Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Sedimentology and Stratigraphy > (S14) Post Depositional Structures and Diagenisis > Flashcards

(S14) Post Depositional Structures and Diagenisis Flashcards

1
Q

What happens to water in sediment when it becomes trapped by overlying sealing layer?

A

Water becomes overpressured

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What happens to overpressured water when a crack in the layer above forms?

A

Has ability to be released at high velocities upwards causing fluidisation of the sediment

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the name for the process by which sediment moves with released pore-water?

A

Fluidisation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the name for the process by which particles are seperated by size?

A

Elutriation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What occurs in deposition after fluidisation occurs?

A

Elutriation of sediment as finer particles are more readily carried away

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the name for the shorter-term process by which a mass of saturated sediment becomes momentarily liquid?

A

Liquefaction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What timescales does liquefaction occur on?

A

Short timescales - Earthquakes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What are the four loose categories of soft-sediment loading?

A

Sediment Instabilities, Liquefaction, Fluidisation, Loading

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What are the two main process of soft-sediment deformation due to sediment instabilities?

A

Slumped beds, Growth faults

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What are the two main processes which can cause sediment failure?

A

Shock, Additional sedimentation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What internal structures may be found in slumped beds?

A

Fold structures with noses of anticlines facing downslope

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the name for the surface left after slumped material is removed?

A

Slump scar

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How is a slump scar preserved?

A

Later infil of sedimentation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What planes can a slump-scar be found on?

A

All three - “spoon-shaped”

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the range of size for a slump scar?

A

Few meters to hundreds meters across

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What depositional environments are slump scars most common in?

A

Deltaic

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

What is the name for a structure which forms during deposition?

A

Syndepositonal structures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Where do growth faults most commonly occur?

A

Delta front successions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What is a growth fault?

A

A surface along which there is relative displacement

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

What is the difference between slumped beds and growth faults?

A

Growth faults affect only part of the succession and overlying beds are unaffected by the fault

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

What is the name for a spoon-shaped fault?

A

Listric fault

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

What are the two main processes of soft-sediment deformation due to sediment liquefaction?

A

Convolute bedding/lamination, Overturned cross-stratification

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is the name for the highly folded layers which are formed by liquefaction?

A

Convolute bedding, Convolute lamination

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

When does convolute bedding/lamination occur? (2)

A

Slight slope OR overlying shear stress

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

What structures are typical of convolute lamination/bedding?

A

Asymmetric folds, where noses point downslope/in direction of flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

What is the two names for shearing of upper part of cross bedding?

A

Recumbent cross-bedding or Overturned cross-bedding

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

When does overturned cross-stratification occur?

A

When sands are deposited in sub-aqueous dunes are loosely packed and thus saturated - easily sheared by a strong current

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What are the three main processes of soft-sediment deformation due to fluidisation?

A

Dish and pillar structures, Clastic dykes, Sand volcanoes and extruded sheets

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is another name for a structure formed by fluidation processes and why is it called that?

A

Dewatering structures, result of expulsion of pore water from a bed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

What is the name for the concave structures formed by upwards release of fluid?

A

Dish structures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

What is the range of size for a dish structure?

A

Few cm to tens cm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

How are dish structures often recognised?

A

Fine clay laminae which is the cause of the local barrier

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

What form do dish structures make in plan view?

A

Polygonal shapes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

What is another name for a pillar structure?

A

Elutriation pipes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

What is another name for an elutriation pipe?

A

Pillar structure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

What is a pillar structure?

A

A vertical water-escape channel which can be a simple tube or vertical sheet

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

What other structure often forms in association to a pillar structure?

A

Dish structures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

What is the name for a subsurface sheet-like vertical body formed from fluidisation?

A

Clastic dykes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

What process often occurs within a clastic dyke?

A

Elutration

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

What is usually the main sedimentary component of a clastic dyke?

A

Fine sand

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

What is the typical width of a clastic dyke?

A

cm to 10’s cm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

What is the common characteristic of a clastic and volcanic dyke?

A

Both cross-cut other beds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

When do clastic dykes form?

A

When a fracture occurs above an overpressured bed and upward rush of pore waters carries sediment into the crack

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

What internal structures may be found in clastic dykes?

A

Some layering parallel to the walls but mostly structureless

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

What is the difference between a clastic dyke and a fissure fill?

A

Fissure fills fill form above to below, clastic dykes fill upwards from pressure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

What are the two main processes which form the fissures to allow for fissure fill?

