Diagenesis

  

Diagenesis

       A sediment is a set of particles suspended in water, the atmosphere or ice, this sediment ended up being deposited under the effect of gravity and often sedimented in successive layers or strata. Sediments can have a post-deposition evolution (after sedimentation); This evolution manifests as a consolidation which is at the origin of the formation of the rocky sedimentary layers. The latter designates all the physico-chemical and biochemical processes by which the sediments are transformed into sedimentary rocks. These transformations usually take place at shallow depth, so under conditions of low pressure and temperature this phenomenois called diagenesis.

  

Rocks cycle 

     In post-sedimentary evolution, it is possible to distinguish a certain number of major processes, which will follow one another over time and lead to more and more important modifications of the original sediment. We distinguish the pedogenesis (development of a soil) which can occur when a sediment has emerged; compaction, which consists first of all in the expulsion of water following the overload caused by the accumulation of sediments and diagenesis which mainly concerns (bio) chemical phenomena of degradation of organic matter and of dissolution and crystallization.

     Sediments are generally loose and rich in water, therefore Diagenesis consists of their chemical, biochemical and physical transformation into compact sedimentary rocks. It deals with problems of degradation and evolution of organic matter and phenomena of cementation, dissolution, recrystallization and replacement affecting the carbonate, siliceous or sulphated phases.

Types of diagenesis:

     Diagenesis can begin on the background applies to processes occurring at the water / sediment interface (halmyrolysis in the marine domain), refers to essentially biochemical modifications, rapid from the start of burial, this diagenesis is said to be early (syn-sedimentary diagenesis or eogenesis); and which includes all the other physicochemical changes, a lot slower (compaction, dissolution, and mineralogical transformations) where to continue when they are buried under other sediments and which includes mesogenesis and telogenesis, this diagenesis is called late (burial diagenesis). progressive and difficult to define with precision, it takes place when the sediment overload becomes significant.

    The diagenetic regime of telogenesis occurs after the tectonic inversion of the sedimentary basin and begins at any point in the previous two regimes. 

 

 


Diagenesis environment

 The factors and mechanisms of diagenesis:

    From the influence of many parameters involved in diagenetic transformations, we can distinguish the following processes:

         Biochemical process: 

    Due to the activity of living beings (bacteria in superficial areas then macro and microbenthos in deeper areas, particularly for sediments deposited in water) or even certain anthropogenic activities which modify the natural evolution of Soils participate in an important way in the superficial diagenetic modifications characterizing the halmyrolysis which make the medium more and more acidic and reducing thus the red iron in oxidizing medium becomes green in reducing medium.

        Physico-chemical process:

Compaction:

     The successive deposition of sediments leads to a progressive (lithostatic) overload which is weakly compensated by an increase in the pore water pressure. The compaction of sediments consists of a reduction, by physical or chemical means, of their original thickness. Mechanical compaction corresponds to a loss of porosity associated with the expulsion of fluids by rearrangement of sedimentary grains while chemical compaction corresponds to processes of dissolution by pressure ("pressure-solution"). All sediments do not react in the same way during compaction: in other words, the reduction in thickness depends on the original composition of the sediment (detrital sludge, limestone, sand, etc.), or even on the existence an early cement (a limestone with marine cement compacts very little, unlike an uncemented limestone sand). This phenomenon is differential compaction. It is responsible for modifications in the geometry and the relative arrangement of sedimentary bodies of different composition.

 


Numerous faults due to compaction in an alternation of lignite and lake limestone. Limestone from Ventenac, Eocene, Minerve, Montagne Noire (France). 

Dissolution:

      Dissolving a substrate or a pre-existing diagenetic phase obviously results in an increase in porosity. This phenomenon operates at various scales, from that of the karstic system to intraparticle porosity.    A dissolution process always involves going through a stage where there is a void: this void can then be filled with internal sediment or cemented. There are different types of porosity according to their dependence / independence with respect to the original structures of the sediment. Pressure-dissolution is a process of dissolution following an increase in pressure at the points of contact between grains. It is this process which is responsible for the development of structures such as stylolites (in pure limestones) and "pressure-dissolution joints" in limestones richer in insoluble matter (by concentration of insolubles along the dissolution surface. preferential). This process can lead to the formation of stylocumulates and nodular limestones.  

                           


Stylolith bringing two different facies into contact (rudstone with corals and crinoids at the top and mudstone at the bottom). Arrows indicate partially dissolved corals at the stylolith.

Recrystallization:

     During burial under other sediments, the pressure and temperature increase favoring the dissolution of minerals which will recrystallize under new environmental conditions (carbonate concretion in detrital sediments, thus in red sandstones we frequently observe the presence of continental carbonates) forming spherulites, nodules, geodes or septaria in Permian sandstones).


Increase in the size of the crystals forming a calcareous ball. A: recrystallized slurry (microsparite); B: fine grain slurry, not recrystallized (micrite).


  Geode 


    Septaria


 

Cementing: 

      Corresponds to the precipitation of material on a substrate and the gradual increase in the crystals thus formed. The cementing results in the gradual disappearance of the porosity. The grains gradually merge under the action of the salts which circulate with saturated fluids and precipitate; the sands give sandstones and the mud clays, pelites or marls. These include the main types of cements for carbonate rocks:

                                  


Different types of sparitic cements A: spariteequigranular (all crystals have the same size) B: microspariquigranular (it could be either cement or a matrix having undergone a neomorphism, see below). C: sparite in blades forming an isopaque fringe on the edge of a cavity. D: fibrous sparite, also in isopaque fringe. E: drusic sparite (the first crystals are small, then their size increases over time towards the center of the cavity). F: syntactic or coaxial sparitic cement on echinoderm plates.

