surface processes Flashcards
1
Q
what is weathering?
A
it is a set of processes that change the physical and chemical character of a rock at the earths surface
2
Q
what influences weathering?
A
tectonics (rate of uplift)
climate (water flux, temperature)
topography/ relief
prior rock history (rock type/ deformation history)
biology
3
Q
what is erosion?
A
it is the physical removal of rock particles, especially the products of weathering.
achieved initially through a set of geomorphological processes dominated by:
- unchannellised hillslope erosion
- mass flows
4
Q
what does a large amount of soil show you?
A
a large amount of soil shows a large amount of weathering,
5
Q
what does a small amount of soil show you?
A
either weathering is very slowly
or
it is being rapidly removed.
6
Q
what is meant by physical weathering?
A
Involves physical disintegration of rock
Application of in situ and ex situ stresses to rock and mineral discontinuities in the unsaturated zone, causing fragmentation
7
Q
physical weathering includes what processes?
A
Volumetric changes in the rock mass caused by variations in temp or pressure
Processes that involve stresses caused by the introduction of material, commonly water, but also salts into void spaces in the rock mass
8
Q
what do we see where there has been high freeze thaw action?
A
cliffs heavily fractured
debris cones/ scree slope at the base of cliffs
9
Q
what is the freeze thaw hypothesis?
A
- Close to 00C ice is less dense than water
- When water freezes in a “closed system” the phase change causes an expansion of 9%
- Expansion causes high stresses on the sides of the rock (or bottle), causing fracture
- Thawing of the ice loosens blocks, causing failure
10
Q
what is meant by hydrofracturing?
A
as temperature decreases the greater the force to break the rock up.
11
Q
what is meant by frost weathering?
A
Volume change cause by freezeing of water is 9%
Occurs as its molecules organise into a rigid hexagonal crystalline network
Freezing of water in rock cavities can shatter host rock
produces scree like talus
12
Q
what is the main factor that decides the impact of frost weathering?
what is the max growth rates of cracks propagating fracks?
A
main factor - number of freeze thaw cycles
max growth rate s about 1cm per day
13
Q
what causes growth rate in frost weathering to increase?
A
when temps are between -5 and -15
when permeabilty of rock is high
water availabilty is high
14
Q
what does this graph show?
A
shows that crack growth rate is strongly controlled by temperature.
maximum growth between -5 and -15 0C
below -20 it is permenently frozen
15
Q
what is meant ice lense growth?
A
growth of ice lenses which pull water to the surface through capillary action.
it requires a closed system.
16
Q
what do these diagrams show us?
arrow represents north
A
From this we can see that the fractures line up on the north south axis. This is because as the sun rises and sets from east to west it will heat one side of the rock and then the other. This means that fractures will form perpendicular to the suns path direction.
17
Q
what is meant by thermal expansion and contraction?
A
Temperature fluctuations cause rocks to change volumeà insolation weathering
Rocks have low thermal conductivity:
T-changes at surface are communicated into the rock mass at a slow rate
T gradient below rock surface is STEEP
E.g. rock surfaces in desert environments; surface temperature may be 60oC but the temperature of 1m might be only 30oC
Expand on heating
Like all materials, rock expands and contracts upon heating
If surface expansion greater then the sub-surface, stresses will occur in the bonds between adjacent mineral grains within the rock
18
Q
stresses may result from large Tchanges at the surface
what stresses are created in these systems?
hot exterior, cool interior –>???
cool exterior, hot interior –>???
A
hot exterior, cool interior –> tensile stess
cool exterior, hot interior –> compressive stress
19
Q
experiments indicate that temperature differences even in extreme conditions are not enough to cause brittle fracture.
even when we have diurnal temp changes between day and night
how is is possible then?
A
due to the sun moving in the sky. one side of the rock is hotter than the other so fractures align perpendicular to suns movement direction?
20
Q
what direction does fracture grow in thermal environments?
A
fracture growth is perpendicular to suns movement.
21
Q
what is meant by unloading?
what types of rocks does it occur for?
A
for homogenous rocks e.g. granite
An intrusive solidifies at equilibrium with the country rock.
Outward pressure balances inward pressure
Erosion unroofs the intrusion. Outward pressure is no longer balanced. Exfoliation occurs.
22
Q
what is meant by sheeting?
A
Sheeting – rock fracturing that develops fractures just below bedrock surface and concordant with it (like an onion peeling)
23
Q
what leads to weathering limited hillslopes being convex?
A
If the ground surface is curved, radial release of the confining pressure will tend to produce curved sheets of rock = exfoliation sheets
24
Q
what impact does hydration have on weathering?
A
some minerals change volume upon hydration
25
what is the use of chemical weathering?
economic geology: laerites, bauxites, and metal enrichment in supergene deposits
sources of inorganic netrients in soils
26
what is chemical weathering?
chemical weathering is the decomposition of a rock through chemical reactions, and the formation of new minerals , and ions in solution.
27
whats the reason for chemical weathering taking place?
the basic reason why chemical weathering takes place is that alomst all rocks were formed at temperatures and pressures very different to those existing at the earth's surface
these mineral are therefore unstable when exposed at the earths surface.
28
what happens during chemical weathering?
chemical weathering takes place with acids and rains
this gives altered rock and mineral fragments and dissolved ions
the altered rock and mineral fragments change to a residual material (quartz) and clays (what's left behind to make soil) which goes into the oceans
it also produces secondary minerals which dissolve with the ions. (what is removed by the water, leaving the material behind)
29
what impact does freeze thaw have on chemical weathering? why?
increases the rate of reactions.
this is due to freeze thaw increasing the surface area of the rock thus more reactions
30
what is meant by residual material?
