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They sometimes are found together with dolostone concretions, barite nodules, or phosphatic nodules.
Septarian concretions, some as much as 6 feet in diameter, occur in the Mancos Shale north of the town of Grand Junction where the land slopes upward toward the base of the Book Cliffs.
The mudstones are dark and contain calcareous concretions, the latter brecciated and with calcareous veins.
Most specimens are contained in concretions and are preserved only because the concretions formed around them.
This spike has been previously regarded as diagenetic, as the samples analysed correspond to limestone concretions.
A nodule is a type of concretion with a rough and knobby surface.
The barite and calcite form excellent euhedral crystals in concretions within the Cretaceous-age Mancos Shale.
Coal balls are a particular type of carbonate concretion that has been long known for the superb preservation of plant material.
Preservation of non-biomineralized tissues within concretions occurs through most of the Phanerozoic, from the Upper Cambrian to the Recent.
These are found rarely in large septarian concretions in the Huron Shale in north-central Ohio near Milan, in Huron County.
Asbestos bodies are asbestos fibers that have been coated with an iron-rich, proteinaceous concretion.
To make matters worse, the concretions can be associated with cycads.
Coal balls are concretions of permineralized peat formed in place.
Large chert concretions appear 8 m above the base of the unit at Muller Canyon whereas at Reno Draw they do not appear until 18 m above the base.
At certain levels, carbonate-rich beds are present, inside which decimetre-scale hard calcareous concretions develop.
The biomineralized shells of trilobites, gastropods and brachiopods are preserved within the concretions.
The bodies of many smaller concretions are surrounded by a shell of fine-grained pyrite.
The authors show that early concretion growth prevented collapse and infilling of voids in the organisms, in the time-interval between organic decay and precipitation of calcite.
Recently, one of us saw a bin of silicified barite concretion fragments in a Utah rock shop that were being sold as cycad specimens.
Eventually, the sandstone slowly eroded away and the hard, erosion-resistant concretions were left on the ground.