Unit 2 - Sedimentology Flashcards
Distribution of porosity and permeability is controlled by ?
Transport and deposition
What is texture?
term to describe size, shape and arrangement of grains, matrix and cement in a sedimentary rock
What does the grain shape depend on ?
- mineralogy of grains and the degree and energy of transportation
What is the fabric?
orientation and packing of grains and their contacts → describes how grains are arranged 3-dimensional in a sediment
What is microporosity ?
pores < 0.5 microns
What is macroporosity ?
pores > 0.5 microns
Reasons for anisotropic permeability?
- 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)
How come that porosity tends to increase with decreasing grain size?
- Finer grains tend to have less regular shapes than coarser grains → often less efficiently packed
- Fine sediments are often better sorted than coarser sediments
Why do clays have an effective porosity <1% ?
Because clay bonded water fills available microporosity
What are the three basic physical processes of transportation and deposition of sediments ?
- Fluid flow
- gravity flow
- deposition from suspension
Describe the transport by fluid flow!
- 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)
What is gravity flow?
- particles are transported down a slope due to gravity acting on the particles → deposition will take place once frictional forces overcome the gravitational forces
What is Deposition from suspension?
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
What are the main types of fluid motion by which grains are transported ?
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”)
The grainsize and volume of sediment, which can be transported by a fluid depends on ?
- velocity and viscosity of fluid
- depth of flow
- turbulence of flow
How do gravity flows work?
sediment accumulates on a slope → if frictional forces < force of gravity → material will move downslope
Name different transport proccesses considered as gravity flow!
- rock falls
- rock slides
- landslides
- slumps
- debris flow
- turbidity currents
→ list is in order of increasing fluid contact
When does deposition take place?
When forces supporting the grains in motion become less than the force of gravity
What is the definition of a bedform?
Features on the sediment surface (distinctive shapes)
What is a sedimentary structure ?
Features within the sediment which are commonly preserved in the rock record
What do bed boundaries represent?
temporary breaks in the deposition or hiatus
contacts between beds can tell us something about ?
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
Bed- and lamina thickness?
Why are antidunes very rarely preserved in rock record?
- because they represent a very high energy turbulent flow → any bedforms developed are rapidly altered
Size of ripples ?
less than 5cm high
Size of megaripples?
5cm-1000cm
The dip of the lee side of a ripple is controlled by ?
The “angle of repose” → maximum slope at which grains of a grain size and sorting can rest without failure of the slope
What kind of forms do the foresets of ripples have?
planar or concave-upwards
How do you call the sedimentary structure deposited by megaripples?
cross-bedding
How do you call the sedimentary structure deposited by ripples?
cross-lamination
What is straight-crested and lunate crested? and what kind of sedimentary structures do they produce?
Why do straight crested ripples or megaripples generally occur at lower current velocities?
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
What happens to the megaripple bedforms, when the current velocity or stream power increases?
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
What is the wave base?
Point where the orbital radius of a water particle reaches 0 → the radius is highest on water surface
What happens if the sediment surface is above the wave base?
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
What can happen when waves impinge on sediment surface?
They can produce wave-ripples (up to 200m water depth)
Describe wave ripples
- 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
What are differences between wave ripples and current ripples?
- wave ripples have lower ripple indices (ripplewavelentgh divided by ripple height)
- crestal bifurcation → meaning: crests usually split along their length
Where is Hummocky cross-stratification (HCS) found?
Only in shallow marine rock record
Decribe Hummocky cross-stratification (HCS)
- 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
Wind - generated bed forms?
- wind- ripples
- dunes
- compound dune - like bedform → draas
measurements of aeolian ripples?
- wavelengths of 0.01-20m
- heights of mm-1m
- grains move mainly by saltation
How are aeolian dunes mainly differentiated?
-by their structure
Aeolian dune and draa morphology?
How come that there are different kinds of flow ?
- Different proportions of sediment to water
- different grainsizes
different slopes
Different sediment gravity flow processes?
what are the most commonly observed debris flows?
mudslides → consists of debris in a muddy matrix
what is a turbidity current?
- mixture of sediment and water, in which the sediment is kept in suspension by the turbulence of the flow
What do currents and waves which transport sediments often do to the underlying sediment?
They often erode it before they deposit their own load
Why is recently deposited sediment often unstable?
Because it is often poorly packed and has a high water content → can easily be deformed by various processes
What is heterogenity of the permeability?
-permeability is highly variable throughout the rock
How are mud clasts formed?
Force of water flow erodes local mud accumulations and carries block of mud downstream
How do you create convolute lamination?
By applying stress to recently deposited beds → may cause shear of the deposited sediment
How do is a pillow structure in beds created?
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
What is diagenesis?
The range of physical and chemical processes and changes, which turn a sediment into a rock (lithification) and continue to alter the rock
Which diagenic processes reduce porosity and permeability?
- compaction
- cementation
- precipitation of authigenic clays
Which diagenic processes increase the porosity and permeability?
- grain or cement dissolution
When clastic sediments are buried, sometimes cementation processes happen, explain why!
- majority of cements are precipitated from pore waters
→ type of minerals precipitated will reflect changes in water chemistry with time
