First, the characteristics of stratigraphic regionalization
The study area is located at the junction of Hunan and Hubei, which belongs to the southeast edge of the upper Yangtze platform and has typical passive continental margin characteristics (Wang Hongzhen, 1978, 198 1,1982; Lai Caigen et al., 1980,1982; Wang Hongzhen,1985; Wang Hongzhen et al., 1986,1990; Zhou Mingkui et al.,1992; Liu Bao et al., 1993) (see figure 1). According to the principles of sedimentary type, biological characteristics, sedimentary thickness, sequence structure and top-bottom interface characteristics (Wang Hongzhen, 1978), the study area can be divided into the following three stratigraphic types from north to south (Hunan Regional Adjustment Team,1986; Zeng qingluan et al., 1987). Its general characteristics are as follows:
(1) is generally along the north (northwest) of the first line of Taoyuan Reshi-Cili Longtan River-Jishou, and the lithology and lithofacies are similar to Yichang in the east of Xiadong. The lower part of Ordovician is pure carbonate rock with a small amount of shale; The upper part is carbonate rock, containing more argillaceous rocks, as well as carbonaceous siliceous graptolite shale. The top layer is Guanyin Bridge. Near Cili, the top of Ordovician-the bottom of Silurian is mostly missing to varying degrees. The biota are mainly trilobites, cephalopods and brachiopods with graptolites in the middle. The overall thickness is 300 ~ 400 meters, which belongs to the sedimentary environment of platform facies area with stable basement, namely Yangtze area.
(2) Represented by Jiuxi-Huangshi and Cili Chenjiahe in Taoyuan, it basically extends from northeast to southwest along the southern slope of Wuling Mountain. The Ordovician sedimentary thickness in this area is relatively large (700 ~ 1000 m). Its lower stratum, from Lianghekou period to Guniutan period, is dominated by argillaceous carbonate rocks, mixed with mud-rock flow deposits such as multi-layer carbonate breccia, and gradually transits to argillaceous-silty deposits. Upper Ordovician, Miaopo period to Wufeng period, similar to Yangtze region, is composed of argillaceous carbonate rocks and carbonaceous graptolite shale, with Guanyin Bridge layer at the top. The biota is mainly Yangtze type and Jiangnan type, which reflects a sedimentary environment of platform margin slope with active sedimentary basement and large subsidence, and belongs to the transitional zone between Yangtze area and Jiangnan area (Wulingshan community).
(3) The Ordovician is a set of siliceous argillaceous, carbonaceous argillaceous and silty plate shale with a small thickness (300 m), and the middle and upper parts are manganese-containing carbonate rocks and near-source turbidites, represented by Xiangtaoyuan-Maobaozi area in Taojiang. Cambrian and Silurian are continuously deposited at the top and bottom of the reservoir, and graptolite is the main biota. Compared with the first two areas, it is generally characterized by a relatively hungry deep slope-basin sedimentary background far from carbonate platforms. This area belongs to the transition zone between Yangtze and South China, commonly known as Jiangnan District (Xuefeng District).
Second, stratigraphic division and correlation
The Xia Dong-Northwest Hunan region on the southeast edge of the Upper Yangtze Platform is one of the classic study areas of Ordovician in South China. The study of stratigraphy can be traced back to the early 1920s and 1930s. Li Siguang (1924), Tian (1933), Wang Yu (1938), (194 1) and other geoscientists have done a lot of pioneering research in this field. Since the founding of New China, many scholars have done many aspects and levels of work here. Such as, Mu Enzhi (1954), (1957, 1962, 1982), Jin (1952) 1989), Antai 'ao. Subordinate units of Hubei Bureau of Geology and Mineral Resources and Hunan Bureau of Geology and Mineral Resources have carried out geological mapping and special research in this area, such as the Three Gorges Strata Research Group of Hubei Bureau of Geology and Mineral Resources, Yichang Institute of Geology and Mineral Resources of the former Ministry of Geology and Mineral Resources, and regional dispatching teams of Hubei and Hunan provinces. After decades of accumulation, biostratigraphy and related research in this field have reached a higher level. Huanghuachang section in Yichang has become the designated stratotype section of Ordovician in China (Lai Caigen et al.,1982; Wang Xiaofeng et al., 1987).
