GE0-3112 Sedimentary processes and products

1. Lecture 12. Deep sea GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 26. Oceanic processes and sediments.

2. 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

3. 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.

4. Lutgens & Tarbuck 2006

5. 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.

6. 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.

7. Equatorial currents (trade winds 0-25˚) • Subtropical gyres (trade winds + westerlies, c. 30˚). • West wind drift.

8. Intertropical zone of convergent trade winds • Arctic and Antarctic convergence (polar front).

9. Subtropical gyres • Coriolis-driven Ekman transport raises water surface c. 1.4 m. • Generates oblique gradient (geostrophic) currents.

10. 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.: • 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.

11. 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

12. 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

13. 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.

14. 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)

15. Deep oceanic currents • 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. • 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’).

16. Paths and transport rates (in 106 m3/s) of NADW (1.8-4˚)

17. 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).

18. 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.

19. Suspended sediment concentration (nepheloid layer in Atlantic Deep Water)

20. 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.

21. Progradational and erosional continental margins

22. Processes affecting ’graded’ slope profile.

23. 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

24. Dag Ottesen 2006

25. Debris flows and debris avalanches off Canary Islands

26. 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.

27. 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.

29. 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

30. Amazon fan morphology and sediments

31. Channel meanders and cutoff

32. Low and high sinuosity channels

33. Fan structure and stratigraphy • Channel-levée complexes (lowstand). • Debris flow deposits. • Onlapping and draping hemipelagic sediments (highstand).

34. 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'.

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

36. Tana delta slope/ submarine fan Corner, unpublished

38. 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.