Genetic types and formation environment of wood fossils (Zheng Shaolin, Zhang Wu)

Wood fossils generally refer to wood that has been petrified in the whole geological history. Therefore, its general concept is relative to living wood. Due to the different causes and sedimentary environments, there are many kinds of wood fossils, but there are three main categories that we have more contact with and have greater research value: infiltrated wood, coal and charcoal, and briquettes.

1. 1. 1 infiltrated mineralized wood

Mineralized wood can be divided into silicified wood, calcified wood and pyritized wood according to the composition of mineralized medium.

1.1.1.1silicified wood

Among the fossil wood specimens collected all over the world, silicified wood is the largest and most common wood fossil. Because its mineralized substance composition is mainly composed of silicon dioxide (SiO2), it is called silicified wood. Silica is a secondary product of volcanic rocks or plutonic rocks, and it is also a product of hot water solution. It is usually combined with chalcedony (SiO2 nH2O) and quartz (SiO2). Under exogenous conditions, silica is amorphous silica gel, some of which are deposited in situ, and some of which are brought to various water bodies or water media. When wood is soaked in an aqueous solution containing appropriate concentration of silica, after a long period of infiltration, all tissues and cells inside the trunk will be filled with silica solution, and after dehydration and fossilization, it will become silicified wood. It has been proved that the content of silica in water medium is a very important factor for the quality of silicified wood. Because the content of silica is too low, it is not easy to form silicified wood, or the wood has rotted before silicification. In this case, even if the wood forms silicified wood, its quality is not good, because the plant body will be full of impurities such as soil and sand, and the cell structure is often not well preserved. If the content of silica is too high, they will have a strong destructive effect on the cells in plants, because the supersaturated silica solution will often produce "agation", which will disintegrate or destroy the cell wall and the organizational structure of each part (Zhang Wu et al., 2005; Zheng et al., 2008).

The formation environment of silicified wood is varied, but it can be mainly divided into two categories: in-situ burial type and off-site burial type. When a dense forest is suddenly attacked by volcanic eruption, because the lava flow of volcanic eruption is accompanied by high temperature and heat wave, those forests near the center of volcanic eruption will be swallowed up or burned by the fiery lava flow and reduced to ashes. However, the forest far away from the eruption center will not only be baked by high-temperature heat waves, but also be buried in the original growth place by falling volcanic ash, turning into upright silicified stumps or fallen trees. The length of well-preserved silicified wood can reach 20 ~ 30 m. For example, the "Wood Fossil Forest" preserved in Shenzhen Huxian Botanical Garden and Shenyang Botanical Garden (figure 1. 1.1.2) and the "Wood Fossil Forest" in Benxi and Chaoyang National Geoparks in Liaoning Province (figure 1.65438). All the silicified wood is produced in tuff interlayer (164 ~ 165 Ma) of the volcanic rock series of the Middle Jurassic Tiaojishan Formation in western Liaoning, China (Chen Wen et al., 2004). These silicified wood belong to in-situ burial type in the place of origin.

In-situ buried silicified wood is widely distributed around the world. For example, in Nanpeng National Park in Western Australia, there are hundreds of upright silicified wood piles with small spires in a sandy land, which were produced in the Pleistocene about 40,000 years ago (figure 1. 1.5). The upright "Sequoia sempervirens" stump in Huangshi National Park was formed in Eocene about 50 Ma ago (figure 1. 1.6).

In the long geological history, whenever storms, earthquakes, tsunamis or transgressions occur, silicified wood buried in different places may cause landslides or mudslides, which may cause a large area of forests to collapse, or wash away forest green spaces due to floods, or uproot or break trees due to hurricanes. A large number of damaged trees were washed away by the flood. After the flood, driftwood was stranded, such as driftwood stranded on the beach on the east coast of Taiwan Province Island (Figure 1. 1.7). If these driftwoods are exposed to the air for a long time, they will disappear due to slow decay. If they are transported to low-lying water bodies and quickly buried by mud and sand, they will become silicified wood under the action of aqueous solution containing appropriate amount of silicon dioxide.

Figure 1. 1. 1 Wood fossils preserved in different places in Shenzhen Huxian Botanical Garden (photo by Zhang Wu)

Figure 1. 1.2 Wood fossils preserved in different places in Shenyang Botanical Garden (photo by Zhang Wu)

The formation environment of silicified wood includes river bed facies, flood plain facies, lake basin facies and intermountain basin accumulation. Compared with the silicified wood buried in situ, the silicified wood buried in different places has no vertical roots and stumps, only a few logs or trees nearly parallel to the stratum. A good example of silicified wood buried in different places is coniferous wood fossils deposited in the conglomerate of Lower Cretaceous Shahe Formation in Shangshidonggou, Yixian County, Liaoning Province, China (Figure 1. 1.8).

