Jump to content


xin tài liệu về điều kiện thành tạo karst và xâm thực dòng sông


  • Bạn không được tạo chủ đề mới
  • Please log in to reply
4 replies to this topic

#1 tearbuttock

tearbuttock

    Newbie

  • Thành viên
  • Pip
  • 2 Bài viết:
  • Joined 21-October 11
  • Reputation: 0
    • Đến từ:đại học mỏ địa chất

    Posted 24 October 2011 - 12:25 PM

    mình đang phải làm tiểu luận mà không có tài liệu các bạn giúp mình với.



    Kính mời các đồng nghiệp xa gần đóng góp cho cuộc thi Chất Động Pangaea lần thứ XIII - năm 2016 Thời gian bắt đầu cuộc thi: 06/03/2016

    Facebook Comments

    #2 ThanhdcB

    ThanhdcB

      Super Member

    • Thành viên
    • PipPipPipPipPip
    • 585 Bài viết:
    • Joined 06-November 08
  • Reputation: 143
    • Yahoo! Status:
    • Gender:Male
    • Đến từ:MDA
    • Interests:Film, Sports, Music...
      http://www.facebook.com/Thanhdcb

    Posted 24 October 2011 - 08:41 PM

    Carbonate Rocks

        

    The carbonate rocks make up 10 to 15% of sedimentary rocks.  They     largely consist of two types of rocks.

        
    • Limestones which are composed mostly of calcite (CaCO3) or         high Mg calcite [(Ca,Mg)CO3], and
              
              

    • Dolostones which are composed mostly of dolomite [CaMg(CO3)2]
              
              

        

    Because carbonate minerals in general are soluble in slightly acidic     waters, they often have high porosity and permeability, making them ideal reservoirs for     petroleum.  For this reason they are well studied.

        

    Limestone can be easily recognized in hand specimen or outcrop because of     its high solubility in HCl.  A drop of such acid placed on the rock will cause it to     fizz due to the generation of CO2 gas. A dolostone, on the other hand,     will not fizz until a fine powder is made from the rock or mineral.  Also, dolostones     tend to weather to a brownish color rock, whereas limestones tend to weather to a white or     gray colored rock.  The brown color of dolostones is due to the fact that Fe occurs     in small amounts replacing some of the Mg in dolomite.

                   Classification
        
    Two classification schemes are in common use by those who work on carbonate     rocks. Although you will use only the Folk classification in lab, you should also     become familiar with the Dunham classification since it is widely used as well.
        

                  
    • Folk Classification- The Folk classification,         which we will use in lab, is shown below. The classification divides carbonates into two         groups.  Allochemical rocks are those that contain grains         brought in from elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical         rocks are those in which the carbonate crystallized in place.  Allochemical rocks         have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts.         These are embedded in a matrix consisting of microcrystalline carbonate (calcite or         dolomite), called micrite, or larger visible crystals of  carbonate, called sparite.         Sparite is clear granular carbonate that has formed through recrystallization of micrite,         or by crystallization within previously existing void spaces during diagenesis.
                      

           Posted Image

        

                      
    • Dunham Classification.  The Dunham classification is based on the         concept of grain support.  The classification divides carbonate rocks into two broad         groups, those whose original components were not bound together during deposition and         those whose original components formed in place and consist of intergrowths of skeletal         material.  The latter group are called boundstones (similar to biolithite of the Folk         classification).  The former group is further subdivided as to whether or not the         grains are mud-supported or grain supported.  If the rock consists of less than 10%         grains it is called a mudstone (potentially confusing if taken out of context).  If         it is mud supported with greater than 10% grains it is called a wackstone.  If the         rock is grain supported, it is called a packstone, if the grains have shapes that allow         for small amounts of mud to occur in the interstices, and a grainstone if there is no mud         between the grains.
                      

           Posted Image

        

                        Textures
        
    Textures of carbonate rocks are extremely variable.  Textures can vary from     those similar to clastic sediments, showing characteristic grain sizes, sorting, and     rounding, to those produced by chemical precipitation.  In carbonates the matrix can     range from fine grained carbonate mud to crystalline calcite or dolomite.  But     carbonates can also show textures derived from the growth of living organisms.  

                  Many limestones (carbonate rocks in general) show characteristics similar     to those of clastic sediments, like sandstones. Sandstones are composed of sand     grains, a mud or clay matrix, and a crystalline cement produced during diagenesis.       Similarly carbonate rocks are composed of allochemical grains (grains produced by     precipitation somewhere else and transported, usually short distances, to the depositional     site), mud matrix, consisting of fine-grained carbonate minerals, and a crystalline cement     of calcite (or dolomite) precipitated during diagenesis.  From the figure shown here,     one can see that the average sandstone and mudrock have similar proportions of analogous     constituents to average sparry (crystalline) limestones and micritic (fine grained     crystalline limestones.  This suggests a similarity of processes involved in the     formation of clastic sediments and carbonate rocks.  
        


         Posted Image              
    • Grains in Carbonate Rocks - The grains that occur in carbonate rocks         are called allochemical particles or allochems.           They are grains often precipitated by organisms that formed elsewhere and         became included         in the carbonate sediment.  Because calcite and aragonite, the main biochemical         precipitates, are soft and soluble in water, the distance of transport is         usually not very far.  Unlike clastic sediments, the degree of rounding and sorting         of the grains may not be a reflection of the energy of the transporting medium, but may be         biologically determined.  For example some organisms produce particles that already         have a rounded shape. If many of the same size organisms die at the same place, then the         grains may be well sorted.  Grains found in carbonate rocks are as follows:
        
      • Whole or broken skeletons of organisms (fossils).  These may range in size from           gravel to fine sand, depending on the organism and the degree to which the grains are           broken by waves or during transport.
                      
      • Ooids.  These are spherical sand sized particles that have a concentric or radial           internal structure.  The central part of each particle consists of a grain of quartz           or other carbonate particle surrounded by thin concentric layers of chemically           precipitated calcite.  The layers or coatings are formed in agitated waters as the           grain rolls around.
      • Peloids.  These are spherical aggregates of microcrystalline calcite of coarse silt           to fine sand size.  Most appear to be fecal pellets from burrowing benthic organisms.            As these organisms burrow through the muddy carbonate-rich sediment, they ingest           material in search of nutritional organic compounds resulting in waste products containing           microcrystalline calcite.  The peloids are much easier seen in thin section than in           hand specimen because of their small size.
      • Limeclasts.  These are fragments of earlier formed limestone or partially lithified           carbonate sediment.  Most are intraclasts, originating           within the basin of deposition.  They may be pieces of partially cemented carbonate           mud that were ripped from the seafloor by storms.  Some appear to be fragments of           partially cemented carbonate mud that originated in intertidal mudflats.  Some may           also be pieces of limestone carried into the basin from nearby carbonate outcrops.
                      
    • Matrix-   The matrix of carbonate rocks consists of either         fine grained carbonate mud, called micrite. Or coarser grained         calcite crystals formed during diagenesis, called sparite.
        
      • The micrite results from recrystallization of carbonate mud during diagenesis or from           direct precipitation of calcite, and causes lithification of the sediment.  The           micrite gives the dull opaque appearance of most limestones as seen in hand specimen. If           the rock consists entirely of fine-grained mud matrix, it implies deposition in a low           energy environment just like in siliclastic mudstones.  Some of the mud may start out           as aragonite needles 5 to 10 mm in length produced by           calcareous algae. But, again this becomes recrystallized to a microspar 5 to 15 mm in diameter during diagenesis.
      • Larger sparry calcite matrix results from diagenesis in the same way that calcite cement           originates in sandstones.
        
