Basic characteristics of regional geological development history in Guangxi

(Guangxi Geology and Mineral Assets Management Center)

Geology is a natural science that studies the history of the earth. Professor Zhang, chairman of the International Union of Geological Sciences, said: "The earth is a book, which has long been recorded in writing. The task of science is to interpret the historical traces of the earth itself." "Every cognitive leap and theoretical innovation in geological science has brought great impetus to social and economic development." It has a great influence on the survival and development of human beings, and understanding the earth is conducive to the development and utilization of resources and the improvement and protection of the environment. Comrade Jiang Zemin once pointed out that the fundamental task of geological science lies in understanding the earth and providing scientific basis for human activities.

Geological survey is a way to understand the history of the earth. Only by repeated practice and research can we improve our understanding and research level and provide basis and promotion for carrying out various geological prospecting work. Since 1956, Guangxi has comprehensively and systematically carried out various geological surveys, accumulated a large number of first-hand materials with scientific value, made many major discoveries and breakthroughs, solved a number of geological problems left over from history, and made important progress on the basis of previous work, but there are still some geological problems to be solved in the future. At present, only the basic characteristics of the history of regional geological development are preliminarily summarized into three aspects.

I. Land and sea changes

The formation of the crust surface in Guangxi is a history of land and sea changes, which mainly witnessed the formation of strata in various geological ages. Seawater invaded Guangxi 2 billion years ago, and has experienced Mesoproterozoic (Bosi Group), Neoproterozoic (Danzhou Group), Sinian, Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Tertiary (Paleogene) and Quaternary * * * 65438. From the point of sedimentation, the sedimentation in each geological period is continuous and orderly, except that the crustal movement temporarily affects sedimentation for a short time, most of the time it is in the process of land-sea change. According to the sedimentary environment of sediments, it can be divided into two environments, namely, marine (water) sedimentary environment and continental (land) sedimentary environment. Before the Indosinian movement (227 million years), it was a marine sedimentary environment, and after the Indosinian movement, it was a continental sedimentary environment. Sedimentary types can be divided into three development stages. Before Devonian (465.438+0 billion years), it basically belonged to the deep-water and deep-water sedimentary stage of geosyncline. After Devonian, it is the sedimentary stage of platform shallow sea and marginal sea; The Indosinian movement was followed by the sedimentary stage of continental basin. According to the characteristics of sediments, geosyncline sediments are mainly clastic rocks, platform sediments are mainly carbonate rocks, and continental basins are mainly red clastic rocks. The sedimentary thickness of strata in each geological era can reach 80,000 m. With the changes of sedimentary era and environment, all kinds of organisms are very rich, from the low-level plant sporopollen in Proterozoic to a large number of fauna in Paleozoic, from Mesozoic to Cenozoic, giant dinosaurs appeared and ancient humans appeared, which developed from low to high in an orderly manner.

1. geosyncline development stage

Proterozoic geosyncline is mainly developed in northern Guangxi, and Caledonian geosyncline in central, eastern and southern Guangxi is mainly shallow sea and semi-deep sea. The sedimentary characteristics are clastic facies dominated by sandstone, shale and glutenite, with simple lithology, clear flysch sequence and obvious seabed scouring characteristics. The reproduction of biota begins at a low level. Proterozoic is dominated by archaspore plants and crustaceans, with as many as 27 genera and 38 species. A large number of graptolite groups were discovered in Ordovician and Silurian in southeastern Guangxi, and 12 and 19 graptolite belts were established. A large number of graptolite groups, belonging to 9 genera and 23 species, were also found in Ordovician in Haiyangshan area. However, the sedimentary environment and sedimentary facies in western Guangxi have changed greatly. Cambrian changed from geosyncline to platform, sedimentary facies changed from clastic rocks to carbonate rocks, and fauna changed to North China trilobites, coccidiosis and brachiopods. A large number of trilobites have been found in Napo area, and 25 genera have been established. Biota changes obviously with the change of geological environment.

