Detrital terrigenous rocks: description, species and classification

Terrigenous accumulations are rocks that were formed as a result of the movement and distribution of debris - mechanical particles of minerals that collapsed under the constant action of wind, water, ice, sea waves. In other words, they are the products of the disintegration of pre-existing mountain massifs, which, due to destruction, were subjected to chemical and mechanical factors, then found themselves in the same basin, turned into a solid rock. Terrigenous rocks account for 20% of all sedimentary accumulations on the earth, the location of which is also varied and reaches up to 10 km deep in the earth's crust. At the same time, the different depth of the rock arrangement is one of the factors determining their structure.

Weathering as a stage of formation of terrigenous rocks

The first and main stage in the formation of clastic rocks is destruction. In this case, a sedimentary material appears, as a result of the destruction of igneous rocks of magmatic, sedimentary and metamorphic origin. First, mountain massifs are subjected to mechanical influence, such as cracking, crushing. Next is a chemical process (transformation), as a result of which rocks are transferred to other states.

When weathering, the substances are separated by composition and move. In the atmosphere goes sulfur, aluminum and iron - in solutions and colloids, calcium, sodium and potassium - in solutions, but silicon oxide is resistant to dissolution, therefore, in the form of quartz, it mechanically turns into debris and is transported by flowing water.

Transportation as a stage of formation of terrigenous rocks

The second stage, in which terrigenous sedimentary rocks are formed, consists in the transfer of the mobile sedimentary material formed by weathering, wind, water or glaciers. The main transport of particles is water. Absorbing solar energy, the liquid evaporates, moving in the atmosphere, and falls out in liquid or solid form on land, forming rivers that carry substances in various states (dissolved, colloidal or solid).

The quantity and mass of transported debris depends on the energy, speed and volume of the flowing water. Thus, fine sand, gravel, and sometimes pebbles and slurries are transported by fast flows, in turn, clay particles transfer. Glaciers, mountain rivers and mudflows carry the bulk of boulders, the size of such particles reaches 10 cm.

Sedimentogenesis - the third stage

Sedimentogenesis is the accumulation of transported sedimentary formations, in which the transferred particles pass from the mobile state to the static one. In this case, chemical and mechanical differentiation of substances occurs. As a result of the first, the particles transferred in solutions or colloids to the basin are separated, depending on the replacement of the oxidizing medium for the reducing and the changes in the salinity of the basin itself. As a result of mechanical differentiation, fragments are separated by mass, size and even method and speed of their transportation. Thus, the transferred particles uniformly precipitate clearly, according to the zonality along the bottom of the entire basin. So, for example, boulders and pebbles are deposited in the mouths of mountain rivers and foothills, on the shore there is gravel, far from the shore - sand (since it has a shallow fraction and the ability to move over long distances, while occupying an area larger than pebbles) The following extends fine silt, often precipitated with clay.

The fourth stage of formation is diagenesis

The fourth stage in the formation of detrital rocks is the stage called diagenesis, which is the transformation of accumulated precipitation into a solid rock. The substances deposited on the bottom of the basin, previously transported, harden or simply turn into rocks. Further in the natural sediment accumulate various components that form chemically and dynamically unstable and nonequilibrium bonds, so the components begin to react with each other. Also in the sediment are accumulated crushed particles of stable silicon oxide, which passes into feldspar, organic sediments and fine clay, which forms a reducing clay, which, in turn, deepening by 2-3 cm, is able to change the oxidizing environment of the surface.

The final stage: the formation of clastic rocks

Diagenesis is followed by catagenesis, a process in which metamorphic rocks are formed. As a result of increasing accumulation of precipitation, the stone undergoes a transition to a phase of a higher temperature regime and pressure. The long-term effect of such a phase of temperature and pressure contributes to the further and final formation of rocks, which can last from ten to one billion years.

At this stage, with a temperature regime of 200 degrees Celsius, there is a redistribution of minerals and the mass formation of new minerals. This creates terrigenous rocks, examples of which are found in every corner of the globe.

Carbonate rocks

What are the terrigenous and carbonate rocks related to each other? The answer is simple. Carbonates often contain terrigenous (clastic and argillaceous) massifs. The main minerals of carbonate sedimentary rocks are dolomite and calcite. They can be either individually or together, and their ratio is always different. Everything depends on the time and manner of formation of carbonate sediments. If the terrigenous layer in the rock is more than 50%, then it is not carbonate, but refers to clastic rocks such as silts, conglomerates, gravelites or sandstones, that is, terrigenous massifs with an admixture of carbonates, whose percentage is up to 5%.

Classification of clastic rocks by degree of roundness

Terrigenous rocks, the classification of which is based on several characteristics, is determined by the roundness, size and cementity of the debris. Let's start with the degree of roundness. It has a direct dependence on the hardness, size and nature of the transport of particles in the formation of the rock. For example, particles carried by the sea are more refined and have virtually no sharp edges. The breed, which was initially loose, is cemented completely. This type of stone is determined by the composition of the cement, it can be clayey, opal, ferruginous, carbonate.

Varieties of terrigenous rocks in terms of the amount of debris

Also terrigenous rocks are determined by the size of the debris. Depending on their size, the rocks are divided into four groups. The first group includes debris, the size of which is more than 1 mm. Such rocks are called large clastic rocks. The second group includes fragments, the size of which falls in the range from 1 mm to 0.1 mm. These are sandy rocks. The third group includes fragments from 0.1 to 0.01 mm in size. This group is called silty breeds. And the last fourth group defines clay rocks, the size of detrital particles varies from 0.01 to 0.001 mm.

Classification of clastic rock structure

Another classification is the difference in the structure of the clastic layer, which helps determine the character of formation of the rock. Layered texture is characterized by the alternate addition of layers of rock. They consist of a sole and a roof. Depending on the type of layering, it is possible to determine in which medium the rock was formed. For example, coastal-marine conditions form a diagonal stratification, the seas and lakes form a rock with parallel stratification, water flows - oblique lamination.

The conditions under which clastic rocks were formed can be determined from the features of the surface of the layer, that is, by the presence of ripple marks, rain drops, drying cracks or, for example, signs of the sea. The porous structure of the stone indicates that the fragments were formed due to volcanogenic, terrigenous, organogenic or hypergenic effects. A massive structure can be determined by rocks of different origins.

Species breed by composition

Terrigenous rocks are divided into polymictic, or polymineral and monomictic, or monomineral. The first, in turn, are determined by the composition of several minerals, they are also called mixed. The second determine the composition of one mineral (quartz or feldspar rocks). To the polymictic rocks include graywacks (they include particles of volcanic ash) and arkoses (particles formed as a result of the destruction of granites). The composition of terrigenous rocks is determined by the stages of their formation. According to each stage, the share of substances in the quantitative ratio is formed. Terrigenous sedimentary rocks, when detected, are able to tell at what time, in what ways the substances moved in space, how they were distributed along the bottom of the basin, what living organisms and at what stage they took part in the formation, and also in what conditions were the formed terrigenous rocks .