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.