    • Insoluble Residues - While minor amounts of clay minerals and quartz         occur in limestones, most of the insoluble residues, (so called because they do not         dissolve in HCl) are grains of nodules of chert.  Such chert mostly originates from         the shells of silica secreting organisms.  These include diatoms, radiolarians, and         some sponges.  Individual grains of chert result from recrystallization of the shells         of these organisms.  Chert nodules can range in size from centimeters         to meters in         length.  Many nodules are concentrated along bedding planes and probably resulted         from dissolution of the siliceous debris and reprecipitation of the microcrystalline         quartz at centers of nucleation located along zones of migration of the fluids, such as         along bedding planes.
                       Structures
        
    Since most limestones are formed by clastic processes, many of the same types of     structures observed in siliciclastic rocks also occur in limestones.  
        

    • Current-Generated Structures.  Structures like cross-bedding,         ripple marks, dunes, graded bedding, and imbricate bedding are common in carbonate rocks,         although they may not be as evident as in siliclastic rocks because of the lack of         contrasting colors of individual beds in carbonates.  Since many shells of organisms         have curved outlines in cross-section (brachipods, pelecypods, ostracods, and trilobites,         especially), when the organism dies it may settle to the bottom with the outline being         concave downward, and latter become filled with carbonate mud.  When such features         occur they can be used as top/bottom indicators.
                      
    • Lamination.  The most common type of lamination in         carbonate rocks is produced by organisms, in particular blue-green algae that grow in the         tidal environment.  These organism grow as filaments and produce mats by trapping and         binding microcrystalline carbonates, as incoming tides sweep over the sand.  This         leads to the formation of laminated layers that consist of layers of organic tissue         interbedded with mud.  In ancient limestones, the organic matter has usually been         removed as a result of decay, leaving cavities in the rock separated by layers of material         that was once mud.  These cavities are called fenestrae (for         a photo, see Figure 16-14, page 305, in Blatt & Tracy).  
              
              Another type of lamination occurs as bulbous structures, termed Stomatolites         (for photo, see Figure 16-15, p. 306, in Blatt & Tracy).  These are produced in a         similar fashion, i.e. by filamentous blue-green algae, but represent mounds rather than         mats.
                      
    • Stylolites.  Stylolites are irregular surfaces that         result from pressure solution of large amounts of carbonate.  In cross-section they         have a saw tooth appearance with the stylolites themselves being made of insoluble         residues or insoluble organic material.  Some studies have suggested that the         stylolites represent anywhere from 25% to as much as 90% of missing rock that has been         dissolved and carried away by dissolution (for a photo, see Figure 16-22, page 314, in         Blatt & Tracy).
              Posted Image               Carbonate Depositional Environments
        Most modern, and probably most ancient, carbonates are predominantly shallow water (depths     <10-20 m) deposits.  This is because the organisms that produce carbonate are     either photosynthetic or require the presence of photosynthetic organisms.  Since     photosynthesis requires light from the Sun, and such light cannot penetrate to great     depths in the oceans, the organisms thrive only at shallow depths.  Furthermore,     carbonate deposition in general only occurs in environments where there is a lack of     siliciclastic input into the water.  Siliclastic input increases the turbidity of the     water and prevents light from penetrating, and silicate minerals have a hardness much     greater than carbonate minerals, and would tend to mechanically abrade the     carbonates.  Most carbonate deposition also requires relatively warm waters which     also enhance the abundance of carbonate secreting organisms and decrease the solubility of     calcium carbonate in seawater.  Nevertheless, carbonate rocks form in the deep ocean     basins and in colder environments if other conditions are right.
        
        For this course, our discussion of carbonate depositional environments will be brief.       See your text for more detailed discussion.  The principal carbonate     depositional environments are as follows:

    • Carbonate Platforms and Shelves.  Warm shallow seas attached the         continents, or in the case of epiric seas, partially covering the continents, are ideal         places for carbonate deposition.  Other shelves occur surrounding oceanic islands         after volcanism has ceased and the island has been eroded (these are called atolls).          Carbonate platforms are buildups of carbonate rocks in the deeper parts of the oceans on         top of continental blocks left behind during continent - continent separation.
              
              Reef building organisms from the framework of most of these carbonate buildups.
    • Tidal Flats. Tidal flats are areas that flood during high tides and are         exposed during low tides.  Carbonate sands carried in by the tides are cemented         together by carbonate secreting organisms, forming algal mats and stromatolites.
    • Deep Ocean.  Carbonate deposition can only occur in the shallower         parts of the deep ocean unless organic productivity is so high that the remains of         organisms are quickly buried.  This is because at depths between 3,000 and 5,000 m         (largely dependent on latitude - deeper near the equator and shallower nearer the poles)         in the deep oceans the rate of dissolution of carbonate is so high and the water so         undersaturated with respect to calcium carbonate, that carbonates cannot accumulate. This         depth is called the carbonate compensation depth (CCD).         The main type of carbonate deposition in the deep oceans consists of the accumulation of         the remains of planktonic foraminifera to form a carbonate ooze.  Upon burial, this         ooze undergoes diagenetic recrystallization to form micritic limestones.  Since         most oceanic ridges are at a depth shallower than the CCD, carbonate oozes can         accumulate on the flanks of the ridges and can be buried as the oceanic crust moves away         from the ridge to deeper levels in the ocean. Since most oceanic crust and overlying         sediment are eventually subducted, the preservation of such deep sea carbonates in the         geologic record is rare, although some have been identified in areas where sediment has         been scraped off the top of the subducting oceanic crust and added to the continents, such         as in the Franciscan Formation of Jurassic age in California.
    • Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as         a result of evaporation, in which case the carbonates are associated with other evaporite         deposits, and as a result of organisms that remove CO2 from the water causing         it to become oversaturated with respect to calcite.
    • Hot Springs. When hot water saturated with calcium carbonate reaches         the surface of the Earth at hot springs, the water evaporates and cools resulting in the         precipitation of calcite to form a type of limestone called travertine.
                       Dolostones
        
    Dolostones are carbonate rocks composed almost entirely of dolomite - (Ca,Mg)CO3.       Although there used to be a common perception that the abundance of     dolostones increased with age of the rock, it is now recognized that although no primary     dolomite bearing rocks are being directly precipitated in modern times, dolostones have     formed throughout geologic time. This is true despite the fact that modern sea water is     saturated with respect to dolomite. Still, most dolostones appear to result from     diagenetic conversion of calcite or high-Mg calcite to dolomite, after primary deposition     of the original calcium carbonate bearing minerals.
        
        Dolomite, and therefore rocks containing large amounts of dolomite, like dolostones, is     easily distinguished by the fact that it only fizzes in dilute HCl if broken down to a     fine powder. Also, dolostones tend to weather to a brownish color rock, whereas     limestones tend to weather to a white or gray colored rock.  The brown color of     dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in     dolomite.In thin section it is more difficult to distinguish from calcite, unless it is     twined.  Unfortunately, sedimentary dolomite is rarely twinned.  In order to     facilitate its identification in thin section, the sections are often stained with     alizarin red S.  This turns calcite pink, but leaves the dolomite unstained.

