What is Limestone?

CHEMICALLY AND BIOCHEMICALL

DEPOSITED SEDIMENTARY ROCKS

The rocks described under this heading are, with a few noted exceptions, made up either completely or largely of materials that have been formed by chemical precipitation (in some cases as a result of evaporation), by biochemical precipitation, or by flocculation of colloidal suspensions. Chemical precipitation occurs when a solution becomes super saturated with a substance because of a change in the chemical properties of the solution Evaporation, when extensive, frequently causes solution concentrations that promote precipitation. Biochemical precipitation is precipitation that takes place either directly or indirectly in response to chemical activities of living organisms. Flocculation is the process whereby colloids, which are essentially molecular sized particles in suspension, are deposited, generally as gels that may be desiccated to solid masses of micro or cryptocrystalline substance.
As already stated in the section dealing with the overall classification of sedimentary and digenetic rocks, chemical and biochemical sediments that have undergone physical transport within the basin where they were formed as well as those that have not moved since original deposition are included in this category. The coverage is such that limestone of both origins are described first followed by chert and siliceous sinter, and than by the common evaporates and anhydrock, much of which may be of diagenetic origin.
Clastic Limestones
Many limestones are made up largely of calcium carbonate-rich fragments that have been transported since they were initially precipitated. On the basis of the size of their constituent fragments, these rocks may be called calcirudites, calcarenites or calcilutites-which correspond, respectively, to the already described detrital rocks called conglomerates, sandstones, and silt-or claystones.
Chemically and biochemically precipitated carbonate-rich fragments that have been transported since they were initially precipitated. On the basis of the size of their constituent fragments, these rocks may be called calcirudites, calcarenites or calcilutites-which correspond, respectively, to the already described detrital rocks called conglomerates, sandstones, and silt-or claystones.
Chemically and biochemically precipitated carbonate grains have a special class name-allochem. As already implied, some allochems have been deposited and subsequently lighified in place; others have been formed and, prior to lithification, transported and redeposited elsewhere within the basin in which they were formed. There are four distinct kinds of allochems that are common: intraclasts, fossils, oolites (sometimes termed ooliths), and pellets. Intraclasts are fragments made up of lithified to semilified fragments of calcite or aragonite mud. Most intraclasts are of sand or pebble size; although they may be of just about any shape, most of the larger ones are roughly tabular. Fossils, as allochems, include both completer and fragmental skeletal remains. Fragmentation may occur as a result of such processes as wave and current transport or passing through certain animals' digestive tracts. Oolites are ellipsoidal masses, typically between 0.2 and 2.0 millimeters in longest diameter, with concentric and/or radial structures. Generally found in tidal environments, most oolites resemble fish roe. Pellets are ellipsoidal masses of the same or smaller size than oolites, but with no apparent internal structures. Many pellets are thought to represent fecal material from diverse carbonate mud-ingesting invertebrates; some may be algal spores or nodes. Some geologists apply the term pelletoids to pellets of unknown origin.
A few, probably less than one percent on a world-wide basis, of the fragments termed intraclasts may have been derived from preexisting limestones by mechanical weathering. If a clast is so identified, it should be called a lithoclast and recognized as terrigenous detritus. Unless certain fossil contents or other characteristic features are contained in such clasis, however, lithoclasts and intraclasts may be indistinguishable. Thus some geologists use the less specific designation lime clast.
In most clastic limestones, transported allochems are surrounded by an extremely fine-grained carbonate matrix called micrite and/or by a coarsely crystalline, typically clear, carbonate cement that is sometimes referred to as sparite. Some limestones are almost wholly microcrystalline carbonate and are properly referred to as merely micrites.
Some geologists combine the root terms for the allochems (intra, bio, oo or pel) and for the matrix materials (mic or spar) plus or minus the appropriate grain-size stem (- rudite, -arenite or -lutite) to name rocks of this category-for example, intramicrudite (meaning a breccia made up of intraclasts within a fine-grained matrix), biosparite (a fossiliferous limestone consisting of fossils cemented by coarsely crystalline calcite) and pelmicrite (a pelletiferous calcilutite). An alternative classification scheme, which also includes names for limestones made up of non transported chemical and biochemical calcium carbonates, is given in Table 5-4. As can be seen, that scheme is based primarily on depositional fabric, with the chief concern being whether the constituent grains were or were not in mutual contact when originally deposited. Also, the terms calcirudite, calcarenite or calcilutite plus qualifying adjectives or clarifying phrases can be used for example, anoolitic calcarenite with a microcrystalline matrix (instead of oomicarenite). We prefer the longer, less cryptic designations.
