OCEANESS 410 14 Sediment Transport in the Ocean

OCEAN/ESS 410 14. Sediment Transport in the Ocean Basins – In Development William Wilcock 1

Lecture/Lab Learning Goals • Know the terminology of and be able to sketch passive continental margins • Understand differences in sedimentary processes between active and passive margins • Know how sediments are mobilized on the continental shelf • Understand how sediments are transported into deep water and be able to explain the difference between turbidites and debrites • Understand the concept of accommodation space and the processes that cause it to vary with time • Understand the concept of eustacy and the processes that control it • Be able to draw a labeled diagram and explain prograding and aggrading sediment deposits • Be able to explain the patterns of sedimentation on a continental margin during a eustatic cycle • Interpreting a sedimentary stratigraphic section in terms of sea level 2 changes- LAB

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Passive Margins Transition from continental to oceanic crust with no plate boundary. Formerly sites of continental rifting 4

Terminology Shelf Break Abyssal Plain Continental Shelf - Average gradient 0. 1° Shelf break at outer edge of shelf at 130 -200 m depth (130 m depth = sea level at last glacial maximum) Continental slope - Average gradient 3 -6° Continental rise (typically 1500 -4000 m) - Average gradient 0. 1 -1° Abyssal Plain (typically > 4000 m) - Average slope <0. 1° 5

Active Margins Plate boundary (usually convergent) Narrower continental shelf Plate boundary can move on geological time scales - accretion of terrains, accretionary prisms 6

Sediment transport differences Active margins - narrower shelf, typically have a higher sediment supply, earthquakes destabilize steep slopes. 7

Sediment Supply to Continental Shelf • Rivers • Glaciers • Coastal Erosion Sediment Transport across the Shelf Once sediments settle on the seafloor, bottom currents are required to mobilize them. • Wave motions • Ocean currents 8

10 largest rivers in world supply 40% of freshwater and sediment to ocean 90% of carbon accumulating in ocean does so on continental shelves 9

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Sediment Mobilization - 1. Waves The wave base or maximum depth of wave motions is about one half the wave length 11

Shallow water waves Wave particle orbits flatten out in shallow water Wave generated bottom motions • strongest during major storms (big waves) • extend deepest when the coast experiences long wavelength swell from local or distant storms 12

Sediment Mobilization - 2. Bottom Currents • Wind driven ocean circulation often leads to strong ocean currents parallel to the coast. • These interact with the seafloor along the continental shelf and upper slope. • The currents on the continental shelf are often strongest near outer margins 13 Aguihas current off east coast of southern Africa. The current flows south and the contours are in units of cm/s

Holocene deposits (<20, 000 y) on passive continental shelves 70% of shelf surfaces have exposed relict deposits Boundary between modern inner-shelf sand modern mid-shelf mud depends on waves 14

Sedimentation on active margins Washington continental shelf 15

Shelf Sedimentation • Coarse grained sands - require strong currents/waves to mobilize • Fine grained muds require weaker currents to mobilize, transported to 16

Sediment Transport from Shelf to Deep Waters 1. Turbidity currents (and hyperpycnal flow) 2. Fluidized sediment flows 3. Debris Flows/Slides 17

Debris Flows and Turbidity Currents 18

Debrites and Turbidites • Debrites – Weakly Inversely graded (upward coarsening) – Thick, but pinch out quickly – Convoluted bedding • Turbidites – Normally graded (upward fining) – Laterally extensive – Thin – Horizontal bedding Lahars and pyroclastic flow deposits, Mt. St. Helens, WA. 19

Debrites and Turbidites • Debrites – Weakly Inversely graded (upward coarsening) – Thick, but pinch out quickly – Convoluted bedding • Turbidites – Normally graded (upward fining) – Laterally extensive – Thin – Horizontal bedding Turbidite in sandstone, unknown location (from http: //uibk. ac. at) 20

Turbidity Current Experiments There is a good movie of a turbidity current available at http: //learningobjects. wesleyan. edu/turbiditycurrents/ 21

Turbidity Currents – Erosion and Deposition 22

Submarine Channels 23

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Accommodation Space available for sediment accumulation based on: • Eustatic sea level Relative sea level • Tectonics • Sediment input 27

Depositional Patterns w. r. t. Accommodation Space 28

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Classical Turbidite 31

10 largest rivers in world supply 40% of freshwater and sediment to ocean 90% of carbon accumulating in ocean does so on continental shelves 32