(1. Physical Geological Data Center of Ministry of Land and Resources, Beijing101149; 2. Geological Survey Institute of Inner Mongolia, Hohhot 010010; 3. First Geological Exploration Institute of China Metallurgical Geological Exploration Engineering General Bureau, Beijing 10 160 1)
The Permian Amushan Formation, which belongs to Maoming Allianz in Dalhan, Inner Mongolia, is a set of littoral-continental clastic rocks dominated by carbonate rocks, which can be divided into two sections from bottom to top: the first section has a terrigenous clastic tidal flat sedimentary system at the bottom and a carbonate (platform) tidal flat sedimentary system at the top; The second section is delta sedimentary system, which consists of pre-delta sedimentary subfacies, delta front sedimentary subfacies and delta plain sedimentary subfacies from bottom to top. The above-mentioned filling sequence reflects that the Amushan Formation has experienced three evolution stages: tidal flat-carbonate (platform) tidal flat-delta, representing a complete transgression-regression sea level change cycle.
Keywords Dalhan Maoming Allianz Banner; Lower Permian; Amushan Formation; sedimentary facies
I. Overview of Amushan Formation
The Lower Permian Amushan Formation (1959) was discovered by the 24 1 team of the former Ministry of Geology in Yinwusu, A Mu, Hongqi Ranch, Lianhe Banner, Maoming City, and has been in use ever since. The Amushan Formation in the study area is mainly distributed in the Yinwusu-Hariza-Xirihada-Shanda area of A Mu in Damao United Banner area, with an arc distribution in the east-west direction and an exposed area of nearly 60 km2, belonging to Tianshan-Meng Xing stratigraphic area, and the stratigraphic regionalization of Inner Mongolia grassland is complete. The lithology is mainly terrestrial clastic rocks mixed with carbonate rocks, and the terrigenous clastic rocks gradually decrease from west to east, while the carbonate rocks gradually increase. The limestone is rich in animal fossils such as brachiopod, coral and sea lily. Clastic rocks contain plant fragments. According to the bottom-up characteristics of limestone, wheat (Guo,1983; Zhao Zeguo, 1965), Pseudoschwagerina (Guo, 1983) and Eoparafusulina (Guo,1983; The outcrop of Han Jianxiu (1979) is divided into three assemblage zones, and the age is Late Carboniferous [1]. According to the international stratigraphic table in 2000 and the regional stratigraphic table in China in 200 1 [2], its age should be early Permian, and the fossils collected this time can basically be compared with it. Based on the above characteristics, the author thinks that the age of Amushan Formation should be divided into Early Permian. Previous studies [1 ~ 5] were basically based on paleontology and stratigraphy, and little research was done on sedimentary facies and sedimentary environment. This time, we will try this. This paper takes Shanda section as the main body and Bayinwusu and A Mu Yinwusu sections as the reference. The sedimentary system and evolution of Amushan Formation in this area are widely representative in this area. Although the rock assemblage type is simple, the sedimentary structures are widely developed and diverse, so it is of great significance to study its sedimentary environment. According to the characteristics of lithologic association, biological association, sedimentary structure and sedimentary cycle, the Amushan Formation in this area is divided into two sections from top to bottom, and the sedimentary sequence is shown in Figure 1.
Figure 1 Sedimentary Sequence Map of Amushan Formation
The bottom of the first member is grayish white medium-coarse grained-medium-fine grained calcareous timely sandstone (which may contain gravel), and the purplish red calcareous mudstone is mixed with medium-thick argillaceous siltstone, which is in unconformity contact with the underlying Chaganbuhe Formation. The middle and upper part is gray thick layered bioclastic limestone, micrite limestone mixed with purple medium-coarse grained feldspathic sandstone and silty mudstone, with a total thickness of about 381m.
The lower part of the second member is interbedded with grey siltstone, argillaceous siltstone and feldspathic sandstone, with a thickness of 231m; ; The middle part is medium-coarse lithic sandstone, feldspar sandstone mixed with siltstone and argillaceous siltstone, with a thickness of 323 m; The upper part is grayish yellow and grayish purple medium-fine grained feldspathic sandstone, lithic sandstone and silty mudstone mixed with siltstone, which is in fault contact with Xibiehe Formation and is about 353 meters thick.
According to the description of outcrop lithology and sedimentary structure, detailed observation of lithofacies and lithofacies combination and comprehensive indoor study, the Ameishan Formation is divided into three sedimentary systems: terrigenous clastic tidal flat, carbonate tidal flat and delta [6 ~ 10].
