Unit 2 - Sedimentology

Question 1

Distribution of porosity and permeability is controlled by ?

Answer 1

Transport and deposition

Question 2

What is texture?

Answer 2

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

Question 3

What does the grain shape depend on ?

Answer 3

• mineralogy of grains and the degree and energy of transportation

Question 4

What is the fabric?

Answer 4

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

Question 5

What is microporosity ?

Answer 5

pores < 0.5 microns

Question 6

What is macroporosity ?

Answer 6

pores > 0.5 microns

Question 7

Reasons for anisotropic permeability?

Answer 7

• 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)

Question 8

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

Answer 8

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

Question 9

Why do clays have an effective porosity <1% ?

Answer 9

Because clay bonded water fills available microporosity

Question 10

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

Answer 10

• Fluid flow
• gravity flow
• deposition from suspension

Question 11

Describe the transport by fluid flow!

Answer 11

• 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)

Question 12

What is gravity flow?

Answer 12

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

Question 13

What is Deposition from suspension?

Answer 13

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

Question 14

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

Answer 14

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”)

Question 15

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

Answer 15

• velocity and viscosity of fluid
• depth of flow
• turbulence of flow

Question 16

How do gravity flows work?

Answer 16

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

Question 17

Name different transport proccesses considered as gravity flow!

Answer 17

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

→ list is in order of increasing fluid contact

Question 18

When does deposition take place?

Answer 18

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

Question 19

What is the definition of a bedform?

Answer 19

Features on the sediment surface (distinctive shapes)

Question 20

What is a sedimentary structure ?

Answer 20

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

Question 21

What do bed boundaries represent?

Answer 21

temporary breaks in the deposition or hiatus

Question 22

contacts between beds can tell us something about ?

Answer 22

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

Question 23

Bed- and lamina thickness?

Answer 23

Question 24

Why are antidunes very rarely preserved in rock record?

Answer 24

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

Question 25

Size of ripples ?

Answer 25

less than 5cm high

Question 26

Size of megaripples?

Answer 26

5cm-1000cm

Question 27

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

Answer 27

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

Question 28

What kind of forms do the foresets of ripples have?

Answer 28

planar or concave-upwards

Question 29

How do you call the sedimentary structure deposited by megaripples?

Answer 29

cross-bedding

Question 30

How do you call the sedimentary structure deposited by ripples?

Answer 30

cross-lamination

Question 31

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

Answer 31

Question 32

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

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What is the wave base?

Answer 34

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

Question 35

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

Answer 35

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

Question 36

What can happen when waves impinge on sediment surface?

Answer 36

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

Question 37

Describe wave ripples

Answer 37

- 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

Question 38

What are differences between wave ripples and current ripples?

Answer 38

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

Question 39

Where is Hummocky cross-stratification (HCS) found?

Answer 39

Only in shallow marine rock record

Question 40

Decribe Hummocky cross-stratification (HCS)

Answer 40

- 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

Question 41

Wind - generated bed forms?

Answer 41

- wind- ripples - dunes - compound dune - like bedform → draas

Question 42

measurements of aeolian ripples?

Answer 42

- wavelengths of 0.01-20m - heights of mm-1m - grains move mainly by saltation

Question 43

How are aeolian dunes mainly differentiated?

Answer 43

-by their structure

Question 44

Aeolian dune and draa morphology?

Answer 44

Question 45

How come that there are different kinds of flow ?

Answer 45

- Different proportions of sediment to water - different grainsizes different slopes

Question 46

Different sediment gravity flow processes?

Answer 46

Question 47

what are the most commonly observed debris flows?

Answer 47

mudslides → consists of debris in a muddy matrix

Question 48

what is a turbidity current?

Answer 48

- mixture of sediment and water, in which the sediment is kept in suspension by the turbulence of the flow

Question 49

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

Answer 49

They often erode it before they deposit their own load

Question 50

Why is recently deposited sediment often unstable?

Answer 50

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

Question 51

What is heterogenity of the permeability?

Answer 51

-permeability is highly variable throughout the rock

Question 52

How are mud clasts formed?

Answer 52

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

Question 53

How do you create convolute lamination?

Answer 53

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

Question 54

How do is a pillow structure in beds created?

Answer 54

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

Question 55

What is diagenesis?

Answer 55

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

Question 56

Which diagenic processes reduce porosity and permeability?

Answer 56

- compaction - cementation - precipitation of authigenic clays

Question 57

Which diagenic processes increase the porosity and permeability?

Answer 57

- grain or cement dissolution

Question 58

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

Answer 58

- majority of cements are precipitated from pore waters → type of minerals precipitated will reflect changes in water chemistry with time