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Geoscience > Unit 2 - Sedimentology > Flashcards

Unit 2 - Sedimentology Flashcards

1
Q

Distribution of porosity and permeability is controlled by ?

A

Transport and deposition

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2
Q

What is texture?

A

term to describe size, shape and arrangement of grains, matrix and cement in a sedimentary rock

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3
Q

What does the grain shape depend on ?

A
  • mineralogy of grains and the degree and energy of transportation
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4
Q

What is the fabric?

A

orientation and packing of grains and their contacts → describes how grains are arranged 3-dimensional in a sediment

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5
Q

What is microporosity ?

A

pores < 0.5 microns

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6
Q

What is macroporosity ?

A

pores > 0.5 microns

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7
Q

Reasons for anisotropic permeability?

A
  • grain orientation and packing arrangements of non-spherical grains
  • presence of larger scale permeability barriers vertically (e.g. bedding planes, changes in grain size or other layering)
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8
Q

How come that porosity tends to increase with decreasing grain size?

A
  1. Finer grains tend to have less regular shapes than coarser grains → often less efficiently packed
  2. Fine sediments are often better sorted than coarser sediments
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9
Q

Why do clays have an effective porosity <1% ?

A

Because clay bonded water fills available microporosity

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10
Q

What are the three basic physical processes of transportation and deposition of sediments ?

A
  • Fluid flow
  • gravity flow
  • deposition from suspension
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11
Q

Describe the transport by fluid flow!

A
  • particles are picked up (eroded), transported and deposited by moving fluids
  • fluids are: air (wind), water (river currents, waves and oceanic currents) and ice (glaciers)
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12
Q

What is gravity flow?

A
  • particles are transported down a slope due to gravity acting on the particles → deposition will take place once frictional forces overcome the gravitational forces
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13
Q

What is Deposition from suspension?

A

particles are in a fluid in which is no movement and therefore no force to keep the particles in suspension → they settle due to gravity

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14
Q

What are the main types of fluid motion by which grains are transported ?

A

Currents (water or air) and waves

→ in both sediment is transported either in suspension (through effects of turbulence) or as a bed load by rolling or bouncing (“saltating”)

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15
Q

The grainsize and volume of sediment, which can be transported by a fluid depends on ?

A
  • velocity and viscosity of fluid
  • depth of flow
  • turbulence of flow
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16
Q

How do gravity flows work?

A

sediment accumulates on a slope → if frictional forces < force of gravity → material will move downslope

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17
Q

Name different transport proccesses considered as gravity flow!

A
  • rock falls
  • rock slides
  • landslides
  • slumps
  • debris flow
  • turbidity currents

→ list is in order of increasing fluid contact

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18
Q

When does deposition take place?

A

When forces supporting the grains in motion become less than the force of gravity

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19
Q

What is the definition of a bedform?

A

Features on the sediment surface (distinctive shapes)

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20
Q

What is a sedimentary structure ?

A

Features within the sediment which are commonly preserved in the rock record

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21
Q

What do bed boundaries represent?

A

temporary breaks in the deposition or hiatus

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22
Q

contacts between beds can tell us something about ?

A

the processes creating the beds → is contact abrupt, sharp, interfingering etc. → in describing a bed it is important to note the nature of both, the top and bottom contacts

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23
Q

Bed- and lamina thickness?

A
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24
Q

Why are antidunes very rarely preserved in rock record?

A
  • because they represent a very high energy turbulent flow → any bedforms developed are rapidly altered
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25
Q

Size of ripples ?

A

less than 5cm high

26
Q

Size of megaripples?

A

5cm-1000cm

27
Q

The dip of the lee side of a ripple is controlled by ?

A

The “angle of repose” → maximum slope at which grains of a grain size and sorting can rest without failure of the slope

28
Q

What kind of forms do the foresets of ripples have?

A

planar or concave-upwards

29
Q

How do you call the sedimentary structure deposited by megaripples?

A

cross-bedding

30
Q

How do you call the sedimentary structure deposited by ripples?

A

cross-lamination

31
Q

What is straight-crested and lunate crested? and what kind of sedimentary structures do they produce?

