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Understanding Earth Sciences > Sedimentology > Flashcards

Sedimentology Flashcards

1
Q

Sedimentology =

A

Study of processes that create sediments and dynamic environments where they accumulate

~75% of rocks are sedimentary rocks

Useful for palaeoclimatology, palaeooceanorgaphy, economic geology, record of life, correlation and dating of rocks

Uniformatarian principles work best with sediments

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2
Q

MOUNTAINS: Benefit/threat

A

+ Source of sediment

  • Extreme events deliver too much sediment
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3
Q

DRAINAGE BASINS: Benefit/threat

A

+Source of sediment

  • Source of contaminants
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4
Q

FLOOD PLAINS: Benefit/threat

A

+ Forms berms to floods
+ Provides nutrients and soil

  • Buries vegetation
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5
Q

BEACHES: Benefit/threat

A

+Forms beaches

  • Lack of sediment leads to erosion
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6
Q

OFFSHORE/MARINE: Benefit/threat

A

+Provides nutrients to benthic habitats

  • Excessive sediment buries habitat and fragments ecosystems
  • Creates turbid water = loss of light for aquatic vegetation
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7
Q

DELTAS AND MARSHES: Benefit/threat

A

+Enables marsh accretion, buffers sea level rise

  • Excessive sediment buries habitat
  • Lack of sediment = inundation due to SL rise
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8
Q

RIVERS: Benefit/threat

A

+Creates spawning habitat

  • Channel deposition increases flood risk
  • Changes in sediment size affect spawning habitat
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9
Q

Stratigraphy =

A

Study of rock layers, usually sedimentary

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10
Q

Sediment yields =

A

Total sediment exported from a drainage basin in a given time (Km2/a)

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11
Q

“Liberation of sediment flux”

A

Due to weathering/erosion:
- chemical or physical weathering e.g. freeze thaw, onion skin weathering, dissolution etc

CAUSED BY:

  • uplift
  • climate
  • lithology (certain rocks erode more than others)
  • vegetation
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12
Q

CLASTIC ROCKS (define)

A

Clasts = grains/pebbles/boulders

Names:
Conglomerates
Gravels
Sandstones
Siltstones
Mudstones
Clays
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13
Q

Mass wasting =

A

Downslope movement of soil/rock under gravity

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14
Q

Creep =

A

Slow downslope movement of soil/rock under gravity, more confined than mass wasting

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15
Q

Landslides =

A

Rapid down and outward soil/rock movement

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16
Q

Types of mass flows (fast to slow)

A

Rock/debris fall

Snow/debris avalanche

Rock/debris slide

Mudflow

Slump

Creep/rock glacier

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17
Q

Thermohaline circulation

A

Bottom water currents

  • water heated at equator
  • travels to poles
  • cools, sinks,
  • travels back to equator

= governs many sedimentary processes
N.B. Global warming could affect this
For example; During the late Cretaceous, chalk (a pelagic sediment formed in shallow warm seas) was deposited in South England
This formed when the oceans were stagnating and the ocean temps were therefore warmer

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18
Q

Sedimentary basins =

A

Bowl-shaped depressions/topographic lows of the Earth’s crust where sediment can accumulate into successions hundreds to thousands of metres thick

Their sedimentary fill provides unique evidence for the environmental conditions during the basin’s lifetime

Main control producing sedimentary basins = TECTONICS

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19
Q

Subsidence =

A

Local/regional scale change in Earth’s crust in form of downward shift relative to e.g. SL

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20
Q

The 5 basin settings

A

RIFT BASINS

INTRACRATONIC BASINS

PASSIVE MARGIN BASINS

FORELAND BASINS

STRIKE-SLIP BASINS

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21
Q

Rift basins

A

Extensional
Crust thinned as stretched
As rift grows, blocks of crust on the border fault slips = low areas and narrow mountain ridges
Can be continental/marine

e.g. E African rift system has many rift basins along its length

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22
Q

Intracratonic basins

A

In the interiors of continents
Round/oval shapes
Long geological histories of slow subsidence
DUE TO THERMAL SAG
- over areas of previous rifting
- hot, stretched crust cools, contracts and sinks

Usually filled with continental sediments but flooding from adjacent oceans can cause large epicontinental seas

