Sediments And Stratigraphy Flashcards
1
Q
Where was the concept of deep time observed?
A
Siccar Point, between Berwick and Dunbar
2
Q
What is Huttons Unconformity?
A
Vertical Silurian rocks overlain by gently dipping denovian rocks
3
Q
Provide examples of sedimentary layers with different dips
A
Siccar Point
Mt Sharp, Mars
Stainmore Fm, Northumberland
4
Q
What is Shaler Outcrop, Mars, an example of?
A
Cross bedding
5
Q
Define sedimentary facies
A
A distinct set of lithological and fossil characteristics of a given sedimentologic unit, typically defined at the bed scale
6
Q
What are the seven characteristics of sedimentary facies?
A
Composition
Texture
Sedimentary Structures
Bedding Geometry, Style and Thickness
Nature of Bedding Contacts
Fossils
Colour
7
Q
What is the purpose of sedimentary facies?
A
Provide a guide for interpreting the environmental setting of sedimentary deposits
8
Q
What are you looking for when describing composition?
A
The mineralogical constituents and their percentages
Quartz, Lithics, Feldspar
9
Q
How do you describe texture?
A
Grain size
Shape
Sorting
Matrix supported vs clast supported
10
Q
How do you describe the sedimentary structures?
A
Types of bedforms and scale
Cross bedding _______ vs trough cross bedding _/
11
Q
How do you describe bedding geometry, style and thickness?
A
Shape (lenticular, wedge shaped etc)
Orientation
Lateral Continuity
12
Q
How do you describe the nature of bedding contacts?
A
Sharp vs gradational
wavy, flat
13
Q
How do you describe fossils?
A
Species type - body vs trace vs plant
Abundance
Life position vs disarticulated
14
Q
What is the issue with describing colour?
A
May be primary, but can also be diagenetic, reflecting chemistry of pore fluids
Ie liesegang rings
15
Q
What’s the difference between planar cross bedding and trough cross bedding?
A
Planar cross bedding bounding surfaces are planar and parallel
Trough cross bedding bounding surfaces are curve-planar and non-parallel
16
Q
What could turn rocks red?
A
Ferric iron (Fe3+) in oxidising conditions
Hematite
17
Q
What could make rocks yellow/brown/tan colours?
A
Hydrated forms of ferric oxide ie goethite and limonite
18
Q
What could be a possible cause for greenish rocks?
A
Ferrous iron (Fe2+) present in reducing conditions
Glauconite, chlorite
19
Q
What could give rocks a greyish black colour?
A
Organic Matter
Anoxic to oxic conditions
Pyrite
20
Q
What should individual facies be?
A
Distinct
Exhaustive
Reproducible
Usable
21
Q
What are we trying to reconstruct from facies?
A
Provenance (source areas)
Transport History
Place of deposition
Alteration since deposition
22
Q
Facies Association is?
A
Groups of genetically related facies in terms of a depositional environment
23
Q
What is facies succession?
A
Facies that change progressively either vertically or laterally
24
Q
What can associated facies be used for?
A
To determine a depositional environment
25
What can facies models be used for?
A standard to compare real examples
A predictor for lateral and vertical changes
A framework and guide for observations
A basis for interpretation
26
What does a sharp contact in bedding suggest?
A sudden change in the energy of an environment
27
What is rheology?
The behaviour and properties of fluids and solids
28
What is the equation for force (and the units)?
F=ma
Newtons or kgms-1
29
What is the value for gravity?
9.81ms-2
30
What is stress, and what’s its equation?
Stress is force per unit area
Normal stress = pressure = force acting normal to an area
Shear stress = t wiggly = force acting tangential to an area
o = F/A (Nm-2)
31
What is Newton’s first law?
Inertia - an object will remain at rest or in uniform motion unless acted on by an external force
32
What is Newton’s second law?
F=ma
33
What is archimedes principle of levers and torque? (Formula)
Work = force x length of lever arm = FL
34
What is it called when grains start to move?