A
32
Q

Why do straight crested ripples or megaripples generally occur at lower current velocities?

A

Because when current velocity increases, the strength and localisation of separation eddies in lee side of bedforms becomes greater → leads to increased and localised erosion of trough → leads to localised embayments of crestline → leads to appearance of concave- downcurrent (lunate) bedforms

33
Q

What happens to the megaripple bedforms, when the current velocity or stream power increases?

A

They become lower and flatter and are eventually replaced by a plane bed on which there is intense sediment transport, with most of the grains moving most of the time → called upper-stage plane bedding

34
Q

What is the wave base?

A

Point where the orbital radius of a water particle reaches 0 → the radius is highest on water surface

35
Q

What happens if the sediment surface is above the wave base?

A

Waves will impinge on the sediment and this will change the behaviour of the waves and of the sediment

→ the orbital motion of the waves changes to elliptical and as the waves enter shallower water, they become steeper, migrate more rapidly onshore and eventually break onto the shore

36
Q

What can happen when waves impinge on sediment surface?

A

They can produce wave-ripples (up to 200m water depth)

37
Q

Describe wave ripples

A
  • generally straight crested
  • vary greatly in size (size is dependent of wave dimensions)
  • ripple wavelength between 0.0009m - 2m
  • ripple height 0.003-0.25m
38
Q

What are differences between wave ripples and current ripples?

A
  • wave ripples have lower ripple indices (ripplewavelentgh divided by ripple height)
  • crestal bifurcation → meaning: crests usually split along their length
39
Q

Where is Hummocky cross-stratification (HCS) found?

A

Only in shallow marine rock record

40
Q

Decribe Hummocky cross-stratification (HCS)

A
  • generated by waves or combined waves and currents
  • caused by current motions during storm acitvity
  • consist of low-relief mound-like hummocks, separated by troughs
  • cross section include both concave-upwards and convex-up laminae, → convex up is very rare
41
Q

Wind - generated bed forms?

A
  • wind- ripples
  • dunes
  • compound dune - like bedform → draas
42
Q

measurements of aeolian ripples?

A
  • wavelengths of 0.01-20m
  • heights of mm-1m
  • grains move mainly by saltation
43
Q

How are aeolian dunes mainly differentiated?

A

-by their structure

44
Q

Aeolian dune and draa morphology?

A
45
Q

How come that there are different kinds of flow ?

A
  • Different proportions of sediment to water
  • different grainsizes

different slopes

46
Q

Different sediment gravity flow processes?

A
47
Q

what are the most commonly observed debris flows?

A

mudslides → consists of debris in a muddy matrix

48
Q

what is a turbidity current?

A
  • mixture of sediment and water, in which the sediment is kept in suspension by the turbulence of the flow
49
Q

What do currents and waves which transport sediments often do to the underlying sediment?

A

They often erode it before they deposit their own load

50
Q

Why is recently deposited sediment often unstable?

A

Because it is often poorly packed and has a high water content → can easily be deformed by various processes

51
Q

What is heterogenity of the permeability?

A

-permeability is highly variable throughout the rock

52
Q

How are mud clasts formed?

A

Force of water flow erodes local mud accumulations and carries block of mud downstream

53
Q

How do you create convolute lamination?

A

By applying stress to recently deposited beds → may cause shear of the deposited sediment

54
Q

How do is a pillow structure in beds created?

A

In a succession, more efficiently packed, coarser and therefore denser beds may overlie less dense beds → unstable situation → response to gravity, the base of a dense bed may bulge down into the underlying bed

55
Q

What is diagenesis?

A

The range of physical and chemical processes and changes, which turn a sediment into a rock (lithification) and continue to alter the rock

56
Q

Which diagenic processes reduce porosity and permeability?

A
  • compaction
  • cementation
  • precipitation of authigenic clays
57
Q

Which diagenic processes increase the porosity and permeability?

A
  • grain or cement dissolution
58
Q

When clastic sediments are buried, sometimes cementation processes happen, explain why!

A
  • majority of cements are precipitated from pore waters

→ type of minerals precipitated will reflect changes in water chemistry with time

59
Q
A
60
Q
A

Geoscience (10 decks)

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