Rock types of common terrigenous clastic rocks

Clastic rocks can be divided into conglomerate (breccia), sandstone, siltstone and argillaceous rocks according to the size of clastic particles.

1. Conglomerate (breccia)

Conglomerate and breccia are collectively called coarse clastic rocks. Conglomerate refers to the rock containing more than 50% round and sub-round gravel (debris with particle size > > 2mm) (Figure 9-5a); Breccia refers to rocks with angular and sub-angular gravel content exceeding 50% (Figure 9-5b). Conglomerate and breccia can be further divided according to the size, composition and genesis of gravel.

Figure 9-5 Conglomerate and Breccia

According to the size of gravel, coarse clastic rocks can be divided into conglomerate or breccia (gravel diameter > 256 mm), coarse conglomerate or breccia (gravel diameter 64 ~ 256 mm), medium conglomerate or breccia (gravel diameter 4 ~ 64 mm) and fine conglomerate or breccia (gravel diameter 2 ~ 4 mm).

According to the composition of gravels, coarse clastic rocks can be subdivided into monocomponent gravels or breccias (more than 75% gravels have the same composition, such as quartzite gravels) and multicomponent gravels or breccias (the content of any gravels is less than 75%).

According to the genesis, coarse clastic rocks can be divided into landslide breccia, flood breccia, river breccia, lake breccia, coastal breccia, turbidite (submarine fan) breccia, moraine breccia and karst breccia.

Coarse clastic rocks can be divided into bottom conglomerate (located at the bottom of the sequence, in unconformity or pseudo-conformity contact with the underlying strata, with well-selected gravels, good roundness and high maturity, representing the product of long-term erosion discontinuity) and interlayer conglomerate (located in continuous sedimentary strata, with no upper and lower sedimentary discontinuities, and the same lithology, usually formed by local rock scouring and deposition).

Common coarse clastic rocks have the following types:

Quartzite conglomerate gravel is mainly composed of quartzite, flint and veined quartz. The gravel grade is medium-fine gravel, which is well sorted, rounded and supported by particles. Common cements are Yingshi, calcite and hematite.

Volcanic conglomerate gravel is mainly volcanic rock or volcanic tuff, with single or complex composition, medium grade of gravel, and medium sorting and grinding. Sand-graded sedimentary matrix (sand base) or mud-sand mixed matrix is often distributed among gravels. The composition of sand base is similar to that of gravels, but there are many quartz and feldspar single crystals. Cement is usually argillaceous, calcareous or iron.

Limestone breccia or conglomerate gravel is mainly limestone or whole limestone, with a large particle size variation, which can be coarse gravel, medium gravel or fine gravel, mostly angular-sub-circular, sorted from good to poor, mostly containing mud base or mixed mud-sand base, and sometimes cemented by calcite.

The composition of composite conglomerate gravel is complex, and common magmatic rocks, sedimentary rocks and metamorphic rocks are mixed. The ratio of stable gravel to unstable gravel is uncertain, but unstable gravel is often more, with moderate roundness and poor sorting. Mud base or mixed base, and the composition of mixed base is also very complicated. There is little chemical cement, and sometimes there is timely cement.

If there is no strong explanation, the mineral composition and structure (and sometimes structure) inside the gravel are not essentially different from the parent rock that provided it. The ancient gravel layer is often an important aquifer. The cement of conglomerate usually contains precious minerals, such as gold, platinum and diamonds. The study of conglomerate can also help us understand the geological background of conglomerate formation. Almost all thick conglomerates were formed after large-scale orogeny. The composition, structure, arrangement and shape of conglomerate can reflect the composition of parent rock, erosion and deposition speed, transportation distance and water flow direction.

2. Sandstone

Sandstone, also known as mesoclastic rock, refers to sedimentary rock containing more than 50% sandy terrigenous debris. Sandstone is one of the most important and studied sedimentary rocks, accounting for about 25% of continental sedimentary strata. The sedimentary environment of sandstone is much wider than that of coarse clastic rocks, mainly deposited in terrestrial environments such as rivers, deserts and lakes, river-sea transitional delta environment, shallow sea to deep sea environment, and forms various vertical sequences with coarse clastic rocks, siltstones, argillaceous rocks and carbonate rocks.

Sandstone has important economic significance. It and carbonate rocks are the two most important oil and gas reservoir rocks. Sandstone is also a huge underground fresh water reservoir, and pure quartz sand or timely sandstone is also a cheap raw material for the glass industry.