Sediment entrainment
35
What are the two types of loads in sediment transport?
Bed load and suspended load
36
What are the types of bedload?
Traction transport - rolling, sliding, impact and creep
Uninterrupted saltation - perfect curve
Interrupted saltation - grain collision
37
What is Fg?
Gravity force
= mg
= density x volume x g
38
What is the true equation for gravity force (Fg)?
Fg = (ps - pf) 1/6 pi D^3 g
Density of sediment minus density of fluid x 1/6 pi x diameter cubed x g
39
What is the point when a grain starts moving called?
Critical entrainment
40
What are the forces acting on a grain?
Fl - lift force l^
Ff - fluid force /
Fd- drag forces -
Fg - gravity force lv
41
What is Bernoullis principle and what force does it correspond to?
Pressure is inversely related to velocity
Lift force - consequence of Newton’s third law
42
What is Ff?
Fluid force
Vectorial sum of lift force and drag force
l^ + - = /
43
How do you calculate Fluid Force
Ff = wiggly t x pi x D^2
Shear stress times surface area
44
How do you define fluid force?
The shear stress operating over the grains surface area
45
What is Shields Criterion?
Ff / Fg = o (o with an h)(theta)
46
What is the fully formed equation for Shields Criterion?
wiggly t / (ps - pf) gD = o (theta)
47
What does Hjulströms curve show?
The critical velocity for movement if quartz grains on a plane bed at water depth 1m.
48
What are the units for Shields Criterion?
Dimensionless
49
Using shields criterion, how can theta be increased?
- increased shear stress
- increased fluid velocity
- decreased grain density
- decreased grain diameter
- decreased g (i.e on Mars)
50
What happens to shear stress in entrainment of mixed grain sizes?
Coarser sizes will move at lower shear stress, and smaller grain sizes will require larger shear stresses, than in a uniformly sized sediment.
-tennis ball over marbles vs marble over marbles
51
Why do large grains move easier over small grains than small grains?
Larger grains have longer lever arms so the torque is greater.
52
What is the angle of internal friction?
a
The angle between the pivot points of two spheres
53
Give an example of entrainment of big and little grains
Placers
54
When does entrainment occur?
When gravity force is less than fluid firce
55
What is the value of theta in shields criterion just before motion begins?
1
56
In Hjulströms curve, why do clay and slit require the same velocity as boulders?
Clay and slit are mud so they stick together and require more force to get into suspension
57
What are the conditions of Hjulströms curve and what does this mean?
Conditions : quartz grains, depth 1m, flat bed
Therefore limited application
58
What are the three controls of sediment settling?
Fb - buoyant force
Fg - gravity force
Fd - drag force
59
What is sediment settling through a fluid affected by?
Grain size (D)
Grain Density (Ps)
Fluid viscosity (u)
Fluid density (Pf)
Gravity (g)
Velocity (U)
60
What is the equation for buoyant force?
Fb = pi/6 x D^3 x pf x g
A sixth of pi x grain size cubed x density of the fluid x gravity
61
In what circumstances would a grain sink?
If the density of the grain is greater than the density of the fluid
If there is a net force between gravity and buoyancy
62
What happens at constant settling velocity?
Drag on the grain balances fge net force between gravity and buoyancy
63
How do you calculate the net force on a grain?
F g-b = pi/6 x (ps-pf) x g xD^3
64
How do you calculate the net force on a grain?
Fd = F g-b = pi/6 x (ps-pf) x g xD^3
Or
Fd = 3pi x D x u x U
65
What is the drag force exhibited on a grain related to?
Viscosity of the fluid
Size of the grain
Grain velocity through the fluid
66
What is stokes law?
U = (ps-pf) x g xD^2
——————————
Qi u
67
What is the purpose of stokes law?
Describing the resisting force in on a particle moving through a viscous fluid and showing that a maximum velocity is reached in such cases
68
What is terminal velocity?
The highest velocity attainable by an object as it falls through a fluid.
69
When does terminal velocity occur?
