limestones

limestones Limestones are sedimentary rocks that contain more than 50 per cent of calcium carbonate, mainly in the form of the mineral calcite, but also as aragonite, which has the same chemical composition but a different crystal structure. After mudstones and sandstones, limestones are the next most abundant type of sedimentary rock. They extend over huge areas of the continents and the continental shelves, and many mountain chains are dominated by them. Limestones are relatively soluble in meteoric water (water derived from the atmosphere) and they give rise to distinctive landscapes, called karst landscapes, as a result of solution at the surface and for hundreds of metres underground.

Modern shallow-water calcium carbonate sediments are initially composed of the orthorhombic mineral aragonite, together with the rhombohedral minerals calcite and high-magnesium calcite. Ancient indurated (hardened) limestones are composed of calcite. Aragonite and calcite have each been the dominant carbonate phase originally deposited at various times throughout geological history, and all limestones do not originate from what was once thought to be a dominantly aragonitic sediment. These temporal changes in mineralogy are considered to be due to changes in the partial pressure of carbon dioxide and the calcium : magnesium ratio of sea water, which in turn are a reflection of deep-seated crustal processes. Limestone and their modern carbonate sediment equivalents exhibit the same variety of grain sizes, textures, and sedimentary structures as siliciclastic deposits and, in addition, others not exhibited by siliclastic sediments. The old terms used to describe siliciclastics have, in some classifications, been retained for carbonates, with the addition of the prefix ‘calci’, for example, gravels are calcirudites, sands are calcarenites, and muds are calcilutites.

Carbonate sediments and limestones are composed of grains and crystals produced by biological and chemical extraction of calcium carbonate from solution. In contrast to siliciclastic deposits, they are normally deposited close to their site of formation within the basin of deposition; that is they are autochthonous, and are rarely transported great distances from their site of origin. The various components which combine together to form modern carbonate sediments and limestones are of diverse origins and sizes.

Gravel-grade material is generally composed of whole disarticulated or broken skeletal fragments together with sand-grade material of whole, disarticulated or disaggregated and broken skeletal debris. Such sediments can contain fragments of early cemented limestones of local origin, known as intraclasts, or of extra-basinal origin, that is, extraclasts. Sand-sized carbonate sediments commonly consist of sub-spherical or spherical grains with cores consisting of carbonate or siliciclastic grains coated with concentric layers of precipitated carbonate. Such spherical grains are known as ooids, and the common name for a rock consisting of ooids is an oolite. Gravel-grade sediment clasts may have coatings of fine carbonate bound by a cyanobacterial film. The resulting particles are called oncoids. Some sand-sized carbonate grains composed of fine-grained carbonate are faecal pellets produced by sediment-ingesting organisms. Grains with a similar structure and form are, however, produced by cyanobacteria and fungi boring into skeletal debris or ooids, destroying the structure, and then filling their pores with fine-grained precipitated carbonate. When the origin of such grains is uncertain, the term peloid is used; pellet is retained if the origin is certain. Individual grains of all other types are sometimes bound together by sticky coating of cyanobacteria or cement to form composite grains which behave hydrodynamically as a single particle—grapestones.

Mud-grade carbonate is composed of whole tests of small organisms, disarticulated, disaggregated skeletal debris, and broken and finely ground skeletal debris, together with precipitated crystals. The exact origin of this material, particularly when it has been recrystallized in ancient limestones, is often very difficult or impossible to ascertain, although precipitated muds usually have a smaller variety of grain sizes than muds produced by other processes.

Carbonate sediments display the same geomorphological depositional landforms (beach-dune barriers, etc.) as siliciclastic deposits. They can also form wave-resistant structures as a result of binding by skeletal organisms (in reefs or accretionary mounds). The term biostrome is often used to describe these organically produced units when they are tabular or sheet-like, and the term bioherm is used when they form positive elevations on the sea floor.

Unlike siliciclastic deposits, carbonate sediments of different geological age are commonly unlike each other because of the change in flora and fauna over time, some disappearing and others appearing. Furthermore, the mineralogy of individual groups of organisms has varied with time.

Carbonate sediments, and their ancient equivalents, that is limestone, form in a wide variety of sedimentary environments, extending from soils on older rocks, or river flood-plains (limestone nodules in such settings being called calcrete) to lakes to a variety of coastal and marine environments. However, unlike siliciclastic deposits carbonates are unable to accumulate in very deep water because they are dissolved by carbon dioxide-rich bottom waters at the so-called carbonate compensation depth (CCD), where the rate of solution of calcium carbonate equals the rate of supply from overlying water. Below this depth carbonate sediment cannot accumulate.

Marine carbonates are by far the most abundant, both today and in the geological record. As they are the result of extraction of calcium carbonate from sea water, they are most abundant in the low-latitude saturated and supersaturated waters of the tropics where there is greater biological productivity and diversity, and where evaporation enhances the chemical precipitation of calcium carbonate.

Carbonates are also important contributors to shelf sediments in high-latitude areas and become dominant whenever the supply of siliciclastic sediment from adjacent land is limited. Important accumulations of shelf carbonates are found in high latitudes, as, for example, off Scotland and Ireland, and also off southern Australia; their importance has become increasingly recognized over the past few decades.