This paper basically follows the existing stratigraphic system in this area (table 1- 1). Ordovician chronostratigraphy, especially stratigraphy, is synthesized according to the division of Lai Caigen et al. (1982) and Wang Xiaofeng and Chen Xu et al. (1996). Graptolite belt and conodont belt refer to Antaiyao (1987), Ni (1987), (1993), Wang Xiaofeng, (1996) and (1996) respectively. Cambrian-Ordovician is temporarily bounded by the bottom of Cordylodus lindstromi belt, and Ordovician-Silurian is temporarily bounded by the bottom of Glyptograptus persculptus belt (Wang Xiaofeng et al., 1987, 1992). The data of Harland et al. (1989), Wang Hongzhen, Li Guangcen (1990) and Wang Hongzhen (1996) are used for the ages of series and step boundaries respectively. The lithostratigraphic division is basically based on Zeng Qingluan et al. (1987), Hunan Dispatching Team (1986) and Wang Xiaofeng and Chen Xu et al. (1996), but this time Datianba Formation and Serenwan Formation were divided in northwest Hunan, and the boundaries of lithostratigraphic units such as Taohuashi Formation were redefined from the perspective of sequence stratigraphy.
Table 1- 1 Ordovician multiple stratigraphic division and correlation table in the southeastern margin of the upper Yangtze River
Three. Sedimentary environment and paleogeographic evolution
The existing research shows that during Ordovician, the Yangtze platform was generally located in the low latitude area of the southern hemisphere near Gondwana continent (Wang Hongzhen et al.,1985; Wang Xiaofeng et al.,1987; Chen, Rong,1992; Liu Baojun et al.,1993; Wang and Chen (1995) made the Ordovician strata in the Yangtze platform mainly carbonate rocks. However, due to the fact that the late paleolatitude is slightly south than the early one (Wang Hongzhen,1985; Wang Xiaofeng et al.,1987; Chen, Rong,1992; Liu Baojun et al.,1993; Wang Hechen, 1995), so it is more influenced by Gondwana continental glaciers and deep cold water background. In the Late Ordovician, this platform showed deep cold water characteristics different from those in the early warm shallow water (Rong Jiayu,1984; Chen,1984; Chen Xu et al.,1986; Rong Jiayu et al.,1987; Zeng qingluan,1991; Lu Hao et al., 1996). However, from western Hubei to central Hunan, from the interior of carbonate platform through its edge to the shelf slope and sedimentary facies area of the basin, the succession of sedimentary environment in this area has become more complicated and diverse in time and space (Hunan Regional Adjustment Team,1986; Zhou Mingkui et al.,1993; Liu Baojun et al.,1993; Wang Xiaofeng et al., 1996). Nevertheless, in general, the study area and the upper Yangtze platform can still be divided into two main sedimentary stages in Ordovician, namely:
1. Normal warm shallow water carbonate platform stage
The time limit of this stage is Lianghekou period-Honghuayuan period. They basically inherited the sedimentary background and pattern since the end of Sinian. At this stage, the study area can be divided into the following three facies areas:
(1) Platform facies area: the vast area from the north of the first line of the Longtan River in Cili to the hot market in Taoyuan, including the eastern part of Xia Dong;
(2) Platform marginal slope facies area: represented by Jiuxi-Huangshi in Taoyuan and Chenjiahe in Cili, a narrow strip extending northeast-southwest along the southern slope of Wuling Mountain;
(3) Deep-water shelf (slope)-basin facies area: represented by Taojiang Xiangtaoyuan-Anhua Maopu Belt, distributed southeast along the southern foot of Xuefeng Mountain.
2. The stage of submerged deep cold water carbonate platform
This stage started from the Dawan period and ended at the end of the Five Peaks. At this stage, the first two facies areas are basically merged into one facies area, and the nodular marl and atrophic marl with deep-water biota are generally developed, showing a typical wide shelf sedimentary environment, but the third facies area still exists. This stage can be divided into two secondary stages: Guniutan stage and Miaopo stage. Before and after the comparison, the latter developed black siliceous carbonaceous graptolite shale representing the background of stagnation and reduction, reflecting that the sedimentary environment has changed greatly.
"The structure controls the basin and the basin controls the sedimentation" (Wang Hongzhen, 1992). Next, the author will roughly reveal the sedimentary environment and paleogeographic changes in the study area by calculating the slope of the ancient slope and the subsidence rate of the sedimentary basement structure.
