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    Planet earth sedimentary_rock_powerpoint_presentation Planet earth sedimentary_rock_powerpoint_presentation Presentation Transcript

    • Sedimentary Rocks
    • SEDIMENTARY ROCKS Sedimentary rocks form from solidification of sediment (particles from pre-existing rocks, chemical or biologic material) “ Sediment” are particles formed at the earth’s surface. Sediment can form in several ways, giving us three main classes of sedimentary rocks: 1. Detrital (or clastic ) sedimentary rocks form from fragments of older rocks. These pieces may be transported some distance by water, wind, or glacial ice, then deposited, buried, and turned into rock. Examples include sandstone and conglomerate . 2. Chemical sedimentary rocks form from chemical precipitation, generally precipitation from a solution of water and dissolved ions. Examples include rock salt and gypsum . 3. Biogenic sedimentary rocks form from the remains of living things. For example, coal is the altered remains of wood, and most limestone and chalk comes from the shells of marine animals.
    • COQUINA (a form of limestone): an example of a biogenic sedimentary rock. CONGLOMERATE: an example of a detrital sedimentary rock. ROCK SALT: an example of a chemical sedimentary rock.
    • This figure shows how clastic sediment of various sizes will, after compaction and cementation, form different types of detrital sedimentary rocks . The process of sediment turning into rock is called lithification .
    •  
    • Clastic Sedimentary Rocks made from the cementation and lithification of particles (sediments or fragments).
    • Clastic Sedimentary Rocks
      • Clastic sedimentary rocks are composed of two parts  the clasts (or sediment) and cement (glue that holds the clasts together.
      • Examining the individual clasts that make up clastic rocks, we can infer a lot of information.
    • Clastic Sedimentary Rocks
      • Grain Size
      • Individual grains in a sedimentary rock can range from boulder size (256 mm diameter) to clay size (0.0039mm). For our purposes know the following:
      • Boulders, Cobbles and Pebbles
      • Sand (very coarse, medium, fine, very fine)
      • Silt
      • Clay (smallest grain sizes)
      • Boulders>Cobble>Pebble>Sand>Silt>Clay>Dissolved Stuff
      • Grain Size tells the ENERGY involved in transporting material.
    • Clastic Sedimentary Rocks: Grain Size
      • As a general rule, grain size can tell us
      • how much ‘weathering’ of minerals has occurred and
      • How much ENERGY was required to transport and deposit the sediments.
      • Weathering: Individual clasts that are smaller in size most likely have been broken down more than bigger stuff. This could either be due to a long time in transit (as sediments are grinding against other sediments and rocks they become smaller and smaller) or due to weak minerals (the weaker they are, the easier they are to erode into smaller sizes).
      • Grain size also tells information about the VELOCITY and ENERGY of the transporting medium. It takes more energy (a higher velocity stream) to move large pebbles compared to clay or silt. Very fine grained (small grain sizes) clastic rocks indicates a very slow moving (if not stagnant) transporting medium.
    • Clastic Rocks: Color
      • Generally speaking, color MAY be an indicator of the oxygen concentrations in the environment where sediments were deposited.
      • RED sediments typically form in well oxygenated environments involving water (assuming iron is present in sediments). i.e. a running stream or floodplain to the stream. Fe in sediments will rust in the presence of water and oxygen yielding a red color in sediments (recall oxidation in weathering).
    • Clastic Rocks: Color
      • BLACK sediments usually mean the opposite  a lack of oxygen.
      • Stagnant water bodies do not mix with the air at the surface. Dead and decomposing organic matter are acted upon by bacteria which subsequently remove oxygen from the water for respiration.
      • Black color usually represents lots of decomposing organic matter in the sediments.
      • What are some examples of where we might find black sedimentary rocks?
    • Clastic Rocks: Color
      • GREEN sedimentary rocks indicate oxygen somewhere in between. For example, on the continental shelf where in the ocean we have oxygen introduced by wave action at the surface (and by photosynthetic stuff living up there) but we still have animals living in sediments to remove oxygen simultaneously.
    • Clastic Rocks: Color
      • WHITE sediments may lack any iron or water (to be rusted) such as in sand on a beach or wind blown desert sand dunes.
