Characteristics of Sedimentary Rocks

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    Characteristics of Sedimentary Rocks

    مُساهمة من طرف GeoMohamad في 2007-07-13, 6:50 am

    Characteristics of Sedimentary Rocks

    Sedimentary
    rocks
    can be categorized into three groups
    based on sediment type. Most sedimentary rocks
    are formed by the lithification of weathered rock
    debris that has been physically transported and
    deposited. During the transport process, the particles
    that make up these rocks often become rounded due
    to abrasion or can become highly sorted. Examples
    of this type of sedimentary rock include conglomerate and sandstone.
    Scientists sometimes call this general group of
    sedimentary rocks clastic.
    The remaining types of sedimentary rocks are created
    either from chemical precipitation and crystallization,
    or by the lithification of once living organic matter. We identify
    these sedimentary rocks as being non-clastic.



    Conglomerate.





    Sandstone.





    All sedimentary rocks are lithified into
    some collective mass. Lithification is
    any process that turns raw rock sediment into consolidated
    sedimentary rock. The process of lithification usually
    produces identifiable layering in these type of rocks
    (Figure 10f-3). Lithification can occur by way
    of:



    • Drying and compaction.
    • Oxidation of iron and aluminum.
    • Precipitation of calcium and silica.







    Figure 10f-3:
    Dipping sedimentary layers of rock, Rocky Mountains,
    Canada.



    The classification of clastic sedimentary
    rocks is based on the particle types found in the rock.
    Some types of clastic sedimentary rocks are composed
    of weathered rock material like gravel, sand, silt, and clay.
    Others can be constructed from the break up and deposition
    of shells, coral and other marine organisms by wave-action
    and ocean currents. Table 10f-1 describes some
    of the main types of clastic sedimentary rocks.


    Clastic
    sedimentary rocks.


    Name
    of Rock

    Fragment
    Type



    Breccia (Image Link)
    Coarse Fragments
    of Angular Gravel and
    Rocks


    Conglomerate
    Coarse Fragments
    of Rounded Gravel and
    Rocks


    Sandstone
    Sand Sized
    Particles that are 90% Quartz


    Arkose
    Sandstone
    composed of 25% Feldspar Grains


    Shale
    Clay Particles


    Siltstone
    Silt Particles


    Mudstone
    Mixture
    of Clay and Silt


    Limestone
    Mixture
    of Shells, Coral, and Other Marine Skeletons




    Arkose.


    Shale.


    ] Siltstone.


    Limestone.



    Earlier it was suggested that there were
    two types of non-clastic sedimentary rocks. One group
    forms through the chemical precipitation and crystallization
    of elements and compounds from solution. Elements such
    as calcium, sodium, potassium, and magnesium are commonly
    released into the environment through a variety of chemical
    weathering
    processes. These elements can then
    become dissolved into aqueous solutions that are often
    transported via runoff, stream
    flow
    , or groundwater flow. If this solution
    enters a basin environment where evaporation exceeds precipitation and
    in-flow, sedimentary evaporites can
    form because of the loss of water from the solution.

    The oceans are almost saturated with dissolved calcium
    carbonate
    . This compound originates from
    the shells of a variety of marine organisms that
    use it for the construction of shells and other hard
    body parts. Because these organisms are surrounded
    in a solution, some of the calcium carbonate dissolves
    into the ocean waters. Under the right circumstances
    the dissolved calcium carbonate can precipitate out
    forming chemically created limestone deposits.
    The formation of dolomite involves
    the chemical modification of limestone deposits by
    a magnesium rich solution.





    Dolomite.



    Name
    of Rock

    Precipitate
    Type



    Halite
    Sodium and Chlorine


    Gypsum
    Calcium, Sulfur, and
    Oxygen


    Silcretes
    Silica


    Ferricretes
    Iron


    Limestone
    Calcium Carbonate


    Dolomite
    Calcium Magnesium Carbonate




    Halite.


    Gypsum.



    Several types of sedimentary rocks are
    formed from the lithification of once living organisms. Limestone deposits
    can be formed by the direct lithification of coral reefs,
    marine organism shells, or marine organism skeletons. Chalk is
    a particular variety of limestone that is composed of
    the skeletons of marine microorganisms like forminifera. Coal and lignite are the lithified
    remains of plants.



    Figure 10f-11Chalk.





    Figure 10f-12Coal.



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