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Concretions in Torysh, Western Kazakhstan
Concretions with lens shape from island in Vltava river, Prague, Czech Republic
Marlstone aggregate concretion, Sault Ste. Marie, Michigan, United States

A concretion is a hard and compact mass formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil.[1] Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word concretion is borrowed from Latin concretio '(act of) compacting, condensing, congealing, uniting', itself derived from concrescere 'to thicken, condense, congeal', from con- 'together' and crescere 'to grow'.[2]

Concretions form within layers of sedimentary strata that have already been deposited. They usually form early in the burial history of the sediment, before the rest of the sediment is hardened into rock. This concretionary cement often makes the concretion harder and more resistant to weathering than the host stratum.

There is an important distinction to draw between concretions and nodules. Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body.

Descriptions dating from the 18th century attest to the fact that concretions have long been regarded as geological curiosities. Because of the variety of unusual shapes, sizes and compositions, concretions have been interpreted to be dinosaur eggs, animal and plant fossils (called pseudofossils), extraterrestrial debris or human artifacts.

Origins

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Concretion rock with white core from the Middle Jurassic of Iran

Detailed studies have demonstrated that concretions form after sediments (small rock particles eroded from a larger rock and carried away to a new location – often by water) are buried but before the sediment is fully lithified (compressed into solid rock) during diagenesis.[3][4][5][6][7][8]

Concretions typically form around a solid core called a "nucleus". This core is often composed of organic material, such as a leaf, tooth, piece of shell or fossil. A mineral solution (that is, a mineral dissolved in water) then precipitates (crystallizes) around the nucleus and cements sediment around it. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens.[9] Some of the most unusual concretion nuclei are World War II military shells, bombs, and shrapnel, which are found inside siderite concretions found in an English coastal salt marsh.[10]

Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth.[11][12] In concentric growth, the concretion grows as successive layers of mineral precipitate around a central core. This process results in roughly spherical concretions that grow with time. In the case of pervasive growth, cementation of the host sediments, by infilling of its pore space by precipitated minerals, occurs simultaneously throughout the volume of the area, which in time becomes a concretion. Concretions are often exposed at the surface by subsequent erosion that removes the weaker, uncemented material.

Appearance

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Samples of small rock concretions found at McConnells Mill State Park in Pennsylvania

Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible[13] to huge bodies three meters in diameter and weighing several thousand pounds.[14] The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost 3 m (9.8 ft) in diameter.[15] Spheroidal concretions, as large as 9 m (30 ft) in diameter, have been found eroding out of the Qasr el Sagha Formation within the Faiyum depression of Egypt.[16] Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.[17]

Composition

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Spherical concretions embedded in sandstone in Anza-Borrego Desert State Park in the United States

Concretions are commonly composed of a mineral present as a minor component of the host rock. For example, concretions in sandstones or shales are commonly formed of a carbonate mineral such as calcite; those in limestones are commonly an amorphous or microcrystalline form of silica such as chert, flint, or jasper; while those in black shale may be composed of pyrite.[18] Other minerals that form concretions include iron oxides or hydroxides (such as goethite and hematite),[19][20] dolomite, siderite,[21] ankerite,[22] marcasite,[23] barite,[24][25] and gypsum.[26]

Although concretions often consist of a single dominant mineral,[27] other minerals can be present depending on the environmental conditions that created them. For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor percentages of pyrite.[28] Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.[29]

Occurrence

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Vaqueros Formation sandstone with concretions
A mosaic of images showing spherules, some partly embedded, spread over (smaller) soil grains on the Martian surface

Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones.[30] They often outwardly resemble fossils or rocks that look as if they do not belong to the stratum in which they were found.[31] Occasionally, concretions contain a fossil, either as its nucleus or as a component that has been incorporated during its growth but concretions are not fossils themselves.[18] They appear in nodular patches, concentrated along bedding planes,[18] or protruding from weathered cliffsides.[32]

Small hematite concretions or Martian spherules have been observed by the Opportunity rover in the Eagle Crater on Mars.[33]

Types of concretion

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Concretions vary considerably in their compositions, shapes, sizes and modes of origin.

Septarian concretions

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"Septaria" redirects here. For the genus of gastropod snail, see Septaria (gastropod).
Moeraki Boulders, New Zealand
A slice of a typical carbonate-rich septarian nodule.

Septarian concretions (or septarian nodules) are carbonate-rich concretions containing angular cavities or cracks (septaria; sg. septarium, from the Latin septum "partition, separating element", referring to the cracks or cavities separating polygonal blocks of hardened material).[34][35] Septarian nodules are characteristically found in carbonate-rich mudrock. They typically show an internal structure of polyhedral blocks (the matrix) separated by mineral-filled radiating cracks (the septaria) which taper towards the rim of the concretion. The radiating cracks sometimes intersect a second set of concentric cracks.[36][34] However, the cracks can be highly variable in shape and volume, as well as the degree of shrinkage they indicate.[37] The matrix is typically composed of argillaceous carbonate, such as clay ironstone, while the crack filling is usually calcite.[36][34] The calcite often contains significant iron (ferroan calcite) and may have inclusions of pyrite and clay minerals. The brown calcite common in septaria may also be colored by organic compounds produced by bacterial decay of organic matter in the original sediments.[38]

Septarian concretions are found in many kinds of mudstone, including lacustrine siltstones such as the Beaufort Group of northwest Mozambique,[39] but are most commonly found in marine shales, such as the Staffin Shale Formation of Skye,[38] the Kimmeridge Clay of England,[40][41] or the Mancos Group of North America.[42]

