Structures in the Field and Contact Relationships

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Structures in the Field and Contact Relationships
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Outline

• Stratigraphic contacts
• Primary Sedimentary Structures
• Intrusive contacts
• Fault Contacts
• Folds

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Stratigraphic SuccessionBasalt Flows- Snake
River Plain
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Stratigraphic SuccessionSedimentary Rocks -
Grand Canyon
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Stratigraphy Laws

• Law of Superposition
• Law of Original Horizontality
• Law of Cross-cutting relationships

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Younger
Older
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Basic QuestionIs the section conformable?

• Continuous deposition?
• No missing time?

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3 types of unconformities

• Disconformity
• Nonconformity
• Angular unconformity

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Unconformities
Disconformity
Angular Unconformity
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Tapeats Sandstone (500 million years)
Cardenas Basalt (1.1 billion years)
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Angular unconformityDeath Valley
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Nonconformity
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Non-conformity Grand Canyon
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• Deposition of sediment
• Folding
• Erosion
• Deposition of sediment on top

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Is the sedimentary succession right-side up?

• Look for
• facing
• indicators

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Primary StructuresPillow Basalts
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Pillow Basalt, Chiricahua Mountains, Arizona
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Sedimentary StructuresStromatolites, Belt Gp., MT
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Graded beds
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Sharp-based debris-flow deposit, Basomari Fm.
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Sedimentary StructuresAncient Mudcracks
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Mud cracks
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Ripples
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Symmetrical oscillation ripples, Ferron Ss.
Member of Mancos Shale, Utah
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Overturned flat-topped oscillation ripples, Twist
Gulch Fm. (Jurassic), Utah
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Eolian foresets with truncated tops
(arrow),, concave-up morphology. Navajo Sandstone
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Cross bedded sandstones
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Load structures in sandstone of
Pleistocene Truckee River delta, near Sparks,
Nevada
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Calcite spar (former pore)
Micrite (former mud)
Brachiopods with geopetal structure, Martin Fm.
(Devonian),SE Arizona.
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Intrusive Contacts

• Dikes
• Sills
• Stocks/plutons
• Batholiths

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Dikes Planar intrusionsLaw of Cross-Cutting
Relationships
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Sills Horizontal planar intrusion
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Batholithslarge areas of intrusive rockroots
of an arc
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Fault Contacts

• Normal
• Thrust, reverse
• Strike-slip

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Thrust Fault
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Folds

• Syncline
• (concave up)
• Anticline
• (concave down)

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