Sediments Flashcards
Sediment
Any geological materials generated at the Earth’s surface
Clastic sediment
Sediment that has been separated from parent rock by errosion.
Split into terrigenous, volcaniclastic and carbonates.
Siliclastic sediment
Clastic sediment that is silicate based - pretty much all of it
Terrigenous sediment
Clastic sediment that originates from land.
e.g. constituents of mudrocks, sandstones, conglomerates
common minerals: quartz, feldspar
Volcaniclastic sediment
ashes and tuffs - ejected from volcanic erruptions
common minerals: micas, quartz, feldspar
Organic sediments
sediment from dead organisms that isn’t rich in calcium carbonate.
e.g. hydrocarbons and peat which turn to oil/gas and coal
Chemogenic sediment
Minerals precipitated inorganically
e.g. ironstones or evaporites
Cementation
Sticking together of unconsolidated sediment.
Water precipitates out different minerals, e.g. calcite, between grains.
Can begin immediately or may require burial
Compaction
Closer packing of grains due to weight of sediment above
Stages in Diagenesis
Mineralogical changes to sediment after burial
Cementation and/or compaction
Recrystallisation
Dissolution and replacement
Uniformitarianism
Assumption that observations of the present can inform us about the past i.e. environments that make characteristic structures were the same in the past
Sedimentary log
Graphical representation of vertical sections of rock, showing how sedimentary signatures change through stratigraphic section.
Outcrops
Show high resolution detail of sedimentary record
Boreholes
Show large scale, regional characteristics of sedimentary rock units, but lack high resolution.
Superposition
Generally, a succession of strata represent a sequence of depositional events
Walther’s law
Sedimentary environments are diachronous.
If 2 sedimentary units are adjacent and there’s no unconformity between them, the vertical transition between them shows a lateral transition in adjacent environments.
Diachronous
adjacent components of a sedimentary environment can be active at the same time
Evaporites
Form when a natural body of water evaporates and leaves behind the salts that were dissolved in it.
Carbonates, chlorides, sulphates
Erosional processes
Pick up grains from previously deposited sediment or from parent rock
Transport
Movement of grains by fluid or gravity
Deposition
Grains deposited in their temporary or final resting place when the fluid carrying them can no longer keep moving the grains
Physical weathering
Weakening and breaking up parent rock
e.g. freeze thaw, temperature changes, salt growth or biological intrusion like growing roots
Chemical weathering
Weakening rock by chemical changes
by solution in groundwater or brine, hydrolysis or oxidation
e.g. feldspar to kaolinite and quartz by hydrolysis; oxidation of pyroxene to magnetite and quartz; dissolution of calcite
Regolith
Unconsolidated sediment that hasn’t been transported
Reynold’s number
Determines if flow is laminar or turbulent which affects grain entrainment, transport and deposition.
Affected by viscosity, velocity and flow diameter
Labile Minerals
Weak, cleaved minerals that are readily converted to clay minerals.
In order of increasing resistance to weathering: Olivine, pyroxene, amphibole, biotitie, muscovite
Quartz grains
Polycrystalline: Different grains sutured together which go into extinction independently.
Unstrained quartz: Single crystal, uniform extinction.
Strained quartz: Single crystal, undulating extinction.
Most common in clastic rocks
Feldspar
Labile minerals, so weather to clay minerals easily
Lithic fragments
polycrystalline fragments of existing rock
Clay minerals
Sheet silicates that form during chemical weathering.
Cohesive and flocculate together as larger aggregates
Kaolinite
e.g. hydrolysis of feldspar
Often forms in warm, humid environments with acidic water
Montmorillonite
Clay mineral
Swelling clay: expands with water
Forms in moderate climates with neutral/alkaline pH or alkaline, arid conditions
Illite
Clay mineral
e.g. from biotite weathering
Forms in soils in temperate climates with acidic groundwater or in arid conditions
Bioclasts
Fragments of biological debris in sedimentary rocks
Maturity of sandstones
Supermature: Contain mostly quartz with only a few lithic fragments, no labile minerals. Good sorting and rounding. e.g. shallow marine, beach and aeolian environments.
