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Some Basic Stratigraphic Principles

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Presentation on theme: "Some Basic Stratigraphic Principles"— Presentation transcript:

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

7

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

18

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


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