geology 3120 - sedimentary...
TRANSCRIPT
![Page 1: Geology 3120 - Sedimentary Structuresgeode.colorado.edu/~structure/teaching_GEOL3120/coursenotes/3120... · Outline • Review the geologic history exercise from last time • Contacts,](https://reader035.vdocuments.us/reader035/viewer/2022062413/5a7a77f37f8b9a2d788baed0/html5/thumbnails/1.jpg)
Geology 3120 Geology 3120 -- Sedimentary Structures Sedimentary Structures
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OutlineOutline
• Review the geologic history exercise from last time
• Contacts, primary structures, and secondary structures
• How to determine which “way is up”…
• Cross- bedding, graded- bedding, reverse graded- bedding
• Determining way up using top surface features - ripples, mudcracks, raindrops, footprints
• Determining way up using bottom surface features - load casts
• Determining way up using features within rocks -geopetal, bioturbation, stromatolites, flame structures, pillow lavas
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Block model for exerciseBlock model for exercise
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Geologic HistoryGeologic History
•• 12 Ma dike12 Ma dike•• 15 Ma dike15 Ma dike•• Normal faultNormal fault•• 20 Ma 20 Ma sedsed•• 22 Ma 22 Ma sedsed•• ErosionErosion•• Thrust faultThrust fault•• FoldingFolding•• Layer BLayer B•• 60 Ma 60 Ma sedsed•• Layer GLayer G•• 70 Ma 70 Ma sedsed•• 80 Ma 80 Ma sedsed
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Contacts, Primary Structures & Secondary StructuresContacts, Primary Structures & Secondary Structures
• Contact - a boundary between rock units• Primary structure - structures that form during lithification• Secondary structure - structures that form after lithificaiton
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Contacts, Primary Structures & Secondary StructuresContacts, Primary Structures & Secondary Structures
• Contact - a boundary between rock units• Primary structure - structures that form during lithification• Secondary structure - structures that form after lithificaiton
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WhicWhich h
way way
is is up?up?
Today Option 1Overturnedsyncline
Option 2“up side down”overturnedanticline
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CrossCross--beddingbedding
Checkerboard Mesa, Zion National Park
Y
Older
YoungerFlow direction
Concave up
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Graded beddingGraded bedding
Y
Older
Younger
• Decrease in depositional energy with sedimentation
• Example: flood deposits, turbidity currents
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Graded Graded
beddingbedding
• Decrease in depositional energy with sedimentation
• Example: flood deposits, turbidity currents
Younger
Y
Older
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Reverse (Inverse) Graded BeddingReverse (Inverse) Graded Bedding
Y
Older
Younger
• Increase in depositional energy with sedimentation
• Example: debris flows (a lot less common than normal graded beds)
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Ripple marksRipple marks
• Symmetric ripples indicate
bi-modal current
•Concave = up
• Asymmetric ripples indicate
unidirectional current
Y
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Mud cracksMud cracks
5 cm5 cm
• Mud cracks
• Desiccation of muddy sediments
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RaindropsRaindrops
Y
• Limited to terrestrial sediments
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FootprintsFootprints
Y
• Limited to terrestrial sediments
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Load castsLoad casts
• Protrusion of material into a layer below
• Load casts indicate the base of a layer, notthe top of a layer
• Determining the current direction may be possible
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GeopetalGeopetal StructuresStructures
• A “natural” carpenter’s level
Shell or cavity in the rock Matrix
Infill material (I.e., calcite)
Y
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BioturbationBioturbationY
• Habitation burrows
• Feeding burrows
• Movement
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StromatolitesStromatolites
Y
• Sharks Bay, Australia
• Cyanobacteria grow upward toward the surface
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Flame structuresFlame structures
Y
• Less dense material intrudes into material above • Caused by rapid loading of turbidite sands
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Pillow lavaPillow lavaY
Upper curved surface
“V “ notch
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ReferencesReferences
Slide 8http://www.utahpictures.com/Checkerboard.html
Slides 9-15, 17Busch, 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 18http://www.discoverwest.com.au/hablin.html
Slide 20http://volcanoes.usgs.gov/Products/Pglossary/PillowLava.html
http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html