(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
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
3
Q
What is the name for the process by which sediment moves with released pore-water?
A
Fluidisation
4
Q
What is the name for the process by which particles are seperated by size?
A
Elutriation
5
Q
What occurs in deposition after fluidisation occurs?
A
Elutriation of sediment as finer particles are more readily carried away
6
Q
What is the name for the shorter-term process by which a mass of saturated sediment becomes momentarily liquid?
A
Liquefaction
7
Q
What timescales does liquefaction occur on?
A
Short timescales - Earthquakes
8
Q
What are the four loose categories of soft-sediment loading?
A
Sediment Instabilities, Liquefaction, Fluidisation, Loading
9
Q
What are the two main process of soft-sediment deformation due to sediment instabilities?
A
Slumped beds, Growth faults
10
Q
What are the two main processes which can cause sediment failure?
A
Shock, Additional sedimentation
11
Q
What internal structures may be found in slumped beds?
A
Fold structures with noses of anticlines facing downslope
12
Q
What is the name for the surface left after slumped material is removed?
A
Slump scar
13
Q
How is a slump scar preserved?
A
Later infil of sedimentation
14
Q
What planes can a slump-scar be found on?
A
All three - “spoon-shaped”
15
Q
What is the range of size for a slump scar?
A
Few meters to hundreds meters across
16
Q
What depositional environments are slump scars most common in?
A
Deltaic
17
Q
What is the name for a structure which forms during deposition?
A
Syndepositonal structures
18
Q
Where do growth faults most commonly occur?
A
Delta front successions
19
Q
What is a growth fault?
A
A surface along which there is relative displacement
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
21
Q
What is the name for a spoon-shaped fault?
A
Listric fault
22
Q
What are the two main processes of soft-sediment deformation due to sediment liquefaction?
A
Convolute bedding/lamination, Overturned cross-stratification
23
Q
What is the name for the highly folded layers which are formed by liquefaction?
A
Convolute bedding, Convolute lamination
24
Q
When does convolute bedding/lamination occur? (2)
A
Slight slope OR overlying shear stress
25
What structures are typical of convolute lamination/bedding?
Asymmetric folds, where noses point downslope/in direction of flow
26
What is the two names for shearing of upper part of cross bedding?
Recumbent cross-bedding or Overturned cross-bedding
27
When does overturned cross-stratification occur?
When sands are deposited in sub-aqueous dunes are loosely packed and thus saturated - easily sheared by a strong current
28
What are the three main processes of soft-sediment deformation due to fluidisation?
Dish and pillar structures, Clastic dykes, Sand volcanoes and extruded sheets
29
What is another name for a structure formed by fluidation processes and why is it called that?
Dewatering structures, result of expulsion of pore water from a bed
30
What is the name for the concave structures formed by upwards release of fluid?
Dish structures
31
What is the range of size for a dish structure?
Few cm to tens cm
32
How are dish structures often recognised?
Fine clay laminae which is the cause of the local barrier
33
What form do dish structures make in plan view?
Polygonal shapes
34
What is another name for a pillar structure?
Elutriation pipes
35
What is another name for an elutriation pipe?
Pillar structure
36
What is a pillar structure?
A vertical water-escape channel which can be a simple tube or vertical sheet
37
What other structure often forms in association to a pillar structure?
Dish structures
38
What is the name for a subsurface sheet-like vertical body formed from fluidisation?
Clastic dykes
39
What process often occurs within a clastic dyke?
Elutration
40
What is usually the main sedimentary component of a clastic dyke?
Fine sand
41
What is the typical width of a clastic dyke?
cm to 10's cm
42
What is the common characteristic of a clastic and volcanic dyke?
Both cross-cut other beds
43
When do clastic dykes form?
When a fracture occurs above an overpressured bed and upward rush of pore waters carries sediment into the crack
44
What internal structures may be found in clastic dykes?
Some layering parallel to the walls but mostly structureless
45
What is the difference between a clastic dyke and a fissure fill?
