Li Shanglin1 Liu Yucai1 Yang Buwang2 Yuan Huabao3 Wang Xiaoyan3 Yu Jingchun1 Long Wenhua2
(1. Physical Geological Data Center of the Ministry of Land and Resources, Beijing 101149; 2. Inner Mongolia Geological Survey, Hohhot 010010; 3. The First Geological Survey Institute of the China Metallurgical Geological Exploration Engineering Administration, Beijing 101601)
Abstract The Permian Amushan Formation under the Inner Mongolia Darhan Maoming'an Union is a set of coastal - Continental clastic rocks intercalated with carbonate rock deposits can be divided into two sections from bottom to top: the bottom of the first section is the terrigenous clastic tidal flat sedimentary system, and the upper part is the carbonate (platform) tidal flat sedimentary system; the second section is The delta sedimentary system consists of prodelta sedimentary subphases, delta front sedimentary subphases, and delta plain sedimentary subphases from the bottom upward. The above filling sequence reflects that the Amushan Formation went through three evolutionary stages: tidal flat - carbonate (platform) tidal flat - delta, which represents a complete transgressive-regressive sea level change cycle.
Keywords Darhan Maoming'an United Banner; Lower Permian; Amushan Formation; sedimentary facies
1. Overview of the Amushan Formation
Lower Permian The "Amushan Formation" was named in 1959 by the 241st Team of the former Ministry of Geology at the Red Flag Ranch Amanyin Wusu in Darhan Maoming'an United Banner (referred to as Damao United Banner)[1], and it has been used to this day. The Amushan Formation in the study area is mainly distributed in the Amuyinwusu-Harizha-Hirhada-Shanda area in the Damao United Banner area. It is distributed in a nearly east-west arc shape with an exposed area of ??nearly 60 km2. It belongs to the Tianshan- In the Xingmeng stratigraphic area, the Inner Mongolia grassland stratigraphic divisions and facies are complete. The lithology is mainly terrestrial clastic rock intercalated with carbonate rock. From west to east, the terrigenous clastic rock gradually decreases and the carbonate rock gradually increases. The limestone is rich in animal fossils such as worms, brachiopods, corals, and crinoids. Clastic rocks contain plant fragments. Previous studies have divided them into three types based on the bottom-up occurrence of Triticites (Guo Shengzhe, 1983; Zhao Zeguo, 1965), Pseudoschwagerina (Guo Shengzhe, 1983) and Eoparafusulina (Guo Shengzhe, 1983; Han Jianxiu, 1979) respectively. The combined zone is dated to the Late Carboniferous [1]. According to the 2000 international stratigraphic table and the 2001 Chinese regional stratigraphic table [2], its age should be the Early Permian, and the fossils quarried this time can basically be compared with it. Based on the above characteristics, the author believes that the age of the Amushan Formation is appropriately assigned to the Early Permian. Previous studies [1-5] were basically based on paleontology and stratigraphy, with few studies on sedimentary facies and sedimentary environment. This time we will make an attempt in this regard. This article focuses on the Shanda section, with reference to the Bayinwusu and Amanyinwusu sections. The Amushan Formation sedimentary system and its evolution in this area are widely representative in the area. Although the rock combination types are simple, sedimentary structures are widely developed and of various types, which is of great significance for studying its depositional environment. According to the characteristics of lithology combination, biological combination, sedimentary structure and sedimentary cycle characteristics, the Amushan Formation in the area is divided into two sections from top to bottom. The sedimentary sequence is shown in Figure 1.
Figure 1 Sedimentary sequence diagram of the Amushan Formation
The bottom of the first section is gray-white medium-coarse- to medium-fine-grained calcareous quartz sandstone (may contain gravel) and purple-red calcareous mudstone. Thick layers of argillaceous siltstone are in unconformable contact with the lower overlying Chaganhebu Formation; the middle and upper parts are gray thick-layered bioclastic limestone and micrite limestone intercalated with purple-red medium-coarse-grained feldspar sandstone and silty mudstone. , the total thickness is approximately 381m.
The lower part of the second member is interbedded with gray siltstone, argillaceous siltstone, and feldspathic sandstone, with a thickness of 231m; the middle part is composed of medium-coarse grained lithic sandstone, feldspathic sandstone interbedded with siltstone, and argillaceous siltstone. , 323m thick; the upper part is gray-yellow, gray-purple medium-fine-grained feldspathic sandstone, lithic sandstone, silty mudstone intercalated with siltstone, and is in fault contact with the Xibiehe Formation, about 353m thick.
