Concretions

Concretions are commonly misunderstood geologic structures. Often mistaken for fossil eggs, turtle shells, or bones, they are actually not fossils at all but a very common geologic phenomenon in all types of sedimentary rock; including sandstone which is made up of compacted sand grains, shale which is made up of compacted mud, siltstone which is made up of a fine grained silt, and limestone which is made up of calcium carbonate precipitated by many marine invertebrates.

Concretions form as minerals within a rock segregate and begin to precipitate within cracks and cavities, or as a sediment builds up in successive layers around a nucleus such as a shell or pebble.

In some cases, the bedding planes, which are the compacted layers of sediment fom the time of deposition and condition of the sediment are preserved within the concretion itself. If the planes pass continuously through the concretion, then it probably formed after the sediment was compacted. If the planes curve around the concretion, it probably formed early after the deposition of the sediment. Also the size of the concretion can shed light on the permeability of the rock in which it occurs. The more permeable the surrounding rock, the larger the concretion.

Learn more about:

Concretions in Sandstones and Siltstones

Shale

Limestone

Resources:

Sedimentary Petrology by Harvey Blatt, 1992.
Sedimentary Rocks by F.J. Pettijohn, 1975.
Principles of Sedimentology and Stratigraphy by Sam Boggs, Jr., 2001.
Mineral Science by Cornelis Klein, 2002.

Written and designed by Eric Chapman.

Concretions in Sandstones and Siltstones

A Kugelsandstein concretion is a spheroidal or disk-shaped carbonate concretion formed in the air gaps within the sediment around a foreign object. It can range in size from 1 cm to 9 m in diameter.

Imatra stones are disk-shaped calcareous concretions often found in silt beds. They are characterized by the presence of bedding planes passing through them giving them a stacked disk appearance. They are flattened parallel to the bedding plane and can range from 2 to 10 cm in diameter.

(Imatra stone found in the Clay Bluff of Block Island, Rhode Island from the Pleistocene. PRI 43564)

Crystal aggregates are large grained crystal-like bodies, generally found in sandstones. They can range in size from 5 to 10 cm.

(Sand crystal aggregate from the Miocene of South Dakota. Image from Sedimentary Rocks by F.J. Pettijohn.)

Concretions in Shale

Calcareous concretions are spherical to oval-shaped calcium carbonate concretions that are flattened parallel to the bedding planes. They can range in size from 3 cm to 8 m in diameter. They most often precipitate around a nucleus of fossilized material including plant matter, shells, or even remains of fish. Often when the concretions erode, they can form odd shapes, sometimes resembling fossils. These are very common in the Devonian rocks found in the Ithaca area.

(Calcareous concretion found in the Hamilton Group of Ludlowville, New York from the Middle Devonian. The nucleus in this case was the fossilized remains of an ammonoid. PRI ACC 1107)

(Calcareous concretion from the Middle Devonian. PRI ACC 1053)

(Large calcareous concretion from central New York. The cracks in this concretion occurred as a result of the collapsing of the dome which often occurs in the larger concretions.)

Septarian nodules are distinctly sphere-like concretions that are characterized by a series of cracks that widen towards the center and die out towards the sides of the concretion. These radiating cracks are often crossed by a series of concentric cracks giving them a "turtle-back" appearance. Dehydration of the concretion creates the cracks which then are filled with another crystalline cement, such as calcite or silica. They can range in size from 10 to 100 cm in diameter and usually are made up of a large component of iron.

(Septarian nodule found in the Windom Shale of Ludlowville, New York from the Middle Devonian. PRI ACC 1056)

(Large Septarian nodule found in Crowbar Point near Cayuga Lake in New York.)

Cone-in-cone concretions occur along the bedding plane as a 2 to 15 cm thick layer. It is composed of an aggregate of upright cones with ribbed or grooved sides, usually made up of minerals like calcite or gypsum.

(Image of cone-in-cone concretion from Sedimentary Rocks by F.J. Pettijohn.)

 

Concretions in Limestone

Chert nodules are highly irregular bodies composed of dense black flint (SiO2) surrounded by a white calcareous coating.

(Chert nodule found in Seneca County, New York from the Middle Devonian. PRI 45566)

(Chert nodule found in the Onondaga Limestone of Dryden, New York from the Lower Middle Devonian. PRI ACC 1058)

Geodes are spherical concretions with a hollow interior, ranging from 2 cm to 1 m in diameter. The outermost layer is generally composed of chalcedony, a brown waxy fibrous variety of quartz. The interior contains a lining of inwardly projecting crystals, usually quartz.

(Calcite geode from Mexico. PRI GD3.055.1A)

(Quartz geode. PRI ACC 1250)