**residual material** - last remnant (cant be weathered easily) thus lots of quartz shows it has been weathered heavily as the others have been weathered away.
31
what is meant by pedosphere?
**pedosphere** - is the outermost layer of the earth that is composed of soil and subject to soil formation processes.
it exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere
the sum total of all the organisms, soils, water and air is termed as the pedosphere
32
with continued weathering what happens to the residual minerals, clays and other secondary minerals?
they accumulate in the pedosphere or are washed into the oceans with dissolved ions.
33
how can we measure the rate of chemical weathering?
by measuring the concentration of chemical species in rivers (the product of chemical weathering being dissolved ions) we can measure the rate of chemical weathering.
34
which two minerals are least and most weathered by chemical weathering?
quartz (least weathered)
olivine (most weathered)
35
what causes chemical weathering?
Water is an extremely important catalyst to the instability of rock-forming minerals when exposed at the earths surface. (important but acid rain is most important)
Rain is acidic --\> due to CO2 in atmosphere. May also contain nitric acids.
Humic acids in some soil waters also important
H+ can substitute for cations in mineral; thus disrupts he mineral structure: cations go into solution.
36
what is the most important agent for chemical weathering?
the fact that the rain is acidic is most important as the H+ is needed to disrupt the mineral structure.
37
what factors influence mobility of ions and their removal?
- temperature
- frequency of precipitation - determines degree of leaching
- ground water movement
- pH - mineral solubility commonly varies with pH
charge (e.g. Fe2+ is more mobile than Fe3+)
38
which is more important for chemical weathering , frequency oor the amount of precipitation? why?
frequency is more important as there needs to be removal of cations so the concentration gradient is high.
39
what is meant by humic acid?
it is produced by biodegradation of dead organic matter. it is not a single acid but a complex mixture of many different acids.
40
what is meant by laterites?
are soil types rich in iron and aluminium.
formed in hot and wet tropical areas.
rusty red colour due to iron oxides.
develop by intensive and long lasting weathering of the underlying parent rock,
41
what does a thick laterite show you?
because there is a thick soil, water takes a long time to move thus this image shows little weathering over a long period of time rather than lots of weathering happening.
42
what are the four types of chemical weathering reactions?
ionic dissolution
carbonation
oxidation and reduction
acid hydrolysis.
43
what is meant by ionic dissolution?
involves action of water as solvent , breaking down ionic bonds in minerals
produces ions in solution and secondary minerals
e.g
**gypsum:**
CaSO4 \<-H2O-\> Ca2+ + SO42-
44
describe the rates of ionic dissolution and thus how we can speed up the reaction?
reaction rate during dissolution slows as water becomes saturated with reactants.
thus the rate or removal of saturated water controls weathering rate (ground water flow needs to be increased)
45
what rock type is dominated by carbonation?
carbonation dominates the weathering of **limestones**
46
what is meant by carbonation?
water and CO2 produce a weak acid (carbonic acid) which dissolves limestone, leading to the typical solutional morphology known as karst.
calcite dissolves in weakly acidic water:
CaCO3 + H2CO3 \<--\> Ca2+ + 2(HCO3)-
H2CO3 comes from CO2 and H2O in the atmosphere,
47
how can we measure the rate of carbonation?
by looking at the concentrations of Ca2+ and HCO3- we can see the extent of carbonation
48
what is meant by oxidation and reduction in chemical weathering?
what does the sudden appearance of haematite in geological history show us?
it involves the gain (reduction) or loss (oxidation) of electrons. products may be more soluble than reactants
e.g.
pyroxene + oxygen \<--\> haematite + silica
thus the sudden appearance of haematite in geological history shows when oxygen was produced.
49
what is meant by acid hydrolysis?
reaction of mineral with acidic agents: produces ions in solution and secondary minerals
involves replacement of metal cations in crystal lattice by hydrogen or hydroxyl ions of water. released cations combine with further hydroxyl ions (H+) to form clay minerals.
H+ replaces cations in minerals structure
50
what is the typical mechanism of silicate dissolution (acid hydrolysis)?
Olivine Mg2SiO4 + 4H+ --\> 2Mg2+ + H4SiO4
51
what are the three main acid sources for acid hydrolysis?
**carbonic acid** - forms from carbon dioxide dissociation in water
**organic acids** (from respiration of carbon dioxide in soils)
**sulfuric acid** (volcanic)
52
describe feldspar hydrolysis
Feldspar (most common mineral on earth’s surface) reacts with free hydrogen ions in water to form a secondary mineral kaolinite and additional ions in solution
Orthoclase + hydrogen ions + water --\> K+ + kaolinite (clay) + silica
53
what is the equation for feldspar hydrolysis?
Orthoclase + hydrogen ions + water --\> K+ + kaolinite (clay) + silica
54
describe how granite is weathered?
The feldspars will undergo hydrolysis to form kaolinite (clay) and Na and K ions
The Na and K ions will be removed through leaching
The biotite and/ or amphibole will undergo hydrolysis to form clay, and oxidation to form iron oxides
The quartz (and muscovite, if present) will remain as residual minerals because they are very resistant to weathering.
55
what is the use of the quartz feldspar ratio?
how does it work?
If mechanical & chemical weathering dominates, feldspar breaks down and quartz remainsIf little chemical weathering occurs but physical weathering dominates, quartz and feldspar can be found in almost the same proportions.
