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6-Understanding Planet Earth > Chapter 6: Sedimentary Rocks > Flashcards

Chapter 6: Sedimentary Rocks Flashcards

1
Q

What do sedimentary rock layers represent? Where are those layers formed?

A
  • The layers record a history of ancient environments.

- The layers occur only in the upper part of the crust.

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2
Q

Sedimentary rocks cover ________

A

underlying basement rock

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3
Q

What is weathering?

A

chemical decay & physical fragmentation of rock at/near the Earth surface

  • Weathering is the response of earth material to a changing environment
  • Intrusive igneous rocks revealed by erosion and now subjected to weathering
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4
Q

What is erosion?

A

incorporation and transport of material by a mobile agent (water, wind, ice)

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5
Q

What is mass-wasting?

A

transfer of rock material downslope under the influence of gravity (rockfalls/rockslides, landslides, slumps, avalanches, debris flow, mud flows, creep, etc.)

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6
Q

Why can’t we dissociate weathering from erosion and mass-wasting?

A

Weathering cannot be totally dissociated from erosion and mass-wasting because, as weathering breaks rocks apart, it facilitates the movement of rock debris by erosion and mass-wasting and by removing the products of weathering, erosion and mass-wasting expose fresh, unaltered rock to weathering.

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7
Q

What is sediment?

A

unconsolidated or loose fragments of rocks or minerals, mineral precipitates and shell fragments.

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8
Q

What is Diagenesis?

A

cementation/lithification

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9
Q

How and where are the sedimentary rocks formed?

A

Sedimentary rocks are formed at Earth’s surface by cementing together weathered fragments of preexisting rock, fragments of shells, organic matter, or mineral precipitates – through a process known as diagenesis, sediment particles become lithified or cemented together

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10
Q

Which layer of the Earth do sedimentary rocks occur and what do they indicate?

A

Sediments and sedimentary rocks only occur in the upper part of the crust and they often preserve evidence of their mode of origin in the nature of the sediment grains that comprise the rock and the cements that bind those grains together

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11
Q

True/False

A

FALSE

Products of weathering do not always remain at the site of origin but are eventually transported by various means to their final resting place, in the ocean basins

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12
Q

How are sediments transported?

A

Sediment is transported in many ways. It may slide down a hillside or be carried by wind, by a glacier, or by flowing water. In each case, when transport ceases, the sediment is deposited in a fashion characteristic of the transporting agent.

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13
Q

What are the sizes of sediments that have been carried by sliding or rolling downhill?

A

When sediment is transported by sliding or rolling downhill, the result is generally a mixture of particles of all sizes.

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14
Q

What is a glacial till?

A

When the glacier eventually melts, the pebbles, boulders and finer particles carried by the glacier are left behind (e.g., glacier erratics) and creates a distinctive layer of sediment, glacial till, at the foot of the glacier.

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15
Q

When does the deposition occur and what does it tell us when the transportation is done by wind or water?

A

If the sediment particles are transported by wind or water, deposition occurs when the flowing water or moving air slows to a speed at which particles can no longer be carried.

The grain size of sediments tells us about the speed of the wind or water…

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16
Q

The words weathering and erosion are synonyms; they can be used interchangeably.

A.True
B.False

A

B

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17
Q

Sedimentary rocks differ from igneous rocks in that sedimentary rocks can never be crystalline and igneous rocks always are crystalline.

A.True
B.False

A

A?

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18
Q

What is physical (Mechanical) weathering?

A

Rocks are broken down or disintegrated by physical processes, but retain the characteristics (mineralogy, composition) of the original parent rock

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19
Q

What is chemical weathering?

A

The constituents of the rock undergo chemical changes

–> decomposition, leaching, and formation of secondary minerals

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20
Q

Physical and chemical weathering provide the raw materials (particles and dissolved ions) for all sedimentary rocks. Geologists define four sedimentary rock classes:

A
  • Biochemical: cemented shells of organisms.
  • Clastic: loose rock fragments (clasts) cemented together.
  • Chemical: minerals that crystallize directly from water.
  • Organic: carbon-rich remains of once living organisms.
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21
Q

What are clastic/detrial sedimentary rocks? How are they formed?

