Biogenic sedimentary rocks

Biogenic sedimentary rocks represent the accumulation of skeletal material, produced by the biochemical action of organisms, to form limestone, chert, and phosphorites, or the accumulation of plant material to form coal. Biogenic sedimentary rocks are economically important as sources of energy, fertilizers, and certain chemicals, and they contain evidence of former life on Earth.

Lithification and Coalification

Numerous organisms, particularly those living in the oceans, produce a shell or some type of skeletal material composed of the minerals calcite or aragonite (polymorphs of calcium carbonate), silica (silicon dioxide), or phosphate minerals, principally apatite. After the organism dies and the soft tissue decays, the skeletal material accumulates as sediment. This sediment then undergoes lithification, a process involving compaction and chemical cementation. The time necessary for lithification is highly variable and ranges from a few decades to millions of years. Biogenic sedimentary rocks composed of calcite or aragonite minerals are commonly referred to as limestones; those composed of silica are cherts, and those composed of phosphate minerals are known as phosphorites or sedimentary phosphate deposits. Limestones and cherts may form as the result of direct chemical precipitation of calcium carbonate or silica from a saturated solution, but the primary mode of formation is biochemical. Limestones are the most abundant of the biogenic sedimentary rocks and, economically, the most important. Iron formations, our principal source of iron ore, were also precipitated biogenically early in the earth’s history. They are discussed in a separate article.

Coal also is a biogenic sedimentary rock. Most coal forms from the accumulation of woody plant material in anoxic (oxygen-lacking) environments, such as stagnant lakes, swamps, and bogs. The formation of coal is referred to as coalification and represents microbiological, physical, and chemical processes whereby the percentage of carbon is increased and the volatile content decreased. Limestones, cherts, phosphorites, and coal are often closely associated and may occur as alternating beds in a sequence of sedimentary rocks.

Phosphorites

Because of their biological affinities, biogenic sedimentary rocks contain important evidence of former life on Earth, a record of organic evolution (changes in organisms through geologic time), and clues to the past environments in which the rocks were formed. Limestones may serve as reservoir rocks for oil and gas, which form from the chemical breakdown of microscopic organisms that accumulate with the sediment at the bottom of the ocean and in some lakes. Phosphates, which are contained in phosphorites, are one of the chief constituents of fertilizers and are widely used in the chemical industry. Important evidence of past swamp communities, including both plants and animals, has been preserved in coal beds.

Phosphorus is one of the essential elements of life, and when chemically linked with other elements to form phosphate minerals (especially apatite) it becomes one of the major constituents of all vertebrate skeletons and some invertebrate hard parts. Phosphate is a primary nutrient in marine waters and, therefore, controls organic productivity. Phosphorites often occur as nodules, which are highly variable in size and shape; they may be several centimeters in diameter and up to a few meters in length. These nodules are usually rich in vertebrate skeletal debris, especially that of fish, and fecal material. Bedded phosphorites are commonly interbedded with limestones. Thin beds of phosphorite are rich in bones, fish scales, and fecal material. Modern phosphorites are forming in areas where cold, nutrient-rich waters rise from the ocean depths toward the surface, such as off the west coasts of Africa and North and South America.

Cherts

Cherts, which are occasionally referred to as flint or novaculite, are not nearly as abundant as limestones; however, chert has two properties that have made it one of the most important rocks to humans. Chert has the same chemical composition as quartz but is cryptocrystalline; that is, its crystals are submicroscopic in size. Like quartz, chert is very hard and it tends to break along smooth, curved surfaces. With a little practice, one can produce a sharp edge by chipping a piece of chert with another hard object. Although early humans used numerous rock types for tools and weapons, chert was the most important for the production of arrowheads, knives, and scrapers. Consequently, chert might be considered to have contributed as much to the rise of civilization as did the development of the steam engine. In addition to chert’s utility, Precambrian cherts (1.8 to 3.4 billion years old) are interbedded with important iron deposits known as banded iron formations. These cherts contain evidence of some of the earliest life to evolve on Earth.

