Sedimentology

Definition

The sedimentology (or sedimentary petrology) is a branch of the Géologie which studies the formation processes of the sedimentary rocks.

Interests

The study of the sedimentary rocks that the Géologue meets with the outcrop, allows to rebuild the Paléoenvironnements, Paléogéographies and Paléoclimats near the formation of this rock. In the major part of the situations the principle of actualism is used. This principle was formulated by Charles Lyell in 1830: the present is the key to include/understand the past . This principle is based on the fact that the formation of a current rock under current conditions translates for a similar rock (past) the last conditions. These conditions are articulated around four processes:

conditions of erosion producing the sediments
transport conditions (by the vectors Water and Wind)
conditions of deposit
evolution of the deposit or Diagenesis

Associated disciplines

the study of the characteristics chemical, mineralogical, and paleontological of the sedimentary rocks is the sedimentary Pétrographie.
the study of the evolution of the characteristics of the sedimentary rocks during time is the Stratigraphie.
the Hydrogéologie is also very related to the sedimentology because the major part of the tablecloths (free or captive) runs through sedimentary rocks.

Sedimentology of facies

The sedimentology of facies is a discipline of ground mainly. It is based on the observation of the figures of deposit on the rocks with the outcrop. Thus one can recognize figures related on the swell or the tide for example. These sedimentary figures, associated with the Facies of the rock, make it possible to carry out paléo-environmental reconstitutions at the time of the formation of these structures (context glacial, lake, marine major…). These figures or structures are divided into two categories:

Primary structures

Primary structures of erosions

They appear on the surface. They are syn-sedimentary structures.

Differentiation according to their sizes

one defines the gutters, the cuillières, the grooves or pot of erosion and the basic figures of bench (plate marks).

Differentiation according to the diagenesis

Traces of current (Current marks)

Traces of affouiements related to the direct activity of a turbulent fluid on the bottom. One distinguishes:

the flutes (traces of affouiement in spindle, flut casts). They can be isolated or grouped and give the direction of the current.
crescents (marks crescent). Cups in the shape of crescent formed around a motionless object. They also make it possible to give the direction of the current.
drains (or gutters, rill marks)
peaks and furrows (rigdes and furrows). Parallel figures within the meaning of the current finishing in frondescent cabbage sheets. They meet especially in turbidities.

Traces of objects or Tools marks

These traces are formed by objects transferred onto the bottom.

continuous or rectilinear Traces. These traces correspond to the hauling of an object (groove) giving a direction of current. Sometimes small lattéraux folds or rafters are associated giving the direction of the current.
simple discontinuous traces: Trace rebound (bounes) in spindle or trace of obstinate (prods) in corner giving the direction of the current.
repeated discontinuous traces: trace bearing (rolls), the most obvious example and that of a Ammonite travelling on the bottom on its ventral hull and traces rebounded or repeated rebound (skips).

Primary structures of deposit

Correspond to a volume of sediment determining the stratification.

the fillings géotrope: of mechanical origin
Of the quite particular factories like the Fossil directed, the pulled up rollers, overlaps of rollers…
the dropstones which can result:

Of repercussions of volcanic ejections in the soft sediment.
Of element transported by a stock of floating tree.
Of element transported by drift ice have suddenly melted.

It results a deformation from it from stratigraphy under the object only.

the other primary structures of deposit which correspond to all the structures with various combinations of litages. The layers forming these structures are gathered in beams so called set.

Beams with flat litage (flat bedding), representing for the majority a fast flow of high energy with a fine granulometry. Some of these beams represent a slow flow but have a coarse granulometry. On each layer exist linéations of offenses (parting lineation) which are lines parallel between them and parallel with the current.

Beams with oblique litage (cross-country race bedding) or FLO. They are very varied and are due to currents or waves. They nottament were nottament studied by MacKee and Weir (1953). They are divided into four types:

simple FLO: very rare.
plane FLO at rather plane erosive base with the layers in tilted plan.
arched FLO (trough cross-country race stratification) at erosive base arched with the arched layers. Allen (1963) defines 15 different types of them.
mammelonnés FLO (HCS for Hummochy cross-country race stratification). Meet in the environments of storm and take orgine is to them in the filling of a form of preexistent erosion or by the accumulation of wrinkle in the broad sense.

