Sediments! ☺
January 19, 2009
Wentworth Scale of Grain Size
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Boulder
Cobble
Pebble
Granule
Sand
Silt
Clay
Benthic Sediments by the Numbers
• Cover 75% of seafloor
• Oozes = >30% biogenous material
– Siliceous (14% of ocean surface area)
– Calcareous (48% of ocean surface area)
• Abyssal clay (38% of ocean surface area)
Siliceous ooze
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SiO2 – opal
Comes from diatoms & radiolarians
Found @ deeper depths
Less abundant han calcareous ooze b/c
silica dissolves readily in ocean water
• Usu. <100 micrometers
Calcareous ooze
• CaCO3
• Comes from coccolithophores &
foraminiferans
• Dissolves under high pressure (@ great
depths) & in basic water, so only found in
shallower water (<4500m)
Calcium carbonate
• Carbonate compensation depth: depth
@ which CaCo3 dissolved @ same rate
as replenished from above. Affected by
CO2 concentrations, pressure,
temperature, pH, productivity of
calcareous-shelled organisms.
• Lysocline: depth @ which rate of CaCO3
dissolution significantly ↑
Types of Sediment
• Lithogenous (rocks, volcanoes, continents)
• Biogenous (remains of living organisms,
usu. plankton)
• Cosmogenous (space)
• Hydrogenous (precip of ions dissolved in
H2O)
Lithogenous: Examples
• Volcanic ash
• Ferromagnesian minerals (iron, silicate,
magnesium compound)
• Igneous rocks
– Extrusive = fine-grained
– Intrusive = coarse-grained
Biogenous: Examples
• CaCO3 (foraminiferans, coccolithophores,
coral reefs)
• Silica (SiO2) (diatoms, radiolarians)
Cosmogenous: Examples
• Nickel-iron spherules
• Tektites
• Silicate chondrules
Hydrogenous: Examples
• Manganese nodules (manganese dioxide,
iron oxide)
• Phosphorite nodules (P2O5) (grow down
into sediment)
• Calcium carbonate (not from coral)
Sediment Deposits
• Turbidites deposited @ edge of continental
margin (form continental rise)
• Phosphorite mined→phosphate (fertilizer)
• Glacial deposits = from continental shelf, many
formed during Pleistocene Epoch
• Rafting = sediments carried from shore by
icebergs & deposited when they melt
• Stromatolites = dome-shaped calcareous
structures in shallow water that had been
secreted in layers by ancient cyanobacteria
Miscellaneous
• Fecal pellets = detritus material from plankton;
responsible for carrying particles from shallow
depths to deep in ocean
• Sediment maturity = ↑ as move away from
source→more sorting of particles, ↓ clay content,
rounding of grains such as sand
• Huljstrom’s Diagram: relationship b/w current
velocity & erosion/transportation/deposition of
different-sized grains
Accretion
• Process by which material is added to a
tectonic plate or landmass (could be
sediment, volcanic arcs, seamounts, etc.)
– Plate accretion (in subduction, sediment
scraped off subducted plate attaches to top
plate; island arcs & seamounts collide w/
continent, add to it.) W. Coast, E. Coast of
Australia, New Zealand.
– Landmass accretion (addit. of sediment to
coastline/riverbank. Alluvium deposits!)
Random Landforms
• Seamount: elevation of sea floor 1000m or
higher
– Guyot: seamount w/ flat top
– Seapeak: seamount w/ peak
Random Terms
• Bioturbation: disturbance of sediment
near water interface by benthic organisms
(burrowing worms)