9.1 INTRODUCTION
The second great group of sedimentary rocks is variously referred to as the chemical or
autochthonous sediments group. This group consists of rocks that form within a depositional basin, as opposed to the terrigenous sands and muds that originate outside the
basin. The chemical rocks are sometimes divided into organic and inorganic groups.
Carbonate skeletal sands are a good example of the first type; evaporites of the second.
Biochemical research shows, however, that there is no clearly defined boundary between
these two groups. For example, apparently spontaneous precipitates of lime mud occur
in response to chemical changes in seawater due to the activity of plankton and bacteria
(see Section 9.2.3.2). It is a matter of semantics whether such sediments are organic or
inorganic in origin. Though the chemical rocks are directly precipitated within a sedimentary basin they can be subjected to minor reworking. There are, therefore, examples
of detrital (i.e., clastic) chemical sediments, but these must be carefully distinguished
from the detrital terrigenous sediments of extra-basinal origin.
Table 9.1 shows the main chemical sedimentary rocks. Of these the carbonates are
volumetrically the most important. The carbonate rocks include the limestones made
of calcium carbonate (CaCO3), and the dolomites, composed of the mineral dolomite
(CaMg(CO3)2). Some pedants prefer to restrict the term "dolomite" to the mineral and
refer to the rock as "dolostone." The carbonate rocks form by organic processes, by direct inorganic precipitation, and by diagenesis. Carbonates are important aquifers and
hydrocarbon reservoirs because of the high porosity which they sometimes contain. Porosity distribution is complex, however, and has merited considerable research. For
these reasons carbonate rocks will subsequently be described and discussed in some
detail.
A second important group of the chemical sediments are the evaporites. These form
both by inorganic crystallization and by diagenesis. The most common evaporite mineral
is anhydrite, calcium sulfate (CaSO4), and its hydrated product gypsum (CaSO4.2H20).
Less common evaporites are rocksalt or halite (NaC1) and a whole host of potassium and
other salts. The sedimentary ironstones are a much less abundant chemical sedimentary
rock. They also form both as precipitates and by diagenesis. The common ferruginous
sedimentary minerals are pyrite (FeS2) and siderite (FeCO3). The sedimentary iron ores,
however, include the oxides goethite and hematite, and chamosite, a complex ferrugi394
The second great group of sedimentary rocks is variously referred to as the chemical or
autochthonous sediments group. This group consists of rocks that form within a depositional basin, as opposed to the terrigenous sands and muds that originate outside the
basin. The chemical rocks are sometimes divided into organic and inorganic groups.
Carbonate skeletal sands are a good example of the first type; evaporites of the second.
Biochemical research shows, however, that there is no clearly defined boundary between
these two groups. For example, apparently spontaneous precipitates of lime mud occur
in response to chemical changes in seawater due to the activity of plankton and bacteria
(see Section 9.2.3.2). It is a matter of semantics whether such sediments are organic or
inorganic in origin. Though the chemical rocks are directly precipitated within a sedimentary basin they can be subjected to minor reworking. There are, therefore, examples
of detrital (i.e., clastic) chemical sediments, but these must be carefully distinguished
from the detrital terrigenous sediments of extra-basinal origin.
Table 9.1 shows the main chemical sedimentary rocks. Of these the carbonates are
volumetrically the most important. The carbonate rocks include the limestones made
of calcium carbonate (CaCO3), and the dolomites, composed of the mineral dolomite
(CaMg(CO3)2). Some pedants prefer to restrict the term "dolomite" to the mineral and
refer to the rock as "dolostone." The carbonate rocks form by organic processes, by direct inorganic precipitation, and by diagenesis. Carbonates are important aquifers and
hydrocarbon reservoirs because of the high porosity which they sometimes contain. Porosity distribution is complex, however, and has merited considerable research. For
these reasons carbonate rocks will subsequently be described and discussed in some
detail.
A second important group of the chemical sediments are the evaporites. These form
both by inorganic crystallization and by diagenesis. The most common evaporite mineral
is anhydrite, calcium sulfate (CaSO4), and its hydrated product gypsum (CaSO4.2H20).
Less common evaporites are rocksalt or halite (NaC1) and a whole host of potassium and
other salts. The sedimentary ironstones are a much less abundant chemical sedimentary
rock. They also form both as precipitates and by diagenesis. The common ferruginous
sedimentary minerals are pyrite (FeS2) and siderite (FeCO3). The sedimentary iron ores,
however, include the oxides goethite and hematite, and chamosite, a complex ferrugi394