ratio of quartz to feldspar. As sediments are reworked, perhaps through two or more
cycles of sediment, they thus tend to mature to pure quartz sands.
Physical maturity, on the other hand, describes the textural changes that a sediment
undergoes from the time it is weathered until it is deposited. These changes involve both
an increase in the degree of sorting and a decrease in matrix content. Thus an index of
the degree of physical maturation might be the ratio of grains to matrix. Total clay content is a useful index of textural maturation, given certain reservations to be discussed
shortly.
Both physical and chemical maturation occur during the history of a sand population,
but they are not closely related (Johnsson and Basu, 1993). Thus a chemically mature
sand may be physically immature and vice versa. This is because chemical composition
is essentially a result of provenance, while textural composition is a result of process.
From the preceding analysis, it would appear that the most appropriate triangular classification to adopt would have stable grains at one apex, matrix at a second and unstable
grains at the third (Fig. 8.10). As a sand population increases in textural maturity it
would move away from the matrix apex. As it improves in mineralogical maturity it
would move away from the unstable grain apex. Since both types of maturation occur
simultaneously, albeit at different rates, the net tendency is for a sediment to move to
the stable grain apex. This may not be achieved in a single sedimentary cycle, but it is
the ultimate destination of any sand.
These concepts of maturity can be used as a basis for sandstone nomenclature
(Fig. 8.11). The total amount of clay material in a sand is obviously the best indicator
of matrix content. Feldspar is a common and often volumetrically abundant unstable
mineral which may be used as an index of chemical immaturity. Quartz is the obvious
choice as the index mineral for the apex of chemical stability.