Lecture Notes: Unconformities in Geology

Introduction to Unconformities

• Unconformities indicate missing time in the geologic record.
  • Two ways missing time occurs:
    • No deposition: No material was deposited; a lack of erosion and weathering.
    • Erosion: Materials were removed from the geologic record.
• Types of Unconformities:
  • Disconformity
  • Non-Conformity
  • Angular Unconformity
• General Tip: If unsure about specific type, refer to it as an "unconformity."

Types of Unconformities

1. Disconformity

• Occurs between two horizontal sedimentary layers.
• Presence of an erosion or non-deposition surface.
• Example:
  • Horizontal layers A, B, C.
  • Disconformity: Wavy erosional surface on top of C before layer E is deposited.
  • Identified by irregular, non-flat surfaces between sedimentary beds.

2. Non-Conformity

• Sedimentary beds overlay an unconformable surface of igneous or metamorphic "hard rocks."
• Example:
  • Hard rock below, sedimentary layers above.
  • The contact between the hard rock and sedimentary rock is the non-conformity.

3. Angular Unconformity

• Flat sedimentary rocks overlie an unconformable surface with angled layers beneath.
• Formation due to faults, folds, tectonic activities, followed by deposition of flat layers.
• Example:
  • Vertical or angled layers below, horizontal layers above.

Video and Practical Assignment

• Video:
  • Explains sediment accumulation, tectonism, volcanism over 100 million years.
  • Highlights processes such as erosion, deposition, and volcanic activity.
• Assignment:
  • Watch video to understand depositional history.
  • Tasks will involve sequencing and identifying geologic layers.

Geologic Sequencing and Relative Ages

• Understanding relative ages:
  • Inclusions are older than the surrounding rock.
  • Faults that cut through layers are younger than those layers.
• Example of sequence: Layers A, B, C, D, E, etc.
  • Includes understanding of intrusions and dikes as younger features.

Application

• When hiking, observe geological features:
  • Look for disconformities, non-conformities, angular unconformities.
  • Apply principles of dikes, sills, and superposition.

Conclusion

• Review definitions of unconformities.
• Apply knowledge to assignments and real-world observations.
• End of lecture; further discussion will continue in next session.