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23
Q

Epicontinental sea =

A

Shallow sea overlying a continent

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24
Q

Passive margin basins

A

Form along margins of continents, NOT tectonic plate boundaries
Usually underlain by former rift with oceanic crust
- long after rifting ceases, thermal relaxation/subsidence continues

Carbonate and clastic sediments 10-20km THICK

E.G. Gulf of Mexico margin along southern US

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25
Q

Foreland basins

A

Adjacent and parallel to mountain belts
Regions of compressional tectonics

  • downward flexing of LITHOSPHERE in response to weight of adjacent mountain belt
  • sediment eroded from mountain belt
  • gradually decrease in thickness away

Sediments transition from deep marine to continental environments
Thicknesses greater than 10km

e.g. Persian Gulf, produced by Zagros Mountains of Iran

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26
Q

Strike slip basins

A

“A horizontal sense of movement along the fault plane”
Fault planes not straight = areas of localised compression/extension
“Pull-apart” basin

Can be filled with continental/marine sediments

E.g. San Andreas Fault, California
North Anatolian Fault, Turkey

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27
Q

Sediment to sedimentary rock, process

A 
Lithification
~diagenesis~
Burial
Compaction
Cementation (new minerals ppt in between)

MUDROCKS

  • lose about 60-70% water = shale
  • can’t lose 100% because contains hydrous minerals which would have to be broken down/changed form
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28
Q

CLASTIC SEDIMENTS =

A

Cemented together fragments and grains derived from pre-existing rocks

  1. Conglomerates/breccia (>2mm)
  2. Sandstone (0.063mm-2mm)
    - quartz arenite (95% quartz)
    - arkose (25% feldspar)
    - litharenite (rock fragment rich)
    - greywacke (more than 15% matrix; n.b. turbidity currents)
  3. Mudstone (<0.063mm)
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29
Q

BIOGENIC SEDIMENTS =

A

Derived from the skeletal remains and soft organic matter of pre-existing organisms

e.g. Great Barrier Reef, Australia = LIMESTONE

Allochems
- grains (skeletal/non-skeletal e.g. ooids)

Orthochems

  • matrix (micrite)
  • cement (sparite)
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30
Q

Ooid formation

A

Small fragment of sediment e.g. piece of shell
Strong currents wash fragment around sea bed = accumulate layers of chemically precipitated calcite from supersaturated seawater

Carbonate sediment composed of ooids = oolite

E.G. Mid Jurassic oolite; decorative stone in World Heritage City of Bath, England

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31
Q

Dolomite

A

Most formed from replacement of limestone

  • chemical reaction of calcite with Mg-bearing groundwater
  • occurs soon after burial

e.g. Dolomite Mountains, N Italy
?? primary precipitate or secondary replacement product??

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32
Q

ORGANIC SEDIMENTS =

A

Organic carbon compounded from relics of plant or animal material that has not completely decayed e.g. leaves/roots/planktonic organisms/algae/spores/pollen

E.g. coal, oil shales, shale gas

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33
Q

CHEMOGENIC SEDIMENTS =

A

Formed from direct precipitation of minerals from a saturated solution e.g. evaporites

  • gypsum
  • halite
  • anhydrite

Crystalline texture
Crystal size varies due to concentration of salts in sea/lagoon/lake depositional setting

Prone to diagenetic modification following burial; dissolution/recrystallisation/deformation

E.g. Zechstein Basin in NW Europe

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34
Q

VOLCANICLASTIC SEDIMENTS =

A

Composed of grains/fragments derived from volcanic activity

Ash (<2mm)
Lapilli (2-64mm)
Bombs/blocks (>64mm, round/angular)

Pyroclastic fall deposits
Pyroclastic flow deposits (ignimbrite)
Epiclastic sediments (erosion of volcanic rocks)

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35
Q

Pyroclastic flow =

A

Fast moving currents of hot gas and rock fragments that move rapidly down volcano slopes in response to gravity

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36
Q

Lahar =

A

Type of volcanic mudflow/debris flow composed of a slurry of pyroclastic material, rocky debris and water

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37
Q

Diagenesis =

A

All physical/chemical/biological processes that occur during burial, prior to metamorphism

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38
Q

What do flow hydraulics depend on?

