geology 3120 - sedimentary...
TRANSCRIPT
Geology 3120 Geology 3120 -- Sedimentary Structures Sedimentary Structures
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
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 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
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
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
WhicWhich h
way way
is is up?up?
Today Option 1Overturnedsyncline
Option 2“up side down”overturnedanticline
CrossCross--beddingbedding
Checkerboard Mesa, Zion National Park
Y
Older
YoungerFlow direction
Concave up
Graded beddingGraded bedding
Y
Older
Younger
• Decrease in depositional energy with sedimentation
• Example: flood deposits, turbidity currents
Graded Graded
beddingbedding
• Decrease in depositional energy with sedimentation
• Example: flood deposits, turbidity currents
Younger
Y
Older
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)
Ripple marksRipple marks
• Symmetric ripples indicate
bi-modal current
•Concave = up
• Asymmetric ripples indicate
unidirectional current
Y
Mud cracksMud cracks
5 cm5 cm
• Mud cracks
• Desiccation of muddy sediments
RaindropsRaindrops
Y
• Limited to terrestrial sediments
FootprintsFootprints
Y
• Limited to terrestrial sediments
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
GeopetalGeopetal StructuresStructures
• A “natural” carpenter’s level
Shell or cavity in the rock Matrix
Infill material (I.e., calcite)
Y
BioturbationBioturbationY
• Habitation burrows
• Feeding burrows
• Movement
StromatolitesStromatolites
Y
• Sharks Bay, Australia
• Cyanobacteria grow upward toward the surface
Flame structuresFlame structures
Y
• Less dense material intrudes into material above • Caused by rapid loading of turbidite sands
Pillow lavaPillow lavaY
Upper curved surface
“V “ notch
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