Primary structures and facing directions
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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
Quick review of Huttonian principals
• Original horizontality
• Superposition
• Cross-cutting relationships
• Inclusion
What are primary structures
• Depositional, intrusive, or volcanic contacts• Sedimentary structures• Biogenic structures• Volcanic structures
Structures formed during deposition or crystallization
Contacts as primary structures• Depositional or volcanic, conformable
• Intrusive
• Depositional or volcanic, unconformableDisconformity, Angular unconformity, Nonconformity
Flat-lying sedimentary contacts
Intrusive contact
Intrusive contacts
Angular unconformity
Angular unconformity
Nonconformity
Nonconformity
Primary structures: “Way-up indicators”
• Sedimentary
• Biogenic
• Volcanic
Why is it important to know facing direction?
1. Can help interpret complicated structures from isolated outcrops
2. Can serve as a predictive tool while mapping
Cross beds: Note tangential contact at base and truncation at top
Ginormous eolian cross-beds
Ripples: Top of beds — can also have ripple casts
Mud cracks: Tops of beds — can also have casts
Graded beds: note sharp and gradual contacts
Graded beds