         Dolostones are almost entirely composed of euhedral and subhedral rhombs of     dolomite.  Although dolostones contain allochems, like limestones, the allochems are     generally recrystallized to dolomite, and rhombs of dolomite can be seen to cut across the     boundaries of allochemical particles.  Some dolostones show no evidence of allochems,     but only contain rhombs of dolomite.  These could either represent limestones that     have completely recrystallized to dolomite, leaving no trace of the original fragments     that made up the limestone, or could represent primary crystals of dolomite.  

         Two mechanisms of dolomitization of limestones have been proposed based on field     and laboratory studies.

    • Evaporative Reflux.  This mechanism involves the evaporation of         seawater to form a brine that precipitates gypsum.  After precipitation of gypsum,         the brine is both enriched in Mg relative to Ca and has a higher density.  If the         brine then enters the groundwater system and moves downward into buried limestones.           This Mg-rich brine then reacts with the calcite in the limestone to produce         dolomite.
    • Mixing of Seawater and Meteoric Water.  This mechanism involves         the mixing of groundwater derived from the surface with saline groundwater beneath the         oceans.  Dolomitization is thought to occur where the two groundwater compositions         mix with each in the porous and permeable limestone within a few meters of the surface.          
              
                      

        

    Other Sedimentary Rocks

        

    Evaporites
        
    Evaporite minerals are those minerals produced by extensive or total evaporation     of a saline solution.  Because such minerals dissolve readily in less saline rich     solutions, like most groundwater and surface water, evaporite rocks rarely outcrop at the     surface except in aid regions.  Evaporite rocks are common, however, in the     subsurface.  Three different environments result in the deposition of evaporites.
        

                  
    • Basins of internal drainage. In arid regions with basins of internal         drainage rainfall in the adjacent areas is carried into the basin by ephemeral streams         carrying water and dissolved ions.  The water fills the low points in the basin to         form a playa lake.  These lakes eventually evaporate,         resulting in the precipitation of salts such as halite, gypsum, anhydrite, and a variety         of other salts not commonly found in marine evaporite deposits, such as trona (NaHCO3.Na2CO3.2H2O),         natron (Na2CO3.10H2O), nahcolite (NaHCO3),         mirabilite (Na2SO4.10H2O), borax (Na2B4O5(OH)4.8H2O),         kernite [Na2B4O6(OH)2.3H2O], and         colemanite (CaB3O4(OH)3.H2O).
              Posted Image              
    • Restricted bays or seas. In areas where there restricted input of fresh         or marine waters into a basin, coupled with extensive evaporation within the basin,         dissolved ion concentrations may increase to the point where form a dense concentrated         solution is formed near the surface.
              Posted Image              
      These dense saline waters then sink within the basin, become oversaturated with respect       to salts like gypsum and halite, and precipitate the salts on the floor of the basin.
            
                      
    • Shallow arid coasts or sabkhas.  Along         shallow arid coastlines where input of fresh water is rare and evaporation increases the         salinity of the marine water, evaporation may increase the salinity of the water to a         point where evaporite minerals like halite and gypsum are precipitated.
              Posted Image              
          Cherts
        
    Chert is a mineralogically simple rock consisting of microcrystalline quartz.       There are three common occurrences of chert.    
    • As nodules and silt-sized grains in carbonate rocks.          Chert nodules, as discussed previously, occur as structureless dense masses within         carbonate rocks.  They range in size from a few centimeters to many meters in         length.  The source of silica is likely silica secreting organisms that include         diatoms (Jurassic to Holocene), radiolaria (Ordovician to Holocene), and sponges (Cambrian         to Holocene).   But, these organisms are not preserved in the chert nodules.          Instead, the remains of these organisms were likely dissolved by fluids flowing through         the rock during diagenesis.  Most chert nodules are found along bedding planes in the         carbonate rocks, likely because these were zones along which fluids that precipitated the         microcrystalline quartz were able to move.          

                            
    • As bedded cherts that formed along tectonically active         continental margins. Bedded cherts occur in association turbidites, ophiolites,         and mélanges (oceanic trench deposits scraped off the seafloor at subduction         zones).  The beds range in thickness from a few centimeters to several meters, and         are interbedded with siliceous shales.  Although thought to represent deep water         accumulations of silica secreting organisms, they may also form in warm nutrient rich         shallow water environments.  Sometimes the remains of silica secreting organisms,         like radiolaria, sponge spicules, or diatoms are preserved in the cherts, but most show a         microcrystalline texture that results from recrystallization during diagenesis.  
              
              

    • Associated with hypersaline-lacustrine deposits.  Although less         common than the previously discussed occurrences of chert, some cherts appear to form in a         hypersaline environment where they are associated with evaporite deposits.  Such         cherts may in fact form as a result of replacement of sodium silicate evaporite minerals         like magadiite by the following chemical reaction:

        

          

            

    NaSi7O13(OH)3.3H2O         => 7SiO2 + 4H2O + Na+ + OH-
             magadiite                                                     quartz

          

        

        

        

    Since mechanisms 1 and 2 generally require the presence of silica     secreting organisms in order to form chert, the occurrence of chert in Precambrian rocks     is problematical because no such organisms existed prior to the early Paleozoic.       Such Precambrian cherts may have actually formed by direct chemical precipitation from     silica oversaturated seawater.


    Posted Image

    #3 ThanhdcB

    ThanhdcB

      Super Member

    • Thành viên
    • PipPipPipPipPip
    • 585 Bài viết:
    • Joined 06-November 08
  • Reputation: 143
    • Yahoo! Status:
    • Gender:Male
    • Đến từ:MDA
    • Interests:Film, Sports, Music...
      http://www.facebook.com/Thanhdcb

    Posted 24 October 2011 - 08:47 PM

    Bạn thử tìm trên mạng thêm về tài liệu geohazard in karst areas ... xem sao, tài liệu về các vùng đồng văn - hà giang; quảng ninh; vịnh hạ long trên cạn _NB và vùng quảng bình với rất nhiều sông và hang động ngầm xem
    good luck!
    Posted Image

    #4 tearbuttock

    tearbuttock

      Newbie

    • Thành viên
    • Pip
    • 2 Bài viết:
    • Joined 21-October 11
  • Reputation: 0
    • Đến từ:đại học mỏ địa chất

    Posted 24 October 2011 - 08:55 PM

    View PostThanhdcB, on 24 October 2011 - 08:41 PM, said:

    Carbonate Rocks

    The carbonate rocks make up 10 to 15% of sedimentary rocks.  They largely consist of two types of rocks.

    • Limestones which are composed mostly of calcite (CaCO3) or     high Mg calcite [(Ca,Mg)CO3], and
              
              

    • Dolostones which are composed mostly of dolomite [CaMg(CO3)2]
              
              

    Because carbonate minerals in general are soluble in slightly acidic waters, they often have high porosity and permeability, making them ideal reservoirs for petroleum.  For this reason they are well studied.

    Limestone can be easily recognized in hand specimen or outcrop because of its high solubility in HCl.  A drop of such acid placed on the rock will cause it to fizz due to the generation of CO2 gas. A dolostone, on the other hand, will not fizz until a fine powder is made from the rock or mineral.  Also, dolostones tend to weather to a brownish color rock, whereas limestones tend to weather to a white or gray colored rock.  The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite.

               Classification
        
    Two classification schemes are in common use by those who work on carbonate rocks. Although you will use only the Folk classification in lab, you should also become familiar with the Dunham classification since it is widely used as well.
        

