Marshall and Pirrie (2013) noted, “concretions are simply patches of cemented sediment. The mineral cement between the grains is different, or more abundant, from that in the host sediment (which may not be cemented at all).” They point out that the most common carbonate concretions are calcite (calcium carbonate) and siderite (iron carbonate), but others (like magnesium carbonate) can occur. An organic carbon source, a source of cations, and a source of an anion (e.g. carbonate) are required. They described the formation of carbonate concretions as 1) sediment deposition, where mineral-saturated water begins to be squeezed out, 2) sediment grains become more compact, clay particles and other platy minerals tend to rotate to a horizontal orientation, 3) porosity decreases, while water permeability is forced horizontally, 4) minerals start to precipitate (cement) in microcrystalline form around a nucleus (and if it forms prior to compaction in sediment, fossil nuclei may be shielded from being crushed), and 5) concretion grows as precipitation continues, replacing water in the rock, until conditions (such as lack of carbonate) force it to stop.
If the sediment isn’t completely lithified when the concretion starts to form (early diagenesis), sedimentary layers will be pliable enough to mold around it as it grows. Potter-McIntyre et al. (2014) noted that there are two patterns of nucleation: 1) Radial growth, where a nucleus of organic matter or ‘a preexisting particle of the precipitating mineral’ becomes the initial site of crystal growth, whether radially or concentric, and 2) Pervasive growth, where crystals nucleate throughout the concretion or just in the rinds (where centers are hollow, or porous, and secondary precipitation fills it in).
Concretion precipitation starts within a complex interplay of microbial, biochemical, and inorganic chemical reactions. Sandstone strata tends to benefit spherical growth and often formed in what were likely shallow marine environments, while mudstone strata (which could be marine, brackish, or freshwater) usually produces elongated concretions. Elongate concretions, from pencil-sized to log-sized, “are thought to form from flowing ground water, with the long axis of the concretion oriented parallel to the ground-water flow direction.” (Mozley 1995)
From a creationist perspective, concretions have formed in Flood sediments and post-Flood sediments (and could have formed in pre-Flood sediments, as there are Precambrian concretions). Carbonate concretions continue to form today in marine, brackish, and freshwater environments (Lloyd and Berelson 2016). A “rather amorphous, rounded nodule” was once discovered in the “marshy sediments of the Wash in Eastern England. . . . Its finders were a little perturbed to discover that the nucleation point of the nodule, and probable source of some of the iron, was an unexploded hand grenade that had landed in the marsh earlier last century” (Marshall and Pirrie 2013). Rapid formation of spherical carbonate concretions (calcite and dolomite), even very large ones, has been determined as “usually within several years” (Muramiya et al. 2020).
