Rocks of clastic (mechanical) origin

Breeds clastic(mechanical) origin are products of mechanical destruction of any parent rocks and are composed mainly of fragments of weathering-resistant minerals and rocks. They are divided according to the size of the fragments into coarse-clastic, medium-clastic (sandy), fine-clastic (silty) and fine-clastic (clayey). Among them, only clayey rocks are products of chemical decomposition of parent rocks, while the remaining rocks are composed of fragments that have not undergone significant weathering. Regardless of particle size, clastic rocks can be loose or cemented.

Loose coarse clastic rocks include varieties with rounded and angular shapes, formed as a result of the accumulation of large fragments. Among them are fragments measuring 1000... 100 mm, called boulders (rounded) or blocks (angular); 100 ... 10 mm - pebbles (rounded) or crushed stone (angular), 10 ... 1 mm - gravel (rounded) or rubble (angular).

Boulders (boulder stone) consist of roughly rounded fragments processed and transported by water or a glacier. According to its genesis, boulder stone can be glacial, river, sea, or lake. Its smaller varieties measuring 120... 300 mm are called cobblestone. Large boulder stone supplied for construction requires preliminary processing into piece material - checker, rubble stone, etc.

Pebbles and gravel are formed similarly to the first, when fragments are transported over long distances by rivers, mountain streams, and also under the influence of sea surf, while acquiring varying degrees of roundness and sorting. The quality of gravel is determined by its genesis, mineral composition, content of clay and organic impurities, etc. The best type of gravel is considered to be glacial, which is less rounded. Gravel is used in reinforced concrete structures, road construction and as a filter material.

Blocks, crushed stone and debris They are accumulations of angular fragments of rocks, heterogeneous in mineral composition. These deposits are especially characteristic of desert and polar regions with intense processes of physical weathering. They are quite widespread in the middle and northern zone of the European part of our country.

Sandy (medium clastic) rocks They are a loose mixture of grains with sizes from 1 to 0.1 mm. They are usually divided according to grain size into coarse-grained with a particle diameter from 1 to 0.5 mm; medium-grained - 0.5 ... 0.25 mm; fine-grained - from 0.25 to 0.1 mm. The sands consist predominantly of quartz, the most resistant mineral to chemical weathering. Pure quartz sands of light color are monomineral rocks. Mixed (polymict) sands consist of a mixture of minerals, in which, in addition to quartz, there are feldspars, micas, amphiboles, etc. Among them, the most widespread are arkosic sands of red or gray color, predominantly of an acidic feldspathic composition, with a small admixture of quartz and other minerals. Marine and aeolian sediments are distinguished by the greatest purity and uniformity of grains; sea ​​and river sands have rounded grains, and glacial sands have angular, most favorable for construction purposes, grain shapes. Clay and silt fractions (0.05... 0.005 mm) are harmful impurities in sand. When assessing the quality of sand as a building material, its mineral and granulometric composition, grain shape, porosity, filtration coefficient, etc. are taken into account. The density of sand is 2.64 g/cm3, and the average density is 1800 kg/m3. They are the main raw materials for the production of ceramics, dinas, glass, concrete and mortar, brick; used for road surfaces and in abrasive production. Distributed everywhere.

Silty (fine clastic) rocks consist of particles ranging in size from 0.1 to 0.01 mm and differ from sand in the content of smaller particles. A representative of these rocks is loess, a light-colored porous (46...50%) rock containing quartz, feldspars, up to 30% calcite and up to 50% clay minerals. The density of loess is 2.5 ... 2.8 g/cm3, and the average density is 1200 ... 1800 kg/m3.

They are characterized by subsidence and easily soak in water. They are used in the cement industry as an additive to concrete, in the production of bricks, tiles, etc. They are widespread in the European part of our country, in the south of Ukraine, in Central Asia, and southern Siberia.

Clayey (fine clastic) rocks consist of more than half of tiny (less than 0.01 ... 0.001 mm) scale-like particles of clay minerals, of which at least 25% have dimensions less than 0.001 mm. Clays are formed during the weathering of feldspathic and some other silicate rocks and consist predominantly of clay minerals such as kaolinite, montmorillonite and hydromica with an admixture of quartz, mica, secondary calcite, opal, etc. Most clays are polymineral, but among them there are the most valuable monomineral ones: kaolinite and montmorillonite varieties. The main factor in the use of clays in construction and the production of building materials is their mineral composition.

Polymineral clays are raw materials for the production of brick and tile products, rough ceramics, alumina, refractories, etc.

Kaolinite clays are composed mainly of kaolinite and are relatively free from impurities of iron oxides. They are white, fine-grained, greasy to the touch, low-plasticity rocks, which are products of the decomposition (hydrolysis) of aluminosilicates by dissociated water containing free hydrogen ions and dissolved CO2.

Kaolinite clays are continental sediments and form under acidic conditions. They are used in the production of porcelain and earthenware products, cement, and fireclay. Deposits of kaolinite clays are located in Ukraine, the Urals, Siberia, etc.

