Sedimentary Structures Lecture Notes

Introduction

• Focus on sedimentary structures within sedimentology.
• Part of the sixth lesson in a playlist.

Key Topics Discussed

• Stratification and Lamination
• Cross Bedding (Planar vs Trough, Herringbone)
• Ripples vs Dunes
• Graded Bedding (Normal and Reverse)
• Growth Bedding

Beds, Strata, and Laminations

• Beds/Strata: Geological layers of rock.
• Laminations: Defined as less than one centimeter in thickness.
  • Beds: Can be thicker.
    • Thick beds: >60 cm
    • Thin beds: 50-60 cm
    • Very thin beds: 1-5 cm
• Beds and strata often synonymous, but "beds" may refer to larger units.
• Regularity of Bedding: Can vary in uniformity and lateral continuity.
• Discontinuous Beds: Example includes lenses formed in specific depositional settings.

Depositional Environments

• Planar Laminations: Characteristic of fine-grained sediments (e.g., silt, shale).
  • Indicative of calm, deep water settings (e.g., lacustrine or pelagic marine).
  • Wave Base: Depth below which sediment isn't moved by waves.
• Varves: Seasonal variations (e.g., ice-proximal lakes).
  • Summer: Coarser sediment brought by melting ice.
  • Winter: Fine sediments settle.
  • Associated with glacial environments.
  • Tidal Rhythmites: Alternating seasonal laminations, rare but indicative of spring and neap tides.

Cross Bedding

• Formation: Migration of ripples or dunes by wind or water.
• Types:
  • Planar Cross Bedding: Formed by linear ripples/dunes.
  • Trough Cross Bedding: Formed by curvy or tongue-shaped ripples/dunes.
• Herringbone Cross Bedding: Alternating directions, common in tidal environments.
• Differences between Ripples and Dunes:
  • Scale: Dunes >0.5 m, Ripples 1-2 cm.
  • Forbidden Wavelength Gap: Stable forms don't exist between these sizes.

Graded Bedding

• Normal Grading: Coarse to fine, common in turbidity currents.
• Reverse Grading: Fine to coarse, less common.
• Turbidite Sequences: Boma sequence, predictable sedimentary sequence.

Growth Bedding

• Chemical vs Detrital
  • Chemical: Precipitation of minerals from solution (e.g., evaporite lakes).
  • Detrital: Transported sediments deposited and lithified.

Mud Cracks

• Form from drying and shrinking of mud.
• Indicate an exposed environment (not underwater).
• Preservation: Often as casts filled by sand.

References

• Principles of Sedimentology and Stratigraphy by Sam Boggs.
• Sedimentary Rocks by F.J. Pettijohn.

Additional Resources

• Videos and playlists are available on the channel for further exploration into topics such as hydrocarbon preservation, depositional environments, and more.