Marlstone

A lithified marl, an indurated sedimentary rock consisting chiefly of the same components as marl, ie. an intimate mixture of clay and silt-sized grains and carbonates (usually calcite). Quantitative classifications vary considerably but typically they are considered to contain 30% to 70% siliciclastics (clay, silt and biogenic silica) and 70% to 30% carbonates (based on Colorado School of Mines: https://www.researchgate.net/profile/Alena_Grechishnikova/publication/321886269/figure/fig21/AS:572983786905605@1513621388394/Chalk-and-marl-classification-system-defined-by-the-Colorado-School-of-Mines-Niobrara.png; and IODP publications: http://publications.iodp.org/proceedings/341/102/images/02_F05.jpg). Other classifications use ranges from 25 to 35% carbonate or siliciclastics, but many rocks called marlstones are probably based on guesstimates of their composition. Using the above link the rocks with more carbonate are called argillaceous limestone, or with less are called calcareous mudstones. With an increasing biogenic silica component it grades into calcareous chert; with increasing sand component into a calcareous sandstone or sandy limestone.

Marlstone is usually pale grey or white; it has a blocky subconchoidal fracture, and is less fissile than shale. It can be formed under marine or more commonly freshwater conditions.

Synonym: marlite

The term was originally applied by Bradley (1931) to slightly magnesian calcareous mudstones or muddy limestones in the Green River Formation of the Uinta Basin, UT, but subsequently applied to associated rocks (including conventional shales, dolomites, and oil shales) whose lithologic characters were not readily determined. Picard (1953) recommended abandonment of the term as used in the Uinta Basin.

Ref: AGI