پتروگرافی سنگهای کربناته 2
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Dolomites
In contrast diagenetic dolomitization is the more important but the understanding of the process is still evolving. This encompasses an ever-increasing number of settings and geochemical models. Whatever the model proposed high magnesium to calcium ratios occur in the waters responsible for this diagenesis. This is particularly true of mixing zone brines with different compositions where dolomitization can become locally important. Although the deep burial diagenetic realm is not as well understood as near-surface conditions, it is apparent that dolomitization, pressure solution compaction, and cementation are associated with the deeper parts of the section. The subsurface fluids responsible for the diagenesis are thought to be derived from a variety of sources (figure above), and most likely from down-dip basinal shales and fine carbonates that expel fluids as they are compacted during burial. The term dolomite may be used as both a mineral name and a name for a carbonate rock containing more than 50% dolomite. Where good evidence exists that the rock was once limestone, the adjective dolomitized may be used. Although staining is the best method to distinguish dolomite from calcite, development of good rhombohedral structure is typical of dolomites and uncommon in calcites. This criterion should be employed with caution, however. Much mud-sized carbonate material can often be dolomite. A subsurface setting for the diagenesis associated with dolomitization is important but this process can occur in the following listed settings:
Arid Supratidal Salt Flats Mineralogical changes are common in Holocene supratidal flats of arid areas including those of the United Arab Emirates (Butler et al 1982; Swart et al, 1987; and Kendall & Warren 1988). Here aragonitic sediments can be dolomitized and evaporites emplaced in these carbonate sediments (figure below). Landward, gypsum followed by anhydrite and halite may be precipitated. The gypsum may form individual displacive crystal laths or layers of mush, whereas anhydrite occurs in contorted layers or as nodules. The origins of the brines associated with this dolomitization is thought to be both marine groundwater influxing from the adjacent Arabian Gulf (Patterson and Kinsman 1977; and Butler et al 1982) and subsurface brines coming from the adjacent Oman Mountains (Wood et al 2001).
Where the sulphates precipitate in standing bodies of water, for instance isolated coastal lagoons or playas, they form horizontal layers that parallel the sediment-water interface. The occurrence of evaporites at the updip side of a carbonate shelf or platform is important to the hydrocarbon industry since the evaporites often form the updip seals to reservoirs developed in dolomitized shelf carbonates. Capillary Crusts of Tidal and Supratidal Flats Near Surface Submarine Sediments of the Margins of Carbonate Banks Flanks and Interior of Carbonate Banks Penetrated by Circulating Marine Waters The second form of dolomitization associated with this setting occurs in coarse-grained reefal sediments (Swart and Melim 2000). It is suggested that the circulation of normal marine water in a relatively open system explains the pervasive character of the dolomitization and the relatively normal Sr concentrations. Near Surface Mixing Zone Remobilization of Magnesium Late Dolomitization Associated with the Late Movement of Subsurface Waters
However it is formed, the dolomite has the potential for creating reservoir quality porosity and permeability in originally tight limestones (left figure). Early dolomitization may preserve porosity by creating a rigid framework that inhibits compaction. In still other cases dolomitization in lime muds may enhance porosity, because dolomites are denser and so consequently take up less volume than the original calcite. Intercrystalline porosity in dolomites is responsible for many Paleozoic reservoirs, a good example is the Mississippian Little Knife field carbonates. Dolomitization may reduce, redistribute, preserve or create porosity. In a few carbonate reservoirs, as in the Jurassic Arab limestones of Ghawar field in Saudi Arabia, replacement dolomite crystals extend into adjacent pores thereby reducing the primary porosity. In many dolomitized reservoirs such as the Jurassic Smackover Formation of Alabama and the Leduc reef carbonates in Alberta, porosity and permeability have beeb redistributed during dolomitization and associated leaching and enhance reservoir character. Porosity that was formed during dolomitization is common in the Mission Canyon and Red River Formations of the Williston Basin. References Shields Gordon and Brady 1999 Ali Fazeli = egeology.blogfa.com Wood. W.W, and Sandford W. E, 2002; Hydrogeology and solute chemistry of the coastal-sabkhas acquifer in the Emirate of Abu Dhabi, Barth & Boer (eds) Sabkaha Ecosystems, Kluwer Academic Publishers, Printed in the Netherlands p 173-185. Ali Fazeli = egeology.blogfa.com
Chertification Chertification is a diagenetic process that converts carbonate sediments into chert. These cherts are composed of microcrystalline quartz that contains abundant water that is dispersed interstitially between the crystals. In addition to replacing limestones, cherts can replace opal and/or dolomites. They may fill fractures and are derived from groundwaters rich in silica. Cherts can form nodules, breccias, beds, dikes, and spheroids. The setting of their formation is variable, as is their rate and timing of formation. They can be form contemporaneously with sediment deposition close to the sediment water/ or air interface form ground waters occur which are related to volcanism and so are rich in silica. They can also be late diagenetic features created during the migration of deepwater brines rich in silica. Thus chertification is a product of diagenesis that can be response to low temperature silica rich waters or the product of metasomatism related to volcanic extrusion and dyke intrusions. They form siliceous rocks that can arguably be chemical sediments or the products of replacement of pre-existing sediments (chertification) and the development of concretions. Chert nodules in limestones have been linked interstitial anoxia, soft-sediment deformation, when the secondary replacement of carbonate by silica can occur. They are also often associated with evaporites. In deeper waters opaline silica is commonly deposited as the tests of marine organisms that include Radiolarian and/or sponge spicules.to form an important component of marine sediments. After deposition, these oceanic sediments can undergo diagenesis and form cherts that replace pelagic limestones. Thus deep-sea cherts are found both in the central Atlantic and the Pacific Oceans. Post deposition erosion and transportation can lead to the development of siliceous turbidites that represent by bedded cherts derived from the vicinity of ocean ridges.
