Editing Concretion (section)

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

===Septarian concretions===
{{redirect|Septaria|the genus of gastropod snail|Septaria (gastropod)}}[[File:MoerakiBouldersSunrise.jpg|thumb|left|[[Moeraki Boulders]], New Zealand]]
[[Image:Septarian Nodule.jpg|thumb|upright|A slice of a typical carbonate-rich septarian nodule.]] '''Septarian concretions''' (or '''septarian nodules''') are [[Carbonate mineral|carbonate]]-rich concretions containing angular cavities or cracks ('''septaria'''; {{abbr|sg.|singular}} ''{{linktext|septarium}}'', from the Latin {{lang|la|septum}} "partition, separating element", referring to the cracks or cavities separating polygonal blocks of hardened material).<ref name=Jackson>{{cite book |editor1-last=Jackson |editor1-first=Julia A. |title=Glossary of geology. |date=1997 |publisher=American Geological Institute |location=Alexandria, Virginia |isbn=0922152349 |edition=Fourth |chapter=septarium}}</ref><ref>{{cite web
|url=http://dictionary.reference.com/search?q=septarian|title=septarian|publisher=dictionary.reference.com|access-date=March 20, 2014}}</ref> 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.<ref name=PotterEtal1980>{{cite book |last1=Potter |first1=Paul Edwin |last2=Maynard |first2=J. Barry |last3=Pryor |first3=Wayne A. |title=Sedimentology of shale: study guide and reference source |date=1980 |publisher=Springer-Verlag |location=New York |isbn=0387904301 |pages=23, 36}}</ref><ref name=Jackson/> However, the cracks can be highly variable in shape and volume, as well as the degree of shrinkage they indicate.<ref>{{cite journal |last1=Pratt |first1=Brian R. |title=Septarian concretions: internal cracking caused by synsedimentary earthquakes |journal=Sedimentology |date=27 February 2001 |volume=48 |issue=1 |pages=189, 193–194 |doi=10.1046/j.1365-3091.2001.00366.x|bibcode=2001Sedim..48..189P |s2cid=140665532 }}</ref> The matrix is typically composed of argillaceous carbonate, such as clay ironstone, while the crack filling is usually calcite.<ref name=PotterEtal1980/><ref name=Jackson/> 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.<ref name="HendryEtal2006">{{cite journal |last1=Hendry |first1=James P. |last2=Pearson |first2=Michael J. |last3=Trewin |first3=Nigel H. |last4=Fallick |first4=Anthony E. |title=Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis: Jurassic septarian concretions, NW Scotland |journal=Sedimentology |date=16 May 2006 |volume=53 |issue=3 |pages=537–565 |doi=10.1111/j.1365-3091.2006.00779.x|s2cid=130767202 |doi-access=free }}</ref>

Septarian concretions are found in many kinds of mudstone, including [[lacustrine]] [[siltstone]]s such as the Beaufort Group of northwest Mozambique,<ref name="MelezhikEtal2007">{{cite journal |last1=Melezhik |first1=Victor A. |last2=Fallick |first2=Anthony E. |last3=Smith |first3=Richard A. |last4=Rosse |first4=Danta M. |title=Spherical and columnar, septarian, 18 O-depleted, calcite concretions from Middle–Upper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids |journal=Sedimentology |date=December 2007 |volume=54 |issue=6 |pages=1389–1416 |doi=10.1111/j.1365-3091.2007.00886.x|bibcode=2007Sedim..54.1389M |s2cid=129030770 }}</ref> but are most commonly found in marine [[shale]]s, such as the [[Staffin Shale Formation]] of [[Skye]],<ref name="HendryEtal2006"/> the [[Kimmeridge Clay]] of England,<ref name=AstinEtal1988>{{cite journal |last1=Astin |first1=T. R. |last2=Scotchman |first2=I. C. |title=The diagenetic history of some septarian concretions from the Kimmeridge Clay, England |journal=Sedimentology |date=April 1988 |volume=35 |issue=2 |pages=349–368 |doi=10.1111/j.1365-3091.1988.tb00952.x|bibcode=1988Sedim..35..349A }}</ref><ref name=Stotchman1991>{{cite journal |last1=Scotchman |first1=I. C. |title=The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England |journal=Sedimentology |date=February 1991 |volume=38 |issue=1 |pages=79–106 |doi=10.1111/j.1365-3091.1991.tb01856.x|bibcode=1991Sedim..38...79S }}</ref> or the [[Mancos Group]] of North America.<ref name=DaleEtal2014>{{cite journal |last1=Dale |first1=Annabel |last2=John |first2=Cédric M. |last3=Mozley |first3=Peter S. |last4=Smalley |first4=P. C. |last5=Muggeridge |first5=Ann H. |title=Time-capsule concretions: Unlocking burial diagenetic processes in the Mancos Shale using carbonate clumped isotopes |journal=Earth and Planetary Science Letters |date=May 2014 |volume=394 |pages=30–37 |doi=10.1016/j.epsl.2014.03.004|bibcode=2014E&PSL.394...30D |doi-access=free }}</ref>