Secondly, terrigenous clastic tidal flat sedimentary system
1. Lithology analysis
At the bottom of the first member of Amershan Formation, there are mainly three kinds of lithology: ① Medium-coarse grained sandstone with granular support structure and high maturity of structure and composition. The composition of sand debris is mainly timely, with a content of about 90%, subangular to circular, with a particle size of 0.5~2mm, a single layer thickness of 5 ~ 10cm and a series thickness of 1 ~ 10m. (2) Fine sandstone, siltstone, granular support structure, and a small amount of miscellaneous foundation support structure, with good separation among single layers, often in interlayer shape, unstable lateral extension, single layer thickness < < 10mm, or lenticular sandwiched in mudstone; (3) Pink calcareous mudstone, with plant fossil fragments and plant stems, has biological disturbance structure and common mud fracture structure. It is a sedimentary rock formed under the conditions of frequent water level changes and intermittent exposure, representing a dry and low-energy supratidal environment.
2. Sedimentary structural features and other facies marks
The tidal flat sedimentary system of this group is mainly composed of sediments in the supratidal zone and intertidal zone. According to sedimentary characteristics, it can be divided into sand flat, mud flat and mixed flat. The sand flat is mainly composed of gray and gray-white calcareous seasonal sandstone with high composition maturity and high structure maturity, with occasional thin siltstone and mudstone layers and developed quicksand bedding. The lithology of mixed flat is represented by fine-grained timely sandstone and siltstone. There are often thin mudstone layers in sandy sediments. This kind of thin mudstone layer is deposited by suspended sediment disturbed by strong tidal current at high tide and low tide or when the flow velocity is minimum at high tide and low tide, and its lateral extension is unstable and often lenticular. Banded bedding, wavy bedding and lenticular bedding are developed, among which banded tidal alternating bedding is the most developed. Ni Ping is mainly composed of pink calcareous mudstone, sandwiched with lenticular sandstone and siltstone, with horizontal bedding, horizontal wavy streaks, common muddy fault structures and biological disturbance structures.
The retrogradation sedimentary sequence of the tidal flat in this group is obviously tapering upward on the profile, starting from the sand flat at the bottom, the mixed flat at the top and the mud flat at the top. The lens of partially filled lenticular tidal channel sandstone is mainly composed of medium-coarse grained sandstone, with erosion structure and typical bidirectional pinnate cross bedding at the bottom.
Thirdly, the tidal flat sedimentary system of carbonate platform
Located in the upper part of the first member of Amershan Formation, the main lithology is: ① micrite limestone, the rock composition is mainly mortar, and the biological debris content is less than 10%, including biota, brachiopods, bivalves and stems of sea lilies, which are semi-self-shaped. From the thin section, it is a structural type mainly supported by mortar, with horizontal bedding or massive bedding, belonging to low-energy environment; ② Bioclastic granular marl, which contains different kinds of bio-fossil particles, is damaged to varying degrees and supported by micrite to a certain extent. Particles are mainly filled with mortar, and a small amount of fine bright calcite is scattered in the mortar matrix, which may be recrystallization mortar or fine biological debris. Millimeter-level horizontal bedding is developed, mostly in thin layers, with weak hydrodynamic energy and inconspicuous tidal current, which mainly forms above the Apollo bottom. ③ Feldspar sandstone, with medium-low composition maturity, grain support, low content of matrix, pore cementation, bottom erosion structure and pinnate cross bedding, belongs to lenticular filling deposition of tidal channel; ④ Calcareous mudstone, containing silt and plant debris, with well-developed biological disturbance structure, represents low-energy deposition under the tide.
The thickness of the sedimentary system is small, the horizontal structure in the rock stratum is developed, and the biological disturbance structure is strong, and the massive bedding and interference wave marks formed by water flow in different directions can be seen. The relative development of terrigenous clastic tidal channel deposits indicates that the platform is close to the coast and the supply of terrigenous materials is sufficient. The basic sequence consists of micrite limestone, bioclastic limestone and mudstone, and its combined feature is carbonate (platform) tidal flat deposition.
Fourthly, delta sedimentary system.
1. Lithology analysis
This group of delta sedimentary system tract facies belt is complete, and the sedimentary thickness is over 907 meters, which is carefully studied. According to the grain size, texture, structure and stratum geometry of the sediments, combined with the research results of Miall[9, 10], the sediments are divided into four lithofacies: ① Sandstone (sp), which is mainly composed of medium to poor medium-coarse grained sandstone, and may contain gravel. It extends horizontally for several meters to more than twenty meters, and is lenticular, with a single layer thickness of 0.3 ~/kloc-0. ② Sandstone (sr), which is mainly composed of fine sand and high roundness coarse-grained sandstone, has stable lateral extension, single layer thickness of 0.6 ~ 60 cm, flat bottom, wavy cross bedding and small plate cross bedding in the layer, and belongs to low-flow deposition; ③ Siltstone (Fm, Fl) is mainly composed of well-sorted siltstone, sandwiched with ordinary mudstone, with a single layer thickness of 3 ~ 10 mm and a series thickness of 0.6 ~ 2 m, with flat bottom, far lateral extension, massive bedding, horizontal bedding and sand bedding, belonging to floodplain deposit; ④ Mudstones (ml, Mb) are mainly composed of clay-grade materials with horizontal bedding, sand bedding and massive bedding.