      • We might not be able to indicate oxygen levels through white sediments, however, we can apply the principle of uniformitarianism.
      • Where do we see white sediments accumulate? Sandy beaches, dunes, deserts.
    • Clastic Rocks: Sorting
      • The sorting (poorly to well sorted) of the rock refers to the number of different grain sizes present in the rock.
      • A well sorted rock would contain predominantly one size of clast (i.e. all medium sand).
      • A poorly sorted rock would contain a range of many different grain sizes.
      • Sorting tells us some information:
      • Well sorted sediments occur when water flow slows down gradually (so that each grain size drops out of suspension at different water velocities). Also, wind can pick up small sizes and transport them (i.e. sand) leaving only the large stuff behind.
      • -> For example, imagine water during a flood—stream velocity is the fastest IN the stream channel. Water eventually tops the banks of the stream and flows onto the floodplain. As the water washes on the floodplain it slows down gradually until it stops. What grain size would be deposited closest to the stream channel? Furthest from the stream channel?
    • Clastic Rocks: Sorting
      • Poorly sorted sediments are deposited rapidly without being selectively separated into sizes.
      • Fast moving water (carrying many grain sizes) may abruptly stop, dumping its sediment load. (i.e. alluvial fan).
      • Glacial ice can carry ALL different sizes and when the ice melts, all the sediments are deposited together.
    • Clastic Rocks: Roundness
      • Roundness refers to the degree of rounding of each individual clast. Sphericity is how closely the grain resembles the shape of a sphere.
      • Roundness increases with an increasing length of transportation (recall abrasion from weathering). Clasts smacking into one another during transport tends to increase the roundness of the grains (i.e. sea glass).
      • Clasts with rough edges are considered angular and have been in transport for a much shorter period of time.
      • We call sediments with angular grains IMMATURE as they have been in transit and have not been ‘worked’ a short time.
    • Clastic Rock Examples
      • Poorly Sorted Rocks
      • Conglomerate (well rounded grains)
      • Breccia (angular grains)
      • Although they are both poorly sorted, what does the difference in roundness of the clasts tell us about the history of these rocks?
    • Conglomerate and Breccia: both are poorly sorted (indicative of rapid deposition). The rounded grains in conglomerate indicate long transportation time and the angular grains in breccia, short.
    • Clastic Rock Examples
      • Sandstones
      • Well sorted and well-rounded usually.
    • Sandstone: Larger grain sizes than silt and clay. Shale left, sandstone right.
    • Sandstone in thin section.
    • Layers upon layers of red sandstone; oxygenated environment of deposition.
    • Red sandstone; red indicates oxygen (iron rust) whereas black (shale/right), indicates very LOW oxygen concentrations.
    • Clastic Rock Examples
      • Siltstones and Shales
      • Well sorted and well-rounded usually but very fine grain sizes.
    • Shale (left) and siltstone (right) both are made of fine grained sediment (shale is made of clay, the smallest particles, therefore the finest grains). The smaller the grain size, the slower the water must be moving in order for it to settle out of solution (or the air).
    • Clastic Rock Examples
      • Limestones
      • Limestones are a group of sedimentary rocks made with calcite CaCO 3 (the mineral that reacts with dilute hydrochloric acid.
      • Limestones can either be chemical, detrital or biogenic in origin. In the next few slides, you’ll see some of the clastic varieties of limestone.
      • In clastic limestones, the same rules to clastic rock interpretation apply. You would look at grain size, sorting, roundess, (not color) to determine the history of the sediments.
      • As a general rule, calcium carbonate dissolves readily in COLD water. Where on earth would we expect to find limestone deposits?
      • Calcite is only deposited in MARINE environments (for the most part)—this is because Ca and CO 3 ions are readily available in marine water.
    • Clastic Rock Examples
      • Micrite is analogous to shale (only shale is made from silicate minerals and thus doesn’t react with HCl). Small clasts indicates slow moving water.
      • Coquina is usually made from the shells of clams and snails that live in marine environments. Upon death, the shells become subject to weathering just like any rock. Shells become fragments (clasts) and can be ‘glued’ together with calcite cement.
