Limestone

Limestone, the third most common sedimentary rock, is composed mostly of calcium carbonate, typically of organic origin. Limestone is usually fossiliferous and thus contains abundant evidence of organic evolution; it is also important as a construction material, groundwater aquifer, and oil reservoir.

Limestone Identification and Importance

Limestones are a diverse group of sedimentary rocks, all of which share a common trait: They contain 50 percent or more calcium carbonate, either as the mineral calcite or as aragonite. Both are composed of calcium carbonate; however, they have different atomic arrangements. Other carbonate minerals may also be present; siderite (iron carbonate) and dolomite (calcium-magnesium carbonate) are especially common. Although carbonate minerals can form other rocks, limestone is easily the most common and important carbonate rock. Many geologists use the terms “carbonate rock” and “limestone” almost interchangeably, because most carbonate rocks are limestones.

Limestone may be of chemical or biochemical (organic) origin, and can form in a wide variety of depositional environments. A limestone’s texture and grain content are often useful clues for determining how and where it formed; however, diagenesis can easily obscure or destroy this evidence. Texture and grain content remain the basis for naming numerous varieties of limestone. These include dolomitic limestone, fossiliferous limestone, and crystalline limestone. Other common varieties include chalk, a very soft, fine-grained limestone; travertine, a type of crystalline limestone that forms in caves; and calcareous tufa, which forms by precipitation of calcium carbonate at springs.

Most limestones contain fossils, and many are highly fossiliferous. Limestones are perhaps our best record of ancient life and its evolutionary sequence. They are important sources for building and crushed stone and often contain large supplies of groundwater, oil, and natural gas. Weathering of limestone helps to develop distinctive landscapes as well.

Limestone Formation

Limestones form in one of three ways: chemical precipitation of crystalline grains, biochemical precipitation and accumulation of skeletal and nonskeletal grains, or accumulation of fragments of preexisting limestone rock. Chemical precipitation occurs when the concentration of dissolved calcium carbonate in water becomes so high that the calcium carbonate begins to come out of solution and form a solid, crystalline deposit. The concentration of calcium carbonate in the water may change for a number of reasons. For example, evaporation, increase in water temperature, influx of calcium or carbon dioxide, and decreasing acidity can all cause precipitation. Crystalline limestone forms in the ocean, in alkaline lakes, and in caves, and also as a precipitate in arid climate soils—a variety known as caliche.

Certain marine organisms are responsible for the formation of many kinds of limestone. Their calcareous (calcium carbonate) skeletons accumulate after death, forming carbonate sediment. Many limestones are nothing more than thousands of skeletal grains joined to form a rock. The organisms that contribute their skeletons to carbonate sediments are a diverse group and include both plants and animals. Among these are algae, clams, snails, corals, starfish, sea urchins, and sponges. Some marine animals also produce nonskeletal carbonate sediments. An animal’s solid wastes, or fecal pellets, may accumulate to form limestones if they contain abundant skeletal fragments or compacted lime mud. Limestones composed of skeletal grains, or of nonskeletal grains produced by living organisms, are organic limestones.

Recycling of preexisting limestones is a third source for carbonate grains. Weathering and erosion produce limestone fragments, or clasts, that may later be incorporated into new limestone deposits. Limestones consisting of clasts are clastic, or detrital, limestones; they are probably the least common of the three types of limestone.

Lithification

The processes that turn loose sediment grains into sedimentary rock are known as lithification. These may include either compaction, cementation, or both. The grains (crystals) in a chemically formed limestone are usually joined together into an interlocking, solid matrix when they precipitate; thus, they do not undergo further lithification. The grains in organic and clastic limestones, however, are usually loose, or unconsolidated, when they first accumulate and so must be lithified to form rock.

Limestones, unlike most other sedimentary rocks, are believed to undergo lithification during shallow burial rather than when deep below the earth’s surface. Some may be lithified within a meter or two of the surface, or even at the surface. Therefore, lithification in most limestones consists of cementation without significant compaction of grains. In most cases, the cement is calcium carbonate. If the spaces, or pores, between the grains become cement-filled without much compaction, cement can be as much as 50 percent or more of the volume of a limestone. The cement forms by precipitation, much like the formation of a crystalline limestone.

