LENS ON SEPTARIAN NODULES

Septarian Nodule, Dan Bowen, 2020 Tucson Gem and Mineral Show

An exceptional polished septarian nodule with an ammonite heart. Septarian concretions or septarian nodules are concretions containing angular cavities or cracks, called "septaria." These septaria can be filled 

The word comes from the Latin word septum; "partition", and refers to the cracks/separations in this kind of rock.

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.

The cracks or patterns you see here 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.

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.

Beautiful smaller septarian concretions are found in the Kimmeridge Clay exposed in cliffs along the Wessex Coast of England. As as you walk the beach, look for exposures of Speeton Clay beds D6 and D7, the bentonite horizons that weather to yellow colouration. Beneath the Speeton Shell Bed cliff exposures is an exposure of Kimmeridge Clay, UK. This outcrop contains concretions that show the characteristic 'turtle-stone' patterns of these septarian nodules.

Photo top: Dan Bowen, Chair, VIPS, 2020 Tucson Gem and Mineral Show.

References: Humberside Geologist No. 14, Humberside Geologist Online, The geology of East Yorkshire coast.http://www.hullgeolsoc.co.uk/hg146t.htm

Dale, P.; Landis, C. A.; Boles, J. R. (1985-05-01). "The Moeraki Boulders; anatomy of some septarian concretions". Journal of Sedimentary Research. 55 (3): 398–406.

Milliken, Kitty L.; Picard, M. Dane; McBride, Earle F. (2003-05-01). "Calcite-Cemented Concretions in Cretaceous Sandstone, Wyoming and Utah, U.S.A." Journal of Sedimentary Research. 73 (3): 462–483. Bibcode:2003JSedR..73..462M. doi:10.1306/111602730462. ISSN 1527-1404.