1. Sediment and Sedimentary Rocks
Chapter 7
Sediment and Sedimentary Rocks

2. Introduction
The two primary types of sediment are detrital and chemical.
Sedimentary rock is simply rock made up of consolidated sediments.
Fig. 7.1, p. 159

3. Introduction
Detrital sediment consists of solid particles that are products of mechanical weathering.
Chemical sediments consist of minerals precipitated from solution by inorganic processes and by the activities of organisms through chemical weathering.
Fig. 7.1, p. 159

4. clay–sized particles)
SOURCE OF SEDIMENTARY MATERIALS
(gravel, sand, silt,
clay–sized particles)
MECHANICAL
WEATHERING
(clay minerals and ions,
compounds in solution)
CHEMICAL
WEATHERING
TRANSPORT
TO SITES OF DEPOSITION
Stepped Art
Fig. 7-1, p. 159

5. Sediment Sources, Transport, and Deposition
Rocks weather, weathered materials undergo erosion, and the material is transported to a new location as sediment.
Transportation of sediment results in rounding and sorting.
Fig. 7.2, p. 159

6. Sediment Sources, Transport, and Deposition
Why are rounding and sorting important in sediments and sedimentary rocks?
Both are important in determining how liquid water, ice, and wind move through sediments and sedimentary rocks.
The amount of rounding and sorting depends on particle size, distance of transportation, and depositional processes.
Rounding and size control, also how important subsurface resources such as groundwater and petroleum, move through sedimentary rocks and sediment

7. Sediment Sources, Transport, and Deposition
Classification of particle sizes in sediments
Gravel includes pebbles, cobbles and boulders
Table 7.1, p. 158

8. Sediment Sources, Transport, and Deposition
Eventually, the sediment comes to rest in a depositional environment.
Depositional environments are areas of sediment deposition that can be defined by their physical characteristics (topography, climate, wave and current strength, salinity, etc.).
Depositional environments provide clues as to how the rock formed and what the geologic past was like.

9. Sediment Sources, Transport, and Deposition
Major depositional settings are continental (including freshwater), transitional (shore or near shore marine), and marine.
Each of these depositional settings includes several specific subenvironments.
Fig. 7.3, p. 160

10. Fig. 7.3, p. 160

11. How Does Sediment Become Sedimentary Rock?
Through the process of lithification, sediment is converted into sedimentary rock.
Fig. 7.5c, p. 162

12. How Does Sediment Become Sedimentary Rock?
Lithification involves two processes:
1. Compaction - The volume of sediment decreases as the weight of overlying sediment causes a reduction in pore space (open space) as particles pack more closely together.
Compaction alone is sufficient for lithification of mud into shale.

13. How Does Sediment Become Sedimentary Rock?
Lithification involves two processes:
2. Cementation is a process that glues the sediments together.
The most common cements are calcium carbonate and silica, but iron oxide and iron hydroxide are important in some rocks.
Compaction alone will not form rocks from sand and gravel. Cementation is necessary to glue the particles together into rocks.

14. How Does Sediment Become Sedimentary Rock?
Fig. 7.4, p. 161

15. Compaction/cementation
Sediment
Process
Rock
Conglomerate
Sedimentary
breccia
Rounded clasts
Angular clasts
Gravel > 2 mm
Compaction/cementation
Sand 2 mm–1/16 mm
Compaction/cementation
Quartz sandstone
(mostly quartz)
Arkose
(> 25% feldspars)
Sandstone
Silt 1/16 mm–1/256 mm
Compaction/cementation
Claystone
Mudstone
Siltstone
Mudrocks
Mostly silt
Silt and clay
Mostly clay
Clay < 1/256 mm
Compaction
Shale if
fissile*
*Fissile refers to rocks capable of splitting along closely spaced planes.
Stepped Art
Fig. 7-4, p. 161

16. Types of Sedimentary Rock
Detrital Sedimentary Rocks are made of solid particles of pre-existing rocks.
Detrital sedimentary particles are classified according to grain (particle) sizes, in decreasing diameter:
Gravel (including boulders, cobbles and pebbles)
Sand
Silt
Clay (or mud).

