Geological Oceanography Section II Lecture 2

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Geological Oceanography Section II Lecture 2

• Depositional Processes Pathways
• 12 February 2008

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Lecture Outline

• Outline of transport processes
• Brief review of ice and wind
• Focus on water
• Brief introduction to fluid flow
• Currents
• Tides
• Waves
• Sediment gravity flows
• What happens to the solution load?
• Himalayas source to sink

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Fundamental Characteristics of Siliciclastic
Sedimentary Rocks

• Grain-size distribution
• Grain shape
• Surface texture
• Roundness
• Sphericity/form
• Stratification
• Bedding and lamination
• Cross stratification
• Irregular stratification
• Bedding plane structures
• Scour marks
• Bedforms

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Grain characteristics provide information on
weathering and transport processesSurface
textureRoundness/sphericity/form
Size each 1 mm maximum diameter
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Bedding plane structures and bedforms can small
or large scale
Aerial photo of a barrier island and lagoon
2 cm
Flow marks on muddy sand
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Relative importance of transport processes

• Aeolian, volcanic (lt1)
• Groundwater (1-2)
• Glacial/ice rafting (7)
• Rivers (90)

Aerial photo of a delta and barrier islands
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Sediment transport

• Glacial/ice rafting
• All sizes glacial till, glacial erratics
• Mass wasting talus, debris flows
• Windblown relatively fine dust, loess
• Water
• Bed/traction load sands and larger
• Suspended load sands and muds
• Density flows gravity driven, rocks and
  sediments lubricated by interstitial fluid (air
  or water)
• Slides, slumps, turbidity currents

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Glacial erosion, transport, deposition
Flying somewhere over Alaska
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Location of mass wasting event in 1959 that
buried campgrounds and created Quake Lake just
outside Yellowstone National Park
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Sediment transport

• Glacial/ice rafting
• All sizes glacial till, glacial erratics
• Mass wasting talus, debris flows
• Windblown relatively fine loess, volcanic ash,
  dust
• Water
• Bed/traction load sands and larger
• Suspended load sands and muds
• Density flows gravity driven, rocks and
  sediments lubricated by interstitial fluid (air
  or water)
• Slides, slumps, turbidity currents

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Loess in the Wind, Matanuska Valley, Alaska
Loess is dust windblown by winds blowing off
glaciers the winds pick up sediments from
glacial outwash fans
http//tvl1.geo.uc.edu/ice/image/propro/32.html
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Nebraska Sand Hills
www.uwsp.edu/.../dutch/VTrips/SandHills.HTM
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Nebraska Sand Hills Loess deposits from
continental glaciation
www.uwsp.edu/.../dutch/VTrips/SandHills.HTM
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Mt. St. Helens - USGS
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Volcanic glass distribution on sea floordownwind
from volcanic sources
After Kennett 1981
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Saharan dust over the North Atlantic
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Illite in the ocean basins is primarily aeolian
near shelves, source may be fluvial
gt50
gt50
lt20
lt20
lt20
gt50
gt50
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Aeolian Sand Dunes
source is typically fluvial or beach deposits
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Sediment transport

• Glacial/ice rafting
• All sizes glacial till, glacial erratics
• Rock falls - talus
• Windblown relatively fine dust, loess
• Water
• Bed/traction load sands and larger
• Suspended load sands and muds
• Density flows gravity driven, rocks and
  sediments lubricated by interstitial fluid (air
  or water)
• Slides, slumps, turbidity currents

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From Press and Slever 1986
Downhill path of sediment transport and deposition
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WHAT MOVES AND HOW?
Bed or suspension load
Suspension load
Solution load
http//csmres.jmu.edu/geollab/fichter/SedRx/sedcla
ss.html
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• Forces acting on a grain
• Lift (buoyancy)
• Drag
• Gravity
• Resultant is fluid force
• What determines bed load vs. suspension load?

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direction of flow
Velocity profile for a steady current flow over a
bed
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direction of flow

• Laminar flow
• Shear stress
• ?0 µ x du/dz
• µ molecular viscosity
• u velocity
• z height above the bed

• Turbulent flow
• Shear stress
• ?0 (µ ?) x dû/dz
• ? eddy viscosity
• û mean horizontal velocity

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How grain size and current velocity influence the
boundary layer

• Viscous sublayer
• Thickness is a function of current velocity
• If grain diameter lt 1/3rd of the thickness of
  viscous sublayer, it remains intact and
  protects grains from suspension
• If grain diameter is gt1/3rd of the thickness of
  the sublayer, it begins to break down
• If grain diameter gt7 times viscous sublayer
  thickness, turbulent eddies are able to penetrate
  between the grains

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How grain size and current velocity influence the
boundary layer
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Shear velocity (U) (cm/sec)
Diameter (mm)
Criteria for initial movement and suspension of
quartz grains in water at 20o C
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Hjulstroms diagram showing critical velocity for
movement of quartz grains on a plane bed at water
depth of 1 m modified by Sundborg (1956)
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Transport as bed load
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Settling velocity of quartz grains in water
Coarse grains
Settling velocity (W)
Stokes Law W D2 applies to fine sand and silt
Diameter (D) in millimeters
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Relationship between bed forms, mean current
speed and grain size
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Direction of flow and direction of bed form
migration
Ripples Megaripples (aka dunes) Antidunes
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Ripple marks
Direction of migration
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Megaripples
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BEDFORMS IN OUTCROP
Direction of flow
Megaripples
Plane (flat) bedding
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Types of motion that move sediments

• Unidirectional
• Rivers and streams
• Oceanic currents
• Longshore
• Wind driven
• Thermohaline

• Unidirectional, reversing periodically
• Tidal currents
• Bidirectional
• Waves

