Shetian Bridge Stage of Upper Devonian in China ()

Shetian Bridge Stage is the lowest stage of Upper Devonian in China. The name of the step was named by Wang Yu and Yu Changmin in 1962. The standard section determined at the time of naming was located in Shetian Bridge, Shaodong County, Hunan Province, and the name of the step originated from Shetian Bridge where the named section was located. Shetian Bridge Stage is roughly equivalent to the Fratian Stage in the international stratigraphic table.

It is difficult to define the bottom boundary of the sequence because of the poor outcrop of the lower strata of the named profile, so it is not an ideal stratotype profile. After research by the research group (Gong et al. ), the candidate stratotype profile of Shetian Bridge steps was re-selected.

(1) Position of candidate stratotype section of Shetian Bridge step

The candidate stratotype profile of Tian Bridge selected by Gong et al. is located on the hillside of Sihongshan (Huangyingshan) about 800 meters east of Du 'an Town, Debao County, Guangxi Zhuang Autonomous Region, with geographical coordinates of 2319' north latitude and 0/06 35' east longitude (Figure 90). The traffic in this section is convenient, and the exposed strata include the fourth terrace of Lower Devonian and the tin terrace of Upper Devonian, among which the Shetian Bridge terrace is the most complete.

Fig. 90 Location Map of Shetian Bridge Graded Section

(2) Description of layered profile of Shetian Bridge (Figure 9 1)

Overburden of Wuzhishan Formation of Upper Devonian

Gray-purple thick limestone, marl and lentiform limestone

Progress and Achievements of Major Dating Strata Research Projects in China (200 1 ~ 2009)

Fig. 9 1 lithostratigraphic histogram, conodont zoning, cyclic strata and digital dating map of Shetianqiao step in Debao, Guangxi.

Gray thin-medium calcareous shale of Devonian Tang Min Formation in the underlying stratum.

(3) The biostratigraphic sequence of Shetian Bridge Stage and its bottom boundary definition.

At present, conodonts, bamboos, ostracods and other fossils have been found in Tianqiao stratigraphic section of Du 'an Society in Debao, Guangxi, especially conodonts, which are the most continuous and abundant, and are produced from bottom to top. Gong et al. divided the conodonts in the stratotype profile into nine conodonts from bottom to top: the upper middle falsiovalis zone, the Pal-matolepis transitans zone and the Palmatolepis puncatata zone. Lower palmar tendinous dragon belt, upper palmar tendinous dragon belt, lower palmar tendinous dragon belt, upper palmar tendinous dragon belt and lingual palmar tendinous dragon belt. Zoning sequence is a conodont, which can almost be compared with zoning sequence, an upper Devonian conodont widely used in the world at present.

With regard to the definition of the bottom boundary of Shetian Bridge, the exact definition of the bottom boundary of Shetian Bridge has not yet been determined, because the lower M falsiovalis belt marked by the first appearance of Middle Devonian falsiovalis has not been identified between the Klapperina disparalis belt at the top of Middle Devonian and the upper Middle Devonian Falsiovalis belt found at the bottom of Upper Devonian. However, according to the careful observation and analysis of sedimentary facies and cyclic strata in the interval near the boundary of stratotype profile, the research group (Gong et al. It is considered that although there is a small amount of limestone debris in the second sequence (375-6e) of the second layer of the profile, the stratum loss caused by scouring will not reach the stratum thickness of a fossil zone. Through further in-depth conodont biostratigraphy work in this interval, it is possible to find out the first appearance of the lower mesoscopic falsiovalis zone and its fossils. At present, before this problem is solved, the research team proposed that the top boundary of the sharp-toothed PARALICHTHYS olivaceus belt with conodonts Klapperina disparilis, PARALICHTHYS olivaceus, PARALICHTHYS olivaceus and Polygnathus dengleri can be temporarily used as the bottom boundary of the steps of Shetian Bridge.

(4) Digital dating of Shetian Bridge steps and their dentate spines.

In this study, Gong et al. tried to use Milankovic's periodic law to digitally date the strata with known orbit-climate change period. According to the measured data (Figure 9 1, Figure 92) and geochemical analysis of large-scale profiles (1∶ 10) in Du 'an, Guangxi, it is found that the sequence structure of stratigraphic cycle is one of the best alternative indicators to reflect Milankovic cycle. Stratigraphic cycle is the product of many factors, and the main controlling factors of high frequency cycle with a period less than one million years are usually climate and sea level change. The key criteria to prove whether the high-frequency cycle is driven by Milankovic cycle force are: the time period and order structure of the high-frequency cycle are consistent with Milankovic cycle (time standard); High-frequency cycles are comparable across stages or regions (spatial standard). Within the framework of fine biostratigraphy and/or isotopic chronostratigraphy, through the careful study of sequence structure and pedigree comparison of high-frequency cycles, we can obtain an important criterion whether high-frequency cycles are driven by Milankovic cycle force in terms of time constraints. The spatial standard can be obtained by comparing the high-frequency cycle and its sequence structure on the basis of fine biostratigraphy (or isotopic chronostratigraphy) in different regions or facies areas.