Figure 1. 1.3 Wood fossils preserved in different places in Benxi National Geopark, Liaoning Province (photo by Zhang Wu)

Figure 1. 1.4 "Wood Fossil Forest" in Chaoyang National Geopark, Liaoning Province (photo by Zhang Wu)

Figure 1. 1.5 Petrochemical Forest in Nanpeng National Park, Western Australia (according to Lu)

Figure 1. 1.6 "Sequoia sempervirens" stump standing upright in Yellowstone National Park, USA (according to the land)

Fig. 1. 1.7 driftwood stranded on the beach on the east coast of Taiwan Province Island, China. Trunks and wood fragments washed away by the forest and brought to the beach by floods due to flash floods (photo by Zheng Shaolin)

Figure 1. 1.8 Silicified logs buried in different positions in the conglomerate layer of Lower Cretaceous Shahe Formation in Shangshidonggou, Yixian County, Liaoning Province, China (photo by Zheng Shaolin)

1. 1. 1.2 Calcified wood

Calcified wood refers to wood fossils formed by diagenesis when wood is soaked in aqueous solution containing carbonate. Carbonate minerals are mostly exogenous and mainly formed by sedimentation. The crystals of carbonate minerals are also called calcite (CaCO3). Calcite is one of the most widely distributed minerals in nature. The aggregate of carbonate minerals has various forms, including dense and massive, such as limestone; Granular, such as marble. Limestone and marble are dissolved by groundwater and enter the aqueous solution during weathering. When wood is soaked in such water, calcium carbonate will precipitate in plant cells and various tissues, which will calcify wood. From the perspective of wood fossil research, the quality of calcified wood is not as good as silicified wood, because silicate amorphous colloidal solution does not form crystals at room temperature and pressure, and has no destructive effect on cell structure; However, carbonate solution can form calcite rhombic crystals or other isomorphic crystals at normal temperature and pressure, which sometimes destroys the cell structure in plants and makes the anatomical structure unclear. However, in most cases, calcified wood can basically maintain a good internal structure after dissection. Therefore, calcified wood is also one of the important sources of wood fossil specimens. Carbonate-containing water bodies are generally concentrated in areas where limestone strata are developed, or in water bodies or water media related to seawater, because seawater is often rich in carbonate minerals.

1. 1. 1.3 Pyritized wood

Pyrite (FeS2) is a mineral with high content in the earth's crust. In sedimentary rocks, especially in coal measures strata under reducing environment, the formation of pyritized wood may be related to the decomposition of plant residues. Pyrite single crystals are mostly hexahedral, and the aggregate is granular, dense, disseminated or spherical; The aphanitic colloid is called collophanite. Pyritized wood is generally not suitable as a research specimen, because it is easy to break when making thin slices, and it must be boiled to strengthen its firmness. In addition, pyrite is easy to form hexagonal crystals at normal temperature and pressure, which may destroy the internal structure of plants. Moreover, pyrite will be transformed into limonite (Fe2O3 nH2O) due to the loss of sulfur ions after weathering, and its color is dark, which is more likely to cause rust-like pollution and unclear anatomical structure. However, in some cases, pyritized wood can also be used. For example, Pan Suixian (1983) found some pyritized briquettes in No.2-3 coal seam of Upper Paleozoic Taiyuan Formation in Shanxi, China. After research, some plant species can also be identified.

1. 1.2 coal and charcoal (silk charcoal)

Coal can be defined as compressed fossil. It is a heterogeneous mixture after a period of compression (Scott, 1987). Generally speaking, the degree of coal metamorphism is directly related to the preservation of plant details. Low-grade coal is superior to high-grade coal in preserving factory details. Higher rank coal means more coal deterioration and higher carbon content. The rank of coal from low to high is: lignite, sub-bituminous coal, bituminous coal and anthracite. Lignite represents the early stage of coal formation, so the plant organs and tissues in lignite are not crushed or rotted, and are usually identifiable. Generally speaking, all parts of fossil wood and plants preserved in lignite can be identified under scanning electron microscope (Alvin and Muir, 1969). For example, in Brandon lignite, a famous early Miocene plant producing area in Vermont, USA, combing separated plant fragments and various structures and restoring them to complete plants is a good example (Haggard and Tiffney, 1997). Bituminous coal is a kind of coal with deep metamorphism, in which the plant part is flatter, but it is also possible to study the plant fragments in bituminous coal. Anthracite is the highest metamorphic type, and its metamorphic degree has reached the level that the original plant materials are rarely recognized.

