A brief history of the research on the generalized Yixian formation in western Liaoning

Grabau( 1923) first named the sedimentary interlayer between the upper and lower porphyrites (which may be called intermediate-acid volcanic rocks now) near Lingyuan (then Jehol Province) in western Liaoning as the Jehol system. In fact, it is what we now call the Daxin house of Yixian Group. Later, the name of the Jehol system gradually evolved into jehol group (Gu Zhiwei, 1962), which included several stratigraphic units.

A real member of Yixian Formation comes from "Yixian Volcanic Rock" built by Japanese scholars (1940) near Yixian. At that time, it only represented a set of volcanic rocks above ancient gneiss or granite and below Mount Kumgang, which is now known as the narrow Yixian Formation. The Paleontology Department of Beijing Institute of Geology (1960) used Yixian Formation (generalized) for the first time in internal scientific research reports. 1959 At the first national stratigraphic conference, most people called "Yixian volcanic rocks" Yixian Formation, which was officially published by Gu Zhiwei on 1962. It should also be mentioned that Mi Jiarong and others (1964) called it Sunjialiang Formation, which originated from the "Sunjialiang Volcanic Rock Layer" in Lin Chaobai (1942) and was established near Sunjialiang Village in Heichengzi Basin of Beipiao. Because it is two years later than the first name of Yixian formation, it is generally considered to be a late synonymous name belonging to Yixian formation, and it has been stopped.

Compared with the generalized Yixian Formation, from the 1960s to the end of 1980s (Table 3- 1- 1), the Liaoning regional survey team put forward a three-point plan for Yixian Formation, Mount Kumgang Formation, Alkaline Field Formation (or Tuhulu Formation) in the western Liaoning regional survey map (1∶ 200,000), resulting in Although there are many schemes for comparison with Yixian Formation in western Liaoning, these group names have been used for a long time.

The Yixian Formation in a broad sense includes the above groups, which were mainly proposed by Chen Piji and Wang Wuli respectively. Chen Piji and others, on the basis of the study of the late Mesozoic continental strata in western Liaoning in 1960s, classified the fossil-bearing sedimentary interlayer of Yixian Formation in Beipiao and Yixian areas as 1980 from bottom to top, and classified the original Huanghuashan Formation (1976) into the generalized Yixian Formation. At the same time, on the basis of a new study of Late Mesozoic strata in western Liaoning in 1970s, Wang et al. named the volcanic rock series above Tuchengzi Formation or Zhangjiakou Formation and below Jiufotang Formation as Yixian Formation in the paper of the Second National Stratigraphic Conference (1979), including three sedimentary interlayers: Daobazi (Jianshangou), Zhuanchengzi and Jingangshan. It was officially confirmed in 1985, 1987, 1989. At the same time, it is considered that the Yixian Formation on both sides of the Daling River can be completely compared, and the so-called "Yushugou Formation" is actually Yixian Formation. In Jinlingsi-Yangshan, Dachengzi, Jianchang and Lingyuan-Sanjiazi basins, the Jingangshan Formation divided by1∶ 200,000 regional adjustment teams (1.965 and 1.976) is actually a sedimentary rock interlayer in the lower part of Yixian Formation, and the Jianchang Formation on it should be in the middle and upper part of Yixian Formation. Since then, researchers using the generalized Yixian group have gradually increased (Table 3- 1- 1).

Table 3- 1- 1 Brief Table of New Division and Evolution of Yixian Formation in Western Liaoning

From the evolution table 3- 1- 1, it is not difficult to see that the 1980s was an era of cross-use and preliminary research between the narrow sense Yixian Formation and the broad sense Yixian Formation.

Since the 1990s, especially in recent years, early birds, hairy and partly feathered theropods, primitive mammals and angiosperm fossils have been found in Sihetun, Huangbanjigou and Jianshangou of Beiyuan. This series of new discoveries involves many important issues in the history of life development, such as birds, mammals, bird flight, the origin and early evolution of angiosperms, which has aroused fierce debates among colleagues with different views in the world and has become the focus of world paleontology and news media for a time.

With the discovery of a large number of rare animal and plant fossils, the increase of horizons and the continuous expansion of regions, it has become an urgent task for stratigraphic research to find out the exact source, horizon, age and correlation of these fossils. However, most of these fossils occur in different sedimentary rock interlayers of volcanic rocks in Yixian Formation, so the Yixian assemblage in a broad sense is the main target layer for research. In 1997, Ren Dong et al. (1997) integrated Yixian Formation and Yixian Basin in Beipiao Basin into a stratigraphic column section, and divided into six lithologic sections, of which 1 ~ 3 was taken from Beipiao area, and the typical section of Mashenmiao-Songbahu Yixian Formation in Yixian area was taken as 4 ~ 6 sections and placed in Beipiao area. In 2000, in the stratigraphic division and comparison between western Liaoning and Inner Mongolia, Wang Xiaolin and others compared the Hugou stratum in Ningcheng Road, which belongs to the Haifanggou Formation of the Middle Jurassic, with the Jianshan stratum in the lower part of Yixian Formation in Beipiao area, and placed the Daxinfangzi stratum of Yixian Formation in Lingyuan on the Jianshan stratum. In addition, Ji Qiang (1999), Lu Chonghai (2000), Li Yousheng (2000) and Sun Ge (200 1) established Yaolugou Formation, Wei Jialing Formation, Chaomidianzi Formation and Jianshangou Formation in Jianchang Basin respectively. 200 1, Li Peixian and others made a new division and comparative study on the Yixian Formation in Beipiao and Yixian Basin, and proposed for the first time that the Zhangjiakou Formation in northern Hebei exists in the lower part of Yixian Formation in both places (namely Wangjiatun Formation in this paper), and there may be Dabeigou Formation above Zhangjiakou Formation. At the same time, they compared Jianshangou Formation in the lower part of Yixian Formation in Beipiao area with Jingangshan Formation in the upper part of Yixian Formation in Yixian area.

It should also be mentioned that there is another view that the northern and southern strata of the Daling River cannot be compared. Pan Guang (198 1) proposed that the volcanic rock of Yixian Formation in the north of Daling River is another new volcanic rock horizon, named Yushugou Formation; Yixian Formation to the south of Daling River belongs to Lan Qi Formation, and Jiufotang Formation above it is regarded as the phase transition of Tuchengzi Formation, and Yixian Formation scattered to the east of Yixian area is also the "Yushugou Formation". On this basis, Chang et al. (1999) think that Yushugou Formation is a new horizon between Jiufotang Formation and Shahai Formation, and that the northern and southern strata of Daling River cannot be correlated.

This round of stratigraphic research has been further improved, which has two remarkable characteristics; First, paleontologists in various scientific research units are the main body, and most of them focus on biostratigraphy, which has accumulated unprecedented rich biological fossil data and greatly improved the level of biostratigraphy; Second, the research team with the team of the Ministry of Geology and Minerals as the main body, especially the new round of1∶ 50,000 regional survey, mainly focused on lithostratigraphic research, carried out multiple stratigraphic division and correlation, provided a large number of lithostratigraphic data, and further improved the level of lithostratigraphic research. However, there are still some problems, that is, the former is rough and insufficient in the study of lithostratigraphy of Yixian Formation, especially in the division and correlation of strata; The latter is not enough in biostratigraphic data.