Concretion
A concretion is a hard, 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 derived from the Latin concretio "(act of) compacting, condensing, congealing, uniting", itself from con meaning "together" and crescere meaning "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
Detailed studies have demonstrated that concretions form after sediments are buried but before the sediment is fully lithified during diagenesis.[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 or fossil. 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.
Appearance
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible
Composition
Concretions are commonly composed of a mineral present as a minor component of the host rock. For example, concretions in
Although concretions often consist of a single dominant mineral,
Occurrence
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 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 or
Types of concretion
Concretions vary considerably in their compositions, shapes, sizes and modes of origin.
Septarian concretions
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
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
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
Cannonball concretions are large spherical concretions, which resemble cannonballs. These are found along the
Hiatus concretions
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.[64] Their significance for stratigraphy, sedimentology and paleontology was first noted by Voigt who referred to them as Hiatus-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 which calcite sea conditions prevailed, such as the Ordovician, Jurassic and Cretaceous.[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 including
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.[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 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
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
Kansas pop rocks
Kansas pop rocks are concretions of either iron sulfide, i.e.
Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either pyrite and marcasite, are nonmagnetic.[77] On the other hand, iron sulfide concretions, which either are composed of or contain either pyrrhotite or smythite, 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
Disc concretions composed of
Gogottes
Gogottes 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.[84]
See also
- Bowling Ball Beach – Part of Schooner Gluch State Beach in Mendocino County, California
- 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, United States of America, United States of America
- Speleothem – Structure formed in a cave by the deposition of minerals from water. CaCO3
References
- ISBN 0662015339.
- ^ Harper, Douglas. "concretion". Online Etymology Dictionary.
- ISSN 1527-1404.
- ISSN 1527-1404.
- ISSN 1365-3091.
- . Retrieved 19 August 2021.
- ISSN 1527-1404.
- ISSN 0016-7606.
- ISBN 0716739054.
- S2CID 129359274. Archived from the original(PDF) on 2019-12-13.
- ISSN 1527-1404.
- S2CID 128897857.
- ISBN 9780199653065.
- ISBN 0131547283.
- ^ "Theodore Roosevelt National Park North Unit Scenic Byway". Archived from the original on 2012-05-08. Retrieved 2012-05-13.
- .
- .
- ^ a b c Prothero & Schwab 2004, p. 118.
- .
- .
- .
- S2CID 140180525.
- ^ Van Horn, F.R.; Van Horn, K.R. (1933). "X-ray study of pyrite or marcasite concretions in the rocks of the Cleveland, Ohio, quadrangles". American Mineralogist. 18 (7): 288–294. Retrieved 10 August 2021.
- .
- .
- doi:10.3233/978-1-61499-603-3-3248.)
{{cite journal}}
: CS1 maint: numeric names: authors list (link - ^ Allaby 2013, "concretion".
- .
- S2CID 129664903.
- ^ Boggs 2006, p. 114.
- ^ "Concretions". Paleontological Research Institution. Retrieved 11 August 2021.
- S2CID 129659655.
- ^ Dvorsky, George (15 February 2019). "The Enduring Mystery of the Martian 'Blueberries' Discovered by Opportunity Rover". Gizmodo.
- ^ ISBN 0922152349.
- ^ "septarian". dictionary.reference.com. Retrieved March 20, 2014.
- ^ ISBN 0387904301.
- S2CID 140665532.
- ^ S2CID 130767202.
- ^ S2CID 129030770.
- .
- ^ .
- ^ .
- .
- ^ S2CID 128897857.
- .
- ^ S2CID 129928697.
- S2CID 126635562.
- ^ Potter, Maynard & Pryor 1980, p. 23.
- S2CID 130385560.
- ^ Pratt 2001, pp. 189–213.
- S2CID 128609480.
- .
- doi:10.1016/j.chemgeo.2004.06.020. Archived from the original(PDF) on 2022-01-30. Retrieved 2021-08-19.
- S2CID 129727119.
- .
- ISBN 0-908678-97-5
- ^ Forsyth, P.J., and G. Coates, 1992, The Moeraki boulders. Institute of Geological & Nuclear Sciences, Information Series no. 1, (Lower Hutt, New Zealand)
- ^ Thyne, G.D., and J.R. Boles, 1989, Isotopic evidence for origin of the Moeraki septarian concretions, New Zealand, Journal of Sedimentary Petrology. v. 59, n. 2, p. 272-279.
