A Comprehensive Study Report on Devonian Donggangling Stage in China —— Suggestions on the boundary and stratotype point of Donggangling Stage.

Yin Baoan Tan Jian Zheng Lu Jipu Tang Hong Zhuan Li Yukun Thassapak Hsu

(Guangxi Institute of Regional Geological Survey, Guilin 54 1003)

Before 1980s, there was no unified name of Devonian strata in the world, and some countries had their own chronostratigraphic names, such as Germany and Belgium. Devonian strata in South China are developed with regional characteristics and have been highly studied. Since the early 1960s, the Devonian strata in South China have been established (Wang Yu, Yu Changmin, 1962), and the Devonian Symposium in South China in 1974 has established a relatively complete chronostratigraphic table (Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences, 1978). And Wang Yu, Yu Changmin (1974), Hou (1978), Yang Shipu (1979), Xian (1980), Bai Shunliang (1. The stratum code of China is Devonian (State Stratigraphic Committee, Hou et al. , 2000) and Notes on China Regional Chronology (Geochronology) (National Stratigraphic Committee, 2002), which reflect the progress of stratigraphic research in this period. In this research process, the Donggangling period of Middle Devonian has also been widely recognized, accepted and applied. However, due to the limitation of historical conditions at that time, there is no concept of "boundary stratotype and boundary point" in chronostratigraphic research at present. Therefore, the National Stratigraphic Committee initiated the construction of China Strata, and requested that the China Strata be revised, improved and newly built, and the relevant stratums and correlation work should be done well. One of its tasks is "research on Donggangling".

The typical areas of Donggang Ridge are Xiangzhou and Wuxuan in China and Guangxi. Since Le Sen-xun named the carbonate deposits of high-yield brachiopod fauna as "Donggangling Formation" on 1928, there has been no unified definition of bottom boundary and boundary stratotype, and even later generations attributed the underlying persistent sections (groups) to Donggangling Stage, while others did not, resulting in the Donggangling Stage of Xiangzhou Donggangling Section. This division is not uniform, and it is more complicated in atypical areas. Unfortunately, until now, Stringocephalus has not been found in the marl section at the bottom of Donggangling Formation in Xiangzhou and Wuxuan areas, and conodonts and slub fossils have not been analyzed, which brings difficulties to the boundary division, boundary stratotype and comparison of different fossils in Donggangling period, and correspondingly brings certain difficulties to stratigraphic correlation and lithofacies paleogeography research in different facies areas.

Liu Jing, Hengxian County, Guangxi is one of the classic areas in the study of Devonian in South China, in which Tang Min Formation of Middle Devonian has both benthos (Wang Yu, Yu Changmin, Fang Dawei, 1965) and plankton (Mu Daocheng,1978; Bai Shunliang et al., 1982), which provides a very favorable condition for studying the definition, boundary stratotype and location of Donggang Ridge. 1985, Su Yibao and Wang Chengyuan discovered five conodonts in the Tang Min Formation in this area: (1) Tortodus kockelianus kockelianus belt; (2) Polygonatum odoratum belt; (3) the spiny snapper belt; (4) CRI state zone of schmid-Togner Qiusi-Herman-Spinnert Qiusi; (5) gypsy moth area. In 1992, Bai Shunliang et al. studied the biostratigraphy of this group in more detail, in which the conodont zone added the Polygnathus hemiansatus zone between the above-mentioned areas 2 and 3. Sun Yuanlin (1992) pointed out that brachiopod fauna existed at the bottom of Tang Min Formation, and Stringo-cephalus appeared. 1999 sun yuanlin and A.J.Boucot further believe that brachiopod Stringocephalus burtini evolved from S.gubiensis (sun yuanlin et al., 1999). Anoxic otomari event layers were found in Sihongshan section in Du 'an, Debao, Guangxi and Liu Jing section in Hengxian. It is located between the toothed thorn belt and the spiny bamboo belt, rich in bamboo stone Nowa-kia otomari, and produces brachiopod Stringocephalus gubiensis.