A

Sediments may be accumulations of materials (mineral grains, rock fragments) that originate and are transported as solid particles derived from both mechanical and chemical weathering. Deposits of this type are termed clastic or detrital and the sedimentary rocks that they form are called clastic or detrital sedimentary rocks.

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22
Q

Give examples of sedimentary rocks that are considered clastic

A

Homblende
Feldspar
Quartz

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23
Q

What are chemical sediments? Give examples

A

derived from material that is carried in solution to lakes and seas by surface runoff and groundwaters and precipitated chemically rather than mechanically

  • Stalactite growing in cave
  • Opaline silica deposited from hot springs
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24
Q

What are biochemical sediments? Give an example

A
  • Produced by living organisms

- e.g. discarded shells of snails and clams made of calcium carbonate shells can form the sedimentary rock limestone

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25
Q

What are organic sediments? Give example

A

Made of organic carbon, the soft tissues of living things

E.g. Coal—altered remains of fossil vegetation

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26
Q

Clastic sedimentary rocks are created by:

A
  • Weathering—generation of detritus via rock disintegration.
  • Erosion—removal of sediment grains from parent rock.
  • Transportation—dispersal by gravity, wind, water, and ice.
  • Deposition—settling out of the transporting fluid.
  • Lithification—transformation into solid rock.
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27
Q

What is lithification and what are the 2 processes that can occur?

A

Lithification—transforms loose sediment into solid rock.

  • Compaction—burial adds pressure to sediment.
  • Cementation—minerals grow in pore spaces.
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28
Q

What is compaction?

A
  • Squeezes out air and water.

- Compresses sediment grains.

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29
Q

What is cementation?

A
  • Often quartz or calcite.
  • Precipitate from groundwater.
  • Glue sediments together.
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30
Q

How are clastic sedimentary rocks classified?

A

Classified on the basis of texture and composition.

  • Clast (grain) size.
  • Clast composition.
  • Angularity and sphericity.
  • Sorting.
  • Character of cement.

These variables produce a diversity of clastic rocks.

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31
Q

In clastic sedimentary rocks the size of particles is related to ____________.

A

the energy of the transporting medium

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32
Q

Transport of gravel (clastic sedimentary rock) requires ________.

A

swiftly flowing rivers

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33
Q

_____ and ______ settle very slowly and accumulate in quiescent environments.

A

Silts
clays

(clastic sedimentary rock

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34
Q

How does transport affect the grain size?

A

the stronger the current, the larger the particle size carried

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35
Q

What is clast size?

A

Clast size is a measure of the size of fragments or grains. Size ranges from very coarse to very fine (gravel, sand, silt, and clay). As transport distance increases, grain size decreases

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36
Q

What is the clast size of gravel?

A

Gravel—very coarse (boulder), coarse (cobble), medium, (pebble), and fine (pea).

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37
Q

What is the clast size of sand?

A

coarse, medium, and fine.

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38
Q

What is the clast size of silt?

A

coarse, medium, and fine.

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39
Q

What is the clast size of clay?

A

coarse and fine

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40
Q

What is clast compostion?

A

Clast composition—the mineral makeup of sediments.

  • May be individual minerals or rock fragments.
  • Composition yields clues about the original source rock.
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41
Q

What is angularity?

A

the degree of edge or corner smoothness.

42
Q

What is sphericity?

A

Degree to which a clast nears a sphere.

  • Fresh detritus is usually angular and nonspherical.
  • Grain roundness and sphericity increases with transport. - Well-rounded—long transport distances.
  • Angular—negligible transport.
43
Q

How is sphericity classified and what does it indicate?

A

Grain roundness and sphericity increases with transport. - Well-rounded—long transport distances.
- Angular—negligible transport.

44
Q

What is sorting? How is it classified and what does it indicate?

A

Sorting is a measure of the uniformity of grain sizes in a sediment population.
Degree of sorting increases with transport distance.

45
Q

How is sorting classified and what does it indicate?

A

Poorly sorted—clasts show a wide variety of grain sizes.

Well sorted—all clasts have nearly the same grain size.

46
Q

What is the character of cement and how does it differ in different rocks?