The wrinkles are defined according to the size: There thus exist Rides in a strict sense (ripples) making less than 5 cm in height and the Dunes which make more than 5 cm in height. The conservation of these wrinkles is variable. They can be strongly or partially truncated by erosion, producing veined, lenticular structures or eyelets (lenticular bedding).

When the form is preserved, the wrinkle can take several forms:

Wrinkles with discontinuous peaks with forms lingoïdes which are wrinkles of current.

Wrinkles with continuous peaks: being divided into dissymmetrical wrinkle of current, watery origin (granodécroissance vertical) or of wind origin (low wrinkle with granocroissance vertical) and the symmetrical wrinkles which are wrinkles of oscillations related to vaguenesses having of the layers in the two directions.

It can also exist an interference of the trains of wrinkles of oscillations forming of the wrinkles with double peak or polygonal network.

The majority of the beams with litage oblique (largest) are indicators of the direction and direction of the current (attention however to the problem of the real and apparent dip).

Secondary structures

Are a modification of the primary structures (Deformation primarily).

rheotropic structures, related to purely physical factors (current of deposit, shock, intersticielle circulation of fluid, gravity…). These structures are more or less précoses (sometimes syn-sedimentary) but they are always antelithification.
biogenetic structures, resulting from the action of living beings (animal or vegetable). One defines the traces of vegetable origin or rhizolite and the traces of animal origin or ichnite. The whole of these structures are gathered under the term of Bioturbation and are the objects of studies of the Paléoichnologie.
physicochemical structures, related on the crystallization and/or the dissolution of certain minerals. They inform about the precosity of the Diagenèse.

Rheotropic structures

It exite five situations of rheotropic structures.

Rearrangements

In section cohesive: one obtains typical structures of slits of withdrawal when the volume of sediment drops. Under water, these slits name slit of Synérèse or slit of dessication if they foment on ground.

One can also find traces of drops of rain or traces of cups (mud curls or mud chips).

Attention, the slits of withdrawal are different from the diastases resulting from the differential compaction at the time of the diagenesis précose.

In section liquefied, a shock in the liquefied sediment is called Thixotropie. The préexisant litage is deformed to see disappeared.

When the litage is preserved, one speaks about convolutes.

Lastly, when the litage is not preserved, of the clastic seams are formed by injection (sedimentary Dike) which can give sand or mud volcanos.

Exhausts of fluid

When the fluid is water, it is upwards formed structures in cups (dish structure) with drive of fine particles.

If the fluid is a gas (air, methane, H2S…), one speaks about fenestrae.

Differences in density between sediments

The difference in density results in a deformation synsédimentaire involving the formation of loadcast and the flammées structures. When the deformation is accentuated, the load can be detached and form pseudo-nodules (load and pillow).

When the deformation is post sedimentary late, it is especially formed visible pockets of cryoturbation in periglacial field.

Drives by gravity

When the sediment is soft, it is formed slumps on the paléopentes. If the sediment is firm, the drives will form soil slip mounds on the paléopentes. When a fault is formed (listric Faille) it takes the name of hydroplastic fault in such a context.

Drives by the current

In such a case, the oblique litage is accentuated to see turned over. To obtain this result, liquefaction is necessary.

Physicochemical structures

the assemblies of crystals outhigenes, is displacifs (mineral hustling the others to set up itself) like the balls of Marcassite or the flowers of Pyrite. That is to say poecilitic (gypsum of pinks of sands, calcite in certain sands).
crystal mouldings: enough running with regard to the mouldings halite cube.
nodules: often isolated with an internal structure (Septaria) or grouped. There exist 2 types of structures:

The nodules are jointed and form a hen netting or chickenwire. The nodules are not jointed and form a enterolitic structure.

Some Stylolithes, resulting from a pressure-dissolution with matter disappearance
preferential Cementing such as the rings of Liesegang (ferruginous cementing related to the capillarity).

Biogenetic structures

to see Paléoichnologie

Industrial applications

The sedimentology has applications in oil geology and for the search and the exploitation for useful substances. Like the exploitation of mines of Potash, Bauxite, Halite or rock salt, of Gypsum etc

Sources

Course of sedimentology, 2nd year of license Géosciences

See

Sedimentary rock

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