1 School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083.

2 State Key Laboratory of Paleobiology and Stratigraphy, CAS, Nanjing 210008. E–mail: suwenbo@cugb.edu.cn

3 Yichang Institute of Geology and Mineral Resources, CGS, Yichang 443003.

4 Beijing Natural History Museum, Beijing 100050.

5 Faculty of Earth Sciences, China University of Geosciences (Wuhan), Wuhan 430074.

6 Test Center of China University of Geosciences (Wuhan), Wuhan 430074.

Key words: K-bentonites. Ordovician-Silurian boundary. Yangtze Platform. Correlation. Tectonomagmatology.

Ordovician–Silurian K–bentonite beds have long been studied with particular attention around the world. Since the 1980’s K–bentonite bed research has made significant achievement, including Ordovician–Silurian event stratigraphy, chronostratigraphy, reconstruction of paleocontinents and faunal evolution. They are however almost all developed in Europe and the Americas (e.g., Bergström et al., 1989,1998; Huff et al., 1992, 1995, 1998; Hayes, 1994; Kolata et al., 1996, 1998, 2001; Peralta et al., 1999; Cingolani et al, 2000).

In China, Ordovician volcanic deposits have been noted since the 1980’s (Fu, 1983; Wang et al., 1983; Ross et al., 1984; Huang et al., 1991; Huff et al., 1995). Recently, Chen et al., (2000) identified two bentonite beds at different levels in Guizhou, South China.

Recently, we have discovered numerous K–bentonite beds in many Ordovician–Silurian Boundary sections in South China and have studied their tectonomagmatic and regional stratigraphic character. In this short paper, we will present some preliminary findings and show the outline of current progress.

In the Yangtze Platform of South China, generally the uppermost Ordovician lithostratigraphic unit is the Wufeng Formation including two members (Wang et al., 1996), the lower Graptolite–bearing Shale Member and the upper Guanyinqiao Member containing Hirnantia Fauna. It is overlain by the lowest Silurian Longmaxi Formation, for which the basal unit is also graptolite–bearing shale. Three Ordovician–Silurian Boundary sections of this type are presented, which represent the upper, middle and lower Yangtze Platform in the area, respectively. The volcanic layers are numbered in ascending order as WHKB, NKB and GKB in each of the three sections (Figure 1).

Figure 1. Integrated stratigraphic correlation of Ordovician–Silurian Boundary K–bentonites in Yangtze Platform. 1. K–bentonite bed; 2.siliceous shale; 3.carbonaceous shale; 4.calcareous argillite; 5.marl; 6.bioclastic marl; 7.limestone; 8.carbonaceous argillite; 9.silty argillite; 10.nodule marl; 11.bioclastic limestone; 12.marl/siliceous lens; 13.shell/coral fossil; 14. parasequence set; OSq: Ordovician sequence; SSq: Silurian sequence. Sequence stratigraphy after Su (1999) and Su et al. (2002); Chrono– and bio–stratigraphy after Rong et al. (1999) and Chen et al. (2000).

At Huanghuachang, 17 bentonite–like layers in the lower and middle part of Wufeng Formation can be observed. Meanwhile, at Wangjiawan, the leading candidate section of the global stratotype for the Hirnantian Substage proposed by Rong et al. (1999), 9 layers were discovered in the upper Wufeng and the lowest Longmaxi Formations. The two sections 15 km apart could be joined up very well based both on precise biostratigraphy (Wang et al., 1983; Chen et al., 2000) and the characteristics of the clay layers (Figure 1, section 1).

The Honghuayuan section recently studied by Chen et al. (2000) is located at Shanwangmiao, Honghuayuan, Tongzi, Guizhou and has been proposed as the auxiliary stratotype section for the Hirnantian Substage by Rong et al. (1999). Originally it was quite a good section but has now been turned into hilly farmland and covered slightly (see the figure of Chen et al., 2000). Thus we had to find a new section at Nanbazi, about 600 m to the southeast of Shanwangmiao. With the well–exposed outcrop in this section, 25 yellowish– white clay layers were also discovered (Figure 1, section 2).