It is commonly thought that concretions grew incrementally from the inside outwards. Chemical and textural zoning in many concretions are consistent with this concentric model of formation. However, the evidence is ambiguous, and many or most concretions may have formed by pervasive cementation of the entire volume of the concretion at the same time.[43][44][38] For example, if the porosity after early cementation varies across the concretion, then later cementation filling this porosity would produce compositional zoning even with uniform pore water composition.[44] Whether the initial cementation was concentric or pervasive, there is considerable evidence that it occurred quickly and at shallow depth of burial.[45][46][47][38] In many cases, there is clear evidence that the initial concretion formed around some kind of organic nucleus.[48]

The origin of the carbonate-rich septaria is still debated. One possibility is that dehydration hardens the outer shell of the concretion while causing the interior matrix to shrink until it cracks.[36][34] Shrinkage of a still-wet matrix may also take place through syneresis, in which the particles of colloidal material in the interior of the concretion become gradually more tightly bound while expelling water.[39] Another possibility is that early cementation reduces the permeability of the concretion, trapping pore fluids and creating excess pore pressure during continued burial. This could crack the interior at depths as shallow as 10 meters (33 ft).[49] A more speculative theory is that the septaria form by brittle fracturing resulting from earthquakes.[50] Regardless of the mechanism of crack formation, the septaria, like the concretion itself, likely form at a relatively shallow depth of burial of less than 50 meters (160 ft)[51] and possibly as little as 12 meters (39 ft). Geologically young concretions of the Errol Beds of Scotland show texture consistent with formation from flocculated sediments containing organic matter, whose decay left tiny gas bubbles (30 to 35 microns in diameter) and a soap of calcium fatty acids salts. The conversion of these fatty acids to calcium carbonate may have promoted shrinkage and fracture of the matrix.[46][38]

One model for the formation of septarian concretions in the Staffin Shales suggests that the concretions started as semirigid masses of flocculated clay. The individual colloidal clay particles were bound by extracellular polymeric substances or EPS produced by colonizing bacteria. The decay of these substances, together with syneresis of the host mud, produced stresses that fractured the interiors of the concretions while still at shallow burial depth. This was possible only with the bacterial colonization and the right sedimentation rate. Additional fractures formed during subsequent episodes of shallow burial (during the Cretaceous) or uplift (during the Paleogene). Water derived from rain and snow (meteoric water) later infiltrated the beds and deposited ferroan calcite in the cracks.[38]

Septarian concretions often record a complex history of formation that provides geologists with information on early diagenesis, the initial stages of the formation of sedimentary rock from unconsolidated sediments. Most concretions appear to have formed at depths of burial where sulfate-reducing microorganisms are active.[41][52] This corresponds to burial depths of 15 to 150 meters (49 to 492 ft), and is characterized by generation of carbon dioxide, increased alkalinity and precipitation of calcium carbonate.[53] However, there is some evidence that formation continues well into the methanogenic zone beneath the sulfate reduction zone.[54][38][42]

A spectacular example of boulder septarian concretions, which are as much as 3 meters (9.8 feet) in diameter, are the Moeraki Boulders. These concretions are found eroding out of Paleocene mudstone of the Moeraki Formation exposed along the coast near Moeraki, South Island, New Zealand. They are composed of calcite-cemented mud with septarian veins of calcite and rare late-stage quartz and ferrous dolomite.[55][56][57][58] The much smaller septarian concretions found in the Kimmeridge Clay exposed in cliffs along the Wessex coast of England are more typical examples of septarian concretions.[59]

Cannonball concretions

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Concretions on Bowling Ball Beach (Mendocino County, California, United States) weathered out of steeply tilted Cenozoic mudstone

Cannonball concretions are large spherical concretions, which resemble cannonballs. These are found along the Cannonball River within Morton and Sioux Counties, North Dakota, and can reach 3 m (9.8 ft) in diameter. They were created by early cementation of sand and silt by calcite. Similar cannonball concretions, which are as much as 4 to 6 m (13 to 20 ft) in diameter, are found associated with sandstone outcrops of the Frontier Formation in northeast Utah and central Wyoming. They formed by the early cementation of sand by calcite.[60] Somewhat weathered and eroded giant cannonball concretions, as large as 6 meters (20 feet) in diameter, occur in abundance at "Rock City" in Ottawa County, Kansas. Large and spherical boulders are also found along Koekohe beach near Moeraki on the east coast of the South Island of New Zealand.[61] The Moeraki Boulders, Ward Beach boulders and Koutu Boulders of New Zealand are examples of septarian concretions, which are also cannonball concretions. Large spherical rocks, which are found on the shore of Lake Huron near Kettle Point, Ontario, and locally known as "kettles", are typical cannonball concretions. Cannonball concretions have also been reported from Van Mijenfjorden, Spitsbergen; near Haines Junction, Yukon Territory, Canada; Jameson Land, East Greenland; near Mecevici, Ozimici, and Zavidovici in Bosnia-Herzegovina; in Alaska in the Kenai Peninsula Captain Cook State Park on north of Cook Inlet beach[62] and on Kodiak Island northeast of Fossil Beach.[63] This type of concretion is also found in Romania, where they are known as trovants.[64][65]

Hiatus concretions

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Hiatus concretion encrusted by bryozoans (thin, branching forms) and an edrioasteroid; Kope Formation (Upper Ordovician), northern Kentucky
Hiatus concretions at the base of the Menuha Formation (Upper Cretaceous), the Negev, southern Israel

Hiatus concretions are distinguished by their stratigraphic history of exhumation, exposure and reburial. They are found where submarine erosion has concentrated early diagenetic concretions as lag surfaces by washing away surrounding fine-grained sediments.[66] Their significance for stratigraphy, sedimentology and paleontology was first noted by Voigt who referred to them as Hiatus-Konkretionen.[67] "Hiatus" refers to the break in sedimentation that allowed this erosion and exposure. They are found throughout the fossil record but are most common during periods in which calcite sea conditions prevailed, such as the Ordovician, Jurassic and Cretaceous.[66] Most are formed from the cemented infillings of burrow systems in siliciclastic or carbonate sediments.

