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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

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2
Q

what influences weathering?

A

tectonics (rate of uplift)

climate (water flux, temperature)

topography/ relief

prior rock history (rock type/ deformation history)

biology

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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
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4
Q

what does a large amount of soil show you?

A

a large amount of soil shows a large amount of weathering,

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5
Q

what does a small amount of soil show you?

A

either weathering is very slowly

or

it is being rapidly removed.

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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

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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

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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

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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
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10
Q

what is meant by hydrofracturing?

A

as temperature decreases the greater the force to break the rock up.

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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

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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

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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

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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

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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.

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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.

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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

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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

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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?

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20
Q

what direction does fracture grow in thermal environments?

A

fracture growth is perpendicular to suns movement.

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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.

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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)

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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

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24
Q

what impact does hydration have on weathering?

A

some minerals change volume upon hydration

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25
Q

what is the use of chemical weathering?

A

economic geology: laerites, bauxites, and metal enrichment in supergene deposits

sources of inorganic netrients in soils

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26
Q

what is chemical weathering?

A

chemical weathering is the decomposition of a rock through chemical reactions, and the formation of new minerals , and ions in solution.

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27
Q

whats the reason for chemical weathering taking place?

A

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.

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28
Q

what happens during chemical weathering?

A

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)

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29
Q

what impact does freeze thaw have on chemical weathering? why?

A

increases the rate of reactions.

this is due to freeze thaw increasing the surface area of the rock thus more reactions

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30
Q

what is meant by residual material?

A

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.

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31
Q

what is meant by pedosphere?

A

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

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32
Q

with continued weathering what happens to the residual minerals, clays and other secondary minerals?

A

they accumulate in the pedosphere or are washed into the oceans with dissolved ions.

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33
Q

how can we measure the rate of chemical weathering?

A

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.

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34
Q

which two minerals are least and most weathered by chemical weathering?

A

quartz (least weathered)

olivine (most weathered)

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35
Q

what causes chemical weathering?

A

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.

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36
Q

what is the most important agent for chemical weathering?

A

the fact that the rain is acidic is most important as the H+ is needed to disrupt the mineral structure.

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37
Q

what factors influence mobility of ions and their removal?

A
  • 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+)

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38
Q

which is more important for chemical weathering , frequency oor the amount of precipitation? why?

A

frequency is more important as there needs to be removal of cations so the concentration gradient is high.

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39
Q

what is meant by humic acid?

A

it is produced by biodegradation of dead organic matter. it is not a single acid but a complex mixture of many different acids.

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40
Q

what is meant by laterites?

A

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,

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41
Q

what does a thick laterite show you?

A

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.

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42
Q

what are the four types of chemical weathering reactions?

A

ionic dissolution

carbonation

oxidation and reduction

acid hydrolysis.

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43
Q

what is meant by ionic dissolution?

A

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-

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44
Q

describe the rates of ionic dissolution and thus how we can speed up the reaction?

A

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)

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45
Q

what rock type is dominated by carbonation?

A

carbonation dominates the weathering of limestones

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46
Q

what is meant by carbonation?

A

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,

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47
Q

how can we measure the rate of carbonation?

A

by looking at the concentrations of Ca2+ and HCO3- we can see the extent of carbonation

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48
Q

what is meant by oxidation and reduction in chemical weathering?

what does the sudden appearance of haematite in geological history show us?

A

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.

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49
Q

what is meant by acid hydrolysis?

A

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

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50
Q

what is the typical mechanism of silicate dissolution (acid hydrolysis)?

A

Olivine Mg2SiO4 + 4H+ –> 2Mg2+ + H4SiO4

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51
Q

what are the three main acid sources for acid hydrolysis?

A

carbonic acid - forms from carbon dioxide dissociation in water

organic acids (from respiration of carbon dioxide in soils)

sulfuric acid (volcanic)

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52
Q

describe feldspar hydrolysis

A

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

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53
Q

what is the equation for feldspar hydrolysis?