When the sum of the drag force and the buoyancy is equal to the downward force of gravity acting on the object
70
What are the four types of bedforms that evolve as flow increases?
Flat beds
Ripples
2-D dune
3-D dune
71
What is the anatomy of a symmetrical bedform?
Water surface in phase with bed surface
Characteristic of alternating flow direction
Symmetrical
72
What is the anatomy of an asymmetrical bedform?
Water surface out of phase with bed surface
Stoss slope, summit, crest, brink, lee slope, trough
73
How to identify stoss vs lee slopes?
Stoss is a long gentle slope on the first side
Lee is short and highly angled on the second side
-stand in lee of building to avoid wind
74
What do migrating bedforms lead to?
Lead to various sedimentary structures which are stable under certain conditions
75
What is a bedform?
Features that form under certain flow conditions
76
What are the characteristics of 2-D dunes?
Planar tabular or wedge shaped cross stratification
Flow ————>
Stoss then lee side
Stratification matches lee slope
77
What is the characteristics of 3-D dunes?
Trough cross-stratification
Curved surfaces
78
What are the seven types of upper plane beds?
Typical ripple pattern
Dunes with ripples superposed
Dunes
Washed out dunes
Plane bed
Antidune standing wave
Antidune breaking wave
79
What are the characteristics of ripples?
(Length, height, ripple index, geometry, flow velocity, flow depth, sediment size)
Length - 0.1-0.2m
Height - few cms
Ripple index - relatively low
Geometry - strongly irregular/short crested
Velocity - low
Depth - less than a few cm
Sediment size - 0.03-0.6mm
80
What are the characteristics of 2D dunes?
(Length, height, ripple index, geometry, flow velocity, flow depth, sediment size)
Length: 10s cm - 100s m
Height: cms - cm-10s of m
Ripple index: relatively high
Geometry: straight/ sinuous and long crested
Velocity: low/moderate
Depth: few dm
Sediment size: >0.3mm
81
What is the Southard diagram?
Bedform stability diagram
82
What happens to bed forms when velocity is low, and when it increases?
At low velocities grain sizes >0.7mm can form ripples
As it increases fine grains upper plane beds, grain sizes >0.2mm form dunes
83
What are the parameters of Southards bedform stability diagrams?
Velocity and grain size based on hundreds of experiments in a lab
Water depth 0.25-0.4m
84
What is Froudes number?
The energy state of a flow
Small u / V/gh
Flow speed divided by the square root of gravity times depth of flow
85
What does the Froude number suggest?
Fr<1 : subcritical waves
Fr=1 : critical waves at the same speed of bulk fluid
Fr>1 : supercritical waves
86
What are standing waves an example of?
Critical energy state
87
What is Reynolds number?
Defines whether a flow is laminar or turbulent
(Density of fluid x flow speed x depth of flow)/viscosity
88
What does the Reynolds number suggest?
Re<500 : laminar flow-flow is dominated by viscous forces
5002000: turbulent flow-flow is dominated by inertial forces
89
What does the mud line represent?
Where the system loses energy and fine sands are replaced with muds
90
What is sediment transport and deposition controlled by?
Tides and waves
91
When do fluvial systems influence coastal settings?
When water and sediment discharge are at large
92
What is the difference between the fair weather wave base and the storm wave base?
Fair weather base is the average day
Storms have a deeper wave base
93
How are tides controlled?
Gravitational forces set up by the alignment of the sun, earth and moon
94
How are tides controlled?
Gravitational forces set up by the alignment of the sun, earth and moon
95
What are, and why do, spring tides occur?
Highest high tide and lowest low tide
Sun and moon in alignment
96
What is, and why do, spring tides occur?
Lowest high tide and highest low tide
When moon is 90 degrees from sun ie perpendicular
97
What are the three bedforms under tides?
Herring bone cross stratification
Mud drapes on cross beds
Reactivation surfaces
98
What are the four stages of bedforms under tides?
1. Dominant current stage
2. First slack water stage
3. Subordinate current stage
4. Second slack water stage
99
What happens during the dominant current stage?