General characteristics of (1) sandstone

Sandstone mostly occurs in stable layers, and the shape of sand body can be mat-shaped, mound-shaped, channel-filled and fan-shaped. The sedimentary structures of sandstone are extremely rich, especially various bedding and ripple structures are very developed. Except that some calcite autogenous particles (mainly bioclastic particles, internal debris and oolitic particles) are generated or transited from limestone, the sedimentary components in sandstone are mainly sandy terrigenous debris and sedimentary matrix. Single crystal debris is the most common terrigenous debris (sand), and some sandstones may also contain quite a lot of drill cuttings. Single-crystal detritus is mainly quartz and feldspar, with a small amount of mica and heavy minerals. Debris mainly includes flint rock, acid extrusive rock, fine-grained schist and gneiss. Sometimes there are neutral or even basic volcanic rocks or volcanic tuffs and argillaceous rocks. The matrix in sandstone is mainly clay and contains fine sand debris, which is called mud matrix or miscellaneous matrix. Some sandstones associated with carbonate rocks may also contain carbonate micrite matrix.

The diagenesis of sandstone is mainly cementation, and the common cements are yingshi, calcite, hematite, glauconite and gypsum. Special cement includes siderite, chlorite, barite and zeolite. Sandstone cemented by sedimentary matrix is also common.

(2) Classification and nomenclature of sandstone

According to the purpose and degree of research, different sandstone classification and naming schemes can be adopted.

Figure 9-6 Classification of Sandstone Composition

According to the grain size of main sand grains, sandstone can be divided into: extremely coarse sandstone (the grain size of main sand grains is 2.0 ~1.0 mm); Coarse sandstone (main sand grain size is1.0 ~ 0.5 mm); Medium sandstone (main sand particle size 0.5 ~ 0.25mm); Fine sandstone (particle size of main sand is 0.25 ~ 0.1mm); Very fine sandstone (main sand particle size 0. 1 ~ 0.05 mm).

According to the content of miscellaneous base, sandstone can be divided into clean sandstone (with little or no miscellaneous base content) and miscellaneous sandstone (with miscellaneous base content exceeding 15%). Miscellaneous sandstone is also called hard sandstone and wacker rock.

According to the composition of sand grains, single crystal quartz, single crystal feldspar and all cuttings are selected as endmembers. Sandstones can be divided into seven types: quartz sandstone, feldspathic sandstone, feldspathic sandstone, feldspar quartz sandstone, lithic quartz sandstone, lithic feldspar sandstone and feldspar lithic sandstone (Figure 9-6).

(3) Main rock types of sandstone

More than 90% of sandstone detritus is single crystal detritus, containing a small amount of flint and siliceous rock detritus. There are few heavy minerals. Cements are usually siliceous, and secondary cementation is also common. The cementation type is mainly pore cementation (Figure 9-7a) and sometimes mosaic cementation (Figure 9-7b). Seasons rich in sandstone are generally deposited in coastal or shallow sea areas by strong chemical weathering of parent rocks (granite, granite gneiss, metamorphic quartzite, etc.). ) under the conditions of stable structure, little topographic fluctuation and warm and humid climate.

Figure 9-7 Sandstone

Feldspar sandstone is mainly composed of clastics and feldspar, and the clastics content of feldspar exceeds 25%. Feldspars in feldspathic sandstone are mainly orthoclase, microcline and acid plagioclase (Figure 9-7c and D). The color is usually red or yellow. Its formation depends largely on the composition of the parent rock. First, there must be feldspar-rich parent rocks, such as granite and granite gneiss. In addition, favorable paleostructure, paleogeography and paleoclimate conditions are needed. In the area with strong tectonic movement, the topography fluctuates greatly, the granite basement uplift suffers strong erosion, and the erosion products accumulate rapidly, forming thick feldspathic sandstone.

Clastic sandstone is mainly composed of clastic time and detritus, and the content of detritus exceeds 25%. The composition of cuttings in cuttings sandstone is varied, which varies with the parent rock, and siliceous cuttings are more common (Figure 9-7e). Clastic sandstone is dark, gray, grayish green and grayish black, and light sandstone is rare. Clastic sandstone is mostly formed in the structural belt or depression basin near the strong structural uplift area, which is formed by the rapid erosion and accumulation of parent rock. Clastic sandstone can be continental or marine.

