Transcript Sediments
Sediments! ☺ January 19, 2009 Wentworth Scale of Grain Size • • • • • • • 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 • • • • 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)