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Some Basic Stratigraphic Principles
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Principle of Original Horizontality
strata are originally laid down in a horizontal orientation (some exceptions) - See text fig. 19.3
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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)
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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
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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
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Conformable contact
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Gradational, conformable contact
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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
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Paraconformity – juxtaposition of unrelated facies
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From Davis, 1982. Depositional Systems. Prentice-Hall.
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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
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Angular unconformity – approx. 160 My time gap
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Angular unconformity – approx. 1 My time gap
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Unconformities Non-conformity
Sedimentary strata overlying nonstratified rocks (metamorphic or intrusive igneous rocks) Erosional contact (sediments are younger than non-stratified rocks)
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Nonconformity – sedimentary rocks overlying unstratified rocks – intrusive igneous or metamorphic
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Nonconformity with clasts in overlying sediment
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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
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Intrusive contact (granite into sedimentary rocks) with altered contact zone
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Intrusive dyke cross-cutting volcanic strata and normal fault
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Sedimentary (clastic or neptunian) dyke (uncommon) – sediments from below intruded into overlying strata during an event of water pressure release.
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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
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Thrust (reverse) fault
Disconformity
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Slickensides Fault Breccia and mineralized veins Drag folds
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