Sand is deposited on Lee slope
Megaripple/ sand wave migration
100
What happens during slack water stages?
Mud is deposited on Lee slope, creating a mud drape
101
What happens during subordinate current stage?
Water goes backwards, creating a reactivation surface and rippled sand
102
What are bedforms affected by?
Tidal range
Tidal current velocities
Sand/mud ratios
103
What are the five types of bedform organisation from high velocity and sand availability to low?
Flaser
Wavy
Lenticular
Wavy
Pinstripe
104
What do surface waves control?
Erosion
Sediment transport and deposition
105
What are the characteristics of a wave?
wave Length
wave Height
wave Celerity (speed)
106
What is the period of a wave?
T = L/C
the time (s) it takes for one wavelength to pass a point on the water surface
107
What are the most important type of waves?
Wind generated waves
108
What does wind speed control (waves)?
Size and energy of the waves, positively correlated with wave height, length and period
109
What is a wave orbital?
The fluid beneath a rising and falling wave that follows a circular path
110
What is the diameter of a wave orbital dependant on?
Height and length of the waves
Depth below water surface
111
What is deep water?
When h > L/2
When the orbitals are small enough that fluid motion is negligable
112
What is the wave base?
Depth h >L/2
The depth below the surface where waves no longer affect the water column
113
What is effective wave base?
The depth below the surface at which the fluid motion due to waves is not sufficient to move sediment on the bed
114
What are transitional waves?
When L/20 < h < L/2
Orbitals become flatter as the approach the bed, and at the bottom the orbitals are flat and the motion of the water is back and forth (oscillating)
115
What are the three types of wave?
Deep water
Transitional
Shallow
116
What are the characteristics of classical wave ripples?
Oscillation
Symmetrical profile
Peaked crests
Broad troughs
Straight, bifurcating crests
117
What can we do with wave ripples?
Wavelengths reflect orbital diameter, therefore water depth
Ripple crests are parallel to wave crests, so are aligned parallel to shoreline at time of formation - palaeoshoreline
118
What can the appearance/direction of ripples be affected by?
Ebb tidal current
119
What causes combined flows?
Storms with onshore winds
Also called riptides
120
What types of cross stratification are formed in combined flows?
Hummocky x-strat
Swaley x-strat
121
What are the characteristics of a combined flow cross stratification?
Convex upward (hummocky) and concave upward (swales) laminae
Low angles
Internal laminae approx parallel to lower bounding surfaces
Best developed in coarse slit and fine sand
122
What are the three causes of waves?
Gravitational waves
Marine landslides ie seismicity
Wind - Shear stress
123
What is a Newtonian fluid?
A fluid that has no strength and does not undergo a change in viscosity as the shear rate increase
Ie plain wtaer
124
What is an eddy?
Highly turbulent water masses
125
What does the Hjulström curve show?
The flow velocity required to move sediment of different grain sizes
126
What is the basic mechanism behind the phenomenon of sediment transport?
Drag force in the direction of the flow
127
True or False:
Facies successions are facies and/or facies associations that change progressively either vertically or laterally
True
128
True or False:
Facies successions are facies and/or facies associations that change progressively either vertically or laterally
True
129
True or false:
Retro gradation is the landward change in position of the front of a delta with time. Retro gradation is most common during periods of sea level fall.
False
130
Hummocky cross stratification is characterised by:
Convex upward lamination that is typically low angle dipping
131
What would typify the deposit of a. Idealised sediment gravity flow in which the mechanism for grain suspension was turbulence?
A bed that showed coarse tail reverse grading
132
Which sandstone types is most likely to form by the mechanical and intense chemical weathering of a granite?
Quartz arenite
133
What is the facies model for a storm dominated shallow marine environment?
From top to bottom:
- coastal plain
- coal
- breaker zone (ridge and runnel rip channels)
- shore face (swaley cross stratification)
- lower shore face (Hummocky cross stratification)
- mid shelf (bioturbated sandy siltstone)
- outershelf (bioturbated mudstone)
134
Give an example of a shallow marine, storm dominated succession.
