Upper Cambrian (Hibiscus) Outward Period ()

Parallelism is a new stage in the upper CAMBRIAN in the world. In recent 10 years, the division of stratigraphic units in CAMBRIAN era has changed greatly. For more than a hundred years, the division of CAMBRIAN units at home and abroad has always adopted a three-point scheme, namely, lower, middle and upper series; The division of ranks has never been carried out systematically before. In China, the division scheme of 10 rank was gradually established and improved from the early 1960s to the late 1970s, and it has been used ever since. However, these steps are basically established in the platform facies area, defined by unit stratotype, and the bottom boundary is unclear. Moreover, the biostratigraphic sequence based on unit stratotype is mainly local trilobites, and the comparability between large areas and continents is not good. In view of this, in order to establish an internationally comparable chronostratigraphic unit of CAMBRIAN, the team led by the expert Peng Shanchi who participated in this project began to turn its attention to the study of CAMBRIAN in slope facies in the 1980s. After nearly 20 years of research, 29 planktonic trilobite fossil belts and 8 conodont belts have been systematically established. On this basis, in 2000, Pengshanchi formally put forward the scheme of dividing the Cambrian into 4 series and 9 orders, and revised and supplemented this scheme. Around 2002, he put forward four series, 10 projects, and recommended them to the international stratigraphic commission. 65438+At the end of February, 2004, the Cambrian branch of the International Stratigraphic Commission voted by a high vote (88% in favor). This plan officially entered the international stratigraphic table, thus ending the three-point plan of the world CAMBRIAN for more than 100 years.

The parallelism stage was put forward under this background. On the basis of many years' important research progress, Peng Shanchi and others officially named the Quaternary Cambrian in the world "Furong System" and the lowest order "Paibi Stage" (formerly called Waergang Stage) when they announced that the boundary stratotype section of the Quaternary Cambrian in China and its lower order were global stratotype sections and pointed to the Cambrian Branch of the International Stratigraphic Commission (GSSP). In August 2003, the International Union of Geological Sciences officially approved this proposal. As a result, the Furong System and Paibi Stage in China officially entered the international stratigraphic table, and the boundary stratotype profile of Paibi Stage became the global standard stratotype profile and point (GSSP), which is the third "golden nail" obtained by China.

Paibi stage, the name of the stage comes from the name of the location of this class profile, that is, Paibi Township, Huayuan County, Xiangxi. Stratigraphic and boundary stratotype profiles are located near Sixin Village, northwest of Paibi Township, Huayuan County, western Hunan Province, with geographical coordinates of 28 23.37 ′ north latitude and109 31.54 ′ east longitude (Figure 3).

The bottom boundary of "Furong System" and "Paibi Stage" is located in the 37th layer of the upper part of Huaqiao Formation in Paibi section. The total thickness of Huaqiao Formation measured on the wall profile is 388. 5m (top accident), which is divided into 39 floors. The lower part of 1 25m is mainly dark gray thin-medium dolomite, calcareous mudstone, striped micrite limestone mixed with gray-dark gray thin-medium silty limestone, and there are usually1layered gravelly limestone upward, with an interval of about 10m, and the thickness of a single layer is 8-30 cm. Then, the interlayer of gravelly limestone gradually increases, and the thickness of its single layer also increases from the bottom of the profile.

The bottom boundary of "Furong System" and "Paibi Stage" is located in the upper 37th layer of this section. Due to the detailed profile measurement, only the paleontological fossils contained in the upper and lower strata near the boundary are described below (Figure 4).

Fig. 3 Traffic Location Map of Layered Section of Row-wall Boundary Line

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

① The meter distance between the collected fossil horizon and the bottom boundary of Huaqiao Formation is 378. 25, the field sample collection number of fossils is in brackets, in which HP, Gs and Gr are trilobite sample numbers, and PC is conodont sample number, the same below.

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

Fig. 4 Stratigraphic stratification and vertical distribution of trilobite fossils near the bottom boundary of the terrace.

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

The bottom boundary point of the parapet terrace (that is, the bottom boundary of the Furong series) is located at the bottom boundary of the parapet section 369. Marked by the first appearance of the spherical trilobite netted bird, this demarcation point is in the continuous series evolution from the spherical trilobite netted bird to the netted bird.

According to the analysis of the obtained paleontological fossil data, three trilobite fossil belts can be established from bottom to top: Glyptagnostus retic-ulatus belt, Initagnostus Inexpextans-Proceptopyge Prouta belt and Corynexochus Plumula-Sinotopygecf belt. Jiangxi zone. The above-mentioned zone fossils have been recognized as important molecules with wide international correlation significance by the Cambrian branch of the International Stratigraphic Commission.

According to the study of carbon and oxygen isotopes, the bottom boundary of the Paibi stage coincides with the beginning of the Cambrian δ 13C positive shift (that is, the step-to-a positive carbon isotope shift), and its peak value lies between the first appearance point of reticulate algae and the first appearance point of trilobite Irvingella, and δ 13C reaches +4‰. The location of this peak in North America roughly corresponds to the peak of biodiversity in the pterodactyl biota of Laurentian, and also corresponds to an important sea level drop represented by the Souk Ⅱ-Sauk Ⅲ discontinuity of Laurentian. This drift is obvious not only in great basin, USA, but also in the strata in the early hibiscus period, including the slope facies area in southern China, Kyrshabakty in Karatau, Kazakhstan and the corresponding strata in Queensland, Australia. (Figure 5)

Fig. 5 Comprehensive Diagram of Primary and Secondary Identification Signs at the Bottom Boundary of Paibi Order

The wall exclusion period is equivalent to the lower part to the middle part of the Changshan period of the Upper Cambrian in the traditional Cambrian chronostratigraphic system established in China platform area.