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Concretion

From Wikipedia, the free encyclopedia
(Redirected fromSeptarian)
For other uses, seeCalculus (medicine),Enterolith,andNodule (geology).

Concretions inTorysh,WesternKazakhstan.
Concretions with lens shape from island in Vltava river, Prague, Czech Republic.
Marlstoneaggregate concretion,Sault Ste. Marie, Michigan,United States.

Aconcretionis a hard, compact mass formed by the precipitation ofmineralcement within the spaces between particles, and is found insedimentary rockorsoil.[1]Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word 'concretion' is derived from theLatinconcretio"(act of) compacting, condensing, congealing, uniting", itself fromconmeaning "together" andcresceremeaning "to grow".[2]

Concretions form within layers of sedimentarystratathat 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 toweatheringthan the hoststratum.

There is an important distinction to draw between concretions andnodules.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 bedinosaureggs, animal and plantfossils(calledpseudofossils), extraterrestrial debris or humanartifacts.

Origins

[edit]
Concretion rock with white core from the Middle Jurassic of Iran

Detailed studies have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified duringdiagenesis.[3][4][5][6][7][8]They typically form when a mineral precipitates and cements sediment around a nucleus, which is often organic, such as a leaf, tooth, piece of shell orfossil.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 areWorld War IImilitaryshells,bombs,andshrapnel,which are found insidesideriteconcretions found in an English coastalsalt 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 hostsediments,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 atMcConnells Mill State Parkin 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 inTheodore Roosevelt National Park,inNorth 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 theQasr el Sagha Formationwithin theFaiyum depressionof Egypt.[16]Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-likeaggregates.[17]

Composition

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Spherical concretions embedded in sandstone in the 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 insandstonesorshalesare commonly formed of acarbonatemineral such ascalcite;those inlimestonesare commonly an amorphous or microcrystalline form ofsilicasuch aschert,flint,orjasper;while those in black shale may be composed ofpyrite.[18]Other minerals that form concretions include iron oxides or hydroxides (such asgoethiteandhematite),[19][20]dolomite,siderite,[21]ankerite,[22]marcasite,[23]barite,[24][25]andgypsum.[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 ofsulfatesbybacteria,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

[edit]
Vaqueros Formationsandstone with concretions
A mosaic of images showing spherules, some partly embedded, spread over (smaller) soil grains on theMartian surface.

Concretions are found in a variety of rocks, but are particularly common inshales,siltstones,andsandstones.[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 was 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 orMartian spheruleshave been observed by theOpportunityroverin theEagle Crateron Mars.[33]

Types of concretion

[edit]

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, seeSeptaria (gastropod).
Moeraki Boulders,New Zealand.
A slice of a typical carbonate-rich septarian nodule.

Septarian concretions(orseptarian nodules) arecarbonate-rich concretions containing angular cavities or cracks (septaria;sg.septarium,from the Latinseptum"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 (thematrix) 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, includinglacustrinesiltstonessuch as the Beaufort Group of northwest Mozambique,[39]but are most commonly found in marineshales,such as theStaffin Shale FormationofSkye,[38]theKimmeridge Clayof England,[40][41]or theMancos Groupof 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 thisconcentricmodel of formation. However, the evidence is ambiguous, and many or most concretions may have formed bypervasivecementation 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 throughsyneresis,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 fromearthquakes.[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 byextracellular polymeric substancesor 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 earlydiagenesis,the initial stages of the formation of sedimentary rock from unconsolidated sediments. Most concretions appear to have formed at depths of burial wheresulfate-reducing microorganismsare 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, increasedalkalinityand 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 ofboulderseptarian concretions, which are as much as 3 meters (9.8 feet) in diameter, are theMoeraki Boulders.These concretions are found eroding out ofPaleocenemudstone of the Moeraki Formation exposed along the coast nearMoeraki,South Island,New Zealand.They are composed of calcite-cemented mud with septarian veins of calcite and rare late-stagequartzandferrousdolomite.[55][56][57][58]The much smaller septarian concretions found in theKimmeridge Clayexposed incliffsalong theWessexcoast of England are more typical examples of septarian concretions.[59]

