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Geology 3120 - Geology 3120 - Unconformities and ContactsUnconformities and Contacts
OutlineOutline
• The “contact” that started it all…
• Steno, Hutton, and Lyell
• Conformities and Unconformities
• The Grand Canyon and Contact Corner
• Intrusive contacts
• Fault contacts
• Shear zone contacts
• Exercise - timing of geologic events
The “Contact”The “Contact”
• Siccar Point, Berwickshire, Scotland
• James Hutton (1788)
• Vertical Silurian rocks overlain by Devonian Old Red Sandstone
Steno’s PrinciplesSteno’s Principles
• Nicolaus Steno (1669)
• Principle of Original Horizontality - sediments are deposited in horizontal layers
• Principle of Superposition - the highest material in a vertical section is the youngest
• Principle of Original Continuity - sediments are deposited in laterally continuous layers
• Also consider cross-cutting relationships - if Rock A crosscuts Rock B, then Rock A is younger than Rock B
BB AA
Hutton and LyellHutton and Lyell
• 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
• Based on observations by James Hutton (1785)
• Rates of geological processes are not necessarily constant
• Catastrophic processes may occur
Conformable contacts and Conformable contacts and
UnconformitiesUnconformities
NonconformityAngular UnconformityDisconformity
Conformity
The Grand CanyonThe Grand Canyon
• Paleozoic sedimentary rocks (248-540 Ma)
• Angular unconformity
• Proterozoic sedimentary rocks (800-1250 Ma)
• Nonconformity
• Vishnu Group (1400-1500 Ma)
The Grand Canyon UnconformitiesThe Grand Canyon Unconformities
Contact Corner (disconformity)Contact Corner (disconformity)
Boulder Creek
Granodiorite
(1700 Ma)
Fountain Formation
(320-300 Ma)
Silver Plume
Dikes
(1400 Ma)
Intrusive ContactsIntrusive Contacts
Igneous - plutons, dikesIgneous - plutons, dikes
Intrusive ContactsIntrusive Contacts
Sedimentary - salt structures & diapirsSedimentary - salt structures & diapirs
Fault ContactsFault Contacts
• Hurricane normal fault, Utah and Arizona (western boundary of Colorado Plateau)
• Slickenline lineations (probably quartz growth) - see cross-section of growth below:
1.
2.
3.
QTZ
Shear Zone ContactsShear Zone Contacts
Exercise - timing of geologic Exercise - timing of geologic
events 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.
15 Ma Dike
Normal Fault
Thrust Fault
Fold
80 Ma
Fold
70 Ma
12 Ma Dike
60 Ma
60 Ma
50 Ma
70 Ma
70 Ma80 Ma70 Ma
20 Ma
Layer G
Layer B
22Ma
Block model for exerciseBlock model for exercise
Geologic HistoryGeologic History
• 12 Ma dike12 Ma dike
• 15 Ma dike15 Ma dike
• Normal faultNormal fault
• 20 Ma sed20 Ma sed
• 22 Ma sed22 Ma sed
• ErosionErosion
• Thrust faultThrust fault
• FoldingFolding
• Layer BLayer B
• 60 Ma sed60 Ma sed
• Layer GLayer G
• 70 Ma sed70 Ma sed
• 80 Ma sed80 Ma sed
15 Ma Dike
Normal Fault
Thrust Fault
Fold
80 Ma
Fold
70 Ma
12 Ma Dike
60 Ma
60 Ma
50 Ma
70 Ma
70 Ma80 Ma70 Ma
20 Ma
Layer G
Layer B
22Ma
ReferencesReferences
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.