                  
    • Folk Classification- The Folk classification,     which we will use in lab, is shown below. The classification divides carbonates into two     groups.  Allochemical rocks are those that contain grains     brought in from elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical     rocks are those in which the carbonate crystallized in place.  Allochemical rocks     have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts.     These are embedded in a matrix consisting of microcrystalline carbonate (calcite or     dolomite), called micrite, or larger visible crystals of  carbonate, called sparite.     Sparite is clear granular carbonate that has formed through recrystallization of micrite,     or by crystallization within previously existing void spaces during diagenesis.
                  

       Posted Image

                  
    • Dunham Classification.  The Dunham classification is based on the     concept of grain support.  The classification divides carbonate rocks into two broad     groups, those whose original components were not bound together during deposition and     those whose original components formed in place and consist of intergrowths of skeletal     material.  The latter group are called boundstones (similar to biolithite of the Folk     classification).  The former group is further subdivided as to whether or not the     grains are mud-supported or grain supported.  If the rock consists of less than 10%     grains it is called a mudstone (potentially confusing if taken out of context).  If     it is mud supported with greater than 10% grains it is called a wackstone.  If the     rock is grain supported, it is called a packstone, if the grains have shapes that allow     for small amounts of mud to occur in the interstices, and a grainstone if there is no mud     between the grains.
                  

       Posted Image

                        Textures
        
    Textures of carbonate rocks are extremely variable.  Textures can vary from those similar to clastic sediments, showing characteristic grain sizes, sorting, and rounding, to those produced by chemical precipitation.  In carbonates the matrix can range from fine grained carbonate mud to crystalline calcite or dolomite.  But carbonates can also show textures derived from the growth of living organisms.  

                  Many limestones (carbonate rocks in general) show characteristics similar to those of clastic sediments, like sandstones. Sandstones are composed of sand grains, a mud or clay matrix, and a crystalline cement produced during diagenesis.   Similarly carbonate rocks are composed of allochemical grains (grains produced by precipitation somewhere else and transported, usually short distances, to the depositional site), mud matrix, consisting of fine-grained carbonate minerals, and a crystalline cement of calcite (or dolomite) precipitated during diagenesis.  From the figure shown here, one can see that the average sandstone and mudrock have similar proportions of analogous constituents to average sparry (crystalline) limestones and micritic (fine grained crystalline limestones.  This suggests a similarity of processes involved in the formation of clastic sediments and carbonate rocks.  
        


    Posted Image              
    • Grains in Carbonate Rocks - The grains that occur in carbonate rocks     are called allochemical particles or allochems.       They are grains often precipitated by organisms that formed elsewhere and     became included     in the carbonate sediment.  Because calcite and aragonite, the main biochemical     precipitates, are soft and soluble in water, the distance of transport is     usually not very far.  Unlike clastic sediments, the degree of rounding and sorting     of the grains may not be a reflection of the energy of the transporting medium, but may be     biologically determined.  For example some organisms produce particles that already     have a rounded shape. If many of the same size organisms die at the same place, then the     grains may be well sorted.  Grains found in carbonate rocks are as follows:
      • Whole or broken skeletons of organisms (fossils).  These may range in size from       gravel to fine sand, depending on the organism and the degree to which the grains are       broken by waves or during transport.
                  
      • Ooids.  These are spherical sand sized particles that have a concentric or radial       internal structure.  The central part of each particle consists of a grain of quartz       or other carbonate particle surrounded by thin concentric layers of chemically       precipitated calcite.  The layers or coatings are formed in agitated waters as the       grain rolls around.
      • Peloids.  These are spherical aggregates of microcrystalline calcite of coarse silt       to fine sand size.  Most appear to be fecal pellets from burrowing benthic organisms.            As these organisms burrow through the muddy carbonate-rich sediment, they ingest       material in search of nutritional organic compounds resulting in waste products containing       microcrystalline calcite.  The peloids are much easier seen in thin section than in       hand specimen because of their small size.
      • Limeclasts.  These are fragments of earlier formed limestone or partially lithified       carbonate sediment.  Most are intraclasts, originating       within the basin of deposition.  They may be pieces of partially cemented carbonate       mud that were ripped from the seafloor by storms.  Some appear to be fragments of       partially cemented carbonate mud that originated in intertidal mudflats.  Some may       also be pieces of limestone carried into the basin from nearby carbonate outcrops.
                  
    • Matrix-   The matrix of carbonate rocks consists of either     fine grained carbonate mud, called micrite. Or coarser grained     calcite crystals formed during diagenesis, called sparite.
      • The micrite results from recrystallization of carbonate mud during diagenesis or from       direct precipitation of calcite, and causes lithification of the sediment.  The       micrite gives the dull opaque appearance of most limestones as seen in hand specimen. If       the rock consists entirely of fine-grained mud matrix, it implies deposition in a low       energy environment just like in siliclastic mudstones.  Some of the mud may start out       as aragonite needles 5 to 10 mm in length produced by       calcareous algae. But, again this becomes recrystallized to a microspar 5 to 15 mm in diameter during diagenesis.
      • Larger sparry calcite matrix results from diagenesis in the same way that calcite cement       originates in sandstones.
    • Insoluble Residues - While minor amounts of clay minerals and quartz     occur in limestones, most of the insoluble residues, (so called because they do not     dissolve in HCl) are grains of nodules of chert.  Such chert mostly originates from     the shells of silica secreting organisms.  These include diatoms, radiolarians, and     some sponges.  Individual grains of chert result from recrystallization of the shells     of these organisms.  Chert nodules can range in size from centimeters     to meters in     length.  Many nodules are concentrated along bedding planes and probably resulted     from dissolution of the siliceous debris and reprecipitation of the microcrystalline     quartz at centers of nucleation located along zones of migration of the fluids, such as     along bedding planes.
                   Structures
        
    Since most limestones are formed by clastic processes, many of the same types of structures observed in siliciclastic rocks also occur in limestones.  
        

    • Current-Generated Structures.  Structures like cross-bedding,     ripple marks, dunes, graded bedding, and imbricate bedding are common in carbonate rocks,     although they may not be as evident as in siliclastic rocks because of the lack of     contrasting colors of individual beds in carbonates.  Since many shells of organisms     have curved outlines in cross-section (brachipods, pelecypods, ostracods, and trilobites,     especially), when the organism dies it may settle to the bottom with the outline being     concave downward, and latter become filled with carbonate mud.  When such features     occur they can be used as top/bottom indicators.
                  
    • Lamination.  The most common type of lamination in     carbonate rocks is produced by organisms, in particular blue-green algae that grow in the     tidal environment.  These organism grow as filaments and produce mats by trapping and     binding microcrystalline carbonates, as incoming tides sweep over the sand.  This     leads to the formation of laminated layers that consist of layers of organic tissue     interbedded with mud.  In ancient limestones, the organic matter has usually been     removed as a result of decay, leaving cavities in the rock separated by layers of material     that was once mud.  These cavities are called fenestrae (for     a photo, see Figure 16-14, page 305, in Blatt & Tracy).  
              
              Another type of lamination occurs as bulbous structures, termed Stomatolites     (for photo, see Figure 16-15, p. 306, in Blatt & Tracy).  These are produced in a     similar fashion, i.e. by filamentous blue-green algae, but represent mounds rather than     mats.
                  