Formation and Preservation Factors

A number of factors control the formation and preservation of carbonate sediments. These include water temperature and pressure, the amount of agitation, concentrations of dissolved carbon dioxide, noncarbonate sedimentation, and light penetration. Cold, deep water with high levels of dissolved carbon dioxide tends to discourage the formation and accumulation of carbonate sediment. Warm, clear, well-lit, shallow water tends to promote formation and accumulation.

Certain periods of geologic history also favored limestone formation. Generally, the greatest volumes of ancient limestones formed when the global sea level was higher than today, so that seas covered large areas of the continents, and when global temperatures were also higher than at present. This combination of factors was ideal for producing thick, extensive deposits of carbonate rocks. Such limestones are exposed throughout the world today and provide a glimpse into Earth’s distant past. Their abundant marine fossils are especially useful to paleontologists and biologists, as they allow them to piece together the sequence of biological evolution for a variety of plants and animals.

Modern carbonate sediments accumulate in ocean waters ranging in depth from less than 1 meter to more than 5,000 meters and at nearly all latitudes. However, most ancient limestones now exposed at the earth’s surface formed in low-latitude, tropical, shallow marine environments; for example, in reefs or lagoons.

One of the world’s largest modern accumulations of carbonate sediment and rock is located in the Great Barrier Reef off the northeast coast of Australia. This reef tract, the largest in the world, contains thick sequences of carbonate sediment deposited during the last few thousand years draped over even older carbonate rocks formed by coral reef organisms in the more distant past. As long as these reefs continue to thrive, carbonate sediment production will also continue. As a result, this mass of limestone will grow even thicker, and the older rock will continue to subside, sinking deeper into the subsurface. This is questionable, however, as the reefs have been damaged by the effects of climate change.

Diagenesis

No matter where they form or what their origin, carbonate sediments are all subject to diagenesis. Diagenesis consists of those processes that alter the composition or texture of sediments after their formation and burial and before their eventual re-exposure at the earth’s surface. Therefore, lithification is a part of diagenesis, and weathering is not.

One of the great mysteries of geology concerns the origin of dolomite, the calcium-magnesium carbonate mineral, and dolostone, the dolomitic equivalent of limestone. Many geochemists believe dolomite and dolostone owe their origin to the diagenesis of limestone. Dolostones are relatively common in the ancient rock record, yet the formation of dolomite by direct crystallization is rare. This creates a dilemma: Where did all this ancient dolomite come from? Many geochemists believe that the answer lies in alteration (diagenesis) of relatively pure limestone to form dolostone, which contains at least 15 percent dolomite. This process can involve the mixing of fresh and marine water, the enrichment of magnesium by evaporation, and the circulation of warm fluids after diagenesis and burial.

Weathering of Limestone

Most rocks contain fractures known as joints. When rainwater enters joints, chemical weathering occurs, and the joints quickly widen. In the subsurface, horizontal and vertical joints widen, as downward-flowing surface water and laterally flowing groundwater dissolve away limestone, creating increasingly large void spaces, or caves, in the rock. The largest and most extensive cave systems in the world, such as Mammoth Cave in Kentucky, form in limestones. The largest caves usually form where multiple joints intersect in the subsurface.

Where subterranean cavities collapse just below the surface, they form sinkholes. Sinkholes may be exposed at the surface or covered by a layer of soil. Some sinkholes grow and subside only very slowly, while others may collapse in one rapid, catastrophic event. They range in size from a few meters wide and deep to sinkholes large enough to swallow several large buildings should they collapse. The resulting irregular, pockmarked landscape is called “karst topography.” Karst topography is easily recognized by the presence of sinkholes, disappearing streams (which flow into sinkholes), caves, and springs.

Carbonate Petrography

Geologists study limestones for a variety of reasons and at a variety of scales. Most early studies of limestones focused on their fossils. Many limestones contain abundant, well-preserved fossils; some are famous for the exceptional quality of the specimens they contain. Visible, or macroscopic, fossils provide evidence for the sequence of evolution of many invertebrate organisms. Fossils are clues to a limestone’s depositional environment as well; however, more detailed information concerning depositional environments can often be gathered by examining limestones at either smaller or larger scales.