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.<ref name=Mozley1996>{{cite journal |last1=Mozley |first1=Peter S. |title=The internal structure of carbonate concretions in mudrocks: a critical evaluation of the conventional concentric model of concretion growth |journal=Sedimentary Geology |date=May 1996 |volume=103 |issue=1–2 |pages=85–91 |doi=10.1016/0037-0738(95)00087-9|bibcode=1996SedG..103...85M }}</ref><ref name=RaiswellFisher2000>{{cite journal |last1=Raiswell |first1=R. |last2=Fisher |first2=Q. J. |title=Mudrock-hosted carbonate concretions: a review of growth mechanisms and their influence on chemical and isotopic composition |journal=Journal of the Geological Society |date=January 2000 |volume=157 |issue=1 |pages=239–251 |doi=10.1144/jgs.157.1.239|bibcode=2000JGSoc.157..239R |s2cid=128897857 }}</ref><ref name="HendryEtal2006"/> 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.<ref name=RaiswellFisher2000/> Whether the initial cementation was concentric or pervasive, there is considerable evidence that it occurred quickly and at shallow depth of burial.<ref name="TynesBoles1989">{{cite journal |last1=Thyne |first1=Geoffrey D. |last2=Boles |first2=James R. |title=Isotopic Evidence for Origin of the Moeraki Septarian Concretions, New Zealand |journal=SEPM Journal of Sedimentary Research |date=1989 |volume=59 |doi=10.1306/212F8F6C-2B24-11D7-8648000102C1865D}}</ref><ref name=Duck1995>{{cite journal |last1=Duck |first1=R. W. |title=Subaqueous shrinkage cracks and early sediment fabrics preserved in Pleistocene calcareous concretions |journal=Journal of the Geological Society |date=February 1995 |volume=152 |issue=1 |pages=151–156 |doi=10.1144/gsjgs.152.1.0151|bibcode=1995JGSoc.152..151D |s2cid=129928697 }}</ref><ref name=DeCraenEtal1998>{{cite journal |last1=De Craen |first1=M. |last2=Swennen |first2=R. |last3=Keppens |first3=E. |title=Petrography and geochemistry of septarian carbonate concretions from the Boom Clay Formation (Oligocene, Belgium) |journal=Geologie en Mijnbouw |date=1998 |volume=77 |issue=1 |pages=63–76 |doi=10.1023/A:1003468328212|s2cid=126635562 }}</ref><ref name="HendryEtal2006"/> In many cases, there is clear evidence that the initial concretion formed around some kind of organic nucleus.{{sfn|Potter|Maynard|Pryor|1980|p=23}}