Clastic limestones may be nearly white, gray, bluish, greenish, reddish, brownish or nearly black in color. They may range from micre-to coarsely crystalline with grains up to a few centimeters across. Some of these limestones are extremely pure calcium carbonate; others are clayey, sandy, dolomitic, glauconitic or some combination of these and thereby grade into calcareous shales, calcareous sandstones, and so forth. Many clastic limestones are fossiliferous and a few consist almost wholly of fossils.
Both natural outcrops and man-made exposures of limestone tend to be rounded because of the relative ease whit which limestone dissolves in even weak natural acids, such as rainwater.
Varieties of clastic limestone
Fossiliferous limestones may be made up of many different kinds of fossils or of predominantly one or a few species. The latter are often given specific names such as crinoidal or coralline limestone.
Coquina consists of very loosely packed and cemented shell fragments (Figure 5-23). Most coquina is white with some of the constituent shells sporadically stained buff or tan. Chalk is made up largely of calcareous powder that consists of various mixtures of microorganisms. Under the microscope chalks may be seen to include such things as calcareous exoskeletons of foraminiferans, plates and discs of algae, plus or minus some minor percentage of siliceous diatoms, radiolarians, and/or sponge spicules. Most chalk is white but some is gray, buff, or flesh colored. Typically consists almost wholly of calcium carbonate oolites. Grained limestones that are typically pale creamy to buff or light gray in color and tend to break along conchoidal fractures. Whether these limestones consist of precipitated calcium carbonate mud, of extremely fine bioclastic fragments, or of some combination of the two, can rarely be determined megascopically.

Table 5-4: Classification of limestones according to depositional texture. Modified after R. J. Dunham, 1962.
(With permission of the American Association  of Petroleum Geologists)

- Particles of clay and fine silt size.
- To be subdivided according to classifications designed to bear on physical texture or diagenesis.
Occurrences of clastic limestones.
Clastic limestones ranging in age from Precambran to Recent occur in thick sequences of marine sedimentary rocks throughout the world. With a little searching in areas underlain by such bedrock, several varieties of limestone can usually be found.
One of the world's most famous fossiliferous limestones is the Eocene aged rock used as the facing stone for the Great Pyramid of Cheops near Giza, Egypt. It is reported that tests of a foraminifer, nummulites, within a matrix of fine-grained comminuted fossil debris make up most of the 2.5 million two-ton blocks that were used.
Chalks are especially common in formations of Cretaceous Age. In fact, the name Cretaceous comes from the Latin word creta, which means chalk. Exemplary Cretaceous chalk formations are the famous white Cliffs of Dover in England; the similar high bluffs of eastern Denmark (e.g., Stevns and MQns klints); the Selma Chalk of Alabama, Mississippi and Tennessee; and the Niobrara and Fort Hays chalk formations of Nebraska and kansas.
One of the best known coquinas-mainly because of its widespread use in introductory geology teaching laboratories-is that from the Pliocene and more recent formations of southeastern Florida. For the same reason, and also because it is used widely as a trimstone, the Mississippian age Indiana Limestone from the vicinity of Bedford in southern Indiana is an especially well known oolitic limestone.
The Jurassic age Solenhofen Limestone of Bavaria is a world famous lithographic limestone. It is known not only because of its former use in high-quality lithography but also because of its remarkable fossil preservation. Approximately 500 species have been discovered including insects, jelly fish and even impressions of fleshy parts and bird feathers.
Uses of clastic limestones
Limestones constitute one of the most widely used rock materials. The following gives the more important uses plus a few exemplary lesser uses. Limestone is used widely as road metal, as aggregate for both macadam and concrete mixes, and as building stone. The already mentioned Indiana Limestone is used for statuary as well as for trimstone. The Middle Ordovician age Holston Limestone from Knoxville, Tennessee, is an example of the many limestones that take a good polish and are marketed under the incorrect term marble (see page 253).Another misnomer is the light buff to gray colored Tennessee marble, which contains striking stylolites, that has been used as sanitary enclosures in so many public lavatories in North America.
Limestone is used as a flux in many open-hearth iron smelters. It is one of the basic raw materials for the manufacture of Portland cement. It is the chief source for chemical lime and is ground for use as agricultural lime. It is used as the inert ingredient in some pharmaceutical materials. It is also ground and pressed to make blackboard chalk.
Nonclastic Limestones
Chemically and biochemically precipitated calcitic and/or aragonitic materials that have not been transported since original deposition comprise the rocks of this subclass. These limestones have been formed in both marine and nonmarine environments.
One group of marine limestones that belongs to this group has already been described under the preceding heading "Clastic Limestones". These are the limestones that are made up of oolites and/or pellets or, in rare cases, even fossils that have not been transported laterally since deposition. Actually, in most cases, the matter of whether these allochems have or have not been transported is difficult, if not impossible, to ascertain. In any case, the descriptive terms applied to the corresponding clastic limestones are generally also used for this kind of nonclastic limestones.