- .
- ^ .
- ISBN 0-477-01438-0
- ^ "Kenai Peninsula Online - Alaska Newspaper -". Archived from the original on 2011-07-08. Retrieved 2010-05-13.
- ^ "Geological Survey Professional Paper". U.S. Government Printing Office. 24 May 1976 – via Google Books.
- ^ S2CID 247665440.
- S2CID 128842746.
- S2CID 28818298.
- S2CID 129664357.
- .
- .
- doi:10.1130/B25618.1.
- S2CID 129502157.
- ^ a b Chan, M.A.; Parry, W.T. (2002). "Mysteries of Sandstone Colors and Concretions in Colorado Plateau Canyon Country" (PDF). Utah Geological Survey Public Information Series. 77: 1–19. Retrieved 18 August 2021.
- ^ S2CID 4393420.
- . Retrieved 18 August 2021.
- S2CID 10139364.
- ^ Hattin, D.E. (1982). "Stratigraphy and depositional environment of the Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of the type area, western Kansas". Kansas Geological Survey Bulletin. 225: 1–108.
- S2CID 250900204.
- S2CID 131123088.
- S2CID 84690956. Retrieved 18 August 2021.
- ^ Sheldon, J.M.A. (1900). Concretions from the Champlain clays of the Connecticut Valley. Boston: University Press. p. 74. Retrieved 18 August 2021.
- .
- .
- ^ Warkentin, B.P., 1967. Carbonate content of concretions in varved sediments. Canadian Journal of Earth Sciences, 4(2), pp.333-333.
- ISSN 0362-4331. Retrieved 2021-07-14.
External links
- Dietrich, R.V., 2002, Carbonate Concretions--A Bibliography, The Wayback Machine. and PDF file of Carbonate Concretions--A Bibliography Archived 2014-12-17 at the Wayback Machine, CMU Online Digital Object Repository, Central Michigan University, Mount Pleasant, Michigan.
- Biek, B., 2002, Concretions and Nodules in North Dakota North Dakota Geological Survey, Bismarck, North Dakota.
- Everhart, M., 2004, A Field Guide to Fossils of the Smoky Hill ChalkPart 5: Coprolites, Pearls, Fossilized Wood and other Remains Part of the Oceans of Kansas web site.
- Hansen, M.C., 1994, Ohio Shale Concretions PDF version, 270 KB Ohio Division of Geological Survey GeoFacts n. 4, pp. 1–2.
- Hanson, W.D., and J.M. Howard, 2005, Spherical Boulders in North-Central Arkansas PDF version, 2.8 MB Arkansas Geological Commission Miscellaneous Publication n. 22, pp. 1–23.
- Heinrich, P.V., 2007, The Giant Concretions of Rock City Kansas PDF version, 836 KB Archived 2016-10-20 at the Wayback Machine BackBender's Gazette. vol. 38, no. 8, pp. 6–12.
- Hokianga Tourism Association, nd, Koutu Boulders ANY ONE FOR A GAME OF BOWLS? and Koutu Boulders, Hokianga Harbour, Northland, New Zealand High-quality pictures of cannonball concretions.
- Irna, 2006, All that nature can never do, part IV : stone spheres Archived 2016-03-03 at the Wayback Machine
- Irna, 2007a, Stone balls : in France too! Archived 2015-07-13 at the Wayback Machine
- Irna, 2007b, Stone balls in Slovakia, Czech Republic and Poland Archived 2008-03-15 at the Wayback Machine
- Katz, B., 1998, Concretions Digital West Media, Inc.
- Kuban, Glen J., 2006–2008. Nevada Shoe Prints?
- McCollum, A., nd, Sand Concretions from Imperial Valley, a collection of articles maintained by an American artist.
- Mozley, P.S., Concretions, bombs, and groundwater, on-line version of an overview paper originally published by the New Mexico Bureau of Geology and Mineral Resources.
- United States Geological Survey, nd, Cannonball concretion
- University of Utah, 2004, Earth Has 'Blueberries' Like Mars 'Moqui Marbles' Formed in Groundwater in Utah's National Parks Archived 2015-08-02 at the Wayback Machine press release about iron oxide and Martian concretions
- Tessa Koumoundouros: These Eerie 'Living Stones' in Romania Are Fantastical, And Totally Real. On: sciencealert. 25 December 2020: About Trovants in Costești, Ulmet and other locations in Romania