Figure 2. The Ga–Zr/TiO2 diagram (after Winchester and Floyd, 1977) of the bentonite beds.

The Gaojiabian section is located at the foot of Lunshan Hill in Jurong of Jiangsu, about 60 km east of Nanjing. In fact, its lower part is the streamside section that Huang et al., studied in 1992. Recently, we re–measured it and established the upper part along a rural road about 200 m away in the west. More than 120 bentonite–like layers, 1mm to 6cm in thickness, have been discovered in this section (Figure 1, section 3). Obviously, it is quite different from either the middle or the upper Yangtze Platform and possibly suggests a location much nearer the center of the eruption.

At the outcrop section, the light color of bluish–white to grayish–white or yellowish– brown, the soapy to waxy texture as well as the fairly leveled bed surface of the K–bentonites appear in strong contrast in the generally black carbonaceous–siliceous shale succession. In thin sections of samples, many of the layers show vitroclastic or residual tuff textures, some contain very fine non–depositional quartz and feldspar grains. The X–ray diffraction analysis shows that they consist primarily of illite, smectite (montmorillonite) as well as mixed–layer illite/smectite (I/S) and that they also contain quartz, K–feldspar, albite and minor accessory phases. Preliminary rare earth element analysis suggests an origin from a potassium–rich magma. Obviously, the clay layers originated from depositional volcanic ash fall or tuff, intermediate to silicic in composition that have undergone burial metamorphism, as is typical for K–bentonite beds. Furthermore, the geochemical data plotted on the classification diagram of Winchester and Floyd (1977) shows that the parental magmas should be trachyandesite to rhyodacite with some rhyolite in composition (Figure 2). Meanwhile, the same data plotted according to the method of Pearce et al (1984) show that the parental magmas range from within–plate (WP) to volcanic–arc (VA) and syn–collision (syn–COL) tectonic settings (Figure 3).

Figure 3. The Y–Nb diagram (after Pearce et al., 1984) of the bentonite beds.

The high–resolution stratigraphic correlation of the K–bentonites between Nabazi in Guizhou and Wangjiawan–Huanghuachang in Hubei has been explored recently based mainly on biostratigraphic data (Wang et al, 1983, 1987; Chen et al, 1999, 2000) and sequence stratigraphy (Su, 1999; Su et al, 2002). With the preliminary integrated study in the Gaojiabian section, it may also be possible to determine initially the correlation between the lower Yangtze and other areas, although a few smaller intervals still need to be examined carefully (Figure 1).

The preliminary results of this research permit the most significant stratigraphic and tectonomagmatic clues as follows:

1. Many K–bentonites near the Ordovician–Silurian Boundary in South China have been discovered and their correlation through the whole Yangtze Platform has been initially determined for the first time;

2. The WP and syn–COL+VA settings of the parental magmas indicate that the K–bentonites should be associated with the vast eruptions caused by the plate/arc collision near the Yangtze Block. Considering similar deposits elsewhere in the world, it is conceivable that they are related closely with the break–up of Gondwanaland in the early Paleozoic;

3. It seems that, at least in South China, the horizon of the end–Ordovician mass–extinction phase coincided with a single, large volcanic eruption (see the WHKB19, NKB19, GKB74 in Figure1). It will be helpful to reveal deeply the coupling on the geospheres during this special period if more detailed integrated investigations can be developed.

It is supported jointly by the NSFC Grant (49802002), the SSER, and the Grant of the State Key Laboratory of Paleobiology and Stratigraphy, CAS (033101, 013101). We are thankful to Profs. Wang Hongzhen, Shi Xiaoying, Wang Xunlian, Rong Jiayu, Chen Xu, Zhan Renbin, Wang Xiaofeng, for their kind help in many ways. Thanks sincerely to Prof. Gerald R. Baum for his help in the research of sequence stratigraphy for years. We also thank Prof. Warren D. Huff for his patient and critically reviewing the manuscript. Last, we wish to express our grateful to Prof. Matilde S. Beresi and her Argentina colleagues for their kind encouragement and respectable efforts.