Carbonate petrography involves the study of limestones for the purpose of description and classification. This usually involves using a microscope to determine a limestone’s grain content—that is, the types of carbonate and noncarbonate grains present and their mineral composition, or mineralogy. Carbonate petrology deals with the origin, occurrence, structure, and history of limestones. This involves petrographic studies of limestone as well as field studies of one or more outcrops. Carbonate stratigraphy applies the concepts of petrology at even larger scales and attempts to determine the physical and age relationships between rock bodies that may be separated by great distances.

Carbonate petrography is commonly performed by observing a thin slice of rock through a light microscope. A small block of the rock is cemented onto a microscope slide, then ground down and polished until the slice is about 30 microns (0.03 millimeter) thick. The slice is then thin enough for light to pass easily through it. Microscopic examination of a thin section can reveal a limestone’s mineralogy and its microfossil or other grain content. Other observable traits include cement types, the presence or absence of lime mud, the purity of the limestone, and the types and degree of diagenesis. Use of special stains along with microscopy can reveal even more details of mineralogy. Stains allow easy identification of particular minerals. For example, Alzarin Red S colors calcite red and dolomite purple, so that their percentages can be determined.

Study of Depositional Environments

The field study of limestone exposures, or outcrops, also provides useful information. Along with the macroscopic fossil studies mentioned previously, geologists can study sedimentary structures to learn about a limestone’s depositional environment. Sedimentary structures are mechanically or chemically produced features that record environmental conditions during or after deposition and before lithification. For example, ripple marks indicate water movements by either currents or waves, and their shape and spacing suggests the depth of water and velocity of water movement. The careful study of sedimentary structures provides methodical observers detailed information.

Outcrops also contain evidence of the lateral and vertical sequence of environments responsible for limestone deposition. By studying the lateral changes in a series of limestone outcrops, it is possible to interpret the distribution of environments, or paleogeography, in an area. For example, a researcher might determine in what direction water depths increased, or where a coastline might have been located. The vertical sequence of limestones at an outcrop indicates the paleogeography through time. By interpreting changes in sedimentary structures and other characteristics, a vertical sequence of limestones may indicate that, during deposition, a lagoon existed initially but gave way first to a coral reef and finally to an open ocean environment.

Many researchers conduct even larger-scale studies of limestone sequences. Using advanced technology developed for locating and studying petroleum reservoirs, geophysicists can produce cross-sections showing limestone distribution in the subsurface. This research technique, sequence stratigraphy, allows geologists to see very large-scale features located thousands of meters below the surface and so provides even better insights into regional paleogeography.

Study by Geochemists, Mineralogists, and Engineers

Geochemists and mineralogists study limestones to determine their mineral composition. Simple techniques might involve dissolving a sample of limestone in stages, using a series of different acids. At each stage, the scientist weighs the remaining solid material. From this, approximate percentages of the limestone’s mineral components can be determined. More advanced techniques can involve the use of X-rays and high-energy particle beams to determine a mineral’s atomic structure and precise composition. Such analysis might, for example, allow a chemist to suggest new industrial uses for a particular limestone deposit.

Engineers also study limestones, usually to determine their suitability as a construction or foundation material. Numerous tests are available; engineering tests generally involve determination of physical and chemical properties such as composition, strength, durability, porosity, permeability, solubility, and density. Results from such tests help to predict the behavior of limestones under certain conditions. For example, testing may indicate how a particular limestone would perform as a building foundation in an area with a humid climate and highly acidic soils.

Principal Terms

calcite: the main constituent of limestone, a carbonate mineral consisting of calcium carbonate

carbonates: a large group of minerals consisting of a carbonate anion (three oxygen atoms bonded to one carbon atom, with a residual charge of 2) and a variety of cations, including calcium, magnesium, and iron

cementation: the joining of sediment grains, which results from mineral crystals forming in void spaces between the sediment

deposition: the settling and accumulation of sediment grains after transport

depositional environment: the environmental setting in which a rock forms; for example, a beach, coral reef, or lake

diagenesis: the physical and chemical changes that occur to sedimentary grains after their accumulation

grains: the individual particles that make up a rock or sediment deposit

lithification: compaction and cementation of sediment grains to form a sedimentary rock

texture: the size, shape, and arrangement of grains in a rock

weathering: the disintegration and decomposition of rock at the earth’s surface as the result of the exertion of mechanical and chemical forces

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