A distinctive feature of hiatus concretions separating them from other types is that they were often encrusted by marine organisms including bryozoans, echinoderms and tube worms in the Paleozoic[68] and bryozoans, oysters and tube worms in the Mesozoic and Cenozoic. Hiatus concretions are also often significantly bored by worms and bivalves.[69]

Elongate concretions

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Elongate concretions form parallel to sedimentary strata and have been studied extensively due to the inferred influence of phreatic (saturated) zone groundwater flow direction on the orientation of the axis of elongation.[70][60][71][72] In addition to providing information about the orientation of past fluid flow in the host rock, elongate concretions can provide insight into local permeability trends (i.e., permeability correlation structure; variation in groundwater velocity,[73] and the types of geological features that influence flow.

Elongate concretions are well known in the Kimmeridge Clay formation of northwest Europe. In outcrops, where they have acquired the name "doggers", they are typically only a few meters across, but in the subsurface they can be seen to penetrate up to tens of meters of along-hole dimension. Unlike limestone beds, however, it is impossible to consistently correlate them between even closely spaced wells.[citation needed]

Moqui Marbles

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Moqui Marbles, hematite, goethite concretions, from the Navajo Sandstone of southeast Utah. The "W" cube at the top is one cubic centimeter in size.

Moqui Marbles, also called Moqui balls or "Moki marbles", are iron oxide concretions which can be found eroding in great abundance out of outcrops of the Navajo Sandstone within south-central and southeastern Utah. These concretions range in shape from spheres to discs, buttons, spiked balls, cylindrical forms, and other odd shapes. They range from pea-size to baseball-size.[74][75]

The concretions were created by the precipitation of iron, which was dissolved in groundwater. The iron was originally present as a thin film of iron oxide surrounding sand grains in the Navajo Sandstone. Groundwater containing methane or petroleum from underlying rock beds reacted with the iron oxide, converting it to soluble reduced iron. When the iron-bearing groundwater came into contact with more oxygen-rich groundwater, the reduced iron was converted back to insoluble iron oxide, which formed the concretions.[74][75][76] It is possible that reduced iron first formed siderite concretions that were subsequently oxidized. Iron-oxidizing bacteria may have played a role.[77]

Kansas pop rocks

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Kansas pop rocks are concretions of either iron sulfide, i.e. pyrite and marcasite, or in some cases jarosite, which are found in outcrops of the Smoky Hill Chalk Member of the Niobrara Formation within Gove County, Kansas. They are typically associated with thin layers of altered volcanic ash, called bentonite, that occur within the chalk comprising the Smoky Hill Chalk Member. A few of these concretions enclose, at least in part, large flattened valves of inoceramid bivalves. These concretions range in size from a few millimeters to as much as 0.7 m (2.3 ft) in length and 12 cm (0.39 ft) in thickness. Most of these concretions are oblate spheroids. Other "pop rocks" are small polycuboid pyrite concretions, which are as much as 7 cm (0.23 ft) in diameter. These concretions are called "pop rocks" because they explode if thrown in a fire. Also, when they are either cut or hammered, they produce sparks and a burning sulfur smell. Contrary to what has been published on the Internet, none of the iron sulfide concretions, which are found in the Smoky Hill Chalk Member were created by either the replacement of fossils or by metamorphic processes. In fact, metamorphic rocks are completely absent from the Smoky Hill Chalk Member.[78] Instead, all of these iron sulfide concretions were created by the precipitation of iron sulfides within anoxic marine calcareous ooze after it had accumulated and before it had lithified into chalk.

Marleka fairy stone from Stensö in Sweden

Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either pyrite and marcasite, are nonmagnetic.[79] On the other hand, iron sulfide concretions, which either are composed of or contain either pyrrhotite or smythite, will be magnetic to varying degrees.[80] Prolonged heating of either a pyrite or marcasite concretion will convert portions of either mineral into pyrrhotite causing the concretion to become slightly magnetic.

Claystones, clay dogs, and fairy stones

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Disc concretions composed of calcium carbonate are often found eroding out of exposures of interlaminated silt and clay, varved, proglacial lake deposits. For example, great numbers of strikingly symmetrical concretions have been found eroding out of outcrops of Quaternary proglacial lake sediments along and in the gravels of the Connecticut River and its tributaries in Massachusetts and Vermont. Depending the specific source of these concretions, they vary in an infinite variety of forms that include disc-shapes; crescent-shapes; watch-shapes; cylindrical or club-shapes; botryoidal masses; and animal-like forms. They can vary in length from 2 in (5.1 cm) to over 22 in (56 cm) and often exhibit concentric grooves on their surfaces. In the Connecticut River Valley, these concretions are often called "claystones" because the concretions are harder than the clay enclosing them. In local brickyards, they were called "clay-dogs" either because of their animal-like forms or the concretions were nuisances in molding bricks.[81][82][83] Similar disc-shaped calcium carbonate concretions have also been found in the Harricana River valley in the Abitibi-Témiscamingue administrative region of Quebec, and in Östergötland county, Sweden. In Scandinavia, they are known as "marlekor" ("fairy stones").[84][85]