A

Earthquake opening, Solution (e.g. Karst)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

What is typical of the deposits formed by fissure fills?

A

Clastic, Breccia common

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

What is the name for a fissure fill which has occurred over multiple phases?

A

Neptunian dyke

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

When do sand volcanoes occur?

A

Liquified sediment brought to the surface in isolated pipes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

How large are sand vocanoes?

A

Few 10’s cm to m’s

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

When are sand volcanoes preserved?

A

Low energy conditions prevent sand from being reworked by currents

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

What is the name for a sheet of sand brought to the surface by a clastic dyke?

A

Extruded sheet

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

How is an extruded sheet recognised in the stratigraphic record?

A

Has to be linked to an underlying clastic dyke

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

What are the two main processes of soft-sediment deformation due to sediment loading?

A

Load casts and Diaprism

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

When does a load cast form?

A

Saturdated, low density body overlain by saturdated mass of higher density - sinks to form downward bulbous structures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

What is the name for a structure where a saturdated underlying low density body is forced up into saturated high density body?

A

Flame structure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
57
Q

What is the name for saturated sand which sinks into saturated mud to become completely isolated from its original layer?

A

Load balls

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
58
Q

What is the common name for the overall load-cast structures?

A

Ball and pillow structures

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
59
Q

Where are ball and pillow structures most common?

A

Base of sandy turbidite beds

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
60
Q

What is the name for the largescale movement of material due to the instability of densities between layers?

A

Diapirism

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
61
Q

Where is dispirism most common?

A

Where density difference between the layers is large and low-density material is relatively mobile

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
62
Q

What is the two main factors which control the bulk density of a rock/sediment layer?

A

Density of minerals, Proportion of material occupied by pores spaces filled with gas/liquid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
63
Q

What are the two main types of diapirism?

A

Salt and Mud

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
64
Q

What are the two important implications of diapirism for sedimentology and stratigraphy?

A

Can create local highs on sea floor - locis for carbonate development, Can create subsurface structures which are traps for hydrocarbons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
65
Q

What is the typical mineral density of halite?

A

2.17 g cm-3

66
Q

How does halite react to a few hundred meters of overburden?

A

Plasticically due to heat/pressure

67
Q

What two conditions does upwards moving halite favour?

A

Thinner overburden, Overburden weakened by faults

68
Q

What is the overall name for the processes involved in diapiric movement of salt?

A

Salt tectonics

69
Q

What can occur when salt breaks through overburden?

A

Mass of halite erupts like very viscous volcanic flow

70
Q

How does mud diapirism occur?

A

Muddy sediment deposited rapidly and maintains up to 75% water, water trapped in muddy layers

71
Q

Where does mud diapirism most commonly occur?

A

Fine, pro-delta facies overlain by sands with progradation

72
Q

What is another name for mud diapirism?

A

Shale diapirism

73
Q

What temperatuer and depth does diagenisis occur up to?

A

Below 250 degrees, Depths of up to 5000m

74
Q

What is the main difference between diagenisis and metamorphism?

A

Metamorphism destroys original fabric

75
Q

What % can compaction reduce the volume of sand by?

A

Up to 10%

76
Q

What may happen to weaker grains such as mica flakes and mud clasts during compaction in a sandstone?

A

Deformed plastically by pressure from stronger grains

77
Q

What is another name for fracturing which may occur in grains under compaction pressure?

A

Cataclasis

78
Q

What is the maximum change in % of water content from a mud deposited in water?

A

From 80% at first depositon to 30% under burial

79
Q

Give an example of a rock which may not compact at all?

A

Boundstone

80
Q

What would happen to sedimentary structures, such as cross-strata, subject to compaction?

A

Angle of strata decreased slightly

81
Q

What is the name for compaction which varies laterally?

A

Differential compaction

82
Q

Give an example where differential compaction occurs

A

A sand bar surrounded by mud - where submarine channel cuts into muds

83
Q

Why is differential compaction less marked in fluvial successions where sand-filled channels are surrounded by overbank mudstones?

A

Fine sediment dries out between flood events - loss of pore waters and thus overburden pressure on two lithologies will be the same

84
Q

Mud layers may become ___ around lenses of sand formed by a ripple/dune bedform during compaction

A

Draped

85
Q

What is the name for the lithification process which occurs on a microscopic scale

A

Pressure solution/pressure dissolution

86
Q

How does pressure solution/dissolution work?

