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.
Recognition of marls
Marl show some of the properties of limestones and mudrocks. Marls are characterized by some fissility, related to the presence of platy clay minerals, which determine the tendency of these rocks to break along planar surfaces. Marls react readily with a water solution of hydrochloric acid 10%, leaving an insoluble muddy residuum that can be observed by cleaning the site of reaction with a piece of fabric or a tissue. At touch, marls feel like dry mud.
Classification of marls
There are several classification schemes available, classifying marls in function or the proportion of carbonate and siliciclastic mud (or even ternary diagrams like carbonate mud – silt – clay). A simple diagram based on the fraction of carbonate and silt/clay present is simple and straightforward for use in the field:
In order to estimate precisely the proportion of carbonate mud and silt/clay, it is necessary to observe marls in thin section. However, it is possible to obtain a qualitative estimation in the field: marls with a higher content of carbonate mud are less fissile, tend to develop a quasi-conchoidal fracture, and give a stronger reaction to HCl (leaving less residuum); on the other hand, marly mudstones are almost as fissile as shales and react with HCl poorly, leaving much more clayey residuum.
Examples of marls
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