GE0-3112GE0-3112 Sedimentary processes and products Sedimentary processes and products

Lecture 12. Deep seaLecture 12. Deep sea

Geoff CornerGeoff CornerDepartment of GeologyDepartment of GeologyUniversity of TromsøUniversity of Tromsø20062006

Literature:Literature:- Leeder 1999. Ch. 26. - Leeder 1999. Ch. 26. Oceanic processes and sediments.Oceanic processes and sediments.

ContentsContents

► IntroductionIntroduction► Coupled ocean-atmosphere systemCoupled ocean-atmosphere system► Surface oceanic circulationSurface oceanic circulation► Deep oceanic circulationDeep oceanic circulation► Contental margin sedimentationContental margin sedimentation► Sumarine canyonsSumarine canyons► Submarine fansSubmarine fans------------------------------------------------------------------------------------------► THESE SUBJECTS WILL BE ADDED LATER:THESE SUBJECTS WILL BE ADDED LATER:► Biological and chemical processesBiological and chemical processes► Pelagic sedimentsPelagic sediments► Palaeo-oceanography (palaeoceanography) Palaeo-oceanography (palaeoceanography) ► Anoxic eventsAnoxic events► Hypersaline oceansHypersaline oceans

Coupled ocean-atmosphere Coupled ocean-atmosphere systemsystem

► Ocean-atmosphere heat engine: Ocean-atmosphere heat engine: redistributes heat (from tropicsredistributes heat (from tropicsto poles).to poles).

► Heating Heating winds winds wind shearwind shear surface drift surface drift and (horizontal) gradient currents.and (horizontal) gradient currents.

► Heating Heating heating/cooling and heating/cooling and evaporation/precipitation evaporation/precipitation density differencesdensity differences vertical gradient currents.vertical gradient currents.

Lutgens & Tarbuck 2006

Physical forces and Physical forces and processesprocesses

► External forcesExternal forces Wind shear Wind shear surface currents. surface currents. Wind shear Wind shear horizontal gradients horizontal gradients Ekman transport. Ekman transport. Coriolis Coriolis deflects moving water masses. deflects moving water masses. Tides Tides weak tidal currents (+ pressure differences?). weak tidal currents (+ pressure differences?).

► Internal forcesInternal forces Thermohaline density differences Thermohaline density differences deep currents. deep currents. Suspended particle density differences Suspended particle density differences turbidity turbidity

currents.currents. Friction. Friction.

Surface oceanic circulationSurface oceanic circulation

►Complex in time and space:Complex in time and space: Latitudinal zonation due to ’heat engine’.Latitudinal zonation due to ’heat engine’. Local and regional differences in Local and regional differences in

evaporation/precipitation, glacial evaporation/precipitation, glacial meltwater, etc.meltwater, etc.

Local ’langmuir circulation’ (horizontal Local ’langmuir circulation’ (horizontal helical eddies).helical eddies).

Periodic storms cause movement and Periodic storms cause movement and mixing to variable depth.mixing to variable depth.

► Equatorial currentsEquatorial currents (trade winds 0-25 (trade winds 0-25˚̊))► Subtropical gyresSubtropical gyres (trade winds + westerlies, c. (trade winds + westerlies, c.

3030˚).˚).

► West wind driftWest wind drift..

► Intertropical zone of convergent trade windsIntertropical zone of convergent trade winds► Arctic and Antarctic convergenceArctic and Antarctic convergence (polar front). (polar front).

Subtropical gyresSubtropical gyres

► Coriolis-driven Ekman Coriolis-driven Ekman transport raises water transport raises water surface c. 1.4 m.surface c. 1.4 m.

► Generates oblique gradient Generates oblique gradient (geostrophic) currents. (geostrophic) currents.

Surface currentsSurface currents► Typically 3 - 4 distinct warm or cold currents Typically 3 - 4 distinct warm or cold currents

encompassing a gyre (e.g. N.Atlantic, Canary, and encompassing a gyre (e.g. N.Atlantic, Canary, and N.Equatorial current around the N.Atlantic gyre).N.Equatorial current around the N.Atlantic gyre).