In warm subtropical and tropical waters of moderate to high salinity, ooids form in turbulent coastal waters and accumulate in shallow water or form beach-dune barriers. The nearshore shelf is dominated by remains of benthonic skeletal organisms, together with some peloids and pellets; there are also variable qualities of carbonate mud, produced by breakdown of skeletal grains as well as chemical precipitation. The remains of planktonic organisms increase away from the shoreline and become dominant in the deeper waters of adjacent marine basins. Hermatypic (reef-building) corals form fringing reefs along coastlines, patch reefs on the shelves, and sometimes barrier reefs along the shelf edge.

The barriers commonly enclose lagoons where pelletal (and peloidal), skeletal, and grapestone sands and carbonate muds accumulate. This association of carbonate sediments forms the so-called chloralgal association of Lees and Buller. Where lagoons have high salinities, hermatypic corals disappear and green algae are rare or unimportant; this is a chlorozoan association. In temperate latitudes, where hermatypic corals and green algae are absent, the sediments are dominated by the remains of molluscs, foraminfera, and red algae, which can form beach-dune barriers similar to those in tropical areas: the foramol association. Ooids and grapestones are, however, absent, although pellets are present but are less important. Carbonate mud is also less important, forming only by skeletal breakdown and not being chemically precipitated.

Limestones and their present-day equivalents are classified using the nature of the component framework grains and their texture. The classifications introduced by Folk in 1959 and 1962, by Dunham in 1962, and the modifications of Dunham's classification by Embry and Klovan in 1971 have superseded all process classifications. Dunham's classification, with its later modifications, is more easily applied to both modern and ancient carbonates than is that of Folk. According to Dunham (Fig. 1), carbonates composed of a supporting framework of grains of various types separated by voids which later may have carbonate cement precipitated in them are termed carbonate grainstones (Fig. 1a, b, c), or packstones if they contain some interstitial mud (Fig. 1d). In contrast, sediments having a carbonate-mud matrix are termed carbonate wackestones (Fig. 1e) if they contain more than 10 per cent of individual grains, or carbonate mudstones (Fig. 1h) if the grain content is less than 10 percent. In 1971 Embry and Klovan added two further terms: carbonate floatstones (Fig. 1f) for sediments containing more than 10 percent of the grains having a grain size greater than 2 mm diameter supported by a mud matrix; and carbonate rudstones (Fig. 1 g) for those with more than 10 percent of the grains having a grain size greater than 2 mm diameter but which are grain supported. Sediments composed of a crystalline mosaic of doubtful, probably diagenetic (post-depositional), origin are merely termed crystalline carbonates (Fig. 1l)

The terms bafflestone, boundstone, and framestone are used where the sediments are bound together by marine grasses, encrusting organisms, or a rigid structure such as a reef, respectively (Fig. 1i, j, k). Grainstones are characteristic of high-energy conditions and mudstones of low-energy, less turbulent conditions, with the intermediate types of sediment forming a continuum between these extreme conditions.

A common facies pattern seen in modern seas and in ancient environments from a shoreline out into a deep marine basin is:(1) lagoonal pelletal (and peloidal) and skeletal sands and muds (peloidal–skeletal packstones, wackestones, and mudstones);(2) beach-dune barrier oolitic and/or skeletal sands (oolitic– skeletal grainstones) with or without reef sediments (coral–algal boundstones);(3) nearshore shelf skeletal sands, muddy sands, and muds (skeletal packstones, wackestones, and mudstones) with patch reefs (coral boundstones) and sometimes barrier reefs; and(4) deep-water carbonate muds with planktonic faunas (carbonate mudstones), sometimes mixed with increasing amounts of siliciclastic mud (marls).Calcium carbonate is a relatively soluble compound and undergoes a wide variety of diagenetic changes after deposition. Grains soon become bound together by the precipitation of fringes of cement around them on beaches, to form beachrock and also on the sea floor at almost all depths where sediment deposition is slow. This produces hard crusts which are bored by bacteria and fungi and support an epifauna. Similar phenomena found in the geological record are termed hardgrounds.

At the present day, cements formed at an early stage are aragonite and high-magnesium calcite in marine sediments, and normal low-magnesium calcite in freshwater deposits and deep-water sediments. During burial the early cements recrystallize, a process called neomorphism, to calcite, and are then overlain by later cements, sometimes produced by solution of the less stable mineral aragonite, by carbonate released by pressure solution at point contacts of adjacent grains, or by solutions moving from adjacent deposits. Fine-grained carbonates recrystallize to form a mosaic of calcite crystals. Aragonitic framework components are usually leached and infilled with a mosaic of calcite. Rarely do aragonitic grains retain their original structure, as they must go through a solution phase before being replaced by calcite.

The diagenetic processes generally produce a rock with less permeability and porosity than the earlier deposit. Occasionally, however, solutions may pass through the sediment and dissolve earlier cements to produce a sediment with high porosity and permeability. Late diagenesis generally greatly reduces both porosity and permeability.

Carbonate sediments are commonly replaced by other minerals; for example, dolomitization can occur at various times in the history of a deposit. Both phosphatized carbonates and silicified carbonates are also common.

Carbonates and limestones are important as raw materials for the building industry (building stone, cement) and for agriculture (lime). Because of their porosity and permeability (which may be primary or may be secondarily produced by waters moving over grains and along fractures) many limestones are important host rocks for water, oil, and gas, for example, in many of the oilfields of the Middle East. They can also act as hosts for mineral deposits, in particular lead and zinc ores.

G. Evans

Bibliography

Bathurst, R. G. (1971) Carbonate sediments and their diagenesis. Elsevier Publishing Company, Amsterdam.
Tucker, M. E. and and Wright, V. P. (1990) Carbonate sedimentology. Blackwell Scientific Publications, Oxford.