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.<ref name=PotterEtal1980/><ref name=Jackson/> Shrinkage of a still-wet matrix may also take place through [[Syneresis (chemistry)|syneresis]], in which the particles of colloidal material in the interior of the concretion become gradually more tightly bound while expelling water.<ref name="MelezhikEtal2007"/> 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 {{convert|10|m||sp=us}}.<ref name="Honslow1997">{{cite journal |last1=Hounslow |first1=Mark W. |title=Significance of localized pore pressures to the genesis of septarian concretions |journal=Sedimentology |date=November 1997 |volume=44 |issue=6 |pages=1133–1147 |doi=10.1046/j.1365-3091.1997.d01-64.x|bibcode=1997Sedim..44.1133H |s2cid=130385560 }}</ref> A more speculative theory is that the septaria form by brittle fracturing resulting from [[earthquake]]s.{{sfn|Pratt|2001|pp=189-213}} 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 {{convert|50|m||sp=us}}<ref name=Astin1986>{{cite journal |last1=Astin |first1=T. R. |title=Septarian crack formation in carbonate concretions from shales and mudstones |journal=Clay Minerals |date=October 1986 |volume=21 |issue=4 |pages=617–631 |doi=10.1180/claymin.1986.021.4.12|bibcode=1986ClMin..21..617A |s2cid=128609480 }}</ref> and possibly as little as {{convert|12|m||sp=us}}. 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.<ref name=Duck1995/><ref name="HendryEtal2006"/>

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 substance]]s 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.<ref name="HendryEtal2006"/>

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.<ref name=Stotchman1991/><ref name=PearsonEtal2005>{{cite journal |last1=Pearson |first1=M.J. |last2=Hendry |first2=J.P. |last3=Taylor |first3=C.W. |last4=Russell |first4=M.A. |title=Fatty acids in sparry calcite fracture fills and microsparite cement of septarian diagenetic concretions |journal=Geochimica et Cosmochimica Acta |date=April 2005 |volume=69 |issue=7 |pages=1773–1786 |doi=10.1016/j.gca.2004.09.024|bibcode=2005GeCoA..69.1773P }}</ref> This corresponds to burial depths of {{convert|15 to 150|m||sp=us}}, and is characterized by generation of carbon dioxide, increased [[alkalinity]] and precipitation of calcium carbonate.<ref name=RaiswellFisher2004>{{cite journal |last1=Raiswell |first1=R. |last2=Fisher |first2=Q.J. |title=Rates of carbonate cementation associated with sulphate reduction in DSDP/ODP sediments: implications for the formation of concretions |journal=Chemical Geology |date=November 2004 |volume=211 |issue=1–2 |pages=71–85 |doi=10.1016/j.chemgeo.2004.06.020 |bibcode=2004ChGeo.211...71R |url=http://eprints.whiterose.ac.uk/404/1/raiswellr6.pdf |access-date=2021-08-19 |archive-date=2022-01-30 |archive-url=https://web.archive.org/web/20220130203933/https://eprints.whiterose.ac.uk/404/1/raiswellr6.pdf |url-status=dead }}</ref> However, there is some evidence that formation continues well into the methanogenic zone beneath the sulfate reduction zone.<ref name=Huggett1994>{{cite journal |last1=Huggett |first1=J. M. |title=Diagenesis of mudrocks and concretions from the London Clay Formation in the London Basin |journal=Clay Minerals |date=October 1994 |volume=29 |issue=4 |pages=693–707 |doi=10.1180/claymin.1994.029.4.22|bibcode=1994ClMin..29..693H |s2cid=129727119 }}</ref><ref name="HendryEtal2006"/><ref name=DaleEtal2014/>

