1. Some Basic Stratigraphic Principles

2. Principle of Original Horizontality
strata are originally laid down in a horizontal orientation (some exceptions)
- See text fig. 19.3

3. Principal of Superposition
in any sequence of strata not severely deformed, those strata that are higher in the succession are younger than those below (younger = more recently deposited)

4. Stratigraphic “way up”
The original orientation of the strata
determined from:
stratigraphic relationships – see text figs
sedimentary structures – see text fig. 19.7
successions of fossils

5. Contacts Between Rock Units
Stratigraphic Contacts (contacts between strata)
Concordant contacts – strata above an below contact are parallel – text fig. 19.3
Conformable contact (conformity) – concordant contact with no discernable break in sedimentation (no time gap)
may be sharp or gradational

6. Conformable contact

8. Gradational, conformable contact

9. Unconformities – contacts with significant time gap
Paraconformity – concordant contact with no physical evidence of erosion but evidence of a time gap (hiatus)
Missing fossil zone
Juxtaposition of unrelated facies
Disconformity – concordant contact with significant evidence of erosion as well as a hiatus (text-fig. 19.4b)
Clasts of underlying strata in overlying beds
Erosional downcutting

10. Paraconformity – juxtaposition of unrelated facies

11. From Davis, 1982. Depositional Systems. Prentice-Hall.

12. Unconformities Angular unconformity – non-concordant (text-fig. 19.4a)
Strata below oriented at a different angle from strata above
strata below are erosionally truncated by unconformity surface

13. Angular unconformity – approx. 160 My time gap

14. Angular unconformity – approx. 1 My time gap

15. Unconformities Non-conformity
Sedimentary strata overlying nonstratified rocks (metamorphic or intrusive igneous rocks)
Erosional contact (sediments are younger than non-stratified rocks)

16. Nonconformity – sedimentary rocks overlying unstratified rocks – intrusive igneous or metamorphic

17. Nonconformity with clasts in overlying sediment

19. Other types of contacts
Intrusive contact – younger rocks (usually intrusive igneous) cut through older rocks by intrusion – cross-cutting relationship (text-fig. 19.6a)
May show evidence of alteration (“baking”) in contact zone
May be fragments of host rock incorporated into intrusion

20. Intrusive contact (granite into sedimentary rocks) with altered contact zone

21. Intrusive dyke cross-cutting volcanic strata and normal fault

22. Sedimentary (clastic or neptunian) dyke (uncommon) – sediments from below intruded into overlying strata during an event of water pressure release.

23. Other types of contacts
Fault contact – two rock units juxtaposed by faulting
May or may not show cross-cutting of either or both units
Commonly shows brecciation, deformation in fault zone
May be striations (slickensides) along fault plane

24. Thrust (reverse) fault
Disconformity

25. Slickensides
Fault Breccia and mineralized veins
Drag folds