396
9 AUTOCHTHONOUS SEDIMENTS
porosity and permeability, coupled with chemical instability, is responsible for the complicated diagenesis of carbonate rocks, and hence for the problems of locating aquifers
and hydrocarbon reservoirs within them.
The following brief account of carbonates first defines their mineralogy, then describes their petrography and classification and concludes by showing the relationship
between diagenesis and porosity development.
The carbonate rocks have generated a vast literature. Key works include Chilingar
et al. (1967a,b), Milliman (1974), Bathurst (1975), Reijers and Hsu (1986), Scoffin (1987),
and Tucker and Wright (1990).
9.2.2 Carbonate Minerals
It is necessary to be familiar with the common carbonate minerals to understand the
complex diagenetic changes of carbonate rocks. Calcium carbonate (CaCO3) is the
dominant constituent of modern carbonates and ancient limestones. It occurs as two
minerals, aragonite and calcite. Aragonite crystallizes in the orthorhombic crystal system, while calcite is rhombohedral. Calcite forms an isomorphous series with magnesite (MgCO3). A distinction is made between high- and low-magnesium calcite, with the
boundary being arbitrarily set at 10 mol%.
Ancient limestones are composed largely of low magnesium calcite, while modern
carbonate sediments are made mainly of aragonite and high-magnesian calcite. Aragonite is found in many algae, lamellibranchs, and bryozoa. High magnesian calcite occurs
in echinoids, crinoids, many foraminifera, and some algae, lamellibranchs, and gastropods (Table 9.2). An isomorphous series exists between calcite and magnesite (MgCO3).
Skeletal aragonite and calcite also contain minor amounts of strontium, iron, and other
trace elements. The relationship between carbonate secreting organisms, the mineralogy of their shells, and their contained trace elements has been studied in detail (e.g.,
Lowenstam, 1963; Milliman, 1974). These factors are important because their variation
and distribution play a controlling part in the early diagenesis of skeletal sands. Dolomite is another important carbonate mineral, giving its name also to the rock. Dolomite is calcium magnesium carbonate (CaMg(CO3)2). Isomorphous substitution of
some magnesium for iron is found in the mineral termed ferroan dolomite or ankerite
Ca(MgFe)(CO3) 2. Unlike calcite and aragonite, dolomite does not originate as skeletal material. Dolomite is generally found either crystalline, as an obvious secondary replacement of other carbonates, or as a primary or penecontemporaneous replacement
mineral in cryptocrystalline form. The problem of dolomite genesis is elaborated on
later.
Siderite, iron carbonate (FeCO3), is one of the rarer carbonate minerals. It occurs, apparently as a primary precipitate, in ooliths. These "spherosiderites," as they are termed,
are found in rare restricted marine and freshwater environments. Spherosiderite is often associated with the hydrated ferrous aluminosilicate, chamosite, in sedimentary iron
ores (see Section 9.4). Siderite also occurs as thin bands and horizons of concretions in
argillaceous deposits, especially in deltaic deposits. It is not uncommon to find siderite
bands contorted and fractured by slumping. Siderite clasts are also found in intraformational conglomerates. These facts suggest that siderite forms diagenetically during
early burial while the host sediment is still uncompacted. Its formation is favored by al-
9 AUTOCHTHONOUS SEDIMENTS
porosity and permeability, coupled with chemical instability, is responsible for the complicated diagenesis of carbonate rocks, and hence for the problems of locating aquifers
and hydrocarbon reservoirs within them.
The following brief account of carbonates first defines their mineralogy, then describes their petrography and classification and concludes by showing the relationship
between diagenesis and porosity development.
The carbonate rocks have generated a vast literature. Key works include Chilingar
et al. (1967a,b), Milliman (1974), Bathurst (1975), Reijers and Hsu (1986), Scoffin (1987),
and Tucker and Wright (1990).
9.2.2 Carbonate Minerals
It is necessary to be familiar with the common carbonate minerals to understand the
complex diagenetic changes of carbonate rocks. Calcium carbonate (CaCO3) is the
dominant constituent of modern carbonates and ancient limestones. It occurs as two
minerals, aragonite and calcite. Aragonite crystallizes in the orthorhombic crystal system, while calcite is rhombohedral. Calcite forms an isomorphous series with magnesite (MgCO3). A distinction is made between high- and low-magnesium calcite, with the
boundary being arbitrarily set at 10 mol%.
Ancient limestones are composed largely of low magnesium calcite, while modern
carbonate sediments are made mainly of aragonite and high-magnesian calcite. Aragonite is found in many algae, lamellibranchs, and bryozoa. High magnesian calcite occurs
in echinoids, crinoids, many foraminifera, and some algae, lamellibranchs, and gastropods (Table 9.2). An isomorphous series exists between calcite and magnesite (MgCO3).
Skeletal aragonite and calcite also contain minor amounts of strontium, iron, and other
trace elements. The relationship between carbonate secreting organisms, the mineralogy of their shells, and their contained trace elements has been studied in detail (e.g.,
Lowenstam, 1963; Milliman, 1974). These factors are important because their variation
and distribution play a controlling part in the early diagenesis of skeletal sands. Dolomite is another important carbonate mineral, giving its name also to the rock. Dolomite is calcium magnesium carbonate (CaMg(CO3)2). Isomorphous substitution of
some magnesium for iron is found in the mineral termed ferroan dolomite or ankerite
Ca(MgFe)(CO3) 2. Unlike calcite and aragonite, dolomite does not originate as skeletal material. Dolomite is generally found either crystalline, as an obvious secondary replacement of other carbonates, or as a primary or penecontemporaneous replacement
mineral in cryptocrystalline form. The problem of dolomite genesis is elaborated on
later.
Siderite, iron carbonate (FeCO3), is one of the rarer carbonate minerals. It occurs, apparently as a primary precipitate, in ooliths. These "spherosiderites," as they are termed,
are found in rare restricted marine and freshwater environments. Spherosiderite is often associated with the hydrated ferrous aluminosilicate, chamosite, in sedimentary iron
ores (see Section 9.4). Siderite also occurs as thin bands and horizons of concretions in
argillaceous deposits, especially in deltaic deposits. It is not uncommon to find siderite
bands contorted and fractured by slumping. Siderite clasts are also found in intraformational conglomerates. These facts suggest that siderite forms diagenetically during
early burial while the host sediment is still uncompacted. Its formation is favored by al-