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