Therefore the ratio of quartz to feldspar is indicative of the dominant weathering process.
56
what is the effect of rising mountains to the removal of carbon dioxide?
The rise of the mountains increases weathering so more CO2 is removed from the atmosphere making the earth cooler. Leads to glaciations.
For every 2 carbon atoms used in weathering only 1 is returned to the atmosphere.
57
how is the material derived from erosional processes eventually disposed of?
material derived from erosional processes eventually disposed of by **the action of erosion by rivers**
58
what is meant by rainsplash?
rainsplash: the detatchment of loose particles by raindrop impact
on a sloping ground, rainsplash results in a net downslope transport of sediment
59
what influences the impact of rainsplash?
the total kinetic energy of rain depends on:
- raindrop size
- distribution
- rainfall intensity
- duration
- frequency
60
what are rills and gullies?
rills and gullies: concentration of flow leads to increased flow depth and velocity. enhanced erosion causes formation of rills
gullies are entrenched stabilised rills (basically large rills)
61
what are the three types of mass wasting?
rockfall
creep
sliding
62
what is meant by rockfall?
rockfall: products of physical weathering fall down steep slopes onto talus slopes/cones
63
what is meant by creep?
**creep**: slowest moving type of mass wasting is called soil creep.
gradual but persistent movements of dry surface soil.
soil particles are lifted and moved by cycles of moistness and dryness, temperature variations and grazing livestock.
Freeze and thaw cycles in soil moisture also contribute to creep through frost heaving. When soil moisture freezes, it causes soil particles to expand out. When it melts though, the soil particles move back down vertically, causing the slope to become unstable.
64
what is meant by sliding?
landslides occur where shear stress is enhanced and material strength is reduced.
typically, this occurs during rainstorms or earthquakes.
65
what factors the critical slope for landsliding depend on?
**shear strength** of hillslope material
**water saturation** (increases the downward component of the weight of the mass of material above the potential failure plane).
**pore pressure** caused by the presence of water in pore spaces, which lubricates the potential plane of failure and reduces shear strength
**vegetation**, such as plant roots, which binds te regolith (soil) and therefore increases its shear strength.
66
what is the relationship between mass removal, isostatic, and river incisions?
removing mass causes tectonic uplift
rivers incise into bedrock
and insure the progressive lowering of the base level for hillslope processes
67
what is the relationship between rock hardness and steepnes of slopes due to weathering?
the stronger the rock the steeper the slopes.
however past a point it fails and we get land slides.
68
what do you need lots of to create a gorge?
you need high rainfall and high uplift.
69
what are the three bedrock erosion processes
**quarrying** or plucking
**impact erosion** - wear/abrasion - by saltating bed load (pebbles hitting rocks causing erosion)
**macro-abrasion** - chipping and block fracture by impact (sand blasting)
70
what is meant by quarrying?
large fragment rocks weakend by weathering.
huge floods then move blocks the size of cars.
71
what is characterisitic of sand blasting?
sand impact makes smoothes surfaces.
72
what is needed to erode landscapes by river in terms of the goldilocks effect?
**water is not enough** to erode landscapes by river
you need sediment particles - we call these 'tools' to actually do the physical erosion
but there is a goldilocks effect...
need some tools
but not too many, or else the river bed is covered and no erosion can take place.
73
what is the relationship between bedrock landsliding and bed rock incision?
bedrock landsliding lowers landscape and provides the tools for river sutting
bed rock incision uses the tools to drive down base level which undercuts the valley sides. causeing more landsliding.
creates a cycle.
74
what is meant by entrainment?
it is the oicking up and moving grains
75
what is sedimentology is all about..
sedimentology is all about entrainment and settling
76
the ability of a fluid to erode a particle depends on...
the ability of a fluid to erode a particle depends on:
fluid density
fluid viscosity
depth of flow
fluid velocity
characteristics of the sediment (size, shape, binding by micro-organisms)
77
what are the two types of flow?
laminar and turbulent
78
describe laminar and turbulent flows and say how they are different.
**laminar**: straight or boundary parallel flow lines
**turbulent:** constantly changing flow lines. net mass transport in the flow direction
**different because:**
flow velocity
bed roughness
type of fluid
79
what is meant by inertial forces?
object in motion tends to remain in motion
- slight changes in path can have huge impact
- perfectly straight flow lines are rare
80
what is meant by viscous forces?
object flows in laminar fashion
viscosity: resistant to flow
- high viscosity fluid: uses so much energy to move its more efficient to resist, so flow is generally straight
low viscosity fluid (air): very easy to flow, hard to resist, so flow is turbulent.
81
what two forces decide the flow?
flow is the fight between inertial and viscous forces.
82
what is reynolds number the ratio of?
reynolds number is the ratio of inertial to viscous forces.
83
what are the forces that resist the motion of a grain?
gravity
interparticle friction
cohesion/ elecrtochemical bonds (clays)
84
describe the shear stress at the beds surface that is exerted by a fluid moving above the bed.
- there is a boudary shear stress.
force per unit area parallel to the bed.
proportional to flow velocity, fluid density, scale/ depth of flow, slope of stream bed
85
what is the drag component a function of?
drag component - function of boundary shear stress
86
what is the lift component related to?
lift component - bernoulli effect
87
what are the two causes of the lift force?
1. a difference in flow velocity between the top and bottom of a grain sets up a vertical pressure gadient
2. turbulent eddying produces local velocity components which act directly upwards close to the bed
88
what happens to the lift force as you move away from the bed?
lift force strength descreases rapidly as you move away from the bed.
