A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Septarian concretions are concretions containing angular cavities or cracks, called "septaria". The septaria are the calcite filled cracks at the centre of the rock, indicating where the centres of the concretions have shrunk, possibly during dehydration during its long transformative journey. Septarian concretions are typically found in sedimentary rocks, such as mudstone and sandstone.
|Septarian from Muddy Creek, Orderville, Kane Co., Utah. |
Credit: Will Hough
Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that radially oriented cracks taper towards the margins of septarian concretions is taken as evidence that in these cases the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of cement precipitated.
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.
Formation of Septarian Concretion
Septarian concretions form in sedimentary rocks, such as mudstone and sandstone. The process that created the septaria that characterize septarian concretions remains unclear. A number of mechanisms have been proposed, including the dehydration of clay-rich, gel-rich, or organic-rich cores; shrinkage of the concretion's center; expansion of gases produced by the decay of organic matter; or brittle fracturing or shrinkage of the concretion interior by either earthquakes or compaction. Another theory is that the cracks are caused by the growth of crystals within the concretion. As the crystals grow, they push against the surrounding material, causing it to crack.
|Calcite inside Septarian concretion. photo: DweadPiwateWoberts|
Septaria usually contain crystals, often calcite, that precipitated from circulating solutions. Siderite or pyrite coatings are also occasionally observed on the wall of the cavities present in the septaria, giving rise respectively to a panoply of bright reddish and golden colors.
Where to Find Septarian Concretions
Septarian concretions can be found all over the world, but they are most common in sedimentary rocks that were deposited in marine environments. Some of the best-known localities for septarian concretions include:
- The Moeraki Boulders in New Zealand
- The Isle of Wight in England
- The Jurassic Coast in England
- The Smoky Hills region of Kansas, USA
- The Mancos Shale region of Utah and Colorado, USA
- The Bearpaw Shale region of Montana and Saskatchewan, Canada
Famous Septarian Concretions
A spectacular example of 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.
Very similar concretions, which are as much as 3 meters (9.8 feet) in diameter and called "Koutu Boulders", litter the beach between Koutu and Kauwhare points along the south shore of the Hokianga Harbour of Hokianga, North Island, New Zealand. 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.
|Septarian Concretion from Kabardino-balkaria, Russia|
Photo: Arsen Bashiev