Primary structures and facing directions

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