Transcript Slide 1

GE0-3112
Sedimentary processes and products
Lecture 12. Deep sea
Geoff Corner
Department of Geology
University of Tromsø
2006
Literature:
- Leeder 1999. Ch. 26.
Oceanic processes and sediments.
Contents
Introduction
► Coupled ocean-atmosphere system
► Surface oceanic circulation
► Deep oceanic circulation
► Contental margin sedimentation
► Sumarine canyons
► Submarine fans
--------------------------------------------► THESE SUBJECTS WILL BE ADDED LATER:
► Biological and chemical processes
► Pelagic sediments
► Palaeo-oceanography (palaeoceanography)
► Anoxic events
► Hypersaline oceans
►
Coupled ocean-atmosphere system
► Ocean-atmosphere
heat engine:
redistributes heat (from tropics
to poles).
► Heating
winds wind shear surface drift and
(horizontal) gradient currents.
► Heating heating/cooling and
evaporation/precipitation density differences
vertical gradient currents.
Lutgens & Tarbuck 2006
Physical forces and processes
► External
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
forces
Wind shear  surface currents.
Wind shear  horizontal gradients  Ekman transport.
Coriolis  deflects moving water masses.
Tides  weak tidal currents (+ pressure differences?).
► Internal
forces
 Thermohaline density differences  deep currents.
 Suspended particle density differences  turbidity
currents.
 Friction.
Surface oceanic circulation
► Complex
in time and space:
 Latitudinal zonation due to ’heat engine’.
 Local and regional differences in
evaporation/precipitation, glacial meltwater, etc.
 Local ’langmuir circulation’ (horizontal helical
eddies).
 Periodic storms cause movement and mixing to
variable depth.
► Equatorial
currents (trade winds 0-25˚)
► Subtropical gyres (trade winds + westerlies, c. 30˚).
► West
wind drift.
► Intertropical
zone of convergent trade winds
► Arctic and Antarctic convergence (polar front).
Subtropical gyres
► Coriolis-driven
Ekman
transport raises water
surface c. 1.4 m.
► Generates oblique gradient
(geostrophic) currents.
Surface currents
►
►
Typically 3 - 4 distinct warm or cold currents encompassing a
gyre (e.g. N.Atlantic, Canary, and N.Equatorial current around
the N.Atlantic gyre).
Flow is intensified on western borders of oceans;
warm western boundary currents up to 10x stronger than cool
eastern currents (max. vel. >1.4 m/s = 5 km/h)
e.g.:
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
►
N.Atlantic Gulf Stream
S.Atlantic Brazil Current
Pacific Kuro Shio (’Black tide’)
Indian Ocean Current
Stronger currents during
glacial epochs on e.g.
Blake Plateau.
Upwelling and counter currents
►
Intertropical convergence zone:
 upwelling of 1 m/day (due to poleward Ekman transport).
 E-flowing counter current and deeper W-flowing counter-counter current
(<1 m/s) (also causes upwelling and eddy mixing).
►
Antarctic (and Arctic convergence):
 descent of cold water accompanied by upwelling.
►
Upwelling where convergent winds cause water flow
divergence:
 cf. intertropical convergence zone and elsewhere.
►
Coastal upwelling occurs where flow is away from the coast
(Ekman/Coriolis transport to left or right):
 Peru
 California
 NW and SW Africa
ENSO
►
El-Niño-Southern Oscillation (ENSO)
 El-Niño = warm water appearance off Peruvian coast
 S. oscillation = atmosphere-ocean feedback process
1) Normally: trade-wind-driven circulation in S. Pacific piles up warm water in the W.
2) During an ENSO event:
 trade winds weaken
 relaxation flow (wave) of warm tropical water from W to E
 warm water replaces cold off S. American coast
 changes to ocean currents, upwelling and precipitation in Pacific and beyond.
► Quasi-periodic (every c. 2-5 years), effects last minimum 2 years, with delayed
effects farther afield by up to 10 years.
► Variable in frequency and intensity; 1982-83 was century’s strongest.
► The southern oscillation tends to switch between two states:
►
►
 El-Niño – warm and dry
 La Niña cool and wet
Lake Tarawera, New Zealand
Deep oceanic circulation
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Global oceanic (thermohaline) circulation system:
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warm Pacific upper water
warm North Atlantic Drift
cold North Atlantic Deep Water (NADW)
Circum-Antarctic Undercurrent/ Antarctic Bottom Water (ABW).
Circulation takes c. 500 years.
Thermohaline circulation system
► Driven
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
by density differences caused by:
surface heating (density decreases)
evaporative loss (density increases)
surface cooling (density increases)
runoff and precipitation (density decreases)
sea-ice formation (density increases)
Deep oceanic currents
►
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Discharge c. 50 x 106 m3/s (50x world’s rivers).
