Maximizing information from fine-grained sedimentary rocks: An inclusive nomenclature for mudstones

A simple nomenclature for fine-grained sedimentary rocks is proposed based on sedimentary structures present and abundance of all materials forming more than 10% of rock volume irrespective of their origin (allochthonous, autochthonous, and diagenetic). A mudstone (sedimentary rock that comprises > 50% grains < 0.063 mm) containing more than 90% of a material is said to be dominated by that component. One containing between 50 and 90% of a particular component is described as being rich in that material, and one containing 10 to 50% is said to be bearing that component. Examples of rock names using this nomenclature are clay-dominated mudstone (> 90% clay), silt- and sand-bearing, clay-rich mudstone (50 to 90% clay, 10 to 50% sand, 10 to 50% silt), and calcite cement-, calcareous nannoplankton-, and clay-bearing mudstone (all components 10 to 50%). Textural information can and should be incorporated into this scheme by prefixing the rock name with descriptions such as bioturbated, and thin-bedded. Such a nomenclature is necessary to fully describe variability within fine-grained sedimentary rocks that hitherto have typically been described using selected constituents of varying significance such as total organic carbon, fissility, parting spacing, gamma-ray signature, and lamination. Using the inclusive nomenclature proposed here will improve opportunities to compare all fine-grained sediments, aid interpretation of depositional and diagenetic controls of variability, and enable these rocks to be more easily integrated into basin-scale facies models.