Miscellaneous sandstone is poorly sorted sandstone mixed with mud and sand. Generally speaking, it contains less time and is mostly angular. It contains feldspar and cuttings in different proportions, and often contains a small amount of mica. Feldspar is mainly plagioclase, and there are many kinds of cuttings. Rich matrix is the basic feature of complex sandstone (Figure 9-7f), and the matrix composition is common in chlorite and hydromica. The formation conditions of composite sandstone are similar to feldspar sandstone or lithic sandstone, that is, it is formed by rapid erosion, transportation and sedimentation, but it can be formed under different climatic conditions. Typical complex sandstone usually accumulates in rapidly settling turbidite or flysch strata.

3. Siltstone

It is mainly composed of silt debris (particle size 0.0625~0.0039mm). The clastic composition of siltstone is relatively simple, mainly in time, and sometimes contains more muscovite. The interstitial materials include calcium, iron and clay. Siltstone often has thin horizontal bedding, and when the sediment contains water, it is easy to liquefy and produce sliding structures such as deformation bedding.

According to the particle size of silt, siltstone can be divided into coarse siltstone (particle size 0.0625 ~ 0.03 12 mm) and fine siltstone (particle size 0.03 12 ~ 0.0039 mm).

According to the composition of the mixture, siltstone can be divided into argillaceous siltstone, iron siltstone and calcareous siltstone.

According to the clastic composition, siltstone can be divided into muscovite powder sandstone, quartz siltstone and feldspar siltstone. Quartz siltstone is more common (Figure 9-8).

Figure 9-8 Siltstone

The color of siltstone varies with the composition of the mixture. Siltstone was formed in the environment of long-distance transportation, calm hydrodynamic conditions and slow sedimentation rate. It can gradually become sandstone or clay rock in the horizontal and vertical directions, forming rhythmic bedding. From the sedimentary environment, siltstone is mostly distributed in deep waters such as floodplain, delta, lagoon, swamp and sea lake.

China is the most developed area of loess in the world, and its thickness ranks first in the world. Loess is light yellow or brownish yellow and semi-consolidated siltstone, which has a series of special properties. It is characterized by uniform texture, easy to powder by hand rubbing, and contains a large number of grotesque calcareous tuberculosis. The mineral composition of loess is mainly seasonal, but there are also feldspar, carbonate and clay minerals. The mixture is mainly calcium.

4. Muddy rocks

Sedimentary rocks with argillaceous particles (particle size < < 0.0039mm) exceeding 50% are called argillaceous rocks, loose unconsolidated argillaceous rocks are called clay, and those solidified into rocks are called mudstone and shale. The total amount of argillaceous rocks and siltstones (fine clastic rocks) accounts for about 60% ~ 70% of the total amount of sedimentary rocks, and they are the most widely distributed sedimentary rocks. Clay (mudstone, shale) has unique physical properties, such as plasticity, fire resistance, sinterability, expansibility, adsorption and so on, and is widely used in industry and agriculture.

General characteristics of (1) argillaceous rocks

Most argillaceous rocks are deposited by mechanical means, and the fine clastic materials in the weathering products of parent rocks are transported to the deposition site by suspension. Clay minerals are the most important mineral components in argillaceous rocks. Clay minerals are very fine, and their crystal size is generally less than1~ 2μ m. There are many kinds of clay minerals, among which kaolinite, hydromica, montmorillonite, chlorite and attapulgite are the most widely distributed.

The chemical composition of argillaceous rocks is mainly silica, alumina and iron oxide. The color of argillaceous rocks depends on the composition of clay minerals, impurity minerals and organic matter and the color of pigments contained. One-component kaolinite clay (mudstone and shale) and hydromica clay (mudstone and shale) are often white, light gray and light yellow. Some clays (mudstone and shale) contain finely dispersed iron oxides and hydroxides, while others are red, purple, brown, yellow or rose. Manganese-containing oxides are brown or black; It is gray or black when it contains dispersed organic matter and iron sulfide; If clay (mudstone and shale) contains more glauconite, chlorite, malachite and sapphire, it is green or blue. Most argillaceous rocks are relatively stable layered and often interbedded with sandstone and siltstone.

(2) Classification and nomenclature of argillaceous rocks

According to the consolidation strength, argillaceous rocks are divided into unconsolidated clay, consolidated mudstone and shale. Mudstone has no foliation, while shale has foliation (Figure 9-9a). According to mineral composition and properties, argillaceous rocks (clay) can be divided into kaolinite clay, montmorillonite clay, sepiolite clay and attapulgite clay.

(3) Structure of argillaceous rocks

According to the relative content of clay, silt and sand, it can be divided into argillaceous structure, silt argillaceous structure and sand argillaceous structure.

