1. Geology 3120 - Sedimentary Structures

2. Outline Contacts, primary structures, and secondary structures
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

3. Block model for exercise

4. Geologic History 12 Ma dike 15 Ma dike Normal fault 20 Ma sed
Erosion
Thrust fault
Folding
Layer B
60 Ma sed
Layer G
70 Ma sed
80 Ma sed

5. 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

6. 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

7. Which way is up? Today Option 1 Option 2 Overturned “up side down”
syncline
Option 2
“up side down”
overturned
anticline

8. Y Cross-bedding Checkerboard Mesa, Zion National Park Younger
Flow direction Y
Concave up
Older

9. Y Graded bedding Younger Older
Decrease in depositional energy with sedimentation
Example: flood deposits, turbidity currents

10. Y Graded bedding Younger Older
Decrease in depositional energy with sedimentation
Example: flood deposits, turbidity currents

11. Y Reverse (Inverse) Graded Bedding Younger Older
Increase in depositional energy with sedimentation
Example: debris flows (a lot less common than normal graded beds)

12. Y Ripple marks Symmetric ripples indicate bi-modal current
Concave = up Y
Asymmetric ripples indicate unidirectional current

13. Mud cracks
Desiccation of
muddy sediments
Mud cracks
5 cm

14. Raindrops Y
Limited to terrestrial sediments

15. Footprints Y
Limited to terrestrial sediments

16. 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

17. Y Geopetal Structures A “natural” carpenter’s level
Shell or cavity in the rock
Matrix Y
Infill material (I.e., calcite)

18. Bioturbation
Habitation burrows
Feeding burrows
Movement Y

19. Y Stromatolites Sharks Bay, Australia
Cyanobacteria grow upward toward the surface

21. Y Flame structures Less dense material intrudes into material above
Caused by rapid loading of turbidite sands

22. Pillow lava Y
Upper curved surface
“V “ notch

24. 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
Slide 20