Texture of sedimentary rocks

Texture is the size, shape and arrangement of grains and their relationship in a rock. Sedimentary textures are of two main types.

1. Clastic (fragmental) texture

Rock is made up of broken grains cemented together. Features:

• Grain size: gravel (> 2 mm), sand (2 to 1/16 mm), silt and clay (< 1/16 mm).
• Grain shape: angular (little transport, breccia) to rounded (long transport, conglomerate); sphericity.
• Sorting: well sorted (similar size) or poorly sorted (mixed sizes).
• Packing and fabric: arrangement of grains and pore space.
• Cement and matrix: silica, calcite or iron oxide cement; fine matrix between grains.

2. Non-clastic texture

• Crystalline: interlocking crystals from chemical precipitation (rock salt, gypsum, chemical limestone).
• Oolitic: small spherical grains (ooliths) in limestone.
• Bioclastic / organic: shell fragments, fossils (coquina, coral limestone).
• Amorphous or cryptocrystalline: chert.

The grain size, sorting and cement control the strength and porosity of the rock.