Chert, which may be any color depending on the presence of impurities, occurs principally as spherical to oblong nodules 2 to 25 centimeters in diameter in limestones and dolostones, or as thin beds 2 to 25 centimeters deep. Nodular cherts are commonly parallel to bedding or stratification. Bedded cherts may extend laterally for great distances and are commonly interbedded with limestones and dolostones. Most geologists believe that nodular chert forms from the replacement of limestone and that bedded chert forms either by the complete replacement of carbonate-rich beds or by the diagenetic alteration of siliceous ooze.

The source of the silica may be the chemical precipitation of silicon dioxide, volcanic ash, or skeletal material. Biogenic silica is opaline, which means that it may contain up to 10 percent water. Only skeletal silica will be considered in this article. The principal biogenic sources of silica for chert are sponges, diatoms, and radiolarians. Skeletal materials from these organisms are common constituents of oceanic sediments. As this siliceous skeletal material accumulates, the opaline silica is diagenetically converted to crystalline opal and reprecipitated as bladed crystals. Continued diagenesis results in the formation of quartz chert, a mosaic of microscopic quartz crystals. The final diagenetic change often obliterates the shape or structure of the original skeletal material.

Sponges, which are abundant in most marine environments, contain as part of their supportive structure microscopic rods of opaline silica called spicules. When the sponge dies, these spicules accumulate and become part of the sediment. During burial, the opaline silica undergoes diagenesis and precipitation of crystalline opal occurs within pores in the sediment. As crystalline opal is converted into quartz chert, chemical replacement of the surrounding sediment (usually calcite) occurs, resulting in the formation of chert nodules.

Radiolarians and diatoms are microscopic organisms with disk-shaped, elongate, or spherical tests (shells) with spines and surface ornamentation composed of opaline silica. Radiolarians occur as part of the marine zooplankton, and diatoms are part of the marine and nonmarine phytoplankton. Radiolarian and diatom oozes accumulate on the deep ocean floor. In time, these beds of silica-rich ooze are diagenetically converted into thin, bedded cherts. Diatoms, in contrast, may occur in great numbers in lakes and accumulate to form diatomaceous earths, or diatomites. Diatomaceous earths have a wide variety of uses, such as in filtering agents, absorbents, and abrasives.

Limestones

Limestones are the most abundant of the chemical sedimentary rocks. A minor amount of limestone forms from the inorganic precipitation of calcite from seawater or the deposition of calcite in caves and around hot springs. The majority of limestones form as the result of biological and biochemical processes that produce aragonite or calcite. Later, this material becomes part of the carbonate sediment. Once deposited, the carbonate sediment is often modified by the chemical and physical processes of diagenesis.

Numerous animal phyla, such as mollusks, brachiopods, echinoderms, bryozoans, coelenterates, and certain protozoans, produce aragonite or calcite as part of their skeletal structure. The skeletal remains of these organisms are important constituents of carbonate sediments and, eventually, of most limestones. Some marine, bottom-dwelling algae secrete aragonite or calcite. These calcareous algae represent a significant contribution to carbonate sediment. Calcareous algae called coccoliths and calcareous foraminifera are found in great numbers in the plankton of the open ocean. Accumulation of this calcareous material at the bottom of the ocean contributes greatly to the formation of chalk, a type of biogenic limestone. Modern marine organisms principally secrete aragonite; consequently, modern calcareous sediments are initially composed predominantly of aragonite. Aragonite, however, is unstable at low pressures and quickly undergoes diagenesis to calcite.

It is not uncommon to find that ancient limestones have been partially or completely transformed into dolostones; this process is known as dolomitization. Dolomitization occurs when calcium carbonate minerals are diagenetically converted into dolomite. Diagenesis may take place soon after the calcite or aragonite has been deposited or a long time after the deposition. The diagenesis is the result of the magnesium-bearing waters (seawater or percolating meteoric water) moving through the carbonate sediment or limestone. One important aspect of dolomitization is that it often leads to the formation of pores, cavities, and fissures, which enable the rock to serve as a reservoir for oil, gas, and water. Recent research suggests that much dolomitization is caused by the circulation of deep, warm fluids long after burial.

Under certain conditions, limestones are relatively easy to dissolve. In areas such as Florida or Kentucky, where limestones are abundant and there is adequate rainfall, cave systems may develop. Regions with extensive cave systems and related features are referred to as karst.