Velocities:
 normally ~0.05 m/s
 maximum 0.25 m/s at W ocean margins (boundary currents) and
topographic constrictions.
►
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Periodic intensification of near-bottom flow during deep-sea
‘storms’, i.e. downward transfer of surface eddy energy.
Curved paths following submarine topography (‘contour
currents’).
►
Paths and transport rates (in 106 m3/s) of NADW (1.8-4˚)
Sediment transport by deep currents
► Boundary
undercurrents cause:
 transport and deposition  contourites comprise alternating
thin v.f.sand, silt and bioturbated mud forming km-thick
’drift’.
 erosion (winnowing)  stratigraphic gaps in deep-sea cores.
► Contourites
(unlike distal turbidites) are well sorted
due to winnowing.
► Deep-sea ’storms’  ripple-like forms, tractional and
current scour features.
► Nepheloid layers comprise sediment in transit (see
below).
Nepheloid layers
► Nepheloid
layers – high concentrations of suspended
sediment.
► Form at bottom and intermediate depths.
► Normally 1-200 m thick (>2 km)
► Mud (<12 μm: clay-fine silt)
► Concentrations: <500-5000.
► Produced by:
 resuspension by deep-sea ’storms’
 enhanced thermohaline currents
 distal turbidites.
► Suspended
sediment concentration (nepheloid layer in
Atlantic Deep Water)
Continental margin sedimentation
► Thick
terrigenous clastic deposits on contintental
slope and rise and inner abyssal plain.
► Some large deltas at the shelf edge (shelf-edge
deltas).
► Steep slopes (~6˚; max. 30˚) disturbed by salt
diapirs, growth faults and slumps.
► Submarine fans at the base of slopes.
Progradational and erosional
continental margins
► Processes
affecting ’graded’ slope profile.
Resedimentation processes
► Slope
instability caused or enhanced by:
Sea-level variations (lowstand-highstand).
Development of gas hydrates.
Alternating coarse (sandy) and fine (mud) sediments.
Pressure fluctuations caused by earthquakes, tsunamis
and internal waves.
 Storms and tides.
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► Slumps,
faults and debris flows
► Turbidity currents
Dag Ottesen 2006
► Debris
flows and debris avalanches off
Canary Islands
Submarine canyons
Occur on shelves, slopes and fans.
► Important conduits for sediment from shelf to deep sea.
► Originate by some or all of following processes:
►
 retrogressive slope failure of slump scars
 fluvial erosion during s.l. lowstands
 erosion by turbidity currents
Several 100 m deep and km’s wide.
► V-shaped profile (± slumps).
► Many ‘headless’ canyons on slope.
► Downcanyon/turbidity flows
(>1m/s) lasting hours/days,
triggered by ocean tides, storms,
etc.
►
Submarine fans
► Located
on the continental slope; large ones
extending to the rise and abyssal plain.
► Fed by submarine canyons and channels; the largest
below deltas.
► Maximum activity during s.l. lowstands; low activity
during present (Holocene) highstand.
► Sensitive to changes in sea-level and runoff, i.e.
sediment supply.
Fan morphology
►
Upper fan
 contains main feeder channel, usually with
levées.
 debris flow lobes may occur.
►
Middle fan
 one main, levée-bound, active channel;
several older distributary channels.
 meandering or braided channels.
 channels terminate or pass into ‘supra-fan
lobes’.
►
Lower fan
 smooth or with many small channels.
 sometimes ending in well-defined terminal fan
lobes.
Walker 1992, after Normark 1978
► Amazon
fan
morphology and
sediments
► Channel
meanders and cutoff
► Low
and high sinuosity channels
► Fan
structure and stratigraphy
 Channel-levée complexes (lowstand).
 Debris flow deposits.
 Onlapping and draping hemipelagic
sediments (highstand).
Turbidite facies on fans
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Typically thick (100s m)
alternating, parallel
sandstones and shales.
Base sharp and often
containing:
 tool marks
 sole marks
 organic marks
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►
Sandstone bed commonly
graded or 'fining-up'
Sandstone bed commonly
contains complete or
partial 'Bouma sequence'.
► Terminal
fans/suprafans
Suprafan lobe of the
Delgada fan.
Terminal lobe complex
formed by progradation
and avulsion
► Tana
delta
slope/
submarine fan
Corner, unpublished
Further reading
► Allen,
J.R.L. 1970. Physical processes of
sedimentation.
 Chapter 1 covers the same ground as Leeder and
explains clearly the principles involved; good
supplementary reading for aquiring a sound grasp of the
physics of fluid dynamics and sedimentation.
Alternatively consult the more encyclopedic:
► Allen,
J.R.L 1984. Sedimentary structures: their
character and physical basis.
 A more encyclopedic alternative to the above if it is
unavailable.