Concretion

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.

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.

[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.

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 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.

[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.

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.

[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.

[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.

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 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.

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.

[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.

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.

They are found where submarine erosion has concentrated early diagenetic concretions as lag surfaces by washing away surrounding fine-grained sediments.

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.

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.

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.

[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.

[84][85] Gogottes [fr] are sandstone concretions found in Oligocene (~30 million years) aged sediments near Fontainebleau, France.