Gogottes

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Gogotte concretion

Gogottes [fr] are sandstone concretions found in Oligocene (~30 million years) aged sediments near Fontainebleau, France. Gogottes have fetched high prices at auction due to their sculpture-like quality.[86]

See also

[edit]
  • Bowling Ball Beach – Beach in Mendocino County, California, US
  • Caliche, also known as calcrete – Calcium carbonate based concretion of sediment in arid and semi-arid soils
  • Champ Island – Island in Franz Josef Land, Russia
  • Diagenesis – Physico-chemical changes in sediments occurring after their deposition
  • Dinocochlea – Trace fossil in the Natural History Museum, London
  • Dorodango – Japanese art form in which earth and water are molded to create a delicate shiny sphere
  • Gypcrust – Hardened layer of soil with a high percentage of gypsum. CaSO4 concretions in arid and semi-arid soils
  • Klerksdorp sphere – Natural nodule-like rock concretions
  • Martian spherules – Small iron oxide spherules found on Mars
  • Moeraki Boulders – Large spherical boulders on Otago coast, New Zealand
  • Mushroom Rock State Park – State park in Kansas, United States
  • Nodule (geology) – Small mass of a mineral with a contrasting composition to the enclosing sediment or rock, a replacement body, not to be confused with a concretion
  • Rock City, Kansas – Park in Kansas, U.S.
  • Speleothem – Structure formed in a cave by the deposition of minerals from water. CaCO3

References

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Concretion

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A concretion is a clearly bounded, compact mass of matter embedded within sedimentary rocks, typically forming through the of minerals from that cements surrounding grains into a structure harder than the enclosing material. The term derives from the Latin concretio, meaning "a growing together," reflecting the cohesive nature of these formations. Concretions often exhibit rounded, oval, lumpy, or irregular shapes and can vary in size from a few centimeters to over 3 meters in diameter, commonly developing around a central nucleus such as a , shell, or grain. These structures arise during , the process of sediment compaction and chemical alteration after deposition, where dissolved minerals in percolating fluids selectively precipitate to bind particles, creating distinct boundaries with softer host rocks like or . Common compositions include carbonates like (forming limestone concretions), silica (chert or flint), and iron oxides such as or , with the latter often resulting from iron-rich solutions in oxidizing environments. In coal-bearing strata, concretions frequently appear as isolated masses in mine roofs, posing potential stability issues due to their hardness contrasting with friable shales. Notable examples include the cannonball concretions of in , which are large, spherical masses up to 1.5 meters (5 feet) in diameter, and the Moqui marbles of southern , small spheres formed in Jurassic sandstones. In , concretions such as large spherical forms or septarian nodules, featuring internal cracking filled with veins, are prominent in the Greenhorn , aiding paleontologists in preserving fossils. These formations not only reveal insights into ancient groundwater chemistry and but also influence resource extraction, such as in where they can complicate roof support.

Fundamentals

Definition

A concretion is a compact, rounded or irregular mass of matter formed by localized of cement within , filling pore spaces between grains. These structures are typically composed of minerals such as , silica, or iron oxides that precipitate from or pore fluids, creating a hard, cohesive body embedded in the surrounding . Unlike primary depositional features, concretions develop post-depositionally through diagenetic processes, where mineral growth occurs selectively around a nucleus, such as a fragment or . Key characteristics of concretions include their clearly bounded form, distinguishing them as discrete bodies within softer enclosing sediments of similar overall composition. They are not fossils themselves but can enclose and preserve organic remains, internal like , or even other concretions. This bounded nature arises from the concentration of cementing material in specific zones, resulting in a harder, more resistant mass compared to the host rock, which aids in their identification during or excavation. Concretions are frequently mistaken for fossil eggs, bones, or turtle shells due to their ovoid or rounded shapes and smooth surfaces, though they lack the organic microstructures characteristic of true fossils. Historical examples include concretions that were once thought to be petrified tortoises, highlighting how their morphology can lead to misidentification without close examination. In reality, these resemblances stem from inorganic mineral aggregation rather than biological origins. The term "concretion" originates from the Latin concretio, meaning "growing together," which reflects the process of mineral particles aggregating and hardening into a solid mass. This etymology, derived through Middle English concret from Latin concrescere (to grow together), underscores the geological mechanism of selective mineral coalescence within sediments.

Formation Processes

Concretions primarily form through the localized precipitation of mineral cements, such as calcite or silica, from supersaturated pore fluids or groundwater within unlithified sediments. This process initiates with nucleation around a central point—often a decaying organic nucleus—and expands outward as minerals displace or replace surrounding sediment grains, filling pore spaces and binding the material into a coherent mass. The precipitation is driven by chemical gradients established during early diagenesis, where fluids migrate through permeable sediments, leading to selective cementation that contrasts with the less indurated host rock. The formation unfolds in distinct stages: initial occurs shortly after burial, when reactive sites promote seeding; this is followed by concentric or radiating growth as layers accrete progressively. Supersaturated conditions arise from the decay of , which releases ions and alters fluid chemistry, facilitating rapid deposition before significant compaction. Growth patterns reflect the of reactants from the surrounding toward the nucleus, resulting in spherical, ellipsoidal, or irregular shapes depending on local permeability and fluid flow. Influencing factors include fluctuations and shifts induced by microbial of organics, which enhance solubility and availability—such as for carbonates or silica from biogenic sources. Microbial plays a key role, with processes like sulfate reduction producing that indirectly promotes carbonate precipitation by consuming protons and raising locally. Ion concentrations from these reactions create zones of , accelerating cementation around the nucleus while adjacent areas remain undersaturated. Concretions typically develop rapidly after deposition, within thousands to a few million years, often completing before full compaction to preserve internal structures. This timeframe aligns with early diagenetic conditions in near-surface, porous environments. Diffusion-reaction models describe this growth quantitatively, simulating transport and kinetics. A basic for the precipitation rate is Rate=k[Ion]n\text{Rate} = k [\text{Ion}]^n, where kk is the rate constant, [Ion][\text{Ion}] is the reactant concentration, and nn is the reaction order (often 1 or 2 for common cements like ). This formulation captures how localized drives cementation, with limiting growth radius based on sediment permeability and reaction speed.