► Flow is intensified on western borders of oceans;Flow is intensified on western borders of oceans;warm warm western boundary currentswestern boundary currents up to 10x stronger than up to 10x stronger than cool eastern currents (max. vel. >1.4 m/s = 5 km/h)cool eastern currents (max. vel. >1.4 m/s = 5 km/h)e.g.:e.g.: N.Atlantic Gulf StreamN.Atlantic Gulf Stream S.Atlantic Brazil CurrentS.Atlantic Brazil Current Pacific Kuro Shio (’Black tide’)Pacific Kuro Shio (’Black tide’) Indian Ocean CurrentIndian Ocean Current

► Stronger currents during Stronger currents during glacial epochs on e.g. glacial epochs on e.g. Blake Plateau. Blake Plateau.

Upwelling and counter Upwelling and counter currentscurrents

► Intertropical convergence zone:Intertropical convergence zone: upwelling of 1 m/day (due to poleward Ekman transport).upwelling of 1 m/day (due to poleward Ekman transport). E-flowing counter current and deeper W-flowing counter-counter E-flowing counter current and deeper W-flowing counter-counter

current (<1 m/s) (also causes upwelling and eddy mixing).current (<1 m/s) (also causes upwelling and eddy mixing).► Antarctic (and Arctic convergence):Antarctic (and Arctic convergence):

descent of cold water accompanied by upwelling. descent of cold water accompanied by upwelling. ► Upwelling where convergent winds cause water flow Upwelling where convergent winds cause water flow

divergence:divergence: cf. intertropical convergence zone and elsewhere.cf. intertropical convergence zone and elsewhere.

► Coastal upwelling occurs where flow is away from the Coastal upwelling occurs where flow is away from the coast (Ekman/Coriolis transport to left or right):coast (Ekman/Coriolis transport to left or right): PeruPeru CaliforniaCalifornia NW and SW AfricaNW and SW Africa

ENSOENSO► El-NiEl-Niño-Southern Oscillation (ENSO)ño-Southern Oscillation (ENSO)

ElEl-Ni-Niño = warm water appearance off Peruvian coastño = warm water appearance off Peruvian coast S. oscillation = atmosphere-ocean feedback process S. oscillation = atmosphere-ocean feedback process

► 1) Normally: 1) Normally: trade-wind-driven circulation in S. Pacific piles up warm water trade-wind-driven circulation in S. Pacific piles up warm water in the W.in the W.

► 2) During an ENSO event:2) During an ENSO event: trade winds weakentrade winds weaken relaxation flow (wave) of warm tropical water from W to Erelaxation flow (wave) of warm tropical water from W to E warm water replaces cold off S. American coastwarm water replaces cold off S. American coast changes to ocean currents, upwelling and precipitation in Pacific and changes to ocean currents, upwelling and precipitation in Pacific and beyond.beyond.

► Quasi-periodic (every c. 2-5 years), effects last minimum 2 years, with Quasi-periodic (every c. 2-5 years), effects last minimum 2 years, with delayed effects farther afield by up to 10 years.delayed effects farther afield by up to 10 years.

► Variable in frequency and intensity; 1982-83 was century’s strongest. Variable in frequency and intensity; 1982-83 was century’s strongest. ► The southern oscillation tends to switch between two states:The southern oscillation tends to switch between two states:

EEl-Nil-Niño – warm and dryño – warm and dry La La NiNiña cool and wetña cool and wet

Lake Tarawera, New Zealand

Deep oceanic circulationDeep oceanic circulation

► Global oceanic (thermohaline) circulation system:Global oceanic (thermohaline) circulation system: warm Pacific upper waterwarm Pacific upper water warm North Atlantic Driftwarm North Atlantic Drift cold North Atlantic Deep Water (NADW)cold North Atlantic Deep Water (NADW) Circum-Antarctic Undercurrent/ Antarctic Bottom Water Circum-Antarctic Undercurrent/ Antarctic Bottom Water

(ABW).(ABW).► Circulation takes c. 500 years.Circulation takes c. 500 years.