Other rocks of this nonclastic limestone subclass are named on the basis of their textures and/or modes of formation.
Reef rocks
In many instances, the remains of active reef-building organisms and/or the products of sediment-binding organisms have been found together since deposition. Included are biohermal reef limestones and algal mat stromatolitic limestones.
Reef limestones are products of communities of marine organisms, most of which have secreted calcareous skeletal matter. Among the organisms that have been primarily responsible for the construction of different reef rocks generally consist largely of a framework of loosely knit, intergrown skeletal matter, most of which is still in its original, respective growth position. The rocks are commonly full of open spaces that are either coated with fine calcite crystals or filled or partially filled with sediment. Many reef rocks appear, at first glance, to be highly fossiliferous breccias.
Stomatolitic limestones exhibit fine laminae, typically a couple of millimeters or less thick, that have the shapes of various convex-upward, hemispherical forms, such as those of inverted shallow bowls and bulbous cabbageheads. The laminae are composed predominately of calcareous clasts, ranging from clay to fine sand size, that have been trapped and bound together by an algal mat. In essence, then, the lamination is merely stratification that has been modified by shallow water algal activities. In most stromatolitic limestones, the laminations are different shades of gray, apparently reflecting different organic matter contents.
Primarily because of their importance to petroleum production, these rocks have been studied rather intensively. As an indirect consequence, several terms have been applied to them. Along with those mentioned in the preceding paragraphs are the more general terms, biolithite and boundstone (see Table 5-4).
Marl
Marl is a variously defined term that is applied to many different materials, Generally it is used for loosely consolidated, earthy-appearing mixtures that consist largely of calcium carbonate and clay. Included are such diverse rocks and deposits as shell and clay mixtures (shell marl), slightly indurated calcareous sands (sandy marl and glauconitic marl), fresh water bog lime, and even poorly cemented argillaceous limestones. We think that if the term is used at all, it should be restricted for use as a gross field designation.
Occurrences of chemically precipitated limestones
Reef rock masses ranging in size from a meter or so up to more that 1000 meters across and more than 100 meters thick occur in many marine sequences of Paleozoic and later age. Some noteworthy examples in North America are the Silurian age Niagaran reefs of the Great Lakes Region, the oil bearing Devonian reefs of Alberta, the Mississippian and Pennsylvania age biohermal reefs of New Mexico, and the Permian complex reefs of the Guadalupe Mountains of Texas and New Mexico. Stromatolitic limestones that range from Precambrian to Recent in age, appear to be best developed in Precambrian and Early Paleozoic age rocks. One frequently cited example is a stromatolitic reef, measuring approximately 65 meters across and 20 meters thick, that occurs near Great Slave Lake in the northwest Territories of Canada.
Most known travertine deposits are of Pleistocene or Recent age. They are relatively common in areas of hot springs (e.g., in the Appalachian Valley and Ridge and the Allegheny Plateau provinces of the eastern United States). In caves, at least some of the precipitation appears to take place as a result of a change in partial pressure of CO2 rather than as a consequence of evaporation.
Marls of diverse compositions are especially common among the loosely consolidated Cenozoic age sediments of the Atlantic Coastal Plain sequence. Bog lime occurs in many fresh water lakes-for example, those of the upper Midwestern United States and adjacent Canada-where some plants (e.g., Chara) produce calcium carbonate in their fruits, leaves and stems.
Uses of nonclastic limestone. Reef rock limestones have many of the same uses already listed for clastic limestones. Travertine, especially that from Tivoli, Italy, has been used as a building material since ancient times-for example, many buildings of ancient Rome, including the exterior of the Colosseum (amphitheatrum Flavium), were made of this rock. It is still being widely used as an interior accent stone. So-called Mexican onyx, most of which is from the area just southeast of Pueblo, Mexico, is carved into ornaments and marketed throughout the world. Marl has been used both as a fertilizer and in the manufacture of Portland cement.
Cherts and Siliceous Sinter
Although much chert appears to be of diagenetic origin, some cherts, like the already described limestones, are of chemical or biochemical origin. Like their biochemical and chemical limestone analogs, these cherts may be either clastic or nonclastic, but this aspect is seldom discernible. Siliceous sinter is analogous to calcareous tufa.
Chemical chert
Some cherts may have been formed as the result of the flocculation of colloidal silica. One present-day occurrence of colloidal silica is on the deep ocean floor of, for example, the central Pacific and in a belt between approximately 45 and 60 degrees south latitude, nearly encircling Antarctica. The general characteristics of chert are described under diagenetic rocks (page 217).