Montmorillonite clays appear during the decomposition of volcanic ash in an alkaline environment. Among them are soda clays that strongly swell in water, with a predominance of the Na cation over the Ca, Mg and K cations, and non-swelling calcium clays, with a predominance of Ca over the Na and Mg cations. The first include bentonites and floridins, rocks of white, grayish-white, pinkish and other colors, characteristic feature which are characterized by strong swelling when moistened with an increase in volume by approximately 16 times or more and high adsorption capacity. Most of these clays have a pronounced plasticity when mixed with water, retaining their given shape when dried, and after firing they form stone-like masses. With an increase in mechanical impurities in clays, their plasticity quickly decreases. Montmorillonite clays are used as excellent adsorbents, as they have high absorption capacity. Their deposits are found in Georgia, Crimea, the Dnieper region, Transcarpathia, and Central Asia.

Cemented clastic rocks were formed by the cementation of loose rocks by a variety of chemicals. The most durable is siliceous cement (secondary quartz, opal, chalcedony), less durable is ferruginous (limonite), carbonate (calcite) and clay cement has low cementing ability. Below is a description of the main representatives of this group.

Breccia are compact rocks consisting of angular fragments of gruss or crushed stone, cemented with some kind of cement. The petrographic composition of these fragments is homogeneous. The angular shape of the fragments ensures good adhesion to natural cements, so breccias with some types of cements have quite high strength and are used as finishing stones. Breccias have a limited distribution.

Conglomerates- accumulations of pebbles, gravel, small boulders, etc., cemented with natural cement, differing from breccias in the diversity of petrographic composition, a wide range of strength from 5 to 160 MPa and a change in average density in the range of 1500 ... 2900 kg/m3. Compared to breccias, conglomerates are less durable, since the rounded clastic material is rather weakly bonded to the cement. The practical significance of these rocks is small, but their characteristic structure (binding loose material) is the prototype of the most common ISC structure. Their weakly cemented varieties are used to obtain ballast, and beautiful ones are used as finishing decorative stones. Thick deposits of conglomerates are known in the Crimea and Central Asia.

Sandstones are formed by cementation of sand grains when various mineral solutions seep through them. Depending on the type of cement, siliceous, calcareous, ferruginous, gypsum, clayey, bituminous and other types of sandstones are distinguished. Their strength is determined by the type of natural cement, the nature of its adhesion to sand grains, and the density of the rock. It varies widely from 1 to 150 MPa and above, and the average density is from 1900 to 2800 kg/m3. The most durable (100... 150 MPa and more) are siliceous sandstones with medium density up to 2800 kg/m3. Clay sandstones are characterized by low strength and are easily destroyed when saturated with water or cyclic freezing and thawing; calcareous sandstones are not water-resistant. In bituminous sandstones, the bitumen that permeates the rock strata accounts for up to 20% of their mass. The color of sandstones depends on the cement: siliceous and calcareous have white and light tones, ferruginous - yellow and reddish, etc. They are widespread in Karelia, Ukraine, the Volga region, etc. and are used to produce wall stone, rubble, crushed stone, as well as decorative finishing material. Their varieties, containing at least 97% silica, are used for the production of acid-resistant materials and raw materials, for the production of refractories, abrasives, etc.

Sedimentary rocks occupy an impressive area of ​​the globe. These include most of all the minerals that our planet is so rich in. Most sedimentary rocks are located on the mainland, continental slope and shelf, and only a small part is located on the bottom of seas and oceans.

Origin of sedimentary rocks

Under the destructive influence of sunlight, temperature fluctuations, and water, solid igneous rocks are weathered. They form fragments of various sizes, which gradually disintegrate into the smallest particles.

Wind and water transport these particles, which at some stage begin to settle, thereby forming loose accumulations on the land surface and at the bottom of water bodies. Over time, they harden, become denser, and acquire their own structure. This is how sedimentation occurs rocks.

Rice. 1. Sedimentary rocks

Like metamorphic rocks, sedimentary rocks are classified as secondary rocks. They lie only on the surface earth's crust, occupying about 3/4 of the area of ​​the entire planet.

Since almost everything construction works conducted on sedimentary rocks, it is very important to know perfectly the properties, composition and “behavior” of this type of rock. The science of engineering geology deals with these and many other issues.

The main feature of sedimentary rocks is layering, unique to each natural compound. As a result of shifts in the earth's crust, the original forms of occurrence of sedimentary rocks are disrupted: all kinds of breaks, cracks, faults, and folds appear.

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Rice. 2. Layering of sedimentary rocks

Rock classification

The deposition process can take place different ways. Depending on its specificity, several main groups of sedimentary rocks are distinguished:

• clastic - formed under the influence of weathering and further transfer of igneous rock particles;
• chemogenic - the result of the isolation and precipitation of substances that are formed from saturated aqueous solutions;
• biochemical - are formed as a result of chemical reactions with the participation of living organisms;
• biogenic - the result of decomposition of the remains of plant and animal organisms.

In nature, mixed groups of sedimentary rocks are often found, the formation of which was influenced by several factors. Thus, one of the striking examples of mixed-type sedimentary rocks is limestone, which can equally be of chemogenic, organogenic, biochemical or clastic origin.

Rice. 3. Limestone

What have we learned?

Sedimentary rocks occupy vast areas of the Earth's surface. They can be located both on land and at the bottom of seas and oceans. Any sedimentary rock is formed from destroyed and modified igneous rocks. The classification of rocks is based on the characteristics of the sedimentation process, which can occur under the influence of many factors.