      • Crystalline LS below forms by CHEMICAL processes.
      • All of these rocks would react with acid.
    • LIMESTONE: Made mostly of calcium carbonate. (Calcite can precipitate from solution, or form by animals). Coquina (left), fossiliferous limestone (right). Fossil LS looks like shale (it does have small grains) but when in doubt, do an ACID test for calcite.
    • Chemical Sedimentary Rocks Rocks that form chemically; precipitates out of solution or water evaporates leaving solutes behind.
    • Chemical Sedimentary Rocks
      • Chemical sedimentary rocks form by directly precipitating out of water or water evaporates leaving the once dissolved minerals behind.
      • Precipitation occurs when there is so much solutes dissolved in water that they just crystallize right there in the water. For example, when making rock candy, you must boil water and continuously dissolve sugar in the water (this supersaturates the solution). Then you let the water cool down to room temperature. The sugar crystals cannot remain dissolved anymore and ‘precipitate’ from the solution. Oftentimes, direct precipitation of minerals out of water is the ‘cement’ that glues clasts together in clastic rocks.
      • Evaporation occurs when water has dissolved stuff in it (i.e. salt). Boil the water so that evaporates—the salts that were dissolved in the water are left behind (as a chemical sedimentary rock).
    • Chemical Sedimentary Rocks
      • Evaporites
    • Rock Salt and Rock Gypsum Evaporites
      • Rock salt contains the mineral halite (NaCl).
      • It forms chemically through evaporation. Salt water stranded on land, in a warm environment, will evaporate leaving the salt beds behind.
      • In a similar situation, fresh water that has been flowing, dissolved materials. If a fresh water body is stranded on land, and evaporates, the silicate mineral rock gypsum will be left behind.
    • Rock gypsum (left) and rock salt (right). Stranded bodies of salt water in warm climates evaporate and leave the salt behind. Gypsum forms in the same way but from fresh water with different elements dissolved in it.
    • Chemical Sedimentary Rocks
      • Precipitates
    • Precipitates
      • Chemical sedimentary rocks that form by precipitation generally develop in the water due to supersaturation.
      • When we discuss the limestone caves (groundwater), the structures (stalactites and stalagmites) are made from dripping water containing calcium ions and carbonate ions.
      • When water drips down into the cave, the water drips down a calcite icicle—precipitating the calcite mineral, layer after layer.
      • Calcite also directly precipitates out of water in warm, shallow marine environments forming crystalline limestone (picture shown in the limestone slides).
    • Biogenic Sedimentary Rocks
      • Form from the remains of a living organism.
      • Note that coquina is a clastic sedimentary rock formed from clasts produced by living organisms (clam and snail shells).
      • Sedimentary rocks, therefore can have a clastic and biogenic origin.
    • Biogenic sedimentary rocks are made by biological organisms. Coal is an example of lots of plant material that becomes pressurized. As pressure increases oxygen, water, hydrogen, nitrogen (etc.) is squeezed from the plant material. (Peat left, lignite right).
    • As other elements are squished out, CARBON is left behind. The more squishing, the more CARBON, the higher grade coal. (Bituminous left, anthracite right). Peat-lignite-bituminous-anthracite are all coal with peat being the least pressurized and anthracite the best quality and highly pressurized coal.
    • Sedimentary Rock Rules
      • Sedimentary rocks are examined to determine the environment in which they were deposited.
      • CLASTIC sedimentary rocks form from lithification/cementation of clasts (pieces of rocks). CHEMICAL rocks form from precipitation or evaporation of water. BIOGENIC sedimentary rocks form with the help of some living organism.
      • In clastic sedimentary rocks:
        • The rounder the grains, the longer the transportation time.
        • The smaller the grains, the longer the transportation time.
        • The smaller the grains, the slower the transport (water/wind).
        • The largest grain indicates the fastest moving water (big grains, fast moving; need a lot of energy to move big stuff).
        • Well sorted means deposition velocity was gradually slowing whereas poorly sorted sediments were essentially ‘dumped’; quick deposition.
        • Color can indicate oxygen content (clastic only). BLACK (low low oxygen), GRAY/GREEN (medium oxygen), RED (high oxygen).