8.1 INTRODUCTION: THE CLASSIFICATION
OF SEDIMENTARY ROCKS
Natural processes tend to separate the various products of weathering and, after erosion
and transportation, deposit sand, mud, and carbonates in many different environments.
Thus sediments are grouped and segregated spontaneously on the earth's surface. The
next two chapters describe how sediments are turned into rock, first the allochthonous
sediments (those that are transported into a sedimentary basin), and then the autochthonous sediments (those that form within a basin).
To begin with, however, it is necessary to review how the various sedimentary rocks
are named and classified. There are two main reasons for such exercises. First, effective
communication requires a uniformity of nomenclature. Secondly, in a particular study,
it is often necessary to differentiate, compare, and contrast rock types. The following
section describes first the problems of sediment classification in general, and then the
classification of the allochthonous rocks in particular. Sediment is "what settles at the
bottom of a liquid; dregs; a deposit" (Chambers Dictionary, 1972 edition). This definition is itself unacceptable to most geologists since it would exclude, for example, eolian
deposits and biogenic reefs from the realm of sedimentology. Similar dilemmas will be
encountered with most other descriptive terms applied to sedimentary petrography.
Essentially, five main genetic classes of sediment can be recognized: chemical, organic,
residual, terrigenous, and pyroclastic (Hatch et aL, 1971). The chemical sediments are
those that form by direct precipitation in a subaqueous environment. Examples include
evaporites such as gypsum and rock salt, as well tufa and some lime muds.
The organic sediments are those composed of organic matter of both animal and vegetal origin. Examples include skeletal limestones and coal. The residual sediments are
those left in place after weathering, and examples include the laterites and bauxites described in Chapter 2.
The terrigenous sediments are those whose particles were originally derived from the
earth, and include the mudrocks, siliciclastic (as opposed to carbonate) sands, and conglomerates. Pyroclastic sediments are the product of volcanic activity. Examples include
ashes, tufts, volcaniclastic sands, and agglomerates. Additional terms to introduce at this
point are clastic, detrital, and fragmental. These all tend to be used in the same w a y -
describing a rock as formed from the lithifaction of discrete particles, a sediment no less.
329
OF SEDIMENTARY ROCKS
Natural processes tend to separate the various products of weathering and, after erosion
and transportation, deposit sand, mud, and carbonates in many different environments.
Thus sediments are grouped and segregated spontaneously on the earth's surface. The
next two chapters describe how sediments are turned into rock, first the allochthonous
sediments (those that are transported into a sedimentary basin), and then the autochthonous sediments (those that form within a basin).
To begin with, however, it is necessary to review how the various sedimentary rocks
are named and classified. There are two main reasons for such exercises. First, effective
communication requires a uniformity of nomenclature. Secondly, in a particular study,
it is often necessary to differentiate, compare, and contrast rock types. The following
section describes first the problems of sediment classification in general, and then the
classification of the allochthonous rocks in particular. Sediment is "what settles at the
bottom of a liquid; dregs; a deposit" (Chambers Dictionary, 1972 edition). This definition is itself unacceptable to most geologists since it would exclude, for example, eolian
deposits and biogenic reefs from the realm of sedimentology. Similar dilemmas will be
encountered with most other descriptive terms applied to sedimentary petrography.
Essentially, five main genetic classes of sediment can be recognized: chemical, organic,
residual, terrigenous, and pyroclastic (Hatch et aL, 1971). The chemical sediments are
those that form by direct precipitation in a subaqueous environment. Examples include
evaporites such as gypsum and rock salt, as well tufa and some lime muds.
The organic sediments are those composed of organic matter of both animal and vegetal origin. Examples include skeletal limestones and coal. The residual sediments are
those left in place after weathering, and examples include the laterites and bauxites described in Chapter 2.
The terrigenous sediments are those whose particles were originally derived from the
earth, and include the mudrocks, siliciclastic (as opposed to carbonate) sands, and conglomerates. Pyroclastic sediments are the product of volcanic activity. Examples include
ashes, tufts, volcaniclastic sands, and agglomerates. Additional terms to introduce at this
point are clastic, detrital, and fragmental. These all tend to be used in the same w a y -
describing a rock as formed from the lithifaction of discrete particles, a sediment no less.
329