mudstones

Mudstone is a clastic sedimentary rock composed mainly of very fine clay- and silt-sized particles, typically smaller than 0.062 mm. It forms from the compaction and cementation of mud that settles in low-energy environments. Unlike shale, mudstone generally lacks well-developed fissility and does not easily split into thin sheets.

Common constituents include clay minerals such as illite, kaolinite, and montmorillonite, along with quartz, feldspar, and variable amounts of organic matter. Cementing minerals—calcite, silica, and iron oxides—bind the grains. The texture ranges from clay-rich to silty, with the possible presence of nodules or concretions.

Depositional settings include lakes, floodplains, deltas, and deep marine basins where suspended clays accumulate in slow-moving water. Diagenesis and compaction consolidate the mud into rock, and chemical cementation can alter the clay mineralogy.

Mudstone is often contrasted with shale (which is fissile and splits readily along bedding planes) and with siltstone (which contains coarser silt-sized grains). In practice, the terms are used variably, and some rocks may be called mudstone or shale depending on local usage.

Economic and practical significance includes roles as hydrocarbon seals or source rocks, as well as use in construction or ceramics where clay-rich rocks are valued. Near the surface, mudstones weather to clay-rich soils that influence drainage and land-use patterns.