A

Orthoclase + hydrogen ions + water –> K+ + kaolinite (clay) + silica

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54
Q

describe how granite is weathered?

A

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.

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55
Q

what is the use of the quartz feldspar ratio?

how does it work?

A

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.

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56
Q

what is the effect of rising mountains to the removal of carbon dioxide?

A

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.

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57
Q

how is the material derived from erosional processes eventually disposed of?

A

material derived from erosional processes eventually disposed of by the action of erosion by rivers

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58
Q

what is meant by rainsplash?

A

rainsplash: the detatchment of loose particles by raindrop impact

on a sloping ground, rainsplash results in a net downslope transport of sediment

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59
Q

what influences the impact of rainsplash?

A

the total kinetic energy of rain depends on:

  • raindrop size
  • distribution
  • rainfall intensity
  • duration
  • frequency
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60
Q

what are rills and gullies?

A

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)

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61
Q

what are the three types of mass wasting?

A

rockfall

creep

sliding

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62
Q

what is meant by rockfall?

A

rockfall: products of physical weathering fall down steep slopes onto talus slopes/cones

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63
Q

what is meant by creep?

A

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.

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64
Q

what is meant by sliding?

A

landslides occur where shear stress is enhanced and material strength is reduced.

typically, this occurs during rainstorms or earthquakes.

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65
Q

what factors the critical slope for landsliding depend on?

A

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.

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66
Q

what is the relationship between mass removal, isostatic, and river incisions?

A

removing mass causes tectonic uplift

rivers incise into bedrock

and insure the progressive lowering of the base level for hillslope processes

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67
Q

what is the relationship between rock hardness and steepnes of slopes due to weathering?

A

the stronger the rock the steeper the slopes.

however past a point it fails and we get land slides.

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68
Q

what do you need lots of to create a gorge?

A

you need high rainfall and high uplift.

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69
Q

what are the three bedrock erosion processes

A

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)

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70
Q

what is meant by quarrying?

A

large fragment rocks weakend by weathering.

huge floods then move blocks the size of cars.

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71
Q

what is characterisitic of sand blasting?

A

sand impact makes smoothes surfaces.

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72
Q

what is needed to erode landscapes by river in terms of the goldilocks effect?

A

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.

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73
Q

what is the relationship between bedrock landsliding and bed rock incision?

A

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.

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74
Q

what is meant by entrainment?

A

it is the oicking up and moving grains

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75
Q

what is sedimentology is all about..

A

sedimentology is all about entrainment and settling

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76
Q

the ability of a fluid to erode a particle depends on…

A

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)

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77
Q

what are the two types of flow?

A

laminar and turbulent

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78
Q

describe laminar and turbulent flows and say how they are different.

A

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

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79
Q

what is meant by inertial forces?

A

object in motion tends to remain in motion

  • slight changes in path can have huge impact
  • perfectly straight flow lines are rare
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80
Q

what is meant by viscous forces?

A

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.

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81
Q

what two forces decide the flow?

A

flow is the fight between inertial and viscous forces.

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82
Q

what is reynolds number the ratio of?

A

reynolds number is the ratio of inertial to viscous forces.

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83
Q

what are the forces that resist the motion of a grain?

A

gravity

interparticle friction

cohesion/ elecrtochemical bonds (clays)

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84
Q

describe the shear stress at the beds surface that is exerted by a fluid moving above the bed.

A
  • 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

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85
Q

what is the drag component a function of?

A

drag component - function of boundary shear stress

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86
Q

what is the lift component related to?

A

lift component - bernoulli effect

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87
Q

what are the two causes of the lift force?

A
  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
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88
Q

what happens to the lift force as you move away from the bed?

A

lift force strength descreases rapidly as you move away from the bed.

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89
Q

what causes the initiation of motion of sand grains?

A

grains move when the combined lift and drag forces become large enough to counteract gravity and frictional forces holding grains in place.