2. Characteristics of sedimentary subfacies and sedimentary structures
The delta sedimentary system can identify three subfacies, namely delta plain, delta front and pre-delta, and each subfacies can be divided into several microfacies units. Among them, plain subfacies and inter-branch channel bay (flood plain or swamp) subfacies, underwater distributary channel, inter-branch channel bay, estuary bar and other front subfacies are typical, and the front delta is mainly a set of fine clastic rocks mixed with silty mudstone and siltstone. The whole delta sedimentary sequence shows the upward thickening progradation profile cycle structure.
(1) delta plain subfacies
Branch channel subfacies distributary channel is mainly composed of medium-coarse grained feldspathic sandstone and lithic sandstone. Sandstones are mostly angular, with poor sorting, low structure and low maturity. Gravels at the bottom are arranged in imbricate shape, with a single particle size less than 1cm, and there are plate-like cross bedding and groove-like cross bedding. There are several positive rhythmic sequences with thick bottom and thin top in the longitudinal direction, which represent the products of lateral migration and deposition in the estuary. The bottom of a single rhythm is usually a scouring surface, and the riverbed is deposited on it under the condition of high kinetic energy scouring, and medium-sized fine sandstone with bad cross bedding or parallel bedding develops, and the surface develops grid-like wave-water interference wave marks, which is the product of superposition of water and waves in different directions [9].
Interbranch floodplain (flood plain or swamp) microfacies are mainly siltstone, mudstone and argillaceous siltstone, with horizontal bedding, wavy bedding and raindrop imprint structure. This is a landmark environmental structure, which still exists after the exposed surface of the flood plain is attacked by rain, and well-preserved plant stem fossils can be seen.
(2) Delta front subfacies
Underwater distributary channel microfacies underwater distributary channel is an underwater extension of plain environment after entering the sea, and sediments are generally well sorted, mainly composed of conglomerate-bearing sandstone, medium-coarse-medium fine-grained lithic sandstone and feldspathic lithic sandstone. The composition of sandstone gravel is mainly chronological, followed by feldspar, sub-angular-sub-circular, and the chronological content is about 75% ~ 80%. The rock is supported by particles and the content of argillaceous matrix is small. There is usually a scouring surface at the bottom of a single sequence, and the sandstone filled on it is lenticular. The maximum thickness of the mirror body is 0.5 ~ 2m, and a few can reach 5m. When the lens body extends horizontally for several meters, it will be pointed out quickly. The lower part of the lens body is gravel or coarse gravel sandstone, with massive and trough-shaped cross bedding and parallel bedding, and gradually transforms into siltstone and argillaceous siltstone, mainly with small plate-like and wavy bedding. The basic sequence is gradually tapering upward, and the erosion surface is the bottom.
The bay microfacies of underwater distributary channel are composed of siltstone, silty mudstone and mudstone, with a small amount of fine sandstone, and single-layer sediments are well sorted. Due to the frequent migration of underwater rivers, sediments between rivers are often eroded and destroyed. Lenses of different sizes often appear in channel sandstone on the profile, with horizontal bedding and lenticular bedding, and biological disturbance structures can be seen.
The microfacies of Hekouba are composed of medium-coarse to medium-fine grained lithic sandstone, feldspathic sandstone and siltstone. Sandstones are usually well sorted, and biological debris such as bivalves can be seen. In the vertical direction, there are two basic sequences: upward thickening and upward thinning. The sedimentary structure of estuary dam is well developed, and there are common wave bedding, parallel bedding, small and medium-sized board staggered bedding and trough staggered bedding.
(3) Pre-delta subfacies
The pre-delta is a fine-grained sedimentary assemblage developed in the seaward direction of the delta front, with simple lithology, mainly siltstone, argillaceous siltstone and mudstone, horizontal bedding and interbedded sand and mud, and strong biological interference.