Middleton and Hampton( 1973) used a lot of detailed practical data to summarize the relationship between the thickness of debris flow in the stratum and the angle of ancient slope, and put forward the following empirical formula:
Tcrit=(K+σntanφ)/ρ 1gsinθ
Where Tcrit is the thickness of clastic flow sedimentary rock, k is the static strength of clastic flow, σn is the normal sedimentary static strength, φ is the friction angle, θ is the slope angle, and ρ 1 is the wetting density of clastic flow. For wetted loose sedimentary rocks, since φ is close to zero (Schofild and Worth, 1968), the above formula can be simplified as:
Tcrit=K/ρ 1gsinθ
And according to D=8.8K/g△ρ, k can be obtained. Where d is the largest spherical gravel diameter that gravity flow can carry, g is the acceleration of gravity, and△ρ is the density difference between debris flow and matrix, so
sinθ=D △ρ/8.8ρ 1Tcrit
For the gravity flow supported by particles, ρ is almost equal to the density of limestone, that is, it is equal to 2.73g/cm3, and ρ ρ can be approximately regarded as 1.73g/cm3, so
sinθ= 0.072d/t critθ= arcsin(0.072d/t crit)
According to this formula, according to the data of the largest equiaxed or nearly equiaxed gravel size in debris flow deposition in the study area, the slope and the static strength of debris flow are estimated, and the results are shown in Table 1-2:
Table 1-2 Table of slope gradient and static strength of debris flow of Ordovician in the study area
Note: HJ is Jiuxi section, HH is Huangshi town section of Taoyuan, and HC is Chenjiahe section of Cili. O 1p is panjiazui formation, and O 1m is madaoyu formation. These three sections belong to Wulingshan Community. HX is the section of Taoyuan (Nanshichong) in Taojiang Township, and O2n is the Nanshichong Formation, which belongs to central Hunan.
As can be seen from the above table:
(1) The slope of the Ordovician ancient slope in the study area is 0. 12 ~ 1.40. They are included in the slope angle range observed in modern times (0. 1 ~ 6.5)(Embrey,1976; Jacobi, 1976), and Gupojiao (0.28 ~ 2.49; 1.07 ~ 2.35), which is consistent on the whole.
(2) The static intensity of debris flow in the study area ranges from102 to104 Pa. This is consistent with the theory of a. m. Johnson( 1970) about the intensity of modern surface debris flow (102 ~ 104 Pa) and Liu Baojun (1990) about the static intensity of CAMBRIAN debris flow in Hunan and Guizhou (102 ~/.
(3) If there is no big deviation in the measured values (it is not excluded that the diameter of the largest equiaxed gravel that can be seen may become smaller due to the limitation of outcrop area, etc. ), it seems that the slope angle of the ancient slope in Jiuxi area of northern Hunan in early Ordovician is larger than that in Taoyuan area of Hunan in late Ordovician. At the same time, according to the current slope calculation value and referring to the ancient water depth (near the intertidal zone) in Maocaopu, a hot city in northern Hunan, the "ancient water depth" of ancient slopes in Jiuxi area in northern Hunan and Taoyuan area in central Hunan can be estimated in an ideal state. The former is mostly 100 ~ 200m, and the latter is about 350~700m. This also explains the problem from another angle: the former belongs to the front slope of carbonate platform, and the latter may belong to the gentle slope belt of outer shelf or basin facies area (Wang Hongzhen,1985; Hunan District Dispatching Team,1986; Zhou Mingkui et al.,1993; Liu Baojun et al., 1993). The former may correspond to the Great Barrier Reef and the edge of the Bahamas platform in the modern tropical-subtropical ocean, while the latter may correspond to the outer side of the East China Sea and the Yellow Sea shelf. At the same time, it also shows that the previous (Gao, 1995) understanding that "the Lower Ordovician has isochronous deposits" in Jiuxi area is questionable, at least it is debatable.
It can be seen that the sedimentary basement in the southeast margin of the upper Yangtze platform from northwest to central Hunan has gradually changed from the front slope of the carbonate platform with a slightly steep slope to the gentle slope of the outer shelf or basin facies area with a relatively gentle slope, which basically inherits the features of the Sinian system and the Cambrian period (Liu Baojun,1991; Liu Baojun et al., 1993). However, due to the submerged carbonate platform (Liu Baojun et al., 1993) and the sedimentary filling after the Hongyuan period, the front slope of the carbonate platform with a slightly steep slope has been transformed into a part of the gentle slope of the shelf. That is, since the Dawan period, the sedimentary basement environment in the study area has changed to some extent.
Von BuBunov (1954) first used the time-sedimentary thickness curve, that is, the average subsidence rate, to represent the subsidence history of sedimentary basins. Although the accuracy and numerical value are slightly lower than the current "stripping method", the final trend is basically the same as the latter (Liu Baojun et al., 1993). Therefore, in the absence of porosity, compaction ratio and other parameters, people can still directly use the measured formation thickness and refer to some boundary ages to get this value. The following figure shows the settlement curve of the Ordovician basement in the study area (figure 1- 1) drawn by the author according to the measured data of four main sections of the Ordovician in the study area and referring to the existing age of the Ordovician in each stage.
