Geology 3120 - Sedimentary Structures
Outline
•
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 exercise
15 Ma Dik e
12 Ma Dik e
Nor m al 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
Thr us t Fault
70 Ma
Geologic History
15 Ma Dik e
12 Ma Dik e
•
12 Ma dike
•
15 Ma dike
•
Normal fault
Nor m al 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
Thr us t Fault
70 Ma
•
20 Ma sed
•
22 Ma sed
•
Erosion
•
Thrust fault
•
Folding
•
Layer B
•
60 Ma sed
•
Layer G
•
70 Ma sed
•
80 Ma sed
Contacts, 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 Structures
•
Contact - a boundary between rock units
•
Primary structure - structures that form during lithification
•
Secondary structure - structures that form after lithificaiton
Whic
h
way
is
up?
Today
Option 1
Overturned
syncline
Option 2
“up side down”
overturned
anticline
Cross-bedding
Checkerboard Mesa, Zion National Park
Concave up
Younger
Y
Flow direction
Older
Graded bedding
Younger
Y
Older
•
Decrease in depositional energy with sedimentation
•
Example: flood deposits, turbidity currents
Graded
bedding
Younger
Y
Older
•
Decrease in depositional energy with sedimentation
•
Example: flood deposits, turbidity currents
Reverse (Inverse) Graded Bedding
Younger
Y
Older
•
Increase in depositional energy with sedimentation
•
Example: debris flows (a lot less common than normal graded beds)
Ripple marks
•
Symmetric ripples indicate
bi-modal current
•Concave
Y
• Asymmetric
ripples indicate
unidirectional current
= up
Mud cracks
•
Desiccation of
muddy sediments
•
Mud cracks
5 cm
Raindrops
Y
•
Limited to terrestrial sediments
Footprints
Y
•
Limited to terrestrial sediments
Load casts
Protrusion of material
into a layer below
•
Load casts indicate
the base of a layer, not
the top of a layer
•
Determining the
current direction may
be possible
•
Geopetal Structures
• A “natural”
carpenter’s level
Shell or cavity in the rock
Matrix
Y
Infill material (I.e., calcite)
Bioturbation
•
Habitation burrows
•
Feeding burrows
•
Movement
Y
Stromatolites
Y
•
Sharks Bay, Australia
•
Cyanobacteria grow upward toward the surface
Flame structures
Y
Less dense material intrudes into material above
• Caused by rapid loading of turbidite sands
•
Pillow lava
Upper curved surface
Y
“V “ notch
References
Slide 8
http://www.utahpictures.com/Checkerboard.html
Slides 9-15, 17
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 18
http://www.discoverwest.com.au/hablin.html
Slide 20
http://volcanoes.usgs.gov/Products/Pglossary/PillowLava.html
http://volcanoes.usgs.gov/Products/Pglossary/ancientseq.html