17. Types of Sedimentary Rocks
Detrital sedimentary rocks are classified on the basis of particle size.
Examples include conglomerate, breccia, sandstone, siltstone, mudstone, and shale
Fig. 7.5a,b , p. 162

18. Types of Sedimentary Rocks
How do conglomerate and sedimentary breccia differ?
Both begin as detrital gravel: conglomerate consists of rounded gravel, breccia consists of gravel with sharp edges
Fig. 7.5a,b, p. 162

19. Types of Sedimentary Rocks
Chemical and Biochemical Sedimentary Rocks
Chemical and biochemical sedimentary rocks are substances derived from solution by inorganic or biochemical processes.
Some have a crystalline texture, meaning they are composed of a mosaic of interlocking crystals.
Others have a clastic texture, meaning that they are made of fragments, like shells that are glued together.

20. Types of Sedimentary Rocks
Chemical and Biochemical Sedimentary Rocks
Table 7.2, p. 162

21. Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Chemical sedimentary rocks are classified on the basis of composition.
Carbonate rocks, such as limestone and dolostone, consist primarily of minerals containing the carbonate ion, such as calcite and dolomite.
Dolostone forms when magnesium replaces some calcium in limestone.
Fig. 7.6b,c, p. 163

22. Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Evaporites
Bedded rock salt (halite) and rock gypsum are chemical evaporite sediments formed by precipitation of minerals during the evaporation of water.
Fig. 7.7, p. 164

23. Types of Sedimentary Rocks
Chemical Sedimentary Rocks
Bedded Chert Marin County, California
Dark chert is flint. Red chert is jasper.
Used by ancient people for tools and to start fires
The origin of chert is highly debated.
Fig. 7.8b, p. 164

24. Types of Sedimentary Rocks
Biochemical Sedimentary Rocks
Coal is a biochemical sedimentary rock composed largely of altered land plant remains.
Fig. 7.9c, p. 165

25. Types of Sedimentary Rocks
Biochemical Sedimentary Rocks
Coal forms from the deep burial, heating and dehydration of peat.
Peat is partially decayed plant remains found in low- oxygen waters in swamps and bogs.
Fig. 7.9a, p. 165

26. Types of Sedimentary Rocks
Biochemical Sedimentary Rocks
With deep burial, heating, and dehydration, peat converts into lignite coal.
Fig. 7.9b, p. 165

27. Types of Sedimentary Rocks
Biochemical Sedimentary Rocks
Further burial, heating, and dehydration converts lignite into subbituminous coal, then bituminous coal, and finally into anthracite, a metamorphic coal.
Anthracite coal is about 98% carbon.
Fig. 7.9c, p. 165

28. Sedimentary Facies
Geologists realize that if they laterally trace a sedimentary layer far enough, it will undergo changes in composition and/or texture.
Bodies of sediment or sedimentary rocks which are recognizably different from adjacent sediment or sedimentary rocks and are deposited in a different depositional (sub) environment are known as sedimentary facies.
Today we recognize modern facies changes when we go from an inland area with rivers to the beach.

29. Sedimentary Facies
Sedimentary facies are used to identify ancient changes in sea level, called marine transgressions and regressions.
Fig. 7.10, p. 166

30. Sedimentary Facies Marine Transgressions and Regressions
A marine transgression occurs when sea level rises with respect to the land, resulting in offshore facies overlying nearshore facies.
Fig. 7.10, p. 166

31. Sedimentary Facies Marine Transgressions and Regressions
The Tapeats Sandstone, Bright Angel Shale, and Muav Limestone of the Grand Canyon, Arizona, show a marine transgression.
Fig. 7.11, p. 166

32. Sedimentary Facies Marine Transgression and Regression
A marine regression, caused when the land rises relative to sea level, results in nearshore facies overlying offshore facies.
Note the difference in the vertical rock sequence that occurs in a transgression versus a regression.
Fig. 7.10, p. 166

33. Offshore Low-energy Near shore High-energy
Limestone
facies
Shale
Sandstone
Time
line
Three Stages of Marine Transgression
Offshore
Low-energy
Near shore
High-energy
Land
surface
Three Stages of Marine Regression
Time
lines
Time
lines
Old land surface
Old land
surface
Stepped Art
Fig. 7-10, p. 166

34. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Textures describe the shape, size and sorting of grains in sedimentary rocks.
Sedimentary structures are larger- scale features.
Fig. 7.12, p. 167

35. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Some sedimentary structures, such as ripple marks, bedding, cross-bedding, and mud cracks form shortly after deposition.
Sedimentary structures are useful in determining the types of environments in which the sediments were deposited.
Sediments are most commonly deposited flat in water. One of the most common is strata or bedding.

36. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Depositional environments are often inferred by comparison of these structures with present-day depositional environments.
A common sedimentary structure is cross-bedding.
Fig. 7.12, p. 167

37. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Cross-bedding preserves layers deposited at an angle.
They are common in depositional environments like sand dunes, shallow marine deposits and stream-channel deposits.
Fig. 7.12, p. 167

38. Reading the Story in Sedimentary Rocks
Sedimentary Structures
How is cross-bedding used to determine ancient current directions?
Understanding how physical features like cross- beds form today can reveal important ancient climate information such as current directions.
Fig. 6.23b-c, p. 158

39. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Graded Bedding: In a turbidity current, largest particles settle first and then smaller particles on top.
Turbidity currents are avalanches of sediment on the sea floor.
Fig. 7.13, p. 168

40. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Ripple Marks: Depositional environment: streams or shallow marine?
Fig. 7.14, p. 168

41. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Current ripple marks: Streams have a current and leave behind asymmetric ripple marks.
Wave-formed ripple marks: Shallow marine ripple marks exhibit a symmetrical shape from the rocking motion of the waves.
Fig. 7.14, p. 168

42. Reading the Story in Sedimentary Rocks
Sedimentary Structures
Mud cracks
Depositional environment: Lagoons and mudflats
Fig. 7.15, p. 169

43. Reading the Story in Sedimentary Rocks
Fossils-Remains and Traces of Ancient Life
Fossils are usually found only in sediments and sedimentary rocks.
They are used by geologists to correlate strata, date rocks, and to interpret depositional environments.
Fig. 7.16, p. 169

44. Reading the Story in Sedimentary Rocks
Determining the Environment of Deposition
How do we know that the Navajo Sandstone formed as a desert dune deposit?
Fig. 7.17, p. 172

45. Reading the Story in Sedimentary Rocks
Determining the Environment of Deposition
How do we know that the Navajo Sandstone formed as a desert dune deposit?
Look at modern environments
Sand textures
Cross beds and asymmetrical ripple marks
Footprints of land animals
Fig. 7.17, p. 172

46. Important Resources in Sedimentary Rocks
Many important natural resources are sedimentary rock deposits. These include:
Sand and gravel
Clay
Coal
Limestone
Phosphates
Placer deposits with gold, uranium and other metals
Evaporites (salts)
Banded-iron formations
Petroleum (oil and natural gas)

47. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Oil and natural gas reserves are found within sedimentary rocks.
Fig. 7.18a, p. 173

48. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Microorganisms die and accumulate in low-oxygen bottom sediments of seas or in some cases large lakes.
The lack of oxygen prevents their total decay.
The organic sediments are buried.
If burial temperatures are just right, oil and/or natural gas forms. The host sediments are lithified into source rocks.

49. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Low-density oil and/or natural gas migrate upward from the sources rocks.
Oil and/or natural gas may reach the surface or accumulate in reservoir rocks underneath traps.

50. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
What are stratigraphic and structural traps? Both are areas where petroleum, natural gas, or both accumulate in economic quantities
Fig. 7.18a p. 173

51. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Stratigraphic traps form because of facies changes in the rock layers (strata).
Fig. 7.18a p. 173

52. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Structural traps form as the result of folding or fracturing (faulting) of rocks.
Fig. 7.18b, p. 173

53. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Oil shale is a fine-grained sedimentary rock that contains kerogen from which liquid oil and combustible gases can be derived.
Fig. 7.18c p. 173

54. Important Resources in Sedimentary Rocks
Petroleum and Natural Gas
Oil shale
No oil or gas is currently mined in the United States because oil and gas from conventional sources are cheaper. However, they will become reserves in the future.
Fig. 7.18c p. 173

55. Important Resources in Sedimentary Rocks
Uranium
Most uranium is used in nuclear reactors. The uranium comes from the minerals carnotite and uraninite.
The richest ores are found in Wyoming, Utah, Arizona and New Mexico in ancient stream (placer) deposits.
Large reserves of low grade ore is found in the Chattanooga Shale, which covers portions of several states.

56. Important Resources in Sedimentary Rocks
Banded Iron Formation
Banded iron formation consists of alternating thin layers of chert and iron minerals, mostly iron oxides.
Nearly all of Earth’s iron ore is mined from ancient banded iron formations.
Fig. 7.19a, p. 176

57. End of Chapter 7