Fissure fills fill form above to below, clastic dykes fill upwards from pressure
46
What are the two main processes which form the fissures to allow for fissure fill?
Earthquake opening, Solution (e.g. Karst)
47
What is typical of the deposits formed by fissure fills?
Clastic, Breccia common
48
What is the name for a fissure fill which has occurred over multiple phases?
Neptunian dyke
49
When do sand volcanoes occur?
Liquified sediment brought to the surface in isolated pipes
50
How large are sand vocanoes?
Few 10's cm to m's
51
When are sand volcanoes preserved?
Low energy conditions prevent sand from being reworked by currents
52
What is the name for a sheet of sand brought to the surface by a clastic dyke?
Extruded sheet
53
How is an extruded sheet recognised in the stratigraphic record?
Has to be linked to an underlying clastic dyke
54
What are the two main processes of soft-sediment deformation due to sediment loading?
Load casts and Diaprism
55
When does a load cast form?
Saturdated, low density body overlain by saturdated mass of higher density - sinks to form downward bulbous structures
56
What is the name for a structure where a saturdated underlying low density body is forced up into saturated high density body?
Flame structure
57
What is the name for saturated sand which sinks into saturated mud to become completely isolated from its original layer?
Load balls
58
What is the common name for the overall load-cast structures?
Ball and pillow structures
59
Where are ball and pillow structures most common?
Base of sandy turbidite beds
60
What is the name for the largescale movement of material due to the instability of densities between layers?
Diapirism
61
Where is dispirism most common?
Where density difference between the layers is large and low-density material is relatively mobile
62
What is the two main factors which control the bulk density of a rock/sediment layer?
Density of minerals, Proportion of material occupied by pores spaces filled with gas/liquid
63
What are the two main types of diapirism?
Salt and Mud
64
What are the two important implications of diapirism for sedimentology and stratigraphy?
Can create local highs on sea floor - locis for carbonate development, Can create subsurface structures which are traps for hydrocarbons
65
What is the typical mineral density of halite?
2.17 g cm-3
66
How does halite react to a few hundred meters of overburden?
Plasticically due to heat/pressure
67
What two conditions does upwards moving halite favour?
Thinner overburden, Overburden weakened by faults
68
What is the overall name for the processes involved in diapiric movement of salt?
Salt tectonics
69
What can occur when salt breaks through overburden?
Mass of halite erupts like very viscous volcanic flow
70
How does mud diapirism occur?
Muddy sediment deposited rapidly and maintains up to 75% water, water trapped in muddy layers
71
Where does mud diapirism most commonly occur?
Fine, pro-delta facies overlain by sands with progradation
72
What is another name for mud diapirism?
Shale diapirism
73
What temperatuer and depth does diagenisis occur up to?
Below 250 degrees, Depths of up to 5000m
74
What is the main difference between diagenisis and metamorphism?
Metamorphism destroys original fabric
75
What % can compaction reduce the volume of sand by?
Up to 10%
76
What may happen to weaker grains such as mica flakes and mud clasts during compaction in a sandstone?
Deformed plastically by pressure from stronger grains
77
What is another name for fracturing which may occur in grains under compaction pressure?
Cataclasis
78
What is the maximum change in % of water content from a mud deposited in water?
From 80% at first depositon to 30% under burial
79
Give an example of a rock which may not compact at all?
Boundstone
80
What would happen to sedimentary structures, such as cross-strata, subject to compaction?
Angle of strata decreased slightly
81
What is the name for compaction which varies laterally?
Differential compaction
82
Give an example where differential compaction occurs
A sand bar surrounded by mud - where submarine channel cuts into muds
83
Why is differential compaction less marked in fluvial successions where sand-filled channels are surrounded by overbank mudstones?
Fine sediment dries out between flood events - loss of pore waters and thus overburden pressure on two lithologies will be the same
84
Mud layers may become ___ around lenses of sand formed by a ripple/dune bedform during compaction
Draped
85
What is the name for the lithification process which occurs on a microscopic scale
Pressure solution/pressure dissolution
86
How does pressure solution/dissolution work?