Chemical Sediments Evaporites emplaced in carbonate sediments (figure below) through supratidal evaporation and from standing bodies of water. Supratidal flats of the Holocene arid areas of the United Arab Emirates (Butler et al 1982; and Kendall & Warren 1988) is a well know example of this.
Traverses of these flats traced landward show the precipitation of gypsum in the near surface sediments with anhydrite occurring upslope on these falts. A halite crust caps the surface. The gypsum may form individual displacive crystal laths or layers of mush, whereas anhydrite occurs in contorted layers or as nodules. The origins of the brines associated with these minerals and the associated dolomitization of the aragonitic marine carbonates is thought to be both marine groundwater influxing from the adjacent Arabian Gulf (Patterson and Kinsman 1977; and Butler et al 1982) and subsurface brines coming from the adjacent Oman Mountains (Wood et al 2001). Where the sulphates precipitate in standing bodies of water, for instance isolated coastal lagoons or playas, they form horizontal layers that parallel the sediment-water interface. The occurrence of evaporites at the updip side of a carbonate shelf or platform is important to the hydrocarbon industry since the evaporites often form the updip seals to reservoirs developed in dolomitized shelf carbonates (Kendall & Warren 1988). Pisolites, Oncoids, and Oncolites
Pisolites, oncoids, and oncolites are enveloped by irregular layers. All these grains are frequently larger than ooids and commonly are over a centimeter in diameter. Pisolites form by the precipitation of calcium carbonate around nuclei trapped in sediment within the vadose zone of soils or marine tidal flats (Figure 24). Oncoids form on the surface of intertidal and supratidal flats where carbonate precipitates from salt water spray and marine flood waters (left figure).
Though these grains are termed oncoids, in limestones they probably cannot be distinguished from pisolites. Finally, other grains resembling pisolites but termed oncolites are formed by accretions of blue-green algae and trapped sediment around a nucleus in moderately protected marine environments (figure below). Variations of this type of grain occur when the coatings are formed by algae and foraminifera or by red algae to form rhodolites.
Neomorphism - Recrystallization or Inversion To quote from Folk: Neomorphism is a broad "term of Ignorance" denoting merely the change from one aspect of calcium carbonate into another, by whatever mechanism. If it is possible to determine the specific mechanism, then that more exact term should be used rather than the broad "neomorphism". Chief processes are:
For details see Folk '65 SEPM SpP#13. Unfortunately, the term "recrystallization" has been used in the past for almost any method which explains the formation of sparry calcite in carbonates--even for simple cementation. Solution-cavity fill, where a large gap in time of in space separates the solution of one type of carbonate and precipitation of another type, is not considered neomorphism; it is no different than simple cementation. Of the processes grouped under neomorphism, inversion of aragonite to calcite is probably the most important and easiest to identify. True calcite to calcite recrystallization is probably rare, although it is certainly important in some localities. Neomorphism of carbonate mud to microspar, etc., is very common-- neomorphism is the proper word here because one does not know if the original mud was aragonite (most likely) of or calcite. Is a very slovenly and all-to-common tendency to ascribe everything one can't immediately understand either to "algae" or vaguely to "recrystallization". You should be able to show proof if recrystalization is your preferred model to explain a particular carbonate fabric. | |||
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