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Clastic, or clastic (Greek klastes - fragment), rocks are formed from fragments of minerals and rocks; most often they accumulate as marine sediments. The classification of clastic rocks is based on the size of the fragments (coarse, sandy, silty), the degree of their roundness (rounded and unrounded) and the presence or absence of cement (cemented and loose). The classification of clastic rocks is shown in Table 2.

Coarse clastic rocks, or psephites (Greek psephos - pebble), consist of fragments with dimensions greater than 2 mm. According to shape and size, they are divided into rounded and unrounded, large, medium and small. Rounded fragments include fragments that have rounded or smoothed edges ( boulders, pebbles, gravel and etc.); unrounded fragments are always acute-angled ( blocks, crushed stone, debris). Psephites with rounded fragments held together by cement are called conglomerates(Fig. 24, a), and those consisting of unrounded cemented fragments - breccias(Fig. 24, b).

Table 2 Clastic rocks.

Breed group	Dimensions of fragments, mm	Loose rocks		Cemented rocks
		rounded	unrounded	rounded	unrounded
Coarse clastics (psephytes)				Boulder conglomerates	Block breccias
		Pebbles, pebbles		Pebble conglomerates
				Gravel conglomerates
Sandy (psammites)		coarse-grained		Sandstones: coarse-grained
		coarse-grained		coarse-grained
		medium grain		medium grain
		fine-grained		fine-grained
Silts		Silts		Siltstones
				Mudstones

Among breccias, several types of different origins are distinguished. To sedimentary include breccias formed as a result of deposition of acute-angled fragments of various compositions in an aquatic environment; landslide breccias contain fragments of various sizes that have the same composition as cement; tectonic breccias bear traces of pressure, broken by cracks; In them, both on the fragments and in the cement, smooth, seemingly polished surfaces – sliding mirrors – are often found.

Sandy rocks, or psammits (Greek psammos - sand). The psammite group includes rocks with fragment sizes ranging from 0.1 to 2 mm. Loose varieties of psammites are called sands, and cemented varieties are called sandstones (Table 2).

Psammites consisting of grains of one mineral - quartz, glauconite, etc., are called oligomictic(Greek oligos - few, miktos - mixed), and consisting of several minerals - polymictic(Greek poly – many, miktos – mixed). Based on the relative size of the grains, psammites are divided into uniformly grainy(sorted) and heterogeneous(unsorted).

Based on their mineral composition, the following main groups of sand rocks are distinguished.

Quartz sands and sandstones, in which, in addition to quartz, feldspars, micas, glauconite, etc. are found as impurities; the cement of such sandstones can be siliceous, clayey, calcareous, ferruginous, phosphorite, etc.

Magnetic and garnet sands and sandstones consist of grains of quartz and glauconite, are rare.

Quartz-glauconitic sands and sandstones consist of grains of quartz (20-40%) and glauconite (60-80%) with a small admixture of mica and other minerals, depending on the amount of glauconite and the intensity of its color, the sands have a more or less bright green color. During weathering, which is accompanied by the decomposition of glauconite and the formation of iron oxides, their color becomes rusty-brown.

Ferrous sands and sandstones usually they are quartz sands and sandstones, the grains of which are covered with a crust of brown ferruginous minerals - goethite and hydrogoethite; The cement of sandstones is also ferruginous, so the color of the rocks is brown - from lilac-brown to rusty brown.

Arkose sands and sandstones are formed during the destruction of granitoids, therefore they contain quartz, feldspars and a small amount of dark-colored minerals - biotite, hornblende, pyroxene; The composition of sandstone cement is varied.

Graywacke – dark gray, greenish-brown or greenish-brown, often densely cemented psammites, composed mainly of grains of dark-colored minerals - amphiboles, pyroxenes, etc. These are typical polymictic formations.

Silts(loose) Andsiltstones(dense) composed of mineral particles ranging in size from 0.1 to 0.01 mm. Silts include loess, sandy loam (silt material with sand), loam (silt material with clay) and some other rocks. Dense siltstones have cement that is slightly different from the cement of sandstones.

Pelites, or clay (Greek peles - clay), is a large group of rocks formed as a result of grinding mineral particles to sizes of 0.01 mm or less, occurring in the process of grinding and chemical decomposition. In terms of their basic properties, pelites differ from clastic rocks: being small in size, pelite particles do not settle to the bottom under the influence of gravity, but form suspensions.

Clays are rocks that form a plastic mass with water that hardens when dried, and when fired, acquires the hardness of stone. When dry, clays are either earthy, loose, easily crumbled and rubbed, or very dense. When saturated with water, this rock swells, softens and turns into a plastic viscous mass, which, with further addition of water, acquires the ability to flow; Due to its hygroscopicity, it is capable of absorbing up to 70% (by volume) of water, and after complete saturation with water it becomes waterproof and does not allow water to pass through. Pure clays are called bold, A with a significant admixture of sand - skinny. Depending on the amount of sand there are different sandy clays or clayey sands; clays with an admixture of calcium carbonate are called calcareous.

Kaolins – white clays composed of kaolinite, formed during weathering of feldspathic rocks. In the weathering crust, kaolins contain impurities of quartz grains, mica flakes and other weathering-resistant minerals that are part of the original rock.

