PETROGRAPHY OF SEDIMENTARY ROCKS, Classification of sedimentary...


Classification of sedimentary rocks

Knowing the composition and structure of sedimentary rocks, their ability to systematize are one of the necessary conditions for the successful use of lithology in the study and development of the Earth's interior.

Classification of sedimentary rocks, compiled by N.M. Strakhov [36] on the basis of the theory of the types of lithogenesis, with additions of N.V. Logvinenko [20] takes into account the material composition and genesis of rocks simultaneously:

1) clastic;

2) clayey;

3) aluminous (allite);

4) glandular;

5) manganese;

6) phosphate;

7) Siliceous;

8) carbonate;

9) salts;

10) caustobioliths.

Clastic, argillaceous, aluminous and ferruginous rocks represent a series of successive decompositions of magmatic, metamorphic and sedimentary rocks. Phosphate, siliceous, carbonate rocks and salts are formed from solutions arising from weathering, and as a result of the vital activity of organisms. Caustobioliths are the products of the vital activity of plants under conditions of humid climate and the result of transformation of organic matter.

Textures of sedimentary rocks

Texture of the sedimentary rock is the mutual arrangement of rock fragments, their orientation relative to each other, the bedding surface and the rock as a whole. There are textures during sedimentation (primary - sedimentation), at the stage of diagenesis and subsequent changes (secondary - diagenetic and catagenetic).

Sedimentary Textures

Among the primary textures emerging at the earliest stages of sedimentary rock formation are all layering phenomena.

Layering is expressed in the alternation of different types of rocks, which are clearly separated from each other. It reflects the hydrodynamics of the transport and deposition media. The sedimentation environment is constantly in motion: the velocity of the near-bottom currents changes, wave activity is manifested, the amount of clastic material is changed, etc. Layering is caused by more or less rhythmic fluctuations in the intensity of certain sedimentation factors.

Three main types of primary stratification are distinguished by morphological features: horizontal, wavy and oblique [14, 15].

Horizontal layering is characterized by an alternation of puffs and layers parallel to the layering plane (Figure 22). It is formed in calm conditions, outside currents and unrest. Thin horizontal stratification can be formed in a calm environment in the bottom layer and depends on the intensity of the sediment input and its mechanical properties.

Fig. 22. Horizontal stratification

Wavy layering is an alternation of puffs that have a curved convex-concave shape (Figure 23). This type of stratification characterizes the excitement, that is, the multidirectional movements of water, which, depending on the strength and magnitude of the waves, form different forms of stratification. Wavy stratification indicates a depth of no more than 70 m, occurs mainly in shallow and coastal-marine, flooded, less often - in floodplain sediments.

Fig. 23. Wavy layering

Slanting layering usually occurs when the water moves in a certain direction and indicates accumulation of precipitation under conditions of high dynamic activity of the medium (Figure 24).

Slices are straight and curved. The slope angles of the pavers with respect to the bedding plane are steep (& gt; 30 °), medium (30-20 °), shallow (<20 °).

The ratio of puffs can be unidirectional (parallel) and multidirectional (wedge-shaped).

Fig. 24. Skew layering

Fig. 25. Varieties of complex layered textures

Layered textures are widely used in sedimentary rocks, which are combinations of several types of layering (Figure 25). Such textures characterize a rapid and abrupt change in the activity of sedimentation media and the formation of a new layering against the background of the previous one, sometimes cutting it off.

Deformation textures are formed in sedimentary rocks with in-situ disturbances of horizontal stratification, caused by different density of unsettled precipitation, sliding along the bottom and paleoscills.

Load and subsidence textures occur when the density of a laminate is unstable, most often when the sand layer is deposited on a less dense water-saturated layer formed by clay material. With any vibrations of the Earth, the clay material liquefies, causing a loss of its strength. The sandy material forms so-called penetration pockets , in the form of elongated spheroids immersed in a clay mass (Figure 26).

Fig. 26. Load and subsidence textures

Sand dikes are associated with sandy rocks. When water-saturated sands are compacted and their volume is reduced, pore water is squeezed out. The extracted water rises upward at any rate with any cracks, grabbing sand grains, and carries them to the surface of the sediment (Figure 27).

Fig. 27. Sand dikes

The texture of landslide , rupture and collapse (Figure 28) are formed when deformed unconsolidated precipitation as a result of motions caused by gravity on paleoscills.

The water-saturated clay sediment is unstable and, in all shakes, begins to move along the paleoscillon, involving layers lying over the sliding surface in this motion. Small-scale folds appear, large folds with steep angles of inclination of the wings. Sliding textures are formed at slow sliding. Sometimes small fracture discontinuities are formed - the fracture textures are formed.

Creep usually grabs several alternating layers. Low-power layers are broken up into separate fragments, which can be separated to a different degree from each other. Formations of caving textures are formed.

Fig. 28. Deformational textures on paleoslopes

Biogenic textures are created by the activity of animals and plants or their remains.

The fossil remains of fauna - fossils - are represented by shells, leaves, internal cores, casts, fragments of skeletons, various fragments and their clusters, as well as small indefinable shell detritus (Figure 29). > Worms and some other organisms that do not have a solid skeleton or shell are not preserved in a fossil state, but in the sediment where they lived, there are traces of their life activity, which are called themfossils (Figure 30) . These textures differ from these organisms in that they can not be processed and repartitioned, while characterizing the sedimentation situation.

Preservation of plant residues (Fig. 31) depends on the conditions of formation of sediment. The finest branches, the leaves remain only in a calm atmosphere at the site of sedimentation. During the transfer, plant residues break down, their magnitude depends on the activity of the transport medium and the distance. Sometimes in the rock there are remains of root systems that violate the primary interlayer texture.

Fig. 29. Fossils

Fig. 30. Ihnofossils

Fig. 31. Vegetable residues

Post-sample (secondary) textures

Diagenetic textures begin to form in the early stage in the form of oolites, small crystals spreads of poorly consolidated sediment throughout the layer. Later, the growth of individual crystals in this soft rock promoted the formation of stellar forms (Figure 32).

The redistribution of diagenetic minerals due to the inhomogeneity of the medium led to the formation of concretions (Figure 33). In the rock, they are located along the bedding planes, and also confined to the boundaries of the section of the puffs. In the presence of layering, the growth of nodules leads to a deformation of the bedding, the puffs flow around the concretion.

The diagenetic formations also include the various piracy pseudomorphs on shells and charred plant residues.

Fig. 32. Early diagenetic minerals

Fig. 33. Diagenetic textures

To catagenetic textures (Figure 34) include a variety of cracks , made by newly formed minerals. In limestones, secondary crystalline calcite is released in open fissures and pores of the rock. Quartz veins, dendritic-like cracks and inclusions made with iron hydroxides are widely developed in sandy-silty rocks.

Fig. 34. Catagenetic textures

The textures of metagenesis are formed by pressure and recrystallization. In clayey rocks, shale and planar textures are formed, characterized by waviness and fine crumple of the finest layers (Figure 35).

Fig. 35. Flickering texture in clay-carbonaceous rock

thematic pictures

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