Based on the detailed study of Du 'an, Debao Sihongshan profile and Guilin Di Yang profile, five-level cyclic stratigraphic units composed of laminae, sequences, sequence groups, supersequence groups and system tracts are identified in the carbonate platform-basin facies cycle strata of Upper Devonian. Among them, the outline of the sequence group is the clearest, intuitive and stable. The so-called stripes are mainly displayed by differences in lithology, color and organic matter content, and their thickness is generally between 1 mm and several centimeters. Sequence refers to the basic cycle unit consisting of two or more layers of strata with different lithology and different colors, which are produced in pairs, with one layer as the main one and the other as the auxiliary one. Sequence group refers to the natural combination of multiple identical or similar sequences, and its top and bottom boundaries are usually natural single-layer planes or series interfaces. Supersequence group is a natural combination of multiple sequence groups with the same or similar lithology, structure, thickness and occurrence. The interface between supersequence groups is usually the transformation surface of sequence groups (sequence structure) or a larger, clearer and more intuitive level than the interface between sequence groups. On the profile, three kinds of supersequence groups with accretion, progradation and retrogression structures can be identified. Supersequence groups usually consist of four or three sequence groups.

Through systematic conodont biostratigraphy and related research, 177 sequences, 48 sequence groups and 12 supersequence groups, with a thickness of 989, have been identified from the Klapperina disparilis belt at the top of the Middle Devonian Donggangling step and the Palmatolepis triangularis belt at the lower part of the Upper Devonian Xikuangshan step in the basin limestone of the Sihongshan section in Du 'an, Debao. 5 centimeters. There are 265,438+0 sequence groups and 6. Five supersequence groups with a thickness of 1568 cm have been identified from the upper Palmatolepis rhenana belt of Shetian Bridge Stage of Upper Devonian to the Palmatolepis triangularis belt of Xikuangshan Stage.

Fig.92 Conodonts zonation, Cyclic Strata and Digital Dating of Shetian Bridge in Di Yang, Guilin, Guangxi.

By comparing Figure 9 1 with Figure 92, it can be clearly seen that among the studied 12 conodont belts, the conodont belts containing the most sequence groups are Lower Palmatolepis rhenana belt and P. linguiformis belt (8 belts); The conodont belt with the least sequence group is the P. jamieae belt (2 belts). In the same conodont zone of two sections, namely, upper rhenana zone, upper P. linguiformis zone and lower, middle and upper P. triangularis zone, there are sequence groups and supersequence groups with the same number and hierarchical structure, that is, a sequence group contains three or six sequences, and the hierarchical structure of sequence groups and sequences is 1: 3 or 65438+. Below the Shetianqiao-Xikuangshan boundary, a supersequence group consists of four sequence groups, and the sequence structure between them is1∶ 4; Above the Shetianqiao-Xikuangshan boundary, a supersequence group consists of three sequence groups, and the sequence structure between them is 1∶3. It shows that there are obvious differences in sequence structure between supersequence group and sequence group above and below the secondary boundary.

According to Belka et al. (1997), the same conodont zone should be isochronous in a biogeographic region, so sequence groups and supersequence groups with the same number and sequence structure in this conodont zone should also be isochronous in different sections. According to the internationally published Devonian chronostratigraphic sequence and its digital dating data, the average time limit for unifying a standard conodont belt in the upper Devonian is 0. 47 mA or 0. 64Ma。 On the profile of Guangxi, the number of sequence groups contained in a standard conodont zone varies from 2 to 8 (Figure 9 1, Figure 92). Therefore, Gong et al. think that the duration of 1 sequence group should be 654.38+ten thousand years. At the same time, Belka et al. (1977) studied the quantitative biostratigraphy and sedimentary facies of the Middle Devonian Eifelian conodonts in Morocco, and pointed out that the sedimentary rate of basin limestone was 2. 5 m/Ma, that is, 25 cm/ 100000a, is in the alternate environment of carbonate platform and basin (very similar to the Devonian paleogeographic pattern in South China). Ratio (2 1. 1 cm) and the estimated average thickness and duration (100000 a) of 48 sequence groups (basin facies limestone) in Du 'an section of Debao, Guangxi (21.100000a).

