Carbonate Sedimentary Rocks

Carbonate sedimentary rocks are a major class of rocks whose composition is dominated by calcium carbonate and calcium-magnesium carbonate minerals — principally calcite, aragonite, and dolomite. Modern carbonate sediments are composed mainly of aragonite, but they also include calcite (especially in deep-sea calcareous ooze) and dolomite. ^[1] The mineralogy and chemistry of carbonate sediments can be strongly influenced by the composition of calcareous fossil organisms present in the sediments. ^[1] For example, many molluscs such as pelecypods, gastropods, pteropods, chitons, and cephalopods, as well as calcareous green algae, stromatoporoids, scleractinian corals, and annelids build skeletons of aragonite, while echinoids, crinoids, bottom-dwelling forams, and coralline red algae are composed mainly of high-magnesian calcite, and planktonic forams, coccoliths, and brachiopods have low-magnesian calcite shells or tests. ^[1] This means that the bulk mineralogy of a carbonate rock is partly a function of which organisms were living in the depositional environment — a detail that carries both environmental and stratigraphic significance.

Mineralogy Through Geologic Time

Although aragonite dominates modern shallow-water carbonate sediments, ancient carbonate rocks older than about the Cretaceous contain little aragonite. ^[1] Aragonite is the metastable polymorph of CaCO₃ — sharing the same chemical composition as calcite but with a different (orthorhombic) crystal structure — and it is converted fairly rapidly under aqueous conditions to calcite. ^[1] Aragonite deposited during the late Paleozoic and early Cenozoic has subsequently dissolved and been replaced by calcite, which is why it is so rarely found in old rocks even where it was originally abundant.

The ratio of dolomite to calcite is much greater in ancient carbonate rocks than in modern carbonate sediments, presumably because CaCO₃ minerals exposed to magnesium-rich interstitial waters during burial and diagenesis are converted to dolomite by replacement. ^[1] The preference for aragonite vs. calcite precipitation has itself varied through Earth history: the oceans during early Paleozoic and middle to late Cenozoic time favored precipitation of calcite, probably because of a lower ratio of magnesium to calcium in seawater during those times. ^[1]

Limestone Textures

Ancient limestones are composed mainly of calcite, which can be present in at least three distinct textural forms: carbonate grains (silt-size or larger aggregates such as ooids and skeletal grains), microcrystalline calcite or carbonate mud (extremely fine crystals texturally analogous to siliciclastic mud), and sparry calcite (coarser-grained crystals appearing clear to translucent in plane light). ^[1] These three components form the basis for most limestone classification schemes, particularly Folk’s and Dunham’s systems.

Early geologists regarded limestones as simply crystalline rocks formed by passive precipitation from seawater, but we now know that many — and perhaps most — carbonate rocks are composed in part of aggregate particles that may have undergone mechanical transport before deposition. ^[1] Folk (1959) introduced the term allochems for these transported carbonate grains to emphasize their non-precipitate origin. ^[1] Carbonate grains typically range in size from coarse silt (0.02 mm) to sand (up to 2 mm), and can be divided into five types — carbonate clasts, skeletal particles, ooids, peloids, and aggregate grains — each distinguished by shape, internal structure, and mode of origin. ^[1]

Sedimentary Structures

Carbonate rocks contain many of the same kinds of sedimentary structures found in siliciclastic rocks, including cross-stratification, laminated bedding, lenticular bedding, convolute lamination, flame structures, load casts, flute casts, groove casts, and mudcracks, as well as trace fossils. ^[1] They may also contain stromatolites and cryptoalgal structures, as well as more carbonate-specific features such as teepee structures (arched or upturned polygons), solution cavities, and stromatactis — irregularly shaped masses of sparry calcite and internal sediment ranging from elongate to globose in shape. ^[1] The fact that carbonates display the same physical transport structures as siliciclastic rocks confirms that carbonate grains behave as particles subject to normal fluid-flow and gravity-flow processes after formation.

Stable Isotopes

The stable isotope composition of carbonate rocks is of considerable interest in palaeoenvironmental studies and for time-stratigraphic correlation. ^[1] Isotopes of oxygen (¹⁸O and ¹⁶O) are particularly useful for these purposes, although carbon, sulfur, and strontium isotopes also have significant utility. ^[1] Stable-isotope studies commonly involve comparison of the ratios of stable isotopes (for example, ¹⁸O/¹⁶O) in a sample to those of a standard. ^[1] Because carbonate minerals precipitate in isotopic equilibrium with ambient water, the ratio of heavy to light oxygen in an ancient limestone can reveal the temperature of the water in which it formed — making carbonates one of the most powerful tools for paleoclimate reconstruction.

Related Pages

• Carbonate Minerals — Full mineralogy table with formulas and crystal systems
• Allochems — The five types of transported carbonate grains
• Micrite — Microcrystalline carbonate mud
• Sparry Calcite — Coarse cement vs diagenetic recrystallization
• Ooids — Coated carbonate grains and their formation
• Limestone Classification — Folk and Dunham classification systems
• Calcium Carbonate Chemistry — CO₂ equilibria and precipitation controls
• Dolomite Problem — The unsolved mystery of dolomite formation
• Dolomitization Models — Hypersaline, mixing-zone, and seawater models

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