    • Stylolites.  Stylolites are irregular surfaces that     result from pressure solution of large amounts of carbonate.  In cross-section they     have a saw tooth appearance with the stylolites themselves being made of insoluble     residues or insoluble organic material.  Some studies have suggested that the     stylolites represent anywhere from 25% to as much as 90% of missing rock that has been     dissolved and carried away by dissolution (for a photo, see Figure 16-22, page 314, in     Blatt & Tracy).
              Posted Image           Carbonate Depositional Environments
        Most modern, and probably most ancient, carbonates are predominantly shallow water (depths <10-20 m) deposits.  This is because the organisms that produce carbonate are either photosynthetic or require the presence of photosynthetic organisms.  Since photosynthesis requires light from the Sun, and such light cannot penetrate to great depths in the oceans, the organisms thrive only at shallow depths.  Furthermore, carbonate deposition in general only occurs in environments where there is a lack of siliciclastic input into the water.  Siliclastic input increases the turbidity of the water and prevents light from penetrating, and silicate minerals have a hardness much greater than carbonate minerals, and would tend to mechanically abrade the carbonates.  Most carbonate deposition also requires relatively warm waters which also enhance the abundance of carbonate secreting organisms and decrease the solubility of calcium carbonate in seawater.  Nevertheless, carbonate rocks form in the deep ocean basins and in colder environments if other conditions are right.
        
        For this course, our discussion of carbonate depositional environments will be brief.   See your text for more detailed discussion.  The principal carbonate depositional environments are as follows:

    • Carbonate Platforms and Shelves.  Warm shallow seas attached the     continents, or in the case of epiric seas, partially covering the continents, are ideal     places for carbonate deposition.  Other shelves occur surrounding oceanic islands     after volcanism has ceased and the island has been eroded (these are called atolls).          Carbonate platforms are buildups of carbonate rocks in the deeper parts of the oceans on     top of continental blocks left behind during continent - continent separation.
              
              Reef building organisms from the framework of most of these carbonate buildups.
    • Tidal Flats. Tidal flats are areas that flood during high tides and are     exposed during low tides.  Carbonate sands carried in by the tides are cemented     together by carbonate secreting organisms, forming algal mats and stromatolites.
    • Deep Ocean.  Carbonate deposition can only occur in the shallower     parts of the deep ocean unless organic productivity is so high that the remains of     organisms are quickly buried.  This is because at depths between 3,000 and 5,000 m     (largely dependent on latitude - deeper near the equator and shallower nearer the poles)     in the deep oceans the rate of dissolution of carbonate is so high and the water so     undersaturated with respect to calcium carbonate, that carbonates cannot accumulate. This     depth is called the carbonate compensation depth (CCD).     The main type of carbonate deposition in the deep oceans consists of the accumulation of     the remains of planktonic foraminifera to form a carbonate ooze.  Upon burial, this     ooze undergoes diagenetic recrystallization to form micritic limestones.  Since     most oceanic ridges are at a depth shallower than the CCD, carbonate oozes can     accumulate on the flanks of the ridges and can be buried as the oceanic crust moves away     from the ridge to deeper levels in the ocean. Since most oceanic crust and overlying     sediment are eventually subducted, the preservation of such deep sea carbonates in the     geologic record is rare, although some have been identified in areas where sediment has     been scraped off the top of the subducting oceanic crust and added to the continents, such     as in the Franciscan Formation of Jurassic age in California.
    • Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as     a result of evaporation, in which case the carbonates are associated with other evaporite     deposits, and as a result of organisms that remove CO2 from the water causing     it to become oversaturated with respect to calcite.
    • Hot Springs. When hot water saturated with calcium carbonate reaches     the surface of the Earth at hot springs, the water evaporates and cools resulting in the     precipitation of calcite to form a type of limestone called travertine.
                   Dolostones
        
    Dolostones are carbonate rocks composed almost entirely of dolomite - (Ca,Mg)CO3.   Although there used to be a common perception that the abundance of dolostones increased with age of the rock, it is now recognized that although no primary dolomite bearing rocks are being directly precipitated in modern times, dolostones have formed throughout geologic time. This is true despite the fact that modern sea water is saturated with respect to dolomite. Still, most dolostones appear to result from diagenetic conversion of calcite or high-Mg calcite to dolomite, after primary deposition of the original calcium carbonate bearing minerals.
        
        Dolomite, and therefore rocks containing large amounts of dolomite, like dolostones, is easily distinguished by the fact that it only fizzes in dilute HCl if broken down to a fine powder. Also, dolostones tend to weather to a brownish color rock, whereas limestones tend to weather to a white or gray colored rock.  The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite.In thin section it is more difficult to distinguish from calcite, unless it is twined.  Unfortunately, sedimentary dolomite is rarely twinned.  In order to facilitate its identification in thin section, the sections are often stained with alizarin red S.  This turns calcite pink, but leaves the dolomite unstained.

    Dolostones are almost entirely composed of euhedral and subhedral rhombs of dolomite.  Although dolostones contain allochems, like limestones, the allochems are generally recrystallized to dolomite, and rhombs of dolomite can be seen to cut across the boundaries of allochemical particles.  Some dolostones show no evidence of allochems, but only contain rhombs of dolomite.  These could either represent limestones that have completely recrystallized to dolomite, leaving no trace of the original fragments that made up the limestone, or could represent primary crystals of dolomite.  

    Two mechanisms of dolomitization of limestones have been proposed based on field and laboratory studies.

    • Evaporative Reflux.  This mechanism involves the evaporation of     seawater to form a brine that precipitates gypsum.  After precipitation of gypsum,     the brine is both enriched in Mg relative to Ca and has a higher density.  If the     brine then enters the groundwater system and moves downward into buried limestones.       This Mg-rich brine then reacts with the calcite in the limestone to produce     dolomite.
    • Mixing of Seawater and Meteoric Water.  This mechanism involves     the mixing of groundwater derived from the surface with saline groundwater beneath the     oceans.  Dolomitization is thought to occur where the two groundwater compositions     mix with each in the porous and permeable limestone within a few meters of the surface.      
              
                  

    Other Sedimentary Rocks

    Evaporites
        
    Evaporite minerals are those minerals produced by extensive or total evaporation of a saline solution.  Because such minerals dissolve readily in less saline rich solutions, like most groundwater and surface water, evaporite rocks rarely outcrop at the surface except in aid regions.  Evaporite rocks are common, however, in the subsurface.  Three different environments result in the deposition of evaporites.
        

                  
    • Basins of internal drainage. In arid regions with basins of internal     drainage rainfall in the adjacent areas is carried into the basin by ephemeral streams     carrying water and dissolved ions.  The water fills the low points in the basin to     form a playa lake.  These lakes eventually evaporate,     resulting in the precipitation of salts such as halite, gypsum, anhydrite, and a variety     of other salts not commonly found in marine evaporite deposits, such as trona (NaHCO3.Na2CO3.2H2O),     natron (Na2CO3.10H2O), nahcolite (NaHCO3),     mirabilite (Na2SO4.10H2O), borax (Na2B4O5(OH)4.8H2O),     kernite [Na2B4O6(OH)2.3H2O], and     colemanite (CaB3O4(OH)3.H2O).
              Posted Image              
    • Restricted bays or seas. In areas where there restricted input of fresh     or marine waters into a basin, coupled with extensive evaporation within the basin,     dissolved ion concentrations may increase to the point where form a dense concentrated     solution is formed near the surface.
              Posted Image              
      These dense saline waters then sink within the basin, become oversaturated with respect   to salts like gypsum and halite, and precipitate the salts on the floor of the basin.
                  
