1. particles
Particles in carbonate rocks can be divided into intra-basin particles and extra-basin particles according to whether they are formed in sedimentary basins. Extrabasin particles refer to terrigenous clastic particles from outside the sedimentary basin, such as gravel, sand, silt and mud. Intra-basin particles are carbonate mineral aggregates with abnormal chemical precipitation controlled by hydrodynamic, biological, biochemical and chemical actions in sedimentary basins. Fokker (1959, 1962) called it "alienated particles", that is, particles formed by abnormal chemical action. Generally, the particles in the basin are simply called "particles".
In carbonate rocks, the common particle types are internal debris, oolitic particles, biological particles, spherulites, algae particles and other particles.
(1) internal debris
Internal detritus is mainly semi-consolidated or consolidated carbonate (mainly calcium carbonate) rocks deposited in sedimentary basins for a short period of time, which are broken, transported, abraded and redeposited by waves or currents. It can also be formed by other actions.
Internal debris can be divided into four grades according to its particle size: gravel debris (> > 2mm), sand debris (2-0.05mm), powder debris (0.05~0.005mm) and mud debris (< 0.005mm). Here, not only the terms of terrigenous clastic rocks, such as gravel, sand, silt and mud, are quoted to name the particles of clastic rocks in carbonate rocks, but also the grain size boundary is the same, which provides convenience for beginners.
The internal debris of gravel grade, that is, gravel chips, has long been recognized by people. The bamboo-like gravel (Figure 9- 12a) widely distributed in the Cambrian-Ordovician bamboo-like limestone in northern China is of this type. This kind of gravel is mostly flat cake-shaped, and its side is often long, like bamboo leaves, so it is often called bamboo leaf gravel. Some bamboo-like gravels have a brown oxide ring on the surface. This kind of bamboo-like gravel is formed in shallow water with strong water energy, and the semi-consolidated or consolidated micrite limestone layer at the bottom of the water is broken, transported, abraded by waves or currents, and even exposed to oxidation and redeposition.
The internal debris of sand size, that is, sand debris, can be easily observed under the microscope and also on the weathered surface of rocks, but it is difficult to identify on the general rock surface. Most of the sand chips are micrite limestone chips with good roundness and good sorting, almost spherical, but there are also sand chips with irregular shapes (Figure 9- 12b).
The internal debris of silt grade, namely silt, also exists widely. Its characteristics are basically the same as sand chips, but the particle size is smaller.
Theoretically speaking, the internal debris of mud, that is, mud debris, must exist; However, this kind of debris is difficult to distinguish from mud crystals produced by chemical precipitation and mud-grade biological particles produced by biology.
As for the cause of internal debris, most people think it is caused by mechanical crushing; But there are also those who advocate the cause of chemical precipitation. According to the existing modern and ancient carbonate clastic data, we can know that there are three generation processes:
1) The high-energy area under the tide is formed by wave breaking, that is, the semi-consolidated limestone layer on the seabed is broken, transported, abraded and redeposited by waves or currents in the high-energy area under the tide to form internal debris. Shoals with strong tidal action, especially those in tidal channels, are favorable areas for the formation of such internal debris.
2) Intertidal zone and supratidal zone are formed by flowing water, that is, micrite calcium carbonate deposits are exposed to the atmosphere, forming mud cracks or mud rolls; These mud cracks and mud rolls are broken, transported, abraded and redeposited by tidal current, that is, internal debris. This kind of internal debris often has an oxidation ring at the edge.
3) Calcium carbonate particles are caked and bonded with each other, that is, the internal debris of Bahamian rocks. For example, in the modern shallow sea saturated with calcium carbonate in the Bahamas, calcium carbonate particles are mutually agglomerated and coalesced to form a "prehnite" in the shape of a grape cluster.
Figure 9- 12 Internal Fragments
(2) Oolitic grains
Oolitic particle is a spherical particle with a core and concentric layer structure, which is very similar to fish eggs (that is, ooidic particles), hence the name. There is also called "oolitic grain", which can also be referred to as "oolitic grain". Oolitic grains are mostly extremely coarse sand to medium sand (2 ~ 0.25mm), common oolitic grains are coarse sand (1 ~ 0.5mm), and those larger than 2mm and smaller than 0.25mm are rare.