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90
Q

what are the 4 forces acting on a grain particle?

do they resist or encourage motion?

A

resist

weight

frictional forces between adjacent particles

encourage

hydraulic lift forces

tangential drag force

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91
Q

what is the relationship between particle movement and moments?

A

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)

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92
Q

why is a greater shear stress needed to move small grains as shown on the shields curve?

A

for grain sizes finer than silt, cohesion holds grains together: greater shear stress is required to move small grains.

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93
Q

for grains that get coarser than sand grains is more or less shear stress needed as shown on the shields curve?

A

for sand sizes and coarser, shear stress needed for entrainment increases with grain size.

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94
Q

the critical threshold for grain movement depends on?

A

particle shape, size, sorting

bed roughness

cohesion.

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95
Q

once a particle has been set in motion its transport path is a funtion of:

A

once a particle has been set in motion its transport path is a funtion of:

  • particle settling velocity
  • current velocity
  • fluid turbulence
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96
Q

is more energy needed to put a particle in motion or to keep it in motion?

A

more energy is needed to put a particle in motion than to keep it in motion

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97
Q

what are the two types of particles found in a river flow?

describe them.

A

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)

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98
Q

what are the three types of bedload transport?

A

traction - rolling/sliding

saltation - jumping over the bed surface

suspension

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99
Q

describe saltation

A

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.

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100
Q

what causes grains to rise during saltation?

A

lift forces

grain splashing

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101
Q

what is meant by grain splashing?

A

when one grain hits another as it lands. this forces othr sand grains to be ejected upwards.

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102
Q

what is meant by suspended load?

A

particles held in continuous suspension by fluid turbulence

  • upward component of fluid flow overcomes gravity
  • finer grain sizes (very fine sand to clay)
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103
Q

how does mode of transport change with increasing flow velocity?

A

transport stage is a function of velocity divided by critical velocity to move sediment

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104
Q

what is meant by wash load?

A

clay sized particles derived from up current source

always remain in suspension.

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105
Q

when does sediment deposition take place?

A

sediment deposition takes place when the flow system can no longer support the grain.

when buoyancy of fluid and drag are less than gravity

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106
Q

which law quantifies settling velocity?

A

stokes law

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107
Q

which grains settle out first?

A

generally the coarsest grains settle out first

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108
Q

what plays a key role in keeping grains aloft?

A

turbulence plays a keyrole in keeping grains aloft.

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109
Q

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?

A

due to a decrease in slope

in deltas by spreading of flow into standing body of water.

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110
Q

what happens if you put a sediment grain in a less dense fluid?

A

it will settle downward and accelerate under the influence of gravity.

but its movement will be restricted by fluid drag.

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111
Q

what happens when fluid drag becomes equal to the weight of the grain?

what is this called?

A

the grain will fall at constant velocity.

called terminal settling velocity or stokes velocity

112
Q

what is settling velocity dependent on?

A

grain size

grain shape

density of grain

viscosity of fluid

density of fluid

113
Q

drag exerted on a grain is proportional to what?

what would this mean in terms of mica?

A

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
Q

accurate predictions of rate of fall of solids through stationary fluid is only possible when?

A

smooth, insoluble fluid sphere

in a newtonian fluid

infinitely wide and deep container.

115
Q

drag coefficient is a complex function of…

A

fluid viscosity and density, grain size and shape, and the speed and trajectory of settling.

116
Q

what is a bedform?

A

a bedform is a deformation of a granular bed due to interaction of the bed with the flow

117
Q

what is the use of bedforms?

A

they make sedimentary structures that enable us to read the rock record for processes and past flow conditions.

118
Q

describe the anantomy of an asymmetrical bedform

A
119
Q

what happens when you have two dunes. one smaller behind a larger dune.

A

the small one moves faster so combines with the large one to make an even larger dune

120
Q

what are the main types of bedform?

A

low flow regime

  • ripples
  • dunes

upper flow regime

  • plane beds - leads to flat lamination
  • antidunes
121
Q

what is the general height and length of ripples?

what does length depend on?

