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Understanding Primary Structures and Facing Directions in Geology

Review different geologic contacts as markers of deformation. Learn types of primary structures like depositional, intrusive, and volcanic contacts. Recognize the importance of facing directions for interpreting structures. Observe features such as cross beds, ripples, mud cracks, and graded beds.

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Understanding Primary Structures and Facing Directions in Geology

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  1. Primary structures and facing directions Goal: To review and recognize different types of geologic contacts so that we can use them as markers of deformation

  2. Quick review of Huttonian principals • Original horizontality • Superposition • Cross-cutting relationships • Inclusion

  3. What are primary structures Structures formed during deposition or crystallization • Depositional, intrusive, or volcanic contacts • Sedimentary structures • Biogenic structures • Volcanic structures

  4. Contacts as primary structures • Depositional or volcanic, conformable • Intrusive • Depositional or volcanic, unconformable Disconformity, Angular unconformity, Nonconformity

  5. Flat-lying sedimentary contacts

  6. Intrusive contact

  7. Intrusive contacts

  8. Angular unconformity

  9. Angular unconformity

  10. Nonconformity

  11. Nonconformity

  12. Primary structures: “Way-up indicators” • Sedimentary • Biogenic • Volcanic

  13. Why is it important to know facing direction? • Can help interpret complicated structures from isolated outcrops • Can serve as a predictive tool while mapping

  14. Cross beds: Note tangential contact at base and truncation at top

  15. Ginormous eolian cross-beds

  16. Ripples: Top of beds — can also have ripple casts

  17. Mud cracks: Tops of beds — can also have casts

  18. Graded beds: note sharp and gradual contacts

  19. Graded beds

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