1. Sedimentary Rocks — The Archives of Earth History?
Disconformity

2. There are actually 3 types of rocks…
Igneous – cool from liquid (magma or lava)
Metamorphic – pre-existing rocks that have been altered by intense temperature or pressure
Sedimentary – form mainly from deposition of sediments

3. Historical geology focuses on sedimentary rocks
•Why???
Only rocks that contain fossils
Indicate ancient depositional environments

4. What is a sediment? Fragment of pre-existing rock (or animal shell)
Why does water off Galveston look murky, while water off Florida looks clear?

5. What kind of rocks do we find around Houston?
Not many rocks!
Lots of unlithified sediment
Why do many houses in Houston have foundation problems?

6. What is a sedimentary rock?
Rock that forms at or near Earth’s surface
3 types
Clastic
Chemically-precipitated
Biogenic

7. How do clastic sedimentary rocks form?
Weathering
Transport
Deposition
Lithification

8. How do other sedimentary rocks form?
Chemical – precipitation of dissolved materials
Biogenic (organic) – accumulations of organic material

9. Environments of Deposition
At or near surface of Earth
Marine
Continental
Transitional (deltas, barrier islands, beaches)

10. Marine Coastal Shelf Deep water

11. Continental Fluvial Desert Lacustrine Glacial Meandering Braided

12. Continental Fluvial Desert Lacustrine Glacial Meandering Braided

13. Continental Fluvial Desert Lacustrine Glacial Meandering Braided

14. Dune Cross-Beds
Large-scale cross-beds in a Permian-aged wind-blown dune deposit in Arizona

15. Continental Fluvial Desert Lacustrine Glacial Meandering Braided

16. Continental Fluvial Desert Lacustrine Glacial Meandering Braided

17. Moraines and Till Origin of glacial drift
Moraines and poorly sorted till

18. Delta Form in oceans or lakes (marine and non-marine)

19. Stream/River-Dominated Deltas
long distributary channels extending far seaward
Mississippi River delta

20. Where would you find different sedimentary rocks?

21. Where would you find different sedimentary rocks?

22. Where would you find different sedimentary rocks?
Grain size is controlled by energy
High energy
River
Beach
Low energy
Lake
Deep ocean
Large grains
Small grains

23. Sorting, Rounding If the size range is not very great,
the sediment or rock is well sorted
If they have a wide range of sizes,
they are poorly sorted
Wind has a limited ability to transport sediment so dune sand tends to be well sorted
Glaciers can carry any sized particles because of their transport power, so glacier deposits are poorly sorted
Grains more rounded with longer transport

24. Rounding and Sorting A deposit of well rounded and well sorted gravel
Angular, poorly sorted gravel

25. Cross-Bedding Tabular cross-bedding forms by deposition on sand waves
Tabular cross-bedding in the Upper Cretaceous Two Medicine Formation in Montana

26. Current Ripple Marks
form in response to water or wind currents flowing in one direction
asymmetric profiles allowing geologists to determine paleocurrent directions

27. Wave-Formed Ripple Marks
As the waves wash back and forth, symmetrical ripples form
Wave-formed ripple marks in shallow seawater

28. Modern Deposition near Houston
Fluvial
Brazos, Colorado, Trinity, San Jacinto Rivers
Transitional
Deltas, barrier islands
Marine
Gulf of Mexico

29. Ancient Environments Important for historical geology
Important for oil companies (need to know where sand was deposited)
Why are we looking at modern depositional environments?

30. Present is the key to the past
Study modern depositional environments to learn about ancient ones
Knowledge of ancient environments helps oil companies and historical geologists

31. Brazos River Longest river in Texas – 1450 km
Highest sediment supply of any Texas river
Originates in New Mexico

32. Where does deposition occur?

33. Point Bars Sediment deposited within the inside bank of a meander loop
Fining upward sequence (grain size decreases)
Coarsest sediment deposited by highest energy

34. Brazos River – Point Bar

35. Brazos River – Cut Bank

36. Coastal – Galveston Island

37. Barrier Islands Formed during sea level rise
Rate of SL rise and rate of sediment deposition approximately equal
Wave-dominated environment

38. Barrier Islands
On broad continental margins with abundant sand, long barrier islands lie offshore separated from the mainland by a lagoon
Barrier islands are common along the Gulf and Atlantic Coasts of the United States
Subenvironments of a barrier island complex:
beach sand grading offshore into finer deposits
dune sands contain shell fragments (not found in desert dunes)
fine-grained lagoon deposits

39. Barrier Island Complex
Subenvironments of a barrier island complex

40. Texas Coast Most Texas beaches relatively fine-grained
Low gradient of rivers like Brazos
Why is this a problem?

41. Brazos Delta
gulf.rice.edu

42. Environmental Interpretations and Historical Geology
Present-day gravel deposits by a swiftly-flowing stream
(Most transport and deposition takes place when the stream is higher)
Nearby gravel deposit probably less than a few thousand years old

43. Environmental Interpretations and Historical Geology
Conglomerate more than 1 billion years old
shows similar features
We infer that it too was deposited by a braided stream
Why not deposition by glaciers or along a seashore?
No evidence for either glacial activity or transitional environment