muds and mudstones

muds and mudstones Mud and its indurated (i.e. hardened) equivalent mudstone are the most common sedimentary rocks in the Earth's crust. The terms ‘mud’ and ‘mudstone’ are loosely used for sediments in which the majority of the particles are less than 63 µm (micrometres) in diameter. The terminology is more precisely defined in Table 1. Silts or siltstones have physical properties more comparable to those of sands than mudstones. Very fine-grained sediments in which more that 66 per cent of the grains are less than 2 µm are called clays or claystones. When subjected to severe compression on burial, some muds become fissile and are then termed shales.

Muds and mudstones are composed of particles produced by the breakdown and alteration of silicate minerals together with silica, calcium carbonate, and organic matter. The particles less than 2 µm in diameter that dominate their physical properties consist mainly of clay minerals. These minerals are hydrated aluminosilicates consisting of sheets of alumina and silica with interchangeable cations and considerable amounts of combined or loosely bound water. Clays are produced by weathering in the area that was the source for the muds, and their composition depends upon the character of the source rocks, the type of weathering they undergo, and changes produced during transport, deposition, and burial.

Table 1. Classification of mudstones

Percentage of clay-size
constituents (< 63 μm)	0–32	33–65	66–100
(Adapted from Potter et al. (1980), Table 1.2)
Non-indurated	Beds > 10 mm	Bedded silt	Bedded mud	Bedded clay-mud
Laminae < 10 mm	Laminated silt	Laminated mud	Laminated clay-mud
Indurated	Beds > 10 mm	Bedded siltstone	Mudstone	Claystone
Laminae < 10 mm	Laminated siltstone	Mud-shale	Clay-shale

When muddy sediments are composed dominantly of calcium carbonate they are called carbonate mudstones. Mixtures of siliciclastic mud and carbonate mud are termed marl.

Muds are richer in organic matter than most other sediments. This is partly because fine-grained organic matter has hydrodynamic properties similar to those of clays, but also because it is preferentially preserved in these impermeable sediments. On burial, organic matter is broken down to yield oil and gas; hence some mudstones are major source rocks for hydrocarbons.

Muds are transported in suspension by surface waters and are widely dispersed from their source area. They settle in areas of low turbulence ranging from river flood-plains, lakes, and sheltered coastal areas to the deepest parts of the ocean.

When initially deposited many muds contain up to 80 per cent water, partly within and also partly bound to the clay minerals, as well as infilling primary voids. Muds are thus very reactive to physical disturbance or differential loading, and they slump and flow easily when subjected to stress. They sometimes flow upwards through overlying sediments to form mud-diapirs. Compaction and subsequent loss of water produce a deposit which is only a fraction of its original thickness. When sedimentation is very rapid, as in a delta, the water is sometimes prevented from escaping and it is sealed in the sediment column to produce zones of very high pore-pressure. These zones are a hazard when penetrated by subsequent drilling for, say, oil.

Muds and mudstones are important as raw materials for industry: for example, bricks, cement, ceramics, and drilling muds. They are a major source of the natural mineral cements of sandstones when their contained water is driven into adjacent porous and permeable beds during compaction, and they are also regarded as the major source of mineralizing solutions. Mudstones are the main source of oil and gas, although by the time the oil or gas is extracted it has usually migrated to porous rocks such as sandstones of limestones. Muds and mudstones give rise to many problems in civil engineering because they are weak and shrink or swell on being dried or wetted.

G. Evans

Bibliography

Potter, P. E.,, Maynard, J. B.,, and and Pryor, W. A. (1980) Sedimentology of shale. Springer-Verlag, New York.