In the weathering crust of rocks containing aluminosilicates - granitoids, etc., specific rocks are often found - bauxite. These are dense rocks, colored red, less often gray, consisting mainly of aluminum oxides, often with an admixture of iron oxides, and having a clastic or oolitic structure.

Mudstones are dense, hard (hardness up to 3) rocks formed as a result of clay diagenesis. The latter lose a number of characteristics, such as plasticity and water absorption.

These classes include the well-known loose rocks - sand, crushed stone, pebbles, gravel; cemented rocks, among which the most famous is sandstone, as well as clayey rocks - clay, loam, sandy loam.

The named rocks differ greatly from each other in composition and properties, but in nature the transition from clastic rocks to clayey rocks is very gradual, with big amount mixed differences, which makes it necessary to consider these classes within one section.

Classification

The section examines five classes of rocks: coarse-clastic, sandy, fine-clastic, clayey and mixed. For brevity, we will agree to call them all clastic and clayey. As can be seen, they are all classified according to size, clast shape, cementation and connectivity.

Sedimentary clastics, clayey and mixed rocks

Structure and particle size, mm	Breed name
	Texture
	Uncemented		Cemented		Messenger
	Angular wreckage	Rounded wreckage	Angular wreckage	Rounded wreckage
1. Coarse clastics: more than 1000	Neoka- data blocks	Lumps	Blocky breccia	Blocky conglomerate
200-1000	Neoka- data boulders (stones)	Boulders	Valunnaya breccia	Boulder conglomerate
10-200	Crushed stone	Pebble	Bre^ia	Conglo- merat
2-10	Dresva (small crushed stone)	Gravel	Small breccia	Gravelite
2. Middle clastic - sandy (0.05-2):	Sands (according to the predominant fraction): gravelly (rough)		Sandstones (by predominant fraction): gravelly (rough)
0,5-1	large		large
0,25-0,5	average		average
0,1-0,25	small		small
0,05-0,1	dusty (thin)		dusty (thin)
3. Fine clastic - dusty: 0.002...0.05	Silt		Siltstone		Loess
4. Micro-grained - clayey: less than 0.002 (0.005)	Clay		Argillite		Clay
5. Mixed	Silty-clayey sand with crushed stone and gravel, pebbles with sandy gravel filler, etc.		Sandy conglomerate, sandy gravel, etc.		Clay, loam, sandy loam

Compound

These rocks consist of products of mechanical and chemical destruction and transformation of other rocks on the surface of the earth. In the vast majority of cases, they are soil-forming material; most of the construction and other environmental management is carried out on them; they are most often called “soil”.

The composition of clastic and clayey rocks consists of three main components - fragments, cement and clayey material.

Clastic material

Clastic material- the main component of clastic rocks is stone material consisting of blocks, boulders, pebbles, gravel, crushed stone, grains of sand forming sand, quartz mineral dust. All this can be represented by various rocky or semi-rocky rocks, and the name of the original rock can only be mentioned - granite crushed stone, limestone pebbles, quartz sand. Cobblestone, rubble, pebbles, cobbles - natural or specially processed and selected stone tens of centimeters in size, used in construction for paving roads and laying foundations.

Based on their shape, there are two main types of fragments - angular and rounded; there are also several transitional types between them.

Stone fragments of various shapes

a - angular; b - rounded (rounded); c - semi-rounded

The widespread moraine is usually called gravelly loam, while the stone inclusions present in it are more likely to be closer to rounded pebbles than to angular crushed stone.

The fragments are angular in shape.They are formed during weathering and breaking off pieces from the bedrock.In nature, this process is most intensively developed on slopes; the resulting debris accumulates at the foot of the slopes, forming stone screes. With horizontal relief, angular fragments remain in place, and the weathering process quickly fades with depth. This is how weathering crusts are formed.

Angular fragments in the composition of weathering crust and stone scree

Scree rocks and weathering crusts, depending on the size of the fragments, are called blocks, crushed stone, gruss, or cartilage. They can serve building material in places of their distribution, although crushed stone, blocks, etc. are actually used in construction. much more often they are artificially crushed stones, mined in quarries using explosions. Based on them, it is possible to obtain more durable materials for construction than when using weathered and cracked natural stone, especially since the majority of the Russian population lives in flat areas where these screes and weathering crusts are practically absent.

Rounded (rounded) fragments acquire this shape as a result of treatment with water (sea surf, rivers, water-glacial flows), and less often - with wind. Boulders are formed from angular blocks, pebbles are formed from crushed stone, and gravel is formed from rubble (fine crushed stone). The smaller the fragments, the more often they are round. For example, sands with angular fragments occur in nature, but are extremely rare. The silty fraction—quartz fragments measuring 0.002–0.05 mm in size—is always round. Due to their small size, they begin to demonstrate colloidal properties - they easily stick together, and when agitated, they slowly settle in water.

