Give a generalized classification of sandstone. State genetic significance of such classification.

Generalized Classification of Sandstone

Sandstone classification is complex, often employing multiple parameters. A widely accepted scheme considers composition, texture, and structural features.

1. Compositional Classification (Based on Mineralogy)

This is arguably the most important classification as it reflects the source rock weathering and transport history. The classification, largely based on the work of Pettijohn (1957), categorizes sandstones based on the dominant framework grains:

• Quartz Arenite: >90% quartz. Indicates a stable source area with extensive weathering, removing less resistant minerals. Often associated with mature sedimentary basins.
• Arkose: >25% feldspar. Suggests a nearby granitic or metamorphic source area with rapid erosion and limited weathering. Feldspar is relatively unstable and easily weathered.
• Lithic Arenite (or Litharenite): >25% rock fragments. Indicates a source area composed of diverse rock types, often with significant tectonic activity and less chemical weathering.
• Greywacke: A specific type of lithic arenite characterized by a matrix-rich composition (more than 15% matrix) and poorly sorted grains. Suggests rapid deposition from turbidity currents in deep-sea environments.

2. Textural Classification (Based on Grain Size, Sorting, and Roundness)

Textural characteristics reveal information about the transport and depositional processes.

• Grain Size: Classified using the Wentworth scale (e.g., fine-grained, medium-grained, coarse-grained). Grain size is directly related to the energy of the transporting medium.
• Sorting: Describes the uniformity of grain size.
  • Well-sorted: Grains are similar in size, indicating prolonged transport and winnowing.
  • Poorly-sorted: Grains vary significantly in size, suggesting short transport distances or deposition from high-energy events.
• Roundness: Describes the degree of grain abrasion.
  • Angular: Grains have sharp edges, indicating minimal transport.
  • Rounded: Grains have smooth edges, indicating extensive transport and abrasion.

3. Structural Classification (Based on Internal Features)

Internal structures provide insights into depositional processes and post-depositional modifications.

• Bedding: Layering within the sandstone. Types include cross-bedding (indicating dune or current migration), planar bedding (indicating quiet water deposition), and ripple marks (indicating shallow water currents).
• Lamination: Very fine-scale layering. Often associated with turbidity currents or tidal environments.
• Concretions: Rounded masses of precipitated minerals within the sandstone, indicating diagenetic processes.
• Mud Cracks: Indicate alternating wet and dry conditions, common in tidal flat or desert environments.

Genetic Significance of Classification

The classification of sandstone is not merely descriptive; it’s fundamentally linked to understanding its origin.

Classification Feature	Genetic Implication
Quartz Arenite	Mature sedimentary basin, extensive weathering, stable tectonic setting, long transport distances.
Arkose	Proximity to granitic/metamorphic source, rapid erosion, limited weathering, active tectonics.
Lithic Arenite	Diverse source rocks, active tectonics, moderate weathering, moderate transport distances.
Greywacke	Deep-sea deposition, turbidity currents, rapid sedimentation, unstable tectonic setting.
Well-sorted, Rounded Grains	Prolonged transport, high-energy environment, mature sedimentary system.
Poorly-sorted, Angular Grains	Short transport distance, low-energy environment, immature sedimentary system.
Cross-bedding	Dune or current migration, indicating a directional flow.

By combining these classifications, geologists can build a comprehensive picture of the sandstone’s provenance, transport pathway, depositional environment, and post-depositional history. For example, a poorly sorted, angular lithic arenite with cross-bedding suggests deposition from a debris flow near a tectonically active mountain range.