    • Shallow arid coasts or sabkhas.  Along     shallow arid coastlines where input of fresh water is rare and evaporation increases the     salinity of the marine water, evaporation may increase the salinity of the water to a     point where evaporite minerals like halite and gypsum are precipitated.
              Posted Image              
          Cherts
        
    Chert is a mineralogically simple rock consisting of microcrystalline quartz.   There are three common occurrences of chert.
    • As nodules and silt-sized grains in carbonate rocks.          Chert nodules, as discussed previously, occur as structureless dense masses within     carbonate rocks.  They range in size from a few centimeters to many meters in     length.  The source of silica is likely silica secreting organisms that include     diatoms (Jurassic to Holocene), radiolaria (Ordovician to Holocene), and sponges (Cambrian     to Holocene).   But, these organisms are not preserved in the chert nodules.          Instead, the remains of these organisms were likely dissolved by fluids flowing through     the rock during diagenesis.  Most chert nodules are found along bedding planes in the     carbonate rocks, likely because these were zones along which fluids that precipitated the     microcrystalline quartz were able to move.          

                            
    • As bedded cherts that formed along tectonically active     continental margins. Bedded cherts occur in association turbidites, ophiolites,     and mélanges (oceanic trench deposits scraped off the seafloor at subduction     zones).  The beds range in thickness from a few centimeters to several meters, and     are interbedded with siliceous shales.  Although thought to represent deep water     accumulations of silica secreting organisms, they may also form in warm nutrient rich     shallow water environments.  Sometimes the remains of silica secreting organisms,     like radiolaria, sponge spicules, or diatoms are preserved in the cherts, but most show a     microcrystalline texture that results from recrystallization during diagenesis.  
              
              

    • Associated with hypersaline-lacustrine deposits.  Although less     common than the previously discussed occurrences of chert, some cherts appear to form in a     hypersaline environment where they are associated with evaporite deposits.  Such     cherts may in fact form as a result of replacement of sodium silicate evaporite minerals     like magadiite by the following chemical reaction:

      

        

    NaSi7O13(OH)3.3H2O     => 7SiO2 + 4H2O + Na+ + OH-
             magadiite                                                 quartz

      

    Since mechanisms 1 and 2 generally require the presence of silica secreting organisms in order to form chert, the occurrence of chert in Precambrian rocks is problematical because no such organisms existed prior to the early Paleozoic.   Such Precambrian cherts may have actually formed by direct chemical precipitation from silica oversaturated seawater.



    ối mẹ ơi

    #5 jolly_9x

    jolly_9x

      Intermediate Member

    • Thành viên
    • PipPipPip
    • 173 Bài viết:
    • Joined 14-April 10
  • Reputation: 105
    • Yahoo! Status:
    • Gender:Male
    • Đến từ:Ha Noi
    • Interests:Hello Every Ones

    Posted 24 October 2011 - 10:28 PM

    View PostThanhdcB, on 24 October 2011 - 08:41 PM, said:

    Carbonate Rocks

    The carbonate rocks make up 10 to 15% of sedimentary rocks.  They largely consist of two types of rocks.

    • Limestones which are composed mostly of calcite (CaCO3) or     high Mg calcite [(Ca,Mg)CO3], and
              
              

    • Dolostones which are composed mostly of dolomite [CaMg(CO3)2]
              
              

    Because carbonate minerals in general are soluble in slightly acidic waters, they often have high porosity and permeability, making them ideal reservoirs for petroleum.  For this reason they are well studied.

    Limestone can be easily recognized in hand specimen or outcrop because of its high solubility in HCl.  A drop of such acid placed on the rock will cause it to fizz due to the generation of CO2 gas. A dolostone, on the other hand, will not fizz until a fine powder is made from the rock or mineral.  Also, dolostones tend to weather to a brownish color rock, whereas limestones tend to weather to a white or gray colored rock.  The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite.

               Classification
        
    Two classification schemes are in common use by those who work on carbonate rocks. Although you will use only the Folk classification in lab, you should also become familiar with the Dunham classification since it is widely used as well.
        

                  
    • Folk Classification- The Folk classification,     which we will use in lab, is shown below. The classification divides carbonates into two     groups.  Allochemical rocks are those that contain grains     brought in from elsewhere (i.e. similar to detrital grains in clastic rocks). Orthochemical     rocks are those in which the carbonate crystallized in place.  Allochemical rocks     have grains that may consist of fossiliferous material, ooids, peloids, or intraclasts.     These are embedded in a matrix consisting of microcrystalline carbonate (calcite or     dolomite), called micrite, or larger visible crystals of  carbonate, called sparite.     Sparite is clear granular carbonate that has formed through recrystallization of micrite,     or by crystallization within previously existing void spaces during diagenesis.
                  

       Posted Image

                  
    • Dunham Classification.  The Dunham classification is based on the     concept of grain support.  The classification divides carbonate rocks into two broad     groups, those whose original components were not bound together during deposition and     those whose original components formed in place and consist of intergrowths of skeletal     material.  The latter group are called boundstones (similar to biolithite of the Folk     classification).  The former group is further subdivided as to whether or not the     grains are mud-supported or grain supported.  If the rock consists of less than 10%     grains it is called a mudstone (potentially confusing if taken out of context).  If     it is mud supported with greater than 10% grains it is called a wackstone.  If the     rock is grain supported, it is called a packstone, if the grains have shapes that allow     for small amounts of mud to occur in the interstices, and a grainstone if there is no mud     between the grains.
                  

       Posted Image

                        Textures
        
    Textures of carbonate rocks are extremely variable.  Textures can vary from those similar to clastic sediments, showing characteristic grain sizes, sorting, and rounding, to those produced by chemical precipitation.  In carbonates the matrix can range from fine grained carbonate mud to crystalline calcite or dolomite.  But carbonates can also show textures derived from the growth of living organisms.  

                  Many limestones (carbonate rocks in general) show characteristics similar to those of clastic sediments, like sandstones. Sandstones are composed of sand grains, a mud or clay matrix, and a crystalline cement produced during diagenesis.   Similarly carbonate rocks are composed of allochemical grains (grains produced by precipitation somewhere else and transported, usually short distances, to the depositional site), mud matrix, consisting of fine-grained carbonate minerals, and a crystalline cement of calcite (or dolomite) precipitated during diagenesis.  From the figure shown here, one can see that the average sandstone and mudrock have similar proportions of analogous constituents to average sparry (crystalline) limestones and micritic (fine grained crystalline limestones.  This suggests a similarity of processes involved in the formation of clastic sediments and carbonate rocks.  
        


    Posted Image              
    • Grains in Carbonate Rocks - The grains that occur in carbonate rocks     are called allochemical particles or allochems.       They are grains often precipitated by organisms that formed elsewhere and     became included     in the carbonate sediment.  Because calcite and aragonite, the main biochemical     precipitates, are soft and soluble in water, the distance of transport is     usually not very far.  Unlike clastic sediments, the degree of rounding and sorting     of the grains may not be a reflection of the energy of the transporting medium, but may be     biologically determined.  For example some organisms produce particles that already     have a rounded shape. If many of the same size organisms die at the same place, then the     grains may be well sorted.  Grains found in carbonate rocks are as follows:
      • Whole or broken skeletons of organisms (fossils).  These may range in size from       gravel to fine sand, depending on the organism and the degree to which the grains are       broken by waves or during transport.
                  