89
what causes the initiation of motion of sand grains?
grains move when the combined lift and drag forces become large enough to counteract gravity and frictional forces holding grains in place.
90
what are the 4 forces acting on a grain particle?
do they resist or encourage motion?
**resist**
weight
frictional forces between adjacent particles
**encourage**
hydraulic lift forces
tangential drag force
91
what is the relationship between particle movement and moments?
for a particle to start moving it must rotate about the pivot point where the particles are in contact
in order for movement to be initiated, the clockwise moments must balance the anticlockwise moments.
(moment = F x D)
92
why is a greater shear stress needed to move small grains as shown on the shields curve?
for grain sizes finer than silt, cohesion holds grains together: greater shear stress is required to move small grains.
93
for grains that get coarser than sand grains is more or less shear stress needed as shown on the shields curve?
for sand sizes and coarser, shear stress needed for entrainment increases with grain size.
94
the critical threshold for grain movement depends on?
particle shape, size, sorting
bed roughness
cohesion.
95
once a particle has been set in motion its transport path is a funtion of:
once a particle has been set in motion its transport path is a funtion of:
- particle settling velocity
- current velocity
- fluid turbulence
96
is more energy needed to put a particle in motion or to keep it in motion?
more energy is needed to put a particle in motion than to keep it in motion
97
what are the two types of particles found in a river flow?
describe them.
**bed-material load**
- bed load - close to the bed
- suspended load - temporarily suspended
**wash load** (fine)
material not present in sediment bed
finest fraction - rarely settles
(like the little particles that make up milk)
98
what are the three types of bedload transport?
**traction** - rolling/sliding
**saltation** - jumping over the bed surface
**suspension**
99
describe saltation
particles launched at moderate to steep angles
have an arching trajectory
descend at small angles
saltation trajectories regular due to greater realative density of particles not effected by eddies.
100
what causes grains to rise during saltation?
lift forces
grain splashing
101
what is meant by grain splashing?
when one grain hits another as it lands. this forces othr sand grains to be ejected upwards.
102
what is meant by suspended load?
particles held in continuous suspension by fluid turbulence
- upward component of fluid flow overcomes gravity
- finer grain sizes (very fine sand to clay)
103
how does mode of transport change with increasing flow velocity?
transport stage is a function of velocity divided by critical velocity to move sediment
104
what is meant by wash load?
clay sized particles derived from up current source
always remain in suspension.
105
when does sediment deposition take place?
sediment deposition takes place when the flow system can no longer support the grain.
when buoyancy of fluid and drag are less than gravity
106
which law quantifies settling velocity?
stokes law
107
which grains settle out first?
generally the coarsest grains settle out first
108
what plays a key role in keeping grains aloft?
**turbulence** plays a keyrole in keeping grains aloft.
109
sediment transported will be entrained by a fluid flow as long as flow is competent enough. e.g. has a flow velocity high enough to keep particle in motion.
what ways can a flow decelerate?
due to a decrease in slope
in deltas by spreading of flow into standing body of water.
110
what happens if you put a sediment grain in a less dense fluid?
it will settle downward and accelerate under the influence of gravity.
but its movement will be restricted by fluid drag.
111
what happens when fluid drag becomes equal to the weight of the grain?
what is this called?
the grain will fall at constant velocity.
called **terminal settling velocity** or stokes velocity
112
what is settling velocity dependent on?
grain size
grain shape
density of grain
viscosity of fluid
density of fluid
113
drag exerted on a grain is proportional to what?
what would this mean in terms of mica?
drag is proportional to the pressure exerted on the grain as it falls and the surface area of the grain.
mica grains will then be at the top as they fall the slowest due to increased surface area.
114
accurate predictions of rate of fall of solids through stationary fluid is only possible when?
smooth, insoluble fluid sphere
in a newtonian fluid
infinitely wide and deep container.
115
drag coefficient is a complex function of...
fluid viscosity and density, grain size and shape, and the speed and trajectory of settling.
116
what is a **bedform**?
a **bedform** is a deformation of a granular bed due to interaction of the bed with the flow
117
what is the use of bedforms?
they make sedimentary structures that enable us to read the rock record for processes and past flow conditions.
118
describe the anantomy of an asymmetrical bedform
119
what happens when you have two dunes. one smaller behind a larger dune.
the small one moves faster so combines with the large one to make an even larger dune
120
what are the main types of bedform?
low flow regime
- ripples
- dunes
upper flow regime
- plane beds - leads to flat lamination
- antidunes
121
what is the general height and length of ripples?
what does length depend on?
H- less than about 4cm
L - \<60cm (L depends on grain size)
122
what conditions do ripples form in?
form in shallow, slow moving flows, low transport rates
123
what grain size do ripples form from?
ripples will only form in medium grained sand or finer
124
how do we change straight ripples to sinuous?
with increasing flow strength, crests become more sinuous
125
what is the height and length of dunes?
what is L dependent on?
H- up to 5m in large rivers
L up to 100s of m; L depends on flow depth.
126
what grain sizes does dunes form in?
dunes form in sediement medium and above.
127
how do dunes become more curve crested and discontinuous?
dunes become more curve crested and discontinuous as shear stress increases.
128
describe the fall velocity of a grain in a dispersion?
it will be smaller the that in an otherwise grainless fluid.
and will be strongly dependent on concentration
in hindered settling , particles settle close enough to each other that their boundary layers interact.
hindered settling can reduce fall velocity to 2 or 3 % of the unhindered value.