Physical Properties

Appearance

Concretions typically exhibit spherical, ellipsoidal, or irregular shapes, with dimensions ranging from a few millimeters to several meters in . Their external surfaces often appear smooth and rounded, providing a polished contrast to the softer, more friable host rock in which they are embedded, though some feature rough or knobby textures due to differential or coatings. Color variations in concretions are influenced by their composition, appearing or gray in calcite-dominated examples, or in those enriched with iron oxides, and black or dark purplish in manganese-rich varieties. Cross-sections commonly reveal concentric layers akin to tree rings, formed by successive precipitation. Internally, they may display radiating , fibrous patterns, or voids and cavities arising from volume shrinkage during consolidation. A key diagnostic trait of concretions is their greater compared to the enclosing , which enables them to resist and more effectively, often resulting in their prominence and isolation within exposed outcrops.

Composition

Concretions are predominantly composed of authigenic minerals that precipitate from interstitial fluids within sedimentary host rocks, with calcite (\ceCaCO3\ce{CaCO3}) serving as the most common cementing agent due to its prevalence in carbonate-rich environments. Other primary minerals frequently include silica in forms such as opal or quartz (\ceSiO2\ce{SiO2}), iron oxides like hematite (\ceFe2O3\ce{Fe2O3}) and goethite (\ceFeO(OH)\ce{FeO(OH)}) that characterize red-hued varieties, siderite (\ceFeCO3\ce{FeCO3}), and pyrite (\ceFeS2\ce{FeS2}). These minerals form the core matrix of concretions, often binding detrital grains from the surrounding sediment. The cementing material in concretions consists of these authigenic minerals that fill pore spaces, enhancing cohesion; s, including and rare earth elements, may also be incorporated via migration of diagenetic fluids, leading to localized enrichments. Such accumulation reflects fluid chemistry and can vary significantly between concretions. Concretion composition generally reflects the host sediment but shows enrichment in the cementing phases, such as carbonates in limestone hosts or siliceous materials in sandstones. Isotopic signatures, particularly δ13C\delta^{13}\mathrm{C} values depleted due to microbial or sulfate reduction, indicate formation under reducing conditions influenced by organic matter decomposition. X-ray diffraction (XRD) and petrographic thin-section analysis are standard methods for identifying mineral phases and quantifying compositions in concretions, revealing details like crystal structure and grain relationships. These compositional attributes contribute to the enhanced durability of concretions relative to host rocks, as the denser cement resists mechanical and chemical weathering.

Geological Context

Occurrence

Concretions are primarily found in sedimentary rocks, where they occur abundantly in formations such as shales, sandstones, and limestones. They are commonly preserved in and strata, such as those in and , reflecting formation during periods of widespread marine and continental sedimentation. Globally, concretions exhibit a widespread distribution across diverse sedimentary basins. Notable examples include the Jurassic in the United States, where siliceous and carbonate varieties are prevalent in fluvial and lacustrine deposits. In , siliceous concretions known as flints are ubiquitous within chalk formations, forming nodular masses in the soft limestone. In , concretions occur in cool-water carbonate sediments, including to marine sequences along the southern margin. These structures are frequently exposed in erosional landscapes, including , riverbeds, and quarries, which reveal them through the differential of surrounding softer sediments. Concretions typically form at shallow burial depths of less than 1 km, during early before significant compaction or deep can disrupt their structure. Their abundance is greater in fine-grained, organic-rich sediments, where provides sites and reduces mobility, combined with stable that facilitates transport for cementation. These diagenetic conditions enhance their preservation in low-energy depositional environments.

Diagenetic Environments

Concretions primarily develop in anoxic marine basins characterized by high organic content, where the accumulation of fuels anaerobic diagenetic reactions. These environments promote the early formation of through interactions in sulfate-rich waters, as ions from facilitate microbial reduction processes that generate , which reacts with available iron to precipitate within the . In contrast, freshwater aquifers support the precipitation of silica or cements in concretions, often under conditions where introduces dissolved ions that supersaturate pore waters during burial. Biological processes play a central role in concretion development, with microbial degradation of releasing and , thereby creating conditions of for mineral precipitation. In marine settings, sulfate-reducing are particularly influential, as they metabolize to produce and , nucleating minerals and enabling early concretion growth near the sediment-water interface. These contribute to negative carbon values (δ¹³C ranging from -17‰ to -22‰) in the resulting carbonates, reflecting the incorporation of biogenic carbon sources. Physical conditions in the host sediments are critical for concretion formation, with low-permeability fine-grained deposits such as shales and mudstones effectively trapping diagenetic fluids and preventing their rapid dispersal. Permeability contrasts between the nucleating site and surrounding matrix drive focused fluid flow toward organic-rich nuclei, concentrating reactants and promoting localized cementation. Such environments, often in clay- to silt-grade marine sediments deposited in relatively deep water, stabilize reaction zones and enhance concretion integrity during early burial. Temporally, concretion formation spans early in the zone, where microbial and reduction dominate, to later stages involving deeper and compaction. Sea-level changes exert a significant influence on fluid chemistry, as fluctuations alter the influx of marine versus meteoric waters, shifting conditions and availability within the column. For instance, transgressive events can introduce -rich , enhancing nucleation, while regressions promote cementation through freshwater incursions. Recent studies from 2023 highlight concretions as valuable recorders of ancient gradients, capturing the temporal evolution of diagenetic environments through isotopic and mineralogical signatures in shallow marine settings. More recent 2024-2025 studies further explore concretion formation in diverse settings, such as iron-manganese concretions recording paleoenvironments in the and burial formation in turbidites, reinforcing their role as diagenetic proxies. These analyses reveal how organic carbon abundance modulates microbial activity and fluid chemistry over burial timescales, providing proxies for past anoxic conditions and environmental shifts.

