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. Concretions are often ovoid or spherical in shape, although irregular shapes also occur. The word concretion is borrowed from Latin concretio, itself derived from concrescere, from con- and crescere.
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, 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.Concretions typically form around a solid core called a "nucleus". This core is often composed of organic material, such as a leaf, tooth, piece of shell or fossil. A mineral solution then precipitates around the nucleus and cements sediment around it. For this reason, fossil collectors commonly break open concretions in their search for fossil animal and plant specimens. 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.
Depending on the environmental conditions present at the time of their formation, concretions can be created by either concentric or pervasive growth. 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 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 are often exposed at the surface by subsequent erosion that removes the weaker, uncemented material.
Appearance
Concretions vary in shape, hardness and size, ranging from objects that require a magnifying lens to be clearly visible to huge bodies three meters in diameter and weighing several thousand pounds. The giant, red concretions occurring in Theodore Roosevelt National Park, in North Dakota, are almost in diameter. Spheroidal concretions, as large as in diameter, have been found eroding out of the Qasr el Sagha Formation within the Faiyum depression of Egypt. Concretions occur in a wide variety of shapes, including spheres, disks, tubes, and grape-like or soap bubble-like aggregates.Composition
Concretions are commonly composed of a mineral present as a minor component of the host rock. For example, concretions in sandstones or shales are commonly formed of a carbonate mineral such as calcite; those in limestones are commonly an amorphous or microcrystalline form of silica such as chert, flint, or jasper; while those in black shale may be composed of pyrite. Other minerals that form concretions include iron oxides or hydroxides, dolomite, siderite, ankerite, marcasite, barite, and gypsum.Although concretions often consist of a single dominant mineral, other minerals can be present depending on the environmental conditions that created them. For example, carbonate concretions, which form in response to the reduction of sulfates by bacteria, often contain minor percentages of pyrite. Other concretions, which formed as a result of microbial sulfate reduction, consist of a mixture of calcite, barite, and pyrite.
Occurrence
Concretions are found in a variety of rocks, but are particularly common in shales, siltstones, and sandstones. They often outwardly resemble fossils or rocks that look as if they do not belong to the stratum in which they were found. Occasionally, concretions contain a fossil, either as its nucleus or as a component that has been incorporated during its growth but concretions are not fossils themselves. They appear in nodular patches, concentrated along bedding planes, or protruding from weathered cliffsides.Small hematite concretions or Martian spherules have been observed by the Opportunity rover in the Eagle Crater on Mars.
Types of concretion
Concretions vary considerably in their compositions, shapes, sizes and modes of origin.Septarian concretions
Septarian concretions are carbonate-rich concretions containing angular cavities or cracks. Septarian nodules are characteristically found in carbonate-rich mudrock. They typically show an internal structure of polyhedral blocks separated by mineral-filled radiating cracks which taper towards the rim of the concretion. The radiating cracks sometimes intersect a second set of concentric cracks. However, the cracks can be highly variable in shape and volume, as well as the degree of shrinkage they indicate. The matrix is typically composed of argillaceous carbonate, such as clay ironstone, while the crack filling is usually calcite. The calcite often contains significant iron 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.Septarian concretions are found in many kinds of mudstone, including lacustrine siltstones such as the Beaufort Group of northwest Mozambique, but are most commonly found in marine shales, such as the Staffin Shale Formation of Skye, the Kimmeridge Clay of England, or the Mancos Group of North America.
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. 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. Whether the initial cementation was concentric or pervasive, there is considerable evidence that it occurred quickly and at shallow depth of burial. In many cases, there is clear evidence that the initial concretion formed around some kind of organic nucleus.
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. 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. 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. A more speculative theory is that the septaria form by brittle fracturing resulting from earthquakes. 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 and possibly as little as. 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 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.
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 or uplift. Water derived from rain and snow later infiltrated the beds and deposited ferroan calcite in the cracks.
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. This corresponds to burial depths of, and is characterized by generation of carbon dioxide, increased alkalinity and precipitation of calcium carbonate. However, there is some evidence that formation continues well into the methanogenic zone beneath the sulfate reduction zone.
A spectacular example of boulder septarian concretions, which are as much as 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. 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.