Clastic Sedimentary Rocks: Physical Geology Overview

How Clastic Sediments Become Sedimentary Rocks

• Lithification

  • Process of converting sediments into solid rock.
  • Compaction: First step, where deposited sediments are buried under more sediments, squeezing pore spaces and pushing clasts together.
  • Cementation: Second step, involves precipitation of minerals from groundwater in pore spaces, binding grains together.
  • Common cement minerals: Quartz, Calcite, possibly Hematite and Clay.

• Microscopic View

  • Sandstone under a microscope shows quartz grains and cement.
  • Grains appear in different shades under cross-polarized light.

Types of Clastic Sedimentary Rocks

• Named based on clast characteristics: grain size, shape, and sorting.

Coarse-Grained Clastic Rocks

• Conglomerate: Rounded clasts, form in high-energy environments (e.g. fast-flowing rivers).
• Breccia: Angular clasts, form in areas with minimal transport (e.g. alluvial fans).

Medium-Grained Clastic Rocks

• Sandstone: Common, organized by conditions of formation.
  • Arenite
    • Clean sandstone with less than 15% silt and clay.
    • Subtypes include Quartz Arenite, Arkosic Arenite (Arkose), and Lithic Arenite.
  • Wacke
    • Contains 15-75% fine particles, more matrix than cement.
    • Subtypes include Quartz Wacke, Feldspathic Wacke, and Lithic Wacke (Greywacke).

Fine-Grained Clastic Rocks

• Mudrock: Composed of 75% silt and clay.
  • Shale: Shows fine layers, breaks into sheets.
  • Siltstone, Mudstone, Claystone: Vary in silt and clay dominance.
• Form in low-energy environments (e.g. lakes, flood plains).

Sediment Maturity

• Refers to sediment characteristics: size, shape, sorting, resistance to weathering.
• Mature Sedimentary Rocks: Quartz Sandstone.
• Immature Rocks: Wacke, Conglomerate.

Practice and Concept Checks

• Exercises available for identifying sandstone types based on composition and characteristics.
• Interactive activities for understanding lithification steps and sedimentary rock classification.