A spectacular example of [[boulder]] septarian concretions, which are as much as {{convert|3|m|ft|abbr=off|sp=us}} 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 (mineral)|dolomite]].<ref name="BolesLandisDale">{{cite journal |last1=Boles |first1=J.R. |last2=Landis |first2=C.A. |last3=Dale |first3=P. |title=The Moeraki Boulders – Anatomy of Some Septarian Concretions |journal=SEPM Journal of Sedimentary Research |date=1985 |volume=55 |pages=398–406 |doi=10.1306/212F86E3-2B24-11D7-8648000102C1865D}}</ref><ref name="FordyceMaxwell">Fordyce, E., and P. Maxwell, 2003, ''Canterbury Basin Paleontology and Stratigraphy, Geological Society of New Zealand Annual Field Conference 2003 Field Trip 8'', Miscellaneous Publication 116B, Geological Society of New Zealand, Dunedin, New Zealand. {{ISBN|0-908678-97-5}}</ref><ref name="ForsythCoates">Forsyth, P.J., and G. Coates, 1992, ''The Moeraki boulders''. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)</ref><ref name="ThyneBoles">Thyne, G.D., and J.R. Boles, 1989, [http://jsedres.sepmonline.org/cgi/content/abstract/59/2/272?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&author1=Thyne&andorexactfulltext=and&searchid=1&FIRSTINDEX=0&sortspec=relevance&resourcetype=HWCIT ''Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand''], Journal of Sedimentary Petrology. v. 59, n. 2, p. 272–279.</ref> The much smaller septarian concretions found in the [[Kimmeridge Clay]] exposed in [[cliff]]s along the [[Wessex]] coast of England are more typical examples of septarian concretions.<ref>{{cite journal |last1=Astin |first1=T. R. |title=The diagenetic history of some septarian concretions from the Kimmeridge Clay, England |journal=Sedimentology |volume=35 |issue=2 |pages=349–368 |doi=10.1111/j.1365-3091.1988.tb00952.x |year=1988 |bibcode=1988Sedim..35..349A }}</ref>

===Cannonball concretions===
[[File:Bowling Balls Beach 2 edit.jpg|thumb|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 {{convert|3|m|ft|abbr=on|sp=us}} in diameter. They were created by early cementation of sand and silt by [[calcite]]. Similar cannonball concretions, which are as much as {{convert|4|to|6|m|ft|abbr=on|sp=us}} 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.<ref name=McBride/> Somewhat weathered and eroded giant cannonball concretions, as large as {{convert|6|m|ft|abbr=off|sp=us}} in diameter, occur in abundance at "[[Rock City, Kansas|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]].<ref>Dann, C., and Peat, N. (1989) ''Dunedin, North and South Otago''. Wellington: GP Books. {{ISBN|0-477-01438-0}}</ref> The [[Moeraki Boulders]], [[Ward Beach#Ward Beach 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 [[Kettle Point Formation|"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<ref>{{cite web |url=http://cookinletconcretions.com/Kenai%20Article.htm |title=Kenai Peninsula Online – Alaska Newspaper – |access-date=2010-05-13 |url-status=dead |archive-url=https://web.archive.org/web/20110708191432/http://cookinletconcretions.com/Kenai%20Article.htm |archive-date=2011-07-08 }}</ref> and on [[Kodiak Island]] northeast of Fossil Beach.<ref>{{cite web|url=https://books.google.com/books?id=bGlonWEE-8YC&q=Fossil+beach+kodiak+concretions&pg=RA1-PA17|title=Geological Survey Professional Paper|date=24 May 1976|publisher=U.S. Government Printing Office|via=Google Books}}</ref> This type of concretion is also found in Romania, where they are known as ''trovants''.<ref>{{cite web |title=trovant |url=https://dexonline.ro/definitie/trovant |work=dexonline.ro |accessdate=October 3, 2024}}</ref><ref>Emma Davies, "[https://www.sciencefocus.com/planet-earth/trovants These ‘living’ rocks can give birth to baby stones]", 8 August 2023, ''[[BBC Science Focus]]''</ref>