Points of contact between mineral grains have higher pressure with compaction and thus with presence of pore waters, mineral gets reprecipitated
87
What are the four stages of grain orientation during compaction events?
Point contacts, Long contacts, Concavo-convex contacts, Sutured contacts
88
When do grains orientate themselves together as long contacts?
Subject to enough pressure that the grains re-orientate themselves elongate fabrics
89
When do grains orientate themselves together as concavo-convex contacts?
When pressure solution between grains occurs
90
When do grains orientate themselves together as sutured contacts?
When subject to very high overburden pressures - most commonly seen in metamorphism
91
What is the name for cements which form at sediment-water or sediment-air interfaces?
Eogenetic cements
92
When do eogenetic cements form?
Synsedimentary or very soon after deposition
93
What is the name for cements which form from chemical changes in sediments buried and saturated?
Mesogenetic cements
94
What is the name for cements which form during uplift?
Telogenetic cementation
95
What proccesses occur to make up cementation at low temperatures? (4)
Dissolution, Precpitation, Recrystalization, Replacement of minerals
96
What two factors increase caronate solubilities?
Decreasing tempearture and increasing acidity (decreasing pH)
97
What two factors determine the processes of dissolution?
Compaction of grain minerals, chemistry of pore water
98
The presence of ___ ___ will increase acidity of pore waters and leaching of compunds from __ __ may also reduce the pH
Carbon dioxide, Organic matter
99
What two factors increase silica solubility?
Warmer water, higher pH (more alkaline)
100
___ silica is more soluble than ___ quartz
Opaline, Crystaline
101
What scale does most quartz dissolution occur at?
Grain boundaries - pressure dissolution, reprecipitated in adjacent pore spaces
102
What is the difference between cementation and matrix?
Matrix is filling of finer grains, cementation is lithification via precipitation during diagenisis
103
Name seven minerals which often are found as cements
Quartz, Chalcedony, Carbonates, Calcite, Aragonite, Dolomite, Siderite
104
What are the three variables affecting the type of cement formed in diagenisis?
Availability of minerals, Temperature, Acidity of pore waters
105
Growth of cement preferentially takes place on a grain of what?
The same composition as the cement
106
What is the name for cement which grows on crystal of the same composition?
Overgrowth
107
What is the name where cement forms a continuous crystal upon precipitation?
Syntaxial overgrowths
108
Where are overgrowths from diagenisis most commonly seen?
Silica-cemented quartz sands
109
What type of overgrowth can form on biogenic fragments of organisms such as crinoids and echinoids, and why?
Sparry calcite overgrowths, because these organisms create single calcite crystals in their structures
110
What is porosity?
The volume of rock not occupied by solid material in proportion to rocks overall volume
111
What is permeability?
The ease at which fluid can pass throug a volume of rock
112
What is primary porosity?
Porosity of a rock at the time of its deposition
113
What is interparticle porosity?
Porosity from space between grains (as opposed to intraparticle porosity)
114
What is intraparticle porosity?
Porosity from voids within the grains - common in shelly organisms with cavities
115
What is secondary porosity?
Forms after deposition - from diagenisis which selectively dissolves parts of rock
116
When can a rock have a high porosity and a low permeability?
When pore spaces are not connected, e.g. in porous sandstone with partial cement or limestone with porosity sealed in organisms chambers
117
When can a rock have a low porosity and a high permeability?
Contains a large number of interconnected cracks
118
What may high-Mg calcite/aragonite become recrystalized to in diagenisis?
Low-Mg Caclite
119
When does recrystalization not occur in diagenic conditions for organisms?
When the organism is made up of low-Mg calcite
120
When does recrystalization of siliceous organisms occur?
When the original structure is amporphous opaline silica - recrystalizes to microcrystaline quartz
121
What is the most common type of feldspar to undergo replacement during diagenisis?
Calcium rich
122
What are the least common types of feldspar to undergo replacement during diagenisis?