Types and Varieties

Septarian Concretions

Septarian concretions are a distinctive variety of concretion characterized by an internal network of polygonal cracks, known as , that are infilled with secondary minerals such as , celestite, barite, or . These cracks typically radiate outward from the center, forming a star-like or polyhedral pattern, and are widest at the core while tapering toward the exterior. The infilling minerals often exhibit crystalline habits, creating geode-like interiors within the otherwise solid concretion body, which is primarily composed of cement like . The formation of septarian concretions begins with the precipitation of early diagenetic around a nucleus in fine-grained s, such as mudstones, shortly after deposition. Once the concretion has lithified, internal shrinkage occurs due to and volume reduction of the enclosed , generating tensile stresses that propagate cracks from the interior outward. These fractures develop post-cementation but prior to significant compaction of the surrounding matrix, allowing fluids rich in dissolved minerals to infiltrate and precipitate the characteristic infills, often in multiple stages. While some mechanisms invoke synsedimentary tectonic stresses, the predominant process involves shrinkage from in organic-rich, reducing environments. In appearance, septarian concretions typically feature a rough, brown or gray rind derived from the host , contrasting with sparkling white, yellow, or translucent crystalline interiors exposed upon sectioning. They range in size from a few centimeters to about 1 meter in diameter, though exceptional examples exceed this. The mineral infills, such as blocky or fibrous celestite, enhance their aesthetic appeal, making them popular for work as decorative geodes. Notable occurrences include the Kayenta Formation and similar strata in southern , , where they weather out as collectible nodules near Orderville, often revealing and fillings. In , they are common in the Formation, particularly in the Member, where they form in marine mudstones and exhibit complex fluid involvement during infilling. These sites highlight their prevalence in sedimentary basins with high organic content.

Cannonball Concretions

Cannonball concretions are large, spherical masses that closely resemble cannonballs due to their near-perfect rounded shapes and uniform internal structure. These concretions form through isotropic growth within homogeneous sedimentary layers, where precipitation expands evenly in all directions from a central nucleus, resulting in symmetrical spheres rather than irregular forms. The formation of cannonball concretions involves the precipitation of cementing minerals, primarily such as or , around an organic nucleus like material or a grain cluster. Mineral-rich percolates through porous sediments, depositing successive layers of these minerals that bind surrounding particles into a cohesive , much like the layering in a pearl. This diagenetic process occurs early in sediment compaction, often in marine or lacustrine environments with stable chemical conditions that promote radial growth. Once formed, the denser, cemented makes them highly resistant to and , causing them to protrude or accumulate as isolated boulders when softer enclosing sediments erode away, sometimes forming conglomerate-like deposits. In terms of appearance, cannonball concretions feature smooth, polished exteriors with diameters typically ranging from 20 to 50 cm, though larger examples up to 1 meter exist in some settings. Their color is predominantly reddish-brown, imparted by the content in the cement, which also contributes to a concentric banding visible in cross-sections. The surfaces may show subtle ridging or polish from abrasion in exposed terrains. Notable occurrences of cannonball concretions include the Paleocene-Eocene Sentinel Butte Formation in , , , where they appear as dense, spherical bodies up to 3 meters in diameter.

Hiatus Concretions

Hiatus concretions are early diagenetic bodies that originate within host sediments but become exhumed and exposed at surfaces or unconformities during periods of non-deposition, thereby incorporating older underlying sediments or fossils into their structure. These concretions serve as markers of sedimentary discontinuities, distinguishing them through their unique history of exposure, biological colonization, and reburial. Unlike typical concretions that remain , hiatus concretions record episodes of submarine or sediment starvation, often featuring borings and encrustations from marine organisms that colonized their surfaces while exposed. The formation process begins with the precipitation of carbonate cement around nuclei in the during early , creating a hardened core resistant to . Subsequently, during a depositional hiatus—characterized by reduced rates or by currents—these concretions are exhumed and lie on the seafloor, where they undergo cementation enhancements, such as phosphatization or ferruginous impregnation, due to interaction with . This exposure phase allows for extensive and epifaunal encrustation, after which renewed deposition reburies the concretions, preserving the evidence of the temporal gap. Such processes highlight their role in indicating breaks in without requiring subaerial exposure. In terms of appearance, hiatus concretions typically exhibit irregular or tabular morphologies, reflecting the irregular surfaces on which they form, and often display dark, weathered coatings from prolonged exposure and oxidation. Their surfaces bear prominent traces of borings from organisms like sponges or worms, alongside encrustations from bryozoans, corals, or serpulids, which add textural complexity. Internally, they may show concentric layering from initial cementation, with secondary infills in voids created during exposure. Notable occurrences of hiatus concretions are documented in Paleozoic limestones across , where they appear at unconformities in carbonate sequences, recording ancient depositional pauses. In the , they are prominent in siliciclastic deposits of southern , featuring diverse encrusting faunas that provide paleoecological insights into shallow-marine environments. examples include horizons in European marine sediments from the to , as well as in formations of , , where they mark hiatuses in coastal plain deposits.

