Geology 3120 Unconformities and Contacts
Outline
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The “contact” that started it all…
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Steno, Hutton, and Lyell
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Conformities and Unconformities
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The Grand Canyon and Contact Corner
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Intrusive contacts
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Fault contacts
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Shear zone contacts
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Exercise - timing of geologic events
The “Contact”
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Siccar Point, Berwickshire, Scotland
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James Hutton (1788)
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Vertical Silurian rocks overlain by Devonian Old Red Sandstone
Steno’s Principles
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Nicolaus Steno (1669)
Principle of Original Horizontality - sediments are
deposited in horizontal layers
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Principle of Superposition - the highest material in a
vertical section is the youngest
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Principle of Original Continuity - sediments are deposited
in laterally continuous layers
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• Also
consider cross-cutting relationships - if Rock A
crosscuts Rock B, then Rock A is younger than Rock B
B
A
Hutton and Lyell
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Charles Lyell (1830)
Principle of Uniformitarianism - “the present is the key to
the past”, or that present processes can be used to infer past
processes
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Based on observations by James Hutton (1785)
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Rates of geological processes are not necessarily constant
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Catastrophic processes may occur
Conformable contacts and Unconformities
Conformity
Disconformity
Angular Unconformity Nonconformity
The Grand Canyon
Paleozoic sedimentary rocks
(248-540 Ma)
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• Angular
unconformity
Proterozoic sedimentary rocks
(800-1250 Ma)
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•
Nonconformity
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Vishnu Group (1400-1500 Ma)
The Grand Canyon Unconformities
Contact Corner (disconformity)
Boulder Creek
Granodiorite
(1700 Ma)
Silver Plume
Dikes
(1400 Ma)
Fountain Formation
(320-300 Ma)
Intrusive Contacts
Igneous - plutons, dikes
Intrusive Contacts
Sedimentary - salt structures & diapirs
Fault Contacts
Hurricane normal fault, Utah
and Arizona (western boundary
of Colorado Plateau)
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Slickenline lineations (probably
quartz growth) - see crosssection of growth below:
1.
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2.
3.
QTZ
Shear Zone Contacts
Exercise - timing of geologic events
The following exercise is designed to introduce you to
how sediments and intrusive rocks can be used to constrain
the timing of deposition, faulting, folding or other deformational
processes.
The figure is a block diagram of a sequence of rocks
that have undergone a period of folding, thrust faulting, normal
faulting and intermittent magmatic events (dike intrusion).
See if you can determine the geologic history of
deposition, intrusion, and deformation by examining the
contact relationships between the sediments, igneous rocks,
and faults.
Block model for exercise
15 Ma Dike
12 Ma Dike
Normal Fault
22Ma
20 Ma
Fold
60 Ma
Layer G
Fold
50 Ma
70 Ma
70 Ma
Layer B
80 Ma
60 Ma
80 Ma
70 Ma
Thrust Fault
70 Ma
Geologic History
15 Ma Dike
12 Ma Dike
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12 Ma dike
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15 Ma dike
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Normal Fault
22Ma
20 Ma
Fold
60 Ma
Layer G
Fold
50 Ma
70 Ma
70 Ma
Layer B
80 Ma
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20 Ma sed
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22 Ma sed
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Erosion
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Thrust fault
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Folding
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Layer B
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60 Ma sed
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Layer G
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70 Ma sed
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80 Ma sed
60 Ma
80 Ma
70 Ma
Thrust Fault
70 Ma
Normal fault
References
Slide 6
Busch, R. M. and D. Tasa, Laboratory Manual in Physical Geology, 3rd. Ed., American
Geological Institute and National Association of Geology Teachers, 260 p., 1990.
Slide 11
Twiss, R. J. and E. M. Moores, Structural Geology, W. H. Freeman & Co., New York, 532 p.,
1992.