In order to obtain the information of wood fossils, coal is made into thin slices, in which wood fragments, spores and pollen grains can be identified. In addition, coal can be embedded with epoxy resin, its surface can be polished and etched in a low-temperature plasma field, and the etched surface can be made into a thin film by tearing, and the biological composition of coal can be determined by an optical microscope or a scanning electron microscope (Winston, 1989). This method can successfully determine the contents of various plants in different types of coal (Winston, 1986). Chemical reagents can also be used to impregnate solid fragments of coal to release plant fragments. Through the inspection of plant composition, the plant species growing in ancient coal-forming swamp were further determined. The application of 13C nuclear magnetic resonance (NMR) technology and pyrolysis-gas chromatography analysis determined the different stages of coal formation (Hatcher et al., 1989). The same technique is also used to identify Cenozoic leaf fossils and wood (Yang et al., 2005). The composition of coal can also be used for paleoecological analysis (Poole et al., 2006).

Charcoal or silk charcoal: Charcoal is produced by incomplete combustion of organic substances, also known as sericite or silk charcoal, and is one of the important sources of ancient plant materials (Cope and Chaloner,1985; Loupias, 1995). The remains of burnt plants can be traced back to the earliest land plants (Glasspool et al., 2004b). There are some techniques for examining charcoal fossils (Sander and Ji,1990; Bastin of the League of Nations,1998; Figueiral et al., 2002), which provides information about burial and paleoecology (Scott et al., 2000), including the past atmospheric composition (Scott and Glasspool, 2006) and the existence of wildfires in paleoecosystem (Uhl et al. , 2004, 2007a; ; Collinson et al., 2007). Well-preserved burnt flowers (Tiffney,1977; Fries and Scarby, 198 1. These discoveries enable us to obtain a lot of information about fossil wood.

Silk charcoal is the result of wildfire activity in geological period. Wildfire is a natural phenomenon in arid-semi-arid ecosystem, and its cause may be related to volcanic activity and lightning in the atmosphere. When the forest fire ends, there will be many unburned plant fragments and carbon blocks. They can retain part of the wooden structure and become silk charcoal after diagenesis and compression. After acid solution leaching, silk charcoal becomes a yellow-brown transparent substance, and the wood structure and the structures of cells and various tissues are clearly visible. He Dechang (1995) obtained many samples of silk charcoal from Huolinhe and Yimin coalfields in Inner Mongolia, China, and determined some genera and species of wood fossils based on them (Figure 1. 1.9), which greatly enriched the Cretaceous wood fossil assemblage in China. However, most silk carbon materials are small stem fragments. Although good anatomical features can be obtained in radial and chord sections, it is usually difficult to obtain wood cross-sectional materials, so it is necessary to make detailed statistics and comparative studies. As for the formation environment of silk charcoal, most of them may be buried in different places, but the possibility of in-situ burial cannot be ruled out. He Dechang (1995) thinks that in Yimin Formation, 16 coal seam is rich in silk charcoal fossils, and the estimated silk charcoal content can reach 20%, which are formed by in-situ plant oxidation, not the result of wood combustion. Recently, Zhang Wu and others found a wood fossil in the protected Wood Fossil Forest Park in Chaoyang City, western Liaoning Province. As a whole, it is silicified, but some of it is carbonized wood. After slicing, the wood structure is well preserved (figure 1. 1. 10). This fact shows that in geological history, there are examples of wood being burnt by lightning or wildfire to form carbonized wood.

Figure 1. 1.9 The transverse, tangential and radial slices of silk charcoal wood show the internal structure of wood.

Figure 1. 1. 10 Charcoal Specimen in Rare Wood Fossil Forest Park, Chaoyang City, western Liaoning Province (according to Zheng Shaolin)

1. 1.3 coal ball

Coal briquettes represent oozed and mineralized peat deposits, which are almost entirely composed of plant parts preserved in calcium carbonate. Some briquettes first discovered in Britain are almost spherical, so they are named briquettes. However, their actual shapes are irregular and their volumes are changing, and their thickness can range from several centimeters to several meters. We know more about the anatomy, morphology and biology of Carboniferous coal marsh plants than any other era, mainly because of the discovery of briquettes. During the Carboniferous period, North America and Europe were close to the equator, and large tropical forests were distributed. Today, they provide rich coal resources for these areas. Associated with these coals are briquettes, which have various forms and appear in coal seams. For coal miners, these carbonate briquettes represent an impurity in the coal seam, which is usually an "obstacle" to coal mining; But for paleontologists, they provide a very attractive source of information. This information can be used to study the biology of plants that grew in peat bogs hundreds of millions of years ago. Some of the oldest briquettes are produced in Upper Namur Stage (Upper Mississippi Substage) in Germany and Czech Republic, and also in Permian coal-bearing strata in China (Li Xingxue et al.,1995; Wang Shijun et al., 2009). They can be studied by slicing and uncovering techniques (figure1.1.11).