Study of Rock Samples

Initially, the study of biogenic sedimentary rocks involves field investigations. Where outcrops of these rocks occur, geologists plot and map their distribution and thicknesses; note changes in the rocks’ character, both vertically and horizontally; observe their association with other types of rocks; and collect samples of both the rock and any fossils that may be present. When drilling for oil, gas, or water, engineers and geologists study the same phenomena.

Samples, which are brought back to the laboratory, are analyzed with a vast array of techniques, including X-ray analysis, electron microprobe analysis, scanning electron microscopy, cathodoluminescence, and thin-section examination with a polarized microscope. Samples containing large fossils are observed with a magnifying glass or dissecting microscope. Some samples, particularly limestones, are dissolved in acid to recover both megafossils and microfossils. When samples have been ground thin enough to allow light to pass through the rock, microfossils can be observed with the help of microscopes. Scanning electron microscopy is particularly useful for observing extremely small fossils, especially radiolarians, diatoms, and coccoliths.

Areas of Research

In general, the chemistry of biogenic sedimentary rocks is simple; however, diagenesis can alter the structure, texture, and mineralogy of a sediment during its deposition, lithification, and burial. The analysis of diagenetic changes, particularly in limestones, is one of the most important avenues of investigation. Limestones are greatly influenced by diagenetic modifications and undergo changes in sediment size, porosity (the spaces between sediment grains), and mineralogy. Early diagenetic changes include the conversion of unstable aragonite—which is produced by most calcium-carbonate-producing organisms—into calcite and changes in the calcite’s magnesium content. (Since the atoms of calcium and magnesium have similar properties, they can often substitute for each other in the crystal structure.) Dolomitization may also occur. The replacement of carbonate minerals by silica, so that chert is formed, is another important aspect of diagenesis. Stages of diagenesis can best be observed by using cathodoluminescence, which bombards a thin slice of rock with an electron beam. This process causes minerals within the rock to luminesce (emit light energy for a short interval after the energy source has been removed). The luminescences indicate various stages of diagenesis and are particularly important in studying the details of cements and crystal growth. X-ray analysis and electron microprobe techniques allow scientists to detect subtle differences in chemistries and the presences of trace elements or rare-earth elements.

Another major area of research is the classification of biogenic sedimentary rocks. The classification of limestones is of primary importance. Limestones have been classified using several criteria, but in general, classifications have focused on chemical and mineralogical composition; fabric features, such as fossils and cements; and special physical parameters, such as porosity. When examining chert, the geoscientist must ask: What was the source of the silica (volcanic ash, skeletal grains, chemical precipitate)? What was the time and rate of conversion of siliceous ooze to chert? What was the environment of deposition?

Economic and Evolutionary Significance

Biogenic sedimentary rocks are among the most important of all sedimentary rocks. Because of their biological affinities, they have preserved an important record of past life on Earth. In addition, some of these rocks were used for toolmaking by early humans; others are important resources for construction materials, fertilizers, and chemicals. Oil and natural gas, which also have biological affinities, are inseparably linked to biogenic sedimentary rocks.

Although not widespread, biogenic phosphorites are economically important, particularly to the fertilizer and chemical industries. Biogenic chert was of major importance in the past. Early humans used chert, or flint, to make tools and weapons. Biogenic limestone is the most common and widespread biogenic sedimentary rock. Economically, limestones are very important. Hydrocarbons (oil and gas) are commonly recovered from porous limestones and dolostones, and in some areas, limestones are reservoirs for groundwater supplies. Limestones often serve as host rocks for important mineral deposits, such as lead. Limestones are quarried as building stone; crushed to form construction materials, such as gravel; or processed into lime and cement. Biogenic limestones are the most important sedimentary rock containing fossils and, therefore, are the most important record of the evolution of life.

Principal Terms

carbonate rocks: the general term for rocks containing calcite, aragonite, or dolomite

chert: multicolored, fine-grained sedimentary rock composed of silica and formed in the ocean

coal: dark brown to black sedimentary rock formed from the accumulation of plant material in swampy environments

diagenesis: the chemical, physical, and biological changes undergone by a sediment after its initial deposition

limestone: light gray to black sedimentary rock composed of calcite and formed primarily in the ocean

lithification: the process whereby loose material is transformed into solid rock by compaction or cementation

opal: a form of silica containing a varying proportion of water within the crystal structure

phosphorite: sedimentary rock composed principally of phosphate minerals

polymorph: minerals having the same chemical composition but a different crystal structure

Bibliography

Birch, G. F. “Phosphatic Rocks on the Western Margin of South Africa.” Journal of Sedimentary Petrology 49 (1979): 93-100. Provides a good starting point for a review of phosphorites. Suitable for college-level readers. Available in most university libraries.

