Mixed siliciclastic and carbonate sediments: a proposed first‐order textural and compositional classification

ABSTRACT Sediments that are composed of mixtures of siliciclastic and carbonate material occur in both the modern and the ancient. To date, there is no commonly‐used classification of this type of sediment. A descriptive, first‐order classification system is proposed here that is applicable to most mixed sediments and their lithified equivalents. The sediments are composed of four components: (1) siliciclastic sand (sand‐sized quartz, feldspar, etc.), (2) mud (mixtures of silt and clay), (3) allochems (carbonate grains such as peloids, ooids, bioclasts and intraclasts > 20 μm in size), and carbonate mud or micrite (< 20 μm in size). These components define a tetrahedra whose subdivision produces eight general classes of mixed sediment. The name of a class reflects both the dominant grain type and the most abundant antithetic component. For example, a rock that contains greater than 50% siliciclastic material, of which most is sand‐sized, and subordinate allochems is termed an allochemic sandstone. Other rock names include micritic sandstone, allochemic and micritic mudstone, sandy or muddy allochem limestone and sandy or muddy micrite. Prefixes and adjectives can be added to these terms to include dolomitic sediments, conglomeratic textures, and biogenic constituents.