Oolitic grains are usually composed of two parts: one is the core and the other is the concentric layer. The core can be internal debris, fossils (intact or broken), terrigenous debris and other substances. Concentric layer is mainly composed of micrite calcite; Oolitic particles in modern marine environment are mainly composed of aragonite. Some oolites have a radial structure.
According to the structural and morphological characteristics of oolitic grains, oolitic grains can be divided into the following types:
The thickness of the normal oolitic concentric layer is greater than the diameter of the nucleus. Generally speaking, oolite refers to this normal oolite, also called concentric true oolite (Figure 9- 13a).
The thickness of its concentric layer is smaller than its core diameter. Some epidermoid cells even have only one concentric layer, that is, a skin shell (Figure 9- 13b).
Multiple oolites contain two or more small oolites in one oolite (Figure 9- 13c). The radioactive oolite has a radioactive structure (Figure 9- 13d).
Fig. 9- 13 oolitic grains
The whole oolite of single crystal and polycrystalline oolite is basically composed of one calcite crystal or several calcite crystals, and its concentric layered structure is only faintly visible or invisible. This oolite is the result of recrystallization.
Negative oolite (hollow oolite) This is an oolite whose interior (most of the core and concentric layers) are selectively dissolved. In fact, it is an intragranular dissolved pore.
Before the appearance of beans, most of the oolitic grains with a diameter greater than 2mm were called beans; However, there is a tendency to limit the bean grain to the product of diagenesis, instead of calling the oolitic grain larger than 2mm as bean grain.
Algal oligosaccharides are ooligosaccharides formed by algae and can be classified as algae particles.
There are two main theories and viewpoints about the origin of oolitic particles: biological theory (algae origin) and inorganic theory (inorganic precipitation). Among them, the inorganic precipitation theory is convincing to link the formation of oolitic particles with its structural characteristics (with cores and concentric layers) and its formation environment (in areas with strong hydrodynamic conditions).
(3) Biological particles
Biological particles refer to the transported and worn biological fossil fragments and complete biological fossil individuals, such as foraminifera, corals, bryozoa, brachiopods and sea lilies. (Figure 9- 14). Those that have not been transported and abraded are mostly caused by the natural disintegration of fossil individuals deposited in situ or the destruction of carnivores.
Biological particles can be called green particles for short, and there are many synonymous terms, such as fossils, fossil particles, organisms, biological fragments, biological bones, bone particles, biological bone components, bone particles, bone fragments, bone shells and so on. Biological particles are one of the most important particle types.
Figure 9- 14 biological particles (according to the electronic teaching plan of the excellent course Sedimentary Petrology of Yangtze University)
(4) small ball
Pellets are fine particles (mostly silt, but also fine sand), spherical or oval particles with no internal structure, and are composed of microcrystalline carbonate minerals (Figure 9- 15A, b). If only from this definition, the internal debris classified as good, spherical, silty or fine sand is spherulite.
Regarding the causes of particles, some people limit particles to the category of fecal particles; Others think that pellets are produced by chemical condensation, that is, Bahamian stone particles; Others advocate the origin of internal debris. Among them, there are some reasons to regard particles as fecal particles. Because in the modern carbonate sediments of the Bahamas, some creatures are producing a lot of fecal particles. The fecal mass has a high content of organic matter and is dark in rock thin slices, which is an important identification feature.
Fig. 9- 15 particles
(5) Algae particles
Algae particles are particles related to algae. Common algae grains include algal ash nodules, algal blocks, algal fragments, algal oolites and so on.
Algae-gray tuberculosis, also known as nucleolith or algae inclusion body, has concentric layer structure. Algae is very similar to flypaper, and the mucus on its surface can capture tiny carbonate sediments, thus forming an irregular growth layer. This growth layer is sometimes discontinuous and sometimes continuous concentric.
Algae blocks also belong to the origin of algae bonded growth particles, but they do not have concentric layer structure, and bonded particles can often be seen in them.
Algae chips are broken algae particles, that is, they are broken by larger algae particles or algae grids.
Algae oolite is a kind of oolite closely related to algae.