A

Points of contact between mineral grains have higher pressure with compaction and thus with presence of pore waters, mineral gets reprecipitated

87
Q

What are the four stages of grain orientation during compaction events?

A

Point contacts, Long contacts, Concavo-convex contacts, Sutured contacts

88
Q

When do grains orientate themselves together as long contacts?

A

Subject to enough pressure that the grains re-orientate themselves elongate fabrics

89
Q

When do grains orientate themselves together as concavo-convex contacts?

A

When pressure solution between grains occurs

90
Q

When do grains orientate themselves together as sutured contacts?

A

When subject to very high overburden pressures - most commonly seen in metamorphism

91
Q

What is the name for cements which form at sediment-water or sediment-air interfaces?

A

Eogenetic cements

92
Q

When do eogenetic cements form?

A

Synsedimentary or very soon after deposition

93
Q

What is the name for cements which form from chemical changes in sediments buried and saturated?

A

Mesogenetic cements

94
Q

What is the name for cements which form during uplift?

A

Telogenetic cementation

95
Q

What proccesses occur to make up cementation at low temperatures? (4)

A

Dissolution, Precpitation, Recrystalization, Replacement of minerals

96
Q

What two factors increase caronate solubilities?

A

Decreasing tempearture and increasing acidity (decreasing pH)

97
Q

What two factors determine the processes of dissolution?

A

Compaction of grain minerals, chemistry of pore water

98
Q

The presence of ___ ___ will increase acidity of pore waters and leaching of compunds from __ __ may also reduce the pH

A

Carbon dioxide, Organic matter

99
Q

What two factors increase silica solubility?

A

Warmer water, higher pH (more alkaline)

100
Q

___ silica is more soluble than ___ quartz

A

Opaline, Crystaline

101
Q

What scale does most quartz dissolution occur at?

A

Grain boundaries - pressure dissolution, reprecipitated in adjacent pore spaces

102
Q

What is the difference between cementation and matrix?

A

Matrix is filling of finer grains, cementation is lithification via precipitation during diagenisis

103
Q

Name seven minerals which often are found as cements

A

Quartz, Chalcedony, Carbonates, Calcite, Aragonite, Dolomite, Siderite

104
Q

What are the three variables affecting the type of cement formed in diagenisis?

A

Availability of minerals, Temperature, Acidity of pore waters

105
Q

Growth of cement preferentially takes place on a grain of what?

A

The same composition as the cement

106
Q

What is the name for cement which grows on crystal of the same composition?

A

Overgrowth

107
Q

What is the name where cement forms a continuous crystal upon precipitation?

A

Syntaxial overgrowths

108
Q

Where are overgrowths from diagenisis most commonly seen?

A

Silica-cemented quartz sands

109
Q

What type of overgrowth can form on biogenic fragments of organisms such as crinoids and echinoids, and why?

A

Sparry calcite overgrowths, because these organisms create single calcite crystals in their structures

110
Q

What is porosity?

A

The volume of rock not occupied by solid material in proportion to rocks overall volume

111
Q

What is permeability?

A

The ease at which fluid can pass throug a volume of rock

112
Q

What is primary porosity?

A

Porosity of a rock at the time of its deposition

113
Q

What is interparticle porosity?

A

Porosity from space between grains (as opposed to intraparticle porosity)

114
Q

What is intraparticle porosity?

A

Porosity from voids within the grains - common in shelly organisms with cavities

115
Q

What is secondary porosity?

A

Forms after deposition - from diagenisis which selectively dissolves parts of rock

116
Q

When can a rock have a high porosity and a low permeability?

A

When pore spaces are not connected, e.g. in porous sandstone with partial cement or limestone with porosity sealed in organisms chambers

117
Q

When can a rock have a low porosity and a high permeability?

A

Contains a large number of interconnected cracks

118
Q

What may high-Mg calcite/aragonite become recrystalized to in diagenisis?

A

Low-Mg Caclite

119
Q

When does recrystalization not occur in diagenic conditions for organisms?

A

When the organism is made up of low-Mg calcite

120
Q

When does recrystalization of siliceous organisms occur?

A

When the original structure is amporphous opaline silica - recrystalizes to microcrystaline quartz

121
Q

What is the most common type of feldspar to undergo replacement during diagenisis?

A

Calcium rich

122
Q

What are the least common types of feldspar to undergo replacement during diagenisis?

A

Sodium rich and potassium rich

123
Q

What does feldspar become replaced with during diagenisis?