A

H- less than about 4cm

L - <60cm (L depends on grain size)

122
Q

what conditions do ripples form in?

A

form in shallow, slow moving flows, low transport rates

123
Q

what grain size do ripples form from?

A

ripples will only form in medium grained sand or finer

124
Q

how do we change straight ripples to sinuous?

A

with increasing flow strength, crests become more sinuous

125
Q

what is the height and length of dunes?

what is L dependent on?

A

H- up to 5m in large rivers

L up to 100s of m; L depends on flow depth.

126
Q

what grain sizes does dunes form in?

A

dunes form in sediement medium and above.

127
Q

how do dunes become more curve crested and discontinuous?

A

dunes become more curve crested and discontinuous as shear stress increases.

128
Q

describe the fall velocity of a grain in a dispersion?

A

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
Q

bedforms with linear crests and a roughly uniform height migrate downstream with a uniform___________

A

bedforms with linear crests and a roughly uniform height migrate downstream with a uniform depth of erosion.

130
Q

describe the air flow over an aeolian dune.

A
131
Q

what is the relationship between bed shear stress and the back of the ripple / dune?

A

bed shear stress increases up the back of ripple/dune

132
Q

why is the deposition at top of face of a dune?

A

downstream of a dune crest flow expands, slows – results in flow seperation

reduction in shear stress at top of avalanche face promotes deposition.

133
Q

describe where there would be deposition and erosion on a ripple.

A

most sand is deposited at crest

most erosion on lee side due to flow seperation.

134
Q

due to deposition on the crest and erosion of the lee side what do we normaly see?

A

avalanching

135
Q

describe ripple and dune formation

A

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
Q

what are the four stages to a bed as velocity is increased?

A

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
Q

what is meant by antidunes?

A

its the opposite to a dune, so deposition on the stoss side and eroded on the lee side.

138
Q

why cant straight and sinuous not be together?

A

they need different flow rates.

139
Q

what is meant by the bedform phase diagram?

A

different bedform states occupy distinct fields on plots of flow variables

140
Q

explain the no movement zone on the bedform stability diagram

A

because the grains are course, ripples are not possible

141
Q

as flow velocity increases what are ripples and dunes replaced by?

A

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
Q

what is meant by 2D and 3D in terms of ripples and dunes?

A

2D - straight crested

3D - sinuous crested

143
Q

sketch the bedford stability diagram

use this to describe changes to bedforms when flow velocity/ shear stress increases.

A
144
Q

what is meant by sedimentrary structures?

A

sedimentary structures are features found within the sedimentary section, and/or on, and/or between, bedding plane surfaces subdividing that section

145
Q

what is the use of sedimentary structures?

A

provide critical clues to sedimet transport and depositional processes.

gives us information on:

lithology

bedding

geometries

fauna

146
Q

under which systems are primary sedimentary structures formed?

A

formed under influence of sme hydrodynamic and or aerodynamic conditions as entrainment, transport and deposition of sediment particles

147
Q

what can we resolve form primary sedimentary structures?

A

process

transport pathway

paleohorizontal

way up

148
Q

what are planar laminae? what creates them?

A

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
Q

why wont you find mud mixed in with sand in planar laminae?

A

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
Q

cross stratification produced by the migration of bedforms depends primarily on which two factors?

A

the 3D geometry of the bedforms

the ratio beween downstream translation and vertical accretion.

151
Q

where do we find cross stratification?

how do we tell them apart?

A

we find cross stratification in both aeolian (desert) and under water

they are hard to tell apart

152
Q

what is meant by aeolian?

A

formed in a desert

153
Q

describe the migration of straight crested bed forms with no climb.

A

surface never builds up.

stays planar.

154
Q

describe the difference between migration of straight crested bedforms where there is no net deposition and with net depositon.

A

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
Q

what creates large scale and small scale cross stratification?

A

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
Q

what are the two types of cross ctratification?