(6) Deformed particles
Primitive particles, such as oolite and internal debris, can be deformed in the epigenetic stage due to pressure dissolution or other mechanical effects, forming various shapes, such as lentil, tadpole and chain. Some can also see their relationship with the original particles. At this time, it can be called deformed oolite, deformed internal debris and so on. Some can't see their relationship with the original particles, so they have to be called deformed particles in general.
The main particles in the basin, such as internal debris, oolitic particles, biological particles, pellets and algae particles, have three forming functions, namely mechanical crushing, chemical coagulation and biological action. Internal debris is basically caused by mechanical crushing, and its deposition is mainly controlled by hydrodynamic conditions. Biological particles are of biological origin. Oolitic particles are a comprehensive product of chemical precipitation and hydrodynamic action. Dung balls are basically the product of biological action. Algae particles are also basically produced by biological action.
2. Microcrystalline matrix (mud)
Microcrystalline matrix (mud) is another structural component corresponding to particles, which refers to mud-grade carbonate particles, equivalent to "mud or clay" in terrigenous clastic rocks. Microcrystalline carbonate mud, microcrystal, mud crystal and mud chip are synonymous terms. According to the composition, it can be divided into "plaster" and "cloud mud". Gypsum is calcite mud (Figure 9- 16a, b), also called microcrystalline calcite mud or microcrystal and micrite. "Yun Ni" is mud made of dolomite.
The boundary between mud and particles is usually 0.005 mm. There are three views on the causes of stucco:
The reason of chemical precipitation is that chemical precipitation produces gypsum. This is how the needle-like aragonite mud in modern marine sediments is produced. Most of this aragonite mud was born in tropical seawater with high salinity.
The reason of mechanical crushing is that mechanical crushing produces gypsum, which mainly refers to the internal debris of mud grade.
The reason of biological action is that gypsum is produced by biological action. There are many needle-like aragonites in calcareous algae (Nostoc commune and Phaeodactylum commune) existing in modern oceans. When these algae die and their organic tissues rot, the acicular aragonite in them will be separated and turned into gypsum on the seabed. The data of isotope O 18/O 16 also prove that these plasters are biogenic.
3. Bright crystal cement
Bright crystal cement mainly refers to crystalline calcite filled between particles. Because the crystal is clean and bright under the microscope, it is called "bright crystal", "bright crystal calcite" and "bright crystal cement". The grain size of bright calcite is generally larger than that of mortar, usually larger than 0.0 1mm or smaller than 0.005 mm Leucite is very similar to cement in sandstone (Figure 9- 17a).
Bright calcite cement is formed by chemical precipitation from water between particles after particle deposition, so it is often called "crystalline calcite" and "crystalline calcite cement". Because it is generated by chemical precipitation between particles, this calcite crystal is often distributed around the surface of particles in a comb or horse tooth shape. This is commonly known as pocket monster cement. It is difficult for Pokemon to fill the intergranular pores with comb cement. The residual intergranular pores not filled by Pocket Monster cement are sometimes still empty, but sometimes filled by the second generation bright calcite cement. This second-generation bright calcite is no longer comb-shell-shaped, but mostly embedded with particles (Figure 9- 17b).
Fig. 9- 16 microcrystalline (micrite) matrix
Figure 9- 17 bright crystal cement
The difference between bright calcite cement and granular mortar lies in:
(1) granularity is different. Glittering particles are larger, while gypsum is smaller.
(2) The cleanliness is different. Bright crystals are cleaner and brighter, while plaster is dirtier.
(3) The morphological characteristics are different. Bright crystal cement often presents comb-like shell-like distribution characteristics, while gypsum is not.
When rocks are recrystallized, gypsum often becomes larger crystals, and the bright calcite cement also changes. At this time, it is difficult or even impossible to distinguish calcite crystals recrystallized from gypsum from bright calcite. At this time, these two non-particulate components are generally called "matrix".
4. Biological skeleton
Biological skeleton, also known as in-situ biological skeleton, is a hard carbonate skeleton composed of coral, moss, algae and other in-situ reef-building organisms. Bioskeleton is an indispensable structural component of carbonate rocks in reefs, so it is also called bioskeleton.