2. Platform development stage

Since the Guangxi movement at the end of Silurian, the sedimentary environment of the Late Paleozoic has changed greatly, returning from geosyncline to form a broad uplift area, changing from deep-water environment to shallow sea and coastal platform environment, with turbulent seawater and a large number of biota. In the early stage, it was conglomerate, sandstone and shale deposited alternately on land and sea, and then it began to change to carbonate deposition on platform, mainly limestone and dolomite, and some were argillaceous rocks, siliceous rocks and coal seams. Because the coastal seawater is rich in oxygen and has strong hydrodynamic activity, it is the most prosperous period of paleontology in Guangxi, with the most species, including benthos and plankton, up to 17, of which Devonian is the most abundant, such as the discovery of more than 500 genera and species in 17, and the discovery of 18 fossil belt in six standard sections. There are more than 30 genera and species of brachiopods, corals, foraminifera and bamboos in Beiliu section, brachiopods, corals and bamboos in Dale section, and 97 fossil assemblage zones in * * * *. The main features of Luofu section are bamboos, ammonites and conodonts floating in deep water. There are 77 genera and subgenera and 580 species and subspecies in Carboniferous and Permian. The platform development stage is also the most favorable period for the mineralization of sedimentary minerals in Guangxi. Devonian is rich in manganese, iron, vanadium, limestone and other minerals, accounting for about 60% of the proven reserves in the whole region. Carboniferous and Permian coal, bauxite, manganese and limestone also have great industrial value.

3. Development stages of continental deposits

Since the Indosinian Movement at the end of Middle Triassic, the crust of Guangxi has generally risen, the seawater in the basin has completely withdrawn, and marine deposition has ended. Since then, under the control of active basement faults, more than 50 large and small basins have been formed in Guangxi, among which Shiwandashan and Guiping Wei Qiao basins are the largest, forming lakes and rivers deposits in inland basins. Because of the strong weathering and oxidation on land and the hot climate at that time, the important feature of sediments is red coarse clastic rocks. During Jurassic and Tertiary (Paleogene), due to the proliferation of plants and organic matter, coal-bearing beds (such as Xiwan and Nanning basins) and oil-gas beds (such as Baise basin) and minerals such as clay (Hepu basin), bentonite (Ningming basin) and gypsum salt were formed in some basins. Further evolution and reproduction of biota. The Triassic was dominated by ammonites and bivalves, and reptiles appeared in the fauna. For example, the Oriental species of Guangxi Xianlong was discovered in Wuming County in Triassic, and the Duckbilled Dragon was discovered in Shiwandashan and Nanning Basin in Cretaceous. Since the Quaternary, important ancient human remains and a large number of vertebrate fossils, such as great apes, giant pandas and saber-toothed elephants, have been found at 10 sites in Laibin, Liuzhou, Liucheng, Guilin and Baise. The most important discoveries are Qilinshan man, Liujiang man, Bailiandong man and Zan Piyan man, which provide important practical data for the study of ancient humans.

Second, magmatic activity.

Magmatic activity is an important part of crustal activity, which often occurs at the same time as crustal movement. It is also an important content in the history of regional development in Guangxi, which is closely related to the formation of endogenetic metal minerals. The important metallogenic period of almost all endogenetic metal minerals is closely related to magmatic activity, because it is the main source of metallogenic materials. Magma customization mainly comes from mantle-derived magma (basic rocks and ultrabasic rocks) in the deep part of the earth and crust-derived magma in the shallow part of the earth, in which crust-derived magma is divided into mantle-derived differentiation, mixed eutectic, crust-derived remelting and crust-derived metasomatism. There are two main ways of magmatic activity: deep intrusion and surface eruption.

