Granulometric grading

It is called granulometric classification or granulometry, to the measurement and grading that is carried out of the grains of a sedimentary formation, of the sedimentary materials, as well as of the soils, for purposes of analysis, both of their origin and of their mechanical properties. , and the calculation of the abundance of those corresponding to each one of the sizes provided by a granulometric scale.

Method of granulometric determination

The size of a grain, clasto or particle, not always easy to determine when they are irregular, is usually defined as the diameter of a sphere of its same volume, and is expressed in millimeters. In larger ridges, the average of the three maximum orthogonal measurements is usually made, even if they are not cut at the same point.
The simplest granulometric determination method is to pass the particles through a series of meshes of different lattice widths (like colanders) that act as grain filters, which is commonly called sieve column. But for a more accurate measurement a laser granulometer is used, whose beam diffracts in the particles to determine its size. Or you can also use gamma obs.

Screening test

For its realization a series of sieves with different diameters are used that are assembled in a column. In the upper part, where the larger diameter sieve is found, the original material (mixed soil or sediment) is added and the sieve column is subjected to vibration and intense rotating movements in a special machine. After a few minutes, the sieves are removed and disassembled, taking separately the weights of material retained in each of them and that, in their sum, must correspond to the total weight of the material that was initially placed in the column of sieves ( Conservation of the Mass).

Granulometric curve

Taking into account the total weight and retained weights, we proceed to make the grain size curve, with the retained percentage values that each diameter has obtained. The grain size curve allows visualizing the homogeneous or heterogeneous tendency of grain sizes (diameters) of the particles.

Sedimentology

From the point of view of sedimentology, the measure of resemblance of the size of the particles or clasts of a sediment is expressed by the terms granoselección, seleccion or sorting (in English). A heterogeneous material, with very varied classe sizes, is considered poorly selected, while a homogeneous material, in which a single grain size predominates, is considered well selected.
On the other hand, the distribution or variation of the average size of the clasts within a layer or stratum is called granoclassification or graded bedding. Thus, a layer that has finer than ceiling materials in the base is said to have increasing or negative granoclassification, and if it is the other way around, the thick materials are in the base and the fines to the ceiling, decreasing or positive granulolation. The terms granoselección and granoclasificación often used sometimes like synonyms.
When a succession of strata shows, from the oldest to the most modern, an increase in the average size of the clasts that form each layer, it is said to be a grain-growing sequence, whereas if it decreases, it is called the grain-growing sequence.