Mature: Contain mostly quartz with some lithic fragments and labile minerals e.g. fluvial environments
Immature: Contain labile minerals and lithics, 40-60% quartz. Poor sorting and rounding. e.g. glacial environments
Properties of grains
Grain size: larger grains require more viscous and faster fluids to move them
Grain shape:
Roundness: a measure of surface roughness, i.e. how much attrition occurred
Sphericity: how close grains are to perfect spheres since spheres are used in most models
Porosity
Dependent on grain size, shape and sorting
Where cement can be precipitated and how fluid can move through the sediment post deposition
Stoke’s Law
Determines settling velocity of a particle
Depends on grain size, densities of the grains and fluid and the fluid’s viscosity
Only accurate for low concentrations of spherical grains in laminar flow
Entrainment
A fluid begins to exert enough force on a grain to begin to move it
Bedload
Sediment transported by rolling or saltation
Suspended load
Sediment transported while suspended in a fluid
Rolling
Moves only due to drag force of fluid parallel to bed
Saltation
Lift generated by Bernoulli effect: Fluid mass constricted as it passes over an grain; velocity increases to conserve constant flow rate; decrease in static pressure above grain, which generates lift. If lift is greater than weight, grain is briefly lifted into the flow. If fluid velocity <= settling velocity, the grain falls back to the bed.
Suspended load transport
Lift generated by Bernoulli effect: Fluid mass constricted as it passes over an grain; velocity increases to conserve constant flow rate; decrease in static pressure above grain, which generates lift. If lift is greater than weight, grain is lifted into the flow. If viscosity and fluid velocity are high enough, the bouyancy >= weight and the grain remains as suspended load
Hjulstrom Diagram
http://www.coolgeography.co.uk/A-level/AQA/Year%2012/Rivers_Floods/Long%20profile/Hjulstrom.htm
Key points:
Logarithmic scale for both axes, in factors of 10
Flow velocity in cm/s from 0.1 to 1000
Grain size in mm from 0.001 to 1000
Grains of mud size are never deposited, but when they clump together due to cohesive properties, they eventually get big enough to be deposited.
Curve for erosion dips down from c. 300 to minimum of 11 (grain size 0.1mm), then grows to c. 1000
Curve for deposition starts at grain size of 0.01mm and grows to
c. 200
Graded beds
Occur due to changes in fluid velocity i.e. different grain sizes are entrained and the critical settling velocity changes.
Normal grading: Fining upwards in a single bed due to decelerating fluid flow
Reverse grading: Coarsening upwards in a single bed due to accelerating fluid flow
Change in grain size over multiple beds
Fining upwards in a series of beds: Decrease in fluid velocity of depositional environments
Coarsening upwards in a series of beds: Increase in fluid velocity of depositional environments
Wackestone
Sandstone with more than 15% matrix
Lithic wacke: >5% lithic fragments, more lithic fragments than feldspar
Feldspathic wacke: >5% feldspar, more feldspar than lithic fragments
Quartz wacke: >95% quartz
Mudstone
More than 75% matrix
Arenite
Sandstone with up to 15% matrix
Quartz arenite: >95% quartz
Lithic arenite: >25% lithic fragments, more lithic fragments than feldspar
Feldspathic arenite / arkose / arkosic arenite: >25% feldspar, more feldspar than lithic fragments
Subarkose: Between 25% and 5% feldspar, more feldspar than lithic fragments
Sublitharenite: Between 25% and 5% lithic fragments, more lithic fragments than feldspar.
Ripple Formation
Flow irregularities in the viscous sublayer (turbulent sweeps) make small grain clusters which cause the streamlines at the bed’s base to detach from the bed’s surface at the top of the clusters.
Where the streamline detaches and reattaches from the bed are the flow separation point and the flow attachment point.
On the stoss side of the ripple, flow contracts and on the lee side, flow expands. On the stoss side, grains roll or saltate up the ripple. On the lee side, there is an increase in static pressure due to the Bernoulli principle and sediment is deposited.
Ripples migrate downstream and previous lee sides are preserved as cross-lamination.
Ripples up to 4cm high. Wavelengths up to 0.5m
Formation is independent of flow depth.
Viscous sublayer
Typically less than 1mm thick
Layer of fluid at base of the bed that is so affected by drag that the fluid velocity is low enough to maintain effectively laminar flow.
Hydraulically smooth:
Grain diameter < thickness
Ripples can form
Hydraulically rough:
Grain diameter < thickness
Ripples can’t form
Stoss side
Upstream side
Lee side
Downstream side
Straight vs. linguoid or sinuous ripples
With time and increasing fluid velocity or depth, straight ripples become sinuous or linguoid ripples.
