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Sedimentology and Stratigraphy > (S1) Terrigenous Clastic Sediments: Gravel, Sand and Mud > Flashcards

(S1) Terrigenous Clastic Sediments: Gravel, Sand and Mud Flashcards

1
Q

Define “detritus”

A

Clasts made of pre-existing rocks

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2
Q

What is another name for detritus

A

Siliclastic Sediments

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3
Q

Sandstones/Conglomerates make up __% of the sedimentary rocks in the geological records

A

20-25%

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4
Q

Mudstone makes up __% of the sedimentary rocks in the geological record

A

60%

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5
Q

How do we define the prefix of a conglomerate

A

The maximum size of clast found, eg pebble conglomerate

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6
Q

What is the name of the grain-size scale used in sedstrat?

A

Wentworth Scale (Udden - Wentworth Grain-size Scale)

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7
Q

What is the phi scale in the Wentworth scale?

A

A numerical representation, where Φ = -log2

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8
Q

How many class devisions are there in the Wentworth Scale, and what are they?

A

Four: Gravel, Sand, Mud and Clay

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9
Q

Why is -ve used on the phi scale?

A

Conventional to have grain-size on a graph decreasing from left to right

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10
Q

Another name for consolidated gravel

A

Conglomerate, named according to dominant grain size

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11
Q

What is a breccia?

A

Conglomerate with angular clasts

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12
Q

What are the two forms of breccia?

A

Tectonic breccia and sedimentary breccia

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13
Q

What is another name for a lithified mixture of rounded and angular clasts?

A

Breccio-conglomerate

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14
Q

What is a monomict?

A

Gravel/Conglomerate made of all same material

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15
Q

What is a polymict?

A

Gravel/Conglomerate made of many different types of material

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16
Q

What is an oligomict?

A

Gravel/Conglomerate made of two or three different types of material

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17
Q

How can we determine the source of the clasts in a conglomerate?

A

Comparison to surrounding bedrock

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18
Q

What is the matrix?

A

Finer material in between clasts in a conglomerate

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19
Q

What is a “sandy conglomerate” and a “muddy conglomerate”?

A

A conglomerate where >20% of the matrix is made of sand or mud respectively

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20
Q

What is an “intra-formational conglomerate” and how does it form?

A

Clasts are same composition as matrix, formed from reworking of lithified sediment soon after deposition

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21
Q

Define “clast supported” and “matrix supported”

A

Clast supported: clasts touching, matrix supported: clasts not touching

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22
Q

In erosional processes, what kind of rocks form spheres?

A

Rocks with equally shaped fracture planes, which form cubic blocks, in turn eroding to spheres

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23
Q

In erosional processes, what kind of rocks form oblate/discoid forms?

A

Lithologies which break into slabs, such as well bedded sandstone/limestone, clasts with one axis shorter than other two

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24
Q

In erosional processes, what kind of rocks form rod/prolate forms?

A

Less common, from metamorphic rocks with strong linear fabrics

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25
Q

In erosional processes, what kind of rocks form bladed?

A

Rocks with a mixture of strong linear fabrics and slab-like lithologies

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26
Q

What is imbrication?

A

Where discoid clasts become preferentially orientated due to flow - e.g. dipping upstream

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27
Q

What is paleoflow?

A

A bedform or indicator which gives an idea of which direction a sediment carrier (river, glacier etc) used to preferentially flow

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28
Q

How does paleoflow differ in an elongated clast between glacial deposition and river deposition?

A

Glacial: long axis is parallel to flow
River: long axis is perpendicular to flow

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29
Q

What is an arenite?

A

Sandstone with less than 15% matrix

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30
Q

What are detrital mineral grains?

A

Mineral grains from pre-existing rock broken down to sand-sized grains

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31
Q

What are lithic fragments?

A

sand-sized pre-existing grains (Not Quartz or Feldspar)

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32
Q

What is the most common mineral in sand/sandstone?

A

Quartz

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33
Q

What mineral is absent from basic igneous?