Thermohaline circulation Thermohaline circulation systemsystem

► Driven by density differences caused by:Driven by density differences caused by: surface heating (density decreases)surface heating (density decreases) evaporative loss (density increases)evaporative loss (density increases) surface cooling (density increases)surface cooling (density increases) runoff and precipitation (density decreases)runoff and precipitation (density decreases) sea-ice formation (density increases) sea-ice formation (density increases)

Deep oceanic currentsDeep oceanic currents

► Discharge c. 50 x 10Discharge c. 50 x 1066 m m33/s (50x world’s rivers)./s (50x world’s rivers).► Velocities:Velocities:

normally normally ~0.05 m/s~0.05 m/s maximum 0.25 m/s at W ocean margins (boundary currents) maximum 0.25 m/s at W ocean margins (boundary currents)

and topographic constrictions.and topographic constrictions.► Periodic intensification of near-bottom flow during Periodic intensification of near-bottom flow during

deep-sea ‘storms’, i.e. downward transfer of surface deep-sea ‘storms’, i.e. downward transfer of surface eddy energy.eddy energy.

► Curved paths following submarine topography (‘contour Curved paths following submarine topography (‘contour currents’).currents’).

► Paths and transport rates (in 10Paths and transport rates (in 1066 m m33/s) of NADW (1.8-/s) of NADW (1.8-44˚̊))

Sediment transport by deep Sediment transport by deep currentscurrents

► Boundary undercurrents cause:Boundary undercurrents cause: transport and deposition transport and deposition contouritescontourites comprise comprise

alternating thin v.f.sand, silt and bioturbated mud alternating thin v.f.sand, silt and bioturbated mud forming km-thick ’drift’.forming km-thick ’drift’.

erosion (winnowing) erosion (winnowing) stratigraphic gaps in deep- stratigraphic gaps in deep-sea cores.sea cores.

► Contourites (unlike distal turbidites) are well Contourites (unlike distal turbidites) are well sorted due to winnowing. sorted due to winnowing.

► Deep-sea ’storms’ Deep-sea ’storms’ ripple-like forms, ripple-like forms, tractional and current scour features.tractional and current scour features.

► NepheloidNepheloid layers comprise sediment in transit layers comprise sediment in transit (see below).(see below).

Nepheloid layersNepheloid layers► NepheloidNepheloid layers – high concentrations of layers – high concentrations of

suspended sediment.suspended sediment.► Form at bottom and intermediate depths.Form at bottom and intermediate depths.► Normally 1-200 m thick (>2 km)Normally 1-200 m thick (>2 km)► Mud (<12 Mud (<12 μμm: clay-fine silt)m: clay-fine silt)► Concentrations: <500-5000.Concentrations: <500-5000.► Produced by:Produced by:

resuspension by deep-sea ’storms’resuspension by deep-sea ’storms’ enhanced thermohaline currentsenhanced thermohaline currents distal turbidites. distal turbidites.

► Suspended sediment concentration (nepheloid Suspended sediment concentration (nepheloid layer in Atlantic Deep Water)layer in Atlantic Deep Water)

Continental margin Continental margin sedimentationsedimentation

► Thick terrigenous clastic deposits on Thick terrigenous clastic deposits on contintental slope and rise and inner abyssal contintental slope and rise and inner abyssal plain.plain.

► Some large deltas at the shelf edge (shelf-Some large deltas at the shelf edge (shelf-edge deltas).edge deltas).

► Steep slopes (Steep slopes (~6~6˚; max. 30˚; max. 30˚) disturbed by ˚) disturbed by salt diapirs, growth faults and slumps.salt diapirs, growth faults and slumps.

► Submarine fans at the base of slopes.Submarine fans at the base of slopes.

Progradational and erosional Progradational and erosional continental marginscontinental margins

►Processes affecting ’graded’ slope Processes affecting ’graded’ slope profile.profile.

Resedimentation processesResedimentation processes

► Slope instability caused or enhanced by:Slope instability caused or enhanced by: Sea-level variations (lowstand-highstand).Sea-level variations (lowstand-highstand). Development of gas hydrates.Development of gas hydrates. Alternating coarse (sandy) and fine (mud) Alternating coarse (sandy) and fine (mud)

sediments.sediments. Pressure fluctuations caused by earthquakes, Pressure fluctuations caused by earthquakes,

tsunamis and internal waves.tsunamis and internal waves. Storms and tides.Storms and tides.

