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Sedimentary Rocks Every stone a story, like a rosary. ---Joan Osborne Weathering The sediments that make up sedimentary rocks are produced by: Mechanical & Chemical Weathering Sedimentary Rocks and Processes Clastic Non-Clastic Sediments are derived from fragments of pre-existing rocks (clasts). All sedimentary rocks not composed of clasts Categorized primarily by clast size Categorized primarily by composition Clastic Rocks e.g., conglomerate, breccia, sandstone, siltstone, shale clast size Formed from clasts (bits and pieces of pre-existing rocks). The major processes in clastic rock formation are: Weathering of clasts from pre-existing rock Transport of clasts Deposition of clasts Lithification of clasts Weathering processes continue to alter the clasts until deposition. In general, both physical and chemical weathering tend to make clasts smaller and rounder. Sorting of clasts by size occurs mostly during transportation. Sedimentary clasts are classified by grain size (the size of the individual clasts) using the Wentworth Scale: Wentworth Scale Boulders Larger clasts require more energy to transport Cobbles Gravel (pebbles) Sand Smaller clasts require less energy to transport http://www.glossary.oilfield.slb.com/ Silt Clay Clast Sorting The farther the clasts have traveled from the source, the more well sorted they tend to be. Clast Rounding As clasts are rolled around during transportation, sharp corners tend to be knocked off, and the clasts become more rounded the farther they are transported. Angular grains are usually found only near the source rock. Deposition Proximal Depositional Environments Near the source rock, the clasts have not experienced much physical or chemical weathering. These areas contain poorly rounded and poorly sorted clasts. Clasts composed of chemically unstable minerals (e.g., feldspar) are most common close to the source rock. Distal Depositional Environments Clasts experience both chemical and physical weathering as they are transported. Thus, the further they have traveled from the source the more altered the clasts are. http://www.flmnh.ufl.edu/alligatorfund/potm-nov04.html http://www.ufz.de/index.php?de=5366 Distal areas contain well rounded and well sorted clasts. Large clasts (e.g., boulders) are rare (they have either been left upstream, or have weathered into smaller clasts). Clasts composed of chemically unstable minerals (e.g., feldspar) are very rare in distal settings. Clay minerals derived from feldspars are common. Clast size indicates ancient relative current velocity weak moderate >=1.8 km/hr (strong currents) Chemical Sedimentary Rocks Clastic Sedimentary Rocks Formed from clasts (bits and pieces of pre-existing rocks) transported, deposited,and lithified Chemical Sedimentary Rocks Formed from precipitation of minerals within the depositional basin. Includes biominerals, evaporites, coal, chert, etc. Evaporites minerals precipitated from evaporating water Siliceous Rocks Silica and opalline silica precipitate either organically or inorganically Coal and Peat Carbon-rich deposits formed from build-up of plant material in swamps Carbonates Variety of limestones composed of calcite, aragonite, and dolomite precipitated either organically or inorganically http://csmres.jmu.edu/geollab/Fichter/SedRx/chembiokey.html Common Minerals: Carbonates - Limestone Biochemical (corals) Bioclastic (chalk, coquina) Inorganic – chemical precipitate travertine tufa oolitic limestone Calcite CaCO3 Aragonite CaCO3 Dolomite (CaMg)CO3 Corals and Snails Chalk Limestone formed from the microscopic calcite skeletons of tiny animals. Siliceous Sedimentary Rocks Microcrystalline quartz and opal precipitated from dissolved ions in sedimentary environments. Can be precipitated organically or inorganically. Microcrystalline quartz can also precipitate in pore spaces to form cement. http://www.diskworks.com/octoberopals/opal1.html Scanning Electron Micrograph (SEM) of tiny, siliceous skeletons of radiolarians, which are a source silica chert Ionic Composition of Seawater Cations Anions Na+ 10,556 Cl- Mg2+ 1,272 (SO4)2- 2,649 K+ 380 (HCO3)- 140 Ca2+ 400 parts per million Common Mineral Salts NaCl - Halite KCl - Sylvite CaSO4•H2O - Gypsum 18,980 Total Ions: H2O: 34,387 ppm 965,613 ppm 35 o/oo normal marine salinity Other Mineral Salts KMgCl3•6H2O - Carnalite MgCO3 - Magnesite CaMg(CO3) - Dolomite CaSO4 - Anhydrite Fossil example of the past activity of organisms mixing sediment --- an example of fossil BIOTURBATION