A

Quartz

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34
Q

What is the Moh’s hardness of quartz?

A

7

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35
Q

Why is the Moh’s hardness of quartz relevant in sedstrat?

A

Due to hardness, found in abundance in the sedimentary record: remains in tact for long periods/distances

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36
Q

Aside from it’s physical hardness, what other property does quartz possess, and why is it relevant in sedstrat?

A

Resistance to chemical breakdown - stable at earths surface

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37
Q

Aside from quartz, what other mineral is common in most igneous rocks?

A

Feldspar

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38
Q

What two important properties does feldspar possess relevant to the sedimentary record?

A

More susceptible to chemical and physical breakdown than quartz

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39
Q

Where is feldspar most likely to be found in the sedimentary record? (Two possibilities)

A

Where transportation pathway is short and chemical weathering is not too intense

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40
Q

What significant mineral to the sedimentary record is platy in shape?

A

Mica

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41
Q

How is mica often found in the sedimentary record and why?

A

In bands: it’s platy shape means that the settling of grains takes longer

42
Q

Most sandstones contain __% heavy minerals with densities of __ g cm-3

A

<1%, 2.85 g cm-3

43
Q

How can heavy minerals be separated from other sediments in a sample?

A

Placed into a liquid with a similar density as the majority of the grains (2.6-2.7 g cm-3)

44
Q

Why is it so useful to determine heavy minerals in a sedimentary sample?

A

Usually characteristic of a particular province

45
Q

Give five examples of heavy minerals which could be found within a sedimentary sample

A

Zircon, Tourmaline, Rutile, Apatite, Garnet

46
Q

Give four examples of misc. minerals which could be found within a sedimentary sample

A

Olivine, Pyroxene, Amphibole, Oxides of Iron

47
Q

What feature does the remaining misc. minerals found within a sedimentary sample usually have?

A

Readiness to chemical breakdown

48
Q

Give an example of a particularly resistant lithic fragment

A

Chert

49
Q

Aside from specific identified minerals, what are the other four potential components of a sedimentary sample?

A

Lithic fragments, Biogenic material, Authigenic material and Matrix

50
Q

Give two examples of biogenic material that could be found within a sandstone sample

A

CaCO3 - Shells

Wood/Seeds preserved in Sandstone

51
Q

What is an authigenic material?

A

Crystallised minerals which grow in depositional environments

52
Q

Give two prominent examples of authigenic minerals

A

Calcium carbonate - in supersaturated conditions

Glauconite - green iron silicate in shallow marine environments

53
Q

What would the typical matrix grain-size be for a sand/sandstone?

A

Silt/Clay

54
Q

What is the difference between matrix and cement?

A

Matrix is deposited with the larger grains, where as cement is chemically precipitated after deposition

55
Q

What is a micaceous sandstone, and when is it used?

A

Mica rich sandstone

56
Q

What is a calcareous sandstone, and when is it used?

A

Calcium Carbonate rich sandstone

57
Q

What is a ferruginous sandstone, and when is it used?

A

Iron rich sandstone

58
Q

What is the name of the classification scheme used in full petrographic analysis of a sand sized sedimentary sample?

A

Pettijohn Classification Scheme

59
Q

What shape could be used to describe the Pettijohn Classification Scheme?

A

Toblerone plot

60
Q

What is the name of a cross section found within the Pettijohn Classification Scheme?

A

QFL triangle

61
Q

What are the two steps in using the Pettijohn Classification Scheme?

A
  1. Determine relative QFL abundance

2. Determine matrix %

62
Q

What is a “wacke”?

A

Matrix of 15-75%

63
Q

How is a “mudstone” officially recognised in petrographic analysis?

A

A matrix of >75% within a sandstone/sand sample

64
Q

What is a “greywacke”?

A

Another name for a Feldspathic Wacke/Lithic Wacke

65
Q

What is the technical definition of a claystone?

A

> 2/3rd of particles are clay sized

66
Q

What is the technical definition of a siltstone?