129
bedforms with linear crests and a roughly uniform height migrate downstream with a uniform\_\_\_\_\_\_\_\_\_\_\_
bedforms with linear crests and a roughly uniform height migrate downstream with a **uniform depth of erosion.**
130
describe the air flow over an aeolian dune.
131
what is the relationship between bed shear stress and the back of the ripple / dune?
bed shear stress increases up the back of ripple/dune
132
why is the deposition at top of face of a dune?
downstream of a dune crest flow expands, slows -- results in flow seperation
reduction in shear stress at top of avalanche face promotes deposition.
133
describe where there would be deposition and erosion on a ripple.
most sand is deposited at crest
most erosion on lee side due to flow seperation.
134
due to deposition on the crest and erosion of the lee side what do we normaly see?
avalanching
135
describe ripple and dune formation
ripples form by grain avalanching
on a bedding surface, grains accumulate due to flow and surface irregularities
small piles of grains produce points of flow seperation, and the creation of eddies
erosion at the point of flow reattachment entrains grains which move up the stoss side of the ripple
grain avalanching occurs when grains become unstables and they avalanche down the lee face of the ripple, creating a foreset.
foresets can often be distinguished from one another due to subtle grain size variations.
136
what are the four stages to a bed as velocity is increased?
stage 1: initiation of grain motion
stage 2: lower flow regime (ripples, dunes)
stage 3: transition:; bed is washed out
stage 4: upper flow regime ( planar bed or anti dunes)
137
what is meant by antidunes?
its the opposite to a dune, so deposition on the stoss side and eroded on the lee side.
138
why cant straight and sinuous not be together?
they need different flow rates.
139
what is meant by the bedform phase diagram?
different bedform states occupy distinct fields on plots of flow variables
140
explain the no movement zone on the bedform stability diagram
because the grains are course, ripples are not possible
141
as flow velocity increases what are ripples and dunes replaced by?
as flow velocity increases ripples and dunes are **replaced by plane beds, and then** by bed waves in phase with surface waves known as **standing waves.**
142
what is meant by 2D and 3D in terms of ripples and dunes?
2D - straight crested
3D - sinuous crested
143
sketch the bedford stability diagram
use this to describe changes to bedforms when flow velocity/ shear stress increases.
144
what is meant by sedimentrary structures?
sedimentary structures are features found within the sedimentary section, and/or on, and/or between, bedding plane surfaces subdividing that section
145
what is the use of sedimentary structures?
provide critical clues to sedimet transport and depositional processes.
gives us information on:
lithology
bedding
geometries
fauna
146
under which systems are primary sedimentary structures formed?
formed under influence of sme hydrodynamic and or aerodynamic conditions as entrainment, transport and deposition of sediment particles
147
what can we resolve form primary sedimentary structures?
process
transport pathway
paleohorizontal
way up
148
what are planar laminae? what creates them?
few mm thick beds
seperated by variations in colour, grain size, composition
due to:
deposition from high flow velocity (beach)
settling from standing body of water with very low flow velocity eg. in a lake.
149
why wont you find mud mixed in with sand in planar laminae?
because the mud doesnt settle at the same rate as sand.
thus intead we get laminaions of different coarseness with mud and sand layers.
150
cross stratification produced by the migration of bedforms depends primarily on which two factors?
the 3D geometry of the bedforms
the ratio beween downstream translation and vertical accretion.
151
where do we find cross stratification?
how do we tell them apart?
we find cross stratification in both aeolian (desert) and under water
they are hard to tell apart
152
what is meant by aeolian?
formed in a desert
153
describe the migration of straight crested bed forms with no climb.
surface never builds up.
stays planar.
154
describe the difference between migration of straight crested bedforms where there is **no net deposition** and **with net depositon**.
A. with no net deposition or erosion on the bed the volume of sediment eroded from the stoss side of the ripple must equal the volume deposited on the lee slope.
B. net deposition on the bed no erosion takes place and the entire ripple form is preserved.
155
what creates large scale and small scale cross stratification?
migratting ripples produce cross laminae in sets only a few cm thick -- small scale cross stratification
migrating dunes produce cross strata that are thicker; sets are cm's to 10s of cm's thick -- large scale cross stratification
156
what are the two types of cross ctratification?
- planar cross strat.
- trough cross strat.
157
what creates planar cross strat?
planar cross strat created by straight crested bedforms
158
what produces trough cross strat?
trough cross strat is produced by sinual crested bedforms
159
describe planar cross bedding
when moving the lee side erodes down into the layer below. thus when they move they create a flat surface.
160
what is the terminology related to cross stratification?
161
what can we say about cross stratification that has large pebbles in it?
wind can not move grains greater than 2mm. thus the pebbles show that the cross stratification was created by water.
162
what is meant by grainflow and grainfall?
**grainflow** - coarse grain - avalanche deposits formed by collapse of crest down lee face.
which shows as thin bands on cross strat that poke out.
**grainfall** - fallout from suspension of fine grained fraction between avalanche events.
thicker layers between grain flow layers.
163
what is meant by toesets?
the toeset is the forward part of the dunes advancing frontal slope.
164
explain the weire shape of trough cross strat.
depending on where you are on the dune it cuts down by different amounts so we get this wierd shape
165
what are the dark lines on the trough cross strat
dark lines represent the **erosional surfaces** (sometimes called smiles)
166
how can we tell the difference between planar and trough cross strat?
trough cross strat has curved lines
planar cross strat has straight lines.
