Stratigraphic unit

The stratigraphic units are cuttings suggested to define sequences of deposit. It acts, by the fine description of the sections of ground which the cuts and the profiles constitute, to propose at the same time a sequencing of each cut or profile but also to define correlations between them. It is the convergence of criteria which makes it possible to propose correlations. Each criterion is carrying information on the modes of installation ou/et the modes of post-dépositionnelles transformations.

Methods of description of the stratigraphic units

The stratigraphic units are described in a fine way with a series of criteria borrowed from the Géologie and the Pédologie. The various components of the stratigraphic units are representative of the processes of deposits and the pedogenic processes. It is only after one objective description that one can begin the process of interpretation and correlation of the stratigraphic units between different the cuts.

Limits between the stratigraphic units

The stratigraphic units are superimposed volumes between which one can define more or less clear and more or less sinuous limits. It is a question of defining the form and the clearness of these limits. The limits can be:

irregular (presence of sinuosities deeper than broad)
corrugated (presence of sinuosities broader than deep)
regular (limit roughly parallel at the surface of the ground)
stopped (the limit between the stratigraphic units is discontinuous, the stratigraphic units are developed in separated cracks or pockets, organization out of nonjointed pockets).
clear (or abrupt)
gradual
.

The Color S

The colors are very important and very significant characters during the description of the cuts. As they are immediately perceptible, they guide us to distinguish the various stratigraphic units before beginning the systematic description which takes place stratigraphic unit per stratigraphic unit. The color is not a misleading character: each modification is significant of a change in the proportions of one or more components or a change of their state. This criterion is often very subjective. Only the use of a charter of color makes it possible to demolish this subjectivity. The Nuancier of Munsel ® is an international reference frame of the colors, whose charter for the ground S exists. The principle of this charter is to propose comparisons between a sample of the stratigraphic unit and pastilles of color distributed on the boards of a booklet according to three criteria:

basic color ( Hoots )
clearness ( been worth )
saturation ( chroma ).

Each board corresponds to a basic color. The pastilles are classified on this board upwards in increasing clearness and from left to right in increasing saturation. The basic colors correspond to the principal colors, the red (R, red ), the yellow (Y, yellow ), the green (G, green ), blue (B, blue ) and the crimson (P, purple ). Intermediaries between these colors exist (YR, PB, etc). A neutral color (NR, neutral ) corresponding to the levels of gray exists too. Each beam of color is cut out in 10 segments: 10R is reddest, 2.5Y the least yellow. The range used for the grounds includes the basic colors following 10R, 2.5YR, 5YR, 7.5YR, 10YR, 2.5Y and 5Y, with the complementary ones for the Gley S (hydromorphic grounds, gray blue or green). Clearness is given according to 7 values 2.5,3,4,5,6,7 and 8, saturation according to 6 values, 1,2,3,4,6 and 8. The pastille which the color approaches the most sample gives the ® this one, for example 10YR4/2 where 10YR corresponds to the basic color 4 with clearness and 2 with saturation. The chartre proposes a literal denomination: very dark grayish brown is in brown French grisâtre very dark. We always give the state of moisture of the stratigraphic unit at the time of the characterization of the color, this one being able to depend on it. Five methods are recognized:

dry (not of detectable moisture)
fresh
wet (malleable sample, absence of interstitial water)
very wet
drowned (presence of interstitial water).

The texture

Each stratigraphic unit includes/understands a whole of components visible with the naked eye, the coarse elements, contained in a material mass finer than one names the Matrice. Texture is defined on the matrix. It is about a total judgment of properties of a material made thanks to tactile feelings (kneading between the fingers), allowing an estimate of the grain-size distribution of a stratigraphic unit. This makes it possible to allot to a material the name of a class texturale (example: sandy silt, muddy clay, etc) by indicating its content in the granulometric big classes of fines, out of sands, silts and clays.

