Marl

An unindurated sediment consisting chiefly of 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 sediments called marls are probably based on guesstimates of their composition. Using the above link the sediments with more carbonate are called lime-muds, or with less are called calcareous muds. With an increasing biogenic silica component it grades into calcareous biogenic ooze; with increasing sand component into a calcareous sand or sandy limestone.

Marl is usually pale grey or white; it can be formed under marine or more commonly freshwater conditions. A calcium-carbonate rich mud containing variable amounts of clays and silt. This may be defined either as calcite-mud or lime-rich silicate-mud depending on the proportion of carbonate to clay.

Note the name is also commonly used in geological literature for lithified marl muds where the name marlstone would be more technically correct.

An old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay and calcium carbonate, formed under marine or esp. freshwater conditions; specif. an earthy substance containing 35% to 65% clay and 65% to 35% carbonate. Marl is usually gray; it is used esp. as a fertilizer for acid soils deficient in lime. In the Coastal Plain area of Southeastern United States, the term has been used for calcareous clays, silts, and sands, esp. those containing glauconite (greensand marls); and for newly formed deposits of shells mixed with clay.

Ref: AGI

Classification Diagram: http://publications.iodp.org/proceedings/341/102/images/02_F05.jpg

ii. A soft, grayish to white, earthy or powdery, usually impure, calcium carbonate precipitated on the bottoms of present-day freshwater lakes and ponds, largely through the chemical action of aquatic plants, or forming deposits that underlie marshes, swamps, and bogs that occupy the sites of former (glacial) lakes. The calcium carbonate may range from 90% to less than 30% .

Synonym of: bog lime

iii. A term occasionally used (as in Scotland) for a compact, impure, argillaceous limestone.

Ref: AGI