Over the years, many geologists in China have made achievements in the study of Middle Devonian strata, especially the National Stratigraphic Committee (2002) first put forward the definition that "the bottom boundary of Donggangling stage is marked by the first appearance of brachiopod Stringocephalus". The Devonian branch of the International Stratigraphic Committee identified the boundary of the Givitian Stage in the Middle Devonian as the first appearance of 1992, which laid a practical and theoretical foundation for studying the boundary division and stratotype points of the Donggangling Stage. On the basis of biostratigraphy, event stratigraphy and sequence stratigraphy, the definition and boundary stratotype of the bottom boundary of Donggangling stage are put forward.

1 Donggangling and the origin of its name

The Donggangling Terrace was established by Wang Yu and Yu Changmin in 1962, and evolved from the Donggangling Formation (1928), representing the late Middle Devonian strata. Named after Haijiaoling (also known as Donggangling) in the east of Xiangzhou County, Guangxi. The top and bottom of Donggangling Formation, a lithostratigraphic unit in this area, are complete, the underlying stratum is (Duan) Formation, and the overlying stratum is Liujiang Formation.

2 Definition of the bottom boundary of Donggangling terrace

As can be seen from the above description, there are three schemes to choose from. One is that the brachiopod Stringocephalusgubiensis first appeared before S.burtini (figure 1). Polygonatum odoratum conodont is a transitional type evolved from Polygonatum odoratum, which has a downward inclined outer front abutment edge to form a. Secondly, spiny teeth began to appear; Finally, there is the bottom of the knotted stone Nowakia otomari belt, which marks the beginning of the otomari incident. The aforementioned Stringocephalus is also included in the interlayer of the otomari event layer. Comparatively speaking, the author chooses the latter.

This paper suggests that the bottom line of Donggang Ridge should be the first appearance of Nowaki-aotomari.

The advantages of definition are:

(1) The slubby stone Nowakia otomari is distributed in a wide range of basin facies siliceous argillaceous, carbonate deposits and platform margin facies carbonate strata; There are many individuals, and the genus and species are easy to identify.

(2) 2) The appearance of Nowakia oto mari, that is, the beginning of the Otomari event (Walliser, 1985, 1995), has the isochronous significance of global events.

(3) Conodont Polygnathus hemiansatus and brachiopod Stringo-cephalus gubiensis first appeared, which are close to the bottom boundary of the event layer where Nowakia otomari first appeared, and can be used as auxiliary signs to identify the underground boundary of Donggang Ridge.

(4) The appearance of brachiopods and their extensions are all contained in Donggang Ridge.

Figure 1 shows the evolution of Schistosoma guppiensis to Schistosoma brucei.

(5) From the point of sequence stratigraphy, the bottom of marl layer at the bottom of Donggangling Formation in Xiangzhou, Guangxi can be compared with the bottom boundary of the strata in Nowakia otomari belt or Strin-gocephalus gubiensis belt, which is consistent with the traditional Donggangling terrace.

(6) The rock features near the boundary of Donggangling terrace are obvious, which is easy to be judged by field work and beneficial to stratigraphic correlation in different facies areas.

3. Reference section of Donggangling boundary stratum

The Middle Devonian in Liu Jing, Hengxian County, Guangxi and Sihongshan, Du 'an, Debao County represent two different sedimentary types, with complete sections and abundant fossils, which are convenient for observing geological phenomena and collecting samples. The traffic is very convenient, and you can drive directly to the profile, which is beneficial to the later research work. It is suggested that the stratigraphic sections of these two places should be selected as reference sections for the boundary stratotype and boundary points of the Middle Devonian Donggangling Stage in China.