Elongate Concretions

Elongate concretions are rod-like or tabular masses of cemented that develop parallel to planes in sedimentary rocks, their asymmetric growth driven by anisotropic permeability that favors directional along preferred pathways. Their formation involves channeled migration of mineral-rich fluids during early , leading to selective cementation in high-permeability zones such as paleochannels; they are particularly common in sandstones where or serves as the primary cementing agent. These concretions typically exhibit a cylindrical or log-like external form, with lengths extending up to several meters and diameters ranging from centimeters to tens of centimeters, often displaying a fibrous internal texture composed of radially elongated crystals that reflect the directional flow during precipitation. Notable occurrences include elongate concretions within sandstones of the in , where they vary from spherical to distinctly rod-shaped forms up to a meter or more in length, and in Tertiary fluvial deposits such as the Tipam Formation in northeastern , representing diagenetic features in ancient riverine environments.

Moqui Marbles

Moqui marbles are small, spherical concretions typically ranging from pea-sized to about 2 inches in diameter, consisting of a core of surrounded by a rind of , primarily , and often featuring a hollow interior resulting from dissolution by acidic . These features distinguish them as nodules formed through diagenetic processes, where the outer shell forms a protective coating that resists while the inner material dissolves over time. The formation of Moqui marbles occurred within the Jurassic Navajo Sandstone formation, where iron minerals leached from the surrounding rock precipitated out of groundwater to create the outer rind during burial diagenesis between 0.3 and 5 million years ago, as determined by magnetic dating techniques in recent studies. Subsequent exposure to acidic fluids post-burial led to the selective dissolution of the sandstone core, hollowing the spheres while preserving the hematite shell. This process is supported by magnetic dating techniques that confirm the iron oxide rind's age and the later internal erosion. In appearance, Moqui marbles exhibit a distinctive rusty red or brownish-black exterior due to the hematite coating, contrasting with a gray or white sandstone interior when not fully hollowed; the hematite also imparts weak magnetic properties to the concretions. They are notably found scattered on the surface of the Escalante Desert in southern , , within areas such as Grand Staircase-Escalante National Monument and near , where erosion of the exposes them. The name "Moqui marbles" derives from local legends of the Moqui (or ) tribe, who believed the stones were used by ancestral spirits for games at night.

Kansas Pop Rocks

Kansas Pop Rocks are small, irregular concretions resembling popcorn, primarily composed of iron sulfides such as (FeS₂) and , occasionally with jarosite or alteration products. These nodules form clusters embedded within chalky sediments and typically range from 1 to 5 cm in size, exhibiting a metallic golden to yellowish-brown coloration depending on . They are renowned for their unique , often developing around a central nucleus like a fragment. The formation of Kansas Pop Rocks occurs through diagenetic processes in the Smoky Hill Chalk Member of the , a marine deposit dating to approximately 84 million years ago. Associated with thin layers—altered —the concretions precipitate when bacterial sulfate reduction in anoxic conditions converts dissolved s to , incorporating iron from or detrital sources. This process highlights the role of in providing reactive components within the chalky matrix. These concretions are notably abundant in western , particularly in Gove County along the Smoky Hill Valley, where in washes and gullies exposes them in outcrops of the . Collectors often gather them for educational demonstrations illustrating sedimentary and mineral precipitation in ancient marine environments.

Claystones, Clay Dogs, and Fairy Stones

Clay dogs, also referred to as claystones in certain geological contexts, are compact concretions primarily composed of , clay, and precipitates that form around a nucleus of organic material. These structures are denser and harder than the surrounding unconsolidated clay sediments, which led early observers in the Connecticut River Valley to name them "claystones." Workmen in local brickyards dubbed them "clay-dogs" due to their frequent animal-like shapes or because they interfered with brick molding processes. Fairy stones represent a similar variety of concretions embedded in clay deposits, often exhibiting irregular, figurine-like forms that evoke associations. These concretions typically develop through pedogenic or lacustrine processes in terrestrial or near-surface environments, where rich in dissolved minerals percolates through fine-grained s. In pedogenic settings, iron oxides such as can act as cementing agents, binding clay particles together during and under seasonally moist conditions; this role of iron minerals is explored further in discussions of concretion composition. The aggregation occurs as mineral precipitates, including and iron oxides, accumulate around organic remnants or nuclei, solidifying the mass over time in low-energy depositional settings like glacial till or lake beds. While the precise mechanisms remain partially understood, analyses of similar iron-rich examples indicate that ambient contributes the necessary materials for cementation, with concretions requiring volumes of surrounding matrix equivalent to several times their own size. In appearance, clay dogs and fairy stones are often elongate, rounded, or anthropomorphic, ranging from a few centimeters to tens of centimeters in length, with colors spanning brown to black due to incorporated iron compounds. They tend to be friable when dry, easily crumbling under pressure, but gain cohesion from the cements. Notable occurrences include glacial clay deposits in the Champlain Formation along the Valley in the , where they weather out of post-glacial marine clays. Similar formations appear in Midwestern glacial clays, such as those in and , while similar calcareous concretions known as fairy stones occur in glacial clay deposits of , , and parts of .