A

Character of cement – minerals that fill sediment pores.

  • Different clastic sedimentary rocks have different cement.
  • Quartz and calcite are the most common cements.
47
Q

What are fine clastics composed of and what happens when they lithify?

A

Fine clastics are composed of silt and clay. Silt-sized sediments are lithified to form siltstone. Clay-sized particles form mudstone or shale

48
Q

What is the difference between shale and mudstone?

A

Shale—breaks into thin platy sheets.

Mudstone—doesn’t break into platy sheets.

49
Q

What is shale composed of and how is it formed?

A

Shale rock consists of silt and clay-sized particles that are compacted together by pressure. Shales account for ~70% of all sedimentary rocks.

50
Q

How is clay found in shale?

A

Clay mineral particles are flat or tabular and usually become tightly packed. They are not prone to cementation because very little water can percolate through them and thus these rocks crumble and weather easily. Shale typically split into layers along well-developed, closely spaced planes

51
Q

What is mudstone?

A

In a more restrictive use of the term, shale must exhibit the ability to split into layers along well-developed, closely spaced planes. If the rock breaks into chunks or blocks, the name mudstone is applied.

52
Q

Describe how a clastic sedimentary rock forms from its unweathered parent rock

A

First, physical and chemical weathering break up and alter the parent rock to form detrital fragments of parent material, dissolved ions, and clay. This sediment of weathered grains is then eroded from the parent surface and transported away from the source by water, wind, or glacial ice (or directly by gravity, in the case of large clasts on a slope). Ultimately, the sediment will settle out of the transport medium. Eventually, loose grains of deposited sediment may become buried under additional sediment, compacted, and cemented to form sedimentary rock

53
Q

How do grain size and shape, sorting, sphericity and angularity change as sediments move downstream?

A

Mechanical forces such as tumbling and abrasion wear on sediments as they are transported downstream. Angular protuberances are especially likely to be broken off. As a result, grain size decreases, with grains becoming more spherical and more rounded (less angular). The speed at which the water in a stream is traveling decreases along its course, and the capacity of the stream to carry sediment is directly related to its rate of flow. So larger grains are deposited upstream from finer grains (grains become sorted as they travel downstream)

54
Q

What are biochemical sedimentary rocks and how are they formed? What are they made out of?

A

Sediments derived from the shells of living organisms.

  • Hard mineral skeletons accumulate after death.
  • Calcite and Aragonite (CaCO3)—limestone.
  • Silica (SiO2)—chert
55
Q

What is coal made out of? (organic sedimentary rock)

A

Unlike any other sedimentary rock, coal is not composed of minerals in the true sense of the word, but of organic matter. A close examination of a piece of coal under the microscope will reveal the presence of various plant remains: leaves, bark, and wood

56
Q

What are organic sedimentary rocks?

A

made of organic carbon, the soft tissues of living things.

57
Q

What is coal?

A
  • Black, combustible sedimentary rock
  • Over 50–90% carbon
  • Fuels industry since the industrial revolution began.
58
Q

What are evaporites and how are they formed?

A

chemical sedimentary rocks

  • rock from evaporated sea or lake water.
  • Evaporation triggers deposition of chemical precipitates.
  • Thick deposits require large volumes of water.
  • Evaporite minerals include halite (rock salt) and gypsum.
59
Q

What is petrified wood? How is it formed?

A

Petrified wood is chert that forms when silica-rich sediment buries a forest. The silica dissolved in the groundwater percolates through the rotting wood and precipitates a very fine-grained (cryptocrystalline) quartz within the wood, gradually replacing the wood’s cellulose and, thus, preserving its original structure

60
Q

The process of erosion does NOT include which of the following?

A.glacial ice plowing sediment away from its original depositional setting
B.gravity-driven tumbling off a cliff
C.removal of debris by wind or running water D.precipitation of cement

A

D

61
Q

Sedimentary rocks formed through cementation of formerly loose grains of sediment derived from weathering of preexistent rock are called _______ sedimentary rocks. A very fine-grained example, which typically splits into thin sheets, is called ________ .