A 
Grain size/density
Grain drag
Grain roughness
Flow velocity
Flow viscosity
Slope
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39
Q

Pebble clustering

A

Peak discharge when the very large grain is deposited in a 1/100 or 1000 year event
Larger grains then clustered on upstream side of large grain
Palaeocurrent indicators
Presence indicative of perennial gravel-bed rivers

40
Q

Beds vs laminae

A 
Beds = >1cm
Laminae = <1cm
41
Q

What to consider within beds/laminae

A

Attitude (i.e. strike/dip)

Thickness of units

Lateral variations

42
Q

Sedimentary structures =

A

Marks/traces/sediment disturbances preserved in sedimentary strata

Not many are preserved - often cancelled out by the same sedimentary process e.g. rain drops = rare, flute casts = common

  • must know preservation potential and interpretation limitations
  • can only preserve once sediment is buried
43
Q

Types of sedimentary structures

A

BP DICE

Biogenic

Post depositional

Depositional

Internal

Chemogenic

External

44
Q

Geopetal structures

A

= “palaeospirit level”

  • original CaCO3 shell of living animal
  • mud infilled empty shell after animal died
  • remaining space filled with crystals during burying

Mud side = lower side

45
Q

DEPOSITIONAL STRUCTURES

A

Bedforms

= develops at the interface of fluid and a moveable sediment-dominated bed as a result of the bed being moved by fluid flow

Ripples/dunes

Can be used to infer flow depth and velocity (and Froude number)

46
Q

Current ripples vs wave ripples

A

Current = unidirectional flow

  • asymmetric cross section
  • down-current (lee) slope steeper than up-current (stoss) slope

Wave = bidirectional flow

  • truncating top
  • tapering base
47
Q

Current ripple development

A

Current ripples 2-5cm high
Up to 40cm wavelength
Migrate along bed in current direction
Sediment eroded from stops side, carried to crest and avalanches down lee side

Series of laminae build up // to lee slope
Successive laminae = cross stratification

Flow current increases = more sinuous ripple crests, eventually break up into linguoid ripples

Increasing current further = dunes

  • up to 1m high
  • 0.5-10m wavelength

Increased even further = dunes washed out
Sediment sweeps over flat surface = planar stratification

Higher speeds = flat bed&raquo_space;> low relief, undulating mounds (ANTIDUNES) - rarely preserved as reworked

48
Q

Types of cross-stratification

A

PLANAR
- 2D bedforms (straight crests)

TROUGH
- 3D bedforms (curved/sinuous crests)

49
Q

Flaser bedding =

A

Alternating periods of moving and slack water e.g. tide-dominated marine settings

= alternating rippled sand and mud layers

50
Q

Wave ripple development

A

Produced by oscillatory motion of waves in shallow marine/intertidal settings

Storms/large storm waves = large-scale undulatory bedroom; HUMMOCKY CROSS STRATIFICATION

  • circular to elliptical hummocks and swales
  • low height (10s of cams)
  • long wavelengths (1-5cm)
51
Q

Wind ripples vs current ripples

A

Better sorting
Rounded grains
Straight crested
Asymmetric

52
Q

EROSIONAL STRUCTURES

A

Bed surface:

  • rain drops
  • prods
  • downcutting

Bed base:

  • sole marks
  • flute casts
  • grooves
53
Q

Flute casts =

A

Heel shape with bulbous upstream end that flares downstream and merges with bedding

Localised erosion of sand-laden currents passing over cohesive muds

Common in submarine environments (sediment laden flows common)

54
Q

BIOGENIC STRUCTURES

A

= action of plants/animals

Bioturbation
- irregular disruption of sediment

Discrete organised markings

  • trace fossils/ichnofacies
  • MORE RELIABLE INDICATOR OF ANCIENT SEDIMENTARY ENVIRONMENTS THAN BODY FOSSILS AS PRESERVED IN SITU

Direct biogenic growth structures
- stromalolites

55
Q

INTERNAL STRUCTURES

A

Graded bedding = grain size distribution in a regular fashion

Normal/inverse

Inverse is less common and occurs in debris flows/aeolian ripples

56
Q

POST DEPOSITIONAL STRUCTURES

A

Seismically active/sudden addition of sediment = instabilities and internal deformation…