===Hiatus concretions===
[[Image:OrdovicianEdrio.jpg|thumb|Hiatus concretion encrusted by bryozoans (thin, branching forms) and an [[edrioasteroid]]; [[Kope Formation]] (Upper Ordovician), northern [[Kentucky]]]]
[[Image:HiatusConcretionsIsrael060910.jpg|thumb|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 deposit|lag surface]]s by washing away surrounding fine-grained sediments.<ref name=Zaton/> Their significance for stratigraphy, sedimentology and paleontology was first noted by Voigt who referred to them as ''Hiatus-Konkretionen''.<ref>{{cite journal |last1=Voigt |first1=Ehrhard |title=Über Hiatus-Konkretionen (dargestellt an Beispielen aus dem Lias) |journal=Geologische Rundschau |date=October 1968 |volume=58 |issue=1 |pages=281–296 |doi=10.1007/BF01820609|bibcode=1968GeoRu..58..281V |s2cid=128842746 }}</ref> "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]].<ref name=Zaton>{{cite journal |last1=Zatoń |first1=Michał |title=Hiatus concretions |journal=Geology Today |date=24 September 2010 |volume=26 |issue=5 |pages=186–189 |doi=10.1111/j.1365-2451.2010.00762.x|bibcode=2010GeolT..26..186Z |s2cid=247665440 }}</ref> 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 worm (body plan)|tube worms]] in the Paleozoic<ref>{{cite journal |last1=Wilson |first1=M. A. |title=Disturbance and Ecologic Succession in an Upper Ordovician Cobble-Dwelling Hardground Fauna |journal=Science |date=3 May 1985 |volume=228 |issue=4699 |pages=575–577 |doi=10.1126/science.228.4699.575|pmid=17736081 |bibcode=1985Sci...228..575W |s2cid=28818298 }}</ref> and bryozoans, [[oysters]] and tube worms in the Mesozoic and Cenozoic. Hiatus concretions are also often significantly [[bioerosion|bored]] by worms and bivalves.<ref name=Wilson>{{cite journal |last1=Wilson |first1=Mark A. |last2=Taylor |first2=Paul D. |title=Palaeocology of Hard Substrate Faunas from the Cretaceous Qahlah Formation of the Oman Mountains |journal=Palaeontology |date=February 2001 |volume=44 |issue=1 |pages=21–41 |doi=10.1111/1475-4983.00167|bibcode=2001Palgy..44...21W |s2cid=129664357 |doi-access=free }}</ref>

===Elongate concretions===
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.<ref>{{cite journal |last1=Johnson |first1=M.R. |title=Paleogeographic Significance of Oriented Calcareous Concretions in the Triassic Katberg Formation, South Africa |journal=SEPM Journal of Sedimentary Research |date=1989 |volume=59 |pages=1008–1010 |doi=10.1306/212F90D9-2B24-11D7-8648000102C1865D}}</ref><ref name=McBride>{{cite journal |last1=McBride |first1=E. F. |last2=Picard |first2=M. D. |last3=Milliken |first3=K. L. |title=Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A. |journal=Journal of Sedimentary Research |date=1 May 2003 |volume=73 |issue=3 |pages=462–483 |doi=10.1306/111602730462|bibcode=2003JSedR..73..462M }}</ref><ref>{{cite journal |last1=Mozley |first1=Peter S. |last2=Goodwin |first2=Laurel B. |title=Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations |journal=Geology |date=1 June 1995 |volume=23 |issue=6 |pages=539–542 |doi=10.1130/0091-7613(1995)023<0539:POCAAC>2.3.CO;2|bibcode=1995Geo....23..539M }}</ref><ref>{{cite journal |last1=Mozley |first1=Peter S. |last2=Davis |first2=J. Matthew |title=Internal structure and mode of growth of elongate calcite concretions: Evidence for small-scale, microbially induced, chemical heterogeneity in groundwater |journal=Geological Society of America Bulletin |date=2005 |volume=117 |issue=11 |pages=1400 |doi=10.1130/B25618.1|bibcode=2005GSAB..117.1400M }}</ref> 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,<ref>{{cite journal |last1=Davis |first1=J. Matthew |title=Oriented carbonate concretions in a paleoaquifer: Insights into geologic controls on fluid flow |journal=Water Resources Research |date=June 1999 |volume=35 |issue=6 |pages=1705–1711 |doi=10.1029/1999WR900042|bibcode=1999WRR....35.1705D |s2cid=129502157 |doi-access=free }}</ref> 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|date=May 2010}}

