ELEMENTS OF OCEANOGRAPHY

 

TOPIC 13 - MARINE SEDIMENTS AND SEDIMENTATION

Overheads:

Major Marine Sediment Types
Lithogenous (terrigenous) - detrital sediment
Biogenous - remains of dead organisms
Hydrogenous - precipitation of dissolved minerals in water
Cosmogenous - dust and meteorite fragments from Outer Space

Sediment Transport to Continental Margins
Sediments reach continental margins by several methods
­ River input
­ Mass wasting
­ Direct runoff
­ Glacial action
Sediments bypass the coastal zone though wind (aeolian)transport

Sediment Transport and Sorting Along Continental Margins
Sediment in the coastal setting is sorted by grain size
­ Gravels stay close to their source areas (river or glaciers)
­ Sands move along coasts by wave action
­ Silts and clays are suspended in the water column by wave action and carried out to the deep sea
Biota may injest fine sediments and make fecal pellets which deposit quickly

Continental Shelves
Shallow platforms (~100m) surrounding land masses
Shelves developed by wave action, longshore currents, glaciers, and submarine channels
Terraces are ancient shelves uplifted or downwarped by tectonics or sea level changes
Passive margin shelves broad
Active margin shelves narrow or nonexistant

Shelf Sediments
Dominated by lithogenous material - sands and some silts
Locally important biogenous components - coral reefs
When sediment supply is low - eroded rock platforms may be exposed
Passive margin shelves may contain old shoreline feature

Sediment Pathways to the Deep Sea
Turbidity currents, slides - lithogenous
Aeolian dust - lithogenous
Cosmic dust - cosmogenous
Pelagic sediment (derived from the open ocean) - mostly biogenous
Hydrothermal - hydrogenous
Submarine volcanic ­ lithogenous

Continental Slope
Approximately equal to shelves in total area
Slopes are wider on passive margins and very narrow on active margins where they lead to trenches
Typical slopes of 6 degrees - gravity an important force in sediment transport
Slopes are areas of mass movement - submarine slides or turbidity currents

Submarine Canyons
Cut shelves and slopes
Conduits for transport of sediment to the deep ocean - turbidity currents
Large canyons associated with large rivers
Submarine canyons terminate in deep sea fans on continental rises
California Active Submarine Canyon model

Deep Sea Fans
Analogous to deltas formed by rivers entering lakes or oceans
Transport tubidity currents out onto continental rise and deep ocean basins
Form the terminus of submarine canyons
Constitute broad continental rises on passive margins
Fill trenches on active margins

Most Common Deep Sea Sediments
Foraminiferal "foram" oozes - comosed mostly of single - celled organisms with carbonate shells
Diatom oozes - single - celled plants with siliceous shells
Radiolarisn oozes - single - celled animals with siliceous shells
Red clay - low sed rate mixture, mostly lithogenous

 

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