Sodium rich and potassium rich
123
What does feldspar become replaced with during diagenisis?
Clay minerals
124
Why might cementation be unevenly disperced through a rock?
Presence of remains of plant/animal creates localised concetrations of organic material
125
What is a name for a irregular cemented patch within a rock?
Nodules
126
What is the name for a symmetrical, rounded or discoud feature cemented in rock called?
Concretions
127
What is another name for a nodule found in a sand bed?
Doggers
128
Concretions formed at particular levels within a succession may ___ to form ___
Coalesce, bands of well-cemented rock
129
What feature makes it evident that concetions in mudrocks form soon after deposition?
Layer within mudstone drapes around concretion and thus mudstone underwent compaction after concretion formed
130
What is the name for the feature whereby some carbonate concretions in mudstones display an array of cracks filled with sparry calcite?
Septarian structures
131
What are the two theories for the formation of septarian structures?
Shrinkage (similar to synereiss) or excess pore fluid pressure
132
When does primary chert form?
As layers associated with deep-water sediments
133
When does secondary chert form?
As concretions or nodules from silica concentration during diagenisis
134
What lithology is secondary chert most associated with and why?
Limestones, because siliceous organisms which were deposited with calcareous sediments precpitate silica
135
The diagenetic origin of secondary cherts can be seen in ___ ___ where structures of organisms originally had ___ hard parts
Replacement fabrics, carbonate
136
What is flint?
A specific name for nodules of chert in Cretaceous Chalk
137
What is the specific name given to nodules of chert in Cretaceous Chalk?
Flint
138
In fine grained sediments, ___ ___ may form around particles of organic matter where it draws oxygen ions from the surrounding material
Reduction spots
139
What colour are reduction spots typically?
Green or grey
140
What is the name for bands of colour formed by concentrations of iron oxides in irregular layers within a rock?
Liesegangen bands
141
How big are liesegangen bands?
mm scale - look like laminae
142
Which is more extensive in clastic diagenesis, eogenetic or mesogenetic cementation?
Mesogenetic cements
143
What does mesogenetic cementation mostly involve in clastic rocks? (3)
Authigenic growth of quartz, calcite and clays
144
Where does the calcite commonly originate from for cementation in clastic rocks?
Aragonitic shelly material deposited with sand/gravel
145
Where does calcite nucleate in cementation of clastic rocks?
Tends to favour carbonate grains but crystals may envekioe a number of grains if sparse
146
What is the name for the cement fabric found where calcite nucleates around clastic rocks during cementation?
Poikilotopic cement fabric
147
What does poikilotopic cement fabric look like in hand specimen?
Can have a shiny surface on parts of sandstone
148
How does quartz cements commonly form in sandstones?
Syntaxial overgrowths, diffusion along grain boundaries are pressure contacts
149
What is a good source of silica in water to form cements? (3)
Volcanic class, Extremely fine quartz dust, Skeletal material from sponges diatoms and radiolaria
150
Silica cements are commonly found only in circumstances where there is a absence of ____ ___. For example __
Calcium carbonate, Quartz rich sands in continental environments
151
When do clay mineral cements form in diagenetic history?
Early or late
152
What formsc clay mineral cements? (2)
Precipitation from pore waters or recrystalization of other clay minerals
153
Where is Illite commonly formed?
Diagenetically at depth (T = >70 degrees) from other clay minerals which formed at the surface
154
At what temperature is Illite commonly formed?
>70 degrees
155
What is the index which can be used to measure burial temperature in clay diagenisis?
Illite crystallinity index
156
Once formed Illite is very ___ and is often ___.
Stable, reworked
157
What clay is formed at lower temperatures than Illite from weathering of volcanic glass, feldspare and other silicate minerals?
Smectite
158
What does smectite transform into at higher temperatures?
Illite
159
What clay is less common as a diagenetic mineral, occuring as part of the formation of illite?
Chlorite
160
What clay is a formed as a product of weathering above the water table, and what clay does it become with burial?
Kaolinite, Illite
