Introduction to Sedimentary Rocks

Sedimentary Rocks Overview

• Comprise ~80% of Earth’s dry surface
• Formed at or near Earth’s surface through:
  • Lithification of loose clasts
  • Growth/lithification of shells or shell fragments
  • Accumulation and alteration of organic matter
  • Precipitation of minerals from solution

Classification of Sedimentary Rocks

• Clastic (detrital): Formed by lithifying loose clasts
• Biochemical: Formed by growth/lithification of shells or shell fragments
• Organic: Formed by accumulation and alteration of organic matter
• Chemical: Formed by precipitation of minerals from solution

Formation of Clastic Sedimentary Rocks

1. Weathering: Disintegration of bedrock
2. Erosion: Separation of clasts from bedrock by gravity, air, water, or ice
3. Transportation: Movement of clasts by gravity, wind, water, or ice
4. Deposition: Clasts settle out from transporting medium
5. Lithification: Transformation of sediment to rock
   • Compaction
   • Cementation (common cements: quartz, calcite)

Transportation and Particle Size

• Strong current: deposition of larger particles (boulders, pebbles)
• Moderate current: deposition of sand-sized particles
• Weak current: deposition of fine particles (silt, clay)
• Current strength sorts materials by grain size
  • Strong currents produce coarser sediment, weak currents produce finer sediment

Effects of Transportation

• As transportation distance increases:
  • Sediment becomes finer grained
  • Grains become smaller and rounder (e.g., Breccia → Shale)
  • Grains become better sorted by size
  • Coarse sediment is left behind as currents lose strength

Mineral Maturity

• As transportation distance increases, less-resistant minerals are removed, increasing the maturity of sedimentary rock.