167
describe the migration of straight crested bedforms with a steep climb?
the stoss side is preserved
168
describe the differences in the preservation in the stoss and lee side for different conditions of bedload transport and suspended sediment fall out rate.
arrows represent movement
169
how are climbing ripple cross lamination formed?
created when the addition of sediment from the current exceeds the forward movement of the ripple, deposition will occur on the stoss side as well as on the lee side.
170
what are climbing ripples indicators of?
climbing ripples are indicators or rapid sedimentation as their formation depends on the addition of sand to the flow.
171
describe wave ripples
symetrical slopes
- no pointy crests due to when the water dissapears the crests collapse.
waves can split in two - biforcation
172
describe the orientation of wave ripples.
what does this allow us to do?
this allows us to see palaeo beach geometry
173
what are wave ripples created by?
wave ripples are created by oscillating currents.
174
what are water waves?
water waves are wind generate oscillatory motions of water
175
what is wave height dependent on?
wave height is dependent on **wind strength** and **fetch (surface area)**
176
what is meant by wave base?
wave base is the maximum depth in which motion created by waves can happen.
177
what is a common mistake to make with wave motion?
the fact that the waves go back and forth is wrong.
they actually rise and fall
178
what is meant by wave orbitals?
when water goes up and down, it creates circular motion pathways called orbitals.
179
describe wave orbitals in shallow water.
in shallow water the orbitals become more elliptical until in is basically back and forth.
thus creates sand waves at the bottom.
180
describe wave orbitals in deep water
in deep water the wave orbitals get smaller in radius.
then at a point the wave orbitals are so small that they have no impact on the sand (wave base)
181
what is the difference between wave ripples and current ripples?
wave ripples are distinct from current ripples due to their symmetry.
182
why do orbitals become more elipticl with increased depth?
orbitals become more elliptical because of their interaction with the water bodies base (sea bed)
183
explain how these sand grains eject
little eddies created causing sand grains to eject from back and forth frow of wave orbitals
184
on a flat surface with low bed shear stress (gentle wind), what will be produced?
**rolling grain ripples - ** (very low amplitude symmetrical ripples with little to no grain movement, dont have sharp crests)
185
in the process of creating wave ripples, what happens with increased bed shear stress?
with increased bed shear stress, rolling grain ripples will evolve into vortex ripples due to vortices produced on either side of the ripple from scouring in troughs and building on crests. this increases ripple height and the isolation of to and fro motion
186
what is the shape of waves created by purely oscillatory flows?
pure wave ripples created by oscillatory flows are smmetrical in shape.
187
how do we identify wave ripples?
generally symmetrical (but can be asymmetric close to beach)
generally straight crested or slightly sinuous
crest often bifurcate
_laminae in ripples dip in both direction away from the crest_
188
what is the major difference that we can use to differ wave ripples to current ripples?
wave ripples have cross lamination that go down on both sides. current ripples have laminations only on one side.
189
if we see wave ripples and the cross lamination is bunched up on one side, what can we say?
on one side the more bunched up laminations show how one wave direction is stronger than another.
190
what is the use of looking at wave ripples?
we can find palaeowater depth (lake/ marine is possible , not deep marine as ripples dont form)
gives us ancient shoreline orientations
reconstruct wave conditions.
191
describe how cross laminations in a wave ripple form.
in bidirectional flow, structures are formed with alternating layers of cross-beds dipping in opposite directions that reflect the alternating paleocurrent.
not common because they require the current to be equal in both directions.
The time period represented by each cross-stratified layer is many years
192
what happens when a river emerges from a mountain?
the gradient shallows upruptly.
the river is no longer confined to a single valley but can now move (alluvial fan)
thus is a decrease in velocity (deposition)
the point that it leaves the mountain is a fixed point and the river changes direction from this fixed point.
193
what do the lines on this image represent?
lines represent former river systems.
194
in alluvial fans does the river system change slowly or abruptly?
in alluvial fans the river system **changes abruptly**.
195
describe alluvial rivers
easily erodible channel boundaries - banks and bed made of sediment
input is greater than output thus can have large sediment storage
channels are shaped by the magnitude and frequency of the floods that they experience, and the ability of these floods to erode, deposit, and transport sediment
196
describe flood plains
raised area is called a levee
197
what are the four main types of channel patterns of alluvial rivers?
meandering rivers
braided rivers
straight rivers
anastomosing
198
describe the major points of a meandering rivers.
cut bank - erosion
point bar - deposition
199
describe the motion of water on the high velocity side of a meandering river
water flow is helical thus causing large amounts of erosion.
200
what would we find in the accretion topography of a meanderig river?
cross laminations
cross bedding
graded bedding
201
what direction do ripples face in a meandering river? why?
the crests of the ripples lie parallel to the banks edge.
this is due to the water moving in a helical fashion.
202
describe the flow and sediment transport of a meandering river and thus how the meander moves.
flow is faster on outside than inner
thus erosion on outer (cut bank) and deposition on inner (point bar)
the meander then shifts across the floodplain due to erosion on outer bank and deposition on inner bank.
203
why do we get several types of bedforms on a pointbar?
decreasing bed shear stress from thalweg to point bar top leads to development of suite of bedforms on pointbar surface and upward fining.
204
what is meant by thalweg?
it is the deepest part of a water course.
205
what is a braided river
complex multichannel systems of low sinuosity which commonly migrate across the braidplain.
characterized by wide, shallow channels
206
what is the major difference between meandering rivers and braided rivers?
meandering rivers are a single spagetti thread
braided rivers are several spaghetti thread systems.
