GE Sedimentary processes and products Lecture 12. Deep sea Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder Ch. 26. Oceanic processes and sediments. 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 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 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.:  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; 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 ► Global oceanic (thermohaline) circulation system:  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 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 ► Discharge c. 50 x 10 6 m 3 /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. ► 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 10 6 m 3 /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 m thick (>2 km) ► Mud (<12 μm: clay-fine silt) ► Concentrations: < ► 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. ► 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, (>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 ► Typically thick (100s m) alternating, parallel sandstones and shales. ► Base sharp and often containing:  tool marks  sole marks  organic marks ► Sandstone bed commonly graded or 'fining-up' ► Sandstone bed commonly contains complete or partial 'Bouma sequence'.

► Terminal fans/suprafans Terminal lobe complex formed by progradation and avulsion Suprafan lobe of the Delgada fan.

► Tana delta slope/ submarine fan submarine fan Corner, unpublished

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