Sedimentary Rocks: Formation, Types, and Composition
Sediments and Sedimentary Rocks
Denudation: A phenomenon in external geological processes where agents (wind, water, ice) destroy rocks near the surface with existing relief on Earth. Erosion leads to sedimentation basins where materials are deposited as sediment, ultimately forming sedimentary rocks.
Sediments and Sedimentary Processes
Sediment: Preexisting rock fragments of various sizes, loose minerals, crystals of substances dissolved in water and precipitated, and the remains of animals and plants.
- Detrital rocks: Sediments consisting of pieces of rocks.
- Biochemical rocks: Sediments that are remains of animals and plants or crystals of soluble salts.
The Transportation of Sedimentary Particles
Particles caused by erosion are transported to different sites by various means: water, air, and ice.
- Particle transport by water: Drag (fragments are pushed and pulled), saltation (pieces repeatedly collide with the bottom), suspension (particles travel within the water body), and solution (water-soluble particles are transported).
- Transport by wind: Drag, suspension, and saltation.
- Transport by ice: Movement on the surface.
Diagenesis
Diagenesis is a set of transformations that sediments deposited in basins undergo, turning them into sedimentary rocks. It comprises two main processes:
- Compaction: Loss of sediment volume, reducing pore size between fragments due to the weight exerted by upper sediment layers.
- Cementation: Union of sediment fragments. Main cements include limestone, silica, and other minerals.
Clay matrix: When sediments are weakly united by a group of microscopic mineral clays.
Plot: The set of particles that make up the bulk of the rock. Matrix: Composed of all finer particles deposited in the pores of the plot. Cement: Material crystallized in the pores of the plot.
Classification of Sedimentary Rocks
Detrital: Formed by fragments of other rocks or minerals eroded, transported, and deposited in a sedimentation basin. They are classified according to fragment size.
- Rudite: Larger than 2mm clasts. Named breccia if sharp and conglomerate if rounded.
- Arenites: Size between 2mm and 0.06mm. Main varieties are orthoquartzites (mostly quartz cement and silica) and molasse (limestone and quartz cement).
- Shales: Composed of microscopic-sized minerals.
Non-detrital: Sediments composed of chemical remains and remains of living organisms. Main groups include:
- Carbonate rocks: Formed by crystallization of carbonates in aquatic sedimentary basins or accumulation of debris formed by calcite (CaCO3) and dolomite (CaMg(CO3)2). If the percentage of calcite or dolomite is greater than 90%, it is called limestone or dolostone, respectively. If less, dolomitic limestone or calcareous dolostone. Equilibrium reaction: CaCO3 + CO2 + H2O = Ca(HCO3)2.
- Chemical carbonate rocks: Travertine (limestone forming stalactites and stalagmites), pisolitic or oolitic limestone (calcite crystallized around sand grains forming small spheres), micritic limestones (microscopic calcite crystals), marl (mixture of limestone and clay), dolomite (rocks similar to limestone, with calcite partly replaced by dolomite).
- Organic carbonate rocks: Reefal limestone (accumulation of reef-building corals), shelly limestone (mollusk shells), calcareous foraminifera (accumulation of shells).
- Silicate rocks: Sedimentary rocks consisting of silica detritus not originating from organisms with skeletons.
- Evaporites: Formed by water evaporation. Their deposition often precedes carbonate rock formation. Main types: evaporitic limestone, gypsum, anhydrite, rock chlorides.
- Coals: Dark and light rocks, fuels originating from the accumulation of plant remains.
- Peat: More modern coal. Over time, organic matter undergoes transformations compressed by overlying clay layers. An oxygen-deprived environment allows anaerobic bacteria to slowly transform organic matter, enriching it in carbon.
- Lignite: Made up of primitive spermatophytes.
- Anthracite: Coal accumulating fern. Older coal has higher carbon content, density, compactness, and heat capacity. Peat (55% carbon, 5000-6000 kcal/kg), brown coal (70%, 6000-7000 kcal/kg), coal (80%, +8000 kcal/kg), anthracite (90%, +8000 kcal/kg).
- Oil: A mixture of liquid and solid gaseous hydrocarbon compounds, dark in color. It is produced from the metamorphosis of sediment accumulated in enclosed seas and buried. High pressure forms a carbon-rich compound, which with silt forms a sapropel. Protopetroleum migrates from the source rock to porous rocks (reservoir rocks) unless it encounters an impermeable rock (oil trap). Structures trapping oil include anticlines, diapirs, lateral zones of saline, porous strata wedges, and faults affecting the reservoir rock.