207
how is a braided river created
caused primarily by weak banks, so that channelscannot incise deeply.
thus with an increase in discharge channel banks erode and the channel becomes wider.
208
compare braided vs meandering
braided systems have relative to meandering:
- wider, shallower channels
- many channels seperated by bars ('multithread channels')
- less fine grained material in channel and on flood plain
- steeper gradient
- weak erodible banks
channels full of bars with lots of cutting and filling of channels
209
what designates if a stream will be a braided system?
a braided river needs:
- coarse load
- steep
- higher shear stress
- more bedload
- less cohesive walls
210
what conditions are needed for a meandering stream?
a meandering stream needs:
- fine load
- shallow
- lower shear stress
- less bedload
- more cohesive walls
211
describe the evolution of a meandering river
they become wider and more sinuous due to the erosion of the cut bank and deposition of the point bar.
212
what do old point bar deposits indicate?
old point bar deposits indicate channel migration.
213
what is meant by the resetting of a meandering river?
when the meandering river becomes too sinuous an oxbow lake is created and it resets by straightening out.
214
what can happen to meandering rivers during flooding.
during floods, rivers sometimes flow over the point bar instead of around the bend. this might form a chute channel.
overtime, ends of the old bend may fill with deposited sediment, cutting off flow from river.
215
what are the uses of ancient river systems?
excellent petroleum reservoirs
excellent aquifers
placer deposits concentrated in fluvial channels... diamonds, gold, uranium.
216
define a delta
**delta**
a mound of sediment deposited where a river channel enters a body of water and supplies more sediment than can be carried away by currents in the water body.
217
what are the components of a delta?
- **delta plain**
sub aerial floodplain
subaqueous - marshes, lakes
**delta front**
area where sediment laden river water enters open water
**pro delta**
at the toe of the delta front.
218
what are the main sedimentary environments of deltas?
219
describe the delta plain
low relief, subaerial portion of delta
channels may divide into smaller, straighter distributary channels
between channels - lakes or floodplains
220
describe the delta front.
area where sediment - laden river enters sea/lake
mouthbars in river dominated deltas
beaches and barrier islands in wave dominated deltas
221
describe the tip of a channel.
more shallow at end than the channel due to deposition of sand. grains get small further away
222
what are the main processes of low sand sedimentation at deltas?
at a river mouth water spreads out from a confined flow thus decelerates, depositing the load
sediment transport is diminished by standing water
coarse sediment near mouth and finer sediment goes further out in suspension
currents in water body may then rework and move deposited sediment.
223
what is meant by a jet?
a jet is the combined discharge of sediment and fresh water issuing from the mouth of a major delta distributary.
224
describe inertia dominated jets
inertial interactions between jet and ambiant fluid
high flow velocities
flow spreads as a turbulent jet.
flow velocity max at centre
long and elongate deposit
becomes less shallow and then has steep bar front.
225
describe frictional dominated jets
where water depth decrease or shallow seaward of river mouth
frictional drag exerted on base of jet by delta slope.
rapid deceleration
thus bar deposition and expansion.
226
when we have river water and basin water density contrasts, what causes differences in density?
salinity
temperature
suspended sediment concentration
227
what is meant by homopycnal jets?
**homopycnal** - density of river and basin water are similar
e.g. river with low suspended sediment concentration flowing into a lake
river and basin water become well mixed
most sediment deposited close to river mouth.
228
what is meant by buoyent jets?
buyant jets due to jets dentsity contrast with ambient fluid
river water spreads as a buoyant layer
as it spreads it decreases in velocity
229
which is denser fresh or salt water?
salt water is denser
230
what is meant by hypopycnal jet?
dominance of buoyancy forces in jet causes outflow to expand as a narrow expanding plume above a salt wedge.
friction at interface of jet and salt wedge causes deceleration and deposition of coarser sediment at mouth bar.
231
what is meant by hyperpycnal jet?
hyperpycnal - density of river water greater than basin water
e.g. river water has high suspended sed concentration or relatively lower temperature.
river water flows **beneath** basin water as a dense bottom-hugging plume - **underflow**
plume propagates along basin floor
sediment can bypass shoreline
232
describe wave dominated deltas.
are characterized by mouth bars reworked into shore parallel sand bodies and beaches.
in a wave dominated setting the sand bar keeps moving forward into the sea and the waves move the sand out of the way due to long shore drift.
233
describe river dominated deltas and their movement
they occur in microtidal settings with limited wave energy, where delta lobe progradation is significant and redistribution of sediment is limited.
a delta shift position due to when the sand bar builds up the gradient goes to small thus the channel changes direction where the gradient will be larger and greatest energy.
234
what is meant by progradation?
progradation refers to the growth of a river delta farther out into the sea over time.
235
what does this diagram show?
shows how the delta moves towards the lake through time
236
in a rock sequence how can we tell that a delta has been migrating seawards?
we see **coarsening up** where finest grains at the bottom and coarser on top
237
describe tide dominated mudflats
new distributaries are formed during times when there's a lot of water around – such as floods or storm surges. These distributaries slowly silt up at a pretty constant rate until they fizzle out
238
what would alterneting layers of sandstone and mustone show you?
layers of mudstone and sandstone show regression and transgression.
239
what environment is given by mudstone?
low energy marine
240
what environment is given by a sandstone?
shallow marine
241
describe the continental margin
242
how are the canyons on continental slopes created?
canyons created due to drainage from transgression and regression
243
what do submarine canyons allow?
submarine canyons allow sand to reach the deep marine.
