mudstone

Mudstone is a fine-grained sedimentary rock composed mainly of clay- and silt-sized particles, typically less than 0.063 millimeters in diameter. Unlike shale, which splits easily along well-developed bedding planes (fissility), mudstone is generally non-fissile or only weakly fissile and rocks of this type often break with a blocky fracture.

Composition and classification vary, but mudstone is commonly dominated by clay minerals such as illite, kaolinite, or mixed-layer clays, with variable amounts of quartz, feldspar, carbonates, and organic matter. The color ranges from gray to brown or red, influenced by iron content and diagenetic cementation.

Formation occurs in low-energy environments where fine sediments can settle and compact, including lakes, floodplains, offshore muds, and deep-water settings. Mudstone may occur as massive units or in thinly bedded sequences and can display lamination. Fossils are often rare, though they can be preserved in some beds.

Diagenesis cements the grains with minerals like calcite, silica, or iron oxides, giving the rock additional strength. Mudstones weather to clay-rich soils and are commonly involved in weathering profiles.

In geology and engineering, mudstones are noted for their low permeability, influencing fluid migration in basins and acting as potential source rocks or seal rocks in petroleum systems. Metamorphically, substantial burial and heat can transform mudstone into slate, phyllite, or higher-grade metamorphic rock. The term is sometimes used interchangeably with claystone or calcareous mudstone, though precise usage varies by region.