Straight ripples have planar sross-lamination
Linguoid and sinuous ripples have trough cross-lamination
Climbing ripples
If the sedimentation rate is high enough, there is no net removal of grains from the stoss side of ripples. A migrating ripple will climb up the stoss side of the ripple in front
Starved ripples
Fixed amount of sediment in a system
The rate of removal of grains from the stoss side = the rate of addition to the crests and avalanches down the lee side
Dune formation
Also form by detachment of streamlines from the bed. But, related to turbulence throughout entire flow.
Dependent on flow depth since this affects the scale of turbulent eddies.
Low flow rates create straight creasted dunes with planar cross bedding.
High flow rates create sinuous creates dunes with trough cross bedding because at the flow reattachment point there are market scour pits.
Lower plane bed
Form when grain diameter > 0.7mm and viscous sublayer becomes hydraulically rough i.e. ripples can’t form
Why dunes aren’t just large ripples
Very few bedforms are c. 10cm high and c. 1m in wavelength. So, ripples don’t just grow until they become dunes
Counter-flow ripples
If flow rate is high enough, a large roller vortex can be created on the lee side of dunes from flow separation. These vortices will cause ripples to migrate up the slope of dunes, antiparallel to the net current.
Upper plane bed
Fluid velocity is high enough to entrain all grains that would have made up other bedforms. Leads to high concentrations of suspended load which dampens turbulence. No turbulence = no ripples or dunes.
Has planar lamination in cross section.
Parting lineation or primary current lineation: Streaks of different grain sizes parallel to flow. Eddies sweep into viscous sub-layer but can’t form ripples.
Froude number
Supercritical flow when FN > 1
Depends on flow velocity and flow depth
Supercritical Flow
Flow velocity > wave velocity
Temporary standing wave forms on the surface that eventually steepen and break upstream
Antidunes
Created by supercritical flow
In phase with the water surface
Have cross bedding that dips up stream
Lower flow regime
Ripples, dunes, lower plane beds are stable
Supper flow regime
Antidunes and upper plane beds are stable
Bedform stability diagrams
Show what bedforms are stable at a given temperature and flow depth.
Flow velocity against grain size
Wave ripples
Waves are orbital motion of water molecules.
If water depth > wave base, this exponentially dies out with increasing depth due to drag.
If water depth < wave base, orbital motion becomes more elliptical and eventually becomes horizontal oscillation due to friction with bed.
Symmetrical ripples generated: Max velocity of oscillation at centre, minimum velocity at extremes. So, grains rolled from centre to edges.
Wave base
Greatest depth at which wave effects are felt = half the surface wavelength
Sedimentary facies
A body of rock that has a certain combination of lithological, physical and biological structures
Depositional sedimentary environments
Net accumulation of sediment
Preserved in geological record
Erosional sedimentary environments
Net errosion
Not preserved in geological record
Accomodation space
Where sediment can accumulate
Isolated from erosional processes
Affected by changes in balance of erosion and deposition; tectonics; sea level change
e.g. deep ocean or half-graben or trenches
Sedimentary basin
Topographic lows generated by subsidence
Extensional basin
Form during 1st stage of rifting
Intracratonic basin
Large areas of subsidence within a continental block.
Large areas but low rates and amount of subsidence.
e.g. Murzuk and Kufra basins in southern Libya.
Those that formed from extinct rifting are due to thermal subsidence. Rifting thins the crust and uplifts hot mantle. So there is a greater geothermal gradient. When rifting stops, the area cools to the stable geotherm and the crust above subsides.
Horst
Sections of an extensional basin that faulted upwards
Graben
Sections of an extensional basin that faulted downwards
Passive margin basins
During rifting, continental crust is injected with basaltic magmas and thins. After the formation of an ocean, the transitional crust forms the continental shelf and slope - where lots of sediment settles.
Pull-apart basins
Created when diverging strike-slip motion of an offset fault.
e.g. Dead Sea
Terrestrial rift valley
Basin formed by extension of continental crust that’s on land
e.g. Death valley
Maritime rift
Accomodation space formed by extension in oceans and seas.
e.g. Gulf of Corinth in Greece
Peripheral foreland basin
Due to loading of a volcanic arc during oceanic subduction. Basin forms on the opposite side of the volcanic arc from the trench.