A
  • planar cross strat.
  • trough cross strat.
157
Q

what creates planar cross strat?

A

planar cross strat created by straight crested bedforms

158
Q

what produces trough cross strat?

A

trough cross strat is produced by sinual crested bedforms

159
Q

describe planar cross bedding

A

when moving the lee side erodes down into the layer below. thus when they move they create a flat surface.

160
Q

what is the terminology related to cross stratification?

A
161
Q

what can we say about cross stratification that has large pebbles in it?

A

wind can not move grains greater than 2mm. thus the pebbles show that the cross stratification was created by water.

162
Q

what is meant by grainflow and grainfall?

A

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
Q

what is meant by toesets?

A

the toeset is the forward part of the dunes advancing frontal slope.

164
Q

explain the weire shape of trough cross strat.

A

depending on where you are on the dune it cuts down by different amounts so we get this wierd shape

165
Q

what are the dark lines on the trough cross strat

A

dark lines represent the erosional surfaces (sometimes called smiles)

166
Q

how can we tell the difference between planar and trough cross strat?

A

trough cross strat has curved lines

planar cross strat has straight lines.

167
Q

describe the migration of straight crested bedforms with a steep climb?

A

the stoss side is preserved

168
Q

describe the differences in the preservation in the stoss and lee side for different conditions of bedload transport and suspended sediment fall out rate.

A

arrows represent movement

169
Q

how are climbing ripple cross lamination formed?

A

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
Q

what are climbing ripples indicators of?

A

climbing ripples are indicators or rapid sedimentation as their formation depends on the addition of sand to the flow.

171
Q

describe wave ripples

A

symetrical slopes

  • no pointy crests due to when the water dissapears the crests collapse.

waves can split in two - biforcation

172
Q

describe the orientation of wave ripples.

what does this allow us to do?

A

this allows us to see palaeo beach geometry

173
Q

what are wave ripples created by?

A

wave ripples are created by oscillating currents.

174
Q

what are water waves?

A

water waves are wind generate oscillatory motions of water

175
Q

what is wave height dependent on?

A

wave height is dependent on wind strength and fetch (surface area)

176
Q

what is meant by wave base?

A

wave base is the maximum depth in which motion created by waves can happen.

177
Q

what is a common mistake to make with wave motion?

A

the fact that the waves go back and forth is wrong.

they actually rise and fall

178
Q

what is meant by wave orbitals?

A

when water goes up and down, it creates circular motion pathways called orbitals.

179
Q

describe wave orbitals in shallow water.

A

in shallow water the orbitals become more elliptical until in is basically back and forth.

thus creates sand waves at the bottom.

180
Q

describe wave orbitals in deep water

A

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
Q

what is the difference between wave ripples and current ripples?

A

wave ripples are distinct from current ripples due to their symmetry.

182
Q

why do orbitals become more elipticl with increased depth?

A

orbitals become more elliptical because of their interaction with the water bodies base (sea bed)

183
Q

explain how these sand grains eject

A

little eddies created causing sand grains to eject from back and forth frow of wave orbitals

184
Q

on a flat surface with low bed shear stress (gentle wind), what will be produced?

A

**rolling grain ripples - ** (very low amplitude symmetrical ripples with little to no grain movement, dont have sharp crests)

185
Q

in the process of creating wave ripples, what happens with increased bed shear stress?

A

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
Q

what is the shape of waves created by purely oscillatory flows?

A

pure wave ripples created by oscillatory flows are smmetrical in shape.

187
Q

how do we identify wave ripples?

A

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
Q

what is the major difference that we can use to differ wave ripples to current ripples?

A

wave ripples have cross lamination that go down on both sides. current ripples have laminations only on one side.

189
Q

if we see wave ripples and the cross lamination is bunched up on one side, what can we say?

A

on one side the more bunched up laminations show how one wave direction is stronger than another.

190
Q

what is the use of looking at wave ripples?