3. 1 Section of Middle Devonian Ping 'en Formation in Sihongshan, Du 'an, Debao, Guangxi (slightly modified according to Guangxi Bureau of Geology and Mineral Resources 1982; Wang Chengyuan's Identification of the Tooth Fossil System)

3. 1. 1 geographical location

Sihongshan section is located on the main road from Debao to Jingxi (2317 ′ 27 ″ north latitude and 0/06 29 ′ 40 ″ east longitude), and it is 8 km away from Debao/KLOC-0, so it can be reached by car (Figure 2).

Fig. 2 Section of Ping 'en Formation in Sihongshan, Du 'an, Debao, Guangxi (part)

3. 1.2 personal data deduction

Research Report on Major Dating Strata in China (2006 ~ 2009)

Research Report on Major Dating Strata in China (2006 ~ 2009)

3. 1.3 Underground Boundary Point of Donggangling Terrace

The conodont Tortodus kockelianus kockelianus located at the top of the 26th layer of Pinggen Formation in the above section. 27-storey slubby stone Nowakia otomari；; According to the 28-layer conodont Polygnathus hemianstus determined by Bultynck (Bultynck, 1987) (Figure 3), the contact point between the 26-layer argillaceous banded limestone and the 27-layer siliceous mudstone is flat, and the bottom of the 27-layer is the bottom boundary point of the Donggangling terrace, which is the beginning of the otomari event (Figure 4).

Fig. 3 Rock characteristics of the 28th layer and the generated penumbra.

Fig. 4 The bottom of the 27th floor is the bottom boundary point of Donggang Lingtai.

The underground boundary point of the Donggangling Stage is located at the lithologic interface, which is not only distributed in the Sihongshan section in Du 'an, Debao, but also in many other areas, such as the Sanchahe section in Napo, Guangxi, where Nowakia otomari is siliceous and argillaceous, and its underlying is limestone with thin-medium thickness. In huangjiang, mangchang and luofu sections of nandan, N.otomari is black mudstone, and the underlying stratum is sandstone. The bottom of Donggangling Formation or Tangjiawan Formation in Xiangzhou and Guilin is roughly equivalent to the marl layer first appeared by N.otomari, and its underlying stratum is mudstone layer; And Liu Jing in Hengxian, Pohong and Shanglin in Tian Yang. , can be identified by lithology. This shows that it is also one of the feasible methods to determine the underground boundary of Donggang ridge terrace and refer to the rock characteristics.

3.2 Profile of Middle Devonian Tang Min Formation in Liu Jing County, Guangxi (slightly modified according to Sun Yuanlin 1992).

3.2. 1 section position

This profile is located in the hill (22 52 ′ north latitude and 65 438+008 52 ′ east longitude) east of Liu Jing, Hengxian County, Guangxi, and can be directly driven to the profile point (Figures 5 and 6).

Fig. 5 Schematic diagram of the profile position of Tang Min Formation in Liu Jing County, Guangxi.

Fig. 6 Section (part) of Tang Min Formation in Liu Jing County, Guangxi.

Brief description

Research Report on Major Dating Strata in China (2006 ~ 2009)

Research Report on Major Dating Strata in China (2006 ~ 2009)

3.2.3 Bottom Boundary Point of Ridge Steps in Donggang

The conodont of 1 layer in the above profile is Tortodus kockelianus kockelianus; Two-layer slubby stone nowakia cf. otomari; ; In the third layer, there are ancient brachiopods and long-fin herring → semi-long-fin herring; Nowakia otomari is common above the fourth floor. Because the two layers of bioclastic limestone containing iron and mud have obvious recrystallization, it is difficult to accurately identify the slubby stone. However, according to the dentate spines in 1 and 3a layers, on top of the fourth layer, Nowakia otomari is included, and its thickness is less than 1 m, so it can be said that the second layer is the first layer of Nowakia oto mari. The lithology of layer 2 to layer 3b can be determined as otomari event layer. Therefore, the bottom boundary point of Donggang Lingtai is set at the bottom of the second floor, that is, the bottom of otomari movable floor (Figure 7 ~ Figure 9).

