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 phenomenon is 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.
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.
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).
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.
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.
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).
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).
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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