A

Majority of particles silt sized

67
Q

What is the name for a rock which has a mixture of minuscule particle sizes?

A

Mudstone

68
Q

What is “fissility”?

A

A characteristic whereby a rock breaks predominantly in one plane

69
Q

Give an example of a fissile rock and the plane it usually breaks on

A

Shale - Parallel to bedding

70
Q

What is the difference between shale and slate?

A

Slate = metamorphic, breaks along the cleavage plane

71
Q

What is the most resistant mineral in silt/siltstone?

A

Quartz

72
Q

Aside from quartz, what other four minerals are often found in silt/siltstone?

A

Feldspar, Muscovite, Calcite, Iron Oxides

73
Q

What is “rock flour”?

A

Silt sized lithic fragments, usually from glacial erosion

74
Q

What type of energy environment is usually associated with silt/siltstone?

A

Low velocity/energy, still waters

75
Q

What grain size is loess usually associated with?

A

Silt/clay

76
Q

When in the geological record is loess usually most prominent?

A

Important during glacials

77
Q

Describe in one sentence how clay minerals form

A

Chemical products of feldspars and other silicates

78
Q

What is the name for the group of clay minerals with two layers?

A

Kandite Group

79
Q

What is the name for the group of clay minerals with three layers?

A

Smectite Group

80
Q

Under what conditions does Kaolinite usually form? (Environment, Bedrock lithologies)

A

Warm, humid environments where acidic waters leech bedrock lithologies such as granite

81
Q

Under what conditions does Montmorillonite usually form? (Environment, pH)

A

Moderate temperature conditions, neutral to alkaline pH or alkaline pH in arid conditions

82
Q

What clay group does Montmorillonite fall into?

A

Smectite Group

83
Q

What clay group does Kaolinite fall into?

A

Kandite Group

84
Q

What physical property does the Smectite Group have, and therefore what is another name for this group?

A

Swelling clays, due to their ability to absorb water within their structure

85
Q

Name the four most prominent clay types

A

Kaolinite, Montmorillonite, Illite, Chlorite

86
Q

Under what conditions does Illite usually form? (Environment, Leeching)

A

Temperate areas where leeching is limited

87
Q

Under what conditions does Chlorite usually form? (Groundwater, Environment)

A

Acidic groundwater conditions, arid climates

88
Q

What particular lithology does montmorillonite, illite and chlorite commonly form from?

A

Weathering of volcanic rocks, particularly volcanic glass

89
Q

What two properties make clay particles cohesive?

A

Electrostatic bonds between particles due to incomplete bonds, strong surface tension of smaller particles

90
Q

How does the cohesive property of clay lead to the formation of deltas?

A
  1. Particles flocculate into smaller aggregates
  2. Deposition is enhanced by saline and the fresh to saline transition
  3. Cohesion keeps particles from becoming re-immobilised in regions of intermittent flow
91
Q

What are the five properties which define the texture of a terrigenous clastic rock

A

Clasts/Matrix, Sorting, Clast Roundedness (i.e. breccia), Clast Sphericity (i.e. parent feature, not feature of distance), Fabric (alignment)

92
Q

What is the main control of the shape of a pebble?

A

Shape of source bedrock, no amount of rounding will change fundamental directions

93
Q

What is the definition of “maturity” of a rock?

A

Extent of change compared to bedrock source

94
Q

Give an example where a sample may be compositionally immature but texturally mature?

A

Volcanic beach - unstable minerals still present in a high energy environment

95
Q

What are the properties of a texturally immature sandstone?

A

> 15% mud (wacke)

96
Q

What are the properties of a texturally submature sandstone?

A

0.5)

97
Q

What are the properties of a texturally mature sandstone?

A

<0.5)

angular clasts

98
Q

What are the properties of a texturally supermature sandstone?

A

<0.5)

rounded clasts

99
Q

What does “labile” mean?

A

Less resistant

100
Q

Under what circumstances is a sandstone compositionally mature?

A

Is an arenite where the proportion of quartz is particularly high

Sedimentology and Stratigraphy (19 decks)

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