Based on the description of field outcrop lithology and sedimentary structure, detailed observation of lithofacies and lithofacies combinations, and comprehensive indoor research, the Amushan Formation can be divided into terrigenous clastic tidal flat, carbonate (platform ) three sedimentary systems: tidal flat and delta [6-10].
II. Terrigenous clastic tidal flat sedimentary system
1. Lithological analysis
At the bottom of the first member of the Amushan Formation, three main lithologies are developed: ① Medium Coarse-grained sandstone has a grain support structure and high structural and compositional maturity. The sand debris is mainly composed of quartz, with a content of about 90%. It is sub-angular to rounded, with a particle size of 0.5-2mm, and a single layer thickness of 5-10cm. The system is 1 to 10m thick, has relatively stable lateral extension, and is a high-energy environment deposit in the intertidal zone; ② Fine sandstone and siltstone, with particle support structure and a small amount of miscellaneous matrix support structure, with good sorting between single layers, often appearing as interlayers, and horizontal The extension is unstable, with a single layer thickness less than 10mm, or a lens sandwiched in the mudstone; ③ pink calcareous mudstone, plant fossil fragments and plant stems can be seen, and bioturbation structures are developed. Mud crack structures are more common, which is caused by frequent water levels. Sedimentary rocks formed under conditions of variable and intermittent water exposure, representing an arid, low-energy supratidal environment.
2. Sedimentary structural characteristics and other facies indicators
The tidal flat sedimentary system of this group is mainly composed of supratidal and intertidal zone sediments. According to the sedimentary characteristics, it is divided into three categories: sand flat, mud flat and mixed flat. The sand flat is mainly composed of gray and gray-white calcareous quartz sandstone, with high composition and structural maturity. It is occasionally intercalated with thin layers of siltstone and mudstone, and develops flowing sand laminations. The mixed flat lithology is represented by fine-grained quartz sandstone and siltstone. There are often thin layers of mudstone in sandy sediments. This thin layer is deposited by suspended sediments that are disturbed by strong tidal currents and deposited when the flow velocity is minimum during high and low tides or flat and dead tide periods. The lateral extension is unstable and often Distributed in a lens shape. Striped bedding, wavy bedding and lenticular bedding are developed, among which striped tidal interactive bedding is the most developed. The mud flat is mainly composed of pink calcareous mudstone, intercalated with lenticular sandstone and siltstone. It develops horizontal bedding, horizontal corrugated laminae, common mud crack structures and bioturbation structures.
The tidal flats in this group are a retrogradational sedimentary sequence that tapers upward on the profile. It starts with a sand flat at the bottom, transitions upward to a mixed flat, and ends with a mud flat at the top. The lenticular tidal channel sandstone lens is partially filled, mainly composed of medium-coarse-grained sandstone, with erosional structures at the bottom and typical bidirectional plume cross-bedding.
3. Carbonate platform tidal flat sedimentary system
Located in the upper part of the first member of the Amushan Formation, the main lithology is: ① Micrite limestone, the rock component is mainly lime mud , the bioclastic content is less than 10, including biological categories such as worms, brachiopods, bivalves and crinoid stems. It is produced in a semi-hedral-heteromorphic shape. From the thin section, it shows a structural type mainly supported by mortar. , developing horizontal bedding or massive bedding, which is a low-energy environment; ② Bioclastic marl contains different types of biological fossil particles, and the particles have been damaged to varying degrees and subjected to a certain degree of micritization. The particles are supported, and the spaces between the particles are mainly filled with mortar. A small amount of fine-grained spangled calcite can also be seen scattered in the mortar matrix, which may be recrystallized mortar or fine biological debris. Millimeter-level horizontal laminae are developed. , mostly produced in thin layers, with weak hydrodynamic energy and insignificant tidal flow effects, and are mainly formed in medium-low energy environments above the wave base; ③ Feldspar sandstone, medium-low compositional maturity, particle support, mixed matrix Low content, pore cementation, erosion structure at the bottom, feathery cross-bedding development, and lenticular filling sediments in tidal waterways; ④ Calcareous mudstone, containing silt sand and plant debris, with developed bioturbation structures, representing subtidal Low energy deposition.
The thickness of this sedimentary system is small, horizontal textures are developed in the rock layers, and bioturbation structures are strong. Massive bedding and interference ripple marks formed by water flow in different directions can be seen. The terrigenous clastic tidal channel deposits are relatively developed, indicating that the platform is close to the coast and the supply of terrigenous materials is relatively sufficient. The basic sequence is an upward shallowing type composed of micrite limestone, bioclastic limestone and mudstone, and its combination is characterized by carbonate (platform) tidal flat sedimentation.
