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Geol341-Structural Geology
Map-Scale Structures
and Contact
Relationships
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Last Lecture
• The downward pull of subducting slabs drives plate motions
• All plate motions are rotations
• 3 kinds of plate boundaries create 3 different structural environments
• Gravity causes deformation at passive margins
• Continents grow by accretion at their edges
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What type of plate boundary is
there along the US East coast?
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Continental Growth
• Material plastered onto the continents at
convergent boundaries
• Example:
– Everything west of Nevada has been added to
North America in last 700 Ma
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Continental Growth
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Today’s Outline
• Stratigraphic contacts
– Unconformities
• Primary Sedimentary Structures
– Younging direction
• Intrusive contacts
– Sills and dikes
– Batholiths
• Fault Contacts
– Normal, thrust, strike-slip
• Folds
– Anticlines, synclines
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1. Superposition: younger layers on top
Red layers deposited over
tan
Tan sediment deposited
over older rock
Third layer is
youngest and is
on top
Young
Old
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Stratigraphic Succession
Basalt Flows- Snake River Plain
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Horizontality of Sedimentary
Successions
North Slope, AK
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Horizontality of Sedimentary
Successions
North Slope, AK
Without vertical exaggeration
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Basic Question:
Is the section conformable?
• Is the sedimentary pile continuous?
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Unconformities
Disconformity
Angular Unconformity
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Angular unconformity
Death Valley
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The Great Unconformity
Grand Canyon, AZ
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1.1-1.2 Ga
1.7 Ga
0.5 Ga
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Angular Unconformity
In seismic data
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Onlap Surface
North Slope, AK
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Fossils & disconformities
Correlation = determine equivalent units
I
H
G
F
E
D
C
B
A
I
H
C
B
A
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Is the sedimentary succession
right-side up?
• How can
we tell?
Which is older. A or B?
AB
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Is the sedimentary succession
right-side up?
• How can
we tell?
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Primary Structures
Pillow Basalts Up
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Sedimentary StructuresStromatolites, Belt Gp., MT
Up
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Cross bedded sandstones
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Intrusive Contacts
They cut older rocks
• Dikes
• Sills
• Stocks
• Batholiths
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Coarse-Grained
Fine-Grained
Intrusive Structures
Dikes
Sills
Stock or
batholith
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Shiprock, New Mexico
Stock
Dike
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Sierra
Nevada
Batholith
Yosemite N.P.
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Cross Cutting RelationshipsYounger intrusive cuts through older folds
Amato photo, Burro Mts, NM
1.6 Ga Amphibolite
1.4 Ga
Dikes
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Geologic history of an area
based on contact relationships
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Orientation of a Plane
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Salisbury Crags, Edinburgh, UK,
BGS photo
Jointed gabbro sill
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Strike and dip on a map
North
34
38
28
30
Blue Fm.
Grey Fm.
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Angular
Unconformity
Map View
Alabama
uK1 =Upper
Cretaceous
IP1 =Permian
20
22
60
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Visualizing
the 3D
Structure6
Cross Section
W E
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Folds
• Syncline
– (concave up)
• Anticline
– (concave down)
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Trend and Plunge
• Used for linear features
• Trend = Azimuth of a line
• Plunge = Inclination of a line
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Does the arrow point at a syncline or anticline?
A. Syncline
B. Anticline
C. Neither
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Sheep Mountain Anticline, WY
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Fault Contacts
• Normal
• Thrust, reverse
• Strike-slip
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Faults in Map View
Teeth Point down dip
Pen
Trias
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Rose Diagram
Bins # Measurements
0-9 0
10-19 2
20-29 5
…
100-109 3
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Rose Diagram Examples
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Software
• Stereonet (free)
• Openstereo (free)
• StereoNet (free)
• Visible Geology (on line app- watch the
tutorial)
• Lambert (iphone app)
• Rockworks (commercial)
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How the compass works
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Magnetic Declination
True
North
Magnetic
North
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USGS Topographic Maps
MN- Magnetic North
- True North
GN- Geomagnetic North
Morgantown Quadrangle
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http://www.ngdc.noaa.gov/geomag-web/#declination
Morgantown Declination
9o West
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Adjusting Your Compass
Front Back
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Key concepts• Stratigraphic contacts
– Unconformities
• Primary Sedimentary Structures– Younging direction, right-side-up?
• Intrusive contacts– Sills and dikes
– Batholiths
– Cross cutting relationships
• Interpreting sequence of events
• Folds– Anticlines, synclines
• Fault Contacts– Normal, thrust, strike-slip