Kalkkikerrostuma

Kalkkikerrostuma is a geological term in Finnish referring to calcite-rich concretionary structures or laminated calcareous layers that form within sedimentary rocks or soils. The term describes both nodular calcite masses and laminated calcareous beds that stand out from the surrounding material.

Formation is a diagenetic process in which calcium carbonate precipitates from pore waters in carbonate-bearing sediments. This precipitation often centers around nuclei such as fossil fragments, plant material, or organic-rich layers, leading to the growth of calcite around these cores. Over time, concentric growth or repetitive mineralization can produce spheroidal, ellipsoidal, or layered forms. The resulting kalkkikerrostumat are typically more resistant to weathering than the host rock and may show colour banding due to impurities like iron, manganese, or organic matter.

Morphology commonly includes nodules embedded in limestone, shale, marl, or chalk, as well as discrete laminated calcareous bands. They can appear as spherical to oblate bodies or as parallel layers, often with a zoned internal structure.

Occurrence and significance: Kalkkikerrostumat are widespread in carbonate-bearing sedimentary successions worldwide and record diagenetic histories and fluid flow within the rocks. In hydrogeology and hydrocarbon geology, calcareous cement can alter porosity and permeability, influencing reservoir quality. They are distinct from primary limestone beds and represent post-depositional mineralization.

Etymology and usage: The name combines kalkk- (calcium carbonate/limestone) with kerrostuma (layering or concretion).