IV. Delta sedimentary system
1. Lithological analysis
The delta sedimentary system tracts of this group are complete and the sedimentary thickness exceeds 907m. The research is relatively detailed. According to the sediment grain size, structure, structure and stratigraphic geometry, combined with the research results of Miall [9, 10], the sediments are divided into 4 lithofacies: ① Sandstone (sp), mainly composed of medium to poorly sorted medium-coarse grains It is composed of sandstone, which can contain gravel. It extends from several meters to more than 20 meters laterally and is produced in a lens shape. The thickness of a single layer is 0.3 to 1.2m. The bottom surface is an erosion structure. The normal bedding is cut, and the layers are plate-like cross-bedding or Trough-shaped cross-bedding, which is a sedimentary product of low flow state or sand wave migration; ②Sandstone (sr), mainly composed of well-sorted silt-coarse-grained sandstone with high roundness, stable lateral extension, single layer It is 0.6-60cm thick, has a relatively flat bottom surface, and has wavy cross-bedding and small plate-like cross-bedding in the layer, which is a low-fluid sedimentation; ③ Siltstone (Fm, Fl), mainly composed of well-sorted siltstone, Common mudstone interlayers, single layer thickness 3~10mm, layer thickness 0.6~2m, the bottom surface is relatively flat, extending laterally far, with massive bedding, horizontal bedding, sand grain bedding, belonging to the flood plain (flood plain) Sedimentation; ④ Mudstone (ml, Mb), mainly composed of clay-grade materials, develops horizontal bedding, sand-grained bedding, and massive bedding.
2. Sedimentary subfacies and sedimentary structural characteristics
Three subfacies, delta plain, delta front and prodelta, can be identified in this group of delta depositional systems. Several microphase units can also be divided. Among them, the branch channels and inter-branch channel bays (floodplains or swamps) in the plain subfacies, the underwater distributary channels, underwater distributary channel bays, and estuary sand bars in the front subfacies are relatively typical, and the prodelta is mainly a A set of fine clastic rocks with silty mudstone intercalated with siltstone. The entire delta sedimentary sequence exhibits a progradational profile cycle structure that coarsens upward.
(1) Delta plain subfacies
Branch channel subfacies The branch channel is dominated by medium-coarse grained feldspathic sandstone and lithic sandstone deposits. The sandstone is mostly angular, has poor sorting properties, and has low structure and maturity. The gravels at the bottom are arranged in an imbricated shape, with a single particle size less than 1cm, and develop plate-shaped cross-bedding and trough-shaped cross-bedding. Vertically, it consists of several positive rhythmic sequences that are thicker at the bottom and thin at the top, representing the product of lateral migration and sedimentation in the river mouth. The bottom of a single rhythm is usually a scour surface, on which are river bed retention deposits under high kinetic energy scour conditions, medium-sized fine sandstone with cross-bedding or parallel bedding, and grid-like waves-water flow interference ripples developed on the surface. The product of superposition of currents and waves in different directions [9].
The microfacies of the floodplain (flood plain or swamp) between branch rivers are mainly siltstone, mudstone and argillaceous siltstone, with horizontal bedding, wavy bedding and rain mark structures. It is a floodplain The exposed surface is an iconic environmental structure preserved after being hit by rain, and well-preserved plant stem fossils can be seen.
(2) Delta front subfacies
Underwater distributary channel microphase Underwater distributary channel is the underwater extension of the plain environment after entering the sea, and the sediments are generally well sorted , mainly composed of conglomerate lithic sandstone, medium coarse-medium fine-grained lithic sandstone, and feldspathic lithic sandstone. The gravel composition of the sandstone is mainly quartz, with a small amount of feldspar, followed by lithic debris, and sub-angular-sub-angular. Round shape, the quartz content is about 75-80%, the rock is supported by particles, and the content of argillaceous impurities in the rock is very small, mostly below 10%. There is usually a scour surface at the bottom of a single sequence, and the sandstone is filled above and produced in a lens shape. The maximum thickness of the lens is 0.5~2m, and a few can reach 5m. It quickly pinches out after extending horizontally for several meters. The lower part of the lens is composed of gravelly or coarse gravel sandstone, with massive and large trough-shaped cross-bedding and parallel bedding. It gradually transitions upward to siltstone and argillaceous siltstone, with mainly small plate-like and wavy bedding. The basic sequence is an upward-thinning type with the erosion surface as the base.
The microfacies of the bay between underwater distributary channels is composed of siltstone, silty mudstone and mudstone, with a small amount of fine sandstone, and the single-layer sediments have good sorting properties. Due to the frequent migration of underwater river channels, inter-channel sediments are often damaged by erosion. Lenses of varying sizes often appear in the channel sandstone on the profile. Horizontal bedding and lenticular bedding are developed, showing bioturbation structures.
