Hi everybody.
I'm currently writing a manuscript and there are discrepancies between authors on the use of "marl" and "clay", some of them suggesting that these are outdated terms that should be avoided in favor of "marlstone" and "claystone". I have found some opinion pieces on the internet supporting this view, but I'm not sure if there is scientific consensus on the matter.
To be clear, I use the terms "marl" and "clay" when referring to unconsolidated (but not recent!) sediment. The use of "-stone" in this context seems counterintuitive to me.
What do you think?
Hello,
Petrological nomenclature of sedimentary rocks is not yet globally standardized, as is the case for igneous and metamorphic rocks (IUGS recommendations). There are certainly national guidelines for sediment terminology. It is also mentioned in every petrography textbook. It is certainly a good idea to cite the source of the nomenclature that was used. I don't know the Spanish terminology, but here in the Czech Republic there is a fundamental distinction between consolidated and unconsolidated types of clastic rocks. You can't take the petrography exam without it. There is sandstone and sand, conglomerate and gravel, clay and claystone and shale, etc. etc. and I am not aware of any controversy. Which is a bit controversial, or both marlstone and marl are vaguely defined. Both belong somewhere in the middle of the range of clays of dust - calcareous sediment, or claystonec or duststones - limestones. Terms such as calcareous clay, calcareous claystone, silty limestone, etc. are usually sufficient.
Translation from (Štelcl J. and Vávra V., Ústav geologických věd, PřF MU, Brno) https://atlas.horniny.sci.muni.cz/sedimentarni/slin_slinovec.html
Marl and marlstone
Marl is an unconsolidated sediment consisting of a mixture of clayey and dusty fractions with carbonate mass. A solidified rock of similar composition is referred to as marlstone. The content of the carbonate component ranges from 20-80%. Rocks with a content of 50–80% carbonates are sometimes referred to as high-percentage marls, or marlstones. Terminologically, such a designation of rocks is not correct, but this term is quite common and these are common types of rocks. Most marls and marlstones are formed in the marine environment, freshwater marls are also known.
In the Czech Cretaceous, the incorrect but quite common term "opuka = opoka, fine calcareous sandstone" is used for sandy shale. These are light yellow-brown or light gray rocks with distinct plate-like separation. Very often they contain abundant mushroom needles, foraminifera shells and glauconite. The correct classification of these common rocks is usually spongilitic sandy-calcareous clay.
The use of terminology in scientific writing is an important aspect to consider, as it can affect the clarity and accuracy of the information being conveyed. In the case of the terms "marl" and "clay," it is true that some authors suggest avoiding them in favor of "marlstone" and "claystone" to avoid confusion.
Marl is a type of unconsolidated sediment that contains both clay and calcium carbonate. Clay is a type of sediment composed of fine-grained minerals. Marlstone and claystone are both lithified versions of these sediments.
In terms of scientific consensus, the International Union of Geological Sciences recommends using the terms "marlstone" and "claystone" to describe lithified sedimentary rocks, rather than "marl" and "clay" which are unconsolidated sediments. However, there is less consensus on the use of these terms when referring to unconsolidated sediments.
Ultimately, the choice of terminology in your manuscript should be based on clarity and accuracy. If using "marl" and "clay" accurately conveys the information you want to present and is not likely to cause confusion, then it may be appropriate to use those terms. However, if you believe that using "marlstone" and "claystone" is clearer and more accurate, then you should consider using those terms instead. It may also be helpful to consult with other experts in your field to get their perspective on the matter.
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