Sedimentary rocks form through a series of processes that shape Earth's surface. From weathering and erosion to transportation and deposition, these steps break down existing rocks and create new ones. The composition of sedimentary rocks tells a story of their origins and journey.

Diagenesis plays a crucial role in transforming loose sediments into solid rock. Through compaction, cementation, and other processes, sediments undergo physical and chemical changes. The resulting rocks provide valuable clues about past environments and Earth's history.

Sedimentary Rock Formation

Processes of sedimentary rock formation

• Weathering breaks down rocks and minerals at or near Earth's surface
  • Physical weathering mechanically disintegrates rocks without chemical changes (frost wedging, exfoliation, abrasion)
  • Chemical weathering decomposes rocks and minerals through chemical reactions (dissolution, oxidation, hydrolysis)
• Erosion removes and transports weathered material by agents such as water, wind, or ice
• Transportation moves eroded sediments from their source to a site of deposition
  • Agents of transportation include water, wind, ice, gravity
  • Particle size and shape influence transportation distance and method
• Deposition occurs when transported sediments settle and accumulate
  • Happens when the energy of the transporting medium decreases
  • Depositional environments include rivers, deltas, beaches, oceans, deserts, glaciers

Role of diagenesis in sediments

• Diagenesis transforms sediments into sedimentary rocks through physical, chemical, and biological changes after deposition
  • Compaction reduces pore space due to the weight of overlying sediments, increasing density and reducing permeability
  • Cementation binds sediment grains together by precipitating minerals (calcite, quartz) in pore spaces, creating a solid rock
  • Recrystallization changes mineral composition or crystal structure without melting, resulting in larger, more stable mineral grains
  • Dissolution removes soluble minerals by groundwater, creating secondary porosity

Factors in sedimentary rock composition

• Composition is determined by the source material and weathering processes
  • Mineralogy reflects the composition of the parent rock and the stability of minerals during weathering
  • Chemical composition is influenced by chemical weathering processes and depositional environment
• Texture is determined by the size, shape, and arrangement of sediment grains
  • Grain size is controlled by the source material, weathering, and transportation processes
    • Clastic sedimentary rocks are classified based on grain size (claystone, siltstone, sandstone, conglomerate)
  • Sorting refers to the degree of uniformity in grain size (well-sorted vs poorly-sorted sediments)
  • Rounding is the degree of smoothness of grain edges, influenced by the duration and distance of transportation
  • Packing is the arrangement and compaction of sediment grains, affecting porosity and permeability of the sedimentary rock

Types of sedimentary rocks

• Clastic sedimentary rocks form from the accumulation and lithification of weathered rock fragments (clasts)
  • Examples include sandstone, conglomerate, breccia, shale
  • Grain size and composition reflect the source material and transportation processes
• Chemical sedimentary rocks form by the precipitation of minerals from aqueous solutions
  • Evaporites form by the evaporation of water, leaving behind dissolved minerals (halite, gypsum)
  • Travertine and tufa form by the precipitation of calcite from groundwater or surface water, often associated with hot springs or caves
• Biochemical sedimentary rocks form by the accumulation and lithification of organic material or by the precipitation of minerals mediated by biological activity
  • Organic-rich rocks form from the accumulation and preservation of organic matter (coal, oil shale)
  • Biogenic rocks form by the precipitation of minerals by organisms (limestone from calcareous algae, coral, and shell fragments; chert from siliceous plankton)