Fig. 7 Far view point of bottom boundary line of Donggangling platform (bottom of floor 2)

Fig. 8 otomari event layers (layers 2 to 3b)

Fig. 9 Boundary line point at the bottom of Donggangling terrace (bottom of the second floor)

Because brachiopods Stringocephalus gubiensis and bamboo stone Nowaki otomari appeared almost at the same time, it can be used as an auxiliary sign to identify the underground boundary of Donggang Ridge in the field.

4 Donggang Ridge Cascade Comparison

4. 1 Donggangling biostratigraphy

At the end of the early Middle Devonian, the sea level in South China and even the whole world changed dramatically. With the fluctuation of sea level, the living space and related environment of organisms have changed, which has led to a biological event, such as obvious species changes of bed corals, four corals and brachiopods (Liao, Ma, 2008). Therefore, the biological features of the late Middle Devonian (Donggangling stage) are different from those of the early Middle Devonian. The fossils of benthos and plankton in the Donggangling period are very rich. Through years of research by paleontologists, we have learned their temporal and spatial changes in detail and gradually established fossil belts (assemblages). The list of fossil belts (assemblages) of the main fossil categories currently selected is summarized as follows (Table 1, Table 2). It is only a supplement: the profile shows that the ancient brachiopod fish belt has almost the same duration as the conodont half-jaw multi-jaw fish belt; The appearance of haddock began in the lower spiny snapper zone. Accordingly, the pomfret can be divided into lower pomfret belt and upper pomfret belt.

Table 1 Devonian Donggangling Zooplankton Fossil Belt (Combination) Comparison Table

Table 2 Comparison Table of Zoobenthos Fossil Zones (Assemblages) in Donggangling Stage of Middle Devonian

The ancient oriole is characterized by a nearly circular outline, a flat lens in side view, and a protruding and straight belly edge; Oblique); Have a straight tilt; The fleshy stem hole is large, oval, extending backward, and the back shell has a lower middle partition, the height of which does not exceed the base of the hinge plate; Adult specimens have high, bifurcated main protrusions at the ends and are inserted by high abdominal septa; However, in young specimens, the main protrusions are a pair of separated rod-shaped protrusions. Compared with Stringocephalus burtini, the outline is similar, but Stringocephalus burtini has a thick side view, a lenticular shape and a slightly curved belly beak, so the back is slightly forward.

4.2 Main lithostratigraphic units

Due to the influence of Haikou movement, regression began in the early Middle Devonian. During the early and late period of this period, sea level decline caused false conformity contact between lithostratigraphic units in South China, and clastic strata with coastal facies characteristics appeared. Since the late Middle Devonian (Donggangling period), the sea level has risen rapidly, lithofacies has re-differentiated, and many sedimentary types have been found. Subsequently, the names of lithostratigraphic units with regional characteristics were established in various places. In recent years, various provinces (autonomous regions) have made a systematic summary, and monographs have been published, so they will not be described one by one. At present, only figure 10 is used to illustrate the general law of lithostratigraphic unit relationship in Donggangling period of Middle Devonian in South China.

Fig. 10 schematic diagram of late middle Devonian lithostratigraphic unit combination sequence in different facies areas of Guangxi.

4.3 Sequence stratigraphy

During the Middle Devonian Donggangling period, the distribution of sedimentary facies along the ancient land from the continental margin to the center of the basin was always coastal clastic facies, platform facies, platform margin facies, slope facies, basin facies and isolated platform facies in the basin. At the same time, strata with different sedimentary types provide strong support for the division of sequence stratigraphy and the establishment of chronostratigraphic and lithostratigraphic framework. The strata in Donggangling period can be divided into three third-order sequences. When the bottom boundary of the first sequence entered the early and late Middle Devonian, the bottom boundary was type I sequence boundary (figure 1 1, figure 12), and the second and third sequences were type II sequence boundary (figure 13, figure 14).

