marls

Marls are sedimentary rocks composed of clay minerals mixed with calcium carbonate. They are calcareous mudstones or claystones in which carbonate minerals occur within a clay-rich matrix. Marl can vary in color from greenish or gray to brown or red, reflecting iron content and organic matter. When the rock becomes more consolidated through diagenesis, it is called marlstone.

Formation and environment: Marl forms in low-energy aquatic settings where fine clay- and silt-sized particles accumulate in the presence of dissolved carbonate, which precipitates as calcite or dolomite. This can occur on continental shelves, in lagoons, lakes, or floodplains. Marl often appears as interbeds with limestone or claystone, recording changes in sediment supply and chemistry over time.

Petrology and properties: The mineral assemblage typically includes clay minerals (illite, montmorillonite, kaolinite) together with carbonate minerals (calcite, dolomite) and minor quartz or organic matter. The rock is generally soft when fresh and can be easily weathered; its color may be altered by oxidation.

Uses and significance: Marl has long been used as a source of lime for agriculture and construction, especially where soils are deficient in calcium carbonate. In geology and stratigraphy, marl layers help interpret past environmental conditions, including episodes of reduced or increased carbonate supply and shifts between marine and freshwater settings.

See also: limestone, claystone, marlstone, calcareous soil.