There are more than 1200 rock masses exposed by intrusive activities in Guangxi, with an exposed area of about 20,000 km2, indicating that the intrusive activities are extensive and intense, with the characteristics of large scale. Daqian black rock mass mostly occurs in anticline and anticlinorium axis, such as Yuanbaoshan, Yueling Mountain and Yangshan Mountain in Beihai, Guangxi, and some of them occur in and around the great fault zone. According to the geological age characteristics of the activity, it has the characteristics of discontinuity and multi-period, and is closely related to structural cycles, such as Bos cycle, Xuefeng cycle, Caledonian cycle, Indosinian cycle, Yanshan cycle and Himalayan cycle. Among them, the granite intrusive body in the Variscan cycle is the largest, with the distribution area of 805 1km2, and the Caledonian cycle (3363km2). The lithology is complex, mainly acidic granitoids, with an exposed area of 65,438 0.9 million km2, accounting for 96% of the intrusive rock area in the whole region, mainly distributed in the vast areas of northern, northeastern and southern Guangxi. The main component is silicon dioxide, usually 60% ~ 65%, with high viscosity, low magnesium and iron content and high potassium and sodium content. The intrusion area of basic rocks and ultrabasic rocks is relatively small, with about 940 large and small rock masses, with an area of 8 17km2, mainly distributed in Bao Si District and Xuefeng District in northern Guangxi. The main basic rocks in western Guangxi are Variscan and Indosinian, and the main component is SiO _ 2, with a content of 45% ~ 50%. The content of SiO _ 2 in ultrabasic rocks is less than 45%, with low viscosity, high magnesium and iron content and low potassium and sodium content.

Volcanic eruption rocks in Guangxi are also frequent. One is the sedimentary process in the basin. After submarine eruption, various layered sediments were formed. Rocks often have pore-shaped, almond-shaped and pillow-shaped structural features, such as layered, pillow-shaped spilite, amphibole, tuff lava, volcanic breccia, lava, etc., especially from the deep part of the earth. One is the eruption activity on the mainland, which forms various lava flows and volcanic deposits. The accumulated thickness of volcanic eruption can reach 9000m, accounting for 1 1% of the total thickness of sedimentary rocks in the whole region, which fully shows that the material source of the crust is supplied by the magmatic activity of the earth. The main ways of volcanic eruption are fissure eruption (along the fault zone) and central eruption (near circle). Judging from the geological age of eruption activity, except CAMBRIAN volcanic activity, it was found in 13 geological ages from Bosi period to Quaternary period, indicating that the regional geological history of Guangxi is active and uninterrupted. According to the existing statistics, there are more than 40 obvious eruption activities. Judging from the previous data, it is almost blank in western Guangxi. Through geological investigation, it is found that a large number of multi-level marine volcanoes erupted from Devonian to Triassic, with a total thickness of 5400m, which is characterized by frequent eruptions, small thickness (usually tens of meters thick) and sporadic distribution. Judging from the rock types, volcanic rocks from Middle Devonian to Permian are mostly basic rocks, and Triassic is mainly intermediate-acid rocks. Volcanic eruption is mainly controlled by basement faults, mostly intermittent faults. After Indosinian movement, it turned into continental volcanic eruption, including rhyolite, rhyolite porphyry, tuff, tuff lava, volcanic breccia, pyroclastic rock and andesite porphyry. Characterized by stomata, almond-shaped and rhyolite. Craters were found in Yandunling and Weizhou Island in Hepu County, and more volcanic deposits and rocks were found.

Third, tectonic movement.