Cement

Some breeds in nature resemble in their build such well-known artificial materials, like hardened cement mortar or concrete, in that they consist of stone fragments held together with cement. It is possible that the idea of ​​​​creating concrete was borrowed by people from nature. Natural cement is similar in composition to some chemical sedimentary rocks. It can be carbonate, siliceous, sulfate, ferruginous and clayey - then it is called clay aggregate. Carbonate cement is similar in composition to chemical limestone and is determined by its reaction with acid. Siliceous is the most durable and hardest of cements; sometimes it has a greasy sheen and does not react with acid. Sulfate is not durable, it can be scratched with a fingernail, and sometimes sugar-like crystals are visible on it. Ferrous cement is recognized by its rusty color. Clay cement is scratched with a fingernail and becomes soaked in water.

The formation of cement is possible in two ways:

• in marine conditions with simultaneous accumulation of chemical sediment along with debris;
• due to the precipitation of chemical material from groundwater within the clastic strata after its accumulation.

Rocks with different types of cement

a - basal cement; b - pore cement; in - contact

Clay minerals

In coarse-grained rocks, clay minerals can play the role of filler between stone particles and actually act as cement. When clay minerals are mixed with sandy and fine-clastic material, so-called clayey rocks are formed - loams, sandy loams and natural clays. Clay minerals acquire the role of the main component, giving the entire mixture the properties of clay rocks, the main of which are moisture capacity, water impermeability and cohesion - the ability to become plastic when moistened and hard when dried.

Structure, granulometric and mineral composition

These characteristics are closely related to each other. The structure of the material is determined depending on the particle size. Particles of a certain size are usually called fractions. The boundaries of the fractions are taken according to GOST 25100-2011 “Soils”, they, with very minor changes, repeat the boundaries accepted in the geological literature, only the names of the fractions differ; geological data are given in parentheses.

Structures and approximate composition of clastic, clayey and mixed rocks

Structure and fraction - particle size	Approximate composition
1. Coarse clastic (psephytes) - larger than 2 mm	Fragments of any rocks
2. Medium clastic - sandy (psammites) - 0.05-2 mm	Quartz predominates, feldspar may be present, and there are very few other minerals
3. Fine clastic - silty (silt) - 0.002-0.05 mm	Quartz - almost the entire faction
4. Micro-grained - clayey (pelites) - less than 0.002 mm (less than 0.005 mm)	Kaolinite, montmorillonite, glauconite and other clay minerals, quartz, limonite
5. Mixed - clastic-sandy, sandy-clayey, etc.	Various mixtures of particles of fractions 1–4

It is known that the finer the material is crushed, the faster it dissolves and enters into chemical reactions. Therefore, among large-sized fragments (blocks, boulders, crushed stone, pebbles), almost all rocks are found with the exception of the most soluble ones - gypsum, anhydrite, rock and other salts. Among the medium-sized fragments one finds mainly quartz, the most weathering-resistant mineral, less commonly feldspar, and even more rarely other minerals. Medium clastic rocks are sands.

Among the fine-clastic (silty) particles, there are almost no other minerals except quartz. Rocks: loess, siltstone, siltstone.

Micrograined rocks are composed of kaolinite, montmorillonite, hydromicas and other clay minerals. The type is pure clay.

Mixed rocks - most often a mixture of sand, silt and clay fractions - these are clays, loams and sandy loams. The terms “sandy-clayey” and “clayey rocks” are widely used, used as synonyms.

The percentage weight content of particles of various fractions is calledgranulometric composition (granular composition). To determine it, a soil sample is passed through a set of sieves with further weighing of each fraction. Next, according to a small set of rules, the breed is given a formally correct name. This applies to unconsolidated coarse, sandy and partly some clayey rocks, which will be discussed below.

Division of coarse and sandy soils

Varieties of coarse soils and sands	Particle size, mm
Coarse:
boulder (blocky)	>200	> 50
pebble (crushed stone)	> 10	> 50
gravel (wood)		> 50
Sands:
gravelly
large	>0,50	> 50
medium size	>0,25	> 50
small	> 0,10
dusty	>0,10	< 75

Correctly naming sandy and clayey soils is an important task in geology and soil science. The type of soil (in fact, the name) determines the various tabular values ​​of the parameters included in the foundation calculations, which is important for designers. Therefore, the granular composition, along with other laboratory properties of soils, is one of the most important indicators of properties and is determined en masse during surveys.

As you can see, everything begins in mountainous conditions with weathering, landslides and shedding of angular stone fragments - sonatural blocks and crushed stone. During the process of weathering (chemical),clay minerals, which are easily carried away by water, and if granites and gneisses, which are very common in nature, are destroyed, then detrital quartz with sandy and silty particles is also formed.

Scheme of formation of clastic rocks

Due to gravity, slope processes, temporary water flows and rivers, angular clastic material reaches the sea coast. Here the material formed due to the destruction of the shore by waves is added to it. In the surf zone, the stone material is further crushed, the fragments are rounded, and boulders, pebbles, gravel, sand and quartz dust - alewrite material. Some of the material dissolves. Waves and sea currents carry sediments to greater depths, where perhaps cementady and transformation into cemented analogues occur - conglomerates, gravelites, sandstones, siltstones.

Similar processes on a smaller scale can occur due to the geological work of mountain rivers, glaciers and water-glacial flows. If there is no rounding phase, then during cementation of angular material,sedimentary breccias.

Tectonic brecciasare formed in zones of tectonic disturbances. Clastic material is produced by the movement of tectonic blocks along fault planes, and cementation is produced by the release of chemical sediment from groundwater that easily circulates through the fractured zone.