The third sequence extends to the early Late Devonian, and the top surface is also a type II sequence interface (Figure 15a, b).

Figure 1 1 The underlying stratum and overlying stratum of the Shiqiao sandstone underwater fan of the Russell Huohong Formation in Luodian, Guizhou are black mudstone (hyperopia).

Figure 12 Characteristics of Underwater Fan of Dashan Sandstone in Mangchang, Nandan, Guangxi

Figure 13 The outcrop characteristics of dolomite at the top of the first sequence of Donggangling period in Tangjiawan Formation, Di Yang, Yangshuo, Guangxi.

Figure 14 The boundary between Tangjiawan Formation and Baqi Formation in Di Yang, Yangshuo, Guangxi, is the interface between the second sequence and the third sequence.

Figure 15a Characteristics of upward shallowing of the top stratum of the third sequence in Donggangling period of Baishazhai Formation in Yangshuo, Guangxi.

Figure 15b The boundary between Baqi Formation and Liujiang Formation in Di Yang, Yangshuo, Guangxi, is also the top boundary of the third sequence in Donggangling period (where the hammer is located).

4.4 Comparison with international chronostratigraphy

The Devonian branch of the International Stratigraphic Committee defined the bottom boundary of the Givi stage of the Middle Devonian as the first appearance of the conodont Polygnathus hemiansatus, while the bottom boundary of the Donggangling stage was the first appearance of the bamboo stone Nowakia otomari. The difference between the two is that the otomari event layer of the former is below it, and the otomari event layer of the latter is above it. Nowakia otomari appeared earlier than Po.hemiansatus, only 1 ~ 3 m away from the formation thickness.

4.5 Geological event analysis

An important sedimentary event can be identified in the Donggangling period, which belongs to a global event, namely the otomari event (Wallser, 1987). Otomari event is an anoxic event, which has obvious characteristics at the peak of Liu Jing section (layer 2 to layer 3b) in Hengxian, Guangxi. The lithology is mainly bioclastic limestone containing iron and mud, containing tumor-like stones, which often turn into reddish-brown mud after weathering. Observing under the rock microscope, it is obvious that calcite is self-shape-semi-self-shape, rhomboid is often wrapped by a layer of limonite, and a few limonite occurs in granular form, which may be pyrite pseudocrystals. It can be seen that limonite is filled inward from the outer wall along the knobby stones (Figure16). In the Sihongshan section of Du 'an, Debao, Guangxi, the otomari event layer consists of siliceous mudstone and siliceous rocks. The Luofu and Mangchang sections in Nandan, Guangxi, and the Wuxiangling section in Nanning, Guangxi, and the Otomari event layer are composed of black mudstone; The otomari event layer in Daxin and Longan of Guangxi is composed of thin microcrystalline limestone mixed with siliceous rocks, and the overlying strata and underlying strata are medium-thick biogenic limestone and sandy limestone. The slope flood in Tian Yang, Guangxi is composed of thin carbonaceous microcrystalline limestone and brachiopod crustacean limestone, which is sandwiched between algal laminae and algal laminae limestone in the lower slope flood of Tangjiawan Formation (Figure 17a, b).

Figure 16 bioclastic limestone containing iron and mud

Figure 17a Characteristics of algal laminae-algal laminae limestone profile in low slope flood of Middle Devonian Tangjiawan Formation in Tian Yang, Guangxi.

Figure 17b Arrows and hammers show carbonaceous microcrystalline limestone and brachiopod crustacean microcrystalline limestone.

This event was caused by large-scale and rapid sea level rise, and it was the largest flood period in sequence stratigraphy. The event occurred between Tortodus kockelianus kockelianus Polygnathus hemiansatus belts, which was consistent with the global otomari event.

Thanks to the author's enthusiastic guidance, help and relevant information during my work and during the writing of this article, which contributed to the completion of this article. I would like to express my heartfelt thanks here.

Take the exam and contribute.

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