Primary structures and facing directions

Goal: To begin to understand how to use primary structures 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 — Bedding probably most useful

• Sedimentary structures• Biogenic structures• Volcanic structures• Metamorphic features

Structures that form during deposition or crystallization of the rock

Why do we care

1. Serve as markers of deformation

2. Can often be used to determine facing direction or “Which way is up” or “Way up”

• Upright beds

• Overturned beds

Contacts as primary structures• Depositional or volcanic, conformable

• Intrusive

• Depositional or volcanic, unconformable• Disconformity• Angular unconformity• Nonconformity

Flat-lying sedimentary contacts

Intrusive contact

Intrusive contact

Angular unconformity

Angular unconformity

Nonconformity

Nonconformity

Primary structures: “Way-up indicators”

• Sedimentary

• Biogenic

• Volcanic

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

Ginormous eolian cross-beds

Cross-beds and erosional channel

Ripples: Top of beds — can also have ripple casts

Flute casts — On bottom of beds

Mud cracks: Tops of beds — can also have casts

Graded beds: note sharp and gradual contacts

Graded beds

Biogenic features

• Trace fossils

• Burrows and roots

• Stromatolites

Trace fossil: Footprints

Stromatolites

Volcanogenic features

• Pillow lavas

• Baked weathering surfaces

Pillow lavas

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