SEDIMENTARY, SANDY, CLAY AND MIXED (SAND-CLAY) ROCKS

Clastic rocks and their classifications

These classes include well-known loose rocks - sand, crushed stone, pebbles, gravel; cemented rocks, among which the most famous is sandstone, as well as clayey rocks - clay, loam, sandy loam.

The named rocks differ greatly from each other in composition and properties, but in nature the transition from clastic rocks to clayey rocks is very gradual, with a large number of mixed varieties, which makes it necessary to consider these classes within one section.

Classification. The section examines five classes of rocks - coarse-grained, sandy, fine-grained, clayey and mixed. For brevity, we will agree to call them all together clastic and clayey. As can be seen, they are all classified according to size, clast shape, cementation and connectivity (Table 3.5).

Sedimentary clastics, clayey and mixed rocks

Table 3.5

Structure and particle size, mm	Breed name
	Texture
	Uncemented		Cemented
	Angular	Rounded	Angular	Rounded
1. Coarse clastics: more than 1000			Blocky	Block conglomerate
	Neo-rocked boulders (stones)		Valunnaya	Boulder conglomerate
		Pebble		Conglomerate
				Gravelite
2. Medium clastic - sandy (0.05-2):	Sands (according to the predominant fraction): gravelly (rough)		Sandstones (according to the predominant fraction): gravelly (rough)
	dusty (thin)		dusty (thin)
3. Fine clastic - dusty: 0.002...0.05			Siltstone
4. Micrograined - clayey: less than 0.002 (0.005)			Argillite
5. Mixed	Silty-clayey sand with crushed stone and gravel, pebbles with sandy gravel filler, etc.		Sandy conglomerate, sandy gravel, etc.		loam,

Compound. These rocks consist of products of mechanical and chemical destruction and transformation of other rocks on the surface of the earth. In the vast majority of cases, they are soil-forming material; most of the construction and other environmental management is carried out on them; they are most often called “soil”.

The composition of clastic and clayey rocks consists of three main components - fragments, cement and clayey material.

1. Clastic material - The main component of clastic rocks is stone material consisting of blocks, boulders, pebbles, gravel, crushed stone, grains of sand that form sand, and quartz mineral dust. All this can be represented by various rocky or semi-rocky rocks, and the name of the original rock can only be mentioned - granite crushed stone, limestone pebbles, quartz sand. Cobblestone, rubble, pebbles, paving stones are natural or specially processed and selected stones tens of centimeters in size, used in construction for paving roads and laying foundations.

Based on their shape, there are two main types of fragments - angular and rounded; there are also several transitional types between them (Fig. 3.12).

Rice. 3.12. Stone fragments of various shapes: A- angular; b- rounded (rounded); V- semi-rounded

The widespread moraine is usually called gravelly loam, while the stone inclusions present in it are more likely to be closer to rounded pebbles than to angular rubble.

1.1. The fragments are angular in shape. They are formed by weathering and breaking off pieces from bedrock.

In nature, this process is most intensively developed on slopes; the resulting debris accumulates at the foot of the slopes, forming stone screes. With horizontal relief, angular fragments remain in place, and the weathering process quickly fades with depth. This is how weathering crusts are formed (Fig. 3.13).

Rice. 3.13.

Scree rocks and weathering crusts, depending on the size of the fragments, are called blocks, crushed stone, gruss, or cartilage. They can serve as a building material in the places where they are distributed, although crushed stone, blocks, etc. are actually used in construction. much more often they are artificially crushed stones, mined in quarries using explosions. Based on them, it is possible to obtain more durable materials for construction than when using weathered and cracked natural stone, especially since the majority of the Russian population lives in flat areas where these screes and weathering crusts are practically absent.

• 1.2. Rounded (rounded) wreckage they acquire this form as a result of treatment with water (sea surf, rivers, glacier flows), less often - with wind. Boulders are formed from angular blocks, pebbles are formed from crushed stone, and gravel is formed from gruss (fine crushed stone). The smaller the fragments, the more often they are round. For example, sands with angular fragments occur in nature, but are extremely rare. Silt fraction - quartz fragments 0.002-0.05 mm in size are always round. Due to their small size, they begin to demonstrate colloidal properties - they easily stick together, and when agitated, they slowly settle in water.
• 2. Cement. Some rocks in nature resemble in their composition such well-known artificial materials as hardened cement mortar or concrete, in that they consist of stone fragments held together with cement. It is possible that the idea of ​​​​creating concrete was borrowed from nature by people. Natural cement is similar in composition to some chemical sedimentary rocks. It can be carbonate, siliceous, sulfate, ferruginous and clayey - then it is called clay aggregate. Carbonate cement is similar in composition to chemical limestone and is determined by its reaction with acid. Siliceous cement is the most durable and hard cement; sometimes it has a greasy sheen and does not react with acid. Sulfate is not durable, it is scratched with a fingernail, and sometimes sugar-like crystals are visible on it. Ferrous cement is recognized by its rusty color. Clay cement is scratched with a fingernail and becomes soaked in water.

The formation of cement is possible in two ways:

• 1) in marine conditions with simultaneous accumulation of chemical sediment along with debris;
• 2) due to the precipitation of chemical material from groundwater within the clastic strata after its accumulation.

