Sedimentary Rocks

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Information about Sedimentary Rocks

Published on April 2, 2009

Author: kcollazo


Sedimentary Rocks : Sedimentary Rocks Sedimentary Rocks : Sedimentary Rocks Sediments = pieces of solid material deposited on the Earth’s surface. Sedimentary Rock : Sedimentary Rock Rocks that are composed of the weathered remains of preexisting rock, or plant and animal remains. Sedimentary rocks commonly originate from sediments laid down in horizontal strata by water or wind. Horizontal layers called “Beds” separated by “Bedding planes”, are a common feature in sedimentary rocks. Slide 4: Horizontal Beds of Sedimentary Rock Beds Bedding planes Sedimentary Rocks : Sedimentary Rocks How is a sedimentary rock formed? Sediments get compacted and cemented together. Sedimentary Rocks : Sedimentary Rocks Clastic – made from fragments of other rocks, that have been transported, deposited, then compacted and cemented together. Shale, sand, conglomerate, siltstone, breccia Classified by the size of the fragments in the rocks Think about it: What environments would create these types of rock? High energy fast moving water (carrying fragments), then water slows down (lake or ocean) and deposits fragments. Cemented Rocks : Cemented Rocks Clastic sedimentary rock – rocks composed of weathered sediments: Pebbles or gravel – usually quartz Sand – usually quartz Clay and silt – weathered feldspars and mica : held together by a natural cement Silica – commonly gray-white in color Calcite – commonly gray-white in color Iron oxide – commonly reddish to yellow brown in color, or by compaction of clay and silt. Slide 9: Conglomerate – cemented sand, silt, and pebble sediments. If large fragments are angular this rock is called a breccia. Sandstone – cemented quartz sand grains. Feels gritty. Unfilled spaces between grains make most sandstones porous and permeable to water. Slide 10: Shale – clay and silt sized particles lithified by dehydration and compaction. Note the cleavage at bedding planes. Thumps when you tap it with a nail and, moistened, it smells like damp earth. Bedding planes Slide 11: Sandstone in the Pinnacle Desert, Australia Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL Sedimentary Rocks : Sedimentary Rocks Chemical – formed when minerals fall out of solution. From evaporation of salt water or from chemical reactions. Rock salt, rock gypsum, some limestones Think about it: Where would these rocks form? Sea, lake, swamps, or underground waters that contain dissolved minerals Chemical Sedimentary Rock : Chemical Sedimentary Rock Rocks formed either as precipitates or as evaporites of dissolved chemical sediments. Mineral salts that accumulate in water become concentrated by evaporation until they precipitate from solution, or Mineral salts are deposited as a result of the total evaporation of the solution. Slide 14: Rock salt, the mineral halite (NaCl), left as an evaporite as a shallow sea evaporated. Alabaster, the mineral gypsum (CaSO4), also an evaporite. Slide 15: Compact (or precipitate) limestone, the mineral calcite (CaCO3), precipitated from sea water as evaporation increased concentration. Many cavern systems are formed in this type of limestone. Sedimentary Rocks : Sedimentary Rocks Organic Rocks – formed from the remains of plants and animals. Shells of marine animals pile up, compact and cement to create fossiliferous limestone (coquina). Where would these form? Oceans Plants pile up and compress over time to form coal. Where would these form? Swamps – large amount of build up of organic material. Organic Sedimentary Rock : Organic Sedimentary Rock Rocks formed from the altered remains of plant and animal material An exception to the definition of rocks as a mixture of minerals (Remember: Minerals are inorganic substances). Organic material exists in sequences of sedimentary rock to the extent that it cannot be ignored as a true rock type. Organic Sedimentary Rock (Cont) : Organic Sedimentary Rock (Cont) Molds, casts, and other traces of ancient plant and animal life, called fossils, are a common feature of sedimentary rock. Some layers of rock, coal and limestone in particular, are formed, almost, entirely of the remains of living things. Slide 19: Coquina – cemented aggregate of geologically modern shell fragments. Fossiliferous limestone – a cemented aggregate of original shell fragments, molds, and casts of ancient marine organisms. Note fossil mold of a shell in this specimen. Slide 20: Peat, a mass of matted together plant material covered by H2O, which impedes decay. H2 and O2 are lost, concentrating carbon. Lignite, so called “brown coal”, a soft coal that forms when peat is compressed and aged, about 40% carbon. Slide 21: Bituminous coal – soft coal formed when lignite is compacted and altered for millions of years, about 85% carbon, this coal is the most commonly mined and used for a fuel. Sedimentary Rock Features : Sedimentary Rock Features Features in sedimentary rock that reflect the sedimentary environment. Not found in other rock types. Features: Stratification Fossils Ripple marks & crossbeds Mud cracks Nodules, concretions & geodes Slide 23: Rock Stratification (layering) Bryce Canyon, UT Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL Slide 24: Ripple marks caused by wave action on the sandy bottom of a shallow bay Almost identical ripple marks on the surface of a sandstone millions of years old. Slide 25: Mud cracks in drying mud Mud cracks preserved on the bedding surface of a shale. Slide 26: Groundwater dissolves hollow spaces in sedimentary rock, typically limestone, and mineral material is deposited inside the hollow with crystal points growing toward the center. Geode Thunder Egg Slide 27: Cross-bedding at Checkerboard Mesa Zion National Park, UT Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL Slide 28: Dinosaur skeleton preserved in sedimentary rock - China Photo used with permission from Mike Jarvis, Naperville Central HS, Naperville, IL

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