Presentation on theme: "GE0-3112 Sedimentary processes and products Lecture 12. Deep sea Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999."— Presentation transcript:
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. 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.
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.
► 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.: 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 ► 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 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
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
► 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
► 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. 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.