      • Ooids.  These are spherical sand sized particles that have a concentric or radial       internal structure.  The central part of each particle consists of a grain of quartz       or other carbonate particle surrounded by thin concentric layers of chemically       precipitated calcite.  The layers or coatings are formed in agitated waters as the       grain rolls around.
      • Peloids.  These are spherical aggregates of microcrystalline calcite of coarse silt       to fine sand size.  Most appear to be fecal pellets from burrowing benthic organisms.            As these organisms burrow through the muddy carbonate-rich sediment, they ingest       material in search of nutritional organic compounds resulting in waste products containing       microcrystalline calcite.  The peloids are much easier seen in thin section than in       hand specimen because of their small size.
      • Limeclasts.  These are fragments of earlier formed limestone or partially lithified       carbonate sediment.  Most are intraclasts, originating       within the basin of deposition.  They may be pieces of partially cemented carbonate       mud that were ripped from the seafloor by storms.  Some appear to be fragments of       partially cemented carbonate mud that originated in intertidal mudflats.  Some may       also be pieces of limestone carried into the basin from nearby carbonate outcrops.
                  
    • Matrix-   The matrix of carbonate rocks consists of either     fine grained carbonate mud, called micrite. Or coarser grained     calcite crystals formed during diagenesis, called sparite.
      • The micrite results from recrystallization of carbonate mud during diagenesis or from       direct precipitation of calcite, and causes lithification of the sediment.  The       micrite gives the dull opaque appearance of most limestones as seen in hand specimen. If       the rock consists entirely of fine-grained mud matrix, it implies deposition in a low       energy environment just like in siliclastic mudstones.  Some of the mud may start out       as aragonite needles 5 to 10 mm in length produced by       calcareous algae. But, again this becomes recrystallized to a microspar 5 to 15 mm in diameter during diagenesis.
      • Larger sparry calcite matrix results from diagenesis in the same way that calcite cement       originates in sandstones.
    • Insoluble Residues - While minor amounts of clay minerals and quartz     occur in limestones, most of the insoluble residues, (so called because they do not     dissolve in HCl) are grains of nodules of chert.  Such chert mostly originates from     the shells of silica secreting organisms.  These include diatoms, radiolarians, and     some sponges.  Individual grains of chert result from recrystallization of the shells     of these organisms.  Chert nodules can range in size from centimeters     to meters in     length.  Many nodules are concentrated along bedding planes and probably resulted     from dissolution of the siliceous debris and reprecipitation of the microcrystalline     quartz at centers of nucleation located along zones of migration of the fluids, such as     along bedding planes.
                   Structures
        
    Since most limestones are formed by clastic processes, many of the same types of structures observed in siliciclastic rocks also occur in limestones.  
        

    • Current-Generated Structures.  Structures like cross-bedding,     ripple marks, dunes, graded bedding, and imbricate bedding are common in carbonate rocks,     although they may not be as evident as in siliclastic rocks because of the lack of     contrasting colors of individual beds in carbonates.  Since many shells of organisms     have curved outlines in cross-section (brachipods, pelecypods, ostracods, and trilobites,     especially), when the organism dies it may settle to the bottom with the outline being     concave downward, and latter become filled with carbonate mud.  When such features     occur they can be used as top/bottom indicators.
                  
    • Lamination.  The most common type of lamination in     carbonate rocks is produced by organisms, in particular blue-green algae that grow in the     tidal environment.  These organism grow as filaments and produce mats by trapping and     binding microcrystalline carbonates, as incoming tides sweep over the sand.  This     leads to the formation of laminated layers that consist of layers of organic tissue     interbedded with mud.  In ancient limestones, the organic matter has usually been     removed as a result of decay, leaving cavities in the rock separated by layers of material     that was once mud.  These cavities are called fenestrae (for     a photo, see Figure 16-14, page 305, in Blatt & Tracy).  
              
              Another type of lamination occurs as bulbous structures, termed Stomatolites     (for photo, see Figure 16-15, p. 306, in Blatt & Tracy).  These are produced in a     similar fashion, i.e. by filamentous blue-green algae, but represent mounds rather than     mats.
                  
    • Stylolites.  Stylolites are irregular surfaces that     result from pressure solution of large amounts of carbonate.  In cross-section they     have a saw tooth appearance with the stylolites themselves being made of insoluble     residues or insoluble organic material.  Some studies have suggested that the     stylolites represent anywhere from 25% to as much as 90% of missing rock that has been     dissolved and carried away by dissolution (for a photo, see Figure 16-22, page 314, in     Blatt & Tracy).
              Posted Image           Carbonate Depositional Environments
        Most modern, and probably most ancient, carbonates are predominantly shallow water (depths <10-20 m) deposits.  This is because the organisms that produce carbonate are either photosynthetic or require the presence of photosynthetic organisms.  Since photosynthesis requires light from the Sun, and such light cannot penetrate to great depths in the oceans, the organisms thrive only at shallow depths.  Furthermore, carbonate deposition in general only occurs in environments where there is a lack of siliciclastic input into the water.  Siliclastic input increases the turbidity of the water and prevents light from penetrating, and silicate minerals have a hardness much greater than carbonate minerals, and would tend to mechanically abrade the carbonates.  Most carbonate deposition also requires relatively warm waters which also enhance the abundance of carbonate secreting organisms and decrease the solubility of calcium carbonate in seawater.  Nevertheless, carbonate rocks form in the deep ocean basins and in colder environments if other conditions are right.
        
        For this course, our discussion of carbonate depositional environments will be brief.   See your text for more detailed discussion.  The principal carbonate depositional environments are as follows:

    • Carbonate Platforms and Shelves.  Warm shallow seas attached the     continents, or in the case of epiric seas, partially covering the continents, are ideal     places for carbonate deposition.  Other shelves occur surrounding oceanic islands     after volcanism has ceased and the island has been eroded (these are called atolls).          Carbonate platforms are buildups of carbonate rocks in the deeper parts of the oceans on     top of continental blocks left behind during continent - continent separation.
              
              Reef building organisms from the framework of most of these carbonate buildups.
    • Tidal Flats. Tidal flats are areas that flood during high tides and are     exposed during low tides.  Carbonate sands carried in by the tides are cemented     together by carbonate secreting organisms, forming algal mats and stromatolites.
    • Deep Ocean.  Carbonate deposition can only occur in the shallower     parts of the deep ocean unless organic productivity is so high that the remains of     organisms are quickly buried.  This is because at depths between 3,000 and 5,000 m     (largely dependent on latitude - deeper near the equator and shallower nearer the poles)     in the deep oceans the rate of dissolution of carbonate is so high and the water so     undersaturated with respect to calcium carbonate, that carbonates cannot accumulate. This     depth is called the carbonate compensation depth (CCD).     The main type of carbonate deposition in the deep oceans consists of the accumulation of     the remains of planktonic foraminifera to form a carbonate ooze.  Upon burial, this     ooze undergoes diagenetic recrystallization to form micritic limestones.  Since     most oceanic ridges are at a depth shallower than the CCD, carbonate oozes can     accumulate on the flanks of the ridges and can be buried as the oceanic crust moves away     from the ridge to deeper levels in the ocean. Since most oceanic crust and overlying     sediment are eventually subducted, the preservation of such deep sea carbonates in the     geologic record is rare, although some have been identified in areas where sediment has     been scraped off the top of the subducting oceanic crust and added to the continents, such     as in the Franciscan Formation of Jurassic age in California.
    • Non-marine Lakes. Carbonate deposition can occur in non-marine lakes as     a result of evaporation, in which case the carbonates are associated with other evaporite     deposits, and as a result of organisms that remove CO2 from the water causing     it to become oversaturated with respect to calcite.
    • Hot Springs. When hot water saturated with calcium carbonate reaches     the surface of the Earth at hot springs, the water evaporates and cools resulting in the     precipitation of calcite to form a type of limestone called travertine.
                   Dolostones
        
    Dolostones are carbonate rocks composed almost entirely of dolomite - (Ca,Mg)CO3.   Although there used to be a common perception that the abundance of dolostones increased with age of the rock, it is now recognized that although no primary dolomite bearing rocks are being directly precipitated in modern times, dolostones have formed throughout geologic time. This is true despite the fact that modern sea water is saturated with respect to dolomite. Still, most dolostones appear to result from diagenetic conversion of calcite or high-Mg calcite to dolomite, after primary deposition of the original calcium carbonate bearing minerals.
        
