Concretions, Bombs, and Ground Water

Peter S. Mozley
Department of Earth and Environmental Science, New Mexico Tech

Concretions are hard masses of sedimentary and, more rarely, volcanic rock that form by the preferential precipitation of minerals (cementation) in localized portions of the rock. They are commonly subspherical, but frequently form a variety of other shapes, including disks, grape-like aggregates, and complex shapes that defy description (Figs. 1, 2, and 3). Concretions are usually very noticeable features, because they have a strikingly different color and/or hardness than the rest of the rock. In some areas this is unfortunate, as the concretions have attracted the unwanted attention of local graffiti artists.

Commonly, when you break open concretions you will find that they have formed around a nucleus, such as a fossil fragment or piece of organic matter. For a variety of reasons, this nucleus created a more favorable site for cement precipitation than other sites in the rock.

Perhaps the most unusual concretion nuclei are found in a modern coastal salt marsh in England. Siderite (FeCO₃) concretions in the marsh formed around World-War-II era military shells, bombs, and associated shrapnel, including some large unexploded shells (Al-Agha et al., 1995). A British geologist studying these concretions realized this only after striking a large unexploded shell repeatedly with his rock hammer (yes, he lived to tell about it)! The concretions formed preferentially around the military debris because it provided an abundant source of iron for the siderite.

In shales, concretions often preserve features of the original sediment - such as burrows, fossils, and sedimentary layering - that cannot be seen in the rest of the rock. Preservation of primary features occurs because concretions usually form relatively early, before sediment compaction and other processes disrupt the original sediment. Thus, cementation of the concretions "freezes" the early sediment structure, forming a rigid mass that resists later alterations. Examining the concretions is the only way of understanding these early features.

Septarian concretions are the most common type of concretion found in rock and mineral shops. (The name septarian originates from the term "little walled," referring to the raised cracks in the outside of some concretions.) In these concretions the fine-grained concretion body (usually composed of calcite, CaCO₃) is cut by a radiating network of fractures filled with coarsely crystalline calcite and other minerals (Figs. 4 and 5). The origin of the fractures is poorly understood, but they may result from internal shrinkage of the concretion body (like the cracks found in seasoned firewood). This shrinkage perhaps is related to dehydration or transformation of a gel-like mineral precursor in the concretion interior (Raiswell, 1971; Astin, 1986). Alternatively, some authors have suggested that they originate as tensile fractures produced in response to burial and compaction (Astin, 1986). The name septarian originates from the Latin word saeptum (enclosure or wall), referring to the raised cracks on the outside of some septarian concretions. If you are interested in hunting for septarian concretions in New Mexico, many are found in the Mancos Shale, a marine Cretaceous unit found in northwest New Mexico.

Recently hydrologists have become interested in elongate concretions. These concretions range in size from pencil- and cigar-like bodies to those that resemble large fallen logs (Figs. 2 and 3). They are thought to form from flowing ground water, with the long axis of the concretion oriented parallel to the ground-water flow direction (McBride et al., 1994). In formations where such concretions are common, measuring concretion orientation can provide a direct measurement of the past ground-water flow orientation over a large area. Such concretions are common in the Santa Fe Group, New Mexicos most important aquifer (Mozley and Davis, 1996). Elongate concretions have even been found in faults cutting the Santa Fe Group, where they record the past flow orientation of ground water in the fault zone (Mozley and Goodwin, 1995).

Acknowledgements

Thanks to Stuart Burley for relating his hair-raising encounter with the Linconshire Wash concretions. This article benefitted from the comments and suggestions of Virgil Lueth.

References

• Al-Agha, M. R., Burley, S. D., Curtis, C. D., and Esson, J., 1995, Complex cementation textures and authigenic mineral assemblages in Recent concretions from the Linconshire Wash (east coast, UK) driven by Fe(0) Fe(II) oxidation: Journal of the Geological Society, London, v. 152, pp. 157-171.

• Astin, T. R., 1986, Septarian crack formation in carbonate concretions from shales and mudstones: Clay Minerals, v. 21, pp. 617-632.

• McBride, E. F., Picard, M. D., and Folk, R. L., 1994, Oriented concretions, Ionian Coast, Italy - Evidence of groundwater flow direction: Journal of Sedimentary Research, v. A64, pp. 535-540.

• Mozley, P. S., and Davis, J. M., 1996, Relationship between oriented calcite concretions and permeability correlation structure in an alluvial aquifer, Sierra Ladrones Formation, New Mexico: Journal of Sedimentary Research, v. 66, pp. 11-16.

• Mozley, P. S., and Goodwin, L., 1995, Patterns of cementation along a Cenozoic normal fault: A record of paleoflow orientations: Geology, v. 23, pp. 539-542.

• Raiswell, R., 1971, The growth of Cambrian and Liassic concretions: Sedimentology, v. 17, pp. 147-171.

Figures

Figure 1 - "Cannon ball" concretions in the Zia Formation, NM. Photo courtesy of Dave Love.

Figure 2 - Complex elongate concretions in the Zia Formation (Miocene), NM. Lens cap for scale.

Figure 3 - Huge elongate concretions in the Zia Formation (Miocene), NM.

Figure 4 - Septarian calcite concretion from the Mancos Shale, NM. Top concretion has been cut on a rock saw to reveal septarian fractures filled with several generations of coursely crystalline calcite cement. Top concretion is 10 cm in diameter. Concretions courtesy of NMBM&MR Mineral Museum.

Figure 5 - Moeraki boulders, Moeraki Formation, Paleocene, South Island, New Zealand. These huge septarian calcite concretions were exposed when the shale host rock was eroded by wave action. Photo courtesy of James Boles.

Originally published in:
LITE GEOLOGY
WINTER, 1995. All rights reserved.