A.Clastic/shale
B.Biochemical/conglomerate
C.Chemical/coal
D.Clastic/sandstone

A

A

62
Q

Imagine a mountain stream whose fast-moving water has carried away all grains except for well-rounded pebbles, cobbles, and boulders. If such stream gravel were lithified, it would turn into . . .

A.Shale    
B.Siltstone    
C.Arkose 
D.Breccia    
E.Conglomerate
A

E

63
Q

What is a bedding plane?

A

Sedimentary rocks are usually layered or stratified in planar, close-to-horizontal beds. The boundary between two beds is a bedding plane.

64
Q

Why does bedding form?

A

Bedding reflects changing conditions during deposition.
Changes in transporting medium (velocity, volume per time).
Changes in sediment source, etc.
These may alter:
- Sediment composition.
- Grain size.
- Sorting, etc.

65
Q

Bedding forms due to changes in:

A
  • Climate
  • Water depth
  • Current velocity
  • Sediment source
  • Sediment supply
66
Q

What are formations and how are they named?

A

Distinct rock units that are so unique that they can be recognized—and mapped—over large regions, are termed formations.
Formations are named for places where they are best exposed. Geologic maps display the distribution of formations.

67
Q

Bedform character is tied to ___________

A

flow velocity and grain size.

68
Q

How are bedforms created?

A

Water or wind flowing over sediment creates bedforms

Under the category of “current depositions”.

69
Q

What are ripple marks? Where are they found?

A

Ripple marks—cm-scale ridges and troughs.

  • Develop perpendicular to flow.
  • Ripple marks are frequently preserved in sandy sediments.
  • Found on modern beaches
  • Found on bedding surfaces of ancient sedimentary rocks
70
Q

What are crossbeds and how are they formed?

A
  • Under the category of “current depositions”
  • Cross beds—created by ripple and dune migration.
  • Sediment moves up the gentle side of a ripple or dune.
  • Sediment piles up, then slips down the steep face.
  • The slip face continually moves down current.
  • Added sediment forms sloping cross beds.
71
Q

When do bed surface markings occur?

A

Bed-surface markings occur after deposition while sediment is still soft

72
Q

What are mudcracks? What do they indicate?

A

polygonal desiccation features in wet mud. They indicate alternate wet and dry terrestrial conditions

73
Q

What are depositional environments?

A

Depositional environments are locations where sediment accumulates. Different environments vary in energy regime, sediment transport, and depositional processes and chemical, physical, and biological characteristics, all of which conspire to create unique sedimentary rocks.
- Environments range from terrestrial to coastal to marine.

74
Q

Where do sedimentary basins form?

A

Sedimentary basins form where tectonic activity creates space

75
Q

Give 4 examples to the sedimentary basins?

A
  • foreland basin
  • rift basin
  • intercontinental basin
  • passive margin basin
76
Q

How thick are sedimentary basins?

A

Sedimentary basins are special places that accumulate sediment. Sediments vary in thickness across Earth’s surface from zero to 20+ km in sedimentary basins.

77
Q

What are rift basins? Where are they found? How are they formed?

A

Divergent (pull-apart) plate boundaries.

  • Crust thins by stretching and rotational normal faulting.
  • Thinned crust subsides.
  • Sediment fills the down-dropped troughs.
78
Q

What are intercontinental basins? Where are they found? How are they formed?

A

Interiors far from margins.

  • May be linked to failed crustal rifts.
  • Continue to subside for millions of years after formation.
79
Q

What are foreland basins? Where are they found? How are they formed?

A

Craton side of collisional mountain belt.

  • Flexure of the crust from loading creates a downwarp.
  • Fills with debris eroded off of the mountains.
  • Fluvial, deltaic, and lake sediments fill foreland basins.
80
Q

What is transgression and regression? What do they depend on?

A

Sea level changes.

  • Sedimentary deposition is strongly linked to sea level.
  • Changes in sea level are commonplace geologically.
  • Sea level rises and falls up to hundreds of meters
  • Changes in climate, tectonic processes
  • Depositional belts shift landward or seaward in response.
  • Layers of strata record deepening or shallowing upward.
81
Q

What are limestones? How are they formed?