===Moqui Marbles===
[[Image:MoquiMarble1.jpg|thumb|Moqui Marbles, hematite, goethite concretions, from the Navajo Sandstone of southeast Utah. The "W" cube at the top is one cubic centimeter in size.|left]]
[[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.<ref name=ChanParry2002>{{cite journal |last1=Chan |first1=M.A. |first2=W.T. |last2=Parry |year=2002 |title=Mysteries of Sandstone Colors and Concretions in Colorado Plateau Canyon Country |journal=Utah Geological Survey Public Information Series |volume=77 |pages=1–19 |url=https://ugspub.nr.utah.gov/publications/public_information/PI-77.pdf |access-date=18 August 2021}}</ref><ref name="Catling2004">{{cite journal |last1=Catling |first1=David C. |title=On Earth, as it is on Mars? |journal=Nature |date=June 2004 |volume=429 |issue=6993 |pages=707–708 |doi=10.1038/429707a|pmid=15201892 |s2cid=4393420 |doi-access=free }}</ref>

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.<ref name=ChanParry2002/><ref name="Catling2004"/><ref>{{cite journal |last1=Chan |first1=M.A. |first2=B.B. |last2=Beitler |first3=W.T. |last3=Parry |first4=J. |last4=Ormo |first5=G. |last5=Komatsu |year=2005 |title=Red Rock and Red Planet Diagenesis: Comparison of Earth and Mars Concretions |journal=GSA Today |volume=15 |number=8 |pages=4–10 |doi=10.1130/1052-5173(2005)015[4:RRARPD]2.0.CO;2 |url=https://www.geosociety.org/gsatoday/archive/15/8/pdf/i1052-5173-15-8-4.pdf |access-date=18 August 2021}}</ref> It is possible that reduced iron first formed [[siderite]] concretions that were subsequently oxidized. [[Iron-oxidizing bacteria]] may have played a role.<ref>{{cite journal |last1=Loope |first1=David B. |last2=Kettler |first2=Richard M. |last3=Weber |first3=Karrie A. |title=Morphologic Clues to the Origins of Iron Oxide–Cemented Spheroids, Boxworks, and Pipelike Concretions, Navajo Sandstone of South-Central Utah, U.S.A. |journal=The Journal of Geology |date=September 2011 |volume=119 |issue=5 |pages=505–520 |doi=10.1086/661110|bibcode=2011JG....119..505L |s2cid=10139364 |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1197&context=bioscifacpub |url-access=subscription }}</ref>

===Kansas pop rocks===

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|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 [[bivalve]]s. These concretions range in size from a few millimeters to as much as {{convert|0.7|m|ft|abbr=on|sp=us}} in length and {{convert|12|cm|ft|abbr=on|sp=us}} in thickness. Most of these concretions are [[oblate spheroid]]s. Other "pop rocks" are small polycuboid pyrite concretions, which are as much as {{convert|7|cm|ft|abbr=on|sp=us}} 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.<ref name=Hattan>{{cite journal |last1=Hattin |first1=D.E. |year=1982 |title=Stratigraphy and depositional environment of the Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas |journal=Kansas Geological Survey Bulletin |volume=225 |pages=1–108}}</ref> Instead, all of these iron sulfide concretions were created by the precipitation of iron sulfides within anoxic marine [[pelagic sediments|calcareous ooze]] after it had accumulated and before it had [[Lithification|lithified]] into chalk.
[[File:Marleka fairy stone from Stensö in Sweden.JPG|thumb|''Marleka'' fairy stone from Stensö in Sweden]]

Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either [[pyrite]] and [[marcasite]], are nonmagnetic.<ref>{{cite journal |last1=Hobbs |first1=D |last2=Hafner |first2=J |title=Magnetism and magneto-structural effects in transition-metal sulphides |journal=Journal of Physics: Condensed Matter |date=25 October 1999 |volume=11 |issue=42 |pages=8197–8222 |doi=10.1088/0953-8984/11/42/303|bibcode=1999JPCM...11.8197H |s2cid=250900204 }}</ref> On the other hand, iron sulfide concretions, which either are composed of or contain either [[pyrrhotite]] or [[smythite]], will be magnetic to varying degrees.<ref>{{cite journal |last1=Hoffmann |first1=Viktor |last2=Stanjek |first2=Helge |last3=Murad |first3=Enver |title=Mineralogical, magnetic and mössbauer data of symthite (Fe9S11) |journal=Studia Geophysica & Geodætica |date=December 1993 |volume=37 |issue=4 |pages=366–381 |doi=10.1007/BF01613583|bibcode=1993StGG...37..366H |s2cid=131123088 }}</ref> 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 ===
Disc concretions composed of [[calcium carbonate]] are often found eroding out of exposures of interlaminated [[silt]] and [[clay]], [[varve]]d, [[proglacial lake]] deposits. For example, great numbers of strikingly symmetrical concretions have been found eroding out of outcrops of [[Quaternary]] proglacial lake [[sediment]]s along and in the [[gravel]]s 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 {{convert|2|in|cm|abbr=on|sp=us}} to over {{convert|22|in|cm|abbr=on|sp=us}} 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.<ref name="Gratacap1884a">{{cite journal |last1=Gratacap |first1=L.P. |year=1884 |title=Opinions Upon Clay Stones and Concretions |journal=The American Naturalist |volume=18 |number=9 |doi=10.1086/273756 |pages=882–892 |bibcode=1884ANat...18..882G |s2cid=84690956 |url=https://www.journals.uchicago.edu/doi/pdf/10.1086/273756 |access-date=18 August 2021|url-access=subscription }}</ref><ref name="Sheldon1900a">{{cite book |last1=Sheldon |first1=J.M.A. |year=1900 |title=Concretions from the Champlain clays of the Connecticut Valley |publisher=University Press |location=Boston |page=74 |url=https://books.google.com/books?id=tUkPAAAAYAAJ&dq=Sheldon,+J.M.A.,+1900.+%27%27Concretions+from+the+Champlain+clays+of+the+Connecticut+Valley.%27%27+University+Press,+Boston.+pp.74.&pg=PA7 |access-date=18 August 2021}}</ref><ref name="Tarr1935a">{{cite journal |last1=Tarr |first1=W. A. |title=Concretions in the Champlain formation of the Connecticut River Valley |journal=Geological Society of America Bulletin |date=31 October 1935 |volume=46 |issue=10 |pages=1493–1534 |doi=10.1130/GSAB-46-1493|bibcode=1935GSAB...46.1493T }}</ref> 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").<ref name="Kindle1923a">{{cite journal |last1=Kindle |first1=E. M. |title=Range and Distribution of Certain Types of Canadian Pleistocene Concretions |journal=Geological Society of America Bulletin |date=30 September 1923 |volume=34 |issue=3 |pages=609–648 |doi=10.1130/GSAB-34-609|bibcode=1923GSAB...34..609K }}</ref><ref name="Warkentin1967a">Warkentin, B.P., 1967. ''Carbonate content of concretions in varved sediments''. ''Canadian Journal of Earth Sciences'', 4(2), pp.333-333.</ref>

=== Gogottes ===
[[File:Gogotte sandstone concretion (5784278101).jpg|thumb|Gogotte concretion]]
{{ill|Gogottes|fr|Gogotte (géologie)}} 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.<ref>{{cite news|last=Haigney|first=Sophie|date=2021-06-18|title=Once Again, Fossils Are Hot|language=en-US|work=The New York Times|url=https://www.nytimes.com/2021/06/18/arts/fossils-private-sale-collectors.html|access-date=2021-07-14|issn=0362-4331}}</ref>
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