The Structure

It is the way according to which arranges itself naturally and durably the elementary particles by forming or not Agrégat S. the aggregate is the result of the natural organization of the components, it in what it is basically different from a fragment which results from the crack of a preexistent object. It is a very important criterion for the Pédologie. All the mechanisms and processes of the pedogenesis (physical, chemical and biological actions) contribute to transform materials with lithological structure (rock and deposits) into materials with pedological structure. If their presence is proven we let us give the size of the aggregates.

Effervescence with HCl

This criterion exclusively carried out at the laboratory, makes it possible to estimate the content of the stratigraphic unit calcium carbonate in the fine ground (matrix). By pouring the Hydrochloric acid (HCl) on a sample of the stratigraphic unit one can observe his reaction to effervescence. Four methods are recognized:

null effervescence: no bubble gets clear;
low effervescence: some visible bubbles (and audible);
strong effervescence: reaction lives;
very strong effervescence: very sharp and instantaneous reaction, very large bubbles.

This test makes it possible to define the variations of the content of Carbonate of calcium in the pedological profiles and the stratigraphic cuts. It is an important criterion to reconstitute the pedological evolutions.

Coarse components

They are all the individualized mineral components (fragments of rocks, poly- or mono Minéraux) visible with the naked eye of the coarse Sable S (0,2 to 2 mm), with the Bloc S (beyond 100 mm), while passing by the Granule S or gravel S (2 to 10 mm) and the Cailloux (10 to 100 mm).

Even if they lost, partially or completely, their original lithic structure by deterioration, they did not acquire a pedological structure. These elements should not be confused with aggregates.

Their description is done independently of that of the finer components (matrix) which only are taken into account in the determination of texture ( cf supra ).

Their abundance (or charges in coarse elements) corresponds to the proportion in volume of the coarse elements in the mass of the stratigraphic unit. This proportion is estimated on the ground for each particle-size range (coarse sands, granules/gravels, stones and blocks). 5 terms of abundance are proposed:

rare elements
few elements
elements (without mention, average charge)
abundant elements
elements very abundant.

Sometimes is defined the opposite case by presenting the proportion taken by the matrix (very little matrix, little matrix, stamps abundant, very abundant).

Their lithological nature is defined in a detailed way: Calcareous chalky, Granite, Flint, Gneiss, Quarzite, etc

The notation of nature is supplemented if possible by the description of their hardness, their gelivity and their state of deterioration.

hardness is apprehended using the hammer. One distinguishes the not very hard elements which break with the hand, the hard elements which break easily with the hammer and very hard which breaks with difficulty with the hammer. Moreover, the sound returned to the shock is significant: a clear sound indicates a great hardness, a noise chechmate or “cartonneux” being the sign of a coarse element to tend.
It is necessary well to distinguish the level from deterioration of the coarse elements. This deterioration can be chemical, physical or mechanical. Blunted and corrosion gives an idea of the chemical weathering (fringe, cortex), the form of deterioration mechanical (round-off, angular), the cups, cracks or the changes of color of physical deterioration (freezing, heating).

blunted the defined one in four methods: blunted (rough), little blunted, blunted and very not blunted.
One distinguishes 4 scales from corrosion (healthy, little corroded, corroded, very corroded), in certain cases corrosion can be ultimate, it remains in these cases there only one phantom of the coarse element.

The form is defined on the one hand by the district (angular with round) and on the other hand by flatness (globulous flat). Physical deteriorations are given in light (crack of surface, fringes rubéfaction, cups thermal, etc).

The Porosity

It is about the volume of ground occupied by the air, water and the living organisms (animal and vegetable). This porosity of the ground is fundamental for the transfers of matters and the biological activity, in particular the facility with which the system racinaire of the plants can penetrate the various horizons of the ground. The vacuums visible with the naked eye or the magnifying glass with hand are described according to their forms, their sizes and their origins.

Particular and various elements

In this unit are distinguished from the elements natural or anthropic (shell S, fragments of root, Oxide iron, coal, Silex, Céramique, Os, etc). Certain particular elements have clean implications in interpretation such as for example the Oncolithe S and the Galet S crystalline lenses.

See too

Géoarchéologie
Micromorphologie
Sedimentology
Stratigraphy

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