Crustal movement is not only manifested as sedimentation and magmatic activity of land and ocean changes, but also as deformation and metamorphism of rocks and strata. The predecessors have done a lot of research on the tectonic movement in Guangxi. 1963, Yin Zanxun and others counted as many as 28 crustal movements named by geologists in Guangxi before. After geological investigation, only 19 times was considered as having more evidence and other evidence was insufficient. Moreover, the nature, characteristics and intensity of movement in each period are different, and they are different with different regional geological conditions. Generally speaking, it has the characteristics of multi-stage and multi-cycle development, and different stages are also different in sedimentation, magmatic activity, rock metamorphism and deformation. During the 19 movement, there were three Proterozoic movements (Bao Si, Xuefeng and Fulu movements), three early Paleozoic movements (Yunnan, Beiliu and Guangxi movements), four late Paleozoic movements (Liujiang, Guizhou, Soochow, Jiangsu and Anhui movements) and five Mesozoic movements (Guangxi, Indosinian and Yanshan movements). Spatial changes show that the crust has the characteristics of orogeny and land reclamation. Bao Si movement, Guangxi movement, Wu Dong movement, Indosinian movement, Yanshan movement and other orogenic movements have strong intensity, which can make the mainland seawater retreat and stop deposition, make strata and rocks undergo severe folds, fractures, metamorphism, deformation and displacement, and make the sea basin become a continent mountain and basin. After the deposition is interrupted, it begins to deposit in a new development stage, which obviously reflects the unconformity contact between the old and new strata. The tectonic movement is mainly characterized by large-scale ups and downs of the crust. The seawater in the basin withdrew to form land, but the stratum did not move vertically. After a period of erosion and erosion, the basin sank again. When seawater begins to deposit again, the new and old strata often show a parallel unconformity contact relationship, but there are intermittent weathered erosion surfaces and weathered deposits between the new and old strata, such as iron oxide aluminum rocks. The crustal movement is unbalanced in spatial distribution and its intensity is different. For example, in most areas of Guangxi orogeny, the Devonian stratigraphic angle is not integrated in the early Paleozoic strata, and breccia exists widely, but there is no orogeny, shrinking basin, continuous deposition of seawater and continuous evolution of graptolite in Qinzhou area. Another example is the Wu Dong Movement, in which the strata in southern Guangxi were unconformity in different periods, while in other areas, the strata were only in parallel unconformity contact, so a large number of weathered residual Fe-Al rocks and bauxite were formed between the early and late Permian.

Orogenic movement is a manifestation of strong crustal movement, which often causes folds and fractures of strata, forming many anticlines, synclines, synclines and large uplift areas of different sizes, thus forming many mountains, rivers, basins and plains in the mainland. The main mountain ranges in Guangxi are Yuanbao Mountain and Miaoer Mountain in northern Guangxi, Haiyang Mountain in northeastern Guangxi, Dayao Mountain in eastern Guangxi, Rongda Mountain in southern Guangxi, Wanwan Mountain and Daming Mountain in western Guangxi. At the same time of strong folds, super-strength fractures and displacements occurred in the strata, resulting in dislocation and displacement in both horizontal and vertical directions. The larger fault distance can exceed 1000 meters. In the historical process of fault occurrence, there are many active and remarkable fault activities, such as regional large faults and deep large fault zones, which have the functions of controlling sedimentation, magmatism, ore guiding and ore controlling in crustal activities. The regional investigation team summarized 24 major fault zones in Guangxi, such as Bobai-Cenxi deep fault zone and Lingshan-tengxian deep fault zone. Some units also summarized many fault zones according to the data. The main features are obvious. They cut strata of different geological ages, with large fault distance and wide fracture bandwidth, including structural metamorphic rocks, silicification and breccia zones, which have guiding, guiding and controlling effects on magmatic activities and mineralization. It is of great significance in the history of regional geological development, and it is our key research object, especially the geological environment and geological disasters closely related to these deep faults, such as seismic activity zones and neotectonic movements, such as 1934 Lingshan earthquake. Especially for large-scale projects under construction, such as reservoirs and dams, when gravity breaks the balance and recombines, we should pay close attention to new tectonic movements and deepen safety research.

The above are the basic characteristics of basic geology in Guangxi, so let's discuss them together. Some geological problems need further practice and research. If there is anything wrong, please criticize and correct me.