Evolution of verb (abbreviation of verb) sedimentary system
As we all know, the northern part of North China Plate has been accompanied by the alternating process of continental margin cracking into ocean and uplifting into land in Caledonian and Hercynian [1 1], and the Amushan Formation was formed under the tectonic background of continental margin cracking and crustal decline. In the early Amushan Formation, seawater invaded from east to west, which made this area in a coastal shallow sea environment. At this time, the supply of terrigenous materials is relatively sufficient, mainly composed of clastic sediments. A set of thick clastic rock series consisting of grayish white medium-coarse grained chronological sandstone, fine grained chronological sandstone, purplish red siltstone and mudstone is deposited on the edge of the basin. The basic sequence group consists of several cyclic sedimentary sequences, the vertical filling sequence shows sand flat-mixed flat-mud flat deposition, and the plane shows sand flat-mixed flat-mud flat deposition law from east to west. In the late period of the first member of Amershan Formation, terrigenous detrital materials decreased correspondingly, and sediments gradually changed from detrital rocks to carbonate rocks, forming continental margin detrital rock deposits dominated by carbonate rocks, characterized by the development of a large number of corals, brachiopods and sea lilies. The basic sequence group is composed of cyclic sedimentary sequences gradually tapering upward, which are carbonate (platform) tidal flat facies deposits.
In the late period of Amershan Formation (the second member), the whole basin uplifted, the water body retreated and became shallow, rivers developed, facies belts migrated to the basin, and a set of terrigenous clastic sedimentary system dominated by lithic sandstone, feldspathic sandstone and siltstone with limestone lenses developed, with low rock maturity. On the whole, the profile structure is upward thickening, with a rhythmic progressive basic sequence of upward thickening and thickening, which represents the process of gradual shallowing of water, and the sedimentary environment gradually transits from shallow sea environment to delta coastal plain, and finally evolves into alluvial plain.
Therefore, the evolution of the depositional system of Amershan Formation in this area is summarized as the basic law of terrigenous clastic tidal flat facies-carbonate platform tidal flat facies-delta facies from east to west.
This paper is written on the basis of1:50,000 regional survey results. Thanks to all the comrades who participated in this project.
refer to
Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region. Regional geology of Inner Mongolia Autonomous Region [M]. Beijing: Geological Publishing House, 163 ~ 165.
2 National Stratigraphic Committee. 2002. Description of Chronostratigraphy (Geological Age) Table of China Region [m]. Beijing: Geological Publishing House, 5 ~ 10, 38 ~ 42.
Han jianxiu. 1982. late carboniferous fauna in A Mu mountain area of Wulanchabu league, inner Mongolia [A]. see: anthology of stratigraphic paleontology (second series) [C]. Beijing: Geological Publishing House, 132 ~ 140.
Guo. 1987. Carboniferous marine system in the northern trough of China [J]. Published by Shenyang Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences,15:121~144.
Bureau of Geology and Mineral Resources of Inner Mongolia Autonomous Region. 1996. Lithology of Inner Mongolia Autonomous Region [m]. Wuhan: China Geo University Press, 198 ~ 203.
Liu Baojun, Zeng Yunfu. 1985. lithofacies palaeogeography foundation and working methods [M]. Beijing: Geological Publishing House, 1 ~ 300.
Li Shanglin. 1996. Sequence stratigraphy of the Middle-Upper Proterozoic Bayun Aobao Group in Inner Mongolia [A]. Geological progress in China (1993~ 1996)[C]. China Ocean Publishing House, Beijing, 672~675.
Research group on sedimentary structure and environmental interpretation. 199 1. sedimentary structure and environmental interpretation [M]. Beijing: Science Press, 1 ~ 75.
[9]miall A D. 1978。 Lithofacies types and vertical profile models in braided river sediments: a review [A]. In: Mikhail AD (ed.). River sedimentology [C]. Journal of Geology, 5:597~604.
miall A D. 1984。 Principles of Sedimentary Basin Analysis [M]. new york: springer, 668~670.
Cheng. 1994. introduction to regional geology in China [M]. Beijing: Geological Publishing House, 56 ~ 345.
Sedimentary characteristics of Permian Amushan Formation under Darhan Muminggan Union, Inner Mongolia
Li Shanglin,,, Yang,,, Yu Jingchun, Long Wenhua
(1. National Geological Sample Center of Ministry of Land and Resources, Beijing101149; 2. Inner Mongolia Geological Survey Institute of Hohhot, Inner Mongolia 010010; 3. The First Geological Survey Institute of China Metallurgical Geological Development Engineering General Bureau, Beijing 10 160 1)
The Permian Amushan Formation, which belongs to Darhan Mominggan Union, Inner Mongolia, is a set of coastal continental clastic rocks mixed with carbonate rocks. From bottom to top, it can be divided into two sections: the first section consists of the lower terrigenous clastic tidal sedimentary system and the upper carbonate tidal (platform) sedimentary system; The second member belongs to delta sedimentary system, which can be divided into outer delta sedimentary subfacies, pre-delta marginal sedimentary subfacies and delta plain sedimentary subfacies from bottom to top. This sequence reflects that the Amshan Formation has experienced three stages: tidal flat, carbonate tidal flat (platform) and delta, and represents a complete A-level vibration cycle between transgression and regression.
Keywords Amushan Formation; Sedimentary facies; Lower Permian; Darhan Muminggan United Banner