Classification of sedimentary rocks

Sedimentary rocks are classified mainly by their mode of origin into three groups, and also by grain size and composition.

1. Clastic (mechanical, detrital) rocks

Formed from broken fragments of older rocks, transported and deposited, then cemented. Classified by grain size:

• Rudaceous (> 2 mm): conglomerate (rounded gravel), breccia (angular fragments).
• Arenaceous (1/16-2 mm): sandstone, arkose, greywacke.
• Argillaceous (< 1/16 mm): shale, mudstone, siltstone, clay.

2. Chemical rocks

Formed by precipitation from solution or evaporation.

• Limestone (chemical), dolomite, rock salt, gypsum, chert, travertine.

3. Organic (biogenic) rocks

Formed from remains of plants and animals.

• Calcareous: fossiliferous limestone, coral limestone, chalk.
• Carbonaceous: peat, lignite, coal.
• Siliceous: diatomite.

 Sedimentary
   |-- Clastic    : conglomerate, sandstone, shale
   |-- Chemical   : rock salt, gypsum, chert
   `-- Organic    : coal, coral limestone