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Exposed intertidal flat
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Changes in current velocity during a complete
tidal cycle.
Changes in rate of bedload transport during same
tidal cycle. Shaded areas are proportional to
total sediment transported.
Incoming
Outgoing
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Exposed intertidal flat
More sand
Channel
Rippled fine, muddy sands
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WAVES AND WAVE MOTION
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Relationship between wavelength and water motion
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Currents associated with wave motion
Wave transport
Rip currents
Longshore current
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Sediment transport to the deep sea
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Classification of subaqueous sediment gravity
flows
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Cartoon of a turbidity current in a tank
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Cable breaks following 1929 Grand Banks
earthquake provided first evidence for high
velocity of turbidity currents timing indicated
velocities 40-55 km/hr
Epicenter
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Bouma Sequence deposition from tubidity current
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Turbidites

• Channel deposits of sand and pebbles
• May be grain-flow deposits rather than turbidites
• Proximal turbidites
• Relatively close to source
• Massive, poorly developed grading
• Classic turbidites
• Characteristic succession known as Bouma sequence
• Distal turbidites
• Thin, fine grained layers with well developed
  cross lamination
• Interbedded with pelagic clays or carbonate oozes

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Summary of routes, processes, and resultant
deposits
Sands gt muds
Sands muds (biogenics some places
Biogenics lt/ muds
Contour currents
Waves, tides, wind-driven currents
Rivers and streams
Density flows
Turbidites
Biogenics gt muds
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What happens to the solution load?
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Non-Clastic Sediments

• Form on seafloor or in water column
• Locally transported
• Primary marine authigenic sediments
• Carbonates
• Evaporites
• Phosphates
• Manganese crusts and nodules

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Biogenic Sediments
Will be discussed later in the semester
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Evaporites

• High aridity needed
• Key minerals
• Halite
• Anhydrite
• Gypsum
• Nitrates
• Borates

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Evaporites

• Volumetrically minor
• Geologically significant
• Great climate indicator
• Dramatic events (Messinian salinity crisis)
• Traps for hydrocarbons
• Gulf of Mexico rich with salt deposits
• flow structures--great producer of oil/gas

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Phosphate

• A primary nutrient in oceans
• Mineralizes on/within sediments in areas of
  upwelling--high organic matter loading
• Phosphorite is composed of authigenic minerals
• Released by organic matter decomposition
• Primarily used in fertilizers
• Florida contains 30 worlds phosphate

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More about phosphorites later in the semester
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Manganese nodules in box core(authigenic
deposits)
10 cm
NOAA Photo Library
Section through Mn nodule
5 cm
Ifremer
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(Kennett 1982)
Distribution of manganese nodules
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Dissolved loads
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The Himalayas A Source to Sink History

• Use magnetics to track the collision
• Use erosional products (Bengal Fan) in the
  northern Indian Ocean to track uplift rates
• Uplift rates are linked to erosional rates

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Mt. Everest (8,798 m)
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BENGALFAN
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Mountains Consume Themselves

• Uplift places rocks into ice-dominated
  environments
• Freeze-thaw weathering
• Glacial erosion
• Mountains make their own weather
• Orographic rainfall effect
• Isostatic unloading ? uplift
• Continues cycle of erosion

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Forced ascent of warm, moist oceanic air over a
mountain barrier produces precipitation and a
rainshadow desert
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Isostatic adjustment
Response of lithosphere to loading and unloading
Unloading (erosion)
Loading (deposition)
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Sediment Classification by Origin
Siliciclastic sediments
Biogenic sediments
From Open University Press
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What you now know about sediment transport

• What are the major transport mechanisms?
• What is the relative importance of each
  mechanism?
• What is the depositional record of each major
  mechanism?
• What happens to the solution load?
• How do mountains consume themselves?

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Reading assignments

• The Open University (either edition) Waves, Tides
  and Shallow-Water Processes.
• Chapter 3. Introduction to shallow-water
  environments and their sediments
• Chapter 4. Sedimentary movement by waves and
  currents
• Chapter 6. Tidal Flats

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Weblinks for glaciers/glacial deposits

• Continental glaciation http//www.isgs.uiuc.edu/m
  aps-data-pub/publications/geonotes/geonote3.shtml
• Loess http//www.iptv.org/exploremore/land/loess_
  hills/loess_hills.cfm
• Glaciers http//www.ux1.eiu.edu/cfjps/1300/glaci
  er_photos.html
• http//en.wikipedia.org/wiki/Glacier

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Weblinks for sediments/sediment transport

• http//en.wikipedia.org/wiki/Sediment
• http//faculty.gg.uwyo.edu/heller/ including
• http//faculty.gg.uwyo.edu/heller/Sed20Strat20Cl
  ass/Sedstrat6/sedlect_6.htm
• http//faculty.gg.uwyo.edu/heller/sed_video_downlo
  ads.htm
• Past Seminars Charlie Paull (Spring 2007)
• http//www.marine.usf.edu/news-and-events/seminars
  -old.shtml

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Some weblinks for evaporites, Mn nodules,
phosphorites

• http//en.wikipedia.org/wiki/Evaporite
• http//en.wikipedia.org/wiki/Manganese_nodule
• http//www.cartage.org.lb/en/themes/sciences/Earth
  science/Oceanography/OceanSediments/Manganesenodul
  es/Manganesenodules.htm
• http//www.cartage.org.lb/en/themes/Sciences/Earth
  science/Oceanography/OceanSediments/Phosphorites/P
  hosphorites.htm
• http//arjournals.annualreviews.org/doi/abs/10.114
  6/annurev.ea.09.050181.001343 (phosphorites)