A

Limestone is a sedimentary rock made almost entirely of calcite or aragonite (CaCO3 polymorphs).
These minerals are the most common materials used by organisms that make seashells.

82
Q

What is the difference between chalk and chert?

A

Chalk – Plankton (Foraminifera;CaCO3)

Chert – Plankton (Radiolaria; SiO2)

83
Q

What are 4 classes for chemical sedimentary rocks?

A
  • Evaporites
  • Travertine
  • Replacement chert
  • Dolostone: CaMg(CO3)2
84
Q

What is travertine? How is it formed and where is it found?

A

Travertine—calcium carbonate (CaCO3) precipitated from ground water where it reaches the surface.

  • CO2 expelled into the air causes CaCO3 to precipitate.
  • Thermal (hot) springs. #Caves—speleothems.
85
Q

What is replacement chert?

A
  • nonbiogenic

- Cryptocrystalline silica gradually replaced calcite, long after limestone was deposited.

86
Q

Give 3 examples of replacement chert and describe them

A
  • Flint—colored black or gray from organic matter.
  • Agate—precipitates in concentric rings.
  • Petrified wood—wood grain preserved by silica.
87
Q

______ environments are those where sediment is deposited above sea level.

A

Terrestrial

88
Q

How are sedimentary rocks formed in glacial environments? Give examples of the sedimentary rocks that can be found

A

In glacial environments, sediments are created, transported, and deposited by the actions of moving glacial ice. Ice carries and dumps every grain size. A common feature of this environment is glacial till, a poorly sorted mixture of all grain sizes, gravel, sand, silt, and clay.

89
Q

How are sedimentary rocks formed in mountain stream environements? Give examples of the sedimentary rocks that can be found

A

In mountain stream environments, water carries large clasts during floods. During low-flow conditions, cobbles and boulders are immobile. Coarse conglomerate is a characteristic of this setting.

90
Q

What are alluvial fans?

A

Alluvial fans are cone-shaped wedges of sediments that pile up where a rapid drop in stream velocity occurs at a mountain front.

91
Q

Give examples to terrestrial environments

A
  • sand-dune environments
  • lake
  • delta
  • mountain stream environments
  • glacial environments
92
Q

How are sedimentary rocks formed in sand-dune environments? Give examples of the sedimentary rocks that can be found

A

Wind-blown, well-sorted sand – aeolian deposition.

  • Dunes move according to the prevailing winds.
  • Result in uniform sandstones with gigantic cross beds.
93
Q

How are sedimentary rocks formed in lake environments? Give examples of the sedimentary rocks that can be found

A

large ponded bodies of water.

  • Gravels and sands trapped near shore.
  • Well-sorted muds deposited in deeper water.
94
Q

How are sedimentary rocks formed in delta environments?

A

sediment piles up where a river enters a lake.

95
Q

________ environments are those where sediment is deposited at or below sea level.

A

Marine

96
Q

How does a marine delta environment form?

A

In a marine delta environment, sediment accumulates where river velocity drops upon entering the sea. Deltas grow over time, building out into the basin. Many sub-environments present.

97
Q

Give examples to marine environments?

A
  • coastal beach sands
  • shallow water carbonate environments.
  • deep marine deposits
98
Q

How are sedimentary rocks formed in coastal beach sands? Give examples of the sedimentary rocks that can be found

A
  • Sand is moved along the coastline.
  • Sediments are constantly being processed by wave action.
  • A common result? Well-sorted, well-rounded medium sand.
  • Beach ripples often preserved in sedimentary rocks.
99
Q

How are sedimentary rocks formed in shallow water carbonate environments? Give examples of the sedimentary rocks that can be found

A
  • Most sediments are carbonates—shells of organisms.
  • Warm, clear, marine water, relatively free of clastic sediments.
  • Protected lagoons accumulate mud.
  • Wave-tossed reefs are made of coral and reef debris.
  • Source of limestones.
100
Q

How are sedimentary rocks formed in deep marine deposits? Give examples of the sedimentary rocks that can be found

A

Fines settle out far from land.

  • Skeletons of planktonic organisms make chalk or chert.
  • Fine silt and clay lithifies into shale.

6-Understanding Planet Earth (18 decks)

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