Carbonate Rock Classification

The final major category of sedimentary rocks that we will look at is the carbonate rocks. Carbonates are composed of limestones and dolomites and constitute 10 to 15 percent of the sedimentary record. The study of carbonates is focused mostly on the study of their texture and structure, as they are composed almost entirely of the mineral calcium carbonate (CaCO₃).

In the field carbonate rocks often form hard resistant layers similar to sandstones that do not weather as easily as the mudrocks. They react to a couple of drops of dilute hydrochloric acid, bubbling and fizzing when the acid is applied. Limestone reacts more readily than dolomite, and often the only way to get dolomite to react with the acid is to apply the acid to a powdered sample.

The texture of carbonate rocks is variable. Limestone textures can resemble those of clastic rocks like rounding and sorting, or they can have textures resembling chemical precipitates like interlocking crystals. Often limestones will show both types of textures. Limestones will often also have textures associated with biological organisms. These textures are characteristic of the growth habit of the organisms. Though texture is often important in determining the depositional environment for a limestone most limestones are described by their grains (or particles), matrix, and cement.

The grains or particles that make up limestones are called allochemical particles or allochems. Allochems are silt, sand, and gravel-sized particles that compose the framework of mechanically deposited limestones. Mechanically deposited particles are those that accumulate from broken fragments of animals, rocks, or other sources. Another method of deposition forms orthochemical particles (orthochems) and these form from the precipitation of minerals directly from the water. There are four types of allochems: fossils, peloids, ooliths, and intraclasts.

Fossils, or bioclasts, are the carbonate remains of living animals. These are the hard parts that make up the corals, bivalves, crinoids, and other animals that lived in the water where the limestone was formed. Bioclasts are identified by their shape, and are described by the animal that they came from, such as crinoids or bryozoans. Sometimes it is difficult to identify the animal as the bioclasts are broken into smaller fragments before being deposited.

Not every animal that lives in the sea leaves behind a hard body part to be deposited, but many animals leave other traces. Peloids, are ellipse-shaped aggregates of microcrystalline calcium carbonate without any internal structure. Peloids are the fecal pellets made by marine animals that burrow through the sediment eating everything small enough for them to ingest.

Go To Page: 1 2 3

Articles in this Topic    Discussions in this Topic