        Dolomite, and therefore rocks containing large amounts of dolomite, like dolostones, is easily distinguished by the fact that it only fizzes in dilute HCl if broken down to a fine powder. Also, dolostones tend to weather to a brownish color rock, whereas limestones tend to weather to a white or gray colored rock.  The brown color of dolostones is due to the fact that Fe occurs in small amounts replacing some of the Mg in dolomite.In thin section it is more difficult to distinguish from calcite, unless it is twined.  Unfortunately, sedimentary dolomite is rarely twinned.  In order to facilitate its identification in thin section, the sections are often stained with alizarin red S.  This turns calcite pink, but leaves the dolomite unstained.

    Dolostones are almost entirely composed of euhedral and subhedral rhombs of dolomite.  Although dolostones contain allochems, like limestones, the allochems are generally recrystallized to dolomite, and rhombs of dolomite can be seen to cut across the boundaries of allochemical particles.  Some dolostones show no evidence of allochems, but only contain rhombs of dolomite.  These could either represent limestones that have completely recrystallized to dolomite, leaving no trace of the original fragments that made up the limestone, or could represent primary crystals of dolomite.  

    Two mechanisms of dolomitization of limestones have been proposed based on field and laboratory studies.

    • Evaporative Reflux.  This mechanism involves the evaporation of     seawater to form a brine that precipitates gypsum.  After precipitation of gypsum,     the brine is both enriched in Mg relative to Ca and has a higher density.  If the     brine then enters the groundwater system and moves downward into buried limestones.       This Mg-rich brine then reacts with the calcite in the limestone to produce     dolomite.
    • Mixing of Seawater and Meteoric Water.  This mechanism involves     the mixing of groundwater derived from the surface with saline groundwater beneath the     oceans.  Dolomitization is thought to occur where the two groundwater compositions     mix with each in the porous and permeable limestone within a few meters of the surface.      
              
                  

    Other Sedimentary Rocks

    Evaporites
        
    Evaporite minerals are those minerals produced by extensive or total evaporation of a saline solution.  Because such minerals dissolve readily in less saline rich solutions, like most groundwater and surface water, evaporite rocks rarely outcrop at the surface except in aid regions.  Evaporite rocks are common, however, in the subsurface.  Three different environments result in the deposition of evaporites.
        

                  
    • Basins of internal drainage. In arid regions with basins of internal     drainage rainfall in the adjacent areas is carried into the basin by ephemeral streams     carrying water and dissolved ions.  The water fills the low points in the basin to     form a playa lake.  These lakes eventually evaporate,     resulting in the precipitation of salts such as halite, gypsum, anhydrite, and a variety     of other salts not commonly found in marine evaporite deposits, such as trona (NaHCO3.Na2CO3.2H2O),     natron (Na2CO3.10H2O), nahcolite (NaHCO3),     mirabilite (Na2SO4.10H2O), borax (Na2B4O5(OH)4.8H2O),     kernite [Na2B4O6(OH)2.3H2O], and     colemanite (CaB3O4(OH)3.H2O).
              Posted Image              
    • Restricted bays or seas. In areas where there restricted input of fresh     or marine waters into a basin, coupled with extensive evaporation within the basin,     dissolved ion concentrations may increase to the point where form a dense concentrated     solution is formed near the surface.
              Posted Image              
      These dense saline waters then sink within the basin, become oversaturated with respect   to salts like gypsum and halite, and precipitate the salts on the floor of the basin.
                  
    • Shallow arid coasts or sabkhas.  Along     shallow arid coastlines where input of fresh water is rare and evaporation increases the     salinity of the marine water, evaporation may increase the salinity of the water to a     point where evaporite minerals like halite and gypsum are precipitated.
              Posted Image              
          Cherts
        
    Chert is a mineralogically simple rock consisting of microcrystalline quartz.   There are three common occurrences of chert.
    • As nodules and silt-sized grains in carbonate rocks.          Chert nodules, as discussed previously, occur as structureless dense masses within     carbonate rocks.  They range in size from a few centimeters to many meters in     length.  The source of silica is likely silica secreting organisms that include     diatoms (Jurassic to Holocene), radiolaria (Ordovician to Holocene), and sponges (Cambrian     to Holocene).   But, these organisms are not preserved in the chert nodules.          Instead, the remains of these organisms were likely dissolved by fluids flowing through     the rock during diagenesis.  Most chert nodules are found along bedding planes in the     carbonate rocks, likely because these were zones along which fluids that precipitated the     microcrystalline quartz were able to move.          

                            
    • As bedded cherts that formed along tectonically active     continental margins. Bedded cherts occur in association turbidites, ophiolites,     and mélanges (oceanic trench deposits scraped off the seafloor at subduction     zones).  The beds range in thickness from a few centimeters to several meters, and     are interbedded with siliceous shales.  Although thought to represent deep water     accumulations of silica secreting organisms, they may also form in warm nutrient rich     shallow water environments.  Sometimes the remains of silica secreting organisms,     like radiolaria, sponge spicules, or diatoms are preserved in the cherts, but most show a     microcrystalline texture that results from recrystallization during diagenesis.  
              
              

    • Associated with hypersaline-lacustrine deposits.  Although less     common than the previously discussed occurrences of chert, some cherts appear to form in a     hypersaline environment where they are associated with evaporite deposits.  Such     cherts may in fact form as a result of replacement of sodium silicate evaporite minerals     like magadiite by the following chemical reaction:

      

        

    NaSi7O13(OH)3.3H2O     => 7SiO2 + 4H2O + Na+ + OH-
             magadiite                                                 quartz

      

    Since mechanisms 1 and 2 generally require the presence of silica secreting organisms in order to form chert, the occurrence of chert in Precambrian rocks is problematical because no such organisms existed prior to the early Paleozoic.   Such Precambrian cherts may have actually formed by direct chemical precipitation from silica oversaturated seawater.


    Đã giúp thì giúp người ta cho chót. Chứ như thế này thằng bé kia đến sang năm không dịch được. Nó vừa mới học xíu đại cương, đến đọc tiếng việt còn chả hiểu nổi. Lôi cái này vào chắc nó nản lần sau không dám vô diễn đàn xin tài liệu nữa. Bó tay!
    LƯƠNG VĂN ÁNH   
      
    Chức vụ: Giám đốc

    AD&D
    CÔNG TY CỔ PHẦN PHÁT TRIỂN CÔNG NGHỆ AD&D VIỆT NAM
    Địa chỉ: Số 11, Ngách 885/17, Tổ I, Phường Yên Sở, Quận Hoàng Mai, Thành Phố Hà Nội

    VPGD: Tầng 1, Toàn nhà N4A - Lê Văn Lương - Thanh Xuân - Hà Nội      
    Mobile: 0977192606 & 0936491486
    Email: Nvkd9x@Gmail.com& Anh.lv@FlexOffice.com.vn
    Webstie: www.flexoffice.com.vn
    Yahoo: Geology_9x & Nvkd_9x
    Sky: Nvkd_9x



    Bài viết tương tự Collapse