Terrigenous accumulations are rocks that were formed as a result of the movement and distribution of fragments - mechanical particles of minerals that collapsed under the constant action of wind, water, ice, sea waves. In other words, these are decay products of previously existing massifs, which, due to destruction, underwent chemical and mechanical factors, then being in the same basin, turned into solid rock.
Terigenic rocks make up 20% of all sedimentary accumulations on the earth, the location of which is also diverse and reaches up to 10 km deep in the earth's crust. At the same time, different depths of the rocks are one of the factors determining their structure.
Weathering as a stage in the formation of terrigenous rocks
The first and main stage in the formation of clastic rocks is destruction. In this case, sedimentary material appears, as a result of the destruction of igneous, sedimentary and metamorphic origin on the surface of rocks. First, the mountains are subjected to mechanical influence, such as cracking, crushing. The chemical process (transformation) follows, due to which the rocks pass into other states.
When weathering, the substances are separated in composition and moved. Sulfur, aluminum and iron go into the atmosphere - into solutions and colloids, calcium, sodium and potassium - into solutions, but silicon oxide is resistant to dissolution, therefore, in the form of quartz, it passes into fragments mechanically and is transported by flowing water.
Transportation as a stage in the formation of terrigenous rocks
The second stage, in which terrigenous sedimentary rocks are formed, consists in the transfer of mobile sedimentary material formed as a result of weathering by wind, water or glaciers. The main transporter of particles is water. Having absorbed solar energy, the liquid evaporates, moving in the atmosphere, and falls in liquid or solid form onto 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 flowing water. Thus, fine sand, gravel, and sometimes pebbles are transported by rapid flows, and suspended particles, in turn, carry clay particles. Boulders are transported by glaciers, mountain rivers and mudflows, 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 a mobile state to a static one. In this case, chemical and mechanical differentiation of substances occurs. As a result of the first, separation of particles transferred in solutions or colloids to the pool occurs, depending on the replacement of the oxidizing medium with the reducing one and changes in the salinity of the pool itself. As a result of mechanical differentiation, fragments are separated by mass, size, and even the method and speed of their transportation. So the transferred particles are evenly precipitated clearly, according to zoning along the bottom of the entire pool.
So, for example, boulders and pebbles are deposited at the mouths of mountain rivers and foothills, gravel remains on the shore, sand (far from the shore, sand (since it has a small fraction and the ability to travel long distances, while occupying an area larger than pebbles), the next is a small 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 conversion of accumulated sediments into hard stone. Substances deposited at the bottom of the pool, previously transported, solidify or simply turn into rocks. Further, various components accumulate in the natural sediment, which form chemically and dynamically unstable and nonequilibrium bonds, therefore, the components begin to react with each other.
The sediment also accumulates 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, can change the oxidizing environment of the surface.
Final stage: debris nucleation
Diagenesis is followed by catagenesis - this is the process by which metamorphization of the formed rocks occurs. As a result of the increasing accumulation of precipitation, the stone undergoes a transition to a phase of higher temperature and pressure. The long-term effect of this phase of temperature and pressure contributes to the further and final formation of rocks, which can last from a dozen to one billion years.
At this stage, at a temperature of 200 degrees Celsius, redistribution of minerals and the mass formation of new minerals occur. This creates terrigenous rocks, examples of which are found in every corner of the globe.
Carbonate rocks
What is the relationship between terrigenous and carbonate rocks? The answer is simple. The composition of carbonate often includes terrigenous (clastic and clay) 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. It all depends on the time and method 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, i.e. terrigenous masses with an admixture of carbonates, the percentage of which is up to 5%.
Classification of clastic rocks by degree of roundness
Terrigenous rocks, the classification of which is based on several features, are determined by the roundness, size and cementation of the debris. Let's start with the degree of roundness. It is directly dependent on the hardness, size and nature of the transport of particles during rock formation. For example, particles carried by the surf are more honed and have virtually no sharp edges.
The rock, which was originally loose, is completely cemented. This type of stone is determined by the composition of cement, it can be clay, opal, ferruginous, carbonate.
Varieties of terrigenous rocks in size debris
Terrigenous rocks are also determined by the size of the debris. Breeds are divided into four groups depending on their size. The first group includes fragments whose size is more than 1 mm. Such rocks are called coarse. The second group includes fragments whose size is 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 silt rocks. And the last fourth group defines clay rocks, the size of clastic particles varies from 0.01 to 0.001 mm.
Classification of clastic rock structure
Another classification is the difference in the structure of the debris layer, which helps determine the nature of the formation of the rock. The layered texture characterizes the alternate addition of rock layers.
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, seas and lakes form a rock with parallel stratification, and water flows form an oblique stratification.
The conditions under which detrital rocks are formed can be determined from the signs of the surface of the layer, i.e., by the presence of signs of ripples, raindrops, drying cracks, or, for example, signs of sea surf. The porous structure of the stone suggests that fragments were formed due to volcanic, terrigenous, organogenic or hypergenic effects. Massive structure can be determined by rocks of various origin.