Biochemical chert.
These cherts, most of which were originally made up of opaline silica, consist preominantly of microscopic diatoms, radiolarian tests and/or sponge spicules. On the basis of microscopic examination these cherts would be called, for example, radiolarian chert or diatomaceous chert (or diatomite).
Diatomaceous earth is a name widely applied to a loosely coherent, chalklike sediment made up of fragmentary and complete shells of diatoms. Diatoms are free-swimming, one-celled aquatic plants that secrete microscopic, opaline-silica shells. To give an indication of their size, it has been calculated that a thumb-sized piece of diatomaceous earth would contain about a quarter of a billion diatom shells. The material is typically white but may be pale yellow, gray, or tan in color. It is generally so light that it will flowat on water. Diatomaceous earth can be distinguished from chalk because it does not effervesce with acid, and from clay by its lack of a clayey odor when breathed on.
Siliceous sinter
Opaline silica is sometimes precipitated by the hot waters issuing from hot springs and geysers. The general name for the resulting rock material is siliceous sinter; that deposited from geysers is often called geyserite.
When pure, opaline silica is white. More commonly, it is variously tinted because of the presence of diverse impurities. Most siliceous sinters occur as incrustations around the orifices of springs or geysers and range from loose to compact and from porous to fairly dense. Deposits of siliceous sinter are in several ways the siliceous analogs of deposits of calcareous tufa. Siliceous sinter occurs along with calcareous tufa in Yellowstone National Part, Wyoming.
Occurrences and uses of cherts
Some of the silicic oozes being deposited in the ocean basins today consist largely of radiolarian tests. The Mesozoic age Franciscan Chert of California is a fine example of a radiolarian chert. Other well-known radiolarian cherts are present in the Lower Carboniferous section of Great Britain and Germany and in the Jurassic rocks of the Austrian Alps.
Diatomaceous earth occurs widely in fresh water lake deposits. In fact, at nearly all latitudes there are swamp deposits that include layers of diatomaceous earth. In addition, diatomaceous earth and diatomite of marine origin comprise a nearly 1600 meters thick Miocene age formation that crops out in the Coast Ranges of southern California. The correlative Monterey Chert of central California is a diatom-rich sediment that has been altered to chert.
Diatomite, mostly from the Lompoc District of Santa Barbara Country, California, is used rather widely as an insulating material, a polishing agent, a filler, and as a filter for many purposes.
Evaporites
Evaporites, as already implied, are formed when an aqueous solution is totally or in a large measure evaporated. Although evaporates have been formed from inland lake water as well as from seawater, most of the really extensive deposits have been formed from seawater. Therefore, evaporates deposited in former lake basins are not described as such in this book. Suffice it to say that each rock formed by lake evaporation is generally  referred to by the name of its predominant mineral constituent for example, glauberite, mirabilite, and trona.
The fundamental constituents of evaporates are the ions which were dissolved in the water that was evaporated. In seawater, the most common ions are sodium Na+1, calcium Ca+2, chlorine Cl-1, sulfate (SO4)-2, and carbonate (CO3)-1 . When evaporation takes place, salts are deposited in a predictable order that is controlled by both the solubilities of the salts and the ever changing composition of the solution. The order for typical seawater  is calcite or aragonite (limestone), gypsum and/or  anhydrite (gyprock and/or anhydrock), halite (rock salt), and then the relatively rare salts of magnesium, potassium, and the other ions present, also in a set order. In general, however, evaporate deposits exhibit relationships that clearly show that interruptions and repetitions occurred during their deposition. In some basins the latter relationships are interpreted by some geologists as related to some cyclic phenomenon.
There are a number of especially noteworthy problems associated with the evaporates, perhaps the most puzzling ones relate to their tremendous thicknesses. Consider, for example, that total evaporation of typical seawater with a depth of more than 100 meters would yield only about 0.08 meters of gypsum and 1.4 meters of halite, yet several known deposits are hundreds of meters thick. To date, the only reasonable subggestion to account for such thicknesses involves basins in which the rate of evaporation is equal to or exceeds the rate of inflowing solution.
Another question relates to the fact that in many evaporate deposits the proportions of the evaporates differ markedly from those that would be logically expected from evaporation of a column of seawater.
Also, there is the already posed question as to whether or not anhydrock may be of diagenetic origin. Briefly stated, some geologists have interpreted certain field relationships, such as the presence of gypsum at shallow depths and of anhydrock deeper down  within the same individual deposits, to indicate that gypsum was the originally deposited sulfate and that the anhydrock was formed by subsequent diagenetic dehydration of the gypsum. The often suggested process involves reflux activity of circulating pore solutions.