1. Sedimentary Rocks
Chapter 6

2. What is a sedimentary rock?
Sedimentary rocks are products of mechanical and chemical weathering
They account for about 5 percent (by volume) of Earth’s outer 10 miles
Contain evidence of past environments
Provide information about sediment transport
Often contain fossils

3. What is a sedimentary rock?
Sedimentary rocks are important for economic considerations because they may contain
Coal
Petroleum and natural gas
Sources of iron, aluminum, and manganese

4. Turning sediment into rock
Many changes occur to sediment after it is deposited
Diagenesis – all of the chemical, physical, and biological changes that take place after sediments are deposited
Occurs within the upper few kilometers of Earth’s crust

5. Turning sediment into rock
Diagenesis
Includes
Recrystallization – development of more stable minerals from less stable ones
Lithification – unconsolidated sediments are transformed into solid sedimentary rock by compaction and cementation
Natural cements include calcite, silica, and iron oxide

6. Lithification

7. Types of sedimentary rocks
Sediment originates from mechanical and/or chemical weathering
Rock types are based on the source of the material
Detrital rocks – transported sediment as solid particles
Chemical rocks – sediment that was once in solution

8. Detrital sedimentary rocks
The chief constituents of detrital rocks include
Clay minerals
Quartz
Feldspars
Micas
Particle size is used to distinguish among the various types of detrital rocks

9. Detrital (or clastic) sediments
They have a clastic (broken or fragmental) texture consisting of:
Clasts (larger pieces, such as sand or gravel)
Matrix (mud or fine-grained sediment surrounding the clasts)
Cement (the glue that holds it all together), such as:
calcite
iron oxide
silica

10. Clasts and matrix

11. Detrital sediments are classified according to their texture (grain size):
Gravel: Grain size greater than 2 mm (Conglomerate & Breccia)
Sand: Grain size 1/16 to 2 mm (Sandstone)
Silt: Grain size 1/256 to 1/16 mm (gritty) (Siltstone)
Clay: Grain size less than 1/256 mm (smooth) (Shale)
Shale and siltstone are called mudrocks

12. Detrital sedimentary rocks
Common detrital sedimentary rocks (in order of increasing particle size)
Shale
Mud-sized particles in thin layers that are commonly referred to as laminae
Most common sedimentary rock

13. Shale containing plant remains

14. Detrital sedimentary rocks
Sandstone
Composed of sand-sized particles
Forms in a variety of environments
Sorting, shape, and composition of the grains can be used to interpret the rock’s history
Quartz is the predominant mineral

15. Sandstone

16. Arkose
The presence of feldspar suggests that little chemical weathering has occurred – this is associated with arid climates

17. Sandstone Close-ups Quartz sandstone (left)
Arkose (sandstone with >10% feldspar)

18. Outcrop of Sandstone

19. Detrital sedimentary rocks
Conglomerate and breccia
Both are composed of particles greater than 2mm in diameter
Conglomerate consists largely of rounded gravels
Breccia is composed mainly of large angular particles

20. Conglomerate

21. Outcrop of Conglomerate

22. Interbedded sandstone and conglomerate

23. Breccia

24. Chemical sedimentary rocks
Consist of precipitated material that was once in solution
Precipitation of material occurs in two ways
Inorganic processes
Organic processes (biochemical origin)

25. Chemical sedimentary rocks
Common chemical sedimentary rocks
Limestone
Most abundant chemical rock
Composed chiefly of the mineral calcite
Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms)
Inorganic limestones include travertine and oolitic limestone

26. Coquina

27. Fossiliferous limestone

28. Oolitic Limestone

29. Travertine forming

30. Chemical sedimentary rocks
Common chemical sedimentary rocks
Dolostone (dolomite)
Typically formed secondarily from limestone
Chert
Made of microcrystalline quartz
Varieties include flint and jasper (banded form is called agate)

31. Chemical sedimentary rocks
Common chemical sedimentary rocks
Evaporites
Evaporation triggers deposition of chemical precipitates
Examples include rock salt and rock gypsum