 

                   


 Microstalactitic meteoric vadose cement (arrow) in a Cenozoic grainstone in the Persian Gulf.

Replacement:

      Implies not only a change in crystallinity, but also a chemical change of a pre-existing substrate. The so-called secondary dolomitization is a frequent example, like silicification. Note that the minerals constituting the fossils can be replaced without their morphology being affected; For example, the formation of dolomite corresponds to the growth of a crystalline network of which the magnesium and calcium reticular planes alternate very regularly. This can only happen by very slow crystallization from a dilute solution or, more quickly, from a concentrated solution with a high Mg / Ca ratio ected.There are various models of dolomitization mentioned above show that dolomites can form in very diverse backgrounds at both early and late stages. However, none of these models can explain, on its own, all the ancient dolomitic masses.

Dolomitized pisoide

 The main diagenetic environments:

- According to the chemical properties of the water, a distinction is made between marine waters (basic, Mg, Sr, etc.) and meteoric continental waters (acidic, little or no Mg and Sr, etc.).

- According to the degree of saturation by marine waters and continental waters, we distinguish a phreatic zone and a vadose zone.

- The groundwater zone represents the zone flooded by the aquifer (total saturation); while the vadose zone represents the aeration zone above the aquifer (partial saturation where H2O passes only).

 


Vadose and phreatic zone

Diagenesis in the continental environment:                                                      

      In this medium, the diagenetic activity is intense. It is due to the absence of Mg and to the often acidic properties of meteoric waters which are unsaturated with carbonates. It is characterized by the dissolution of aragonite and magnesian calcite and recrystallization in the form of stable calcite sparitic cement. Recrystallization can lead to the complete disappearance of the primary organogenic structures. In the superficial vadose zone, the cements resulting from vertical percolations are irregular and asymmetrical. In the phreatic zone, cementation, regular and relatively rapid, leads to crystals of large size and of comparable appearance (isopacs).

     Strictly continental limestone cementations are also characterized by the development of a pedological crust on limestone rocks, stalactites, stalagmites, pisolites, tuffs and precipitation travertines.

Diagenesis in the inter and supratidal environment:

     In the marine domain, physicochemical variations (T, pH, salinity, etc.) in the water on the surface of the sediment or immediately below, linked to hydrodynamic or biological actions (bacteria, plant metabolisms) can create conditions favorable to such diagenesis (hardened bottom, hard-ground).

     In this environment, the diagenesis is more dynamic and more varied. Thus, the increase in T ° C, evaporation and the supply of fresh water cause the evolution of pore water. The main diagenetic phenomenon is the lithification of beach sands (beach sandstone or beach rock) which develop under weak sedimentary cover (qlq tens of cm) by cementation of acicular aragonite or micritic highly magnesian calcite This phenomenon is common in tropical to sub-tropical regions (latitudes: 0 and 45 °). We can also cite the occasional presence of asymmetric cements and cavities (fenestrae structures or bird-eyes), the destruction of algal films or the existence of shrinkage figures (desiccation cracks).

 

 

 Keystone Vugs

     Early cementing, on the surface of the supratidal or coastal zone, in the case of a “Bahamian” platform, such as the vast old carbonate platform of the Middle Lias in Morocco (High Atlas, Middle Atlas, southern riffs etc.). The carbonate platform extends from Algeria and Tunisia to Oman, along the south-Tethyan margin. A good current example of sedimentation and diagenesis is given by the coastal deposits (tidal and storm) platiers and "chotts" (cyclic sedimentaton) at the bottom of the Gulf of Gabès in Tunisia.

Diagenesis in the infratidal underwater environment:

      In this environment, interstitial waters are the same as marine waters. Mineralogical transformations are absent. Diagenesis is characterized by aragonite cementation and magnesian calcite which determines the formation of aggregates, hardened bottoms or hard-grounds (especially on shoals), as well as the consolidation of reef sets. However, in the Persian Gulf, off Qatar for example, lithification is active on the platform between 20 and 40m deep, where the moderately agitated marine waters are supersaturated with carbonates, and the sands are not very mobile.

Diagenesis in deep marine water:

     It is sometimes manifested by micritic carbonate cementations, particularly when the deposition is very slow and the bottom waters are relatively warm and salty, as in the trenches of the eastern Mediterranean or in the Red Sea. Calcareous crusts and nodules of very magnesian calcite, aragonite, or even protodolomite, can thus encompass by cementing various sedimentary debris, up to around 3500 m deep, and present significant local variations, size, thickness, fabric and chemical composition.

Interest of the study of diagenesis:

- Identification of paleorivages (Transgression, Regression).

- Estimation of the thickness of the vadose zone and consequently of their elevation (relief).

- Determination of the depth of the water table (relief and climate: Example: arid and semi-arid climates -> the water tables are ± deep).

        Conclusion :

     Of very diverse and complex magnitude and modalities, the diagenetic phenomena depend on the initial composition, texture and granularity of the sediment, the exchange of fluids between the water-sediment interface and the interstitial medium as well as between successively accumulated sediments, the porosity and even more so the permeability of the material subjected to lithifcation. The methods of diagenesis also depend on the evolution of temperature and pressure related to burial or tectonic constraints. The field of diagenesis extends to around 200 ° C for maximum pressures of around 4 kb and depths of up to 15 to 20 km depending on the geothermal gradient. Beyond begins the field of metamorphism.

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