244
what are sediment gravity flows?
sediment, or mixtures of sediment and fluid that move as mass flows under the action of gravity.
flows move solely due to gravity and dont necessarily need help from overlying medium
thus it works by gravity working on the individual grains.
245
what is key for sediment gravity flows to occur?
for movement to occur, grains must be dispersed.
246
what is a turbidity current?
a turbidity current is a member of the class of dense bottom flows that includes thermohaline bottom flow (e.g. straits of gibralter)
247
how does a turbidity current work? what is its driving force?
the prescence of a dilute suspension of sediment in the water of a tubidity current renders it slightly heavier than the ambient water.
gravity pulls the sediment downslope, and the sediment then pulls the water with it.
they obtain their driving force from the extra weight of sediment in suspension.
248
how can turbidity currents move on very small gradients?
because they move on density differences thus can travel on very small gradients
249
why do turbidities often pulsate?
pulsates due to equillibrium between the fluid and sediment mixture.
250
draw a diagram of a turbidity current
251
where does deposition occur in turbidity currents?
sediment deposition occurs at base of the flow
252
how does a turbidity current decelerate?
sediment deposition coupled with the intake of water means the density of the flow decreases, and thus decelerates
253
describe the morphology of a turbidity current
1) head: 1.5 - 2 times thicker than rest, intense turbulence which may result in erosion; prescence of lobes and clefts
2) body: flows faster than the head, and will commonly partially overtake the head, consuming itself in the process
3) tail/ wake: the more dilute rear end of the turbidity current
254
how is a river different to a turbidity current?
a **river** flows under the influence of gravity **acting on the water**. the water then drags the sediment with it.
a **turbidity current** flows downslope under the influence of gravity **acting on the sediment.** the sediment then drags the water with it.
a **river** is sediment laden water **under air**
a **turbidity current** is sediment laden water **under sediment free water.**
255
why do all turbidity currents die?
they must die as eventually the amount of sediment in suspension must aproach zero.
256
can we see turbidity currents?
not really, difficult to see them and they dont happen very often.
257
what triggers turbidities?
storms (transport of sediment on to the shelf and then onto the slope.
sudden deltaic slope failures (excess pore pressure, earthquakes)
hyperpycnal flows (river plunging)
retrogressive slope failure
258
how can we use turbidities to measure past earthquakes?
number of deposits give frequency
thickness of deposit governs size of earthquake
C14 dating gives age.
259
what is the Bouma sequence?
an idealized tubidite sequence shows a deposit with 5 distinct units, that records the decay of flow strength through time and transition from upper to lower flow regime.
at the bottom we would get the coursest grains where the hed passes first and at the top we would get the silt and mud as final deposits where the tail passes.
260
what are sole structures?
seimentary structures created from the erosion of a surface of a cohesive sediment bed are called sole structures
- fluid flow over sediment that has already been deposited can result in partial or localised removal of sediment from the bed surface.
sole structures are preserved infills of these scours, infilled with overlying sediment.
preserved at the sole of the turbidity bed
261
why are sole structures important?
way up indicators on deformed rocks
paleaocurrent indicators
262
how do you work out the flow direction from flute casts?
the deepest part is upstream and tapers of to beome shallower
263
how are flute casts created?
created by lots of eddies digging down into the sides
264
what are tool marks?
- often have sharply defined edges
- produced by objects carried in turbidity flow
- occur as isolated casts or in patterns covering bedding surfaces.
265
what are the two types of tool marks?
**continuous**
- groove casts (elongated marks produced by an object being carried along a bed) can give orientation due to making a straight line.
- chevron marks (crenulated marks produced by objects saltating along the bed surface
**discontinuous**
- prod marks and bounce marks
- skip marks
266
what does it mean if tool marks are pointing in opposite direction?
this is wherenthe flow has bounded off something and then goes in the opposite direction
267
describe the stages of the bouma sequence
5 sections A-E fining upwards
indicating decreasing energy through time and therefore decreasing sediment velocity and deposition rate
it is rare for all parts of the sequence to be fond in one location
A,B - coarse
C,D - sand silt
E - mud
(decrease in velocity thus smaller grains)
268
describe the thickness of turbidite beds.
close to the source area the beds are thicker
further along it gets thinner as most sand has already been deposited.
269
turbidity currents can initiate on slopes as little as \_\_\_\_\_
0.2o
270
what speeds and distances can turbidity currents travel at?
what can they contain?
in excess of 70 mph
or as slow as a few mph
can travel distance of hundreds of kms
their deposits contain a large proportion of the earths oil supply
271
describe th movement of turbiditys on a passive margin.
sediment moves down the continental slope onto the continental rise by sediment gravity flows
- usually sediment travels via submarine canyons ( currents are purely erosional due to its high speeds)
at the mouth of canyon spreads out to form submarine fan. (due to canyon widening and gradient is less steep)
denser than sea water - mix of sediment and water.
272
what happens as turbidity current exit submarine canyons?
they deposit huge submarine fans.
sinuous channels (meandering)
273
describe how turbidite currents flow down turbidite channels
current is not confined to the channel. they can be higher than the channel and can be in several different channels
if a channel bends then there is superelevation on the outside of the bend. thus this side of the bend is higher than the other.
274
describe turbidite beds
commonly deposited in deep basin
successions of rock contain the record of hundreds of turbidity currents
turbidites are commonly very tabular and very extensive outcrop. this is because the individual turbidity currents cover vast areas, (what makes them good oil reservoirs)
275
what are turbidites seperated by?
turbidites are seperated by muds