  1. Convoluted bedding = tectonic forces cause unstable wet sediment to move; beds above and below not folded; = “seismite bed”
  2. Flame structure = wet mud sediment and rapid sedimentation on top = compress and break into overlying sediment for fluid to escape; always point UP

Also desiccation = evaporation of water/drying up = shrinkage and cracks develop in polygon shapes

57
Q

CHEMOGENIC STRUCTURES

A

A type of post depositional structure

Concretions = secondary diagenetic features found as globular mass which stand out from the outcrop and are poorly cemented, formed by the precipitation of mineral cement between particle spaces

Septarian nodules = concretions containing cracks
- mystery

58
Q

Source area =

A

Locality where rocks have been eroded to provide the sediment

59
Q

How can sediment composition indicate source type?

A

Conglomerate - pebbles of certain substances indicate that these were present in source area

Arkose - likely from granitic-rich area

Well rounded quartz - erosion of pre-existing sandstones
- quartz resistant to erosion to more rounded = more cycles of erosion/transportation/deposition

60
Q

Types of depositional environments

A

CONTINENTAL

  • glacial
  • alluvial
  • aeolian
  • lacustrine
  • fluvial

MARGINAL MARINE
- deltas

MARINE

  • continental shelves
  • reefs
  • deep sea
61
Q

Glacial environments

A

= ice transports and deposits sediment, shapes the landscape as rocks and sediment carried by the glacier smooths and polishes the underlying rock OR large boulders cause STRIATIONS
Ice melts and deposits TILL = poorly sorted and undifferentiated material

Presently 10% of land surface is covered with glacial ice
Earth has been glaciated to different degrees at different times

62
Q

Alluvial environments

A

Alluvial fan = broad, fan/cone-haped sedimentary deposit when streams emerge from a point source of higher topographic relief

  • gradient decreases = drops coarse
  • reduces channel capacity = changes direction

2-15km radius

Poorly sorted with coarsest sediments closest to mountain front (proximal)
Finest further (distal)
63
Q

Ancient alluvial debris flow - characteristics

A

Poorly sorted
Angular
Often matrix supported

Conglomerates
Breccia
Sandstones
(Less) mudstones

64
Q

Desert =

A

Area of intense aridity less than 250mm rain per year and vegetation less than 15% of surface

65
Q

Aeolian environment

A

Desert
Well rounded grains and frosted surface produced by frequent grain collisions during wind movement
Red due to iron oxide coatings

Large scale cross bedding
Absence of finger-grained sediment
Highly porous = :) water/hydrocarbon reservoirs

66
Q

Ventifact =

A

Pebble whose surface has been polished/etched/grooved/faceted by wind-driven sand in arid environments

Can be used to determine paleo-wind directions

67
Q

Lacustrine environment

A

Can be fresh/saline, shallow/deep

Coarse sediment near margins
Fine towards lake centre - finely laminated mudstones/varves

Ephemeral lakes = semi arid climate; evaporitic sediments; mud cracks; fossil fish

68
Q

Varve =

A

Shows annual cycle of deposition, layer of sandstone/mudstone

69
Q

Fluvial environment

A

Controls sediment supply to all others!
Range of grain sizes; conglomerate/sandstone/mudstone

Can have meandering/braided/straight channels

70
Q

Braided river characteristics

A 
Multi-thread channels
High energy
Steep valley gradients <0.5%
Large/variable discharges
Non cohesive banks
71
Q

Meandering stream characteristics

A 
Single channel
High sinuosity
Migrate
Point bar deposition
- FINING UP POINT BARS
72
Q

Fining up point bars/point bar succession

A

In a meander a helical overturn flow develops
Causes coarse material at bottom and finer at top
= finding up sequence in lateral accretion depositions

73
Q

Sea level and continental shelf relationship

A

Low sea level = wide continental shelf

High sea level = diminishing continental shelf

74
Q

Delta =

A

Shoreline proturbance formed at the point where a river flows into an ocean basin or large standing body of water

Transports sediment laden water from a channel to an unconfined environment (unique)

75
Q

Deltaic environment

A

Delta top/plain = where river meets ocean

  • sub environments e.g. distributary channels, floodplains, swamps, lakes
  • sandstones/mudstones/coals with abundant plant material preserved