► Slumps, faults and debris flowsSlumps, faults and debris flows► Turbidity currentsTurbidity currents

Dag Ottesen 2006

►Debris flows and debris avalanches off Debris flows and debris avalanches off Canary IslandsCanary Islands

Submarine canyonsSubmarine canyons► Occur on shelves, slopes and fans.Occur on shelves, slopes and fans.► Important conduits for sediment from shelf to deep Important conduits for sediment from shelf to deep

sea.sea.► Originate by some or all of following processes:Originate by some or all of following processes:

retrogressive slope failure of slump scarsretrogressive slope failure of slump scars fluvial erosion during s.l. lowstandsfluvial erosion during s.l. lowstands erosion by turbidity currents erosion by turbidity currents

► Several 100 m deep and km’s wide.Several 100 m deep and km’s wide.► V-shaped profile (V-shaped profile (± slumps).± slumps).► Many ‘headless’ canyons on slope.Many ‘headless’ canyons on slope.► Downcanyon/turbidity flowsDowncanyon/turbidity flows (>1m/s) lasting hours/days,(>1m/s) lasting hours/days,

triggered by ocean tides, storms,triggered by ocean tides, storms,etc.etc.

Submarine fansSubmarine fans

► Located on the continental slope; large ones Located on the continental slope; large ones extending to the rise and abyssal plain.extending to the rise and abyssal plain.

► Fed by submarine canyons and channels; the Fed by submarine canyons and channels; the largest below deltas.largest below deltas.

► Maximum activity during s.l. lowstands; low Maximum activity during s.l. lowstands; low activity during present (Holocene) highstand.activity during present (Holocene) highstand.

► Sensitive to changes in sea-level and runoff, i.e. Sensitive to changes in sea-level and runoff, i.e. sediment supply. sediment supply.

Fan morphologyFan morphology► Upper fanUpper fan

contains main feeder channel, usually contains main feeder channel, usually with levwith levées.ées.

debris flow lobes may occur.debris flow lobes may occur.► Middle fanMiddle fan

one main, leone main, levvée-bound, active channel; ée-bound, active channel; several older distributary channels.several older distributary channels.

meandering or braided channels.meandering or braided channels. channels terminate or pass into ‘supra-channels terminate or pass into ‘supra-

fan lobes’. fan lobes’. ► Lower fanLower fan

smooth or with many small channels.smooth or with many small channels. sometimes ending in well-defined sometimes ending in well-defined

terminal fan lobes.terminal fan lobes.

Walker 1992, after Normark 1978

►Amazon fan Amazon fan morphology morphology and sedimentsand sediments

►Channel meanders and cutoffChannel meanders and cutoff

►Low and high sinuosity channelsLow and high sinuosity channels

► Fan structure and stratigraphyFan structure and stratigraphy

Channel-levChannel-levée complexes ée complexes (lowstand).(lowstand).

Debris flow deposits.Debris flow deposits. Onlapping and draping Onlapping and draping

hemipelagic sediments hemipelagic sediments (highstand).(highstand).

Turbidite facies on fansTurbidite facies on fans► Typically thick (100s m) Typically thick (100s m)

alternating, parallel alternating, parallel sandstones and shales.sandstones and shales.

► Base sharp and often Base sharp and often containing:containing: tool markstool marks sole markssole marks organic marksorganic marks

► Sandstone bed Sandstone bed commonly graded or commonly graded or 'fining-up''fining-up'

► Sandstone bed Sandstone bed commonly contains commonly contains complete or partial complete or partial 'Bouma sequence'. 'Bouma sequence'.

►Terminal fans/suprafansTerminal fans/suprafans

Terminal lobe complex formed by progradation and

avulsion

Suprafan lobe of the Delgada fan.

► Tana delta Tana delta slope/slope/

submarine fansubmarine fan

Corner, unpublished

Further readingFurther reading

► Allen, J.R.L. 1970. Physical processes of Allen, J.R.L. 1970. Physical processes of sedimentation.sedimentation. Chapter 1 covers the same ground as Leeder and Chapter 1 covers the same ground as Leeder and

explains clearly the principles involved; good explains clearly the principles involved; good supplementary reading for aquiring a sound supplementary reading for aquiring a sound grasp of the physics of fluid dynamics and grasp of the physics of fluid dynamics and sedimentation. Alternatively consult the more sedimentation. Alternatively consult the more encyclopedic:encyclopedic:

► Allen, J.R.L 1984. Sedimentary structures: Allen, J.R.L 1984. Sedimentary structures: their character and physical basis. their character and physical basis. A more encyclopedic alternative to the above if it A more encyclopedic alternative to the above if it

is unavailable.is unavailable.