Figure 1- 1 Comparison of Ordovician basement settlement curves in the study area
I-Taojiang Xiang Tao Garden; Ⅱ—Yichang Huanghuachang; Ⅱ —— Taoyuan Hot City —— Maocaopu; Ⅳ—Jiuxi in Taoyuan
The following features can be seen from the figure:
1. Difference of basement settlement rate in different areas
On the whole, the area with the largest and fastest subsidence is Wulingshan community represented by Jiuxi section, that is, the slope area on the edge of the platform. Secondly, the Bamianshan community where the Reshi-Maocaopu section is located belongs to the platform facies area, but it is close to the front slope of the platform, which is roughly equivalent to the hinge zone. Thirdly, the eastern part of the canyon where the Huanghuachang section is located belongs to the internal phase area of the platform. The area with the smallest and slowest subsidence is the central Hunan area where the Xiangtaoyuan section is located, which belongs to the slope-basin facies area of the outer shelf. This shows that the formation of facies areas and the division of stratigraphic areas are actually determined by the stability of sedimentary basement first.
2. The evolution of basement subsidence in different areas has gone through several stages.
(1) Lianghekou-Honghuayuan period: it belongs to the intense subsidence stage. The settlement rate varies from 4 to 25 m/ma, and the order is Jiuxi > Reshi > Huanghuachang > Xiangtaoyuan. It reflects that the upper Yangtze platform and its edge where the study area is located may be in a state of thermal subsidence, extension or rift (Miall,1990; Ainse Le,1992; Liu Baojun et al., 1993), and may eventually lead to the whole platform area and carbonate platform being submerged (Liu Baojun et al., 1993). During this period, not only the typical flysch turbidite (Hunan Diaodui, 1986) appeared in the slope (Jiuxi area in northwest Hunan) and basin facies area (Xinhua area in central Hunan), but also the gravity flow accumulation of carbonate breccia (Lei et al., 1996) appeared in the eastern part of the canyon in the platform internal facies area.
(2) Dawan-Guniutan stage: it belongs to weak subsidence stage, and the subsidence rate is reduced to 1.9 ~ 7.3m/ma, which is much smaller than the previous stage in general, but the order has changed, and it is Jiuxi > Xiangtaoyuan > Huanghuachang > Hot Market. Among them, the speed of the first two is very close, but the settlement of Xiang Tao Garden exceeds the previous period. After that, the amplitudes of both are much lower than those in the previous period. It reflects the thermal subsidence, extension or rift state of the upper Yangtze platform and its edge, which is much weaker than the previous period and may be adjusted. Therefore, the upper Yangtze platform and its margin were exposed to sea level in its final stage and suffered from different degrees of erosion (Wang Xiaofeng et al., 1996).
(3) Miaopo-Linxiang stage: extremely weak subsidence stage. The settlement rate is very low, 0.7 ~ 1.2m/ma. The four areas are very similar, but the Jiuxi section is slightly smaller. It reflects that the study area may be in a balanced state with little tectonic subsidence and overall stability, and it is likely that the sea level will rise rapidly and substantially in the early stage, resulting in anoxic events, so that its representative product, black graptolite shale, is almost all over the original facies area (Hunan Regional Investigation Team,1986; Zeng qingluan et al., 1987). Although it was improved in the later period, both the clastic coast and the carbonate platform are still far from the provenance, belonging to a relatively stable reducing sedimentary environment, which is conducive to the formation of relatively uniform nodular marl and shrinking marl (Chen Xu et al., 1986). Finally, it may be gradually exposed or close to exposure, so dolomite appears in some places (Liu Yongyao et al., 1984).
(4) Five-peak period: Generally speaking, it belongs to the weak subsidence stage, but there are great differences in different places. The sedimentation rate can be increased from 2m/Ma to 12m/Ma. Among them, the area of the hot stone area is difficult to estimate due to insufficient erosion in the later period, and the other order is Jiuxi > Xiangtaoyuan > Huanghuachang. It reflects that there may be new thermal subsidence extension in this area, such as near-source turbidite (Xu, 1980) developed in Taojiang-Anhua area in central Hunan. At the end of the period, due to the extrusion state (Liu Baojun et al., 1993), the Hunan-Hubei-Guizhou border region, represented by Reheng area, was partially uplifted and eroded (Mu Enzhi,1954; Hunan Dispatching Team, 1986).