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Marl

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.

marl outcrop

Layers of marl of the Palombini shales. Capo Vita, Island of Elba, Italy.

Marl
Mixed siliciclastic – carbonate rock
Composition:
• calcite
• clay minerals
quartz
• dolomite

Varieties:
• limey marl, clayey marl
• marly mudstone
• marly limestone

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:

marl classification

Classification of marls based on the proportion of siliciclastic and carbonate mud. After Compton (1962).

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

marl outcrop

Outcrop of marls, showing the typical fissility. Palombini shales. Capo Vita, Island of Elba, Italy.

marl hand sample

Hand sample of marl from the same outcrop shown above. Palombini shales. Capo Vita, Island of Elba, Italy.

marl and limestone

Marls interlayered with carbonate-rich layers of marly limestone. The layers of marly limestone are less fissile and stand out because they are more resistant to erosion. Palombini shales – Calpionelle limestones transition. Capo Vita, Island of Elba, Italy.

marl and limestone

Marls (more fissile) interlayered with marly limestones from the same outcrop. The thick grey layer at the top consists of limestone. Palombini shales – Calpionelle limestones transition. Capo Vita, Island of Elba, Italy.

Posidonomya alpina fossils in marl

Sample of marl with fossils of Posidonomya alpina, a pelagic bivalve of the Middle Jurassic. Width: about 5 cm. Calcari e Marne a Posidonia Fm. Sassorosso, Garfagnana, Italy.

References
Adams, A.E., & McKenzie, W.S. (1998). A color atlas of carbonate sediments and rocks under the microscope. Wiley, 1st edition.
Dunham, R. J. (1962). Classification of carbonate Rocks according to depositional texture. In: Ham, W. E. (ed.), Classification of carbonate Rocks: American Association of Petroleum Geologists Memoir, p. 108-121.
Flugel, E., & Flügel, E. (2004). Microfacies of carbonate rocks: analysis, interpretation and application. Springer Science & Business Media.
Folk, R.L. (1959). Practical petrographic classification of limestones: American Association of Petroleum Geologists Bulletin, v. 43, p. 1-38.
Folk, R.L. (1962). Spectral subdivision of limestone types, in Ham, W.E., ed., Classification of carbonate Rocks-A Symposium: American Association of Petroleum Geologists Memoir 1, p. 62-84.
James, N. P., & Jones, B. (2015). Origin of carbonate sedimentary rocks. John Wiley & Sons.
Murray, R. C. (1960). Origin of porosity in carbonate rocks. Journal of Sedimentary Research30(1), 59-84.
Scholle, P. A. & Ulmer-Scholle, D. S. (2003). A Color Guide to the Petrography of carbonate Rocks: AAPG Memoir 77, 474 p.
Scholle, P. A., Bebout, D. G., & Moore, C. H. (Eds.). (1983). Carbonate depositional environments: AAPG Memoir 33 (No. 33). AAPG.

        

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