Fig.1.1.1/briquette cross section: showing the pith, secondary xylem cylinder and cortex of the stem.

Some paleobotanists have commented on the formation of briquettes (Falcon-Lang, 2008). Stopes and Watson( 1908) studied it, but so far they have not fully understood its formation process. Fresh peat or partially decayed plants have been infiltrated by a solution containing carbonate (fibrous calcite) before being fully compacted. Because some briquettes are related to marine limestone, some people think that these plants grow in low-lying swamp areas near the sea. This hypothesis provides paleogeography applicable to the Carboniferous Central America and the North American continent. During storms or transgressions, coal marshes are submerged by seawater, which provides a source of calcium carbonate for the infiltration and mineralization of plants (Mamay and Yochelson, 1962). This hypothesis explains the existence of plant and marine animal residues in some briquettes, and shows the mixed nature of fresh water and salt water organisms.

Scott and Rex( 1985) proposed that not all coal balls were formed in the same way, and put forward a non-marine formation model, which believed that mineralized fluid came from the infiltration of groundwater with high carbonate content. Scott et al. (1996) investigated the origin of coal balls in Europe, America and China in Carboniferous and Permian, and incorporated some complicated methods, and thought that their origin should depend on the area and position of coal balls in coal seam. According to the carbon isotope, they found that some briquettes were related to the infiltration of the mixture of seawater and fresh water through peat, and most briquettes were formed in at least some freshwater basins affected by marine factors. There is a doubt that the formation of briquettes is a highly specialized process, because after Carboniferous-Permian, we knew nothing about briquettes (Taylor et al., 2009).

Strictly speaking, briquettes themselves are not wood fossils, because in briquettes, except for xylem fragments of some plant stems, most of them are composed of vegetative organs of plants, such as leaves, roots, rhizomes and reproductive organs of ferns, such as spikes of microspores, male and female cones, spikes of fruits, seeds and sporopollen of gymnosperms. In the stems of gymnosperms, only xylem belongs to the research category of wood fossils. The accumulation environment of the original materials of coal briquettes in China is the coastal swamp, and the coal briquette plants represent the ancient coastal peat swamp and the plant burial groups washed into the peat swamp. This gathering environment may be related to the formation of many briquettes (fields, etc. , 1995). The composition of mineralized substances in coal briquettes is also varied. Most briquettes are closely related to marine sediments. Because seawater is rich in carbonate, most briquettes are mainly calcified. However, under special circumstances, pyritized briquettes can also be formed (Pan Suixian, 1983). In western Europe, silicified briquettes were also found in Stephanian briquettes in the Late Carboniferous, such as Grand Croix, France. However, the briquettes in this area were produced in conglomerate, and their strata were hundreds of meters higher than the coal-bearing strata, which was generally considered as secondary accumulation (fields, etc. , 1995).

Figure1.1.1.12 The cross section of metamorphic silicified wood shows the residual pith, secondary xylem cylinder and secondary stress pulse injected along the direction of tree rings and wood rays.

Figure 1. 1. 13 shows the residual secondary xylem fragments in the silicified wood.

In addition, it is necessary to explain a little bit about "redeposited silicified wood fossils" and "metamorphic wood fossils": the former refers to wood fossils that are transported to new sedimentary sites by running water after mineralization, weathering and denudation, and then re-deposited to form new rocks. This kind of denuded, transported and redeposited wood fossils must be fully explained in the study, because they can not be used as evidence of horizon age, let alone as a basis for discussing geographical environment. The latter mainly refers to the phenomenon that after the formation of wood fossils, the rock strata containing wood fossils are affected by local dynamic metamorphism (such as tectonic movement) or thermal metamorphism (such as magmatic activity or volcanic eruption, dike intrusion), which leads to the recrystallization of the mineralized material components of wood fossils after remelting. Some primitive wood structures may be unrecognizable after deterioration, and the research value of this wood fossil is generally low. However, some still retain the local wood structure, and the annual rings and tracheids can be clearly seen on the cross section. Due to the serious damage of wood structure, it is generally impossible to accurately identify and classify this metamorphic wood fossil. However, after deterioration, a small amount of rare metal elements may be mixed and dyed into various colors, and some original structures of wood still exist or remain, forming beautiful patterns and patterns, which have certain ornamental value. Therefore, they can be developed and utilized as "strange stones" (figure 1. 1. 12, figure 1.66).