Cannonball concretions

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Concretions onBowling 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 theCannonball Riverwithin 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 bycalcite.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 northeastUtahand centralWyoming.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"inOttawa County, Kansas.Large and spherical boulders are also found along Koekohe beach nearMoerakion the east coast of the South Island ofNew Zealand.[61]TheMoeraki Boulders,Ward Beach bouldersandKoutu Bouldersof New Zealand are examples of septarian concretions, which are also cannonball concretions. Large spherical rocks, which are found on the shore ofLake HuronnearKettle Point, Ontario,and locally known as"kettles",are typical cannonball concretions. Cannonball concretions have also been reported fromVan Mijenfjorden,Spitsbergen;nearHaines Junction,Yukon Territory,Canada;Jameson Land,EastGreenland;near Mecevici, Ozimici, andZavidoviciin Bosnia-Herzegovina; in Alaska in theKenai PeninsulaCaptain Cook State Park on north ofCook Inletbeach[62]and onKodiak Islandnortheast of Fossil Beach;[63]

Hiatus concretions

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Hiatus concretion encrusted by bryozoans (thin, branching forms) and anedrioasteroid;Kope Formation(Upper Ordovician), northernKentucky.
Hiatus concretions at the base of theMenuha Formation(Upper Cretaceous), theNegev,southernIsrael.

Hiatus concretions are distinguished by their stratigraphic history of exhumation, exposure and reburial. They are found where submarine erosion has concentrated early diagenetic concretions aslag surfacesby washing away surrounding fine-grained sediments.[64]Their significance for stratigraphy, sedimentology and paleontology was first noted by Voigt who referred to them asHiatus-Konkretionen.[65]"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 whichcalcite seaconditions prevailed, such as theOrdovician,JurassicandCretaceous.[64]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 includingbryozoans,echinodermsandtube wormsin the Paleozoic[66]and bryozoans,oystersand tube worms in the Mesozoic and Cenozoic. Hiatus concretions are also often significantlyboredby worms and bivalves.[67]

Elongate concretions

[edit]

Elongate concretions form parallel to sedimentary strata and have been studied extensively due to the inferred influence ofphreatic(saturated) zonegroundwaterflow direction on the orientation of the axis of elongation.[68][60][69][70]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,[71]and the types of geological features that influence flow.

Elongate concretions are well known in theKimmeridge Clayformation 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 theNavajo Sandstonewithin 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.[72][73]

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 containingmethaneorpetroleumfrom underlying rock beds reacted with the iron oxide, converting it to solublereduced 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.[72][73][74]It is possible that reduced iron first formedsideriteconcretions that were subsequently oxidized.Iron-oxidizing bacteriamay have played a role.[75]

Kansas pop rocks

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Kansas pop rocks are concretions of either iron sulfide,i.e.pyriteandmarcasite,or in some casesjarosite,which are found in outcrops of theSmoky Hill Chalk Memberof the Niobrara Formation withinGove County, Kansas.They are typically associated with thin layers of altered volcanic ash, calledbentonite,that occur within thechalkcomprising the Smoky Hill Chalk Member. A few of these concretions enclose, at least in part, large flattened valves of inoceramidbivalves.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 areoblate 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 rocksare completely absent from the Smoky Hill Chalk Member.[76]Instead, all of these iron sulfide concretions were created by the precipitation of iron sulfides within anoxic marinecalcareous oozeafter it had accumulated and before it hadlithifiedinto chalk.

Marlekafairy stone from Stensö in Sweden.

Iron sulfide concretions, such as the Kansas Pop rocks, consisting of eitherpyriteandmarcasite,are nonmagnetic.[77]On the other hand, iron sulfide concretions, which either are composed of or contain eitherpyrrhotiteorsmythite,will be magnetic to varying degrees.[78]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 ofcalcium carbonateare often found eroding out of exposures of interlaminatedsiltandclay,varved,proglacial lakedeposits. For example, great numbers of strikingly symmetrical concretions have been found eroding out of outcrops ofQuaternaryproglacial lakesedimentsalong and in thegravelsof theConnecticut Riverand its tributaries inMassachusettsandVermont.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 theConnecticut 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.[79][80][81]Similar disc-shaped calcium carbonate concretions have also been found in theHarricana Rivervalley in theAbitibi-Témiscamingueadministrative region ofQuebec,and inÖstergötlandcounty, Sweden. InScandinavia,they are known as "marlekor" ( "fairy stones" ).[82][83]

Gogottes

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Gogotte concretion

Gogottes[fr]are sandstone concretions found inOligocene(~30 million years) aged sediments nearFontainebleau,France. Gogottes have fetched high prices at auction due to their sculpture-like quality.[84]

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. CaSO4concretions 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, areplacement body,not to be confused with a concretion
  • Rock City, Kansas– park in Kansas, United States of America, United States of America
  • Speleothem– Structure formed in a cave by the deposition of minerals from water. CaCO3

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