Gogottes

Gogottes are rare siliceous concretions renowned for their intricate, sculpture-like forms, featuring tubular or branching structures with smooth, textures that evoke natural artistry. These formations develop in sandy fluvial environments, where fine-grained sands from ancient river systems provide the host material for silica cementation. Primarily occurring in the Formation of the , , gogottes represent a unique subset of concretions distinguished by their flowing, organic shapes rather than uniform or geometric patterns. The formation process begins with the deposition of sands, approximately 30 million years ago, followed by diagenetic cementation through silica precipitation from . Mineral-rich waters percolate through the uncemented sands, depositing that progressively crystallizes into and eventually , creating tightly bound masses. In particular, cracks within the drying fluvial sediments become filled with this , enhancing the porous and convoluted internal structure while preserving the external surfaces. This periglacial-influenced silicification, active during cold periods, results in horizontal or irregular concretionary bodies that contrast with the surrounding loose sand. Typically white due to their high silica content and porous nature, gogottes can measure up to 1 meter in length, with surfaces exhibiting a sparkling, undulating quality from the aligned crystals. Their natural elegance has led to their collection and use in , often displayed in museums and private collections for their resemblance to abstract sculptures. Notable occurrences are concentrated in the historic sand quarries of , southeast of , where the concretions were historically extracted alongside building sands for royal chateaux. Similar siliceous concretions, though less ornate, appear in Tertiary opal fields of , such as those in , where silica precipitation in sedimentary hosts yields comparable gem-bearing masses.

Significance

Paleontological Role

Concretions play a crucial role in by facilitating the exceptional preservation of fossils, particularly through their function as geochemical traps that isolate organic remains from destructive processes. The primary preservation mechanism involves rapid early cementation, where minerals such as or precipitate around decaying , sealing it from oxygen and microbial activity. This process creates anoxic interiors that prevent oxidation and further decay, allowing soft tissues to be mineralized or compressed before they disintegrate. The timing of this cementation, often occurring shortly after , enhances preservation by limiting diffusive loss of decay byproducts like ions. Common inclusions within concretions include soft-bodied organisms that are rarely fossilized elsewhere, such as , , and worms. For instance, the Mazon Creek nodules from the period in contain remarkably detailed impressions of these delicate structures, preserved as compressions or carbon films within concretions. Recent 2025 research on Mazon Creek has revealed three contemporaneous ecosystems preserved in these concretions, including evidence of specialized predation through bromalites (fossilized stomach contents), highlighting diverse trophic interactions in a 300-million-year-old tropical delta environment. Similarly, some concretions from the Gogo Formation and concretions from the La Voulte-sur-Rhône Lagerstätte yield Burgess Shale-like assemblages of soft-bodied invertebrates, offering insights into otherwise unpreserved faunas. The scientific value of concretions lies in their ability to provide high-fidelity snapshots of ancient ecosystems, capturing contemporaneous assemblages that reveal and ecological interactions. Isotopic of preserved , such as lipids within the concretion matrix, enables reconstruction of paleodiets and environmental conditions, including water chemistry and trophic levels, through ratios like δ¹³C and δ¹⁵N. These features make concretions invaluable for understanding evolutionary transitions and biotic events. However, concretions present challenges in paleontological study, as their external surfaces often obscure internal fossils, necessitating careful preparation techniques like mechanical splitting or chemical etching to reveal contents without damage. Misidentification can also occur, with concretions sometimes mistaken for fossils themselves due to their nodular shape.

Recent Research Insights

Recent studies have underscored the pivotal role of microbial activity in concretion formation, particularly through processes like sulfate reduction that drive precipitation. A comprehensive 2023 review synthesizes evidence showing that sulfate-reducing facilitate by generating and ions during degradation, with isotopic signatures confirming biogenic origins in various concretion types. Metagenomic sequencing of cores from concretions has further revealed diverse bacterial communities, including and other anaerobes, persisting within mineral matrices and indicating early diagenetic microbial mediation. Advancing this, a 2025 study published in iScience demonstrated that concretions can preserve ancient microbial DNA and proteins, including bacterial peptides and even human contaminants in modern samples, highlighting their potential as molecular archives. This research indicates that sediment concretions protect biomolecules from degradation, with uneven preservation across sample types, further supporting the role of microbial diagenesis in creating closed chemical systems for long-term organic preservation. Concretions increasingly serve as robust environmental proxies, capturing signals of ancient variability in sedimentary records. published in 2023 highlights how concretions in shallow marine settings preserve evidence of seeps, where anaerobic oxidation of by microbial consortia leads to authigenic formation and associated shifts from sulfidic to more oxygenated conditions. These features reflect transient paleoenvironmental perturbations, such as fluctuations in sea-level or bottom-water oxygenation, offering high-resolution archives for reconstructing transitions. A 2025 study on iron-manganese concretions from the further records centennial- to millennial-scale environmental shifts, linking hypoxia events to anomalies like the Thermal Maximum and Medieval Climate Anomaly. Technological innovations have enhanced the analysis of concretion interiors and formation conditions. Synchrotron X-ray computed microtomography, applied in post-2020 studies, provides non-destructive, high-resolution imaging of pore networks and zonations within concretions, revealing growth patterns influenced by migration and microbial biofilms. Complementing this, clumped thermometry (Δ47) has been refined for systems, yielding formation temperatures of 17–35°C for diagenetic concretions and enabling precise pore-water δ18O reconstructions without assumptions about composition. These advancements address longstanding gaps in understanding microbial contributions to concretion genesis, previously underrepresented in older , and establish geochemical profiles from concretions as inputs for paleoclimate modeling. For instance, reaction-transport simulations incorporating concretion data from 2023 investigations simulate diagenetic responses to environmental forcings like or warming, linking ancient dynamics to modern climate projections. Iron-manganese concretions, in particular, record centennial-scale marine environmental shifts, supporting models of biogeochemical under changing climates.

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