The estuary sand bar microfacies is composed of medium-coarse to medium-fine grained lithic sandstone, feldspathic sandstone, and siltstone. Sandstone generally has good sorting properties, and fragments of bivalves and other biological fossils can be seen. Vertically, two basic sequences, thickening upward and thinning upward, are developed. The sedimentary structure of the estuary sand bar is developed, and wavy bedding, parallel bedding, small and medium-sized plate cross-bedding and trough cross-bedding are common.
(3) Prodelta subfacies
The prodelta is a fine-grained sedimentary association developed in the seaward direction of the delta front. The lithology is relatively simple, mainly siltstone, Argillaceous siltstone and mudstone develop horizontal bedding and sand-mud interbedded bedding, etc., and the bioturbation is relatively strong.
5. Evolution of sedimentary systems
As we all know, the northern edge of the North China Plate has been accompanied by the processes of alternate disintegration of the continental margin into oceans and uplift of continental margins during the Caledonian and Hercynian periods [11] , the Amushan Formation was formed under the tectonic background of continental margin breakup and crustal decline. In the early stage of the Amushan Formation (section 1), seawater invaded from east to west, leaving this area in a coastal and shallow marine environment. At this time, the supply of terrestrial materials was relatively abundant, mainly clastic sedimentation. A set of thick clastic rocks composed of gray-white medium-coarse-grained quartz sandstone, fine-grained quartz sandstone, purple-red siltstone and mudstone was deposited near the edge of the basin. System, the basic sequence group is several cyclic sedimentary sequences. The vertical filling sequence shows sand flat-mixed flat-mud flat deposition, and the plane shows the sand flat-mixed flat-mud flat deposition pattern from east to west. In the late Stage 1 of the Amushan Formation, terrestrial clastic materials decreased accordingly, and the sediments gradually transitioned from clastic rocks to carbonate rocks, forming mainly carbonate rocks with continental margin clastic rocks to develop a large number of corals and brachiopods. It is characterized by crinoids and crinoids. The basic sequence group is composed of overlapping cyclic sedimentary sequences that taper upward, and is a carbonate (platform) tidal flat phase deposit.
In the late period of the Amushan Formation (Second Member), the basin rose as a whole, the water receded and became shallower, rivers developed, facies belts migrated toward the basin, and a set of terrigenous clastic sediments dominated by river deltas developed. The system is composed of lithic sandstone, feldspathic sandstone, siltstone intercalated with limestone lenses, and the rock maturity is low. The cross-section structure generally shows an upward coarsening type, with a rhythmic upward coarsening and thickening progradational basic sequence, representing the gradual shallowing process of the water body, and the depositional environment gradually transitioned from a shallow marine environment to a delta coastal plain, and finally to alluvial Plains evolution.
Therefore, the evolution of the sedimentary system of the Amushan Formation in this area can be summarized from east to west as the basic law of terrigenous clastic tidal flat phase - carbonate platform tidal flat phase - delta phase.
This article is written based on the results of the 1:50,000 district survey. I would like to express my gratitude to all the comrades who participated in the project.
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Sedimentary Characteristics of Lower Permian Amushan Formation in Darhanmuminggan Lianheqi, Innermongolia
Shanglin Li, Yucai Liu, Buwang Yang, Huabo Yuan, Xiaoyan Wang, Jingchun Yu, Wenhua Long
(1.National Geologicalsample Center, ministry of Land and Resources, Beijing 101149; 2.Geologicalsurvey Institute of Innermongolia, Hohhot , Innermongolia 010010; 3. Chinametallurgical Geological Exploitation Engineering General Bureau First Geologica
lsurvey Institute, Beijing 101601)
Abstract In Darhanmuminggan Lianheqi of Innermongolia, lower Permian Amushan Formation is aset ofsediments of littoral land-facies clastic rocksmixed with carbonate rock.Form the bottom to the top it can be divided into two sections: the first section consists of the lower terrigenous clastic tidalsedimentarysystem and the upper carbonate tidal(platform)sedimentarysystem; the secondsection is attributed to deltasedimentarysystem, which bottom-up can be redivided into off-deltasedimentarysub-facies, front-delta-marginsedimentarysub-facies, and delta plainsedimentarysub-facies.The order reflects Amushan Formations successfully experienced three stages of formation as of tidal flat, carbonate tidal flat(platform), and delta, which represents an integrated sea level vibration cycle between sea intrusion and sea regression.
Key words Amushan Formation; sedimentary facies; lower Permian; Darhanmuminggan Lianheqi