Sedimentary Rock Classification and Formation

Sedimentary Rock Formation and Diagenesis

Sedimentary rocks are solid materials deposited from a fluid; the sediment originates from the weathering of surface rocks. Diagenesis refers to the set of chemical, physical, and biological changes that occur after deposition. During this process, recrystallization and lithification (compaction and cementation) take place.

Detrital Sedimentary Rocks

The primary constituents of these rocks are clay minerals and quartz clasts, with sedimentation occurring in a solid state.

Shales

Shales are rocks composed of 75% clay and silt-sized particles. They represent 50% of all sedimentary rocks and are typically found in lakes, floodplains, and rivers. From a geotechnical perspective, they are somewhat weak and poorly cemented because they are not fully lithified. They exhibit fissility, which means the clays are able to separate into thin layers along parallel planes.

Sandstone

Sandstone consists of sand-sized particles. Its characteristics include:

• Selection: The variety of different grain sizes present.
• Roundness: Greater transport distance results in greater roundness.
• Composition: Includes feldspar, dark silicates, and quartz.

Mineral Classification of Sandstone

1. Quartzarenite: Composed of 95% quartz grains.
2. Arkose: Contains 25–100% feldspars.
3. Graywacke: Grains with a 15% matrix.

Rudites: Clusters and Gaps

These rocks have a gravel-sized particle profile and are poorly selected because the gaps between clasts are filled with sand, gravel, silt, or clay. Angular clasts (breccias/gaps) and rounded clasts (conglomerates) occur in fluvial and coastal environments or due to slope movements.

Chemical Sedimentary Rocks

These rocks result from the sedimentation of dissolved material being transported.

Limestones

Limestone is the most abundant chemical rock, formed by calcite through biochemical processes. The structure of carbonate rocks is made up of grains, matrix, and cement.

• Travertine: A limestone formed by rapid precipitation; it is a porous, light, and soft-colored rock containing plant impressions and thin laminations. Deposits from ambient temperature waters are called tufa, while those from thermal waters are called travertine.
• Dolomite: Composed of the mineral dolomite; it occurs when magnesium (Mg) in the water replaces part of the calcium (Ca) in the limestone.

Evaporites, Siliceous Rocks, and Coal

• Evaporites: Precipitation produced by the evaporation of water in which minerals are dissolved. The most frequent are gypsum and halite.
• Siliceous Rocks: Very compact and hard rocks composed of silica, such as flint, jasper, and agate.
• Carbon (Coal): Formed by the burial of plant material over millions of years. The stages include:
  • Peat: Partial decomposition in swamps.
  • Lignite: Shallow burial.
  • Bituminous Coal (Hull): Lower proportion of water and volatiles.
  • Anthracite: A black metamorphic rock that is bright and hard.

Sedimentary Facies and Ripples

Facies

A facies is a set of characteristics of a sediment that reflect the conditions of a particular environment or the specific materials involved.

Ripples

Ripples are small sand undulations that form on the surface of a layer of sediment by the action of water or air.