General Process involved (in order) 1. Weathering of rocks 2. Erosion- removal of material formed by #1(above) 3. Transport of material- by rivers, glaciers, wind 4. Deposition- particles settle or precipitate out. Sediment formed. 5. Burial- layers of sediment accumulate, compacting older layers 6. Diagenesis- pressure, heart, chemical reactions, "lithify" (harden) sediments to form sedimentary rocks. (1) and (2) yield both broken grains (detrital or clastic material) as well as dissolved material. Products in clastic part depends on starting rock type and intensity of weathering Table 5.1 Feldspars alter relatively easily to clay minerals; if sediment contains feldspar, it denotes low amount of weathering. At highest weathering amounts, only quartz and clay minerals survive. Give rise to two main types of clastic sedimentary rocks- sandstones (quartz dominant) and shales(clay dominant) Biochemical sediments form from seawater- undissolved mineral remains of organisms, as well as minerals precipitated from sea water
Transportation Most sediments are transported by water or air currents. Rivers play a mjaor role. Strength of current important. Strong ones carry both coarse and fine detritus. Moderately strong currents carry fine detritus (mainly clay) and some coarser material (silt), but not coarse sand, which is deposited. Weak currents carry only the finest detritus (fine clay) and when these are checked, deposit fine muds (deep oceans) Distance of transportation affects both size and angularity of clastic particles Fig 5.3 When transportation stops, sedimentation begins. As current carrying particles of all sizes slows, biggest particles settle out first, smallest ones last (gravity) Tendency of variations in current velocity to segregate sediments according to size is called sorting Well sorted sediments has particles all of about same size. Poorly sorted sediment has particles of many diff sizes Fig 5.2 Chapter 5 Friday, February 23, 2007 12:42 PM Oceans and lakes may be regarded as mixing bowls. Input of material from rivers, rain, glaciers. Lose water by evaporation. Over short periods (<100 years), amount of water in oceans remains constant. But over long periods (m.y.) sea level may alter dramatically (100 m or more) e.g. glacial epochs Ocean water shows "salinity" caused by dissolved material. Amount of salinity is balanced by addition of "salts" by rivers, and removal by precipitation. Of main interest to us: Calcium (bi)carbonate Sodium Chloride Sedimentary Basins Cover>10,000 km2, and contain thick accumulations of sediments and Sedimentary rocks. They are prime sources of oil and gas.
- Spring '07
- Geology, Sedimentary rocks, Fig, sedimentary rocks Fig