Concretions are hard, compact geological formations embedded within softer sedimentary rock layers. They are created through a complex, localized chemical process that occurs after the initial sediment has been deposited. Often mistaken for fossils or meteorites due to their distinct shapes, concretions are fundamentally a concentration of mineral cement within the host rock. This article explains the geological mechanism that transforms loose sediment into these dense structures.
Defining the Necessary Materials and Setting
The formation of a concretion requires two primary components: a permeable host rock and a mobile cementing agent dissolved in fluid. The host rock is typically a fine-grained sedimentary layer, such as shale, mudstone, or sandstone, containing abundant pore space. This porosity allows for the movement of mineral-rich groundwater or pore water through the sediment.
The cementing agent is a dissolved mineral that will eventually solidify to bind the sediment grains together. Common agents include calcium carbonate (forming calcite concretions) or various iron compounds like iron oxides (creating structures such as Moqui Marbles). Silica and iron sulfides, such as pyrite, can also act as the binding material, depending on the specific chemistry of the environment.
The Chemical Mechanism of Concretion Growth
The chemical process begins with nucleation, the establishment of a localized microenvironment. The initial trigger is often organic matter, such as a shell fragment or small fossil, buried within the sediment. As this material decays, it creates a chemical gradient that alters the local acidity and oxygen levels of the surrounding pore water.
This chemical change causes the dissolved cementing minerals to reach a state of supersaturation near the nucleus. Once the concentration of dissolved ions exceeds the water’s capacity to hold them, the second stage, precipitation, occurs. The mineral solidifies out of the solution and begins to crystallize in the open pore spaces of the sediment.
The final stage is growth, where the concretion expands outward by accretion. Mineral-rich fluids continue to flow toward the chemically active zone, depositing successive layers of cement. This growth front consumes the surrounding sediment, cementing the grains together and hardening a spherical or ovoid volume of rock over thousands to millions of years.
Analyzing Concretion Shapes and Internal Structures
Concretions exhibit a variety of shapes, ranging from small, spherical masses to flattened discs and elongated tubular forms. The final shape is determined by the homogeneity and permeability of the host rock and the direction of groundwater flow. Uniform mineral deposition in a homogenous sediment typically results in a spherical shape.
A unique internal structure is found in septarian concretions, characterized by internal cracks, or septa, filled with secondary mineral crystals. These cracks form after the initial mass has been cemented but before the entire rock layer is fully lithified. The core shrinks, possibly due to dehydration, causing the formation of radiating fractures.
Groundwater later infiltrates these cracks, depositing a second generation of minerals, most commonly calcite or barite, to fill the fissures. These mineral-filled veins create a distinctive polygonal or honeycomb pattern when the concretion is broken open. Irregular or tubular shapes often develop when the cementation front follows linear pathways, such as ancient root systems or animal burrows.