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===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, 193β194 |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=537β565 |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 MiddleβUpper Permian lacustrine siltstones in northern Mozambique: evidence for very early diagenesis and multiple fluids |journal=Sedimentology |date=December 2007 |volume=54 |issue=6 |pages=1389β1416 |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=349β368 |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=79β106 |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=CΓ©dric 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=30β37 |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=1β2 |pages=85β91 |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=239β251 |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=151β156 |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=63β76 |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=1133β1147 |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=617β631 |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=1773β1786 |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=1β2 |pages=71β85 |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=693β707 |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 β Anatomy of Some Septarian Concretions |journal=SEPM Journal of Sedimentary Research |date=1985 |volume=55 |pages=398β406 |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. 272β279.</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=349β368 |doi=10.1111/j.1365-3091.1988.tb00952.x |year=1988 |bibcode=1988Sedim..35..349A }}</ref>
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