A

Clay minerals

124
Q

Why might cementation be unevenly disperced through a rock?

A

Presence of remains of plant/animal creates localised concetrations of organic material

125
Q

What is a name for a irregular cemented patch within a rock?

A

Nodules

126
Q

What is the name for a symmetrical, rounded or discoud feature cemented in rock called?

A

Concretions

127
Q

What is another name for a nodule found in a sand bed?

A

Doggers

128
Q

Concretions formed at particular levels within a succession may ___ to form ___

A

Coalesce, bands of well-cemented rock

129
Q

What feature makes it evident that concetions in mudrocks form soon after deposition?

A

Layer within mudstone drapes around concretion and thus mudstone underwent compaction after concretion formed

130
Q

What is the name for the feature whereby some carbonate concretions in mudstones display an array of cracks filled with sparry calcite?

A

Septarian structures

131
Q

What are the two theories for the formation of septarian structures?

A

Shrinkage (similar to synereiss) or excess pore fluid pressure

132
Q

When does primary chert form?

A

As layers associated with deep-water sediments

133
Q

When does secondary chert form?

A

As concretions or nodules from silica concentration during diagenisis

134
Q

What lithology is secondary chert most associated with and why?

A

Limestones, because siliceous organisms which were deposited with calcareous sediments precpitate silica

135
Q

The diagenetic origin of secondary cherts can be seen in ___ ___ where structures of organisms originally had ___ hard parts

A

Replacement fabrics, carbonate

136
Q

What is flint?

A

A specific name for nodules of chert in Cretaceous Chalk

137
Q

What is the specific name given to nodules of chert in Cretaceous Chalk?

A

Flint

138
Q

In fine grained sediments, ___ ___ may form around particles of organic matter where it draws oxygen ions from the surrounding material

A

Reduction spots

139
Q

What colour are reduction spots typically?

A

Green or grey

140
Q

What is the name for bands of colour formed by concentrations of iron oxides in irregular layers within a rock?

A

Liesegangen bands

141
Q

How big are liesegangen bands?

A

mm scale - look like laminae

142
Q

Which is more extensive in clastic diagenesis, eogenetic or mesogenetic cementation?

A

Mesogenetic cements

143
Q

What does mesogenetic cementation mostly involve in clastic rocks? (3)

A

Authigenic growth of quartz, calcite and clays

144
Q

Where does the calcite commonly originate from for cementation in clastic rocks?

A

Aragonitic shelly material deposited with sand/gravel

145
Q

Where does calcite nucleate in cementation of clastic rocks?

A

Tends to favour carbonate grains but crystals may envekioe a number of grains if sparse

146
Q

What is the name for the cement fabric found where calcite nucleates around clastic rocks during cementation?

A

Poikilotopic cement fabric

147
Q

What does poikilotopic cement fabric look like in hand specimen?

A

Can have a shiny surface on parts of sandstone

148
Q

How does quartz cements commonly form in sandstones?

A

Syntaxial overgrowths, diffusion along grain boundaries are pressure contacts

149
Q

What is a good source of silica in water to form cements? (3)

A

Volcanic class, Extremely fine quartz dust, Skeletal material from sponges diatoms and radiolaria

150
Q

Silica cements are commonly found only in circumstances where there is a absence of ____ ___. For example __

A

Calcium carbonate, Quartz rich sands in continental environments

151
Q

When do clay mineral cements form in diagenetic history?

A

Early or late

152
Q

What formsc clay mineral cements? (2)

A

Precipitation from pore waters or recrystalization of other clay minerals

153
Q

Where is Illite commonly formed?

A

Diagenetically at depth (T = >70 degrees) from other clay minerals which formed at the surface

154
Q

At what temperature is Illite commonly formed?

A

> 70 degrees

155
Q

What is the index which can be used to measure burial temperature in clay diagenisis?

A

Illite crystallinity index

156
Q

Once formed Illite is very ___ and is often ___.

A

Stable, reworked

157
Q

What clay is formed at lower temperatures than Illite from weathering of volcanic glass, feldspare and other silicate minerals?

A

Smectite

158
Q

What does smectite transform into at higher temperatures?

A

Illite

159
Q

What clay is less common as a diagenetic mineral, occuring as part of the formation of illite?

A

Chlorite

160
Q

What clay is a formed as a product of weathering above the water table, and what clay does it become with burial?

A

Kaolinite, Illite

Sedimentology and Stratigraphy (19 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions