1. Today: Origin and Interpretation of Sedimentary Rocks
Wed: Origin and Interpretation of Metamorphic Rocks

2. Sedimentary Rocks — Why Care?
They are the repository of Earth’s history, especially the history of how the surface environment (including life) changed over time. To extract this history, we need to be able to “read” this rock record.
They hold all of the oil, gas,coal, and water — locating these materials requires an understanding of the physical characteristics of different sedimentary rocks and how those characteristics change in 3-D.

3. Coconino Sandstone

4. Coconino Sandstone up close

5. thin slice of Coconino sandstone seen through microscope
un-cemented quartz sand grains from a sand dune

6. So how are sedimentary rocks made?

8. Physical & Chemical Weathering → Sediment & Dissolved Ions

10. hincks

11. Sedimentary Structures in the Coconino

12. This is the modern environment where large cross-beds form
Wind Direction

13. Fig. 5.21abc
W. W. Norton
So cross beds are tilted in the direction of flow (downstream)

14. Fig. 5.21d
Stephen Marshak

15. ripples Stephen Marshak
Uniformitarianism — depositional environments of today are physically the same as ancient environments, so clues from the modern can be used to infer ancient environments from sedimentary rocks, or “the present is the key to the past”
ripples
Stephen Marshak

16. Ancient ripples
Stephen Marshak

17. Mud cracks
Stephen Marhsak

18. Ancient mudcracks
Stephen Marhsak

19. Modern Alluvial Fan, Death Valley
Fig. 5.24c
© Martin Miller

20. Ancient Alluvial Fan Deposits

22. OOIDS imply shallow (<3m), tropical marine depositional environment
(made of calcite — CaCO₃)
imply shallow (<3m), tropical marine depositional environment

23. Monte Bove in Italy
800 meters of oolitic limestones
Ooids with cement, seen in a thin section, through a microscope
2 mm

24. So, the question is — how can we explain such a thick pile of shallow-water sediments?
We need to think about isostasy and sea level change in our search for an explanation of how we can get such thick piles of sediment, all of which appear to be deposited in fairly shallow water.

26. Isostasy

27. Isostasy

28. Equilibrium level for wooden block
time 1
time 4
time 1
time 4

30. Add a Weight Increase Density Thin Crust Raise Sea Level
So, the question remains — how to get such a thick pile of shallow-water sediments?
Add a Weight
Increase Density
Thin Crust
Raise Sea Level

31. As sea level goes down, previously deposited sediment will be eroded
In our case, crustal thinning makes sense because the sediments in question were deposited during and after continental rifting.

32. Summary on Making and Interpreting Sedimentary Rocks
Make sediment
Transport & deposit sediment
Create space to allow sediment to accumulate
Compact and cement sediment
Depositional environments are interpreted based on studies of modern environments where sediment is deposited.

34. Metamorphic rocks