9.2 CARBONATES
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Oncoids are a third type of coated grain (Fig. 9.2B). These are several centimeters in
diameter and irregularly shaped. The laminae are discontinuous around the grain. Oncoids are formed by primitive blue-green algae growing on a grain and attracting carbonate mud to their sticky surface. Intermittent rolling of the grain allows the formation
of discontinuous lime-mud films. In contrast to ooids therefore, pisoids and oncoids
are indicators of low-energy environments. A rock composed of oncoids is termed an
"oncolite."
9.2.3.2 Micrite
Carbonate mud is termed mierite. The upper size limit of micrite is variously taken as
0.03-0.04 mm in diameter. Micrite may be present in small quantities as a matrix within
a grain-supported carbonate sand, or may be so abundant that it forms a carbonate mudrock, termed "micrite" or "calcilutite" (Plate 6A). Most modern lime muds are composed of aragonite; their lithified fossil analogs are made of calcite.
Several processes appear to generate lime mud. The action of wind, waves, and tides
will smash shell debris and ultimately may abrade them into their constituent crystals.
Fecal pellets may be similarly disaggregated. Biological action is also efficient at breaking up carbonate particles to form crystal muds. This includes the parrot fish which eats
coral, shell-munching benthonic and burrowing invertebrates, and especially some bluegreen algae. These form pits within skeletal grains and lead to the micritization of the
grain surface. Grains so softened will tend to break up and release the micrite. The calcareous algae, such as Halimeda, also secrete aragonite needles within their mucilagenous tissues. On death the mucilage rots to release the aragonite needles.
Some evidence suggests that direct inorganic precipitation of aragonite muds may
sometimes take place. In the Bahamas Banks and the large gulf between Iran and Arabia, "whitings" have been described by Milliman et al. (1993) and Wells and Illing (1964),
respectively. These are temporary cloudy patches of lime mud disseminated in seawater.
They are attributed to spontaneous abiogenic precipitation of aragonite from seawater.
Some geochemical data are inconsistent with this interpretation, however, and there are
other explanations for these phenomena.
Micrite can also form as a cryptocrystalline cement in certain circumstances. Because
of this it must be used with care as an index of depositional energy. In conclusion, it
would appear that lime mud, micrite, can form by a variety of processes.
9.2.3.3 Cement
The third component of carbonate rocks is cement. By definition this applies to crystalline material that grows within the sediment fabric during diagenesis. The most common
cement in limestones is calcite, termed "spar" or sparite (Plate 7D). Other cements in
carbonate rocks include dolomite, anhydrite, and silica.
It is the custom to restrict the term "cement" to the growth of crystals within a pore
space. This has also been termed "drusy crystallization." This type of spar is genetically distinguishable from neomorphic spar which grows by replacement of preexisting carbonate (Folk, 1965). Neomorphism is itself divisible into two varieties. Polymorphic transformations are those which involve a mineral change, as in the reversion of
401
Oncoids are a third type of coated grain (Fig. 9.2B). These are several centimeters in
diameter and irregularly shaped. The laminae are discontinuous around the grain. Oncoids are formed by primitive blue-green algae growing on a grain and attracting carbonate mud to their sticky surface. Intermittent rolling of the grain allows the formation
of discontinuous lime-mud films. In contrast to ooids therefore, pisoids and oncoids
are indicators of low-energy environments. A rock composed of oncoids is termed an
"oncolite."
9.2.3.2 Micrite
Carbonate mud is termed mierite. The upper size limit of micrite is variously taken as
0.03-0.04 mm in diameter. Micrite may be present in small quantities as a matrix within
a grain-supported carbonate sand, or may be so abundant that it forms a carbonate mudrock, termed "micrite" or "calcilutite" (Plate 6A). Most modern lime muds are composed of aragonite; their lithified fossil analogs are made of calcite.
Several processes appear to generate lime mud. The action of wind, waves, and tides
will smash shell debris and ultimately may abrade them into their constituent crystals.
Fecal pellets may be similarly disaggregated. Biological action is also efficient at breaking up carbonate particles to form crystal muds. This includes the parrot fish which eats
coral, shell-munching benthonic and burrowing invertebrates, and especially some bluegreen algae. These form pits within skeletal grains and lead to the micritization of the
grain surface. Grains so softened will tend to break up and release the micrite. The calcareous algae, such as Halimeda, also secrete aragonite needles within their mucilagenous tissues. On death the mucilage rots to release the aragonite needles.
Some evidence suggests that direct inorganic precipitation of aragonite muds may
sometimes take place. In the Bahamas Banks and the large gulf between Iran and Arabia, "whitings" have been described by Milliman et al. (1993) and Wells and Illing (1964),
respectively. These are temporary cloudy patches of lime mud disseminated in seawater.
They are attributed to spontaneous abiogenic precipitation of aragonite from seawater.
Some geochemical data are inconsistent with this interpretation, however, and there are
other explanations for these phenomena.
Micrite can also form as a cryptocrystalline cement in certain circumstances. Because
of this it must be used with care as an index of depositional energy. In conclusion, it
would appear that lime mud, micrite, can form by a variety of processes.
9.2.3.3 Cement
The third component of carbonate rocks is cement. By definition this applies to crystalline material that grows within the sediment fabric during diagenesis. The most common
cement in limestones is calcite, termed "spar" or sparite (Plate 7D). Other cements in
carbonate rocks include dolomite, anhydrite, and silica.
It is the custom to restrict the term "cement" to the growth of crystals within a pore
space. This has also been termed "drusy crystallization." This type of spar is genetically distinguishable from neomorphic spar which grows by replacement of preexisting carbonate (Folk, 1965). Neomorphism is itself divisible into two varieties. Polymorphic transformations are those which involve a mineral change, as in the reversion of