Marls are sedimentary rocks with mixed composition, consisting in part of carbonate sediment (carbonate ooze) and in part of fine-grained siliciclastic sediment (clay and silt). The most abundant carbonate mineral in marls is calcite, even though dolomite and aragonite can also be present, whereas the silicate fraction consists of clay minerals and other detrital components, like quartz, feldspar, micas, etc. Marls can also contain oxides, carbonaceous material, and sulphides, and can be fossiliferous.

Marly sediments deposit in sedimentary environments with calm waters that allow the slow settling of very fine-grained sediment, like open seas and oceans or lakes. The carbonate fraction of a marl derives in part from inorganic processes, such as precipitation of calcite from water or cementation of the sediment, in part represent fossils of microorganisms, like nanoplankton, single-celled organisms with a calcite/aragonite shell. The siliciclastic (silt/clay) fraction of the sediment is supplied by various processes: it might represent the (distal) very fine-grained sediment transported by rivers to a marine or lacustrine basin, or aeolian dust, transported by winds and depositing into the ocean.