Blatt, Harvey, Robert J. Tracy, and Brent Owens. Petrology: Igneous, Sedimentary, and Metamorphic. 3d ed. New York: W. H. Freeman, 2005. Provides easily understood background material and has an extensive bibliography for each category of sedimentary rocks.

Boggs, Sam, Jr. Petrology of Sedimentary Rocks. New York: Cambridge University Press, 2009. Begins with a chapter explaining the classification of sedimentary rocks. Remaining chapters provide information on different types of sedimentary rocks. Multiple chapters describe siliciclastic rocks and discuss limestones, dolomites, and diagenesis.

Boyd, Austin Jarl, et al. "3.7 Billion Year Old Detrital Sediments in Greenland Are Consistent With Active Plate Tectonics in the Eoarchean." Communications Earth & Environment, 15 Apr. 2024, www.nature.com/articles/s43247-024-01376-w. Accessed 25 July 2024. Chernicoff, Stanley. Geology: An Introduction to Physical Geology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2006. Reviews the scientific understanding of geology and surface processes. Includes an online address that provides regular updates on geological events around the globe.

Ham, W. E., ed. Classification of Carbonate Rocks: A Symposium. Tulsa, Okla.: American Association of Petroleum Geologists, 1962. This collection of ten papers is the standard reference for limestone classification. Suitable for college students. Available in most university libraries.

McBride, E. F. Silica in Sediments: Nodular and Bedded Chert. Tulsa, Okla.: Society of Economic Paleontologists and Mineralogists, 1979. This collection of sixteen papers provides the most complete discussion available of chert. Each paper contains an extensive bibliography. Suitable for college-level readers.

Middleton, Gerard V., ed. Encyclopedia of Sediments and Sedimentary Rocks. Dordrecht: Springer, 2003. Cites a vast number of scientists, also listed in the author index. Subjects range from biogenic sedimentary structures to Milankovitch cycles. An index of subjects is provided as well. Designed to cover a broad scope and a degree of detail useful to students, faculty, and geology professionals.

Oldershaw, Cally. Rocks and Minerals. New York: DK, 1999. This small, fifty-three-page volume offers color illustrations and is of great use to new students who may be unfamiliar with the rock and mineral types discussed in classes or textbooks.

Prothero, Donald R., and Fred Schwab. Sedimentary Geology: An Introduction to Sedimentary Rocks and Stratigraphy. 2d ed. New York: W. H. Freeman, 2003. A thorough treatment of most aspects of sediments and sedimentary rocks. Well illustrated with line drawings and black-and-white photographs, it also contains a comprehensive bibliography. Chapters 11 and 12 focus on carbonate rocks and limestone depositional processes and environments. Suitable for college-level readers.

Reading, H. G., ed. Sedimentary Environments: Processes, Facies, and Stratigraphy. Oxford: Blackwell Science, 1996. A good treatment of the study of sedimentary rocks and biogenic sedimentary environments. Suitable for the high school or college student. Well illustrated, with an index and bibliography.

Riding, Robert E., and Stanley M. Awramik. Microbial Sediments. Berlin: Springer-Verlag, 2010. A compilation of articles discussing various biological sources of sediment. Each article includes references. Includes subject indexing. Designed for postgraduates and professional researchers.

Tucker, Maurice E. Sedimentary Rocks in the Field. 4th ed. New York: John Wiley & Sons, 2011. Presents a concise account of biogenic sedimentary rocks and other sedimentary rocks. Classification of sedimentary rocks is well covered. Depositional environments are only briefly discussed. References are well selected. Suitable for undergraduates. Wang, Liuewi, and Hou, Deyi. "Pastistone: An Emerging Type of Sedimentary Rock." Earth-Science Reviews, Dec. 2023, www.sciencedirect.com/science/article/abs/pii/S0012825223003094. Accessed 25 July 2024.