According to the morphology of clay mineral aggregate, it can be divided into worm-like structure, oolitic structure and bean-like structure. Worm-like structures mainly exist in kaolinite mudstone, and the rocks contain coarse worm-like crystals formed by recrystallization of kaolinite (Figure 9-9b), with a diameter of 2 ~ 3 mm and a length of 20 mm Oolitic and pea-like structures are structures in which clay particles are condensed around a core during deposition. The particle size less than 2mm is called oolitic particles, and the particle size greater than 2mm is called bean particles.

Figure 9-9 argillaceous rocks

(4) Main rock types of argillaceous rocks

Calcium carbonate is widely distributed in calcareous mudstone and shale.

Iron mud rocks and shale contain iron minerals. Because the sediments are rendered by trivalent iron in an oxidizing environment with continental arid and semi-arid climate conditions, they form the so-called "red bed".

The silica content of siliceous mudstone and shale can reach more than 85%. Because diatom, sponge and radiolarian fossils are often preserved in siliceous mudstone and shale, it is generally believed that the source of siliceous matter is related to biology and may also be related to volcanic ash erupting from the seabed.

Carbonaceous shale contains a lot of plant fossils and carbonized organic matter, which is black and can be dyed. The ash content exceeds 30%. It is the product of lakes and swamps. Usually in coal measures strata, it constitutes the roof and floor of coal seam.

Black shales are black because they contain more organic matter or finely dispersed iron sulfide. The appearance is similar to that of carbonaceous shale, but the difference is that it is not touched by hands. It is formed in a relatively closed bay and lake environment with oxygen deficiency and abundant H2S.

Oil shale contains a certain amount of kerogen (> 10%). Colors are light yellow, yellow brown, dark brown, brown black, black and so on. It is characterized by being lighter and more flammable than ordinary shale, and emitting asphalt smell and oil droplets. Oil shale belongs to the category of shale, but it has the characteristics of sapropelic coal. Some people call it "high ash sapropelic coal", which is formed by the remains of lower organisms such as algae and plankton in closed bays or lakes and swamps under the reduction condition of air isolation.

Kaolinite clay is called kaolin for short. It is a fine-grained clay with kaolinite minerals as the main component, and its structure is pure. It was first discovered in gao ling cun, near Jingdezhen, Jiangxi, China, and was named after the first place name in China. Kaolin is white and light gray in appearance, and yellow, gray and black when it contains impurities. Dense lumpy or loose soil, with a sense of satiny and less hardness than nails. The relative density is 2.4 ~ 2.6. It is sticky after drying and has strong water absorption. The fire resistance reaches 1770 ~ 1790℃. Low plasticity, low adhesion, good insulation and chemical stability. Pure kaolin is white after calcination, and its whiteness can reach 80% ~ 90%. It is an important raw material for ceramics, papermaking, rubber and other industries.

Montmorillonite clay is also called bentonite, swelling rock and bentonite. It is a fine-grained clay with montmorillonite as the main mineral component. The appearance of bentonite is generally white, pink, light gray and light yellow, and it can be gray green, purple brown and other dark colors when contaminated by impurities. Blocky or earthy, slippery. Loose soil has a dull luster, and dense blocks have a waxy luster. Hardness 1 ~ 2, soft. The relative density is 2 ~ 3. The volume expands after water absorption, and the maximum water absorption is 8 ~ 15 times of its volume. High plasticity, good adhesion, suspension gelation in aqueous solution, and cation exchange characteristics. Bentonite is one of the important industrial mineral raw materials.

Sepiolite clay is a kind of clay with sepiolite as the main component. Appearance is yellowish brown, dark gray, grayish white, etc. The soil is soft and light, with hardness 1 ~ 2 and relative density of 2.4 ~ 2.65. It's sticky and plastic, and it feels slippery with your hands. After adding water, it can be made into paste, and after drying, it can be hammered to leave hammer marks. Mainly used as adsorbent to purify, decolorize and refine oil, grease, wax, resin, beer and water. Another important application is to make salt-resistant and high-temperature resistant drilling mud, which is used in salt-bearing formations and offshore oil drilling.

Attapulgite clay is a kind of clay with attapulgite as the main component. Its appearance is the same as ordinary clay, especially it is very similar to montmorillonite clay, and they are often a natural pair. Its field identification sign is that its appearance is soil-like, blue-gray, gray-white, duck egg blue, the soil is smooth, sticky and plastic when wet, light in weight and small in shrinkage after drying. Put it in water, it hisses and sounds, it collapses into pieces, but it doesn't expand. Attapulgite clay, like sepiolite clay, has thermal stability, salt tolerance, adsorption and high decoloration ability, and is widely used in industry.