Rocks with the most common types of cementation are shown in Fig. 3.14.

Rice. 3.14. Rocks with different types of cement: A- basal cement; b - pore cement; V- contact

3. Clay minerals. In coarse rocks, clay minerals can act as a filler between rock particles and actually act as cement. When clay minerals are mixed with sandy and fine-clastic material, so-called clayey rocks are formed - loams, sandy loams and natural clays. Clay minerals acquire the role of the main component, giving the entire mixture the properties of clay rocks, the main of which are moisture capacity, water resistance and cohesion - the ability to become plastic when moistened and hard when dried.

Structure, granulometric and mineral composition. These characteristics are closely related. The structure of the material is determined depending on the particle size. Particles of a certain size are usually called fractions. The boundaries of the fractions are taken according to GOST 25100-2011 “Soils”, they, with very minor changes, repeat the boundaries accepted in the geological literature, only the names of the fractions differ; geological data are given in brackets (Table 3.6).

Table 3.6

Structures and approximate composition of clastic, clayey and mixed rocks

Structure and fraction - particle size	Approximate composition
1. Coarse clastic (psephytes) - larger than 2 mm	Fragments of any rocks
2. Medium clastic - sandy (psammites) - 0.05-2 mm	Quartz predominates, feldspar may be present, other minerals are very few
3. Fine clastic - silty (silt) - 0.002-0.05 mm	Quartz - almost the entire faction
4. Micro-grained - clayey (pelites) - less than 0.002 mm (less than 0.005 mm)	Kaolinite, montmorillonite, glauconite and other clay minerals, quartz, limonite
5. Mixed - clastic-sandy, sandy-clayey, etc.	Various mixtures of particles of fractions 1-4

It is known that the finer the material is crushed, the faster it dissolves and enters into chemical reactions. Therefore, among large-sized fragments (blocks, boulders, crushed stone, pebbles) almost all rocks are found with the exception of the most soluble ones - gypsum, anhydrite, rock and other salts. Among the medium-sized fragments, one finds mainly quartz, the most weather-resistant mineral, less commonly feldspar, and even more rarely other minerals. Medium clastic rocks are sands.

Among the fine-clastic (silty) particles, there are almost no other minerals except quartz. Rocks: loess, siltstone, siltstone.

Micrograined rocks are composed of kaolinite, montmorillonite, hydromicas and other clay minerals. The rocks are pure clay.

Mixed rocks - most often a mixture of sand, silt and clay fractions - these are clays, loams and sandy loams. The terms “sandy-clayey” and “clayey rocks” are widely used, used as synonyms.

The percentage by weight of particles of different fractions is called granulometric composition (by personnel). To determine it, a soil sample is passed through a set of sieves with further weighing of each fraction. Next, according to a small set of rules, the breed is given a formally correct name (Table 3.7). This applies to unconsolidated coarse, sandy and partly some clayey rocks, which will be discussed below.

Table 3.7

Division of coarse and sandy soils

Correctly naming sandy and clayey soils is an important task in geology and soil science. Various tabular values ​​of the parameters included in foundation calculations depend on the type of soil (in fact, on the name), which is important for designers. Therefore, the granular composition, along with other laboratory properties of soils, is one of the most important indicators of properties and is determined en masse during surveys.

Origin of clastic rocks shown schematically in Fig. 3.15.

As you can see, everything begins in mountainous conditions with weathering, landslides and shedding of angular stone fragments - this is how natural blocks And crushed stone During the process of weathering (chemical), clay minerals, which are easily carried away by water, and if granites and gneisses, which are very common in nature, are destroyed, then detrital quartz with sandy and silty particles is also formed.

Rice. 3.15.

Due to gravity, slope processes, temporary water flows and rivers, angular clastic material reaches the sea coast. Here the material formed due to the destruction of the shore by waves is added to it. In the surf zone, the stone material is further crushed, the fragments are rounded, and boulders, pebbles, gravel, sand And quartz dust- material silts. Some of the material dissolves. Waves and sea currents carry sediments to greater depths, where perhaps cementation and transformation into cemented analogues occur - conglomerates, gravelstones, sandstones, siltstones.

Similar processes on a smaller scale can occur due to the geological work of mountain rivers, glaciers and water-glacial flows. If there is no rounding phase, then during cementation of angular material, sedimentary breccias.

Tectonic breccias are formed in zones of tectonic disturbances. Clastic material is produced by the movement of tectonic blocks along fault planes, and cementation is produced by the release of chemical sediment from groundwater that easily circulates through the fractured zone.

Artificial pebbles, artificial beach. If it is necessary to increase the area of ​​the natural pebble beach, crushed stone is transported to the coast and dumped into the surf zone. The rate of rounding of the debris depends on the strength of the original rock and usually takes several months, after which the beach is again ready for use. An artificial beach must be regularly replenished with crushed stone and protected from erosion, since in nature there are constant processes of grinding pebbles and carrying them away with sea currents. Increasing the area of ​​sandy beaches is carried out in a similar way, but protecting them from erosion is even more difficult.

Texture of clastic, sandy and mixed rocks. The rocks of this group have a wide variety of textures and composition due to the diversity of the rocks themselves (Table 3.8).