A

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
Q

describe how cross laminations in a wave ripple form.

A

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
Q

what happens when a river emerges from a mountain?

A

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
Q

what do the lines on this image represent?

A

lines represent former river systems.

194
Q

in alluvial fans does the river system change slowly or abruptly?

A

in alluvial fans the river system changes abruptly.

195
Q

describe alluvial rivers

A

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
Q

describe flood plains

A

raised area is called a levee

197
Q

what are the four main types of channel patterns of alluvial rivers?

A

meandering rivers

braided rivers

straight rivers

anastomosing

198
Q

describe the major points of a meandering rivers.

A

cut bank - erosion

point bar - deposition

199
Q

describe the motion of water on the high velocity side of a meandering river

A

water flow is helical thus causing large amounts of erosion.

200
Q

what would we find in the accretion topography of a meanderig river?

A

cross laminations

cross bedding

graded bedding

201
Q

what direction do ripples face in a meandering river? why?

A

the crests of the ripples lie parallel to the banks edge.

this is due to the water moving in a helical fashion.

202
Q

describe the flow and sediment transport of a meandering river and thus how the meander moves.

A

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
Q

why do we get several types of bedforms on a pointbar?

A

decreasing bed shear stress from thalweg to point bar top leads to development of suite of bedforms on pointbar surface and upward fining.

204
Q

what is meant by thalweg?

A

it is the deepest part of a water course.

205
Q

what is a braided river

A

complex multichannel systems of low sinuosity which commonly migrate across the braidplain.

characterized by wide, shallow channels

206
Q

what is the major difference between meandering rivers and braided rivers?

A

meandering rivers are a single spagetti thread

braided rivers are several spaghetti thread systems.

207
Q

how is a braided river created

A

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
Q

compare braided vs meandering

A

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
Q

what designates if a stream will be a braided system?

A

a braided river needs:

  • coarse load
  • steep
  • higher shear stress
  • more bedload
  • less cohesive walls
210
Q

what conditions are needed for a meandering stream?

A

a meandering stream needs:

  • fine load
  • shallow
  • lower shear stress
  • less bedload
  • more cohesive walls
211
Q

describe the evolution of a meandering river

A

they become wider and more sinuous due to the erosion of the cut bank and deposition of the point bar.

212
Q

what do old point bar deposits indicate?

A

old point bar deposits indicate channel migration.

213
Q

what is meant by the resetting of a meandering river?

A

when the meandering river becomes too sinuous an oxbow lake is created and it resets by straightening out.

214
Q

what can happen to meandering rivers during flooding.

A

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
Q

what are the uses of ancient river systems?

A

excellent petroleum reservoirs

excellent aquifers

placer deposits concentrated in fluvial channels… diamonds, gold, uranium.

216
Q

define a delta

A

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
Q

what are the components of a delta?

A
  • 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
Q

what are the main sedimentary environments of deltas?

A
219
Q

describe the delta plain

A

low relief, subaerial portion of delta

channels may divide into smaller, straighter distributary channels

between channels - lakes or floodplains

220
Q

describe the delta front.

A

area where sediment - laden river enters sea/lake

mouthbars in river dominated deltas

beaches and barrier islands in wave dominated deltas

221
Q

describe the tip of a channel.

A

more shallow at end than the channel due to deposition of sand. grains get small further away

222
Q

what are the main processes of low sand sedimentation at deltas?

A

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
Q

what is meant by a jet?

A

a jet is the combined discharge of sediment and fresh water issuing from the mouth of a major delta distributary.

224
Q

describe inertia dominated jets

A

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
Q

describe frictional dominated jets

A

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
Q

when we have river water and basin water density contrasts, what causes differences in density?

A

salinity

temperature

suspended sediment concentration

227
Q

what is meant by homopycnal jets?

A

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
Q

what is meant by buoyent jets?

A

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
Q

which is denser fresh or salt water?

A

salt water is denser

230
Q

what is meant by hypopycnal jet?

