Clastic terrigenous rocks: description, types and classification

2024 Author: Henry Conors | [email protected]. Last modified: 2024-02-12 02:43

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, these are the decay products of pre-existing mountain ranges, which, due to destruction, were subjected to chemical and mechanical factors, then, being in the same pool, turned into solid rock.

Terigenous rocks make up 20% of all sedimentary accumulations on earth, the location of which is also diverse and reaches up to 10 km in the depth of the earth's crust. At the same time, different depths of rocks are one of the factors that determine 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. Whereinsedimentary material appears as a result of the destruction of rocks of igneous, sedimentary and metamorphic origin exposed on the surface. First, mountain ranges are subjected to mechanical influence, such as cracking, crushing. Next comes the chemical process (transformation), as a result of which the rocks pass into other states.

When weathering, substances are separated by composition and move. 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 mechanically passes into fragments and is transported by flowing waters.

Transportation as a stage in the formation of terrigenous rocks

The second stage, in which terrigenous sedimentary rocks are formed, is 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 on land, forming rivers that carry substances in various states (dissolved, colloidal or solid).

The amount and mass of transported debris depends on the energy, speed and volume of flowing waters. So fine sand, gravel, and sometimes pebbles are transported in fast streams, suspensions, 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, the separation of particles transferred in solutions or colloids to the pool occurs, depending on the replacement of the oxidizing environment by 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 by the method and speed of their transportation. So the transferred particles are evenly deposited clearly, according to the zonality along the bottom of the entire basin.

than pebbles), fine silt, often deposited with clay, extends next.

The fourth stage of formation - diagenesis

The fourth stage in the formation of clastic rocks is a stage called diagenesis, which is the transformation of accumulated sediments into solid 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 non-equilibrium bonds, so the components begin toreact with each other.

Also, crushed particles of stable silicon oxide accumulate in the sediment, which turns 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: birth of clastic rocks

Diagenesis is followed by catagenesis - a process in which the metamorphism of the formed rocks occurs. As a result of the increasing accumulation of precipitation, the stone undergoes a transition to a phase of a higher temperature regime and pressure. The long-term action 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, at a temperature of 200 degrees Celsius, there is a redistribution of minerals and the massive formation of new minerals. This is how terrigenous rocks are created, examples of which can be 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 (detrital and clayey) massifs. The main minerals of carbonate sedimentary rocks are dolomite and calcite. They can be both separately and together, and their ratio is always different. It all depends on the time and method of formation of carbonateprecipitation. If the terrigenous layer in the rock is more than 50%, then it is not carbonate, but refers to such clastic rocks as silts, conglomerates, gravelstones or sandstones, that is, terrigenous massifs with an admixture of carbonates, the percentage of which is up to 5%.

Classification of clastic rocks by degree of roundness

Clastic rocks, the classification of which is based on several features, are determined by the roundness, size and cementation of the fragments. Let's start with the degree of roundness. It has a direct dependence on the hardness, size and nature of the transport of particles during the formation of the rock. For example, particles carried by the surf are more refined and have virtually no sharp edges.

Rock, which was originally loose, is fully cemented. This type of stone is determined by the composition of cement, it can be clay, opal, ferruginous, carbonate.

Varieties of terrigenous rocks by size of fragments

Also, terrigenous rocks are determined by the size of the fragments. Depending on their size, the rocks are divided into four groups. The first group includes fragments whose size is more than 1 mm. Such rocks are called coarse-grained. The second group includes fragments, the size of which is in the range from 1 mm to 0.1 mm. These are sandstones. The third group includes fragments ranging in size from 0.1 to 0.01 mm. This group is called silt rocks. And the last fourth group defines clay rocks, the size of clastic particles varies from0.01 to 0.001mm.

Clastic structure classification

Another classification is the difference in the structure of the clastic layer, which helps to determine the nature of the formation of the rock. The layered texture characterizes the sequential addition of rock layers.

They consist of a sole and a roof. Depending on the type of layering, it is possible to determine in what medium the rock was formed. For example, coastal-marine conditions form a diagonal layering, seas and lakes form a rock with parallel layering, water flows - oblique layering.

The conditions under which the clastic rocks were formed can be determined from the signs of the layer surface, that is, by the presence of signs of ripples, raindrops, drying cracks, or, for example, signs of the sea surf. The porous structure of the stone indicates that the fragments were formed as a result of volcanic, terrigenous, organogenic, or supergene influences. The massive structure can be defined by rocks of various origins.

Rock variety by composition

Clastic rocks are divided into polymictic or polymineral and monomictic or monomineral. The former, in turn, are determined by the composition of several minerals, they are also called mixed. The latter determine the composition of one mineral (quartz or feldspar rocks). Polymictic rocks include greywackes (they include particles of volcanic ash) and arkoses (particles formed as a result of the destruction of granites). Composition of terrigenousrocks are determined by the stages of their formation.

According to each stage, its own share of substances in a quantitative ratio is formed. Terrigenous sedimentary rocks, when discovered, are able to tell at what time, in what ways substances moved in space, how they were distributed along the bottom of the basin, what living organisms and at what stage took part in the formation, and also in what conditions the formed terrigenous rocks were located.