Gong et al. especially quoted Berger et al. (1992, 1994) for astronomical calculation results of orbital forces in geological history. Records show that the ratio of eccentric period to inclined period in Devonian is 1: 3. 1, and the ratio of eccentric period to precession period is 1: 5. The ratio of long eccentric period to eccentric period is 1∶4. This proportional relationship is very consistent with the sequence structure of 1∶3 or 1∶6 between sequence groups and supersequence groups in two research sections in Guangxi, and with the sequence structure of 1∶4 between supersequence groups and sequence groups in the above-ground rhesus monkey belt and the long-toothed monkey belt in Shetianqiao terrace.

Gong et al. think that the above research results (Figure 9 1, Figure 92) can be used for the digital dating of the upper Devonian Shetian Bridge Step and the 12 conodont belt from the upper middle Devonian fal-siovalis belt to the upper Palmatolepis triangularis belt (Figure 9 1 ~ Figure 93), as well as the Shetian Bridge Step. Figure 93 shows that in a time span of 5. The fastest time for the emergence of new conodonts is in the lower, middle and upper triangular conodont belts behind the boundary between the Setianqiao terrace and the tin terrace, and it only takes 0. 2Ma for a new species to appear. 3 ma; The slowest time for new species to appear is tongue-shaped tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth Tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth tooth According to the appearance speed of new conodonts, the biological evolution process from Shetianqiao period to Xikuangshan period can be divided into background period, mass extinction period and recovery period, and the appearance rate of new species is 2 ~ 3 /Ma respectively. 1 ~ 2/ ma and 3 ~ 4/ ma. At present, experts have different understandings of this division. However, the data obtained from existing studies show that the slowest period of new species is consistent with the peak period of mass extinction.

(5) Strata correlation of Shetian Bridge Stage

Other fossil belts or standard fossils roughly in the same period as Shetian Bridge Stage are: the ammonite Manticoceras belt; Ostracoda assemblage Bertil-lonella cicatrocosa belt → B. reichi belt → Protozoa striata belt → E. splerdens belt; Cyrto-spirifer-Tenticospirifer assemblage zone of platform brachiopod; Coralline column algae-Tabulupyllum assemblage zone; Micro-vertebrate Huaningthysoma-lobes → Phoebodus AFF. fastigutus→p . digul us； Wood moth, wood moth, wood moth; Bryozoa → bryozoa composite belt in the hole; Spore redeposition zone; Ancient plants such as Lepidoptera rhombifolia-ancient fern belt, etc.

The lithostratigraphic units that are roughly the same as Shetian Bridge Stage are Guangxi Liujiang Formation, Gubi Formation and Guilin Formation, and the lower part is Da Liujiang Formation and Da Rongxian Formation. Shetianqiao Formation in Hunan and northern Guangdong; Lower part of Pengzhong Formation, Guanshan Formation and Leigutai Formation in Jiangxi; Lower part of Wu Tong Formation in Anhui and Zhejiang; Tuqiaozi Formation-Shawozi Formation in Longmenshan, Sichuan, Zoige Formation in Gansu and Mokuohe Formation in Diebu; Wangchengpo Formation, Dushan, Guizhou; Tianwa Formation, Longyan, Fujian; Wang Guan Gou Formation in the Western Zhou Dynasty; The lower part of Shazijing Formation in Lijiang, Yunnan, the lower part of Zaige Formation in East Yunnan and the lower part of Guangguang Formation in Southwest Yunnan; Huangjiadeng Formation in western Hunan and Hubei; Lengshuihe Formation in the border area of Shaanxi and Hubei; Dahelihe Formation, Handaqi District, Heilongjiang Province; Cailunguoshao Formation in central Inner Mongolia and Xiadaminshan Formation in Wuer area; Lower part of Zhongning Formation in Zhongwei area of Ningxia; Lower part of Shaliushui Group in Qilian Mountain, Gansu Province; Heishangou Formation in the southern margin of Qaidam Basin in Qinghai and the lower part of Yakushan Formation in the northern margin; Lower part of Ke 'ankuduke Formation in East Junggar, Kaxiong Formation and Zhulumute Formation in West Junggar, Xinjiang; The middle part of the meandering slope group in the Himalayas and the lower part of the New Territories Group in the New Territories of Hong Kong.

Fig. 93 Comparison of digital dating and conodont evolution rate of Shetian Bridge Stage and its conodont zone in Upper Devonian.