A

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
Q

what is meant by hyperpycnal jet?

A

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
Q

describe wave dominated deltas.

A

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
Q

describe river dominated deltas and their movement

A

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
Q

what is meant by progradation?

A

progradation refers to the growth of a river delta farther out into the sea over time.

235
Q

what does this diagram show?

A

shows how the delta moves towards the lake through time

236
Q

in a rock sequence how can we tell that a delta has been migrating seawards?

A

we see coarsening up where finest grains at the bottom and coarser on top

237
Q

describe tide dominated mudflats

A

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
Q

what would alterneting layers of sandstone and mustone show you?

A

layers of mudstone and sandstone show regression and transgression.

239
Q

what environment is given by mudstone?

A

low energy marine

240
Q

what environment is given by a sandstone?

A

shallow marine

241
Q

describe the continental margin

A
242
Q

how are the canyons on continental slopes created?

A

canyons created due to drainage from transgression and regression

243
Q

what do submarine canyons allow?

A

submarine canyons allow sand to reach the deep marine.

244
Q

what are sediment gravity flows?

A

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
Q

what is key for sediment gravity flows to occur?

A

for movement to occur, grains must be dispersed.

246
Q

what is a turbidity current?

A

a turbidity current is a member of the class of dense bottom flows that includes thermohaline bottom flow (e.g. straits of gibralter)

247
Q

how does a turbidity current work? what is its driving force?

A

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
Q

how can turbidity currents move on very small gradients?

A

because they move on density differences thus can travel on very small gradients

249
Q

why do turbidities often pulsate?

A

pulsates due to equillibrium between the fluid and sediment mixture.

250
Q

draw a diagram of a turbidity current

A
251
Q

where does deposition occur in turbidity currents?

A

sediment deposition occurs at base of the flow

252
Q

how does a turbidity current decelerate?

A

sediment deposition coupled with the intake of water means the density of the flow decreases, and thus decelerates

253
Q

describe the morphology of a turbidity current

A

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
Q

how is a river different to a turbidity current?

A

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
Q

why do all turbidity currents die?

A

they must die as eventually the amount of sediment in suspension must aproach zero.

256
Q

can we see turbidity currents?

A

not really, difficult to see them and they dont happen very often.

257
Q

what triggers turbidities?

A

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
Q

how can we use turbidities to measure past earthquakes?

A

number of deposits give frequency

thickness of deposit governs size of earthquake

C14 dating gives age.

259
Q

what is the Bouma sequence?

A

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
Q

what are sole structures?

A

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
Q

why are sole structures important?

A

way up indicators on deformed rocks

paleaocurrent indicators

262
Q

how do you work out the flow direction from flute casts?

A

the deepest part is upstream and tapers of to beome shallower

263
Q

how are flute casts created?

A

created by lots of eddies digging down into the sides

264
Q

what are tool marks?

A
  • often have sharply defined edges
  • produced by objects carried in turbidity flow
  • occur as isolated casts or in patterns covering bedding surfaces.
265
Q

what are the two types of tool marks?

A

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
Q

what does it mean if tool marks are pointing in opposite direction?

A

this is wherenthe flow has bounded off something and then goes in the opposite direction

267
Q

describe the stages of the bouma sequence

A

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
Q

describe the thickness of turbidite beds.

A

close to the source area the beds are thicker

further along it gets thinner as most sand has already been deposited.

269
Q

turbidity currents can initiate on slopes as little as _____

A

0.2o

270
Q

what speeds and distances can turbidity currents travel at?

what can they contain?

A

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
Q

describe th movement of turbiditys on a passive margin.

A

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
Q

what happens as turbidity current exit submarine canyons?

A

they deposit huge submarine fans.

sinuous channels (meandering)

273
Q

describe how turbidite currents flow down turbidite channels

A

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
Q

describe turbidite beds

A

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
Q

what are turbidites seperated by?

A

turbidites are seperated by muds

geophysics (9 decks)

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