In terms of density, rocks can be dense, porous, micro- and macroporous, fractured and weathered. Among the rocks of this group, only well-cemented breccias, conglomerates, gravelites, sandstones and siltstones have dense textures. All uncemented rocks are porous due to the spaces between fragments and particles - boulders, pebbles, crushed stone, gravel, sand, silt, etc. Microporous - all clayey rocks due to micropores invisible to the naked eye.

The porosity of unconsolidated clastic and clay rocks can be 20-35% and exceed 50% in loess. Widely used terms (dense clay, dense sand, etc.) are relative and indicate the minimum porosity of these rocks, amounting to 10-25% of the volume. For sandy and clayey rocks, porosity is measured during surveys and is an indicator by which the compression of these rocks at the base of structures is calculated.

According to the relative arrangement of particles, clastic rocks, like most sedimentary rocks, are layered and non-layered. Highly compacted layered varieties are sometimes called schistose because of their external resemblance to a group of metamorphic schists. In contrast, sedimentary shale rocks become wet.

Based on the connections between particles (this characteristic can also be attributed to the structure), clastic rocks are defined as unconsolidated (loose, loose), cemented and cohesive (loose). The term “connected” is used in relation to sandy Table 3.8

Textures and some features of the composition of sedimentary clastic, clayey and mixed (clastic-clayey) rocks

Type of texture	Characteristic
1. Texture determined by density
1.1. Dense	Pores are not visible, water is not absorbed into a dry sample - cemented clastic rocks
1.2. Microporous	Inherent in clayey rocks. The exact porosity is determined in the laboratory. Some samples are light
1.3. Porous, finely porous, cavernous	Pores are visible to the naked eye. This is typical for weakly cemented and uncemented rocks.
1.4. Macroporous	The term is used only for loess that has not only microporosity, but also visible pores with a diameter of about 1 mm, called macropores, visible to the naked eye.
1.5. Fissured	There are cracks in the rock
1.6. You are the wind	Cracks and voids in the rock are widened as a result of weathering processes. The breed is weakened
2. Textures determined by the relative position of particles in the rock
2.1. Layered: a) macrolayered	Visible only in outcrop due to changes in color, composition, and composition of the rock
b) finely layered	Sometimes visible in samples
c) schistose	Thin, fine layering of clayey rocks of a refractory and hard consistency. Samples are broken into slab blocks according to bedding
2.2. Non-layered	Rocks do not have layering - loess, moraine
3. Textures determined by the bonds between particles
3.1. Cemented	Rock particles are held together by cement
3.2. Uncemented (loose, loose)	The rock particles are not held together
3.3. Connected (loose)	Inherent in clayey rocks. The rock is connected due to colloidal bonds between particles. The rock is plastic when soaked, becomes hard when dried, but is neither a monolithic nor a granular material

clayey rocks. They are neither rocky nor granular material. They are plastic and fluid when moistened and become almost hard when dry.

Hydrogeological and engineering-geological properties of cemented clastic rocks. Cemented rocks can be either dense, impenetrable, or porous, permeable to water - it all depends on the ratio of the spaces between the fragments and the amount of cement. They can also be fractured, and if the cemented rock contains carbonate or sulfate components, karst may develop, which further increases permeability. These rocks have the usual properties of rocky and semi-rocky rocks. As a base they are quite strong and incompressible. Only sandstones and siltstones are widely used as crushing materials for crushed stone, although coarse-grained rocks can also be used. To obtain beautiful facing tiles, marble breccias are used, and sandstones and siltstones are used to obtain tiles laid on the floor. Strong, well-cemented sandstone is even used for making steps, as it provides a good rough surface. Thin-layered sandstone varieties do not need to be sawed - they produce natural tiles of irregular shape and are suitable for laying on paths.

Hydrogeological and engineering-geological properties of unconsolidated clastic rocks. All unconsolidated rocks have good permeability, water abundance, and form aquifers that are suitable and convenient for exploitation. The larger the fragments, the greater the permeability, the greater the filtration coefficients (see Part II, Table 8.1). Pebbles, crushed stone, and gravel are second only to highly porous, fractured and karst rocks in their permeability.

Sands are also permeable rock. The sizes of sand grains vary from 0.05 to 2 mm. Also, the filtration coefficient varies tenfold - it is maximum in gravelly sands and minimum in silty sands.

Sands are the most common among unconsolidated clastic rocks. They often lie on the surface, forming groundwater aquifers. Sands are often found in the section, and when overlain by clayey rocks, they form interstratal aquifers of fresh water. For the purposes of construction design, coarse soils and sands in accordance with GOST 25100-2011 are classified according to their particle size distribution, degree of water saturation, porosity and some other indicators determined in the laboratory.

The presence of clay or organic aggregate greatly reduces the permeability of unconsolidated rocks. Pebbles with clay filler essentially turn into low-permeability rocks. The permeability of clayey sands with organic matter decreases tens of times compared to similar rocks without filler. As a base and medium for structures, unconsolidated rocks usually do not present any difficulties, with the exception of dusty and fine sands that are capable of exhibiting quicksand properties and frost heaving. Boulders, blocks, pebbles, crushed stone, gravel - a weakly compressible base.