Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.

[4][5] Shales are typically gray in color and are composed of clay minerals and quartz grains.

[6] Black shale results from the presence of greater than one percent carbonaceous material and indicates a reducing environment.

Clay minerals of Late Tertiary mudstones are expandable smectites, whereas in older rocks (especially in mid-to early Paleozoic shales) illites predominate.

The transformation of smectite to illite produces silica, sodium, calcium, magnesium, iron and water.

Black shales, which form in anoxic conditions, contain reduced free carbon along with ferrous iron (Fe2+) and sulfur (S2−).

There is evidence that shale acts as a semipermeable medium, allowing water to pass through while retaining dissolved salts.

Filter feeders produce an estimated 12 metric tons of clay pellets per square kilometer per year along the U.S. Gulf Coast.

[20][21] Early stages of diagenesis, described as eogenesis, take place at shallow depths (a few tens of meters) and are characterized by bioturbation and mineralogical changes in the sediments, with only slight compaction.

The particles become strongly oriented into parallel layers that give the shale its distinctive fabric.

[27] Lithification follows closely on compaction, as increased temperatures at depth hasten deposition of cement that binds the grains together.

Pressure solution contributes to cementing, as the mineral dissolved from strained contact points is redeposited in the unstrained pore spaces.

Common in some Paleozoic and Mesozoic strata, black shales were deposited in anoxic, reducing environments, such as in stagnant water columns.

[30][32][33] Fossils, animal tracks or burrows and even raindrop impressions are sometimes preserved on shale bedding surfaces.

[8] The lack of coarse sediments in most shale beds reflects the absence of strong currents in the waters of the depositional basin.

[36] The organic matter in shale is converted over time from the original proteins, polysaccharides, lipids, and other organic molecules to kerogen, which at the higher temperatures found at greater depths of burial is further converted to graphite and petroleum.