Mesaverde Fm, Wyoming
135
What are the three types of shallow marine clastics?
Zoophycos - shallow water, bioturbated vertically
Cruziana
Skolithos - deep water. Bioturbated horizontally
136
What is bioturbation?
Groups of trace fossils formed during feeding, dwelling, and escape behaviour of benthic organisms
137
What is the importance of stratigraphy?
Archive earths history, which only sedimentary rocks archive
138
What are the four types of sedimentary basin?
Foreland basin
Rift basin
Intercontinental basin
Passive margin basin
139
What are the four types of sedimentary basin?
Foreland basin
Rift basin
Intercontinental basin
Passive margin basin
140
How are foreland basins formed?
Weight of a mountain belt pushes down the crusts surface
141
How are rift basins formed?
Downward slip on faults produces narrow troughs
142
How are intercontinental basins formed?
Basin in the interior of a continent, perhaps over an old rift.
143
How do passive margin basins form?
Subsidence occurs over thinned crust at the edge of an ocean basin
144
What controls accommodation space?
Global sea level change
Sea floor (tectonic uplift/subsidence)
Changes in rate of sediment accumulation
145
What is transgression?
Flooding due to sea level rise
Sediment belts shift landward, strata deepen upwards
146
What is retrogradation?
Linked to transgression
Process of environment shifting backwards
147
What is regression?
Exposure due to sea level fall
Depositional belts shift seaward, strata shallow upwards
148
What is progradation?
Environments shifting seaward
Linked to regression
149
What is Walther’s law?
Beds that occur in conformable vertical successions of strata were deposited in laterally adjacent environments
150
What happens during uplift and subsidence in terms of erosion and deposition?
Uplift of mountains leads to downcutting of rivers and leads to lots of erosion
Subsidence leads to lots of deposition and leads to increased accommodation space
151
What is a parasequence?
Small scale units resulting from a small amplitude, short term oscillation in the balance between sediment supply and accommodation space
152
What is a flooding surface?
A transgression event marked by a distinct surface which caps the facies succession
153
What is the facies model for a meandering river?
From top to bottom:
Floodplain - vertical accretion - caliche nodules, desiccation cracks, root traces
Point bar- lateral accretion - ripple cross lamination, trough cross lamination
154
What is the facies model for braided river?
Coarser cross bedding ie pebbles
Point bar planar cross lamination
Characteristic of glaciation, or tectonically active areas
155
What is the facies model of a delta?
Sandstone unit after sandstone unit with a coal horizon
Plant fragments may be present
156
What is the facies model of a shoreline?
Hummocky and Swaley cross stratification
Storm deposits
2D planar cross bedding associated with beaches
157
What is the facies model of a turbidite?
From top to bottom:
- pelagic and hemipelagic mud
- laminated silts
- cross laminated sands, ripples
- parallel laminated sands
- massive sand and granules, rapidly deposited under upper flow regime
- scoured base
158
What is an aggradational sequence?
Where there is no change in the parasequence, vertical
159
How do you identify flooding sequences?
Sharp contacts
Bioturbation
160
What is chemostratigraphy?
Study of the chemical variations within sedimentary sequences to determine stratigraphic relationships
161
What would typify the deposit of an idealised sediment gravity flow in which the mechanism for gran suspension was turbulence?
A decimetre thick graded bed
162
In ppl, what is it called if you rotate the stage and the mineral changes colour?
Pleochroism
163
What pleochroism is present in biotite mica?
Deep brown, straw yellow
164
How do you describe relief?
High
Moderate
Low
165
What is fracture?
Curved breaks or lines of weakness within mineral grains
Ie olivine and garnet
166
What is cleavage?
Straight planar lines of weakness which relate to weakly bound directions
167
What is the fancy word for angular crystals and rounded crystals, and in between?
Angular - euhederal
Middle - subhedral
Rounded p- anhedral