Delta front = where sediment carried by distributary channels is deposited

  • sands with cross bedding/ripples/bioturbation
  • region of rapid deposition = seaward migration

Prodelta = farther offshore
- organic-rich, laminated and bioturbated mudstones

Sands of mouth bar/delta front migrate over finer sediments of pro delta = thick (20-40m) coarsening upwards units

76
Q

Continental shelf environment

A

Continental shelf = staging post between erosion of the continents and final deposition in deep ocean
Water depth varies 5-500m
Complex mix of tides/waves/currents

Widespread sands/muds/CARBONATES e.g. oolites in agitated waters

77
Q

Carbonate shelves - requirements

A

Warm temperature >15 degrees C

Low pressure

Agitation

Absence of silt/clay

78
Q

Autochtonous =

A

Forms where deposited

79
Q

Allochthonous =

A

Material exported to adjacent environemnts

80
Q

Ooid =

A

Spherical grain with a nucleus (a mineral grain or biogenic fragement) and mineral cortex accreted around it <2mm in diameter

81
Q

Reef environment

A

Reef = Intergrown organismal skeletons and sediment

  1. Fringing reef
  2. Smaller patch reefs in open lagoons
  3. Atoll reefs in open ocean on submerged volcanic islands
  4. Barrier reef

FOREREEF

FRAMEWORK AND CREST

BACKREEF

Components:
Coral (10%)
Carbonate mud (micrite)
Skeletal elements
Cement
82
Q

FOREREEF =

A

Steep/talus slope

Agitated waters where waves are breaking

83
Q

REEF FRAMEWORK AND CREST =

A

Main site of coral growth
Highest range of organisms
Highest wave energy

84
Q

BACKREEF =

A

Sheltered from wave energy by reef front
Low diversity
Likely to be hyper saline

85
Q

Deep sea environments

A

Greater than 500m

TURBIDITY CURRENTS

Further offshore dominated by pelagic sediments:
Lithogenous
Biogenous
Hydrogenous
Cosmogenous
86
Q

Turbidity currents =

A

Main agent for transporting shallow water sediment to deep waters

High density, sediment laden fluids

87
Q

Submarine canyon =

A

Turbidity currents trigger flow and erodes surfaces forming a canyon with a fan at the bottom

88
Q

Bouma sequence

A

SANDS/LARGER GRAINS
- slow energy drop = graded

PARALLEL LAMINATED SANDS

  • upper flow regime
  • traction = flute casts
  • CONVOLUTE LAMINATIONS AND FLAME STRUCTURES/SEISMITE BEDS

CROSS LAMINATED SANDS

  • lower flow regime
  • enough energy for saltation

PARALLEL LAMINATED SILTS
- slight current

MUD OFTEN BIOTURBATED
- suspension settling with no current

89
Q

Lithogenous pelagic sediments

A

Terrigenous muds
RED CLAY

GREY MUD

From rivers and deserts

90
Q

Biogenous pelagic sediments

A

Calcareous oozes

  • coccolithophore
  • foraminifers
  • “periplatform ooze”

Siliceous oozes
- radiolarians
- diatoms
N.B. Unlike carbonate, surface waters not supersaturated wrt silica so dissolution occurs more rapidly
Fecal pellets bring down faster than dissolution so they survive

91
Q

Cosmogenous pelagic sediments

A

Cosmic dust found in red clay
Common in South Pacific

Iron nickel and magnetite
50-200microns diameter
~300x10^3 tonnes fall on earth’s surface each year

92
Q

Hydrogenous pelagic sediments

A

Formed directly from seawater in the pelagic zone (an oxygen environment)
Ion exchange and precipitation

E.g. ferromanganese nodules

93
Q

Determining the environment e.g. ripples/cross bedded

A

Organism lived in moving water

Sediment moved by currents

94
Q

Determining the environment e.g. broken shelves

A

High energy/storm conditions with waves pounding on beach

95
Q

Determining the environment e.g. absence of corals

A

Murky water with high % suspended particles and low levels of light

96
Q

Determining the environment e.g. organic material well preserved

A

Low levels of oxygen

Organism unlikely to have lived in these conditions but well preserved here

97
Q

Types of sediments

A

Clastic

Chemogenic

Biogenic

Organic

Volcaniclastic

Understanding Earth Sciences (8 decks)

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