32. Rock Salt

33. Bonneville Salt Flats

34. Layered gypsum

35. Chemical sedimentary rocks
Common chemical sedimentary rocks
Coal
Different from other rocks because it is composed of organic material
Stages in coal formation (in order)
1. Plant material
2. Peat
3. Lignite
4. Bituminous

36. Successive stages in coal formation

37. Classification of sedimentary rocks
Sedimentary rocks are classified according to the type of material
Two major groups
Detrital
Chemical

38. Classification of sedimentary rocks
Two major textures are used in the classification of sedimentary rocks
Clastic
Discrete fragments and particles
All detrital rocks have a clastic texture
Nonclastic
Pattern of interlocking crystals
May resemble an igneous rock

39. Identification of sedimentary rocks.

40. Sedimentary environments
A geographic setting where sediment is accumulating
Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)

41. Sedimentary environments
Types of sedimentary environments
Continental
Fluvial - dominated by erosion and deposition associated with streams
Glacial
Eolian (Wind)
Marine
Shallow (to about 200 meters)
Deep (seaward of continental shelves)

42. Sedimentary environments
Types of sedimentary environments
Transitional (shoreline)
Deltas
Estuaries
Barrier island system
Beaches
Lagoons
Tidal flats and marshes

43. Continental (left) and marine (right) depositional environments

44. Sedimentary environments
Sedimentary facies
Different sediments often accumulate adjacent to one another at the same time
Each unit (called a facies) possesses a distinctive set of characteristics reflecting the conditions in a particular environment
The merging of adjacent facies tends to be a gradual transition

45. Sedimentary facies

46. Marine Transgressions
A marine transgression occurs when sea level rises with respect to the land
During a marine transgression, the shoreline migrates landward the environments paralleling the shoreline migrate landward as the sea progressively covers more and more of a continent

47. Marine Transgressions
Each laterally adjacent depositional environment produces a sedimentary facies
During a transgression, the facies forming offshore become superposed upon facies deposited in nearshore environments

48. Marine Transgression The rocks of each facies become younger
in a landward direction during a marine transgression
One body of rock with the same attributes
(a facies) was deposited gradually at different times
in different places so it is time transgressive
meaning the ages vary from place to place
younger shale
older shale

49. Transgressions and Regressions

50. Sedimentary structures
Provide information useful in the interpretation of Earth history
Types of sedimentary structures
Strata, or beds (most characteristic of sedimentary rocks)
Bedding planes that separate strata
Cross-beds – formed by currents

51. Grand Canyon showing strata

52. Bedding Sedimentary rocks generally have bedding or stratification
Individual layers less than 1 cm thick are laminations
common in mudrocks
Beds are thicker than 1 cm
common in rocks with coarser grains

53. Sedimentary structures
Types of sedimentary structures
Graded beds - an upward gradual decrease in grain size
Ripple marks- can be symmetrical or asymmetrical; formed by currents or waves
Mud cracks – polygonal pattern of cracks produced on the surface of mud as it dries

54. Graded Bedding

55. Graded Beds

56. Wave Ripples

57. Ripple marks

58. Recent mudcracks

59. Ancient mudcracks

60. Asymmetrical ripples and cross-bedding

61. Cross-Bedding
To view this animation, click “View” and then “Slide Show” on the top navigation bar. 61

62. Cross-stratification, upper Paleozoic rocks, Birmingham, AL

63. Great Sand Dunes National Monument, Colorado
What is the prevailing wind direction?

64. X-beds in Navajo Sandstone
What formed these x-beds?
In what direction was the wind blowing?
Wind

65. Fossils: Evidence of past life
By definition, fossils are the traces or remains of prehistoric life now preserved in rock
Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and almost never in igneous rock)

66. Fossils: Evidence of past life
Geologically fossils are important for several reasons
Aid in interpretation of the geologic past
Serve as important time indicators
Allow for correlation of rocks from different places

67. Natural casts of ammonites

68. Natural cast of a trilobite.

69. Dinosaur footprint in limestone

70. End Chapter 6